Atlas of migmatites

Atlas of

Migmatites

About the cover: Front cover Metatex1te m1gmatite developed from a foliated granodionte show1ng leucosome 1n dilat1on structures. Laag Mountain area, Bntish Columb1a. Photograph by Paul McNeill. Rear cover D1atex1te m1gmat1te w1th schlieren, developed from a pelitic protolith. Quetico Subprov1nce. Ontano.

The Mineralogical Association of Canada gratefully acknowledges the financial contribution of rhe following organizations: Ca nadian Geological Foundation Natural Resources Canada

@

Atlas of

Migmatites Edward W. Sawy er

The Canad ian Miner alogis t Special Publica tion Mln er a logi c.aJ

Ass o ciatio n o f Can a d a

Association

min~ralogique

du canada

ritC·Cf itC NRC Research Press

9

© 2008 National Research Cou ncil of Canada All rights reser ved. o part of this publication m ay be reproduced in a retrieval system, or t ra nsmi tted by any m ea ns, electronic, mechanical, photocopyi ng, recording or otherwise, without the prior w rillen permission of the ational Resea rch Council of Ca nada, Ottawa, Ontario KIA OR6, Ca nad a. Printed in Ca nada on acid -free paper.@ ISBN 978-0-660- 19787-6 ISS 17 17-6387 N RC o. 46331

Library and Archives Canada Cataloguing in Publication Sawyer, Edward W illiam, 1951Atlas of migm atites/ E.W. Sawyer " RC Resea rch Press" Issued by: Nationa l Research Cou nci l Ca nada Co-published by Mineralogical Association of Ca nada Includes bibliographical references ISB 978-0-660-19787-6 l. M igmatite. 2. Migmatite - Pictorial works. 3. Pet rology. I. ationa l Research Council Canada II. Mineralogical Association of Canad a

Ill. Tit le. Q£475.M5S38 2008

552'.4

C2008-980022-2

NRC Monograph Publishing Program

Editor: P.B. Cavers (University of Western Onta rio) Editoria l Board: W.G.E. Caldwell, OC, FRSC (University of Western Ontario); M.E. Cannon, FCAE, FRSC (University of Ca lga ry); K.G. Davey, OC, FRSC (York Universi ty); M.M. Ferguson (University of Guelph); S. Gubins (Annual Reviews); B.K. Ha ll, FRSC (Dalhousie Un iversity); W.H. Lewis (Washington University); A.W. May, OC (Memor ial University of ewfound land); B.P. Dancik, Editor-in-Chief, RC Research Press (University of Alberta) Inquiries: Monograph Publishing Program, RC Research Press, National Research Council of Can ada, Ottawa, O ntario K IA OR6, Canada. Web site: http://pubs.nrc-cnrc.gc.ca Correct citation for this publication: Sawyer, E.W. 2008. Atlas of Migmatites. The Canadian Minera logist, Special Publication 9. N RC Research Press, Ottawa, On tario, Ca nada. 371 p.

Also in this series Encyclopedi a of Mineral Names W H . Blackburn & W H . De nnen Special Publication I ( 1997)

Glossary of Mineral Synonym s J. de Foures tier Special Publication 2 ( 1999)

Atlas of Micromorphology of Mineral Alteration and W eathering J. Delvigne Special Publication 3 ( 1998)

N ew Minerals 1995-1999 (2001) J.A. Mandarino Special Publication 4 (200 I)

The H ealth Effects of C hrysotile Asbestos: Contributio n of Science to Risk-Managem ent Decisions R.P. N o lan, A.M. Langer, M. Ross, F.J. Wick & R.F. Martin, eJ s. Special Publication 5 (200 I)

Mineral Species Discovered in Canada and Species Named after Canadians Las: l6 Ho rvath Special Publication 6 (2003)

Mineral Species First D escribed from Greenland O le V. Petersen and O le Johnsen Special Publication 8 (2005)

Available from: Mineralogical Association of Canada 490, rue de Ia Couronne Q uebec, Q C G l K 9A9 Canada www.mincralogicalassociatio n.ca

Editor, T he Canadian Mineralogist Robert E Martin

Table of Contents Preamble Preface

.................................................................................................................................................................................................................... xiii

........................................................................................................................................................................................................................... xiv

Acknowledgements Introduction

................................................................................................................................................................................... xv

............................................................................................................................................................................................................... 1

1. The scope of this atlas .................................................................................................................................................................................. 1 2. General terminology and definitions needed for work on migmatites ........................................ 2 2.1 The heritage of migmatite terminology ........................................................................................................................................... 2 2.2 A definition of migmatite .......................................................................................................................................................................... 3 2.3 Descriptive terms and definitions for the principal parts of a migmatite .................................................................... 4 Terms specific to the neosome ...................................................................................................................................................... 5 Terms for the other parts of a migmatite ................................................................................................................................. 7

3. Migmatites: the processes and morphologies ............................................................................................................... 8 3.1 The first-order morphological division of migmatites .............................................................................................................. 8 3.2 Temperatures, degree of partial melting, and fraction of melt .......................................................................................... 9 3.3 The partial-melting process ...................................................................................................................................................................... 9 3.4 A special case: melting under lithostatic stress conditions (so-called “static melting’) .................................... 10 3.5 The general case: melting under differential stress (so-called “dynamic melting”) ........................................... 10 3.6 Definitions of metatexite and diatexite .......................................................................................................................................... 12 3.7 The second-order morphological divisions in migmatites ................................................................................................... 13 Morphologies characteristic of metatexite migmatites ................................................................................................. 14 Patch migmatites ........................................................................................................................................................... 14 Dilation-structured migmatites ............................................................................................................................ 15 Net-structured migmatites ..................................................................................................................................... 15 Stromatic or layer-structured migmatites ..................................................................................................... 16 Transposition and the morphology of metatexite migmatites .................................................................... 17 Morphologies characteristic of diatexite migmatites ...................................................................................................... 17 Schollen or raft-structured migmatites ........................................................................................................... 17 Schlieric migmatites .................................................................................................................................................... 18 Diatexite migmatites ................................................................................................................................................. 18 High strain and the morphology of diatexite migmatites ............................................................................... 19

3.8 Migmatite morphologies outside the metatexite–diatexite division ............................................................................ 19 Fold-structured migmatites .................................................................................................................................... 19 Vein-structured migmatites ................................................................................................................................... 20 3.9 Descriptive terms that should be abandoned ............................................................................................................................ 20 Bedded migmatites ..................................................................................................................................................... 20 Agmatite ............................................................................................................................................................................ 20 Ptygmatic migmatites ................................................................................................................................................ 20 Ophthalmite migmatites ......................................................................................................................................... 20

4. Metasomatism and migmatites ..................................................................................................................................................... 21 4.1 Influx of aqueous fluid into hot rocks causing partial melting ........................................................................................... 21 Large-scale influx of fluid ................................................................................................................................................................. 21 Small-scale influx of fluid ................................................................................................................................................................. 22 4.2 Metasomatism and changes in the fertility of rocks ............................................................................................................... 22 4.3 Morphology of migmatites affected by infiltration metasomatism ............................................................................... 23

5. Microstructures in migmatites ...................................................................................................................................................... 23 5.1 Mineral paragenesis ..................................................................................................................................................................................... 23 5.2 Quantitative analysis .................................................................................................................................................................................. 24 The grain-contact method ............................................................................................................................................................. 24 Crystal-size distributions ................................................................................................................................................................. 25 Studies of grain size, aspect ratio, and orientation ......................................................................................................... 25 5.3 Diagnostic microstructures in migmatites .................................................................................................................................... 26 Microstructures produced in partial-melting experiments .........................................................................................26 Microstructures in the residual rocks, and evidence for partial melting ............................................................ 27 Microstructures in the melt-rich parts of migmatites; evidence for crystallization of the melt .......... 28 Magmatic and submagmatic foliations ..................................................................................................................................... 29 Melt inclusions ........................................................................................................................................................................................ 29 Cordierite–, garnet–, and orthopyroxene–quartz intergrowth microstructures ...................................... 30 Symplectitic intergrowths of quartz and plagioclase with mica ............................................................................... 31 Composition and zoning of plagioclase ................................................................................................................................... 31 Biotite composition and microstructures .............................................................................................................................. 32 Contact between leucosome and melanosome in metatexite migmatites .................................................... 33 Microstructure of schlieren in diatexite migmatites ....................................................................................................... 33 Microstructure of biotite-rich selvedges in migmatites ................................................................................................ 33

6. Whole-rock geochemistry in migmatite studies .................................................................................................... 34 6.1 A possible sequence of processes and some relevant questions .................................................................................. 34 6.2 Reference-point compositions ............................................................................................................................................................ 36 Determining protolith compositions (the starting material) ..................................................................................... 37 Determining the “melt” composition ...................................................................................................................................... 37 Residual rocks ........................................................................................................................................................................................ 40 Mineral compositions ......................................................................................................................................................................... 41 6.3 Diagrammatic representation ............................................................................................................................................................... 42 Matched triplet sets of samples .................................................................................................................................................. 43 General sets of samples ................................................................................................................................................................... 43

7. Migmatite-like rocks ..................................................................................................................................................................................... 47 7.1 Rocks formed by subsolidus segregation ....................................................................................................................................... 48 7.2 Models for the process of subsolidus segregation ................................................................................................................... 48 7.3 P–T conditions at which subsolidus segregation occurs ....................................................................................................... 48 7.4 The relationship between subsolidus segregation and migmatites ............................................................................... 49 7.5 Small-scale features of subsolidus segregations ......................................................................................................................... 49 The constituent parts ........................................................................................................................................................................ 49 Mineralogy of subsolidus segregation ...................................................................................................................................... 50 Microstructure ....................................................................................................................................................................................... 50 7.6 Outcrop-scale morphology ................................................................................................................................................................... 50 Stromatic, or layered, subsolidus segregations .......................................................................................... 50 Dilatant structures ....................................................................................................................................................... 51 Fleck structures ............................................................................................................................................................. 51 7.7 Rocks formed in syntectonic plutons and plutonic complexes ........................................................................................ 51 Syntectonic injection of magma ................................................................................................................................................... 51 Syntectonic crystallization of felsic plutonic rocks ........................................................................................................... 52 7.8 Vein complexes ............................................................................................................................................................................................... 52 7.9 Rocks formed in syntectonic plutonic and vein complexes compared with migmatites ................................. 53 Similarities ................................................................................................................................................................................................. 53 Differences ............................................................................................................................................................................................... 53

8. Working with migmatites .................................................................................................................................................................... 53 8.1 First-level map units .................................................................................................................................................................................... 54 8.2 Second-level map units ............................................................................................................................................................................ 54 8.3 Other considerations for mapping migmatites ........................................................................................................................... 55

9. Appendices .............................................................................................................................................................................................................. 56 9.1 Checklist of observations for each outcrop of migmatites ................................................................................................. 56 Observations on the neosome and paleosome ............................................................................................................... 56 Petrological observations in the study of migmatites ..................................................................................................... 57 Structural observations in the study of migmatites ......................................................................................................... 57 Way-up criteria in migmatites ....................................................................................................................................................... 57 Sampling of migmatites ...................................................................................................................................................................... 57 9.2 Glossary .............................................................................................................................................................................................................. 58

10. References .............................................................................................................................................................................................................. 62

The photographs

............................................................................................................................................................................................. 79

A. Some examples of migmatites ..................................................................................................................................................... 79 B. The parts of a migmatite ..................................................................................................................................................................... 83 Neosome and paleosome .................................................................................................................................................. 85 Neosome with leucosome and melanosome ........................................................................................................ 89 Neosome without distinct leucosome or melanosome ................................................................................. 99 Neosome in open-system migmatites ....................................................................................................................... 111 Variations within neosome .............................................................................................................................................. 120 From leucosome to leucocratic dikes in migmatites ....................................................................................... 128 Selvedges in migmatites ..................................................................................................................................................... 136

C. Metatexite and diatexite, the first-order division of migmatites ................................................... 142 Migmatites from the contact aureole of the Ballachulish Igneous Complex .................................... 144 Migmatites from the contact aureole of the Duluth Igneous Complex ............................................. 148 Upper amphibolite facies, regional migmatites from Saint-Malo, France .......................................... 150 Upper amphibolite facies, regional migmatites from the Opatica Subprovince, Quebec ....... 152 Granulite-facies, regional migmatites from the Ashuanipi Subprovince, Quebec ........................ 154

D. Second-order morphologies in migmatites ............................................................................................................... 156 The start of partial melting ............................................................................................................................................. 159 Metatexite migmatites with a patch structure .................................................................................................... 165 Metatexite migmatites with a nebulitic structure ............................................................................................. 169 Metatexite migmatites with leucosome in dilatant structures ................................................................... 171 Metatexite migmatites with a net structure .......................................................................................................... 181 Metatexite migmatites with a layered or stromatic structure associated with low strain ..... 190 Metatexite migmatites with layered or stromatic structure due to transposition ....................... 194 The transition from metatexite to diatexite migmatites .............................................................................. 201 Diatexite migmatites with schollen and with schlieren structures ......................................................... 207 Diatexite migmatites with schlieren structures .................................................................................................. 213 Diatexite migmatites ............................................................................................................................................................ 217 Diatexite migmatites at high strains .......................................................................................................................... 223

E. Other morphologies of migmatite ........................................................................................................................................ 225 Syn-anatectic folding: fold structures in migmatites ....................................................................................... 226 Migmatites with a vein structure ............................................................................................................................... 236

F. Microstructures characteristic of migmatites ......................................................................................................... 242 Results from quenched deformation-melting experiments: a starting point .................................. 246 Subsurface contact-aureoles: the Glenmore plug, Scotland ..................................................................... 250 Erupted, partially melted xenoliths: El Joyazo, Spain ....................................................................................... 252 Subsurface contact-aureoles: the Rum Igneous Complex, Scotland ................................................... 256 Shallow contact-aureoles: the Traigh Bhàn na Sgùrra sill on Mull, Scotland .................................... 258 Shallow- to medium-depth contact-aureoles: the Duluth Igneous Complex ................................. 262 Deeper contact-aureoles: the Ballachulish Igneous Complex, Scotland ............................................. 274 Regional migmatite terranes: the Ashuanipi Subprovince .......................................................................... 278 Regional migmatite terranes: the Opatica Subprovince ............................................................................... 282 Microstructures in residual rocks ............................................................................................................................. 284 Crystallization-induced microstructures in the melt-derived parts of migmatites: leucosome and leucocratic veins ........................................................................................................................... 292 Crystallization-induced microstructures in the melt-rich parts of migmatites: diatexite migmatites ...................................................................................................................................................... 305 Microstructures formed by flow in diatexite migmatites ............................................................................. 312 Inclusions of melt quenched to glass in minerals ............................................................................................... 316 Cordierite–, garnet–, and orthopyroxene–quartz intergrowth microstructures ....................... 320 Biotite–quartz and biotite–plagioclase symplectitic intergrowth microstructures ..................... 324 Biotite–sillimanite and biotite aggregates replacing garnet or cordierite .......................................... 328 Plagioclase .................................................................................................................................................................................. 329 Contact between the leucosome and melanosome in metatexite migmatites ............................ 330 Microstructure of schlieren in diatexite migmatites ........................................................................................ 343 Microstructure of biotite-rich selvedges in migmatites ................................................................................. 347

G. Migmatite-like rocks ................................................................................................................................................................................ 351 Layer-parallel or stromatic subsolidus segregations ........................................................................................ 352 Fleck segregations ................................................................................................................................................................ 354 Syntectonic plutons: rocks that resemble metatexite migmatites .......................................................... 356 Syntectonic plutons: rocks that resemble diatexite migmatites ............................................................... 362 Arrays of felsic veins that look like migmatites .................................................................................................... 366

Ada~

of MlgiTl.HHcs

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Pre am ble

It is with pleasure that I mtroduce the latest addition to the list of Spec1al Publications produced by the M1neralog1cal Association of Canada. This Atlas of Migmatites contributes d1rectly to the educational mandate of our Assoc1at1on. The book deals w1th migmatites, a very widespread group of rocks, especially common in the Archean crust that makes up a major proportio n of our country. Everyone has learnt a b1t about m1gmat1tes, but by and large, the subject matter "falls between the cracks" in our university curricula. Typ1cally, an upper-level undergraduate course in metamorphic petrology does set the stage for an understanding of anatect1c react1ons in terranes that have undergone metamorphism at conditions of upper amphibolite or granulite faoes. Students 1n such a course do learn about the concept of anatex1s and dehydration-Induced melt 1ng, but really not much about the extraordinarily complex array of products of such anatex1s, the 1mportance of fract1onal crystallization of the anatectic liquid, and the fate of assemblages of residual m1nerals. Furthermore, contact-related anatexis usually is not discussed. On the other hand, students in an upper-level course 1n igneous petrology do deal w1th products of partial melting in the crust, and can speak at length about the physical propert1es of a silicate magma and the results of its fractional crystallization. But they deal w1th ready-made plutons. Such students do not develop a good understanding of the steps that precede the formation of a pluton, where small batches of leucosome coalesce and rise through a deforming mass of neosome and resister litholog1es. As IS made clear below, Professor Sawyer has remed1ed the situat1on by writing the first authoritative treatise about the petrology of m1gmatites s1nce the work of Karl Richard Mehnert ( 1913 1996), of Berlin, published 40 years ago. He has abandoned the purely descript1ve approach of Mehnert 1n favor of an openly genet1c approach: a m1gmat1te 1s a rock that is the product of partial melting. If melting can be shown not to have taken place, the rock IS simply not a mlgmatite. Furthermore, he has recogn1zed the importance of

anatectic reactions not only 1n amphibolite-facies rocks, but also in the realm of the granul1te facies. He has exam1ned key occurrences of m1gmatites throughou t the world, and presents here 1n a systematic way the results of his scrutiny on all scales. Thus, the reader IS shown photographs of key exposures of m1gmat1tes 1n the field and petrographic details in 273 photographs, each with a deta1led capt1on, supplied by the author m 68% of the cases, the rema1nder being contributed by respected colleagues also active in the charactenzat1on of m1gmat1t1c assemblages. F1eld examples are presented from 12 countries, with the author personally involved 1n field studies 1n s1x of those. In th1s atlas, the author emphasizes the latest contributions in the area of migmat1te-related research, e1ther 1n a reg1onal settmg or 1n a contact aureole. Society-run, not-for-pr ofit organizations like the Mineralogical Assoc1ation of Canada find 1t very challenging to undertake such a publicat1on, ow1ng to the h1gh costs of product1on. In this 1nstance, the Assooat1on is most fortunate to have coproduced the volume w1th NRC Research Press. On behalf of the MAC. I thank Suzanne Kettley, Mark Bo1leau, and D1ane Candler for the1r valuable 1nput and their key roles in creating this book. In addit1on, I acknowledge the Involvement of P1errette Tremblay, Dwector of Publications of the Association, in ensuring close commumcatlons w1th N RC Research Press throughou t the preparation of this book. A lso, P1errette applied for and obta1ned funds from the Canad1an Geolog1cal Foundation and Natural Resources Canada, part of which paid for the pnnt1ng of a poster to publicize th1s book. I thank Vicki Loschiavo, who entered the ed1torial corrections on the master files of text and capt1ons as I progressed through the book. I am grateful to have had this opportun ity to help bnng th1s major contribut1on in the area of m1gmat1te research to fruition.

Robert F. Martin Editor, The Canadian Mineralogist

xiv - - - - - - - - - - - - - - - - - - - - -

Preface

Migmatites are some of the most confus1ng-look1ng, yet aesthet1cally pleas1ng rocks. The most stnk1ng cons1st of light-colored quartzofeldspath1c segregat1ons 1n a darker host, w1th the light segregat1ons show1ng a d1verse and often spectacular range of appearances, 1n some cases in swirls, 1n some others cross-cutt1ng, and in yet others discrete, or diffuse, or folded, or conta1n1ng high-grade metamorphic minerals like garnet and pyroxene. Migmatites justify their age-old name; they are "mixed rocks." Migmatites commonly look "squishy." This is no coincidence, for they are interpreted to be rocks frozen in the act of part1ally melting. A mixture of melt, disaggregated minerals, and unmelted rock is mechanically highly heterogeneous. It 1S no wonder that m1gmat1tes show such a w1de range of textures and structures. M1gmat1tes are of crucial 1mportance 1n understanding the genes1s of the huge volumes of gramt1c magma that are found 1n batholiths and that make up much of the continental crust. Migmatites are widely cons1dered to be examples of assemblages where such magma was caught in the act of being generated and escaping. M1gmatites thus furnish a un1que perspective on a fundamental process 1n the evolution of the Earth.

The arnval of this Atlas of Migmatites IS very timely. It 1s the first book devoted to m1gmatltes s1nce Mehnert's 1conic 1968 text. In the 1nterven1ng 40 years. dramatic strides have been made in the understand1ng of these complex rocks. Ed Sawyer is without quest1on the world's leading expert on migmatit1c textures and structures. In th1s atlas, he provides a wide range of superb field and thin -section photographs of migmatites from around the world, both his own and those of others, each carefully described and interpreted. The photographic part of the atlas is preceded by a substantial Introductory section in which he clarifies the dizzying range of descnptive terms for migmatites, both new and old. Although migmat1tes w1ll rema1n among the most complex of rocks to understand, th1s atlas goes a long way to mak1ng them more understandable.

David R.M. Pattison Univers1ty of Calgary

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Ackn owled geme nts

I would like to start by expresstng my gratttude to Mtke Brown and Richard Whtte for thetr encouragement and thetr reviews of the text. They pointed out my errors, omissions, and tracts of muddled wnting; the changes that they suggested have resulted tn a far better text. The rematntng faults are enttrely mine. Many people took the time to send me thetr photographs of migmatites for this atlas. The choice of which ones to use was far from easy, but each photograph, whether tncluded or not, has had an influence. I thank everyone who sent me a photograph or electronic image for his/her contribution. My perceptton of m tgmatttes and what happened wtthtn them has been influenced by seeing and dtscussing mtgmatites with many col leagues from around the world. To have seen such a wide range of mtgmatttes has been tnvaluable, and I am grateful for the hospitality and generosity of all t hose who have shown me their field areas. In particular, and more or less in chronologtcal order: Ned Chown (Grenvtlle Front), Ron Vernon and Bill Collins (Wuluma and Mount Hay), Dante! Lamothe and Alatn Leclair (Ashuanipi Subprovince), Mark Severson and Steve Hauck (aureole of the Duluth Igneous Complex), Mike Brown (Satnt-Malo), Gary Solar (Matne), Abdelali Moukhstl (Eastmain area), Geoff Clarke and Richard White (Mount Stafford, Broken Hill and Wuluma), Fernando Bea (Pena

Negra), Oltvier Vanderhaeghe (Masstf Central), and Dtrk van Reenen (Ltmpopo Belt). There has always been an easy exchange of t houghts and views wit hin the migmatite community, and next I would like to thank Marian Holness, Tracy Rushmer, Bernardo Cesare, Roger Powell, Alfons Berger, and Claudio Rosenberg for illumtnating some of the connections bet ween what one can see in migmatites and the various processes that have happened to them. In most endeavors, progress comes through sustained and focused effort, together with some serendipit y, of course. My work on migmatites, and ultimately this atlas, have been no different and would not have been possible without conttnutty of fund ing. Therefore, I am grateful for the philosophy behind the discovery grant program of the Natural SCiences and Engineenng Research Council of Canada, which funds long-term, cunostty-driven research programs. Finally, I thank Pierrette Tremblay and Robert F. Marttn for the tnvttation to write a book on migmat ttes and for thetr encouragement throughout, and to Robert for all his hard work tn reading and editing.

Edward W . Sawyer Universit e du Quebec

aChicoutimi

Int ro du cti on

I

I

TH E SCOPE O F T H IS ATLA S Migmatites are spectacular. complex-looking rocks that can insp1re, fasc1nate. or confuse geolog1sts. All migmatites v1ewed in an outcrop represent the sum of a senes of processes t hat acted 1n parallel. or sequentially. and the 1nfluence of vanous local factors (see the photogr aphs in section A) . In order to begin to understand the complexity and seem1ngly endless vanety in m1gmat it es, and to prov1de the reader context for the subJect of this book, some of the key factors and processes that make individual migmatites the way they are should be outlined at the outset. First. t here IS the pet rolog1cal process of par tial melt1ng. If the tempera ture becomes sufficiently high (above about 650°C), rocks may beg1n to part1ally melt. Some rocks have compos1t1ons that will produce more melt than others at a given temperature, a propert y of the rock called fertilIt y. Thus. one can Immediately see that comple xity beg1ns at this stage. because some rocks 1n a sequence will make more melt than others; some w ill not melt. The highest tempera ture atta1ned also has an effect. If the tempera ture only just surpasses the solidus. the m1gmat1te will conta1n a few small patches of melt scattered about in t he most fertile rock. If the max1mum temperature was. say. another 250° h1gher. then melting in the fertile layers might be pervasive, and well advanced 1n other less fertile rocks. Thus, the two m1gmatites would look completely different. even if generat ed from exactly the same sequence of rocks. Second. the nature of t he proto lith has an influence too. For example, the m1gmatite produced from a sequence of rocks that contains a single t h1n fertile layer (say a pelite) enclosed in a t h1ck sequence of 1nfertile rocks (say quartzite) will look completely different from one generated from a sequence that conta1ns a thin quartzit e in a thick sequence of pelites. Similarly. a m1gmatite generated from an 1sotrop1c rock (e.g.. a gran1te) wdl not look the same as one generated from a sequence of th1n ly bedded shales and sandstones.

Th1rd. deformation dunng part1al melt1ng has a maJor effect on the morpho logy of migmatites. Only rarely (e.g.. in some contact aureoles) do migmatites form under condit1ons where t here is essentially no deformation. Generally. migmatites are deformed w h1le they cont ain melt . Deform at ion of heterogeneous rocks produces vanations 1n the different ial st ress from place to place, and this sets up pressure gradients that dnve the movement of matenal. T here are likely to be vanations 1n viscos1ty or competence from rock to rock 1n the protolit h. but once partial melt ing of t he JUxtaposed rocks starts. much larger vanat1ons are cre ated. The part 1al melt has a viscos1ty that is much lower than that of nonmelted rocks; t herefore, dunng deformation, the melt 1n the rock moves more rapidly down pressure gradients t han the solid fract1on. and collect s 1n lowpressure s1tes. D eformat1on thus creates petrolog1cal diversit y. because t he melt is separated, or segregated. from the solids. which enhances the mechan1cal ext remes 1n the m1gmat1te. Deformat1on becomes concentrated, or partitioned. int o the weakest places; these are located where there 1s melt. Consequently. stra1n becomes more heterogeneous. and the m1gmatltes develop more and more complex and deforme d geometnes. Finally. t he penod of time t hat a migmatite has had to form also exerts an 1nfluence. The heat1ng and cooling cycle in contact aureoles is rapid (days to tens of thousands of years). such that melt may quench to glass or crystallize to granophyre, and pnmary microstructures, such as the shapes of the pores that t he melt occupied. are preserved because recrystallization did not reset microstructures. Strains are small. and deformation 1s restricted to small cracks and shear zones, so that the 1n1t1al geometncal relat1ons 1n the m1gmatite are generally preserved. In contrast. t he slow heat1ng and cooling (millions to tens of m1llions of years) of reg1onal metamorphic terranes mean that melts crystall1ze slowly. and subsequent recrystallization extens1vely modifies , or even eliminates, the pnmary microstructures. Moreover. large stra1ns can accumulate over long penods of t1me. so that the orig1nal geomet ry of the m1gmat1te 1s extensively folded. boudinaged, and transposed.

INTRODU CT IO N

2 ----------------------------------

Th1s book conta1ns two parts. One IS a summary of the advances 1n the understanding of m1gmat1tes and related rocks, including partially melted xenoliths of crustal rocks, that have been made s1nce Mehnert's ( 1968) influential book. The other consists of seven senes of photographs, each with a substantial explanatory caption, that Illustrate the aspects of migmat1tes covered 1n the first part. The two parts are complementary, and in the text, I refer to the 1llustrat1ons throughout, but the two parts could also be read separately, with the text part as a genenc discusSIOn of migmatites, and the Illustrated part as a senes of

2. GENERAL TERM INOLOGY AN D D EFINITIONS N EED ED FOR WO RK ON MIGMATITES

2.1 The heritage of migmatite terminology

The debate over what m1gmat1tes represent and the processes involved 1n the1r formation has been closely connected with the prevailing v1ews of how high-grade reg1onal Sect1on 2, on th1s page. beg1ns with the term1nology assometamorphism occurs. and how gramtes and gran1tic oated w1th m1gmat1tes; the various constituent parts of magmas are formed. Hutton (1795) believed that partial m1gmatltes are dealt with first, then the divers1ty 1n outcrop melt1ng changed pelit1c sed1ments and crystalline sch1sts appearance as a function of deformation, melt fract1on. and 1nto gne1sses. Lyell ( 1855) went a step further and considparent rock-type are descnbed. The terms necessary for ered that granites form where the part1al-meltmg process the study and descript1on of m1gmat1tes are defined; text was stronger and more complete. However, the present 1S shown in bold type to 1ntroduce a term where 1t is first terminology d1d not start to develop until later. One of defined, whereas text in italics is used to ind1cate a new the first terms specific to crustal melting is diatexis, introdefinition. Next, the processes that control the morphologduced by Gurich (1905) to describe cases where "partial" ICal d1vers1ty of migmatites are considered. so as to denve melting is complete, or where 1t has occurred throughout a a class1ficat1on scheme that 1s useful as a mapp1ng tool, and rock, to lead to the format1on of granite. Sederholm ( 1907) as a means for understand1ng why migmat1tes have a parmade the obv1ous comment that 1n most cases where a ticular morphology. A two-t1ered scheme of class1ficat1on term is requwed to explain the process of mak1ng gran1te. 1s proposed. All parts of migmatites and types of m1gmafus1on was not complete; he Introduced the term anat1tes ment1oned sect1on 2 are systematically Illustrated 1n texis, meamng melting, or remelting. of rocks, to cover the sect1ons B. C. D, and E. The weight g1ven to the Imporent1re range from 1nc1p1ent to complete melt1ng. tance of metasomat1sm and the 1ngress of H 0-nch flu1ds 1n the format1on of m1gmat1tes has changed considerably In the late mneteenth and early twent1eth centuries. maps1nce m1gmat1tes were first defined; current results and ping in the Precambrian sh1elds of Fennoscandia and North th1nk1ng are summanzed 1n sect1on 4. The interpretation of Amenca revealed many places where metasedimentary microstructures has always been central to metamorphic rocks appeared to pass 1nto gran1te. These transitional petrology and to the study of m1gmat1tes 1n part1cular. In zones attracted a great deal of mterest. Not surprisingly, recent years, the repertoire of microstructures that can be there appeared 1n the literature many opinions as to what used to identify a specific process has grown cons1derably, processes were involved; each hypothesis reflected an Indiand 1ncludes some microstructures derived from exper- vidual's own field experience. Iments as well as from natural rocks; both are discussed One school of thought. wh1ch saw Intrusions of gran1t1c 1n sect1on 5 and shown 1n sect1on F. Because the acqu1s1magma as the source of heat for the transformations. t1on of chem1cal compositions of whole rocks and m1nerals 1s now a rout1ne part of reg1onal mappmg. a bnef d1scus- developed from work done at the edges of granite plutons by Barro1s (1884). Lacroix (1898, 1900). M1chei-Levy s1on of what to sample, and the use of geochem1cal data (1893), Greenly (1903), and Sederholm (1907. 1923, 1n 1dentify1ng the processes contribut1ng to m1gmat1tes. are 1926). Sederholm (1907) described the manner 1n whiCh be not should and gu1de a th1s however, 6; g1ven 1n sect1on an older, foliated gran1te underwent "refus1on" and was regarded as an exhaust1ve survey of what can be done with into 1ntrus1ve masses by the 1njection of the Hango "reborn" compos1t1onal data on rocks and mmerals. Some rock types that are not formed by part1al melt1ng can resemble m1gma- (Finland) gran1te, which 1mparted what he called a "new eruptiv1ty" to it. Because he believed that the rocks 1n the t1tes; thus, 1n section 7, I d1scuss rocks that are commonly Hango area were produced by a comb1nation of processes m1staken for migmatites; examples of these are shown 1n section G . Finally, sections 8 and 9 deal with the prob- and not s1mply by partial melt1ng. Sederholm ( 1907) introduced the term migmatite to describe them. He defined lems of mapp1ng of m1gmat1tes; top1cs range from what to m1gmat1te as "the m1xture of two genet1cally d1fferent conof interpretation the of matter tncky show on maps to the .. . one 1s intrus1ve relat1ve to the other ... To stituents "generat1ons" of leucosome.

deta1led Interpretations applied to 1ndividual migmat1tes.

Ad,,, of 1\.t.gmanrc,

---------------------------------3

th1s group belongs the gneiss1c granit1c rocks wh1ch show 'net structure' charactenst1c of 1nc1p1ent meltmg. frequently 'blind-ending veins', breccia-like granites with innumerable fragments of more-or-less completely d1ssolved older rocks, and finally some stnped gran1tes 1n wh1ch only the still preserved parallel structure ind1cates a fa1nt remnant of the original properties of the rock." Sederholm ( 1907) called the m1gmat1te-forming process palingenesis. and although 1t spec1fically 1ncluded part1al melt1ng and d1ssolut 1on, he also regarded magma inJection and its assoCiated ve1ned and brecCiated rocks as fundamental to the process. The ve1ned rocks were later def1ned by Sederholm (1923) as arterite , and the brecCiated rocks as agmatite. In subsequent work, Sederholm ( 1926) placed more emphasis on the roles of ass1milat1on and the act1ons of fluids, for wh1ch he Introduced the term ichor, 1n the formation of m1gmat1tes. An alternative view of m1gmat1tes was developed by geologists work1ng 1n reg1onal metamorphic terranes. One of them, Holmquist ( 1916). found h1gh-grade gneisses that contain many small patches and ve1ns of granit1c material. As there were no gran1tes nearby, he Interpreted the patches and ve1ns to be the collect1on s1tes for part1al melt exuded from the m1ca-rich parts of the host gneiss. Holmquist gave these m1gmat1tes the name venite to emphas1ze their 1nternal ong1n (Holmqu1st 1916. 1920. 1921) and to distmgu1sh them from Sederholm's artentes. w1th thew veins of InJected material. The absence of nearby granites indicated to him that the heat source for the part1al melting lay deeper 1n the Earth. like that 1nferred for reg1onal metamorphism 1n general. Thus, Holmquist (1916) Introduced the term ultrametamorp hism 1n order to indicate the h1gher-than-normal degree of metamorphism requ1red for part1al melt1ng (i.e., anatexis). The term metatexis was introduced later by Scheumann ( 1936) to mean the general process of partial melt1ng 1n the continental crust, and he Intended that it replace Sederholm's terms anatex1s and m1gmat1te. However, the ex1st1ng term "anatexis" was perfectly adequate, and both metatex1s and d1atex1s disappeared from use. leaving anatex1s to mean part1al melt1ng of the continental crust. The dispute over the origin of gran1te (see the discuss1ons by Read 1957) also Influenced thought on m1gmatites. Bowen's ( 1928) demonstration that a gran1t1c magma IS the product of the extreme fractional crystallization of basaltiC magma, and the subsequent find1ng that there IS far more gran1te than could poss1bly be made by th1s mechan1sm, st1mulated new 1nterest in part1al melting of the continental crust. However. the lack of a structural model to expla1n how large volumes of gran1t1c magma move in the crust led to the so-called "space problem" and to the

idea that bodies of granite form in situ. Debate then turned to whether or not granite has a magmatic ong1n, and among the nonmagmatic protagonists, to the "wet" versus "dry" granitization d1spute. S1nce m1gmatites were commonly seen 1n the field 1n an Intermediate pos1tion between reg1onal metamorphic rocks and granite, op1nion as to thew origin also diverged. The legacy of that controversy was the development of a nongenetlc descnpt1ve termmology for m1gmatites. first by Dietnch and Mehnert (1960), and finally by Mehnert (1968). S1nce the publication of Mehnert's book 1n 1968. a very large maJority of work done po1nts to the fundamental role of partial melting in the formation of migmat1tes. There are few studies of subsolidus processes in m1gmat1tes, but they show that the leucocrat1c subsolidus segregat1ons present 1n the m1gmat1tes had formed prior to the onset of partial melting (e.g.. Blom 1988). Other authors have shown that metasomatism 1s not a significant factor in the formation of m1gmat1tes. although the 1nflux of aqueous flu1ds into rocks that are close to the1r solidus temperatures may provoke partial melting and the format1on of m1gmat1tes (e.g.. Patt1son 1991, Harns et al. 2003, Johnson et al. 2003). Thus. the modern view of migmatites corresponds closely to Holmquist's concept of ultrametamorph1sm, and to Sederholm's concept of anatex1s, but IS far from the concept of palingenes1s, or the vanous metasomatiC and subsolidus processes proposed dunng the granit1zation debate. In modern usage. m1gmat1te IS synonymous w1th anatex1te. Consequently, 1t IS now h1ghly debatable whether a general defin1t1on of m1gmat1te needs to be nongenet1c; 1n th1s book. I use a genetic. partial-melting-based definition and descnptlve term1nology for m1gmat1tes. Any defin1t1on of m1gmat1te should certa1nly 1nclude reference to basic characteristics 1n the field, the most important of wh1ch are that ( I) migmatites occur only in the h1gh-grade parts of reg1onal metamorphic terranes (upper amphibolite and granulite fac1es) and contact aureoles (pyroxene and san1din1te hornfels facies), and (2) because migmatites contain parts where melt1ng occurred, parts where melt was extracted. parts through wh1ch melt m1grated. parts where melt collected, and parts that d1d not melt at all. they are morphologically complex rocks at almost every scale of observat1on. from the m1croscop1c to the macroscop1c.

2.2 A

definition of migmatite

Mehnert (1968) defined a m1gmat1te as"... a megascop1cally compos1te rock cons1sting of two or more petrographically different parts, one of which IS the country rock generally 1n a more or less metamorph1c stage. the other 1s of pegmatitiC, aplit1c, granitic or generally pluton1c appearance." For

I NTROD U CTION

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a nongenet1c defin1t1on, the use of the terms "pegmatitic", the terminology used to differentiate the old (nonpartially melted) from the newly created (t.e .. part1ally melted) parts "aplit1c", and "gran1t1c" was unfortunate, as the three carry migmatites 1s discussed. and then the terms that are used in unmistakable genetic implications. A second problem 1s describe the features and variat1ons withtn these broad to the 1mprecise refe rence to metamorphic grade. Ashworth at a smaller scale are outlined. This level of terivisions d ( 1985) rectified both of these problems in his defin1t1on of minology is, therefore. applicable to the fine-scale, detailed migmatite as "a rock fo und in medium-grade to high-grade of migmatites in a small outcrop (less than description metamorphic areas, that is pervasively inhomogeneous 2 ), or large hand-samples. m several on a macroscop1c scale , one part be1ng pale-coloured and consistently of quartzofeldspath1c or feldspath1c compoSition." Unfortunately, th1s nongenet1c definit1on does not Descriptive terms and exclude Sederholm's agmatites, rocks that Brown (1973) definitions for the principal had already argued are intrus1on brewas and not m1gmaparts of a migmatite t1tes. There is further advantage to be ga1ned by exclud1ng agmat1tes. The phrase "... two genetically different constitu- lmag1ne that one 1s able to compare a large outcrop ents ..." 1n Sederholm's ( 1907) defin1t1on of m1gmatite becomes (say 400 m') of m1gmatite w1th the rocks that ex1sted 1n Irrelevant 1f agmat1tes are excluded because for part1al the same outcrop before partial melt1ng occurred. Some melt1ng (and 1ndeed subsolidus segregation and metasoma- rocks. because they have su1table bulk compositions, will tism), the newly generated parts of a m1gmatlte ought to be have been affected by the part1al melt1ng. whereas othpetrogenetically related. Hence, the following revised definition ers, of unsuitable compositions. will not. Those rocks newly formed by part1al melting are called neosome, meaning is proposed. "new rock"; the term IS defined as follows. Migmatite: a rock found tn med1um and high-grade metamorphic areas that can be heterogeneous at the miCroNeosome: the parts of a migmat1te newly formed by, or scopic to macroscopic scale and that cons1sts of two. or more, reconstituted by, partial melting. The neosome may, or may not. petrographically different parts. One of these parts must have have undergone segregation tn wh1ch the melt and solid fracformed by partial melting and contam rocks that are petro- tions are separated. genetically related to each other (called the neosome) and to the1r protolrth through partial melt1ng or segregauon of A general charactenst1c of neosome IS that 1t has a coarser gra1n-s1ze (e.g., F1gs. Bl B4) than the rest of the m1gmathe melt from the solid fraction. The partially melted part Furthermore. the structure (e.g.. foltat1on, folds. t1te. typ1cally contams pale-colored rocks that are quartzolayenng) and microstructure (shape. s1ze, and onentat1on feldspathic, or feldspathlc, in compos1t1on. and dark-colored of gra1ns) that ex1sted pnor to part1al melt1ng are progresrocks that are ennched 1n ferromagneslan m1nerals. However. sively degraded as the degree and extent of part1al melting the partially melted part may simply have changed mmeralogy, 1ncrease. and eventually replaced by a new microstructure microstructure, and grain s1ze without developmg separate l1ght created by the neosome-form1ng processes. A neosome or dark parts. displays a wide range of morphology; as the examples in Note that the proportions are not spec1fied and that some, section B of th1s book show. the melt and sol1d fractions but not necessarily all, of the light-colored parts have to be have segregated in some (F1gs. BS B14). but not 1n others petrogenet1cally related to the other parts. There are four (F1gs. BI5-B25). The neosome 1n many m1gmat1tes 1s 1n Situ 1n found be may all not although m1gmat1te, a to parts bas1c (e.g.. the spat1ally focused neosome described by Wh1te et any part1cular m1gmatite, especially 1f the outcrop exam1ned al. 2004). but 1n migmat1tes where the melt fract1on was 1s small (less than several m ); scale IS an 1mportant factor h1gh. the neosome may have moved. If the port1ons (e.g.. 1n the study of m1gmat1tes. There are parts where part1al beds or compos1t1onal layers) of the rock from wh1ch the meltmg has occurred, parts from wh1ch the melt fract1on neosome formed can be 1dent1fied 1n the pre-m1gmat1te has been removed, parts where the melt fract1on has state. then these parts are called the protolith (Johannes collected. accumulated. or been InJected. and of course. 1985), or the pa rent rock (Ashworth 1985). Th1s means. there are parts that d1d not melt at all. Each of these parts of course, that the protolith cannot be present in a mlgmahas 1ts own w1de range of morpholog1es (or structure). tite; it will have been converted to neosome. m1neral assemblage, bulk compos1t1on. and microstructure Naming of the non-neosome parts of a migmatite has been (also called texture) ; consequently, there 1s a term1nology to by the Inconsistent use of terms, and by the use confused define and describe each. In the next section. I deal with the of inappropnate terms. Mehnert ( 1968) used the term definitions for the constituent parts of a migmat1te; these are illustrated in section B ofth1s book (F1gs. Bl B54). First. "paleosome," mean1ng "old rock" throughout the text of h1s

2.3

Ada .. of MlgmcHitC'

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book for the non-neosome part of a migmatite, but th1s usage is 1ncons1stent w1th the definition ("parent rock of a migmatite") given 1n the append1x of h1s book. To Ashworth

(1985), such a defin1tlon means that paleosome cannot be present 1n a migmatite: 1t must all have been converted to neosome by defin1t1on. Some authors (e.g., johannes and Gupta 1982) have used the term "mesosome" for the non-neosome part of a migmatite, but that IS problematical too, as the non-neosome part need not be mesocratic, and moreover, 1n many cases, the neosome has mesocratic parts. The root of the problem IS that there IS no clear opinion on which "o ld rock" IS the relevant one. If it IS the "old rock" of the pre-partial- melt1ng state. then paleosome refers to the litholog1es that will become neosome, but this usage is redundant. as adequate terms, i.e .. protolith or parent rock. already ex1st for that part. The alternative. that "old rock" refers to that part of the m1gmat1te not affected by part1al melt1ng and whiCh. therefore. contains only structures that pre-date the part1al melt1ng. IS far more useful. Hence, the follow1ng defin1t1on 1s proposed.

the development of leucocratic and melanocrat1c rocks 1n a neosome IS nearly always the result of well -defined petrological processes (e.g.. part1al melt1ng, segregation of the melt from the solid fraction, and fr act1onal crystallization) that have combined to create migmatites. this nomenclature has proven to be very useful and IS easily transferred to a genet1c scheme. The onset of partial melting changes a one-phase (solid) protolith to a two-phase (melt + solid) neosome. The melt fraction has a lower VISCOSity and density than the residual solid. and consequently, the two parts of a neosome can become separated. or segregated. The part of a m1gmatite from which the melt fraction has been removed can be defined as follows. Residuum : the part of the neosome that is predominantly the sol1d fracuon left after partial meltmg and the extraction of some, or all, of the melt fraction. Microstructures md1cotmg partial meltmg may be present.

Residuum is a general term; there is no particular reference to rock color or to m1neral assemblage. For some bulk compositions. the res1duum may be dom1nated by light-

of grams) IS e1ther unchanged, or only slightly coarsened, compared to that m s1milor rocks JUSt outs1de the reg1on affected by anatexiS.

colored m1nerals. such as feldspar or quartz; typically, because these m1nerals were so abundant 1n the protolith. they dominate the residuum as excess phases with respect to the melt-producing reaction. However, part1al melt1ng of the common crustal rock-types typically generates residua 1n which ferromagnes1an minerals are maJor constituents. Consequently. res1dua are most commonly melanocrat1c. and these are g1ven the special term melanoso me.

Paleosome ex1sts because its compos1t1on was such that 1t did not partially melt and did not become neosome (Olsen 1985); however, as some of the photographs 1n the atlas show, 1t 1s not always easy to decide what has melted and what has not. Further subd1v1S10n of the paleosome may be possible. and even useful. 1n mapp1ng some m1gmatites. If. in companng the pre- and post-partial-melt1ng states. certa1n

Melanoso me: the darker-colored port of the neosome m a m1gmat1te that 1s nch 1n dark mmerals such as b1ot1te, garnet. cord1ente. orthopyroxene. hornblende, clmopyroxene, and even oliVine. The melanosome 1s the solid, res1duol fractiOn (1.e.. residuum ) left after some. or all, of the melt fraction has been extracted. Microstructures md1cotmg port1ol meltmg may be present.

Paleosom e: the non-neosome part of a m1gmat1te that was

not affected by part1ol melting, and m wh1ch structures (such as foliations, folds. layenng) older than the partial meltmg ore preserved. The microstructure (s1ze. form, and onentatlon

litholog1es pers1st unchanged 1nto the h1ghest-grade parts of the migmat1te. then these paleosome lithologies can be called resisters (Read 1957) or r efractory. Layers of calc-silicate. quartzite, and metamaf1c rocks are common resister lithologies 1n migmatite terranes. The neosome and paleosome parts of m1gmatites are shown specifically 1n Figs. Bl B4 of section B, but as the other figures 1n sect1on B show, there IS no "typiCal" or "class1c" morphology to e1ther the neosome or the paleosome part of migmatites; their morphology is h1ghly varied.

Terms specific to the neosome The nongenet1c terminology of Mehnert (1968) IS based on changes in the relative proportions of light- to dark-colored m1nerals that occur as a m1gmat1te forms. Because

The complement to res1duum is, of course, derived from the anatectic melt. and th1s part of a neosome IS called the leucosom e. Leucosom e: the l1ghter-colored port of the neosome m a m1gmat1te. cons1stmg dommontly of feldspar and quartz. The leucosome 1s that part of the m1gmot1te denved from segregated partial melt; 1t may cont01n microstructures that ind1cote crystallization from a melt. or a magma Leucosome may not necessarily hove the composition ofon anatectiC melt; fractional crystallization and separation of the fractionated melt may hove occurred.

The size. physical form. onentation. and grain s1ze are not factors 1n determ1n1ng what IS res1duum. leucosome, or

INTRODUCT ION

6 --- -------- -------- ------ ------ -- - -

melanosome, and the photographs in sect1on B show some of the range of d1versity present 1n m1gmatites. However, these parameters should all be recorded. as they may be important in fully descnb1ng all the types of leucosome and of melanosome (or res1dua) 1n a migmat1te. and could be of s1gn1ficance 1n determ1n1ng exactly how each rock 1n a migmatite formed. The sol1d, res1dual fract1on of a neosome can, 1n most orcumstances , be regarded as being in place (i.e., tn s1tu), but the melt phase is potentially mob1le, a fact recogn1zed 1n the older literature by use of the term "mobilizate" for the melt-derived parts of a m1gmatite. Therefore, a set of terms that convey how far the fract1on of anatectic melt has moved from where 1t formed to where 1t crystallized are useful 1n mak1ng a more complete field-based description of the 1ndiv1dual products of melt1ng in a m1gmat1te. and for the subsequent understand1ng of the petrogenetiC relationships between the melt products and the rocks around them. The following terms are proposed and adopted throughout the book.

product of crystallization of on anamelt that has segregated anatectic on of port tectic melt, or at the Site where the rem01ned has (rom 1ts res1duum, but melt formed. In situ leucosome: the

In-source leucosome: the product of crystallization of on

anatectic melt, or port of on anatectiC melt, that has m1grated away (rom the place where 1t formed, but IS still w1thm the confines of 1ts source foyer. Leucocratic vein or dike: the product of crystallization of on anatectic melt, or port of on anatecuc melt, that has m1grated out of 1ts source foyer and has been tnjected mto another rock, wh1ch maybe nearby, or farther away, but IS still m the reg1on affected by the anatectic event. Granite (or granodiorite , tonalite, etc.) dike or sill : the product of crystallization of a (efs1c melt that has m1grated

out of 1ts source reg1on completely, and IS InJected mto host rocks of lower metamorphic grade or mto nonmetomorphosed rocks. Each of the four can be recogn1zed by a comb1nat1on of morpholog1cal and geochemical charactenst1cs, and by the petrogenetiC relationsh1p between the melt-denved part and 1ts host. In s1tu leucosome 1s n contact w1th 1ts own res1duum, wh1ch typ1cally forms a melanosome around it (see, for example, Figs. BS and Btl). Contacts between the leucosome and melanosome are generally diffuse on a m1llimeter to cent1meter scale. The composition of an m s1tu leucosome corresponds to an in1tial anatectic liqu1d or, if some melt was lost after crystallization had started, to

a cumulate composit ion denved from an in1t1al melt. The petrogenetic relat1onship between the leucosome and host 1s qu1te specific; the leucosome is denved from the anatectic melt with respect to which the melanosome is the res1duum. However. if some loss of melt has occurred, an excess of res1duum relative to leucosome can be expected. An in-source leucosome IS derived from melt that has moved and, therefore, may be d1scordant and have sharp borders w1th 1ts host (e.g., Fig. B27); many occurrences, however, are stromatiC (i.e., layered; see below) , and have some d1ffuse and some sharp contacts. An 1n-source leucosome may have a composition corresponding to an 1nitial anatectic melt, a cumulate or a fractionated anatectic melt. Because 1t IS denved from a melt that has moved, an 1n-source leucosome 1s detached from its own residuum, but as 1t remains 1n 1ts source layer; 1t IS hosted by another. s1m1lar res1duum (melanosome) that was formed from the same source layer. Thus, there IS a petrogenetic connection between the leucosome and 1ts host (e.g., similar Mg-number, mineral assemblage, or ISOtopic compos1t1ons). but they are not exact complements. The res1dual host may, for example, have formed from a slightly lower. or h1gher. degree of part1al melting, or may have expenenced a h1gher, or lower, degree of melt loss. Leucocrat1c ve1ns or dikes typ1cally have sharp contacts. 1rrespect1ve of whether they are discordant or concordant to the structure 1n their host (e.g., Fig. B28). There IS no dwect petrogenetiC relat1onsh1p between the leucocratic vein or dike and 1ts host rock; the age of crystall1zat1on 1n the former should be cons1stent with the metamorphic age 1n the latter. however. Leucocrat1c vems or dikes generally have compositions that ind1cate derivat1on from a fractionated anatectiC melt, but some have compositions of 1nitial melts. and others, compositions of cumulates. Gran1te d1kes typically have sharp contacts w1th the1r host and. because they were InJected 1nto cool hosts, may have fine-grained border zones (chilled marg1ns). There is no petrogenetic relat1onsh1p between a gran1te dike and 1ts country rock. The dike may be a product of crustal anatexis, or 1t may be denved from the crystallization of a felsic. 1ntermed1ate. or even mafic magma. Gran1t1c dikes commonly have a compos1t1on consistent w1th crystallization from a fract1onated melt. Well-defined bands of leucosome are very common in migmat1tes, but as the photographs 1n the book show. a leucosome may also be poorly defined and may form somewhat nebulous patches. Furthermore. 1t 1s qu1te common for several different forms of leucosome to be present 1n the same outcrop of migmatlte (e.g.. Oliver and Barr 1997). Most outcrops are relatively small, and on these. 1t may not

Aria> of Migmatites

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be possible to trace leucocratic veins, or dikes, back to t heir

Magma: a s11icate liquid that contains crystals, wh1ch might

source layers and to examine t heir relationship with 1n situ leucosome. However, the root zones of leucocratic veins

hove crystallized from the melt (llqwdus phases), be the solid products of the melting reaction (also called peritectic products by some authors), or be minerals in excess.

and dikes can be fou nd in some larger outcrops (see Figs. B43-B48), and these show a characterist ic petrological continuity from in situ leucosome t hrough in-source leucosome to the leucocratic dikes and, commonly, with a strong structural cont rol (i.e., by shear zones or folds) on the initial locat ion of the dikes. The best-known form that the residual part of a migmatite can take is the melanosome that occurs along the margin of 1n situ leucosome (e.g., Figs. BS-BI4) . Far less att ention has been paid to t he other, and far more common, morphologies that the melanosome can adopt in migmatit es; it may occur as patches, as continuous layers, and in some cases, as irregularly shaped bodies (e.g., Kenah and Hollister 1983, Sawyer 2001, Wh ite et al. 2004). Some residual rocks formed in migmatites in granulit e-facies terranes are not particularly melanocratic, although they are very w1despread (e.g., Guernina and Sawyer 2003). The neosome in many migmatites has segregated into clearly defined leucosome and melanosome. However, if the melt and residual solid fractions do not separate, leucosome and melanosome do not form, or are far less evident. For these nonsegregated examples, t he term "neosome", used alone, suffices. Examples of a neosome that has not developed leucosome and melanosome parts through segregat ion are shown in Figs. B15- B26. The recogn ition of neosome and it s leucosome and melanosome part s is, generally, a simple matter in migmatites developed from mesocratic or melanocratic protoliths (e.g., metagreywackes, metapelites, metadiorites, and metamafic rocks). It may be a much more difficult matter if the protolith is leucocratic (e.g., some granites, tonalites, trondhjemit es, and metapsammites); the dilution of the mafic minerals by feldspar and quartz means that subsequent changes in modal proportions produce only subtle changes in color, which may be difficult to detect in the field. For such rocks, distinguishing the neosome from the paleosome in the field may best be accomplished by using changes in microstructure, such as fabric or grain size, rather than changes in mineral modes or color. Examples of this approach will be found in the atlas (e.g., Figs. BIS and Bl6). Two furthe r terms specific t o the neosome remain to be defi ned. Melt: a silicate liqwd without crystals.

Terms for the other parts of a migmatite In morphologically complex migmatites, t here are, inevitably, some part s that are neit her leucocratic nor melanocratic. Mesosome has been used as a descriptive term for these, but its usefulness in any genet ic sense is very limit ed . In part, t his is because of inconsistent usage in the past. For Henkes and Johannes ( 198 1), mesosome corresponded to the protolith, whereas for Ashworth (1985) , it was the paleosome. A far more serious problem is that mesocratic rocks can occur in either the neosome or the paleosome; hence, the petrogenetic significance of mesosome is inherently less precise than that attached to leucosome or melanosome, wh ich are, by definition, par ts of the neosome. For this reason, I suggest that mesosome should not be used. Because mesocratic rocks can occur in the neosome or paleosome (or both), it is necessary to record where the particular mesocratic rock is located in the migmatite (e.g., in the paleosome, or in the neosome) . If the melt and solid fractions do not separate in a neosome. it will not develop leucosome and melanosome, and overall, the neosome wil l be mesocrat ic. However, in such cases, a coarser grain-size and a more isotropic fabric will typically have replaced the microstructure and fabric t hat pre-dated anatexis, and w ill aid in ident ifying what is neosome. As more work is done on migmatites, it is becoming clear that some example of 1n situ and in-source leucosome and many leucocratic dikes, or veins, in migmatite terranes are separated from their host rocks by a narrow rim that is compositionally, mineralogical ly, and microst ruct urally different from the host (see Figs. B49-B54). These compositional rims are not the residuum left after extraction of anatectic melt; therefore, the term used for them is se lvedge. Field observations from many migmatite terranes indicate that selvedges are most commonly developed around leucocratic veins and dikes; the injected melt thus does not seem to have been in chemical equilibrium wit h its host. Consequently, three main mechanisms have been proposed for the formation of selvedges: (I) reaction between the host rock and an aqueous nuid exsolved as the melt that produced the leucosome or leucocratic dike crystallized, (2) reaction between minerals in the adjacent wallrock and the injected anatectic melt, and (3) diffusional exchange of components between the anatect ic melt (or the crystallized leucosome or leucocratic dike) and its host in response to activity gradients.

INTRODUCTION

8 ---------------------------------

Selvedges can be leucocrat1c, mesocrat1c, or melanocrat1c. However, a very common type cons1sts of a th1n (mmwlde) b1otite-nch (or hornblende-nch 1n metamafic rocks) melanocrat1c nm adjacent to leucocrat1c dikes and some cases of in-source leucosome; these are called m afic selvedges. Mafic selvedges are also common at the margin of granitic dikes and sills intruded into low-grade country rocks; mafic selvedges are. therefore. not found only 1n m1gmat1tes.

3. MI GMATITES: THE PROCESSES AND MORPHOLOGIES The next step IS to introduce the terms that describe the overall appearance of a migmat1te at a scale larger than a small outcrop. Mehnert ( 1968) introduced 13 morphological terms (agmatitic. diktyon1t1c. schollen, stromatic, surre1tic. folded, ptygmatic, veined, ophthalmitic, stictolit hic, schlieren, nebulitic, and homophaneous) for th1s purpose. His scheme was not explained 1n terms of petrological processes, and so, overall, it has proved unsatisfactory for three mam reasons. Fwst, this scheme does not prov1de a way of understand1ng how one morphology of a migmatite is related to another. Second, 1t does not prov1de any 1ns1ght 1nto the petrolog1cal processes that contributed to the format1on of m1gmat1tes. Consequently, the scheme has turned out to have little pract1cal use 1n making a map of a migmatite terrane. Th1rd, the terms were designed to be nongenet1c, but 1n pract1ce most of the morpholog1es are h1ghly ongin-specific, i.e., they occur exclus1vely as a result of partial melt1ng. The overall morphology of migmatites will be considered next, with the intention of determining wh1ch factors control their appearance at the scale of a large outcrop. Th1s approach leads to a much clearer p1cture of how different morpholog1es of m1gmat1te form and are related to one another, 1n addit1on to prov1ding a workable bas1s for mapping migmatites.

3.1

The first.. order morphological division of migmatites

Field stud1es from a great many anatect1c terranes (Breaks et al. 1978, Brown 1979, Jamieson 1984, Weber et al. 1985, Sawyer and Barnes 1988, W1ckham 1987a, Bea 1991, Obata et al. 1994, Sawyer 1998, Oliver et al. 1999, Solar and Brown 2001, Johnson et al. 2001a, White et al. 2005) reveal a similar change in migmat1te morphology with 1ncreas1ng metamorphic grade. Examples from several m1gmat1te terranes are

shown in sect1on C of th1s book. In the lower-grade parts of anatect1c terranes, paleosome IS dominant 1n the migmatlte (i.e., the proport1on of neosome to paleosome 1s low), and old, pre-partial-melt1ng structures such as bedding, compositional layering, foliat1on, and folds are widely preserved 1n it. The neosome part 1s charactenzed by narrow bodies of leucosome of various orientations, bordered by melanosome, with its residual mineralogy and bulk composition. From a rheolog1cal aspect. the bulk behavior of these m1gmatites d1ffers little from solid rocks that are not partially molten, although the rocks are weaker. Toward the h1gher-grade parts of anatect1c terranes, there 1s a change 1n m1gmat1te morphology; neosome becomes the dom1nant feature. It is s1gn1ficant that 1n many of these higher-grade m1gmat1tes. leucosome IS far more abundant than res1dual matenal. The latter typ1cally occurs as schlieren of mafic m1nerals 1n the leucosome, together w1th schollen or rafts of paleosome and melanosome. Overall, paleosome 1s not abundant 1n h1gher-grade m1gmat1tes, and may even be absent. Charactenst1cally, pre-part1al-melt1ng structures are absent (except where they are preserved as schollen or rafts); they were destroyed during neosome formation and replaced by syn-anatectic structures, typically a magmatiC or submagmatic-foliation, or a flow banding. In terms of rheology, the neosome 1n those m1gmatites was magma-like. The trans1t1on from one morphology to the other IS gradual 1n some m1gmat1te terranes. The progress1ve change 1n the morphology from the lower-grade to the h1gher-grade parts of m1gmat1te terranes is systematically covered in section D of the book. However, the passage from one to the other is abrupt 1n many terranes, and commonly tectonic, as 1t coinodes w1th doma1ns of h1gh shear strain (e.g., Brown and Solar 1998a, Solar and Brown 2001). The abrupt nature of the many contacts suggests that some type of threshold behav1or may be controlling the changeover. Exactly the same change in the morphology of migmatites occurs between the outer and 1nner parts of contact aureoles affected by part1al melt1ng (Flood and Vernon 1978, Pattison and Harte 1988. Grant and Frost 1990, Hobson et al. 1998, Barnes et al. 2002, Johnson et al. 2003) . Th1s two-fold morpholog1cal diVISion of migmat1tes readily falls 1nto the old scheme of metatex1s and diatexis. The paleosome-dominated types were regarded as having formed from low degrees of part1al melt1ng. and were called metatexites, whereas the neosomedominated ones were Interpreted to be the result of nearly complete fus1on, and called diatexites (e.g., Mehnert 1968, Brown 1973, Ashworth 1985). The reasons why only these two basic types of m1gmatites (usefully called metatexite and diatexite) form needs to be understood before the terms can be properly defined. To do th1s, 1t 1s necessary

Atla, of Mtgm
- - - - - -- -- - - -- -- - - - - -- - - -- - - ----9

to examine the results from recent petrolog1cal stud1es of migmatites and from studies of the physical properties of partially molten rock.

3.2 Temperature, degree of partial melting, and fraction of melt Est1mates of the metamorphic temperature reached in d1atex1te m1gmat1tes (Brown 1979. 1995; Weber et al. 1985; Perc1val 1991 ; Guern1na and Sawyer 2003; Sawyer 1998; Solar and Brown 200 1; Obata et al. 1994; Ol1ver and Barr 1997; Johnson and Brown 2004) are typically between 750 and 900°C. These temperatures are commonly a little h1gher than 1n nearby metatexites (Grant and Frost 1990; Obata et al. 1994, fig. II; Oliver et al. 1999, fig. I). In some terranes, however, there is no s1gn1ficant difference 1n the metamorphic t emperatures recorded in metatexite and diatex1te migmat1tes. In some cases, the transition from metatex1te to d1atex1te m1gmat1te may be due to the influx of hydrous flu1ds, wh1ch promoted more melt1ng, where the d1atex1te migmat1tes subsequently formed (e.g., White et al. 2005) . The temperatures reached in diatexite m1gmat1tes are clearly much lower than requ1red for complete fus1on (e.g., 1150- 1250°C. V1elzeuf and Holloway 1988). At complete fus1on, the composition of the anatect1c melt must, necessarily, be the same as that of 1ts protolith. The composition of some diatexite migmatites does indeed co1ncide w1th that of the protolith, but th1s IS Invariably because the anatectiC melt d1d not separate from 1ts res1duum (Sawyer and Barnes 1988; Sawyer 1996, 1998; Milord et al. 2001; Solar and Brown 200 I) , and not because of complete fusion. Petrological and geochemical modeling indicates that diatexlte m1gmat1tes can form w1th as little as 16% part1al melt1ng, but that more commonly, 30 60% part1al meltIng occurred (Sawyer 1998, Milord et al. 2001, Johnson et al. 2001b). S1milar studies on leucosome- melanosome pairs show that the degree of part1al melt1ng (F) 1n metatex1te m1gmatltes IS generally less than 20 30% (Sawyer 1987, 1991; Barbey et al. 1990, 1996). H1gher degrees of part1al melting (60-70%) found 1n certa1n layers in some metatexlte migmat1tes have been attributed to the local influx of aqueous fluids, or to "wet" melt1ng (Weber et al. 1985). Petrolog1cal (Brown 1979, Johnson et al. 2001b) and geochemical (Sawyer 1998, Milord et al. 200 I, Solar and Brown 200 I) studies show that there is a considerable variation 1n the fraction of melt (M ) 1nferred to have been pres1 ent within diatex1te m1gmat1tes. Some parts contained a h1gh fraction (up to 100 vol.%) of anatectic melt (or a melt denved from 1t by fract1onal crystallization), whereas other parts, typically those that are richer 1n res1dual m1nerals. contamed very little melt (<5 vol.%); yet other parts of d1atex1te m1gmat1tes are nch 1n the m1nerals that crystallized

from the anatectiC melt, most commonly plagioclase. The proport1on of leucosome in metatexite migmatites is typically less than 20 vol.% (Weber et al. 1985, Symmes and Ferry 1995, Barbey et al. 1996, Kohn et al. 1997, Johnson et al. 200 Ia, Solar and Brown 200 I. Guermna and Sawyer 2003) and may 1nd1cate that metatexite m1gmatltes cont a1ned a lower fraction of melt than d1atexite migmatites. Caution must be exerosed, as the leucosome commonly does not have the composition of an 1nit1al anatectiC melt (see below, sect1on 6.2). Outcrops of metatexite m1gmatite with greater t han 20 vol.% leucosome have been described, and are attributed to veining by granitic melt (Pattison and Harte 1988, Symmes and Ferry 1995, Guern1na and Sawyer 2003) or to the early crystall1zat1on of feldspar 1n places where large volumes of melt have passed through the crust (Brown 2004). A large discrepancy between the degree of part1al melt1ng (F) and the fract1on of melt (M,) ex1sts for some metatexlte and d1atex1te m1gmat1tes, where M. < F indicates a net loss of melt, and where M. > F 1nd1cates a net gain of melt. Discrepancies indicate that melt was redistnbuted w ithin m1gmat1tes during anatex1s (Sawyer and Barnes 1988, Barbey et al. 1990). Because the degree of part1al melt1ng 1n d1atex1tes 1s typ1cally less than 60 vol.%, a redistribution of melt 1s the most likely mechan1sm by which the diatexites that are close to I 00 vol.% melt, or mostly leucosome, may have formed (Weber et al. 1985; Sawyer 1996, 1998; Brown and Rushmer 1997).

3.3 The partial..melting process Where the input of heat has caused the temperature to rise sufficiently that melt1ng starts, small 1solated pockets and tubes of melt fo rm at the JUnctions between the reactant phases (Mehnert et al. 1973). The addit ion of more heat causes further melting, and where the tubes and pockets of melt have grown sufficiently to link and form an Int erconnected network, the rock becomes permeable (Waff and Bulau 1979). Melt can then move out of (i.e., segregation of the melt can beg1n), int o or through, the solid framework. This Important step was called the permeability threshold by Maal0e ( 1982), and the liqu1d percolation threshold (LPT) by Vigneresse et al. (1996). Less than 2 vol.% melt is needed to ensure that suffioent gra1n-boundanes contain melt, so that fels1c systems become permeable (e.g., Deii'Angelo and Tullis 1988, Laporte et al. 1997, Lupulescu and Watson 1999); Rosenberg and Handy (2005) suggested that almost all of the gra1n boundaries w ill conta1n melt where the fract1on of melt reaches 0.07. Fo r the segregation of melt to actually occur, there has to be both a driving force and a dilatant sit e (s1nk) in which the melt can collect (Sawyer 1994).

IN T RODUC TI ON

10 -----------------------------

The greater v1scos1ty of fels1c melts means that grav1tat1onal forces are far less effect1ve at dnv1ng large-scale segregation of melt in the continental crust (Wickham 1987b) than they are m segregating basaltic melt from the mantle (McKenz1e 1984) . Most part1al melting in the continental crust was synchronous with tectonic deformation; therefore, segregation and migration of the melt fraction in migmatites were probably driven by differential stress (Mclellan 1988; Barbey et al. 1990; Brown 1994; Sawyer 1991, 1994; Brown et al. 1995; Rutter 1997; Brown and Rushmer 1997; Vanderhaeghe 1999; Marchildon and Brown 2001) . Part1al melt1ng under conditions of lithostatic stress 1n wh1ch grav1ty is the only dnv1ng force for melt segregation IS probably very rare m the continental crust. Nevertheless. it represents an end-member case to be cons1dered first.

3.5 The general case: melting under differential stress (so .. called "dynamic melting")

Crustal rocks are an1sotrop1c; therefore, dunng tectonic processes, they deform heterogeneously, and dilatant structures form in the more competent layers. Furthermore, local differences in differential stress create pressure gradients (Rob1n 1979). Once sufficient melt has been produced that permeability IS ach1eved 1n the matnx. any additional melt generated moves down the pressure grad1ents and collects in nearby low-pressure s1tes. Further deformation produces more dilat1on and increases the pressure gradients, and together these dnve more movement of melt. Where the crust melts under d1fferent1al stress, the fraction of melt at the s1te of melt1ng should not normally exceed that required for permeab1lity to be ma1ntained, because A special case: melting any excess would move away to nearby low-pressure sites under lithostatic stress conditions (Sawyer 1994). In many reg1onally developed migmatite (so .. called "static melting") terranes, the cont1nuous segregatiOn of melt from its resid uum appears to be a relatively nondestructive process. For At low melt fractions, near the onset o f part1al melting example, some granulite-fac1es m1gmatites that now conand under conditions of purely lithostat1c stress. gravity largely of strongly melt-depleted rocks still contain sist 1s 1nsuffic1ent to drive the v1scous gran1tic melt out of the sedimentary structures (Waters and well-preserved framework of solids 1n which it formed over the period of and Sawyer 2003) . Guemna 1984, Whales a typ1cal anatectiC event, say 30 40 My (e.g., Rubatto et al. 200 I). Consequently, the anatect1c melt and res1dual The movement of melt 1n m1gmat1tes can be d1v1ded solids remam together, the neosome 1s 1sotrop1c and 1nto three stages (e.g., Petford 1995. fig. 5: Rutter 1997; w1thout leucosome or melanosome. The distribution of Vanderhaeghe 2001). F1rst. 1t may move a short d1stance, neosome 1n the m1gmat1te depends on how much melt1ng up to several tens of centimeters. by porous flow. The melt occurred, and whether melt1ng occurred at spec1fic s1tes. moves from where 1t formed on gra1n boundanes. through or was pervasive. As more melting occurs. the fraction an Interconnected network cons1st1ng of m1croscop1c, of melt eventually becomes sufficiently large that the conmelt-filled tubes located along gra1n edges and melt-filled tacts between all the solid gra1ns disappear; th1s marks the grain boundanes and 1nt o small-scale (mm to perhaps em) onset of a magma-like (very weak) rheology. For a matnx low-pressure s1tes nearby. These may be dilatant fo liation 1nitially consisting of uniform, ngid spheres, th1s occurs at and bedding planes, shear bands, 1nterboudin partitions, or about 26 vol.% melt (MacGregor and Wilson 1939, Arzi ductile fractures. The porous flow stage moves melt away 1978). Under perfectly lithostatic conditions, little hapfrom its immed1ate po1nt of ong1n, and concentrates it in pens as the volume of melt reaches and then exceeds th1s various small-scale s1tes still with1n the confines of the origivalue, wh1ch was referred to as the melt-escape threshold nal source. (MET) by Vigneresse et al. ( 1996) and the solid-to-liqu1d trans1t1on (SLT) by Rosenberg and Handy (2005); the melt The second stage marks the onset of a reg1me of flow and res1dual solids rema1n Interspersed. Holness ( 1999) 1n clearly defined channels. In the 1n1t1al phase of chanreported that m the absence of deviatonc stresses. the melt neled flow, the melt moves tens of cent1meters through and solid fract1ons d1d not separate from each other 1n a mesoscop1c network of linked channels. Thus, the melt the shallow contact-aureole of the Rum Igneous Complex, has moved farther away from 1ts po1nt of generat1on and even though the melt fract1on locally reached 0.95. Typ1cally, has accumulated 1n larger (em ·m scale). more stable lowa new, coarser, 1sotropic fabnc develops 1n the neosome pressure s1tes, but st1ll w1thin 1ts source. The s1tes 1n wh1ch once the melt fraction crystallizes; m1gmat1tes that the melt accumulates are commonly dilatant foliat1on or display these characteristics occur 1n some deeper contact- bedding planes and interboudin partitions. The channeledaureoles, but very rarely in regional metamorphic terranes flow network is made up of em-scale linked segment s (see below, Figs. AI, Bl7, and B19). However, if gravity- consisting of various dilatant structures, the exact type induced separation of dense minerals or convect1on could and orientation of wh1ch depend on local circumstances, occur (a matter of some contention; see Scaillet et al. such as the distnbut1on of rock types and the style of 1998). then some separation and onentation of m1nerals

3.4

may occur.

Ad.t, of 1\.ttgmautc,

II

deformation. The melt rema1ns trapped 1n these dilatant sites until 1t IS forced to move aga1n. Upon crystallization, this part of the channel network w ill become an array of 1n-source leucosome. The last phase of the channeled-flow reg1me is called the t ransfer stage. It occurs once the melt t hat accumulated in the second-stage mesoscopic network migrates out of 1ts source ent1rely. The flow of melt IS through a tert1ary network of a few commonly discordant, macroscopic channels (fractures, veins, or dikes) over distances of tens to hundreds of meters. Th1s segment of the channeledflow reg1me produces leucocrat1c ve1ns or dikes, and 1f the transfer d1stances are sufficiently large, gran1t1c dikes. Thus, many of the bodies of leucosome seen 1n migmat1tes mark e1ther the channels through wh1ch melt flowed, or the sites in which anatectic melt collected. Examples of flowpath geometries are given by Brown et al. ( 1999), Sawyer

(2000, 2001), Guern1na and Sawyer (2003), March1ldon and Brown (2003), and Brown (2004). Although some melt-filled sinks and channels may be longlived, others are trans1ent. S1mak1n and Talbot (2001a, b) used a numencal model of an 1sotrop1c protolith to show that melt-rich "ve1ns" can grow or atrophy, depending upon their onentation relat1ve to the max1mum and min1mum pnnopal stresses. Sawyer (200 I) argued that melt channels would collapse and d1sappear 1f the supply of melt stopped before crystallization began . S1milarly, the direction of melt flow may change. Simak1n and Talbot (2001a, b) found that 1f a ve1n array IS reonented, or the onentat1on of the stress field changes, the direct1on of melt flow may reverse: change of melt-flow direct1on has also been demonstrated from field studies (Sawyer et al. 1999). Rob1n ( 1979) cons1dered the m1grat1on of melt during layerparallel extension and found that low-pressure s1tes develop perpendicular to the extens1on dwect1on 1n the most v1scous (1.e., competent) layer, wh1ch attracts melt from the nearby less viscous layers. The most viscous layers are most commonly paleosome, but if the res1duum develops to the extent that 1t conta1ns a large modal proport1on of strong minerals, such as garnet or pyroxene, then the res1dual layers (melanosome) could start to attract melt, rather than lose 1t. Th1s model has been applied to m1gmatites (Patt1son and Harte 1988, Brown et al. 1995) and relates princ1pally to the segregation of melt into the space between baudIns 1n competent layers and the growth of some stromat1c m1gmat1tes. From the discussion above, 1t IS relatively easy to understand how metatexite migmat1tes form dunng melt1ng of a heterogeneous continental crust under differential stress. In contrast, 1t is not clear how d1atexite migmat1tes can

form. Two 1mportant top1cs that are central to the formation of diatex1te m1gmatites need to be cons1dered next: (I) the replacement of pre-anatectic structures 1n m1gmatites by syn-anatect1c flow-structures, and (2) the generation of large-doma1ns that contain h1gh fractions of melt in m1gmat1tes. Rosenberg and Handy (2005) have re-exammed data from deformation expenments conducted on part1ally melted crustal rocks. They found that the maJor (90%) decrease 1n strength occurs where t he fraction of melt reaches about 0.07. Rosenberg and Handy called th1s drop 1n strength the "melt connect1v1ty trans1t1on" (MCT) and suggested that 1t occurs at the po1nt at wh1ch wtually all of the grain boundaries 1n the part1ally molten rock become meltbeanng. The deformation mechan1sms that operate at the high stra1n-rates 1n experimental stud1es 1nclude 1nter- and intra-granular cracking, rigid rotation of grains, and sliding along pockets of melt (Rosenberg and Handy 2005). At the slower stra1n-rates of natural deformatiOn, other mechanisms, such as grain-boundary sliding accommodated by diffus1on, intracrystalline plastic1ty, and the dissolution of gram gra1n contacts 1nto the melt (e.g., H1rth and Kohlstedt 1995: Park and Means 1996, 1997; Rosenberg and Handy 2000) may operate. Alt hough the flow rates ach1eved by shear stresses at low fract1ons of melt (ca. 0.07) may be lower than those attainable 1n a magma (suspens1on of crystals in a melt), the macroscopic effect 1s, nevertheless, similar. The pre-partial-melt1ng structures 1n the rock are progressively destroyed as a new flow-1nduced structure develops, w1th the result that the neosome created IS a diatexlte m1gmatite. Sawyer ( 1996, 1998) has suggested that 1n some instances, a diatexite m1gmatite that resu lted from low degrees of partial melt1ng and conta1ned low fract1ons of melt (M <0.2) may have formed 1n th1s way. Stevenson ( 1989) developed a model for the m1grat1on of melt dunng layer-parallel compreSSIOn. In h1s model, decreases 1n pressure 1n the least competent layers (i.e., t he layers conta1n1ng most melt) create a pressure gradient that brings more melt 1n from the surrounding, more competent layers. Thus, a melt-nch layer grows by a s1mple feedback-type mechan1sm. Stevenson's model IS partiCUlarly interest1ng because it IS a way of concentrat1ng melt in vanous layers to achieve melt fract1ons well above the liqu1d-percolat1on threshold (LPT) in the general case of melt1ng under differential stress. High fract1ons of melt may also be ach1eved 1n a m1gmat1te 1f melt IS produced at a rate greater than the rate at wh1ch 1t m1grates away (Sawyer 1994). One mechanism for triggenng such rapid melting could be the influx of an aqueous flu1d (e.g., Wh1te et al. 2005). Whether the high fract1on of melt 1n a m1gmat1te undergoing synchronous melt1ng and deformat1on arises from rapid melting, or from melt

I NTRODUCTION

12--------------------------------

m1grat1on as outlined by Stevenson ( 1989), the effects on the morphology of the m1gmatlte are essentially the same. Once melt has accumulated to the po1nt that suffic1ent gra1n-to-gra1n contacts 1n the matnx have disappeared, the partially molten rock becomes a suspens1on of crystals 1n a melt. i.e., a magma, which will then flow in response to the shear stresses imposed on it by local or far-field tectonic forces. Where magma flow occurs, the pre-partial-melting structures in the matrix are destroyed and are replaced by magmat1c flow structures, thus forming a diatex1te migmatlte. Many examples of diatexite m1gmatite, especially those that have bulk compos1t1ons 1nd1cating that they contained a h1gh. or very h1gh. fract1on of melt. formed 1n th1s way (Sawyer 1998, Milord et al. 2001). Est1mat1ng the fract1on of melt requ1red to change from a rock-dom1nated to melt-dom1nated rheology has attracted a great deal of attent1on. Th1s transition has been termed the rheolog1cal critical-melt percentage (RCMP) by Arz1 (1978). the cnt1cal melt fraction ( CM F) by van der Molen and Paterson ( 1979), and the melt-extraction threshold (MET) by Vigneresse et al. ( 1996) . Recently, Rosenberg and Handy (2005) have called it the solid-to-liquid transition (SLT). but they and others (Takeda and O bata 2003) have suggested that this rheolog1cal threshold 1s much less important than previously considered. Estimates place th1s threshold at melt fract1ons between 0.26 and 0.4. Studies of the microstructures 1n d1atex1te m1gmat1tes clearly 1ndicate that many flowed as suspens1ons of crystals 1n a melt. and some acqu1red the1r microstructures 1n a submagmatic state as the melt crystallized. Nevertheless, some d1atex1te m1gmat1tes may have lost the1r pre-anatectiC structures and developed flow fohat1on at far lower fractions of melt (<0.2) through the other mechan1sms of deformation outlined above. Thus. from the comb1nation of how a high fract1on of melt was ach1eved and the poss1ble mechanism of flow involved, there are five end-member ways of forming a diatexite migmatite. (I) In a closed system where F, and hence M,, are well below the sohd-to-hquid transition, by gra1n-boundary sliding, melt-enhanced diffus1on and d1ssolut1on processes, leading to bulk flow (1.e .. flow of a solid conta1ning a small volume of melt). (2) In a closed system, by an increase in temperature (or possibly a drop 1n pressure), so that F, and hence M , nse above the sohdto-hquld trans1tion, leading to the creat1on of a suspens1on of crystals 1n an anatectiC melt, and hence magma flow. (3) In a locally open system (the system could be closed on a terrane scale, however), by the movement of anatectiC melt w1thin the migmatite, so that M is locally increased above the solid-to-liquid trans1tion, and magma flow can occur. (4) In an open system, by t he intrusion of an external granitic magma into the migmatite, which augments the locally derived anatectic fract1on so that the net M1 rises

above the solid-to-hqu1d trans1t1on and enables magma flow to occur. Th1s external magma m1ght be denved from deeper in the same source. or 1t m1ght be denved from a completely different source-volume. (5) In an open system, by the add1tion of an aqueous fluid into rocks already at a high temperature, which enables rapid fluid-present part ial melting to occur, so that Fand M 1are locally increased to above the solid-to-liquid trans1t1on. and magma flow can occur. In outcrop, the diatex1tes may look very similar, but petrological (1ncluding microstructural) and geochemical studies generally allow a spec1fic process to be determ1ned. In the closed systems, M, corresponds to F determined petrographically or geochem1cally from the rocks; these could be called primary diatexite migmatites. However, in the open systems, F determ1ned from the rocks 1n the m1gmat1tes would not correspond to M ; these could be called secondary diatexite migmatites.

3.6 Definitions of metatexite and diatexite For the same protolith compos1t1on, metatexite migmatites generally form at lower temperatures than diatexite migmatites; therefore, the passage from metatexite to diatexite could be v1ewed as one of 1ncreas1ng degree of partial meltIng w1th temperature, but th1s IS an 1ncomplete v1ew. F1eld, petrolog1cal, and geochem1cal results from migmatites, and the theoretical cons1derat1on of melt flow in partly molten rocks d1scussed above, suggest that movement of melt is also a s1gn1ficant mechan1sm 1n leading to elevated fract1ons of melt 1n m1gmat1tes where melt1ng and tectonic deformat1on were synchronous. The fract1on of melt (M.) may be 1ncreased to I, or decreased to the liqu1d-percolat1on threshold, by m1gration of the melt. even 1f the degree of partial melting (F) was relat1vely low, as Sederholm noted. Only in a closed system does M1 F. Therefore, the first-order morphological div1sion in migmatites is best considered in terms of fraction of melt rather than the degree of part1al melting. Th1s represents a change in emphaSIS, from the degree of part1al melting to the fraction of melt present, and a d1vergence from the orig1nal reason that the terms "diatex1te" and "metatex1te" were Int roduced. However, both terms are too well established and too useful to be d1scarded. Accordingly, the defin1t1ons of diatex1te and metatex1te are changed slightly from those g1ven by Mehnert (1968). Brown (1973). and Ashworth (1985), and go somewhat beyond the etymological roots of their anginal definit1on g1ven by Gunch (1905) and

=

Scheumann ( 1936).

Metate xite : a m1gmat1te that is heterogeneous at the outcrop scale. and m wh1ch coherent pre-partial-melting structures ore w1dely preserved m the poleosome (where

13

8

URS ,----~------------. MetaDiatexite texite 1 .... 1 Transitional ............

NU P+---~~----~--~----~----~ 0 0.2 0.4 1.0 0.6 0.8

b

Q)

c

0

z 0

...J

!

dUata1t

I c/ '

-~

2ro c

2

~

Q)

1:l

0

ro

~

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.t:. 0>

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f

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Fraction of Melt 0.4 0.6 nebulites

patch

~

c

·~

0.2

0

net

1 I

d;ate,;te

li ~i

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st
diatexite I

possible limit of transitional zone

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Fig. I . Classification scheme for migmatites. (a) The fi rst-order division of migmatites into metatex itc and diatcxite migmatites is considered a funct ion of the fraction of melt and the properties of the solid grains in the partial ly melted rock. The passage from metatex ite to d iatcxite migmatite occurs abruptly at a fraction of melt eq ual to 0.26 if the solid crystals arc treated as un iform , rigid sphe res (URS model). However, for a model in which the solid particles arc nonun iform in size and shape ( UP model), there is a tra nsition zone between mctatcx itc and diatcxite migmatites that may extend from 0.16 to 0.6 in terms of fraction of melt. (b) The second-order morphologies of metatcx ite and diatcx ite migmatites arc shown on a plotofsyna natcctic strain versus fraction of melt. The diaoram is shaded for the U RS model, but the vertical dash;d lines indicate where the bounda ries arc for the transitional zone in a U P model. The terms shown, except diatexitc, should be used as a prefix to either mctatexitc or diatcxite as appropriate (e.g., patch mctatexitc migmatitc or schollc n d iatcxitc migmatitc).

the microstructure appears unchanged) and. poss1bly 1n the melanosome (res1duum) part of the neosome. where the fraction of melt was low. The neosome part IS generally segregated tnto leucosome and melanosome. but neosome 1n wh1ch melt and res1duum d1d not segregate may also occur.

suggested by Lejeune and R1chet (1995) and Renner et al. (2000). extending tn terms of fractton of melt from about 0.16 to approxtmately 0.6 between the metatexite and diatexite fields.

Diatex ite: o m1gmat1te tn wh1ch neosome

3.7 The second.. order morphological

IS dommant, and melt was pervas1vely d1stnbuted throughout. Pre-part1al· melttng structures are absent from the neosome, and are commonly replaced by syn-anatect1c-{Jow structures (e.g.. magmatic or submagmat1c foliations. schl1eren). or by 1sotrop1c neosome. The neosome IS vonable tn appearance. reflecttng a large range in the fraction of melt. and 1t can range from predommantly /eucocrat1c to predommantly mesocratlc (e.g.. unsegregated melt and res1duum) to predommantly melanocrat1c. Paleosome occurs as rafts or schollen. but may be absent.

The t ransition from metatexite to diatextte ts abt·upt 1n some terranes and transttional in others. The first-order morphologtcal dtvtslon of mtgmat1tes tnto metatextte and diatex1te is represented on Fig. Io as a funct1on of the fractton of melt (abscissa) and the charactensttcs of the solid part1cles (ordinate). The upper part of the diagram shows the change from metatextte to d1atex1te at a fraction of melt of 0.26. based on the model that the solid particles in the rock undergotng partial melttng are un1form. ng1d spheres (URS) . In rocks. the crystals are uniform ne1ther in s1ze nor aspect ratio; a transttlon therefore exists between the loss of soltd solid contacts at the local scale and the w1despread loss of solid- solid contacts 1n the bulk sample. Thus. the lower part of the d1agram IS for the general case tn wh1ch the rock undergotng parttal melting contatns a range of part1cle stzes and shapes. In such a nonun1form parttcle model (NUP). there ts a trans1t1onal reg1on. as

divisions in migmatites

In this section, I describe the morphologies of migmatites at scales 1nvolvtng 1nd1v1dual leucosome and melanosome on the one hand. and the overall scale at whtch metatexite and diatex1te morphologies are determ1ned on the other. All the morpholog1es d1scussed below are shown 1n sect1on D of the book in the same order as here 1n the text. The morphology of a m1gmatite nght at the onset of partial melting ts likely to be controlled by the physicochemical factors that determ1ne the exact place where the first melt and. therefore, tn s1tu neosome are formed. Thts early morphology is probably best conserved tn envtronments where dtfferential stresses are least. e.g.. contact aureoles. However. dtfferential stresses act1ng as the 1n s1tu neosome grows wtll cause the melt fraction to mtgrate to dilatant structures 1n the m1gmatite. Consequently. further changes in the outcrop-scale morphology of migmatites are not solely due to the petrologtcal process of part1al melt ing; they resu lt fro m the tnterplay of two factors. (I) The proportion of melt 1n d1fferent parts of the m1gmat1te. Melt w1ll be redistributed 1n response to differential stresses; thus. the amount of melt present may range from zero to 100 vol.%. (2) The way the rocks respond to d1fferenttal stress while melt 1s present. The locatton of di latant sttes is controlled by the distribution of the competent layers and by the way tn wh1ch stra1n occurred. Part of

INTROD U CTI ON

14 ------------------------------

th1s response will be 1nfluenced by the 1n1t1al. pre-anatectic distnbut1on of competent and Incompetent litholog1es, but 1t w1ll also be controlled by the structures that form durIng deformat1on and by the locus of melt collection. As the melt fract1on increases, stra1n is increas1ngly partitioned 1nto the melt-bearing parts (Vigneresse and Tikoff 2000) of the migmat1te. Consequently, the morphology of migmatites may be very heterogeneous: a migmatite v1ewed on a 1-m outcrop m1ght not be described in the same way when

and the flow of matenal through. an orogen along discrete zones or channels located 1n the m1ddle and lower part of the crust. The m1gmatites from reg1ons where this type of crustal-scale channel flow has occurred are likely to have strongly layered morpholog1es, but there may still be small domains where other morphologies can be recognized (see

seen on a I 0 OOO-m 2 outcrop. F1gure I b shows how the second-order morpholog1es, wh1ch w1ll be described below, are related to the first-order metatex1te-diatex1te subdiv1s1on of m1gmat1te, and to each other by vanation 1n relat1ve syn-anatect1c "stra1n." Stra1n is not used 1n a quant1tat1ve manner: it IS s1mply used qualita-

below, Figs. A2. A3, 023. and 024) .

Morphologies characteristic of metatexite migmatites Very low fract1ons of melt and low-stra1n condit1ons. near the ongin 1n Fig. lb. corresponds to the onset of melt1ng: m1gmatites that have lost melt could also be near the ongin. Although the very low fract1on of melt precludes any eye-catch1ng morphologies from develop1ng (e.g., Figs.

tively to differentiate relatively more deformed rocks from less deformed rocks. Most of the morpholog1cal divers1ty 1n metatex1te migmatites denves from the anisotropy inherent in the paleosome (and protolith), rather than the magnitude of the syn-anatect ic strain or fraction of melt present. High strain generally results 1n simpler morpholog1es, because all parts of the migmatites are strongly attenuated or transposed, and paleosome, leucosome, and neosome tend to become parallel, and the overall morphology IS stromatiC, or layered. Thus, the cnteria that are wholly stra1n-related, such as the lateral cont1nuity of leucosome, do not prov1de a good bas1s on wh1ch to define metatex1te m1gmat1tes, or on which to distingu1sh metatex1te from d1atex1te m1gmat1tes. In contrast, the morpholog1cal vanat1ons that occur w1th1n d1atex1te migmat1tes are almost wholly a funct1on of the fract1on of melt present; more melt necessarily means less paleosome and more neosome. The effect of syn-anatect1c strain on the morphology of diatexite m1gmat1tes is subtle. Because magmas are weak, flow structures and foliations are 1n1tiated in them at comparatively low shear stresses: h1gher shear strain simply results 1n better preferred onentatlons for the enclaves and m1nerals (lldefonse et al. 1992, Arbaret et al. 1997). not 1n a s1gn1ficant change 1n the morphology. The second-order morpholog1cal terms should be used as a prefix to the first-order one. e.g.. stromat1c metatex1te m1gmat1te and schollen (see below) d1atex1te m1gmat1te. F1gure lb 1nd1cates that most of the second-order morphologieS are charactenst1c of relat1vely low-"stra1n" rocks. However. recent results from numerical models (e.g., Beaumont et al. 2004. ]am1eson et al. 2004) 1ndicate that many m1gmatite terranes exhumed from the deep parts of orogens are likely to have expenenced very high synanatectic stra1n. The weakening that crustal rocks undergo as they part1ally melt results 1n the partit1on of stra1n 1nto,

01-04). the rocks formed at the onset of melt1ng are very important. as they mark the lower-grade lim1t to areas of migmatlte format1on. The field ev1dence for the onset of partial melting IS commonly overlooked because the first signs of partial melt1ng 1n outcrops are subtle. Reactant minerals become slightly rounded where melting has begun, and a sugary texture can develop 1n lithologies not too rich in mica. If the melting react1on produces a distinctive solid product, that m1neral m1ght be recogn1zable. The first evidence of part1al melt1ng 1n some m1gmat1tes is the development of fine-gra1ned. quartzofeldspath1c films. which weather white. along some gra1n boundanes: the films represent crystallized melt (Holness and Clemens 1999. Sawyer 1999, Holness and Watt 2002). A pmk colorat1on at the edge of muscovite gra1ns was found by Holness and Watt (2002) to mark the onset of part1al melt1ng 1n pelit1c rocks in a contact aureole 1n Scotland.

Patch migmatites are formed where slightly higher fractions of melt have been generated, and melt1ng has occurred at d1screte s1tes. so that small, scattered patches of nonfoliated 1n situ neosome develop (Fig. I b). Examples of patch metatexite migmatites can be seen in F1gs. 07- 0 I 0 and also 1n F1gs. Bl. B2. BIS. and B30. Paleosome 1s dominant, and the patches of neosome are generally round or oval 1n shape. Such neosome was called "bhnd endIng" by Sederholm (1907) and "bhnd patches" by Platten ( 1983). and IS charactenst1c of the 1nc1p1ent stages of partial melt1ng 1n the1r hosts (Weber and Barbey 1986, Mclellan 1988. Grant and Frost 1990. Hobson et al. 1998, Sawyer 1991. Timmermann et al. 2002, Slagstad et al. 2005). Consequently, the first appearance of patchy neosome 1n migmat1te terranes IS useful1n locat1ng the "melt-in" 1sograd for the lithology m wh1ch 1t occurs. However. it may be difficult to 1dentify the neosome if the paleosome IS a coarse-grained. nonfoliated, fels1c plutonic rock. Oriented, tabular bodies of neosome can arise if melting is confined to thin layers. or to certain planes. At higher degrees of partial

Atht~

of Migm a cth.'~

IS

melting, the neosome grows, or 1t becomes more plent1ful; lobed shapes can develop by coalescence. If coalescence results 1n the creat1on of large patches of neosome that have diffuse borders, then the term ne bulite can be used (see Figs. 011 and 012). Typically. in patch m1gmatites, the melanocrat1c and leucocratic parts are uniformly distributed throughout the neosome because the melt and res1duum have not separated; hence. overall the neosome IS commonly mesocratiC. However; vanat1ons result 1f the melt and residuum do segregate. In one case, the leucosome may be surrounded by a rim of melanosome, whereas in another case a leucocratiC nm. or moat, may surround a melanocrat1c core, wh1ch 1s commonly a single crystal (Weber and Barbey 1986). Th1s latter type of patch neosome (e.g.. F1gs. B30, D I 0) occurs in the granulite-facies parts of some migmatite terranes (Waters and Whales 1984. StOwe and Powell 1989, Powell and Downes 1990, Sawyer et al. 1999, Waters 200 I, White et al. 2004); mass-balance est1mates for these patches show that many have lost a considerable volume of melt (M 1 < F). White et al. (2004) described patches of neosome that occur in metapelitic rocks from the upper-amphibolitefacies to granulite-fac1es trans1t1on and that 1nvolve garnet produced by an 1ncongruent melt1ng react1on involving the breakdown of b1ot1te and sillimanite. The 1nability of garnet to eas1ly form nucle1 1n these rocks has meant that the location of the melt1ng react1on 1tself and of the other products of react1on was determ1ned by the locus of nucleat1on of t he garnet. Th1s s1tuat1on results 1n a m1gmat1te that contains patches of m situ neosome cons1st1ng of a large garnet porphyroblast surrounded by the other solid products of react1on (e.g.. K-feldspar) and the melt. Patch m1gmatltes are best preserved where syn-anatect1c strain IS low. Such environments 1nclude inc1p1ent melt1ng where rocks are stronger, and layers are competent (e.g.. Slagstad et al. 2005). Hence, the patch m1gmat1tes extend to greater stra1n than nebulltic m1gmat1tes on F1g. lb. The ex1stence of even a small differential stress deforms patch m1gmat1tes into h1gh-aspect-rat1o shapes (Timmermann et al. 2002), and dnves the segregation of melt. As the m s1tu patches of neosome 1n the migmat1tes described by Wh1te et al. (2004) become larger and more abundant. d1fferent 1al stresses cause the melt and solid fractions to separate as the melt moves into dilatant structures. Consequently, the morphology of the migmatites changes from patch t o net-structured, or stromatiC. Thus, even low synunatectlc stra1n results 1n the loss ofsome. or most. of the melt fraction, and the m s1tu patches of neosome are dominated by res1dual minerals. At higher syn-anatectic stra1n, patch m1gmatites no longer exist, and net, or stromatiC (also called layered) morpholog1es form 1nstead.

Net. dilat1onal, and stromatic descnbe three metatex1te migmat1te morpholog1es in which the distribution and form of the neosome (the leucosome 1n part1cular) reflect different ways 1n wh1ch the an1sotropy due to the layenng 1n the protolith responded to strain dunng part1al melt1ng; some examples are shown 1n F1g. 2. S1nce these morpholog1es are defined w1th reference to thew pre-melting structures, they are divis1ons of metatexite m1gmatites. The order of the three types along the "stra1n" ax1s of Fig. lb IS somewhat arbitrary; all may form at low and 1ntermed1ate strains. However, at high stra1ns, the leucosome 1n dilat1onal and net types tends to become parallel, 1n which case the term "stromatiC" or "layered metatex1te m1gmatite" IS more appropriate.

Di lation-structured migmatite s (surre1t1c structure of Mehnert 1968) are dist1nct1ve, as can be seen in F1gs. D 13 022. The leucosome 1s located in dilatant (1.e., low-pressure) structural sites. such as the spaces between boudins. 1n pressure shadows. or 1n fractures (F1g. 2) 1n the more competent layers of the m1gmatite. Typically. the dilat1onal sites are comparat1vely short and restricted to part1cular layers and do not form an outcrop-scale net structure. This morphology 1mplies the ex1stence of a competency contrast between the vanous layers 1n the migmat1te wh1le layer-parallel extens1on occurred. Typ1cally, t he competent layers are the less fertile (e.g.. greywacke 1n a pelite). or the resister l1tholog1es (e.g., calc-s11icates. quartz1te. metamafic rocks. or pegmat1tes). but they could also be newly formed res1dual layers nch 1n strong minerals, such as garnet or pyroxene. Dilat1onal structures may also form owing to strain incompatibilities across the foliation planes (Platt and V1ssers 1980), as 1n the m1gmat1tes of southern Bntanny (Jones and Brown 1990, Brown and Dallmeyer 1996, Brown 2004) and northwestern Australia (Oliver and Barr 1997). Other types of dilational s1tes can develop in, and around, the hinges of folds developed 1n rocks with a strong planar an1sotropy (e.g., Coll1ns and Sawyer 1996). Net-structure d migmatites (diktyon1t1c structure of Mehnert 1968) are a very common type of metatexlte m1gmatite, and Sederholm ( 1907) cons1dered the1r development to be an 1nd1cator of part1al [Y)elt1ng; examples are shown 1n Figs. 023 030. The essential feature of this morphology 1s that the leucosome occurs 1n two or more systematic sets. such that the1r intersection creates a net-like pattern outlining lozenge-shaped, or polygonal blocks of darker rock. At the early stages of melt1ng. the leucosome IS narrow w1th a h1gh aspect-rat1o and IS bordered by melanosome; the centers of the lozenges are paleosome. The net-like pattern is very commonly the result of one or more sets of extens1onal shear bands 1n which the leucosome IS located. It IS 1dent1fied by curvat ure of the foliat1on from the host to, or some way into, the leucosome. The

INTRODUCTION

16 ------------------------------

d

--------- ---~; ' /

..........

----

Fig. 2. Schematic representation of some of the structural sites in which the leucosome in Jilational metatexite migmatites ca n be expected to occur. Solid areas anJ lines represent leucosome, whereas the dasheJ black lines are the traces of bedJing or foliation. (a) Leucosome in inrerboudin partitions that develop in the competent layers of migmatites; t hese layers may be paleosome resisters, or even melano ome. Some boudins contain sma ller intern al boudins. (b) Leucosome located in extensional shear bands; both synthetic and antithetic examples are shown. (c) Leucosome located in an asymmetrical fol iation boudin. (d) Stromatic leucosome orienLed parallel to the principal plane of ani otropy, which may be either bedding or foliation. (e) Leucosome located in a reverse shear, cutting the short limb of an asymmetrical fold. (f) Leucosome associated with parallel folds; leucosome in the fold hinges is located in the space between dilated bedding planes; shorter Jomain s of leucosome arc locateJ in cx tcn ional fractures developed on the outside of the foiJed competent layers and are not ax ial-planar, but tend to be radially d ispo ed. Leucosome that is located parallel to a fold's axial plane occurs in the less competent layers. anatectiC melt thus seems to have m1grated a short distance by porous flow from the host 1nto the shear bands as the bulk rock underwent layer-parallel extens1on (e.g.. Mclellan \988; Sawyer \991; Brown \994, 2004; Oliver et a\. \999). Deformation experiments on part1ally molten analogue materials confirm th1s (Rosenberg and Handy 2000); the onset of partial melting causes deformation to become concentrated 1nto shear bands, and the melt then m1grates to the shear bands. M1grat1on of melt 1nto shear bands, or expuls1on from them. depends on whether the shear band 1s shortemng or lengthening (Mancktelow 2002). A set of leucosome bands commonly IS onented parallel to the compos1t1onal layering or foliat1on. The onentat1on of all sets of leucosome bands, fohat1ons, and shear zones together w1th the1r direct1ons of movement, should be recorded 1n the field so as to determine the local k1nematics of the syn-anatect1c deformation. W1th further partial melting, the paleosome is changed to a gneiss 1n which folia of res1dual m1nerals alternate with quartzofeldspathic layers, and the leucosome bands no longer have obvious melanosome around them (e.g..

Oliver and Barr \997) . The rock 1n the lozenge between leucosome bands progressively becomes neosome, and is generally composed of melanocrat1c residuum . However. 1f melt is injected into these layers, then the lozenge may be mesocratic in color. With increased fract1on of melt, netstructured migmat1tes pass, v1a a transitional stage, into raft or schollen diatex1te m1gmatites (F1g. \b).

Stromat ic or laye r-st ructured migmatites have numerous thm and laterally persistent bands of leucosome that are oriented parallel to the major plane of an1sotropy 1n the paleosome. Examples are shown 1n Figs. 031-040. A s1ngle layer IS called a stroma (plural stromata). The planar anisotropy IS, typ1cally, the compos1t1onal layenng (bedding or 1gneous layenng), or a foliat1on. Generally, a band of leucosome has melanosome on both s1des; 1n some cases. however, the melanosome 1S only on one s1de. and 1n yet others. there is no melanosome. Because of the large number of leucosome domains and the regularity of the1r spaong, stromatic migmatites have attracted a lot of attent1on. M1chei-Levy ( 1893) suggested

17

that they are the result of repeated InJections of magma between the layers, or foliation planes: hence, he Introduced the term Itt par lit to describe this morphology. The find1ng 1n certa1n layered m1gmatites that (I) seem1ngly layer-parallel bands of leucosome actually cross-cut the layenng locally, stepp1ng from one plane of an1sotropy to the next if traced out across large outcrops (e.g., Sawyer et al. 1999), and (2) adJacent melanocratic bands do not have appropnate res1dual compos1t1ons (Jung et al. 1999), support the melt-InJection ong1n. Many invest1gators have proposed an m s1tu origin; Holmqu1st (1916) and Brown et al. (1995) have suggested that the segregations of melt are denved from adJacent rocks. In contrast, Johannes (1983o) and Johannes et al. (1995) have argued that the layered structure results from the part1al melt1ng of only those layers w1th su1table bulk compos1t1ons (i.e., the fertile layers): the separation of biotite 1nto narrow zones between the layers created the melanosome. Petrographic and geochemical studies have demonstrated that 1n some layered m1gmat1tes, the melanocratic part is the residuum left after the extract1on of a partial melt, which corresponds to the composition of the adjacent leucosome (Sawyer 1991, Oliver and Barr 1997), which supports an orig1n by m Situ part1al melt1ng. The InJeCtion and m s1tu layered m1gmatites can form in the same port1on of the fract1on of melt stra1n space (Fig. I b) as net and dilatant types.

Transposition and the morphology of metatexite migmatites Park ( 1983) considered that stram dunng part1al melt1ng could generate a layered structure (see Figs. D35 040) in m1gmat1tes by transpos1t1on. T1mmermann et al. (2002) and Slagstad et al. (2005) descnbed a m1gmat1te from the Grenville Prov1nce 1n wh1ch stra1n part1t1on1ng resulted 1n the preservation of small, equant bodies of leucosome 1n a competent host, but transposed them to form layered (high aspect-ratio) leucosome in adJacent, less competent rocks. The competent porphyroblasts of garnet w1th short (a few m1ll1meters to centimeters 1n length), but elongate doma1ns of leucosome 1n the1r pressure shadows (e.g., Williams et al. 1995, Brown et al. 1999) and hence grow1ng parallel to the foliation, prov1de another example. Some m1gmat1tes show a curvature of the structural fabncs (such as foliat1on or compos1t1onal layering) and lowaspect-ratio leucosome into a part that exhibits stromatic leucosome and a more regular, and finer-scale compos1 t1onal banding than the adJacent nonlayered m1gmat1tes. The layered structure 1n these m1gmatites formed by the attenuation and transposition of ong1nally more equantshaped accumulations of melt, or partially crystallized 1eucosome, in outcrop-scale syn-anatect1c shear zones. At a much larger scale, metatex1te m1gmat1tes that develop

w1th1n crustal-scale shear zones (includ1ng zones of channel flow in the continental crust) typ1cally have a pronounced layered, or stromatiC, morphology due to transpos1t1on and progress1ve deformation, most ev1dent by abundant, parallel and laterally pers1stent bod1es of leucosome (F1gs. A2, A3) . The layered structure may be so strong as to obscure any previous morphology that the metatexite migmatite may have had; 1n th1s case, the name "layered metatex1te m1gmat1te" should be suffioent. Although all pre-anatectic structures 1n these m1gmatites may have been destroyed (by transposition, for example), they should not be called diatex1te migmatites (unless a high fract1on of melt can be demonstrated to have ex1sted 1n them): they should be v1ewed pnmanly as tecton1tes. However, leucosome generally is present, and more rarely neosome (i.e., leucosome + melanosome), which postdate the ma1n layered structure, and these may have any of the morphologies typical of the lower-stra1n part of Fig. Ib. Ev1dently, a layered or stromat1c morphology m metatex1te m1gmat1tes can form in several d1fferent ways, and over a wide range of conditions of syn-anatectic stra1n.

Morphologies characteristic of diatexite migmatites At low syn-melting stra1ns, a fract1on of melt above about 0.26 eliminates the contiguous crystal-based framework, and the whole m1gmat1te becomes nonfohated (i.e., isotropic) and unsegregated neosome; for th1s stage, the term "nebulitiC diatexite migmat1te" should be used (F1g. lb). However, under rare circumstances, nebuht1c migmatites may also form at much lower fractions of melt instead of a patch metatexite migmat1te. This will occur 1f the part1al melt1ng occurred, or spread, throughout the whole volume of the protohth, rather than at a few d1screte s1tes. Pervas1ve melting leads to the widespread development of neosome 1n which the residual and remaining solid minerals can undergo extensive recrystall1zat1on and gra1n growth because of the presence of melt along the grain boundanes. Nebulit1c migmat1tes are preserved only 1n places where the syn-anatectic stra1n was very low. A partially molten rock contain1ng a melt fract1on of more than 0.26 IS very weak, and 1n general. differential stress creates shear stresses, and flow of the magma readily occurs. Consequently, the nebulitic migmat1te develops a fol1at1on due to flow, and then another of the morphological terms shown on Fig. I b becomes appropriate.

Schollen or raft-structured migmatites are characterized by the raft-like form of the remnants of paleosome, resister litholog1es. or melanosome displayed 1n the neosome. Th1s type of migmatite is common at the transition from metatexite to diatex1te migmatites (e.g., Solar and Brown 2001), as well as 1n the lower-melt-fract1on

INTRODUCTION

18 ------------------------------

part of diatex1te m1gmat1tes (Fig. Ib). Examples of schollen diatex1te m1gmatites from the transition can be seen 1n Figs. 041-046. The proport1on of paleosome, melanosome, and res1ster litholog1es IS h1ghest at the start of the trans1t1on zone, where the lateral cont1nu1ty of these starts to become disrupted. The layers and rafts (also called schollen) of paleosome are large and have high aspect-ratios, and some show rounding at the ends, but there is generally httle rotat1on of the rafts. Farther into the trans1t1on zone, and toward the diatexite doma1n, there IS a progresSive decrease in the s1ze, aspect-rat1o, and number of rafts or schollen (F1gs. 047-052) . Typ1cally, the rafts are more rounded, rotated, and d1spersed 1n leucocrat1c neosome. The leucocratiC portions generally have a now fohatlon defined by the onentat1on of platy minerals, such as the feldspars and m1cas. T he m1gmat1tes called inhomogeneous d1atex1tes (Mehnert 1968, Brown 1979), or heterogeneous d1atex1tes (Solar and Brown 2001), have schollen structures and are typical of the transit1on from metatex1te to diatexite.

Schl ieric m igmatites have well-developed flow-1nduced structures indicated by trains of platy or elongate minerals, most commonly biotite, but also s1lliman1te. plagioclase, orthopyroxene, and amph1bole, that are called schlieren (the s1ngular 1s sch/iere). The microstructure of schlieren is shown later, in section F, Figs. F97 FIOO. Rafts or schollen of paleosome, resister litholog1es. and melanosome may be present. but they are far less abundant than 1n schollen m1gmat1tes, as can be seen 1n Figs. 053 056. The passage from schollen m1gmatite to schlienc m1gmat1te IS ach1eved by an 1ncrease 1n the melt fract1on, or neosome:paleosome rat10 (Fig. I b); stra1n has little effect. Small outcrops of schollen and schlienc migmatites tend to show a parallelism between success1ve bands of leucosome, schlieren, and raft-rich layers. However, larger outcrops invanably show reg1ons where the foliation and compositional banding or layering are truncated at a low angle by groups of s1milar-looking, although generally slightly more leucocratlc layers; Mehnert (1968) referred to these as interpenetrating forms. These structures are ev1dence of syn-magmatic shear zones, or now discont1nu1t1es 1n the part1ally molten rock, and are probably due to the partitioning of stra1n 1nto the zones w1th the highest fract1on of melt. Folds may develop 1n these shear zones. 1n wh1ch case they typ1cally have a sheath-like geometry.

Diatexite migmatites, 1n Fig. lb. occupy the h1gh fractionof-melt part of the fraction of melt versus strain plot; thus, they are dominated by neosome (F1gs. 057 062), and relics of paleosome are rare, or absent. Pre-partial-melt1ng structures, such as foliation, folds, and bedding, occur only

in the scattered schollen of paleosome. However, a foliation defined by the onentat1on of platy or tabular m1nerals. most commonly plag1oclase and m1ca, acqu1red dunng now 1n a magmatic or submagmat1c state, IS v1rtually ubiqu1tous 1n the neosome. A now band1ng due to layers of different mineralogy, grain size, or microstructure also may be present. and schlieren may be present but are scarce. Oiatex1te migmatites are gradational from schollen and schlieric migmatites through an 1ncrease 1n melt fraction (or neosome: paleosome rat1o), and from nebulites through an increase 1n syn-anatect1c strain (F1g. lb). wh1ch account for the1r char actenstiC now-Induced foliation and banding. As diatexlte m1gmat1tes tend to show subdued, or subtle. variat1ons 1n outcrop, the term "homogeneous diatex1te" has been applied to them (Mehnert 1968, Brown 1979, Solar and Brown 2001). Unfortunately, 1t IS not always the same property that is be1ng described as homogeneous. For Mehnert, homogeneous meant lack1ng a foliat1on, and he coined to term "homophaneous" spec1fically for these diatexites. Brown ( 1979) also used homogeneous in a structural sense, w hereas Solar and Brown (2001) used it in more of a morphological context to mean lacking schollen and schlieren. O ther investigators have found significant petrological diversity within, and among, d1atex1te m1gmatites in individual anatectic terranes and have, accordingly, chosen to subdiv1de d1atex1te migmatites on a petrolog1cal basis, e.g., the melt-nch and res1dual diatex1tes of Sawyer ( 1998) and the leucocrat1c, mesocratic and melanocrat1c d1atex1te m1gmat1tes of Milord et al. (2001). Almost all d1atex1te migmatites. includ1ng the schollen and schlienc vaneties, conta1n leucocrat1c patches and ve1ns. cons1st1ng of quartz, K-feldspar. and plag1oclase that appear to postdate the magmatiC foliation and now structures in thew host. In all the cases where the compos1t1on of these veins and patches has been 1nvest1gated, the leucosome represents a fractionated melt (also called residual melt) . 01atexite m1gmatites contain sufficient melt that as they cool and crystall1ze under applied differential stress. the crystals 1n them become aligned, g1v1ng the rock a foliat1on (see section 5.3) . A framework oftouch1ng crystals develops where crystallization reduces the fraction of melt below about 0.4. If deformat1on cont1nues. the framework then deforms heterogeneously, and this creates dilatant structures to wh1ch the evolved (fractionated) melt. located 1n the 1nterst1ces of the enclos1ng framework of crystals. m1grates; analogous structures are well known from crystallizing gran1tes (e.g.. Cuney et al. 1990, Pons et al. 1995, John and Stunitz 1997). Geometncally, therefore, these masses of leucosome resemble those 1n metatexite m1gmatites and those shown in Fig. 2. but they differ 1n morphology in that the melt depleted halos around them are not generally melanocratic, because they are dominated by feldspars.

19

High strain and the morphology of diatexite migmatites H1gh shear stram tends to generate better alignments of m1nerals, rafts (or schollen) of paleosome matenal and of the schlieren 1n d1atex1te m1gmat1tes. Deformation dunng crystallization can lead to the segregation of the residual melt and to transposition (e.g., Figs. 063 and 064); the resulting alignments commonly produce well-developed parallel planes of fol1at1on or compos1t1onal band1ng.

3.8 Migmatite morphologies outside the metatexite-diatexite division Two of the migmat1te structures defined by Mehnert (1968), vein and folded, are different from the other types described so far because they occur 1n both metatexite and d1atex1te m1gmat1tes. The pnnc1pal control on the morphology of fold structures 1n m1gmat1tes IS the mechan1sm by which the folds formed, and this 1s linked to the fract1on of melt present, and to the form, or geometry, of the anisotropy present 1n t he migmatite. In metatexite migmat1tes, where paleosome dominates, folds might form by processes such as buckling, but 1n d1atex1te m1gmatites, where neosome dom1nates. the folds may form because of flow nstabilit1es (i.e .. pass1ve flow folds). Ve1n m1gmat1tes. however, are s1ngular; thew morphology IS not controlled by melt fract1on. but by the ability of the1r host to fracture, e1ther by brittle or by ductile fracture, and by the t iming of the vein1ng process relative to the rest of the anatectic h1story.

Fold-structured migmatites, which develop at low fract1ons of melt, 1n the early stages of anatex1s, generally have morpholog1es that are controlled by the relat1ve difference in competency between the layers in the paleosome. In relatively Incompetent rocks, the folds in the paleosome are s1milar types, but if there are more competent layers, buckle (parallel) folds may develop (see Figs. EI-E8) . Dunng folding, some of the melt 1n a migmat1te m1grates nto dilatant sites that develop as the folds grow, resultng 1n leucosome 1n the fold h1nges and parallel to and between the folded layering, 1n ax1al planar shears, or 1n shears developed on the fold limbs. If competent layers are present, then the melt will also occupy the array of rad1al fractures that may develop around the fold h1nges 1n these layers (Coll1ns and Sawyer 1996). At th1s stage, then, the control on the d1stnbut1on of leucosome IS s1m1lar to the dilatant metatex1te m1gmat1tes discussed above. Several 1nvest1gators (e.g., Allibone and Norns 1992, Coll1ns and Sawyer 1996) have noted that there IS generally more leucosome in the fold hinges than on the fold limbs; this find1ng

could Indicate that limbs are regions of net loss of melt, and the h1nges. of net ga1n of melt, wh1ch IS confirmed by the analogue expenments of Barraud et al. (2004). Metatex1te m1gmat1tes 1n wh1ch bodies of leucosome are oriented parallel to the axial planes of syn-anatect1c folds appear to be very common (e.g., Edelman 1973, Hand and Dwks 1992, Brown 1994, Vernon and Paterson 2001), but they are quite diverse 1n appearance. In the maJonty of the examples that have been described, the bod1es of leucosome are thin, have smooth marg1ns and h1gh aspect-rat1os. However, 1n some cases, the leucosome accumulation IS thicker and more equant, with irregular outlines; these are typically also coarser gra1ned. Melanosome IS evident around the leucosome 1n some cases, but absent in others. In some migmat1tes. the leucosome 1n the axial plane is K-feldspar-nch compared to the leucosome elsewhere (e.g., jam1eson 1984). Such K-nch leucosome presumably was derived from anatect1c melts w1t h evolved compos1t1ons ow1ng to fractional crystallization. Why concentrations of leucosome should be located in axial surfaces has long been a controversial point amongst structural geologists workIng on migmatites; Brown and Rushmer (1997) and Vernon and Paterson (200 I) d1scussed th1s matter. Skelton ( 1996) suggested that the flow of melt on the limbs of a fold 1s parallel to the layenng and not across 1t; thus, the melt from both flanks necessanly accumulates at the nearest fold-h1nge, where layer-parallel flow IS no longer possible. Consequently, the melt e1ther accumulates in the hinge, or else must escape across the layering, that IS, 1n a direction necessarily subparallel to the axial plane of the fold. Where exam1ned closely 1n the field, many bod1es of leucosome descnbed as ax1al planar are, 1n fact, not so; they are onented so as to cross both fold limbs. Some Investigators (e.g., Edelman 1949, Kranck 1953, Wynne-Edwards 1963, Mclellan 1984) have noted that where the fract1on of melt reaches and exceeds the meltescape threshold (0.26 0.4), there is a progression from planar, cylindncal folds to noncylindrical, nonplanar disharmonic and convoluted types of folds, with decollement surfaces between some layers. However, h1nge directions commonly rema1n systematiC, and subparallel to the flow lineation; sheath-fold geometries commonly develop. It is important that a study of the microstructures be made to determ1ne whether the folding occurred when the rocks contained a h1gh fract1on of melt, or whether 1t occurred later, at lower fract1ons of melt when the rocks were crystallizing, or whether folding occurred ent1rely 1n the solid state, after all the melt had crystallized. The latter, of course, are folded migmatites and not fold -structured migmat1tes.

IN T RO D UCT IO N

20 -----------------------------

Ve in-structured migmatites (Figs. E9 E14) conta1n one or more generations of d1scordant. leucocratic ve1ns (of gran1tic, granodiont1c, or tonalit1c compos1t1on), wh1ch are typ1cally supenmposed on an earlier metatexite or diatexlte m1gmatite morphology. All of the sets of ve1ns, or only one of t hem, may be deformed, but commonly the youngest set consist s of undeformed planar fracture-fill1ngs that postdate the peak of anatexis and the peak production of a melt fraction 1n thew host. lnterest1ng microstructural relationships have been reported from ve1n migmatites. Some ve1ns contain trains of mafic m1nerals onented parallel to thew walls, whereas others have hehc1t1cally preserved structures from the1r wallrocks. Collect1vely, these microstructures may indicate inJeCtion mto part1ally molten wallrocks and eros1on of the ve1n walls as the melt flowed 1n the ve1n (Brown 1994, Sawyer et al. 1999, Solar and Brown 200 I). W1de borders w1th res1dual compositions are rarely developed around d1scordant leucocrat1c ve1ns. On the other hand, narrow mafic selvedges at the edges of the veins are very common features, and these may have formed as a result of a react ion between the wall rocks and the injected melt. or with an aqueous flu1d derived from the melt. The lack of a mass balance between the volume of melt in the vein and the volume of matenal 1n the adJacent mafic selvedge (or residual border) IS ev1dence that the melt was not denved from 1ts immediate host. but was InJected. It is important to determ1ne that the melt 1n the ve1ns belongs to the same anatectiC event as the m1gmat1tes; 1f the ve1ns belong to a younger event. then the term "vein-structured m1gmat1te" does not apply; 1t 1S a ve1ned migmat1te.

3.9 D escriptive terms that should be abandoned Be dde d migmatite was used by Greenfield et al. (1996) and White et al. (2003) to describe migmatites 1n wh1ch the neosome, and the leucosome 1n particular, are confined to certa1n layers of a part1cular bulk compos1t1on, generally metapelite; examples are shown 1n Figs. AI, Bl7, and Bl8. The examples descnbed 1n the literature d1splay a considerable range in proport1ons and morphology of the neosome, and espeoally the leucosome. Some examples 1n wh1ch the proport1on of neosome 1s relat1vely small have leucosome matenal located 1n dllatant structures (e.g., between boud1ns) or onented parallel to the layenng and foliat1on, and assoc1ated w1th prom1nent melanosome. However, 1n others the layer has been ent1rely converted to neosome in which the melt and the res1duum, typ1cally. have not segregated. Nevertheless, for both of these, an 1n Situ, closed-system origin for the neosome is plausible in which melting occurred in only those layers that were fertile; these are, therefore, exactly analogous to the origin

of stromatic metatex1te migmat1tes descnbed by johannes and Gupta (1982). johannes (1983a). and johannes et al. (1995). In contrast. some of the layers have a very high proportion of leucosome, which suggests that melt was added. i.e.. open-system behavior, unless perhaps the bulk composition was optimal for a very high fraction of melt to form. For these examples with a h1gh fraction of melt. the bed or layer may have acted as a dilatant s1nk 1nto which melt from the outs1de has moved. At a sufficiently high fract1on of melt. magmat1c flow can occur in the layer 1f 1t is deformed (e.g., Sawyer 1996). At Mount Stafford, 1n Australia, shear stresses were low and ne1ther flow nor segregation of the melt from the solid fract1on occurred (Greenfield et al. 1996, White et al. 2003); consequently. the neosome has an 1sotrop1c appearance (e.g., F1g. B17). The term "bedded m1gmat1te" is 1nappropnate, as it does not refer to any process act1ve during the format1on of these migmatites. Furthermore, structures such as bedding or compositional layering that are older than the partial melting are still preserved in a coherent fashion; therefore, these are, necessarily, metatexite migmatites. Consequently. the term "bedded migmat1te" ought to be abandoned.

Agmatite was defined by Sederholm (1923) as a rock w1th "fragments of older rock cemented by gran1te", and was regarded by h1m to be a type of m1gmatlte. However, rocks match1ng th1s descnpt1on can be found around Igneous 1ntrus1ve bodies 1n low-grade or unmetamorphosed country-rocks. Brown ( 1973) argued that agmatites are not m1gmat1tes. and should be called 1ntrus1on brecc1as. Consequently, the term "agmat1te" ought to be abandoned.

Ptygmatic migmatites. The format1on of ptygmat1c folds reflects a particular combination of ve1n and host-rock properties and strain conditions that are not specific to migmat1tes. Moreover, it is common that not all the bands of leucosome in a particular outcrop d1splay ptygmat1c foldIng. Therefore, the term "ptygmat1c" should be reserved to describe the style of folding displayed by some bands of leucosome. and not used to descnbe the overall morphology of a m1gmatite.

Ophthalmite mig m at it es, as defined by Mehnert (1968). are charactenzed by the presence of augen. or lent1cular aggregates of newly formed m1nerals. Many of the augen would now be called porphyroclasts. and the lenticular aggregates v1ewed as d1srupted. coarse-gra1ned quartzofeldspathic layers. In addition, the rocks typically have a strong planar fabric and, 1n some cases, oblique subsidiary foliat1ons. which, taken together, are geometriplanes, and shear bands, planes. cally equ1valent to

·c

·s·

Ada.., of

~11gma t 1tc..,

----~------------------------ 21

respectively. Thus. the structure of ophthalmite migmatites closely resembles that found 1n many coarsely crystalline rocks deformed to h1gh stra1ns under h1gh. but subsolidus. temperatures and that have subsequently undergone some recrystall1zat10n. Therefore, the ophthalm1te struct ure commonly reflects post-anatectic, not syn-anatectic, processes. Consequently, ophthalm1tes are not migmatites, although their protolith may have been a m1gmat1te. and they are better called augen gneisses or blastomylon1tes.

metasomatism is now cons1dered to be an 1mportant, ub1qu1tous, or necessary process 1n crustal anatex1s, although there are other v1ews (e.g.. L1tvonovsky and Podladch1kov 1993). Nevertheless, there are some m1gmat1tes where 1nfiltrat1on metasomat1sm has played a sign1ficant role, and these fall into two broad groups: (I) the ingress of aqueous fluid into hot rocks and (2) metasomatism prior to anatexis that changed (improved) rock fertility.

4.1 Influx of aqueous fluid into

4. METASOMATISM AND MIGMATITES In the first half of the twentieth century. three lines of reasoning contributed to the 1dea that flu1ds are fundamental to the generat1on of granites and migmat1tes. First. the op1n1on of Bowen (1915, 1928) that gran1t1c magma could only be formed at the final stages of the fractional crystallization of a basaltiC magma influenced many geologists to believe that the gran1t1c rocks found abundantly in shield areas may not be magmatic. Second, observations from many terranes appeared to show gradat1onal contacts between metamorphic rocks and gran1tes, and were Interpreted as indicating an m s1tu. nonmagmatic transformation. The v1ew that granites were formed by an 1n Situ transformation was strengthened by the find1ng of fragments and screens of metasedimentary rocks in gran1tes that still retained the onentat1on of the rocks outside the granite (Misch 1949, P1tcher 1952). Third, whole-rock geochemical data appeared to show a systematic 1ncrease of gran1t1c components w1th 1ncreas1ng depth (Lapadu-Hargues 1945) and the existence of compos1t1onal fronts 1n the metamorphic rocks and migmatites around gran1tes; this was 1nterpreted as evidence of the movement of granite-making matenal (and of gran1t1zation) in the crust. One school (e.g., Pernn and Roubault 1937, 1939) argued that the movement of matenal occurred by diffus1on 1n the solid (dry) state, whereas the other (e.g., Sederholm 1926, Backlund 946, M1sch 1949) argued that aqueous flu1ds moved the matenal. More recently, Korzhinsk1 ( 1971) called these two alternatives diffusio n m etasomatism and infiltratio n met asomatism, respectively.

hot rocks causing partial melting Large .. scale influx of fluid The extensive development of migmatites with a high proportion of leucosome denved from leucocrat1c tonalit1c, trondhjem1t1c, granod1ontlc or granitic protoliths IS common 1n many Archean and Proterozoic sh1eld areas. These m1gmat1tes are problematical. The absence of muscovite and the small amount of b1otite or hornblende (or both) in these rocks preclude the generation of a large fraction by dehydration melting. Consider the hornblende-free m1gmatites first; five modal percent biotite y1elds about

8 vol.% granit1c melt. Furthermore, the general absence of cordiente, garnet and orthopyroxene from these m1gmat1tes also precludes volatile-phase-absent Incongruent melt1ng of b1ot1te. Consequently, the high fract1on of melt 1n these migmat1tes is generally attributed to an influx of H 20 close to, or just above, the solidus temperature, which enabled H 20-saturated (congruent) melt1ng of quartz + sodic plag1oclase + K-feldspar to occur (e.g., Kenah and Hollister 1983, Mclellan 1988, Nedelec et al. 1993, Sawyer

1998). Many of the m1gmat1tes denved from fels1c, plutomc protoliths conta1n hornblende 1n the melanosome; th1s could be a product of melt1ng reactions 1nvolv1ng the breakdown of biot1te. Recent expenmental studies (Gard1en et al. 2000) indicate that the formation of product (res1dual) hornblende from the breakdown of biotite requ1res the addit1on of H,O, and hence volatile-phase-present melting. In some cases, the abundance of the large-1on lithophile elements (LILE) IS greater than can be attributed to the protolith, such that they may have been Introduced with the flu1d, generating potass1um-rich m1gmat ites from rather potass1um-poor protoliths.

Subsequent systematiC study of the whole-rock compoSitions of metasedimentary rocks rang1ng from low grade to h1gh grade (e.g., Shaw 1954, 1956: Haack et al. 1984:

The source of the flu1d. or how it enters the rock, IS not understood, but the protoliths of most of these m1gmatite terranes were calc-alkaline rocks formed 1n pluton1c arcs (although the anatexiS may have occurred many m1ll1ons of

Sawyer 1986) has shown that crustal metamorphism IS close to be1ng ISOchemical, except for the loss of H 10 and C02. Consequently, neither diffusion nor infiltration

years later: e.g., Slagstad et al. 2005). and both the associated subducted material and the underly1ng metasomatized mantle wedge are possible sources for the flu1d and the

I N T RODUCT IO N

22 -------------------------------

LILE. A s1milar process seems to occur presently in act1ve island arcs: H 10 entenng the base of the arc fluxes partial melt1ng of hot. mantle-denved mafic rocks there, and generates the more fels1c magmas found at h1gher levels (e.g., Hansen et al. 2002). Part1al melt1ng fluxed by the large-scale influx of H20 into hot rocks has been suggested for the generat1on of upper-amphiboltte-faCtes migmat1tes (Yardley and Barber 1991, Johnson et al. 2001b, White et al. 2005) and for lower-granulite-faCies metatex1te and diatexlte m1gmat1tes (Otamendi and Pat1f\o Douce 200 I) derived from pelttes and aluminous metagreywackes, respect1vely. The aqueous flu1ds that fluxed melttng were probably released by prograde metamorphic dehydrat1on-type reactions occurnng 1n adJacent rocks, wh1ch. because of the1r bulk compos1t1ons. had slightly h1gher soltdus temperatures (Fornelli et al. 2002, Wh1te et al. 2005) . Castro (2004) has proposed that K-nch granodiontic magmas are generated from older tonalit1c cont1nental crust where H 20-nch intermediate (e.g., andes1tic, ferrodiont1c or monzodioritic) magma generated from the hybndized parts of a mantle wedge, suitably modified by flu1ds released from subduct1on zones, are inJected into the lower crust. The H, 0 liberated from the crystallization of the Intermediate magma comes into contact with the adjacent hot. dry tonaltte and promotes flu1d-present part1al melt1ng. The result1ng anatectic melts are ennched 1n K and Rb (and possibly the other LILE) transported from the Intermediate magmas 1n the hydrous fluid. The CaO content of the anatectic melt IS increased to levels typ1cal of granod1orite by the dissolution of m1crogranular enclaves cons1sting of the Intermediate magma. Therefore, these m1gmat1tes show not only the usual paleosome neosome relationships. but also ev1dence of the interaction between anatectic melt and the intermediate magma, such as mtllimeter- to centimeter-scale lobate interface structures and m1crogranular enclaves, and of the synchronous intrust on of dikes of 1ntermed1ate composition tnto the protoltth.

SmaU.. scale influx of fluid Results from studtes on a hand-sample scale, us1ng many different techn1ques 1ncluding neosome protolith mass balance (Olsen 1982. 1984. 1985: Olsen and Grant 1991: Patt1son 1991), whole-rock composttions (Weber and Barbey 1986), flu1d tnclustons (Olsen 1987. Whttney and lrvtng 1994), reactton progress (Symmes and Ferry 1995). and constderation of melttng react1ons (Johnson et al. 200 Ib), have establtshed that some m1gmat1tes formed when a local tnflux of H 20 enabled already hot rocks to partially melt. The whole-rock and mass-balance methods commonly show that the fluid also introduced K. Rb, and other elements tnto the rocks.

One common scenano IS the Intrusion of anatectic magma 1nto the parts of a m1gmatite terrane where metamorph1c temperatures were above the soltdus, yet Insufficient for stgntficant dehydratton melttng of b1ot1te to have occurred in the muscovite-free rocks. Aqueous fluids (and heat) re leased by the crystalltzatton of the tntrustve body tnfiltrates into the hot country-rocks and enables H20-present melting to occur, generattng what Finger and Clemens (1995) have called secondary m igmatites. Several investigators (e.g.. Symmes and Ferry 1995, Harris et al. 2003, Johnson et al. 2003) have shown that fluids generated by dehydratton reacttons in the outer part the contact aureoles have m1grated 1nward. up the temperature grad1ent. and enabled H 0-present melt1ng to occur close to the intrustve body: H 0-absent parttal melt1ng occurs still closer to the 1ntrus1ve contact. In some tnstances, the flu1ds have transported trace elements, such as B. Ba, and Li, which serve to reduce the temperatures required for part1al melttng to start (e.g.. Wh1te et al. 2003). The breakdown of tourmal1ne dunng upper-amphibolite-facies anatexis may also lead to a boron-rich melt and a borondepleted, restdual crust (Kawakami 200 I, Kawakami and Ikeda 2003). Instead of, or tn addttton to, local-scale flutd tnfiltratton, Butck et al. (2004) suggested that more extens1ve parttal melttng may occur at relat1vely low temperatures 1n contact aureoles tf the flu1d from the low-grade protolith rematns tn the system once part1al melt1ng starts (i.e., the system is closed to loss of volatiles). Th1s scenano IS plausible dunng raptd heattng 1n the innermost parts of h1gh-temperature contact-aureoles and, tn effect. renders the protolith more ferttle than 1f heat1ng had been slower and if the flutd had had suffiCient t1me to mtgrate away. Buttner et al. (2005) proposed that a stmdar process may have occurred 1n regionally metamorphosed rocks 1n Argent1na. There, fluids generated by prograde dehydration-type reactions tn the greenschtst and amphiboltte faCies rematned tn the rocks and fluxed htgher degrees of parttal melttng in granulttefaCtes rocks.

4.2 Metasomatism and changes in the fertility of rocks If the bulk compos1t1on. and hence the fertility, n parts of a metamorphtc terrane are changed as a result of Infiltration metasomatism that occurred before anatexis, then the d1stribut1on of anatectiC m1gmatites will reflect th1s. Sawyer ( 1991) noted that the first stgn of anatexts and m1gmatites in mafic protoliths was tn strongly foliated metamafic rocks, and not in the masstve equtvalents. The foliated metamafic

Ada-. nf ~i•gmatttc!<.

----~------------------------23

rocks represented an earlier, pre- to syn-partial-melting shear zone that had been hydrated by a flUid that tntroduced H 0, K 0, Rb, Ba, and Cs. Thus, the shear-zone rocks were made more ferttle than the adJacent masstve metamafic untts. and so melted first.

4.3 Morphology of migmatites affected by infiltration metasomatism Where the tnflux of flutd has caused partial melttng, the resulttng mtgmatttes could have any of the morphologtes characteristic of metatexite and dtatextte migmatites. However, there are two additional factors to consider, which could affect the appearance of the migmatites that form in outcrop. (I) Melttng occurred where the aqueous fluid was present. and hence the overall morphology of the migmattte should m~rror the path taken by the tnfiltrated fluid. In many cases. the fluid tnfiltration was along foliatton planes or lithologtcal layenng, and thts ts reflected by the stromattc (layered) or bedding-confined morphologtes of the resulting migmatites. However, net geometries can also be generated (e.g.. Pattison 1991) tf flutd tngress was through a network of fractures (Ftgs. B31, B32). Dilatancy pumptng tn shear zones (Barr 1985. johnson et al. 2001b) can move aqueous flutds tnto hot rocks, and the geometry of the resulttng anatecttc leucosome should be related to the pattern of dilatant structures that formed, and hence to shear-zone kmemattcs; thus the morphology of the migmatite could be qutte vaned. (2) The degree of parttal melting ts a function of how much flutd was avatlable tn a gtven volume of rock. Furthermore, permeability may differ from layer to layer. Thus, on a local scale, there can be greater variatton tn the degree of partial melting from place to place, and correspondtngly in migmatite morphology, than is generally the case for dehydratton melttng. The layers that generated much melt could become enttrely neosome. whereas adjacent layers may not have melted at all, etther because of refractory bulk composttions, or because they were impermeable.

5. MICROSTRUCTURES IN MIGMAT IT ES In recent years, there has been a constderable advance tn understanding how mtcrostructures form in solid and partially molten rocks. In thts sectton, I identify some of the key mtcrostructures and quantttattve analyttcal techntques that may be used to tdentify the processes that have contnbuted to the formation of mtgmatttes. Most of the mtcrostructures discussed tndtcate the former presence of melt, and hence constttute petrographtc cntena for dtsttngutshtng migmatites from the migmattte-like rocks formed by subsolidus segregatton or other processes (see section 8) . Photomicrographs that tllustrate the mtcrostructures dtscussed here are shown tn sectton F of the book, and are arranged m the same order. In order to fully understand the mtcrostructures. the mtneral parageneses have to be tdentified, and the sequence of overprinting parageneses must be determtned. Once thts has been accomplished, the petrogenests may be tnterpreted by reference to quantttattve petrogenetic gnds. In recent years. the tnstght that can be gatned from mtneral parageneses has been greatly tncreased through equtlibnum thermodynamtc modeling of the spectfic bulk compostttons of the samples betng tnvestigated. Thts has become possible because Internally consistent thermodynamic datasets are now reasonably comprehenstve (e.g., Holland and Powell 1998), and models for the acttvtty composttton relattonshtps for the minerals of 1nterest 1n anatexts have been improved (e.g.. Holland and Powell 2001, NCKASH; Wh1te et al. 200 1, NCK FMASH; Tinkham et al. 2001, MnNCKFMASH). In addit1on. software such as Thermocalc (Powell and Holland 1988, Holland and Powell 1998) enables equilibrium phase-diagrams for the speetflc composition of the bulk rock, or equilibnum domatn, of tnterest to be constructed as a so-called pseudosect1on or as a chemical potent1al diagram. Thts book does not contam any general phase-d1agrams for part1al melt1ng for th1s reason.

5.1

Mineral paragenesis

The approach 1s the same as that used by metamorphtc petrolog1sts 1n general. All mtnerals are 1dent1fied. and the way in whtch they are spatially related IS documented (i.e., whether m1nerals only occur as tnclusions 1n others, whether grains are corroded or not, whether all m1nerals are tn mutual contact somewhere in the rock, etc.) . This tnformation is gathered systematically across the migmatite terrane to det erm1ne the sequence of mtneral parageneses.

INTRO DU CTI O N

24 -----------------------------

Generally, th1s informat1on is arranged 1n order of 1ncreas1ng metamorph1c grade, together w1th 1nformat1on on the bulk-rock compositions in wh1ch they occur. Then, a petrogenetiC grid or an appropnate pseudosect1on is used to identify a sequence of metamorphic react1ons that may account for the mineral parageneses found, and the measured modal abundances of the minerals. In most cases, the greatest d1versity of m1neral parageneses and microstructures 1S to be found in the res1dual rocks and paleosome. Res1ster lithologies are commonly competent, and along with stra1n shadows, should not be 1gnored: they may conta1n m1nerals and microstructures that could reveal the earlier part of the P-T-t h1story destroyed 1n rocks where stra1n was h1gher. Some recent examples of the use of m1neral parageneses and microstructures to reconstruct the sequence of melting react1ons are prov1ded by Hartel and Pattison (1996), Ra1th and Harley (1998), Greenfield et al. ( 1996), Fitzsimons ( 1996), Cesare (2000), White et al. (2003) , and Johnson et al. (2003) . These 1nvest1gators show that the careful examination of the microstructural or textural relationships among minerals in migmatites rema1ns an extremely important part 1n understanding their development. In the past. the conditions of part1al melt1ng were e1ther calculated us1ng suitable geothermometers and geobarometers. or est1mated from a petrogenetic gnd (e.g.. Grant 1985. Vielzeuf and Holloway 1988, Grant and Frost 1990, Spear et al. 1999). As more stud1es of partial melting have been done, the range of bulk compos1t1ons and react1ons 1nvest1gated has expanded and now 1ncludes metapelites (Vielzeuf and Holloway 1988, le Breton and Thompson 1988, Pat1no Douce and Johnston 1991, Carnngton and Watt 1995, Patlfio Douce and Hams 1998. P1ckenng and Johnston 1998, Koester et al. 2002, Sp1cer et al. 2004), metagreywackes (Conrad et al. 1988: Vielzeuf and Mantel 1994: Gardien et al. 1995, 2000: Pat1no Douce and Beard 1995, 1996: Stevens et al. 1997: Mantel and V1elzeuf 1997: Castro et al. 1999: Nair and Chacko 2002: Grant 2004). amph1bolites (Rushmer 1991: Rapp and Watson 1995: Wolf and Wyllie 1991, 1994: Spnnger and Seck 1997: Skjerhe and Pat1no Douce 2002: Selbekk and Skjerlie 2002). tonalite (Rutter and Wyllie 1988: Skjerhe and Johnston 1992, 1996: Smgh and Johannes 1996a, b: Pat1no Douce 2005). trondhJemite (Johnston and Wyll1e 1988), gran1te (Holtz and Johannes 1991, Castro et al. 2000, l1tv1novsky et al. 2000, Acosta-V1gil et al. 2006), and fels1c to Intermediate volcanic rocks (Conrad et al. 1988). The cond1t1ons of melting 1n migmatites are now commonly deduced by d1rect comparison with the experimental results done on similar bulk compositions.

The development of a thermodynamiC model for Silicate melts (Holland and Powell 200 I, Wh1te et al. 200 I) has made 1t poss1ble to calculate petrogenetiC grids and pseudosections for 1nd1v1dual rocks 1n the reg1on of P T-X space where part1al melting occurs, and to adjust thew bulk com positions for the loss of some, or of all, of the granitic melt (White and Powell 2002). This powerful new approach is being applied to part1al melting: the consequences of melt extraction on mineral assemblages, m1neral modes, and mineral compositions 1n pelit1c. sem1pelitic, and psammitK rocks may now be invest1gated 1n a quantitative manner. Recent examples of th1s apphcat1on are g1ven by White and Powell (2002). Wh1te et al. (2003 2005), Johnson et al.

(2003), and Johnson and Brown (2004).

5.2 Quantitative analysis All methods of quant1tat1ve analys1s have the potential problem that the microstructure preserved 1n the rock might not be that wh1ch developed when anatectiC melt was present. The possibility that the microstructure has been partially, or wholly, modified subsequently must be carefully evaluated. Deformation tends to reduce the grain size and to produce changes 1n gra1n shape and grain orientation. Recrystallization tends to increase grain size, to eliminate the small grains, and to produce more equant gra1ns (1.e., to reduce the total area of gra1n boundanes). In addition. retrograde metamorph1sm may Introduce new m1nerals. Fortunately, the gra1n-scale petrographic features result1ng from deformat1on. recovery, and recrystallization are well documented (N1colas and Po1ner 1976, Wh1te 1977, Vernon 1981 ), and are easily 1dentified 1n th1n sect1on, so that later microstructural modificat1on can be recognized in most cases. The papers by Berger and Kalt ( 1999) and Berger and Roselle (200 I) offer introductions to this theme, as well as to quantitative analytical methods.

The grain,contact method This method IS based on the assumption that rocks that crystallized from a melt under cond1tions of hydrostatic stress should develop a random spatial d1stnbut1on of the constituent mineral spec1es. Rocks that crystallized 1n the solid state. or from now1ng magma. or rocks subsequently deformed penetrat1vely. all have mineral d1stnbut1ons that depart from randomness. Contacts between like m1nerals can be e1ther more (aggregate distnbut1on) or less (dispersed distribution) common than 1n the random case. The gra1n-contact method, descnbed by Ashworth (1976). Mclellan (1983), and Ashworth and Mclellan (1985), involves determ1n1ng the frequency of contacts between the various mineral species in a rock, and then conducting a statistical test on 1ts randomness. Symmes and Ferry (1995) applied the techn1que to doma1ns of leucosome 1n

At!." nf M 1gma t1tes

----------- ----------- --------- 25

a

Quartz

E

.s Q)

N

(j)

c

"(ij

(3 0. 1

0.01 0

5

10

Metamorphic Grade (distance in km)

b

P lag ioclas e

E

.s

EtfiD

Q)

N

0

(j)

-~

0.1

0

(3

0

I:ID

0

Fig. 3. Change in the gra in size of metamorphic minerals versus grade Juring 3.5 kbar regional metamorphi m in the Quctico Subprovince, Canada; (a) quartz and (b) plagioclase. There is a ystcmatic increase in the grai n size from the greenschi t facies to the sta rt of melting in the upper amphibolite facies. The grain size in the mctatcxite migmatites increases at about the amc rate as in rocks below the "melt-in" i ograd. A very marked increase in the grain izc of both quartz and plagioclase coincides with the appearance of diatcx itc migmatite and affects both the residual rocks in mctatcxite migmatites and the diatcxite migmatites. However, it does not affect the paleo omc and resister lithologies (triangles), which did not melt, and which occur as schollcn in the diatcxitc migmatitc. The increase in grain size for these rocks follows the same subsolidus, grain-growth trend as that defined by the unmcltcd greenschist- and amphibolite-facies rocks. Data a rc provided for metagrcywackc (plagioclase + biotite + quartz psammitcs) and mctapclitic rock . Symbols, circles and ~quarcs: open, rocks from the greensch ist and a mphibolite facies; half-filled, residual part of mctatexite migmatites; filled, diatexite migmatites. Triangles, unmelted, massive psammitic, palcosomc lit hologies forming schollen in the diatcxitc migmatites.

8:J

0.01

0

5

10

Metamorphic Grade (distance in km)

the aureole of the Onawa Pluton 1n Maine. They found that leucosome from the sillimanite zone had dispersed distributlons but that leucosome from the 1nnermost zone had random distributions. The former were Interpreted as of subsolidus ong1n, and the latter, as of anatect1c ong1n.

Crystal~size

distributions

The study of crystal-s1ze distnbut1ons (CSD) has the potential to reveal 1nformat1on about the crystallization h1story (nucleat1on and gra1n growth) of rock-form1ng minerals. The technique is most commonly used to investigate rap1dly cooled volcan1c rocks, where subsolidus modifications to the gra1n s1ze and shape are believed to be m1n1mal; the result1ng s1ze-dlstribut1on patterns thus largely reflect cryst allizatiOn from the magma. In slowly cooled rocks (e.g., plutons and metamorphic terranes). processes such as surface-energy-driven and stra1n-energy-driven gra1n-boundary migration tend to produce rather umform gra1n-s1zes, and the crystal -s1ze d1stnbution is peaked. More complex histones of Simultaneous coarsening, nucleation, and growth lead to a reduction 1n the number of small grains and an increase 111 the number of larger grains (e.g.. Higgins 1998). Berger and Roselle (2001) found a vanety of

crystal-s1ze distributions 111 the Bayerische Wald migmatites. Euhedral cord1ente from diatexite migmatites exhib1ts a linear crystal-s1ze distribution cons1stent with crystallization 111 a melt. In contrast. samples of leucosome show more complex patterns. The K-feldspar from quartz K-feldspar leucosome has a concave-upward pattern, indicating an overproduction of large gra1ns, wh1ch could be due to the latent heat of crystallization. or to homogeneous nucleation. Plagioclase crystals from nearby areas of leucosome have linear crystal-size distributions, but with a defic1ency of small grains, attributed to heterogeneous nucleation or gra1n coarsening. Berger and Roselle (200 I) illustrated the potent1al range of problems that can be addressed 111 stud Ies of crystal-s1ze d1stribut1on applied to m1gmat1tes. and 1dent1fied some of the problems 111 apply1ng the techn1que to these rocks.

Studies of grain size, aspect ratio, and orientation Most quantitative stud1es (e.g., Ashworth 1976, Johannes and Gupta 1982, Mclellan 1983) have shown that grain s1ze increases more abruptly after melting begins (see Fig. 3); th1s IS attributed to the fact that the melt phase w1ll a1d

INTRODUCTION

26 ------------------------------

diffus1on. and hence Ostwald npemng. Furthermore. a systematically smaller gra1n-s1ze from leucosome to melanosome to paleosome IS typ1cal, although grain s1ze in each generally 1ncreases w1th metamorphic grade. However, reg1ons where melt has been extracted can show a decrease in grain s1ze (Dougan 1983). Berger and Kalt ( 1999) found that differences between the extent of latt1ce-preferred orientation and shape-preferred orientation of b1ot1te, cord1erite, and plagioclase are related to whether the rocks were melt-nch (leucosome), or meltpoor (melanosome and paleosome), and whether they have expenenced melt segregation, and hence either submagmatiC or solid-state deformation. M1lord and Sawyer (2003) compared residual gra1ns of b1ot1te 1n schlieren w1th those 1n the host diatex1te and anatectiC granite. They found that nakes of biOtite In the schlieren are generally larger, have greater aspect-rat1os. show a marked absence of the small grains. are better onented, and d1splay a slightly narrower range of compositions, with marginally higher Mg number and Ti0 2 contents, than biotite from the host. They attributed these features to recrystallization of the biotite after 1t had been concentrated 1nto the schlieren.

5.3 Diagnostic microstructures in migmatites Because m1gmatites conta1n parts where melt1ng occurred, parts from wh1ch melt was removed, parts through wh1ch melt has passed, parts where melt has collected, and 1ndeed parts that did not melt. the type of microstructures (and also m1neral assemblage and bulk compos1t1on) preserved in each may be qu1te d1fferent. The melanosome should be dom1nated by microstructures 1nvolv1ng minerals that were present at the t1me of melt1ng, e1ther those in excess or the solid products of Incongruent melt1ng react1ons. In contrast, the leucosome, and other melt-nch m1gmatites (e.g., diatexites), are dom1nated by m1nerals and microstructures that result from the crystallization of the anatect1c melt or magma. The microstructure of the paleosome should refiect h1gh-temperature solid-state metamorph1sm. The presence of glass is generally considered suffic1ent ev1dence for the former presence of a melt. or melts, 1n a rock. The problem in slowly cooled m1gmat1tes IS that the melt would normally have crystallized, and not quenched to glass. Fortunately. there are microstructural cntena that can be used to 1nfer the former presence of melt; in some cases, 1t IS possible to determ1ne whether the microstructures formed during melt1ng, or later, as the melt fraction crystallized. The microstructures produced in quenched melting experiments (e.g., section F 1n the book, Figs. FI- F4) serve as a good start1ng po1nt for an interpretation of the microstructures 1n m1gmat1tes. and will be outlined first.

Microstructures produced in partial.. melting experiments AnatectiC melt first forms a th1n film along grain boundarIes and edges, and small triangular and tetrahedral, cuspate pockets at the corners among the reactant minerals (e.g., Mehnert et al. 1973). The reactant m1nerals become progressively more corroded, or embayed, as the fraction of melt increases, and commonly form small, rounded inclusions in the melt (e.g., Busch et al. 1974, Holyoke and Rushmer 2002, Acosta-Vigil et al. 2006) . The products from the 1ncongruent melt1ng of ferromagnes1an minerals, e.g., orthopyroxene from the breakdown of biotite, can occur as needles 1n the melt close to the reactant m1neral. The melt tends to form pools 1n many expenments: jurew1cz and Watson (1984) showed that pools of melt form during part1al melt1ng and that the pools have triangular, cuspate margins w1th concave edges because the reactant minerals adjacent to the pool of melt become rounded as they dissolve 1nto the melt phase. In contrast. when a pool of melt crystallizes, the shape of 1ts marg1n changes: 1t becomes more rectangular, or blocky, in outline because the crystals grow into the pool of melt and have crystal faces (Jurewicz and Watson 1985, Wolf and Wyllie 1991). In some cases, these crystals grew ent1rely from the melt, and 1n others they are s1mply overgrowths on former reactant phases (Acosta-Vigil et al. 2006). A common feature of the quenched part1al-melting expenments IS that the crystals that grew 1n the melt have crystal faces agamst the glass (e.g.. Gard1en et al. 1995). However. the aocular hab1ts and skeletal morphology of some crystals may be due to the rap1d rate of cooling imposed on the expenment. Virtually all deformation-melting expenments were performed at confin1ng pressure and stra1n-rate condit1ons such that intergranular and intragranular fractures and shear zones developed and accommodated most of the deformation. In these expenments, much of the melt that was generated IS located 1n shear zones and fractures that develop in the sample (e.g., Rushmer 1995, Rutter and Neumann 1995, Holyoke and Rushmer 2002). Such conditions may not be applicable to anatex1s deep 1n the continental crust. although they may apply to part1al meltIng in some shallow contact-aureoles. In summary, the follow1ng key microstructures develop as part1al melt forms in experiments: (I) convex, tnangular or tetrahedral, cuspate patches of melt develop at the corners where the grains of the assemblage of reactant mmerals touch, (2) narrow, cusp-shaped films of melt penetrate along the gra1n boundanes 1nto the host away from pockets or pools of melt, (3) reactant m1nerals have corroded and em bayed shapes. (4) pools or pockets of melt in the matrix have a rounded, cuspate outline, (5) the solid products of reaction (also called pentect1c) have crystal faces where they grew 1nto melt. and ( 6) if cracks or shears form in the

:\tl;>- of :1,1ogmatlte'

----------------- -------------- 27

part1ally molten material, then they are occup1ed by melt. The follow1ng key microstructures form dunng the crystallizatiOn of the melt: (I) the m1nerals that crystallize from the melt tend to be euhedral, (2) the m1neral products of the part1al-melt1ng react1on (1.e., the pentect1c phases) that grew part1ally. or wholly, 1n the melt commonly develop faces, (3) the pools or patches of melt have a blocky outline, (4) an overgrowth develops on the reactant minerals, and (5) an alignment of tabular or pnsmat1c crystals 1s developed 1n the places where now of the melt occurred.

Microstructures in the residual rocks, and evidence for partial melting Generally, the melt fraction that forms 1n a migmat 1te 1S segregated into s1tes that become patches of leucosome. However, some melt (a few percent) remams 1n the residuum, where 1t occup1es grain corners, or forms films along gra1n faces. Moreover, the residual parts of migmat1tes contain the ev1dence with wh1ch to 1dentify the melt-produong react1ons and to deduce the conditions of metamorphism; thus, they are the best place to start a study of migmat1tes. There 1s a progressive change 1n the microstructure of m1gmat1tes from very shallow contact-metamorphism through deeper contact-aureoles to reg1onal metamorph1c terranes that renects the systemat1c increase in the durat1on of the part1al-melt1ng event, and a decrease 1n the cooling rate. Fast cooling near the surface results 1n supercooling of the melt and a quench to glass, but as the degree of undercooling decreases with depth, there is a change from fine-gra1ned to coarse-gra1ned granophyre 1n progressively deeper contact-aureoles to coarse-gra1ned, polycrystalline aggregates 1n the migmatites formed 1n regional metamorphic terranes. The microstructures found in rap1dly cooled part1al melts formed in subsurface contact-metamorphism (e.g., Knopf 1938, Wyllie 1961, Frankel 1950, Kaczor et al. 1988, Knesel and Dav1dson 1999, Holness et al. 2005) are essent1ally 1dent1ca. to those found 1n quenched part1al-melt1ng expenments (see Figs. F5 and F6) . Planar and cuspate, 1ntragranular f ms of glass, more extens1ve patches of glass, and even ve1ns of glass are commonly reported . Typ1cally, vanet1es of glass of several different colors and compos1t1ons are found with1n ind1v1dual samples. The glass conta1ns corroded rel1cs of reactant quartz and feldspar (e.g., F1g. F5), together w1th newly grown prismatic tridym1te. 13-quartz, orthopyroxene (e.g., Fig. F6), cord1ente. and magnet1te. Glass also IS found in some pelit1c and fels1c plutonic rocks that were partially melted at mid-crust depths (5 7 kbar), but then brought rap1dly to the surface as xenoliths 1n erupting lavas (Le Ma1tre 1974, Bacon 1992, Cesare et al. 1997, Cesare and Ma1nen 1999). Some of the microstructures 1n these

xenoliths (see Figs. F7 Fl 0) are like those produced 1n many part1al-melt1ng experiments. As an example, Fig. FlO shows corroded gra1ns of b1otite 1n wh1ch euhedral sp1nel (a react1on product) IS located in the embayments and is separated from the reactant b1otite by a film of brown glass (Grapes 1986, Cesare 2000). Platten (1982) found very fine-grained granophyre 1n partially melted quartz1te from a shallow (0.5 kbar) contact-aureole. The granophyre forms small cuspate patches between rounded reactant phases. and some patches conta1n corroded quartz and feldspar. Thus, the microstructures in slightly deeper aureoles (Holness and Watt 2002, Holness and Isherwood 2003), and 1n some xenoliths (Bouloton and Gasquet 1995), where cooling was a little slower, are also s1milar to those found 1n melting expenments, except that the melt has crystallized to granophyre (see Figs. Fll and F12). Cracks that were filled with melt are a common feature 1n the part1ally melted rocks from shallow contact-aureoles (e.g., Figs. Fl3 Fl5), and commonly develop 1n relict K-feldspar and quartz that were adjacent to grains of reactant muscovite. Ne1ther glass nor granophyre occurs 1n deeper and more slowly cooled contact aureoles. Part1ally melted rocks 1n the contact aureole of the Duluth Igneous Complex formed at a pressure between 1.5 and 2 kbar, and contain a w1de range of microstructures (e.g., Figs. Fl7 F28). Granophyre occurs 1n some ver y quartz-rich rocks of the Biwabik Formation (e.g., Fig. F21 ), but not in the metapelitic rocks. Metapeht1c rocks from the aureole contain small (0.1 20 mm), Isolated, ell1pso1dal leucocrat1c patches of neosome (e.g., Figs. F17 F20) in add1t1on to small veinlike accumulations of leucosome (<5 mm w1de). The leucocratic patches have a cuspate outline because the adjacent matnx quartz, feldspar, and b1ot1te gra1ns are corroded. Moreover, the patches comprise a few large anhedral gra1ns of quartz and alkali feldspar or plag1oclase, wh1ch conta1n rounded inclus1ons of quartz, b1ot1te. and plag1oclase and tiny euhedral 1nclus1ons of plag1oclase, cordierite. and orthopyroxene. The m1crostructure suggests that the patches of neosome were sites where b10t1te melted Incongruently; then, during slow cooling, the melt crystal lized to g1ve small prismatiC crystals f1rst, and then, as the temperature neared the sol1dus. coarse-gra1ned quartz and feldspar (Sawyer 2001). Sim1lar microstructures are found 1n the metapelitic rocks of the 3 kbar aureole around the Ballachulish Igneous Complex (Pattison and Harte 1988. Harte et al. 1991), and 1n the aureole of the Onawa pluton (Marchildon and Brown 200 I, 2002). Thus, the charactenst1c shape of the pools of melt (F1g. F29), and the corroded form of the reactant minerals (Fig. F30), still rema1n in many deeper contact-aureoles, but the former melt 1s now represented by early-crystallized euhedral grams set 1n coarse-gra1ned

I NTRODUCTION

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quartz, plagtoclase, and K-feldspar that crystallized later. Unfortunately, the mtcrostructural evtdence for the former presence of melt ts easily overlooked tf the amount of melt present was small, or where the protoltth ts a quartzite. Holness and Clemens (1999) were able to establish that small amounts of melt existed tn the Appin quartzite up to 500 m from the Ballachulish Igneous Complex by using several microstructures (see Figs. F31 and F32), such as (I) feldspar grains with cuspate extensions along quartz quartz contacts, (2) narrow tratns of feldspar blebs along quartz-quartz contacts, and (3) fine-gratned quartz. albtte, and K-feldspar along the gratn boundanes.

1996, Sawyer 2001). Key mtcrostructures are (I) small strings of connected gra1ns of quartz, plagioclase, and 1n some cases, K-feldspar, along the gratn boundaries of larger gratns in the matrix (t.e., the "stnng of beads" miCrostructure descnbed by Holness and Clemens 1999), (2) the presence of monomtneralic films of these minerals on the matrix grains, and (3) small, cuspate gratns of these minerals between large grains in the matrix (e.g., Fig. F32) . Figures F39 F46 show examples of the more typical microstructure found tn many restdual rocks. Most have undergone a constderable degree of recrystallization that has greatly modified the ong1nal stze and shape of the gra1ns. Thus, the ev1dence for the former presence of melt, such as cuspate-shaped pore space that was filled with melt (pools of melt), ts largely erased.

The general belief (e.g., Hibbard 1987, Ashworth 1985) that the combtned effects of deformation, recrystallization (gratn growth), and retrograde metamorphtsm would have elimtnated the mtcrostructures generated dunng parttal melting tn slowly cooled, regtonal metamorphtc terranes ts Microstructures in the melt,rich parts of provtng to be incorrect. For example, microstructures (Figs. migmatites; evidence for crystallization of F33-F36) that resemble those produced 1n parttal-melttng the melt experiments have survived tn melt-depleted rocks in the The melt-rich parts of mtgmatites are the most likely to Ashuanipi Subprovince, a regional granulite-facies (6 kbar, contain microstructural evtdence for the crystallization of 875°C) terrane in northern Quebec (Sawyer 1999, 200 I: the anatectic melt; Figs. F47 F58 show typical examples Guernina and Sawyer 2003). Metagreywacke rocks there from the leucosome domains, and Figs. F59 F65 illustrate contain small domains (0.5 mm across) tn which large trreg- the microstructures from dtatextte migmatttes. Plagtoclase ularly shaped grains of K-feldspar (mostly), quartz. and and K-feldspar are the most volum1nous products of crysplagioclase have cuspate extenstons that penetrate along tallization of felstc anatectiC melts. and tn most cases, the gratn boundaries between the adJacent smaller. equant plagioclase starts to crystallize before quartz and K-feldspar. gratns tn the matnx. These large tnterstit1al crystals conta1n Consequently, most mtcrostructures used for the identifismall, rounded, or em bayed tnclus1ons of quartz, plagtoclase, cation of crystallized melts 1n m1gmat1tes tnvolve etther the and btot1te, tn addition to large gra1ns of orthopyroxene. feldspars, or the shape of crystals. whtch have crystal faces (Ftgs. F33, F34). These domatns Platten ( 1983) studied part1ally melted semi pelitic schists are tnterpreted to be s1tes where btotite, plag1oclase, and close to the Barnamuc tntrustve complex in Scotland. He quartz reacted to produce orthopyroxene and melt; thus. noted two features that are essentially tgneous in crosssome of the mtcrostructure that formed at the peak metacutting pink leucosome: (I) the plagioclase and K-feldspar morphtc temperature is actually preserved in these rocks. have crystal faces where tn contact with interstitial quartz, Most (>80%) of the melt produced drained away. leaving (2) small gratns of quartz. plagioclase, biotite, and and a small amount (<3 vol.%) that became the coarse-grained cordiente located tn patches of granophyre, and in the K-feldspar, quartz, and plagioclase. coarse-gratned parts of the leucosome, are commonly Kenah and Hollister ( 1983) showed stmtlar microstruc- euhedral. These featu res 1ndtcate that the leucosome crystures preserved around small, remnant pockets of melt tallized from an anatecttc melt. or from a magma (see F1gs. left after most of the melt that was generated had been F47 and F48). Stmtlar criteria for tdent1fy1ng mtnerals that expelled from partially melted Terttary plutonic rocks crystalltzed from a melt were proposed by Cuney and tn Bnt1sh Columbia. Very s1milar microstructures (F1gs. Barbey ( 1982) and later by Pattison and Harte ( 1988) and F37, F38) have also been reported tn mtgmat1tes devel- Vernon and Collins (1988). oped from pluton1c protoliths 1n Archean (Sawyer 2001), Some patches of leucosome, many of the narrow vetns Mesoproterozoic (Ttmmermann et al. 2002, Slagstad et al. 2005), and Phanerozotc (Brown 2002) regtonal anate- of leucosome, and doma1ns the diatexite migmatites tn the contact aureole around the Duluth Igneous Complex ctic terranes. In some cases, the former presence of melt consist of large (I 2 mm), anhedral grains of unstrained gra1ns small of can still be inferred from the distribution quartz, plagioclase, and K-feldspar that contain inclusions of of quartz, plagioclase, or K-feldspar in the matrix of larger much smaller euhedral plagtoclase. cordiente, and orthopyresidual grains, even after some later re-equilibratton of the roxene cryst als (see Figs. F23 F27) . Moreover, as shown gra1n boundaries has occurred (e.g., Hartel and Pattison

Atl." of Mtgmarue'

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1n Figs. F28 and F52. the pnsmatic 1nclus1ons are commonly strongly aligned (Sawyer 200 I). Together, these microstructures 1ndicate that the small pnsmatic grams crystallized 1n. or from. a melt. and were aligned by now of that magma (melt + entra1ned crystals). before the melt finally crystallized.

of 1nterst1t1al quartz that represent the places where the last melt crystall1zed. There are three reg1mes, based on fract1on of melt present. that are of interest; the magmatic (M > 0.45), the submagmat1c (0 < M < 0.45), and the subsolidus (M, 0): the submagmat1c reg1me has been subdivided still further (Vigneresse et al. 1996, Rosenberg 200 I).

The microstructures in leucosome and the melt-rich migmatites from reg1onal metamorphic terranes are different from those 1n contact aureoles. The gram s1ze IS considerably larger and more un1form, and far fewer of the minerals have crystal faces. N evertheless, the presence of euhedral crystals of feldspar 1n leucosome (Figs. F56, F57) and 1n diatexite m1gmatites (Figs. F59, F64) that have crystal faces against later-crystallizing minerals, such as quartz or K-feldspar. IS the most widely used microstructure for infernng the former presence of a melt, or of magma, 1n reg1onal m1gmat1tes. Euhedral, or 1dioblast1c, unzoned ferromagneSian mmerals that have few 1nclus1ons have also been used to 1nfer crystallization in the presence of a melt (StOwe and Powell 1989, Florence et al. 1995, Braun et al. 1996, Barbey et al. 1999, Berger and Kalt 1999, Sawyer et al. 1999, Alcock and Muller 2000, Berger and Roselle 2001, Johnson et al. 2001a). However, other cntena are needed to determine whether the m1neral is the product of an mcongruent melt1ng react1on (1.e., a peritectiC phase) and developed crystal faces because it grew 1n a melt, or whether the mineral crystallized from the melt (i.e., l1qu1dus phase). The presence of euhedral accessory phases, such as zircon, in leucosome and diatexites also has been used as evidence for crystallization from a melt (e.g., Barbey et al. 1995).

In the magmat1c state, platy or tabular m1nerals are suffiCiently widely spaced to rotate passively as the magma 1s deformed and to become aligned; local1nteract1on between

A zonal d1stnbut1on of m1nerals w1th1n a leucosome that IS s1m1lar to that expected from 1n s1tu fract1onal crystallization of felsiC melt has also been used to 1nfer crystallization from a melt or magma (Sawyer 1987, Kohn et al. 1997). In particular, Kohn et al. ( 1997) noted that myrmekite occurs only at the edges of the leucosome they studied, and muscov1te, only 1n the center: they interpreted th1s as the progress1ve 1nward crystallization of a melt 1n wh1ch the last volume of melt 1n the center finally became sufficiently hydrous to crystallize muscovite.

Magmatic and submagmatic foliations Platy or tabular crystals suspended 1n a magma become aligned dunng now of the magma, and form a foliat1on. It •s 1mportant to 1dent1fy such magmatiC foliations 1n mlgma'tes. espec1ally 1n the field. but the problem 1s to d1st1ngu1sh them from tecton1c foliations formed 1n the solid state. Critena for 1dent1fying magmatiC foliations have been developed (Blumenfeld and Bouchez 1988, Paterson et al. 1989, Arbaret et al. 1996). In essence, these methods compare the degree of intracrystalline deformation in the foliat1on -def1n1ng m1nerals w1th that 1n the large doma1ns

=

rotat1ng crystals can occur, but 1s not s1gn1ficant (Paterson et al. 1989). There are microstructural cnteria w1th wh1ch to 1dent1fy magmatiC foliations: the m1nerals that crystall1zed from , or were in the magma, must have a preferred onentation, and the quartz crystals and domains in the matnx around them should be equant, and there should be no Intracrystalline deformatiOn or recrystallization (e.g., undulose extinction, deformation bands, polygonal subgrams, kinks, bent cleavage-planes, or ev1dence of gra1n-boundary m1grat1on. such as lobate boundanes) in e1ther the foliationdefining minerals, or in the 1nterst1t1al quartz. Examples of the microstructure developed during the flow of magma in diatex1te migmatites are shown in Figs. F66- F69. As the fraction of melt decreases, 1nteract1ons between rotat1ng crystals becomes progressively more common, and th1s forms clusters of 1mbncated or "tiled" gra1ns (Blumenfeld and Bouchez 1988; lldefonse et al. 1992, 1997; Arbaret et al. 1996, 1997) and the submagmat1c regime begins. A framework of crystals develops, and once formed, the framework may become deformed (Bouchez et al. 1992). A submagmatic foliation is one 1n wh1ch a framework of aligned crystals formed and may have become deformed before the interstitial magma crystallized. Hence, the microstructural critena for ldent1fy1ng a submagmat1c foliat1on are the presence of Intracrystalline deformation-Induced features in the foliation-defining and framework-forming m1nerals, but not in the interstitial domains of quartz, which rema1n equant 1n shape. Intracrystalline deformat1on in both the foliationdefinmg m1nerals and the quartz domams, wh1ch may have become elongate, 1nd1cates deformat1on in the solid state. Deformation 1n the solid state is generally characterized by a decrease in grain s1ze, and at high stra1ns, by the development of mylon1te. Unfortunately, microstructures formed 1n either the magmat1c or submagmat1c states are eas1ly destroyed 1f deformation cont1nues 1nto the subsolidus reg1me.

Melt inclusions Melt can be trapped as inclusions in the minerals that grew 1n, or crystallized from, a melt: examples are shown 1n Figs. F70 F73. Therefore, t he 1dent1ficat1on of melt 1nclus1ons

INTRODU CTI O N

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1n m1nerals 1n the neosome IS clear ev1dence for the presence of melt. Rap1d cooling, as occurs 1n rap1dly exhumed, partially melted crustal xenoliths (Cesare et al. 2003), will quench melt inclusions to glass. Melt 1nclus1ons that have quenched to glass may conta1n structures due to shnnkage, wh1ch resemble bubbles (Lowenstern 1995). Glass IncluSions are 1mportant, as they may enable the composition of natural anatectic melt s to be determ1ned. However, with slower cooling, the melt crystallizes, and the former melt 1nclus1ons may be m1staken for solid m1neral 1nclus1ons and be overlooked. The homogemzat1on temperature at wh1ch the crystals 1n former melt 1nclusions d1sappear 1nto a s1ngle phase (i.e., melt) has been used to infer the temperatures at wh1ch the magmas were trapped as 1nclus1ons 1n their hosts. Most notably, th1s approach has been applied to the study of volcanic and shallow pluton1c rocks.

Cordierite-, garnet-, and orthopyroxenequartz intergrowth microstructures The 1ntergrowth in large poikiloblasts of garnet and, to a lesser extent. of cordierite and orthopyroxene, w1th quartz and surrounded by feldspar, is a very common microstructure 1n neosome (Powell and Downes 1990: Tracy and Rob1nson 1983: Brown 1983: Waters and Whales 1984: Waters 1988, 2001: Barbey et al. 1990: Braun et al. 1996: Fitzs1mons et al. 1996: Sawyer et al. 1999: Oliver et al. 1999: Alcock and Muller 2000: Wh1te et al. 2004). Field observations (see examples 1n F1gs. B20 B25) indicate that th1s microstructure (F1gs. F74 F77) does not occur in the neosome developed from H 0-saturated melt1ng, or muscov1te-breakdown reactions (Tracy and Rob1nson 1983), but that 1t IS charactenst1c of the biot1te-dehydrat1on-melt1ng reactions that occur 1n the h1gher-grade parts of m1gmatite terranes. The deta1ls of this microstructure are remarkably similar for all reported occurrences. (I) The doma1ns of neosome have a rather d1ffuse margin w1thout a melanosome. (2) The neosome tends to be leucocrat1c rather than mesocratic 1n appearance, probably because the melanocrat1c minerals are concentrated into po1kiloblasts, and do not 1nclude b10t1te. (3) Domains of neosome have a patch, or more commonly a net-like morphology composed of Irregularly shaped (Tracy and Robinson 1983. Wh1te et al. 2004), or planar and regularly spaced segments (Braun et al. 1996). (4) Host and neosome have d1fferent m1neral parageneses: hosts contain plag1oclase. b10t1te. and quartz, with or w·thout sillimanite, whereas the neosome conta1ns plag1oclase, K-feldspar, quartz and one or more of cord1er1te, garnet or orthopyroxene, and ilmen1te or magnet1te. B1ot1te IS generally absent. but 1t may occur as a replacement of garnet. cord1erite, or orthopyroxene. The absence of b1ot1te in the neosome suggests an Incongruent b10t1tedehydrat1on-melting reaction 1n wh1ch cordiente, garnet. and orthopyroxene are the solid products of reaction.

(5) The bulk composition of the neosome IS commonly the same as the host. a result suggesting m s1tu format1on of the neosome 1n a closed system (e.g., Braun et al. 1996), which 1s the reason 1t IS not truly leucocrat1c. However, 1n some cases the neosome has an excess of ferromagnes1an components that IS readily expla1ned by the loss of some, or virtually all, of the melt (Fitzs1mons 1996, Sawyer et al. 1999, Waters 2001, Wh1te et al. 2004): such neosome is melanocrat1c. (6) The ferromagnes1an poikiloblasts or porphyroblasts are 1ntergrown with blebs, patches (see F1gs. F74 and F75), and lamellae of quartz (e.g., Waters 2001, Wh1te et al. 2004) and, more rarely, with feldspar (Barbey et al. 1999) also. In rare cases, the poikiloblasts have a dist1nct core that conta1ns sillimanite Inclusions, but no quartz blebs. Vernon and Collins ( 1988) Interpreted such s1lliman1te-beanng cores to be pre-neosome relics. Powell and Downes (1990) and Wh1te et al. (2004), 1n contrast. suggested that the cores too could have grown dunng the melt1ng reaction. (7) In many cases, the leucocratic part of the neosome 1s zoned. The ferromagnes1an core 1s surrounded by an inner leucocrat1c annulus rich 1n K-feldspar, which IS interpreted to be the solid product of the incongruent melt1ng react1on. The outer zone consists of quartz, K-feldspar, and quartz: 1t IS Interpreted to have been derived from the anatect1c melt. (8) Generally, the quartz 1n the po1kiloblasts connects to a th1n mantle (called a moat by some authors) of quartz that surrounds the po1kiloblasts and separates it from feldspar 1n the neosome (see F1gs. F76 and F77) . Models proposed for the format1on of these Intergrowths are based on k1net1cs. pnnopally the difficulty of nucleating and grow1ng the solid assemblage produced, or on the slow diffus1on of Si and AI 1n melts. StUwe and Powell (1989), Powell and Downes (1990) , and Wh1te et al. (2004) regarded the locat1on of the neosome as controlled by the location of the nuclei of ferromagnesian m1neral produced (garnet 1n thew examples) , wh1ch expla1ns the lack of a nm of melanosome. Barbey et al. ( 1999) proposed that the dissolution of b1ot1te 1nto the melt to make cordierite resulted 1n a local deficit 1n AI, wh1ch was compensated for by the 1ncongruent d1ssolution 1nto the melt of nearby feldspar, the resu 1t1ng quartz be1ng 1ntergrown w1th the cordiente. Waters (200 I) suggested that the microstructures result from the k1net1c control 1mposed by S1 and Ai, the most slowly d1ffus1ng spec1es: the growth of garnet results 1n a local excess of S1 at 1ts edge and 1n the incorporation of quartz. Brown (2004) suggested that local s1tes of lower stress may control the 1n1tial nucleation of the product m1nerals and the accumulation of melt that 1s produced. Importantly, all the models and petrolog1cal observat1ons mdicate that th1s type of microstructure can be used to infer b1ot1te dehydration melt1ng.

Atlas of Migmatites

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Hornblende dehydration melting in mafic protoliths may produce neosome w1th similar types of microstructures: dark cores consisting of pyroxene and garnet surrounded by a leucocratic mantle, or moat, of plagioclase and quartz. However, more work is required on migmatites derived from mafic protolit hs.

Symplectitic intergrowths of quartz and plagioclase with mica On cooling, there is the potential that minerals in a migmatlte will react with the remaining melt. There are two types of react ion: (I) fina l melt reacting w ith the residual solid and (2) aqueous fluid reacting with residual solid. Both involve the formation of new hydrous minerals. Reactions between the final increments of melt left in the rock and the residual solid necessarily occur at temperat ures at or above the solidus, and so are of interest here. One objective of this section is to identify the microstructures associated with this type of reaction. The flu id for solid - aqueous fluid reactions can be derived from the crystallization of the anatectic magma in which case the reactions are a near-solidus phenomenon. As an alternative, the fluid could come from an external source, and the process could take place at subsolidus temperatures, which could extend down through the amphibolite to the greenschist facies : these low-temperature reactions will not be cons1dered further. Thompson ( 1983) and Powell ( 1983) argued that the preservation of low-a(H 20) mineral assemblages in high-grade rocks is due to the formation and subsequent escape of anatect1c melt. The removal of H20 with the melt lim1ts the extent to which rehydrat1on react1ons can take place (Spear et al. 1999, Brown 2002) as the migmatite terrane cools and the melt crystallizes. Although large volumes of melt can be generated 1n anatectic terranes, not all that melt is extracted from the source to make plutons at h1gher levels in the crust: small amounts of melt remain 1n pockets and along the boundaries between the m1nerals in the residual rocks (Guernina and Sawyer 2003), and a small amount of melt also remains in the leucosome (Cagg1anelli et al. 1991). Th1s minor amount of res1dual melt reacts with the residual minerals during the final stages of crystallization and produces new microstructures in both the residual and melt-nch parts of the m1gmatite. Although of minor abundance generally, the microstructures (F1gs. F45 F48) are a characteristic symplectite-like intergrowth of skeletal or th1nly bladed biotite with quartz or plagioclase (Ashworth 1976, jones and Brown 1990, Brown and Raith 1996, Raith and Harley 1998, Kriegsman and Hensen 1998, Sawyer 1999, Guernina and Sawyer 2003), quartz and K-feldspar (Nyman et al. 1995), or alkali feldspar and plagioclase (Waters 2001): the intergrowth locally

replaces small amounts of garnet, cord1erite, or orthopyroxene. Because these are replacement microstructures, and as the product assemblage is essentially the same as the reactant assemblage in some prograde biotitedehydration reactions, many authors have interpreted them as resulting from the reaction of anhydrous solid minerals with the remaining anatectic melt. Commonly cited reactions are Grt ± Crd + Kfs + melt = Bt + PI + Qtz ± Sil and Opx + melt = Bt + PI + Qtz. However, Waters (200 I) expressed a different opinion, and proposed that this particular microstructu re should be considered together with other: less obvious microstructures (e.g., Fig. F82) in the rocks that involve biotite and sill iman1te (see examples in Buttner et al. 2005) . These cases suggest that a more complex overall reaction occurred as the melt was crystalliz1ng in the rock, although the melt was not directly involved in creating any particular microstructure. Many low-grade migmatites in which melting occurred through the breakdown of muscovite contain analogous late-stage microstructures in which K-feldspar is replaced by an intergrowth of skeletal muscovite and quartz, gen erally associated with myrmekite (Ashworth 1972, Brown 1979, Ashworth and Mclellan 1985, Weber et al. 1985). The symplectite-like microstructure is generally interpreted to be the result of the slow d1ffusion of Si and AI, and hence the AI:Si rat io of the reactant IS inherited by the products and controls the nucleation of the product assemblage. Ashworth (1972) proposed a reaction between melt and K-feldspar, such as Kfs + melt + Sil + PI = Ms + Qtz + PI" to account for the microstructures [the superscripts indicate reactant () and product (2) plag1oclase, respectively]. Symplectite-like microstructures between the m1cas and quartz seem to indicate the reaction of res1dual minerals with melt: the extent to which these microstructures have developed depends upon how much melt was present. Therefore, mapping the d istribution of microstructures produced at late stages of crystallization and those microstructures formed under high-temperature subsolidus conditions may enable the loci of net loss of melt to be distinguished from those where melt has accumulated (or was injected): the former should exh1bit a predominance of microstructures indicative of reaction between the res1dual melt and solid, whereas the latter should contain more extensive evidence of rehydration involving reaction between the residual solids and an aqueous fluid (e.g., Spear et al. 1999, Brown 2002, White et al. 2005).

Composition and zoning of plagioclase Bowen ( 1913) showed that during the melting of plagioclase, the albite component fractionates 1nto t he melt, and the remaining plagioclase 1s more calcic. Consequently, the

I NTROD U CTI ON

32 ------------------------------

compos1t1on of plag1oclase was proposed as a test for the ong1n of the leucosome 1n m1gmat1tes (e.g., Yardley 1978). The leucosome generated by part1al melt1ng should have plagioclase more sodic than that 1n the assoc1ated melanosome and protolith. Some investigators found the plagioclase in the leucosome to be systematically more sodic than those in the melanosome (e.g., W eber et a\. \985. Weber and Barbey \986, Alcock and Muller 2000) . More commonly, the compositions of plagioclase 1n the leucosome and 1n the melanosome were found to be very s1mdar (e.g., Harme 1962, Dougan 1979, Henkes and Johannes 1981. Gupta and Johannes 1982. Kenah and Hollister \983). even 1n leucosome where other cnteria po1nt to an anatectic ong1n. The euhedral crystals of plag1oclase 1n d1atex1te migmatites that represent the accumulat1on of plag1oclase crystallized from the "1nlt1al" anatectiC melt 1n the Ashuanip1 and Opatica subprov1nces of Quebec are more calc1c (around An, ) than the euhedral crystals of plagioclase (around An,.) that occur in K-feldspar-nch diatexite m1gmatites that formed from evolved diatexite magmas. Similarly, large euhedral crystals of plagioclase that are part of the framework (e.g., Figs. F59 F64) of early-crystallized minerals in diatexite migmatites (and in some examples of leucosome. e.g., Fig. F56) are more calcic than the interstitial plag1oclase that crystallized later. Johannes (1978, 1980, 1983b) and Acosta-Vig1l eta\. (2006) have shown that the kinetiCS of d1ssolut1on of Intermediate plag1oclase at the temperature (<800°C) at wh1ch granitic melts start to form are very slow 1ndeed. This leads to metastable melting. and the plag1oclase 1n the leucosome and 1n the melanosome need not have d1fferent compos1t1ons. Th1s effect may be 1mportant 1f melt separates rap1dly from 1ts res1duum. Thus, the composition of plag1oclase IS not now considered a reliable cntenon to evaluate the origin of leucosome in migmatites. However. plagioclase is st ill a crucial mineral in the interpretation of m1gmatites, but emphas1s now rests on its morphology, the microstructures of wh1ch it is a part, and on the nature of 1ts

ev1dence of magmatiC growth. However. the presence of oscillatory zon1ng. truncated growth-zon~ng, and complex 1nternal boxlike, skeletal, or honeycomb structures in plagioclase IS regarded as evidence of d1sequdibnum during growth (Wiebe 1968. Hibbard 1981), and as characteristic of regimes where magma m1xing occurred. Plag1oclase crystals containing disequilibrium structures are wel l known from granites (e.g., Barbarin \990, Grogan and Reavy 2002). Petrographic studies of the Vanscan pera\um1nous anatectic complexes in Spa1n have revealed the presence of plagioclase crystals conta1n1ng d1sequ1hbnum microstructures in some of the m1gmatite bodies 1n these complexes. Plag1oclase crystals w1th s1milar complex patterns of internal zoning are also found 1n leucosome from the m1gmat1tes of the Glen Elg reg1on of southeastern Austral1a (see Fig. F83) . Expenmental work by Castro (2001) has shown that complex patterns of 1nternal zon1ng can result from the 1nteract1on of a mantle-denved mafic magma and a crust-denved fels1c magma. Thus. detailed study of the zoning and internal morphology of plagioclase may be a very useful way of d1stingu1shing migmatites that are wholly crust al material from those with an admixed component of mantle-derived magma (e.g., Miller and Wooden 1994).

Biotite composition and microstructures Biotite-dehydration react1ons occur over a temperature 1nterva\ that can exceed I 00°C because of vanous solid solut1ons 1n the b1otite (Monte\ and Vielzeuf 1997). Experimental stud1es show that bes1des the well-known 1ncrease 1n T10 1 (Robert 1976), Mg number and fluonne content also 1ncrease across the react1on 1nterval (Stevens et a\. 1997: Pat1no Douce and Harns \998: Pat1no Douce and Beard \995, \996; Patliio Douce and Johnston 1991: Pickering and Johnston 1998) and extend the stability field of biotite to more than I 000°C. The AI content of biotite tends to decrease relat1ve to the starting composit1on of the biotite 1n bulk compos1t1ons lacking an aluminosilicate mineral. and to 1ncrease 1n those where one is present. Thus, there are several compos1t1onal trends 1n biotite that

compos1t1onal zoning.

can be used to determ1ne metamorphic condit1ons (e.g..

Plagioclase from most m1gmatites shows rather weak and 1rregular (nonconcentnc) compos1t1onal zoning: both normal and reverse trends have been described. The strongest compos1t1ona\ zon1ng 1s generally found 1n plag1oclase from the melanocrat1c res1dual rocks, particularly 1f the protolith was a mafic rock. The pattern of compos1t1onal zon1ng 1n res1dual plag1oclase IS strongly dependent on whether other Ca-bearing phases were present 1n the residuum, and the stage at which they grew (Skjerlie and Johnston \996,

Patino Douce eta\. 1994).

Patino Douce and Beard 1996) . Strongly developed concentric zon1ng that parallels the outline of a plag1oclase crystal IS generally cons1dered as

On the bas1s of morpholog1cal and microstructural differences. several d1fferent populat1ons. or generat1ons, of b1ot1te have been 1dent1fied 1n many migmatites. In some cases. there is no compos1t1onal difference between the morpholog1cal types (e.g.. Solar and Brown 200 I) . However; in others. there are compos1t1onal differences that can be used to explore the metamorphic history of the m1gmatite. For example, in m1gmat1tes that underwent biotite dehydration melting, the residual b1otite. with the highest Mg number. T10 2, and F contents, commonly occurs as small, rounded, reddish 1nclus1ons in other phases, whereas the

AtJ.l.., of Magmatltc'

----------- ----------- -------- 33

larger crystals of biotite 1n the matrix of these rocks have lower Ti0 2 contents and lower Mg numbers (e.g., Braun et al. 1996, Fitzsimons 1996, Barbey et al. 1990, Perc1val 1991), 1nd1catlng that they formed, or re-equ1librated, at lower temperatures. In contrast, the largest, bladed crystals of b1ot1te 1n migmatites, where b1otite dehydrat1on melt1ng was incipient, have the highest Mg number and are residual (Brown 1979, Milord et al. 2001), and the smaller grains, w1th much lower Mg numbers (around 20) and TiO contents, grew from the anatectic melt (Sawyer 1998, M1lord et al. 200 I, Milord and Sawyer 2003) . Large po1kilitic crystals of biotite in leucosome from the lower-crust migmatites of southern Italy have the same morphology and Mg number as b1otite in the melanosome, and were interpreted by Fornelli et al. (2002) as residual. In contrast, small grams of b1ot1te 1n the leucosome are less magnes1an and Interpreted as possibly crystallized from the anatectic melt. Kenah and Hollister ( 1983) found that b1ot1te (and hornblende) from leucosome have lower Mg numbers than those from adjacent melanosome, probably renect1ng a greater proport1on of magma-grown biot1te in the leucosome they exam1ned.

Contact between leucosome and melanosome in metatexite migmatites Observations made 1n the field on the nature of the contacts between a leucosome and its melanosome are extremely 1mportant in determ1n1ng whether the leucosome IS m s1tu, 1n source, or s1mply a leucocrat1c ve1n. Cnt1cal details are whether the contacts are sharp or diffuse, concordant or discordant, and whether there IS a nm of biot1te between the two. In spite of the great s1gnificance attached to the contacts for the Interpretation of leucosome, very little has been reported on thew microstructure. The contacts 1n part1ally melted quartz1tes from rap1dly quenched contact-aureoles show a petrological continu1ty of either glass (Platten 1982) or granophyre (Fig. F84) from the leucosome 1nto the residuum. Th1n films of glass, or granophyre, extend along the gra1n boundanes of the quartz grains from the leucosome 1nto the res1duum for a distance of several gra1n-diameters (Fig. F84), which supports an in s1tu interpretation for the leucosome. The contacts between leucosome and melanosome in metapelit1c rocks from rap1dly cooled contact-aureoles are more complex than 1n the essentially monom1neralic metaquartzites, but are also irregular at the scale of several grain-diameters (Figs. F85 F87). The outline of the large crystals 1n the m s1tu leucosome suggests that melt d1d extend 1nto the res1duum along gra1n boundanes and that lobes of melt may have almost completely surrounded p1eces of the residuum (e.g., F1g. F85) . At a larger scale, cont1nuity of microstructure 1n the leucosome from a segment w1th one onentat1on to a segment w1th another (e.g.,

Fig. F88) can be used to 1nfer that all segments of a leucosome contained melt at the same time. Alteration of ferromagnesian minerals (e.g., cordiente in Fig. F88) 1n the residuum close to the leucosome may 1ndicate that H, 0 was exsolved from the melt as 1t crystallized. Highly Irregular contacts are commonly preserved between leucosome and melanosome components of m s1tu patches of neosome formed at the start of part1al melt1ng 1n reg1onal granulite-faCies metamafic (F1gs. F89 F92) and metapelitiC (F1gs. F93, F94) rocks. However, recrystallization in slowly cooled rocks generally eliminates the evidence for former films of melt. Gra1n s1ze typ1cally mcreases through a melanosome toward 1ts contact w1th leucosome, but the concentration of stra1n (wh1ch reduces gra1n s1ze), or the abundance of sill1man1te (which 1nhib1ts grain growth) near the interface, can result in a smaller gra1n-size and straighter contacts (Figs. F95, F96).

Microstructure of schlieren in diatexite migmatites Schlieren are planar aggregates of platy, tabular, or acicular m1nerals and are common 1n many d1atex1te migmatites. The presence or absence of schlieren 1s part of the nomenclature for nam1ng the subtypes of diatex1te m1gmat1te, but very little IS known about their microstructure or how they developed. All the examples shown 1n th1s book (F1gs. F97FIOO) are of the common biot1te schlieren, but schlieren composed ma1nly of plag1oclase, hornblende, orthopyroxene or silliman1te also occur in m1gmat1tes. The crystals of biotite that comprise the schlieren (see Figs. F97 FI 00) are typ1cally strongly Imbricated, or tiled, in the same sense as the 1mbncat1on of the tabular m1nerals (1.e., feldspar) that define the magmat1c foliat1on 1n the host diatex1te. Thus, schlieren were formed during the now of the d1atex1te. Schlieren range in width from less than the length (<2 mm) of a component crystal of b1ot1te (e.g., Figs. F97, F98). to aggregates (>IS mm) tens of crystals wide (e.g.. F1g. FIOO, and examples 1n Milord and Sawyer 2003). In most cases, the crystals of b1ot1te 1n the schlieren are larger, have a greater aspect-ratio and a stronger preferred onentation than biot1te 1n the host, a feature that Milord and Sawyer (2003) attnbuted to the ease of recrystallization 1n the essentially monom1neralic schl1eren. Many schlieren are s1gn1ficantly enriched 1n accessory m1nerals such as apat1te and zwcon, but where these minerals occur 1n the schlieren and how they came to be concentrated there rema1n unknown.

Microstructure of biotite~ rich selvedges in migmatites B1ot1te-rich selvedges that develop around leucocratic veins and gran1t1c dikes are another common feature 1n m1gmat1tes for whiCh the conditions and mechamsms of formation

IN TRO DU CTI O N

34 -----------------------------

are poorly understood. Typically. they occur as narrow (<5 mm) zones 1n the host rock that form at the contact with a leucocratic vein, or gran1tic dike. In most cases, biotite selvedges show a rap1d gradation 1n microstructure and m1neral proport1ons from the country rock: the proportion of quartz and plagioclase decreases, and there IS a concomitant increase in the modal proportion and grain size of b1otite (Figs. FIOI-FI04) toward the contact. The 1ncrease in gra1n s1ze of biotite 1s probably facilitated by easier diffusion 1n the essent1ally monom1neralic selvedge. Generally, the crystals of biotite 1n the selvedge are onented parallel to the contact w1th the ve1n or dike. However, the onentatlon of b1otite 1n many selvedges around leucocrat1c ve1ns has been locally modified by the bulbous growth of crystals of quartz and plag1oclase outward from the leucocrat1c ve1n (e.g.. Fig. FI02). which may be interpreted as evidence for gra1n growth after the leucocratic vein has solid1fied.

6. W H O LE-ROCK G EOCH EMISTRY IN MIGMATITE STU D IES Most m1gmat1tes can be recognized simply by the1r distinctive morphologies. These morpholog1es are. to some degree. the result of deformation that occurred during anatex1s. However, the underlying processes that change a protolith 1nto a m1gmat1te are petrological, not structural. Therefore, the bas1c requ1rement in understanding migmatlte formation 1s the 1dent1fication of these petrologiCal processes, and one way that this can be done is by examIning whole-rock compositions. Th1s book IS not the place for an exhaustive discussion of the geochemistry of migmatites. Nevertheless, some cons1derat1on of how whole-rock geochem1stry can be used to understand these processes 1s useful. From a practical po1nt of v1ew. this summary also serves to outline which rocks should be sampled for follow-up geochem1cal study wh1le mapp1ng m1gmat1tes.

6.1

A possible sequence of processes and some relevant questions

A migmatite represents the sum of many processes. How far each process has been able to progress and the sequence 1n which they occur are major factors in determining the characteristiCS of a part1cular migmat1te. The sequence of processes that make m1gmatites and gran1tes 1n three different scenanos of part1al meltmg IS shown on Fig. 4. In the first (Fig. 4a), partial melt1ng occurs under the special condit1ons of lithostatic stress ("stat1c melting": see section 3.4) only, and melt forms and later crystallizes without segregation from 1ts complementary solid fract1on. In th1s scenano, anatexis occurs 1n a closed system, and the degree of part1al melting (F) 1s, therefore. equal to the fraction of melt (M.) in the rock. These condit1ons are most likely to occur 1n contact aureoles. If only a small amount of partial melting (dashed line 1n F1g. 4a) occurs, either because the protolith was not very fert1le, or the metamorphiC temperatures were not very high. then a patch migmatite is formed. However, 1f a larger degree of partial melt1ng occurs (black line in Fig 4a), then a nebulit1c diatex1te m1gmatite forms: since anatex1s takes place 1n a closed system. th1s 1s a primary d1atex1te.

In the general case of anatex1s 1n the crust (Fig. 4b), partial melt1ng occurs under condit1ons of d1fferent1al stress ("dynamiC melt1ng") . Any melt fract1on generated 1n excess of that requ1red to establish permeability (generally <5%) in the melt1ng rock is segregated and collects elsewhere. 1.e., the part1ally molten rock is dra1ned, and anatex1s 1s an open-system process. at least on the scale of a hand sample and an outcrop. Overall, the m1gmat1tes formed under these conditions develop by a four-step process. The first is the onset of part1al melt1ng. Once the part1ally melted rocks become permeable. this step is followed by a second one, the extraction of the excess melt and its accumulation elsewhere. Th1s process generates two rock types. A res1dual metatex1te migmat1te develops at the site of part1al melt1ng: Whole-rock composition 1s commonly used to class1fy 1t conta1ns a low fract1on of melt. even 1f the total degree rocks. and as a means of discnm1nating between the tec- of partial melting was substantial (typ1cally 30 60%), and tonic sett1ngs 1n wh1ch rocks such as greywacke. basalt and has a melt-depleted bulk compos1t1on. A diatex1te magma gran1te formed . Ne1ther use has much application to m1g- forms at the s1te where the segregated melt accumulates: mat1tes. M1gmat1tes are class1fied by morphology. and they because the fract1on of melt there is h1gh (much greater 1nherit thew compos1t1onal traits from the1r protoliths. The than the degree of partial melt1ng) and IS not m situ, this protoliths may have become m1gmat1tes 1n a completely can be called a secondary diatex1te magma. Four lines are d1fferent tecton1c sett1ng to that 1n wh1ch they formed. shown 1n Fig. 4b connecting the residual metatex1te to the Whole-rock geochemistry is used pnnc1pally to 1nvest1gate secondary d1atex1te m1gmatite: the first corresponds to the processes that occurred during the formation of mig- loss of melt at the onset of permeability (lowest fraction matites. to define the compositions of the protolith and the of melt), and the others show segregat1on after some accuvarious parts of the neosome, and to ident1fy the reg1ons of mulation of melt in the metatex1te. The thwd step affects matenal loss or material gam. the diatexite magma dunng anatex1s and 1nvolves flow of

Atla~

of ~h g m atltc..,

----------- ----------- --------- 35

a l protolith

c

Fraction of Melt 02

0.4

0.6

0.8

Fraction of Melt

1.0

0.2

0.4

0.6

0.8

1.0

partial meltmg

a> u

en

part1al melting

·x a> -ro c

solidification

E i=

f""d;;'r~r·~··.--".,..---:"-:---::---r;;;~;i~-~~;;;,~·;·;~i1

i!!'.~~:!~~~~

<(

b

magma flow and segregallon residual solids melt fract1on

Fraction of Melt

protolith

02

0.4

06

0.8

~?.r.~:!~!~.~~!.'~~--j

1.0

partial melting fract1onal crystallisation

crystals/

en

·x

'melt

Ill ftl®it*dll l

2t1l c

<(

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flow of magma and segregallon

res1dual sohds -

I me~

E

i=

c

o~

·-

Cl

:..=

Cl)

-t1l ·-c en ~

ro t> -., en-o ~~­

u ~

Fig. 4. Diagrams to illustrate the relationship between physical processes and the generation of migmatites (metatexitc a nd diatex ite) and granite during a natexis in three different scena rios. (a) The special case of partial melting under condi tions of lithostatic stress (so-called "sratic melting"). These conditions arc most likely to occur in contact aureoles. (b) The genera l case of partial melting under differential stress (so-ca lled "dyna mic melting"), which applies to regional a natexis in orogenic settings, such as melt-depleted gra nulite terranes and their associated gra ni tes. (c) A hybrid scenario in which melt acc umu lates in the source before the processes that sepa rate the melt from the solid fraction become active, i.e., a n atexis begins under condit ions of hydrostatic stress, but ends in a differenti al stress regime. This scena rio b most likely to apply to regional anatectic settings where la rge volumes of melt were able to form rapidly. Legend: pa le grey, mctatexite migmat itc; dark grey, diatexite magma I diatexi tc migmatitc; coarse dotted pattern, diatex ite magma I migmatite with a very high fract ion of melt fraction that may resemble granite in terms of composition and microstructure; grey, c ross hachured patte rn, fractionated gra nitic magma that, generally, crystalli:es an igneous microstructure. the diatex1te magma. Th1s generates a magmat1c foliat1on and creates schlieren as well as some separation of the melt from the solid. The final step occurs after anatex1s and IS dominated by fract1onal crystallizat1on of the secondary diatex1te magma, wh1ch generates cumulate diatex1te and melt-ennched diatex1te, each w1th 1ts own d1st1nct1ve composition and microstructure. Granitic magma is a product of these processes. Th1s scenano applies to reg1onal anatex1s 1n orogen1c settings, notably the generat1on of melt-depleted granulite terranes and the1r associated granItes, e .g., the Ashuan1pi Subprovince 1n Canada.

Figure 4c dep1cts a hybnd scenano 111 wh1ch the melt fraction IS able to accumulate at the s1te of part1al melting before the differential stress reg1me starts; th1s scenano IS equivalent to "stat1c melting" followed by "dynamic melt1ng." Consequently, as the degree of part1al melt1ng progressively Increases, the anatectiC rocks pass through a metatexite migmatite stage before becom1ng a dlatexite magma. The diatex1te magma 111 th1s case formed m s1tu. 111 a closed system: therefore, 1t IS called a primary diatexite magma. The fract1on of melt at wh1ch the d1atex1te magma IS formed depends on the mechanism by wh1ch flow occurred. Assuming magmatic flow (i.e., a suspension of crystals 111 an anatect iC melt), then a melt fract 1on

INTRODUC TI ON

36 ------------------------------

1n excess of 0.16 (NUP model) or 0.26 (URS model) 1s requ1red; the upper lim1t of the diatex1te field 1s the degree of part1al melting atta1nable 1n reg1onal metamorphism; th1s is not normally more than 70%. Under certain circumstances, the diatexite field m1ght extend beyond the usual 0.26 0.70 range in terms of fraction of melt. Other mechanisms of deformation that produce bulk flow (see section 3.5) could generate diatexite migmatites at fractions of melt as low as 0.07; this potential extension of the d1atexite field 1s labeled "deform mechan1sm" on Fig. 4c. In some circumstances, such as ultra-high-temperature metamorphism, or H,O-fluxed melting, the degree of part1al melt1ng may approach I 00% and extend the d1atex1te field to h1gh fractions of melt. Flow of the diatex1te magma 1n response to shear stresses leads to the development of schlieren and magmatiC foliations, as well as some separat1on of the melt from the material entra1ned 1n 1t, to form res1dual d1atexite and melt-nch diatexite magma. The th1rd, and final step, 1s fract1onal crystallization of the melt-nch d1atexite; th1s generates a fractionated magma of granitiC composition that commonly develops a granitic microstructure on crystallization and cumulate rocks. This third scenario is most likely to apply to regional anatectic settings where large volumes of melt were able to form rap1dly. Some petrolog1cal quest1ons, but by no means all, relevant to the processes that form migmatites are outlined next. The first process is partial melt1ng of the protolith, and some relevant questions are as follows. (I) What was the protolith compos1t1on? (2) Are the migmat1tes the result of an open system or of closed-system (commonly called rn s1tu) processes? (3) What 1s the compos1t1on of the neosome, and of 1ts const1tuent parts? (4) Wh1ch react1on generated the melt, what was the degree of part1al melt1ng (F), and how does it compare to the fraction of melt (M,) that the migmatite contained, which may possibly be denved from an estimate of the fraction of leucosome 1n the outcrop? The next process is porous flow of the melt out of the source, i.e., melt segregat1on; here, the quest1ons may 1nclude the follow1ng. (I) D1d the melt segregate from the res1duum. and 1f so, what proport1on of the melt remained 1n the res1duum? (2) How much res1duum was entra1ned 1n the melt? (3) How variable IS the compos1t1on of the res1duum and what caused th1s vanation (d1fferent protol1ths. different melt1ng react1ons. d1fferent F. different degrees of segregat1on of the melt)? (4) Over what distance was the porous flow act1ve. over centimeters or tens of centimeters? The third step is transport and flow of the melt. or magma, in channels; questions relevant to th1s stage include the following. (I) Are there compositional vanations that can be attributed to processes occurnng dunng flow. such as the

separation of suspended crystals? (2) Do the leucosome. ve1ns. or d1atex1te m1gmat1tes that represent the bodies of former melt. or magma, that flowed in channels have compositions that are different compared to those of bodies of melt that rema1ned in Situ? (3) D1d the wallrock contaminate the magma dunng flow? The fourth step is crystallization, and it can, of course. begin at any time after partial melt1ng has started. Cooling may be so rapid that separat1on of melt from its residuum before complete crystall1zat1on may not be possible 1n some contact aureoles. Some pnncipal quest1ons are as follows. (I) D1d fract1onal crystallization occur. 1n other words. does the leucosome. or diatex1te. have the compos1t1on of an anatectic melt? (2) Did the entra1ned res1duum separate before or dunng crystallization? (3) Where d1d the separat1on of the entra1ned residuum take place? Finally. post-anatectic processes. such as retrograde metamorph1sm. rehydrat1on. and vein1ng can change the bulk compos1t1on of a migmatite, or its parts. Therefore. 1t is 1mportant that any altered samples be identified The following sections pertain to two factors that seriously affect the way in which whole-rock compositions can be used to investigate m1gmatites. First. in order to interpret the trends in whole-rock compos1t1on that arise from a petrogenetic process. some reference po1nts are essential; the most commonly used are the protolith and neosome compositions (ideally both the melt and the res1duum, but most commonly JUSt the melt). How well these parameters can be constra1ned determ1nes what kmds of petrological questions can be addressed. and what types of quant1tat1ve modeling can be applied. Secondly. it IS generally very difficult to find samples affected by JUSt one process. The investigation of any Individual process requ1res finding ways to isolate its geochemical s1gnature from the rest. Typically. this entails selecting specific combinations or ratios of elements.

6.2 Reference..point compositions Morpholog1cal and compos1t1onal heterogeneity IS typ1cal of m1gmat1tes; hence. all 1ts parts should be sampled, and the spat1al and temporal relationships among samples should be recorded. Compos1t1onal gradients are another common problem w1th migmat1tes. espec1ally 1n the rocks denved from melt-nch port1ons of m1gmatites and 1n the res1dual rocks. Care should be made to sample the full w1dth of bod1es of leucosome and melanosome so as to 1nclude the entire compositional gradient; th1s IS especially important if detailed mass-balance calculations are contemplated. It is common practice to separate the melanosome from t he leucosome for analysis. but 1t must not be forgotten that

At!"' o f hf&gmatl t<'

------------ ------ ------------ --- 37 Quartz

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Orthoclase

Fig. 5. Compositio n of l33 samples of leucosome from mctatexitc migma tites. (a) Quartz - alkali feldspa r - plagioclase mcsonorm plot with fields after Srreckeiscn (1976). (b) Qua rtz- albite - orthoclase mcsonorm plot. Symbols: dot , leucosomcs from metapclitic and metapsamm itic (plagioclase + biotite + quartz) protoliths; squares, eight samples of leu cosome from a single outcrop, mctapclitic protolith; tri angles, leucosome from mctama fic protol iths. The leucosome from metapclitic and mctapsamm itic protoliths ranges in composition from syenogranite ro tonalite, although most samples a rc monzograni tic or granodioriti c in composition. In contrast, leucosome derived from mctamafic rocks range in composition from monzodiori te to ton alite, but most arc ronalitic. The leucosome collected from a single outcrop of metapelite show essentially the same range of composition as the entire set of leucosome samples from mctapelitic a nd metapsamm itic protoliths. The variability in composition of the leucosome thus is not simply a function of the bulk composition of the prorolith ami the P- T conditions at which anate xis occurred; processes that occur during c rystallizatio n of the melt also may be imporra m. such a methodology results In the loss of tnformatton on the composttton and compostttonal gradients at. and close to, the contact. Some problems speofic to protolith, melt, and residuum are dtscussed next.

D etermini ng protolith compositions (the starting material) At the tnoptent stage of parttal melttng, the rock between scattered patches of neosome tn a fertile layer could be constdered as betng very close to the protolith composttion. It may also be posstble to trace a parttcular parttally melted layer, or sequence of layers, laterally to lower metamorphic grades and to sample them JUSt below the "melt-tn" tsograd that marks the onset of parttal melting. Determtntng what the protoltth was ts a challenge tn the htghest-grade parts of anatecttc terranes where virtually all the rocks are parttally melted and neosome abounds. It is an error to use the rematntng paleosome (nonmelted and typtcally mesocrattc) relics to represent the protolith composttton. These mesocrattc layers of paleosome are preserved specifically because they have infertile bulk composttions. and so escaped parttal meltmg. There are three potential opttons in obtaining an esttmate of the protolith composttion. (I) Collect a very large bulk sample of the whole neosome; thts works well if the mtgmattte formed

in a closed system. and where the segregatton dtstance was small, as in some contact aureoles. (2) Find, and sample, stmilar rocks tn an adJacent part of the terrane not affected by parttal melting. The problem then becomes whtch rocks to sample. Hydrous rocks progressively devolatilize durtng prograde metamorphtsm, and could stmultaneously lose some major and trace elements. Thus, sampling the htghest-grade nonmelted rocks could provtde a better approxtmat ton of the protoltth composttton tn a regional metamorphic terrane. Contact aureoles. and some reg1onal metamorphic terranes 111 wh1ch low-grade country rocks were rapidly heated and melted, may be an exception if the nutds and any trace elements released by prograde devolat11izat1on rema1n 111 the rocks and were tncorporated wholesale 1nto the m1gmatites. (3) The least satisfactory opt1on, best cons1dered as a last resort, IS to adopt a compiled average composition (e.g., average upper continental crust) for the protohth; these averages 1nvanably 1nclude bot h ferttle and tnfertile rock types, and so preclude any mean1ngful mass-balance modeling.

D ete rmining the "melt" composition Melt compos1t1on here refers to that generated at partial melt1ng. Because this melt composition serves as the start1ng point 111 1nterpret1ng leucosome and diatex1te petrogenes is, 1t IS in some cases called the "init ial" anatectic

IN T RO DU CT ION

38 --- --- --- --- --- --- --- --- --- ---

"prim ary melt" is melt composition (note that the term the mantle). The generally reserved for partial melts of ctic leucosome. most w1dely used method to 1dent1fy anate al" melts is to or diatex1te m1gmatites, that may be "1n1ti for haplogratics cotec the of compare its position to that Typically. plots. Or AnAb nltiC melts on Ab Or- Qtz or t-perweigh or s CIPW norms are used, but mesonorm n Brow 1961, e cent plots also have been used (e.g., Wylli y Barbe nes 1986, 1979, Johannes 1983b. Gupta and Johan tad et al. 2005). et al. 1990, Symmes and Ferry 1995. Slags the cotect1c 1n A composition that plots on. or close to. Or- Qtz H 0, the systems Ab Or- Qtz H 0 or Ab Anthe natuwh1ch at 0) corresponding to the P- T a(H 2 be1ng an le samp the w1th ral melts formed, IS consistent "initial" anatectic melt.

wh1ch the leucocrystallization of the anatectic melt from some was derived. nt of "1nitial" anaAn est1mate of the maJor-element conte can be obta1ned tectic melts from more complex systems start1ng materials from partial-melt1ng expenments that use hth, and perproto red 1nfer the to r with compos1t1ons s1mila at which those to close t1ons formed at P- T-a(H 20) condi ced produ tally nmen expe the natural melt1ng occurred. The and FeO. , i0 T in 1ons 1 melts show rathe r lim1ted vanat O KL and 0, Na 2 MgO contents, but the vanat1on 1n CaO, comlith proto the contents may be large and are related to l melt1ng, and partia of re eratu temp and ure, position, press ut. an H L 0 fluid whet her melt1ng occurred with, or witho onents, parcomp le volati ure meas to present. The ability s is another glasse tal rimen expe ticularly Na 20, correctly in os1t1on comp the of ate factor that seriously affects any est1m g the nd1n 1thsta Notw of initial anatectic melts (Grant 2004). osicomp the1r g possible analyt1cal prob lems in determin1n of on os1t1 comp tions, I present in Fig. 6 a compilation of the lt1ng part1al-me glasses (i.e., quenched melts) obta1ned from ng composistart1 of ty vane a on ucted experiments cond ut H 20 w1tho ts nmen expe to in tions. Figures 6a e perta and ts), nmen expe ng" melti added (so-called "dehydration with out d carrie ts nmen F1gs. 6( and 6g pertain to expe e, although not H 20 added to the expenmental charg resulting melt (sonecessarily w1th enough to saturate the of leucosome or called "H,O -fluxe d" melt1ng). Samples nal field as approd1atex1te plott1ng 1n the same compositio compositions of pnate experimental glasses may represent

was haplograTh1s approach is satisfactory 1f the melt smaller s1ze of m syste d close a nltic and crystallized 1n these cony sat1sf not do than the sample. For rocks that should be plots Or n ditions, the Ab - Or- Qtz and Ab-A the melt e chang used w1th caut1on. Addit1onal components tic lines cotec the composition, and hence the pos1t1on of a posire, ermo (e.g., Mann1ng and P1chavant 1983). Furth sarily exclude a tion far from the cotectic does not neces xite migmat1tes magmatic ong1n for leucosome (or diate des an "1n1t1al" exclu bly proba it for that matter). although talliz ation , crys l iona melt compos1t1on. During fract from t he rated sepa the phases that have crystallized are Samples melt. " remaining "frac tiona ted" or "evo lved crystals, ed -form that represent accumulations of the early plagioclase apex, "1nit1al" melt. for example. plagioclase, plot nearer to the and Barbey 1982, ctiC melt have or to the plagioclase quartz jo1n (Cuney Once potential samples of "in1t1al" anate melt ted" tiona "frac of les samp the ents should be Sawyer 1987), whereas been identified, their trace -element cont the rd towa ple, exam th1s n 1 s of contamplot 1n the oppos1te dwect1on. investigated for confirmat ion. The poss1b1lit1e clase ortho the or JOin, crystallization quartz apex, the quartz- orthoclase ination with the res1duum and of fract1onal spar K-feld of ns on results 1n apex. Leucosome that conta1ns accumulatio should be invest1gated. Fract1onal crystall1zat1 ermo re, conFurth ble. possi ents comare elem also ls trace the crysta of z or quart an enrichment 1n the cu mulate s, phase ual res1d by melt ct1c m1nerals, anate ulated 1n1t1al accum the tamination of patible with the early-crystallized and plots. cular part1 these for trace oclase the plag1 of and notably quar tz and an enrichment in the fractionated melts The t1c. cotec a from sigaway the plot Thus, that can result 1n samples elements Incompatible w1th these m1nerals. n show are some leuco lie of d 1ns shoul that it compos1t1ons of some doma nature of a "1n1t1al" melt compos1t1on IS and clase plagio ar feldsp field onated melt 1n Fig. 5, on quartz - alkali between the cumulate field and the fract1 osition of the tituent versus a cons le pat1b 1ncom Ab-Q tz-O r mesonorm plots. The comp on bivanant plots of an samm1tic prometap and pelitic meta versus Sr), or from Rb ed or denv 0 Na some s leuco 2 compat1ble one (e.g.. K 0 versu from tonalite and ite. tonal to er 1987) . te Sawy gran1 .. (e.g from ms ds diagra exten toliths on rare-earth element (REE) metamafrom ed deriv r REEs some othe leuco the for and te to monz odion where Eu 1s the compatible element is some leuco a of n ositio arises comp ion the posit ver. al fic protoliths. Howe are Incompatible (see F1g. 7). Th1s centr h lit proto the of n the ositio to comp tal bulk p aren not related simply to the because the "1nitial" anatectic melt is range A rred. occu ng melti More d melt. or the P- T conditions at which crystal-nch cumulate and to the fractionate found w 1th1n sin and fractionlate cumu the of ons of leucosome compositions IS commonly os1t1 comp Importantly, the Sb and Sa gs. F1 1n es the "1n1t1al" melt gle outcrops; for example, the squar ated melt show the same relationship to a s1ngle outcr op from ns ositio comp an equ11ibnum as d some forme leuco melt sent repre 1rrespect1ve of whether the They lith. proto elit1c metap a melt, or from ibnum ed equil deriv ted of migmatite melt. a trace-element-undersatura vanation w1de Th1s ite. ces tonal to produ te ng gran1 melt1 from um show a range a disequilibrium melt. If disequilibn onal fract1 to uted attrib IS some leuco of 1n the compos1t1on

Aria' of Mtgmatltc'

------- ------- ------- ----------39 Quartz

Quartz

Pelite

a

Greywacke

b

\ ..: :~ :,.\ ,.~~~ ·.. o• ,. • o.. .

... •

Albite

Orthoclase

Albite

Orthoclase

Quartz

c

Quartz

Felsic rocks Dehydration melting

Mafic rocks

d

Dehydration melting

6

• 0 0

Albite

Orthoclase

Albite

Orthoclase

Fig. 6. Composition of glass (quenched partial mel ts) obtained from partial-meltin g experiments shown as a function of composition of starting material (protolith) and the type of melting. (a) Part ial melting of pelitic protolit hs with out H ,O added (i.e., dehydration melting). ote the wide range of compositions. (b) Partial melting of greywacke (psam~itic) protoliths without H ,O added; note the tight cluster of compositions corresponding to monzogranite. (c) Pa rtial mel ting of felsic pr~tolith s (granite, tonalite, etc. ) withou t added H ,O; the composit ional range is similar to greywackes. (d) Partial melting of mafic protoliths without H ,O added; ;,,ost compositions of glass fall on the qua rtz-albite join, but some a re granodioritic. (e) Glass compositions for H ,O-added partial-mel ting experiments of pelitic, greywackc, felsic, and mafic protolit hs, shown on a quartz- albite- orthoclase plot. (j) All the glass compositions from a to d shown on a quartz - alkali feldspar - plagioclase mesonorm plot, shown with the fie lds from Streckeiscn (1976). The glass compositions range from syenogranite to tonalite, but most arc monzogranite. (g) Glass compositions for H ,O-added (i.e., H ,O-fluxed) partial-melting experim en ts on pelitic, greywacke, felsic, and mafic protoliths, shown o~ a quartz - alkaii feldspar - plagioclase mesonorm plot, along with the fields of St reckeisen ( 1976). The glass compositions range from syenogranite to tonalite, but most lie in the granodiorite field. Legend: squares, P < 3.5 kbar; circles, P 4- 10 kbar; diamonds, P 10- 16 kbar; triangles, P > 20 kbar; filled symbols, T < 925°C; open symbols, T > 925°C. Data sources: metapelites (Vielzeuf and Holloway 1988, Lc Breton a nd Thompson 1988, Patino Douce and Joh nston 1991, Pati no Douce and Harris 1998, Pickering and Johnston 1998, Koester et a!. 2002); mctagreywackcs (Conrad et a!. I 988; Viclzeuf and Monte! 1994; Gardien et a!. 1995, 2000; Patino Douce and Beard 1995, 1996; Stevens eta!. l 997; Monte! and Viel:euf l 997; Cast ro et al. 1999; Grant 2004); felsic rocks (Castro eta!. 2000; Conrad eta!. I 988; Johnston and Wyllie 1988; Litvinovsky eta!. 2000; Rutter and Wyll ie 1988; Skjerlie and Joh nston 1992, 1996; Singh and Johan nes 1996a, b); mafic rocks (Rushmer 1991; Rapp and Watson 1995; Wolf and Wyllie 1991 , 1994; Springer and Seck 1997; Skjerlie and Patino Douce 2002; Selbekk a nd Skjerlie 2002).

INTRODUCTION

40 --------------------------------

Quartz

Quartz

All rock types

e

All rock types

f

.. Orthoclase

Albite

Alkali Feldspar

Plagioclase

Quartz

All rock types

g

H,O-added melting

3a

Fig. 6 1'

•

Alkali Feldspar

Plagioclase

an "1nit1al" melt with very low concentrations of Incompatible elements, then fractional crystallization of that melt will yield a cumulate with still lower levels. If no "mitial" anatectic melts can be identified from the

have different compos1t1ons because the composit1on of the source has changed, and the temperature of partial melt1ng has 1ncreased. AnatectiC melts of diverse composit ions start to appear as soon as part1al melting beg1ns in other rocks or layers of different bulk composition. Thus, it

m1gmat1te samples collected, then a glass compos1t1on from an appropriate expenmental study [same or Similar starting composit1on and P T o(H 0) cond1t1ons as the m1gmat1tes] could be used as a reference-po1nt com position. However, us1ng the compos1t1ons of glasses from

IS not a s1mple task to determ1ne the composition of the "1nitial'' anatectic melt from which a part1cular batch of leu-

partial-melt1ng expenments has the disadvantage that trace-element contents have generally not been reported for them, although recent advances in analyt1cal techn1ques w1ll enable the trace-element contents of experimental

Rocks that conta1n the res1duum left after part1al melt1ng can occur in many forms 1n m1gmatites and 1n assoCiated autochthonous gran1tes (see. for example, Bar.bey 1991). In the lowest-grade parts of migmat1te terra-~es. the resid-

glasses to be determined rout1nely.

ual rocks are commonly, but not exclus1vely. found 1n the melanocratic zones around bod1es of leucosome. Care should be taken to distinguish melanosome (i.e., meltdepleted residuum) from mafic selvedges formed by reaction between a leucosome, or ve1n, and its wallrocks. The

A unique composit1on of "1n1t1al" anatectic melt may only exist at the very start of anatexis, when the first rock partially melts, or if the proto lith was homogeneous. Success1ve Increments of anatectiC melt derived from a s1ngle source

cosome, or diatexite m1gmatite. was denved.

Residual rocks

Atla> of M 1gmat1tc;

- - - - - - -- - -- - - - - - 41 1000

1000 QU ETI CO LEUCOSOMES Pelitic protolith

~

-oc

2

100

·.::: '0

c 0 r. 0

0

r. 0

W ULUMALEUCOSOM ES Pelitic and semi-pelitic protol iths

10


100


a.

a.

E

E

ro

ro

(/)

10

(/)

a

b

0.1 La Ce

Nd SmEu

Tb

Yb Lu

La Ce

10

Nd SmEu

Tb

Yb Lu

100 MOU NT HAY LEUCOSOMES

2

2

·.:::

·.::: c 0 r. 0

'0

'0

c 0 r. 0


a. E

ro

10


a.

0.1 GRE NVI LLE FRO NT LE UCOSOMES Mafic protolith

(/)

c 0.01

E

ro

(/)

d 0.1

La Ce

Nd SmEu

Tb

Yb Lu

La Ce

Nd SmEu

Tb

Yb Lu

Fig. 7. Chondrite-no rmalized ra re-ea rth eleme nt (REE) pa tterns fo r le ucosome in fou r migma titc te rra nes. (a ) U ppe ram phibolite -facies a natexis, T 700-800°C, P 3-4 kba r. (b) G ranulite -facies a n atex is, T 825-875°C, P 6-7 kba r. (c) U ppe r-amphibolite- to lowe r-gra nulite-facies an atexis, T 800-850°C, P 8- 10 kbar. (d) G ranu lite-facies an atexis T 825-875°C, P ca. 5 kba r. All the le ucosome samples a re from me tatexi te migmati tes. Al th o ugh they fo nned from diffe re nt pro to liths tha t unde rwe nt a na texis at diffe re nt pressures a nd te mpera tu res, the overall REE c h aracteristics o f the le ucosome suites are very simila r. T he re a re three types o f REE pa tte rns wit hin the le ucosome d ata. (1 ) Most samp les o f le ucosome have a REE pattern with a strong, positive Eu anoma ly, bu t lowe r abu ndances o f the total REE compa red to the other pa tterns. (2) A few samples of le ucosome have REE patterns with a negative Eu a no ma ly, and in gen e ral, the high est abunda nces of t he REE. (3) A few samples of le ucosome (shown as bo ld lines) h ave inte rmedia te- leve l REE a bund a nces and a smooth pa tte rn wi th o ut Eu a nom aly. The prese nce o f the t h ree types of REE pa tterns within each of the leucosome suites is inte rpre ted as strong evid e nce t ha t fraction al crystallizatio n played a majo r ro le in the evolutio n of th e a na tectic melt from which the le ucosome in a migma tite te rrane was deri ved. The leucosome samples wit h a smooth pa tte rn are inte rpre ted to h ave crystallized from me lts that h ad com positio ns close to th at of the ini tial a na tectic melt; le ucosome samples with a positive Eu anomaly are inte rp reted to he feldspa r accumula tion s de ri ved fro m prima ry me lts, a nd the le ucosome sa mp les with a n ega ti ve Eu a no maly a re in te rpreted to have c rystallized from evolved a na tectic melts tha t had alread y cry ta llized substa ntial amo unts of feld spa r, p rincipally plagioclase.

latter are generally narrow and composed largely of biotite or hornblende. In h1gher-grade metatex1te migmatites, such as those found 1n melt-depleted, granul1te-faoes terranes (Guern1na and Sawyer 2003), conspicuously melanocrat1c rocks are rare, but v1rtually all the rocks are melt-depleted, and hence residual 1n bulk compos1t1on (Barbey and Cuney 1982, Maccarrone et al. 1983, Caggianelli et al. 1991). In places where d1atex1te m1gmat1tes are abundant, res1dual rocks may be rare, and a deta1led geochem1cal study may

reveal a deficit of res1duum, i.e., a surplus of anatectiC melt (e.g., Obata et al. 1994).

Mineral compositions Many of the processes of 1nterest 1n m1gmatites can be modeled as the addition of one or more minerals to a particular start1ng composition, or thew subtract1on from that start1ng compos1t1on. Therefore, the compos1t1ons of

INTRODUCTION

42 ------------------------------

m1nerals present 1n migmat1tes are a very useful add1t1on to whole-rock geochemical diagrams because they may Indicate the vectors of compos1t1onal change associated w1th the processes that occur in migmat1tes (e.g.. contamination of anatectic melt by residual biotite). W1th the 1ncreas1ng availability of ion probe and laser-ablat1on techniques, contents of major and trace elements are becoming routinely available for the main minerals in migmatites. These data may prove useful in identifying the provenance of residual minerals in leucosome and diatexite m1gmat1tes. as well as 1n providing further constraints on models of petrogenetic processes in migmatites, such as part1al melting, melt extraction, and fract1onal crystallization.

6.3 Diagrammatic representation Most geological processes yield suites of samples that define trends, or fields, on compos1t1onal d1agrams. because the process has advanced farther in some samples than in others. To interpret such trends, the composition of the st arting (i.e., pre-process) material, and either the endproduct composition, or the changes in mineral assemblage and modal proportions that are involved in the process, are required. Useful quantitat1ve modeling can only be done w1th this information. Changes in composit1on are best shown 1n diagrams that d1splay the greatest d1spersion of the product rock types about the start1ng compos1tion. Bivariant d1agrams in which an element that characterizes one of the products of the process cons1dered (e.g .. residuum) 1s plotted against an element that charactenzes the other (e.g.. anatectic melt) are generally suffic1ent. Both the maJor elements and trace elements should be treated. Res1dual rocks are most commonly ennched 1n Fe Mg silicate minerals relative to the protolith, but they may also be enriched in accessory minerals, plagioclase, K-feldspar, and even quartz, depending on the particular composition of the protolith. The Fe-Mg s1licate fraction is commonly represented by the sum (FeOr + MgO + T10). where FeO represents all the 1ron expressed as FeO, but 1n some Circumstances that cho1ce of components IS Inappropriate. If T10 1s pnncipally in m1nerals other than an Fe Mg silicate (e.g.. rutile. rather than b1ot1te). it may be better to cons1der 1t separately from (FeO + MgO). because mmerals that form very small gra1ns could be segregated w1th the melt. much like inherited zircon, for example. The presence of Fe ox1de and Fe sulfide m1nerals 1n the protolith means that some, and poss1bly most, of the Fe IS not 1n the silicate m1nerals. In that circumstance, MgO alone may be the best way of depicting the ferromagnes1an silicate fraction in the residuum Alternatively. the whole-rock FeO content may be corrected by subtracting the amount of Fe in the sulfide and Fe oxide minerals, using a combination of modal

abundance and m1neral compositions determ1ned with an electron-microprobe analyzer. Trace elements that can be used to represent the Fe Mg silicate m1nerals fract1on include Sc and Co; Cr, N1, and V may be used where sulfide and Fe oxide minerals are absent. The b1ot1te component of the residuum could, for example. be represented by Rb. Residua rich in sillimanite, cordierite, or spinel could be represented by Al 20 3• Similarly, CaO may be used for residua rich in hornblende, clinopyroxene, and plagioclase; Sr may also be used for 1nvestigat1ons of plagioclase-rich residua. Many investigators have shown that a large part of the total budget of some trace elements, 1nclud1ng the REE, Y. Th, U, Hf. Zr, Nb. and Ta, IS located 1n accessory m1nerals, and the behav1or of these m1nerals dunng partial melt1ng largely controls the way trace elements are distributed between the anatectic melt and the res1duum (Sawka 1988; Bea 1991, 1996a. b; Bea et al. 1994; Watt and Harley 1993). In some Circumstances, the accessory m1nerals dissolve completely into the melt. resulting 1n a res1duum depleted in the trace elements assoCiated with those particular accessory phases, but in others. the accessory minerals were either in excess, or did not dissolve (e.g.. not enough time), or could not dissolve (e.g.. armored by other phases) into the melt, and an enrichment of the associated trace elements in the res1duum results. However, recent results from ultra-h1gh-temperature (UHT) metamorphic terranes suggest that accessory phases may control less of the traceelement budget in very-h1gh-temperature anatex1s. Hokada and Harley (2004) and Cond1e et al. (2004) found that the major phases 1n rocks formed at ultra-h1gh temperatures can contain significant amounts of h1gh-field-strength elements, such as Zr and the REE, 1n sufficient amounts that the major phases. because of their h1gh modal abundance, can control the budget for many of these trace elements in the whole rock. The bulk compositions of some protoliths result in residua that are very rich 1n K-feldspar. or in quartz; K20 and Si02, respectively, may be an appropriate way to differentiate rocks denved from such res1dua from those derived from the melt fract1on. Anatectic melts generally range from gran1te to tonalite trondhJemite in compos1tion (see Figs. 5 and 6). although potass1c anatect1c melts of syenogran1te compos1t1on may form from protoliths such as plag1oclase-poor pehtes (Pickenng and Johnston 1998. Grant 2004). Generally, the anatectiC melts are ennched 1n K20, Na,O, and S102 relative to their protoliths. The trace elements Cs, Ba. and Li are partitioned int o the melt under most circumstances of partial melting in the continental crust (Icenhower and London 1995. 1996). Hence, these major ox1des and t race

_A_tl_"_''_'f_M~ig_rn_a'-"-"-'--------------------------------- 43

elements are commonly used to represent the anatectiC melt, or melt-denved rocks. in b1vanant plots. Plagioclase is generally one of the first minerals to start to crystallize from melts of granitic compos1t1on, and 1n the early part of the crystallization h1story, 1t may be the most voluminous product. Thus, a plagioclase cumulate formed by fract1onal crystallization may be represented by Na ,0. CaO, or Sr. The rema1n1ng evolved melt is ennched 1n the components 1ncompat1ble w1th plag1oclase; the rocks crystallized from such a melt could be represented by Si0 • KzO. 2 Cs. Ba, L1, Pb, Sn, and even U. The crystalltzat1on of accessory phases. e.g., z1rcon, monaz1te. xenot1me. and apat1te, from anatectic melts common ly controls the high-fieldstrength elements (H FSE) and REE contents of the rocks (e.g., leucosome and some diatex1te m1gmatites) denved from the anatectiC melt (Miller and M1ttlefehldt 1982, Sawka 1988, Wark and Miller 1993. Bea et al. 1994) and should be Investigated. The papers by Bea (1991, 1996o, b) and Bea et al. ( 1994) are 1mportant resources for the traceelement contents of both accessory and maJOr phases, and are useful for those wishi ng to model partial-melt1ng and crystallization processes. Comb1nat1ons of elements may generally be found to produce a w ide d1spersion of the sample points in the compos1t1onal space of peltt1c, greywacke. and mafic bulk compos1t1ons. However, 1n cases where the compos1t1on of the protolith and the anatectic melt are s1m1lar (e.g., part1al melting of granite, arkoses, and some psammites), compoSitional changes can be very small and, consequently, are very d1fficult to illustrate diagrammatically.

Matched triplet sets of samples Sample sets 1n wh1ch a part1cular protolith and the "1n1t1al" anatectiC melt (leucosome) and residuum (melanosome) derived from it have been systematically sampled are few. Because of the known relat1onsh1p among the samples, these tnplet sets of samples offer unique opportuntt1es to study the details of the processes that occurred in migmatites. They can be used for mass-balance techniques, such as the 1socon method of Grant (1986; but see also Baumgartner and Olsen 1995), to investigate whether meltIng occurred 1n an open or a closed system, and to calculate the volume and compos1t1on of the matenal that was added. or lost (e.g .. Olsen et al. 2004). The degree of part1al meltIng (F) can also be constrained, and the vanous conceptual models for partial melting (e.g.. equilibnum batch-meltIng, fractional melting, disequiltbnum melt1ng. etc.) can be tested by try1ng to repltcate the measured compos1t1ons of the leucosome considered to represent the "1n1tial" melt, although selecting which distribution coefficients to use 1s qutte another problem (see comments by Icenhower and London 1995), and one outs1de the scope of thts book.

Matched samples collected from different parts of 1ndiv1dual bod1es of leucosome could be used to investigate fract1onal cryst allization, or the unmixing of t he melt fraction from the residuum. The problem can also be inverted to 1nvest1gate the extent to which contam1nat1on by the wallrocks has occurred dunng the formation of the leucosome, for example, by the movement of melt through the network of channels that dra1ned melt from 1ts source (see Fig. 4b, for example). Mult1-element vanat1on diagrams (the so-called "spidergrams") in wh1ch the products are normalized to the starting composition are a good way of showing compos1t1onal vanations with1n matched sets of samples.

General sets of samples In many migmatites, particularly if the proport1on of neosome 1s h1gh, or 1f the melt fract1on has moved long (meter-scale) distances, it 1s 1mpossible to be certain that the leucosome sampled is genet1cally related to it s adjacent. or even the nearest. melanosome; moreover, determ1n1ng what was the protolith may be Impossible, whtch precludes sampling it. Although these general sets of samples are not suited for studies of the kinds of problem outlined in the prevtous sectton, they are. if a large numbe~ of samples are collected, 1deal to examtne the complete range of petrological processes that occur in making mtgmatites; recent examples of th1s approach were provided by Sawyer ( 1998, 1999). Sawyer et al. (1999), Mtlord et al. 2001, Solar and Brown (200 I). Villaseca et al. (200 I). Fornelli et al. (2002). and Guernina and Sawyer (2003). Two types of diagram are parttcularly useful (Fig. 8). The first. MgO or (FeO~ + MgO) versus Kp or Si0 • can be 2 used to examtne partial melttng and the unmixing of melt from the restduum. The second, CaO or (CaO + Na 0) versus K,O or StO,. ts useful where plagtoclase 1s a maJOr residual phase; with it, one can investigate the processes that occurred dunng the crystallization of the anatectiC melt. such as fractional crystallization 1n whtch an earlycrystalltzed mtneral. typtcally plagioclase, but 1n some cases K-feldspar, accumulates in one part of the system, and the complementary fractionated melt crystallizes elsewhere. Cumulate plagtoclase could be represented by CaO or (CaO + Na20) , K,O could represent e1ther the K-nch fractionated melt or cumulate K-feldspar. and SiO, could be used to represent the fracttonated melt. In order to 1nterpret the scatter and trends that a large set of general samples from a mtgmatite terrane generally show tn com posttton space. some reference po1nts and trends need to be establtshed. Ideally, these are the compos1t1ons of the sampled protolith and the anatectic melt (or melts). If the compositton of the melt (from the leucosome) is unavailable. then a suitable composition of an expenmental melt (;.e.. glass) 1s the next best opt1on. The compositional range of fels1c anatectic melts obtained from a parttcular

INTROD U CTION

44 ----- ----- ----- ----- ----- ----- -

type of prot olith (e.g., greywacke, pel ite, or felsic) in partialmelting expenmen ts below 900°C and at pressures below I0 kbar is relatively small (F1gs. 6a-c); major dive rsity 1n the compos1t1on of anatectiC melts starts after the hydrous phases have disappeared. From a knowledg e of the meltIng react1ons and P- T a(H,O) cond1t1ons unde r which the migmatite s fo rmed, and the type of prot oilth involved (e.g., pelite, greywacke, mafic, gran1te, or tonalite), glass composit ions can be obta1ned from the literat ure with wh1ch to define a "field of anatectiC melt." The compos1t1ons of both nat ural melt (from leucosome ) and expenme ntal melt are shown in Fig. 8 for compariso n. Any samples of leucosome, or d1atexite migmat1te. that lie 1n (or close to) the expenmen tal melt field may be "initial" melts, and should be fu rther Investigated us1ng the1r trace-elem ent (1.e., REE) contents; these samples could be useful in defin1ng the trace-elem ent characten st1cs of the melt. The leucosome compos1tions from metatexit e migmatites derived from mafic protoliths (Figs. 8a, b) plot closer to expen mental glasses (W) that were generated by H,O-adde d partial melting, than to the glasses (D) obtained from expenments w1th no H 20 added. This may mean that melting 1n these m1gmat1tes was 1n fact H20-fluxed. Leucosom e compositions from the Quetico (Ontario) m1gmatite terrane

(Figs. 8c, d) partially overlap the compositional field for expenmen tal glasses generated from dehydrati on melt1ng (i.e., w1thout H,O added). Fo r samples of diatex1te m1gmat1tes (F1gs. 8e-j), the appropna te experimen tal glass compositi ons from partial melt 1ng w ithout H20 added 1nvanably co1ncide w1th those samples of d1atex1te w1th the lowest MgO, (FeOT + MgO) and CaO contents, but w1th h1gh K, 0 and Si02 contents, which from thew petrograp hic charact eristics were essentially anatectic melt. W 1th the trend between melt and protoilth compos1t1ons established, the melt-depl eted or residual rocks (e.g., melanosome and some diatex1te m1gmat1tes) are 1dent1fied by their position along this trend on t he opposite s1de of the protolith to t he melt (F1g. 8). However, F1g. 8 shows that the compositi onal fie ld of t he prot olith is much larger than the compos1t1onal field of the "in1t1al" anatectic melts (i.e., excluding leucosom e that is t he prod uct of fract1onal crystallization) 1n most m1gmat1te terranes; consequently, the compositi onal field of the res1dual rocks IS large r still. Neverthel ess, assum1ng a representa tive sampling of melanosome, 1t could be argued that a t1e line from the densest cluster of melanoso me samples to the melt compos1t1on should pass through the most abundant composit1on of

tion of general sets of samples Fig. 8. Examples of plots of compatible versus incompati ble elements used in the interpreta migmatites from the Grenville Front from migmatite terranes. (a) (FeOT + MgO) versus SiO! and (b) CaO versus iO z for n of hornblend e and the fornear Ch ibougamau, Quebec; anatexis at T 800-850°C, P 8- 10 kbar, involving the breakdow SiO z and (d) (CaO + azO) versus mation of a tonalitic melt with clinopyroxene and garnet. (c) (FeOT + MgO) versus C, P 3-4 kbar, involving the brea k700-800° T at anatcxi> Si0 2 for migmatites from the Q uetico Subprov ince, Ontario; am pies of di atexitc plor in the some that Note down of biotite and sillimani te and the formation of melt plus cordieritc. the palcosomc (resister) liththat and fields, protolith field, whereas others lie between the protolith and anatectic melt (CaO + Nap) versus Kp ([) and ologies have compositions t hat fall outside the protolith field. (e) MgO versus Kp P 5-7 kbar, principally via the H 10fo r migmatites from the Opatica Subprov ince, O ntario: anatexis at T 750-800° C, versus K!O for migmatites from prcsenr melting of quartz, plagioclase, and K-feldspar. (g) MgO versus K20 and (h) CaO + quart: + plagiocla>c = melt + the Ashuanip i Subprov ince, Q uebec: anatexis at T 825-875°C , P 6-7 kbar, through biotite the melt had composit ions of about orthopyroxene + ilmenite or magnetite . The plagioclase that initially crystallized from tely A n 10 . Many samples plot a long approxima is and melt A n 10, whereas the last plagioclase crystalli zed from more evolved however, that trend also coincides plot, this On the trend of melt + residuum (composed of pl agioclase + orthopyroxene). MgO) versus Kp and (j) CaO + with the accumu lations of plagioclase cry tallizcd from the anatectic melt. (i) (FcOT the breakdown of musinvolving versus SiO! for migmati tes from Saint-Malo, France; anatex is at a T of 750°C, P 4- 7 kbar, arc explained on the figures) cm•ite, a nd with biotite genera lly stable. \XIhole-rock data arc shown as points (the symbols ), which arc located at the tips and arc combined wit h mi neral composit ions (determi ned by electron-microprobe analysi to the resid uum and a re drawn of the arrow (sec text for further explanatio n). Solid arrows pertain to minerals belonging the composition of the minerindicate arrows dotted The radiating out from a representative composit ion of the protolith. compositio n of the anatectic al iti in tive representa a from als that crystallize d from the anatectic melt, and they arc drawn residuum, plagioclase accu, protolith , melt. The positions of anatect ic melt (al so marked AM where space is restricted) melts contamin ated with for trend mulations, and fractionat ed melts arc labeled on the diagrams. Labeled arrows: CA M, 0 represents glass compositions from residuu m; A P, trend for plagioclase accumulat ion; FM, trend for fractionat ed melts. xcd experiments. dehydratio n-melting experimen ts, and \XI represents glass composition from HzO-flu

A ri a., o f MJgmatitc"

--------- --------- --------- ---- 45

25

e ~

~ 0

~

20

T Palaeosome

20

• Leucocratic vem

0 ~ 15 + t-

~ 10 u...

5

'


X Experiment glass

~ 010 (I]

ox>xx··.Q;>

~-~X

to garnet

0

j

PI An30

W

. . . . .cf.:.c:_xe.._ ,.

melt-product ~ ........::···.....(A

5

···.X anatectic W • melts

60 Si02 (wt %)

PI An4o; __ D

()

0

trend for accumulation of plagioclase

b

0 50

trend for melt contaminated ...1-j!;;;,.,_~protolith restdual jwith residuum

"'+-+

· by residuum

a 40

T

~15

+ Protohth

trend for anatectic melt contaminated

Metatexite migmatites Grenville Front mafic protolith

Metatexite migmatites Grenville Front mafic protolith

TT

0 In situ leucosome () Neosome unsegregated

25

..

Melanosome

anatectic melts

0

70

40

80

25

50

60 Si02 (wt %)

70

80

8 to • Grt

20

to melt-product

Metatexite migmatites Quetico subprovince pelitic protolith

residuum

\ ...... plagioclase An20

7

~ 0

~

I

0

0

Ol

:a

N

(I]

z

+ 10

t-

+

<1>

0(I]

0

u...

0

5

5

o

60

65 70 Si02 (wt %)

1.7

~

3 to Crd

2

Ol

:a

residuu$

......

8

80

n.~-CAM

~

j 0

N

(I]

z

/

/

___rj,

fractionated melts

+

() ·-·t:Jo ...o/" • • to Qtz ~ ~ ············' ·0.··············•····*·····~~ P_!... ··· ····· ............ ··· x~~:~d~c

• ••

3

75

80

4

K20 (wt %)

5

6

Diatexite migmatites Opatica subprovince leucotonalite protolith

9

•

0(I] 0

8 7 6 5 4

~rs

f 3

2

65 70 Si02 (wt %)

60

0

+

~

to Qtz

Metatexite migmatites Quetico subprovince pelitic protolith

+

to

~ loB~ X

++

10

0

0

55

D Diatex1te, melt-rich

X

~

75

D1atex1te, res1duum-nch ~ D1atex1te () Patch metatexite • Anatectic granite

~ ti+~otolith

•

#..·

0

•

I

0

~ X 0 X X X X to K-feldspar X

11

loBI

Diatexite migmatites Opatica subprovince leucotonalite protolith

c8~ 0 ~ ..~·Ar:,r><··..........

./ ./

d

0

accumulation

\ ...of plagioclase

........

4

FM 55

0

+

0

~ 15

·o

••

6

7

0

.· toQtz

2

~--

Xx X 4 3 K20 (wt %)

fractionated melts·.,

5

6

7

INTRO DUCT I O N

46 7

6

toOpx

5

~

•

4

0 Ol :::!!

res1dual PI An33

Diatex ite migmatites Ashuanipi subprovince psammitic protolith

~

6

•

5 ~

3

~~

2

•

9

~~~

Ia

"

CA~ !41

to 8t

2

"~-~--~~~1'"

h 0

-~--~

7

6

5

4

3

2

0

~

~ 3 u

4j

~~~--~--~~~~~~-

0

4

1

diatexite + Anatectic granite

~7)1t....rf#A~~ e

0~

~ Undifferentiated

residuum

0

I

Diatexite migmatites Ashuanipi subprovince • psammitic protolith

3

K20 (wt %)

•

I

0

Ol

8

~

0

I 0

to Crd

:::!! 10

7

6

2

0 Undifferentiated diatexite + Anatectic granite


5 4 K20 (wt %)

Diatexite migmatites St. Malo pelitic protolith

to 8t

Diatexi te migmatites St. Malo pelitic protolith

20

C1l

u

+ 1-

+

0

•

C1>

u..

5

toQtz

& Pt

0

.~0

~ El ~ 0 0 .. .........t@. ....Q .....0. .. ... .O• AM)('P~ +X

AP

0~...... .........

0

2

0

8

0

--~

~--~--~--~--~~~~~~

0

2

3

4

5

6

7

8

0 40

K20 (wt %)

t1on of protoli th for the m1gmat1te terrane. The onenta ls m1nera wh1ch by lled contro IS trend the melt-res1duum of n functio a is this and m, residuu are abundant in the reboth the compos1t 1on of the protoli th and the pressu place. took g melt1n part1al tempe rature cond1t 1ons at wh1ch proA set of vector s (solid arrows ) can be drawn for each s vector these tolith compos1tion to each res1dual mineral; from d should contain the residuum compos1t1on denve the shows only 8 Fig. clanty, of sake the For that protol ith. prototyp1cal one from ls m1nera l res1dua vector s to the mineral lith composition. The t1p of the arrow rests on the arrow the at not is 1t1on compos1t1on; 1fthe m1neral compos labeled 1s arrow the (e.g.. 1t lies off the plot), however, then d from a "to." The melanocratic parts of the m1gmatite denve ed in deplet metamafic protol1th shown in F1gs. Sa and b are relat1ve plagioclase, but are enriched 1n garnet and diops1de l ong1n. res1dua a with tent consis IS wh1ch th, to the protoli te elonga 1s t1tes m1gma alo Sa1nt-M the The protol ith field for but . vector vite musco the along y 1n a directi on approximatel b1ot1te the samples of res1duum from Sa1nt-Malo fol low the breakthe from d trend (F1gs. 81, J), because anatex1s resulte l. m1nera stable down of muscov1te, and b1ot1te remained a

•

••

FM

50

Kfs

60 Si02 (wt %)

70

80 Fig. 8

meltHowever, the residuum trend for b10t1te dehydrat1on e, yroxen Ing must lie 1n the direct1on of e1ther the orthop and , cordie rite, or garnet vect ors, plus plag1oclase. quartz um res1du the le, examp For t. presen are K-feldspar, 1f they ± b1otite ± e yroxen orthop + clase compos1t1ons (plag1o 1p1 Ashuan the 1n tites m1gma es quartz ) for the granulite-faci plagioSubprovince lie betwe en the orthop yroxen e and res1dua e-nch yroxen clase vector s on Figs. Bg and h. Orthop and can comm only be dist1ngu1shed from cord1entegarnet-nch ones. the posiIf the proto lith field IS well defined, 1n addit1on to ith, protol the to relative tion of the res1duum and melt ied, identif be can or then open- or closed -system behav1 ses and this 1s part1cularly useful 1n 1nterpret1ng the proces the (e.g., that occurr ed 1n m1gmat1tes. Samples of neosome patch unsegregated neosome on F1gs. Sa and b, and the of field the Within plot that and Be Figs. metate xlte on sysclosed a 1n formed have may ths t he1r respective protoli should tem, and are most probably generated in sttu, which ng. sampli of time the at be determ1ned by field observation e (defin and Be In contra st. the d1atexite m1gmatites in Figs.

n

Atl.t.., of ~ltgmatttc"

--------- --------- --------- ----47

a field that 1s far larger than that of the proto lith, a feature that may 1nd1cate open-system behav1or. In part1cular. some of the d1atexite samples plot outside the protolith and In, or close. to t he field of anatectic melt, whereas others plot beyond the protollth on the s1de oppos1te that of the anatectiC melt; these two groups of d1atex1te samples are interpreted to have gained melt (i.e.. they are melt-rich diatexlt es) and to have lost melt (i.e .. they are residuum-nch diatex1tes). respect1vely. In contrast. the patch metatexltes and some of the d1atexite m1gmat1tes from the same migmatite terrane (Figs. Be, 0 plot within the protolith field and can, therefore. be Interpreted to have ne1ther lost. nor ga1ned, anatectiC melt. 1.e., to have formed 1n a closed system.

cases, leucosome (e.g.. Figs. 8d 0 and d1atex1te m1gmat1tes (Figs. 8g j) have compos1t1ons cons1stent w1th crystallizat ion from a fract1onated melt. Figure 8 shows that most of the small leucocrat1c ve1ns or dikes and small bodies of anatectic gran1te that are a common. although not necessarily abundant. component of m1gmat1te terranes, have compositions indicating that they crystallized from an evolved anatectiC melt.

The pos1t1on of m1gmat1te samples relat1ve to the res1duum. protollth. and melt fields has also been used to 1dent1fy samples Infiltrated (Sawyer et al. 1999) or ve1ned (Guern1na and Sawyer 2003) by leucocratic melt. A position to the K"O-ennched s1de of the melt protolith t1e-line could 1ndicate that the InJected melt was a fract1onated one.

M1gmat1tes are produced by part1al meltmg, and th1s IS why they have complex structures and striking appearances. The anatectic melt has a lower viscosity t han nonmelted rock, and th1s enables the melt to segregate from 1ts res1duum and to collect 1n low-pressure s1tes dunng deformation. Th1s separat1on creates the light-colored (leucosome) and darkcolored (residuum) parts of migmatites. The differences in the fraction of melt from place to place 1n a migmat1te results 1n a w1de range in rock strength (or competency). Consequently, m1gmat ites deform heterogeneously and develop complex morphologies. as shown in the Illustrations of th1s book. The presence of the segregated leucocratiC doma1ns 1n the darker. more mafic-lookmg host accounts for the etymolog1cal root of migmat 1te, mean1ng "mixed rock."

The vanous processes such as contamination and fractional crystallization that affect the anatectiC melt can also be investigated on these diagrams. Some trends or vectors useful in Identifying processes are shown as dotted arrows emanat1ng from a s1ngle "typ1cal" 1n1t1al compos1tion of anatectic melt in F1g. 8; 1t should not be forgotten that s1milar vectors apply for all the compos1t1ons of anatectic melt. Using the melt-protolith res1duum tie-line as a reference. melt-denved rocks, such as diatex1te migmat1tes and leucosome, that have become contaminated by the res1duum. or perhaps by part1cular residual phases, can be distingu1shed by a trend (CAM) toward the res1duum. e.g.. in Figs. 8a. b. and g. Plag1oclase 1s one of the first m1nerals to crystal lize abundantly from a gran1t1c magma. and Fig. 8 conta1ns a vector from the compos1t1on of a representative melt to the plagioclase that crystallized from 1t. Samples that lie along th1s trend (AP) may represent accumulat 1ons of plag1oclase. Note that the plag1oclase 1n the res1duum and the cumulate plag1oclase 1n the melt-denved rocks do not. generally. have the same compos1t1on. Many of the doma1ns of leucosome (Fig. 8d, but see also F1gs. 5 and 6) and d1atexite migmatites (F1gs. 8g j) 1n migmat1te terranes are accumulations of plag1oclase that crystallized from the anatectiC melt. If the plagioclase that crystallized first IS separated, then the remain1ng melt has a more evolved. or fractionated, bulk composition relat1ve to that of the 1n1tial melt. Rocks that crystallized from evolved melts are typ1cally ennched in K-feldspar and quartz. and lie along a fractionated melt trend (FM) that lies between these two m1nerals. In some

7. M IG MAT ITE-LIK E ROCKS

However. there are processes other than part1al melt1ng that can produce rocks that have light-colored patches. layers, and ve1ns 1n a darker host and complex morpholog1es. One 1S subsolidus segregat1on 1n wh1ch the mobile (generally fels1c) components are separated from the less mobile (generally mafic) ones in a rock, and another is the 1nject1on of ve1ns of fels1c magma into a host of another composition. as occurs 1n syntectonic plutons and ve1n complexes. Unfortunately. some 1nvest1gators have focused only on the "mixed rock" aspect of light-colored ve1ns in a darker host when call1ng thew rocks m1gmat1tes rather than cons1der the other requ1rements, that m1gmat1tes are produced by part1al melting and are. therefore, found (only) in mediumand h1gh-grade metamorphic areas. Consequently, some rocks have been called m1gmat1te that are not. Therefore. as an a1d to the 1dent1ficat1on of migmatites. some of the rocks that are commonly m1staken for migmat1tes and some cnteria by wh1ch they can be d1st1ngu1shed from m1gmat1tes. are descnbed 1n th1s sect1on. Examples of rocks that can be mistaken for m1gmat1tes are illustrated in section G of the book.

INTROD UCTION

48 ---- ---- ---- ---- ---- -------- -

7.1

Rocks formed by subsolidus segregation

Metam orphic differe ntiatio n IS the general term, not speofic to any grade of metamo rphism. used to describe the moveme nt of matenal 1n a rock dunng metamo rphism; the process, or process es, whereb y the materia l is mobilized 1s not spec1fied . For Holmqu1st (1920) and Eskola (1932). metamo rphic differentiation was accomplished by the move ment of anatectiC me lt 1n h1gh-grade rocks, wherea s Turner (1941) argued that 1n low-grade chlorite schists, it was achieved by the movem ent of material through an 1ntergranular pore-flu id. Subsol idus segreg ation , or subsoli dus differe ntiatio n. refers specifically to the movem ent o f material in met amorph ic rocks by processes not 1nvolv1ng a part1al melt. Many investigators (e.g.. Eskola 1939, Turner 1941. Ramber g 1952, Williams 1972. Rob1n 1979) have noted t he close assooat1on in low-gra de metamo rphic rocks of lightcolored , quartz-n ch segregat1ons w1th dark-colored. micarich bands. Virtually all investigators show that planar subsolidus segrega tions grew dunng penods of penetra tive deforma tion (e.g.. Barr 1985; Mclellan 1983. 1984. 1989; Sawyer and Robin 1986). wh1ch suggests that they result from a stress-d nven phenom enon.

7.2 Models for the process of subsolidus segregation

There have been many 1nvestlgat1ons (e.g.. Stromga rd 1973; Rutter 1976; Rob1n 1978. 1979; Fletcher 1982; van der Molen 1985; Wheele r 1987 ) of stress-Induced chem1ca l transfer that address the mechan1sm of subsolidus segregation 1n rocks. Some investigators cons1der gra1n aggregates of one m1neral speoes, wherea s others cons1der layers of different compos1t1ons. In an aggregate of grains subject to a d1fferent1al stress. these d1fferences 1n stress exist along grain interfaces because of the vanat1on 1n the onentat1 on of the gra1n bounda nes relat1ve to the pnncipal directio n of compre ssive stress. Such vanation establishes chem1cal potent1al grad1ents that dnve the "d1ssolut1on" of "soluble" matenal from the po1nts of highest stress. and its movem ent to the low-stress Interfaces. where 1t "prec1p1tates" and leads to the growth of a new mineral or m1nerals. In polym1neralic rocks. quartz, the feldspars. and carbona te are generally more susceptible to stress-In duced chemiCal transfer, and hence are more "mobile" or "soluble," than the micas. amphiboles, and accesso ry phases. wh1ch are regarde d as "1nsoluble."

Rob1n (1979) develop ed a model for subsolidus segregation bet ween layers of d1fferent rat1o of mica to quartz, and hence of different compet ency or viscos1ty. The model IS equ1valent to a sequenc e containing beds of different modal m1neralogy. Dunng deformation. differences 1n mean stresses betwee n layers set up gradien ts 1n chemical potent1al, wh1ch drive the movem ent of the more "mobile" matenal (quartz and feldspar) to the most compet ent layers (richest in strong minerals, e.g.. quartz and fe ldspar). 1n a process that accentu ates the 1n1t1al contras ts 1n competency inhente d from sedimen tary depositi on (Sawye r and Rob1n 1986). The gram and the layer models show that a driv1ng force for d1ssolut1on and movem ent of matenal ex1sts 1f the gra1ns are subject to differential stress, but there are different mechan1sms of transpo rt by wh1ch the "mobile" matenal can move from s1tes of h1gh to low stress (Walthe r and Wood 1983). From slowest to fastest, these are ( I) diffuSion through m1neral grains, (2) diffus1on along dry gra1n boundar ies. (3) d1ffus1on through a flu1d film along a gra1n boundary, (4) solut1on 1n, and diffus1on through , a stat1c pore-flu1d, and (5) solut1on 1nto an 1ntergranular pore-fluid that IS advecting. Models have also been develop ed for the formatio n of subsolidus segregations under cond1t1ons of hydrostatiC stress (e.g.. Fisher 1973. 1978; joesten 1977) to describe the diffusion -control led growth of reaction-rim microstructures and the growth of concret1ons. The dnv1ng forces 1n these 1sobanc and isothermal models are the chemical potential gradients result1ng from small-scale vanat1ons 1n the m1neral assemblages.

7.3 P-T conditions at which

subsolidus segregation occurs

Subsolidus segregations form at low metamo rphic grades and are commo nly reporte d from metased imentar y rocks 1n greensch1st-fac1es metamo rphic terrane s (e.g.. Turner 1941, Vidale 1974, Sawyer and Rob1n 1986). The conditions of maximum grade of metamo rphism at which they form are well constra1ned 1n several reg1onal metamo rphic terrane s. The subsolidus segregations that g1ve nse to stromatic, m1gmatite-like sch1sts 1n central Massachusetts formed at tempera tures betwee n 550 and 625°C (Tracy 1985). In northea stern Scotland . the highest-grade subsolidus segregations occur between the kyan1te and sillimanite 1sograds. which corresp onds to tempera tures betwee n 550 and 600°C and a pressur e of 6 kbar (Mclellan 1989); Barr (1985) est1mated that the tempera tures were a lit tie h1gher. 580-64 0°C. but still subsolidus. In the Quetico Subprovince of Canada, the h1ghest-grade subsolidus segregations formed 1n rocks that reached tempera tures close to

•

•

Atl.h of ~1•gmatite'

----------- ----------- --------- 49

•

650°C and pressures of 3-4 kbar (Sawyer and Rob1n 1986). very close to where the first scattered, anatectic neosome starts to appear 1n metapelitic rocks. Blom ( 1988) studied amphibolite- and granulite-fac1es rocks 1n F1nland. and found that the processes of subsolidus segregation operated only 1n the amphibolite-fac1es part of the reg1on's metamorphic history, between 500 and 640°C (at about 5 kbar); above the amphibolite fac1es. they were simply heated up to about 800°C w1thout any further growth. The max1mum temperatures at wh1ch subsolidus segregations formed by diffusion-controlled processes in a hydrostatic stress reg1me are not as well constra1ned. Trumbull ( 1988) reported that small. equant, subsolidus segregat1ons 1n b1ot1te and b1ot1te + hornblende gne1sses formed at a temperature between 600 and 700°C. L1ndh et al. (1984) reported temperatures of 550-600°C for almost identical. equant, diffusion-controlled segregations.

7.4 The relationship between subsolidus segregation and migmatites All the data presently ava1lable 1ndicate that stress-dnven subsolidus segregations grow 1n the low- and med1um-grade parts of metamorphic terranes, and not in the h1gh-grade parts. Therefore, rocks formed by stress-driven subsolidus segregat1on do not fall w1thin Ashworth's (1985) defin1t1on of m1gmat1te, and because they are not formed by partial melt1ng, they do not fall w1th1n the defin1t1on of m1gmat1te proposed 1n th1s book; consequently, they are not considered to be m1gmat1tes. Subsolidus segregations cons1st1ng of quartz-nch veins and pods surrounded by melanocrat1c selvedges (see next sect1on for a fuller descnption) may be found 1n many mlgmatite terranes. What then is the relationship between the process of subsolidus segregat1on and m1gmat1tes? Blom ( 1988) suggested that subsolidus segregations are not expected 1n m1gmat1tes and should, therefore, be regarded as part of the pre-anatectic history of the rock, i.e., they are part of the paleosome. Why should subsolidus segregations seem to stop growing when stress gradients still ex1st? An analogy with deformation-mechan1sm maps may prov1de an 1ns1ght. Space on the maps (e.g., Ashby 1972, Po1ner 1985) IS d1v1ded on the bas1s of the fastest, that 1s, dominant, but not exclus1ve, mechan1sm of deformation, because it controls the rate at wh1ch bulk deformat1on can proceed. For processes occurring 1n high-grade metamorphic rocks. the 1mportant parameter may be rate of transport; 1t controls the rate of petrolog1cal change. Once part1al melt1ng beg1ns, the melt offers by far the fastest rate of transport by which petrologiCal changes can be accomplished, because diffu sion 1n a melt 1s faster than 1n a solid. Moreover, melt is

eas1ly advected. The process of subsolidus segregat1on may not cease, but it IS simply too slow to contribute to the petrological changes that occur once melt1ng starts. Poss1bly. subsolidus segregat1on could occur 1n those layers not undergo1ng part1al melt1ng, but if these layers become fiUid-absent because of melting nearby, then transport 1n them will be greatly 1nhib1ted, and only short-range diffusion-induced segregat1on of the type described by Fisher (1973. 1978) and Trumbull (1988) may operate, produong small. scattered leucocrat1c patches called flecks. Because the process of subsolidus segregation separates the "mobile" quartz and feldspar from the "1mmobile" m1cas, 1t can produce rocks that resemble m1gmat1tes. Furthermore, s1nce subsol1dus segregations that formed at lower metamorphic grades are commonly preserved 1n the paleosome parts of migmatites, 1t is clearly desirable that subsolidus segregations be distinguished from neosome 1n m1gmatite. In the next sect1on, I outline the pnnc1pal features of subsolidus segregations that enable th1s to be done.

7.5 Small.. scale features of subsolidus segregations The mineralogical, microstructural. and morpholog1cal characteristics of subsolidus segregations are best known for those developed from metapelitic and metagreywacke protoliths. Consequently. these will serve as the type example.

The constituent parts Two bas1c types of 1nternal structure have been described for subsolidus segregations developed 1n metapelite and metagreywacke protoliths. The most common type IS formed by stress-1nduced chem1cal transfer; they are th1n. laterally extens1ve, h1gh-aspect-ratio segregations. They have an 1nner leucocratic part that is bordered by an outer melanocratic nm, beyond wh1ch IS the country rock (Mclellan 1983, 1989; Sawyer and Rob1n 1986; Blom 1988). In most cases, the central, leucocrat1c part IS comparatively homogeneous and dom1nated by quartz, but others are compositionally zoned (typically a quartz-rich core and a more feldspathic outer part) ow1ng to the accret1on of successive Increments of d1fferent composition dur·ng prograde metamorphism. The dark-colored borders, called melanocrat1c selvedges, are rarely un1form; typ1cally, there IS a progressive decrease in the proportion of quartz and plag1oclase that produces a progress1ve darken1ng toward the leucocrat1c core. The second type 1s much rarer and IS formed without differential stresses. These diffusioncontrolled subsolidus segregations are typ1cally small (<20 mm) and pod-shaped; they have a melanocrat1c core surrounded by a leucocrat1c mantle, w1th the envelop1ng country- rock beyond that (Lindh et al. 1984, Trumbull 1988).

INTRODUC TION

50 ------------------------------

The rat1o of leucocrat1c to melanocrat1c parts 1n the stressInduced segregat1ons 1s a funct1on of volume fraction of "mobile" m1nerals. A protolith with more quartz and feldspar will have a greater ratio of leucocrat1c to melanocrat1c parts in the segregation than a protolith with a smaller volume fraction of "mobile" m1nerals. For diffus1on-controlled segregations formed under hydrostatic conditions. the ratio is fixed by the overall reaction (e.g.. Lindh et al. 1984,

phases produced rema1n 1n the melanocrat1c part. but the "mob1le" phase produced appears 1n the part that IS leucocratic. Curiously, all the reported cases (L1ndh et al. 1984, Sawyer and Rob1n 1986, Trumbull 1988) seem to 1nvolve the same basic reaction: Fe-nch b10t1te == magnetite + K-feldspar + fluid, although 1n some cases. hornblende also is a product phase.

Trumbull 1988).

Mineralogy of subsolidus segregation Ve1ns formed by subsolidus segregat1on 1n the lowestgrade clastiC metasedimentary rocks cons1st almost entirely of quartz. but the plagioclase to quartz rat1o 1n the vems increases systematically w1th metamorphic grade from the lower greenschist to the m1ddle amphibolite fac1es (V1dale 1974, Sawyer and Robin 1986). Furthermore. Sawyer and Rob1n ( 1986) noted an increase 1n the plag1oclase to quartz ratio 1n success1ve increments of matenal added to Individual segregations as they grew dunng conditions of ris1ng metamorphic temperature. The changing plagioclase to quartz ratio might be due to the depletion of quartz near the segregation, which increases the transport distance of quartz and so reduces its effect1ve "mobility." or to an 1ncrease in the solubility of plagioclase faster than that of quartz as metamorphic temperature nses. Most 1nvest1gators have found the same m1neral parageneses 1n each part of the segregation as 1n the protolith; the modal proportions d1ffer greatly, however. In general. there 1s very little vanat1on in the composit1on of minerals from the vanous parts of the segregat1on and the adJacent protolith. Th1s property contrasts with part1al melting, where melanosome and leucosome 1n m1gmat1tes commonly have different assemblages of m1nerals. In the majority of subsolidus segregations. the leucocratic parts conta1n mostly quartz + plagioclase, some cons1st of quartz + K-feldspar; consequently, they tend to plot far from the cotect1c l1nes 1n the system Ab An Or Qtz H.O on CIPW norm d1agrams. The melanocratiC borders are characterized by an ennchment of "msoluble" m1nerals. typ1cally b1ot1te and the accessory m1nerals. relat1ve to the protohth. Garnet. staurolite, hornblende. magnet1te, andalus1te. kyanJte. and sillimamte also may occur 1n the mafic selvedges. Orthopyroxene has not been reported from subsolidus segregations. D1fferent assemblages of m1nerals can develop 1n the leucocratlc and melanocratic parts of the subsolidus segregat1ons 1f the melanocratic part becomes depleted 1n one of the "mobile" minerals, generally quartz. Different assemblages can also develop if a metamorphic dehydration-type reaction occurs as the segregation grows and the "1mmob1le"

Results from mass-balance studies, whether based on modal analyses or on whole-rock compositions, show that subsolidus segregations generally form m s1tu by an essentially 1sochemical process (Sawyer and Rob1n 1986, Sawyer and Barnes 1988, Mcl ellan 1989, Blom 1988, Trumbull 1988). Relat1ve to the protolith. the melanocrat1c selvedge represents the in Situ res1due of 1nsoluble or 1mmob11e matenal from which the soluble or mobile components that compnse the leucocrat1c part were extracted.

Microstructure The leucocratic parts are coarser gra1ned than the melanocratic domains, and they are coarser than the protolith, unless reduced by later mylonitization or cataclasis. Although the leucocrat1c parts are coarse-gra1ned, their microstructure IS metamorphic and dominated by equant grams; they do not conta1n pegmat1te. or other 1gneous textures and microstructures (Mclellan 1983). Minerals 1n the melanocrat1c selvedges generally have a strong preferred onentat1on; quartz and plag1oclase have tabular shapes controlled by the preferred onentat1on of the more abundant b1otlte. The overall microstructure IS, therefore, lepidoblast1c or nematoblastic. and IS progressively more strongly developed toward the melanocrat1c part of the segregat1on, 1.e., w1th progress1ve loss of mobile mmerals (mostly quartz) .

7.6 Outcrop ..scale morphology The morphology of subsolidus segregat1ons is cont rolled by the1r bulk rheology. which rema1ns that of solid rock. Th1s represents the major difference bet ween rocks formed by subsolidus segregation and m1gmat1tes. Structures due to magmat1c flow thus do not develop. and older structures are w1dely preserved. Furthermore, because of their quartz-nch m1neralogy. the leucocrat1c parts of subsolidus segregations are the most competent part of the rock; consequently, they commonly become boud1naged dunng growth, 1n contrast to the leucosome 1n m1gmat1tes.

Stromatic, or laye red, subsolidus segregations are charactenzed by th1n (mm em wide) but laterally exten sive segregations (meters or more) located 1n the plane of mechanical anisotropy 1n the host, e1ther the foliation planes or the compositional layering (see Figs. G I and G2) . The

Atl<.1.., of

M1gmar1tc~

- - - - - - - - - - - - - - - - 51

segregat1ons compnse of two parts: a quartz-nch, ve1nlike core and an outer melanocratic border. The segregations are concordant to host-rock foliat1on or bedding, and not discordant (Yardley 1978; Barr 1985; Mclellan 1983, 1984; Sawyer and Barnes 1988). S1ngle-layer segregat1ons are common, but the most strik1ng layered structure develops when numerous th1n, parallel segregat ions develop 1n protoliths that e1ther were orig1nally thinly bedded or else conta1n a closely spaced cleavage or fol1at1on. Layered subsolidus segregat1ons generally do not form 1n th1ck sequences of homogeneous rocks. The overall appearance of the outcrops, therefore, depends on the distribut1on of layers of a suitable protolith.

Dilatant structures, such as 1nterboud1n part1t1ons. can be favorable places for subsolidus segregations to develop. Typically, the "mobile" quartzofeldspathic constituents m1grate to the interboudin part1t1on, leav1ng the "immobile" m1nerals to form a melanocratlc border 1n the adJacent wallrock. Hence, the leucocrat1c parts can be d1scordant to the foliat1on 1n the host, but are quite short. The formation of this type of segregat ion is analogous to pressure shadows (Stri:imgard 1973). The appearance of outcrops conta1n1ng subsohdus segregat1ons and of metatex1te m1gmat1tes 1n wh1ch leucosome IS located in dilatant structures can be s1milar; they can be d1st1ngu1shed by differences in the microstructure and mineral assemblages. Fleck structures are small, round subsolidus segregations (F1gs. G3, G4) that commonly have a melanocratic core surrounded by a leucocrat1c nm, or mantle (L1ndh et al. 1984, Trumbull 1988). Trumbull (1988) suggested that flecks only develop 1n protoliths of certain bulk compositions (notably K-ennched), and are randomly distributed 1n the1r host. Thew locat1on 1s not d1rectly controlled by planes of mechanical an1sotropy, such as fol1at1on. The equant shape of the flecks and the lack of a penetrative fol1ation in some hosts suggested to L1ndh et al. (1984) that the fleck structure IS an example of diffus1on-controlled subsol1dus segregation formed 1n the absence of differential stress; hence, they resemble patch-type m1gmatites. Past use of the term fleck has been reserved for segregations of subsolidus orig1n; this practice should be continued.

7.7 Rocks formed in syntectonic plutons and plutonic complexes The InJeCtion of one magma 1nto another of d1fferent composition can produce a wide variety of magma-mingling and magma-m1xing structures that have been reported from a large number of pluton1c complexes (e.g., Mason 1985, Barbarin 1988). Some of these rocks can be m1staken for

diatex1te m1gmat1tes 1f the lobate and cuspate boundanes to the enclaves are not found; commonly, they may be lost as a result of high stra1n. The inJeCtiOn of one magma 1nto another of different composition that has already crystallized sufficiently (but not completely) that 1t will fracture dunng Intrusion of the second can produce net-ve1n arrays (e.g., Morogan and S0rensen 1994) that may resemble some net-structured metatexite m1gmat1tes. The w1dest range of m1gmat1te-like 1gneous rocks are found where magmas of fels1c or intermediate compoSition were either InJected into, or crystallized in, places of act1ve penetrative deformation. These correspond to reg1mes of syntectonic emplacement and syntectonic crystallization of magma. The s1m1lanty 1n appearance between 1gneous rocks produced under these circumstances and migmatites is not surpnsing. A crystall1z1ng pluton IS a twophase system (melt + solid) like a m1gmat1te, and the melt fract1on can separate from the solid 1f there IS an applied stress. Once the magma has crystallized sufficiently (M < 0.4) that a ng1d skeleton of crystals is formed, subsequent deformation of the magma produces structures (see Figs. G5-G8) that resemble those that develop where partially melted rocks are deformed dunng anatex1s (e.g., Fig. 2). The po1nt at wh1ch a ng1d framework of crystals is formed as magma crystallizes IS referred to as the ngid percolation threshold (RPT) by Vigneresse et al. (1996). In m1gmat1tes, the part1cular geometry and morphology of the leucosome are controlled by the planar an1sotropy of the protolith and the paleosome, wh1ch may be of pnmary (e.g., bedd1ng) or tectonic orig1n, and by stra1n. The location of fels1c ve1ns in syntectonic plutons is also controlled by the anisotropy of the host and stra1n, but in th1s case. the an1sotropy is generally a foliat1on acqu1red by sheanng in the submagmatic state.

Syntectonic injection of magma Subparallel bands of l1ghter- and darker-colored rocks that resemble stromatiC m1gmat1tes can form where magmas of different compos1t1ons are 1njected 1nto zones of h1gh shear stra1n in plutons. Neves and Vauchez ( 1995) reported that a stromatic-like alternation of rocks of different compos1t1ons and microstructures developed only where a synmagmat1c shear zone traverses the plutomc complex they studied 1n Braz1l. They attnbuted the stromatiC layenng to the attenuation and disruption of successive, compositionally d1fferent batches of magma as they were 1ntruded into the shear zone. Rocks that resemble stromatiC m1gmat1tes can also form by the repeated dik1ng of a partially crystallized, darker magmatiC host-rock by a more felsic magma during noncoax1al

INTROD U C TI O N

51 ------ ------ ------ ------ ------ -

deformat1on (see F1gs. G9 and GIO). Sheanng 1n the magmatic and submagmatic states orients the platy and tabular mmerals 1n the darker magma. and th1s fabnc can then control the onentat1on of subsequent dikes of the more felsic magmas (e.g.. john and Stunitz 1997) to produce compos1t1onally layered rocks that resemble stromatiC m1gmat1tes. In some examples, the fels1c magma is genetically d1fferent from 1ts host. However. 1n others 1t IS the fractionated. res1dual melt expelled from 1t s darker host by shear-1nduced compaction of its crystal framework in the submagmat1c state (Cuney et al. 1990, Pons et al. 1995). If the shear zone rema1ned act1ve after complete crystallization, then the magmatiC and submagmat1c fabrics 1n the darker host and the felsic dikes are overpnnted by solidstate mylon1t1c fabncs. and all the felsic dikes show some degree of rotation 1nto parallelism with the t rend of t he shear zone. However, 1f the shear zone ceased to be act1ve before complete crystall1zat1on occurred, then the darker host and early light-colored felsic dikes preserve submagmatlc fabncs. and the late d1kes cross-cut the earlier ones and have more igneous microstructures (e.g., the Skagit Gne1ss and rocks 1n the nearby Chelan area ofWash1ngton). Babcock and M1sch (1989) showed a CIPW norm plot 1n which the late. cross-cutting d1kes plot on the cotectic line, whereas the lighter-colored. stromatiC layers plot nearby, but are displaced slightly toward the albite apex. This relationship could be used to 1nfer that the lighter-colored layers 1n the Skagit gne1ss are 1n fact magmatiC, and result from the syntectonic crystallization of a magma that resembled the late dikes from wh1ch a res1dual, evolved melt was expelled after the RPT was reached, leav1ng behind lightcolored. plag1oclase-dom1nant fels1c dikes.

Syntecto nic crystallization of felsic plutonic rocks A vanety of nonstromatlc m1gmat1te-llke structures can develop 1n compos1t1onally un1form gramtic plutons undergoing noncoaxial deformation during the final stages of crystallization. For magmas of gran1t1c compos1tlon. most crystall ization occurs over a relatively narrow Interval of temperature. JUSt above the solidus. Consequently, the fraction of melt decreases sharply near the solidus, and a crystal framework containing the remaining melt in its interstices forms (Bryon et al. 1994). At fract1ons of melt below 0.25, the crystal framework deforms, partly by crystalplastiC mechan1sms, and partly by fracture 1n response to applied stresses. so that small (em-scale) dilatant structures (shear bands. extens1on gashes. boudm) form in specific onentat1ons relat1ve to the foliat1on and pnnopal stresses (Pons et al. 1995, Sawyer 2000).

Fract1onated, res1dual melt that IS 1n the 1nterst1ces of the crystal framework is forced out as poros1ty IS destroyed and the crystal framework compacts. and m1grates 1nto the dilatant low-pressure s1tes that are growmg nearby. Hence. the segregation of residual melt can be an essentially m Situ process. JUSt as tn many mtgmat1tes. The volume of melt that ts segregated and forced to mtgrate out of the framework of crystals by deformat1on must decrease as crystallization progresses; therefore, the morphology of the segregations of residual melt changes also. In the early stages, when the local volume of res1dual melt that has been squeezed out of the framework IS large, the domatns of expelled melt closely resemble nebulites and diatexite m1gmatltes (e.g., Figs. Gil Gl4). As the volume of restdual melt declines. the orientation and morphology of the tndivtdual structures that are filled wtth res1dual melt change and are analogous t o some types of leucosome that form tn metatexite mtgmat1tes at the beg1nntng of parttal melttng (e.g.. Figs. G5 G8). If the melt-filled dtlatant sttes become JOined, then the resulttng array of light-colored felsic vetns can resemble the net and ddatant types of metatexite migmatite. Locally. pooling of expelled restdual melt may result tn the development of foliatton-parallel vetns (Sawyer 2000).

7.8 Vein complexes In some geologtcal settmgs. metasedimentary or plutonic rocks have become vetned wtth felstc melts and magmas denved from lower structural levels. These rocks can look very much like some m1gmat1tes (Ftgs. G 15- G20), partiCularly tf the lighter-colored, fels1c veins have narrow maf1c selvedges, formed owtng to reactton of the wallrock wtth the magma. or w1th an aqueous flutd evolved from 1t. and are mistaken for melanosome. The felstc melts may be denved by parttal melttng of much deeper rocks or by the syntectontc expulsion of restdual melt from deeper crystallizing plutons. However, because the felsiC melts and magmas are not genet1cally related to their hosts, these rocks do not meet the definttion of mtgmattte. Ustng the same type of reason1ng as Brown (1973), these InJected rocks should be called vein complexes. If the host rock was a m1gmat1te, then the addttton of fels1c vetns or dikes would make the term "vetned migmat1te" applicable.

Atl'" of

~hgmatlt~'

------------ ------------ --------- 5)

7.9 Rocks formed in syntectonic

•

plutonic and vein complexes compared with migmatites Similarities Multiple injections of magma 1n syntectonic pluton1c complexes can produce rocks with a number of m1gmat1te-like charactenst1cs. (I) L1ght-colored ve1ns 1n a darker host can resemble e1ther leucosome in a paleosome, or leucosome and melanosome. (2) If the light-colored veins have mafic selvedges formed by reaction with their wall rocks, then the rocks could be m1staken for a migmat1te 1f these are Interpreted as leucosome and melanosome, respectively, and the host rock IS treated as paleosome. (3) The alternation of parallel and laterally pers1stent light and dark layers may superficially resemble stromatic metatexite migmat ites. (4) The attenuat1on of structures due to magma mingling and the development of flow banding 1n zones of h1gh synmagmatic stra1n can resemble enclaves of paleosome. or melanosome, 1n schlienc and schollen d1atex1te m1gmat1tes. The segregation of fels1c res1dual melt by syntectonic deformation at a late stage of pluton crystall1zat1on can be a local, in Situ, process that generates 1solated. lighter-colored fels1c ve1ns bordered by melt-deplet1on halos. or more extensive arrays of linked light-colored fels1c ve1ns with1n a broader, melt-depleted zone. Both are morphologiCally and geometncally s1m1lar to leucosome located 1n dilatant and net-structured metatexite m1gmatites.

Differences The most obv1ous d1fference between the migmat1te-like rocks from syntectonic plutons and true migmatites rests w1th the genet1c relat1onsh1p between their const1tuent parts. The defin1t1on of m1gmatite requ1res that there be a genet1c connect1on through part1al melting. In metatexlte m1gmat1tes. the melanocrat1c rocks contain petrographic ev1dence for a part1al-melt1ng react1on, and geochem1cal and petrographic ev1dence to 1nd1cate that the leucosome (or d1atexite migmatite) IS, or was, denved at least 1n part from that anatectiC melt. In contrast, the darker-colored rocks from m1gmat1te-like rocks 1n syntectonic pluton1c complexes do not conta1n petrographic ev1dence for partial melting reactions, and although the light-colored. fels1c rocks m1ght be genet1cally related to the darker rocks, 1t is not through partial melting. In most cases, the fels1c rocks have compositions that lie at the far end of the fractional crystallization trend of the assooated darker-colored rocks, compatible w1th an ong1n as expelled res1dual melt. not as a part1al melt. In other cases of syntectonic diking, the light and dark parts are related by magma m1x1ng; 1n some, they are not genet1cally related at all.

Complexes hav1ng mult1ple syntectonic dikes have some additional characteristics that are unl1ke stromatiC migmatites. (I) Most of the fels1c ve1ns are not surrounded by zones of melanocratic melt-depleted rocks, or they have melt-deplet1on halos that are far too small to be compatible w1th 1n Situ segregation of melt. (2) Many felsic ve1ns are, 1n detail, cross-cutting. (3) The typical width of stromatic fels1c ve1ns 1S much greater than the w1dth of the leucosome doma1ns 1n stromat1c m1gmatites. (4) A s1gn1ficant proportion of the fels1c ve1ns 1n most complexes have true pegmat1t1c textures. whereas a true pegmatit1c texture 1s rare 1n migmatite leucosome. (5) Overall, many migmatltelike complexes of syntecton1c dikes have a far h1gher rat1o of fels1c ve1ns to darker host than most migmat1tes. Together; these critena 1nd1cate an ong1n by dik1ng rather than m s1tu melt1ng and segregation processes.

8. WORKING W ITH MIGMATITES The final sect1on of th1s book deals with the problems of work1ng w1th and mapp1ng m1gmat1tes, and these are addressed 1n two parts. The first deals w1th the quest1on of what divisions to depict on a map, and the second is concerned with what should be noted at each outcrop durIng field work that is of use 1n later, follow-up work 1n the laboratory. This matenal IS condensed and presented as a checklist 1n Appendix 9.1. Migmat1tes have been stud1ed for many reasons. In the recent past. many studies were 1n some way or another related to the petrological quest1ons of the ong1n of gran1t1c magmas and the processes by wh1ch the continental crust became compos1t1onally differentiated ; these questions remain of great 1nterest. In geodynamiCS, there is presently much Interest 1n the rheology of the cont1nental crust. and the effect that weak layers 1n 1t have on the evolution of orogens and on the morphology of mounta1n cha1ns and plateaus. As zones of partially molten rocks are cand1dates for weak layers 1n modern orogens, new stud1es on m1gmat1tes will no doubt focus on structural and rheolog1cal aspects as a means to study the distnbution and geometry of former weak layers and zones of so-called "channel flow" in anoent. eroded orogens. N o matter what the purpose IS for study1ng m1gmat1tes, some part of the work wdl likely 1nvolve exam1n1ng and mapp1ng them 1n the field. The task IS to deode what IS cnt1cal to observe and map 1n the field, what needs to be sampled, and which aspects of the rocks that crop out at the surface should be portrayed on a map. Not only does th1s enta1l showing the locat1on

INTRODUCTION

54 -----------------------------

and dtstnbutton of the vanous mtgmatites and the adjacent rocks. but also the tnternal features and relationshtps withtn the mtgmatites. The chotce of what untts to show is crittcal to the usefulness of the map. and although that depends to a large extent on the size of the map area and the scale of the final map, the principal ingredient to success ts a clear understanding of what the purpose and objectives of the study are before field work begins.

8.1

First..level map units

prefix to the mtgmattte type to g1ve map units such as pelittc metatextte, pelitic diatexite: psamm1t1c metatextte. psammttiC diatextte: granittc metatextte and granittc diatextte. Some workers have disttngutshed the parts of mtgmatites derived from cordtente-beanng melt from those derived from garnet- and orthopyroxene-bearing melts (Johnson et al. 200 Ib), or separated btottte diatexites from orthopyroxene-bearing diatexites (Makttte 200 I). Choosing map units on the basis of the percentage of leucosome and leucocrattc veins present ts not recommended for first-level maps. because of the posstbtlity that some of the '"leucosome" ts in fact later vetn matenal. superimposed on some other m1gmatlte morphology. as dtscussed tn the sect1on on vetn-structured mtgmatttes. The disttnctton between metatextte and dtatextte. a dtvtston that already tncorporates the fraction of melt present 1n the mtgmattte,

For regtonal-scale maps. the map untts are normally the petrologtcally dtsttngutshable rock-types. For migmatites. these should be the first-order morphologtcal divistons, t.e .. metatexite and dtatextte wtth. tf necessary. a transitional mtgmattte untt between them. Other units that could be shown tnclude the protolith (or whatever IS on the lowIS preferred. grade stde of the mtgmatttes) and the nonmigmattte rocks on the htgh-grade side, most commonly, plutontc rocks of one sort or another. Paleosome, or resister. ltthologies Second..level map units could be differentiated and shown withtn the mtgmatites. if This level of map untts ts intended to add a further level scale permits. Resister lithologies can be an effective way of detail to the metatextte and diatexite divisions already of showing some types of pre-anatectic structures in migmade on the regional scale, but they could be the primary matttes, notably folds, and how their geometry may have dtvistons on a map legend under some Circumstances. For changed from the non-anatectiC rocks to the mtgmatites example. tn the detailed mapptng of smaller areas, the and from low to high grade wtthtn the mtgmatttes. The first sctentific objeCtives and the opportun1t1es are different appearance of metatextte mtgmatttes and the passage from compared to those tn regional mapptng. Small areas in mtgmetatextte to diatexite are easily identtfied and mapped matite terranes are typtcally mapped tn detail to examtne ustng the definttions of both gtven earlier. Whether or not some speCific petrological or structural problem. such as these are parallel to tsograds tn the lower-grade rocks how the local vanations tn morphology of mtgmatites are may provtde cnttcal tnformatton tn determintng why meltspattally related to the structures (e.g.. Oliver and Barr 1997, ing began or whether the metatextte dtatextte transttion Brown and Solar 1998a. b). because deformatton appears ts structurally controlled or not. However, the upper limit to control the mtgratton and accumulatton of melt. The of diatexite mtgmatttes can be dtfficult to define, espeCially mapptng may be requtred as the basis for a petrogenetiC if, as in many regional migmattte terranes. the dtatextte or geochemtcal study in whtch case it should be concerned mtgmatites tend to grade into anatecttc granttes that are with obtaintng as much information as possible on how and largely m sttu (autochthonous granite). PlaCing the boundary where each individual sample collected fits into the overall tn such cases ts subjective, and depends on the individual mtgmatite. in other words. to provtde context. mapper's tolerance to uniformtty 1n diatextte, or to heterogenetty in granites. The upper limit of diatextte mtgmatttes For mapping of areas where the variatton in migmattte tn contact aureoles is commonly much easter to map. even morphology is related to anisotropy tn the protolith or the where agatnst granite, because migmatites 1n contact aure- paleosome. or to deformat1on style and stratn tntenstty. as oles generally have a much finer gra1n-size than the rocks well as to fraction of melt present. then the second-order tn felstc plutons, or mtgmatites from reg1onal metamorphtc morphologtcal terms tn Fig. Ib (plus vetn-structured mtgterranes. matlte) are very useful. If used together wtth the systematiC mapptng of regtonal structures. such as folds. shear zones, In terranes where the protoltth ts compostttonally diverse. and large boudins. the distnbutton of these mtgmattte morfurther dtvtstons of both the protoltth and migmatites may phologies ts a powerful ;ud tn understandtng the movement be possible. If a parttcular protolith generates migmatttes of melt wtthtn anatectic terranes and o rogenic belts: they of sufficiently different appearance or mtneral assemblage lead to the tdentificatton of the domatns where net loss of that they can be dtstinguished and mapped separately, then melt or net gain of anatectic melt occurred. Zones of relathat should also be shown on the map. The additional infortively high syn-anatecttc strain may be disttnguished from matton concerning the protohth type can be added as a

8.2

A ri,,, of

~Hgmarue'

----------------- --------------55

t regions of lower-strain by the morphology of the migmatites present: for example, Brown and Solar ( 1998a, b) found that stromattc mtgmatttes are charactensttc of regtons of htgher stram. At these scales. vanatton 1n the proportton of leucosome from outcrop to outcrop may be a very useful parameter to show, although tt would be equally useful to know whether this was the "initial" anatectic melt, an evolved or fracttonated melt, or an accumulatton of earlycrystalltztng phases from the anatectic melt. For the most detailed level of mapptng done at the scale of tndivtdual outcrops, simple untts that are dtrectly related to petrology, e.g., leucosome for melt product, melanosome for residuum, and paleosome for nonparttally melted rocks tn a migmatite, can be particularly effective in revealing the spattal detatls of the processes that combtne to form mtgmatttes (e.g., Oltver and Barr 1997; Sawyer et al. 1999; Sawyer 2001: Guerntna and Sawyer 2003: Marchildon and Brown 2002, 2003). In order to best understand how the mtgmatttes formed, tt ts very tmportant that the spattal relationship between these petrological features and the tectonic structures tn the migmatites are shown.

8.3 Other considerations in mapping migmatites Determtmng the relattve chronology of events tn mtgmatttes is an tmportant first step in understandtng how they formed . In most cases, the timing of events tn migmatites is determined by the cross-cutting relattonshtps between domatns of leucosome or more commonly by determtntng the age (i.e., the generatton) of the structural element tn whtch the leucosome ts located. These structures mtght be foliation planes, axial surfaces, shear bands, or fold hinges. It ts not surpristng, therefore, that the sequenttal nomenclature used in structural geology (e.g., D, 0 2 . .. \ S2 •.. F and F2, etc.) has influenced the way the formation of leucosome, and by extenston, regtonal parttal melting, have been vtewed. The erectton of a chronology of sequenttal ''generattons" of leucosome over a large area, because tt occuptes a stmilar structural site withtn a mapped area. must be done wtth constderable cautton; the formatton of leucosome might be diachronous from place to place or even of completely different age between one structural level, or domain, and another. Some investigators have gone as far as to ascnbe each "generation" of leucosome tn a regional anatecttc terrane to a separate melttng event. The common observatton of one domatn of leucosome cutttng another, and each located tn a different ''generatton" of structure tn contact aureoles where there was only one anatectic event, strongly suggests that thts approach may not the best way to tnterpret the temporal informatton obtained from outcrops tn regtonal migmattte terranes.

Conductton is the pnnctpal mechantsm of heat transfer tn the conttnental crust. Because heat IS conducted very slowly in rocks, the Earth's crust ts slow to heat up and slow to cool. Virtually all the numencal models of crustal heattng stnce that of England and Thompson (1984) show slow cooling in the deep crust, except where tt has been exhumed rapidly, e.g., tn metamorphtc core complexes. The slow response-ttme of the continental crust means that tt ts very unlikely that the complex timing deduced from the relattonshtp between domatns of leucosome tn mtgmatttes ts the result of the metamorphtc temperature repeatedly cycling back and forth across the solidus to generate multiple partial-melting events. Recent geochronologtcal studtes from granulite-factes metamorphic terranes (e.g., Zaleskt et al. 1999, Rubatto et al. 2001, Willigers et al. 2001) show that metamorphic temperatures rematned above the grantte solidus between 30 and SO My. This suggests that once formed, anatecttc melt can extst tn the mtddle and lower crust for a very long penod of ttme. Moreover, 30 SO My IS much longer than most deformatton events tn orogens; thus, tt is to be expected that during the period when the middle and lower crust is hot and contains melt, the stress onentattons and hence deformatton patterns will change. The sequence tn whtch each "generatton'' of leucosome forms could reflect each melttng reactton that occurred on the prograde P-T path: for example, the first could have formed by H 20-present melting, the next by muscovtte dehydration melting, and a thtrd by a biotite-dehydrationmelting reaction. Melting could be further spread out in time (and temperature) because each protolith layer has a different composttion. However, the problem wtth thts explanatton is that all "generattons" of leucosome coextsted as anatecttc melt during t he prograde part of the P T traJectory, and yet rematned tn thetr separate sttes; netther does thts explanatton allow for commonly inferred differences tn competency between the different "generattons" or domams of leucosome. An alternattve, and perhaps more likely explanatton, ts t hat each successtve "generatton" of leucosome stmply records the next of many eptsodes tn which the melt fractton segregated from the solid fractton, and these are brought about by changes tn the local pressure gradients dunng progresstve deformatton. The segregation of melt dunng the prograde part of the metamorphtc htstory (T > solidus) tnvolves separattng the melt fractton from the restduum and the subsequent mtgratton of the anatectic melt down local pressuregradients with little or no crystallizatton; much of the network of channels through whiCh the melt moved at this stage may be lost (Sawyer 200 I). However, as the melt starts to crystallize, it becomes a magma consisting of fractionated melt + crystals; typtcally thts occurs once

INTRODUCTION

56 --------------------------------

could have formed from a melt generated by H 0-present partial melt1ng. and InJected 1nto country rocks that had not yet melted. It pre-dates part1al melting 1n the hosts but belongs to the same anatectiC event. hav1ng formed at a very different t1me with1n it: the host melted later when the isotherms moved h1gher 1nto the crust. In this interpretation, the felsic matenal forms leucocratic veins but not leucosome: 1t was competent and fine-grained because it crystallized in cool rocks before the m1gmatite formed. Examples of fine-gra1ned. layer-parallel, and discordant leucocratiC VeinS formed from a melt generated by nuidpresent part1al melting and 1njected 1nto the rocks JUSt below, and JUSt above. the "melt-in" 1sograd occur at SaintMalo. France (Brown 1979, Weber et al. 1985. M1lord et al. 2001). and 1n the Quet1co Subprov1nce of Ontano. If the "leucosome" has a melanocrat1c selvedge and 1s quartz-nch. leucosome from that of a magma (crystals suspended 1n a then it may be a subsolidus segregat1on. not anatectic 1n melt) to that of a framework of crystals. s1mply by removongin. Alternatively, such early "leucosome" may be leucoIng the evolved melt from 1t: the effect of th1s IS to create a cratic ve1ns that do not belong to the anatectic event that mechan1cally competent leucosome w1thout the temperacreated the m1gmatite: 1n this case. they should be signifiture having to go below the solidus. Secondly, the m1gration cantly o lder than the leucosome 1n the m1gmatite. of the expelled melt fraction to the new low-pressure site creates a new leucosome there that is largely melt and. therefore. of low viscosity: these new domains of leucosome may have a different onentat1on determ1ned by the stresses that led to the new ep1sode of deformation, an F" fold 1n our example. Th1s segregat1on process dunng coolAPPENDICES Ing and crystallization may be repeated several times: each subsequent deformat1on event creates a new low-pressure site to wh1ch the melt m1grates to form the next "generaChecklist of observations tion" of leucosome. and 1t makes the older "generations" of for each outcrop of migmatites leucosome st1ffer because of the loss of melt from them. The purpose of the observat1ons outlined below 1s to propIn th1s scheme. the oldest batches of leucosome should be plag1oclase-nch (1.e .. they are accumulations of plagio- erly ident1fy and name the m1gmat1te, and to gather field information crit1cal to Interpreting the petrological and clase), and the youngest should have the most evolved. geochemical data that will come from later stud1es 1n the or fractionated, compositions. and th1s 1s generally what is laboratory. observed in m1gmatite terranes. In general, the network of

temperature starts to decline. If at any t1me after cooling and crystallization have started. a new ep1sode of deformation occurs (perhaps F1 folds start to form), both the distribution and magnitude of the pressure gradients will change, and new low-pressure s1tes are created 1n the migmatite. The melt fraction in the magma will then be dnven to migrate again to the new, lowest-pressure sites and will leave the minerals that have already crystallized behind: they become the first "generation" of leucosome and mark the older, low-pressure sites. Not1ce that at this stage. melt segregat1on IS not about separat1ng anatectic melt from its res1duum: it 1s now the separation of the evolved melt fract1on from the solid fract1on that has crystallized from the parental anatectiC melt. Th1s segregation process does two th1ngs. First. 1t 1ncreases the VISCOSity of the ex1St1ng

9.

9.1

leucosome in regional migmat1tes marks where melt was crystalliZing as temperature decilned. However, the array of leucosome in rapidly cooled contact-metamorphic aureoles may record the distnbut1on of melt that ex1sted at temperatures much closer to the peak. The oldest "generation" of leucosome in many m1gmat1te terranes 1s located parallel to the compositional layerIng (commonly attenuated bedding). wh1ch IS itself parallel to the pnnopal foliat1on 1n the rock: together. these loo are commonly described as a compos1te S /S band1ng. Charactenst1cally, th1s "leucosome" 1s th1n. conta1ns few mafic m1nerals, and importantly. has a very fine graJn-s1ze that is much less than in the other domains of leucosome in the migmatite The Interpretation of th1s generation of " leucosome" depends on its petrographic features. If it is plag1oclase-nch and does not have a melanosome. then it

Observations on the neosome and paleosome (I) If both neosome and paleosome can be Jdent1fied. note their relat1ve proportions. Dec1de whether 1t 1s a metatexJte or diatex1te m1gmat1te: ;t IS a metatex1te if paleosome dom1nates and contiguous pre-anatectiC structures are present. but a diatex1te 1f neosome dom~nates and pre-partialmelting structures are lost. (2) For metatex1tes. (a) 1f the neosome has segregated. note the form, s1ze. location. and proport1ons of the leucosome and the melanosome 1n the outcrop. and note any cross-cutting relationships between the doma1ns of leucosome. Is there petrological continuity between doma1ns of leucosome? Decide what type of metatexite on the bas1s of the form of the domains of leucosome. The relat1onsh1p

Ada~

of 1\.figmatitc~

-------------------------------5?

between the host and former segregated melt is next: IS the leucosome m sttu or 1n-source, or 1s 1t a leucocrat1c ve1n or dike? Th1s information can be cntiCal to the inter-

differences 1n relat1ve competence between the different "generations" of leucosome.

pretation of geochem1cal data. (b) If the neosome has not segregated, note how 1ts mineralogy, microstructure, and gra1n s1ze compare with its host. Dec1de whether or not

The purpose of these structural observations IS to diStinguish pre-part1al-melt1ng from syn-partial-melting

this IS a patch metatexit e migmatite. (c) For the paleosome, identify the rock types present, their mineralogy and microstructures. and 1dentify the structures that were present

structures and to compare the1r d1stnbut1on. ldent1fy the structures that formed after partial melting. Distinguish melt-rich rocks from melt-poor ones. Finally, interpret the structural and tectonic history of the m1gmat1tes.

before partial melting.

Way~up (3) For diatex1te m1gmatites, (a) 1f the neosome is unsegregated and nonfoliated, it is a nebulit1c d1atexite. (b) If the neosome has now structures or a magmatic foliation, exam1ne the form of the paleosome or melanocratic material: dec1de if the diatexite m1gmatlte could be a schollen or a schlieric type. (c) If possible, note whether the d1atex1te is leucocratiC or melanocratic relat1ve to the "typ1cal" diatexlte: describe any leucocratic ve1ns 1n 1t. Dec1de whether 1t IS a melt-rich, residuum-nch, or cumulate-rich d1atex1te migmat1te.

Petrological observations in the study of migmatites The mmeralogy, microstructure, and grain s1ze of each part of the neosome and paleosome are extremely useful and should be recorded: note anyvanat1ons between and with1n the different doma1ns of leucosome and melanosome. Field observations should a1m to allow one to deduce P T conditions and melt1ng reactions, to 1dent1fy wh1ch parts of the outcrop were melt, solid, and part1ally melted solid, and to 1dent1fy the reg1ons of melt loss (res1dual rocks) and of melt ga1n (vein migmatites, some d1atex1tes). They will prov1de a possible a1d 1n the later quant1ficat1on of M. to compare with F. It 1s also important to 1dent1fy the parts of the m1gmatite where no loss, or gain, of melt occurred, 1.e., where the format1on of migmat1te was essentially a closedsystem process.

Structural observations in the study of migmatites (I) Record, descnbe, and measure all the structural elements (foliations, lineations, folds, shear zones, d1splacement senses. etc.), noting thew locat1on and their geometrical and t1m1ng relationships w1th1n each part of the migmat1te. (2) Identify the parts w1th magmatiC, or submagmat1c, foliat1ons and structures. (3) ldent1fy wh1ch parts of the m1gmat1te terrane represent low, relat1ve syn-anatect1c stra1n and whiCh are h1gh, and those where there 1s sign1ficant transposition; the parts of h1gh relative stra1n or transposition tend to be dom1nated by layered or stromatiC structures in both metatex1te and diatexite m1gmatites. (4) Determine

criteria in migmatites

Determ1mng the way-up d1rect1on, or the structural fac1ng direct1on. 1n rocks 1s v1tal for understanding the structural and tectonic h1story of deformed terranes. Unfortunately, the structures used to determine these geometnc properties are progressively lost dunng part1al melt1ng. Consequently, tecton1c reconstruction of m1gmat1te terranes is very difficult, espec1ally 1n d1atex1te m1gmat1tes. Fortunately, structures that form during the movement and crystallization of anatectiC melts and that can be used to det ermine the paleovertical, and hence way-u p in migmatites, have been described by Burg and Vanderhaeghe (1993) and Vanderhaeghe (1999, 2001). However, caution may be required 1n us1ng some of the way-up cntena, as the local pressure-grad1ents down wh1ch melt moved could have been lateral, or even downward-directed. Branch1ng arrays of leucosome indicate now direction, but the direction of now may not have been vertical.

Sampling of migmatites The ObJeCtives of the study need to be clear before sampling beg1ns 1n order to ensure that appropriate matenal 1s collected. Res1dual matenal 1s taken for the determinat1on of the P-T conditions, react1on h1story, react1on progress (i.e., F), and for microstructural evidence of partial melting. On the other hand, samples from the melt-denved rocks (leucosome and diatex1te) are best for the study of processes such as now of melt, fract1onal crystallization, and crystal accumulation. Onented samples are required for some types of quant1tat1ve microstructural analys1s. GeochemiCal samples should be fresh and from parts of the m1gmat1te that have been fully 1dent1fied 1n the field; ideally. they should 1nclude some from the protolith. One purpose of the samples is to define the key referencepoints (protolith, melt, and res1duum) that are necessary for a geochemical 1nterpretat1on of the dataset. These are also required for most quant1tat1ve modeling, such as est1mat1ng F, making mass-balance calculations, and 1nvest1gat1ng the vanous petrogenetic processes. Well-constrained samples serve to gu1de the interpretation of samples from the more difficult outcrops.

INTROD UC T ION

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9.2 Glossary Anatexis: a general term used to descnbe part1al meltIng of the cont1nental crust (in th1s case) w1thout spec1fic reference to the degree of partial melting; hence, anatex1s applies to all stages. from incipient part1al melt1ng right up to complete fusion . Crustal anatexis is generally accompa-

nied by deformation, which enables many other processes to occur, such as segregation of the melt from the solid, m1grat1on of the melt (by porous flow and by channeled flow), fract1onal crystallization, and magma flow. Anatexite: a rock formed by anatex1s and the assoCiated

processes; an under-used synonym for m1gmat1te. Bedded migmatite : a term for the overall morphology of a m1gmat1te 1n wh1ch partial melt1ng and the formation of neosome are confined to certain beds, or layers. The term should be abandoned. See Layer-confined m1gmat1te. Degree of partial melting (F) : the we1ght fract1on of melt produced in a rock by melt-producing reactions. Diatexis: defunct term meaning complete fus1on, now replaced in common usage by the more useful term

"anatex1s." Diatexite migmatite: a m1gmat1te 1n which neosome 1s dom1nant and melt was pervasively d1stributed throughout. Pre-part1al-melting structures are absent from the neosome. and are commonly replaced by syn-anatect1c flow structures (e.g., magmatiC or submagmatic foliations. schlieren), or by 1sotropic neosome. The neosome 1s d1verse in appearance. reflecting a large range 1n the fraction of melt. It can range from predominantly leucocratic to predominantly mesocratic (e.g.. unsegregated melt and res1duum) to predominantly melanocrat1c. Paleosome occurs as rafts, schollen, and enclaves. but may be absent. In certain Circumstances, it may be useful to d1v1de dia-

texltes 1nto two types. (I) Pnmary d1atexite: a diatexite m1gmat1te formed 1n a closed system, where the melt fractiOn present is equal to the degree of part1al melt1ng. (2) Secondary d1atex1te: a d1atexite m1gmatite in wh1ch the fraction of melt required to pass the solid-to-melt trans1t1on was reached by the mgress of melt (i.e., in an open system where the fract1on of melt present exceeds the degree of part1al melt1ng).

Dilation-structured migmatite: a type of metatex1te m1gmatite in wh1ch the locat1on of the doma1ns of leucosome, or neosome, IS controlled by the d1stnbut1on of

dilat1onal structures that develop 1n the competent layers as the migmat1te is deformed. Dynamic melting: anatex1s under differential stress conditions. This is the general condition during anatexis of anisotropic rocks undergoing tectonic deformation (for the

opposite, see Stat1c melt1ng). Evolved melt: the fraction of melt left dunng. or after, the process of fract1onal crystallization. The evolved melt 1s depleted 1n the elements compatible w1th, and ennched 1n the elements incompatible with, the crystall1zed and sepa-

rated mmerals. Synonym: fractionated melt. Fold-structured migmatite : m1gmatlte that was folded

while 1t conta1ned melt. Fraction of melt (M,): the fract1on of melt that was pres-

ent in a migmatite, or part of a m1gmatite. Fractional crystallization : crystallization of a magma in which the melt becomes separated from the m1nerals that have crystal lized, result1ng in a change 1n composition of the remaining melt (it becomes evolved). The separation of the melt fraction from the crystals may be brought about by either growth of a certain m1neral at a spec1fic site (e.g.. growth of a m1neral on the walls of a dike) or the relative movement of the crystals of a part1cular m1neral (or

m1nerals) relative to the rema1n1ng melt (e.g.. the accumulation of plagioclase). Granite (or granodiorite, tonalite , etc.) dike (or sill ): the product of crystall1zat1on of a fels1c melt that has migrated out of 1ts source region completely, and is injected into host rocks of lower metamorphic grade, or

into unmetamorphosed rocks. Initial melt: an anatectic melt formed by a partial-melt1ng reaction, w1thout modificat1on of 1ts compos1t1on by other processes such as fract1onal crystall1zat1on, contamination,

or alteration.

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Layer-confined or bed-confined migmatite: term for a migmatite 1n which partial melt1ng and the formation of neosome are confined to certain beds, or layers,

1n wh1ch the bulk compos1t1on was appropriate (fertile) for part1al melt1ng. The term 1s scale-dependent. as 1t conSiders only indiv1dual fert1le layers 1n a m1gmatite; thus, 1ts use cannot be recommended. The neosome v1ewed at the scale of an individual layer may d1splay a w1de range of morphologies (some layers reta 1n their pre-anatectic structures, whereas 1n others they are completely overpnnted by syn-anatect1c structures and microstructures). However, viewed at the appropriate scale, i.e., one that 1s representative of the whole m1gmatite, layer-confined or bed-confined m1gmat1tes are clearly metatex1te migmat1tes because pre-anatectiC structures, such as layenng or bedding, are preserved. Leucosome: general term for the lighter-colored part of the neosome 1n a m1gmat1te. cons1sting dom1nantly of feldspar and quartz. The leucosome 1s that part of the mlgmatite derived from segregated part1al melt; it may conta1n microstructures that 1ndicate crystallization from a melt, or

a magma. Leucosome may not necessarily have the composition of an anatectiC melt; fract1onal crystallization and separation of the fractionated melt may have occurred. Because the anatectiC melt can be mobile 1n m1gmat1tes. leucosome may or may not be found m s1tu; additional terms are used to 1nd1cate how far the melt from wh1ch the leucosome was denved moved w1th1n the m1gmat1te.

In situ leucosome: the product of crystallization of an anatectic melt. or part of an anatectic melt, that has segregated from 1ts residuum. but has remained at the s1te where 1t formed. In-source leucosome : the product of crystallization of an anatectic melt. or part of an anatectic melt. that has

migrated away from the place where it formed, but IS still with1n the confines of 1ts source layer. Leucocratic vein or dike : the product of crystallization of an anatectic melt. or part of an anatectiC melt, that has migrated out its source layer and been inJected into another rock, wh1ch may be nearby. or farther away. but IS

st1l 1n the reg1on affected by the anatect1c event. Magma: a sil1cate liquid that conta1ns crystals, which may have formed from the melt (liqu1dus phases). be the solid products of the melt1ng react1on (so-called pentect1c products). or be m1nerals 1n excess.

Magmatic foliation : preferred onentat1on of m1nerals 1n a rock derived from a magma, acquired during flow when

the crystals suspended 1n the magma were free to rotate w1th no. or very little, 1nteract1on w1th nearby crystals, I.e., when the magma contamed less than 55% crystals. See SubmagmatiC foliat1on. Mafic selvedge: a part1cular variety of selvedge in which

mafic m1nerals, most commonly b1ot1te or hornblende, predominate and form a thin rim (a few millimeters w1de) around leucosome. leucocratic ve1n, or gran1te dike. Melanosome : a residuum that is composed predominantly of dark-colored m1nerals. Definition of melanosome: the darker-colored part of the neosome 1n a migmat1te that

IS nch 1n dark m1nerals such as b1ot1te. garnet. cord1ente. orthopyroxene, hornblende, clinopyroxene, and even o livine. The melanosome IS the solid, res1dual fract1on (i.e., it 1s res1duum) left after some, or all, of the melt fract1on has been extracted. MICrostructures 1ndicat1ng part1al melt1ng may be present. Melt: a silicate liqu1d w1thout crystals. Mesosome: a part of a m1gmatite that 1s intermediate 1n color between leucosome and melanosome. Because mesosome can occur 1n the neosome or 1n the paleosome, it should be specified from which part of the migmatite the mesosome ong1nates. Mesosome should be used only 1n a descnpt1ve sense. Metatexis: defunct term for part1al melting 1n the con tinental crust; the term "anatexis" has precedence and should be used. M etatexite migmatite: a m1gmat1te that IS heterogeneous at the outcrop scale, and 1n wh1ch coherent pre-partial-melting structures are w1dely preserved 1n the paleosome (where the microstructure appears unchanged), and possibly 1n the melanosome (res1duum) part of the neosome, where the fract1on of melt was low. The neosome part IS generally segregated 1nto leucosome and melanosome, but neosome in which melt and res1duum did not segregate may also occur. Microstructure : term used to describe the onentation, d1stnbut1on (i.e., spat1al arrangement), relat1ve s1ze, and the internal features of the gra1ns that comprise a rock.

INTRODUCTION

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Migmatite : a rock formed by part1al melt1ng; a synonym 1s anatex1te. Defin1t1on of m1gmat1te: a rock found in med1um- and high-grade metamorphic areas that can be heterogeneous at the microscopic to macroscop1c scale, wh1ch consists of two or more petrographically different parts. O ne of t hese parts must have formed by part1al melting and contains rocks that are petrogenetically related to each other (called the neosome) and to their protolith through partial melt1ng and the segregation of the melt from the solid fraction. The partially melted part typically conta1ns pale-colored rocks that are quartzofeldspathic, or feldspath1c, 1n composition, and dark-colored rocks that are ennched 1n ferromagnes1an m1nerals. However, the part1ally melted part may simply have changed m1neralogy. microstructure, and gra1n s1ze w1thout develop1ng separate light or dark parts. Migmatites are classified and described on the bas1s of thew morphology us1ng a two-t1er system. The pnmary-level divis1on leads us to metatex1te m1gmat1tes and diatexite m1gmatites.

Nebulite migmatite: a type of m1gmatite in wh1ch the neosome is diffuse and difficult to differentiate from the paleosome.

Neosome : the parts of a migmat1te newly formed, or reconstituted, by part1al melt1ng. The neosome may or may not have undergone segregation in wh1ch the melt and solid fract1ons are separated.

Net-structured migmatite: a type of metatex1te migmatite 1n wh1ch the neosome, or more commonly the leucosome, or leucocrat1c vems, form a net-like pattern enclos1ng paleosome or residuum. In some cases. th1s pattern anses because the domains of leucosome or neosome have very Irregular forms, but 1n most cases, 1t anses because the doma1ns of neosome or leucosome follow two or more systematic orientations.

Paleosome: term meaning "old rock" in a m1gmatite; unfortunately, previous usage is not consistent on which rocks these are. Here, paleosome IS defined as the nonneosome part of a m1gmatite that was not affected by part1al melting, and 1n wh1ch structures (such as foliat1ons, folds. layenng) older than the part1al melt1ng are preserved. The microstructure (s1ze. form, and onentat1on of gra1ns) IS e1ther unchanged, or only slightly coarsened, compared to that 1n s1m11ar rocks JUSt outs1de the region affected by anatex1s.

Parent melt: the anatectiC melt from wh1ch the other melts 1n a m1gmat1te were derived, most commonly by fract1onal crystallization; generally, the parent melt is an imtial melt, although there are except1ons.

Parent rock: synonym for protolith. Patch migmatite: a type of metatex1te migmatite 1n which the neosome occurs tn SIW as small, discrete patches. Although typ1cal of the onset of anatex1s, it also occurs in many granulite-facies m1gmat1tes.

Protolith: the rock from wh1ch the neosome 1n a m1gmat1te was denved. Protolith cannot be present 1n a migmat1te because 1t has been converted to neosome. Thus. protolith can only ex1st 1n the lower-grade rocks that d1d not undergo anatex1s. Synonym: parent rock.

Raft migmatite : synonym for schollen m1gmatite. Residuum : the part of the neosome that 1s predominantly t he solid fraction left after part1al melt 1ng and the extraction of some, or all, of the melt fraction . Microstructures indicating partial melt1ng may be present. Some residua are melanocratic; those rich 1n quartz or feldspar can be leucocratiC or mesocrat1c. See also Melanosome.

Resisters: rocks, typ1cally competent ones, 1n the paleosome that are especially res1stant to microstructural change. Quartz1tes, calc-silicates, and metamafic rocks are common res1sters. Schliere: a th1n layer compns1ng aligned platy, tabular, or pnsmatic m1nerals 1n a diatex1te m1gmat1te. Commonly, a schl1ere cons1sts ma1nly of b1otlte. but examples nch 1n slllimanite, orthopyroxene. hornblende, or plag1oclase also may occur. Schlieren is the plural.

Schlieric migmatite : a d1atex1te m1gmatite characterIZed by the presence of schlieren, but few schollen, or rafts. of paleosome matenal.

Schollen migmatite : a d1atex1te migmatite that conta1ns enclaves (called schollen, or rafts) of paleosome matenal or, more rarely, res1duum. In many terranes. the schollen show a progress1on from blocky to rounded or elongate shapes. and a progressively better alignment 1n the d1rect1on of a higher fraction of melt and syn-anatect1c stra1n. Raft m1gmat1te is a synonym.

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Secondary migmatites: a term used to describe migmatites formed around a pluton as a result of partial melt1ng tnggered by the 1nflux of H20 from the crystallizing pluton 1nto the country rocks. Segregation : term used to describe the overall process 1n which anatectic melt is separat ed from the residuum in a neosome. However, not all examples of neosome have undergone segregat1on of the melt. Selvedge : general term for a nm, or border zone, that 1s not res1duum and is compositionally, m1neralog1cally, or microstructurally different from the host and that occurs around a component of a migmatite. Selvedges can be leucocratic, mesocratic, or melanocratic; the most common are mafic and are composed of b1ot1te; see Mafic selvedge. Static melting: anatexis that occurs under conditions of 11thostat1c stress w1thout deviatonc stress; cond1t1ons that approximate th1s state may occur 1n some contact aureoles, or perhaps tn competent layers (see Dynamic melting). Stromatic migmatite : a type of metatexite migmatite 1n which the neosome (leucosome and melanosome), or just the leucosome, occurs as laterally continuous, parallel layers onented along the compositional layenng (e.g.. bedd1ng) or fohat1on. Stromat1c m1gmat1tes are common 1n high-stra1n, strongly transposed env1ronments, such as crust-scale shear zones, but not restncted to t hem. Submagmatic foliation : preferred onentat1on of minerals 1n a rock acqutred when the magma 1t formed from conta1ned suffic1ent crystals that 1nteract1ons occurred w1dely between them as they rotated 1n the flow, i.e., a foliation formed when the fract1on of melt was greater than zero, but less than 0.45. lnteract1ons between crystals init ially form a "t1l1 ng" microst ructure, then a framework of crystals t hat may subsequent ly become deformed before the magma sol1d1fied. See Magmat1c foltat1on. Vein-structured migmatite: a metatex1te or d1atex1te m1gmat1te 1n wh1ch leucocrat1c ve1ns are consp1cuous and abundant. Ultrametamorphism : term used to describe t he higherthan-normal degree of metamorph1sm requtred for part1al melting in t he cont1nental crust.

INTRODUCTION

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INTRODUCTION

76 -------------------------------

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A tla, of M tgmatt tcs

----------------- -------------- 77

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Sciences 36, 945 966.

Ada~

of M i g matitc ~

----------- ----------- --------79

The Photog raphs

M igmat1tes are the product of the process of partial melting, but their morphology IS to a considerable extent determined by other factors. The most Important among these are (I) the nature of the protolith, 1.e., what rock types 1t conta1ned and how they were distributed, (2) whether or not the rocks were being deformed when they contained partial melt, as th1s deformation dnves the separat1on of the melt from the solid fract1on, and also changes the shape of the rocks through folding, sheanng, and transpos1t1on, and (3) t1me, as the length of the Interval between melting and solidification is critical, e.g., it determines how far melt can separate from its residuum and how far the microstructure progresses to a fully equilibrated state. Thus, no two m1gmat1tes are the same; the study of m1gmat1tes 1s not like the study of fossils, and there 1s no equ1valent to a "type specimen". Compare, for example, petrological spec1mens in museums; samples of dionte look much the same from one collect1on to another, but the samples labeled m1gmat1te are all utterly different. The older literature on m1gmat1tes does contain l1ne draw1ngs that represent idealized versions of the relationships between melanosome, leucosome, and paleosome found 1n migmat1tes. However, one can scour outcrop after outcrop of m1gmat1te and find noth1ng that resembles these so-called "class1c relat1onsh1ps". What 1s found 1n outcrops, or in thin section, is vanety. The line drawings are a construct, a simplified interpretation of the connection between morphology and process obta1ned by the study of naturally complex m1gmat1tes. The photographs 1n th1s book Illustrate what IS actually 1n m1gmat1tes. The photographs 1n the following sections of th1s book are 1ntended to g1ve the reader an appreciation of the range of appearance and microstructure that can be found 1n m1gmat1tes. All the sect1ons have a bnef Introduction and all, except the first, have further subdivisions to h1ghlight specific key top1cs, for example, neosome or diatexite m1gmat1te or melt 1nclus1ons. The photographs have been selected w1th two objectives in m1nd; the first photograph 1n a sect1on shows a representative example of the top1c at hand, and the subsequent ones illustrate the range of variations that can occur.

A. SOME EXAMPLES OF M IGMATITES [FIGS. AI-A3] The photographs 1n this short first section are 1ntended to 1ntroduce the reader to the range of morphology that m1gmat1tes can have. The three m1gmat1tes shown were derived from protoliths that were compositionally layered, but it is whether or not the m1gmatites were deformed dunng anatexis that largely determmes the appearance 1n the three cases.

SOME EXAMPLES OF MI G MATITE S

80 ------------------------------Figure

I

Fig. A I. The absence of differential stresses dunng anatexis means that there 1s little dnv1ng force for the separat1on of the less v1scous and less dense melt from the res1dual solIds. Consequently, the m1gmat1tes that result. such as the one shown 1n th1s figure, have a very different appearance to the migmatites that form when there are differential stresses, such as those shown in Figs. A2 and A3. In this migmat1te, partial melting occurred 1n the absence of differential stresses and, moreover, affected only the metapellt1c beds. because the metamorphic temperatures were t oo low for melting to start in the 1nterlayered beds of psamm1te. As a result, the pellte beds have changed color, become much coarser gra1ned. and lost any pre-anatect1c fine-scale structures that they may have had. DIStinct leucosome and melanosome d1d not form in the pelit1c beds because there was no dnv1ng force for the effect1ve separat1on of the melt from the solid (i.e., no movement of the melt occurred). Thus. overall this m1gmat1te cons1sts of neosome. wh1ch was derived from the metapelitlc beds, and paleosome, wh1ch are the dark psamm1t1C beds that d1d not part1ally melt. However, the neosome 1n the center does show slight separation of the melt from residuum at a scale of I 2 em.

Location: Mount Stafford, Australia. Rock type : metatexite migmat1te cons1st1ng of alternating layers of neosome and paleosome; pelit1c protolith Interbedded w1th psamm1te. partial melt1ng at T ca. 775 785°C. P 3.3 4.0 kbar, on the boundary between local metamorphic zones 3 and 4. Image and caption : R1chard Wh1te. Further read1ng: Greenfield, J.E., Clarke, G.L., Bland, M. & Clarke, D.L. ( 1996): In s1tu m1gmatite and hybrid diatexite at Mt. Stafford, central Australia. journal of Metamorphic Geology 14 ,413 426. Greenfield, j .E., Clarke. G.L. & Wh1te, R.W. ( 1998): A sequence of part1al melt1ng react1ons at Mt. Stafford, central Australia. journal of Metamorphic Geology 16, 363~378. White, R.W., Powell, R. & Clarke, G.L. (2003) : Prograde metamorphic assemblage evolut1on dunng part1al melting of metasedimentary rocks at low pressures: migmatites from Mt. Stafford, central Australia. journal of Petrology 44, 1937~1960.

A rias of Migmatites

------------------------------- 81 Figure Fig. A2. Exhumation o f upper amphiboltte fwes from a m id -crust depth in the thtckened crust in Bnttsh Columbta began with crustal extension 1n the early Tertiary. Partial melting and the upward migrat ion of melt occurred during exhumat ion and led to a major weakening of the crust, which contributed to t he rise of metamorphic core complexes and domes, and to the late collapse of the orogen. The weak, partially melted rocks 1n the core complexes, the mtgmatttes, were highly stratned as the core complexes developed and were exhumed by detachment faulttng. The mtgmatttes 1n the photograph are from the Thor-Odin dome and have a morphology that is characteristic of rocks that were highly stratned whi le they contained melt. T he most obvious feature is that t he maJority of the leucocratic domains are parallel to the strong tectonic foliatton and the attenuat ed compoSitional bandtng 1n the mesocratic, grey paleosome host. The preponderant morphology of th1s mtgmattte IS stromattc. Closer inspectton tndicates that there are two types of leucocratiC stromata, a set of narrow (<3 em) greytsh ones (A) that do not appear to have etther a conspicuous melanosome or a mafic selvedge assoCiated wtth them, and a later group of wider, sl ightly discordant wh ite domains (B) that commonly do have a mafic selvedge. The older grey set may have formed m situ and, therefore, be leucosome, but the later set was InJected and, therefore, best termed leucocratic vetns. Shorter and thtcker leucocrat1c bodtes (C) that are htghly discordant to the foliatton occur 1n extensional structures and small oblique shear zones throughout the mtgmattte. Such leucocrat1c domains have a semiregular spactng between 50 em and I m, and commonly have a consptcuous melanosome or a mafic selvedge around them; they may also be leucosome. Melt may have flowed from the stromatic leucosome to the discordant leucosome as t he migmatite was deformed. In order to test thts possibility, it would be necessary to determine whether there is petrological cont inuity from the stromat iC to the dtscordant leucosome in order to establish whether they contained melt at t he same time.

Location: Thor- Odtn dome, Bnttsh Columbta, Canada.

Rock type: stromatic metatextte mtgmattte, parttally melted at upper-amphtbolite-factes conditions. The whtte area in the upper left IS snow. Scale: hammer near the center of the photograph for scale. /mage: Ohvter Vanderhaeghe.

Further reading: Vanderhaeghe, 0., Teysster, C. & Wysoczanskt, R. (1999): Structural and geochronological constraints on the role of parttal meltmg dunng the formation of the Shuswap metamorphic core complex at the latitude of t he T hor Odin dome, British Columbia.

CanadJOnjournal o(Earth SCiences 36,917 943. Vanderhaeghe, 0. & Teyssier, C. (2001): Partial melting and flow in orogens. Tectonophystcs 342, 451 472.

SOME EXAMPLES OF MIG MATITES

82 -----------------------------Figure

3

Fig. A3 . Rocks at Sand R1ver 1n the Limpopo Mob1le Belt of southern Afnca have been deformed and part1ally melted 1n the deep parts of an orogen at least once and possibly three t imes. Many of the high-grade rocks there are h1ghly strained. This photograph shows a migmatite wh1ch is mostly paleosome that has strongly attenuated banding cons1st1ng of layers of trondhJemltiC, granodiont1c, and mafic composition. The more competent layers in the paleosome are boudinaged. Laterally persistent, centimeter- and millimeter-wide leucocratic bands (A) occur in the incompetent trondhjemitic and granodioritic layers that d1d not boudinage. but they are so strongly attenuated that 1t 1s not posSible to determine whether they are felsic dikes or leucosome matenal. However, there are networks of concoarser-gra1ned SpiCUOUS, leucosome assooated w1th each of the boudinaged layers. The appearance of the leucosome in these layers depends on structural pos1t1on. Leucosome located in pressure shadows and interboudln partitions are thickest, have the coarsest grain-size and are not foliated Traong along the leucosome doma1ns reveals that they become finer grained, th1nner. and foliated where located on the long s1des of boudins, or are 1n shear zones, 1.e., they are strongly attenuated where not 1n protected structural s1tes. Some anatect1c melt thus has moved from foliat1on planes and collected between the boudins. There are a few bodIes of leucosome with a coarse gra1n-s1ze and no foliat1on 1n dilatant fractures that cross several layers. Their presence indicates that anatectic melt was still present as the deformation ended. This migmatite and that shown in Fig. A2 w ere both highly strained while melt was present. and they have similar overall morphologies in wh1ch stromatic leucosome dominates. However, the appearance of the

two migmat1tes is very different, and there are two reasons for th1s. F1rstly, the Thor Od1n m1gmat1te conta1ned much more melt, and so has more and wider bodies of leucosome. Secondly, the paleosome 1n the Sand R1ver migmatites 1s leucocratlc, and consequently, the leucosome IS far less conspicuous. Location: Sand R1ver Gne1sses. Causeway locality, South Africa. Rock type: metatexite m1gmatite; trondhJemite, granodiorite, and mafic protoliths. Anatexis 1n the granulite facies, T 800 850°C, P 7 I 0 kbar. Scale: the pocket knife is II em long. Image: E.W. Sawyer.

Atlas of M igmatitcs

----~------------------------ 83

B. TH E PA RTS O F A MIGM AT IT E Neosome and paleosome [Figs. 81-84] T he parts of a migmatite created by and during anatexis are cal led neosome. The part not affected by partial melting, which commonly retains some charactenstics of the pre-partial-melting state, is called the paleosome. The photographs in this section illustrate some of the charactenstiCs used to tdenttfy and descnbe neosome and paleosome.

t

Neosome with leucosome and melanosome [Figs. 85-814] The best-known constituents of neosome are leucosome and melanosome, and they form when the melt fraction generated by partial melt ing becomes separated, or segregated, from the solid fraction. The leucosome is denved from the melt fract ion, whereas t he melanosome represents the solid products of the melting reactton, typically as a result of Incongruent melting, and 1t occurs with or without excess reactant phases. Some of the common relationships between leucosome and melanosome that are found tn neosome are shown on the followtng photographs. The examples also show some of the morphologtcal vanety dtsplayed by leucosome and melanosome; such vanety IS 1n large measure due to protoltth antsotropy and the way 1n whtch it responds to deformatton.

Neosome without distinct leucosome or melanosome [Figs. 815-826] Not all examples of neosome develop conspicuous and separate domains of leucosome and melanosome, and t his can make the recogn ition of anatexis and therefore mtgmatites difficult. However, in most cases, there is little difficulty tn recogniztng the neosome. One particular morphology of neosome (Ftgs. B20 B25) appears to be diagnostiC of reacttons involvtng dehydratton melttng of btottte, and perhaps of hornblende as well.

Neosome in open-system migmatites [Figs. 827-834] Mtgmatites that form without the additton, or loss, of material are said to have formed 1n a "closed system". If material was added t o, or removed from, t he migmatite during anatexis, then the migmatite formed in an "open syst em". O nce a low-viscosit y phase (melt) has formed during anatexis, open-system behavior very commonly ensues, because the melt fraction has a low vtscostty and is able to move out of the more viscous solid residuum. Scale IS an important factor in deciding whether the system was "open" or "closed". For practtcal purposes, tf all the melt and residual parts can be identtfied at the scale of a hand spectmen, or the outcrop, then one can regard the processes of formtng the mtgmatite as a "closed system". However, if the scale of the separatton between the melt and restduum 1s greater than the scale of observatton, one regards the migmattte-forming processes as havtng occurred tn an "open system". Migmatites that formed in closed systems are easier t o underst and because the relative proporttons of the melt and residuu m are preserved in the neosome. In opensystem migmatites. the relative proportions of the melt and residuum have been changed. Migmatttes, or regions withtn migmatites, that have lost melt are recognized by thetr htgher proportion of rest dual material relative to melt. or melt products (e.g., leucosome). Conversely, mtgmatites that have had melt added to them can be recognized by a pauctty of residuum. Thts group of photographs show some examples of "open-system" mtgmatites.

Variations within neosome [Figs. 835-842] In many migmatites, there 1s a considerable range 1n the morphology. microstructure and composition within the leucosome and melanosome they contatn. However, it is the leucosome that shows the most obvious variability within individual domains, between generations of leucosome, or between the structural sites they occupy in the migmat1te. Thts sectton illustrates some of thts vanability.

THE PA RT S O F A MIGMATITE

84 ------------------------------

From leucosome to leucocratic dikes in migmatites [Figs. 843-848] The process of melt segregation 1n m1gmatites starts with the movement of melt from the grain boundaries where it formed toward channels. The first channels into which melt flows are located in the source area where the melt formed; they are surrounded by melt-deplet ed rocks t hat are, typically, melanocratic. The locat ion of channels is controlled by such factors as t he nature of the rock's anisotropy, and the way in which t he rock is deformed. T he channels serve as both the sites w here melt may collect and as conduits through which melt flows. Where the melt m the channels crystallizes, 1t is preserved as the leucosome 1n a migmat1te. For anat ectic melt to leave 1ts source area and reach higher levels 1n the crust, the flow of melt must become focused into progress1vely fewer, but larger, channels. The larger channels do not have a melanosome around them because t hey are no longer fed from t he adjacent rocks. In addition, t he channels are commonly discordant: consequently, what they contain no longer is cal led leucosome; rather, they are leucocrat ic veins, or w here larger, leucocrat ic dikes. Some examples of the transit ion from leucosome to leucocratic dikes in migmatites are shown in this series of photographs.

Selvedges in migmatites [Figs. 849-854] In many migmatites, there are narrow. compos1t1onally and microstructurally distinct rims around the leucosome and leucocratic ve1ns; these are called selvedges. The compositionally distinct rims t hat are t he res1dual material left after the extract ion of the anatectic melt t hat makes up some, most, or all of the associated leucosome could be called melt-depleted selvedges. However, t he residual material is most commonly melanocratic, and the t erm melonosome is reserved exclusively for t hese. Therefore, the term selvedge applies to t he narrow, compositionally distinct rim t hat occurs around leucosome and leucocrat ic veins and t hat is not formed by t he extraction of anatectic melt . There is no requirement that selvedges be melanocratic. However, the most common type of selvedge is melanocratic, and consists of a very thin biot ite- or hornblende-rich nm around leucocratic veins; such a rim is called a mafic selvedge, not a melanosome. Mafic selvedges m migmatite terranes are most commonly developed around late, discordant leucocrat ic veins or dikes; they can also develop around granitic dikes intruded into host rocks t hat are not migmat it es (see Figs. Gl3, GIS, G l6, and G19).

A tlas o f M igmatitcs

------------ ------------ --------- 85 Figure

I

•

Fig. B I. The migmat1te shown consists of scattered patches of neosome in the paleosome. The paleosome IS a mesocrat1c plagioclase quartz-K-feldspar orthogneiss w1th <5% biotite + sillimanite, and is recognized by the presence of a foliation t hat predates the partial melting. The small patches of neosome cons1st of a melanocratic (garnet + hercynite) part. generally 1n the center, and an outer leucocrat1c nm with the assemblage K-feldspar + quartz + plagioclase. Thus, the neosome appears to have undergone segregation of the melt from the residual solids. N ote that some patches of neosome do not have a melanocrat1c core, probably because the plane of the rock surface does not pass near the center of those patches. Unlike their host, the patches of neosome do not have a foliation, and they are generally equidimensional and somewhat irregularly distributed. However, a band of neosome to the left of the lens cap IS subparallel to the foliat1on 1n the paleosome, and 1ts origin may be due to the infiltration of a fluid along the direction of the foliation plane in the host.

Locotton: Baume migmatite, Ardeche, France. Rock type: patch metatexite m1gmatlte; quartzofeldspath1c gneiss protolith partially melted under upper-amphibolite-facies conditions, T 700 ± sooe, P 3 kbar. Scale : the lens cap is 6 em across. Image: Pierre Barbey. Further read1ng: Weber, C. & Barbey. P. ( 1986) : The role of water, mixing processes and metamorphic fabric in t he genesis of the Baume migmatites (Ardeche, France). Contributions to Mmerology and Petrology 92, 481 491.

NEOSOME AND PALEO SOME

86 ------------------------------Figure

Fig. 82 . This m1gmatite has a compos1t1onally un1form and weakly foliated paleosome 1n wh1ch scattered patches of light-colored neosome have a somewhat 1rregular shape that might be related to fractures that developed in the paleosome at the t1me of part1al melting. The paleosome is a medium-gra1ned garnet-hornblendeplagioclase metagabbro; the neosome has the mineral assemblage plagioclase + quartz + clinopyroxene + garnet and is much coarser grained. Hence, a possible reaction to describe melting is hornblende + plagioclase clinopyroxene + melt. The minerals are rather uniformly distributed in the neosome 1n the upper left. However, the larger patch of neosome in the center IS more complex, as the res1dual ferromagnes1an m1nerals (mostly clinopyroxene) are at the marg1n 1n some places, and 1n the center elsewhere. Because the paleosome conta1ns hornblende, 1t 1s darker colored than the res1dual part of the neosome, Illustrating that color is not 1nvariably reliable as a bas1s for dist1ngu1shing paleosome from res1duum, and hence for genetic 1nterpretat1ons in m1gmat1tes.

=

Locat1on: Central Metasedimentary Belt. Grenville Prov1nce near Wakefield, Quebec, Canada. Rock type: patch metatexite migmat1te; metagabbro protolith, anatex1s at conditions of the upper amphibolite to lower granulite faoes, T ca. 800°C, P 6- 9 kbar. Scale: the ruler is IS em long. Image :

E.W. Sawyer.

A tl as of M igmatite;

----------------- -------------- 87 Figure

3

Fig. 83. Only the fine-grained, grey. lens-shaped body, which can be called either a raft. scholle. or enclave (labeled P). to the nght of t he center IS paleosome in this m1gmatite: all the rest IS neosome of one sort or another. The paleosome scholle is fine-gra1ned and mesocrat1c: 1t represents the rema1ns of a plag1oclase + quartz + b1ot1te res1ster lithology (a psammite or metagreywacke) that did not melt, but 1t did undergo boudinage. The rest of the metasedimentary rocks, of pelitic or semipelitic composition, underwent partial melting and has been modified . The neosome 1s morphologically complex. as some parts (top of the photograph) have a layered appearance. whereas others are more 1rregular in form. Because the melt fraction has been segregated from its res1duum to various extents. the neosome is compositionally complex also. The meltrich parts of the neosome are evident as leucosome (L). The parts rich in residual minerals are melanocratic (M), but the parts of the melanosome enriched in biotite are far darker colored than those parts where garnet IS abundant. Furthermore, t here are coarse-grained. mesocratic parts to t he neosome; in these parts, the melt d1d not completely segregate from the residuum. This migmatite illustrates that no specific genet ic connotat ion should be attached to t he parts of a migmatite t hat are mesocratic.

Locat1on: N emiscau Subprovince, Quebec, Canada. Rock

type: schollen diatex1te migmatite; siliciclastic metaturbidIte protolith partially melted at T 750-820°C, P 4 5 kbar. Scale: the ruler IS IS em long. /mage: E.W. Sawyer.

NEOSOME A N D PALEO SOME

88 -----------------------------Figure

Fig. 84. This migmatite cons1sts almost entwely of neosome; 1t IS a diatex1te m1gmat1te. Paleosome occurs only as scattered, dark, rounded rafts or schollen in the neosome (e.g., between the hammer and the ruler) . Although leucocratic relative t o t he paleosome, the neosome cannot be descnbed as leucosome because 1t conta1ns too h1gh a proportion of ferromagnesian minerals; biotite and orthopyroxene in t his case. The neosome is characterized by a coarse grain-size and by the loss of structures, such as bedding, that predate the partial melting. The very uniform appearance of this migmatite could anse because ( I) the protolith had a very homogeneous bulk composition, so that the degree of part1al melt1ng was everywhere similar. or (2) there was very little separat1on of the melt fraction from the solid fract1on, and consequently, the neosome d1d not develop consp1cuous leucosome and melanosome. However, some places are a little lighter colored, or a little darker colored, than others. Several dark bands (S), which are concentrations of residual b1ot1te and orthopyroxene (the melt1ng reaction in these rocks was b1otite + plagioclase + quartz = orthopyroxene + ilmen1te + melt) are present at the lower right. These thin bands, ribbons, and foliae of melanocratic material are called schlieren.

Locat1on: Ashuan1p1 Subprovince, Quebec, Canada. Rock type: d1atexite m1gmatite; Sllioclast1c metaturbidite protolith, granulite-fac1es anatexis; T 825-875°C and P 6- 7 kbar: Scale: the hammer is 40 em long. Image: E.W. Sawyer.

A tlas of M igmati tes

----------------- -------------- 89 Figure

S

t

•

Fig. 85. The protolith to this migmatite is a foliated metamafic rock, and the neosome shown has a concentrically zoned structure: there is a rim of melanosome, and an 1nner domain of leucosome. The leucosome is comparatively un1form: 1t cons1sts of coarse-grained leucotonalite. The melanosome, however, is not un1form. It consists of a clearly v1sible hornblende-rich 1nner part that is essentially devoid of plag1oclase and quartz, and a wider and less obvious outer part in which the abundant plagioclase and quartz progressively increase outward to the level found 1n the host, wh1ch is the paleosome where 1t d1d not partially melt. It may not be easy to locate preosely where the outer edge of the melanosome IS in the field: consequently, petrographic examination of thm sections may be required to do this if mass-balance calculations are contemplated. Geochemical mass-balance of th1s and other areas of neosome from the outcrop indicates that the neosome formed in a closed system, i.e., the melt fract1on that created the leucosome came from the adjacent melanosome, which therefore represents a melt-depleted halo around the leucosome. Anatex1s in a closed system 1n which the melt and residuum have separated, but remained in place, is referred to as m situ; hence, this is an example of an m situ leucosome.

Location: Abitib1 Subprovince south of Ch1bougamau, Quebec, Canada. Rock type: metatexite migmatite; metamafic protolith, anatex1s at T 800-850°C and P 8 I 0 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer. Further readmg: Sawyer. E.W . ( 1991 ) : D1sequ1libnum melt1ng and the rate of melt res1duum separation during migmatization of mafic rocks from the Grenville Front, Quebec. journal of Petrology 32,701 738.

MELA N OS OME N EOSO M E WIT H LEU COSO ME A N D

90 --- --- --- --- --- --- --- --- --- --- -Figure

6

gly foliated horn sparK-feld tetitani grey. and compositionally banded. . some paleo e diont meta blende biotit e- quar tz- plagioclase 1n doma large A nt. Ther e are two types of neosome prese d 1n a dilata nt of low-aspect-rat1o 1n s1tu neosome IS locate o the foliat1on and struc ture that has developed obl1q ue t . Th1s neosome the compositional band 1ng 1n the paleosome melanosome. consists of an inner leucosome and an outer of the leucoall not but . most d wh1ch IS developed aroun gra1ned than er coars are some . Bot h part s of t he neosome or the bandon fohat1 the paleosome. They do not have t he postdate fore, there Ing present in the adJacent paleosome and, in tonalit1c comp othese structures. The leucosome port1on IS spar), and some of Sition (plag1oclase + quar tz+ mino r K-feld quar tz are euhethe plag1oclase crystals that occu r w1thin ate crystallization dral . a micro struct ure Interp reted to ind1c o the leucosome t ging belon me from a melt. The melanoso lende and. in hornb + has the mineral assem blage plagioclase K-feldspar and z quart places. hornblende + plagioclase (i.e., darker, in and ider, are absent). Furth ermo re, it is much w this mela noso me some places t han in others; conse quent ly, t its leucosome is more irregular and less sym metri cal abou neoso me consists t han that in Fig. BS. The second type of

Fig. 86. This migmatite consists mostly of a stron

quart z leucosome of thin band s (< I 0 mm) of plag1oclase + nosome. Because w1t h a very narro w hornblende-nch mela the migmat1te it occurs parallel t o t he foliat1on and g1ves bed as stromatic. a layer ed appearance. 1t can be descn -size than the gra1n er small a has some T he strom at1c leuco the two are e wher but ture. struc leucosome 1n the dilatant r is transiothe the to one 1n contact, the transit1on from rs of the borde me tional. In ne1ther case can the melanoso t-ratio spec low-a stromatic leucosome be traced into the were both that leucosome. T hese relationships could mean part1ally molte n at the same time. ugam au, Locat1on: Abit ibi Subprovince south of Chibo metaat1te; m1gm Q uebe c. Canada. Rock type: meta tex1te kbar. 8-10 P and C diorit e proto lit h. anatexis at T 800- 850° er. Scale: the ruler IS IS em long. Image: E.W . Sawy

A rias of Migmatites

- -- - - -- - - - - - - - - - 91 Figure

7

Fig. 87. The paleosome part of th1s m1gmat1te 1s a biotite quartz hornblende plag1oclase gne1ss 1n wh1ch folded grey leucocrat1c plagioclase + quartz layers I 5 mm w1de w1thout melanocratic borders IS ev1dent. The neosome part (around the pen) consists of quartz plag1oclase K-feldspar leucosome enveloped by melanosome, which consist mainly of hornblende. T he concentration of hornblende to form the melanosome suggests that hornblende was either not involved in the melting reaction and was in excess during partial melting, or that hornblende was a solid product of reaction. The presence of inclusionfree, presumably melt-precipitated hornblende crystals 1n the leucosome suggests that hornblende was 1ndeed involved in the melt1ng reaction, wh1ch may have been biOtite + plag1oclase + quartz + hornblende hornblende1 + melt. Poorly aligned nakes of poikiloblastiC biOtite In the melanosome probably formed by reversal of the melt-produCing react1on dunng crystallization of the leucosome.

=

Locat1on: Arunta Inlier, Australia. Rock type: metatexite m1gmat1te; gneiss protohth of 1ntermed1ate compos1tion part1ally melted under conditions of the upper amphibolite to lower granulite fac1es. Scale: the pen IS 14 em long. image and capt1on: Tony I.S. Kemp.

N EOSOME W ITH LEUCOSOM E AND MELANOSOME

92 ------------------------------Figure

8

Fig. 88. A band of quartz + plagioclase + K-feldspar leucosome about 15 mm w1de can be seen 1n the center of th1s photograph. Although 1t 1s not ev1dent 1n th1s close-up, the leucosome 1s oriented parallel to the compositional layenng (bedd1ng) 1n the m1gmatite; therefore, the leucosome is descnbed as stromatic. There are prominent. narrow (about 4 mm) borders of melanosome contain1ng large crystals of garnet, orthopyroxene and biot1te developed along both sides of the leucosome. The very low modal content of quartz and feldspar in the melanocratic nms is consistent with the interpretation that they contain the residuum left after the extraction of the melt that subsequently crystall1zed to become the leucosome. The grey mesocratiC part of the m1gmatite beyond the melanosome 1s a quartz + plag1oclase + biotite + garnet + K-feldspar ± orthopyroxene metapsammite, wh1ch on the bas1s of 1ts m1neral assemblage has undergone part1al melting and IS mostly residuum. Some porphyroblasts of garnet 1n the mesocrat1c part have a narrow leucocratic rim, and these are interpreted to be K-feldspar (a solid product of the melt-producing reaction) and crystallized melt. Therefore, the mesocratic part of this migmatite is neosome, not paleosome. All t he biotite in t he melanosome and some in the mesocratic part replace garnet and orthopyroxene.

Th1s replacement may be due to the loss of melt, a reaction between anhydrous res1dual phases and melt during cooling, or to react1on with an aqueous nUid.

Location: Wuluma Hills, Arunta Inlier, Australia. Rock type: metatexite m1gmatite; psammite protolith, partially melted at T 825-875°C. P ca. 5 kbar. Scale: the ruler 1s graduated 1n em and mm. Image: E.W . Sawyer.

Atlas of Migma[i[es

------------ ------------ ---------93 Figure

9

Fig. 89. The neosome 1n this m1gmatite occurs 1n a layer of med1um-grained (2-4 mm) garnet- plagioclase-hornblende metamafic sch1st. on which the scale rests. The neosome 1s coarse grained (5 12 mm), and cons1sts of buff-colored, irregularly shaped leucosome of tonalite composition that conta1ns minor amounts of garnet and clinopyroxene. and a brown1sh purple. plag1oclase hornblende- clinopyroxene garnet melanosome. If it is assumed that the host layer has a composition similar to the proto lith of the neosome, then the 1nferred reaction responsible for partial melting was hornblende + plag1oclase melt + clinopyroxene + garnet. The leucosome occup1es dilatant structures that formed as the host layer (protoltth) underwent layerparallel extension. Contacts between t he melanosome and leucosome are diffuse: melanosome IS absent where the leucosome shows a sharp contact w1th 1ts host metamafic sch1st. Some of the melt thus was able to migrate a short distance away from its m s1tu melanosome when brittle fracturing occurred. The leucosome contains large. euhedral crystals of clinopyroxene. whereas the clinopyroxene 1n the melanosome IS generally xenoblastic or sub1d1oblasttc: the clinopyroxene in the leucosome thus crystallized 1n a melt. Many clinopyroxene crystals have a narrow rim of hornblende.

=

Locat1on: Ab1t1b1 Subprov1nce south of Chibougamau. Quebec. Canada. Rock type: metatexite m1gmatite: metamafic protolith, anatex1s at T 800- 850°C and P 8-10 kbar. Scale : the ruler is IS em long. Image: E.W. Sawyer.

N EOSOME WITH LEUCOSOME AND MELANOSOME

94 ------------------------------Figure

I0

Fig. 8 I 0. Th1s strongly foliated metatex1te m~gmat1te from

Sillimanite 1n the rest of the melanosome. This microstructure

southern Bnttany has a complex neosome. The grey, fine-

1s interpreted to have fonmed dunng the incongruent melting of biotrt:e. Consequently, the b1otlte bands were present before

grained, sch1stose layers (P) conta1n the m1neral assemblage quartz+ oligoclase+ b1otrt:e (some layers conta1n m1nor amounts of garnet) and probably underwent little, 1f any, partial meltIng; they are possibly quartzofeldspath1c paleosome. The coin rests on one of several coarser-grained layers that contain the

+ biot1te ± garnet ± ± Sillimanite and may be residual; these were derived

m1neral assemblage plagioclase + quartz cordierit e

from more alumino us metapelitic beds. Domains of leucosome

dehydration melt1ng began. Therefore, the b1ot1te bands are interpreted to have developed at the very beg1nn1ng of anatexis, at which stage H 10-present melt1ng reactions occurred and did not involve b1otite. T he biot1te bands are an integral part of the melanosome and are not mafic selvedges. Most of the leucosome domains in th1s migmat1te are oriented parallel to the compositional layering and foliation. Thus. this is a stromatic

up to a few centimeters wide occur w1th1n the coarser-gra1ned

met atexite migmat1te. However; leucosome 1s present between

layers and are either trondhjem1tic (quartz + b1ot1te) or granrt:ic (quartz

+ oligoclase + K-feldspar + oligoclase + bio-

the boudins. The migmat1te thus was defonmed wh1le melt was still present.

tite) 1n compos1tion; both may conta1n small amounts of garnet and s1lliman1te. Many of the leucosome areas have consp1cuous, coarse-gra1ned melanosome from I to 10 mm w1de around them that conta1ns the mineral assemblage b1ot1te + cordiente + sillimanrte

± garnet ± quartz ± plag1oclase. The assemblage IS

cons1stent with a residuum denved by the loss of anatect1c melt

Location: Port Navalo area, southern Bnttany, France. Rock type: stromat1c metatex1te m1gmatite; quartzofeldspath1c and aluminous metasedimentary protolith, anateXIS at T ca. 800°C, p ca. 8 kbar; then decompression to 4 kbar and cooling. Scale: the coin IS 24 mm across. Image and capt1on: Mike Brown.

from a pelitic protolith. A n unusual feature of the melanosome

Further readmg: jones, K.A. & Brown. M. (1990): High-

1n th1s migmatite 1s the development of prom1nent biot1te-rich

temperature

bands within it. T hese consist almost ent1rely of decussate crys-

development of regional m1gmat1tes: an example from southern

tals of biotite up to 2 em long. T he coarse-grained biotite-rich

Brittany, France.Journal ofMetarnorph1c Geology 8, 551-578.

bands could be part of the melanosome, or they could be a mafic selvedge that fonmed later by reaction between the leucosome and its host. The microstructures assooated with the bands prov1de the critical 1nfonmat1on. Some b1ot1te in the bands 1S part1ally replaced by a nm of cordiente that separates it from

Marchildon,

'clockwise'

N. &

P T paths and melting in the

Brown, M .

(2003): Spatial distribu-

tion of melt-bearing structures 1n anatectic rocks from southern Brittany, France: implications for melt transfer at grainto orogen-scale. Tectonophysics 364, 215

235.

Aria:, of Migmatites

----------------- ------------- 95 Figure

I I

Fig. B II. Migmatites in which the neosome forms parallel layers are called stromatic; single layers, as in this example, are called stroma. The prom1nent layering in the paleosome formed when the metamafic protolith underwent noncoax1al sheanng before and dunng part1al melting. wh1ch attenuated the pre-exist1ng compositional layering. The neosome IS parallel to the layering and consists of two parts: a white leucosome of leucotonalite composition (plagioclase + quartz), and a hornblende-rich. plagioclasedepleted melanosome. The leucosome contains subhedral crystals of plag1oclase that have crystal faces against quartz. and large euhedral crystals of randomly oriented hornblende. Both microstructures 1nd1cate crystallization from a melt. This stromatic neosome is unusual 1n that melanosome is developed only on one side of the leucosome and not on both. as is more common. Mass-balance calculations using the whole-rock major- and trace-element compositions 1ndicate that the neosome probably formed m a closed system and is, therefore. an example of in s1tu melting and subsequent segregat1on of the melt from t he residual solids. This is an example of an 1n Situ neosome; t he leucosome part may also be called an rn situ leucosome.

Location: Abitibi Subprovince, south of Chibougamau, Quebec, Canada. Rock type: metatexite m1gmatite. metamafic proto lith part1ally melt ed at T 800-850°C and P 8 I 0 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer. Further reading: Sawyer, E.W. (1991): Disequilibnum melt1ng and the rate of melt-residuum separation dunng m1gmatization of mafic rocks from the Grenville Front, Quebec.

journal of Petrology 32. 701 738.

NEOSOM E WITH LEUCOSOME AND MELAN OSO M E

96 ------------------------------Figure

I

Fig. 812. The neosome 1n th1s m1gmat1te 1s much coarser gra1ned than the hornblende + plag1oclase + garnet paleosome upon wh1ch the scale rests. Most of the larger doma1ns of neosome are located m and around shear zones and 1nterboudin partitions. The melanosome is composed of hornblende + garnet + clinopyroxene with m1nor plagioclase, and the leucosome mostly cons1sts of plagioclase + quartz, but locally contains large, euhedral crystals of light green clinopyroxene. The melting reaction may have been hornblende + plagioclase clinopyroxene + melt. A lthough the leucosome and melanosome are 1n contact, the distribution of melanosome is 1rregular and asymmetncal relat1ve to the leucosome. The darkest parts of the melanosome are nch 1n hornblende, wh1ch appears to replace clinopyroxene. The hornblende m the melanosome thus results from a late- or post-anatex1s rehydration react1on. Small amounts of leucosome also are present parallel to the foliation planes 1n the paleosome. Some of th1s leucosome has a consp1cuous melanocrat1c border (mafic selvedge) resulting from the late replacement of clinopyroxene by hornblende.

=

Location: Central Metasedimentary Belt, Grenville Provmce near Wakefield, Quebec. Canada. Rock type: metatex1te migmat1te: metagabbro protohth. anatexis under condit1ons of the lower granulite fac1es. Scale: the ruler 1s IS em long. Image: E.W. Sawyer.

A tlas of Mig ma tites

---------------------------------- 97 Figure

I3

r

f

Fig. 813. Th1s photograph shows a th1n (10 em) layer of plag1oclase ·garnet-clinopyroxene metamafic rock w1th quartz-plag1oclase in situ and 1n source tonalit1c leucosome that occurs w1th1n a garnet-beanng plag1oclase· hornblende metabasiC rock, v1sible in t he upper nght. The photograph 1S taken normal to the plane of t he layenng, and shows that most leucosome 1s disposed in an array of linked extenSIOn-induced fractures. There are also small, round halos of leucosome, however, about 8 mm across. surrounding some of the garnet crystals. The borders of the leucosome are not planar; rat her, they are h1ghly irregular on the gra1n scale, wh1ch suggests that they have not been subsequently deformed. The proto lith of this layer is 1nterpreted to have had JUSt the opt1mal bulk compos1t1on for part1al melt1ng to have consumed v1rtually all its hornblende to generate abundant garnet and clinopyroxene, both of which are very strong m1nerals. Consequently, as melt1ng progressed, and the melt fract1on was removed, th1s layer became more competent than its ne1ghbors and developed many extenSional fractures. The last increments of anatectiC melt in the layer migrated into t he extensional fractures as they developed and, when temperat ures declined, crystallized there t o form t he leucosome. A possible melting reaction is hornblende + plagioclase = melt + garnet + clinopyroxene .

•

Locat1on: Ab1t1b1 Subprov1nce south of Chibougamau, Quebec. Canada. Rock type: metatex1te m1gmatite w1th dilatant structures; met amafic protohth, anatex1s at T 800-850°C and P 8 I 0 kbar. Scale: the ruler is IS em long. /mage: E.W . Sawyer.

NEO SOME WITH LEUCO SOME AND MELANOSOME

98------------------------------Figure

14

Fig. 814. This migmat1te was denved from metapelite and cons1sts of only two parts, leucosome and melanosome; 1t 1s all neosome. However, the adJacent outcrops conta1n psamm1t1c, calc-silicate, and mafic resister litholog1es that can be traced laterally for some distance; thus, 1n the larger context, th1s 1s a metatex1te migmat1te. The m1gmatlte 1s polyphase, as it has experienced anatexis and deformation in the Archean (twice) and aga1n in the Paleoproterozoic. Hence, the bodies of leucosome display a complex history of deformation and have been boudinaged and folded at various times. The domains of leucosome 1n the photograph are located parallel to the foliat1on and compos1t1onal layenng denved from the original bedding; they are thm, and have a high aspect-ratio. Hence. the m1gmat1te can be descnbed as stromatic. The leucosome cons1sts of quartz, plag1oclase, and K-feldspar 1n vanous proport1ons. and conta1ns only minor amounts of garnet and b1otite. The melanocratic, metapehtic rocks between the doma1ns of leucosome are nch in biotite and garnet. consistent w1th a melt-depleted, or residual, bulk composition. However, the metapelitic layers show differences in modal mineralogy, grain size, and microstructure from layer to layer. These differences probably reflect the initial variation in protolith composition from bed to bed.

Locat1on: Be1t Bndge Complex. Verbaard Farm, South Afnca. Rock type: stromatic metatex1te m1gmat1te; pelit1c protolith, anatexis at T 800 850°C, P 7 I 0 kbar. Scale: the

pocket kn1fe

IS

II em long. Image: E.W. Sawyer.

Atl,ls of Migmatites

------------------ --------------- 99 Figure

• •

Fig. B IS . Migmat ites derived from felsic igneous protoliths such as granites, leucotonalites and trondhjemites can be very difficult to recogn1ze in the field because the neosome has a compos1t1on. m1neralogy. and microstructure s1milar to those of the paleosome. The paleosome 1n th1s m1gmat 1te (P) IS a grey. foliated leucotrondhJemlte. part of a late Archean. calc-alkaline plutonic arc in the Superior Province of Canada. The neosome (N ) forms lighter. pinkish and buff-colored. rather nebulitic patches that are slight ly coarser grained and. most notably. devo1d of a foliat1on. relat1ve to the paleosome. Petrograph1c and geochem1cal ev1dence 1nd1cates that no segregation of melt from residual solids occurred as the neosome patches formed; hence. there is no leucosome and no melanosome, 1.e., the products of anatexis rema1ned 1n s1tu. Locally, there are thin. pink leucocratic veins (LV) parallel to the foliation in the paleosome; these represent 1nject1ons of anatectic melt (1.e.. they are not m situ or in-source leucosome) . The whitISh streaks onented parallel to the scale are scratches on the surface made when cleanng the outcrop.

Location: Opat ica Subprovince. Quebec, Canada. Rock type : patch metatexite migmatite; leucotrondhjemite protolith part1ally melted under upper-amphibolite-faoes conditions, T ca. 750°C, and P 5 7 kbar. Scale: the ruler 1s 15 em long. Image: E.W. Sawyer: Further readmg: Sawyer, E.W. (1998): Format1on and evolution of granite magmas dunng crustal reworking: the significance of diatexites. Journal of Petrology 39. 11 47 1167

NEOSOME W ITHOUT DISTINCT LEUCOSOME OR MELANOSOME

100 ----------------------------Figure

6

Fig. 816. The protolith for th1s m1gmat1te was a strongly foliated. coarse-gra1ned garnet- and orthopyroxene-bearIng monzogranite to syenogramte that conta1ned abundant megacrysts of K-feldspar. The gramte occurs as a large d1ke in a th1ck sequence of mafic granulites. The small degree of partial melting at this locality has generated scattered lent icular patches of neosome (N ) located in crenulation bands that are oriented at a high angle to the pre-existing foliation . The neosome in th1s migmatite is very difficult to see largely because it is practically the same color as the host. One reason for this uniformity IS the very felsic composition of the paleosome. The res1duum left after the extraction of the small amount of melt generated 1n th1s migmatite is dominated by K-feldspar. plag1oclase, and quartz and. consequently. not particularly ennched 1n ferromagnes1an minerals relat1ve to the melt-denved port1on. Hence. there is no distinct leucosome, nor melanosome, to assist the eye in detecting the neosome. The best criterion to identify neosome in such circumstances is 1ts truncat1on of older structures in the paleosome, the foliat1on 1n th1s example.

Location: Mount Hay. Arunta Inlier. Australia. Rock type : metatex1te m1gmat1te: foliated charnock1te protolith. partial melt1ng at T 825 875°C. P 6-7 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

A tlas of M igmat ites

Figure

17

Fig. B 17. T he center of this photograph shows a pelitic layer that has undergone significant partial melting. It is bordered at the top (beneat h the pen knife) and t he bottom by psammitic layers in which there was little or no partial melting. As the entire pelitic layer has been affected by partial melting, it can be called neosome, and three components can be identified w ithin it. The melanocrat ic part consists of scattered dark grey andalusite porphyroblasts (A); these are surrounded by a dark rim, or corona, of cordierite + spinel symplectite that also contains minor amounts of K-feldspar and plagioclase. The light grey mesocratic component (R) of the neosome consists principally of cordierite and K-feldspar with minor amounts of quartz and biotite, and is interpreted to be t he residuum left after partial melting of a pelitic protolith and the removal of most of the anatectic melt generated. Pink leucocratic material consisting mostly of K-feldspar and quartz is scattered unevenly throughout the neosome; it forms a rim around the andalusite porphyroblasts, as well as rather nebulous patches and small veins in the mesocratic part of the neosome. Because the K-feldspar is commonly prismatic and occurs in larger crystals of quartz (see Figs. F47 and F48), t he leucocratic part of the neosome is interpret ed to have crystallized from a melt, and can be termed leucosome. A lthough the melt and residual fract ions did separate, the striking aspect of this migmatite is that the scale of segregation was millimetric only, and the leucosome and

residuum remained intimately associated, a relationship suggestive of anatexis in the absence of differential stresses. A dark-colored layer separates the pelite from t he psammite; this selvedge has formed as a result of interaction between t he psammite and the anat ectic melt in t he pelite. Location: Mount Stafford, Australia. Rock type: unsegregated neosome in a metatexite migmatite; the protolith was a pelitic layer in a thinly bedded sequence of psammite and pelite, partial melting at T ca. 700°C, P 3.2 kbar in local metamorphic zone 3. Scale: the pen knife is I I em long. Image and caption : Richard White.

Further reading: Greenfield, j.E., Clarke, G.L., Bland, M. & Clarke, DL (1996): In Situ migmatite and hybrid diatexite at Mt. Stafford, central Australia. journal o(Metamorphic Geology 14,4 13-426. Greenfield, J.E., Clarke, GL & White, R.W. ( 1998) : A sequence of partial melting reactions at Mt. Stafford, central Australia.Journal o(Metamorphic Geology 16, 363- 378. White, R.W., Powell, R. & Clarke, G.L. (2003): Prograde metamorphic assemblage evolut ion during part ial melting of met asediment ary rocks at low pressures: migmatites from Mt. Stafford, central Australia. journal o( Petrology 44, 1937- 1960.

101

102

SOME NEOSOME WITHOU T DISTINC T LEUCOSOME OR MELANO

Figure

18

Fig. B 18. The m1gmatite 1n th1s photogr aph developed 1n a rock that contained layers of different compos ition and texture; consequently, the morpho logy of the neosome d1ffers subtly from layer to layer: The pen rests on neosom e that developed from a layer of alum1nous metapelite that contained large porphyroblasts of andalus1te; a s1m1lar layer occurs at the bottom of the photograph. The aluminous metapelite layers underwent a s1gn1ficant degree of part1al melt1ng. and the neosom e that developed 1s complex. as it conta1ns two types of leucocratiC and melanocratic doma1ns. The leucocrat1c doma1ns are rich 1n K-feldspar and quartz. On the bas1s of the microstructures that they conta1n, they are Interpre ted to have contained anatect1c melt; therefore, they can be termed leucosome. Fine-gra1ned leucosome (FL) occurs as narrow layers a few millimeters w1de that alternate w1th th1n layers of fine-gra1ned melanosome that IS nch 1n cordient e and Interpre ted to be the res1duum; they are melanosome. Coarser-grained leucosome (CL) forms somewh at Irregular layers that locally overgrow, and even truncate, the alternation of fine-gra1ned leucocrat1c and melanocratic layers. The coarse leucosome conta1ns melanocratiC patches (A) that are up to 2 em across. These are porphyroblasts of alum1nosilicate (aggregates of Sillimanite crystals that partially, or complet ely, replace andalus1te) that have been part1ally. or wholly. replaced by a symplect1t1c intergro wth of cordierit e and sp1nel where there was no quartz. The protolith to th1s neosome was very s1milar to that of the m1gmat1te shown 1n F1g. Bl7. but perhaps because the degree of partial melt1ng was a little h1gher 1n th1s m1gmat1te, the melt fract1on

1n the neosome began to segregate from the residuum and formed the Irregular and slightly discordant layers of coarsegrained leucosome. However. the process did not advance sufficiently to separate the leucosome from the melanosome by more that a few mill~meters . Consequently. at the outcrop scale. the leucosome and res1duum remain interspersed in this m1gmatite. The finer-grained and more prom1nently layered neosom e that occup1es the center of the photograph (below the tip of the pen) was denved from a subaluminous. cordierlte-nch metapel1te that conta1ned a far lower fract1on of melt. and did not conta1n porphyroblasts of alum1nos11icate. The millimeter-scale layenng 1n th1s neosome IS due to the alternat1on of leucocrat1c layers nch 1n K-feldspar and quartz with melanocratic layers that are rich in cord1ente ; the layering 1n the neosome 1s Interpre ted to m1m1c the fine-scale sedimentary layering of the protolith . The neosome 1n each of the layers 1n th1s m1gmat1te IS quite different 1n terms of 1ts gra1n s1ze and the proport ion of leucocratiC matenal that 1t conta1ns; such charactenstics are controlle d by the compos it1on and the metamorphi c textures that developed 1n the protolith .

Locat1on : Mount Stafford, Australia . Rock type: part1ally segregated neosome 1n a metatex1te m1gmat1te; th1nly bedded pelit1c protolith, partial melt1ng at T ca. 750°C. P 3.2 kbar. on the boundar y between local metamo rphic zones 3 and 4. Scale: the pen is 16 em long./mage and caption: R1chard White.

Further read1ng: see Fig. B 17.

Arias of Migmat ites

Fig. 8 19. Close-up of a part1ally melted metapelite 1n wh1ch the melt did not separate s1gn1ficantly from the residuum. The result IS a migmat1te 1n wh1ch the neosome IS w1thout consp1cuous leucosome or melanosome. However, 1t has a much coarser grain-size and a different microstructu re t han a metapelite. The dark-colored w1sps are the rema1ns of psammit ic beds t hat have become folded. N ot e the absence of either a foliat ion or evidence for folding w ithin t he petite-derived neosome. The melt ing react ion in t his quartz-bearing metapelite w as biot ite + sillimanite + quartz = cordient e + K-feldspar + melt. Location: Mount Stafford, Aust ralia. Rock type: nebuht1c diatex1te m1gmat1te; pelit1c protolith, part1al melt1ng at T ca. 700°C, P 3.2 kbar, on the boundary between local metamorphic zones 2 and 3. Scale: the ruler IS 15 em lang./mage: E.W. Sawyer.

Further readtng: see Fig. B 17.

103

NEOSOME WITHOUT DISTINCT LEUCOSOME OR MELANOSOME

104 -----------------------------Figure

10

solid and melt products of the melt1ng react1on. It can be Fig. 820. Th1s photograph shows an outcrop of metatexdiv1ded into leucosome and melanosome only w1th some lte m1gmatite that contams patches of coarser-grained, difficulty because the two are 1nt1mately distnbuted. The nonfoliated neosome 1n a matnx of finer-gra1ned, foliated, of the neosome that consist entirely of quartz, plaparts leucogranite gneiss. The neosome cons1sts of wreguand K-feldspar could be termed leucosome. Other gioclase, lar patches and veins. some of wh1ch have locally JOined of large 1rregular poikdoblasts. or aggregates, consist parts together to form a vague, net-like pattern. Neosome 1s of garnet intergrown with quartz, domains that could be readily distinguished from the host gneiss by its coarser described as melanosome. grain-size, and by its truncation of the biotite-defined layering in the gneiss. The neosome contains conspicuous Location : Broken Hill area, New South Wales, Australia . crystals of garnet in a l1ght-colored, quartzofeldspathic Rock type: net-structured metatexite m1gmatite; quartzomatnx: the garnet crystals tend to occur toward the center feldspathic protolith, anatexis 1n the upper amphibolite of the patches of neosome and are commonly mtergrown facies, T ca. 800°C, P 4 5 kbar. Scale: the ruler is 15 em w1th quartz. There are no rims of melanosome around the long. /mage: E.W. Sawyer. neosome. There 1s no garnet 1n the host gne1ss, and there 1s no b10t1te 1n the neosome. Th1s is an 1mportant observation: Further readmg: Braun, 1., Ra1th. M. & Rav1ndra Kumar, G.R. from 1t. one can infer that the melt1ng react1on 1nvolved the ( 1996): Oehydrat1on-meltlng phenomena 1n leptyn1t1C 1ncongruent breakdown of b1ot1te 1n a reaction s1m1lar to gne1sses and the generat1on of leucogran1tes: a case study b1ot1te + plag1oclase + quartz = garnet + K-feldspar + melt. from the Kerala Khondahte Belt. southern lnd1a. Journal of In that case then, the fol1ated leucogneiss represents the Petrology 37, 1285 1305. assemblage of reactant m1nerals and could, consequently, White. R.W., Powell, R. & Halpin, j.A. (2004): Spatiallybe v1ewed as both the protolith and the paleosome that focussed melt formation in aluminous metapelites from had temperatures would have melted, if the metamorphic Broken Hill, Australia . Journal of Metamorphic Geology 22, been higher. Because the leucogranite gneiss predominates, anatexis was not particularly advanced 1n this migmatite. 825- 845. The neosome, on the other hand, represents both the

A tlas of Migmatites t

Figure

2

Fig. 821. The light-colored neosome 1n th1s m1gmat1te has 1n a b1otite gne1ss of sem1pelittc bulk composition, and cons1sts of streaks and patches of coarse-gra1ned quartz and feldspar with large crystals of garnet. There IS ne1ther melanosome nor leucosome. The neosome has ind1st1nct or slightly diffuse boundaries. The minerals 1n the neosome have no shape-pr eferred onentation, and there is no foliation. T he streaks of neosome are parallel and follow, approximately, the foliation in t he host paleosome, giving an overal l stromatic, or layered, appearance to the migmatite. Some neosome 1s locally discordant. developed

Locat1on: Namaqualand, South Africa. Rock type: stromatic metatex1te m1gmat1te; semipelitic protohth part1ally melted 1n the granulite faoes. Scale: the hammer head IS 10 em across. Image and caption: Dave Waters. Further readmg: Waters. D.J. (1988): Part1al melt1ng and the format1on of granulite facies assemblages 1n Namaqualand, South Afnca. journal of Metomorph1c Geology 6, 387 404.

lOS

NEOSOME W ITHOUT DISTINCT LEUCOSOME OR MELANOSOME

106 Figure

ll

Locatron: Sand R1ver Gne1sses. Causeway locality, South Fig. 822. The neosome tn th1s photograph has develAfnca. Rock type: metatex1te m1gmat1te; leucocratiC, orthooped at the end of a boud1n denved from a res1ster layer gneiss protolith, part1ally melted 1n the granulite facies. of slightly darker-colored 1ntermed1ate gne1ss (paleosome) Scale: the pocket kn1fe IS II em long. Image: E.W . Sawyer. hosted by a grey leucocrat1c gne1ss. The lighter-colored gne1ss has locally undergone partial melt1ng and has generated neosome that dom1nantly cons1sts of feldspar and quartz, but t hat also contains conspicuous, euhedral crystals of pyroxene and some of amphibole. Two charact eristics in particular ser ve to distinguish the neosome in this migmatite from the paleosome; it is considerably coarser grained, and it does not have a foliation. Although the neosome is lighter in color than the paleosome and could, therefore, be described as leucocratic, 1t should not be called leucosome because 1t contains a h1gh proportion of ferromagnes1an m1nerals that are the product of the melt1ng reaction. The melt fraction thus d1d not segregate from the solid fract1on (res1duum), and hence the term leucosome IS lnappropnate for this migmatite; neosome 1s correct. Th1s photograph illustrates a small part of an extens1ve pavement on which pre-anatectic compositional layenng and cross-cutting dike relations can be seen in the orthogneiSS paleosome; hence, this is a metatexite migmatite.

r

t

t

A tlas o f M igmatites

Figure

23

t

•

I t

Fig. 823. This migmat1te cons1sts of two ma1n parts, one dark-colored and the other light-colored, and is a good example of a case in which the leucocrat1c and melanocratic parts present do not correspond wholly to the usual genet1c relationship found between leucosome (melt-denved part) and melanosome (res1dual part) 1n m1gmat1tes. The dark-colored part IS a sill1man1te-rich, alum1nous metapelit1c schist or gne1ss that conta1ns the mineral assemblage garnet + cordierite + sillimanite + biot1te + quartz + plag1oclase + llmen1te. The light-colored part conta1ns large, prom1nent poikiloblasts of garnet in a med1um-gra1ned leucocratiC matnx that is rich 1n K-feldspar and quartz. The large po1kiloblasts of garnet occur only in the leucocrat1c matenal. whereas cordierite occurs only 1n the dark metapelit1c gne1ss. The leucocrat1c matenal is inferred to have formed 1n Situ around the grow1ng poikiloblasts, and IS dom1nantly composed of peritectiC K-feldspar, together w1th some K-feldspar, quartz and plag1oclase that crystallized from the anatect1c melt. Hence, the light-colored part of th1s m1gmat1te conta1ns both the products of a pentect1c react1on (K-feldspar and garnet), whiCh would be consid ered part of the res1duum assemblage, and the products of the crystallization of t he anatectiC melt (K-feldspar, quartz, and plag1oclase). Consequently, the leucocrat1c port1on of the migmatlte represents neosome that has not undergone

much, if any, separation of melt from residuum. The aluminous metapelite that makes up the dark (melanocratic) part of the outcrop 1s not paleosome, and ne1ther IS 1t wholly res1duum, as 1t does not contain the garnet porphyrob1asts or the pentect1c K-feldspar: 1t may most closely correspond to the protolith. However, there IS no doubt that the light color of the neosome. due pnnc1pally to the elim1nat1on of d1ssem1nated biotite, IS the dominant characteristiC of this migmat1te in outcrop: consequently, these domains have been referred to as "leucosome".

Locat1on: Round H1ll, Broken H1ll area, Australia. Rock type: metatex1te m1gmat1te: aluminous metapelite protolith partially melted at T 800°C and P 5- 6 kbar. Scale: the hand lens IS 3 em long. Image and caption: Richard Wh1te. Image previously published as fig. 2a 1n White et al. (2004) and reproduced with the permiSSIOn of Blackwell Publ1sh1ng. Further readmg: White. R.W., Powell, R. & Halpin, J.A. (2004): Spat1ally-focussed melt format1on 1n aluminous metapelites from Broken Hill, Australia . Journal of Me!Omorph1c Geology 22, 825 845.

107

NEOSOME WITHOUT DISTINCT LEUCOSOME OR MELANOSOME

108 -----------------------------F1gure

24

Fig. 824. This m1gmatite is from the same area as that B23. It too conta1ns dark-colored 1lmeniteplag1oclase-quartz-b1otite silhman1te cordiente garnet aluminous metapelite, but there are some lighter-colored layers 1n it that represent th1n, alum1num-poor beds. Domains of leucocratiC neosome are abundant. and they contain conspicuous poikiloblasts of garnet. However. there are many more small grains of garnet scattered throughout the neosome in this migmatite than in that shown in Fig. B23. As the garnet poikiloblasts do not occur in the host aluminous metapelite, the aluminous metapelite in th1s case may correspond more closely to protolith, rather than residuum or paleosome. The neosome

in

F1g.

is inferred to have formed m s1tu around the growing poikiloblasts of garnet and IS dominantly composed of peritectiC K-feldspar with lesser amounts of K-feldspar, quartz, and plagioclase that crystallized from an anatectiC melt. A t1ght F antiform folds both the neosome and the compo2 Sitional layering in the alum1nous metapelite; an 52 foliation defined by an alignment of b10t1te and s1llimamte occurs in the aluminous metapelite and is oriented parallel to the axial plane of t he F2 fold. In contrast. the 52 structure in the neosome is a fracture cleavage, and this is consistent with neosome already being 1n the solid state during the Dl event.

Locat1on: Round Hill, Broken Hill area, Australia. Rock type: folded metatex1te m1gmat1te: alum1nous metapelite protolith, anatexis at T 800°C and P 5-6 kbar. Scale: the pen is 14 em long. Image and capt1on: R1chard White. Image previously published as fig. 2b 1n Wh1te et al. (2004) and reproduced w1th the permiSSIOn of Blackwell Publishing.

Further readmg: same as for Fig. B23.

A tl as of Migmatites

Figure

Fig. 825. Th1s photograph shows a m1gmat1te developed 1n a strongly foliated gamet-K-feldspar-plagioclase- s1lhman1tequartz biot1te alum1nous metapelite gne1ss. The neosome cons1sts of several diffuse, almost skeletal, coarse-grained patches that have a core of p1nk garnet 1ntergrown w1th quartz and K-feldspar. The garnet-nch cores are surrounded by a nm cons1st1ng of roughly equal proportions of K-feldspar, quartz, and plagioclase. Locally, some coarsegrained b1ot1te replaces garnet and K-feldspar 1n the neosome. The neosome 1s interpreted to be the m s1tu product of the 1ncongruent melting of b1ot1te by a react1on such as b1ot1te + silliman1te + quartz + plag1oclase garnet + K-feldspar + gran1tic melt. Some of the patches of neosome have a h1gh proportion of garnet and K-feldspar (the solid products of the melting react1on. cons1dered "peritectic"

=

phases by some authors), whiCh suggests that these reg1ons of neosome have lost the1r anatect1c melt. This interpretation IS supported by the presence of coarse-gra1ned. gran1t1c leucosome w1thout garnet (e.g., center of the top edge) located 1n dilatant structures with1n the gne1ss, wh1ch are Interpreted to have crystallized from the segregated anatectic melt. T hus, t he migmat1te in th1s photograph shares

the diffuse and patch morphology of the neosome shown 1n Fig. B 15, but also shares the charactenst1c of neosome formed by the Incongruent meltmg of b1ot1te shown 1n Figs. B20-B24, or of amphibole. Location:

Sa1nt-Fulgence, Grenv1lle Province, Quebec, Canada. Rock type: patch metatexite m1gmatite: alum1nous pehte protolith, granul1te-faoes anatex1s T 800- 850°C, P 5 8 kbar. Scale: the w1dth of the photograph corresponds to 16 em. /mage: E.W. Sawyer.

109

NEOSOME W ITHO UT D ISTINCT LEUCOSOME OR MELANOSOME

110 -----------------------------Figure

826

Fig. 826. The leucosome 1n this m1gmat1te IS qUite evident. Some domains are parallel to the bedd1ng (stromatiC morphology), whereas others are discordant (d1latant morphology), but most of them have somewhat d1ffuse borders. In contrast. there are no conspicuously melanocratlc parts tn the migmat1te. such as melanosome around a patch of leucosome. However, the m1neral assemblage (plagioclase + biotite + quartz + garnet), modal proportions, and the whole-rock compos1t1on of the rock around the leucosome indicate that it IS the res1duum left after the extraction of melt from a protolith of semipel1t1c bulk composition. Therefore, the m1gmat1te shown 1n this photograph cons1sts ent1rely of neosome. The melt fract1on. or part of 1t, is represented by the leucosome, but the rest of the rock 1n the photograph IS not paleosome, desp1te 1ts med1um (mesocrat1c) color. It 1s the res1duum left after the melt was segregated.

Locat1on: Nem1scau Subprovmce, northern Quebec, Canada. Rock type : metatextte mtgmattte: sem1pelite protolith partially melted under lower-granulite-faCies condtttons, T ca. 800°C. P ca. 5 kbar: Scale: the ruler IS IS em long.

Image : E.W. Sawyer.

A rias of Migmat ites

Fig. 827. Thts photograph shows the neosome developed tn the neck regton of a boudtnaged layer 1n a metamafic paleosome (P). The macroscopic aspects of the three stages of melt movement that occur in mtgmatttes are well illustrated. Therefore, this example serves as an introduction to "open-system" mtgmatites. The domatns of leucosome t race out a complex pat tern of linked segments that are parallel to the layering, o rthogonal to t he layenng, and 1n a conJugat e set oriented at about 4S0 to t he layenng. The onentations of t he dilat ational sites are consistent with layer-parallel ext enston occurnng during anat exis. However, the locat1on of melanosome (M) IS more restncted. It generally forms layers oriented parallel. or subparallel, to the foliat1on. Thts arrangement suggests that certain sublayers have lost much more melt than others: tndeed, some may not have lost melt. The expelled melt moved through the hnked system of dilatant structures and may ulttmately have dratned out from the layer at the boudin neck. Thus, 1n general, leucosome represents melt frozen 1n t he channel system, and not necessanly the sites where melttng occurred . Some isolat ed, Irregularly shaped domains of leucosome w ith melanosome around t hem (just less than half way down on t he right-hand edge) are 1n s1tu domains of leucosome, and are loci w here melt has col lected after a centimeter or so of porous flow. The domains

of leucosome that form the linked array are examples of Insource leucosome. N ote that some parts of these domatns do not have a prom1nent melanosome. The large tnangu lar doma1n of leucosome 1n the center of the boudin neck m1ght have been part of the system of larger channels t hat belonged to t he tertiar y stage of melt movement t hrough wh ich melt moved out of its source layer and fed leucocratic dikes elsewhere in t he migmat it e. T he top end of the scale rests on a pegmatite vein, not leucosome.

Location: Abitibi Subprov1nce. south of Chibougamau, Quebec, Canada. Rock type: metatex1te m1gmatite: metamafic protolith partially melted at T 800 8S0°C and P 8 10 kbar. Scale: the ruler is IS em long. /mage: E.W. Sawyer. Further readmg: Sawyer, E.W. (1991): Disequilibrium melttng and the rate of melt res1duum separat1on dunng migmatizatlon of mafic rocks from the Grenv1lle Front, Quebec. journal of Petrology 32 , 70 1- 738. Sawyer, E.W . (200 1): Melt segregat1on 1n the cont1nental crust: distribut ion and movement of melt in anatect ic rocks. journal of M etamorphic Geology 19, 29 1 309.

Ill

NEOSOME IN OPEN -SYSTEM M IGMATITES

112 ---------------------------Figure

828

Fig. 828. At first glance, th1s outcrop appears to show a metamafic rock conta1ning many d1scordant fels1c ve1ns 1n fractures; therefore, one may 1nfer that 1t IS not a m1gmatlte. However, m a few places, the presence of 1n Situ neosome (N), cons1st1ng of coarse-grained melanosome that conta1ns minor, th1n (<2 mm) leucosome, indicates that part1al melting and separation of the melt fraction from the solid have occurred and that t he rock is indeed a migmatite. Most of the rock can be considered to be paleosome. In general (e.g., just above and to the left of the scale), the passage from paleosome to melanosome is gradational, without the presence of leucosome, and this suggests the loss of melt on a local scale. The strik1ng aspect of this outcrop is the abundance of medium-gramed, leucocrat1c ve1ns that cross the paleosome, melanosome and m Situ neosome. These ve1ns lack adjacent melanosome, wh1ch suggests that they result from fractunng and InjeCtion of melt. Thus, the history of th1s migmatite 1nvolved part1al melting, then loss of melt, and finally, addit1on of melt.

Locat1on: Ab1tib1 Subprovince, south of Ch1bougamau, Quebec, Canada. Rock type: ve1ned metatexite m1gmat1te; metamafic protohth part1ally melted at T 800- 850°C and P 8- 10 kbar. Scale: the ruler 1s 15 em long. /mage: E.W. Sawyer.

Further readmg: see F1g. B27.

t

t

A tl a., of Migmati tes

Figure

19

t

• t

Fig. 829. Th1s m1gmat1te was denved from the same layered metamafic rock as in Fig. B27, but at a locality that underwent a h1gher degree of part1al melting and a much greater loss of melt. The migmat1te IS stromatic in that w1de, dark bands w1th the m1neral assemblage hornblende + plagioclase + garnet + clinopyroxene alternate w1th th1n, light-colored plag1oclase-nch layers. The bulk composition of the m1gmatite is melt-depleted relat1ve to the likely protolith composition, confirming its residual appearance. The outcrop contains a linked array of very narrow, d1scordant stnngers of leucosome (indicated by the arrows) that conta1n discontinuous pockets and scattered 1nd1v1dual crystals of subhedral plagioclase, and very little quartz. These stringers of 1n-source leucosome cons1st of accumulations of plagioclase, and are interpreted as the collapsed (deflated) remains of a channel system through which the melt generated during anatexis moved out of its residuum. This is a melt-depleted migmatite. The preservation of the compos1t1onal layenng that predates the partial meltIng Indicates th1s to be a metatex1te m1gmat1te. In choos1ng another descnptive term for this migmatite, one has to decide which trait to emphasize. If morphology is important. then "stromatic metatexite migmatite" might be appropriate. However; if the bulk composition of the migmatite is important, then "melt-depleted" or "residual" would be appropnate.

Locatton: Abitibi Subprov1nce, south of Chibougamau, Quebec, Canada. Rock type: melt-depleted metatexite migmatite; metamafic protol1th part1ally melted at T 8008500C and P 8 10 kbar. Scale: the ruler IS IS em long. /mage: E.W. Sawyer. Further reading: see F1g. B27.

113

NEOSO M E IN OPEN-SYSTEM MIG MATITES

114 - - - - - - - - - - - - - - - Figure

30

Fig. 830. Thts close-up photograph shows a patch of neosome developed tn a grey garnet b1ot1te plagtoclase quartz alumtnous metapsammtte. The neosome has two parts. An tnner portton conststs of a stngle, large brown poikiloblast of garnet that is 1ntergrown wtth quartz (see Ftg. F74 for an example of the mtcrostructure of the garnet quartz intergrowths), and an outer part that consists predominant ly of medium-grained K-feldspar, wtth a small proportton of quartz and plagtoclase. The neosome is interpreted to be the solid products (i.e., garnet + K-feldspar) of a melt-producing, btotite-breakdown reactton. The absence from thts neosome of a leucocratic part that has a gran1t1c composttton leads to the 1nterpretat1on that the melt fractton generated by the incongruent breakdown of btottte has mtgrated away. Consequently, the rematntng neosome

conststs essenttally of restduum. Location: Wuluma Hills, Arunta Inlier, Australia. Rock type: melt-depleted patch metatextte mtgmattte: alumtnous psammite protolith, partially melted at T 825 875°C. P ca. 5 kbar. Scale: the ruler IS graduated in em and mm. /mage: E.W. Sawyer.

t

t

A ri a> of Migmatites

Figure

t

• •

•

Fig. 831 . The large-scale morphology of this outcrop shows a medium-grained, grey schistose rock that cont a1ns a few thin ve1ns of gran1te. The gran1te veins occur 1n two sets that are discordant to the foliation, and a locally preserved compositional layenng 1n the host. Together, both sets of ve1ns comb1ne to form a very open, net-like pattern. Furthermore. the granite ve1ns have no melanosome, or mafic selvedge, assoc1ated w ith them. Consequently, the veins may be thought of as injected along two sets of fractures in the host. Hence, at the scale of th1s photograph, there IS no ev1dence for part1al melt1ng. and 1t 1s not at all ev1dent that th1s IS an outcrop of m1gmat1te. The ev1dence for part1al melt1ng comes from the close-up, detailed exam1nat1on of the contacts between the ve1ns and the host. as shown in the next figure (Fig. B32) .

Location: Broken Hill area, New South Wales, Australia. Rock type: metatexite migmat1te: quartzofeldspathic protolith (Hores gne1ss). anatexis in the upper amph1bohte fac1es, T ca. 800°C, P 4 5 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

liS

NEO SOME I N OPEN -SYST EM MIGMATITES

116 -----------------------------Figure

31

'

•

1

, s'•' . . ". . _ . , . CENTIMETRE • 0! ,, Fig. 832. Th1s close-up shows the area close to the left edge and ust above half way up 1n Fig. B31 The country rock to the · gran1t1c ve1n" 1s garnet biotite quartz K-feldspar plag1oclase sch1st. or perhaps gne1ss, 1n wh1ch garnet occurs as scattered porphyroblasts that atta1n 6 mm across. The garnet porphyroblasts do not have a quartzofeldspath1c nm, wh1ch suggests that they grew as a result of a subsolidus prograde react1on, and not as a result of a melt-produong react1on. Thus, the grey host rock does not appear to have undergone part1al melting. Turning to the leucocratic "ve1ns," there are four key observations. (I) The contacts w1th the grey host are diffuse, not sharp, and therefore a relat1onsh1p inconsistent with a ''ve1n'' of granitic melt or magma InJected into subsolidus rocks, but consistent w1th 1n Situ formation. (2) The size and d1stnbution of garnet porphyroblasts 1n the "ve1ns" are s1m1lar to those 1n the grey host: the gra1ns of garnet 1n the "veins" are thus nhented from the host. and as the "veins" formed, they overgrew or engulfed the gra1ns of garnet. (3) The central parts of the "ve1ns" are quartzofeldspath1c. more leucocratlc, and have a gra1n s1ze greater than at the marg1ns or in the host: th1s suggests a greater fraction of melt 1n the center of the "veins" than at the marg1ns. (4) In many places, the garnet porphyroblasts located in the "ve1ns" are completely. or extensively, replaced by biotite, although t here are places where t h1s has not occurred; overall, more H10 seems to have been available to alter anhydrous minerals w here t he "ve1ns" formed, compared to the matnx. These observations. together w1th those made at the larger. outcrop scale (Fig. B31). lead to the 1nterpretat1on that the "ve1ns" are in fact 1n Situ stnngers of leucosome formed by

H. 0-fluxed part1al melt1ng. When the host rocks were at a h1gh temperature. just below the1r solidus, an aqueous flu1d entered them through a system of fractures and caused HL0-fluxed partial melt1ng of the quartz + feldspar assemblage 1n the host. Such H 0-fluxed partial melt1ng of quartz+ plag1oclase + K-feldspar 2 1n the protolith also expla1ns the lack of melanosome assooated w1th the leucosome. The narrowness, together w1th the w1de spaong of the stnngers of leucosome, suggests that comparatively little flu1d entered the rocks. so that part1al melting was only able to occur adjacent to the fracture. The degree of partial melt1ng decreased to zero 1n the wallrocks a cent1meter or two from the fracture. The garnet porphyroblasts that were present where partial melting occurred were replaced by b1otite. either as the 1ngress of fluid occurred, or as t he anatectic melt crystallized. Because of the small amount of aqueous fluid Introduced and the corresponding small extent of partial melt1ng that it caused, the distnbution of the leucosome closely m1rrors the fracture network through wh1ch the aqueous flu1d was transported. The occurrence of part1al melt1ng 1n th1s outcrop 1s the ev1dence required to call th1s a m1gmat1te. In that case, the grey host rock to the tn Situ leucosome IS paleosome. However. at the t1me of partial melting. the grey host rock plus the aqueous flu1d was the protolith. Location: Broken Hill area. New Sout h Wales. Australia. Rock type: metatex1te migmat1te; quartzofeldspathic metavolcanic protolith (Hores gne1ss). anatex1s 1n the upper amphibolite facies, T ca. 800°C. P 4 5 kbar: Scale: the ruler 1s IS em long. /mage: E.W. Sawyer:

Atl as of M igmat ites

Figure

33

t

Fig. 833. The morphology of the rocks 1n the right half of the photograph under the maps 1nd1cate a metatexite m1gmat1te w1th a layered or stromatiC structure that IS almost trans1t1onal to a diatexite migmat1te. The layered structure in the migmatite IS due to alternat1ons of leucosome, melanosome, and mesocrat1c paleosome. Petrographic and geochemical analyses of these rocks 1nd1cate that the degree of 1n s1tu part1al melt1ng 1n the fert1le layers was only about 20 vol .%, but some of the leucocrat ic layers may have formed from InJected anatectiC melt. The pen rests upon the most remarkable feature of th1s m1gmat1te, a quartz plagtoclase K-feldspar-kyantte-b tottte garnet granofels that, because tt truncates the layering 1n the host metatexite migmatite (e.g., in center of the photograph), could be interpreted as a dike tntruded after the formatton of the layering tn 1ts host. A petrographtc (see Fig. F46 for a photomtcrograph of thts rock) and geochemtcal analysts of the garnet granofels tn the dike are consistent wtth t he rock betng the residuum left after more than 50% partial melt ing of a pelit1c protoltth and the almost complete separation of the melt fract1on from the soltd. The question IS, how did the res1duum (wh1ch IS normally cons1dered as rema1n1ng m Situ) from a high degree of partial melting get to be 1n a host that experienced a far lower degree of partial melt ing? One scenano IS to move the res1duum. For th1s to happen, a magma

(melt + res1duum) would have to have formed elsewhere, where the comb1nat1on of rock fertility and metamorphic temperature enabled >50% dehydrat1on-1nduced melting to occur, and then be 1ntruded into the layered metatexite migmat1te. In th1s scenano, the melt fract1on could have separated e1ther after emplacement of the magma mto th1s outcrop, leav1ng beh1nd the entra1ned restdual matenal and a l1ttle anatectic melt trapped between the garnet crystals, or the melt fract1on may have progressively separated from the res1duum as the magma moved through the transferdike system from 1ts source reg1on. It may even be poss1ble that the garnet-nch res1duum 1tself was capable of granular flow (garnet crystals are "rounded" 1n shape) 1f all gra1n boundaries were in contact with melt. and if it was able to intrude as a dike; th1s may occur w1th as little as 7% melt present (Rosenberg and Handy 2005). In a second scenano, the res1duum is produced m s1tu. T his could be accomplished if the layered metatexite had stopped dehydration melting and was cooling, but was still at a temperature above its solidus for H 20 -present melting. The 1ntroduct1on of H 0 2 along a fracture then may provoke further melting through H 20-present react1ons; the degree of melt 1ng that occurs depends on how much H 20 was Introduced into the rock. Separat1on of the melt fraction results in residual rocks with a cross-cutt1ng geometry, s1mply because they trace out the

117

NEOSOME IN OPEN-SYSTEM MI GMATITE S

118 ------------------------------

fracture through wh1ch the H 20 entered the rock. In e1ther scenano, th1s m1gmatite has lost melt and is an example of an open-system migmatite.

Locat1on: Abaukoma, Yaounde. Cameroon. Rock type: dike-like residuum in a metatexite migmatite; metapelitic and metagreywacke protoliths, anatexis at T 800- 850°C. P I 0- 12 kbar. Scale: the pen is 15 em long. Image: Pier re Barbey. Previously published as fig. 3a in Barbey et al. ( 1990) and reproduced with the permission of Oxford University Press.

Further reading: N zenti, J.P., Barbey, P., Macaud11ere, J. & Soba, J. (1988): Origin and evolut1on of the late Precambnan highgrade Yaounde gneisses. Precambnan Research 38,91 - 109. Barbey, P., Macaud1ere, J. & Nzent1, J.P. (1990): Highpressure dehydration melting of metapelites: ev1dence from migmatites of Yaounde (Cameroon). Journal of Petrology 31 ,

401 - 427.

Arias of Migmatites

F1gure

34

• t

• •

Fig. 834. The paleosome 1n th1s m1gmat1te IS a light grey b1ot1te quartz feldspar gneiss. The neosome compnses leucocratic layers, conta1n1ng quartz. feldspar. and m1nor amounts of b1otite, that are lined on each s1de by very narrow, melanocrat1c, biotite-nch and mlcroclinepoor borders. Result s of mass-balance calculations on these m1gmat1tes (Olsen 1982, 1984, 1985) indicate that the domains of leucosome are too large relative to the1r melanocratic borders to consist wholly of melt derived from t he melt-depleted melanosome; hence, one may conclude that the neosome formed in an open system. Some doma1ns of leucosome contain as little as 5% fels1c material added from an external source to the locally derived melt, whereas other domains of leucosome conta1n as much as 90% added material. Thus, the neosome 1n th1s m1gmat1te formed by a comb1nat1on of mechanisms; some fels1c matenal was Introduced from an external source as ve1ns. and some partial melt of local origin segregated from the host 1nto the ve1ns, thereby forming the melanocrat1c borders. It IS the presence of the local melt that makes t h1s a mlgmatite. A foliat ion defined by biotite 1n the grey gneiss is oriented parallel to the t race of the axial planes of the folds that deformed the leucosome.

Locat1on: Colorado Front Range, U.S.A. Rock type: metatexlte m1gmat1te; b1ot1te- quartz feldspar gne1ss protolith metamorphosed at T 650 700°C and P 4 6 kbar. Scale: the coin 1s 19 mm across. Image and capt1on: S.N. Olsen. Further reading: O lsen, S.N. (1987): The composition and role of the fl uid in migmatites: a fluid inclus1on study of the Front Range rocks. Contributions to M1neralogy and Petrology 96, 104- 120. Olsen, S.N. & Grant. j.A. ( 1991) : lsocon analysis of migmatizatlon in the Front Range Colorado, U.S.A. journal of Metamorphic Geology 9, 151 164.

119

VARIATIONS W ITHIN NEOSOME

120 ----------------------------Figure

35

Fig. 835. Th1s polished slab of a m1gmatite has been chemIcally etched and stained to color the K-feldspar yellow. The sta1ning process reveals that K-feldspar IS not uniformly distnbuted throughout the leucosome; some parts lack K-feldspar altogether. The polished surface also reveals thin, melanocrat1c, biotite-rich layers located between the leucosome and the mesocratic, finer-grained paleosome. These melanocratic layers are far too thin to have supplied all the quartzofeldspathic material present in the leucosome, although they may have supplied some of it. This relationship suggests that some of the leucosome material has been intruded into the migmatite sample. An alternative possibility IS that the melanocratlc layer did not supply any matenal to the leucosome at all, but formed as a result of the 1nteract1on between the InJected melt (or an aqueous flu1d denved from the crystallization of the leucosome) and the adJacent paleosome. In th1s case. the b1ot1te-nch layer could be called a mafic selvedge.

Locat1on: Baume m1gmat1tes, Ardeche, France. Rock type: metatex1te migmat1te; quartzofeldspath1c gne1ss protolith partially melted under upper-amphibohte-fac1es condit1ons at T 700 ± 50°C and P 3 kbar. /mage: P1erre Barbey.

Further reading: Weber, C. & Barbey, P. ( 1986): The role of water, mixing processes and metamorphic fabric in the genesis of the Baume migmatites (Ardeche, France). Contributions to Mineralogy and Petrology 92, 481 - 491.

t

Atlas of Migmatites

Figure

36

t

• •

• t

Fig. 836. This stromatic migmatite shows some differences in morphology and microstructure among the various constituent batches of leucosome. The isoclinally folded leucosome in the center is finer gratned than the others, and has rather smooth margtns agatnst the paleosome. Although the leucosome at the top also has smooth margins, it IS nottceably coarser grained than the folded one. Finally, the thtckest leucosome at the bottom has the largest grain-size, and has margins that are conspicuously irregular in outline. Other important features within the outcrop are the followtng: (I) the isoclinally folded leucosome is an F2 feature and rotates the S fohatton, (2) elsewhere, the leucosome at the top cuts across the ma1n 52 layering or foliation, and (3) the coarse leucosome at the bottom grades into a small pegmatitic pod that contatns rotated enclaves that preserve F3 structures. On the basis of these observations, the differences among batches of leucosome may indicate that they belong to different generations relative to the deformation events that have occurred in the mtgmatite. As an alternattve, 1f they are all of the same generation, then the dtfferences in grain stze and mtcrostructure may indicate that each had a different fraction of melt present at the t ime of deformation.

Locat1on: Glenelg River Complex, Victoria, Australia. Rock type: metatexite migmatite; quartzofeldspathic schist protolith, partially melted at T 650-680oC. P 4 6 kbar. Scale: the pencil IS IS em long. Image and capvon: Tony I.S. Kemp. Further readmg: Kemp, A.I.S. & Gray, C.M. (1999): Geologtcal context of crustal anatexis and granit1c magmatism in the northeastern Glenelg River Complex, western Victoria. Australian journal of Earth Sc1ences 46 , 407-420. Kemp, A.I.S. & Hawkesworth, C.j. (2003): Granitic perspective on the generatton and secular evolutton of the conttnental crust. In The Crust. Treatise on Geochemistry 3 (R. Rudntck, ed.). Elsevier Sctence, Oxford, U.K. (349 41 0) .

121

VARIATIONS WI T HI N NEOSOME

122 ----------------------------Figure

37

Fig. 837. Th1s close-up shows two different types of neosome 1n a migmat1te w1th a metamafic paleosome. The neosome at the center of the photograph conta1ns large, euhedral crystals of clinopyroxene 1n a leucocrat1c matnx of finer-gra1ned quartz and plag1oclase. The neosome IS Interpreted to be the m s1tu product of a small degree of part1al melt1ng from the incongruent breakdown of amphibole 1n the metamafic host. Locally, orthopyroxene is present with the clinopyroxene. Larger areas of pink-colored leucosome fiank the m situ clinopyroxene-bearing neosome. The pink leucocratic veins are granitic in compos1t1on and conta1n quartz. K-feldspar, and plagioclase w1th m1nor amounts of biotite and garnet. The leucocrat1c ve1ns are surrounded by a halo, or mafic selvedge, 1n which the metamafic host has been modified; hornblende replaces clinopyroxene (e.g., hornblende nm on clinopyroxene 1n the m s1tu neosome). and some hornblende in the metamafic rock IS replaced by b10t1te. The metamafic rock occurs as a large (10m) boudm in pelit1c metasediments. The metapelites have undergone a greater degree of partial melting, and have lost much of the melt they generated. They now have a res1dual assemblage of minerals, quartz + plagioclase + biotite + garnet + si llimanite + K-feldspar ± cordierite (e.g.. Fig. B38). During regional deformation, the metamafic rock was considerably more competent than the surrounding metasedimentary

rocks and developed a senes of fractures, and some gran1t1c melt from the metapelites m1grated 1nto these. An aqueous fiu1d was exsolved from the p1nk leucocrat1c veins as they crystallized, and as a result. the metamafic rock around them underwent rehydration and some K-metasomat1sm that produced the hornblende nms and the b1otite. Province, Quebec, Canada. Rock type : metatexite migmatite; metamafic protolith, anatexis at lower-granulite-facies conditions, T 800-850°C and P 5 8 kbar. Scale: the ruler is marked in

Locat1on : Saint-Fulgence, Grenville

em. Image: E.W. Sawyer.

t t

t

Atlas of Migmat ites

Figure

38

• • •

• t

• t

•

Fig. 838. The grey part of this m1gmatite contains two rock types: a K-feldspar garnet-silliman1te-biot1te plagioclase quartz metapelite (Pe) that has prom1nent dark foliae nch 1n b1otite and sillimanite, and more mass1ve, lighter-colored garnet- biotite- plagioclase- quartz metasedimentary material (Ps) of init1ally psammopelitic composition. The modal mineralogy of the metapelite layers ind1cates that they are res1dual after the loss of anatectic melt , but it is uncertain whether the psammopelitic layers have lost melt. T he morphology of the portions derived from the anatectic melt 1n the different layers of this migmatite is Interesting. The metapelitic layers contain large, prominent porphyroblasts of garnet and cordiente (Crd), some of which are partially surrounded by a narrow rim, or moat, of leucosome. Some of the garnet crystals, but especially the cordierite, are poikiloblasts intergrown with quartz and feldspar; this microstructure suggests that the leucosome and garnet (or cordierite) are the melt and solid products, respectively, of the incongruent melting of biotit e. A possible react1on for these rocks IS biotite + sillimanite + quartz + plagioclase garnet (or cordierite) + K-feldspar + melt. The lent1cular shape of the garnet, or cordierite, plus leucosome pods, their sharp

=

contacts with the residual met asedimentary material, and the low ratio of leucosome t o garnet (or cordierite), suggest that these domains have lost melt dunng the sheanng

event experienced by the migmatite. Lens-shaped domains of leucosome (L) that have sharp edges, but lack either garnet or cordierite, also occur with1n the metapehtic layers; these may represent some of the escaped melt. The absence of a foliation in the leucosome, and the presence of crystal faces on cordiente and feldspar where aga1nst quartz, suggest that melt1ng and sheanng wer e synchronous. The neosome (N) in the psammopelitic layers consist s of very small euhedral crystals of garnet in a quartz + plagioclase + K-feldspar groundmass. The melt1ng react1on thus was essent ially the same as that which occurred 1n the pelite layers. However, the morphology of the neosome is different; the ratio of melt to garnet was much h1gher, and the domains of neosome have a diffuse marg1n. Both these features are consistent with in s1tu formation of the neosome and no loss of melt. Thus, the melting react ion was essentially the same in both types of layer, but the final morphology of the migmatlte that developed in each layer is controlled by the relative competency of each lit hology.

Location: Sa1nt-Fulgence, Grenville Province, Quebec, Canada. Rock type: metatex1te migmatite; metapel1tlc protolith, anatexis at lower-granulite-facies conditions T 800- 850°C and P 5 8 kbar: Scale: the ruler is IS em long. /mage: E.W. Sawyer:

123

VARIATION S W ITHIN NEOSO M E

124 ----------------------------Figure

39

Fig. 839. The light grey rock 1n the lower part of the photograph IS a b1ot1te orthogne1ss paleosome, whereas the dark-colored metamafic rock 1s 1n part paleosome and 1n part melanosome. The doma1ns of leucosome hosted by the metamafic rock have the same m1neralogy. essentially plagioclase + quartz, w1th minor amounts of hornblende and pyroxene. The po1nt of interest 1n this m1gmatite is the d1fference in grain-size of the various bodies of leucosome. The coar sest grain-size is shown by the leucosome (LI) located m the 1nterpartition space between the boudins. A finer gra1n-size exists in the four offshoots of leucosome (L2) that occur parallel to the layenng w1th1n the two bou dins. These offshoots have an Irregular outline, and at least one of them is in contact w1th melanosome, evidence that suggests a local derivat1on for some of the melt that made the leucosome. The doma1ns of leucosome (L3) with the finest gra1n-size occur on the outs1de edge of the boud1naged layer; they have the smoothest edges, and locally, the crystals 1n them have a shape-defined preferred onentat1on. Close exam1nation of all these domains of leucosome shows that there are no cross-cutting relations among them; rather there is a smooth passage from one to t he next over which the grain-size changes rapidly. Thus, one can conclude that all cont ained melt at the same time. Consequently, it is unrealistic to regard their solidification as occurring at different t1mes.

or that the differences 1n gra1n-s1ze are the result of each havmg a different rate of cooling. However, the spaces that the domains of leucosome occupy may have started to form at different t1mes, and may well have undergone different histones of deformation as the melt w1th1n was crystallizing. For example, the leucosome 1n the 1nterpart1tion space has the coarsest grain-size because that s1te was dilatant during crystallization, whereas the doma1ns of leucosome w 1th the finest grain-size were located in sites where the deformation varied during crystallization and the growing crystals were periodically deformed and their gra1n-s1ze reduced. D1fferences in grain-s1ze and microstructure from one type of leucosome to the next m1ght not 1nd1cate that the various types belong to d1fferent generations; the1r local structural posit1on should always be cons1dered.

Locatton: Be1t Bndge Complex, Baklykraal Farm, South Afnca. Rock type: metatex1te m1gmat1te; metamafic protolith, partially melted 1n the granulite faoes. Scale: the pocket knife 1s

II em long. /mage: E.W . Sawyer. Further readmg: Marchildon, N . & Brown, M . (2003): Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt transfer at grain- to orogen-scale. TectonophysiCS 364 , 215- 235.

A tl as of Migm atite~

Figure

Fig. 840. Th1s m1gmatite IS made up ent1rely of neosome. and shows a w1de range of leucocrat1c through mesocrat1c to melanocratic parts. Coarse-gra1ned melanocrat1c rafts in the upper center and left contain the mineral assemblage orthopyroxene + garnet + clinopyroxene, with a very small amount of plagioclase: these rafts (R) are Interpreted to be res1duum left after the extraction of anatectiC melt. The lower nght comer conta1ns the most leucocrat1c part of the m1gmat1te: a coarse-grained hornblende-pyroxene quartz- plagioclase tonalitic leucosome (L) 1n wh1ch retrograde hornblende part1ally replaces clinopyroxene that is Interpreted to be crystallized anatectiC melt. Coarse-grained leucosome can be traced throughout much of the m1gmatite in a vague. net-like pattern that encloses vanous types of mesocratic rock. The thinnest segments of the leucosome network contain the h1ghest proport1ons of clinopyroxene and orthopyroxene. and may have crystallized from melt that had a substantial res1dual component entra1ned in 1t. Many of the pyroxene crystals are euhedral. which suggests that t hey crystallized in the melt. The rounded mesocratic domain (M) below the center of the photograph conta1ns the mineral assemblage plag1oclase + clinopyroxene + hornblende ± garnet ± orthopyroxene. It has the smallest grain-s1ze (lobe on the nght) and the h1ghest contents of hornblende and plagioclase. This mesocratic domain in the m1gmatite may be closest in composition to the metamafic protolith. However. parts of th1s domain are locally coarser gra1ned and ncher 1n pyroxene and, consequently. are Interpreted as res1duum. Therefore, this mesocratic

doma1n may be a raft of paleosome. or less fert1le protolith. that has been somewhat modified dunng part1al melt1ng. but did not become as melt-depleted as the melanocrat1c rafts above it. Much of the rest of the migmatite can also be described as mesocratic. but 1t has a substantially d1fferent morphology and microstructure; rt does not occur as rafts (also called schollen). Rather. rt is the matnx (M2) that conta1ns the melanocrat1c and mesocrat1c rafts. The mesocratic matnx decreases in grainsize and changes its mineral assemblage from orthopyroxene + quartz + clinopyroxene + plag1oclase near the leucosome to hornblende + clinopyroxene + plag1oclase farther away. Th1s gradat1on 1n the mesocratlc matnx and the network of leucosome it contains are Interpreted to represent the partially melted source and the network of channels by which anatectic melt was segregated from 1t. The extract1on of melt from the mesocratlc domain was Incomplete. as 1t did not progress sufficiently to form melanocratic residuum. but enough melt was extracted so that coarse-gra1ned leucosome could develop. The presence of schollen in the migmatite suggests that sufficient part1al melt was present that layers of more competent matenal (e.g.. res1duum and less fertile units) were disrupted and rotated in the matrix; hence, th1s IS a d1atexite m1gmatite. Locatton: Ashuanipi Subprovince, Quebec. Canada. Rock type: schollen diatexrte; metamafic protolrth. granulite-facies anatexiS. Scale: the st1cker wrth the number IS 2.5 em wide. /mage: E.W. Sawyer:

125

VAR IATIONS WITHIN NEOSOME

126 --------- --------- --------- --Figure

41

Fig. 841 . This migmat1te conta1ns coherent layers (30 50 em thick) of different composition that are interpreted to have ong1nally been bedd1ng. The grey-colored layers have h1gh modal proport1ons of cord1ente and are interpreted to have lost most of the anatectic melt that formed 1n them, even though they are not melanocrat1c. The parts of th1s m1gmatite that were derived from anatectic melt exhibit a w1de range of morphologies. Some anatectic melt collected parallel to the bedding and formed high-aspect-rat1o doma1ns of leucosome. Elsewhere, anatectic melt collected 1nto smaller, lens-shaped bodies located 1n shear bands and formed leucosome that is approximately parallel to the ax1al planes of small-scale folds; these are onented approximately parallel to the ruler: Both

that the rat1o of melanocrattC core to leucocratic nm differs from one neosome to the next. Re-Integration of the melanocrat1c and leucocrat1c parts of the patches of neosome shows that some are the product of closedsystem tn Situ partial melt 1ng. However, most have too large a melanocratic component relat1ve to the stoich1ometry ofthe

these types of leucosome have a relatively small proportion of ferromagnesian minerals 1n them, wh1ch indicates that they represent accumulations of anatectiC melt that had segregated from the residuum. However, the po1nt of Interest here 1s the scattered, irregularly shaped patches of neosome that cons1st of a conspicuous melanocratic core surrounded by a leucocratic nm. The melanocratic cores cons1st of one, or perhaps a few large crystals of orthopyroxene that are 1ntergrown w1th elongate blebs of quartz, whereas the leucocrat1c rims contain quartz, pla-

any particular type of leucosome or neosome morphology. This fact makes it difficult to choose an add1t1onal descnptive term for it; neither patch, stromatic, nor net-structured IS really representative of 1ts overall appearance 1n outcrop.

gioclase, and K-feldspar. It is ev1dent from the photograph

melt1ng reaction 1n these rocks; 1n these cases. the neosome formed tn sttu, but lost much of 1ts anatectiC melt, leav1ng 1t with an overall residual bulk composition. This type of meltdepleted neosome 1n patches IS common 1n the higher-grade, granulite-facies parts of migmat1te terranes. The melt lost from the neosome may have contributed to the two types of high-aspect-ratio leucosome present elsewhere in the outcrop. Th1s metatex1te m1gmat1te is not domtnated by

Location: Wuluma Hills, Arunta Inlier, Austral1a. Rock type: metatex1te migmat1te; Al-poor psamm1te protolith, partially melted at T 825 875°C, P ca. 5 kbar. Scale: the ruler is 15 em long. Image: E.W. Sawyer:

t

Atlas of Migmatites

Figure

l

t t

t

Fig. 842. The protolith to th1s m1gmat1te was a leucogranodionte that conta1ned mafic layers (probably mafic dikes). During the subsequent anatexis, the leucogranodiorite partially melted, but the mafic layers did not: they were res1sters. Where the fraction of melt in the leucogranod1onte was suffic1ent that it could flow. a diatex1te m1gmat1te was formed, and the res1ster layers were disrupted and became enclaves in the neosome, as in this image. Diffusional exchange between the anatectic melt and the fragments of the mafic resister occurred. Some hornblende at the edges of the resisters was convel'ted to biotite, and the adjacent neosome was enriched in plagioclase, possibly by transformation of K-feldspar to plag1oclase, which formed a white rim around the layers of res1ster. Elsewhere, the resister layers became dlsaggregated, and the adjacent neosome IS contam1nated w1th xenocrystic hornblende. This outcrop illustrates that there can be a considerable range in the composition of t he meltderived parts of migmatites aris1ng from contamination, or diffus1ve exchange w1th res1ster lithologies; th1s poss1b1lity should be cons1dered when sampling.

Locat1on: OpatiCa Subprovince, Quebec, Canada. Rock type: diatex1te m1gmat1te: leucogranod1orite protolith, anatexis at T ca. 750°C, and P 5 7 kbar. Scale: the ruler is 15 em long. /mage: E.W. Sawyer. Further read1ng: Sawyer, E.W. ( 1998): Format1on and evolution of gran1te magmas dunng crustal rework1ng: t he s1gn1ficance of diatexites.Journa/ of Petrology 39, 1147 1167.

127

FROM LEUCOSOME TO LEUCOCRATIC DIKES IN MIGMATITES

128 ----------------------------Figure

Fig. 843. Th1s granuhte-fac1es m1gmat1te has prominent domains of melanosome that follow the foliat1on and compositional layenng 1n the mafic gne1ss precursor. The abundant areas of leucosome, however, d1splay a more complex and h1erarchical arrangement. Th1n bod1es of 1n s1tu leucosome (LI ) are assoCiated directly with the melanosome and are oriented either parallel. orthogonal, or oblique to the layering; these form an interconnected network. Some more continuous, layer-parallel domains of in-source leucosome (L2) with a thin melanosome occur outside t he most melanocratic layers. and appear to have been fed by the network of th1nner, orthogonal and oblique doma1ns of leucosome. These layer-parallel domains then connect to much wider bod1es (L3). wh1ch cross the layering 1n the m1gmat1te. The absence of melanocrat1c borders to these w1de, cross-cutting bod1es suggests that they did not attract melt from the1r hosts, but are s1mply condu1ts for the migration of melt out of the source reg1on. Hence, the wide felsic bod1es should be called leucocrat1c vem and not leucosome.

Location: Kapuskas1ng Structural Zone, Ontano, Canada. Rock type: metatex1te m1gmat1te; mafic gne1ss protolith partially melted at granul1te-fac1es condit1ons. T ca. 850°C. P II kbar. Scale: the Brunton compass is 8 em across. /mage: D.R.M. Patt1son.

Further readmg: Hartel, T.H.D. & Pattison, D.R.M. (1996): Genesis of the Kapuskasing (Ontario) migmatitic mafic granulites by dehydration melting of amphibolite: the import ance of quartz to reaction progress. journal o( Metamorphic Geology 14,591 6 11. Sawyer, E.W. (2001): Melt segregation in the continental crust: distribut1on and movement of melt in anatectic rocks.

journal o( MetamorphiC Geology 19, 291-309.

t

A ria-. of M igm a t it e>

Figure

4

• • •

Fig. 844. The init1al compositional heterogeneity. due to the Intrusion of mafic and felsic 1gneous rocks that comprised the protollth of this migmat1te, has been strongly attenuated by h1gh strain synchronous w1th anatexis. Domains of leucosome are espeoally ev1dent in the mafic parts of th1s m1gmat1te. because the color contrast between paleosome, melanosome, and leucosome parts IS greatest there. The leucosome exhibits a range of morphologies, which correlates with 1ts structural position. Bodies of leucosome oriented parallel to the compositional layenng (LI) and the follat1on have the h1ghest aspect-ratio and could be called stromatiC leucosome. Most bod1es of leucosome (L2) are located e1ther between boud1ns (lowest aspect-rat1o) or 1n sets of conJugate extensional shears and fractures (intermediate aspect-rat 1o). However, most stri king in this outcrop IS the combination of all domains of leucosome to form an ordered, branched array. The thinnest domains of leucosome form the m sttu parts of the network, whereas the thickest doma1ns of leucosome (L3) 1n the array clearly cross-cut the layenng 1n the paleosome and melanosome at the lower right of the photograph. If

the melt in this leucosome (L3) was derived locally, then th1s is an in-source leucosome; if it is not, then it IS a leucocratic vein. Detailed exam1nat1on of the contacts between the different segments of leucosome 1nd1cates the absence of abrupt changes 1n gra1n s1ze or compos1t1on; there IS continUity throughout the array. Th1s find1ng IS Interpreted to mean that the ent ire array of leucosome conta1ned melt at the same time and, hence. that melt migrated out of the m1gmatite through the array along progressively fewer and wider channels. finally migrat1ng through the I 0-cm-wide channel marked by the leucosome in the lower right. A Similar, w1der leucocrat1c ve1n on the left may represent the prox1mal end of another array of leucosome that dra1ned melt from the part of t he m1gmat1te farther to the left. Location : Sand River Gne1sses. Causeway locality, South Africa. Rock type: metatexite m1gmatite; metamafic and metagranodioritic to tonalltic protolith, partially melted 1n the granulite fac1es. Scale: the pocket kn1fe IS II em long. Image: E.W. Sawyer.

129

FROM LEUCOSOME TO LEUCO C RATIC D IKES IN MIG MATITES

130----------------------------Figure

4S

Fig. 845. Migmatites in southern Brittany are located south of the transcurrent Sout h Armorican Shear Zone. Shearing during anatexis is probably the reason why the southern Britt any migmatites have a predominantly stro matic morphology (see also Fig. BIO). Typical ly, thin bodies of feldspar-rich leucosome oriented parallel to the foliation alternate with biotite-rich melanocratic layers and rarer, fine-grained biotite-quartz-plagioclase schists. However, not all the bodies of leucosome in the migmatites are parallel to the foliation and compositional layering. In many places, the stromatic migmatites contain cross-cutting bodies of leucosome located in interboudin partitions and extensional shear structures, and are also cut by discordant veins of leucogranite that may be several meters t o t ens of meters long. Although the cross-cutting relationships indicate a clear progression in "structural" age from stromatic leucosome through discordant leucosome to granitic vein, there is no progression in "petrological" age. T his photograph shows t he contact relat ionships between a granitic vein I 0- 15 em wide and the host stromat ic metatexit e migmatite. The granit ic vein truncates the layering defined by the melanosome and t he biot ite-quartz-plagioclase schist. but it does not appear to t runcate the st romatic leucosome. The passage from st romatic leucosome (most evident in the wider domains) to discordant granite vein is petrologically

continuous; there is no mineralogical or microst ructural discontinuit y between the material in the leucosome and that in the vein. T his relationship indicat es that when the granitic vein formed, the stromatic leucosome still contained melt. The stromatic leucosome, the discordant leucosome, and the granit ic veins together constituted a cont inuous melt-bearing network in the migmatite. Since the stromat ic domains of leucosome are associated with melt -depleted melanosome, but the granitic veins are not, anatectic melt is interpreted to have moved subhorizontally a short distance, from where it formed along channels now represented by t he stromatic leucosome, to t he steeply dipping cross-cutting structures now represented by t he leucogranitic dikes, through which it was then able t o escape and feed higher-level granit ic plutons.

Location: Port N avalo area, southern Brittany, France. Rock type: metatexite migmatite; quartzofeldspathic and aluminous gneiss protolith partially melt ed at T ca. 800°C. P ca. 8 kbar, then decompressed to 4 kbar, followed by cool ing. Scale: the coin is 23 mm across. Image and caption: Mike Brown. Image previously published as fig. I0. 11(b) in Brown (2006) and reproduced w it h the permission of Cambridge University Press.

At hh of Mig mati te~

• •

Further readmg: Jones, K.A. & Brown. M. (1990): Hightemperature "clockwise" P-T paths and melting 1n the development of reg1onal m1gmat1tes: an example from southern Bnttany, France. journal of Metamorphic Geology 8 , 551 - 578 .

•

Marchildon, N . & Brown, M. (2003) : Spatial distribution of melt-bearing structures 1n anatectic rocks from southern Bnttany, France: implications for melt transfer at grain- to orogen-scale. Tectonophysrcs 364, 215 235. Brown, M. (2006): Melt segregat1on from lower continental crust of orogens: the field ev1dence. In Evolut1on and D ifferentiation of t he Cont inental Crust (M. Brown & T Rushmer eds.). Cambridge Un1vers1ty Press, U.K.

(33 1-383).

13 1

FROM LEUCOSOME TO LEUCOCRATIC DIKES IN MIGMATITES

132 -----------------------------Figure

6

conta1ned far less melt. or were solid. so that the melt 1n Fig. 846. Th1s outcrop of the southern Brittany m1gthe vem could not 1nteract w1th melt 1n the wallrocks. mat1tes show features that may have formed dunng the wan1ng stages of the system shown 1n Fig. B45. The Locatton: Port NavaJo area. southern Bnttany. France. Rock m1gmat1te 1S dom1nated by th1n stromatic leucosome and 1ts type : metatex1te m1gmat1te; quartzofeldspath1c and alumiadJacent melanosome, but also conta1ns d1scordant bodies nous gne1ss protolith partially melted at T ca. 800°C. P ca. of leucosome and dikes of leucogranite (under the ruler) . 8 kbar. then decompressed to 4 kbar, followed by cooling. The d1scordant leucosome and leucogran1tic ve1ns comScale: the ruler is 15 em long. Image: E.W. Sawyer. monly display a change in the contact relationships with the host stromatic migmatites along their length. Most Further read1ng: see Fig. B45. of the discordant bodies of leucosome appear to root in small shear zones, shear bands, or asymmetncal boudin structures, where they have d1ffuse contacts w1th the host (discordant leucosome on the nght) stromat1c m1gmat1te. Farther along, the d1scordant bodies of leucosome w1den and develop sharp marg1ns aga1nst the layers of melanosome. but are petrologically continuous w1th the stromatiC leucosome. as shown 1n Fig. B45. However. a few ve1ns of leucogranite show a d1fferent relat1onsh1p along the1r length. and cross-cut both the stromat1c leucosome and the border of melanosome 1n the host m1gmat1te (ve1n on the left). and may even contain fragments of the wallrock. These part1cular discordant veins thus are late and formed as the migmatites solidified ; t hey are rooted 1n melt-bearing layers, but then propagated into parts of the migmat1te that either

A tl as of M igmati tcs

Figure

7

Fig. 847. Th1s photograph is taken look1ng down 1nto the core of a meter-scale S-shaped synform, and shows the root of two gramtic dikes. The core of the fold (top nght of the photograph) consists of metapelite (Pe) with abundant leucosome, whiCh ind1cates that 1t contained a h1gh fraction of melt, and the outer part (center left) cons1sts of a th1ck layer of mass1ve quartz-nch metapsamm1te (Ps). The metapsammite has undergone little or no partial melt1ng, and can be regarded as a paleosome resister lithology in the m1gmatite. In contrast, the pelite has undergone extens1ve partial melting and has developed melanocratic garnet- and orthopyroxene-nch res1dual rocks (R). The mass1ve metapsammite served as an impermeable layer above the pelite, and enabled anatectic melt to accumulate in the pelite layer under 1t, 1n both the synform and adJacent ant1form h1nges. Locally, the melt fract1on became suffioently volum1nous 1n the pehte that the foliat1on and melanocrat1c res1dual layers 1n it became d1srupted as the folds tightened. Thus, the morphology that developed in the pelite layer at the fold h1nge is transitional from metatex1te to diatex1te m1gmat1te. An extens1ve dark nm (S) that consists pnnopally of late biot1te occurs at the boundary between the melt-nch pelite layer and the massive psamm1te. Th1s rim, together w1th the extensive replacement of garnet and orthopyroxene by b1ot1te in the res1dual rocks, prov1de further ev1dence

that anatect1c melt accumulated 1n the fold hinges. Some of the melt 1n the h1nges was locally denved (g1v1ng rise to 1n sttu and 1n-source leucosome) , but some may also have m1grated up to the hinge from a lower position on the fold hmbs (to form leucocrat1c veins or patches). The late b1ot1te IS Interpreted to have formed when H,O. released as the accumulated melt crystallized, hydrated the res1dual minerals. Some of the accumulated melt was able to escape from the pelite layer and cross the massive metapsammite layer through the I O-cm-w1de dike (it could also be called a ve1n) that crosses the m1ddle of the photograph (01). The dike IS rooted 1n the synform and IS onented subparallel to the axial plane of the fold. A second leucocrat1c dike (02) that penetrated no more than 50 em 1nto the metapsammlte occurs to the nght, and confirms that the source of the melt was the pellte layer; hence, the term leucocrat1c dike or ve1n IS appropriate. An 1ncrease in pressure on the magma 1n t he pehte layer as the fold tightened may have enabled the formation and propagation of the fracture through which some of the magma was able to escape. The dike contains several large crystals of garnet, some of wh1ch have a nm of b1otite; some aqueous nu1d thus was released as the magma in the dike crystallized and reacted with the garnet to form the biotite rim. The scale rests on a shear zone that is located on the steep, nght nank of the synform.

133

FROM LEUCOSOME TO LEUCOCRATIC DIKES IN MIGMATITES

134 -----------------------------

Location: Wuluma Hills. Arunta lnl1er. Australia. Rock type: leucocrat1c dike in a metatexite m1gmatite; metapelit1c protohth, partially melted at T 825 875°C. P ca. 5 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer. Further read1ng: Sawyer, E.W., Dombrowski, C. & Collins, W .J. (1999): Movement of melt dunng synchronous reg1onal deformation and granulite-facies anatexiS, an example from the Wuluma Hills, central Australia. In Understand1ng Gran1tes; 1ntegrating New and Classical Techn1ques (A. Castro, C. Femandez & J.-L. Vigneresse, eds.). Geolog1cal

Sooety, Speoal Publication 168,221 237.

Atlas of M igmatites

Figure

48

Fig. 848. Overall v1ew of the m1gmat1te outcrop shown 1n Fig. B47. taken from above and look1ng down the h1nge of the folds. The two dikes (D I and 02) that emerge from the pel1t1c layer 1n the synform can be seen m the left half of the photograph. JUSt to the right of the scale. Part of the complementary ant1form is just v1s1ble at the extreme right. Tight, minor folds can be seen 1n the metapelite; the layenng that IS folded involves a combination of biotite-rich pelite. leucosome, and biot ite + garnet + orthopyroxene melanosome material.

Locauon: Wuluma Hills, Arunta Inlier. Austral1a. Rock type: leucocrat1c dike in a metatexite m1gmat1te; metapelit1c protolith. part1ally melted at T 825 875°C. P ca. 5 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

135

SE LVEDGE S IN MIGMATITES

136 ----------------------------Figure

49

Fig. 849. A prominent mesocratiC selvedge IS developed 1n th1s m1gmat1te between the discordant, feldspar-rich leucocratic vein and 1ts host. The dom1nant p1nk1sh grey port1on of th1s outcrop is medium-gra1ned neosome that was derived from a semipelit1c protolith. The grey neosome has two parts. A darker-colored part w1th the m1neral assemblage quartz + plagioclase + K-feldspar + biotite + garnet ± cordierite is interpreted to be residual; the lighter, K-feldspar + quartz + plagioclase granitic parts occur as small, diffuse, and elongate patches. These pink1sh stringers are interpreted to be 1n situ leucosome. The neosome is 1ntruded by a discordant, quartz-poor leucocratic vein that has the mineral assemblage K-feldspar + plag1oclase + garnet + quartz + biotite; the b1ot1te replaces garnet. Prom1nent mesocratic selvedges that are I 2 em wide occur between the host and the leucocrat1c ve1ns. Contacts between the selvedges and the p1nk1sh grey host are gradational, and their grain s1zes and m1crostructures are s1milar, wh1ch suggests that the selvedge may have formed from, or overprinted, the host. The selvedges are not enriched in ferromagnesian minerals relative to the host, but do appear to be enriched in quartz. Consequently, they are not

interpreted as res1dua. Thus, the selvedges are interpreted as a diffusion halo between the host and the leucocratlc vein 1ntruded 1nto 1t. Poss1bly. a grad1ent 1n silica act1v1ty between the quartz-rich host and the intruded quartzpoor magma ex1sted and drove the diffusion of silica from the host toward the leucocrat1c vein, producing the mesocratic selvedge.

Location: Wuluma Hi lls, Arunta Inlier, Australia. Rock type: selvedge around leucocratic vein in a metatexite migmatite; metapelitic protolith, partially melted at T 825-875°C, P ca. 5 kbar. Scale: the ruler IS IS em long. /mage: E.W. Sawyer.

A d a\ of Migma rire,

Figure

SO

• •

Fig. 850. This photograph shows several bodies of garnet-bearing. quartz K-feldspar leucosome developed in a med1um-grained ilmenite- plagioclase quartz- bJOtJtesJihmanJte cord1ente garnet gneiss. All the doma1ns of leucosome are separated from the1r host by a very narrow. melanocrat1c nm that 1s nch in biot1te. The nm 1s probably a retrograde feature that formed as the melt in the leucosome crystallized ; hence, it is a mafic selvedge and not a melanosome. The domains of leucosome in this photograph differ markedly from others present 1n m1gmatites of the same area (shown 1n Figs. B23 and B24) 1n that they do not conta1n large poikiloblasts of garnet. Th1s difference 1s interpreted to Jnd1cat e that the leucosome in th1s 1mage may have formed 1n a different way to the others 1n the area. H ere, the leucosome probably formed by the physical segregation of anatectic melt from the adjacent gneiss. Thus, it IS essentially wholly denved from anatectic melt. whereas the leucosome shown in Figs. B23 and B24 formed by the 1n s1tu growth of the solid products garnet and K-feldspar and, consequently. was mostly solid as 1t grew, w1th a lesser proportion of anatectic melt. React1on between the evolved melt generated, or an aqueous nUid exsolved as the leucosome crystallized, and residual minerals such as cordierite in the host formed the biotite-rich mafic selvedge.

Location: Round Hill. Broken Hill, Australia. Rock type: metatexite migmatite; sem1pelitic protolith, partial melt1ng at T 800°C and P 5 6 kbar. Scale : the pen is 14 em long. Image and capt1on: R1chard Wh1te. Further readmg: Wh1te. R.W.. Powell, R. & Halp1n, J.A. (2004): Spatially-focussed melt formation 1n aluminous metapelites from Broken H ill, Australia. Journal of Metamorphic Geology 22, 825 - 845.

137

SELVEDGES IN MIGMATITES

138 ----------------------------Figure

BS

Fig. BS I. Leucocrat1c ve1ns 1ntruded into migmat1te terranes dunng the wan1ng stages of anatex1s commonly develop th1n mafic selvedges. This photograph shows such a th1n (I 3 mm wide). b1ot1te-nch mafic selvedge developed between a discordant leucocratiC ve1n and 1ts grey metasedimentary host. The leucocrat1c ve1n, a peralumi nous leucogranite, contains the m1neral assemblage quartz + plagioclase + K-feldspar + biotite + garnet. with minor amounts of muscovite, apat1te, and tourmaline. The wholerock composition of the leucocratic vein suggests that it crystall1zed from a fractionated anatectiC melt denved from a metapelltic protolith. Part1al melt1ng has probably not occurred 1n the b1ot1te- quartz plag1oclase psamm1t1C sch1st that hosts the gran1t1c ve1n. However. the Interbedded pelit1c layers do conta1n a res1dual m1neral assemblage: s111iman1te + K-feldspar + b1ot1te + cord1ente + quartz + plag1oclase (e.g.. Ftg. F43). The mafic selvedge IS only a few gra1ns w1de and cons1sts predomtnantly of b1ot1te that has a much coarser grain-size than the b1ot1te 1n e1ther the ve1n or the host schist. Garnet crystals occur locally w1th1n the selvedge, and closely resemble the garnet crystals in the vein. The mafic selvedge is slightly thicker 1n concavities at

the edge of the ve1n. The b10t1te-rich rim is too narrow to be the complementary extract1on zone of the melt from wh1ch the leucocratic ve1n crystallized; hence. 1t is not a melanosome. Moreover, the host probably did not part1ally melt. The mafic selvedge is interpreted to have formed as a result of react1on between the evolved felsiC melt from which the vein crystallized and the host schist. Location: Quetico Subprov1nce, Canada. Rock type: leucocratic vein with mafic selvedge 1n a psammitic layer; metamorphic conditions T 700 800°C and P 3- 4 kbar. Scale: the ruler is 1n centimeters and millimeters. Image: E.W. Sawyer.

Aria> of 1\ligma t itc~

•

+

Fig. 852. The dark grey rock 1n th1s photograph IS a garnetdiopside plag1oclase hornblende metamafic rock. It contains a few thin (< 10 mm) leucocratic ve1ns that are subparal-

plag1oclase caused by the aqueous fluid from the felsic melt. Melanocratic bands also occur subparallel to the layenng 1n the

lel to the compos1t1onal layenng. However. the most strik1ng feature of the outcrop 1S the array of subparallel lenses and

metamafic schist and appear to form lateral bndges between cha1ns of small lenses of leucotonalite. The dark color of these

elongate patches of coarse-gra1ned leucotonalite that are onented at about 70° to the compos1tional layenng. Examination of nearby outcrops reveals that the lenses of leucotonalite are

bands IS due to the replacement of dlops1de by hornblende, and IS interpreted as ev1dence that t he exsolved aqueous

oriented parallel to the ax1al planes of a senes of t1ght folds. A melanocrat1c rim IS developed around each lens. Thus, at first sight, the outcrop appears to conta1n leucosome w1th melanosome and could, consequently. be cons1dered a metatexite m~gmat1te. However. the dark nms are narrow and cons1st predominantly of coarse-gra1ned hornblende and biotite. There are two generat1ons of hornblende: one IS of s1milar morphology to the hornblende in the metamafic rock farther away from the leucocrat1c lenses, and is Interpreted to be prograde, whereas the other replaces d1ops1de and IS retrograde. B1otite part1ally replaces hornblende close to the leucocrat1c lenses. These lenses are 1nterpreted to have crystall1zed from a fels1c melt 1ntruded 1nto the metamafic rock parallel to the ax1al planes of local folds, and the dark rim around them seems to be mafic selvedge formed by reaction between the metamafic host and the felsic melt, or an aqueous flu1d exsolved from it during crystallization. Many of the larger lenses of leucotonalite exhlb1t a w1de, lighter-colored selvedge or halo outs1de the

mafic selvedge. This feature is attributed to the alterat1on of

fluid was locally able to infiltrate parallel to the foliation of the metamafic rock. The widespread development of retrograde hornblende has obscured any melanosome that may have ex1sted 1n the metamafic host. Hence, 1t IS not certa1n whether the metamafic part of the outcrop 1s a m1gmat1te. However. the grey rocks in the top nght corner of the photograph are a metatexite m1gmatlte: the silliman1te-garnet b1ot1te quartzplagioclase pelitic schists contain patches of neosome that have cores of poikiloblastic garnet intergrown with quartz that are surrounded by a plag1oclase + K-feldspar + quartz leucosome, cons1stent w1th the Incongruent melting of b1otite. Some of the garnet po1klloblasts conta1n scattered inclus1ons of s1ll1mamte. Location: Central Metasedimentary Belt, Grenv1lle Prov1nce.

Rock type: lenses of leucotonalite w ith mafic selvedges 1n a metatex1te mlgmatite: metagabbro protolith, lower granulite-facies conditions near Wakefield, Quebec, Canada.

T ca. 800°C, P 6-9 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

139

SELVEDGES I N MIGMATITES

140 - - - - - -- - - - - - - - - - Figure

Fig. 853 . This complex migmatite prov1des a good example of a mafic selvedge. The prom1nent layenng 1n the m1gmat1te is pre-anatectic; the light grey layers are b1ot1te plag1oclase quartz psamm1t1c schists (Ps), the med1um grey layers are garnet cordierite-quartz-K-feldspar plagioclase-sill1man1te biotite pelit1c schists (Pe), and the dark grey layers are biotit e plagioclase hornblende sch1sts (H s) t hat were poss1bly mafic volcanic rocks, or sills. The evidence for partial melting and, therefor e, for considering this a migmatite, can be found in the pelitic layers. At the bottom left, stromatic, p1nkish grey granitic leucosome (LI) occurs in a grey sch1st that has the m1neral assemblage plag1oclase + s1111man1te + b1ot1te + cord1ente + K-feldspar + quartz, w1th m1nor garnet. The leucosome IS Interpreted to be denved from the anatectic melt of an alum1nous pelite v1a a reaction such as b1ot1te + sill1mamte + quartz + plag1oclase

=

melt + cord1erite + garnet + K-feldspar, and the host schist IS Interpreted t o be the res1duum. but 1t 1s not melanocratiC. Another pelit1c layer, located 1mmediately above the dark grey layer that crosses the center of the photograph, contains addit1onal evidence for partial melting. Diffuse patches of neosome (N ) have a core consisting of quartz + garnet surrounded by quartz + plagioclase + K-feldspar in a leucocratic rim, which suggests that the neosome formed in situ by a similar melting reaction, but in a slightly more ironnch pelite. Conspicuous, narrow mafic selvedges (MS)

composed of b1ot1te occur at the margms of a folded, p1nk leucocratic ve1n 5- 10 em w1de that occurs 1n the upper half of the photograph. Although the ve1n IS hosted by pelite, the mineral assemblage 1n the mafic selvedge is not s1milar to the residua assooated w1th e1ther the stromatic leucosome or the patches of neosome. Consequently, the mafic selvedge 1s interpreted to result from react1on between an inJected vein of granit1c magma and its pelitic host, and not to be a residuum generated by partial melting. This interpretation is supported by observations in adjacent outcrops, where a t hin mafic selvedge occurs adjacent to d1scordant pink granitic veins. Because the stromatiC leucosome is folded by the mesoscale tight folds ev1dent 1n the photograph, the patches of neosome are equant and have d1ffuse edges, even though located 1n the core of the folds. Both types of leucosome have igneous microstructures, and the fold1ng is cons1dered to have occurred wh1le anatectiC melt was present in the rock. As another 1nterest1ng feature 1n th1s m1gmat1te, garnet developed at contacts between the mafic and pelit1c layers, but not at the contacts between mafic and psammitic layers.

Locat1on : Sa1nt-Fulgence. Grenville Province, Quebec, Canada. Rock type: leucocratlc vein w 1th mafic selvedge in a metatexite migmatite; metasedimentary protolith, granulitefacies anateXIS at T 800 850°C, p 5 8 kbar: Scale: the ruler is marked 1n em. Image: E.W. Sawyer.

Atl ~1;

Figure

of Migmatite;

S

Fig. 854. Th1s photograph 1s a close-up of the 1ntenor of a meter-wide granitic dike, one of several that are on ented parallel to local shear zones and the axial surfaces of mesoscale folds developed in metatexite m1gmatites. In general. these dikes are Interpreted to be part of the transfer network of condu1ts through wh1ch anatect1c melt moved from the migmat1tes to small gran1te plutons grow1ng in larger dilatant sites nearby. Because these dikes are still with1n their broad source-volume, they belong to the category of leucocrat1c dikes. The photograph shows several th1n (I 1.5 em) leucocrat1c veins, onented roughly parallel to the scale, and each has very narrow b10t1te-nch mafic selvedges. The selvedges are interpreted to have formed as a result of interaction between with veins and their host. A stnking feature of the host is that it conta1ns numerous b1ot1te-nch foliae that are regularly spaced about 1.5 em apart, the same spacing as the mafic selvedges at the edges of the leucocratic veins. Both the thin veins and their host dike rocks have granitic bulk compositions, similar gra1n-s1zes, and s1m1lar microstructures. The host dike thus probably was Itself constructed by the mult1ple Injection of many th1n leucocratic veins, each of which developed a mafic selvedge. The two more prominent leucocratic veins just happen to be the last increments of melt injected into the dike. If the th1n ve1ns represent the entire h1story of now of the larger

dike. then the narrowness of the 1nd1v1dual ve1ns may mean that th1s part1cular dike was not a successful conduit for the transfer of melt. Alternatively, the repeated dik1ng may simply be a record of the waning pulses of melt in the wider dike.

Locauon: Wuluma H1lls, Arunta Inlier, Australia. Rock type: leucocrat1c vein with mafic selvedge, metatexite m1gmatite; metapelitic protolith, partially melted at T 825 875°C. P ca. 5 kbar: Scale: the ruler is IS em long. /mage: E.W. Sawyer.

141

142

~E~X~I~T~E. IT~E~A~N~D~D~I~AT X~ ----------~~~~--~M~ET~A~T~E~

THE FI RST-ORDER DIVISION OF MIGMATITES

C. METATEXITE AND DIATEXITE, THE FIRST- ORDER DIVISION OF MIGMATITES The photographs in this section show the striking change tn the morphology of migmatites that takes places between the lowest- and the highest-grade parts of most migmatite terranes. T he same changes occur whether the migmatttes formed in a regtonal, or tn a contact metamorphtc setting.

Migmatites from the contact aureole of the Ballachulish Igneous Complex [Figs. CI-C4] The Ballachulish Igneous Complex (BIC), tn the Scotttsh Htghlands, was emplaced tnto Dalradtan metasedtmentary rocks at about 425 Ma. The metasedtmentary rocks were regionally deformed and metamorphosed to the greenschist facies before t he intrust on of the BIC. Intrusion occurred at a pressure o f 3 ± 0.5 kbar (corresponding to a depth of about 10 km) and a contact-metamorphic aureole. locally up to 2 km wide, developed around the BIC. Rocks tn the inner part of the contact aureole were parttally melted.

Further readmg: Fraser, G.L., Patttson, D.R.M. & Heaman, L.M. (2004) : Age of the Ballachulish and Glencoe Igneous Complexes (Scottish Highlands), and paragenests of ztrcon, monazite and baddeleytte tn the Ballachulish aureole. journal

of the Geologrcal Society of London

16 1, 447 462.

Harte, B., Pattison, D.R.M. & Ltnklater, C.M. (199 1): Field relations and pet rography of partially melted pelite and semi -pelitic rocks. In Equilibrium tn Contact Metamorphism: the Ballachulish Igneous Complex and its Aureole (G. Voll , J. Topel, D .R.M. Pattison & F. Seifert, eds.) . Springer-Verlag. Hetdelberg, Germany ( 181-209). H olness, M. & Clemens, j D. ( 1999): Partial melting of the Apptn quartzite driven by fracture-controlled H 20 tnfiltratton tn the aureole of the Ballachulish Igneous Complex, Scotttsh Highlands. Contnbutrons to Mmeralogy and Petrology

136, 154-168. Patttson, D .R.M. & Harte, B. (1997): Geology and evolutton of the Ballachulish Igneous Complex and aureole. Scowsh

journal of Geology 33, 1-29.

Migmatites from the contact aureole of the Duluth Igneous Complex [Figs. CS, C6] Mafic rocks of the Duluth Igneous Complex (DIC), 1n northern Minnesota, were intruded over a very narrow time period around I 098 Ma, and produced a considerable contact-aureole in the Archean and Proterozoic country rocks. T he inner part of the aureole locally reached the pyroxene hornfels facies, and anatexis ts particularly evident in the pelit ic and semipelitic rocks of the Virginia Formation, on the western stde of the DIC. The depth of tntrusion was relatively shallow; estimates of the pressure of the contact metamorphtsm are tn the range 1.5 2 kbar.

Upper amphibolite facies, regional migmatites from Saint-Malo, France [Figs. C7, CB] The Satnt-Malo Terrane conststs largely of metasedimentary rocks of Bnovenan age; greywacke and pelittc compostttons predominate. These rocks were partially melted during an upper-amphibolite-facies metamorphtc event at 536 ± 14 Ma. Melting occurred through the breakdown of muscovtte, and biotite generally rematned stable. Diatexite migmatites occur in the central part of the terrane and are partial ly sur rounded by an envelope of metatextte migmatites. N onmelted Bnoverian metasedtments are well exposed south of the migmatttes in the estuary of the Rance.

Further readmg: Brown, M. (1979) : The petrogenests of the St-Malo migmattte belt, Armoncan Masstf, France, with particular reference to the dtatexttes. Neues jahrbuch fur Mtneralogre, Abhandlungen 135, 48 74. Milord, 1., Sawyer, E.W. & Brown, M. (200 1): Formation of diatexite migmattte and grantte magma during anatexts of semi-pelitic metasedimentary rocks : an example from St. Malo, France. journal of Petrology 42 , 487-505. W eber, C., Barbey, P, Cuney, M. & Martin, H . ( 1985): Trace element behaviour during migmatization. Evidence for a complex melt-residuum flutd interaction in the St. Malo migmatitic dome (France) . Contnbuuons to Mmerology and

Petrology 90, 52 62.

A tlas of Migmatites

•

Upper amphibolite facies, regional migmatites from the Opatica Subprovince, Quebec [Figs. C9, C IOJ The Opat1ca Subprovince, in central Quebec, consists principally of tonalit e trondhjemit e-granodiorite (T TG) plutonic rocks with ages between 2825 and 2702 Ma, 1nto w h1ch plutons belonging to a monzodiorite tonalit e granodiorite suite were int ruded between 2697 and 2693

Sawyer, E.W. ( 1994): Melt segregat1on in the continental crust. Geology 22, 1019 1022.

trondhjemite granodiorite suite 1n the subprov1nce. and to the generation of anatectiC leucogran1tes between 2681 and melt1ng occurred under conditions of the

m1gmat1tes and 2678 Ma. Partial

Sawyer, E.W. (2001): Melt segregation 1n the cont1nental crust: distribution and movement of melt in anatect1c rocks.

upper amphibo-

Journal of Metamorphic Geology 19. 291 309.

center of the

Further readmg: Davis, W.J., Machado, N., Gariepy, C., Sawyer, E.W. & Benn, K. (1995) : U Pb geochronology of the Opat1ca tonalite gneiss belt and 1ts relat 1onsh1p to the Abitibi gr eenstone belt, Supenor Province, Quebec. Canad1an journal of Earth Sciences 3 2 , 113-127. Sawyer, E.W. ( 1998): Formation and evolution of granite magmas dunng crustal reworking: the Significance of diatexJtes.Journal of Petrology 39, 1147 1167. Sawyer, E.W. & Benn, K. (1993): Structure of the h1gh-grade Opat1ca Belt and adJacent low-grade Ab1t1bl Subprovince, Canada: an Archaean mountain front. journal of Structural Geology 15, 1443 1458.

r

Granu lite-facies, regional migmatites from the Ashuanipi Subprovince, Quebec [Figs. Cll, Cl2]

•

T he migmatites in the Ashuan ipi Subprovince, Quebec Labrador, were derived from A rchean siliciclastic

•

Percival, J.A ( 199 1) : Granulite-fac1es metamorphism and crustal magmatism in the Ashuan1pi com plex, Quebec Labrador, Canada. Journal of Petrology 32, 1261 1297.

Ma. A subsequent high-grade metamorphic event led to widespread partial melt1ng of rocks of the tonalite

lite faoes (T ca. 750°C, P 5-7 kbar), and probably mvolved the innux of aqueous nuid.

• •

Further reading: Guern1na, S. & Sawyer. E.W. (2003): Largescale melt-deplet1on 1n granulite terranes: an example from the Archaean Ashuan1p1 subprov1nce of Quebec. Journal of Metomorph1c Geology 21 , 18 1 20 I.

sedimentary mat erial deposited by turbidity-current act1on. The beds 1n the sedimentary sequence range up to a meter th1ck, but the typ1cal th1ckness IS I0 20 em. Some beds were graded during depositiOn, and th1s has resulted 1n a compos1t1onal gradat1on w1th1n the beds; the tops have a broadly pelitic composition, whereas the bottoms are greywacke (psamm1t1c). Plag1oclase-nch metagreywacke w1th a simple plag1oclase + quartz + bJotJte + quartz m1neral assemblage dom1nates (ca. 85%) the sedimentary pi le and was volumetrically the principal protolith to the m1gmatites. Partial melt1ng 1n t hese rocks occurred through the biotite-dehydration reaction: biot1te +

=

plagioclase + quartz orthopyroxene + melt ± ilmenite. The average degree of part ial melting was about 3 1 vol.%.

143

MIGMATITES FROM TH E CONTACT A UREOLE

144 -----------------------------0F THE BALLAC HULI SH IGN EOU S COMPLEX Figure

Fig. C I. One of the first 1ndicat1ons of the onset of

Further reading: Pattison, D .R.M. & Harte, B. (2001): The

part1al melt1ng in the field is the presence of very narrow (<2 mm) feldspar-nch leucocrat1c ve1ns 1n pelitic and semipelit1c hornfelses. Th1s photograph shows that the leucocratlc ve1ns are located parallel to the foliation (indicated by the pencil) , and in orientations that are oblique to the layering in the hornfelses. Locally, the veins form a netlike pattern in their hosts. Very narrow biotite selvedges are present around some of t he veins. In thin section, the veins

Ballachulish Igneous Complex and Aureole: a Field Guide. Ed1nburgh Geolog1cal Sooety Gu1debook Series, Edinburgh,

show K-feldspar grains w ith crystal faces in contact with "xenomorphic-interst itial" quartz, a microstructure indicative of crystallization from a melt. The ma1n macroscopic charactenst1c of th1s migmatite is the cont1nuity of preanatect ic structures (bedding and foliat 1on) 1n the paleosome; hence, it is a metatexite m1gmatite.

Locot1on: east s1de of the aureole. Rock type: metatex1te m1gmat1te; t he protolith was interbedded pelite and semlpelite of the Appin Phyllite, anatexis at T 650 730°C. Scale: the pencil is IS em long. /mage: D.R.M. Patt1son. Previously published as Photo 20 in Pattison and Harte (200 I) and is reproduced w it h t he permission of The Edinburgh Geological Society.

U.K. 148 p.

--Atlas of Migmatites

Figure

l

• • • •

• •

Fig. C2. Migmatites with a very distinctive appearance are produced at slightly higher grades from protoliths consisting of thin pelitic and semipelitic beds. T he photograph shows a sawn slab of this type of migmatite, which has been etched and stained to reveal the K-feldspar. The competent layers, typically rich in cordierite, have been extended and become trains of rectangular boudins, whereas the less competent units (partially melted semipelitic layers) have deformed by ductile flow and remain more or less continuous, but are thin ned. Coarser-grained leucocratic material extracted from the partially melted semipelitic layers occupies the dilatant spaces between the rectangular boudins, but also occurs as veins that cross the layering. Although t he proportion of leucosome is locally higher, and this migmatite is more strained than t hat shown in Fig. C I, the pre-anatectic structures remain continuous. Thus, this too is a metatexite migmatite.

Location: east side of contact aureole, I 00 m from contact. Rock type: metat exit e migmatite; the prot olith was t hinly bedded pelite and semipelite of the Appin Phyllite, partially melted at T 650 - 730°C Scale : the lens cap is 65 mm across. Image : D .R.M. Pattison. Previously published as fig. 3 in Pattison and Harte ( 1988) and is reproduced with t he permission of Blackwell Publishing. Further reading: Pattison, D.R.M. & Harte, B. ( 1988): Evolution of structurally contrasti ng migmatites in t he 3-kbar Ballachulish aureole, Scotland. journal ofMetamorphic Geology 6 , 475 494.

145

MIGMATITES FROM THE CONTAC T AUREOLE

146

OF THE BALLACHULISH IGNEOU S COMPLEX

Figure

Fig. CJ. The migmatites located 1n the parts of the aureole that reached the highest metamorphic temperatures, and consequently generated the h1ghest fract1ons of melt, have a very different appearance. Th1s cut and sta1ned slab shows rectangular boudins generated by the pull1ng apart of the more competent (more v1scous, or st1ffer) metasedimentary layers enclosed 1n an anatectic melt-nch matnx. In contrast to Fig. C2, t he blocks of t he competent rock have been moved apart and rotated such that the onginal orientation and cont inuity of the pre-partial-melting structures (bedding and foliation) are disrupted and destroyed; hence,

th1s is a d1atexite m1gmatite. Location: screen with1n the 1ntrus1ve body, east s1de of the complex. Rock type: d1atex1te m1gmat1te; protolith was th1nly bedded pehte and sem1pelite of the Appin Phyllite, part1ally melted at T 750- 800°C. Scale: the co1n 1s 19 mm

1n d1ameter. Image: G.L. Fraser.

-A tlas of Migmat ites

t

Figure

•

• • • • •

•

• • •

t

Fig. C4. The migmatites closest to the intrusive body on its west side show the greatest disruption of the pre-partial melting layering. and were mapped as the "chaotic zone" by Pattison and Harte ( 1988). The outcrop appearance of the "chaotic zone" migmatites is a jumbled mass of psammite, semipelite, and pelite blocks (i.e .. schollen or rafts) wit hin a coarser-grained, lighter-colored semipelitic matrix. Some schollen have recti linear outlines, but ot hers are rounded. The matrix is generally without a foliat ion or compositional

the aureole, the temperat ure of melt ing was relatively low (ca. 650- 700°C) . This situat ion is attributed t o t he innux of H 20 at peak metamorphic conditions released from the crystallization of t he underlying intrusive body.

layering, but where present, it appears to bend around the schollen in a manner suggestive of now. The semipelitic

D.R.M. Pattison. Previously published as fig. 13 in Pattison and Harte ( 1988) and is reproduced w ith t he permission of Blackwell Publishing.

matrix has a granular microstructure and contains patches of int erstitial-xenomorphic quartz and K-feldspar that, together w ith the now-like foliation, st rongly suggest that the matrix contained a substantial fract ion of anatectic melt. However, the semipelitic bulk composition of the matrix, and the scarcity of both leucosome and melanosome in the "chaotic zone," suggest that the melt fraction did not segregate from the solid resid uum in the mat rix. The fraction of melt in many parts of the "chaot ic zone" migmatit e was sufficiently high that bulk now of the migmatite could occur. The st ronger layers (with a lower fraction of melt) were disrupted during the now and became t he schollen in the portions with a higher fraction of melt. Even though the "chaotic zone" experienced the highest extent of melting in

Location: "Chaotic Zone" on the western side of t he intrusive body. Rock type: diatexite migmatite; the protolith was pelite and semipelite of t he Leven schist; anatexis at T 650- 700°C. Scale: the lens cap is 65 mm across. Image:

Further reading: Pattison, D.R.M. & Harte, B. ( 1988) : Evolution of st ruct urally contrasting migmat it es in the 3-kbar Ballachulish aureole, Scotland. journal ofMetamorphic Geology 6, 475-494.

147

E :...:O:...:L:-=..:..:E :...:R T_A_:_:U C....:. O_N__T_A:...: I 48 _______M_IG__M_ A__T_ I_T-=-E--5_FR_O____M_T__H__E:.._C..::.....:. OF T H E DULUT H IGNEOU S C OMPLEX

Figure

Fig. CS. Th1s migmat1te 1s typ1cal of those developed 1n the

outer part of the anatectiC zone. The original sedimentary layenng 1n the hornfels derived from the V1rg1n1a Formation rema1ns ev1dent and can be traced laterally across outcrops: hence, this a metatex1te m1gmatite. Two sets of th1n discordant domains of leucosome cross the compositional layering in the hornfels and locally form a net-like pattern A systematic sense of displacement across the oblique domains of leucosome suggests that they occupy a conJugate set of extens1onal shears related to shorten1ng perpendicular to the layering. This IS possibly due to loading of the footwall by the DIC as 1t was emplaced. Another set of somewhat th1cker, stromatic doma1ns of leucosome that are onented parallel to the bedding also 1s evident. All the leucosome matenal 1s fine-gra1ned (0.5 mm). but still much coarser grained than the paleosome (0. 1 mm). Although not vis1ble 1n outcrop. the doma1ns of leucosome are bordered by a narrow melanocrat1c selvedge that results from the alteration of cordiente 1n the wallrock (see Fig. F88). After local segregation of melt from the wallrocks 1nto nearby low-pressure s1tes (the extensional shears) to create the leucosome, crystallization of the melt appears to have released some H20, which then reacted with cordierite in the wallrock.

Location: Linwood Lake, area 35m from the western contact w1th the DIC. Rock type: net-structured metatexite m1gmat1te; peltt1c and sem1peltt1c protolith; anatex1s at T ca. 750°C. Scale: the ruler 1s IS em long. Image:

E.W. Sawyer.

Atlas of Migmatites

Figure

6

6

Z ._CENTIMETRES t I ~~

Fig. C6. The migmatites from the highest -grade parts of the Duluth aureole no longer contain recognizable and coherent structures that predate the partial melting and are, therefore, diatexite migmatites. Like the one in this photograph, they are typical ly very uniform, mediumgrained (0.5-2 mm), mesocratic rocks that locally contain small elongate fragments (schollen) of hornfels that are, in places, aligned. In many cases, the migmatites have a foliation defined by the parallel orientation of tabu lar minerals, typically plagioclase. cordierite, and orthopyroxene: typically, t here are no leucocratic segregations or mafic schlieren developed. Locally, deflect ions in the trend of the foliation define small shear zones that formed in the magmatic stat e. Furthermore, cordierite, plagioclase, and K-feldspar commonly have crystal faces against abundant pockets of coarser-grained, equant but irregularly shaped quartz, K-feldspar and plagioclase. The microst ructures indicate that these migmatites contained a significant fract ion of melt , and that they developed their foliation by the alignment of crystals in a flowing magma. The modal proportions of plagioclase, quartz, biotite. cordierit e, K-feldspar, and orthopyroxene, together with the wholerock composition of most of the diatexite migmatites in the Duluth aureole, indicate that partial melting of the pelitic and semipelitic protoliths, and subsequent flow of t he

resu lting diatexite magma, occurred without any significant segregat ion of the melt fraction from the residuum fraction. In other words, most of these diatexite migmatites represent a closed anatectic system.

Location: Linwood Lake, area 5 m from the western contact with the DIC. Rock type: diatexite migmatite: pelitic and semipelitic protolith: anat exis at T 800-825°C. Scale: the ruler is IS em long. Image: E.W. Sawyer.

149

ISO

UPPER AMPHIBOLITE FAC IES . REGIONAL ------------------------~---MIGMATITES FROM SAINT-MALO . FRANCE

Figure

7

Fig. C7. Neosome 1n the m1gmat1tes located close to the melt-1n 1sograd on the southeastern s1de of the Sa1nt- Malo m1gmat1te terrane, 1n Bnttany, formed first 1n the muscoVIte-nch semipelit1c layers. The bod1es of neosome are stromat1c and cons1st of thin, coarse-gra1ned muscov1te + K-feldspar + plagioclase + quartz leucosome bordered by b1ot1t e -rich melanosome . The start of partial melti ng in the migmat it es at Saint- Malo involved the breakdown of muscovite; biotite remained stable and was concentrated 1n the melanosome. Beds of psammitic and calc-silicate compos1tion were not as strongly affected by partial meltIng, and hence these are paleosome resister litholog1es at th1s stage. Consequently, the cont1nu1ty of structures that predate the partial melt1ng, sedimentary layenng 1n th1s case, 1s preserved 1n these low-grade m1gmat1tes, and hence th1s 1S a metatex1te m1gmatlte.

Locatton: La Landrais, Sa1nt-Malo m1gmat1te terrane, France. Rock type: metat ex1te migmat1te; Bnoverian sem1pelitic protolit h from just on the high-grade s1de of the "melt-1n" 1sograd, T ca. 650°C, P 4- 7 kbar. Scale: the ruler 1s 15 em long. /mage: E.W . Sawyer.

Atlas of M igmatites

Figure

8

Fig. C8. Regional metamorphic temperatures were higher farther into the Saint-Malo migmatite terrane; consequently, anatexis was far more extensive. A featu re common of virtually all of these migmatites is the absence of any laterally coherent structures that predate the partial melting; hence, these are diatexite migmatites, and they consist largely of neosome. The relative proportions of leucocratic to melanocratic material in the neosome, and the ratio of neosome to paleosome, vary considerably from outcrop to outcrop. In the example shown here, t he neosome is mesocratic overall, and consists of w ider leucocrat ic layers, or bands, that alternate with rather discontinuous, curviplanar biotite-rich foliae, called schlieren. In addition, the remains of a large schollen of a resister lithology also can be seen. Location: Saint-Malo migmatit e t errane, France. Rock type: mesocratic diatexite migmatite; semipelitic and pelitic protolith, partially melt ed at T ca. 750°C, P 4-7 kbar. Scale: t he side of t he yellow box is 17 em long. /mage: E.W. Sawyer.

lSI

152

MIGMATITE S

UPPER AMPHIBOLI TE FACIES , REGIONAL ------ ------ ----------- -----FROM THE OPATICA SUBPROVI NCE, QUEBEC

Figur·e

9

Fig. C9. A continuous foliat ion, or com posit ional layering, that predates the partial melting can be seen in all the migmatites developed 1n the lowest-grade parts of the anatectic zone in t he Opatica Subprovince; thus t hey are metatexite migmatites. Two principal morpholog1es of metatexite migmat it e have developed from the tonalit etrondhjemi te-granodio rite plutonic protoliths. Patch migmatites (see Figs. BIS and 012) formed in the Opatica Subprovince w here there was no penetrative deformation of the protolith during part1al melting, r.e., they occur 1n low-strain environments. This photograph shows another morphology. which developed as the protolith was sheared during partial melting. The light-grey and darker grey layers are a compositional banding that existed before partial melt1ng and are due to changes in the ratio of plagioclase to quartz + K-feldspar. These layers were deformed into north-vergent, asymmetrical folds dunng partial melting. Light-grey, coarser-gra 1ned, diffuse areas of neosome that do not have a foliation are located in shear planes on the limbs of the folds or subparallel to the axial surfaces of the folds. The neosome is slightly lighter colored than t he rock around it, but not really leucocratic. The diffuse edges suggest t hat t his is in situ neosome. The outcrop is also cut by thin, pink leucocratic veins that tend to have diffuse margins; some of these veins are folded. Commonly, the ve1ns

occur in shear zones located on the long limbs of t he asymmetrical folds. The pink veins have a composit ion suggestive of a probable derivation from a similar protolith; they may represent injections of anatectic melt. Locat1on: Opatica Subprovince, Quebec, Canada. Rock type: metatexite migmatite; tonalit e protolith, partially melted under conditions of the upper amphibol1te facies, T ca. 750°C, and P 5-7 kbar. Scale: t he ruler is IS em long. /mage: E.W. Sawyer.

-----Atlas o f M igmatites

Figure

I0

t

Fig. C I 0. Migmatites in the center of t he anatectic region in t he Opatica Subprovince do not contain cont iguous st ructures t hat predate t he partial melting; the structures they cont ain are essentially syn-anatect ic. This example shows lens-shaped schollen of grey, fo liated biotite- quartz- plagioclase tonalite in a pink, coarsegrained biotite-quartz- K-feldspar- plagioclase granite host. Some schollen are fragments of resister lithologies, but most have bulk composit ions indicating that t hey are melt-depleted and hence residua. The microst ructure in t he leucocratic matrix varies considerably from place to place. In many places, as in t his image, the microstructure is isotropic and granit ic, but elsewhere, there are domains in which tabular crystals (plagioclase, K-feldspar, and biotite) are aligned, forming a magmatic fo liat ion with o r without biotite schlieren. In places, coarse-grained granitic veins have intruded finer-grained rocks, but elsewhere (e.g., just below t he center of the photograph), coarse-grained rocks simply occur as diffuse pat ches in finer-gra ined ones. The leucocrat ic part of this migmatite contains some K-feldspar crystals that are euhedral, and others that have rational crystal faces at mut ual contacts with quart z, text ures that are indicative of crystallizatio n from a melt. These observations suggest that in the center of the Opatica Subprovince, t he fraction of melt was sufficiently high that the partially

melt ed rocks were able t o flow, and th us destroy the preanatexis structu res t hey contained . The only pre-anatexis structures that remain are those preserved in t he entrained schollen or rafts; hence, this is a diatexite migmatite and represent s a more advanced stage of anatexis than t hat recorded in Fig. C9.

Location: Opatica Subprovince, Quebec, Canada. Rock type: diatexite migmatite; metatonalite protolit h, anatexis under conditions of t he upper amphibolite facies, T ca. 750°C, and P 5-7 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

153

154

GRANULITE-FACIES , REGIONAL MIGMATITES

----------------------------FROM THE ASHUAN I PI SUBPROVIN CE, QUEBEC Figure

I I

Fig. C II . This m1gmatite 1s dom1nated by grey metasedImentary rocks and has a relatively m1nor proport1on of leucosome. Since the streaks of leucosome are mostly located parallel to the bedding and foliat1on, the migmatlte could be called stromatic. The mineralogy and whole-rock compos1t1on of the metasedimentary rocks 1n this migmatite are not consistent with an interpretation that they are paleosome, but indicate that they are the residuum left after the extraction of about 30 vol.% anatectic melt of granitic composition. This degree of melt loss far exceeds the volume of leucosome present in the outcrop; hence, anatexis and the format1on of th1s m1gmatite were the result of an open-system process at the scale of the outcrop. The preservation of structures and layering that predate partial melt1ng in the residuum after the loss of such a large volume of melt 1ndicates that the melt was contmuously drained from the res1duum dunng anatex1s. If the melt had accumulated 1n the rock up to a large fraction (> I 0%), then bulk flow would have occurred by one process or another, and that would have destroyed the layering. Because all the layers were fertile, and the melt-segregation process allowed only a small fraction of melt to exist in the rock at any one t ime, the resulting migmatite is essentially all neosome, but a residual one in which pre-anatect ic structures are preserved. Large areas of fertile rocks undergoing this type of

partial melting and segregat1on process could be the source of large volumes of granitiC magma 1n the continental crust; for the Ashuamp1 Subprovince, melt-depleted migmatites 3 such as this are est1mated to have produced 640 000 km of gran1t1c magma. In assigning th1s m1gmat1te a further descriptive term, a decision has to be made as what characteristic needs to be emphasized; two possibilities are its morphology and its bulk composition. Location: Ashuanipi Subprovince, Quebec, Canada. Rock type : residual metatexite migmatite; metaturbidite protolith, with partial melting at T 825-875°C, P 6-7 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

Atlas of Migmatites

Figure

•

Fig. C l2 . In this migmatite, schollen (or rafts) of meltdepleted metasediment ary rocks identical to the resid ual parts of t he metatexite migmatites (e.g., Fig. Cl l) occur in a coarser-grained, lighter-colored host; in th is case, then, the rafts are the residual part of the neosome and not paleosome. However, in some out crops, the schollen include ultramafic, mafic, and vein quartz resister lithologies, and these are all that remains of t he paleosome. Microstructures that indicate the flow of magma (i.e., melt containing crystals) and crystallization of a melt are common in the matrix around the schollen. Examples of such microst ructures include the tiling of tabular crystals of feldspar, the local development of euhedral crystals of feldspar and orthopyroxene, and of minerals wit h crystal faces against large interst it ial crystals of equant, xenomorphic quart z. The most important characteristic of t his migmatite, the one that identifies it as a diatexite migmatite, is the absence of structures that predate the partial melting in the mat rix around the schollen. T he matrix is neosome also, and the absence of discrete melanosome and leucosome in it indicates that the melt fract ion and solid fraction did not become separated during anatexis. Rat her, they flowed together as magma.

Location: Ashuanipi Subprovince, northern Quebec, Canada.

Rock type: schollen diatexite migmatite; metaturbidite protolith part ially melted at T 825- 875°C, P 6- 7 kbar. Scale: t he ruler is IS em long. /mage: E.W. Sawyer.

ISS

SEC O N D -O RDER MORPHOLOG IES I N MIG MATIT ES

156 --------------------------~

D. SECOND-ORDER MORPHO LOGIES IN M IGMATITES The second-order morphologies in migmat1tes can be related to the melt fraction present 1n the m1gmat1te and, 1n a general way, to stra1n, or how the protollth responded to stran With the photographs 1n th1s sect1on, 1t w1ll be possible first to examine the range of different morphologies that can be found 1n metatex1te m1gmat1tes. where paleosome dommates, and 1n wh1ch structures that

Metatexite migmatites with a nebulitic structure [Figs. 011, 012] The term nebufltrc has been used 1n two ways to descnbe m1gmatites. The term is most commonly used to describe doma1ns of neosome that have an 1ndistinct. diffuse or "fuzzy" boundary w1th a volumetrically dominant paleosome, so that the contact between them is typICally interpreted to be gradat1onal. This usage applies to metatexite m1gmatites. The second usage applies to the descript1on of larger reg1ons where fa1nt. ghostlike relics of discontmuous. but not disrupted. paleosome can be seen through the "cloudiness" of the neosome. Th1s use refers

predate part1al melt1ng are preserved. Then, the morphologieS found 1n d1atex1te m1gmat1tes, where neosome predom1nates, and 1n whiCh coherent pre-anatectic structures are destroyed and replaced by structures formed

to d1atexite migmatites. Nebulit1c m1gmatites have a homogeneous microstructure (i.e .. no foliat1on. or magmat1c flow structure. and have not segregated into leucosome and melanosome). Consequently, they are mterpreted to form 1n envwonments of very low syn-anatect1c strain.

during anatex1s.

The start of partial melting [Figs. 01 - 06]

Metatexite migmatites with leucosome in dilatant structures [Figs. 0 13-022]

An important part of mapping in metamorphic terranes is the determination of isograds. In high-grade metamorphic terranes, the "melt-in isograd" marks the beginn1ng of the zone of anatexis. It IS where migmatites beg1n to appear. However, noticing the first signs of part1al melting 1n the field IS not easy. The photographs 1n th1s sect1on show

Where anisotropic rocks are deformed, differences in the viscosity of the layers can result in stra1n-rate discontinuities along the layer boundaries that are accommodated by the formation of dilatant structures; boudinage and foliation boudinage are two well-known examples. If partial melt is present in the rock dunng deformation, then it will

examples of the incipient stages of anatexis. Because part1al melt1ng has barely started 1n these rocks, structures that pre-date partial meltmg, for example, bedd1ng, compositional layering, foliations, and folds, rema1n prominent in the paleosome; hence, these are metatexite migmat1tes, albeit

m1grate to the dilatant s1tes, where the pressure in the rock is lowest. If the melt rema1ns there and crystallizes, it will form leucosome. Leucosome located in dilatant sites is very common in migmatites and exhib1ts a very wide range of morphology that is pnncipally controlled by the nature of the anisotropy. Leucosome in dilatant sites related to the

w1th a very small proportion of neosome.

development of folds will be dealt with later.

Metatexite migmatites with a patch structure [Figs. 07-0 I0] Patch migmatites are a type of metatexite migmatite 1n which the neosome occurs as patches in the paleosome. The patches of neosome can be sharply defined or they can be diffuse or nebulose. Similarly, the neosome may, or may not, have segregated into leucosome and melanosome. Patch migmatites are most commonly found in the lowestgrade parts of migmatite terranes, and are preserved in the parts of an anatectic terrane where the syn-anatectic strain was low (see also Fig. Bl).

Metatexite migmatites with a net structure [Figs. 023-030] A net structure is formed in a metatexite migmatite where two or more sets of leucosome segregations intersect to create a net-like pattern of leucosome enclosing rhomb- or lozenge-shaped domains of paleosome or. in some cases, melanosome. A net structure is very common in metatexites, and its development appears to be a necessary step in the extraction of large volumes of melt from anatectic terranes and the creation of large areas of melt-depleted migmatites.

A tlas of M igmati res

Metatexite migmatites with a layered or stromatic structure associated with low strain [Figs. 031-034] A stromat ic or layered morphology results w here the neosome in a migmatite occurs as parallel, laterally continuous bands in two dimensions and parallel sheets in the third. The bodies of neosome must also be parallel to the principal planar structure in the host. Most commonly, this is a foliation, but it can also be com positional layering, e.g.. bedding in a sequence of t hinly bedded pelites, semipelites, and psammit es. The t erm stromatic usually is applied to the leucosome in a migmatite because it is the most prominent part of the neosome. However, t here is no reason why a migmatite consisting of parallel bodies of melanosome, say after the loss of melt, cannot also be called stromatic. In this sect ion, I show examples of migmatites with a stromatic morphology that is an initial, low-strain feat ure of the migmatite. In the next section, I wi ll show examples with stromatic morphology arising from high strain during anatexis.

The transition from metatexite to diatexite migmatites [Figs. 041-046] Metatexite migmatites are characterized by the widespread preservation of t he structures that existed before anatexis. In contrast, diatexit e migmat ites are characterized by the general absence of coherently preserved structures t hat predate t he partial melting, i.e., diatexite migmat ites are largely, or wholly, neosome. The destruction and overprinting of old structures in migmatites are progressive, and in many migmatite terranes, there is a transitional zone between metatexite and diatexit e migmatites in which paleosome is sequentially disrupted and consumed into the neosome. O ne of the feat ures of this t ransition is the preservation of fragments of the metatexite migmatite as abundant enclaves, which are called schollen, or rafts; hence, these transitional rocks are called schollen diatexites or raft diatexites. In ot her migmatite terranes. the change from metatexite to diatexite migmatite is abrupt. This first group of photographs shows examples of the early stages in the formation of diatexite migmatites.

Metatexite migmatites with layered or stromatic structure due to transposition [Figs. 035-040]

Oiatexite migmatites with schollen and with schlieren structures [Figs. 047-052]

A feature of st romatic. or layered, migmatites from zones of high st rain is the intensity and continu ity of the layering,

migmat ites in t he direction of higher metamorphic grade and increased fract ion of melt away from t he metatexite to diat exit e transit ion. The proportion of rafts, or schollen, decreases, and correspondingly, the pro portion of leucocratic neosome increases. Schollen of metatexite material in t he neosome tend to become progressively more lenticular in shape and are no longer angular, whereas schollen composed of resister lithologies, such as calc-sil icates or vein quartz. are commonly rounded and equidimensional. Schlieren, t he t hin persistent layers of dark minerals found in diatexite migmat it es (and in some granites), start to become prominent in the mat rix and commonly define flow lines around t he schollen.

and the remarkable regularit y in the widt h and spacing of the domains of leucosome. A microstructural examination of the leucosome is especially important. If the leucosome contains magmat ic or submagmatic microstructures, then the term layer or stromatic migmatite is just ified. However, if the microstructures all formed in the solid state, t hen the rocks may have acq uired their layered structure after the melt in them had completely crystal lized; in t hat case, the term migmat ite. although possibly appropriate for t he protolith of the highly strained rock, is not appropriate for t he rock overall. It should be named accordingly (e.g., prot omylonitic migmatite, mylonitized migmat ite) .

There is a systematic change in the morphology of diatexite

157

SECOND-ORDERMORPHOLOGIES IN MIGMATITES

158 ----------------------------

Oiatexite migmatites with schlieren structures [Figs. 053-056]

Oiatexite migmatites at high strains [Figs. 063, 064]

The morphology of diatex1te m1gmatites may appear to become less complex as the proport1on of schollen decreases. However. th1s may well be a misconception, and many diatex1te migmatites exhibit s1gn1ficant local variations 1n color, grain size. microstructure, and composition, most commonly in the form of discontinuous bands, or layers. The most w1dely known type of compos1t1onal variation are

Where the melt fract1on in a diatex1te m1gmatite 1s suffiCiently h1gh that crystals are in a dilute suspension, magma flow can occur w1thout 1nteract1on between the solid crystals, and high strains are not fully recorded. As the melt fraction decreases, the crystals start to interact and to create a framework, and th1s can be deformed and preserve a record of the stra1n. Diatexite m1gmatites that had a framework of crystals enclosing melt-filled pockets, where deformed at h1gh stra1ns. can develop strongly layered morpholog1es compns1ng bands of the deformed crystalframework material and leucosome bands cons1sting of the melt expelled from the framework as 1t collapsed.

schlieren, the thin layers of dark m1nerals found in d1atex1te m1gmatites and 1n some gran1tes. Commonly. schlieren are nch 1n b1ot1te, but they can also conta1n other platy or rodshaped m1nerals such as plag1oclase. S1lliman1te. amphibole. and orthopyroxene. Schlieren generally show a considerable ennchment 1n accessory m1nerals. It 1s likely that a number of different processes can produce schlieren. They all requ1re that the host was 1n1tially a magma, however. Mechanisms that have been suggested 1nclude, amongst others, flow sorting, filter press1ng, and eros1on of the rafts or schollen.

Oiatexite migmatites [Figs. 057-062] The highest-grade parts of many migmat1te terranes contain portions of outcrops, whole outcrops. or larger areas 1n wh1ch only syn-anatect1c or younger structures are present. In these rocks. the proportion of neosome IS very h1gh, that of paleosome is correspondingly m1nor, and schollen and schlieren are few, or even absent. Consequently. changes 1n microstructure and compos1tlon that account for the morphological vanat1ons 1n these m1gmatites tend to be very subtle, but these are not homogeneous, or uniform. migmatites. Furthermore, many examples of diatexite migmatite contain leucosome and leucocratic veins of late anatectic age. There is no prefix term to descnbe the morphology of these rocks; they are the quintessential diatexite migmatites. Diatexite migmat ites can be mesocratic, leucocratic, or melanocratic, and these variations in color have been found to correspond to the bulk composition of the protolith, the degree of enrichment 1n the melt fract1on and in the res1duum fraction, respectively. Consequently, a descriptive term used to describe the compos1t1onal, mineralogical, or microstructural vanat1ons w1th1n an outcrop of diatex1te m1gmat1te, or over a larger reg1on, may be useful.

A tlas of Migmatites

Figure

I

Fig. D I. Subtle ev1dence that partial melt1ng has started 1n

Locotton: Mount Hay, Arunta Inlier. Australia . Rock type:

th1s granulite-fac1es metamafic rock is v1sible 1n the center of the image. Th1n. wh1te 1ntergranular nms are developed around the solid product and reactant phases. The wh1te nms consist of quartz and plag1oclase, and represent crystallized fi lms. and small pockets, of melt that formed on the

nebulit1c patch metatex1te m1gmat1te; metamafic protohth, partial melt1ng at T 825- 875°C, P 6 7 kbar. Scale: the ruler

grain boundaries between the react ant minerals. As the melt did not separate from t he residuum, the neosome composition should represent the protolith composition. The melting reaction was hornblende + plag1oclase + quartz

=

orthopyroxene + melt. The thin leucocratic bands parallel to the ruler are subsolidus segregat1ons that predate anatex1s. At the mop1ent stage of anatexis shown by th1s m1gmat1te, the degree and extent of part1al melt1ng are very m1nor. so that at the outcrop scale. the m1gmat1te preserves all of the features of its unmelted lower-grade equ1valents; thus, 1t 1s a metatexite m1gmat1te. As the edges of th1s neosome are very difficult to define prec1sely, and the neosome occup1es a discrete, small volume 1n the paleosome, t he migmatite could, if des1red, be described as a nebulit ic patch metatexit e migmat ite.

IS

IS em long. Image: E.W. Sawyer.

Further readtng: H arte, B., Patt1son, D.R.M. & L1nklater, C.M.

( 199 1): Field relations and petrography of partially melt ed pelit e and semi -pelit ic rocks. In Equilibrium in Contact Metamorphism : t he Ballachulish Igneous Complex and its Aureole (G. Voll, j. Topel. D.R.M. Pattison & F. Seifert, eds.). Springer-Verlag. Heidelberg. Germany ( 181 - 209).

159

THE START O F PARTIAL ME LTIN G

160 -----------------------------

Fig. 02. Dunng 1nc1p1ent melt1ng of th1s metamafic rock, the t1ny fract1on of melt that was generated m1grated to the low-pressure sites adjacent to the competent crystals of garnet. The sol1d product of the Incongruent breakdown of hornblende was clinopyroxene, and th1s mineral occurs in the groundmass, and hence rema1ned at the site where the melt formed. The melting reaction was possibly hornblende + plagioclase + quartz = clinopyroxene + melt ± garnet. Most, if not all, of the garnet in this iron-rich metamafic rock was present before partial melting occurred; rocks in the lower amphibolite facies that have similar bulk compositions contain abundant porphyroblasts of garnet. The metamafic rock IS Fe-nch because 1t crystallized from a melt that was the evolved product of the fract1onal crystallization of a more prim1t1ve parental basalt magma, one that had crystallized magnes1an oliv1ne and pyroxene.

Locatton: Ab1tib1 Subprovince, south of Ch1bougamau, Quebec, Canada. Rock type: metatex1te m1gmat1te; metamafic protolith part1ally melted at condit1ons JUSt below the "orthopyroxene-in" 1sograd, T 800- 850°C, P 8- 10 kbar. Scale: the ruler is IS em long. /mage: E.W Sawyer.

A tlas of Migmat ites

Figure

l

• •

Fig. 03 . Incipient partial melting in t his plagioclasehornblende metamafic rock has produced very small, elongate patches of neosome. The neosome contains

Locatton: Saint-Fulgence, Grenville Province, Quebec, Canada. Rock type: patch metatexite migmatite; metamafic prot o-

small subidioblastic crystals of clinopyroxene in a tonalitic, plagioclase + quartz groundmass. This assemblage is interpreted as indicating that the neosome represents the in situ,

T 800-850°C, P 5-8 kbar. Scale: t he ruler is 15 em long.

nonsegregated products of the incongruent breakdown of hornblende by a reaction such as hornblende + plagioclase + quartz clinopyroxene + melt. The domains of neosome are 5-10 em long, spaced about 75 em apart, and are all elongate in the same direction. In this particular metamafic rock, there are no obvious structures associated with the orientat ion of the domains of neosome. However, the orientation of the domains of neosome in the metamafic layers

=

does correspond to t he orientation of the axial planes to small asymmetrical, Z-shaped folds developed in t he adjacent metapelitic beds. Although partial melting was incipient in the metamafic layers, it had reached a far more advanced stage in the pelitic layers, which have lost significant amounts of anatectic melt and now have assemblages of residual minerals and appropriate melt-depleted bulk compositions (see Fig. B38).

lith, anatexis at conditions of the lower granulite facies, Image: E.W. Sawyer.

161

THE START OF PARTIAL MELTING

162 ----------------------------Figure

4

Fig. 04. The onset of part1al melt1ng 1n coarse-gra1ned fels1c rocks can be difficult to detect because the paleosome and neosome are very s1milar 1n color. mineralogy. and m1crostructure. This example from the "melt-1n" 1sograd shows a small. diffuse leucocrat1c patch. or lens. located along a small, isolated shear zone, vis1ble as a curvature of the pre-partial-melting foliation and compositional banding in the paleosome. The neosome is recognized by 1ts slightly coarser grain-size. diffuse margins, and because it has essentially no foliation, characteristics that together 1ndicate m s1tu formation. A th1n vein of anatect1c melt has 1ntruded the paleosome JUSt below the patch of in sttu neosome. The preservat1on of pre-anatectic structures and the dom1nance of paleosome 1ndicate th1s to be a metatexlte m1gmatite. Furthermore. the form of the neosome means that the descnpt1ve term "patch" could be applied also. The ve1ns become more abundant, and the patches larger. in the dwection of 1ncreas1ng metamorphic grade. Locat1on : Opatica Subprovince, Quebec, Canada. Rock type: patch metatexite migmatite: tonal1te gneiss protolith, partial melting at conditions of the upper amphibolite facies,

T ca. 750°C, P 5- 7 kbar. Scale: the ruler is 15 em long. Image: E.W. Sawyer.

Atlas of Migmatites

F1gure

DS

•

•

• •

Fig. 05. Part1al meltmg 1n these peht1c m1gmatites occurred as the rocks were undergo1ng m1nor extens1on parallel to the layenng. Th1s migmatite IS from JUSt 1ns1de the "melt-1n" 1sograd in the contact aureole of the Duluth Igneous Complex (DIC) and, therefore, shows a m1gmatite formed 1n a sett1ng of very low degree of partial melting. The anatectic melt generated 1n the rocks has migrated a few millimeters from the grain boundaries where it formed into small extensional shear zones nearby, and created the network of millimeter-wide bands of leucosome. At the microscopic scale, the bands have diffuse margins suggestive of local segregation of the melt, rather than the inJection of anatectic melt into fractures, and a granular microstructure. which may be the result of rap1d cooling 1n the contact aureole. Because the bedding, a structure that ex1sted before anatex1s, 1s preserved 1n a coherent manner, th1s 1s a metatexite migmat1te. The arrangement of the doma1ns of leucosome, albeit very thin, in this migmat1te has created a net-like pattern, and such morphology 1s very common 1n metatex1te migmatites; th1s could also be descnbed as a net-structured metatexite migmat1te. The fine gra1n-s1ze of this migmatite is characteristic of migmatites developed from low-grade metasedimentary protoliths in shallow contact-metamorphic aureoles (see also Fig. D6).

Location: L1nwood Lake area 50 m from the western contact of the Duluth Igneous Complex, Mmnesota, U.S.A. Rock type: metatexite m1gmat1te; pelite and semipellte protolith, anatexis at T 700-750°C, P 2 kbar: Scale: the scale is

15 em long. Image: E.W. Sawyer. Further readmg: Holness, M. & Clemens, J.D. (1999): Partial melting ofthe Appin quartzite driven by fracture-controlled H 20 infiltration in the aureole of the Ballachulish Igneous Complex, Scottish Highlands. Contnbuuons to Mmeralogy and Petrology 136, 154-168.

163

THE START OF PARTIAL MELTING

164 Figure

6

N~

: t. -m . ("')

,z -

::! · - m.

" 3: . -i

. ;n m ~

- CJ)

Fig. 06. This migmat1te records the early stages of partial melt1ng in a metasedimentary enclave w1th1n the Kuna Crest Granodiorite 1n the western part of the Tuolumne Complex, Yosem1te National Park, California. The metasedimentary protolith consisted of interlayered S1lioclast1c rocks ranging in composition from pelite to quartzite, but psammite predominates. The metasedimentary rocks were intruded by numerous thin granitic dikes, probably from the nearby El Capitan granite, some IS Ma before they were incorporated into the younger Kuna Crest Granodiorite as an enclave and experienced minor partial melting. The photograph IS dominated by paleosome that consists of grey, quartzofeldspathic metapsamm1tic schist that contains layers of pale buff quartzite 1- 2 em th1ck. Both are cut by th1n, sl1ghtly discordant gran1tic (G) layers that have th1n biotitench mafic selvedges. The ev1dence for part1al melt1ng 1n the enclave is the local presence of 1nconsp1cuous, th1n, diffuse bodies of pale grey neosome (N) that have not segregated into leucosome and melanosome parts. The neosome has a slightly coarser grain-size than the grey metapsammite paleosome, and this may account for its slightly lighter color.

The neosome appears to be located 1n shears developed on both the long and short limbs of asymmetncal crenulation folds prom~nent 1n the center of the photograph. Thus, they are oriented approximately parallel to the axial surfaces of the folds. Location: May Lake area, Tuolumne Complex, Yosemite Park, U.S.A. Rock type: metatexite migmatite; metapsammite protolith containing felsic veins. Scale: the ruler is

IS em long. Image : E.W. Sawyer.

A tla s o f Migma tites

Figure

165

D7

V294 Fig. 07. Th1s photograph shows the first m1gmat1tes that appear 1n an Archean prograde metamorphic sequence that runs from greensch1st through amph1bohte to the granulite fac1es. The neosome occurs as patches that are much coarser gra1ned and more heterogeneous than the surrounding paleosome, a composit ionally layered plagioclase hornblende metamafic rock. The preservation of compositional layenng that predates anatexis indicates that this is a metatexite migmatite. The patches of neosome consist of a plagioclase + quartz leucocratic port1on and a clinopyroxene + hornblende + garnet melanocratic part. Clinopyroxene generally forms the largest crystals. and 1n many cases. 1t conta1ns inclusions of hornblende. Thus, the melt1ng react1on may have been s1milar to hornblende + plagioclase + quartz garnet + clinopyroxene + melt. The largest patch of neosome has a uniform d1stnbut1on of leucocratiC and melanocratic constituents. and 1t has a diffuse border. However. the patch of neosome 1n the lower right has a very narrow, out er melanocrat1c nm. and a more leucocratic core, wh1ch contains large crystals of clinopyroxene. As the bulk geochemical composit ion of t he patches IS similar to that of t he paleosome, t he patches of neosome

=

~ 15 7

•I

1n th1s migmatite appear to have formed m sttu. The differences 1n 1nternal structure between adJacent patches of neosome must represent local d1fferences 1n the degree to wh1ch the melt and res1duum were able to separate.

Location: Abitib1 Subprovince, south of Chibougamau. Quebec, Canada. Rock type: patch metat exite migmat1te; metamafic protolith, partial melt 1ng at condit ions close to t he transition from t he amphibolit e t o granulite facies, T 800- 850°C. P 8- 10 kbar. Scale: t he ruler is IS em long. Image: E.W . Sawyer.

METATEXITE MIGMATITES WITH A PATCH STRUCTURE

166 - - - - - - - - - -

Fig. 08. The grey paleosome of this m1gmatite IS a medium-gra1ned garnet- .and sllllmanite-beanng biotitequartz-plagioclase sem1peht1C sch1st. It conta1ns several small, coarse-gra1ned patches of neosome, each a few centimeters across, that contain the m1neral assemblage plagioclase + K-feldspar + quartz + cordiente + b1otite ± garnet. Biotite in the patches of neosome part1ally replaces cordierite or garnet (or both). The melt ing reaction was melt possibly biotite + quartz + sillimanite + plagioclase + cordierite + K-feldspar ± garnet. Most of the patches of neosome have parts that are rich in ferromagnesian min erals and others that are nch 1n quartz, plag1oclase, and

=

K-feldspar. However, the distnbut1on of the leucocrat1c and melanocratic parts within the neosome vanes considerably from one patch to the next. The manner and extent to wh1ch the melt fraction separated from the solid fraction thus varied from neosome to neosome. Some patches of neosome are essentially equant 1n shape, but others, located in mor e micaceous layers, are elongate along the d1rection of the foliation. Anisotropy 1n the protolith thus 1nfiuenced how individual patches of neosome grew. Both the grey paleosome and the patches of neosome are cut by

planar, discordant leucocrat1c vems that are rich 1n quartz and K-feldspar. These veins are Interpreted to be denved from a fractionated anatectiC melt that Intruded the mlgmatite when 1t was close to 1ts solidus temperature. Location: Lac Sa1nte-Mane. Central Metasedimentary Belt, Grenville Prov1nce, Quebec, Canada. Rock type: patch

metatexite migmatite: metapelitic protolith, partial melting at T ca. 750-800°C, P 5 7 kbar. Scale: the ruler is IS em long. Image : E.W. Sawyer.

A tlas of Migmat ites

Figure

167

9

• • • •

Fig. 09. Thick quartzofeldspathic layers, such as beds of metagreywacke, are more competent, and hence record lower int ernal strains than the more micaceous interbedded pelitic layers. Furthermore, because of their bulk composi tion, partial melting in the quartzofeldspathic layers t ypically starts at higher temperat ures than in the pelitic ones. This patch migmatite formed in a feldspathic, Al-poor psammitic layer where partial melt ing was in its early stages. The migmat it e consists of a light-grey biot ite-quartz-plagioclase schist paleosome that hosts equant patches of neosome; these have a slight preferred orientation parallel t o the local 53 foliation (parallel to the scale). T he patches of neosome are distinctive in that t hey have a dark inner part consisting of several orthopyroxene crystals, and a leucocrat ic outer part consisting of quartz, plagioclase, and K-feldspar; there is no biotite in t he patches of neosome. Commonly, the orthopyroxene contains elongate inclusions of quartz. T he inner part of t he leucocratic rim, close t o the orthopyroxene, is richer in K-feldspar t han t he out er part. The melting react ion in the psammitic layer was probably biotite + quartz + plagioclase orthopyroxene + melt Melting was more advanced, and the st rain higher, in the adjacent pelitic layers; hence, migmatites with a stromat ic, or layered, morphology developed t here. The melting reaction in the

=

pelitic layers probably included sillimanite, and may have been biotite + sillimanite + quartz + plagioclase garnet + cordierite + K-feldspar + melt.

=

Location: Wuluma Hills, Aru nta Inlier, Australia. Rock type: patch metatexite migmatite; Al-poor psammite protolith, w it h anatexis at T 825-875°C. P ca. 5 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

METATEXITE MIGMATITES W ITH A PATCH STRUCTURE

168 - - -- - - - - - - - - - - Figure

I0

Fig. D I 0. T h1s photomicrograph shows a variat1on 1n the morpholog1cal detail of the patch migmat1tes developed m quartzofeldspath1c, Al-poor psammit1c beds (d F1g. 09). The melanocrat1c cores of these patches of neosome consist of a s1ngle, large, 1d1oblastlc crystal of orthopyroxene that 1s surrounded by a narrow, leucocrat1c nm composed of quartz + plagioclase + K-feldspar. Some patches have a w1der nm than others, and this may 1nd1cate the loss of melt from those patches that have a narrow rim. The peculiantles of local weathenng have h1ghlighted some of the leucocrat ic nms by makmg them p1nk1sh wh1te, but most are dull grey and inconspicuous. The m1neral assemblage 1n the fine-gra1ned paleosome around the patch is quartz + plag1oclase + b1otite. The absence of b1ot1te from the patches of neosome suggests that they formed by a meltIng react1on 1nvolv1ng the 1ncongruent breakdown of b1ot1te. such as b1ot1te + plag1oclase + quartz orthopyroxene + melt. The patches of neosome 1n th1s photomicrograph are a max1mum of 3 em across, but s1m1lar patches can attain more than 30 em across elsewhere 1n the Wuluma Hills area. Adjacent, slightly more alum1nous beds conta1n patches of neosome that have a very similar appearance. but they conta1n cordierite, or cord1erite + garnet. in add1t1on to orthopyroxene (see Fig. F77).

=

Locat1on: Wuluma H1lls. Arunta Inlier, Australia. Rock type: patch metatexite m1gmat1te; Al-poor psamm1te protolith, part1ally melted at T 825 875°C. P ca. 5 kbar. Scale: the ruler is IS em long./mage: E.W. Sawyer.

A tl as of Migmatites

Figure

I I

• •

• • • •

Fig. D II . The thin, fine-gra1ned layers in various tones of pale grey are granodiont1c, trondhJemltiC, and intermediate orthogne1sses that compnse the paleosome in th1s migmatlte. Deformation, some of it prev1ous to anatexis and some of 1t synchronous w1th it, has attenuated the felsic orthognelsses and generated the banding 1n them; the more competent and generally thicker intermediate layers have become boudinaged. Partial melt1ng of the orthogne1sses has generated small patches of neosome t hroughout t his m1gmat1te. The domains of neosome are coarser gra1ned than the paleosome, and can be distinguished by the relatively large gra1ns of ferromagnes1an m1nerals they contain, and because they are ne1ther foliated nor compositionally banded. Some doma1ns of neosome are located in obvious low-pressure s1tes. such as at the ends of boudins or in shear bands. Others, however, occur as variously shaped patches (e.g., near the pocket knife) that do not appear to be related to any specific structure. Thus, part1al melt1ng is interpreted to have occurred during. and after, the penetrative deformation that affected the paleosome. Furthermore, the absence of d1screte doma1ns of melanosome in the migmatlte Indicates that the melt fract1on did not separate from the res1duum, wh1ch is consistent w1th t he format1on of the neosome after most of the penetrative deformation. These part1cular doma1ns of neosome (labeled N) can

be called nebulit1c because they have do not have sharply defined borders, and 1n many places appear to be gradational from the paleosome (see also Fig. 01).

Locatron: Sand R1ver Gne1sses, Causeway locality, South Afnca. Rock type: metatex1te m1gmat1te w1th nebulit1c neosome; orthogne1ss protolith, anatex1s under granulite-facies conditions. Scale: the pocket knife is II em long. Image: E.W. Sawyer.

169

METATEX ITE MIGMATITES WITH A NEBULITIC STRUCTURE

170 -----------------------------Figure

11

Fig. D 12. This metatex1te m1gmatlte IS from JUSt w1thm the "melt-1n" isograd; 1t formed by anatex1s of a mass1ve felsic protolith 1n a local environment where the syn-anatect1c stra1n was relatively low. The neosome 1n th1s m1gmatite occurs as light. buff-colored, oval patches of granitic composition hosted by a grey, foliated leucotrondhjem1t1c paleosome. Because the patches of neosome (N ) have a diffuse margin and do not have the foliation ev1dent in the paleosome (P), they are somewhat nebulous in appearance; hence, this patch metatexite migmatite might also be called a nebulite. The neosome has a slightly coarser gra1n-size than the paleosome; the res1dual phases thus recrystallized to a larger gra1n-s1ze while melt was present. Petrographic and geochemical data 1ndicate that melt has not segregated from the residual solids dunng the formation of the patches of neosome; hence, no leucosome or melanosome was produced, 1.e., the products of anatexis have remained strictly m s1tu.

Locot1on: Opat1ca Subprov1nce, Quebec, Canada. Rock type: nebulitic patch metatex1te migmat1te. leucotrondhjemitic protolith, anatex1s at cond1t1ons of the upper amphibolite facies, T ca. 750°C. P 5- 7 kbar. Scale: w1dth of the field of view is 45 em. /mage: E.W. Sawyer.

Further reading: Sawyer, E.W. (1998): Formation and evolution of granite magmas dunng crustal reworking: the significance of diatexites.Journof of Petrology 39, 11 47- 1167.

,

• • •

Atlas of Migmatites

F1gure

13

• •

• • •

• t

Fig. D 13. This photograph shows the morphology of metatex1te m1gmat1tes formed 1n a pre-anatexiS to synanateXIS stnke-slip shear zone 1n a metamafic protoltth. The compos1t1onal layering remains prominent 1n the paleosome. The neosome cons1sts of melanosome most commonly developed parallel to t he compositional layering, and leucosome t hat occurs at a number of different structural sites: (I) along extensional (normal) shears form1ng a conjugate set oriented at about 30 40° to t he layering, (2) 1n the space between boudins t hat have developed in the most competent layers (commonly layers of melanosome), and hence oriented approximately normal to the layenng, and (3) 1n thin films of leucosome ly1ng parallel to the compos1t1onal layering. Not1ce, however, that most of these are also closely related to the melanocratic layers (M) . Thus, the leucosome is located 1n the dilatant s1tes that formed as the metamafic paleosome and then the layers of res1duum (melanosome) underwent layer-parallel extenSIOn. The melt thus was mobile dunng synchronous part1al melting and noncoaxial strain.

LocatiOn: Abitibi Subprov1nce, south of Ch1bougamau Quebec, Canada. Rock type: metatex1te m1gmat1te, metamafic protolith, anatex1s at cond1t1ons near the transition from amphibolit e to granulite faCies, T 800 850°C, P 8 I0 kbar. Scale: ruler is 15 em long. Image: E.W. Sawyer.

171

METATEXITE MIGMATITES WITH LEUCOSOME

172 ------------------------------IN DILATANT STRUCTURES Figure

14

Fig. D 14. This m1gmatite consists of two parts. The mel anocratlc portion is dominant and cons1sts of metamafic layers that have strongly melt-depleted bulk compos1t1ons relative to the likely metamafic protolith, and they are 1nterpreted to be the residuum left after the extract1on of anatectic melt. The mineral assemblage 1n the metamafic layers 1s hornblende + clinopyroxene + garnet + plagio clase: t he color difference between layers is due to minor changes 1n the modal proportions of the same minerals. The migmatite contains a conspicuous set of regularly spaced domains of leucosome IS 25 em long that are oriented e1ther perpendicular to the layering, or between 30 and 45° to 1t. The leucosome occupies structures that formed as a result of extension parallel to the layenng 1n the metamafic rock. All these bodies of leucosome are tonahtiC 1n composition (plag1oclase + quartz) and coarse-gra1ned. Some of the bodies of leucosome conta1n angular, spalled-off fragments of wallrock. There is no d1screte melanocratiC nm around the leucosome, but th1s is not surpris1ng g1ven the res1dual nature of the entire host. Another set of domains of tonalitic leucosome occurs parallel to the layering of the met amafic rock. Although these are laterally persistent, t hey are t hin (<5 mm) and inconspicuous.

Locat1on: Ab1tib1 Subprovince, south of Ch1bougamau. Quebec, Canada. Rock type: dilat1on-structured metatexlte m1gmat1te, metamafic protolith, anatex1s at cond1t1ons

close to the trans1t1on from amph1bolite to granulite facies, T 800 8S0°C. P 8 10 kbar. Scale: the outcrop is 4 m across. /mage : E.W. Sawyer.

A tl as o f M igmatites

Figure

I

Fig. D I S. The paleosome to th1s m1gmat1te cons1sts of th1ck (up to 3 m), compos1t1onally d1fferent layers of metamafic rock, wh1ch are interpreted to have ong1nally been mass1ve flows of basalt. A finer-grained mafic dike (upper left quadrant) crosses the layers of metabasalt. Th1n bodies of leucosome are locat ed parallel to t he compos1t 1onal layering and t he foliation and in t he pressure shadows around garnet crystals. Larger, irregularly shaped bodies of leucosome (e.g., near t he dike) occur in dilatant sites and mark t he posit ion of incipient boudinage of the com petent layer. These areas of leucosome have a melanocrat ic border. w hich 1ndicates a local derivat ion of the melt that became the leucosome. The m1gmatite also conta1ns a set of leucocrat1c veins, wit h exact ly the same compos1t1on as the others, but located (e.g.. just above the ruler) 1n planar-s1ded, dilatant fractures that systematically cross the outcrop at about 45° to the layenng; these have no obv1ous nm of melanosome around them.

Location: Ab1tib1 Subprov1nce. south of Ch1bougamau, Quebec, Canada. Rock type: dilation-structured metatexlte m1gmatite, metabasalt protolith, anatex1s at cond1tlons close to the transition from amphibolite to granulite faoes, T 800- 850°C, P 8- 10 kbar. Scale: t he ruler is IS em long. /mage: E.W. Sawyer.

173

METATEXITE MIGMATITES WITH LEUCOSOME

174

IN DILATANT STRUCTURES

F1gure

16

Fig. D 16. The paleosome 1n th1s m1gmat1te cons1sts of thm layers of metamafic rock 1n a hornblende-b1ot1te granod1orite v1sible at the top of the photograph. Neosome occurs only 1n the metamafic layers of the m1gmatite. and 1t has a very distmct1ve and charactenst1c morphology. The neosome is segregated into well-defined leucosome and melanosome. The area of leucosome has a wide base that is rooted at the contact between the metamafic and the host granodiorite, and tapers to an apex that points toward the center of the layer of metamafic rock. In some cases, the triangular domains of leucosome on opposite walls of the mafic layer have grown sufficiently large that they meet, and leucosome bndges the ent1re layer of metamafic rock. The melanosome IS a dark-colored halo around the doma1ns of leucosome. It IS depleted 1n quartz and feldspar. and consequently ennched 1n amph1bole relative to the metamafic rock farther from the neosome. The metamafic rock 1s Interpreted to have been much more competent than the host granodiorite. Thus. dunng deformation. bnttle fractures developed 1n the metamafic rock as a result of strain part1t1on1ng. Because deformation and anatexis were synchronous 1n this migmat1te. the growth of the brittle fractures provided a convenient low-pressure site into which the anatectic melt forming in the metamafic rock could migrate, and hence the doma1ns of leucosome

were formed. The melanosome halo, therefore, Indicates the region 1n the metamafic rock from wh1ch the melt was denved.

Location: Mt. Odin, Monashee complex, Bntish Columb1a, Canada. Rock type: dilatant-structured metatexite m1gmatite: partial melt1ng in the upper amphibolite faoes. Scale : the pencil is 14 em long. Image and caption: Paul McNeill.

Atlas of M igmatites

Figure

17

Fig. D 17. The dilatant structures into which melt can migrate, to form domains of leucosome, need not be large. In this example, the anatectic melt has collected in a series of dilatant sites produced as garnet crystals in the metamafic paleosome were pulled in a direction parallel to t he foliation in the host. Consequently, many segments of leucosome are tabular in shape and have their long axes at a high angle to the foliation in the paleosome. However, the overall impression is that the domains of leucosome are oriented parallel to the layering.

Location: Georgian Bay, Grenville Province, Canada. Rock type: metat exit e migmatite; the metamafic protolith partially melted at conditions of the upper amphibolite to granulite facies. Scale: the sunglasses are 14 em across. /mage: O livier Vanderhaeghe.

!

175

METATEXITE MIGMATITES WITH LEUCOSOME

176

IN DILATANT STRUCTURES

Figure

Fig. D 18. The ma1n features of th1s outcrop are the layers of Intermediate to mafic orthogneiss. wh1ch conta1n two sets of leucocrat1c ve1ns of tonalite to granod1onte compoSition. A consp1cuous set of leucocrat1c veins IS located 1n

the bas1s of the proportions of leucosome and melanosome in the outcrop, th1s m1gmat1te probably conta1ns more anatectic melt than was generated 1n s1tu. Th1s must. therefore. be v1ewed as a s1te where anatectiC melt was able to accu-

dilatational structures, and the other set. much less obvious (and thinner). is located parallel to the compositional layenng and the foliation. In the bottom half of the photograph. the dilatational sites are related to s1mple boudinage of melanocr atic layers and to extension fractures that cross

mulate. The pressure gradients generated by the format1on of the dilatant structures may have effectively sucked melt into this migmat1te from nearby part1ally melted rocks.

several layers, whereas in the upper part of the photograph. they are associated with a more h1ghly d1srupted mafic layer. Because most of the leucocrat1c ve1ns have sharp contacts with their host. and do not have a melanocratic nm, it 1s not Immediately ev1dent that th1s IS a m1gmatlte; 1t could be a ve1ned gneiss. However, the w1dest, dark grey layer 1n the top nght conta1ns a band of neosome (N) compns1ng small, Irregularly shaped vems of tonalite that have small melanocratic patches rich 1n amphibole assooated w1th them; this band IS interpreted to have been denved from barely segregated anatectic melt and res1duum. Hence, the outcrop is 1ndeed a migmatite. Similar patches of unsegregated neosome occur in the lower left corner, in the melanocratic layer that has undergone boudinage. Th1s melanocratic layer is, therefore, interpreted as residuum, and the leucocratic material in the boudin interpartitions, as leucosome. On

Location: Thor Odin dome. British Columbia, Canada. Rock type: metatexite m1gmatite; mafic and intermediate orthogneiss protolith , part1ally melted at conditions of the upper amphibolite fac1es. Scale : the co1n 1n the center is 2.65 em across. /mage: Ohv1er Vanderhaeghe.

A tl as of 1\ligm a rires

Figure

9

•

•

Fig. 019. The doma1ns of leucosome 1n th1s metatexlte m1gmat1te are located 1n sem1regularly spaced dilatant structures that have developed 1n a fol1ated metamafic paleosome. From the geometry of minor folds 1n the paleosome, the dilatancy appears to have occurred 1n several ways. In some cases, the small symmetncal folds suggest that regular boudinage occurred; however, elsewhere the curvature of the foliation suggests an asymmetrical foliation boudin, or flanking fold next to a zone of oblique shear. N o matter how exactly t he dilatancy occurred, the neosome that formed 1n and around it is very s1milar throughout the m1gmat1te. Two parts. although not necessanly well separated, are evident 1n all the areas of neosome. The most prom1nent part IS a coarse-gra1ned leucosome, wh1ch contains the assemblage plagioclase + quartz + clinopyroxene. Typ1cally. the leucosome occup1es the dilatant structure 1tself. and has a part1al envelope of coarse-gra1ned melanosome w1th the m1neral assemblage clinopyroxene + hornblende + garnet. In most cases, the melanosome IS 1n the wal lrock around the leucosome. Th1s JUxtaposition suggests t he m s1tu format ion of t he neosome in and around the dilat ant structure. However, in some cases, the melanosome occurs as fragments in t he leucosome. Th1s pattern ind1cates a fu rther episode of dilatancy after some of the melanosome has developed. The paleosome is considerably

• •

finer gra1ned than the neosome. and conta1ns the assemblage hornblende + plag1oclase + clinopyroxene + garnet + quartz. The large, 1dioblast1c crystals of clinopyroxene present 1n the melanosome and parts of the leucosome are interpreted as the solid products of reaction that grew 1n the melt produced by a react1on that 1nvolved the incongruent breakdown of hornblende.

Location:

Central Metasedimentary Belt , Grenville Province Quebec, Canada. Rock type: met at exite migmat ite; metamafic protolith, granulite-facies anatex1s T 800 850°C,

P 5 8 kbar. Scale: the ruler 1s IS em long. Image: E.W. Sawyer.

177

178

METATEXITE MIGMATITES WITH LEUCOSOME

--------------------------~-­ IN DILATANT STRUCTURES

Figure

20

this po1nt. the assemblage started to boudinage as a whole Fig. 020. Th1s migmat1te conta1ns well-developed comand created the larger length-scale boudins. The s1te may positional layenng that can be traced across large (I 00 m) then have ceased to lose melt and may have switched to outcrops. The portion shown here compnses Jeucotonailte. become a zone of net accumulat1on of melt. mesocrat1c metamafic and melanocrat1c metamafic layers. The layers of metamafic rock on wh1ch the pocket kmfe Location: Sand R1ver Gne1sses, Causeway locality, South rests conta1n consp1cuous sublayers, some of wh1ch are Africa. Rock type: ddatant structured metatexite migmatite; particularly dark and interpreted to be the most residual felsic and mafic orthogneiss protoilth, anatex1s in the granin composition. Furthermore. the metamafic rocks were ulite facies, T 800- 850°C. P 7 I0 kbar. Scale: the pocket evidently the most competent ones 1n the migmatite. as knife is II em long. Image: E.W. Sawyer. they have undergone symmetrical and asymmetrical bou dinage on the layer and the sublayer scales. and th1s has created a boudin-w1thin-boud1n structure. Small bodies of leucosome consist1ng of plag1oclase + quartz occupy the interboud1n part1t1ons developed 1n the darkest sublayers. At a larger scale, the pattern IS somewhat different. and segments of prominent layer-parallel doma1ns of leucosome link w1th those in the large interboudin part1t1ons between the layer-scale boudins. The melt-depleted melanocratiC sublayers and their associated network of leucosome can be interpreted to have developed as the sequence became progressively melt-depleted during deformation. At some stage during melt depletion, the proportion of residual pyroxene and hornblende in the individual sublayers had increased sufficiently that the overall package of sublayers became stronger than the adjacent mesocratic layers. At

Atla of Migmatites

4 4

Figure

11

• •

• • • •

Fig. 021. The grey-weathenng layers 1n th1s m1gmat1te are Al-poor. orthopyroxene-beanng. blot1te-plag1oclase quartz psammitlc schists that have undergone relatively minor amounts of partial melt1ng (see Figs. D9 and 10 for s1milar rocks from the same area) . The darkest-weathering layers 1n the m1gmatite were orig1nally sem1pelites; they now conta1n the mineral assemblage orthopyroxene + quartz + cordierite + biotite ± garnet , and are feldspar-absent. This assemblage of minerals and the whole- rock major- and trace-element compositions suggest that these layers are the severely depleted residuum left after the extraction of anatect1c melt. In most circumstances, the Al-poor psammltes are more competent than the semipelltes. However, 1n th1s case, the extraction of substant1al amounts of anatectic melt from the sem1pelitic layers has created a res1duum w1th 70 modal % or more of orthopyroxene + quartz. and both are strong minerals. Consequently, as anatex1s advanced and the sem1pelitic layers progress1vely lost melt, they systematically became more competent and started to develop extens1onal shears and to undergo boud1nage 1n response to late layer-parallel extension. Anatectic melt was then able to migrate into these layers as the dilatant sites developed, t o produce t he array of small accumulations of leucosome visible in the most residual layers of this migmatite.

Location : Wuluma Hills, Arunta Inlier. Australia. Rock type: metatexite m1gmat1te; sem1pelit1c protolith, partially melted at T 825-875°C, P ca. 5 kbar. Scale: the ruler IS graduated 1n em. /mage: E.W. Sawyer:

179

180

METATEXITE MIGMATITES W ITH LEUCOSOME

-----------------------------IN DILATANT STRUCTURE S Figure

22

Fig. 022. Th1s metatex1te m1gmatite has a strong planar an1sotropy. Within the grey paleosome. th1s an1sotropy 1s due to variat1on 1n the compos1tion of orig1nal beds, but 1t 1s also due to the segregat1on of melt parallel to the planar anisotropy 1n the protolith, creat1ng parallel sheets of leucosome and mafic selvedge. Consequently. the pnncipal morphology of the migmatlte is stromatiC, or layered. Latestage extension, roughly parallel to the layering. resulted in the formation of two normal-sense shears and their associated flanking folds, in the slightly darker layer running across the center of the photograph. Melt then located in the layer migrated into the dilatant shears, and its crystallization products formed the leucosome. Bot h cross-cutt1ng bodIes of leucosome are diffuse at the1r t1ps. and the one on the nght appears to have diffuse contacts w1th the layenng 1t transects. The stromatic leucosome thus may not have been solid at the time the layer was extended.

Location: Port Navalo area. southern Bnttany. France. Rock type: metatex1te m1gmat1te denved from quartzofeldspath1c and aluminous gne1sses. part1al melt1ng at T ca. 800oC. P ca. 8 kbar. then decompressed to 4 kbar, followed by cooling. Scale: the ruler IS IS em long. Image: E.W . Sawyer.

Further readmg: jones, K.A. & Brown, M. (1990): Hightemperature "clockw1se" P T paths and melting in the development of regional migmatites: an example from southern Brittany, France. journal of Metamorphic Geology 8.

551 - 578. Marchildon, N . & Brown, M. (2003): Spat1al distribution of melt-beanng structures in anatectic rocks from southern Brittany, France: 1mplicat1ons for melt transfer at grain- to orogen-scale. Tectonophysics 364. 215-235.

A tlas of Migmat ites

Figure

23

Fig. 023 . The paleosome of this migmat it e is a dark grey, wellfoliated hornblende granodiorit e; minor changes in color indicate only a weak compositional layering. Large equant to lens-shaped areas of neosome occupy the dilatant spaces creat ed during the formation of both symmetrical and asymmetrical boudins. The absence of a strong composit ional layering and the presence of a strong foliation suggest that these areas could be interpreted as examples of foliat ion boudinage. The neosome in these structures does not have a prominent melanocratic border around t hem, which could be t aken to indicate that the melt in that case was externally derived. However, the areas of neosome have a diffuse margin, which is consistent with in situ partial melting. Furthermore, they contain scattered, large crystals of amphibole, which could be the solid products of the melting reaction . The presence of t he amphibole also is consistent wit h in situ melting and very limited separation of the melt fract ion from residuum. Hence, these bodies are better cal led neosome rather than leucosome. T hin sheets of leucosome wit h a sharp contact and very few mafic minerals occur parallel to the foliation; rarely, they have a narrow, discontinuous melanosome. T he term leucosome is appropriate for these because the melt fraction and residual fractions did become segregat ed. Overall, this migmatite has two orientations of melt-bearing structures, and the intersection of these leads to t he local development of an incipient net structure.

Location: Laag mountain area, Monashee com plex, British Columbia, Canada. Rock type: dilation-structured metatexite migmatite; hornblende granodiorite protolith, partial melting in t he upper amphibolite facies. Scale: the card is 8.5 em long. Image and caption: Paul McNeill.

181

ME TATEXI T E MIGMATITES WITH A NET STRU C TURE

182---------------------------Figure

14

Fig. 024. Th1s photograph IS from the same area as Fig. 023. It shows the net-structured m1gmat1te that developed where the proportion of melt in the migmat1te was somewhat higher. T he outcrop clearly shows two trends of meltfilled structures, one parallel to the foliation and the other discordant at 30-40° to the foliat1on, that together create the net-like morphology. The areas of leucosome that are parallel to the foliat1on have sharp borders and conta1n a very low proport1on of mafic m1nerals. In contrast, the obliquely discordant structures contain a s1gn1ficantly h1gher proport1on of mafic minerals, typically large euhedral crystals of amphibole, and have a diffuse border, which suggests an m situ origin. In most parts of t he outcrop, the bodies of leucosome oriented parallel to the foliation are thin and volumetrically minor, wh1ch could 1nd1cate either that the bulk of the outcrop has only JUSt begun to partially melt, or that these reg1ons have already lost melt. However. part of the photograph shows several prominent, w1de areas of leucosome that are parallel to the foilat1on, but fewer areas of leucosome accumulation in symmetrical and asymmetrical boudinage structures than elsewhere. The overall distribution of melt-filled structures in this migmatite has created a linked network of channels that could have drained melt from the m1gmatite, or a1ded melt 1n passing through the crustal level represented by th1s m1gmatite.

Location: Laag mountain area, Monashee complex, British Columbia, Canada. Rock type: d1lat1on-structured metatexlte m1gmat1te; hornblende granod1onte protolith, part1al melting 1n the upper amphibolite facies. Scale: the card is 8.5 em long. Image and caption: Paul McNeill.

Atlas of Migmatites

Figure

15

Fig. 025 . The migmatite in this phot ograph contains two set s of leucosome domains that together create an incipient net-st ruct ured st romatic migmatite. One set is developed parallel to the strong tectonic foliation and atten uated composit ional layering in the host rock; such leucosome can be described as stromatic. The other set consists of lens-shaped bodies of leucosome that are oblique to the fo liation and occupy small, generally extensional shear zones; these bodies of leucosome can be described as discordant. The stromatic bodies of leucosome are generally thin, about 0.5 em wide, and fine-grained, but many of t hem thicken and become coarser grained near the bodies of discordant leucosome. The contacts between t he stromatic and discordant domains are gradat ional in many places, and interpreted to mean that bot h sets were interconnected and contained anatectic melt at the same time. However, there are places where the discordant areas of leucosome t runcat e t he stromat ic o nes. This complex relat ionship is interpreted t o represent a li nked system in which melt generated in the host migrated along the tectonic fo liation and accumulat ed in t he shear zones; t he gradational contacts preserve this linkage. The discordant bodies of leucosome truncate t he stromat ic ones where the shear zones that accommodated the melt pro pagated into regions of the host where the melt fl ux was much lower, o r where

it had ceased ent irely. The truncat ed domains of stromat ic leucosome in that region were no longer conduits for the transfer of melt.

Location: Tolst ik Peni nsu la, Karelia, Russia. Rock type: incipient net-structured metatexite migmatit e; mafic to intermediate prot o lith, anatexis in the upper amphibolite to lower granulite facies. Scale: the lens cap is 5.5 em across. Image and caption: Mike Brown. Image previously published as fig. 5c in Brown (2007) and reproduced with the permission of the Geological Societ y, London.

Further reading: Brown, M. ( 1994): The generation, segregation, ascent and emplacement of granitic magma: the migmatite-to-crustally-derived gran ite con nection in thickened o rogens. Earth Science Reviews 36, 83- 130. Brown, M. (2007): Crustal melting and melt extraction, ascent and emplacement in orogens: mechanisms and consequences. journal of the Geological Society of London, 164, 709- 730. Sawyer, E.W . (2001): Melt segregation in t he continental crust: dist ribut ion and movement of melt in anatectic rocks. journal of Metamorphic Geology 19, 291 - 309.

183

METATEXITE MIGMATITES WITH A NET STRUCTURE

184 -----------------------------Figure

26

Fig. 026. Bedding 1s well preserved in the lowest-grade part of the anatectiC doma1n at Mount Stafford, Australia, and part1al melting was cons1derably more advanced 1n the pelit1c layers than in the psamm1t1c, or sem1pelit1c, ones. The scale rests on a psamm1tic bed, and above it IS a crenulated and partially melted pelit1c bed. Most of the leucosome in it, however; is not crenulated. There are two principal orientations for the bodies of leucosome, and consequently, the migmatite consists of a net-like pattern of leucosome that encloses rhombic port1ons of crenulated paleosome and melanosome. Hence, this metatexite migmatite has a net, or diktyonitic, structure. This particular net-structured m1gmatite IS a little unusual in that ne1ther of the onentatlons of leucosome 1s parallel to the foliat1on 1n the pelite. The more continuous set of leucosome IS subparallel (1.e., crosses at a low angle) to the crenulated foliat1on 1n the pelite layer, whereas the other follows shear planes that are subparallel to the ax1al planes of the asymmetncal crenulatlons, or follow shears on either the short or long limbs of the folds.

Location: Mount Stafford, Australia. Rock type: net-structured metatex1te m1gmat1te: metapelitiC protolith, Mt. Stafford metamorphic zone 2, T 650 675oC. P 3.2 kbar. Scale: width of the field of v1ew is 75 em. /mage: E.W. Sawyer. Further readmg: White, R.W., Powell, R. & Clarke, G.L.

(2003): Prograde metamorphic assemblage evolut1on during partial melting of metasedimentary rocks at low pressures: migmatites from Mt. Stafford, central Australia.

Journal of Petrology 44 , 1937 1960.

A tlas of Mig ma t ites

Figure

27

Fig. 027. T he most striking examples of net structu re are developed in migmatites that have a paleosome w ith a strong planar anisotropy, e.g., pelit ic and semi pelitic metasedimentary mat erial. In most cases in which net -struct ure migmatites are formed from felsic plut onic protoliths (e.g., granites, tonalites and trondhjemites), a planar anisotropy was created in the protolith before, or during, anatexis. Consequently, the paleosome in these migmatit es shows structural evidence of high to very high strain. T he paleosome in t he migmat it e shown in t his photograph is a strongly fol iated, leucocratic granodiorit e and trondhjem ite that cont ains boudins and lenses of more mafic rocks t hat are the relics of dikes disrupted during the st rong, noncoaxial shearing event t hat generated t he foliation. The paleosome is crossed by a net work of neosome domains 1- 3 em wide that have a leucocratic margin and markedly more melanocrat ic cent er. T he cen tral part of each neosome contains large, euhedral crystals of orthopyroxene, which have lat er been part ially alt ered. T he ferromagnesian minerals in t he center of the neosome are interpreted to be the solid products of t he incongruent melt ing of biot ite, and the leucocrat ic rim is interpreted to be derived from the anatectic melt. The photograph shows t hat the proportion of leucocratic to melanocratic mat erials in t he neosome varies considerably from place to

place. Some parts, such as to the right of the largest mafic boudin, have a higher fract ion of melanosome. This can be explained if t he array of neosome domains served as the system of channels t hrough which anatectic melt flowed w it hin and, ult imately, out of t he migmat ite. T he part s of the network t hat are t he most enriched in melanosome have lost more melt than the other parts. T his variation reflects differences in t he different ial st ress from one part of t he out crop t o another.

Location: Sand River Gneisses, Causeway localit y, South A frica. Rock type : net-structured met atexite migmat it e; felsic orthogneiss protolith, granulite-facies anatexis. Scale: t he pocket knife is I I em long. Image: E.W . Sawyer.

185

METATEXITE M IGMATITES WI T H A NET STRUCTURE

186 Figure

28

Fig. 028. A net-structured metatex1te m1gmat1te can develop from relatively homogeneous protoliths. but they are easily overlooked. The example 1n this photograph developed from a massive and compositionally umform leucogranodioritic to tonalit1c protolith. The areas of neosome that form the net-like array are I 3 em w1de, and have a relatively uniform grain-size and distnbution of minerals throughout (compare w1th the migmatite 1n Fig. D27 only a few tens of meters away). Overall, therefore, the neosome is not very different in either grain size or composition from the paleosome 1n this migmatite, and this makes 1t difficult to decide, from evidence at the outcrop alone, whether the neosome represents unsegregated melt and res1duum. or anatectic melt (in which case it should be called leucosome). The absence of a strong planar anisotropy 1n the paleosome further contributes to the d1fficulty for the eye to pick out a net-like array of neosome domains 1n a m1gmat1te where there 1s little contrast in color, grain size, or microstructure between neosome and paleosome. This is a common problem in migmatites denved from granites, granodiorites, tonalities, and trondhjemites (see also Fig. Dl2) .

Locatron: Sand River Gne1sses. Causeway locality, South Africa. Rock type: net-structured metatex1te migmat1te; fels1c orthogneiss protol1th, granulite-fac1es anatex1s. Scale: the pocket kn1fe 1s II em long. /mage: E.W. Sawyer.

A tlas of M igmat itcs

Figure

19

Fig. 029. The dtstributton of net-structured mtgmatttes ts necessanly restncted to places where there are at least two sets of tntersecttng domains of leucosome. The paleosome tn the mtgmattte deptcted here is compostttonally banded, tntermediate gnetss that has been folded and has undergone parttal melt ing. The resu lting metatexite migmattte has a net structure in the hinge regions of the mesoscale folds. One set of leucosome domains is located parallel to the axial plane of the folds, specifically, in small shear zones on the flanks of minor folds locat ed in the hinge region of the larger folds. T he second set has developed along the compostttonal banding in the paleosome, but prtnCipally in the regton of the fold htnges. The intersectton of both sets of leucosome domains generates a net structure tn the core of the mesoscale folds. The leucosome ts conttnuous tn both mtneralogy and mtcrostructure from one onentatton to the other, suggesting that together the two sets of domains represent a connected network t hat contatned a continuous anatectic melt. A partial melanocratic rim ts developed around many of the leucosome domains, and its ongin is an tmportant matt er t o consider. If the rim is the residue from parttal melting, then it indicates that a certain proportion of the melt was derived locally, and t hat is t he crit ical evidence needed to identify t his rock as a migmat it e. However. sharp contacts locally between the leucocratic and mesocrattc

•

parts of the rock tndicate the tnjectton of some melt, but posstbly thts was simply redistributton of melt wtthin the outcrop. Therefore, the oppostng vtew ts that all the felsic vetns are from an external source, and that the dark rim is a mafic selvedge that results from rehydratton of the wallrocks following crystallization of the veins. In t hat case, the outcrop is an inject ed stockwork of felsic veins and not a migmat tte. In either scenario, t he hinge regions of the folds are the sites where the felsic melt moved th rough t he syst em represented by the out crop. Location: Arunta Inlier, Australta. Rock type: net-structured metatextte migmatite; tntermedtate gnetss protolith, partially melted under condtttons of the upper amphibolite faCies. Scale: the pen ts IS em long. /mage and caption: Tony I.S. Kemp.

187

METATEXITE MIGMATITES WITH A N ET STRU CTURE

188 ----------- ----------- ------Figure

30

Fig. 030. One of the sets of leucosome domains that contributes to the net structure in th1s metatex1te migmat1te IS oriented parallel to t he foliat1on and compositional layenng in the melanosome and paleosome. This set of leucosome can be called stromatic. The second set of leucosome domains 1s located 1n dilatant structures that are e1ther 1nterboudin part1tions orient ed perpendicular to the foliat1on (e.g., each side of the lens cap). or extensional shear planes onented oblique to the foliation (e.g., top right). Petrographic continu· ity (1.e., the same modal proportions of quartz, plag1oclase. and K·feldspar, the same gra1n-s1ze. and the same microstructure) between the segments of leucosome located 1n the different structural s1tes in this migmat 1te implies t hat some proportion of t he leucosome crystallized from a melt-bearing network that was cont1nuous throughout the migmatite. Th1s m1gmat1te 1s located 1n the South Armoncan Shear Zone, wh1ch had a dextral transcurrent sense of movement. Qualitative observat1ons on the net work of leucosome domains using adjacent, mutually perpendicular, flat outcrop surfaces reveals that the leucosome network is anisotropic relative to the subhonzontal elongat1on-1nduced lineat1on on the foliat1on planes. T he anisotropic nature of the stromatic

sets of leucosome suggests that the permeability 1n the plane of the foliat1on was larger parallel to the lineat1on than perpendicular to it. It may then be inferred that t he melt in the migmatite flowed in t he plane of the foliation and parallel to the l1neation, and then flowed into the 1nterboudin partitions and extensional shear surfaces in response to grad1ents 1n melt pressure that developed as these dilatant structures formed.

Location: Port Navalo, Brittany. Fr ance. Rock type: net-st ructured metatexite m1gmatite: pelitic and sem1pelitic protolith, granulite-fac1es anatexis at T ca. 800°C and P 9 kbar. w1th a stepped retrograde cooling path to 4 kbar. Scale: the lens cap is 6.5 em across. Image and caption: Mike Brown. Image previously published as fig. 3b in Brown (2005) and reproduced w1th the permiss1on of the Geological Sooety, London.

Further readmg: Brown, M . (2004): T he mechanism of melt extract ion from lower cont inental crust of orogens. Transactions of the Royal Sooety of Edmburgh: Earth Soences 95,35- 48.

Atlas of M ig matites

Brown, M. (2005) : Synergistic effects from melting and deformation: an example from the Variscan Belt, western France. In Deformation Mechanism, Rheology and Tectonics: from Minerals to Lithosphere (D. Gapais, J.-P Brun & PR Cobbold, eds.). Geological Society, Special Publication 243,

205-226. Jones, K.A. & Brown. M. (1990): High-temperature "clockwise" P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France.

journal ofMetamorphic Geology 8, 55 1- 578. Marchildon, N . & Brown, M. (2003): Spat ial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt transfer at grain- to orogen-scale. Tectonophysics 364, 215 235.

189

I 90 ______M_E=._T_A_T_EX_I_T_E_M_IG=._M_A_T_IT_:__E:..:S_W_:_:IT_:__H:....:.A...:....::.:LA...:._Y:...::E:.:..:R.::.ED=--=O:..:..:R STROMATIC STRUCTURE ASSOCIATED WITH LOW STRAIN

Figure

3I

Fig. 031 . One of the first mechantsms to be proposed as an explanation for layered, or stromat1c leucosome, is the repeated injection of melt parallel to bedd1ng or foliation (i.e., l1t-par-l1t injection). If the InjeCtion 1s perfectly parallel to the layering, th1s mechan1sm can be difficult to detect. In this example of a stromatic migmatite, the very thin, fine-grained, leucocratic layers are an earlier generation of quartzofelspathic veins. The coarser-grained sheets of leucosome seem to be parallel to the layering, and hence are stromatic also. However, trac1ng individual sheets of "leucosome" across the outcrop reveals that many cross the layering at a very shallow angle (<5°); two places are indicated on the photograph. Th1s discordance can be interpreted as ev1dence that they were denved from 1ntrus1ons of anatect1c melt, and d1d not form m Situ. Consequently, they are e1ther in-source leucosome or, more likely, leucocratlc verns, but are not rn s1tu leucosome.

Location: Wuluma Hills, Arunta Inlier, Australia. Rock type: stromatic metatex1te m1gmat1te; metapelit1c protolith, anatexis at T 825 875°C. P ca. 5 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

Further reading: Sawyer, E.W., Dombrowski, C. & Collins, W .J. (1999): Movement of melt dunng synchronous regional deformation and granulite-faoes anatexis, an example from the Wuluma Hills, central Australia. In Understanding Granites: Integrating New and Classical Techniques (A. Castro, C. Fernandez & J.-L. Vigneresse, eds.). Geological Sooety, Speoal Publ1couon 168, 221 237.

A tl as of Migmat ites

Figure

32

Fig. 032. This stromatic migmatite consists of layers of leucosome (quartz + plagioclase + K-feldspar), bordered by biotite- muscovit e-sillimanite melanosome and hosted by a paleosome consisting of a mesocratic, grey quartzofeldspathic schist. T he leucosome contains pristine igneous microst ruct ures in thin section, which suggests that the stromatic geometry was not acquired by high strain in a submagmatic or later state. The inferred melt ing react ion in the prot olith is muscovite + plagioclase + quartz + K-feldspar + H 20 melt. progressing t o muscovite + plagioclase + quartz + H 20 sillimanit e + melt as K-feldspar became exhausted during progressive anatexis. N ote the slight upward bulging of the leucosome across the com positional layering in its melanosome (main leucosome, center of the image); the other contact of the leucosome is planar. Such bulging st ruct ures have been termed "caulinowers" and used to determine the "way up" in migmatites. The high proportion of leucosome relative to melanosome in this migmatite suggests that some anatect ic melt from elsewhere has been added to the in situ melt that formed the

=

=

init ial stromatic leucosome.

Location: Glenelg River Complex, southeastern Australia.

Rock type: stromat ic metatexite migmatite; pelit ic, semipelitic and psammitic schist protoliths, with anatexis in the upper amphibolite facies. Scale: the lens cap is 6.5 em in diameter. Image and caption: Tony I.S. Kemp.

Further reading: Burg, J.-P. & Vanderhaeghe, 0. (1993) : Structures and way-up criteria in migmatites with applications to the Velay Dome (French Massif Central). Journal of Structural Geology I 5, 1293- 130 I. Kemp, A.I.S. & Gray, C M. (1999): Geological context of crustal anatexis and granit ic magmatism in t he northeastern Glenelg River Complex, western Victoria. Australian

journal of Earth Sciences 46, 407- 420.

191

192

METATEXITE MIGMATITES WITH A LAYERED OR STROMATIC STRUCTURE ASSOCIATED WITH LOW STRAIN

Figure

3

Fig. 033. Use of the term stromatic has become broadened to 1nclude metatex1te m1gmat1tes 1n wh1ch the bodies of leucosome are not laterally very extens1ve. but occur as numerous. h1gh-aspect-rat1o lenses onented parallel to one another, and to either the foliat1on or the compositional layenng 1n their host. Th1s stromatiC m1gmat1te IS one of these; 1t has many parallel. thin. high-aspect-ratio doma1ns of leucosome that are sinuous and. locally. bifurcating. The leucosome is trondhjemitic (plagioclase + quartz) 1n composition. and surrounded by a metamafic rock w1th the assemblage hornblende + garnet + clinopyroxene + plagioclase that is locally coarser gra1ned than some small pockets of metamafic material ncher 1n hornblende and plagioclase (e.g .. left of the coin). Thus. most of th1s m1gmat1te IS Interpreted to be melanosome (i.e .. res1dual 1n compoSition). Some port1ons of the leucosome contain large euhedral crystals of pyroxene. and other port1ons. garnet. These are the solid products of react1on of the Incongruent breakdown of hornblende that were not separated from the anatectiC melt, and may have grown to a large s1ze because of the nuxing effects of the melt. The h1gh modal proportion of garnet and pyroxene in the melanosome. and the relat ively low proportion of leucosome, suggest that much of the anatectic melt generated in th1s migmatite has escaped.

Location : Kapuskas1ng Structural Zone. Ontario. Canada. Rock type: stromatiC metatex1te m1gmat1te; mafic gne1ss protolith part1ally melted at granulite-fac1es condit1ons. T ca. 850oC. P II kbar. Scale: the coin 1s 24 mm across. Image: D.R.M. Patt1son. Previously published as fig. 2d in Hartel & Patt1son ( 1996) and reproduced w1th the perm1ssion of Blackwell Pubilsh1ng.

Further reading: Hartel, T.H.D. & Pattison. D.R.M. (1996): Genesis of the Kapuskasing (Ontario) migmatitic mafic granulites by dehydrat1on melting of amphibolite: the importance of quartz to react1on progress. journal of Metamorphic Geology 14. 591 - 611.

Ada~

Fig. 034. Th1s photograph shows the contrasting appearance of stromatic leucosome developed from d1fferent metasedimentary protoliths. In the lower half of the photograph, sheets of med1um-grained leucosome with the m1neral assemblage plagioclase + quartz + K-feldspar ± garnet ± orthopyroxene are in a fine-gra1ned host that contains the assemblage plagioclase + biotite + orthopyroxene + quartz + garnet. The host contains corroded plag1oclase, quartz and b1ot1te grains located in larger xenoblastic quartz and plag1oclase crystals. This microstructure suggests that the melting reaction was perhaps b1otite + quartz + plag1oclase orthopyroxene + garnet + melt. Therefore, the host 1s interpreted to be the res1duum left after the removal of most of the melt from a plagioclase + quartz + b10t1te metagreywacke protollth, and the stromat1c leucosome 1s the product of the segregated melt. The leucosome 1n the upper part of the photograph 1s much coarser grained and contains the m1neral assemblage plag1oclase + quartz + K-feldspar + garnet. 1n a host that conta1ns the m1neral assemblage plag1oclase + quartz + K-feldspar + biot1te + garnet + cord1ente. Th1s more aluminous assemblage is interpreted to be the meltdepleted residuum from a metapelitic protolith, and the leucosome in it is derived from t he anatect1c melt. The melting reaction in the metapelite may have been

=

of Migmatites

193

=

b1otite + sillimanite + quartz + plag1oclase garnet + cord1ente + K-feldspar + melt. If the garnet 1n the leucosome IS the reaction product of the incongruent breakdown of b1ot1te. then this 1s not stnctly leucosome matenal; neosome would be a better term. Moreover, the position of the stromatic neosome could have been determined by the locus of garnet nucleation, rather than by the locus of collection of the melt fraction. Locat1on: N orthern Ashuanip1 Subprovince. Quebec, Canada. Rock type: stromatiC metatexite migmatite; metapelitic and metagreywacke protoliths, anatex1s at T ca. 850°C, p 6- 7 kbar. Scale: the ruler IS 15 em long. Image: E.W. Sawyer.

METATEXITE MIGMATITES WITH LAYERED OR

----------------------194 - - - - - -STROMATIC STRUCTURE DUE TO TRANSPOSITION Figure

35

Fig. 035. The stromatic m1gmat1tes at Port Navalo are located 1n the transcurrent South Armoncan Shear Zone, and are typ1cal of stromat1c migmatites 1n zones of h1gh stra1n. The subvertical compositional layenng defined by the neosome (i.e., melanosome and leucosome) 1S parallel to the compositional layering 1n the paleosome, wh1ch in turn is parallel to the foliation defined by tabular m1nerals in both the paleosome and melanosome. The domains of leucosome are thin and laterally persistent. and are bordered by a thin mafic selvedge. Microstructures. such as rational faces on feldspar against quartz are common, and 1ndicate that the stromatic structure was acquwed with melt present as the rocks were sheared. The bod1es of leucosome are nch 1n plag1oclase. Th1s fact, taken together with the very high proport1on of leucosome 1n the outcrops, suggest that anatex1s was an open-system process 1n th1s m1gmat1te. A possible interpretation IS that large volumes of anatectic melt were channeled upward through the South Armoncan Shear Zone, and that the amount of leucosome in the outcrop presents the collapsed relic of the onginal conduit system made visible by the presence of the minerals (mostly plagioclase) that crystallized first from the magma and were left behind after the remaining evolved melt had migrated away.

Location: Port N avalo area, southern Bnttany, France. Rock type: stromatic metatex1te m1gmat1te, quartzofeldspath1c, and aluminous gne1ss protolith, anatex1s at T ca. 800°C, P ca. 8 kbar, then decompressed to 4 kbar and cooling. Scale: the ruler 1s IS em long. /mage: E.W. Sawyer. Further readrng: Marchddon, N. & Brown, M. (2003): Spatial dist ribution of melt-bearing structures in anatectic rocks from southern Brittany, France: Implications for melt transfer at grain- to orogen-scale. Tectonophysics 364, 215-235 .

A rias of M igma ti tes

Figure

36

Fig. 036 . The effect of stra1n and transpos1t1on on the aspect rat1o of bodies of leucosome can be seen 1n th1s exposure of migmatite that contains, 1n 1ts state of lowest strain (center of the 1mage near the lens cap), doma1ns of leucosome with an aspect rat1o up to 2. The leucosome 1n t he adjacent zones of higher st rain (top and bottom of the image) have an aspect rat io of up to I 0, and are stromatiC in the sense that t hey are elongate parallel t o the foliation . The leucosome contains quartz, plagioclase, and K-feldspar. It is set in a melanocratic matrix that contains sillimanite, b1otlte, muscovite, plagioclase, garnet, and locally, K-feldspar that IS believed to be the res1duum after part1al melt1ng of silic1clast1c turbidites via the react1ons muscov1te + plag1oclase + quartz + H 20 = s1lliman1te + b1ot1te and muscov1te + plag1oclase + quartz + H 20 s1llimamte + K-feldspar + biot1te.

=

Location: western Maine, U.S.A Rock type: inc1p1ent stromatiC metatexite m1gmatite: pelitic and sem1pelitlc sch1sts as protolith, partially melted 1n the upper amphibolite faoes: T 750-800°C, P 4 5 kbar. Scale: the lens cap 1s 6.5 em across. Image and capt1on : Gary Solar. Image prev1ously published as fig. 4e 1n Solar & Brown (200 I) and reproduced w1th the permission of Oxford Univers1ty Press. Further readmg: Solar, G.S. & Brown, M. (200 I) : Petrogenesis of m1gmatites in Maine, USA: possible source of peraluminous leucogramt e 1n plutons? journal of Petrology 42 ,

789- 823.

195

196

METATEXITE MIGMATITES W ITH LAYERED OR

-----------------------------ST ROMATIC STRUCTURE DUE TO TRAN SPOSITION Figure

]

Fig. 037. The final stage of deformation on the lower flank of the fold 1n this stromat ic m1gmat1te occurred after the m1gmat1te had crystallized, as 1t reduced the grain size and produced a protomylonite with feldspar porphyroclasts (bottom of the image) . Elsewhere in this stromatic m1gmatite, both the format1on of the layered structure of the leucosome and its subsequent folding occurred wh1le melt was present (i.e., before the m1gmat1te fully crystall1zed) and the m1gmat1te was subject to h1gh noncoaxial stra1n. As ev1dence for th1s 1nterpretat1on, note that the bodies of quartz + plag1oclase + K-feldspar stromatiC leucosome are th1ckest 1n the fold hinges, w here the melt fraction collected preferentially, and furthermore, note the presence of cross-cutting (center of the image), but compositionally similar, doma1ns of leucosome oriented parallel, or subparallel, to the ax1al plane of the fold. A poss1ble sequence of events IS that. like at Port Navalo, th1s m1gmat1te acquired its stromatiC character 1n a shear zone, and as crystalltzat1on progressed, the flow reg1me was perturbed, and the stromat1c layering became folded, moving the rema1ning melt into the hinge regions of the folds. The evidence that this is indeed a m1gmat1te, and not just a rock made by multiple injections of felsic ve1ns 1nto a banded Intermediate gneiss. 1s as follows: (I) the presence of patches of diffuse neosome elsewhere 1n the outcrop, and (2) m1neral assemblages 1n the melanosome and leucosome that suggest that the melt 1ng react1on 1n the 1ron-nch protolith may have been b1ot1te + plagioclase + quartz = magnet1te + 1lmenite + melt. wh1ch proceeded under conditions of relatively h1gh fugaCtty of oxygen. Location: Arunta Inlier, Australia. Rock type: stromatiC metatexite migmat1te; banded intermediate gneiss protolith, and anatexis at T ca. 850°C. P <5 kbar. Scale: the lens cap is 6.5 em across. Image and caption : Tony I.S. Kemp.

A tlas of Mig ma tites

Figure

I

38

Fig. 038. T he morphology of the leucosome fract ion in t he psammit ic layers of this migmat it e is notably different from t hat in the pelitic layers; t he layering is interpreted t o be t ransposed bedding. The light grey layers (just to the left of t he hammer and in the upper left) are foliat ed layers of psammite and have domains of leucosome w ith a relatively low aspect-rat io locat ed in extensional shears and bet ween boudins oriented at moderat e t o high angles to t he foliation. The cont acts between t he leucosome and t he psammite host are diffuse, which suggests in situ segregation of anat ectic melt into dilatant sites during extension along the layering. The dark grey layer s are strongly foliated pelites, and the majorit y of t he domains of leucosome in them have a stromatic morphology w it h melanocratic borders that are rich in biotite. Many of t he st romat ic domains of leucosome display pinch-and-swell st ruct ures, wh ich are indicative t hat they were st retched parallel to the layering as they crystal lized. There are also a few domains of leucosome located in oblique ext ensional shears in the pelit ic layers. H owever, t he shear surfaces in the pelit ic layers are oriented at lower angles to the foliat ion, and the domains of leucosome in t hem have a higher aspectratio t han those found in t he adjacent psammit ic layers. T he orientat ion of the extensional shear surfaces with r espect to t he foliat ion and the layering suggests that the psam mite layers were more competent than the pelit ic ones. T herefore, t he greater tendency for stromat ic morphology may be due to a partition of the strain into t he w eaker pelitic layers of t he migmatit e, w hich t ransposed t he leucosome fract ion t here.

Location: Thor- O din dome, Brit ish Columbia, Canada. Rock type: stromat ic metatexite migmat ite ; pelitic and psammitic felsic protolit hs, partially melted at condit ions

of the upper amphibolit e facies. Scale: not e t he hammer Image: O livier Vanderhaeghe. Previously published as fig. Sa in Vanderhaeghe et al. ( 1999) and reproduced w it h the permission of the Canadian Journal of Earth Sciences.

Further reading: Vanderhaeghe, 0., Teyssier, C & W ysoczanski, R. ( 1999): St ruct ural and geochronological constraint s o n t he role of partial melt ing during t he formation of t he Shuswap met amorphic core complex at t he lat itude of t he T hor- Odin dome, British Co lumbia. Canadian journal of Earth Sciences 36,9 17- 943.

197

METATEXITE MIGMATITES WITH LAYERED OR

I 98 ----------------------------sTROMATic STRUCTURE D UE TO TRAN SPOSITI ON Figure

39

Fig. 039. Th1s photograph shows the same m1gmatites as 1n Figs. B23 and B24, but from an outcrop where a much h1gher degree of syn-anatectic strain (dunng D 1n the local structural framework) has resulted 1n the development of a completely different morphology. The hght-colored layers in t his migmatite are neosome, not leucosome, as they contain conspicuo us porphyroblasts of garnet and less conspicuous crystals of K-feldspar, which are the solid products of the melt-producing reaction, as well as leucocratic material that has crystallized from the anatectic melt itself. The layers of dark rock that alternate with the neosome are 11menite-plagioclase- quartz- b1ot1te sillimanite cordierlte sch1st or gne1ss. The schist IS not properly descnbed e1ther as paleosome, because 1t has part1ally melted, nor as residuum, because it lacks the garnet. Perhaps, the most relevant term for it IS melanocrat1c neosome. Dunng noncoaxial shearing, the aggregates of garnet and K-feldspar 1n the neosome acted as competent bodies. and the anatectic melt developed in, or m1grated to. the low-pressure s1tes around them. Thus, on th1s section, which 1S subparallel to the mineral lineation 1nd1cated by alignment of sillimanite needles, the morphology is quite clearly that of a stromatic migmatite.

Location: Round Hill, Broken Hill area. Australia. Rock type: stromat1c metatex1te m1gmat1te: alum1nous metapelite protolith partially melted at T 800°C and P 5-6 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

Further reading: Wh1te. R.W., Powell, R. & Halp1n,j.A. (2004): Spatially-focussed melt format1on in aluminous metapelites from Broken Hill, Australia. journal of Metamorphic Geology 22, 825- 845.

Atlas of Migma tites

Figure

40

+ t

• t t

Fig. 040. The Round Hill area 1n Austral ia has some aston1sh1ng migmat1tes: th1s example is remarkable for 1ts morpholog1cal s1milanty to pressure-solut1on seams that are a common feature of low-grade pelit1c sch1sts. This m1gmat1te has two pnnopal const1tuents: a light-colored, coarse-gra1ned neosome and a dark-colored sch1st that IS best cons1dered as a melanocrat1c neosome (see the caption to Fig. 039 for more explanat1on). T he foliation 1n the sch ist part belongs to S" of the local structural sequence. The light-colored neosome can be div1ded into two parts: one conta1ns garnet. K-feldspar, and quartz, and the other. a leucocrat1c granit1c part w1thout garnet. The garnet crystals define a prom1nent Internal, layered structure 1n each area of neosome that 1s orthogonal to the external S2• Moreover, the garnet-defined layenng can be traced from one neosome to the next across the outcrop: 1t ma1nta1ns the same onentat1on. This is compell1ng evidence that a penetrative fabnc, or layering. ex1sted 1n the m1gmatite before s2formed, and has been entirely erased from the schist (melanocrat1c neosome) layers. The garnet-nch portions form the w1dest parts of each neosome, whereas the garnet-absent granitic part occurs either as a wedge-shaped tail to, or as a narrower neck between, adjacent garnetbearing parts. Since the domains of light-colored neosome are lent icular In form and elongate In the Si direCtiOn, the

• •

garnet-nch parts are interpreted to have been more competent than t he leucocratic parts when the neosome was be1ng deformed, dominantly by layer-normal shortening. Th1s 1s cons1stent w1th the petrolog1cal interpretation of the neosome by Wh1te et al. (2004) 1n wh1ch the cores are the solid products of the Incongruent breakdown reaction of b1ot1te and Sillimanite, and the gran1t1c parts are the products of crystallization of the anatectic melt produced by the react ion. The preservation of crystal faces on feldspar 1n the light-colored neosome, and the general absence of plastic deformat1on features 1n the quartz and feldspar of the light-colored neosome, and 1n the m1nerals of the sch1st (melanocratiC neosome) layers, 1nd1cate that deformation occurred while melt was present 1n the rock, 1.e., dunng anatexis. Thus, the curvature of the foliation 1n the ilmenite-plagioclase quartz biotite-sill iman ite- cord1ente sch ist and the increase 1n t he modal proportion of the mafic m1nerals v1sible 1n the sch1st around the competent garnetnch port1ons of the neosome are consistent w1th the loss of volume from the sch1st layers. Thus, the overall structure of t h1s m1gmatite IS exactly analogous to the structure assooated with pressure solution 1n low-grade metasedimentary rocks. In such rocks, the mica-nch microlithons are interpreted to have lost a mobile quartzofeldspathic component during dominantly layer-normal compression, and

199

METATEXITE MIGMATITES WITH LAYERED OR

200 ----------------------------STROMATIC STRUCTURE DUE TO TRANSPOSITION

the competent. typically quartz-nch layers, to have ga1ned some, or all, ofthe mobile material typ1cally as overgrowths, or pressure-shadow fillings, around the most competent m1nerals. However, 1n this migmatite, the mob1le matenal conta1ned the components that made anatectic melt.

Locat1on: Round Hill, Broken Hill area, Australia. Rock type: metatexite migmatite; aluminous metapelite protolith partially melted at T 800°C and P 5-6 kbar. Scale: the width of the field of view is I m. /mage: E.W. Sawyer. Further reading: Rob1n, P.-Y.F. ( 1979): Theory of metamorphic segregation and related processes. Geoch1m1ca et Cosmoch1m1ca Acta 43 , 1587-1600. Wh1te. R.W., Powell, R. & Halpin, J.A. (2004): Spatiallyfocussed melt format1on 1n alum1nous metapehtes from Broken Hill, Australia. journal of MetamorphiC Geology 22,

825 845.

Atlas of Migmatites

---------------------------- 20 1 t

Figure

41

• •

• • ;

• i

Fig. 041 . The neosome in this migmat1te shows very little segregation of the melt fraction from the residual fract1on; hence, it 1S mesocratiC, and medium-gra1ned because part ial meltmg has occurred in a contact aureole and not in a regional metamorphic setting. The local segregation of the melt from the residuum is indicated by the small leucocratic domains within the neosome. However, the most st riking aspect of t his migmatite is the large number of fragments of paleosome, called schollen, or rafts, which are present in t he neosome. In t he cent ral part of t he image, t he schollen are small and tend to be elongate. but toward the top left and bottom right. the schollen are much larger. In this particular migmat1te, the size and shape of the schollen reflect whether the protolith was thinly (small elongate schollen) or thickly (larger schollen) bedded. The m1gmatite shown in th1s photograph 1s largely neosome, and the paleosome occurs only as schollen 1n the neosome. Although some of the schollen have been rotated, others, most notably the elongate ones, have not, and the onentation of the bedding IS more or less preserved through the neosome. There may not have been sufficient neosome developed to provide sufficient space for the longer schollen t o have rotated. Hence, this migmatit e can be considered as transitional from metatexite to diat exit e migmat it e. It is dominated by

neosome formed from the fertile beds, but some continuIty of older, pre-partial-melt 1ng structures IS retained by the relics of the less fertile and sterile beds.

Location: Saint-Cast, Brittany, France. Rock type: schollen diat exite migmatite; semipelitic schist protolith, T ca. 700°C, P ca. 3 kbar. Scale: the ru ler is IS em long. Image: E.W. Sawyer.

THE TRANSITION FROM METATEXITE

-~----202 --------------------TO DIATEXITE MIGMATITES

Figure

42

Fig. 042. Thts photograph shows the abrupt transttton from a metatextte mtgmattte (top left) to a dtatextte mtgmatite (bottom nght). Schollen or rafts from a thick layer of metagreywacke appear to be detached from the metatexite and progressively transformed from tabular to rounded to lenticular shapes as they are incorporated into the diatextte mtgmatite. Smaller, tabular schollen are detached from the edges of a much larger one tn the center of the photograph. The orientation and offset along melt-filled fractures in the

schollen. the asymmetry of schollen shapes, and the trends of schlieren in the leucocratic matnx all tndicate now of the melt-nch part of the migmatite. Thus. this transttion from metatextte and diatextte mtgmatttes was dynamtc, the dtatextte magma flowed past the metatextte and tn effect eroded and intruded it. Most of the melt in this diatextte migmattte was contributed by the metagreywacke layers. whtch underwent tncongruent melting accordtng to the reactton btottte + plagioclase + quartz orthopyroxen e + melt. The schollen contatn the assemblage plagtoclase + orthopyroxen e + btotlte + quartz, which is the residuum left after the extraction of the anatectic melt. They also contain abundant gratn-scale microstructural evidence of parttal melttng (see Figs. F35-F38). The coarse-grained leucocrattc matnx contains the assemblage plagioclase + quartz + K-feldspar + btotite + orthopyroxen e, and represents the melt. The tiling of subhedral to euhedral

=

crystals of plagtoclase and K-feldspar; and the presence of rattonal faces on orthopyroxen e. plagtoclase and K-feldspar against quartz, both indicate a substantial fraction of melt in the leucocrattc portton of the mtgmattte. However; local vanatlons tn the mtcrostructure and whole-rock bulk composttton tndicate that some parts of the melt-rich material have a large component of cumulate plagtoclase, whereas others have a large component of evolved melt.

Location: Ashuantpi Subprovince. Quebec, Canada. Rock type: schollen diatextte mtgmattte: metagreywacke protoltth, anatexts in the granulite factes, T 825- 875°C, P 6 7 kbar. Scale: the ruler ts IS em long. Image: E.W. Sawyer: Further readmg: Guerntna, S. & Sawyer; E.W. (2003): Largescale melt-depletion tn granulite terranes: an example from the Archaean Ashuantpt subprovtnce of Quebec. journal of Metamorphic Geology 21 , 181-201. Peroval, J.A. (1991): Granultte-faoes metamorphtsm and crustal magmatism tn the Ashuanipi complex, Quebec Labrador; Canada. journal of Petrology 32, 1261-1297. Sawyer; E.W. (2001): Melt segregatton tn the continental crust: distribution and movement of melt in anatectic rocks. journal of Metamorphic Geology 19. 291 309.

~ ~

Atla~ of Migmatite:.

----------- ----------- --------- 203

f

f

Figure

• • ~

•

• • •

•

Fig. 043. Large regions of diatexite migmatite developed from metamafic protoliths are very rare, probably because the metamorphic temperatures recorded from most of the exposed parts of the continental crust are not high enough to have caused a pervas1ve h1gh degree of part1al melt1ng 1n such a protolith. However. the local-scale development of m1gmatites trans1t1onal from metatexite to diatex1te 1s more commonly seen 1n metamafic protoliths. This photograph shows part of a small outcrop (ca. IS m 1) 1n which the old, layered structure in a metatex1te migmatite (upper part of the photograph) IS progressively disrupted (lower part) and erased. The metatex1te port1on cons1sts of foliated melanocrat1c layers that have been boud1naged, and of leucocrat ic matenal, w h1ch occurs parallel to the layering, in the spaces between boudins and in vanous fractures. The leucocratic material 1S tonalit1c 1n compos1t1on

the w1de leucocrat1c ve1n, where there are no coherent melanocratiC layers. Rather. the m1gmatite contains many small rounded, rotated, and partially disaggregated remnants (rafts or schollen) of melanocratic material engulfed in leucocrat1c matenal. The proport1on of leucocrat1c material is greater 1n the d1atexite port1on than 1n the metatex1te, and 1t 1s also petrographically different. It has a much h1gher proport1on of mafic minerals, and these tend to occur as larger aggregates than 1n the metatexite. The tonalit1c melt thus seems to have been contaminated with matenal denved from the disaggregatlng melanocratlc schollen. The trans1t1on from metatex1te to d1atex1te migmatite in th1s outcrop 1nvolved an 1ncrease 1n the fract1on of melt. wh1ch assisted the breakup, rotation, and disaggregation of the melanosome and paleosome layers to y1eld a migmatite devoid of structures older than the anatexis.

and was largely denved from anatectic melt, but it conta1ns a variable proport1on of mafic matenal. In some places, the mafic material occurs sparsely as 1nd1v1dual euhedral crystals of

Location: Beit Bndge Complex, Baklykraal Farm, South Afnca. Rock type: transition from metatexite migmatite to schollen

pyroxene and hornblende that could e1ther be the solid product of the melt1ng react1on, or else have crystallized from the

diatexite m1gmatite; metamafic protolith, subjected to granulite-facies anatexis. Scale: the pocket kn1fe IS II em long. Image:

anatectic melt. H owever. 1n many places, the mafic matenal 1s more abundant, and appears to cons1st of fragments denved

E.W. Sawyer:

from nearby melanocratic layers. These portions w ere derived from contaminated anatectic melt. The migmatite is substantially d1fferent in the lower part of the photograph, below

THE TRAN SITION FROM METATEXITE

204 ------------------------~~~

TO DIATEXITE MIGMATITES

Figure

44

Fig. 044. This migmat1te 1s quite complex. but most of it IS neosome. The only part that has not part1ally melted, and hence can be called paleosome. IS the rather un1form. medium-grey scholle (labeled P): It IS a plagioclase + quartz + b1otite Al -poor metagreywacke. All the other schollen, of various shades of grey, have undergone part1al melting and coarsening of their gra1n size. In addit1on, most have lost some of their melt fraction: accordingly, they are called neosome (no loss of melt) or melanosome (loss of some melt). The darker and coarser-gra1ned schollen conta1n the assemblage plagioclase + quartz + biotite + cordierite ± K-feldspar ± sillimanite. wh1ch is interpreted to be res1dual. The schollen have elongate. lent1cular shapes, commonly with a narrow, b1otite-nch nm that is Interpreted to result from react1on between the melt around a scholle and the res1dual cordiente m 1t. All the schollen are enclosed 1n leucocrat1c material that contams the assemblage plagioclase + K-feldspar + quartz ± cordiente, and microstructures (e.g., crystal faces on feldspar crystals 1n contact with quartz, euhedral crystals of cordierite) that 1ndicate crystallization from a granitic melt; hence, 1t can be called leucosome. Locally, the leucosome contains abundant curviplanar schlieren that are rich in biot1te and accessory minerals, such as apatite and zircon. In places, groups of schlieren curve into, or are truncat ed by. other groups

of schlieren, which IS indicat1ve of vanat1ons 1n flow rate within the leucosome magma. The elongation of the schollen and of the schlieren IS parallel to the bedding 1n the nearby metatex1te m1gmat1tes. The locat1on of th1s diatexite m1gmat1te was thus possibly controlled by the presence of particularly fertile beds 1n the protolith, or by the migration of anatectic melt 1nto certain bedding-parallel sites. Two leucocrat1c veins in the migmatite are younger and cross the flow structure in the diatexite. A narrow, irregular band of leucosome (bottom right) is located in a small shear band and has d1ffuse margins. The diffuse contacts between the narrow leucocratic ve1n and the leucosome suggest that 1t IS a late-stage segregation of melt expelled from t he part1ally crystallized leucosome. A wide (>25 em) coarse-gra1ned leucocrat1c vein 1n the top left has sharp contacts and a very narrow mafic selvedge of biotite particularly where 1t is 1n contact w1th the schollen; this leucocrat1c vein was injected when the m1gmat1te was near its solidus temperature. Location: Quetico Subprovince, Ontario, Canada. Rock type: schollen diatexite migmat1te: metapelitic protolith, partially melted in the upper amphibolite facies, T 700- SOOoC. P 3- 4 kbar: Scale: the ruler is IS em long. Image: E.W. Sawyer.

A tlas of Migmati tes

------------------------------ 205 Figure

45

Fig. 045. The Vaasa Migmatite Complex 1n western F1nland

a foliat1on that ts parallel to the compos1t1onal band1ng; both

conta1ns a trans1t1on from a garnet cordiente K-feldspar-

are attributed to flow of the matrix as magma. The presence

plag1oclase quartz b1ot1te gne1ss to m1gmatite. Th1s 1mage

throughout the matnx of scattered euhedral crystals of plagio-

shows the morphology of rocks that are transrt1onal from

clase, or plagioclase crystals w1th stra1ght faces against quartz, is further ev1dence for the pervas1ve presence of melt 1n the matrix

metatexite to diatexrte migmatite. The outcrop appears to be morphologically complex because it has numerous domains

around the enclaves. Thus, the transit1on from metatexite to dia-

of d1fferent composition, grain size, and microstructure. There are several rounded, light greenish grey enclaves composed of

texite in the Vaasa Migmatite Complex can be Interpreted as a change in the distribution of the melt fraction. Where the rock

calcareous resister lithologies (e.g., near the center); these are

developed a pervasive fraction of melt between t he grains, flow

paleosome. However, all the rest of the outcrop is neosome

was able to occur and entrained enclaves able to rotate, so that

of one sort or another. Dark-colored lens-shaped enclaves

the continuity of pre-anatectic structures was lost. and a diatex-

(e.g.. near the top) are common t hroughout; these are frag-

ite m1gmatite formed. T he compositional banding in the matrix

ments derived from the metatexrte m1gmat1te. The maJonty of

between the enclaves IS Interpreted to be due to variat1ons 1n

these enclaves are from the miCa-nch, res1dual port1ons of the

the proportion of anatect1c mett to res1dual crystals from band

metatexrte that were left after the extract1on of anatectiC melt;

to band. As the proport1on of enclaves and the small-scale van-

they are melanosome. However, some enclaves also conta1n th1n stromatic veins of leucosome. Much darker enclaves (h1gher

atlons in the proport1on of mett to res1duum decreased, the schollen diatexites in the Vaasa M1gmatite Complex pass 1nto

contents of garnet) with a more granular microstructure (e.g..

diatexite m1gmatites (see Fig. 061).

near the pen) appear to have been part1cularly prone to disaggregation and are surrounded by numerous small fragments. All

Location: Kauhava area, Svecofenn1an doma1n. Finland. Rock type:

the enclaves occur in a banded mat1ix in which layers of different grain size, microstructure, and modal mineralogy can be distinguished. Some ofthese bands are conspicuously leucocratic (e.g.. left of the pen cap) and contain scattered crystals of garnet. but most are mesocratic, w ith a higher modal proportion of gar-

schollen diatexite migmatite; metapeilt1c protolith, granulite-

T ca. 800°C, P 4 5 kbar. Scale: the pen is 15 em long. Image: Hannu Makitie.

facies anatexis at

Further reading: Makitie, H. (2001): Eastern margin of the Vaasa M igmatite Complex, Kauhava, west ern Finland: preliminary

net. biot1te, cordierite, and locally, orthopyroxene (e.g., left half

petrography and geochemistry of the dlatex1tes. Bulletm of the

of the 1mage). However, all parts of the matrix contain doma1ns

Geologtcal Socrety o{F1nland 73, 35 46.

where the platy or tabular minerals (m1cas or feldspars) define

T HE TRANSITION FROM METATEXITE

206 -----------------------------TO D IATEXITE MIGMATITES

Fig. 046. This m1gmatite shows a trans1tion from a stromatiC metatexite to a diatex1te m1gmatite. It contains a relatively high proportion of schollen (or rafts) set in a coarse-grained. gran1tic matnx. All of the schollen are of stromatic metatexite (i.e .. neosome). which suggests an early stage of diatexite formation; 1n the more advanced stages of diatexite format1on. most schollen of neosome have typically been assimilated. and paleosome (resister) lithologies predominate in the remaining schollen. The pro portion of schollen is a little higher in the left half of the photograph. and the pre-ex1sting structure outlined by the stromatiC leucosome can be traced. more or less. from scholle to scholle. Some schollen are lenticular 1n shape. and the1r internal domains of stromatic leucosome have s1gmoidal forms. wh1ch suggest that the schollen were separated by small shear zones. Other schollen conta1n t1ght to open folds that are truncated by the edges of the scholle. In the left half of the photograph. the matnx between the schollen is narrow. and locally conta1ns d1scontinuous. biotite-rich schlieren that are generally parallel to the long sides of the schollen. However. at the ends of the schollen. the schlieren truncate the sigmoidal internal structure of the schollen and. consequently. appear to occupy shear surfaces between schollen. The migmatite is slightly different in the right half of the photograph; there. the proportion

of schollen to matnx IS lower, and there IS more vanat1on in the orientation of the 1nternal structures between schollen. which suggests that the schollen were able to rotate in the matrix. Locally. biotite-rich schlieren in the matrix outline disharmoniC flow-related folds. These relationships are interpreted to indicate that melt was able to migrate along small shear zones that developed in a stromatic metatexite migmatite during folding. Progressive infiltration of melt enabled some shear-bounded blocks of metatexite to detach and become schollen in a diatexite migmatite. As the melt infiltrated further into the metatexite migmatlte. the schollen that were detached first were no longer at the relatively stagnant edge of the d1atexite magma. and consequently were able to rotate 1n the flowing diatexite magma and to become d1spersed. Location: Saint-Malo. Bnttany. France. Rock type: schollen d1atexite migmatlte; metasedimentary protolith. Scale: the co1n is 2.0 em across. Image and caption: M1ke Brown. Further readmg: Brown. M. ( 1979): The petrogenesis of the St-Malo migmatite belt. Armorican Massif. France, with particular reference to the diatex1tes. Neues Jahrbuch fur Mineralogie. Abhandlungen 135. 48 74.

A tlas o f Migmatites

-------------------------------207 Figure

47

Fig. 047. T his complex migmatite contains a high proportion of neosome, and there are no coherent or contiguous structures of pre-anat ectic age; hence, this is a diatexite migmatite. Paleosome occurs only as several large, equant blocks called schollen (labeled So) of resister lithologies. There are many smaller melanocratic schollen, up to about 15 em across in the neosome, and most of these contain sillimanite + biot ite + garnet + quartz + plagioclase + muscovite and are interpret ed as residuum. The leucocrat ic matrix contains the mineral assemblage quartz + plagioclase + muscovite + K-feldspar + biotite, and contains abundant schlieren (labeled Sl) rich in biotite and sillimanit e. The schlieren define a foliation and flow banding t hat wraps around the larger schollen. The migmatite thus contained a sufficiently high fraction of melt that it could flow. This migmatite could be called either a schollen or a schlieren diatexite, since both are present. There is clear evidence of sequent ial segregation of melt from the meltrich parts of the migmatite t hat host the schollen and the schlieren. Coarser-grained and generally more leucocratic mat erial (LI) occupy posit ions correspond ing t o pressure shadows at the corners of the schollen. Melt thus was segregated from the crystals during t he main st age of magma flow. Furthermore, the sigmoidal curvature of the schlieren reveals a sinistral shear zone (just below and t o t he right

of the hammer), which contains a diffuse, schlieren-poor leucosome (L2) along its center, and which t runcates the curved schlieren. This is interpret ed as a lat e-stage shear zone formed in the diatexite, which had already developed a rigid framework of crystals with interstitial residual melt. T he melt was expelled as the crystal framework was deformed in the walls of the shear zone, and became concentrat ed in the dilatant center of the shear zone. There is evidence of later segregation of melt, which occurred when crystallization was nearly com plete, i.e., the leucocratic vein to t he right, which occupies a brittle fractu re oriented orthogonal t o t he layering and perpendicular to the direction of elongation in the outcrop.

Location: Tu mbledown Anatectic Domain, western Maine, U.S.A. Rock type: schollen diatexite migmatite with schlieren; pelitic and semipelitic metasedimentary protolit h, anatexis in the upper amphibolite facies; T 750-800°C, P 4- 5 kbar. Scale: the hammer handle is 40 em long. Image and caption : Gary Solar. Further reading: Solar, G.S. & Brown, M. (200 1): Pet rogenesis of migmat ites in Maine, USA: possible source of peraluminous leucogranite in plut ons? Journal of Petrology 42 ,

789- 823.

DIATEXITE MIGMATITES W ITH SCHO LLEN

208 -----------------------------AND WITH SC HLIEREN STRU CTURES Figure

48

Fig. 048 . This phot ograph shows a diatexite migmatite with both schollen and schlieren. T he two prominent schollen are metamafic resist er lithologies that contain the assemblage orthopyroxene + plagioclase + ilmenit e. They were probably mafic dikes in the metapelitic protolit h, i.e., they are paleosome. However, the main point of int erest in t his migmatite is the strong morphological variation present w it hin different parts of the neosome. The neosome material between the two schollen (I) is leucocratic and coarse-grained, (2) contains the assemblage plagioclase + quartz + K-feldspar + biotite + cordierite + garnet , (3) generally lacks a prominent foliation, although it has some diffuse, darker wisps that are a little richer in biotite and, locally, domains where t he feldspar crystals form a t ile microst ructure, and (4) contains large regions wher e euhedral crystals of plagioclase form a framework wit h interst itial quartz. Together, these characteristics indicate that this neosome crystallized from a granit ic melt that contained very few residual minerals and records a relat ively low level of syn-anatectic shear strain. This region of neosome and the two paleosome schollen constitute a single structural domain within the migmatite t hat is enveloped by a different neosome that appears to have fiowed around it. The enveloping neosome ( I) is medium-grained, (2) contains the assemblage plagioclase + quartz + biotite + garnet + cordierite + sillimanite ± K-feldspar. (3) has a strong foliation

due to t he alignment of idioblastic and euhedral cryst als of plagioclase, biotit e, sillimanite, and locally, cordierit e, and (4) contains a compositional banding due to the alternation of leucocratic bands w ith mafic schlieren of biotite + sillimanite + garnet composit ion and diffuse, elongate lenses of fine-grained quartz + biotite + sillimanite + garnet + cord ierite + plagioclase melanosome. Together, these characteristics indicate that this neosome was derived from a magma rich in residual crystals, contained scholl en of melanosome, and experienced high shear strain while in magmatic and submagmatic states. Thus, the morphological differences between areas of neosome in this migmatite are a direct function of their structural posit ion in the nowing diat exit e magma. High shear strains in t he partially crystallized, residuum-contaminated diatexite magma drove the segregat ion of fractionated anat ectic melt out of t he banded neosome, and the melt collected into t he low-pressure sit e between the two paleosome schollen, where it became the coarse-grained and residuum-poor neosome.

Location: Lac Kenogam i, Grenville Province, Quebec, Canada. Rock type: schollen diatexite migmatite; metapelitic protolit h, anatexis at T 800- 850°C, P 4-7 kbar. Scale: the ruler is IS em long. /mage : E.W. Sawyer.

Ada; of M igmatite>

----------- ----------- -------- 209 Figure

49

Fig. 049. Schollen d1atex1te m1gmat1tes are best known from metasedimentary protoliths, but they also form from fels1c igneous protoliths. Th1s example, derived from a leucotonalite and leucotrondhjemite protolith, contained a h1gher fract1on of melt than the previous examples shown, and the schollen are fewer and have a more lent1cular shape. Some ofthe schollen have a very h1gh aspectratio, and thew tapered ends pass laterally 1nto biot1te-nch schlieren. Many of the schollen have melt-depleted (i.e., res1dual) bulk compos1t1ons that, because of the nature of the protolith, are dom1nated by plag1oclase. the pnnopal mineral present 1n excess, but they are also ennched 1n the accessory minerals. The compos1tions and microstructures of other schollen suggest they are layers of plag1oclase accumulated from the crystallizing anatectiC melt that have become d1srupted and Incorporated 1nto the flow1ng magma. A few schollen are res1ster litholog1es. The pmk host to the schollen 1s a leucogran1te in compos1t1on. with the assemblage K-feldspar + plagioclase + quartz + b1ot1te. However, the leucogran1te host conta1ns a w1de vanety of microstructures: some parts contain a strong magmatiC, or submagmat1c, foliat1on defined by the onentation of tabular K-feldspar and plag1oclase, whereas other parts are coarser grained and isotropic and occur as layers, patches, or irregular ve1nlike bod1es. Some of the d1fferent

microstructural types of leucogran1te that occur as layers, or as w1spy bands, are truncated by other more cont1nuous bands, or by biotite-nch schlieren. Such variations are typical of many diatexite m1gmatites; the banding and schl1eren are ev1dence for the flow of a compos1t1onally heterogeneous diatex1te magma, and the leucogramte patches and ve1ns prov1de ev1dence of the late segregation of the melt at a t1me when the diatexite magma was in an advanced stage of crystallization. Fractionated melt in the intercrystalline pores was expelled as the framework of euhedral gra1ns of feldspar began to compact 1n response to sheanng; the melt that was expelled collected 1nto brittle fractures and other dilatant structures, to produce the late veins and patches of coarse-gra1ned, Isotropic leucogranite present 1n the outcrop. Location: Opatlca Subprov1nce, Quebec, Canada. Rock type: schollen diatexite m1gmatite: leucotonalite and leucotrondhJemite proto lith, anatexis at condit1ons of the upper amphibolite facies, T ca. 75ooe, and P 5 7 kbar. Scale: the hammer IS 45 em long. Image: E.W. Sawyer.

DIATEXITE MIGMATITES WITH SC HOLLEN

210 -----------------------------AND WITH SCH LIEREN STRUCTURES Figure

50

Fig. 050. Th1s m1gmat1te has a very high proport1on of leucocratic neosome, and 1ts morphology is typ1cal of the reg1ons 1n anatectic terranes where melt has accumulated. The upper part of the outcrop conta1ns mesocratic schollen of metagreywacke res1ster layers with the s1mple assemblage plagioclase + quartz + biotite and schlieren. These are layers that did not melt and were disrupted and incorporated into the layer of anatectic melt. The central and lower parts of the outcrop contain melanocratic lenticular schollen that conta1n the mineral assemblage plagioclase + quartz + cordiente + biotite + K-feldspar + silliman1te. It repre sents the res1duum left after the melt1ng react1on: b1ot1te + s1ll1man1te + quartz + plagioclase cord1ente + K-feldspar + melt. Very th1n schlieren of b1ot1te are present 1n the leucocratic layers, and some schlieren (labeled Sl) seem to hnk some of the lens-shaped melanocrat1c schollen. The asymmetncal shape of most of the res1dual schollen and the curvature of the schlieren define the shear sense of now (dextral) In the migmatlte. The presence of euhedral crystals of cord1erite and feldspar, and of equant doma1ns of stra1n-free quartz 1n the leucocrat1c parts of the diatex1te migmatite, indicates that now occurred in the magmatic and submagmatic states Several periods of melt segregation and emplacement can be recognized. A syn-anatectic shear zone separates the domain of resister schollen from that

=

of res1dual schollen. and conta1ns a sl1ghtly coarser-gra1ned, more leucocrat1c center, which probably represents fractionated melt expelled from the walls of the shear zone. The lower edge of the doma1n that conta1ns the resister schollen and associated schlieren 1s truncated (indicated by the arrows a and b) by the syn-anatect1c shear zone. A later leucocrat1c ve1n has intruded the migmat1te JUSt above the scale; the irregular contacts of this vein suggest that it was injected before the host had cooled below its solidus. The last stage is represented by a coarse-gra1ned, leucocrat1c vein onented orthogonally to the layering in the res1ster schollen doma1n. Location: Quet1co Subprov1nce, Ontano, Canada. Rock type: schlienc d1atex1te m1gmat1te; metapelitlc and metagreywacke protolith, upper-amphibolite-facies melt1ng. T 700 800°C. P 3- 4 kbar. Scale: the ruler 1s 15 em long.

/mage: E.W . Sawyer.

Further readmg: Sawyer, E.W . (1987): The role of part1al melt1ng and fract1onal crystallization 1n determining discordant migmat1te leucosome compositions. journal of Petrology

28,445 473.

A tlas of M igmat itcs

- - - - - - - - - - - - - -- - 211

Fig. 051. This migmatite is essent1ally all neosome, but it exhibits a considerable range 1n microstructure, modal m1neralogy, and color. Although there are no layers, rafts, or schollen of paleosome, there are ghostlike schollen of mesocrat1c neosome. The lightest-colored parts can be called leucosome. The melanocrat1c (darkest) parts of the m1gmatite have two forms. The most prom1nent occur as selvedges adhenng to quartz feldspar pods around which the flow structure 1n the host IS deflected. These selvedges are Interpreted as fragments of early, coarse-gra1ned leucosome, wh1ch together w1th their melanocratic border were d1srupted as part1al melttng advanced and as the flow of the result1ng diatexite magma occurred. Also present are thin biot1te-rich streaks w1thout attached leucosome: these are biot1te schlieren. Furthermore, there are many larger mesocratic domains (below and to the right of the lens cap) that contain a layering and appear to be attenuated, from which one can infer that they were weak rather than strong. These domains could represent layers of partially melted protolith that were e1ther less fertile than the other layers and, therefore. did not partially melt as much, or they are layers 1n wh1ch the melt did not segregate much from the residuum. The result is that these layers were not sufficiently competent to form well-defined rafts or schollen. Rather, they form darker w1spy layers and streaks in

the migmatite. Hence, this outcrop could be viewed as representing an advanced stage tn the transition from a metatexite to diatex1te migmat1te 1n which the mesocratic parts have partially melted, and record the last vestiges of the orig1nal compositional layenng 1n the protolith.

Location: Glenelg River Complex, Australia. Rock type: schlteric diatex1te migmat1te; quartzofeldspath1c protolith, partial melting in the upper amph1boltte faetes. Scale: the lens cap IS 6.5 em 1n diameter. Image and captton: Tony 1.5. Kemp. Further readtng: Kemp, A.I.S. & Gray, C.M. (1999): Geolog1cal context of crustal anatex1s and granitic magmatism in the northeastern Glenelg R1ver Complex, western Victona. Australtan journal o( Earth Setences 46 , 407 420.

DIATEXITE MIGMATITES WITH SCHOLLEN

212------------------------~~~

AND WITH SCHLIEREN STRUCTURES

Figure

52

The schollen are fragments of coarse-gra1ned. stromat1c metatex1te that have a prom1nent Internal layenng due to the alternation of thin bands of leucosome w1th melanosome.

that define a foliat1on, equant quartz gra1ns w1th only minor ev1dence for gra1n-boundary m1gration, and large crystals of plagioclase that have rat1onal faces aga1nst quartz, suggest that the matnx crystallized from a melt. and that the foliation

Doma1ns of leucosome contain the assemblage plagioclase + quartz + K-feldspar + muscov1te. and the melanosome consists of biotite + plagioclase + quartz. The schollen are lenticular in shape and strongly aligned. Internal layering is

1s magmatic. The parallel onentat1on of the schollen, biot1te schlieren, and magmatic foliat1on may, therefore, have developed in the diatexite magma (granitic melt with fragments of metatexite migmatite and crystals of biotite and feldspar) as

parallel to the long axis of the schollen, except in the scholle with the lowest aspect-rat1o, where the internal layenng is sigmoidal. The matrix that encloses the schollen is mesocratic and broadly gran1tic. with the mineral assemblage plag1oclase + quartz + K-feldspar + muscov1te + b1otite, but 1t has both

a result of gradients 1n the flow rate (i.e., shear-strain gradi ents). The leucocratic domains in the matrix may represent either a fractionated leucocrat1c melt, or a melt devoid of entra1ned crystals of res1dual b10t1te that was able to migrate

Fig. 052. This m1gmatite cons1sts ent1rely of neosome.

melanocratic and leucocratic parts. Thin, b1ot1te-nch doma1ns define schlieren (SI) that are parallel to the elongat1on of the schollen, but the b1ot1te schlieren are much less prom1nent and more irregular where there are no schollen present, e.g., 1n the lower right. Coarser-gra1ned, leucocratlc doma1ns 1n the mesocratic matrix occur as (I) lenses onented parallel (LI) to the biotite schlieren and to the d1rection of elongation of the schollen and (2) small domains (L2) located at the ends of low-aspect-ratio schollen, a pos1tion that is analogous to pressure shadows around competent porphyroblasts. Microstructures in the mesocratic matrix, such as the alignment of large crystals of feldspar and b1otite

to low-pressure sites as the framework of crystals 1n the partially crystallized diatex1te deformed toward the end of 1ts crystallization.

Locat1on: Sa1nt-Malo, Bnttany, France. Rock type: schollen diatexite m1gmat1te: pelit1c to sem1pelitic protolith, part1al melting in the upper amphibolite fac1es, at T ca. 750°C, P 4-7 kbar. Scale: the co1n is 2.2 em 1n d1ameter. Image: Mike Brown.

Further read1ng: Brown, M. ( 1979): The petrogenesis of the St-Malo migmatite belt, Armorican Massif, France, with particular reference to the diatexites. Neues jahrbuch rur M1neralog1e, Abhandlungen 135,48 74.

A tlas of Migmat ites

------------------------------ 213 Figure

53

Fig. 053. Most of the neosome in this migmatite is mesocratic and contains the assemblage cordierite + biotite + K-feldspar + quartz

+ plagioclase; sillimanite occurs as inclusions in the cordierite. T he bulk composition of the neosome is the same as that of the likely protolith, collected from just below the "melt-in" isograd. Moreover; the mesocratic neosome contains prismatic cordierite and K-feldspar, has tiled, unstrained crystals of plagioclase, and has a grain size that is S-1 0 times that of the scattered schollen of paleosome (nonmelted plagioclase + quartz + biotite schist). Therefore, this migmatite is largely partially melted protolith in which significant separat ion of the melt from its residuum did not occur; at least not on the scale of the I0-kg sample used for determining its whole-rock composition. The melting reaction was probably biotite + sillimanite + quartz + plagioclase= cordierite + K-feldspar

+ melt. T he principal compositional banding in this migmatite results from the concentration of biotite to various degrees during flow as a magma. Hence, some schlieren are darker than others. As there are no structures preserved that predate anatexis, and as biotite schlieren are the dominant morphological element, this is a schlieric diatexite migmatite. Scattered, elongate schollen contribute to the banding. The migmatite also contains some K-feldspar crystals that are up to 5 em across. A diffuse patch of coarse-grained diatexite

sense at the stage at which this diatexite migmatite was in a magmatic state.

(just to the right of the scale) lacks the strong compositional banding typical of the rest of the diatexite and results from

Location: Quetico Subprovince, Ontario, Canada. Rock type:

the late injection of a mesocratic magma that contained large cryst als of K-feldspar into the already banded diatexite magma.

facies conditions, T 750- 800°C, P 3-4 kbar: Scale: the ruler is

A leucocratic K-feldspar-bearing magma was also injected int o

15 em long. /mage: E.W . Sawyer:

the diatexite after it had become compositionally banded, and this formed the prominent coarse-grained granitic vein that is

Further reading: Milord, I. & Sawyer, E.W (2003): Schlieren formation in diatexite migmatite: examples from the St. Malo migmatite

oriented subparallel to t he compositional banding and crosses the center of the phot ograph. N ote the abundance of features

terrane, France.Journal ofMetamorphic Geology 21 , 341- 362.

such as the asymmetrical shape of the schollen, the asymmet-

Milord, 1., Sawyer, E.W. & Brown, M. (2001): Formation

rical shape of the leucosome-filled shadows around schollen and other competent objects, sigmoidal schlieren and trun-

of diatexite migmatite and granite magma during anatexis

cated flow-banding, which can be used to determine the shear

schlieric diatexite derived from pelitic protoliths; amphibolite-

of semi-pelitic metasedimentary rocks: an example from St. Malo, France.Journal of Petrology 42 , 487- 505.

DIATEXITE MIGMATITES W ITH SCH LI EREN STRUCTURES

214 - - -- -- - - - - - - - - Figure

54

Fig. 054. Th1s diatexite m1gmat1te has no schollen, but it has abundant, subparallel, biotltench schlieren. The schlieren are smuous and comparatively short (<25 em), and they range 1n w1dth from
Arias o f M ig matit es

------------------------------- 215 Figure

55

Fig. 055 . Thts diatexite mtgmattte contains only a few small. ellipttcal. and ghostlike remams of schollen. Alternattng btottte-nch and quartzofeldspathtc layers tn the schollen closely resemble the melanosome and leucosome found tn the neosome of metatextte mtgmatttes elsewhere tn the Satnt- Malo terrane. The leucocratic host contatns the assemblage plagioclase + quartz + K-feldspar + muscovtte + biotite and, locally, domains where tabu lar crystals of plagioclase are tmbricat ed (i.e., tile microstructu re) and have rattonal faces against tntersttt tal quart z. Taken together, these features tndtcate that the leucocrattc part of thts mtgmattte crystallized from a magma of grantttc composttton as 1t was sheared dunng flow. T he most promtnent features of thts mtgmattte are the dtsconttnuous btottte-nch bands that 1t contatns. Some of these could be called schlieren: the posstbility that the shorter, low-aspect ones could be pteces of the btottte-nch melanosome denved from dtsaggregated schollen should be constdered, however.

Location: Satnt-Malo mtgmattte terrane, France. Rock type: schlieren dtatextte mtgmattte; semtpelittc protolith, parttal melttng at T 750 800°C, p 4 5 kbar. Scale : the ruler IS IS em long. Image: E.W. Sawyer. Further readmg: Milord, I. & Sawyer, E.W. (2003): Schlieren formation tn diatexite mtgmattte: examples from the St- Malo migmatite terrane, France. journal o( Mecamorph1c Geology 21 , 341 - 362.

D IAT EXI TE MIG MATITES WI TH SC H LIE RE N ST RUCTU RES

216------------------------------Figure

56

Fig. 056. There are two main components to this diatexite migmatite. The first contains schlieren and irregularly shaped melanocratic patches (e.g., in t he lower right near the ruler), and the second is distinguished by its large crystals of cordierite (up to 4 em across) and a lack of schlieren. Schlieren are up to 20 em long and composed of biotite and sillimanite, with minor amounts of cordierit e and ilmenite. The melanocratic patches consist of the assemblage biotite + cordierite + sillimanite + ilmenite + plagioclase, and are interpreted t o be residuum left after t he incongruent melt ing of biotite; they are melanosome. The melting reaction was probably biotite + sillimanite + quartz melt + cordierite + ilmenite, and the protolith was pelitic. The

=

leucocratic matrix that contains the melanocrat ic patches and schlieren consists of the assemblage plagioclase + quartz + K-feldspar + biotite + cordierite (i.e., a granitic to granodiorit ic bulk composition) and is interpreted to have been derived from the anatectic melt generated in making the melanosome. A layering due to the alternation of leucocratic and melanocrat ic bands is locally folded. The second component has a granitic bulk composition, with an assemblage of plagioclase + quartz + K-feldspar + cordierite + biotite, which suggests derivation from an anatect ic melt extracted from a metapelitic source. In places, t he second component contains enclaves of the schlierenbearing component , indicating that the latter is older, and was more coherent Elsewhere (e.g., above the scale), faint

traces of schlieren persist int o the cordierite-rich component, and resu lt in a diffuse, or nebulitic, contact between the two. The presence of schlieren and melanosome implies that the parent magma contained crystals and fragments of residual material, but the magma that generated t he cordierite-rich component did not. Folding of the schlierenbearing portion could begin once crystallization reached the rigid percolation threshold (ca. 55% crystals) and could have red istributed, or expelled, some of the melt residing in the framework of crystals. Places that lost sufficient melt by compaction to reach the particle-locking threshold (ca. 72% crystals) behaved as if they were solid, but other places in the migmatite still contained as much as 50 vol.% melt. The redistributed melt was depleted in the residuum component, and hence very leucocratic. Whether the magma that produced the cordierite-bearing component of the diatexite was simply redistributed, or was introduced from a more distant source, it incorporat ed the parts of the schlieren-bearing component with > 72% crystals as rigid enclaves. However, where substantial amounts of melt remained, the contact between the two was not sharp.

Location: Pefia Negra, Spain. Rock type: schlieren diatexite migmatite; pelit ic protolith , part ial melting at T ca. 750°C, P 4-7 kbar. Scale: t he ruler is IS em long. Image: E.W. Sawyer.

A tlas of M igmat ites

-------------------------------217 Figure

5

Fig. 057. The Ashuanipi Subprovince contains a number of large bodies of diatexite that show a transition from metatexite migmatites and schollen diatexites at their borders, to more uniform interiors. T his example illustrates a mesocratic orthopyroxene biotite- K-feldspar quartz-plagioclase diatexite migmatite from the interior of a kilometer-size body of diatexite migmatite. Small lenticular aggregat es of biotite in the rock define a weak foliation in w hich some tabular crystals of feldspar also are aligned. The larger, aligned crystals of feldspar are plagioclase, whereas K-feldspar, quartz, the smaller cryst als of plagioclase, and ort hopyroxene are interstitial; this rock has a bulk composition close to that of the protolith. However, in other diat exite migmatites from the same body, t he plagioclase, and some of the K-feldspar, form larger tabular crystals that are aligned and define a magmatic foliation ; the whole-rock composit ion of these rocks is generally closer to that of a granitic, anatect ic melt. The orthopyroxene in diat exite migmatites t hat have a granitic bulk composition generally shows a much greater degree of replacement by biotite than the orthopyroxene in the diatexite migmatites that have composit ions more like that of the protolith.

Locat1on: Ashuanipi Subprovince, Quebec, Canada. Rock type: mesocratic diatexite migmatite; metagr eywacke protolith , anat exis at T 825-875°C, P 6-7 kbar. Scale: the ruler is 15 em long. image: E.W. Sawyer.

DIATEXITE MIGMATITES

218 ---------------------------Figure

58

Fig. 058. This d1atex1te migmat1te conta1ns small, scattered schollen of res1dual metasedimentary matenal cons1st1ng of the assemblage plag1oclase + b1otite + o:-thopyroxene + quartz, and a few mafic schlieren, w1thin a leucocratlc, weakly foliated matnx of granodiont1c bulk compos1t1on. The pnncipal react1on respons1ble for part1al melting in the protolith was biotite + quartz + plag1oclase orthopyroxene + melt ± ilmemte. The matrix is coarse-

=

grained, and the microstructure in much of it consists of a framework composed mostly of weakly aligned euhedral crystals of plagioclase and a small proportion of euhedral crystals of K-feldspar. Anhedral quartz and K-feldspar, together w1th a m1nor amount of b10t1te. fill the angular Interstitial spaces in the feldspar framework. Th1s microstructure is Interpreted to have formed dunng the crystallization of a gran1t1c to granodiont1c anatect1c magma under relat1vely stat1c condit1ons. However, there are some local variations in the microstructure of the m1gmat1te. One of the more common types cons1sts of small, round patches 1n the d1atexite (on the bottom edge, JUSt left of center) m which the grain size is noticeably coarser, the rock 1s more leucocratic, and the large euhedral crystals of feldspar are predominantly K-feldspar. These patches are interpreted as

the s1tes where the last stages of crystallization occurred from a magma that had a more evolved (i.e., fractionated) bulk compos1t1on than the init1al magma from wh1ch rest of the diatex1te crystall1zed. The diatex1te magma had, therefore, undergone fract1onal crystallization and had prev1ously crystallized a maJor amount of plag1oclase. Location: Ashuampi Subprovince, Quebec, Canada. Rock type: mesocratic diatexite migmat1te; metagreywacke protolith, anatexis at T 825 875°C. P 6 7 kbar. Scale: the st1cker (top left) 1s 3 em long; width of the photograph corresponds to 115 em. Image: E.W. Sawyer.

• A ri as of Migm a rires

------------------------------ 219

•

Figure

59

()

() ()

()

()

.,.... @.

]

,, .

()

()

0 ()

()

•

~o,too

7

EF52 ·CENTIMETRES 01

Fig. 059. Partial melttng of semtpehttc rocks in the SatntMalo terrane tnvolved the breakdown of muscovtte through reacttons such as: muscovite + plagtoclase + quartz + H 0 = melt. and muscovtt e + plagtoclase + quartz = melt + K-feldspar + sillimantte + b iottte. Btottte was not generally a reactant and rematned stable. Therefore. changes 1n the proportion of biotite can be used to infer whtch parts of the diat exit e migmatites are residual (btot tt e content tncreased) or melt-enriched (biotite cont ent diluted) relattve to the protolith. Under t hese circu mstances, mapptng the dtatexttes as melanocratic, mesocrat1c, or leucocratic IS a useful field technique for t racing reg1ons of melt loss and melt gain. Thts particular example of dtatextte mtgmattte contains large crystals of feldspar. some of whtch have rattonal faces agatnst quartz, tndtcattng crystallization tn a melt. However, the diatextte IS melanocrattc, has the mtneral assemblage plag1oclase + quartz + K-feldspar + biottte, and a bulk composit ion that tndtcates constderable enrichment 1n the residual components and b1ot1te relat1ve to an anatectic grantte derived fro m a pehte. Euhedral crystals of plagioclase and biot it e are oriented and define the fol iation. Some cryst als of plagioclase have rattonal faces against quartz, and the domains of q uartz are equant. Thus, the fol iation IS Interpreted to be magmatic: however, the

microstructure has a htgher proportton of stratght grainboundanes and lacks the tnterlocktng gratns typtcal of granites. Schlie ren of btotite are oriented parallel to the foliatton . A key problem to resolve for restdual dtatexite m1gmatites is how the restdual composition was attatned, and whether this happened before, or dunng. the bulk now, or magma-now processes t hat destroyed t he pre-anatect tc st ructures in t he protolith and paleosome. Location: Saint-Malo migmatite terrane, Brittany, France. Rock type : me lanocratic dtat exite or, alternatively, residual dtatexite; pelittc and semtpehttc protolith. anatex1s at T ca. 750°C, P 4-7 kbar. Scale: the ruler 1s IS em long. /mage: E.W. Sawyer. Further reading: Mtlord, 1., Sawyer. E.W. & Brown, M. (2001): Formatton of dtatextte mtgmattte and grantte magma dunng anatexts of semi-pelittc metasedimentary rocks: an example from St. Malo, France. journal of Petrology 42, 487 505.

DIATEXITE MI GMATITES

220 ---------------------------Figure

60

Fig. 060. Thts diatextte mtgmatite contatns scattered small schollen and could, therefore. be called a schollen dtatextte. Most of the schollen are small fragments of btotite-rich melanosome and are probably the remnants of larger rafts derived from stromatic mtgmatlte, such as that just left of the lens cap, which have disintegrated. Other rafts are of resister lithologies, notably calc-silicate rocks and quartz veins. However, the main characteristic of this migmatite is the uniformity of the matnx. which appears to have neither mafic schlieren, leucocratic segregations nor a flow banding in it. Although the bulk composition is leucogranitic and the mineral assemblage is plagtoclase + quartz + K-feldspar + muscovtte + btottte. the microstructure ts not grantte-like tn that many groundmass grains are polygonal rather than interlocktng. Euhedral and subhedral crystals of plagtoclase and mtcrocline (up to 5 mm tn length) are generally oriented and define a foliatton, together wtth the larger crystals of btotite present; the quartz domains and grams are equant and generally devotd of strain features. Therefore, the foliation is mterpreted to be of magmatic origin. Although the magma that produced this diatexite migmatite was subject to gradients in shear strain, which produced t he mineral alignments, no segregation of the melt fraction from the solid appears to have occurred. Either the shear stresses were very weak, or they

ended before about 50% crystalltzatton and the ngid percolation threshold was reached. Muscovtte, however, is not onented and appears to have crystallized late. This diatextte mtgmattte ts constderably more leucocratic than that shown in Ftg. 059, also from the Satnt-Malo migmattte terrane; the difference tndtcates that dtatexite migmatttes can be open systems. The htgher proportion of residuum in the migmatite in Fig. 059 tndicates the loss of some melt from it. whereas this migmatite contained a much higher fraction of anatectic melt. either because the residuum had already been separated or because 1t was diluted by the addit ion of anatectic melt. If the latter is correct. then one can infer that thts mtgmattte occuptes a structural stte where anatectiC melt could accumulate.

Locatton: Saint-jacut-de-la-Mer, Satnt-Malo mtgmattte terrane. Brittany. France. Rock type: leucocrattc dtatextte or, alternatively, leucocratic schollen dtatextte; pelitic and semtpelittc protolith, anatexts at T ca. 750°C. P 4-7 kbar. Scale: the lens cap is 6.5 em across. Image: Mike Brown. Previously published as fig. 3c tn Brown (1995) and reproduced with the permisston of the Societe geologique de France.

A tlas of Mig matites

---------------------------- 221 Figure

61

Fig. 061. Th1s very un1form, coarse-gramed K-feldspar b10t1te quartz plag1oclase diatexite m1gmat1te lacks both schollen and schlieren, although it does have a weak foliat1on. The pnnopal variation 1n the diatex1te v1s1ble 1n th1s outcrop-scale photograph results from scattered porphyroblasts or phenocrysts of K-feldspar. Plag1oclase has crystal faces against quartz, and this microstructure is interpreted to indicate that the plagioclase crystallized from a melt. Some of the biotite in the diatexite is intergrown with quartz or plagioclase (or both), and th1s microstructure suggests that the biotite + quartz may have replaced res1dual orthopyroxene (see, for example, Figs. F78 FB I). Orthopyroxene occurs commonly, although not abundantly, 1n other. compositionally less evolved diatex1te m1gmat1tes 1n the Kauhava area .

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Location: Kauhava area, Svecofenn1an doma1n, Finland. Rock type: d1atexite migmat1te: metapelit1c protolith, granulitefaCies anatex1s at T ca. 800°C, P 4 5 kbar. Scale: the pencil is 14 em long. Image: Hannu Mak1t1e. Prev1ously published as fig. 2a 1n Mak1t1e (200 I), and reproduced w1th the permission of the Geolog1cal Society of F1nland. Further readmg: Makitie, H. (200 1): Eastern margin of the Vaasa Migmatite Complex, Kauhava, western Finland: preliminary petrography and geochemistry of the diatexites. Bulletin of the Geological Sooety of Finland 13, 35-46.

D IATEXITE MIGMATITES

222 ---------------------Figure

61

Fig. 062. Thts close-up photograph shows a plagtoclase

+ quartz + K-feldspar + muscovtte + biot1te dtatextte mtgmattte of granitic bulk composition 1n which the miCrostructure and the distribution of mtnerals are almost isotroptc. The prinopal vanat1ons 1n the d1atex1te mtgmatite are (I) scattered small phenocrysts of K-feldspar that are only slightly coarser-grained than the matnx, (2) a very weak fo liation due to the alignment of some of the biotite, and (3) several small (ca. 10 mm) enclaves constst tng of crenulated aggregates o f btottte. sillimanite, and muscovite. The thin, dark, fine-grained layers onented roughly parallel to the scale and located near the center of the photograph are mylonite seams that formed as a result of stmstral strike- slip sheanng after the d1atex1te mtgmatite had crystallized. Locat1on: foreshore at Satnt-Malo, Satnt-Malo mtgmatite

terrane, France. Rock type: diatexite mtgmattte; pelit1c and semtpehtic protohth, anatexts at T ca. 750°C, P 4- 7 kbar. Scale: the ruler is 15 em long. Image: E.W. Sawyer:

A tl as o f Migmatites

------------------------------ 223 Figure

63

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Fig. 063. Th1s photograph shows a parallel layered structure that developed 1n a 4-m-wlde shear zone cross1ng an otherw1se rather un1form diatex1te m1gmat1te that has a magmatic foliation. The host diatexite has been transformed by the shear zone 1nto alternat1ng bands of (I) coarse-gra1ned leucosome 1n wh1ch magmatic textures. such as feldspar w1th crystal faces against quartz. are present and (2) finer-gra1ned layers with a h1gher proportion of mafic m1nerals (biotite + orthopyroxene) and a microstructure that resembles a cumulate-like arrangement of tabular crystals of plagioclase. a very small proport1on of which show fractures healed by plagioclase of slightly more sodic compos1t1on. Th1s layered structure IS Interpreted to have formed at the late stages of crystallization of the host diatex1te body. Dunng cooling. the synmagmatiC shear stram responsible for the foliat1on 1n the host d1atex1te became progressively focused 1nto the narrow1ng melt-beanng parts of the diatex1te body. The ex1st1ng framework of crystals 1n these narrow melt-beanng domains was progressively

deformed. 1ts gra1n s1ze reduced, and the crystals fractured. The pr·nopal effect was to compact the framework by reduong the Intracrystalline pore-space and, 1n so do1ng. to dnve out the melt trapped there. to form adjacent melt-nch and mafic-m1neral-poor leucocrat1c layers. The truncations of the layenng present are exactly analogous to foliat1on cut-offs observed 1n mylon1tes.

Locatton: Ashuan1pi Subprovince, Quebec, Canada. Rock type: h1gh-strain layered diatexite migmat1te; T 825 87SoC. P 6 7 kbar. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

DIATEXITE MIGMATITES AT HIGH STRAINS

224 ----------------------------Figure

64

Location: Glenelg R1ver Complex, southeastern Australia. Fig. 064. Diatexite migmat1te w1th a strong layered type: schl1enc diatex1te m1gmatite; quartzofeldspathic Rock appearance due to alternations of th1ck bands of leucoprotohth, anatex1s 1n the upper amph1bolite facies. Scale: some and thin nbbons of mesocratic paleosome. Some of the penc1l is 15 em long. Image and capt1on: Tony I.S. Kemp. the d1spersed megacrysts of alkali feldspar and fragments of quartz in the diatex1te m1gmatite have an augen-like Further reading: Kemp, A.I.S. & Gray, C.M. ( 1999): Geological appearance and an asymmetrical shape, which indicates a context of crustal anatexis and granitic magmatism in the dextral sense of shear. The megacrysts and fragments of northeastern Glenelg River Complex, western Victoria. quartz may be the remains of an older, d1srupted vein of Austral1an Journal of Earth Sciences 46, 407-420. granitic pegmatite. The presence of finer-grained trails around some of the fragments of quartz and of K-feldspar megacrysts suggests the presence of high strain in a submagmatic state. However. the paradox is that features of solid-state stra1n are not present 1n the d1atexite; th1s would suggest that the shearing was related to magmatiC now. Thus, this migmat1te highl1ghts one of the key problems that needs to be resolved 1f microstructures 1n migmatites are to be fully evaluated; to what extent are the features of intercrystalline deformation that are acqu1red by m1nerals as the crystal framework deforms erased by slow cooling at a high temperature, perhaps with melt or meltderived aqueous fluid present? Put another way, can the microstructural history of migmatites be reset?

• A rias of M ig mat ites

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E. OTHER MO RPHOLOGIES O F MIG MATITE

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Some morphologies are not specific to eit her metatexite or diatexite migmatites. T he photographs in this section illustrate the two most common of these morphologies .

Syn-anatectic folding: fold structures in migmatites [Figs. E 1-ES] It is very important to distinguish migmatites that were folded w hile they contained anatectic melt (i.e., migmat ites with fold structures) from migmatites t hat were folded after they had completely crystal lized (i.e., folded migmatites) . T he presence of melt in a rock creates weak domains that can control the geometry of t he folds (and other structu res such as shear zones) t hat subsequently develop, and thus can affect t he final morphology of a migmatite. Once the melt fraction in a migmatite starts to cr ystallize and the fraction of melt in the leucosome part. or in a diatexite migmatite, declines below t he rigid percolation threshold (less than ca. 45% melt). a framework of crystals w ill have formed, and an incr ease in strength will occur. When crystallization progresses further to t he part icle-locking t hreshold (<28% melt left) . leucosome and diatexites both have attained a significant fraction of their solid -state strengths, and may at that point become t he strong layers in a migmatite .

Migmatites with a vein structure [Figs. E9-EI4] Migmat it es commonly become veined by the intrusion of felsic magma in anatectic or orogenic events long after t he one in which they were formed . However, it is vein ing during the anatect ic event that formed the migmatites that is of interest here. Even with that restriction, veining could occur at any stage between incipient melting, when the fraction of melt is low, t hrough to peak temperatures, w hen migmatites contain a larger fraction of melt. back to near-solidus conditions when the fraction of melt is, again, very small.

SYN-ANATECTIC FOLDING : FOLD STRUCTURES

--226 - - - - - - - - - - - IN MIGMATITES F1gure

I

Fig. E I . This m1gmat1te has a well-developed layering cons1st1ng of (I) a mesocrat1c plag1oclase + hornblende paleosome. (2) a melanocrat1c hornblende + plagioclase res1duum from partial melt1ng. and (3) a plag1oclase + quartz leucosome that represents the product of the former anatectiC melt: hence, th1s IS a stromatic metatexite m1gmatite. Some features 1n th1s outcrop are typ1cal of metatexite migmatites that were folded while they contained melt. Many layers that conta1ned melt show a slight th1cken1ng 1n the fold-h1nge reg1on relat1ve to the fold limbs. For example. the neosome layer (N) at the lower left has leucosome and melanosome components that are Intimately m1xed 1n the fold core, but wh1ch tend to become separated and th1nned on the fold limbs. Another. more complex layer of neosome (on wh1ch the scale rests) IS a little d1fferent: 1t has well-developed layers of melanosome that may have mfiuenced the d1stribut1on of melt durIng the fold1ng. The leucosome layer assoc1ated w1th 1t IS th1ckest on the long limb of an asymmetncal synform, and not m the fold core. However, the leucosome on the fold limb contains several thin screens of boud1naged melanosome, suggestive of dilation w ith1n the competent layer of hornblende-rich melanosome as it was folded. This created local space into w hich the anatectic melt could migrate. The core of the largest fold conta1ns several short. thin streaks

of leucosome (L) that are onented parallel. or subparallel, to the ax1al plane of the fold: the streaks have roots m the layer-parallel leucosome. Th1s pattern 1s cons1stent w1th the retent1on of some melt 1n the fold-h1nge reg1ons dunng fold1ng. If the melanosome marks the place where anatect ic melt was formed. then the best evidence for the overall movement of anatectiC melt away from the fold limbs 1s the presence of melanosome there w1thout any assooated leucosome. The fract1on of melt preserved in this migmatite as leucosome is much lower than the degree of partial melting estimated by petrolog1cal means, 1.e., this migmatite had lost a s1gn1ficant proport1on of its melt before it crystallized. Therefore. one can suppose that the melt produced on the limbs of the fold has migrated out of the m1gmat1te through the core of folds. Location: Ab1tib1 Subprovince. south of Chibougamau. Quebec, Canada. Rock type: stromatiC metatex1te m1gmat1te with fold structures: metamafic protolith. T 800- 850°C. P 8-10 kbar. Scale: the ruler 1s IS em long. /mage: E.W. Sawyer.

t

A tl as of M igmati tes

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Fig. E2. This complex metatexite migmatite had a higher fraction of melt present during folding than the previous example. The dark, competent layer (P) at the base of t he out crop is only gently folded, and t he melt-rich layer of neosome (N) above it has acted as a plane of decollement above which the layers were more t ightly folded and deformed. The folds are asymmetrical; flat-lying long limbs are separated by steep, thinned short limbs. The competent. dark layers are thicker in the antiformal hinge than in the synformal one. Most melt present during the folding was located parallel to the layering, and occurs either as thin bodies of stromatic leucosome in the competent layers (LI) or as thicker layers of neosome (unsegregated melt + residuum mixtures) between the competent ones. The layers of leucosome (e.g., L I) and neosome are generally thickest in the fold hinges, both antiformal and synformal, and thinnest on the steep limbs of folds, indicating that melt moved from the fold limbs to the hinges during folding. Variously oriented fragments of the darkcolored competent layers in the neosome provide further evidence for magma flow in the hinges during the folding. A small number of high-aspect-ratio domains of leucosome (L2) are located in the synformal hinge and parallel to the axial plane of folds; these areas of leucosome contain very little residual material and are indicative of the late-stage

segregation of melt into the axial planes. Domains of leucosome are also located in other sites on t he fold limbs, such as symmetrical and asymmetrical boudins and conjugate, normal-sense extensional shears. Some domains of leucosome are located along reverse-sense shears, however. Location: Mt. Hay, Arunta Inlier, Australia. Rock type: metatexite migmatite with fold structures; metapelitic and semipelitic protolith, partial melting in the granulite facies, T 825-875°C. P 6-7 kbar: Scale: the ruler IS IS em long. /mage : Marianne Bonnay.

Further reading: Collins, W .J. & Sawyer, E.W. (1996) : Pervasive granitoid magma transfer through the lower-middle crust during non-coaxial compressional deformation. journal o( Metamorphic Geology 14, 565- 579. Sawyer, EW., Dombrowski, C. & Collins, W.J. (1999): Movement of melt during synchronous regional deformation and granulite-facies anatexis, an example from the Wuluma Hills, central Australia. In Understanding Granites; Integrat ing New and Classical Techniques (A Castro,

C Fernandez & J.-L. Vigneresse, eds.). ·Geological Society. Special Publication 168, 22 1-237.

SYN-ANATECTIC FOLDING : FOLD STRUCTURE S

228 ----------------------------IN MIGMATITES Figure

3

Fig. El. This metatex1te migmat1te had an 1ntermed1ate gne1ss1c protolith and was folded when anatectiC melt was present; the v1ew is down plunge. Most bodies of leucosome in the core of the fold (near the hammer) are parallel to the compositional layering in the paleosome, and are notably thicker in t he fold hinges compared to the limbs. Few of the leucosome domains in the core area have a melanocratic border. The melt thus migrated there from elsewhere. There are far fewer domains of leucosome in the layer outside the fold core, but they are onented parallel to the ax ial plane of the fold and have a conspicuous melanosome around them, 1ndicat1ng that the neosome probably formed m s1tu. The doma1ns of leucosome in t he top nght are interesting; most are planar (st romat1c) and do not have a melanosome. The one that has melanosome appears to be made up of a senes of short, closely spaced segments that are parallel to the ax1al plane of the fold. Thus, it 1s not stromatiC 1n the same sense as the others. Some shearing parallel to the ax1al plane has occurred in the core of the fold, and has resulted 1n solid-state recrystallization in the affected doma1ns of leucosome. Deformation thus out last ed anatexis at this locality.

Locat1on: Arunta Inlier, Australia. Rock type: metatex1te migmat1te w1th fold structure; Intermediate orthogneiss protolith, anatex1s 1n the upper amphibolite to lower granulite fac1es. Scale: the hammer is 60 em long. Image and caption: Tony I.S. Kemp.

A ri a> o f Migmatites

----------------- -------------- 229 Figure

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Fig. E4. T his metat ex1t e migmat ite developed 1n lnterlayered mafic (Ma), psamm1t ic (Ps, grey and brown colors), and pelitic (Pe) rocks crossed by a few fels1c dikes. It is an example of how layenng can 1nfluence the pos1t1on, morphology, and compos1t1on of the leucosome. The pelit1c layers were Incompetent, and flowed dunng the folding. Thus, they are th1n, or even absent, on the limbs of the fold and th1ckest 1n t he fold hinges. In contrast, t he psammit ic and met amafic layers w ere compet ent and produced parallel folds, and underwent boud1nage on the long limbs of these folds. The psamm1tic layer was the least affected by part1al melt1ng. It conta1ns very sparse areas of leucosome or melanosome. The metamafic layers experienced more extens1ve partial melt 1ng and developed an internal layenng 1n wh1ch t he most melt-depleted parts are the darkest. Part1al melting was most extens1ve 1n pelitic layers, and these now have a (biot1te + garnet) -nch residual compos1t1on. Dunng folding, the part1al melt generated near the fold h1nges 1n the pelitic layers migrated along these layers and formed tnangular bodies of gran1t1c leucosome (e.g., Ll) 1n the h1nges of bot h the ant1formal and the synformal folds. The melt also m1grated along the pelite layers on the long limbs of t he folds, and occupied t he space bet ween t he boudins in t he adjacent mafic layer, where

it cryst allized and formed t nangular bodies of leucosome (L2) . Both types of leucosome conta1n large, dist inctive po1kiloblasts of garnet, wh1ch, as the sol1d products of the 1ncongruent breakdown of biotite, are ev1dence that such leucosome was denved from the pelitic layers. Furthermore, these bod1es of leucosome have a magmatic microstructure; hence, they crystallized after the folding and the boud1nage. Very thin, stromatic bodies of leucosome also occur in the pelitic layers, and these contai n large crystals of garnet and plag1oclase that have pressure shadows and finer-gra1ned tails. These domains of leucosome thus were part1ally crystalline dunng the folding. and underwent gra1n-s1ze reduction. The competent layers conta1n leucosome 1n s1m1lar structural pos1t1ons; stromatic bod ies of leucosome ly1ng parallel to the layenng are slightly thicker in t he fold h1nges. and are boudinaged on the fold limbs. Melt t hus moved 1nto the space created as planes parallel t o the layenng opened, and the leucosome was part1ally solid dunng the fold1ng, probably through loss of melt. Small, equant bod1es of leucosome occur 1n the dilatant spaces bet ween boud1ns 1n the competent layers. The leucosome in the metamafic layers 1s tonalit1c in composit ion and does not conta1n garnet, features cons1stent with a derivation from t he met amafic protolit h. Hence,

SYN-AN ATECTIC FOLDING: FO LD STRUCTURES

230 ----------------------------IN MIGMATITES

anatect1c melt m1grated pnnCipally with1n, and not across. the vanous layers 1n th1s migmat1te. Lastly, a complexly shaped area of leucosome (L3) occurs on the outs1de of the d1srupted layer of psammite. It is interpreted to occupy the low-pressure site created dunng folding and breakup of the psammit e layer. This leucosome contains garnet where it is in contact w ith metapelite. but is garnet-free where it is in contact with the metamafic rock, again indicating local derivation of the melt in this migmatite.

Locat1on: Sand River Gneisses, Causeway locality, South Afnca. Rock type: metatex1te m1gmat1te w1th fold structures; mterlayered protolith, anatex1s in the granulite facies. Scale: the pocket kn1fe is II em long. Image: E.W. Sawyer.

Atlas of Migmatites

----------------- -------------- 231 Figure

S

Fig. ES. For some geologists, the image of a migmatite with fold structure is that shown in this photograph. However, migmat1tes of this kind are uncommon and generally restncted to small (meter-scale) doma1ns in which the strain h1story IS complex. Common examples illustrate progressive deformation in wh1ch structures are rotated and overprinted dunng a single event of deformat1on, such as in shear zones, or in places where later folding is superimposed on earlier structures, producing interference. The domains of leucosome 1n this particular migmatite do not systematically have a melanosome. In places, the melanosome 1s absent. but elsewhere 1t IS thin, and locally, such as on the 1ns1de of fold closures, it IS w1de. The doma1ns of leucosome are characteristically thin, have a granular microstructure, and a grain size of I 2 mm. The folds are disharmonic and convolute. The fold hinges show a considerable range in onent ation, both 1n plunge and azimuth. However, in some Similar examples, the onentat1on of fold h1nges is systematiC, and dome or bas1n structures are the result of sect1ons cut through the noses of sheath folds.

Locat1on: Glenelg River Complex, southeastern Australia. Rock type: migmatite with fold structure in semipelitic sch1sts; upper amph1bohte facies. Scale: the lens cap is 6.6 em across. /mage and caption: Tony 1.5. Kemp.

SYN -ANATECTIC FOLDING : FOLD STRUCTURES

232 -----------------------------IN MIGMATITES Figure

Fig. E6. There are three pnnopal litholog1es 1n th1s metatexrte m1gmatite. A grey layer of biotrte plag1oclase quartz psammite extends across the center of the photograph. and 1n the bottom nght, there 1s a darker grey layer of hornblende gne1ss. Both lithologies are unaffected by anatexis and could be conSidered paleosome 1n th1s m1gmat1te. The third lithology is pelit1c. but dunng anatexis, the pelit1c layers were transformed into neosome with well-defined leucocratic and melanocratic portions. Tight asymmetnc folding w1th a wavelength of about 25 em has affected all three lithologies, but in the pelit1c layer;

the folds are notably convolute and polycllnal. Two different leucocrat1c components can be Identified 1n the pelit1c layers: (I) the thin. fine-gra1ned leucocratlc layers (LI) have sharp borders and a thin dark rim Interpreted to be a mafic selvedge, and (2) the coarser-grained. light-grey, b10tlte-plag1oclase K-feldspar quartz doma1ns of leucosome (L2) have more diffuse edges and a w1dergamet- sillimanite b10t1te melanosome. Some of the fine-gra1ned leucocrat1C layers are 1soclinally folded and form intrafolial folds 1n the layenng; these have been reonented by the t1ght asymmetncal folds. The doma1ns of coarser-grained leucosome occur as lenses, some of which are located along shear planes on the limbs of the t1ght asymmetrical folds. From these observations. the fine-gra1ned leucocratic layers are Interpreted to be granitic veins intruded 1nto the pelite; these layers subsequently developed a mafic selvedge by reaction with the host. then 1n the solid state. The ve1ns and host were 1soclinally folded and later refolded dunng anatexis. The bodies of coarser-

gra1ned leucosome may have formed m SJtu, by partial melt1ng of the pelite, synchronously w1th the asymmetrical folding. In add1t1on to a convolute and polyclinal geometry, there are also many shear zones and discontinuities across wh1ch the onentat1on of the stromatic layenng changes abruptly. Further complexity results from the format1on of shear-bounded lobes of stromat1c neosome that resemble escape structures; these have 1ntruded across adJacent layers and, 1n one case, 1nto the psammite layer. This suite of structures suggests that there were some very weak domains 1n the pelite layer; possibly because they contained partial melt. The overall style of folding in th1s metatexite migmatite is typ1cal of that 1n many h1gh-grade m1gmatites. particularly where all the layers were relatively weak (contrast w1th Figs. E2 and E4, 1n wh1ch some layers were strong and resulted 1n

the formation of parallel folds) .

Location: Thor· Odin dome. Bnt1sh Columbia, Canada. Rock type: stromatiC metatex1te m1gmat1te with fold structures; pelit1c and psamm1t1c felsic protoliths. part1al melt1ng at T ca. 800°C. P 4- 7 kbar. Scale: hammer for scale. Image: Oliv1er Vanderhaeghe. Further read1ng: Vanderhaeghe, 0.. Teyss1er; C. & Wysoczansk1, R. (1999): Structural and geochronological constraints on the role of part1al melt1ng dunng the formation of the Shuswap metamorph1c core complex at the lat1tude of the Thor Odin dome, Bntish Columb1a. Canad1an journal of Earth Soences 36,

917- 943.

A tlas o f M igma t ites

------------------------------- 233 Figure

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Fig. E7. Neosome predominates in t h1s migmat it e, and paleosome occurs only in r elics, possibly boudins, in a finegrained biotite-quartz-plagioclase psammitic schist. The neosome is st romat ic, but cont ains t wo t ypes of stromat ic leucosome, although t hey have t he same plagioclase + quartz + K-feldspar assemblage. One type IS in contact w ith mesocrat ic garnet- cordiente-biotite-plagioclase- quartz pelitic schist; the schist could be residual in com posit ion, and the leucosome may have formed by in s1tu part ial melting. The second, less common type is dist inguished by t he presence of a narrow, biotite-rich mafic selvedge; in places, such leucosome (L) and 1t s mafic selvedge are discordant and can be t raced from the pelit ic layers across into the remnants of psammit e. T his stromatic leucosome is interpreted t o be have been injected, and its mafic selvedge, to be the result of react1on with the host rock. T he outcrop displays complex fold ing. T he style of folding of some bodies of leucosome (e.g., middle of the left edge of the photograph) could be called ptygmatic. In general, however, the folds in t he layer ed neosome are nonplanar, and t he st yle IS convolute; there is a cons1derable range 1n the orient ation of t races of the axial plane of folds, but some of this may be due to local refolding. Many bodies of leucosome are thicker in the fold hinges, and in places, there are thin, d1ffuse domains of leucosome oriented approximately

parallel to t he local direct ion of the axial plane. The mlgmat it e may, t herefore, have been folded when it was close t o its solidus, and only a very small fraction of melt was left that could be moved, or segregated, during the folding. T he 10-cm-w ide leucocratic vein on the left cuts across the stromatic layering in t h1s port1on of the migmat it e. It can, however, be traced some 25 m across the outcrop. In one direct ion, it dies out in a sharply defined discordant brittle fracture, but in the other, it s border s become increasingly diffuse aga1nst the adjacent bodies of stromatic leucosome in the host, and 1t is root ed in a large layer-parallel lens of leucosome locat ed in t he core of a mesoscale ( 10-m) antiform. The discordant leucocrat1c vein is, therefore, an example of local, structu rally controlled accumulation of melt and its subsequent escape along a propagat ing fracture. This m1gmatite IS difficult to class1fy. T he neosome can be described as stromatic or layered, but at the scale of the photograph, there are no coherent pre-anatect iC structures visible (alt hough at a larger scale, individual, thick units of psammite could be mapped). Consequent ly, the migmat it e is not adequately described as a metatexite. T he absence of coherent pre-anatectic structures is a distinguishing featu re 1n diatex1te m1gmatites, but t his migmatite lacks other charact eristics, such as magmatic flow structures. It is t hus not properly described as a d1atexit e migmatite either. It could

SYN - ANATECTIC FOLDING : FOLD STRUCTURES

234 -----------------------------IN MIGMATITES

be descnbed as a trans1t1onal stage between metatexJte and d1atexite m1gmatite. If all the stromatiC leucosome w1th a mafic selvedge was in fact 1ntruded 1nto the m1gmat1te, wh1ch at the time contamed only a small proport1on of in situ leucosome, then this would be a vein-structured migmatite derived from a metatexite migmatite. Location: Nemiscau Subprovince, Canada. Rock type: transitional metatexite to diatex1te migmat1te with fold structures; metasedimentary protohth, anatexis 1n the upper amphibolite fac1es. Scale: the ruler IS IS em long. Image: E.W. Sawyer.

A tl as of M igma t ites

------------------------------- 235 Figure

'

'•

Fig. E8. Thin bod1es of leucosome (
host. The curvature may, therefore, be related to late com-

parallel to t he compositional banding, believed to have been bedd1ng, 1n the mesocratic neosome. However. some areas of

the two large doma1ns of leucosome conta1ned less t han 25% melt, had passed the part1cle-lock1ng threshold, and

"leucosome" (B) are much wider (about 5 em) and seem to

were the more compet ent layers. The morphology of this fold-st ructured migmat ite is not iceably different from t hat in

cut t he compositional layer ing at a low angle: these doma1ns

pression across the ax 1al plane of the fold, at a stage when

of " leucosome" generally have a mafic selvedge, but t here are

Figs. El , E2, and E4; the bodies ofleucosome generally have dif-

some contacts without t hem. T he wider bodies of leucosome may have crystallized from a melt that was injected, or alter-

fuse contacts, and t here are no dilatant structures 1nto which melt has migrated. There may well have been a lower contrast

natively. t hey may result from 1n situ partial melting fluxed by the 1ngress of H 20 along fracture planes. Together. the t hin

in viscosity between the metagreywacke layers and t he leucosome 1n this migmatite.

and th1cker bod1es of leucosome give the m igmat1te a layered or stromatic morphology. and both were tightly folded 1n the presence of partial melt. A grey leucocrat1c ve1n (C) w1th a

Location: Abaukoma, Yaounde, Cameroon. Rock type: m1gmatite transitional from metatex1te to diatex1te w1th fold struc-

different microstructure crosses the outcrop JUSt below the pen. Th1s ve1n has th1n mafic selvedges where 1t crosses the

tures: metagreywacke protolith, anatexis at T 800- 850oC.

metagreywacke, but tends to have diffuse contacts w1th the

Barbey.

thicker. folded bod1es of leucosome. The vein is onented subparallel to the axial planes of t he folds and appears to truncate the folded layers of leucosome; 1t represents an 1nject1on of another felsic melt along a sheared limb. However. the local presence of diffuse margins (e.g., around the splay in t he vein) and cont acts w1th the wider domain of leucosome 1nd1cate that 1t might have been injected while the host still conta1ned some 1nt erstlt1al melt. N ote that bends in t he leucocrat1c vein coincide w ith adJacent. large domains of leucosome 1n the

P 10 12 kbar. Scale: the pen IS IS em long. Image: Pierre

Further reading: Barbey, P., Macaud1ere, J. & Nzent1, J.P (1990): H 1gh-pressure dehydration melting of metapelites: ev1dence from migmat1tes of Yaounde (Cameroon). journal of Petrology 31 ,40 1-427. N zent i,

J.P. Barbey, P, Macaudiere, J. &

Soba, J. ( 1988): O rigin

and evolution of the lat e Precambrian high-grade Yaounde gne1sses. Precambrian Research 38, 91 109.

MIGMATITES WITH A VEIN STRUCTURE

236 Figure

9

Fig. E9. The ruler rests on a slightly rusty-colored magnet1te K-feldspar b1ot1te cord1ente quartz-plag1oclase metapelitic layer that contains a few doma1ns of K-feldsparplagloclase-q uartz leucosome. The cordiente 1n the metapelite conta1ns rounded inclus1ons of s11ilman1te, quartz and biotite that are not in mutual contact. Therefore, the rusty layers are Interpreted as the res1duum from the part1al melt1ng of a peilte by a react1on such as b1ot1te + s1iliman1te + quartz + plagioclase = melt + cordiente + K-feldspar, and subsequent segregat1on of most of the anatectiC melt fract1on. These assemblages prov1de the ev1dence that th1s outcrop IS 1ndeed a migmatite. The more numerous grey layers are b1otite quartz-plag1oclase metapsamm1te, and because they show no evidence of hav1ng partially melted, they are cons1dered paleosome. The outcrop contains promtnent dtscordant ve1ns of leucogranite that have a narrow, b1otite-nch mafic selvedge. Locally, the mafic selvedge crosses the layering (bedding) and is developed at contacts w1th the pelittc and the psamm1t1c layers. Hence, the selvedge IS cons1dered to be the result of a reaction between the granitic melt and the host rock. The leucograntttc ve1ns are peralum1nous. and thetr whole-rock maJor- and trace-elemen t compos1t1on is compat1ble w1th derivation from an anatectic melt from a pelitic protolith, one stmilar to that present in the outcrop. However, the

ve1ns are depleted 1n the heavy rare-earth elements, which suggests that they formed from a melt that had garnet as a residual phase. Moreover, the major- and trace-elemen t contents suggest they were denved from a htgher degree of parttal melting than occurred in the peilt1c layers present in this outcrop. As cord1erite, and not garnet, IS residual tn the pelittc layers of thts mtgmat1te, the melt from whtch the d1scordant leucogramt1c ve1ns were derived was generated deeper in the crust. where garnet was stable, and the degree of parttal melt1ng htgher. Thus, a case can be made that the magma from wh1ch the leucocratic vetns crystallized belongs to the same anatect1c event that formed their mtgmat1te host; hence, thts IS a vein-structure d m1gmattte and not a ve1ned migmatite.

Locatton: Quet1co Subprovince, Ontano, Canada. Rock type: ve1n-structured metatex1te m1gmat1te; metapelitic protolith, anatexis at T 700 800°C, P 3-4 kbar. Scale: the ruler IS IS em long./mage: E.W. Sawyer.

Atlas of M igmatites

------------------------------237 Figure

I0

Fig. E I 0 . At first glance, this looks like a stromatic migmatite with prominent subparallel veins of leucosome in a grey, medium-grained semipelitic host. Close inspection of the "leucosome" fraction reveals t hat they cross the foliation and compositional layering in t he host by a few degrees and have narrow, biotite-rich mafic selvedges and not mel anosome. Some domains (e.g., the lowermost one in the phot ograph) have a pegmatitic microstructure. Al l these features are inconsist ent with formation of leucosome in s1tu. Examination of the grey host rock shows that most of it is a uniform biotite-quartz- plagioclase psammitic schist that does not appear t o be residual in composition and, t herefore, not t o have melted. T here are a few barely visible diffuse, nebulitic quartzofeldspathic patches developed in t he outcrop in which the grain size is larger. These features indicat e t han an incipient stage of partial melting was in fact reached in a few scat tered places in the host. The rock thus is properly called a migmatite. However, the stromat ic morphology is the result of t he injection of granit ic melt, some of it quite evolved in com posit ion, int o a host t hat had just barely melted. Thus, the mesocrat ic host is mostly paleosome, and the "leucosome" fraction is correct ly considered as leucocratic veins or dikes.

Locat1on: Wu luma Hills, Arunta Inlier, Australia. Rock type: vein-struct ured metatexite migmatite; psammitic protolith, partially melt ed at T 800- 850°C, P ca. 5 kbar. Scale the ruler is IS em long. Image: E.W. Sawyer.

M IGMATITES WITH A VEI N STRUCTURE

238 ----------------------------Figure

I I

Fig. E I I . Vems of peraluminous gran1te are intruded subparallel to the foliation and compositional layenng 1n a stromatiC m1gmatite formed from a pelit1c protolith by the dehydration-induced melt1ng of a muscov1te-bearing assemblage. The stromatic m1gmatite has a plagioclase + quartz + muscovite (trondhjemitic) leucosome and b1ot1te + quartz+ Sillimanite + garnet melanosome. The gran1t1c veins contain K-feldspar and have compositions cons1stent with derivation from pelit ic schist similar to the protolith of the stromatic migmatite. H owever, the stromatic leucosome consists of the accumulated plagioclase crystall1zed from the anatectiC melts, whereas the granitic sheets and ve1ns have a slightly more evolved composition. but have still lost evolved fractionated melt. That melt IS bel1eved to have nsen to h1gher levels in the crust and to have formed gran1te plutons.

Location: western Ma1ne, U.S.A. Rock type: vein-structured stromatiC metatex1te m1gmatite; upper amph1bohte fac1es, T ca. 750°C, P 4 kbar. Scale: the lens cap 1s 6.5 em across. Image and caption : Gary Solar. Further reading: Solar, G.S. & Brown, M. (2001): Petrogenesis of migmatites in Maine, USA: possible source of peraluminous leucogranite in plutons? journal o{ Petrology 42,

789 823.

Atlas of Migmatites

------------------------------239 Figure

I2

Fig. E 12 . The veined migmatite in this outcrop was created when a series of thin, subparallel veins were injected into a metatexite migmatite host. The metatexite migmatite is compositionally layered at a scale of 10- 50 em. T he layers

were probably bedding. and contain the mineral assemblage biotite + sillimanite + garnet + quartz, wh ich is interpreted to be the residuum after partial melting of a pelitic prot olith. The domains of quartzofeldspathic leucosome oriented parallel to the layering are interpreted t o represent part of t he anatect ic melt derived from the pelit e; t hese domains of leucosome have a mafic selvedge. T he steeply dipping structure in the metatexite migmatite is cut by a large number of thin, less steeply dipping plagioclase + quartz veins, most of wh ich do not have a mafic selvedge. The local presence of mafic selvedges may indicate that the veins were injected while their host was still hot, and close to the solidus. Some of t he leucocratic veins are of uniform thickness, but many consist of t rains of lenticular segments. The lowtemperature retrogression of the assemblages of residual minerals in the migmatite, caused by an influx of aqueous fluids, can be seen as the greenish blue alteration associated w ith an array of fract ures that traverse the outcrop.

Location : western Maine U.S.A. Rock type: veinst ructured metatexite migmatite; upper amphibolite facies, T ca. 750°C, P 4 kbar. Scale: note the person at the top right. /mage and caption: Gary Solar. Further reading: see Fig. EII.

MIG MATITES WITH A VEIN STRUCTURE

240 ----------- ----------- ------Figure

I3

Fig. E 13. This d1atex1te m1gmat1te has a very diffuse compositional band1ng, and a rather umform, mesocrat1c appearance because very little segregat1on of the melt

from its resid uum has occurred. Some of t he compositional banding IS the result of the presence of quartz-rich, subsolidus segregat1ons 1nto ve1ns (A) and thew b1ot1tench selvedges 1n the protohth; these lower-grade relics are, therefore, paleosome 1n th1s migmat 1te. An asymmetncal lens-shaped raft of paleosome has a biotite-rich mafic selvedge at its edge closest to the pen, and contains an internal compositional layenng due to th1n quartz-nch ve1ns. A very leucocratic melt has segregated into the pressure-shadow region to the nght of the enclave. The posit ion and shape of th is segregation are cons1stent w it h the overall dextral sense of shear determined from foldIng of the ma1n banding and the s1gmo1da. curvature of the leucocrat1c bands. Most of the leucocratlc bands 1n th1s m1gmatite have no assoCiated melanosome. As some are clearly oblique to the weak modal band1ng in the mesocratiC neosome, they are Interpreted to result from the 1nJect1on of leucocrat1c ve1ns 1nto the d1atex1te. Some of the ve1ns (B) have d1ffuse margins, and thus probably were InJected 1nto the diatexite wh ile 1t still contained anatectiC melt. H owever, other veins have sharp contacts. Melting 1n these r ocks occurred through the breakdown of muscov1te:

=

muscov1te + plag1oclase + quartz + K-feldspar + H 2 0 melt. followed by muscov1te + plag1oclase + quartz+ H 20 melt + sill1man1te. As b1ot1te was not 1nvolved . changes in biotite concentration act, therefore, as a good t racer

=

to ident1fy the locat1on and the processes involved in melt segregat1on.

Locat1on: Glenelg R1ver Complex, southeastern Australia. Rock type: vem-structur ed diatex1te migmat ite; quartzofeldspathlc gneiss protolith, anatex1s in the upper amphibolit e faoes, T ca. 700°C, P ca. 5 kbar. Scale: the pen 1n 14 em long. Image and capt1on: Tony 1.5. Kemp. Further reading: Kemp, AI.S. & Gray, C. M. (1999) : Geological context of crustal anatexis and gran1t 1c magmatism 1n the northeastern Glenelg R1ver Complex, western Victoria. Australian journal of Earth Soences 46 , 407-420.

A tl as of Migmat ites

- - - - - -- - - - - - -- - - 241 Figure

14

•

Fig. E 14 . The schollen in this diatexite migmatite are orthopyroxene-quartz-biotite-plagioclase schists, and represent the residuum left after partial melting of a biotite-quartz-plagioclase psammitic schist protolith. Some partial melt may have remained in the residuum to form the fraction of st romat ic leucosome present in the schollen, although some of this material represents injected granit ic veins, not generated tn s1tu. The leucocratic matrix around t he schollen is granodiorit ic in composition, and has the mineral assemblage orthopyroxene + biotite + K-feldspar + quart z + plagioclase. Tabular crystals (euhedral orthopyroxene and feldspar) in the matrix are aligned and define a magmatic foliation. Furthermore, tra1ns of platy biotite define mafic schlieren in the matrix that are parallel to the magmatic foliation. The schollen diatexite is crossed by a number of thin, generally planar leucocratic veins, or dikes (e.g., upper left and upper right). These veins are leucogranitic in composit ion (i.e., more K-feldspar and less orthopyroxene than t he matrix) and do not have melanosome or, in general, a mafic selvedge associated wit h them. In most places, t he contact between the veins and t he granodioritic matrix is gradational, but in the places where the schlieren are truncated, it appears t o be sharp. The leucocratic veins and the granodiorit ic mat rix were thus part ially molten at the same t ime, i.e., the leucocratic veins

are part of the anatectic event in this migmatite. T he wholerock composition of the leucocratic veins and t he matrix indicat es that they are petrogenetically related; the veins are the evolved residual melt derived by fractional crystallization of anatectic melt, and the granodiorit ic matrix is t he remaining plagioclase-enriched cumulate.

Location: Ashuanipi Subprovince, Quebec, Canada. Rock type: vein-struct ured schollen diatexite migmatite; biotite quart z-plagioclase protolith, anatexis at T 825-875°C, P 6-7 kbar. Scale: the ruler is IS em long. /mage: E.W. Sawyer:

MICROSTRUCTURES CHARACTERISTIC O F MI GMATITES

242 -----------------------------

F. MICROSTRUCTURES CHARACTERISTIC OF MI G MAT ITES Investigators in recent years have identified certain microstructures that can be used to 1nfer the presence of partial-melting reactions. the former presence of the melt itself, or the crystallization from a melt. In this section, I Illustrate some of those microstructures currently used 1n the study of migmatites. The first few figures establish a cont1nu1ty of microstructures found in rapidly quenched part1al-melt1ng experiments w1th those found 1n nature 1n progress1vely more slowly cooled natural melts. Then, 1n the next series of figures. I look at microstructures related specifically to dehydration melt1ng and the trapp1ng of melt.

Results from quenched deformation-melting experiments: a starting point [Figs. FI-F4] Many of t he microstructures preserved 1n rapidly quenched experiments in which rocks have been deformed while partially molten closely resemble those found 1n some migmatites. In recent years, expenmental techn1ques have advanced greatly. and 1t IS now possible to deform rocks while they are part1ally molten 1n order to ascerta1n how part1ally molten rocks deform 1n nature. and how part1al melt IS extracted. Th1s group of figures Illustrates some of the microstructures produced 1n four deformat1on part1al-melt1ng expenments. In all the back-scattered electron (BSE) 1mages. the react1on products (RP) are the bnghtest grey and very fine-gra1ned, and typ1cally occur 1n a matrix of melt, whiCh is an 1ntermed1ate grey. B1ot1te (Bt) grains also appear light grey, similar to the react1on products, but are very coarse-grained and show the (00 I) cleavage planes. Plagioclase (PI) and quartz (Qtz) are much darker and have a similar appearance, although quartz is slightly darker. The black areas 1n the images are cracks that result from decompress1on of the sample at the end of the experiment. The foliat1on 1n the gne1ss IS onented parallel to the bottom of each 1mage.

Subsurface contact-aureoles: the Glenmore plug, Scotland [Figs. FS, F6] The part1al melting that has occurred around near-surface intrusions, or lava flows, has expenenced a un1que combination of melting at very low pressures and very rapid rates of cooling. The most rap1dly cooled of the partially melted rocks are glassy.

Erupted, partially melted xenoliths: El Joyazo, Spain [Figs. F7-F I0] The rapidly quenched microstructures found in part1ally melted xenoliths and enclaves brought up from the middle crust by volcamc eruptions are perhaps the closest link between crustal anatex1s and partial-melt ing expenments.

Subsurface contact-aureoles: the Rum Igneous Complex, Scotland [Figs. F II, F 12] The melt in rocks from subsurface contact-aureoles that experienced somewhat slower rates of cooling does not quench to glass, but crystall1zes to a granophync miCrostructure. These rocks may also show ev1dence that the 1nitial miCrostructure has been modified by recrystall1zat1on.

Shallow contact-aureoles: the Traigh Bhim na Sgurra sill on Mull, Scotland [Figs. F13-FI6] The Tra1gh Bhan na Sgurra sill on the island of Mull, Scotland, was a major condu1t for magma flow during the Tertiary. Most of the sill experienced instantaneous injection of basalt ic magma, which then solidified in s1tu, whereas locally, magma cont1nued to flow for up to 5 months. The mafic rocks 1n the reg1ons of long-lived flow have no ch1lled marg1ns, and are surrounded by a w1de contact-aureole 1n which s1gn1ficant degrees of part1al melting occurred 1n the psammit1c country-rocks. Melt1ng was generally H 20absent and due to the breakdown of muscov1te or due to dry melt1ng 1n the system quartz feldspar. The volume 1ncrease assoc1ated w1th muscov1te-dehydrat1on melt1ng resulted 1n the formation of abundant microcracks filled w1th melt. The pressure of metamorph1sm was only about

0.6 kbar. The four images are of part1ally melted psamm1tes from th1s contact aureole.

Shallow- to medium-depth contact-aureoles : the Duluth Igneous Complex [Figs. F17-F28] Anatexis in the aureole of the Duluth Igneous Complex occurred at a depth equ1valent of 1.5 2 kbar. The cooling rate was sufficiently slow that the melt did not quench to glass, but was able to crystallize completely in most cases. albeit to a fine gra1n-s1ze. However. there 1s granophyre locally. The first five 1mages are from res1dual rocks 1n the contact aureole that show m1crostructures formed during part1al melt1ng. The next five 1mages are from melt-nch parts of rocks (i.e .. diatexite m1gmat1tes and leucosome) in the aureole, and show microstructures related to the crystallization of the anatectic melt.

Aria; of Migtna[i[e;

------------------------------ 243

•

•

Deeper contact-aureoles: the Ballachulish Igneous Complex, Scotland [Figs. F29-F32]

Microstructures in residual rocks [Figs. F39-F46]

•

The rate of cooling recorded in part1ally melted rocks from contact aureoles decreases steadily w1th depth 1n the Earth's crust. Contact aureoles such as that around the Ballachulish

Most res1dual rocks are ennched 1n ferromagnes1an minerals that are the solid products of the Incongruent melt1ng of b1ot1te, or hornblende, and hence they are melanocrat1c. However, residual rocks can also be enriched 1n the minerals that were present in excess with respect to the melting

•

•

•

Igneous Complex, 1n Scotland, contain microstructures that have been sign1ficantly modified by recrystallization durIng the slower cooling. Consequently, these migmatites provide an important intermediate step 1n understanding the sequence of microstructures that develop 1n part1ally melted rocks. from those produced 1n quenched expenments and rap1dly cooled shallow intrus1ve bodies on the one hand, to those found 1n very slowly cooled regional migmatite terranes on the other. The first two images are from partially melted rocks of pelitiC and semipelitic bulk composit1ons, and 1n these. dehydrat1on melting was 1mportant. The follow1ng 1mages are of a part1ally melted psamm1te, the App1n Quartz1te. from the high-temperature 1nner part of the aureole. and part1al melt1ng 1n this rock was driven by the fracture-controlled Infiltration of H 20 .

Regional migmatite terranes: the Ashuanipi Subprovince [Figs. F33-F36] The Ashuan1p1 Subprov1nce of northern Quebec containS 90 000 km of anatectiC rocks, a large proport1on of which are SiliCiclastic metasedimentary un1ts that have meltdepleted bulk compositions. Microstructures that indicate the former presence of anatectic melt are espec1ally common, although not abundant. 1n the psammitic (metagreywacke) rocks. However, microstructures 1n migmatites from reg1onal anatectiC terranes show a much greater degree of modification by recrystallization and the slow growth of minerals than the microstructures in migmatites from contact aureoles.

Regional migmatite terranes: the Opatica Subprovince [Figs. F37, F38] Part1al melting 1n the Opatica Subprovince occurred 1n a typical protohth of Archean tonalite trondhjemite granite (TTG) su1te that had been strongly foliated well before anatex1s. Most of the rocks of the TTG suite are leucocratic w1th very low biotite or biotite + hornblende contents. The microstructures all show considerable textural mod1ficat1on due to slow cooling. Nevertheless. the s1milant1es w1th those found 1n contact aureoles are ev1dent.

react1on, such as quartz or plag1oclase, and this can result 1n res1dual rocks that are not noticeably melanocrat1c. The most fam1har form of res1dual, 1.e., melt-depleted rock IS the melanosome adjacent to a leucosome. However, res1dual rocks can also occur as layers. bands. patches or as qUite extensive regions 1n m1gmatites, and in some cases, without doma1ns of leucosome nearby. Residual rocks are volumetncally the largest component 1n many migmatite terranes, the anatectiC melt fract1on having m1grated to h1gher levels 1n the crust, and there formed gran1te plutons.

Crystallization-induced microstructures in the melt-derived parts of migmatites: leucosome and leucocratic veins [Figs. F47-F58] Doma1ns of leucosome are formed 1n the structures in wh1ch melt collected, or flowed through, 1n m1gmatites. However, there may have been a substantial fract1on of res1dual matenal (refractory minerals plus the solid products of 1ncongruent melt1ng. i.e., pentect1c phases) entrained with 1t. The figures in th1s sect1on Illustrate some of the microstructures found in leucosome, and show, not surpns1ngly, that there is considerable similarity w1th microstructures found in plutoniC 1gneous rocks, most particularly cumulus microstructures.

Crystallization-induced microstructures in the melt-rich parts of migmatites: diatexite migmatites [Figs. F59-F65] D1atexite migmatites at one stage conta1ned a substantial amount of melt, although they likely also had entra1ned solid matenal that could cons1st of refractory m1nerals, the solid products of Incongruent melting, or the early products of crystallization. The fract1on of melt was commonly dom1nant (i.e., >50 vol.%), but not 1n every case, as flow can start at quite low fractions of melt, depend1ng on the flow mechanism involved. The figures in this section Illustrate some of the microstructures charactenst1c of diatexlte m1gmat1tes; readers will note some s1m1lanty w1th microstructures 1n pluton1c 1gneous rocks.

MIC ROSTRUC TURES C H A RACTERIS TIC OF MI GMATITE S

244 ----- ----- ----- ----- ----- ----- -

Microst ructure s formed by flow in diatexit e migmat ites [Figs. F66-F69 ] A characteristic feature of d1atexite IS the presence of structures that ind1cate flow of magma, such as flow-band1ng, schlieren, onented elongate enclaves, and mineral alignments. Because of the s1ze of these features and the coarse grain-size of many regionally developed diatex1te m1gmat1tes, 1t IS commonly difficult to apprec1ate the structures resulting from magmatic flow in th1n sect1ons. Hence, this sect1on starts w1th two examples of fine-grained d1atex1te migmatites from the contact aureole at Duluth.

Inclusio ns of melt quenche d to glass in mineral s [Figs. F70-F73 ] Surface Irregularities on minerals e1ther growing from or grow1ng 1nto a melt can lead to the entrapme nt of small pockets of melt 1n the crystal as it grows. If the crystal and 1ts melt-filled 1nclus1on are then rap1dly cooled, the melt will quench to glass. With slower cooling, the melt 1n inclus1ons has t1me to 1nteract w1th 1ts host and to crystallize. Melt inclusions are important in the study of m1gmatites. because analyz1ng them IS one way to directly determ1ne the composition of the anatectiC melt. Glass inclusions found in the m1nerals of part1ally melted enclaves brought to the surface in volcan ic eruptions are, perhaps, the best way of sampling anatectiC melts produced at m1ddle and deep levels of the crust.

Cordier ite-, garnet- , and orthopy roxenequartz intergro wth microst ructure s [Figs. F74-F77 ] T he h1gh-grade parts of many anatectic terranes cons1st of m1gmat1tes derived from pelitic or semipelit1c protoliths that conta1n doma1ns of neosome w1th a dist1nct1ve 1ntergrowth microstructure. The neosome is, overall, lighter colored and coarser gra1ned than 1ts host, and has two parts. One consists of large crystals of a ferromagnesian m1neral intergrown with quartz, whereas the other part IS essentially quartzofeldspathic and leucocratic. Such neosome can occur 1n different morpholog1es, but typ1cally, 1t 1s ve1n- or patch-like in shape, and a d1agnostic feature is that it does not have a melanocratiC border, or nm. The ferromagnes1an mineral IS most commonly garnet, but exactly analogous microstructures w1th cordierite or orthopyro xene also occur. The m1neral assemblage 1n the host corresponds to the reactant assemblage, and that 1n the neosome, to the product assemblage of the melt-produc1ng reaction. Th1s type of neosome and 1ts characteristic intergrow ths of minerals appear to be diagnost1c of b1otite-dehydrat1on-melt1ng

reactions. Mineralogical and whole-roc k compositions of the neosome 1ndicate that most have lost melt, and typ1cally a substantial fract1on of t he melt. Analogous microstructures m1ght be expected dunng dehydration melt1ng of hornblende in a mafic protolith.

Biotite- quartz and biotite- plagioc lase symplec titic intergro wth microst ructure s [Figs. F78-F81 ] Symplect1t1c 1ntergrowth microstructures 1nvolv1ng b1ot1te and quartz or plagioclase can be fou nd in many migmatites, although they are rarely abundant. Typ1cally, they part1ally replace some of the ferromagnes1an residual m1nerals such as garnet, cordiente, and orthopyro xene. The replacement also 1nvolves a K-bearing phase, commonly believed to be the last-rema1n1ng anatectiC melt 1n the m1gmatite, although some stud1es have suggested that it IS K-feldspar. Hence, biot1te quartz and b1ot1te plag1oclase symplect1tes are generally regarded as late anatectic microstructures, formed wh1le a small amount of melt st1ll rema1ned in the m1gmat1te.

Biotite- silliman ite and biotite aggrega tes replacin g garnet or cordier ite [Fig. F82] Coarse-grained aggregates of biotite, or of biot1te + sill1man1te, are common 1n some migmat1tes developed from metapelitic (i.e., aluminous) protoliths . These aggregates are generally regarded as a late-stage alterat1on of garnet or cord1erite that formed after much of the anatect ic melt had already m1grated away, and as the last rema1n1ng melt crystallized.

Plagiocl ase [Fig. F83] The plag1oclase crystals reported from most m1gmat1tes derived from crustal rocks show very little compositional vanat1on w1th1n crystals, and the opt1cal zon1ng that they contain is generally rather subtle. However, it is becom1ng ev1dent that 1solated crystals of plag1oclase w1th a complexly zoned calcic core are a feature of anatectic terranes 1n which there has been an 1nput of mantle-de nved magma. Therefore, the zon1ng patterns 1n the plag1oclase crystals found 1n m1gmatites should be noted.

A tlas of M igmat ites

------------------------------245

Contact between the leucosome and melanosome in metatexite migmatites [Figs. F84-F96] Th1s group of photomicrographs shows some details of the contact and spat1al relationships found between the leucosome and melanosome in neosome from metatexite migmatites.

Microstructure of schlieren in diatexite migmatites [Figs. F97-F I00] Schlieren are thin, planar aggregates of platy, tabular. or ac1cular m1nerals 1n diatex1te m1gmat1tes and some granItes. B1ot1te IS the mineral most commonly associated w1th schlieren, but examples have been reported that are dom1nantly amphtbole, orthopyroxene, stllimantte, or plagioclase. Many schlieren are also ennched in the accessory phases, commonly because they occur as 1nclus1ons 1n the maJOr phases, most notably biot1te. but 1n some schlieren, most of the grains of the accessory phases occur outs1de the major minerals. The microstructure of schlieren have not been widely studied or well understood, but there appears to be considerable variation. Two pnncipal origins have been proposed to explain the development of schlieren in migmatites: (I) accumulations of biotite resultIng from the imp1ngement of grains rotat1ng 1n a magma and (2) attenuated fragments of res1dual matenal denved from d1saggregat1ng schollen.

Microstructure of biotite-rich selvedges in migmatites [Figs. FIOI-FI04] Many ve1ns and dikes of felsic compos1t1on have a narrow selvedge of mafic minerals separating them from the host rock. T he mafic selvedges that are developed on leucocrat ic veins injected 1nto the paleosome lithologies in migmat it es are probably the easiest to not ice in t he field, and all t he examples shown are of t his t ype. In some cases, a few of the quartz and feldspar crystals located at the edge of the leucocratic veins have undergone an increment of late growth out into the mafic selvedge, to produce somewhat bulbous projections. The microstructures present in mafic selvedges, like those in schlieren, appear to be qu1te diverse, but they are, in general, pooly known.

RESULTS FROM QUENCHED DEFORMATION-MELTING

246 ------------~-=~~~~=-~~ A STARTING POINT EXPERIMENTS:

Figure

I

Fig. F I. This experiment 1nvolv1ng a low fract1on of melt was conducted at 900°C and conta1ned less than I% partial melt. The mica flakes between the two gra1ns of plag1oclase shown have partially reacted, and the1r gra1n s1ze has been reduced. The adjacent quartz and plag1oclase grains appear to have flowed along w1th the fine-grained products of reaction that formed at the edge of the biotite gram. The reaction products (RP) are too small to distinguish individually, but are generally light-colored and located close to biotite. Locat1on: Deformation and melting experiment BPQ-0 38. Rock type : b1otite-quartz plag1oclase gne1ss, deformed at T 900°C, P I0 kbar, stra1n rate I x I0 s , run duration 36 h. Scale: bar IS 500 IJm long. BSE image. Image and caption: Caleb Holyoke Ill. Th1s image was published as fig. Sa 1n Holyoke & Rushmer (2002), and IS reproduced w1th

=

the perm1ss1on of Blackwell Pubhsh1ng.

Further readmg: Holyoke, C.W., Ill & Rushmer, T. (2002): An expenmental study of gra1n scale melt segregation mechanisms in two common crustal rock types. journal of Metamorphic Geology 20, 493 512.

Atlas of Migmatites

------------------------------ 247 Figure

l

Fig. F2 . This experiment also contained a low fract ion of melt; it was performed at 920°C and contained about I% melt. This BSE image is a close-up of t he boundary between biotite (Bt) and quartz (Qtz) . Solid reaction products (RP) and glass (Liq), which is quenched partial melt, occur along the boundary between the quartz and biotite grains in the gneiss. N ot ice the strong alignment of the solid products of reaction (which shows as light grey) parallel to the biotitequartz boundary. This relationship suggests that the zone of melt plus reaction products t hat developed on the grain boundary was weak and t hat shear strain was concentrated there and rotated the grains in the melt In some places, t he glass (Liq) occurs w ithin the quartz crystal, and indicates that the anatectic melt entered pre-existing fractures in t he quartz, which was a st rong mineral in this experiment. Location : Deformation and melt ing experiment BPQ-040. Rock type: biotite-quartz-plagioclase gneiss, deformed at T 920°C, p I 0 kbar, strain rate I X I 0 5 s ' run duration 6 h. Scale: bar is 50 1-1m long. BSE image. Image and caption: Caleb Holyoke Ill. T his image was published as fig. 8b in Holyoke & Rushmer (2002), and is reproduced with the

=

permission of Blackwell Publishing. Further readmg: see Fig. FI.

248

RESULTS FROM QUENCHED DEFORMATION-MELTING EXPERIMENTS: A STARTING POINT

Figure

Fig. FJ. In this expenment the fraction of melt was h1gh; part1al melting at 950°C produced 20 vol.% melt. At th1s content of melt, the sample fa1led by cataclas1s, or by melt-assisted granular now, and most of the stra1n IS partit1oned 1nto the zones of d1saggregat1on formed during sample failure. The partial melt in the sample m1grated to these disaggregated zones as it moved to the edges of the sample, where it accumulated between the sample and its jacket. Thus, deformation in the sample is heterogeneous, and some regions are left relatively undeformed. This photomicrograph shows the microstructure that develops in the relatively unstrained reg1ons: note the rounded and corroded edges of the reactant quartz (Qtz), plagioclase (PI) and biotite (Bt) graJnS. Pools of glass (L1q). wh1ch 1s the quenched part1al melt, occur around much of the biOtite, and along many gra1n boundanes between quartz and plagioclase. The light grey nms around the biOtite, and elsewhere 1n the sample, are small accumulations of solid products of react1on.

Location: Deformation and melt1ng expenment BPQ- 045. Rock type: b1ot1te- quartz plag1oclase gne1ss, deformed at T 920°C, P I 0 kbar, stra1n rate I x I0 5 s , run durat1on

=

I mm long. OptiCal m1croscope, crossed polanzers. Image and caption: Caleb Holyoke Ill. T his 1mage was published as fig. Bd 1n Holyoke & Rushmer (2002), and 36 h. Scale: bar

IS

is reproduced with the perm1ssion of Blackwell Publishing.

Further readmg: see Fig. Fl .

A tlas of M igmat itcs

------------------------------- 249 Figure

4

+

Fig. F4. Resu lts of an experiment t hat contained a high fraction of melt (35%) run at 950°C. The image shows a large crystal o f biotite (Bt) surrounded by glass (Liq) that contains many small crystals of various reaction-products (RP). The biotite crystal was originally located between large grains of quartz (Qtz) and plagioclase (PI). The biotit e, quartz, and plagioclase all have a rounded outline, indicating that t hey have partially react ed. The incongruent breakdown of biotite produced melt (now quenched to glass) and t he nu merous small crystals of orthopyroxene and spinel that are in the glass. Crystals of the reaction products are largest in the center of the glass domains, close to the corroded biotite, and they do not have a strong preferred orientation. However, smaller crystals of the reaction products occur abundantly in a band located close t o, and parallel with, the edge of t he quartz grain. A large proportion of these small crystals are aligned, and this is due to flow of the crystal-bearing melt as the sample was shortened. T herefore, t his band of crystals is analogous to the schlieren seen in some migmatites. Some of the reaction products in the glass at the left-hand edge of the image appear as skeletal crystals; such crystals are a feature also seen in the reaction products in some anatectic rocks from low-pressure contact-aureoles, e.g., the Duluth Igneous Complex.

Location : Deformation and melting experiment BPQ- D32. Rock type : biotite quartz plagioclase gneiss, deformed at

T =950°C, P I0 kbar, strain rate I x I0 · S s ', run duration 36 h. Scale: bar is 200 f_Jm long. Optical microscope, crossed polarizers. Image and caption : Caleb Holyoke Ill. This image was published as fig. 9d in Holyoke & Rushmer (2002), and IS reproduced with t he permissio n of Blackwell Publishing. Further readmg: see Fig. FI.

250

- A~U~R~E~O~L~E~S: T~ C~ A~ T~ N~ ~ ~O ~F~A~C~E~C ~R SU S~U~B~ ----------------~

THE GLEN MORE PLUG , SCOTLAND

Figure

S

Fig. FS . Glassy rocks extend almost I m from the con tact of a dolente plug 30 m 1n diameter at Glenmore, Ardnamurchan int rus1ve complex, Scotland. That extent of melted rocks around such a t~ny 1ntrus1ve body 1nd1cates that the plug represents a long-lived conduit of magma. This first photomicrograph is of part1ally melted arkose that underwent a relat1vely low degree of part1al melt1ng in the outer part of the contact aureole. Relict quartz is clear and unclouded, with a corroded outline. The relict feldspar is now sieve-textured and t urbid, or cloudy-looking; the dark elongate patches are former grains of biotite. Brown glass forms a rim around the relict quartz and occup1es the site of the onginal gra1n-boundary between the quartz and adJacent feldspar. T he width of the glass nm 1ndicates the extent to which quartz and feldspar have been consumed 1n generat1ng melt. The small elongate needles that have nucleated on the quartz. whiCh locally form a fringe, are orthopyroxene.

Location: Glenmore dolente plug. Ardnamurchan, Scotland. Rock type : part1ally melted arkose. Scale: the field of v1ew is 0.9 mm. Plane-polanzed l1ght. /mage and caption: Marian Holness.

Further readmg: Butler. B.C.M. (196 1): Metamorphism and metasomatism of the Mo1ne Senes by a dolerite plug at Glenmore, Ardnamurchan. Mmeraloglcal Magazme 32,

866- 897. Holness, M.B., Dane, K., Sides, R.. Richardson. C. & Caddick, M. (2005): Melting and melt segregat1on in the aureole of t he Glenmore Plug. Ardnamurchan. journal of Metamorphic

Geology 25, 29 44.

A d a;, of Migmatites

---------------- -------------- 25 1 Figure

•

Fig. F6. The photomicrograph shows a partially melted arkose from Glenmore in which there was a high degree of partial melting, and the rock contained a larger fraction of melt. The fused arkose 1n th1s sample cons1sts largely of brown glass. The glass conta1ns corroded relics of clear quartz, s1eve-textured feldspar (i.e., the areas of fine-gra1ned crystals w1th brown glass), and biotite (the dark elongate patches near the center of the photograph). Elongate, needlelike crystals in the glass are orthopyroxene, and a product of the partial -melting react1on.

Location: Glenmore dolente plug, Ardnamurchan, Scotland. Rock type: partially melted arkose. Scale: the field of v1ew is 0.9 mm. Plane-polanzed light. /mage and caption: Marian Holness.

252

S: H~ T~ ~L~I_ ~X~E~N~O ED T~ Y_M~E~L~ IA_L~L~ T~ ~·~P_A~R~ _E_D ----------~E~R~U_PT

EL JOYAZO, SPAIN

Figure

7

Fig. F7. Close-up v1ew of a part1ally melted res1dual enclave erupted 1n a daCite lava. The enclave IS full of blu1sh cord1er1te, black biotite, wh1te s1lhmanite, and euhedral crystals of red garnet. In thin sect1on, all these m1nerals are intergrown with glass and contain glass 1nclus1ons. The glass, of course, represents quenched anatectiC melt. The detailed 1mages 1n the three following figures are from enclaves such as this. Locat1on: El joyazo, southeastern Spain. Rock type: residual pelite enclave in dacite volcanic rock: partial melting at T 850 ± 50°C, P >5 kbar, but cooled rapidly by eruption. Scale: the field of view is 4.0 em. Image and caption: Bernardo Cesare.

Further readmg: Cesare, B. (2000): Incongruent melt1ng of biot1te to spinel in a quartz-free rest1te at El joyazo (SE Spain): textures and reaction charactenzat1on. Contributions

to Mmeralogy and Petrology 139, 273-284.

Cesare, B. & Ma1nen, C. (1999): Flu1d-present anatex1s of metapelites at El joyazo (SE Spain): constra1nts from Raman spectroscopy of graph1te. Contnbuvons to Mmeralogy and

Petrology 135,41-52. Cesare, B., Salvioh Mariani, E. & Venturelli, G. (1997): Crustal anatexis and melt extraction in restitic xenoliths at El Joyazo (SE Spain). Mmeralog1col Magaz1ne 61 , 15-27.

A tlas of Mig ma tites

------------------------------- 253 Figure

Fig. F8. Microstructures involv1ng b10t1te from a domain that corresponds to part1al melt1ng 1n a quartz-present and AI 2Si05-free system. The biotite crystals show a th1n react1on-nm consisting of orthopyroxene. 1lmen1te. and glass. The reaction-nm is discontinuous, and appears to be developed ma1nly at contacts between b1ot1te and quartz (top and nght). In contrast, the boundaries between biotite and cordierite (bottom left) seem to be in chem1cal equilibrium. The inferred melting reaction, biot it e + quartz orthopyroxene + ilmenite + melt, could not be balanced with the chemical composit ions determined from t he minerals that comprise this microstructure.

=

Location: El joyazo. southeastern Spam. Rock type: biot1te from an enclave of res1dual pelite in a dac1te, part1al melt1ng at T 850 ± 50°C, P >5 kbar, but cooled rap1dly by erupt1on. Scale: the field of view IS 0.65 mm w1de. Plane-polanzed light. Image and capt1on: Bernardo Cesare. Further readmg: as for Fig. F7.

ERUPTED, PARTIALLY MELTED XEN OLITHS:

254 -----------------------------EL JOYAZO. SPAIN Figure

Fig. F9. Microstructures 1nvolv1ng b1ot1te from a doma1n that corresponds to part1al melt1ng 1n a quartz-free system. The b1ot1te crystals are surrounded by a grey, fine-gra1ned m1xture (mix) of fibrolitlc sillimanite and glass (former melt). All the biotite crystals are 1rregular 1n shape, and have deep embayments, wh1ch indicates that they have been corroded. Furthermore, the crystals d1splay well-developed, clear reaction-rims. Each rim consists of brownish green hercynitic spinel and laths of opaque (i.e., black) ilmenite in a colorless (i.e., white) glass. Notice the euhedral, or subhedral, form of the spinel where 1t IS 1n contact with the glass, which represents the former melt. On the basis of compositions of the m1nerals in this microstructure and 1n its surroundings, Cesare (2000) has balanced the possible biotite-melt1ng react1on as b10t1te + plag1oclase ilmen1te + melt + b1ot1te 1 + hercymte m1x + garnet

=

+ +

plag1oclaser Location : El joyazo, southeastern Spa1n. Rock type: biot1te from an enclave of residual pellte 1n dac1te, part1al melting at T 850 ± 50°C, P >5 kbar, but cooled rapidly by eruption. Scale: the field of view is 0.65 mm wide. Plane-polarized

light. /mage and caption: Bernardo Cesare. Further reading: as for Fig. F7.

A tlas o f M igmat ites

------------------------------ 255 Figure

0

Fig. F I 0. Back-scattered electron image (BSE) of a corroded grain of biotite from a quartz-free domain in an enclave of residual pelite. T he biotite is hosted by an aggregate of finely intergrown, fibrolitic sillimanit e and glass (labeled "mix"). The outline of the biotite has deep embayments, which indicates t hat it has been corroded, and hence was unstable. A well -developed reaction-rim occurs around the biotite and contains crystals of hercynitic spinel (Spl) and laths of ilmenite (lim) in a glass (melt); the ilmenit e appears white in this BSE image. Crystals of both spinel and ilmenite are euhedral or subhedral, and are int erpreted as products grown in the melt formed by the incongruent breakdown of biotite. The composition of the outermost rim of the biotite is richer in both MgO and T i02 (Bt 2) than t he core (Bt1) of the crystal. The possible biotite-melting reaction can be balanced as biot ite + plagioclase 1 + mix + garnet = melt + ilmenite + biotite2 + hercynite + plagioclase2. Location : El joyazo, southeastern Spain. Rock type: biotite from an enclave of residual pelite erupted in dacit e lava, partial melting at T 850 ± 50°C, P >5 kbar, but cooled rap-

idly by eruption. Scale: the field of view is 0.65 mm wide. Plane-polarized light. Image and caption : Bernardo Cesare. Further reading: as for Fig. F7.

:

SUBSURFACE CON TACT-AUREOLES 256 ----------------------------COMPLEX . SCOTLAND THE RUM I GNEOUS

Figure

I I

Fig. F II . Part1ally melted Lew1sian gneiss conta1ning quartz and plag1oclase, with pyroxene aggregates after hornblende (not all are obv1ous from the photograph). Th1s amphlbole-beanng gne1ss has been contact-metamorphosed at low pressure by the Rum Igneous Complex. Part1al melt1ng occurred locally 1n the gneiss where 1t was 1n close proximity to the mafic intrus1ve body. The example shown in this photomicrograph contains a thin rim of granophyre developed along t he boundary between quartz and plagioclase crystals; the onentation of this slide is such that most of the quartz crystals in the photomicrograph are wh1te. This granophyre 1s of spec1al Interest in that much of the quartz in it forms elongate plates; these are paramorph1c after tndym1te. Generally, quartz 1n granophyre from part1ally melted rocks occurs as rounded and elongate blebs. Location: Pro1mh Lochs, Isle of Rum, Scotland. Rock type: partially melted Lew1s1an gne1ss. Scale: the field of v1ew IS 4.25 mm. Cross-polanzed light. Image and capt1on: Manan Holness. Further reading: Holness, M.B. & Isherwood, C.E. (2003): The aureole of the Rum Tert1ary Igneous Complex, Inner Hebndes, Scotland. journal of the Geolog1cal Sooety of London 160, 15- 27.

A tl as of Migmarires

------------------------------- 257

Fig. F 12 . The center of the photomtcrograph ts occupted by a large gratn of feldspar that ts full of tnclustons. Nearby are two large. rounded grains of quartz. The boundary between these quartz and feldspar gratns ts decorated by a "stnng of beads" microstructure conststtng of small gratns of feldspar: these are interpreted to have crystallized from a thtn rim of anatectic melt that separated the larger feldspar and quartz grains. This microstructure ts interpreted

as a slowly cooled version of the rather more common granophyric nms that develop on grain boundanes during local tn sttu parttal melttng followed by more rapid cooling. Locatton: Rum Igneous Complex, Scotland. Rock type: parttally melted Lewtstan gnetss. P ca. ISO bar. Scale: the field of vtew ts 0.9 mm. Cross-polanzed ltght. Image and captton: Manan Holness. Further readmg: as for Fig. FII.

N A:...:. ...:.:...: B:H G:...:.H..:....::. :...:.I:...: :..::A _:_R S :._T:...:.H..:..;E:_T T-...:_A.:..::U:...:.R.:..::E:...:O:...:L:..=E.:..: C...:. :...:.= TA ~C=-0=--:...::N...:. 258 __.:..:SH_:.:A~LL:.:O:...:.W NA SGURRA SILL ON MULL, SCOTLAND

Figure

I3

Fig. F 13. This sample of psammite did not conta1n much m1ca, and the melt present at the peak temperature was denved predominantly from the quartz and feldspar 1n the rock. In th1s cathodoluminescence (CL) 1mage. each feld-

spar grain 1s surrounded by a bnghtly lum1nescent rim of granophyre from which cracks emanate; they are 1nferred to have been filled with melt. and now form a completely interconnected network. Note that the final generat1on of cracks (black in this image) cut across the features related to part1al melting and anatect iC melt. Location: Isle of Mull, Scotland. Rock type: partially melted psamm1te 44 em from the contact w1th the dolente s1ll. Scale: 1ndicated on the 1mage. Image and caption: Manan Holness. Th1s image was published as fig. 13d 1n Holness & Watt (2002), and 1s reproduced w1th the permiSSIOn of Oxford Univers1ty Press.

Further readmg: Holness, M.B. & Humphreys. M.C.S. (2003): The Tra1gh Bhan na Sgurra s1ll, Isle of Mull: flow localisation in a maJor magma condUit. journal of Petrology 44, 1961 1976. Holness. M.B. & Watt. G.R. (2002): The aureole of the Traigh Bhan na Sgurra sill, Isle of Mull: reaction dnven microcracking during pyrometamorphism. journal of Petrology 43 , 511 - 534.

1 ~

Atla; of Migma tites

----------- ----------- -------- 259

i

•

Figure

14

•

• • ~

1

• •

Fig. Fl4. This cathodoluminescence 1mage of a psamm1te shows elongate bright reg1ons that are the s1tes of reacted muscovite grains. Note also the dark, rectangular grains of retrograded, euhedral cordierite with1n domains representIng the former gra1ns of muscov1te. The muscov1te was set 1n quartz, which IS dark in this image. The 1mage shows an abundance of bright. linear features onented at a high angle to the foliation, wh1ch 1n the sample is defined by the onentation of the muscovite. These bright linear features are cracks generated by the volume change assoc1ated with the breakdown of muscovite. These cracks have been filled w1th anatectic melt: they are exactly analogous to those formed experimentally by Connolly et al. ( 1997). Location: Isle of Mull, Scotland. Rock type: part1ally melted psamm1te 80 em from contact w1th the sill. Scale: 1nd1cated on the 1mage. Image and coptton: Marian Holness. Th1s image was published as fig. 9 in H olness & Watt (2002). and is reproduced with the permission of Oxford University Press.

Further readmg: Connolly, J.A.D., Holness, M.B., Rubie, D.C. & Rushmer, T. (1997): React1on-1nduced micro-crack1ng: an expenmental investigation of a mechan1sm for enhancing anatectic melt extraction. Geology 25, 591 - 594.

SHALLOW CONTACT-AUREOLE S: TH E TRAIGH BHAN

260 ------------~---------------

NA SGURRA SILL ON MULL. SCOTLAND

Ftgure

IS

Fig. F 15. T his photomicrograph

IS

a close-up of reacted

gra1ns of muscov1te in a psamm1te. The muscov1te 1s now replaced by mullite, b1ot1te. K-feldspar. sp1nel. and some pnsmatic gra1ns of cord1ente. The cord1ente IS now ent1rely replaced by a yellow product of alterat1on. The s1tes of the reacted muscovite (parallel to the short s1de of the photomicrograph) were linked by melt-filled fractures, which are now filled wit h brownish granophyre. Most of the fractures in this image contained anatectic melt. but th1s is only really apparent on back-scattered electron or cathodoluminescence images. The large colorless gra1ns are quartz.

Location: Isle of Mull, Scotland. Rock type: part1ally melted psamm1te 80 em from contact w1th the sill. Scale: the field of v1ew 1s 0.9 mm wide. Plane-polanzed hght. /mage and caption: Marian Holness. Further readrng: see Fig. Fl3.

Atlas of Mig ma tites

------------------------------ 261 Figure

16

Fig. F 16. Th1s part1ally melted psamm1te IS h1ghly fractured and cons1sts pnnc1pally of large, colorless gra1ns of rounded quartz (e.g., left s1de of the photomicrograph). The fractures are qu1te w1de and show the growth of quartz on

thew walls. Possibly, the melt rema1ned stat1c 1n the fractures for some t1me, and the fracture walls moved back (retreated) as the quartz dissolved into the melt. When cooling began and the melt started to crystallize, the quartz that grew out of the melt formed euhedral overgrowths of quartz on t he walls, trapping melt in small inclusions. The dark, ovo1d doma1ns on the right are the s1tes of reacted gra1ns of muscov1te; note the colorless plates of tndymlte that are now replaced by quartz. associated w1th the reacted muscov1te. Location: Isle of Mull, Scotland. Rock type: part1ally melted psammite from the contact w1th the sill. Scale: the field of v1ew 1s 0.9 mm w1de. Plane-polanzed light. /mage and capuon: Manan Holness.

262

SHALLOW- TO MED IUM-DEPTH CONTACT-AUREOLES: THE DULUTH IGNEOUS CO MPLEX

Figure

17

Fig. F 17. This pelit1c rock was collected from near the "melt-in" isograd > 140 m below the contact 1n the footwall of the Duluth Igneous Complex. Th1s 1mage was taken us1ng a one -wave, quartz accessory plate. Depend1ng on the orientat1on of grains, this e1ther adds, or subtracts, about 570 nm to the color value. Thus, gra1ns that would otherWISe be first-order grey are changed to between first-order red and second-order blue, and consequently, individual grains in a mat rix become easier to distinguish, as they are no longer all grey. Locally, the rock contains small irregular patches of quartz or feldspar (three are 1ndicated with arrows) that contain rounded 1nclus1ons of quartz, plagioclase, and rarely, b1ot1te; these patches are Interpreted as former pockets of anatectiC melt and can be thought of as m s1tu m1croleucosome. Th1s rock conta1ns a h1gh modal percentage of b10t1te, no orthopyroxene and only minor amounts of cordiente, wh1ch 1nd1cates that partial melt1ng was at an incipient stage. The presence of b1otite along many of the grain boundaries makes 1t difficult to find cuspate extensions of quartz or feldspar between the corroded gra1ns of the matnx.

Location: Duluth Igneous Complex, Mmnesota, U.S.A Rock type: metatex1te m1gmat1te; Virgima Formation metapelite protolith, part1al melt1ng at the "melt-1n" 1sograd, T ca. 700-725°C. Scale: the width of the photograph IS 2.5 mm. Cross-polanzed light and quartz plate 1nserted. /mage: E.W . Sawyer.

A tlas of M ig matitcs

Figure

• • •

•

• • • • • • '

18

Fig. F 18. PelitiC rocks w1th res1dual bulk compos1t1ons occur 1n metasedimentary xenoliths of the V1rg1n1a Format1on from the nonte of the D uluth Igneous Complex. This image was taken using a one-wave quartz accessory plate so that some of the former pools of melt appear yellow and are, therefore. easier to see (others are blue. red. and purple, but harder to see). These former pockets of melt have cuspate shapes (A) w1t h t h1n, tapenng extens1ons penetrating along adjacent grain-boundaries between the reactant minerals. Moreover, the matnx minerals 1n cont act w1th the yellow doma1ns are rounded, and thus Interpreted as evidence that they are corroded. The former pockets of melt contain small, rounded gra1ns of quartz, plagioclase, and biotite. Most of t he orthopyroxene crystals (h1gh relief) are sub1dioblast1c, but a large one (B) grown 1nto a melt pool has rat1onal faces and IS, therefore, ldioblastic. The microstructures 1ndicate that the melt1ng reaction in t h1s res1dual rock was biotite + plag1oclase + quartz orthopyroxene + melt + ilmenite. The melt ing was well advanced, and little of the reactant biotite and quartz rema1ns; most of the b1ot1te present 1n the rock forms large bladed porphyroblasts and grew later from t he melt.

=

Locat1on: Duluth Igneous Complex, M1nnesota, U.S.A Rock type: xenolith of Virg1nia Format1on metapelite located about 25 m 1nto the intrus1ve body, anat exis at T > 900°C. Scale: t he widt h of the phot ograph is 2.5 mm. Cross-polarized light and quartz plate 1nserted. Image: E.W . Sawyer.

263

264

SHALLOW- TO MEDIUM-DEPTH CONTACT-AUREOLES: THE DULUTH IGNEOUS CO M PLEX

Figure

19

Fig. F 19. Microstructures 1ndicat1ve of part1al melt1ng can also be found 1n many of the other litholog1es around the Duluth Igneous Complex. Irregular doma1ns of quartz and feldspar contain1ng rounded 1nclus1ons of quartz, b1otite. plag1oclase, and cordiente mark the s1tes of partial melt1ng in this graph1te + Fe-ox1de-beanng metapelite. There are several such doma1ns in this photomicrograph, and the most prominent one 1s the yellow area just right of the center. Others are Indicated w1th arrows. Note the basal sect1ons of large, irregularly shaped crystals of biotite 1n the rock: these are interpreted to have grown e1ther from the anatect1c melt, or from a react1on between the melt and m1nerals such as K-feldspar or cordierite. The bulk compos1t1on of the metapelite, and the low pressure of metamorph1sm 1n the aureole of the Duluth Igneous Complex, mean that K-feldspar and cordiente are abundant 1n many of the pelitic hornfelses, and the earliest generation of both m1nerals formed before part1al melting began.

Locotton: Duluth Igneous Complex, M1nnesota. U.S.A Rock type: metatex1te m1gmatite: graph1te + Fe-ox1de-beanng metapel1te protolith, anatex1s at T ca. 750 800°C. Scale: the w1dth of the photograph IS 2.5 mm. Crossed polarizers and quartz plate. /mage: from a thin section provided by Steve Hauck.

Atlas of Migmatites

-------------------------------265 Figure

10

• •

• • •

•

•

•

• • •

• • • • • • •

Fig. F20. The onset of partial melting in sulfide-rich (> I 0 modal %) pelitic rocks produces microstructures in the silicate portion that are very similar to those in normal pelit ic rocks. The ferromagnesian silicate minerals are far more magnesian, however, and have compositions very close to the Mg end-members. Equant domains of anhedral quartz and plagioclase several t imes larger t han the grains in the mat rix occur throughout t he rock (three are indicated by arrows) and are interpreted to be crystallized pockets, or pools, of anatect ic melt. Many of the large crystals of quartz and feldspar that comprise the domains contain small, rou nded inclusions of quartz, plagioclase, and biotite . These inclusions are interpreted to be the corroded remnants of the reactant minerals. Most of the former pools of melt have cuspate forms; some thin, tapered offshoots extend along the boundaries of rounded matrix grains .

Location: Duluth Igneous Complex, Minnesota, U.S.A. Rock type: metatexite migmatite; the protolith was the bedded pyrrhotite unit, Virginia Formation, anatexis at T ca. 800°C. Scale: the width of the photograph is 2.5 mm. Crossed polarizers and quartz plat e insert ed. /mage : from a thin section provided by Steve Hauck .

266

SHALLOW- TO MED IU M-DEPTH CONTACT-AUREOLES: T HE DU LUTH IG N EOUS CO MPLEX

Figure

21

Fig. F2 1. Part1al melting has also affected the (clinopyroxene + magnet1te)-bearing quartz-nch rocks of the B1wabik Format1on, which occur 1n the footwall of the Duluth Igneous Complex. Th1s photograph shows an example where 1nc1p1ent partial melting has led to the corros1on of the quartz at grain junctions and to the development of films of anatectic melt on some grain boundaries. Upon freezing, the films of melt on the grain boundaries have crystallized to feldspar and quartz. Locally, some larger pools of melt have developed (A), and they crystall1zed to a characteristic micrographic intergrowth (granophyre) of quartz and feldspar (A). Melt that occup1ed smaller pores (B) crystallized only plag1oclase. The anatectiC melt 1n rap1dly cooled m1gmatites quenches to glass (see F1gs. FS and F6), whereas in very slowly cooled migmatites such as those formed by reg1onal metamorphism, 1t crystallizes to produce normal 1gneous microstructures. However. 1n the reg1me of Intermediate rates of cooling. typ1cal of contact aureoles 1n the I 3-kbar range, the anatect1c melt commonly crystallizes to granophyre. as in this example (see also Fig. Fll) .

Locat1on: Duluth Igneous Complex. M1nnesota. U.S.A. Rock type: metatex1te m1gmat1te; magnet1te-beanng quartzIte protohth. anatex1s at T ca. 900°C. Scale: the width of

the photograph IS 6 mm. Cross-polanzed hght. Image: E.W. Sawyer.

A tlas of Mig ma tites

------------------------------267 Figure

ll

•

• •

•

• • • ~

• • • •

•

4

Fig. F22. In th1s part1ally melted 1mpure quartz1te from the B1wab1k Format1on, some of the quartz gra1ns (Qr) are rounded ow1ng to dissolution into the anatectiC melt, and they contain abundant inclus1ons and cracks: these are Interpreted to be grains of residual quartz. However, other quartz gra1ns have straight edges, and t h1s quartz IS Interpreted to have grown from t he anatect ic melt. possibly as an overgrowth on a grain of residual quartz. Some crystals of quartz (Qm) are rectangular and may indicate that t he quartz grew as bet a quartz. Large grains of feldspar (F), some of which display wispy films of sod1c plagioclase of exsolut1on ong1n, occur between the quartz crystals. The large, grey crystal of feldspar near the center of the photograph conta1ns both rounded and rectangular crystals of quartz. The boundary of th1s gra1n IS h1ghly 1rregular: locally. 1t extends between quartz crystals and term1nates 1n cuspate proJeCtions at quartz triple junctions (arrow). Feldspar crystals 1n the upper right and lower left are 1n extinction pos1t1on and, therefore, appear black, but they too extend between quartz gra1ns and have cuspate terminat1ons. Th1s microstructure is interpreted to indicate that the feldspar in th is m1gmatite cryst all ized from t he anatectic melt that was Interstitial to the quartz grains.

Location: Duluth Igneous Complex, M1nnesota, U.S.A. Rock type: metatex1te m1gmat1te: 1mpure quartz1te protohth, anatexis at T ca. 900°C. Scale: the w1dth of the photograph IS 2.7 mm. Cross-polanzed light. /mage: E.W. Sawyer.

268

SHALLOW- TO MEDIUM-DEPTH CONTACT-AUREOLES: THE DULUTH IGNEOUS COMPLEX

Figure

23

Fig. F23. In the melt-nch parts of m1gmat1tes, the former patches of melt have a microstructure dom1nated by crystallization of the melt, and not by the dissolut1on of reactant m1nerals. This photom1crograph from a diatex1te m1gmat1te shows a region of melt that conta1ned small. euhedral (rectangular) gra1ns of plag1oclase. equant euhedral grains of cordiente. some w1th a hexagonal shape show1ng sectortwinning (Crd), euhedral (rectangular) grains of untw1nned K-feldspar, and crystals of blade-shaped biot1te: there 1s also a prominent and somewhat skeletal gra1n of orthopyroxene that is partially replaced by b1ot1te and other products of alterat1on. Most of the melt that contamed these crystals crystallized to large subhedral gra1ns of quartz and K-feldspar. Th1s particular photomicrograph shows mostly the quartz. The abundance of rectangular feldspar and cord1ente crystals g1ves th1s domam of former melt a dist1nct1ve blocky outline, wh1ch contrasts w1th the cuspate outline of pockets of melt formed dunng part1al melt1ng. Locat1on: Duluth Igneous Complex, M1nnesota. U.S.A.

Rock type: diatexite m1gmatite: pel1t1c protoilth, 130 m from the footwall contact: T 850 900°C. Scale: the w1dth of the photograph is 2.5 mm. Cross-polarized light. Image: E.W. Sawyer.

Atlas of Mig mat ites

------------------------------269 Figure

24

Fig. F24. The last parts o f the granit ic melt to crystallize are dominated by quartz and K-feldspar. In this part of a melt pocket. it is the quartz that crystall ized last, and it formed large irregularly shaped grains that enclose plagioclase (PI), K-feldspar (Kfs), cordierite (Crd), and biotite (Bt), i.e., all the other minerals. K-feldspar, which occurs as several large, euhedral crystals (e.g., t he grain at the center of the photomicrograph), and biotite bot h appear t o have crystallized relatively late. The quartz, K-feldspar, and biotite all contain small, euhedral inclusions of plagioclase and cordierite, although t he inclusions are fa r more abundant in the quartz. The common occurrence of rat ional faces on the crystals and the euhedral habit of most of t he cryst als together indicate that t his microst ructure formed as the anatectic melt crystallized.

Location: Duluth Igneous Complex, Minnesota, U.S.A Rock type: diatexite migmat ite; Virginia Fo rmation pelit e protolith, 103 m fro m the contact, T ca. 850- 900°C Scale: the width of the photograph is 2.5 mm. Cross-polarized light Image: E.W. Sawyer.

270

SH A LLOW- TO MEDIUM - DEPTH CONTACT - AUREOLES: THE DULUTH IGNEOUS COMPLE X

Figure

25

Fig. F25. T he two prominent, rectangular crystals 1n th1s photomicrograph are euhedral phenocrysts of K-feldspar. and both conta1n abundant 1nclus1ons. The d1stnbut1on of the Inclusions 1n the light grey crystal at the lower nght is zoned. An 1nner reg1on contains an abundance of small. round 1nclus1ons of reddish brown biot1te. and these are Interpreted to be partially dissolved reactant biot1te. An adjacent zone conta1ns several larger 1nclus1ons of subhedral b1otite that poss1bly grew in the anatectic melt. and rounded reactant quartz. Both of these zones are surrounded by a reg1on w1th JUSt a few small 1nclus1ons of subhedral crystals of cordierite, 1nterpreted to have grown from the melt. The outermost zone 1s best developed at the ends of the crystal, and conta1ns abundant euhedral and subhedral crystals of cord1ente and plag1oclase, plus some rounded inclus1ons of quartz and iron oxide. The other phenocryst of K-feldspar has a similar outer zone 1n which euhedral Inclusions of cordiente are concentnc to the crystal outline. However, the 1ntenor of th1s K-feldspar 1s different; it contam large subhedral crystals of b1ot1te w1th quartz, which resembles the groundmass outs1de the phenocryst; poss1bly. the K-feldspar was hollow and conta1ned melt. which crystal lized later. The groundmass around the K-feldspar crystals comprises large, anhedral crystals of quartz that conta1n abundant Inclusions of euhedral plag1oclase and

cord1ente, and larger subhedral 1nclus1ons of b1otite. The zone in the K-feldspar phenocryst that conta1ns the rounded 1nclus1ons of b10t1te may have grown as a solid product of the melt1ng react1on. and may have been the substrate upon which the K-feldspar that grew from the anatectic melt nucleated. and then trapped euhedral crystals of cord1ente as 1t grew. The marg1ns of both crystals of K-feldspar part1ally envelop much larger euhedral crystals of plagioclase. cordiente. and b1ot1te that are shared with the large subhedral gra1ns of quartz that are adjacent. This relat1onsh1p may 1nd1cate that the last increment of K-feldspar growth occurred as quartz crystallized from the last melt 1n the migmat1te.

Location: Duluth Igneous Complex, M1nnesota. U.S.A Rock type: diat exite migmat1te; Virginia Formation petite protolith, I 03 m from the contact; T ca. 850 900°C. Scale: the width of the photograph IS 3.5 mm. Cross-polanzed hght. Image: E.W. Sawyer.

Ada., of Migmatitc;,

Figure

1

Fig. F26. As an H,O-undersaturated gran1t1c melt progressively crystall1zes, the H 0 content 1n the remain1ng melt fract1on 1ncreases 1f most of the m1nerals that crystallize first, such as plag1oclase. are anhydrous. Th1s photomicrograph shows a doma1n where the crystallization of the last Increments of now H,O-nch melt has resulted 1n the growth of blades of muscov1te (Ms. yellow and blue Interference colors), and also to the replacement of earlier-formed orthopyroxene (Opx) by small bladed crystals of biotite and muscovite. The extent of the former pocket of melt IS Indicated by the quartz (wh1te): the melt also L

conta1ned abundant. small euhedral crystals of plag1oclase, K-feldspar, and cordiente. together with some larger, bladed crystals of b1ot1te. Note (see F1gs. Fl7 F28) that the crystals with the largest grain-size in the m1gmat1tes at Duluth are generally the minerals that were the last to crystallize. typically quartz, plag1oclase, and K-feldspar. Location: Duluth Igneous Complex, Minnesota, U.S.A. Rock type: diatex1te m1gmat1te; Virg1n1a Format1on pelite protolith, 97 m from the contact; T ca. 850 900°C. Scale: the w1dth of the photograph 1s 2.5 mm. Cross-polanzed light. Image: E.W. Sawyer.

------------------ -271

272

SHALLOW- TO MEDIUM - DEPTH C ONTACT-AUREOLES : THE DULUTH IGNEOUS COMPLEX

Figure

27

Fig. F27. Some of the m1gmat1tes in the contact aureole at the Duluth Igneous Complex (DIC) contain crystals that have a skeletal, or hollow, form; skeletal crystals are a common feature 1n many raptdly quenched parttal-melting expenments. Thts photomicrogr aph, of a diatextte migmat1te denved from a pelitic protolith, shows two different morphologtes of cordiente. Most of the cordiente grams (Crdl) occurs as very small (<0.25 mm across), equant, euhedral crystals, some of which show a welldeveloped sector twtnntng. S1m1lar small crystals of cordiertte also occur abundantly as tnclusions 1n plagioclase, K-feldspar, and quartz. Consequently, these small crystals are Interpreted to have crystallized 1n the anatect1c melt. The center ofthe photograph shows a rare, large (ca. 1.5 mm), isolated phenocryst of cordtente (Crd2) that has grown in the most leucocratiC and coarsest-gratn ed part of the dtatextte mtgmatite. Because this leucocratic doma1n conststs predominantly of large anhedral crystals of quartz, plagtoclase, and K-feldspar, 1t IS mterpreted to be one of the last places 1n the dtatexite to have crystallized. The phenocryst 1s skeletal, 1t has angular embayments, and 1ts core contains quartz, plagtoclase. K-feldspar, and b1ot1te that resemble the ground mass outstde the phenocryst. lnteresttngly, there are some small fan-shaped clusters of muscovtte (Ms) 1ns1de the cordiente, whtch suggests that the mtnerals in the core

of the cordierite crystallized from an evolved melt that was parttally enclosed by the cordtente. This fact, and the presence of straight crystal faces against both quartz and feldspar, lead to the 1nterpretat1on that the cordiente grew from the melt, and that 1ts skeletal, or embayed, morphology is a growth feature, and not due to its dissolution. In their most melanocratiC parts, some dtatextte mtgmatttes 1n the DIC aureole contain cordtente that has a thtrd morphology. Such cordierite is generally not twinned and forms subtdioblasttc crystals up to I mm across that conta1n equant Inclusions of btottte and quartz. These crystals are interpreted to be residual cordierite that formed by metamorphic reacttons pnor to partial melttng.

Location: Duluth Igneous Complex, Mtnnesota, U.S.A. Rock type: d1atex1te m1gmat1te; Virgtnia Formation pelite proto lith, I03 m from the contact; T ca. 850 900°C. Scale: the width of the photograph 1s 3.5 mm. Cross-polarized light. Image: E.W. Sawyer.

A d as o f Mig ma tites

---------------- -------------- 273 Figure

18

Fig. F28. Th1s photomicrograph shows elongate crystals of orthopyroxene and plagioclase t hat are strongly aligned 1n a narrow zone onented parallel to the bottom-left-totop-nght d1agonal of the photograph. The strong preferred onentat1on of the elongate grains occurs 1n the center of th1s 1ncipient 1n-source leucosome, and is due to flow of magma that conta1ned the elongate crystals, 1.e., th1s 1s an example of the flow of a crystal-rich magma. Smaller, equant crystals of subhedral plagioclase, K-feldspar, and cordierite occur between the larger, better-onented crystals, and were not aligned because of thew equant shapes. Large, Irregularly shaped reg1ons of quartz, K-feldspar. and m1nor plag1oclase occur around and between the elongate (aligned) and equant (not aligned) subhedral crystals of plagioclase and orthopyroxene, and are Interpreted to be the places w here the last of t he interstit ial melt crystallized after flow of the magma had ceased. Some patches of interstitial melt were surrounded by euhedral crystals that had grown from the melt. and the1r charactenst1c blocky outline IS preserved (two patches are mdicated by arrows). The orthopyroxene 1n th1s rock IS a solid product of the Incongruent melting of b1otite (some authors use t he term "peritectic phase" for minerals formed in th1s way), and because t hey grew in t he presence of an anatect ic melt, some crystals have wel l-developed faces. In contrast, most

of the plag1oclase present cryst allized from the melt and 1s, therefore, considered to be a melt-crystallization product. 1.e., a liqu1dus phase. The leucosome is cons1dered to be 1ncip1ent because the doma1n does not have clearly defined edges, and is not ev1dent in the hand sample. Locat1on: Duluth Igneous Complex, Mmnesota, U.S.A. Rock type: InCipient leucosome 1n a residual metatexite migmatite; Vi rginia Formation pelit e protolit h, xenolit h T > 900°C. Scale: the w 1dth of the photograph IS 2.5 mm. Cross-polanzed light. /mage: E.W. Sawyer.

274 ______________________D~E_E_P_E~R_C~O~N_T_A~C~T~-~A~U~R~E~O~L~E~S: THE BALLACHULISH IGNEOUS COMPLEX

Figure

Fig. F29. Th1s photomiCrograph 1s from a th1n section taken from a nebulitic metatex1te m1gmat1te derived from a semipelitic protolith. The quartz has an 1nterst1t1al to poikilit1c morphology and surrounds subhedral to euhedral gra1ns of K-feldspar and, to a lesser extent. cord1ente. The K-feldspar 1s h1ghly altered, and so appears patchily dark, and somew hat streaked. The cord1ent e gram on the extreme nght IS alt ered largely t o a sh1mmer aggregate. The outline of several K-feldspar grains shows clear evidence of a definite shape o f t he crystals. T he quartz, wh1ch IS clear (white) to yellow1sh brown in color, has an 1nterstit1al, pore-fill1ng form between the gram of K-feldspar and cord1erite. but 1t 1S po1k11itic overall and 1n the field of v1ew cons1sts largely of two cryst als: one 1n the lower left and center has wh1te bwefnngence-colors. and the other has yellow1sh colors. The K-feldspar 1s considered to have crystallized from the anatectic melt earlier than the quartz. which crystallized around the K-feldspar (and cord1ente) crystals as 1t filled the rema1ning " pore space"; hence. 1t has both a poik1lit1c and an 1nterst1tial hab1t. Locat1on: Ballachulish Igneous Complex. Scotland. Rock type : nebulitic met atexite m1gmat1te ; sem1pelitic proto lith. Scale: t he width of the photograph IS 1.5 mm. Cross-polarized light. Image and capt1on: Ben Harte and Cla1re M. Linklater.

Further readtng: Harte. B., Patt1son. D .R.M. & L1nklater, C.M. ( 1991): Field relations and petrography of part1ally melted pehte and sem1-pelit1c rocks. In Equilibnum 1n Contact Metamorphism: the Ballachuhsh Igneous Complex and 1ts Aureole (G. Voll, j. Topel, D.R.M. Patt1son & F. Seifert, eds.) . Spnnger-Verlag, He1delberg. Germany (181-209).

Atlas of Migmatites

----------------------~-------275

Figure

30

•

Fig. F30. This photomicrograph is from a sample of nebulitic metatexit e migmatite developed from a semipelit ic protolith. It shows quartz grains 1n a matrix of K-feldspar.

The K-feldspar is highly altered ; it appears dark and streaky. The shapes of the quartz grains range from roundish to subhedral, and there are small segments of t he grains that are possibly crystal faces. The quartz is interpreted to be principally rounded, or corroded, res1dual grains formed by the partial melting of quartz-rich rocks. The flat-faced segments on the quartz crystals are considered to be portions that grew later as the melt crystallized. T he original crystal of K-feldspar had a morphology interpreted as interstitial, and is considered to have crystallized largely from the melt, which formerly occupied the pore space. Location : Ballachulish Igneous Complex, Scotland. Rock type : nebulitic metatex1te migmatite; semipelitic protolith. Scale: the width of the photograph is 1.5 mm. Cross-polarized light. Image and caption: Ben Harte and Claire M. Linklater. Further readmg: same as Fig. F29.

DEEPER CONTACT- AUREOLES :

------ ------ ------276--- ------THE BA LLAC H ULI SH IGNEOUS CO MPLEX Figure

3I

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.,

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.... Q tz

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Fig. FJI . T h1s 1mage shows microstructures typ1cal of the Appin Quartz1te (wh1ch actually conta1ns abundant feldspar (Fsp). and so may be better described as an arkose) from close to the intrusive body. which has melted almost completely. The quartz gra1ns are recogn1zed by thew clanty. or transparency, and are roundish, whereas 1n contrast, the feldspar gra1ns are slightly cloudy, or turb1d, and have highly cuspate forms; the feldspar- quartz dihedral angles are low. The feldspar is typical of that grown in melt pocket s. and its shape suggests that it has replaced the melt. Locat1on: Ballachulish Igneous Complex, Scotland. Rock type: part1ally melted App1n Quartz1te one meter from the contact. T ca. 775°C, P 3 kbar. Scale: the field of view is 0.9 mm w1de. Cross-polarized light. Image and capt1on: Marian Holness.

Further readmg: Holness, M.B. & Clemens, J.D. (1999): Partial melt ing of t he Appin quartzit e driven by fracturecontrolled H 20 Infiltration in the aureole of the Ballachulish Igneous Complex, Scottish H1ghlands. Contnbuuons to Mineralogy and Petrology 136. 154- 168. Holness. M. B. & Watt G.R. (200 1): Q uartz recryst allisat ion and fiuid now during contact metamorphism: a cathodoluminescence study. Geoflwds I , 2 15- 228.

~

..

' ,,

A ria; of Mig matites

---------------- --------------277

:12

Figure

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.

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Fig. F32. T he quartz grains in this photomicrograph have undergone sign1ficant growth. The ti ny, triangular grains of feldspar (Fsp) that used to be located at three-gra1n JUnctions are now entwely enclosed w1th1n a s1ngle gra1n of quartz (Qtz). The feldspar (turb1d) IS h1ghly cuspate 1n shape, and has some elongate, tapenng extens1ons along the boundanes between the adjacent gra1ns. Location: Ballachulish Igneous Complex, Scotland. Rock type: partially melted App1n Quartzite I m from the contact, T ca. 775°C, p 3 kbar. Scale: the field of VIeW IS 0.9 mm Wide. Cross-polanzed light. Image and captton: Manan Holness. Further readmg: see Fig. F31.

REG IONAL MIGMATITE TERRANES:

--------- --278 --------- --------THE ASHUANIPI SUBPROVINCE Figure

i3

Fig. F33 . Th1s sample contains irregularly shaped doma1ns of K-feldspar (domains of quartz and plagioclase are rarer) that are up to four t1mes the s1ze of the equant. polygonalshaped groundmass minerals, quartz, and plag1oclase and bladed biotite. The largest domains are shown in orange for clanty, but there are others of different colors as well. Typ1cally, the Irregularly shaped doma1ns have cuspate outlines, because the adjacent m1nerals are rounded owing to dissolution; Mehnert et al. (1973) produced very similar microstructures in part1al-melting expenments. In places, th1n, tapered extensions from the doma1ns follow the gra1n boundanes between adJacent minerals. The microstructure present at the t1me of partial melting thus is preserved and d1d not undergo textural re-equilibrat1on e1ther when melt was still present or subsequently in the subsolidus state (e.g., Holness et al. 2005, see Further reading below). Most of the cuspate domains conta1n rounded Inclusions of quartz, plag1oclase, and b1otite. An orthopyroxene crystal located 1n the center of the photomicrograph has welldeveloped, rational faces where it is in contact with a K-feldspar domain. Th1s microstructure is interpreted to 1nd1cate a s1te where part1al melt1ng occurred by the reaction: biotite+ quartz+ plagioclase= melt+ orthopyroxene + Fe-oxide. The distribution of melt in this rock was, therefore. controlled by where the melt1ng react1on occurred, 1.e., by the locat1on of b1ot1te in the rock. Consequently, the former pools of melt are, 1n general, oriented parallel to the

foliation. In other places, thin films of m1nerals representing former melt occur along grain boundaries and at grain JUnctions. From po1nt counting and a mass-balance calculation, th1s particular rock is Interpreted to have a residual bulk composition resulting from the loss of 15.8% melt, but 2.1% melt was retained in the pools, grain boundaries, and grain JUnctions. The microstructures that 1nd1cate the former presence of melt in this rock are unusually large (up to I mm, or four times the grain size) for migmatites formed m a reg1onal metamorphic terrane; more typical examples are shown 1n Figs. F35 and F36.

Locat1on: Ashuanipi Subprovince, Quebec, Canada. Rock type : residual part of a metatexite m1gmat1te; metagreywacke (psamm1te) protolith, anatex1s 1n the granulite faoes at T 825-875°C, P 6 7 kbar. Scale: the long s1de of the photograph corresponds to 3.5 mm. Cross-polarized light and quartz accessory plate 1nserted. Image: E.W. Sawyer.

Further reading: Holness, M .B., Cheadle, M.J. & McKenz1e, D. (2005): On the use of changes in dihedral angle to decode late-stage textural evolut1on 1n cumulates. journal of Petrology 46, 1565- 1583. Sawyer, E.W. (2001): Melt segregation in the continental crust: distribution and movement of melt 1n anatect1c rocks. journal of MetamorphiC Geology 19, 291 - 309.

A tl as of Migmatites

----------------- -------------- 279 Figure

4

Fig. F34. Same image as shown in F1g. F33, but in crosspolarized light without an accessory plate. The microstructures indicative of the former presence of melt are much harder to see w1thout the aid of the quartz accessory plate. Using the accessory plate confers a cons1derable advantage m distinguishing, and hence 1dentify1ng, microstructures that conta1n d1fferently onented gra1ns of m1nerals that all have low, first-order interference-colors, such as quartz, plagioclase, and K-feldspar.

Location: Ashuanip1 Subprovince, Quebec, Canada. Rock type: residual part of a metatex1te m1gmatite; metagreywacke (psamm1te) protolith. anatex1s 1n the granulite facies at T 825 875°C. P 6-7 kbar. Scale: the long s1de of the photograph corresponds to 3.5 mm. Cross-polanzed light. /mage: E.W. Sawyer.

280 __________________R~E~G~IO~N--A~L~M~IG~M~A~T~IT~E~T~E~R~R~A~N~E~S: THE ASHUANIPI SUBPROVI NCE

Figure

J5

Fig. F35 . Th1s res1dual metagreywacke conta1ns a few scattered 1rregularly shaped doma1ns of K-feldspar, quartz and plag1oclase in contact w1th rounded quartz, plag1oclase, and espeCially biotite. The doma1ns are very small (<0.15 mm), comparable to the grain s1ze, and not easy to find. The ones that are most obv1ous in th1s 1mage are orange 1n color, but others are blue or violet; some are indicated by arrows. The domains are Interpreted as the sites where partial melting occurred via the reaction: biotite + quartz + melt + orthopyroxene + Fe-ox1de. The plag1oclase domains of former melt 1n th1s rock are much smaller, relat1ve to the grain size of the matrix, than those 1n Figs.

=

F33 and F34, and have a geometry that IS more suggestive of films of melt located along gram boundanes, rather than discrete pools of melt 1n the matnx. The distnbut1on of the former films of melt 1s strongly linked to the location of b1ot1te, the hydrous reactant mineral present 1n the rock; hence, melt formed preferentially 1n the plane of the foliat1on .

Location: Ashuan1p1 Subprov1nce, Quebec. Canada. Rock type: res1dual part of a metatex1te migmat1te;

metagreywacke (psammite) protolith. anatex1s 1n the granulite faCies at T 825 875°C, P 6 7 kbar. Scale: the w1dth of the photograph corresponds to 4.0 mm. Cross-polarized light plus quartz accessory plate. /mage: E.W. Sawyer. Further reading: Guernina, S. & Sawyer, E.W . (2003) : Largescale melt-depletion 1n granulite terranes: an example from the Archaean Ashuan1pi subprov1nce of Quebec. Journal of Metamorphic Geology 21 , 18 1 201.

Atlas of Migmatites

------------------------------281 Figure

16

•

Fig. F36 . The thin sect1on shown 1n th1s photomicrograph has been cut parallel to the foliat1on plane to show the shape of the former pools of melt m the plane of the foliat1on. The center of th1s image shows several Irregularly shaped, cuspate minerals between t he rounded matrix grains of biotite (Bt), quartz (Qtz), and plag1oclase (PI). T here is no orthopyroxene in this photomicrograph, but it occurs elsewhere in t he thin section. Interstitial films of K-feldspar, plagioclase, and quartz are identified by arrows. The rounded outline of the quartz, biotite, and plagioclase gra1ns near the K-feldspar is interpreted to be the result of dissolution during the melt-produc1ng metamorphic react1on. On the bas1s of th1s interpretation and the presence of orthopyroxene elsewhere 1n the th1n sect1on. part1al melt1ng 1s 1nferred to have occurred v1a the react1on: b1ot1te + quartz + plag1oclase melt + orthopyroxene + Fe ox1de. K-feldspar does not occur on the mutual boundanes between adjacent grains of plag1oclase or between adJacent gra1ns of quartz; 1t occurs adJacent to b1ot1te. Consequently, the irregularly shaped grains are Interpreted to have crystallized from anatectic melt that formed at triple-grain junctions, or along gra1n boundaries adJacent to the grains of reactant biotite. The distnbution of melt in the foliation plane of this rock is thus controlled by where the melting reaction occurred. The K-feldspar has a

=

cuspate outline and thm tapered proJeCtions along the gra1n boundanes between adJacent res1dual m1nerals. The microstructure that developed at the t1me of part1al melting thus has not been mod1fied to any Significant degree subsequently. The presence of 1ntergranular K-feldspar. quartz, and plagioclase suggests that the anatectic melt that crystallized might have had a granit ic bulk composition, and this IS broadly confi rmed by det ermining the mineral modes of t he intergranular patches by point count1ng. Location: Ashuanipi Subprovince, Canada. Rock type: res1dual part of a metatex1te m1gmatite; metagreywacke (psammite) protolith, anatex1s 1n the granulite fac1es. T 825 875°C. P 6 7 kbar. Scale: the w1dth of the photograph corresponds to 2.5 mm. Cross-polanzed light plus quartz accessory plate. /mage: E.W. Sawyer.

Further readmg: see Figs. F33 and F35.

282

S: N~E~ A~ R~ R~ E~T~E~ T~ ~T~I~ ~A IG~M M~ ~I~O~N_A_L~ EG R~ ----------------~

THE OPATICA SUBPROVINCE

Figure

17

=

melt. and proquartz + plagioclase + K-feldspar + H 20 Fig. F37. This is a photomi crograph of a biotite quartzd. consume was r K-feldspa gressed unt1l all the reactant plagioclase leucoton ahte that underwe nt part1al melt1ng and lost most of the melt fraction that was generate d. In the Locot1on: Opatica Subprovi nce. Quebec, Canada. Rock type : upper left quadrant , a large purple, rounded gra1n of quartz melt-dep leted res1duum: leucoton alite protolith . anatex1s at lies 1n a group of large crystals of plag1oclase; the quartz T ca. 750oC. P 5- 7 kbar. Scale: the width of the photogra ph 1s enclosed by a th1n film of K-feldspar. wh1ch separate s 1t correspo nds to 4 mm. Cross-po lanzed light plus quartz from d1rect contact w1th the ne1ghbonng gra1ns of plagioaccessor y plate. /mage: E.W. Sawyer. clase. The K-feldspar 1n th1s rock occurs only as films such as th1s between quartz and plagioclase; it does not form large Further readmg: Sawyer. E.W. (1998): Formatio n and evosubhedra l crystals like quartz and plag1oclase. Furtherm ore, lution of gran1te magmas dunng crustal rework1ng: the the distributi on of K-feldspar 1s not restncte d to the VICins1gn1ficance of d1atex1tes. journal of Petrology 39, 11 47 1167. ity of biot1te. The lower nght quadrant of th1s 1m age shows Sawyer, E.W. (2001): Melt segregat1on 1n the continen tal rounded (i.e.. corroded ) crystals of quartz (yellow1sh) crust: d1stribution and moveme nt of melt 1n anatectiC rocks. and plagioclase (blue) that are separate d by a thin film of tw1nned plag1oclase (orange yellow). This microstr ucture journal of MetamorphiC Geology 19. 291 - 309. is very much like that shown in many quenche d partialmelting expenme nts 1n wh1ch a thm film of glass separate s the corroded reactant phases. In this case, the plane of the th1n sect1on cuts through the part of the melt pocket that crystallized to plagioclase. Biotite does not appear to be corroded 1n th1s rock. or to be spat1ally assoc1ated w1th the K-feldspar films, wh1ch suggests that the melt1ng reaction may not have Included biot1te as a reactant. Melt1ng poss1bly occurred by a flu1d-present react1on such as

A tlas o f Mig ma tit es

------------------------------~ 283

Fig. F38. Th1s photomicrograph shows a th1n, threebranched film of K-feldspar locat ed along the gra1n boundary between rounded (corroded) gra1ns of quartz and plagioclase 1n a b1ot1te quartz-plagioclase leucotonalite. To the left, a smaller film of K-feldspar encloses a rounded gra1n of plag1oclase that is still in optical continu1ty with a larger grain of plagioclase nearby. This microstruct ure resembles those found in quenched partial-melt ing expenments as well. and is int erpreted as evidence for part1al melting. Considering t he whole t hin sect ion, t he fi lms of K-feldspar occur between matrix grains of plagioclase and quartz, and not w1th biotite. Hence, the melting may have occurred by a react1on such as quartz + plagioclase + K-feldspar + H 10 melt. Films of plag1oclase and quartz are rare: 1n general, the gram boundanes between the crystals of residual quartz and plag1oclase contain small rounded grams of quartz (indicated with an arrow) and plag1oclase. These small gra1ns are Interpreted to be the rema1ns of melt films that have subsequently recryst allized. Thus, the distribution of films of K-feldspar and the small recrystallized gra1ns of quartz and plag1oclase can be used to 1nfer t he former dlstnbution of melt in the rock (e.g., Sawyer 200 I).

=

Location: Opat1ca Subprovince, Canada. Rock type: meltdepleted res1duum. leucotonahte protolith. T ca. 750°C, P S- 7 kbar. Scale: t he 1mage IS 3 mm w1de. Cross-polanzed light plus the quartz accessory plate. /mage: E.W. Sawyer.

MICROSTRUCTURES IN RES IDUAL ROCKS

284 ----------- ----------- ------Figure

39

Fig. F39. Th1s photomicrograph shows some of the features typ1cally seen 1n the res1dual rocks adjacent to areas of leucosome. At the out crop scale, t h1s m1gmat1te conta1ns narrow(< I em), parallel bands of leucosome t hat, toget her w1th the1r adjacent domains of melanosome, are onented parallel to the bedd1ng, result1ng 1n a stromatic metatex1te m1gmat1te. The 1mage shows the full w1dth of the melano -

those nearest the leucosome, are part1ally enclosed by coarse-grained b1ot1te, wh1ch suggests that b1ot1te has part1ally replaced garnet. The only occurrence of cordiente (slight ly yellow because of alteration) in t he melanosome 1S 1n, and around, a shallow embayment in t he largest crystal of garnet close to the leucosome: the s1gn1ficance of th1s relat1onsh1p 1s unclear. There is no silhmamte, or K-feldspar,

some: t he leucosome (plag1oclase + quartz + K-feldspar) is just visible at right edge of the image, and the plagio clase + quartz + b1ot1te + garnet paleosome at the other. The modal proportions of quartz and plag1oclase (colorless m1nerals) decrease in a systematiC fashion throughout the melanosome toward t he leucosome, and concom1tant with th is, t here is an 1ncrease in the modal proport1on of

1n the melanosome: the part1al-melt1ng reaction may have been biotite + sillimanite + quartz + plagioclase + H 20 melt + garnet. Biot1te and cor d1ente could both result from 1nteract1on between the residual crystals and the rema1nmg anatectiC melt 1n the melanosome following the m1grat1on

biotite. Moreover, the average grain-s1ze of b1otlte 1n the melanosome Increases toward the leucosome (1.e., from left to nght across the photomicrograph). In contrast, the gra1n size of quartz and plagioclase remains approx1mat ely constant, except for t he domains where biotite is especially abundant: t here, quartz and plag1oclase crystals are smaller. Garnet IS un1formly distributed 1n th1s part1cular melanosome, although the largest gra1n IS located closest to the leucosome. Garnet grains close to t he leucosome contain small, rounded inclusions of biotite, a feature absent from garnet 1n the paleosome. Many gra1ns of garnet, espec1ally

=

of the melt to t he leucosome.

Locauon: Nemiscau Subprovince, Q uebec, Canada. Rock type: melanosome from a stromatiC metatexite m1gmat1te: sem1pelit1c protohth, upper amphibolite or lower granulite faoes (no orthopyroxene). Scale: t he long s1de of the photomicrograph corresponds to 12 mm. Plane-polarized light. Image: E.W . Sawyer.

Further readmg: Wh1te, R.W. & Powell, R. (2002): Melt loss and the preservation of granulite fac1es mineral assemblages. journal of Metamorphic Geology 20, 621 632.

Atlm, of Migmatite;

---------------- --------------285 Figure

10

Fig. F40. T his phot omicrograph shows the same field of view as Fig. F39, but is taken in cross-polarized light to better show the vanation 1n the gra1n s1ze of quartz and plag1oclase. Locat1on: Nem1scau Subprovince, Quebec, Canada. Rock type: melanosome 1n a stromat1c metatex1te m1gmat1te; semipelitic protol1th, upper amphibolite or lower granulite facies (no orthopyroxene). Scale; the long s1de of the photomicrograph corresponds to 12 mm. Cross-polanzed light. Image; E.W. Sawyer.

MICROSTRUC TURES IN RES I DUAL ROCKS

286 ------- ------- ------- ------- Figure

41

.,.....

•

• Fig. F41 . The edge of the leucosome in this photomicrograph IS clearly defined by the abrupt change in the modal abundance of fer romagnesian minerals and by the change in the grain s1ze of quartz and feldspar; the contact IS comparatively planar. In outcrop, the migmatite cons1sts of only two parts. the leucosome representing the products denved from anatectiC melt and the res1duum; there IS no paleosome. The areas of leucosome are not bordered by a conspicuous melanosome; rather, the melt-depleted region complementa ry to the leucosome is the entire layer hostIng the leucosome, and such layers can be as much as 50 em wide. The melt-producing reaction 1n this migmatite was b1otite + quartz + plag1oclase melt + orthopyroxen e +

=

ilmenite (or magnet1te). These layers are remarkably un1form 1n chemical composition, mineralogy. and microstructure . They have melt-depleted maJor- and trace-element compositions, consist of the assemblage plag1oclase + b1otite + orthopyroxen e + quartz + ilmenite, and locally contain

the plagioclase + quartz + K-feldspar leucosome is smaller at its marg1n compared to the intenor, possibly because the plagioclase crystallizing from the melt at the margins of the leucosome was able to nucleate on plagioclase crystals 1n the wallrock. Hence, there are more, but smaller, crystals of plagioclase at the margins of the leucosome. The neosome shown 1n this photomicrogra ph is typ1cal of the leucosome residuum relat1onsh1p in the metatex1te migmatites denved from aluminum-poo r metagreywackes in the Ashuanipi Subprov1nce; orthopyroxen e is the green1sh grey to pinkish mineral w1th h1gh relief

Location: Ashuan1pi Subprov1nce. Quebec, Canada. Rock type: metatex1te m1gmat1te; alum1num-poor metagreywacke protolith, granulite facies. T 825- 875°C, P 6 7 kbar. Scale: the width of photograph corresponds to 13.5 mm. Planepolanzed light. Image: from a thin sect1on lent by Robert Theriault.

microstructures ind1cat1ve of the former presence of melt (see Figs. F33 F36). The leucosome in the photomicrogra ph IS not bordered by melanosome that becomes progresSively more enriched 1n ferromagnes1an m1nerals toward 1t. However, the photomicrogra ph (see F1g. F42 for the same

Further readmg: Guemna, S. & Sawyer, E.W. (2003) : Largescale melt-depletion 1n granulite terranes: an example from the Archaean Ashuanipi subprovince of Quebec. journal of Metamorphic Geology 21 , 181 -20 I.

th1n section in cross-polarized light) shows that there is a very slight 1ncrease 1n the gra1n s1ze of plag1oclase Immediately adjacent to the leucosome and that the modal content of quartz is lowest next to the leucosome. The gra1n size of

Sawyer, E.W. (2001) : Melt segregation in the continental crust: d1stribut1on and movement of melt in anatectiC rocks. journal ofMetamorph1c Geology 19, 291 309.

A tl as of Migmat ites

------------------------------ 287 Figure

42

Fig. F42 . The photomicrograph shows the same field of view as Fig. F41. but 1n cross-polanzed light to show the microstructures 1ndicative of recovery and recrystallization 1n the quartz. Gra1n-boundary readjustments 1n the quartz and feldspar dunng and after deformat1on have mod1fied some of the pnmary gra1n-boundary relat1onsh1ps (e.g.. stra1ght faces on plagioclase aga1nst quartz) formed when this leucosome crystallized from anatectic melt. Location: Ashuanipi Subprovince, Quebec, Canada. Rock type: metatex1te migmat1te; alum1num-poor metagreywacke protol1th. granulite fac1es. T 825-875°C, P 6 7 kbar. Scale: the w1dth of the photograph corresponds to 13.5 mm. Cross-polanzed light. Image: From a th1n sect1on lent by Robert Thenault.

MIC ROSTRU CTURE S IN RES IDUAL ROC KS

288 ------ ------ ------ ------ ------ Figure

:1.3

Fig. F43. Th1s photomicrograph IS from a meter-w1de layer of Archean pelit1c sch1st. The pelite layer contains a few small doma1ns of leucosome that are located 1n small shear bands and 1n irregular dilatant sites that are oriented approx1mately orthogonal to the layering. The borders of the leucosome domains are diffuse, and t hey do not have a consp1cuous melanosome around them. If compared to the average of 30 lower-grade pelit1c sch1sts from t he same sequence, the whole-rock major- and trace-element compos1tion of the peht1c sch1st can be 1nterpreted as being res1dual after the loss of some anatectic melt. The textural relationships among the m1nerals 1n th1s rock confirm its residual nature. The m1neral assemblage IS plag1oclase (PI) + b1otite + quartz + cordierite + K-feldspar + ilmen1te; s1111man1te 1s present, but IS not part of the paragenesis. Cordierite (Crd) forms porphyroblasts up to 7 mm long that are elongate 1n the plane of the foliat1on: most of the porphyroblasts conta1n ac1cular inclus1ons of Sillimanite and small rounded inclusions of quartz and of b1ot1te. However, the b10t1te. quartz, and sdliman1te Inclusions are never in mutual contact; t hey are separat ed by cordierite. Some mclus1ons of quartz conta1n small needles of sillimanite. Silliman1te does not occur in the sch1st outside the cord1ente poikiloblasts. Th1s spatial relat1onsh1p among t he m1nerals suggests that the melt-producing react 1on

=

melt + was b1ot1te + silliman1te + quartz + plag1oclase cordierite the of Most Ilmenite. + cord1ente + K-feldspar is fresh, w1th only very m1nor alterat1on to chlonte at the ends of some porphyroblasts; the removal of t he anatec· t1c melt and the H, 0 1t conta1ned effectively dehydrated the res1duum so that very lit tle rehydration of the cordierite could occur as metamorphic temperatures declined beyond the solidus. M1nor amounts of texturally late muscovite (Ms) occur 1n the schist; it may be the product of late, near-solidus react1on of cord1ente + K-feldspar w1th t he last rema1ning, Interstitial anatectiC melt.

Locat1on: Quet1co Subprov1nce, Ontario, Canada. Rock type : residuum in a metatex1te m1gmatite: pelit ic protolith, T 700- 800°C, P 3 4 kbar: Scale: the long s1de of the photomicrograph corresponds t o 2.5 mm. Cross-polarized light. Image: E.W. Sawyer. Further reading: Sawyer, E.W. (1987): The role of part1al melt1ng and fractional crystallization 1n determin1ng discordant m1gmat1te leucosome compos1t 1ons. journal of Petrology 28, 445-473.

A d a> of Migma tites

----------- ---------- ------- 289 Figure

44

Fig. F44. In outcrop. the cord1ente + K-feldspar + biotite + quartz pelit1c sch1st from wh1ch th1s sample was collected contains scattered domains of K-feldspar + quartz + plagioclase leucosome oriented parallel to the foliation. Smaller but compositionally s1milar domains occur in extens1onal shear bands. Because the leucosome does not amount to more than I 0% of the outcrop, the two onentatlons create a very open, net-like array of leucosome 1n outcrop. This photomicrograph is from a sample of pelitic sch1st collected from between the net-like array of leucosome doma1ns. Both cordiente (Crd) and K-feldspar (Kfs) form abundant (20 modal % each) rounded po1k1loblasts that conta1n abundant Inclusions of biotite and round. corroded quartz. The foliat1on 1n the sch1st is defined by the preferred onentat1on of b1otite. and wraps around the cord1ente + K-feldspar domains. creating an augenlike microstructure; some penetrative deformat1on thus occurred in this migmatlte after the poikiloblasts had formed. Sillimanite 1s not present in th1s particular sample, but 1n the adJacent schist. a small proportion of the cordierite poik1loblasts conta1ns aggregates of sillimanite needles that outline small crenulation-type folds, but the sillimanite inclusions are not 1n contact with quartz and biotite. also present as inclusions in the cord1ente. Hence, a possible melt-produc1ng reaction for the m1gmat1te 1n the photomicrograph was b1ot1te + s1lliman1te + quartz melt + cordiente + K-feldspar. Because cord1ente IS w1dely

=

preserved 1n th1s res1dual rock and not replaced by b1ot1te. muscovite. or sill1mamte. H,0 generated from the dehydration melt1ng of reactant b1otite must have left the res1duum 1n the anatectic melt. Hence, from its mineralogy and the lack of retrograde rehydrat1on. this sample of m1gmat1te 1s Interpreted to be melt-depleted res1duum . Location: Central Zone of the Damara Belt, Namib1a. Rock type: res1duum in a metatexite migmatite: plagioclase-poor. alum1nous pelite protolith. anatex1s 1n the lower granulite fac1es, T ca. 800oC. P 3 4 kbar. Scale: the long side of the 1mage corresponds to 5.5 mm. Cross-polanzed light. Image: E.W . Sawyer:

Further readmg: P1ckering. J.M. & Johnston. A.D. (1998): Flu1d-absent melt1ng behav1or of a two-m1ca metapeiltes: experimental constra1nts on the on gin of Black Hills granite. journal of Petrology 39, 1787 1804. Wh1te. R.W. & Powell. R. (2002): Melt loss and the preservation of granulite facies mmeral assemblages. journal of Metomorphtc Geology 20. 621-632. Wh1te. R.W., Powell, R. & Holland, T.B J. (2001): Calculation of part1al melt equ1libna in the system Na 20 - Ca0 K 0 FeO MgO- AI,Q SiO H.O (NCKFMASH). Journal of Metomorphtc Geology 19. 139 153.

MICRO STRUCTURES IN RESIDUAL ROCKS

290 ------- ------- ------- ------- --Figure

's

•

•

•

+ Fig. F45 . Th1s photomicrograph shows a h1ghly residual, granulite-faoes rock that 1n outcrop occurs as th1ck, lightcolored layers and as 1solated boudins in syn-anatectic shear zones. Ne1ther mode of occurrence has many spat ially assooated doma1ns of leucosome. Th1s sample was collected > 20 em from the nearest domain of leucosome. The m1neral assemblage 1s cordiente + orthopyroxen e + biot1te + silliman1te + sp1nel. Cordiente IS easily Identified by 1ts prominent, yellow pleochroic halos, and the orthopyroxene, by its h1gh relief A few t1ny, isolated. equant gra1ns of quartz also are present in some of the b1otite aggregates. but quartz represents no more than t race amounts. B10t1te occurs as large indiv1dual gra1ns. or as aggregates generally 1nterst1t1al to the cord1ente and part1ally enclosing some orthopyroxene grains. Very small round 1nclus1ons of b1ot1te occur 1n the cordiente: 1n contrast. the inclusions of b1ot1te m the orthopyroxene are larger and more Irregular 1n shape. However, both types are interpreted as relics of reactant b1ot1te. The photomicrograph shows t hat some cordiente-cord1erite and cordierite orthopyroxen e boundaries are almost black. This coloration reflects the presence of various very fine-gra1ned m1nerals. Green hercyn1te occur s as a dust ing of tiny crystals along some orthopyroxen e cordient e contacts (A) and, rarely, as tiny aggregates in cord1ente: the sp1nel IS poss1bly a product of quartz-absent prograde reactions. Aggregates of very small elongate crystals of biotite occur along some

cordiente and orthopyroxen e grain boundanes; these are commonly onented at a h1gh angle to the gra1n edges and might be a late alteration of cord1erite and orthopyroxene. There are also cord1ente cordiente boundanes that are dark brown (B): th1s IS due to a very fine mtergrowth of biot1te and sillimanite in which the orientation of the t1ny aocular crystals 1s controlled by the crystallographic onentat1on of the cord1ente. and not the onentat1on of the grain boundary. Melt 1ng involv1ng biotite, quartz, sillimamte. and plagioclase IS Interpreted to have proceeded to the extent that quartz, sillimanite, and plagioclase were all consumed, leaving a residuum dominated by cordiente + orthopyroxene. Some b1ot1te may be res1dual. but the aggregates of large gra1ns of biotite assoc1ated with the quartz blebs. the aggregates of small grains of biotite. and the b1ot1te s1lliman1te Intergrowths are all Interpreted as the result of a react1on between the res1dual mmerals cord1erite and orthopyroxene, and a small amount of mtergranular melt left on the gra1n boundanes after VIrtually all of the melt generated by anatex1s had migrated away and contributed to the leucosome elsewhere in the m1gmat1te.

LocatiOn: Wuluma Hills, Arunta Inlier, Australia. Rock type: res1duum 1n a metatex1te m1gmat1te; aluminous metapelite protolith, granulite-faoes anatex1s, T 825 875°C, P ca. 5 kbar. Scale: the long side of the image corresponds to 7 mm. Plane-polanzed light. Image: E.W. Sawyer.

• +

• + +

+

• •

A tlas of Migmatites

- - - - - - - - - - - - - - - - 291 Figure

6

~

~

• •

•

• •

Fig. F46. Rocks that have undergone a high degree of partial melting and loss of melt are dist1nCt1ve because of their very h1gh modal proport1on of residual m1nerals. The extremely melt-depleted nature of the pelit1c migmatite 1n th1s photomicrograph is indicat ed by the abundance of large, zoned crystals of garnet . The colorless (white) inter-

•

stit ial areas between the garnet crystals consist of quartz, K-feldspar, and plagioclase (An10 40 ), and are interpreted to be derived from t he crystallization of anatectic melt that was interstitial to the garnet crystals. Large crystals of biotite partially replace the garnet , and they may have formed

• •

partial melting. The h1ghly unusual field-relat1ons of this particular rock are examined in F1g. B33. Location: Abaukoma, Yaounde, Cameroon. Rock type: residuu m in a met atexite migmatite; metapelitic prot olith, anatexis at T ca. 850oe, P I 0- 12 kbar. Scale: the long side of the photograph corresponds to 25 mm. Plane-polarized light. Image: Pierre Barbey.

Further reading: Barbey,

P.,

Macaudiere,

J.

& N zenti,

J.P.

( 1990): High-pressure dehydrat ion melt1ng of metapelit es: ev1dence from migmatites of Yaounde (Cameroon). journal

by react1on between the residual garnet and the small amount of anatectic melt trapped between the garnet crys-

of Petrology 31 ,401-427.

tals. The rock also has a high modal abundance of accessory mmerals: th1s m1ght also be cons1stent with a h1ghly residual nature. Geochem1cal mass-balance calculations based

Nzent1, J.P., Barbey, P., Macaudiere, J. & Soba, J. (1988): Ong1n and evolution of the late Precambnan h1gh-grade Yaounde gneisses. Precambnan Research 38, 91 109.

on the whole-rock composition of th1s rock and a plausible pelitic protol1th are consistent with the garnet-nch rock be1ng the melt-depleted residuum from a pelitiC protolith that has undergone extensive part1al melting, and has lost at least 50 vol.% anatectic melt. However, 1t is not clear whether such extensive partial melting occurred by dehydration melt1ng of biotite at metamorphic temperatures in excess o f 850°C, or whet her it occurred at somewhat lower temperatures (750 800°C) through H 20-fluxed

292

C RYSTALLIZATION- INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

47

Fig. F47. Mount Stafford rs located rn the Proterozorc Arunta Inlier of central Australia. The area around Mount Stafford consists largely of Paleoproterozorc metasedrmentary rocks rn which a sequence of five metamorphrc zones that range from the greenschist to t he granulite facies have been mapped. The area rs intnguing because granulite-faoes anatexrs has occurred at shallow levels rn the crust, correspondrng to pressures of about 3.2 kbar. and has many features that suggest contact metamorphism. For example, partial melting in the interbedded metapsammites, metasiltstones (protolith for cordiente granofels) and alumrnous metapelites has, in places (zones 3 and 4), occurred without segregatron of the melt fractron from the solid fractron and generated some unusual migmatites (see Figs. AI and Bl7) that have essentrally rsotropic neosome, r.e., wrthout foliatron or magmatrc flow-rnduced structures. However. leucosome is developed in some migmatites. An optically contrnuous quartz crystal (light grey) encloses euhedral crystals of K-feldspar withrn a leucosome; thrs microstructure rs typrcal of the crystallization of a granrtrc magma. The K-feldspar has perfectly developed crystal faces against the interstitial quartz; contacts between K-feldspar crystals are less well developed; however. they are, rn general, also planar: N umerous

grain boundaries, rs very unusual for a regronal metamorphic terrane, and illustrates that the migmatites at Mt. Stafford crystallized rn an envrronment of low strarn. Location: Mt. Stafford, Arunta Inlier; Australia. Rock type: leucosome; semipelitrc protolith, anatexis at T 780- 810°C. P 3.5 4 kbar: Scale: the long srde of the image corresponds to 3 mm. Cross-polanzed light. /mage and captron: Rrchard Whrte. This rmage was published as fig. 2g in W hite et al. (2003) and is reproduced wrth the permrssron of Oxford University Press.

Further readrng: Greenfield, j.E., Clarke, G.L., Bland, M. & Clarke, D.L. (1996): In srtu migmatite and hybrid diatexite at Mt. Stafford, central Australia. joumal of Metamorphrc Geology 14, 413 426. Vernon, R.H., Clarke, G .L. & Collins, W.J. ( 1990): Local, mrd-crustal granulite facies metamorphism and meltrng: an example from the Mount Stafford area, central Australia. In H igh Temperature Metamorphism and Crustal AnateXIS u.R. Ashworth & M. Brown, eds.). Mineralogical Society, Series 2. Unwrn Hyman, London, U.K. (272-319).

small inclusions of quartz occur in the core of many crystals of K-feldspar: Most probably the intenor of the K-feldspar grains formed dunng the subsohdus breakdown of coexrsting musco-

W hrte, R.W., Powell, R. & Clarke, G.L. (2003): Prograde metamorphic assemblage evolution during partial melting of metasedrmentary rocks at low pressures: mrgmatrtes from Mt.

vite and quartz. The absence of strain features rn the quartz, such as undulose extinction, or evidence for the mobility of

Stafford, central Australia.Jouma/ ofPetrology 44, 1937 1960.

Atlas of Migmatites

------------------------------ 293 Figure

48

Fig. F48. An optically continuous crystal of K-feldspar (light grey, faintly striped mineral) encloses euhedral crystals of quartz (hexagonal outlines) in a leucosome from metamorphic zone 4 at Mt. Stafford. Some of the quartz crystals appear black because they are viewed along the c crystallographic axis and seem, therefore, isotropic. Biotite crystals are bladed in shape, and have some straight faces (i.e., they are subhedral) where in contact with t he K-feldspar. This microstructure is interpreted to indicate that both biotite and quartz crystallized from the anatectic melt forming t he leucosome, and before the K-feldspar crystall ized from the remaining melt.

Location: Mt. Stafford, Arunt a Inlier, Austral ia. Rock type: leucosome; semipelite protolit h, anatexis at T 780 810°C, P 3.5- 4 kbar. Scale: t he lo ng side of the image corresponds to 2.2 mm. Cross-polarized light. Image and caption: Richard White. This image was published as fig. 2h in White et al. (2003) and is reproduced with the permission of Oxford University Press.

Further reading: as for Fig. F47, Greenfield, J.E., C larke, G.L. & White, R.W. (1998): A sequence o f partial melting reactions at Mt. St afford, central Austral ia. Journal of Metamorphic Geology 16, 363- 378

294

CRYSTALLIZA TION-INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

49

F1gure

9

A tl as of Migmatites

------------------------------295

Fig. F49. This mosaic shows an in situ lens of leucosome (about 20 m m long) locat ed bet ween two boudins in a metasedimentary rock of the Biwabik Format ion, from t he foot wall of t he Dulut h Igneous Complex. The host is thinly bedded iron-formation in which t he bedding is manifested by changes in the modal proportions and grain size of magnetite, plagioclase, quartz, and clinopyroxene. T he bedding is oriented subparallel to t he short side of the figures. The leucosome has very irregular margins at t he grain scale, a featu re t hat is common in bodies of in s1tu leucosome. T he central part of the leucosome is characterized by large crystals of plagioclase that contain conspicuous graphic intergrowths of quartz. However, in a few plagioclase crystals, the graphic intergrowth is wit h K-feldspar (microcline) and not quart z. Most of the quartz and plagioclase at the ends of the lens of leucosome occurs as separat e anhedral, equant grains; a graphic intergrowth of quartz in plagioclase t here is very rare, and is also very poorly formed. Because quart z is the more abundant mineral at the ends of t he leucosome domain, and plagioclase more abundant in the center, t here is a com posit ional gradient along t he leucosome. N evertheless, overall it has a granodiorit ic bulk composition. Some of t he magnet it e and clinopyroxene crystals t hat project into t he leucosome from t he wallrocks have cryst al faces against t he plagioclase in t he leucosome. These minerals are t hus interpreted to have grown into a space filled with melt. In contrast. t he opposit e ends of

t hese same crystals have curved and interlocking grain boundaries, consistent w it h growth in a solid, i.e., t he wall rock. The grain size of the crystals in t he wallrock shows a systemat ic incr ease toward t he leucosome. This is most readily seen around t he w idest part of t he leucosome, w here all the minerals show an increase in grain size by a factor of 5 I 0 toward the leucosome. The increase in grain size may occur because t here was anat ectic melt along a significant proportion of t he grain boundaries close to the leucosome, and this enabled a more rapid grainboundary migrat ion, and hence grain coarseni ng to occur t here. Magnet it e crystals that are complet ely enclosed within t he leucosome are eit her euhedral or skeletal in form, whereas those in the wallrocks have irregular or rounded grain-boundaries. The presence of a granophyric microstruct ure, and of crystals w it h skeletal shapes in the leucosome, are bot h features that indicate a rapid to moderat ely rapid rate of cooling for the anat ectic melt in t he contact aureole of the Duluth Igneous Complex. Locat1on: Duluth Igneous Complex, Minnesota, U.S.A. Rock type: m Situ leucosome in a metatexite migmat it e; Fe-oxide-rich metasedimentary protolith, T ca. 900oC. P 1.5-2 kbar. Scale: t he width of t he photograph is 10 mm. Left: cross-polarized light. Right: cross-polarized light plus t he quart z accessory plat e. Image: E.W. Sawyer.

296

CRYSTALLIZAT ION-INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

FSO

Fig. FSO. One of the first macroscop1c s1gns of partial melt1ng in the contact aureole around the Duluth Igneous Complex (DIC) is the development of small (
Locat1on: Duluth Igneous Complex. M1nnesota, U.S.A. Rock type: tn Situ leucosome 1n a patch metatex1te migmat1te; Virgin1a Format1on metapelite, T ca. 750°C, P 1.5 2 kbar. Scale: the w1dth of the photograph IS 9 mm.

Cross-polarized hght. /mage: E.W. Sawyer:

A tla> of Migm at ites

---------------- --------------297 Figure

5I

Fig. F51 . This 1n-source leucosome, from about halfway between the onset of partial melt1ng and the appearance of diatexite m1gmat1tes. IS typ1cal of the larger bod1es of leucosome from the outer, lower-grade part of the contact aureole of the Duluth Igneous Complex. Like most of the leucosome 1n the outer part of the aureole, this one IS 1n an extensional shear zone (see F1g. F88 for an example) and, therefore, IS slightly discordant to the foliation in the fine-grained host, a quartz biotite-plagioclase K-feldspar cordierite pelitic schist. Comparing th1s leucosome with that in Fig. F53 shows that there are s1gn1ficant petrological differences between the outer and 1nner aureoles. The ma1n difference 1S the widespread occurrence of bladed muscov1te (Ms) 1n the leucosome of the outer aureole, and the narrow, dark-colored nm around the leucosome (most clearly evident on the left side) due to alterat ion of cordierite in the adjacent wallrock; the dark rim is not melanosome. Both of these features 1ndicate that the melt from wh1ch the leucosome crystallized 1n the outer aureole was ncher 1n H 0 than the melt in the 1nner aureole. The bulk compos1t1on of the leucosome suggests that 1t is not the product of fractional crystallization. Therefore, the higher H 10 contents may be due to the occurrence of H 20-fluxed melting in t he out er aureole, whereas dehydration melting predominated in the inner aureole. Another

difference is t hat the doma1ns of leucosome in the outer aureole are significantly finer-grained relative to their width than those 1n the 1nner aureole. Consequently, they have a d1fferent microstructure dom1nated by small anhedral gra1ns that have 1rregular gra1n-boundaries.

LocatiOn: Wyman Creek, Duluth Igneous Complex, M1nnesota, U.S.A. Rock type: in-source leucosome, mctatex1te m1gmatite; pelitic protolith, pyroxene hornfels fac1es. T ca. 750°C, P 1.5 2 kbar. Scale: the long s1de of the photomicrograph corresponds to 9 mm. Cross-polanzed light. Image: E.W. Sawyer:

298

CRYSTALLIZATION-INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

Sl

Fig. F52. T his photomicrograph shows part of a small (2 em long) elongate pat ch of m situ leucosome in a residual metapelite that was collected from a xenolith located close to the contact w1th the Duluth Igneous Complex. Consequently, th1s occurrence of leucosome IS from the very h1ghest grade part of the contact aureole. The patch of leucosome contains ev1dence that the melt in it nowed, and hence t hat 1t s elongate shape was acquired before t he leucosome solidified. The melt in the leucosome contained a few scattered, equant crystals of orthopyroxene (h1gh relief), but 1t also contained many small, elongate, euhedral crystals of plagioclase. The plag1oclase crystals locally show a strong alignment that 1s parallel to the long ax1s of the patch of leucosome. This alignment IS Interpreted to have occurred when t he small lens of magma in the pelite was deformed and became elongated. The last part of the microstructure t o form developed when the melt conta1ning the aligned m1nerals crystallized and formed the large xenomorph1c gra1ns of quartz. K-feldspar. and plagioclase that now host the aligned smaller crystals. The lack of any opt1cal ev1dence for crystal-plast1c deformation 1n the last-cryst allized minerals, such as undulose extinct ion or the formation of polygonal subgrains. 1ndicates that the final stages of cryst allization of the melt occurred

after the deformation that elongated the leucosome. The photomicrograph shows the contact between large, late-cryst allized crystals of quartz (blue) and K-feldspar (yellow). Locat1on: Duluth Igneous Complex, M1nnesota. U.S.A. Rock type: m Situ leucosome 1n a res1dual metatex1te m1gmat1te; V1rg1n1a Formation pelite protolith, T > 900°C, P 1.5-2 kbar. Scale: t he w1dt h of the photograph is 2.5 mm. Crosspolarized light and t he quartz plate inserted. Image: E.W. Sawyer.

Atl as o f M igmatites

------------------------------ 299 Figure

53

Fig. FS3. Thts example of leucosome is from the transttion from metatextte to dtatextte mtgmattte. The photomicrograph shows a fine-gratned pelit1c hornfels (upper right) that has a foliatton defined by the onentatton of btotite. The foltatton ts crossed at an angle of about 20° by a 4-mm-wtde, dtscordant domain of leucosome (lower left) . There ts no melanosome next to thts segment of the leucosome, which suggests that the leucosome ts not m sttu. Consequently, it should be called a leucocratic vein. However. as the leucosome can be traced back a few centimeters to where tt becomes parallel to the foltatton and has a melanosome that ts enriched in cordiente, at least that part of the leucosome can be constdered to be tn sttu. Thus, the term leucosome does seem appropriate. The miCrostructure of the leucosome IS domtnated by large. anhedral crystals of quartz (Qtz), whtch extend from the margins of the leucosome to tts tntenor (cf Fig. F51). Close examtnatton shows that the contact of the leucosome appears to be somewhat diffuse. and quartz crystals tn the leucosome can be traced to the wallrock contact, and locally beyond it into the wallrocks whtle maintatntng opttcal continutty. The melt from which the leucosome crystallized contatned abundant small crystals, and these are now preserved as tnclustons in the large crystals of quartz. Abundant large, dark blutsh grey or black (in extinction posttton) equant inclustons tn

the quartz are K-feldspar (Kfs), and these are tnterpreted to have crystallized from the melt. Small, dark, hexagonal crystals wtth sector twtnntng are euhedral cordtente (Crd), and thts too probably grew from the melt. Plagtoclase is abundant and occurs mostly as small anhedral gratns tnterstttial to the large crystals of quartz and K-feldspar. Some plagtoclase exhibits lamellar twtns. Btottte occurs as small rounded tnclusions in quartz and feldspar and as large, randomly oriented bladed crystals: the former are interpreted to be relics of residual btottte, and the latter, as late btottte grown from the melt. There ts no muscovite. The photomtcrograph is not representattve of the leucosome as a whole, as tt ts somewhat richer tn quartz: thts part was selected because it shows a greater range tn tnterference colors, maktng tt easter to disttngUtsh the indivtdual crystals. Locatton: Wetlegs, Duluth Igneous Complex, Mtnnesota, U.S.A. Rock type: leucosome in a metatexite mtgmatite: peltttc protoltth, pyroxene hornfels facies, T ca. 800°C, P 1.5 2 kbar. Scale: the long stde of the photomtcrograph corresponds to 4.5 mm. Cross-polanzed ltght. Image: E.W .

Sawyer.

300

CRYSTALLIZATION-INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

F

Fig. FS4. Th1s photomicrograph shows a K-feldspar-nch leucocrat1c vein 4 mm wtde 1n a parttally melted. sulfidebearing quartz + phlogop1te + plagtoclase + K-feldspar + cordierite alumtnous metasediment. The contacts between the leucocrat1c ve1n and its host are Irregular. possibly because the host was itself partially molten when the ve1n was inJected. The vein has the mtneral assemblage K-feldspar + quartz + plagioclase. with minor amounts of muscovite and tourmaline. The microstructure 1n the leucocratic vein is dominated by equant shapes of crystals, and is different from that in leucosome from lowertemperature mtgmatites (cf Ftgs. F50 and F51). Small crystals of quartz and plagioclase are prevalent at the border, whereas large (2 mm) crystals of K-feldspar are preponderant 1n the center: However, 1n places (e.g.. at the lower left). the center of the vein conta1ns large crystals of muscov1te that are associated with an 1ntergrowth of tourmaline (light grey) and K-feldspar (dark blue grey). Some of the large crystals of K-feldspar 1n the center of the ve1n have straight contacts against quartz. a microstructure generally interpreted as evidence for crystallization from a melt. Leucosome and leucocratic veins of syenogranitic composition are relatively common in the migmat1tes around the Duluth Igneous Complex. Some of t hese are undoubtedly primary anatectic melts derived from plagioclase-poor

pelitic protoliths (see Further reading). However, most are the result of the fract1onal crystallization of the anatectiC gramt1c melts. From the presence of the tourmaline and muscov1te. one can infer that th1s leucocratic ve1n crystallized from an anatectic melt that had an evolved bulk composition, possibly due to the crystallization and fractionation of plagioclase. Location: Wetlegs. Duluth Igneous Complex, Minnesota, U.S.A Rock type : in-source leucosome. sulfide-bearing metatexite migmatite; pelit1c protolith. pyroxene hornfels facies, T 800- 850oC. P 1.5 2 kbar. Scale: the long stde of the photomicrograph corresponds to 9 mm. Crosspolanzed light. /mage: E.W. Sawyer. Further reading: Grant, j.A (2004): Ltqutd compos1tions from low-pressure expenmental melting of pelitic rock from the Morton Pass, Wyomtng. USA journal o( Metamorphtc Geology 22. 65-78.

Atlas of Migma[itc

----------- ------------------- 301 Figure

5

Fig. FSS. Although the mineral assemblage 1n th1s leucocratlc ve1n, K-feldspar + quartz + plag1oclase + muscov1te + tourmaline. is the same as that in Fig. F54, it s microst ruct ure is very different. The const ituent minerals of this vein are generally not equant, and have a far great er t endency to show irregular and embayed shapes. In places, however, cryst al faces are developed where K-feldspar or plag1oclase are against quartz. Tourmaline (white mineral near t he cen ter of the image) is rare and occurs as irregularly shaped cr ystals intergrown with K-feldspar and quartz. Muscovit e occurs as large grains, locally intergrown w1th quartz, and as a minor replacement of K-feldspar. The difference in microstructu re may be due to t he greater width of this vein (15 mm), or to its location farther out in t he aureole, and therefore, its format ion in lower-temperature migmatites. The presence of tourmaline and muscovite 1ndicates that th1s leucocrat ic vein crystallized from a melt that may have had an evolved. or fractionat ed, composition. Locat1on: Wetlegs. Duluth Igneous Complex, Minnesota, U.S.A Rock type: leucocrat ic ve1n in sulfide-bearing metatexite m1gmatite; pelitic protohth, pyroxene hornfels facies, T ca. 750- 800°C, P 1.5- 2 kbar. Scale: the long side of the photomicrograph corresponds t o 9 mm. Cross-

polarized light. /mage: E.W. Sawyer.

302

CRYSTALLIZATION-INDUCED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES: LEUCOSOME AND LEUCOCRATIC VEINS

Figure

56

Fig. F56. This photomicrograph. from a domain of leucosome IS mm wide in a migmatite derived from a plagioclase-rich pelite protolith (similar to that 1n Fig. F43). shows features common to leucosome from many regional migmatite terranes. Equant. subhedral to euhedral crystals of plag1oclase form a framework of touching crystals. The interstices in the framework of feldspar crystals are filled w ith quartz, as in this photograph, or in other cases, by quartz + K-feldspar. Some of t he plagioclase crystals have st raight, rational faces against the quartz, a feat ure that is commonly taken to indicate crystallization from a melt. However, some of the other plag1oclase crystals in the framework have lobate contacts w1th quartz. Straight plagioclase plag1oclase contacts are commonly the result of a planar. rational face on one of the crystals and simple impingement by t he other, but many plagioclaseplagioclase contacts are irregular. The bulk composition of a plagioclase-rich leucosome, such as this one, can commonly be modeled as a plag1oclase cumulate derived by the fractional crystallization of an anatect1c melt of granitic composition. Evidently, if an evolved melt has been expelled in making a cumulate leucosome, such as this one, then other bodies of leucosome (see Fig. F57) or

veins somewhere in the migmatite should be derived from t his evolved. expelled melt and have appropriate mineral assemblages, microstructures, and bulk compositions (see Further reading). Location: Quetico Subprovince, Ontario, Canada. Rock type: 1n-source leucosome w1th plag1oclase cumulate microstructure and composition from a metatexite migmatite: pelit 1c protolith, T 700 - 800°C, P 3 4 kbar. Scale: the long side of the photomicrograph corresponds t o 5.5 mm. Crosspolanzed light. /mage: E.W. Sawyer.

Further readmg: Sawyer, E.W. (1987): The role of part1al melting and fractional crystallization in determining discordant migmatite leucosome compositions. journal o( Petrology 28,445 473.

A tlas of Mig mat ites

------------------------------- 303 Figure

7

Fig. F57. N ot all feldspar frameworks in leucosome con sist of plagioclase. T his phot omicrograph shows another example of leucosome, 25 mm w ide, from the Q uetico Subprovince, but in t his instance, t he feldspar framework is constructed of subhedral and euhedral cryst als of microcline. The interst ices in the microcline framework are filled w ith quartz, and in places, t he microcline has straight crystal faces against t he quartz, evidence t hat its precursor (a disordered feldspar) cryst allized from a melt. This leucosome also contai ns some subhedral and euhedral crystals of plagioclase (not iceably cloudy owing t o alteration to small crystals of w hit e mica) w hich, alt hough smaller t han the microcline cryst als, are nevert heless also part of the feldspar framework in t he leucosome. Plagioclase crystals in the feldspar framework rarely contain inclusions of quartz or K-feldspar, whereas t he K-feldspar crystals in t he framework commonly contain inclusions of bot h quartz and plagioclase. Bodies of K-feldspar-rich leucosome in metat exit e migmat it es derived fro m aluminum-poor psammitic and plagioclase-rich pelit ic protolit hs can commonly be modeled as cryst allizing from the fract ionated, or evolved, melts generated by t he fract io nal cr yst allizat ion of plagioclase

from an anatectic melt of granit ic composit ion. Thus, t his leucosome could have crystallized from the evolved melt gener at ed by a cumulate process t hat made a plagioclaserich leucosome of the t ype shown in Fig. F56. Location: Quet ico Subprovince, Ontario, Canada. Rock type: st romat ic, in-source leucosome in a met at exit e migmat ite; pelitic prot olith, anatexis at T 700- 800°C, P 3- 4 kbar. Scale: the long side of t he photomicrograph corresponds to 9 mm. Cross-polarized light. /mage : E.W . Sawyer.

304

CRYSTALLIZ ATION-INDU CED MICROSTRUCTURES IN THE MELT-DERIVED PARTS OF MIGMATITES : LEUCOSOM E AND LEUCOCRAT IC VEI N S

Figure

58

Fig. F58. This leucocratic vein is hosted by a biotite quartz plagioclase psammitic schist (greywacke layer) that has not undergone partial melting, and hence it is interpreted t o have been injected. The overall bulk composition of the leucocratic vein is gran1t1c to syenogran1t1c, which suggests t hat it crystallized from a melt t hat had an evolved bulk composit ion. Compared to the leucosome in Fig. F57, which has a sim1larly h1gh content of microcline, t his example is richer in biot ite; possibly the biotite is residual. The feldspar crystals tend to be rectangular, and locally, they have st raight boundaries against quartz, which is interpreted to 1ndicate that they crystallized from a melt. Location: Quet ico Subprovince, Ont ario, Canada. Rock type: stromat1c, leucocratic ve1n in a metatex1te migmat1te; pelitic prot olith, anat exis at T 700- 800°C, P 3- 4 kbar. Scale: the long s1de of t he photomicrograph corresponds to 14 mm. Cross-polarized light. /mage: E.W. Sawyer.

Atla~

of Migmat ites

----------------- -------------- 305 Figure

59

Fig. F59. The equant, euhedral to subhedral crystals of plagioclase in this diat exit e migmatite fo rm a very o pen framework 1n wh1ch the cryst als touch, almost exclusively at their corners. The interstitial space is close to the theoret1cal maximum for a framework, and IS filled mostly w1th quartz. There IS a minor amount of t itan1ferous biotite and trace amounts of K-feldspar also. Almost all of the plagioclase grains have crystal faces against quartz, which is interpret ed as evidence for growth in a melt. T he preservation of this delicate framework and the equant but 1rregular shape of the interstitial quartz suggest that there was little synmagmatic or postmagmatiC stra1n. However, the undulose ext1nct1on in some of the quartz indicates minor post-solidification strain. The plag1oclase is unlikely to be residual in o rigin, because in the residual, meltdepleted metagreywackes, it has a grain size of about 0.5 mm, w hereas in the diatexite migmatites, t he grains are 1-5 mm across. C rystals of residual plagioclase in t he anatectic melt may have provided s1tes for the growth of plag1oclase from the melt.

Locat1on: Ashuanipi Subprovince, Quebec, Canada. Rock type: diatexite migmatite ; plagioclase + quartz + biOtite metagreywacke protolith, anatexis at T 825 850°C, P 6 7 kbar. Scale: the long s1de of the photomicrograph corresponds to 12.5 mm. Cross-polarized light. /mage: E.W.

Sawyer.

306

CRYSTALLIZA TION-I NDUCED MICROSTRU CTURES IN THE MELT-RICH PARTS OF MIGMATITES: D IATEXITE MIGMATITES

Figure

•

0

• • • • • • •

Fig. F60. Th1s photomicrograph shows a d1atex1te migmatlte that has a very dense framework structure compnsed of equant subhedral to euhedral crystals of plag1oclase. Virtually all the plagioclase crystals are in contact along their faces rather than JUSt at their corners; consequently, there is very little interstitial space. That interstitial space is filled w1th quartz, K-feldspar, and biotite. Locally, the crystals of plag1oclase have straight, rational faces against the Interstitial quartz, suggesting that they crystallized in a melt. The pore-space IS partially lined with a th1n film of K-feldspar in some places, but 1t 1S primarily filled with quartz. The irregular form of biotite and its location at grain junctions suggest that 1t crystallized from the melt 1n the interstitial pore-space. In outcrop, th1s diatexite m1gmatite contains biotite schlieren and onented, elongate rafts, or schollen, of res1dual metagreywacke, wh1ch together indicate a high shear strain 1n the diatexite magma. The plag1oclase in the framework is not strongly oriented, nor do the plagioclase crystals appear to be deformed; hence, the flow must have occurred while the diatexite conta1ned a s1gn1ficant fraction of melt. Subsequently, a large proportion of the fractionated melt generated by the crystallization of the plag1oclase was expelled. The result is a plag1oclase-dom1nated diatexite with a bulk composition that can be modeled by the accumulat1on of plagioclase.

Location: Ashuanipi Subprovince, Quebec, Canada. Rock type: plag1oclase-nch diatex1te m1gmat1te; plag1oclase + quartz + biotite metagreywacke protolith, partial melting at T 825- 850°C, P 6 7 kbar: Scale : the long side of the photomicrograph corresponds to 12.5 mm. Cross-polanzed light. Image: E.W. Sawyer.

A tl as of M igmat ites

-----------------------------307

• Figure

I

• 1

• f f

• •

• Fig. F61 . The plagioclase framework in this diat exit e migmat it e is very open, like that in Fig. F59, with most plagioclase grains in contact at the corners only. About half of t he interstit ial space has been filled by a large, single crystal of K-feldspar (now microcline); quartz and biotite fill t he rest of the interst it ial space. T he bulk composition of this rock is granitic. Some interstices show t hat the space was first lined with a thin film of K-feldspar, and then filled with quartz. Overall, this microstructure indicates that plagioclase crystallized as euhedral and subhedral grains first, and formed the framework structure before K-feldspar and then quartz began to crystallize and fill in t he pore space. A few crystals of plagioclase have rational faces against either K-feldspar or quartz, indicative of crystallization from a melt. The overall microstructure can be interpreted as result ing from the crystallization and accumulation of plagioclase before the other phases formed.

Location: Ashuan ipi Subprovince, Quebec, Canada . Rock type: diat exit e migmatite ; plagioclase + quartz + biotite metagreywacke prot olith, anatexis at T 825-850°C, P 6- 7 kbar. Scale: the long side of the photomicrograph corresponds t o 7 mm. Cross-polarized light. Image: E.W. Sawyer.

308

CRYSTALLIZATION- INDUCED MICROSTRUCTURES IN THE MELT-RICH PARTS OF MIGMATITES: DIATEXITE MIGMATITES

Figure

l

Fig. F62 . The open framework made up of subhedral to euhedral crystals of feldspar 1n this diatex1te migmat1te includes of both K-feldspar (now microcline), which predominates, and plagioclase. The bulk composition of this m1gmatite IS monzogranite t o syenogranite. As the protolit h was a plagioclase + quartz + biotite schist, this bulk composition suggest s derivat1on from an evolved, or fractionated, anatectic melt that had already crystallized some plagioclase. Such a melt could have been expelled from a diatexite migmatite like that shown 1n Figs. F59 or F60. Most of the space between the feldspar grains 1n the framework is filled by large crystals of quartz, but in places, it is filled with an aggregate of small, equant, anhedral crystals of sodic plagioclase, microcline, and quartz. Elsewhere, irregularly shaped crystals of biotite, or aggregates of biotite, fill the space between the framework of feldspar grains. The overall microstructure in this diatexit e m1gmatite Indicates the crystallization and accumulation of K-feldspar and plagioclase from a melt, or a magma. At the outcrop scale, the m1gmatite contains schl1eren and onent ed schollen (rafts) of residual metagreywacke, and both t hese features indicate flow of the diatexite in the magmatic state. T he cryst als of

feldspar that form the open framework structure in this d1atex1te m1gmatite are not strongly aligned; the orient ation of the schollen and the formation of the schlieren thus must predate completion of the framework. Location: A shuan1p1 Subprovince, Quebec, Canada. Rock type: diatexite migmatite; plagioclase + quartz + b1otite metagreywacke prot olith, anatexis at T 825- 875°C, P 6- 7 kbar. Scale : the long side of the photomicrogr aph corresponds to 13.5 mm. Cross-polarized light. /mage: E.W. Sawyer.

Atlas of Migmatites

----------------- -------------- 309 Figure

3

Fig. F63. Crystal frameworks are created w here sufficient crystals are present in a magma t hat t hey begin to impinge on one another. Plagioclase is typically the first major phase to crystallize abundantly in magmas of gran1t1c composit ion. and hence 1t commonly forms the framework. However, other minerals can also be part of a framework structure. This photomicrograph shows a biotite orthopyroxenequartz-plagioclase diatexite migmat ite in which the crystal framework consists mostly of euhedral plagioclase, but also contains euhedral crystals of orthopyroxene (Opx) that are of a similar size and shape to the plagioclase. The interstices of the framework are filled by quartz and biotite. Both the plagioclase and the orthopyroxene have crystal faces against the interstitial quartz. wh1ch can be interpreted as evidence of growt h in, or from, a magma. Grains of orthopyroxene in t he residual rocks associated with this diatexite migmatite are small (<0.5 mm) and xenoblast ic; therefore, the 4-mm-long euhedral crystals in this photomicrograph are Interpreted as a cotectic phase, and not as mherited residual gra1ns. The orthopyroxene shows only minor replacement by biotite along the edges and corners. This finding. and t he general absence of K-feldspar in t his rock, indicate t hat t he crystallizing magma had lost most of t he evolved melt fraction, and with it, H 20. Had t he evolved melt and its H 20 remained, then t oward the

final cryst allization of the magma, the exsolved H 20 would have led to the replacement of the orthopyroxene by hydrous phases, such as b1otite. Indeed, the orthopyroxene in diatexite migmatites that have a more evolved bulk composition and a crystal framework containing both K-feldspar and plagioclase is generally extensively altered to b1ot1te, or to biotite + quartz symplect1te, as shown in Figs. F80 and F81. Location: Ashuan ipi Subprovince, Quebec, Canada. Rock type: diatexite migmatite; plagioclase + quartz + biotite metagreywacke protolith, T 825- 875°C, P 6- 7 kbar. Scale: the long s1de of the photomicrograph corresponds to 13.5 mm. Cross-polarized light. /mage: from a thin section lent by Robert T heriault.

310

CRYSTALLIZATION-INDUC ED MI CROSTRUCTURES IN THE MELT-RICH PARTS OF MIGMATITES : DIATEXITE MIGMATITES

Figure

Fig. F64. Many dratexite migmatrtes did not develop an obvrous framework of crystals. Thrs example contarns scattered, euhedral crystals of plagioclase, which locally form small aggregates, or clumps of crystals. The rndivrdual crystals rn some of the aggregates are arranged rn an imbricate or tiled pattern that is rnterpreted to result from the rmprngement of crystals that were rotating in a magma undergorng shearing. In much of thrs thrn sectron, there is no framework of large euhedral or subhedral cr ystals, rndicating that the diatexite magma was not carryrng many large crystals. The mrcrostructure of thrs diatexrte consists mostly of equant, but commonly irregularly shaped crystals of quartz, plagroclase, K-feldspar, muscovrte, and brotrte. Partral meltrng at Sarnt-Malo pnncrpally progressed through the breakdown of muscovite; biotite remained stable rn most of the mrgmatites there. Consequently, much of the biotite rn the diatexrte mrgmatite is resrdual and did not crystallize from the anatect ic melt. Furthermore, melting reactrons rnvolvrng the breakdown of muscovrte generated melts wrth relatrvely high contents of H 20. In the late stages of crystallizatron, muscovite was a liquidus phase. In thrs dratexrte migmatrte, a small amount of K-feldspar had crystallized before muscovite appeared on the liqurdus.

However, some migmatrtes at Sarnt-Malo have substantral amounts of K-feldspar, whereas rn others, muscovite formed instead of K-feldspar. Location: Sarnt-Malo mrgmatrte terrane, Bnttany, France. Rock type : diatexrte mrgmatite; semipelitrc protoltth, anatexts at T ca. 750°C, P 4 7 kbar. Scale : the long side of the photomiCrograph corresponds to II mm. Cross-polanzed light. /mage: E.W. Sawyer. Further readmg: Brown, M. (1979): The petrogeneSIS of the St-Malo mrgmatrte belt, Armoncan Massrf, France, with partrcular reference to the diatexites. Neues jahrbuch (i.ir Mmerologre, Abhandlungen 135, 48 74. Milord, 1., Sawyer, E.W . & Brown, M. (2001): Formation of diatexrte migmatrte and granrte magma dunng anatexrs of semi-pelitic metasedimentary rocks: an example from St. Malo, France. journal o(Petrology 42 ,487 505. Weber, C., Barbey, P., Cuney, M. & Martin, H. (1985): Trace element behaviour during migmatization . Evidence for a complex melt resrduum flurd interactron rn the St. Malo migmatrtrc dome (France). Contnbuttons to Mmerology and Petrology 90, 52-62.

A tl as of Migmatites

Figure

6S

Fig. F65 . Depend1ng on the earlier history of crystal lization, some diatexite migmatites formed from crystal-poor anatectic melts that had previously crystallized much plagioclase. These d1atex1te m1gmatites have fractionated, or evolved, bulk compos1t1ons. Th1s photograph IS of such a b10t1te- plag1oclase quartz-K-feldspar diatex1te m1gmatite of syenogranite to gran1te compos1t1on from the Opatica Subprovince. In outcrop, it is a pink-colored, coarse-grained, leucocrat ic rock dominated by K-feldspar, a proportion of which forms euhedral crystals up to 10 mm long. Although the rock conta1ns biot1te schlieren and elongate schollen (or rafts) of paleosome and res1dual matenal, 1t does not have a foliation due to aligned feldspar, or al1gned biotite. Eit her the microstructure has been reset or the flow indicated by the schlieren and oriented schollen occurred when t he magma contai ned few crystals. T he photomicrograph shows some large, subhedral crystals of plagioclase (composition An " ) with a cloudy interior, but most of the feldspar 1s clear miCrocline. K-feldspar occurs as large euhedral to subhedral crystals that with plagioclase form an open framework structure, as large crystals that fill some of t he interstices 1n the framework structure, and as smaller irregularly shaped gra1ns t hat together w it h quartz, biotite, and a vermicu lar intergrowth (myrmekite) of quartz and sodic plagioclase (An 8 0 ), fill the rema1ning interstitial

space. T he plagioclase in the K-feldspar-rich diatexites of the Opatica Subprovince is consistently more sodic (An 1s 8) than the plagioclase (An" , ,) that comprises the feldspar framework in the K-feldspar-poor, plagioclase-nch diatexite migmatites. The anatect1c melt 1n the Opat1ca Subprov1nce thus underwent fractional crystallization as the diatex1te migmat1tes were formed. Location: Opatica Subprovince, Quebec, Canada. Rock type: diatexite migmatite; plagioclase + quartz + K-feldspar + b1ot1te leucotonalite or trondhjemite protolith, anatexis 1n the upper amphibolite faCies, T ca. 750°C, P 5 7 kbar. Scale: the long s1de of the photomicrograph corresponds to 11.5 mm. Cross-polarized light. /mage: E.W. Sawyer.

Further reading: Sawyer, E.W. (1998) : Formation and evolution of granite magmas during crustal reworking: t he significance of diatexites. journal of Petrology 39, 1147 1167. Sawyer. E.W. (2001): Melt segregation in the contmental crust d1stribution and movement of melt 1n anatectiC rocks. journal of Metamorphic Geology 19, 291 - 309.

311

MI C RO STRU C TURES FO RM ED BY FLOW

~==~ --~~==~ 312 --------------IN DI ATEXITE MIG MATITES

Figure

66

Fig. F66. The tabular, subhedral crystals of biotite (0.7 mm) in this diatexite migmatite have a very strong preferred orientation that defines the foliation in the rock. The matrix around the biotite consists of much smaller (ca. 0.2 mm). interlocking, equant crystals of plagioclase, quartz, and K-feldspar, with still smaller crystals of euhedral cord1erite scattered throughout. Locally, the matrix contains somewhat larger, euhedral, lath-shaped crystals of plagioclase (e.g., in the bottom right corner), and the long axes of these are oriented parallel to the foliation defined by the large crystals of biotite. The diatexite magma consisted of abundant large flakes of biotite, scattered large laths of plagioclase, and small, euhedral crystals of cordierite (the product of the incongruent breakdown of biotite) in an anatectic melt of granitic composition. The orientation of the biotite and similar-sized plagioclase crystals is interpreted to have resulted from rotation during shearing and flow when the diatexite was a magma. The cordierite crystals are small (0.07 mm) and have a low aspect-ratio. and so any preferred orient ation t hey may have is not evident. Because most of the crystallization occurs near the solidus 1n a melt of granitic composition. the late-crystallizing minerals cannot grow freely and tend to develop interlocking

shapes. The magmatic-flow stage of the diatexite may have ended abruptly owing to a sharp increase in viscosity as the magma neared solidification. LocatiOn: Duluth Igneous Complex, Minnesota. U.S.A. Rock type: diatexite migmatite; pelite protolith, anatexis in the pyroxene hornfels facies. T 800-850°C, P 1.5 2 kbar. Scale: the long side of the photomicrograph corresponds to 5.5 mm. Cross-polarized light. /mage: E.W. Sawyer.

A tlas of Mig matites

---------------------------- 313 Figure

7

Fig. F67. Th1s cordierite- b1ot1te- K-feldspar- quartz plag1oclase diatexite migmat1te illustrates the ma1n cnteria for identify1ng a magmatic foliation. The large bladed crystals of biotite (0.5 0.7 mm long) are aligned and define a foliation that 1s approximately parallel to the long s1de of the photomicrograph. The largest of the plagioclase crystals (about 0.5 mm lo ng) in the diatexite also are tabular in shape, and t heir long axes are subparallel to the orientation of the large grains of biotite. Most of the small crystals (0.15 mm) in the groundmass (cordierite, plagioclase, K-feldspar, and quartz) are equant and, therefore, do not appear to contribut e to the foliation. The large and somewhat embayed porphyroblast of cordierite in the center of the photomicrograph is slightly elongate in a direct1on subparallel to the b1otite, although t he cord1erite could also be cons1dered as a pa1r of better-oriented, elongate grains that have become Joined. In contrast. the large, equant but highly 1rregular crystal of quartz (wh1te in the center of the p1cture) that envelops the porphyroblast of cordierite shows no tendency to be elongate and, moreover, displays a uniform pattern of extinction, indicating that it has relatively little internal strain. The microstructure in this diatexi te migmatite is interpreted to indicate that t he elongate crystals of biot ite, plagioclase, and cordierite became oriented as a result of flow gradients in the diatexite magma.

They developed in response to an applied shear stress. The large intergranular crystal of quartz crystallized from one of the last pockets of melt remaining after the tabular crystals had become oriented to create the foliat1on. The absence of undulose extinction in the quartz means that no s1gn1ficant penetrative deformation occurred after this diatexite had fully crystallized. Location: Duluth Igneous Complex, Minnesota, U.S.A Rock type : diatexite migmatite; pelite protolith, anatexis in t he pyroxene hornfels facies, T 850 900°C, P 1.5-2 kbar. Scale: the long side of the photomicrograph corresponds to 2.7 mm. Cross-polarized light. /mage: E.W . Sawyer.

3 14

M ICROSTRUC T URES FOR M ED BY FLOW

--------------------------~-

IN D IATEXITE MIGMATITES

Figure

8

Fig. F68. T his photomicrograph shows three layers in a diat exit e migmatite; they have the same biot it e + quartz + plagioclase + K-feldspar + orthopyroxene + cordierite assemblage, but the modal proportions and grain size differ among the layers. The cordierite-rich layer in the lower right is coarse grained, and because it has the lowest content of biotite, it appears the lightest in color. A coarse-grained orthopyroxene-rich (the high-relief mineral) layer on the left side of the phot omicrograph has a slightly higher content of biot ite, whereas t he fine-grained, orthopyroxene-rich layer that separates t hem has the highest content of biot it e and appears the darkest. These layers probably reflect an original compositional layering (i.e., bedding) in the metasedimentary protolith. T he biotite in this migmatite displays two principal morphologies: ( I) small bladed crystals that commonly define a foliation and (2) large, subhedral , randomly oriented (basal sections are common) crystals that commonly have an irregular outline (several are in the top left corner) . The first type is interpreted to be residual biotite, and the second type, which is present in virtually all the diatexite migmatites at Duluth, is interpreted to have grown during crystallization of the diatexite magma. T he coarser-grained layers mostly contain the late generation of large, irregularly oriented biot it e, but this biotite became oriented close to the contact

with the fine-grained layer. In contrast, the fine-grained layer contains mainly the small residual crystals of biot it e, and t hey define an S-shaped, sigmoidal foliation. The finegrained layer is interpreted to be derived from a less fertile layer of the protolith, and so generated less anatect ic melt than adjacent layers. This layer recorded strain th roughout its history. T he fertile layers next to it consumed their reactant biotite and progressed to the magmatic state, and thus had the record of their early strain history erased when t hey became a magma. T hey carried the less-melt ed layer as a scholle, and during t ransport, it was ablated at its margins (magmatic erosion) . When cooling started, the large crystals of biotite began to grow from the diatexite magma, and because of t he gradient in shear st rain close t o the scholle, those large cryst als of biot ite near t he scholle became oriented and recorded the sense of the late magmat ic shear around t he scholle. Most of t he diatexite crystallized after the magma was sheared and did not develop a magmatic foliation. Location: Duluth Igneous Complex, Minnesota, U.S.A Rock type : diatexite migmatite; pelit ic hornfels protolit h, pyrox-

ene hornfels facies, T 800- 850°C, P 1.5-2 kbar. Scale: the long side of t he photomicrograph corresponds t o 9 mm. Plane-polarized light. Image: E.W. Sawyer.

"""""' Arlao, o f Migma[i[cs

----------- ---------- ------- 315 Figure

69

Fig. F69. Partial melttng and the formatton of migmatttes at Satnt-Malo occurred largely through the breakdown of muscovtte and was, consequently, mostly in the stability field of biotite. Therefore, a large proportion of the btottte tn the migmatttes ts of restdual ongtn and can be used as a marker for an tnvesttgatton of the magma-now htstory and the processes tnvolved; tt can also be used to tdenttfy sttes of crystal accumulatton tn the mtgmatttes. This photomtcrograph shows the parallel alignment of large, bladed crystals of btotite and of subhedral, tabular plagtoclase, respecttvely of restdual ongtn and early products of melt crystallizatton, onented when the magma that contatned them was subJect to a shear stratn. Subsequent crystallization produced large, equant crystals of quartz and microcli ne and smaller, equant crystals of plagtoclase and biottte between the onented crystals of btottte and plagtoclase. Because most of the crystalltzatton occurs near the solidus in melts of grantttc compostt ion, the late-crystalliztng mtnerals cannot grow freely and cannot be rotated into the fo liation. Thus, they tend to develop tnterlocktng shapes. In thts way. magmatte now may have ended abruptly tn thts migmattte owtng to a sharp tncrease in vtscostty as the magma neared solidification. As anhydrous phases progressively crystallized. the H 20 content of the remaining melt increased, and at the late stages of crystallization. an aqueous nuid was exsolved.

It may have reacted wtth the mtnerals already crystallized. Both these effects are common at Satnt-Malo, where muscovtte ts typtcally one of the last phases to crystallize from the melt (e.g., Fig. F64) and, as shown tn this photomicrograph, biotite is parttally replaced by muscovtte. Locauon: Saint-Malo mtgmattte terrane. Bnttany. France. Rock type: diatexite mtgmatite; semtpelitic protoltth, anatexts at T ca. 750°C, P 4 7 kbar. Scale: the long side of the photomtcrograph corresponds to II mm. Cross-polanzed light. Image: E.W. Sawyer. Further readmg: Brown, M. ( 1979): The petrogenesis of the St-Malo migmattte belt, Armorican Masstf, France, wtth particular reference to the diatexttes. Neues Jahrbuch {Dr Mmeralog1e, Abhandlungen 135, 48- 74.

INCLUSIONS OF MELT QUENCHED TO GLASS IN MINERALS

316 ----------------------------Figure

0

'• Fig. F70. Details of the mineral inclus1ons 1n a garnet porphyroblast from a metapelitlc enclave that was rap1dly quenched when erupted in a dacitic lava. Most of the inclus1ons in the garnet crystal are quartz, but one of the quartz 1nclusions contains a glass Inclusion, and this indicates the former presence of anatectic melt in the rock. The metapelit1c enclave contains the mineral assemblage biotite + garnet + sillimanite ; quartz is virtually absent. and is only present as inclusions in the other minerals. This indicates that the quartz in the matrix of the pelite was totally consumed by the melt ing reaction, and hence that the mineralogy of the enclave is residual. Locot1on: El joyazo, Spain. Rock type: melt inclus1on in quartz 1n garnet; met apelite pr otolith melted at T 850 ± 50°C, P > 5 kbar, but quenched by eruption. Scale: the w1dt h of th1s plane-polarized light photograph is 1.35 mm. Image and capuon : Bernardo Cesare. Further readrng: Cesare, B. (2000): Incongruent melting of b1otite to spinel in a quartz-free restite at El joyazo (SE Spain): textures and react ion characterization. Contributions to Mineralogy and Petrology 139, 273 284.

Cesare, B., Salvioli Manan1, E. & Venturelli, G. (1997): Crustal anatexis and melt extraction in restitic xenoliths at El joyazo (SE Spam). Mrneralogical Magazrne 61 , IS 27.

A tlas of Mig matites

Figure

71

Fig. F71. N umerous slightly elongate and equant inclusions of glass form parallel arrays in a plagioclase crystal in a metapelitic enclave brought rapidly to the surface by an eruption of dacitic lava. In some places, the glass inclusions have a negative crystal shape, but most are outlined by two parallel faces, which correspond to specific crystallographic directions in the plagioclase host; the ends of many inclusions are somewhat irregular. The glass is clear and colorless, and shows no signs of having either crystallized or devitrified. Virtually all of the melt inclusions contain a conspicuous "shrinkage bubble," which formed as the glass contract ed on cool ing. The size and number (i.e., total volume) of the shrinkage bubbles in a melt inclusion depend upon cooling rate (Lowenstern 1995) . Furt hermore, the bubbles may eit her be empty or contain volatiles, such as H 20, that were dissolved in the melt. Location: El Joyazo, Spain. Rock type : glass inclusions in plagioclase; metapelite protolith, melted at T 850 ± 50 °C, P > 5 kbar, quenched by eruption. Scale: the w idth of photomicrograph is 0.65 mm. Plane-polarized light. Image and caption: Bernardo Cesare.

Further reading: Lowenstern, j .B. ( 1995): Applications of silicate melt inclusions to the study of magmatic volatiles. In Magmas, Fluid and O re Deposits (J.F.H. T hompson, ed.).

Mineralogical Association of Canada Short Course 23, 71-99. See Fig. F72 for more reading.

317

IN CLUSIO N S OF MELT QUENCHED TO GLASS IN MINERALS

318 ----------------------------Figure

72

• ..

Fig. F72. The glass inclusion near the center of t his photomicrograph is 60 1Jm long and located in a cordierite crystal from an enclave of residual pelit ic material in a volcanic rock. The inclusion has the bulk com posit ion of a peralumi nous rhyolite, and is typical of pelite-derived leucogranites in general; it represents the anatectic melt that was t rapped as the host cordierit e crystal grew. The melt inclusion contains three conspicuous "shrinkage bubbles". Location: Mazarr6n, southeast Spain. Rock type : glass inclusion; pelite protolith, partially melted at T 850 ± 50°C, P 2- 3 kbar. Scale: the field of view is 0.3 mm wide. Planepolarized light. Image and caption; Bernardo Cesare. Further reading: Cesare, B., Marchesi, C., Hermann, j. &

G6mez-Pugnaire, M.T. (2003): Primary melt inclusions in andalusite from anatectic graphitic metapelites: implicat ions for the position of the AI 2Si05 triple point. Geology 31 ' 573-576.

..

•

•

•

•

Atlas of M igmatites

------------------------------- 319 Figure

3

Fig. F73. Th1s photograph shows a large and irregularly shaped glass inclus1on (55 IJm long) in a plag1oclase crystal from an enclave of residual metapelite 1n a volcan1c rock. The glass 1nclus1on has a rhyolitic compos1t1on. and contains a double shnnkage bubble that indicates the presence of a fluid, probably derived from the H 20 that was d1ssolved 1n the anatect1c melt. T he plagioclase crystal also contains two more large and irregular ly shaped glass inclusions that have conspicuous shrinkage-induced bubbles, but these bubbles are too small to check if they also are double. Furthermore, the plagioclase cont ains innumerable much smaller Inclusions. many of wh1ch are square or rectangular 1n outline, and almost all of them contain tiny bubbles.

Location: Mazarron, southeast Spain. Rock type: glass IncluSions in plagioclase; pelitic protolith, part1ally melted at T 850 ± 50°C, P 2- 3 kbar. Scale: the field ofv1ew IS 0.35 mm w1de. Plane-polarized light. Image and caption: Bernardo Cesare.

Further readmg: as for Fig. F72.

320

CORDIE RITE - , GARNET-, AND O RT HOPYROXEN E-

----------~----~-----------

QUARTZ INTERGROW T H MICROSTRUCTU RES

Figure

74

Fig. F74. Close-up of the intergrowth between quartz and garnet from the core of a patch of neosome in which a leucocrat ic (quartz + K-feldspar + plagioclase) rim and a melanocratic (garnet) core developed in a biotite + plagioclase + quartz semipelite host. Figure 830 is an outcrop photograph of th is type of intergrowth between quartz and garnet. The mineral assemblage in the host corresponds to the reactants, and the neosome, to the products in a biotite-dehydration-melting reaction similar to biotite + quartz + plagioclase + sillimanite = garnet + melt + K-feldspar. The reaction may possibly have terminat ed because all the sillimanite in the semipelit ic protolith was consumed. N ot e that the quartz appears to comprise just two main crystals (one yellow and one blue in this image) in t he host garnet (purple). The partial melting is Paleoproterozoic in age, but the undulose extinction in the quartz is due to much younger deformation associated with t he Paleozoic Alice Springs orogeny. Many of the patches of neosome occur in small shear zones; on the basis of their major- and trace-element compositions, t hey have lost some of their melt fraction.

Location; Wu luma Hills area, Arunta Inlier, Australia. Rock type: neosome in a metatexite migmatite; semipelitic protolith, anatexis at T 825-875°C, P ca. 5 kbar. Scale: the long side of the photograph equals 9 mm. Cross-polarized light and quartz accessory plate. Image : E.W. Sawyer.

Further reading; Barbey, P., Marignac. C, Mantel, J.-M., Macaudiere, J., Gasquet, D. & Jabbori, J. ( 1999): Cordierite growth textures and the conditions of genesis and emplacement of crustal granitic magmas: the Velay Granite Complex (Massif Central, France) . journal of Petrology 40,

1425-1441. Waters, D.j. (200 I): The significance of prograde and retrograde quartz-bearing intergrowth microstructures 1n partially melt ed granulite-facies rocks. Lithos 56, 97- 110.

A tla> of M igmat ites

---------------- -------------- 321 Figure

75

Fig. F75. This photomicrograph shows t he 1nternal part of anot her neosome in a biotite + plagioclase + quartz semipelit1c host from the same locality as Fig. F74. In this example, the morphology of the garnet- quartz mtergrowth IS somewhat d1fferent. The quartz occurs as deep embayments 1n the garnet and as equant, o r slightly elongate, bleb-like 1nclusions with1n it. Some of the larger inclusions of quartz also contain small roundish crystals of partially altered plagioclase. The garnet IS surrounded by a rim of quartz, wh1ch is, in t urn, surrounded by an annulus of K-feldspar that conta1ns a few small gra1ns of plag1oclase and quartz. The K-feldspar and garnet in the neosome are Interpreted to be the solid products of an Incongruent melt1ng reaction such as biot1te + sillimanite + quartz + plagioclase melt + garnet + K-feldspar. Sillimanite is no longer present in the host and is considered to have been entirely consumed by the melt-producing reaction; biotite is absent from the neosome and only found in the host.

=

Location: Wuluma Hills area, Arunta Inlier, Australia. Rock type: neosome from a metatexite m1gmat1te; sem1pelit1c protolith, part1al melt1ng at T 82S 875°C, P ca. 5 kbar. Scale: t he long side ofthe photograph equals 14 mm. Crosspolarized light. /mage: E.W. Sawyer.

CORDIERITE - . GAR N ET-, AND ORTHO PYROXENE-

322--------------~----~~~--­ QUARTZ INTERG ROWT H MICROSTRUCTURES

Figure

6

Fig. F76. Occurrences of a garnet- quartz intergrowth are very common in the neosome of the Wuluma Hills migmatites; intergrowths between orthopyroxene and quartz, and between cordierite and quartz, also occur, however: The orthopyroxene cryst al (yellow) in th is intergrowt h contains two large, rounded inclusions of quartz (orange and pink) and is almost completely surrounded by a very narrow rim of small crystals of quartz (most ly blue), part of which also forms a deep embayment in the orthopyroxene. The quartz and orthopyroxene are su rrounded by a single grain of perthit ic K-feldspar (red and purple). Although not visible in this photograph, the K-feldspar is, in turn, surrounded by a rim of medium-grained quartz + plagioclase + K-feldspar that constitutes t he leucosome part of t he neosome. The o rthopyroxene and t he large K-feldspar grain surrounding it are interpreted as t he solid products of the incongruent reaction: biotite + quartz + plagioclase o rthopyroxene + K-feldspar + melt. They const it ute the residuum part of the neosome. and it is. of course. in Situ. The outer granit ic part of the neosome may have been largely derived from the crystallization of the anatectic melt produced by t he reaction, and can therefore be considered as the in Situ leucosome portion.

=

Location: Wuluma Hil ls area, Arunta Inlier, Australia. Rock

type: neosome in a metatexite migmatite; Al-poor psammite protol it h, anatexis at T 825- 875°C, P ca. 5 kbar. Scale: t he long side of t he phot ograph corresponds to 9 mm. C ross- polarized light and t he quartz accessory plate. Image: E.W. Sawyer.

Atlas of M igmat ites

------------------------------- 323 Figure

7

Fig. F77. Although most of the mtergrowth miCrostructures conta1n only one ferromagnes1an phase. some conta1n

anatectic melt in the neosome. Thus. the b1ot1te crystallized at a late stage of m1neral growth 1n the neosome and

several. The core of this neosome is a large (IS mm) crystal of cord1erite (yellow. bottom left). wh1ch is enclosed by a thin (up to 2 mm thick). discontinuous nm of quartz (mostly blue and purple) that locally fills embayments 1n the cordierite. Th1s cordierite + quartz core is enclosed by an annular zone containing garnet, K-feldspar. and idioblastic orthopyroxene with lesser amounts of biotite. quartz. and plagioclase. T his zone is in turn enclosed by an outer,

formed dunng cooling and subsequent to the loss of some melt from the neosome.

quartz + plag1oclase + K-feldspar leucocrat1c rim (not present in the m1crograph) that is of broadly gran1t1c compos1t1on and is in contact w1th the host. a biotitequartz orthopyroxene-plagioclase-cordiente sem1pelit1c sch1st. Locally. th1s leucocrat1c outer zone IS h1ghly stra1ned and attenuated. The outer granit1c zone of the neosome 1s Interpreted to have been derived from the crystall1zat1on of anatectiC melt and can be viewed as an in s1tu leucosome. In contrast. the 1nner zones with cord1ente. garnet. orthopyroxene. and K-feldspar are interpreted to be the solid products of the partial-melting reaction . and are the residuum portion of the neosome. The biotite present 1n the zone that surrounds the cordierite crystal is mostly only on the margins of the garnet and the ort hopyroxene. Hence. this b1ot1te is interpreted to have formed from both garnet and orthopyroxene by reaction with the last rema1ning

Location: Wuluma Hills area. Arunta Inlier. Australia. Rock type: neosome in a metatexite migmatite; pelitic protolith, anatexis at T 825-875°C, P ca. 5 kbar: Scale: t he long side of the photograph equals 9 mm. Cross-polarized light and quartz accessory plate. /mage: E.W. Sawyer. Further reading: Barbey. P.. Marignac. C.. Monte!. J.-M .. Macaudiere. j .. Gasquet. D. & Jabbori. J. (1999): Cordierite growth textures and the cond1t1ons of genes1s and emplacement of crustal gran1t1c magmas: the Velay Granite Complex (Massif Central. France). journal or Petrology 40, 1425- 1441. Waters. D.j. (2001 ): The s1gn1ficance of prograde and retrograde quartz-beanng intergrowth microstructures 1n part1ally melted granulite-faCies rocks. L1thos 56, 97- 11 0. White, RW.. Powell, R. & Halp1n. J.A. (2004): Spatially focussed melt formation 1n aluminous metapelites from Broken Hill, Australia: the importance of m1neral textu re development in melt formation. journal or MetamorphiC Geology 22. 825 845.

324

BIOTITE - QUART Z AND BIOTITE - PLAGIO C LASE

---------------------~----ROWTH MICROS TRUCTU RES

SYMPLE CTITIC INTERG

Figure

8

Fig. F78. This photomicrograph 1s from a melt-depleted, semipelitic layer in a granulite-facies metatex1te m1gmatite. It shows a symplect1t1c 1ntergrowth (Sym) of late-generation biotite + quartz replacing orthopyroxene (Opx). The garnet (bottom left) 1n the rock is not affected, and contains several rounded grains of biotite Interpreted as belonging to the older, reactant assemblage. The colorless, low-reli ef matnx consists of perth1t1c K-feldspar (Kfs) + plagioclase + quartz. Garnet, orthopyroxene, and K-feldspar are interpreted to be the products of various incongruent melt1ng react1ons Involving the breakdown of biotite and, initially, sillimanite. Most of the melt generated by the breakdown of the b1otite has been removed, and a residual plagioclase+ K-feldspar+ garnet+ quartz+ orthopy roxene + biotite assemblage remains. The symplectite microstructure formed when the small amount of anatectiC melt trapped in the matrix reacted with the residual orthopy roxene in a

=

reaction such as melt + orthopyroxene b1ot1te + quartz. The reaction that formed the symplectite 1s essent1ally the reverse of the partial-melting reaction responsible for generating the orthopyroxene.

Location: Ashuanipi Subprovince, Quebec, Canada. Rock type: residual part of a metatex1te m1gmat1te; sem1pelite protolith, anatexis at T 825-87 5°C, P 6-7 kbar. Scale: the long side of the photograph equals 2.7 mm. Plane-polanzed light. /mage: E.W. Sawyer. Further readmg: Waters, D.j. (2001): The s1gnificance of prograde and retrograde quartz-beanng 1ntergrowth microstructures in partially melted granulite-fac1es rocks. Lithos 56, 97 110.

Atlas of M igmatites

------------------------------ 325 9

Figure

.·

-----

- -· 4fll"

• f

•

Fig. F79. A symplectit e (Sym) of q uartz + bladed biotite and larger, more equant crystals of biotite replace both garnet (Grt) and orthopyroxene (Opx) in this metapelite layer from a melt-depleted metatexite migmatite. T he domain that contains t he symplectite is bordered by a leucocrat ic region t hat consists principally of perthite (faintly mott led appearance) and minor amounts of plagioclase and quartz. The format ion of t he symplectite is interpreted to be t he result of a react ion between t he last-remaining anatectic melt in the residu um and t he residual garnet, orthopyroxene, and K-feldspar (Kfs). The reaction responsible is inferred to have been melt + garnet + orthopyroxene + K-feldspar = biot ite + quartz. It occurred after almost all of the melt generated d uring anatexis had migrated away.

Location : Ashuanipi Subprovince, Canada. Rock type: melt-depleted (residual) part of a metatexite migmatite; metapelite prot o lith, anatexis at T 825- 875°C, P 6- 7 kbar. Scale: the long side of t he photograph equals 14 mm. Planepolarized light. /mage: E.W. Sawyer.

J

326

BIOTITE-Q UARTZ AND BIOTITE- PLAG IOCLASE SYMPLECTITIC INTERGROWT H MI CROSTRUCTURES

Figure

80

Fig. F80. Th1s example, from a d1atex1te m1gmat1te. shows a rectangular. b1ot1te-nch doma1n replaong an ld1oblast1c, rectangular crystal of orthopyroxene. The orthopyroxene 1S Interpreted either as the product of Incongruent melting of b1otite. or as a product of crystallization of the anatectic melt. The replacement of the orthopyroxene crystal by b1ot1te and quartz appears to have occurred in two stages. T he b1ot1te forms thin plates in a symplectit e w1th quartz in the portion where corroded relics of t he orthopyroxene st1ll rema1n. H owever, the b1otite and quartz are conspicuously coarser gra1ned and generally not mtergrown 1n the reg1on where no relics of the orthopyroxene rema1n. The orthopyroxene crystal 1n the photomicrograph IS part of a large-scale framework structure in the diatexite that conSISts pnncipally of large subhedral crystals of plag1oclase (top left) and K-feldspar (bottom right). Smaller anhedral crystals of K-feldspar, plag1oclase, quartz, and biotite are 1nterst1t1al to t he framework structure, and are believed to have crystallized from the melt trapped within the crystal framework. The overall miCrostructure of the diatexite migmat 1t e IS, therefore, Interpreted to be magmat1c. On the bas1s of the abundance of K-feldspar, th1s diatex1te IS Interpreted to have crystallized from a relat1vely evolved or fract1onated magma. The symplectite microstructure is Interpreted to be the result of a react1on between the remainmg melt 1n

the d1atex1te, or an H 0-nch flu1d exsolved from the melt as 1t crystallized, and the orthopyroxene crystals through a react1on such as melt + orthopyroxene ::: biot1te + quartz + plag1oclase. The replacement of orthopyroxene by a symplectlte of biotite + quartz IS rare in the plagioclaserich. cumulate diatexite m1gmat1tes from the Ashuan1pi Subprov1nce, but IS very common 1n the more evolved, K-feldspar-nch ones. Th1s may s1mply reflect a h1gher content of H 20 in the evolved magmas. This microstructure suggests that the large, rectangular aggregates of b1ot1te present 1n some orthopyroxene-free d1atex1te m1gmat1tes 1n the Ashuamp1 Subprov1nce may 1n fact be pseudomorphic after orthopyroxene.

Location: Ashuan1p1 Subprovince, Quebec, Canada. Rock type: diatex1te m1gmat1te; metagreywacke protohth, anateXIS 1n the granulite fac1es, T 825 875°C, P 6- 7 kbar. Scale: the long s1de of the photograph corresponds to 3.5 mm. Cross-polarized light. /mage : E.W. Sawyer.

Atlas of Migmarires

------------------------------- 327 Figure

I

Fig. F81 . The photomicrograph shows a cluster of orthopyroxene crystals in the perthite + plagioclase + quartz matrix of a diatexite migmatite. The origin of the orthopyroxene is uncertain; it could be the product of the incongruent melting of biotite and, hence, interpreted as residual crystals in the diatexite magma, or alternatively, it could have crystallized from the diatexite magma. The matrix around the orthopyroxene consists of large, subhedral crystals of K-feldspar (Kfs) and plagioclase (PI) with small interstitial crystals of quartz and minor amounts of fe ldspar. On the basis of its bulk composition, the diatexite is interpreted to have crystallized from a moderately evolved anatectic melt. T he orthopyroxene cryst als have been partial ly replaced by a biotite + quartz symplect it ic intergrowth. Symplectite is not generally developed at the contacts between the large subhedral crystals of K-feldspar and the o rthopyroxene; rather, it appears to be concentrated in regions where o rthopyroxene is in contact w ith the small grains of interstitial quartz and feldspar. This spatial relationship suggest s t hat the symplectite resu lts from reaction bet ween the o rthopyroxene crystals and t he interstitial melt , or an H20-rich fluid exsolved from it.

Location: Ashuanipi Subprovince, Canada. Rock type: diatexite migmatite; metagreywacke protolith, anatexis in the granulite facies; T 825~875°C, P 6~7 kbar. Scale: the long side of the photograph corresponds to 2.7 mm. Planepolarized light. /mage: thin section provided by Robert Theriault.

BI O - ; : : J r iTE- SILLI MANITE AND BIOTITE AGGREGATES REP LACING GARNET OR CORD IERITE

'8 ----.:.....=:.._____

Figure

Fig. F82 _ _ _ . The large crystal of cordtente (Crd) 1n thts residual d - - . i atexite shows incipient replacement by btottte and sillima nite around the edges. Sillimantte crystals at the contact bc:::::z:> ar matrix are relatively small, but some of the sillimanite _ c r ystals (Sil) in the biotite are larger and more equant in t h eir form. The cordierite can be identified by the yellovv - - - ' p leochroic halo around small, high-relief Inclusions, and by t he characteristic fractures across the grains. The cordi c:::::::::::> C? r ite in this rock is a product of the incongruent breakd _ own of biotite and sillimanite and was replaced, following u ---;1..,_::.::::... h e loss of some of the melt fraction, as the dtatexite crys ;;;;;;;;;;.t a llized. A reaction between the last tncrements of melt lef ~t in the rock and the surfaces of the crystals of residual co ,.----r~ rdiente generated the biotite+ sillimantte fnnges around the ~ cordierite. See Waters (200 I) for an alternattve interpretati _ ·i i on of the replacement of garnet or cordtente by aggregates c=:of biot1te and sillimamte. Locot1on: ac Kenogami, Grenville Province, Quebec, Canada. Ro ~~ck type: residual diatextte migmatite; metapelittc protolith, a .-. ~ n atexis tn the lower granulite facies. Scale: the long side o r f' the photograph corresponds t o 2.5 mm. Planepolarized li g g ht. /mage: E.W . Sawyer.

::c:z:-

Further reading: Buttner, S.H., Glodny, j., Lucassen, F., Wemmer, K., Erdmann, S., Handler, R. & Franz, G. (2005): Ordovician metamorphism and plutonism 1n the Sierra de Quilmes metamorphic complex: tmpltcattons for the tectontc setting of the northern Sierras Pampeanas (NW Argenttna). Lithos 83, 143 181. Waters, D.j. (200 I): T he significance of prograde and retrograde quartz-bearing intergrowth microstruct ures in partially melted granulit e-factes rocks. Ltthos 56, 97- 110.

Atlas of M igmatites

------------------------------329 Figure

3

Fig. F83. This plagioclase crystal from the center of a large domain of leucosome exhibit s an unusual, mottled zonat ion (An 47 23 ) in its core, and has concentric overgrowt hs of a more sodic plagioclase (An 31 26). The host migmatites (mostly diatexite at t he location) are intruded by sheets of quartz gabbro and meladiorite, and contain partially hybridized enclaves of this mafic material. As such, plagioclase cores are not encountered in the leucosome remote from the mafic intrusive bodies; the complex zonat ion in this plagioclase is attributed to the presence of t he mafic intrusive bodies in the diatexite. The complexly zoned plagioclase may (I) be inherited by contamination of t he anatectic diat exit e magma by the relatively calcic mafic magma or (2) result from the remelting of early-formed crystals of plagioclase in the anatectic magma as a consequence of heating by t he intrusion of the mafic magmas into the diatexite migmatite, followed by thermal re-equil ibration and precipitation of more sodic plagioclase from t he diatexite magma as concentric zones on the core. In either case, the presence of plagioclase crystals w ith a com plexly zoned and more calcic core provides evidence for the presence of mafic magma in the crust-derived anatect ic one.

Location Glenelg River Complex, Victoria, Aust ralia. Rock type plagioclase from leucosome. Scale: the long edge of the photograph represents 2.5 mm. Cross-polarized light Image and caption: Tony I.S. Kemp. Further reading: Castro, A (200 I): Plagioclase morphologies in assimilat ion experiments. Implications for disequilibrium melt ing in t he generat ion of granodiorite rocks. Mineralogy and Petrology 71 , 31 - 49. Kemp, AJS. (2004): Petrology of high-Mg. low-T i igneous rocks of the Glenelg River Complex (SE Australia) and t he nature of their interaction with crustal melts. Lithos 78, 119 156.

33Q ______C_O_N_TACT BETWEEN THE LEUCOSOME AND MELANOSOME IN METATEXITE MIGMATITES

Figure

4

Fig. F84. Thts photomicrograph shows the edge of an 1n sttu leucosome developed at the start of partial melting in a quartz-nch layer of the Btwabik Formation tn the footwall of the contact aureole around the Duluth Igneous Complex. Because the protoltth ts a whtte quartztte, there 1s restduum 1n th1s mtgmattte, but no melanosome. The melt-nch (left) and restduum-rich (right) parts of the neosome are better distingutshed by thetr dtsttnctive mtneralogies and especially thetr microstructures. The part that formed from the anatectic melt consists mostly of a graphic intergrowth of feldspar and quartz in the leucosome and the larger tntergranular domains in the residuum. but is a film of plagtoclase 1n the smallest tntergranular pores (A) and along gratn boundanes (B) that extend 1nto the residuum. The residual part conststs mostly of large corroded grains of quartz and some grains of opaque Fe-oxtde. The boundary between the melt-derived and the residual parts of the neosome IS highly Irregular at the grain scale; the meltderived part can be traced along gra1n boundanes between the residual quartz grains for a dtstance of at least four gratn diameters tnto the matnx of the quartztte. Gratns of restdual quartz become surrounded by anatectiC melt and t hen detached from the residuum and incorporated int o the melt-derived part; t his pattern is very similar t o that shown at the outcrop scale by some trans1t1ons fro m met atexite

to diatextte mtgmatites (see Figs. D41 -D43). Note that recrystall ization has occurred along some of the former melt quartz contacts, and that the quartz has developed small lobes, whiCh result 1n a cuspate outline. Anatecttc melt etther dtd not develop, or did not penetrate along, all of the quartz-quartz gratn boundanes (C). and several two- or three-gratn aggregates of quartz can be seen. Locatton: Du luth Igneous Complex, Minnesota, U.S.A. Rock type: metatexite migmattte; quartzite protolith, pyroxene hornfels facies, T 850 900°C. P 1.5-2 kbar. Scale: the long stde of the photomicrograph corresponds to 7 mm. Crosspolanzed light. Image: E.W. Sawyer.

A tl as of Migmatites

----------------- -------------- 331 Figure

85

Fig. F85. Th1s photomicrograph shows part of a larger (about IS mm across) diffuse leucosome generated in a pelitic layer that 1s much closer to the contact of the Duluth Igneous Complex than that shown 1n F1gs. F86 and F87. Consequently, the partial melting 1n th1s rock was at a higher metamorphic temperature and consumed much of the reactant b10t1te. Thus, the fract1on of melt that was produced was greater, and the res1duum richer 1n cordierite and K-feldspar, the solid products of the incongruent breakdown of b1ot1te. Much of the melt remained where 1t formed and enabled the residuum to recrystallize to a much coarser gra1n-s1ze and to develop a relat1vely ISOtropic microstructure typ1cal of contact metamorphic rocks. Some of the melt that did segregate formed th1s patch of m situ leucosome: 1t has an irregular and very diffuse outline, and in contrast to that in Fig. F86, has no biotite-rich melanosome around it. In this case. there is an enrichment of both cordierite and K-feldspar around the leucosome, but th1s nm of res1dual material IS not melanocratic and, therefore, 1nconsp1cuous 1n both the outcrop and the hand sample. The fine-scale sed1mentary layering v1s1ble 1n many rocks of the outer part of the m1gmat1te aureole IS no longer v1sible in this one, it was lost during grain growth and recrystallization. The complete absence of a foliation indicates that no flow of the melt w1th its crystals occurred. Thus, th1s IS a nebulitic m1gmat1te that conta1ns scattered

small patches of leucosome where some anatectic melt was able to segregate. The leucosome contains small polygonal, (commonly) hexagonal sector-twinned crystals of cordierlte, and somewhat larger rectangular crystals of K-feldspar and plagioclase, set 1n a matnx of large anhedral crystals of quartz, K-feldspar, and untw1nned plag1oclase. Th1s microstructure indicates that the leucosome crystall1zed from a melt. The euhedral habit of both cordierite and K-feldspar in the leucosome indicates that they crystallized 1n a melt, but the1r exact ong1n IS uncerta1n. They could be either the solid products of the 1ncongruent melting of b1ot1te that happened to grow 1n the anatectic melt. or they could have crystallized later from the anatectic melt 1tself as 1t cooled. The large anhedral crystals of quartz, plagioclase. and K-feldspar were the last minerals to form in the leucosome, and certainly crystallized from the melt. Large. bladed crystals of biotite occur throughout all parts of the migmat1te. and they are interpreted to be either a late product of crystallization of the melt, or to have formed when the final Increments of melt that rema1ned reacted w1th the res1dual cordiente. Location: Duluth Igneous Complex, Minnesota, U.S.A. Rock type: nebulit ic migmatite: metapelitic protolith, pyroxene hornfels facies, T 800 850°C, P 1.5- 2 kbar. Scale: the long s1de of the photomicrograph corresponds to II mm. Crosspolanzed light. Image: E.W. Sawyer.

0 332 ______C:..::O_N...:...T...:...A.:...:C:.....T_ B=--E=--T_W_::_E::..:EN~T...:...H:..::E...:...L:..::E:..::U:..::C:...:O:...:S:...:O:...:.M..:..:E::..:A:....:.:....:N-=-

MELANOSOME IN METATEXITE MIG MATI TES

Figure

86

Fig. F86 . This small. rounded patch of neosome has developed in a plagioclase-poor quartz b10t1te-cord1ente K-feldspar metapelite close to the "melt-in" isograd. wh1ch marks t he onset of partial melting 1n the aureole around the Duluth Igneous Complex. The whole-rock composition of th1s sample IS the same as similar rocks JUSt below the "melt-in" 1sograd, which suggests that no anatectiC melt has been lost or ga1ned and, therefore, that the patch of neosome formed in Situ. The patches of neosome in t his rock are too small (<5 mm across) for practical extraction and separation of the leucosome part from the melanosome. The separat1on of the partial-melt fract1on from 1ts res1duum has formed a small doma1n of leucosome I 3 mm acr oss that contains the assemblage quart z + K-feldspar + plag1oclase + b1otite + cordierite. The photomicrograph shows the leucosome to be surrounded by melanosome (cf. the neosome 1n F1g. F85) that is enriched in b1otite relat ive to the adjacent paleosome. Biotite may not have been involved in t he reaction that caused partial melt ing. but has grown to a noticeably larger grain-s1ze than b1ot1te in the paleosome. The leucosome part contains two populat1ons of biotite: small crystals that may be residual 1n ong1n and a few much larger; bladed crystals of biot it e that may have crystallized from t he melt or may have formed by react ion

between the melt and the residual phases. The contacts between the leucosome and melanosome parts of this neosome are well defined, but not sharp on the gra1n scale.

Locat1on: Duluth Igneous Complex, Minnesota, U.S.A. Rock type: patch metatex1te m1gmatite: Virgin1a Format1on pelite protolith, part1al melting 1n the pyroxene hornfels fac1es. T ca. 750°C, P 1.5 2 kbar. Scale: the long side of the photomicrograph corresponds to 4.5 mm. Plane-polarized light. Image: E.W. Sawyer.

A d as of Migmarires

------------------------------- 333 Figure

87

Fig. F87. The photomicrograph shows the same patch of neosome as 1n Fig. F86, but 1n cross-polanzed light 1n order to show the m1crostructure 1n the leucosome port1on. The quartz, K-feldspar, and plagioclase in the leucosome are much coarser-grained (0.25 mm) than the same m1nerals in the adjacent paleosome (>0.15 mm) . The largest w h1t e and grey crystals in the leucosome are quart z, and the smaller, rectangular grains are euhedral crystals of plagioclase and K-feldspar: Tiny (<0.07 mm, too small to see in this phot omicrograph) inclusions of cordierite are common in t he quartz, and because of their euhedral form, they are interpreted to have crystallized from the melt. Cord1ente and K-feldspar in the paleosome are the products of subsolldus metamorphic reactions and form larger (0.15 mm) xenoblastlc crystals that contain rounded inclus1ons of quartz and plag1oclase. Some of the leucosome matenal 1n the core of other patches of neosome 1n this and adJacent rocks contains small blades or radiating aggregates of muscov1te at 1ts center: the last melt to cr ystallize in these patches of leucosome presumably was enriched 1n H 20 and crystallized muscov1te in addition t o, or in place of, K-feldspar.

Locat1on: Duluth Igneous Complex, M1nnesota, U.S.A. Rock type: patch metatexite migmat1te: Virg1n1a Format1on pelite protolith, partial melting in the pyroxene hornfels faoes, T ca. 750°C, P 1.5- 2 kbar. Scale: the long s1de of the photomicrograph corresponds to 4.5 mm. Cross-polanzed light. /mage: E.W. Sawyer:

CON TACT BE T WEE N TH E LEUCOSOME AND

334

MELANOSO ME IN METATEXITE MIGMATITES

Figure

8

Fig. F88. This mosaic shows domains of leucosome from just inside t he " melt -in" isograd of t he contact aureole at t he D uluth Igneous Complex . Some key structural and pet rological feat ures are well illustrated in this migmat ite. T he bedding in the met asedimentary hornfels is distinguished by t he layer s of different grain size and color; the darker layers are richest in biotite and Fe-oxide minerals. Most of t he metasediment contains the assemblage cordierite + K-feldspar + quart z + plagioclase + biotite and shows no evidence for anatexis; it is, t herefore, paleosome. The grain size of the leucosome (up to 0.4 mm) is considerably greater t han that of t he metasediment (0.1 mm) . Leucosome is locat ed in small extensional shear zones that formed when a relatively more competent, cordierite-rich, semipelit ic bed was extended along its layering. A slight sigmoidal curvat ure of the foliation indicat es dext ral movement for t he shear zones as the layer was extended. Most cont acts between the leucosome and the metasediment are diffuse, and the leucosome thus seems t o have formed

in situ. There are several significant pet rological features in this migmat ite. Because t here is no syst ematic difference in t he modal proportion of cord ierite in the host rock close to the leucosome compared t o farther away, cordierite was probably not a product of t he melt-producing reaction; it had formed, together with K-feldspar, by an earlier

subsolidus reaction. T he slightly darker color of t he host rocks immediat ely around t he leucosome is t he result of alteration of the cordierit e; cord ierite fart her (>2- 3 mm) from t he leucoso me is unaltered. T he domains of leucosome are composed predominant ly of quartz, plagioclase, K-feldspar, and mino r amount s of muscovit e; t he large brown grains in t he leucosome and the adjacent host-rocks are tourmaline. Together, these petrological feat ures suggest that the rocks did not undergo fluid-absent melt ing of biotite. If t hey did partially melt , t hen it was probably by t he flu id-present melting of quartz, plagioclase, and K-feldspar; moreover, t he fluid contai ned boron. If partial melting did occur in these rocks, then the leucosome is in situ or in-source, but if melting did not occur, then the term leucocratic vein applies. In either case, it is likely t hat t he Hp released as t he melt cr ystallized migrated from t he leucosome, hydrat ed t he cordierit e in the adjacent wallrocks, and enabled growth of tourmaline there.

Location: Duluth Igneous Complex, Minnesot a, U.S.A Rock type: net-structured metatexite migmatite; pelit ic protolit h, pyroxene hornfels facies, T ca. 750°C, P 1.5- 2 kbar. Scale: the long side of the photomicrograph corresponds to 45 mm. Plane-polarized light. /mage: E.W. Sawyer.

Atlas of M igmat ites

------------------------------ 335 Figure

9

Fig. F89. This phot omicrograph shows part of a small in situ pat ch of neosome formed in a mafic granulite where

among the leucosome, the melanosome, and the paleosome

orthogonal to the weak layering and foliation in the mafic granulite paleosome. The part of the neosome shown in

is variable in this migmatite. Parts of the neosome conform to the general case for in situ formation and have the melanosome between t he leucosome and the paleosome (see Fig. F86). However, in places, the melanosome assumes the contrary position and is in the center of the neosome, with the leucosome in contact w ith t he paleosome. Melt ing may have begun at several places, and t hese subsequently coalesced to produce the irregular dist ribution.

the center of th is image is diffuse and is especially difficu lt to see in the cross-polarized light image (Fig. F90). The

Location: Mount H ay, Arunt a Inlier, Austral ia. Rock type:

neosome comprises a plagioclase + quartz leucosome and a coarse-grained (0.5- 1 mm), clinopyroxene-dominated melanosome, both of which have a granoblast ic-polygonal

pat ch metatexite migmatite; metamafic protolith, anatexis in the granulite facies, T 825 - 875°C, P 6- 7 kbar. Scale: the long side of t he photo is equal to I I mm. Plane-polarized light. Image: E.W. Sawyer.

the degree of partial melting was very low. T herefore, t his image shows a t ypical relationship between t he leucosome and melanosome that can be seen in many examples of neosome formed at t he onset of anatexis. The patch of neosome is about 3 em long and oriented

microst ructure owing to subsequent text ural re-equilibration, which is typical of regional metamorphic t erranes. T he plagioclase + hornblende + clinopyroxene paleosome exterior to t he neosome is noticeably finer-grained (ca. 0.3 mm), and has a more isotropic distribut ion of its constituent minerals than the neosome. The difference in grain size and dist ribut ion of minerals that results from anatexis is a key factor in distinguishing neosome from paleosome in migmatites w here the degree of partial melt ing was low. The boundary between the leucosome and melanosome is highly irregular. T he spat ial relationship

CONTACT BETWEEN THE LEUCOSOME AND

------------336 ----------------MELANOSOME IN METATE X ITE MIGMATITES Figure

0

Fig. F90. Same as Fig. F89, but t aken in cross-polarized light to better show the coarser grain-size of the neosome relative t o the paleosome. Location: Mount Hay, Arunta Inlier, Australia. Rock type: patch metatexite m1gmat1te; metamafic protolith, anatex1s 1n the granulite fac1es, T 825-875°C, P 6 7 kbar. Scale: the long side of the photo 1s equal to I I mm. Cross-polarized light. /mage: E.W. Sawyer.

A tlas of Migmat ites

------------------------------ 337 Figure

I

Fig. F91 . At t he outcrop scale, the neosome in this migmatite consists of well-defined portions of leucosome and melanosome. H owever, this phot omicrograph shows that the cont act between the two is very irregular at the grain scale, a feature generally regarded as more consistent with in situ formation of neosome than the injection of an anat ectic melt. A mass-balance calculation for this migmat it e using the bulk composition of the leucosome, its adjacent melanosome, and t he assumed prot olith supports an in situ origin for the neosome, that is, segregation of melt from its residue occurred w ithout loss of melt t o an external sink. T he protolit h was a hornblende + plagioclase + cl inopyroxene + garnet metamafic rock, and t he neosome that resulted from partial melting comprises a clinopyroxene + garnet + plagioclase + hornblende melanosome and a plagioclase + quartz leucosome. The garnet and clinopyroxene have a greater modal abundance and form larger cryst als in the melanosome compared to the protolith (not visible in the phot omicrograph). The clinopyroxene in the melanosome has experienced minor ret rogression and has, locally, developed a t hin rim of dark green hornblende.

Location: Abitibi Subprovince, Superior Province, Quebec, Canada. Rock type : met atexite migmatite; metamafic protolith, anatexis in the uppermost amphibolit e facies, T 800-850°C and P 8-10 kbar. Scale: the long side of t he photomicrograph corresponds to 7 mm. Plane-polarized light. /mage: E.W. Sawyer.

CO N TACT BETW EEN T HE LEU CO SO ME AND

338 ----------------------------MELA N OSOM E I N METATE X ITE M IG MATITES Figure

2

Fig. F92. The photomicrograph shows the same migmatite as in Fig. F91, but it is taken in cross-polarized light. T his

Location: A bit ibi Subprovince, Superior Province, Quebec, Canada. Rock type: metatexite migmatite; metamafic

reveals the far larger grain-size in the leucosome relative to t he melanosome. Furthermore, some lobate grainboundaries on t he quartz and undulose extinction are evident in both t he quartz and t he plagioclase in the leucosome. T hese features are the result of subsolidus, intracrystalline plastic deformation, w hich has removed all the microstructural crit eria normally used to infer crystal-

prot olith, anatexis in the uppermost amphibolite facies, T 800- 850°C and P 8- 10 kbar. Scale: the long side of the photomicrograph corresponds to 7 mm. Cross-polarized light. Image: E.W. Sawyer.

lization from a melt. Some small, irregularly shaped cryst als of plagioclase in the melanosome are conspicuously zoned, and generally have a more calcic rim; these grains are interpret ed to be residual plagioclase. Microstructures such as t his, which have been highly modified by subsequent deformat ion and recrystallization so that t hey no longer retain evidence t hat t hey crystallized from a melt, are t ypical of t he leucosome from regional metamorphic terranes.

Further reading: Sawyer, E.W. ( 199 1): Disequilibrium melt ing and the rate of melt-residuum separation during migmatizat ion of mafic rocks from the Grenville Front, Quebec. Journal of Petrology 32, 70 1-738.

A tl as of M igmati res

-------------------------------339 Figure

93

' Fig. F93 . T his photomicrograph shows t he contact between a stromatic leucosome and its melanosome in a metatexite migmatite that was derived from a metapelit ic protolit h. The contact between the leucosome and t he melanosome is abrupt at bot h the outcrop scale and t he hand-sample scale, but it is irregular on t he grain scale. T he melanosome consists of the assemblage biotite + garnet + sillimanite + K-feldspar + quartz + plagioclase, and is interesting because it exhibits an int ernal mineralogical and textural zonation. Most of the garnet occurs close to the contact w ith the leucosome, whereas t he sillimanite is locat ed farther away and is not actually in contact with t he leucosome. The biotite nearest to the leucosome is considerably coarser-grained t han biot ite fart her away and may, in part, replace the garnet. The leucosome is granitic in composition, and has t he mineral assemblage quartz + plagioclase + K-feldspar + minor biotite.

Location: Central Metasedimentary Belt, Grenville Province, Quebec, Canada. Rock type: stromat ic metatexite migmatite; metapelitic protolith, anatexis in the lower granulit e facies. Scale: t he long side of the photomicrograph corresponds to 9 mm. Plane -polarized light. Image: E.W. Sawyer.

CONTACT BETWEEN THE LEUCOSOME AND

------------340 ---------------MELANO SOME IN METATE X ITE MIGMATITES Figure

4

Fig. F94 . The photomicrograph shows the same rock as 1n Fig. F93, but with crossed polarizers to show that the leucosome has a much coarser grain-size than the melanosome, even after 1ts gram size has been reduced by deformation. However, the grain s1ze of the m1nerals 1n the melanosome decreases systematically away from the leucosome. Deformation, recovery, and recrystallization have affected the m1nerals in the leucosome and have eradicated all the microstructural evidence that might have indicated crystallization from an anatectic melt. The quartz in the leucosome has undergone some grain-boundary m1grat1on in response to deformation, and this has resulted 1n the curved, bulg1ng grain-boundanes. Some plag1oclase crystals show curved alb1te-law tw1ns and undulose extinct1on. Locat1on : Central Metasedimentary Belt. Grenville Province, Quebec, Canada. Rock type: stromatic metatexite migmatite; metapelit1c protolith, anatexis 1n the lower granulite

facies. Scale: the long side of the photomicrograph corresponds to 9 mm. Cross-polarized light. /mage: E.W. Sawyer.

A tlas of Migmatites

------------------------------ 341 Figure

95

Fig. F95. Th1s photomicrograph shows the contact between a stromatic leucosome and its residuum 1n a metatexite migmatite formed from a plag1oclase-poor aluminous metapelite in a regional metamorphic terrane. The contact between the granitic leucosome, containing quartz + plagioclase + K-feldspar + biotite + garnet. and the adjacent residuum, containing sillimanite + garnet + K-feldspar + biotite + quartz + ilmenite, is sharp and planar in both the hand sample and the photomicrograph. The irregularity of the contact is on the scale of several graindiameters. like those from other migmatites (e.g.. Figs. F84, F9 1. and F93), but in this case, the grain size of the sillimanite is only 0. 17 mm. Hence, the contact appears to be planar principally because of t he fine grain-size of the sillimanite. The rock adjacent to the leucosome is dominated by sillimanite and is, therefore, not darker than nearby paleosome: it is res1duum. Large crystals of garnet, in places embayed and partially replaced by aggregates of large crystals of biotite, occur within the sillimanite. The grains of garnet are generally not in direct contact with the leucosome, however. Hence, the mineralogical zoning in the residuum next to the leucosome is the converse of the situation in Figs. F93 and F94. A lthough not show n in t he photomicrograph, t he residuum farther away from t he leucosome contains less sillimanite. and more biotite and K-feldspar: the latter,

along with the garnet. represents the solid products of the mcongruent breakdown of biotite through a react1on like b1ot1te + sillimanite + quartz melt + garnet + K-feldspar. Much of the biotite in the leucosome occurs as large grains

=

along the boundaries between feldspar grains, or as irregularly shaped grains that appear to have grown interstitially with respect to quartz and feldspar.

Location: Mount Hay, Arunta Inlier, Australia. Rock type: stromatic metatexite migmatite: aluminous metapelite protolith, melted in the granulite facies. T 825-875°C. P 6-7 kbar. Scale: the long side of the photograph is equal to 9 mm. Plane-polarized light. /mage: E.W. Sawyer.

A da., of Mi gmatite'

---------------- ------------- 343 Figure

7

... I

/ .

.•

'

I ,

Fig. F97. Th1s photom1crograph shows part of a s1ngle, biot it e-nch schliere I 2 mm wide in a K-feldspar (now m1crochne) + quartz + plag1oclase + b1otite diatex1te m1gmat1te denved from an evolved melt. Schlieren 1n th1s m1gmat1te occur 1n a band about 30 em w1de that can be traced several meters. Most 1ndiv1dual schl1ere are th1n (millimetnc) b1ot1te-nch fol1ae that extend for only 30-60 em. Locally, there are w1der (up to 10 mm) and longer schl1eren, but they are less b1ot 1t e-rich (t hey contain biotite + plag1oclase). The microstructu re of the t h1n schliere 1n the photomiCrograph IS typ1cal : 1t IS no more than a few b10t1te gra1ns w1de, and locally w1dens where 1t encloses crystals of plag1oclase. It conta1ns crystals of b1ot1te that have a h1gh aspect-ratiO and that are arranged 1n a systematic, lmbncat e or tiled manner, w1t h thew long axes at a low angle to t he schliere. This schliere may have been assembled as biotite flakes, forced to rot ate by the shear 1mposed on the diatexite magma; they impinged on thew neighbors and formed 1mbncate clusters of gra1ns. There are small clusters of b1ot1te that appear to be 1nc1p1ent schliere next to the pnnc1pal one. Interest ingly, these mop1ent schliere are separat ed from the pnncipal schliere, and each other, by a layer of plagioclase that IS just a single subhedral crystal (ca. I mm) of plag1oclase wide. T his microstructure is reminiscent of the I 0-mm-wide biot1te + plag1oclase schlieren

present elsewhere 1n the rock, composed of a net-like arrangement of b1ot it e filaments that enclose subhedral crystals of plagioclase.

Location: OpatiCa Subprov1nce. Quebec, Canada. Rock type: b1ot1te schliere 1n a diatex1te m1gmatlte; leucotrondhJemlte protolith, T ca. 750°C, P 5 7 kbar. Scale: the width of the photomicrograph corresponds to II mm. Plane-polanzed l1ght. Image : E.W. Sawyer.

Further reading: Milord, I. & Sawyer, E.W. (2003): Schl1eren format1on 1n diatex1te m1gmatite: examples from the St. Malo m1gmat1te terrane, France. journal of Metamorph1c Geology 21 , 341 362.

342

CONTACT BETWEEN THE LEUCOSOM E AND

------ ------ ----------- -----MELANOSO ME IN METATEXITE MIGMATITES Figure

96

Fig. F96. Thts photomicrograph, of the same field of vtew as Fig. F95. is taken 1n cross-polarized light to show that the leucosome 1s much coarser-gratned than 1ts assoc1ated residuum. A lthough the quartz and feldspar 1n the leucosome show undulose extinctton and some evidence of mtgration of grain boundaries. some crystals of plagtoclase and perthitic K-feldspar are subhedral and retain rational faces against quartz. a microstructure that IS commonly used to infer growth from a melt. Locatton: Mount H ay. Arunta Inlier, Australia. Rock type: stromattc metatexite migmattte; aluminous metapelite protolith, melted 1n the granulite faoes. T 825 875°C, P 6 7 kbar. Scale: the long side of the photograph is equal to 9 mm. Cross-polanzed light. /mage: E.W. Sawyer.

MICROSTRUCTURE OF SCHLIEREN

344 ----------------------------I N D IAT EXITE M IGMATITES Figure

98

Fig. F98. The ma1n part of this schliere IS composed of very elongate crystals of biotite that are arranged in an 1mbricate or t1led pattern. Consequently, the schliere IS, 1n general, only a few grains wide. However, the b1ot1te that forms the bridge between the two parallel schl1ere has a lower aspect -rat io and is not organ1zed in an Imbricate fashion. It also appears to be just one crystal of biotite wide. Crystals of biotite in t he large subhedral crystals of plagioclase located between the two schlieren are oriented parallel to the schlieren, i.e., orthogonal to the bridge of biotite. Thus, the lower aspect-ratio of the biotite in the bridge could exist because the biot ite there (I) crystallized from melt located between the adjacent subhedral crystals of plagioclase or (2) recryst allized from smaller gra1ns originally onent ed parallel to the schlieren. It IS 1nterest1ng to note that the tabular crystals of plagioclase 1n the host also are imbncated or tiled in the same sense as the biotite. This microstructure suggests that at the time the b1ot1te schliere was forming, the magma also contained the tabular plagioclase crystals. Both the biotite and the plag1oclase were aligned by the shear imposed on the diatex1te magma.

Location: Opatica Subprov1nce, Quebec, Canada. Rock type: biotite schlieren 1n a d1atex1te m1gmat1te: leucotrondhjemite protolith; upper-amphibolite-facies anatex1s, T ca. 75ooe, P 5-7 kbar. Scale: the w1dth of the photomicrograph corresponds to 9 mm. Plane-polarized light. Image: E.W. Sawyer.

A tlas of M igmatites

------------------------------- 345 Figure

9

Fig. F99. Some of the biotite schlieren that occur in the melt-rich diatexite migmatit es in the Opatica Subprovince have a conspicuous, narrow, white rim . This photomicrograph shows a biotite- rich schliere that has a plagioclase + quartz border 3 mm wide on each side that separates it from the coarse-grained K-feldspar + quartz + plagioclase leucocratic diatexite. The grain size of quartz and plagioclase decreases across the rim t oward t he schliere. The origin of the white, K-feldspar-free rim is problematical. The bulk composition of the diatexite can be modeled as an evolved melt. but the biotite schliere and its w hite plagioclase + quartz border do not resemble an anatectic melt in terms of composition. It can be modeled as an accumulation of these minerals, although how this cou ld happen remains unknown. The change in microstructure and mineral assemblage could indicate that a local diffusion of components, such as K, may play a part in the formation of some schlieren.

Location: Opatica Subprovince, Quebec, Canada. Rock type: biotite schliere in a diat exite migmatite; leucot rondhjemite protolith, T ca. 750°C, P 5- 7 kbar. Scale the length of the photomicrograph corresponds to II mm. Cross-polarized light Image : E.W. Sawyer.

346

_ URE OF SCHLIEREN _T _C _U _ ST_R _________M_IC_ RO IN DIATEXITE MIGMATITES

Figure

100

.-....

~ . . ...

.-

.- '

-:-

~

. ).;.·

~·

'\

\ ..' .,.,.

•

Fig. F I 00. This disc-shaped schliere IS about 20 mm across and 2 mm w1de: it occurs in a diatex1te migmatite that was derived from a pelitic protolith. It contains the mineral assemblage biot1te + silliman1te + garnet + quartz + accessory phases and trace amounts of plag1oclase. The central part of the schliere cons1sts pnmanly of b1otite that encloses pnsmat1c Sillimanite and 1dioblastlc crystals of gar-

net; minor amounts of quartz occur as small. Irregularly shaped grains between some of t he biotite flakes. Domains of quartz + S1lliman1te + plagioclase occur with biot1te at the border of the schliere. K-feldspar and cordiente are confined to the d1atex1te host. although they are locally 1n contact w1th the schliere. Most of the b1ot1te crystals 1n the schliere have a h1gh aspect-rat1o. In the th1cker parts of the schliere, however, the biotite microstructure is decussate, whereas in t he thinner parts, it is imbncat e or t iled. These relationships suggest that this schliere may be an attenuated p1ece of res1dual material, rather than the result of the accumulation of b1ot1te crystals.

Location : Lac Kenogam1, Grenville Prov1nce. Quebec, Canada. Rock type: b1otite schliere in a diatexite migmatlte; pelitic protolith, anatex1s 1n the lower granulite fac1es: T ca. 800°C, p ca. 5 kbar. Scale: the long Side of the photomicrograph corresponds to II mm. Plane-polarized light. /mage: E.W. Sawyer.

A tl as of Migmat ites

------------------------------347 Figure

I0 I

Fig. F I 0 I . A thin (0.3 mm) mafic selvedge composed of biotite occurs between a 2.5-cm-wide K-feldspar- plagioclase- quartz leucocratic vein and its host biotite-quartz-plagioclase psamm1t1c schist (metagreywacke) . The leucocratic vein is discordant to the compositional layering, interpreted to be bedding, and the foliation in the psammitic host. The absence of K-feldspar, sillimanite, cordierite, and patches of neosome is evidence that the psammite probably did not undergo partial melting. However, thin pelitic layers elsewhere in the outcrop did partially melt, as they contain neosome, cordierite, and K-feldspar. Therefore, the psammite layers can be considered to be paleosome in the migmatite. Note that the crystals of biotite in the mafic selvedge are strongly aligned and oriented parallel to t he contact with the leucocratic vein, and not parallel to the fol iation in the host. The biotite cryst als also are slightly longer (0.6 mm) than those in the host (0.4 mm). Two grains of plagioclase (indicated by arrows) at the edge of the leucocratic vein have grown after the mafic selvedge formed and deflected it. T he feldspar, mostly K-feldspar, in the center of the vein shows more intense alteration to white mica than the plagioclase at the edges of the vein.

Location: Quetico Subprovince, Ontario, Canada. Rock type: mafic selvedge to a leucocratic vein in a metatexite migmatite; met agreywacke host, anatexis at T 700-800°C, P 3 4 kbar. Scale: the long side of the photomicrograph corresponds to 10 mm. Cross-polarized light. /mage: E.W. Sawyer.

MIC ROSTRUCTURE OF BIOTITE- RICH

348-----------------------------SELVEDGE S IN MIGMATITES Figure

101

Fig. F I 02 . This photomicrograph shows a mafic selvedge I mm wide developed between an apatite- and garnetbearing quartz-plagioclase- K-feldspar leucocrat ic vein IS mm wide and its quartz- biotite- plagioclase metagreywacke host. The mafic selvedge is only a few grains of biot ite wide and devoid of quartz or plagioclase; quartz and plagioclase thus may have been removed as the mafic selvedge formed. C rystals of biotite in the selvedge are not iceably larger, and like t hose in the host, do not have much of a preferred orientation. Two large grains of plagioclase located at t he edge of t he leucocrat ic vein have undergone a late increment of growt h t hat may have occurred after the mafic selvedge formed, as bot h cryst als appear to have grown across t he biotite t hat forms t he selvedge. The material for this lat e e pisode of grain growt h possibly was derived fro m the redistribution of the plagioclase compo nent as the mafic selvedge fo rmed.

Location: Quetico Su bprovince, O ntario, Canada. Rock type: mafic selvedge to a leucocratic vein in a metat exit e migmatite; metagreywacke host, anatexis at T 700-800°C. P 3- 4 kbar. Scale: t he long side of t he photomicrograph corresponds to 14 mm. Plane-polarized light. /mage: E.W. Sawyer.

A tlas of Migmat ites

Figure

I 03

Fig. F I 03. In this example, the host is a psammit ic metagreywacke poor in both AI and K. Consequent ly, it contains the mineral assemblage plagioclase + biotite + quart z + hornblende. The disco rdant K-fe ldsparplagioclase- quartz leucocratic vein that is int ruded int o t he psammit e has a markedly d iffe rent microst ruct ure fro m t hat in Fig. FIOI. Quartz was o ne of the last mi nerals to crystallize in the interstices of t he framework st ruct ure fo rmed by t he early cryst allizat ion of euhedral and su bhe dral K-felds par and plagioclase. Hence , the cryst als of quartz have a blo cky o utline, as t here are many straight contact s with the fe ldspar cryst als, consistent with crystallizatio n from a melt. The leucocratic vein is II mm w ide. A narrow mafic selvedge 1.8 mm wide t hat contains both biot ite and ho rnblende has fo rmed between t he vein and it s host. In contrast t o many mafic selvedges, t his one shows a more gradual increase in t he grain size of both biotite and ho rn blende (Hb) fro m t he ho st toward t he edge of t he vein; both are coarser by a factor of about five relat ive to t he host. There is also a more gradual depletion of quartz and plagioclase across t he mafic selvedge.

Location: Quetico Subprovince, O ntario, Canada. Rock type: mafic selvedge to a leucocrat ic vein in a metatexite migmat ite; metagreywacke host, anat exis at T 700- 800°C, P 3- 4 kbar. Scale : t he lo ng side of the photomicrograph corresponds to I I mm. Cross-polarized light. /mage: E.W. Sawyer.

349

MICROST RUCT URE OF BIOTITE-RICH

350

SELVEDGES IN MIGMATITES

Figure

104

Fig. F I 04 . Th1s photomicrograph shows the mafic selvedges at both s1des of a discordant K-feldspar-quartz plag1oclase leucocrat1c ve1n I 0 mm wide InJected 1nto a hornblende b1ot1te quartz plag1oclase met agreywacke schist. Plag1oclase (cleavage and cloudy alterat1on) was the first maJOr mineral t o crystallize in the ve1n and was typ1cally euhedral or subhedral in shape. Q uartz (white featureless mineral) was the last major phase to crystallize, and consequently, it has an irregular shape and blocky out line that was inherited from the shape of t he melt-fi lled pore-space within the framework of feldspar cryst als. Bot h mafic selvedges are narrow and show a five- to ten-fold 1ncrease in grain size over that of typ1cal crystals of hornblende and b1ot1te in the host. In general, the biotite in the mafic selvedge IS Oriented parallel to the contact of the ve1n. but locally there are gra1ns, or aggregates of grains, that have other orientations. Most commonly. the differently or~ented gra1ns are spat1ally assooated w ith subhedral gra1ns of plag1oclase that appear to have had a late, outwardly directed 1ncrement of growth (or perhaps overgrowt h) into the mafic selvedge. The late increment of plagioclase growth from the edge of the vein may be related to the eliminat 1on of small crystals of plagioclase (and quartz) from t he innermost part of the mafic selvedge. Inclusions of accessor y phases and t heir

surround1ng pleochroic halos are much larger. and more consp1cuous, 1n the b1ot1te and hornblende of the selvedge, compared to those 1n the same mmerals in the host rock.

Locat1on: Quet1co Subprovmce, Ontario, Canada. Rock type: mafic selvedge to a leucocratic vein in a metatexite migmatite: metagreywacke host. anatexis at T 700 800°C, P 3 4 kbar: Scale: t he long s1de of the photomicrograph corresponds to 14 mm. Plane-polarized l1ght. Image: E.W. Sawyer.

A tl as of Migmatites

----------------- ------------- 351

G. MIGMATITE-LIKE ROCKS Many rocks that are not m1gmat1tes have been m1staken for t hem and shown on maps as migmat it es. In th is section, I show some of the rock types t hat are commonly mistaken for m1gmatites: I start with subsolidus segregations, and follow with syntectonic plutons, and end with arrays of felSIC

ve1ns.

Layer-parallel or stromatic subsolidus segregations [Figs. G I, G2] Segregations consisting of laterally persist ent quartz or quartzofeldspathic layers flanked by biotite-rich selvedges are common 1n many low-, medium-, and h1gh-grade metamorphiC terranes. Typ1cally. the segregations are located parallel to the compositional layenng or to the foliation 1n the host: consequent ly, the ones 1n higher-grade hosts, in particular, can resemble stromatic metatexite migmat ites. Numerous st udies have shown t hat the segregat ions are t he result of stress-1nduced chemical t ransfer of the more mobile quartzofeldspathic components, leaving the insoluble minerals, such as b1ot1te, beh1nd to form the dark selvedge.

Fleck segregations [Figs. GJ, G4] The t erm (leek is reserved for equant. centimeter-sized segregations, typ1cally formed under hydrostatic conditions by subsohdus, diffus1on-controlled growth; partial melting 1s not 1nvolved. Typ1cally, the segregations have a globular form, although more complex shapes can anse from the coalescence of adjacent flecks. Characteristically, flecks have a melanocratic core surrounded by a leucocratic, quartzofeldspathic mantle.

Syntectonic plutons: rocks that resemble metatexite migmatites [Figs. GS-G I 0] Magmas pass from a suspension of crystals 1n melt to a framework of crystals containing Interstitial melt and, finally, to a solid as t hey cryst allize. W hen a framework of touchi ng crystals w1th interstitial melt (a stage called the rigid percolation threshold) forms, the crystall izing magma is in the same state as rocks 1n the early stages of anatexis when a framework of res1dual crystals conta1ns

anatectic melt. T herefore, deformation of a pluton at this stage of crystallization can produce rocks with a morphology that 1s remarkably s1milar to some types of m1gmat1te. Furthermore, 1f parts of a pluton that conta1n magmas of d1fferent compos1t1on are deformed to high, synmagmatic shear stra1ns, then these reg1ons can develop a strongly layered appearance w it h bands of distinct composit1on, grain size, and microstructure, w hich can resemble some stromatic m1gmatites.

Syntectonic plutons: rocks that resemble diatexite migmatites [Figs. GII-GI4] D1atexite migmatites conta1n a high fraction of melt relative to paleosome: therefore, similar-look1ng rocks might be expected to form in environments where the ratio of magma to solid rock was h1gh also. The margin of felsic intrus1ve bodies where xenoliths are entrained 1n magma IS one such sett1ng.

Arrays of felsic veins that look like migmatites [Figs. G I S-G20] The injection of felsic veins into mesocrat1c, high-grade hosts can produce rocks that resemble many m1gmatites, particularly if a melanocrat1c nm is developed around the ve1ns. In these figures, I show some examples of ve~ned rocks that could be m1staken for m1gmatites.

LAYER-PARALLEL OR STROMATIC

352 ----------------~----------SUBSOLIDUS SEGREGATIONS

Figure

I

Fig. G I. This photograph shows folded subsolidus segregations in a lower-amphibolite-facies metapelite and metagreywacke host. The segregations are located parallei to the compositional layering, wh1ch is interpreted to be bedd1ng, and have a prominent Internal structure. The intenor of the segregation is leucocrat1c and consists principally of quartz and plagioclase; however, it can be subdivided 1nto plagioclase-rich and quartz-rich layers plus a few thin seams of biotite. There are biotite-rich selvedges on either side of the segregation, and petrographic and geochemical mass-balance estimates show that they formed by a closed-system segregat1on of more mobile components (quartz and plagioclase) from the less mobile (biotite) . Subsolidus segregations typically develop a quartzrich core and plagioclase-rich nm as they grow, as first quartz and then plagioclase become mobile. Consequently, the internal layering of the thick part of the segregation in the photograph could mean that it IS 1n fact made up of several closely spaced, thinner layer-parallel segregations. The compositional layering v1sible in low-grade subsolidus segregat1ons means that they are less likely to be mistaken for leucosome in stromatic migmat1tes. However, if the zoned structure did not develop, or has been subsequently

lost through recrystallization at higher metamorphic ternperatures, then subsolidus segregations can resemble plag1oclase-rich leucosome.

Locatton: Quetico Subprovince, Ontario Canada. Rock type: plag1oclase-quartz subsohdus segregation, lower amphibolite faoes, T 600°C, P 3 4 kbar. Scale: the ruler is IS em long. Image: E.W. Sawyer.

Further reading: Sawyer, E.W. & Robin, P.-Y. F. (1986): The subsolidus segregat1on of layer-parallel quartz feldspar ve1ns 1n greenschist to upper amphibolite fac1es metasediments. journal of Metamorphic Geology 4 , 237-260.

Atlas of M igmarires

------------------------------- 353 Figure

2

Fig. G2. The outcrop 1n this photograph conta1ns Interbedded metagreywacke and metapelite on the hightemperature s1de of the "melt-in" 1sograd mapped 1n metapelites. It is unlikely that th1cker. light-colored beds of metagreywacke in the photograph have undergone partial melting; they are not sufficiently aluminous, and contain the paragenesis plagioclase + biotite + quartz. The ruler lies close to a vein of coarse-grained granite (K-feldspar + plagioclase + quartz+ garnet+ muscovite assemblage) that has intruded the metasedimentary rocks; 1n places, it has developed a very narrow, biotite-rich mafic selvedge. An offshoot from th1s vein of granite occupies a dextral shear zone and truncates a group of four or five layer-parallel subsolidus segregation veins (A); another subsolidus segregation (B) occurs on the other side of the shear zone. Each has a narrow, mafic selvedge that conta1ns the assemblage b10t1te + plagioclase + accessory m1nerals. These ve1ns have about the same plag1oclase:quartz rat1o (ca. 0.4). but a coarser grain-size and a more homogeneous distribution of the quartz and plagioclase than the subsolidus segregation veins in Fig. G I. These differences are attributed to recrystallization at the higher temperatures experienced by the rocks of this outcrop. rather than growth at the higher temperatures. Although the subsolidus segregations have a leucocratic core and a melanocratic rim like the neosome

1n many migmat1tes. there are some s1gmficant differences 1nd1cating that they did not form by anatex1s. (I) The leucocratic parts do not conta1n K-feldspar; K-feldspar is expected if a melt is derived from the metapelite. (2) The mafic selvedge portions do not contain a mineral assemblage compatible with Incongruent melt1ng involving the breakdown of biot1te. Location: Quetico Subprovince, Ontario Canada. Rock type : metatexite migmatite containing granite veins and subsolidus segregations; metapelite and metagreywacke protolith, upper amphibolite facies, T 700 750°C, P 3 4 kbar. Scale : the ruler is IS em long. /mage: E.W. Sawyer. Further reading: Blom, K.A. (1988): Subsolidus mlgmatization 1n h1gh-grade meta-tuffs (Kurkljlirvl, southwest F1nland). L1thos 2 1. 263-278. Sawyer. E.W. & Barnes, S.-J. (1988): Temporal and compositional differences between solidus and anatectic m1gmatite leucosomes from the Quetico metasedimentary belt, Canada. journal of Metamorphic Geology 6, 437 450.

FLECK SEGREGATIONS

354 ----------------------------Figure

3

Fig. GJ. Photograph of a polished slab of biotiteplagioclase-quartz K-feldspar gneiss (or schist) that contains well-developed fleck structures. All the flecks on this slab have a magnetite core and a quartzofeldspath1c mantle. Most of the flecks are about 8 mm long and slightly elongate in shape, with the long ax1s onented parallel to the foliation in the host rock. In some nearby rocks, the flecks are conspicuously elongate ellipsoids, with their long axes oriented parallel to the mineral lineation in their host. Therefore, the formation of flecks either predates, or was synchronous with, the penetrative deformation 1n the rocks. Flecks are not randomly d1stnbuted in this rock; some foliat1on-parallel domains conta1n more, or larger, flecks than others, and th1s is attributed to m1nor variations 1n composit ion from one layer to the next that favor the development of flecks in some layers over others. Note the close macroscopic resemblance of these fleck structures to some types of patch migmatites, for example, those shown 1n Figs. 09 and D I 0. Flecks w1th cores cons1sting of hornblende and of tourmaline also have been reported.

Locat1on: Wet Mounta1ns, Colorado, US.A. Rock type : flecked gneiss; biotite gne1ss protolith, metamorphic conditions T 600 700°C, P 3 5 kbar. Scale: tape in centimeters. Image: Bob Trumbull. Further reading: Trumbull, R.B. (1988): Petrology of flecked gneisses in the northern Wet Mountains, Fremont County, Colorado. Geological Sooety of America, Bulletin I 00,

247 256.

•

Arias of Migma t ites

Figure

4

Fig. G4. T he flecks in th is biot ite schist have hornblende in the core and are surrounded by a mantle of quartz, plagioclase, and K-feldspar. Although the rock contains some isolat ed flecks, it also contai ns discontinuous bands formed by t he coalescence of many closely spaced flecks. Overall, t he bands are oriented parallel to t he foliat ion in the host, but in detail t he bands are locally discordant to t he biotite foliae (e.g., just below t he start of the scale). It is int erest ing to note t hat a proportion of the hornblende cr ystals are elongate in the direction perpendicular to t he length of the fleck bands and to t he foliat ion in the host. Some of the fl ecks in this rock are equant in shape, but others are lenticular and weakly sigmoidal in shape. Fleck formation t hus seems to have cont inued after t he deformat ion ended in t his rock. In contrast to t he magnetit e flecks in Fig. G3, t he hornblende in t hese flecks is ( I) intergrown wit h quartz and feldspar, (2) commonly irregular in shape, (3) not invariably centered in t he fleck, and (4) commonly present in clust ers, rather t han as single cr yst als. The out crop appearance of the hornblende flecks, and especially t he band of coalesced flecks, closely r esembles neosome in some migmat it es, most not ably t hose formed by t he incongruent melting of biotite in w hich t he melt and solid

reactant fractions have not separated (cf Figs. B 19, 09, and D I 0). The most noticeable difference is the fine grain-size of the quartzofeldspathic mantle of the flecks. Location : Wet Mountains, Colorado, U.S.A Rock type:

flecked gneiss; biotite gneiss protolit h, metamorphic conditions, T 600-700°C, P 3-5 kbar. Scale: t he tape is in centimet ers. /mage : Bob Trumbull. Further reading: see Fig. G3.

355

SYNTECTON IC PLUTONS: ROCKS THAT

356 -----------------------------RESEMBLE METATEXITE MIGMATITES Figure

s

Fig. GS. The outcrop is part of a syntectoniC pluton that IS about 20 km across. The most abundant lithology IS grey tonalite, most of which has a foliation obta1ned 1n the magmatic or submagmatic state. At some places, the tonalite still was a magma when it was intruded by mafic magma. and th1s resulted in the formation of trains of rounded, pillow-shaped mafic enclaves. Elsewhere, the tonalite had crystallized to a greater extent, and the injection of mafic magma into it formed dikes, some of which, as in this outcrop, were later disrupted to form scattered, subangular mafic blocks in a foliated tonalite host. A leucocratic tonalitic to granodioritic magma also was present. and formed conspicuous wh1te layers parallel to the foliat1on 1n the tonalite, and around the blocks of mafic material. Close Inspection of the outcrop reveals the presence of an array of Interconnected grey veins of nonfoliated tonalite that enclose blocks of the foliated tonalite; the contacts between the ve1ns and the host are gradational. In addition, there are (e.g., at the left end of the mafic enclave 1n the center) tranSitional contacts from foliated tonalite through nonfoliated tonal1te to the leucotonalite-granodionte, 1ndicating that all must have contained some melt at the same t ime. This array of diffuse veins enclosing a foliated host closely resembles some net-structured metatexite migmatites, and is the principal reason why this outcrop could be mistaken for a

m1gmatite. However, there IS no petrolog1cal evidence that the rocks in this outcrop underwent partial melting. Locally. metamafic rocks and silic1clast1c metasediments form the wallrocks to the pluton, and they contain lower-amphi bolite-facies assemblages of m1nerals. wh1ch also suggests that the pluton did not undergo anatexis. Furthermore, the reg1onal "melt-in" isograd is located at least 20 km away. Consequently, the transitional contacts are interpreted to indicate that the network of tonalite and leucotonalitegranodiorite veins were derived from a residual (i.e., a fractionated or evolved) melt that was expelled from the foliated tonalite part of the outcrop as it was deformed dunng the final stages of its crystallization. The youngest ve1ns in the outcrop are granitic in composition, and are dist1nguished by their p1nk color and pegmatitic microstructure (comb-structure of feldspar crystals grown in from the vein walls and the quartz-rich center); these ve1ns are 17 Ma younger than the ages of crystallization of the tonalite pluton. Location: Ouescapis Pluton, Opatica Subprovince. Quebec, Canada. Rock type: syntectonic tonalite pluton, lower amphibolite facies. Scale: the ruler is IS em long. /mage: E.W. Sawyer.

A tl as of Migmarircs

--------------- ------------- 357 Figure

6

Fig. G6. Th1s phot ograph shows part of a syntect onic, quartz monzodiorit e pluton that has been intruded by a mafic magma as it crystallized. In the lower part of the outcrop, the mafic magma, which was less competent than 1ts monzodiorite host when it was deformed, has been attenuated into thin, dark-colored w1sps that resemble schlieren in diatexite migmatites. In contrast. the mafic dike in the center of the photograph was more competent than it s host. and was disrupted into a train of boudins. The monzodiorite was in a submagmatic state (i.e., a crystal framework with as much as 60 vol.% of interstitial melt) when the boudins formed, and s1nce the regions between the boudins are domains of low pressure during the boudinage process, they sucked melt out of the interstices in the framework of feldspar and amphibole cryst als. The melt that segregated from the monzodiorite mat rix had an evolved com posit ion and produced small veins of leucogranodiorite between the boudins when it crystallized. The morphology of this part of the syntectonic pluton cons1sts of leucocratic veins located between mafic boudins in a mesocrat1c host and, therefore, resembles some metatexite m1gmatites (e.g., Figs. B39, 018, 019, and 025). However, in this case, there is no petrographic evidence for partial melting in either t he

mafic layers or in t he monzodiorite. The composition of the leucocrat ic rocks is consistent with a fract ionated melt rema1ning after the host monzodiorit ic magma had largely crystallized. Location: Canet pluton, Opatica Subprovince, Quebec, Canada. Rock type: syntectonic monzodionte pluton, greenschist facies. Scale: the ruler is IS em long. Image: E.W. Sawyer.

358

------------~S_ Y~ N_ T~E~C_T~O_N_I~C_P~L~ U_ T~ O_ N~S~:~ R~ O~ C~ K~S~T~H~A~ T

RESEMBLE METATEXITE MIGMATITES

Figure

7

Fig. G7. Thts photograph shows an outcrop in the 2824 ± 3 Ma Lac Rodayer pluton, a typtcal Archean pluton of the tonalite trondhjemtte-granodtorite su1te. Three types of tonalite can be distinguished in the outcrop by subtle differences in color, grain size, and mtcrostructure. The bottom left corner of the photograph has the darkest rock, and 1t is a biotite-hornblende quartz plagioclase tonalite. T his grades to a slightly lighter-colored biotite hornblende- quartz- plagioclase tonalite that is also slightly coarser-grained, and occupies the top half of the outcrop. Both of the grey tonalites form patches wtth somewhat rounded shapes, and are enveloped by a leucocrat1c K-feldspar hornblende biotite quartz plagtoclase tonalite that 1S coarse-gratned; th1s IS prevalent at the bottom nght. The regton of 1nterest is to the right of the scale. There, the leucocratiC tonalite forms an array of diffuse vetns that divtdes the darkest tonalite 1nto several blocks. Overall, therefore, this outcrop contains leucocrattc, mesocrat1c, and melanocratic parts, and moreover, the leucocratic parts are diffuse vetns 1n the melanocratic part, a relattonshtp not unlike that 1n some migmatites. The best evtdence that the relationships in th is outcrop are not due to anatexis comes from nearby outcrops, where magma-mingling featu res are obvious. However, all the rocks 1n this outcrop contain euhedral crystals of plagioclase that have a

complexly zoned core, a feature that appears to be charactensttc of magma mtngling, not anatex1s. The presence of K-feldspar and a htgh content of tncompatible elements in the leucocratic parts suggest that the magma from which they formed was denved by the fracttonal crystallization of a tonalitic magma. The onentation of the leucocratic veins suggests t hat t he fractionated melt may have been expelled from the host t onalites during minor deformation at a point when this part of t he pluton was almost solid. Location: Lac Rodayer pluton, Opatica Subprovince, Quebec, Canada. Rock type: syntectonic tonalitetrondhjemite-granodiorite pluton, amphtbolite facies. Scale: the ruler is IS em long./moge: E.W. Sawyer.

A d a~

of M igma[i[cs

------------------------------ 359 Figure

8

Fig. G8. This photograph shows a gran1te with the mineral paragenesis K-feldspar + plagioclase + quartz + biotite + amph1bole that was emplaced into a dextral. strike-slip shear zone and was strongly deformed as 1t crystallized. Abundant tabular plag1oclase and K-feldspar crystals 1n the magma created a framework structure conta1n1ng 1nterst1t1al melt. Deformation of th1s framework of crystals as the pluton cooled first onented its crystals into a strong foliation. and t hen created shear bands, asymmetrical boudins. and other dilatant struct ures in it. D unng this process. the loss of interst 1t 1al pore-space by compact1on and the creation of low-pressure s1tes at the dilatant structures generated local pressure-grad1ents, which caused the melt to move from the pores to the dilatant structures. in a process identical to the format 1on of leucosome in dilatant struct ures in migmatit es. The photograph shows a segregation of the residual melt in an asymmetrical boudin structure. The resemblance to leucosome in some migmatites is clearly ev1dent (see Figs. B6, DIS. Dl9. D22, and D23); the segregation is coarse-grained. leucogranit1c 1n compos1t1on, has diffuse borders and a granitic microstructure. Although it has no melanocrat1c border. prinopally because the granite has a low modal content of ferromagnes1an minerals. t here is a depletion halo three centimeters wide around the segregation. This halo is depleted in material of the

same compos1t1on as the segregation compared to the matrix I 0 and 30 em away from the segregation. and is correspondingly enriched in the proportion of plagioclase and K-feldspar that it conta1ns. The halo indicates a local origm for the melt 1n the segregat1on. Factors that dist1ngu1sh th1s segregat1on from leucosome in a m1gmatite are the absence of any ev1dence for a part1al-melt-producing reaction 1n the host and the highly evolved bulk composition of the leucocratic segregation, which is more compatible with the final product of a long sequence of fractional crystallization in the pluton rather than anatex1s of the granite. Location: Near Tadoussac. Grenville Province, Quebec, Canada. Rock type: leucogranite segregation 1n a syntectonic hornblende gran1te pluton. Scale: the ruler is IS em long. Image: E.W. Sawyer.

360

SYNTECTON IC PLUTONS: ROCKS THAT

--------~~~~~~~~~~-

RESEMBLE METATEXITE MIGMATITES

Figure

9

Fig. G9. Many syntectonic plutons contain parts that are strongly banded on a centimeter scale and that resemble migmatites with a stromatic morphology. In this example, thin bands of leucotonalite and various bands of metatonalite are interlayered with mesocratic grey tonalite; two aspects are particularly suggest1ve of migmat1tes. (I) Some of t he most leucocrat1c layers are in contact w1th very melanocratic ones (e.g., near the lens cap); this is reminiscent of the leucosome- melanosome pairs in migmatites in which movement of anatectic melt to the leucosome created the adJacent melanosome. (2) The honzontal, discordant leucocratic ve1n 1n t he center of the photograph occupies a small shear zone, and appears to have gradational contacts wit h it s host rocks at both ends, a feature that could indicate local derivation of the vein. However, other evidence aga1nst this rock be1ng a m1gmatite is compelling. ( I) Some of the leucocratic veins, 1ncluding the coarsegrained ones, are discordant to the layering, and many have no melanocratic rim at all, w hereas others have but a very narrow one. The leucocratic ve1ns thus seem to have been intruded into a tonalite that already had compositional layenng, and react1on between the ve1ns and the1r wallrocks seems responsible for the mafic selvedges. (2) There is no evidence for partial-melt ing reactions in the rocks that would establish the required petrogenetic link between the

melanocratic layers and the leucocrat1c ones, and between both these and the mesocratic grey tonalite. The features of this outcrop could be synmagmat1c and principally due to t he injection of leucocratic and melanocratic material in veins into a crystallizing grey tonalite, with some reaction between the ve1ns and the host to create melanocrat1c nms. The ve1n complex underwent h1gh shear stra1n during crystallization; this rotated and attenuated the veins into near-parallelism, folded some and, in the later stages of crystallization, led to the expuls1on of residual melt mto dilatant structures to form leucosome-like ve1ns. Some ve1ns of coarse-gra1ned material were more competent than thew hosts and underwent boud1nage; they could have been solid, but they may simply have had a smaller fraction of melt. Location: Pyhasalm1 area, F1nland. Rock type: Svecokarelian syntectonic tonal1te pluton. Scale: the lens cap IS 6 em in diameter. /mage: S.-J. Barnes.

A tlas of Migmat ites

Figure

I0

Fig. G I 0. The outcrop in t his photograph contains three rock types: ( I) fine-grained leucogranodiorit e layer s, (2) medium-grained, mesocrat ic plagioclase + quartz + biotite t onalite gneiss layers, and (3) rare, discontinuous, biotiterich melanocratic layers locat ed between some of t he leucocrat ic and mesocrat ic layers. This outcrop has been interpreted as a migmatite in which partial melt ing of the mesocratic tonalite gneiss formed t he leucocratic and melanocratic layers. H owever, t hree factors argue against such an interpretation. First, t here appears to be far too much of the leucocratic mat erial relative t o t he melanocrat ic fract ion; in other words, there is t oo much "melt " and not enough "residuum" in t he out crop. Secondly, in order t o produce a K-feldspar-bearing melt from t he tonalite gneiss, a melt ing react ion involving the incongruent breakdown of biotite seems necessary, and there is no evidence for one. Thirdly, t he whole-rock composit ion of the leucocratic layers resembles leucogranodiorit e veins in a nearby hornblende monzodiorite to granodiorite pluton, and these leucogranodiorit e veins formed by fractional crystal lization of the major magmatic phase of the pluton. In addition, the tonalit e gneiss layers have t he same major- and t raceelement composit ions as a nearby pluton of t onalite. Mapping indicates t hat this migmat it e-like rock formed w here abundant dikes of leucogranodiorite emanating from

a syntectonic monzodiorit e to granodiorit e pluton have intruded the margins of a tonalit e pluton. Both the host and t he dikes were t hen rotated int o parallelism and att enuat ed by a synmagmatic shear zone, and then folded. T he biotite-rich selvedges formed as a result of react ion between t he two different lithologies. T he high degree of recrystallization present in t hese rocks indicat es that deformation in t he shear zone continued unt il ver y close t o the solidus. However, there are some coarser-grained diffuse regions t hat contain euhedral crystals of feldspar locat ed w ithin small subsidiary shear zones, and these are t he sit es w here the last melt accumulat ed.

Location: Border between t he Abitibi and Opatica subprovinces, Quebec, Canada. Rock type: sheared vein com plex. Scale : the ruler is IS em long. /mage: E.W. Sawyer.

361

SYNTECTONIC PLUT ONS: ROCKS

362 ---------------------------T H AT RESEMBLE DIATE XITE MI G MATI TES Figure

I I

Fig. G II . Many A rchean and Proterozoic syntectonic t onalite- granodiorite plutons have portions that resem-

latter, but are extended by t he dextral shearing; hence, they are older than t he microstructure in the leucogranodiorit e.

ble migmatites. T his phot ograph shows the termination of one such domain, and Fig. G 12 shows it s interior. The lower and upper parts of t his photograph consist of tonalite; it

Small, dark, irregular patches in the leucogranodiorite are fragments from the mafic enclaves that have been alt ered

is weat hered brownish grey at t he bottom, but is fresh at t he top. T he t onalite is foliat ed, and has a cent imet er-scale composit ional layering due to heterogeneit ies in t he original magma, lat er attenuated by shearing in the magmatic or submagmatic state. The mafic enclaves are considered to be the remains of a layer of mafic magma int ruded contemporaneously with t he tonalit e. However, t hey could be xenolit hs. The migmat it e-like appearance is due to t he presence of an elongate, t apered body of leucogranodiorite t hat crosses the layering and partially engulfs t he mafic enclaves. The composit ional layering in the tonalite curves on both sides of the leucogranodiorite, indicating that it occupies a dextral shear zone. T he composit ional layering of t he tonalite fades into the leucogranodiorit e, which gives it a diffuse aspect that resembles some nebulit ic and diatexite migmatites (e.g., Fig. BIS). T he microstructure ofthe leucogranodiorit e is magmat ic and essentially isotropic, and indicates t hat it crystallized after t he shearing. Lens-shaped bodies of coarse-grained granit e occu r in t he leucogranodiorite and t he t onalit e, and postdat e t he layering in t he

to biotite. Pelit ic schists in the country rocks are of staurolite + andalusit e grade; hence, partial melt ing of the tonalite is unlikely to have occurred, and t hus this is not a migmatite. Gradational contacts between the leucogranodiorite and it s host indicate t hat bot h cont ained melt at t he same time. Consequent ly, the leucogranit e is interpreted t o be derived from t he fractionated residual melt drawn in from the host tonalit e as t he dextral shear zone formed. The zone of dilatancy likely formed once t he tonalit e had cryst allized sufficiently (about 55%) to pass the rigid percolation t hreshold.

Location: La Grande Subprovince, northern Quebec, Canada. Rock type: syntecton ic segregation of residual melt in a tonalite pluton , lower amphibolite facies. Scale: the ruler is IS em long. Image: E.W . Sawyer.

A tlas o f M ig mat it es

------------------------------ 363 Figure

11

Fig. G 12. The tonalit e cou ntry-rock is visible in the top left and bottom right corners of t his photograph. T he compositional layering in the bottom right can be traced to the left.

but becomes progressively more indist inct toward the ruler, and this part resembles a nebulit ic migmatite. T he center of the picture consists of leucogranodiorit e, wh ich cont ains numerous mafic enclaves. Many of the enclaves were frozen in the process of disaggregation. The morphology that results from the mafic fragments scattered within leucogranodiorite resembles that of some diatexite migmatit es (see Figs. 042, 044, 047, and 050). Mafic enclaves and fragments have a melanocratic rim along contact s with the leucogranodiorite. Melanocratic rims are also found in migmat it es, but in this rock, biotite replaces hornblende, which suggests a reaction between the leucogranodiorite and t he enclave, rather t han t he extraction of melt from the latt er. The leucogranodiorit e between the largest enclaves is isotropic and has no trace of the layering in the tonalite; therefore, th is is newer material. Consequently. this portion of the pluton is interpreted to have become dilatant during the late stages of its crystallization. The evolved melt residing in the pores of the tonalite then migrated into t his dilatant region and intruded both the mafic enclaves and the tonalite. T he influx of melt into this area increased the local fluid pressure and partially disaggregated the mafic

layer and the surrounding t onalite, w hich creat ed a diatexite-like rock. Other effects were reaction of the mafic rocks w ith H 20-rich leucogranodiorit ic melt (biotite rim replacing hornblende) and t he partial destruction of the old structures in the t onalite (e.g., near t he ruler). Location: La Grande Subprovince,

northern Quebec. Canada. Rock type: syntectonic segregation of residual melt in a tonalite pluton. lower amphibolite facies. Scale: the ruler is I 5 em long. Image : E.W. Sawyer.

364 __________________S~Y~N~T~E~C~T~O~N~I~C~P~L~U~T~O~N~S~:~R~O~C~K~S THAT RESEMBLE DIATEXITE MIGMATITES

Figure

I3

Fig. G 13. This photograph shows a border phase of a felsic intrusive body. T he wall rocks are at the bottom of the photograph, and have been veined and broken into angular blocks. W ith1n a short distance 1nto the felsic intrusive body, the xenoliths are smaller, and many are not1ceably rounded, whereas others have become fragmented and partially d1saggregated. In other words, they show signs of magmatic erosion. Most of t he fragments of wall rock have developed a dark, coarse-grained reaction rim or rind; the larger enclaves still retain t he original mineralogy and microstructure of the wallrock in the core. However, t he

smaller enclaves are coarsely recrystallized throughout. The felsic magma around the enclaves has become strongly contaminated with cryst als and larger fragments from t he disaggregat ed enclaves. The morphological similarit ies with diatexite migmatites are strong (cf Figs. D42- D44); in part icular, t he dark-colored enclaves and the ri m around the enclaves and along felsic veins in the wallrocks resemble the melanosome, 1.e., the melt-depleted res1duum 1n migmatites. However, there is no ev1dence for a felsic fraction derived from a melt-producing reaction in these enclaves, and the dark rim can be explained by react1on of t he felsic melt wit h t he wallrock. Potassium from the melt diffuses into t he enclave and converts amphibole t here t o biotite,

and t he Ca released by t he conversion of the amphibole to biot it e diffuses out into the felsic magma and generates plag1oclase t here. Locauon: near Biella, northwestern Italy. Rock type: border phase of a felsic 1ntrus1ve body. Scale: The w1dth of the 1mage corresponds to 1.4 m. /mage: E.W. Sawyer.

Atla~

Figure

of Migmatitc~

--------------- - 365

14

Fig. G 14. Small plutons, dikes, or sills of evolved leucogranite or aplite commonly develop a layered structure. In some cases, the repeated sw1tch1ng back and forth, or oscillation, of conditions, such as the activ1ty of H 20, dunng crystallization produces repeated differences 1n gra1n size, microstructure, or m1neralogy that result 1n the development of a fine-scale layenng. However, 1n other examples, very light-colored quartzofeldspath1c layers alternate w1th somewhat darker layers that are richer in biotite; this type of layering may be due to the presence of two magmas. This photograph shows the 1nterior of a s1ll of leucogranite that IS hosted by a sequence of pelit1c to psamm1t1c metasediments. The leucogranite cons1sts of alternat1ng very light-colored layers that are gran1t1c in compos1t1on and conta1n
result IS a prominent but w1spy layering that contains flow structures and truncated layers and that, overall, strongly resembles some schollen and schlieric diatexite m1gmatites.

Location: Eastmain, Nem1scau Subprovince, Quebec, Canada. Rock type: layered leucogran1te; host metasedimentary rocks 1n the m1ddle to upper amphibolite fac1es. Scale: the ruler IS IS em long. /mage: E.W. Sawyer.

A RRAYS O F FE LSIC VE IN S T HAT LOOK LI KE M IGMATITE S

366 ----------------------------Figure

S

Fig. G 15. The principal rock-t ype in this out crop is a grey biotite

granodiorite in which the foliation is parallel t o a weak compositional layering. The granodiorite contains numerous veins of pink granite, most of which are parallel to the foliation and compositional layering, but some are discordant. The granitic veins are folded, and by using the vergence of adjacent folds, a second-order fold closure can be identified in the cent er of the outcrop. Both parallel and similar fold forms are present. There are also examples of "eye" structures (i.e., closed loop struct ures), but these are not interpreted as sheath folds, because the original attitude of the veins cannot be deciphered. The granite veins on the far right are much thinner than elsewhere, and have a layered, or stromatic, morphology in which individual veins exhibit a pinch-and-swell structure, which indicates that t hey have been attenuated. Many of t he veins in the outcrop have a narrow mafic selvedge : the mafic selvedge is narrowest where the veins are the most attenuat ed. The presence of boudins, pinch-and-swell and parallel folds all suggest that the granitic veins were more competent than the granodiorite host during the deformation. As one explanation for t his scenario, the veins were completely cryst alline when the folding occurred. The presence in this outcrop of a mesocratic host (the biotite granodiorite) that cont ains younger and spatially related leucocratic

are not residuum) but rather injected, t hen this is not a mig-

and melanocratic portions is reminiscent of the relationships

mat ite. The felsic material should be interpreted as a complex

seen between t he constituent parts of many metatexite mig-

of granit ic veins, and the mafic selvedges, as the product of the

matites. If the grey granodiorite had undergone partial melting, then the melanocratic border around t he granitic veins might

local alteration of the granodiorite by the granite. However; if some of the granitic material in the veins was produced in situ

be interpreted as the residuum left after the extract ion of an

by melting, then this is a migmatite, but an open-system one in

anatectic melt that became the granitic veins. However; the

which the main mechanism responsible for creating t his mor-

proportion of melanocrat ic rock relative to the amount of

phology is the injection of a granitic magma in veins.

pink granite present in the outcrop seems to be too small to account for the in situ formation of all the granite present (see also Fig. B34). Therefore, it seems likely that most, and possibly all, of the granite veins have been injected. If t he granite in the veins was not locally derived (i.e., the melanocratic rims

Location: Saturday Peak, Monashee Complex, British Columbia, Canada. Rock type: granite veins in a biotite granodiorite. Scale: the hammer in the center is 40 em long. Image: Paul McN eill.

l

A rias of M igmat itcs

------- -------------- ------- --- 367 Figure Fig. G 16. This photograph shows an outcrop of foliated granodiorite that conta1ns numerous parallel gran1t1c veins. The gran1te ve1ns have a very narrow. melanocrat1c nm. Most of the granitiC veins are thin and oriented parallel to each other and to the ma1n compos1t1onal layenng 1n the host granod1onte: because most of the ve1ns are laterally pers1stent, they g1ve the outcrop a layered morphology. Many of the granitic veins exhibit a pinchand-swell structure, or have w1der port1ons that correspond to partitions between asymmetncal foliat1on boudins 1n the host granodiorite. However, some granitic ve1ns are discordant to the layenng. wh1ch suggests that they are dikes, whereas a few others occur as 1solated, lent1cular bodIes 1n asymmetncal boud1nage structures. Locally. gran1tic ve1ns outline 1sochnal fold h1nges. The rat1o of the melanocrat1c border to gran1t1c ve1ns IS low 1n th1s outcrop, wh1ch, taken w1th the discordant nature of some ve1ns. suggests that much of the gran1t1c matenal may have been Injected as dikes. Thus. the s1tuation is the same as w1th the rocks shown 1n the prev1ous figure (G 15): 1n order for the rocks 1n th1s outcrop to be called m1gmatite. there has to be some evidence that the granodiorite, and the granitic ve1ns plus their assoc1ated melanocratiC rims, are genetiCally linked by part1al melt1ng. If th1s cannot be shown, then the outcrop IS a gran1t1c ve1n complex hosted by the granodiorite. The macroscopic charactenst1cs of the gran1t1c ve1ns 1n th1s outcrop Indicate that when they were deformed, they were more competent than the host granodiorite. Although the rocks shown 1n th1s figure and the prev1ous one have formed 1n essentially ident1cal ways, thew final morpholog1es are strikingly different. This is because the rocks shown in this figure were subject to h1gher stra1ns, wh1ch rotated and attenuated (transposed) the gran1t1c veins far more. and consequently generated their strongly layered appearance.

Location: Mount Od1n area. Monashee complex, Bnt1sh Columbia. Canada. Rock type: sheared dike complex. Scale: the head of the ice axe is 35 em wide. Image : Paul McNe1ll.

ARRAYS OF FELS IC VEINS THAT LOOK LIKE MIGMATITES

368----------------------------Figure

17

Fig. G 17. Melanocrat1c layers of garnet silliman1te b10t1te sch1st alternate w1th leucocratic quartzofeldspath1c layers 1n th1s outcrop to produce a morphology that closely resembles a stromatic metatexite migmat1te. The compos1t1on of the melanocratic layers is very un1form over the ent1re outcrop, and is consistent w1th the 1nterpretat1on that 1t IS res1duum left after the extraction of an anatectiC melt. T he leucocratic layers, however, are problematic, as they exhibit a wide range of microstructures and modal compoSition. Some parts are fine-grained, and contain the mineral assemblage plag1oclase + quartz + b1ot 1t e ± K-feldspar, whereas the most leucocratic parts are richer 1n K-feldspar and, 1n places, d1splay a feldspar quartz 1ntergrowth microstructure. The absence of garnet and S1ll1man1te from all the leucocrat1c layers IS not what one would expect if the leucocrat1c layers had resulted from the anatect1c melt generated 1n mak1ng the b1ot1te + sill1man1te + garnet residuum from a metapelitic protolith. Some of the leucocratiC layers are folded and others are boud1naged, wh1ch IS the result of deformation dunng, or after, thew crystall1zat1on.

Location: Val Strona, Italy. Rock type: deformed fels1c dikes. Scale: the ruler is IS em long. Image: E.W. Sawyer.

Ada~

of Migmatites

----------------- -------------- 369 Figure

18

Fig. G 18. Th1s photograph of another part of the outcrop shown in Fig. G l7 shows ev1dence indicat1ng that th1s rock is 1n fact not a migmat1te. Here, some of the leucocratlc layers are as much as 30 em w1de (e.g., bottom nght), and show clearly that the leucocratic layers were denved from two different magmas. One gave nse to a grey b1ot1te-K-feldspar quartz plag1oclase granodionte or tonalite with a fine to medium grain-s1ze. whereas the other produced white and very leucocratic rocks that have a coarser gra1n-s1ze and, locally, a pegmatltic microstructure. The leucocrat1c body 1n the bottom nght of the photograph shows that the more leucocrat1c magma 1ntruded the other and that both enclose fragments of the melanocratlc b1otlte silliman1te garnet sch1st as xenol1ths. Because no genetic l1 nk due to partial melt1ng can be established between the leucocratic and the melanocratic rocks, the rock 1s not a migmatite. The relat1onsh1ps observed in some of the leucocratic rocks suggest that the morphology of th1s rock IS the result of dik1ng of a melt-depleted garnet s1lhman1te b1ot1te sch1st by two fels1c magmas and

thew subsequent deformation.

Locat1on: Val Strona, Italy. Rock type: deformed fels1c d ikes. Scale: t he ruler is IS em long. /mage: E.W. Sawyer.

A RRAYS O F FELSIC V EI NS TH AT LOO K LIKE M IG MAT ITE S

370 -----------------------------Figure

19

Fig. G 19. Metasedimentary schists derived from siliciclastic turbidite sequences are a common component of cratons around the world. In most Precambrian terranes, Al-poor rocks containing the assemblage plag1oclase + quartz + b1ot1te, denved from greywacke protoliths, dom1nate in the metasedimentary belts. For example, the Archean metasedimentary subprov1nces 1n Quebec and the Proterozoic ones in the Narvik area of Norway each contain >80% greywacke, whereas metapelites are a minor constituent. Partial melting in these Al-poor rocks does not normally begin until the assemblage biotite + plagioclase + quartz becomes

soooc.

unstable, at temperatures of about However, in many t erranes, these rocks take on the appearance of stromatic migmatites in the middle amphibolit e facies because of the presence of thin granit ic veins parallel or subparallel to the bedding or foliation. This photograph shows a single granitic vein t hat contains the mineral assemblage plagioclase + quartz + K-feldspar + biotite + garnet. Similar granit ic layers and larger bodies (dikes and small plutons) nearby do not contain garnet , but some have muscovit e, whereas others contain spodumene. Some of the feldspar crystals in the granite layers have rational faces against quartz, and this is taken as evidence for crystallizat ion from a melt. The edges of the granitic vein are not smooth; they have millimeter- to centimeter-scale indent at ions and irregularities that are not necessarily the result of boudinage or pinch-andswell st ructures (see Figs. FI02 and FI04). A very narrow,

mafic selvedge around the granite veins is ubiquitous, and it consists ma1nly of biotite; however, the selvedge in this photograph contains a few crystals of garnet that are identical to that in the granite vein. In general, the mafic selvedges in th1s type of migmatite-like rock do not contain sillimanite, cordierite, orthopyroxene, garnet. or K-feldspar, but muscovite may be present. The major- and trace-element composition (e.g., h1gh Li, Cs, Th, U, and Kp) of the granite vein in the photograph indicates that it is the product of a highly fractionated anatectic melt derived from a metapelitic or metagreywacke protolith. The absence of mineral assemblages or microstructures Indicating that a melt-producing reaction occurred in any of the surrounding rocks, or in the mafic selvedges, indicates that these rocks do not consist of migmatite. They are int erpreted as veined rocks, albeit veined by a felsic melt that was derived from a protolith similar in bulk composition to the host. However, the partial melting occurred at greater depth, where temperatures were sufficient. As the anatectic melt migrated upward, it crystallized and became progressively more fractionated, and was finally inject ed into and crystallized in an amphibolite-facies subsolidus host-rock.

Location: Nemiscau Subprovince, Quebec, Canada. Rock type: granite vein in a metagreywacke, middle amphibolit e facies, T ca. 600°C. P 3- 5 kbar. Scale: t he scale is 15 em long. /mage: E.W. Sawyer.

Ada, of Mig ma tites

----------------- -------------- 371 Figure

Fig. G20. The h1gher proportion of granit ic veins makes the intrusive ongin of this migmatite-like rock clear. Nevertheless, the outcrop resembles some diatex1te migmatites (cf Figs. 043, 045, 049, and 054). A layer of the biotite quartz plag1oclase sch1st occurs at the right of the photograph. The schist does not conta1n orthopyroxene, nor does 1t have any microstructures ind1cat1ve that it partially melted. However, 1t had reached t he m1ddle amphibolite fac1es when t he granite sheets were emplaced into it. A narrow mafic selvedge cons1sting mostly of b1ot1te occurs between the gran1te and its host. Other less cont1nuous melanocrat1c patches occur with1n the granIte, and these may mark the pos1t1on of other gran1te host contacts that have been subsequently overpnnted by granite veins. These relics of mafic selvedges consist almost entirely of biotite (no garnet, cord1ente. orthopyroxene, or K-feldspar 1s present). The mafic selvedge is interpreted to be the result of a react1on between the wallrock and the granitic magma, or more likely of an aqueous flUid exsolved from it. The whole-rock compos1t1on of the gran1te sheets 1n this outcrop 1ndicates that they formed from an evolved melt derived from a pelitic or greywacke source. The U Pb zircon ages of the granite sheets IS 2672 ± 2 Ma, about 4 Ma younger than U-Pb monaZite ages from nearby migmat it es. Consequently, these are Interpreted as rocks from

below the "melt-in" isograd that have been veined by an evolved melt that escaped from a deeper anatectic zone. Locatton: N em1scau Subprov1nce, Quebec, Canada. Rock type: gran1te ve1ns or dikes 1n metagreywacke. middle amphibolite fac1es. Scale: the ruler 1s 15 em long. Image: E.W. Sawyer.

THE PARTS OF A MIGMATITE rocks that will partially melt (the fertile rocks)

I

PROTOLITH

I

rocks that have partially melted

rocks that did not partially melt (the infertile rocks)

~

~

NEOSOME

PALEOSOME

segregation of the melt fraction from the solid fraction

I melt fraction

I

\ solid fraction

\

Leocosome

Residuum

types of leucosome

types of residuum

in situ leucosome in source leucosome leucocratic dike I vein

no segregation of the melt fraction from the solid fraction

melanosome

Unsegregated neosome