Atlas Atlas of Migmatites

__g _ y g Atlas Atlas of Migmatites

Migmatites are highly heterogeneous rocks found in high-grade metamorphic environments; they are commonly encountered in the continental crust. Until now, many geologists have been deterred from working with migmatites because of their complex appearance and an unhelpful non-genetic nomenclature. In Atlas of Migmatites, Dr. Edward Sawyer provides genetically based deﬁnitions and a system of nomenclature with which it will be possible to describe and map migmatites effectively and to understand how combinations of factors and processes produce a bewildering morphological diversity.

Migmatites are produced by partial melting; to aid the reader in the identiﬁcation of migmatites, the author describes and illustrates microstructures that can be used to infer the presence of melt or a melt-producing reaction. He also describes how geochemical data can be used to infer petrological processes involved in migmatite development. Atlas of This book includes the results from two decades of research in whole-rock geochemistry, partial melting, microstructural analysis and experimental Edward W. Sawyer deformation of partially molten rocks. It contains information from an outcrop through to a grain scale. Exceptionally well illustrated, with 272 colour plates Migmatites and accompanying detailed captions, the Atlas provides descriptions and analyses of migmatites not previously available. The Canadian Mineralogist Special Publication 9 Edward W. Sawyer Special Special Mineralogist The Canadian Publication Special Special Mineralogist The Canadian Special Special Mineralogist The Canadian Publication Publication 9 9 9

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 About the cover:

Front cover Metatexite migmatite developed from a foliated granodiorite showing leucosome in dilation structures. Laag Mountain area, British Columbia. Photograph by Paul McNeill.

Rear cover Diatexite migmatite with schlieren, developed from a pelitic protolith. Quetico Subprovince, Ontario.

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

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Edward W. Sawyer

The Canadian Mineralogist Special Publication 9

NRC Research Press

All rights reserved. No part of this publication may be reproduced in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.

Printed in Canada on acid-free paper.

ISBN 978-0-660-19787-6 ISSN 1717-6387 NRC No. 46331

Library and Archives Canada Cataloguing in Publication

Sawyer, Edward William, 1951– Atlas of migmatites/ E.W. Sawyer

“NRC Research Press” Issued by: National Research Council Canada Co-published by Mineralogical Association of Canada Includes bibliographical references ISBN 978-0-660-19787-6

1. Migmatite. 2. Migmatite — Pictorial works. 3. Petrology. I. National Research Council Canada II. Mineralogical Association of Canada III. Title.

QE475.M5S38 2008 552’.4 C2008-980022-2

NRC Monograph Publishing Program

Editor: P.B. Cavers (University of Western Ontario)

Editorial Board: W.G.E. Caldwell, OC, FRSC (University of Western Ontario); M.E. Cannon, FCAE, FRSC (University of Calgary); K.G. Davey, OC, FRSC (York University); M.M. Ferguson (University of Guelph); S. Gubins (Annual Reviews); B.K. Hall, FRSC (Dalhousie University); W.H. Lewis (Washington University); A.W. May, OC (Memorial University of Newfoundland); B.P. Dancik, Editor-in-Chief, NRC Research Press (University of Alberta)

Inquiries: Monograph Publishing Program, NRC Research Press, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada. Web site: http://pubs.nrc-cnrc.gc.ca

Correct citation for this publication: Sawyer, E.W. 2008. Atlas of Migmatites. The Canadian Mineralogist, Special Publication 9. NRC Research Press, Ottawa, Ontario, Canada. 371 p.

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 Also in this series Encyclopedia of Mineral Names W.H. Blackburn & W.H. Dennen Special Publication 1 (1997)

Glossary of Mineral Synonyms J. de Fourestier Special Publication 2 (1999)

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

New Minerals 1995-1999 (2001) J.A. Mandarino Special Publication 4 (2001)

The Health Effects of Chrysotile Asbestos: Contribution of Science to Risk-Management Decisions R.P. Nolan, A.M. Langer, M. Ross, F.J. Wicks & R.F. Martin, eds. Special Publication 5 (2001)

Mineral Species Discovered in Canada and Species Named after Canadians László Horváth Special Publication 6 (2003)

Mineral Species First Described from Greenland Ole V. Petersen and Ole Johnsen Special Publication 8 (2005)

Available from: Mineralogical Association of Canada 490, rue de la Couronne Québec, QC G1K 9A9 Canada

www.mineralogicalassociation.ca

Editor, The Canadian Mineralogist Robert F. Martin

Preamble ...... xiii

Preface ...... xiv

Acknowledgements ...... xv

Introduction ...... 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

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 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

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 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

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 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

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3955947/frontmatter.pdf by guest on 26 September 2021 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

Preamble

It is with pleasure that I introduce the latest addition to the anatectic reactions not only in amphibolite-facies rocks, but list of Special Publications produced by the Mineralogical also in the realm of the granulite facies. He has examined Association of Canada. This Atlas of Migmatites contributes key occurrences of migmatites throughout the world, and directly to the educational mandate of our Association. The presents here in a systematic way the results of his scru- book deals with migmatites, a very widespread group of tiny on all scales. Thus, the reader is shown photographs of rocks, especially common in the Archean crust that makes key exposures of migmatites in the field and petrographic up a major proportion of our country. Everyone has learnt details in 273 photographs, each with a detailed caption, a bit about migmatites, but by and large, the subject mat- supplied by the author in 68% of the cases, the remainder ter “falls between the cracks” in our university curricula. being contributed by respected colleagues also active in the Typically, an upper-level undergraduate course in metamor- characterization of migmatitic assemblages. Field examples phic petrology does set the stage for an understanding of are presented from 12 countries, with the author person- anatectic reactions in terranes that have undergone meta- ally involved in field studies in six of those. In this atlas, the morphism at conditions of upper amphibolite or granulite author emphasizes the latest contributions in the area of facies. Students in such a course do learn about the con- migmatite-related research, either in a regional setting or in cept of anatexis and dehydration-induced melting, but a contact aureole. really not much about the extraordinarily complex array of products of such anatexis, the importance of fractional Society-run, not-for-profit organizations like the crystallization of the anatectic liquid, and the fate of assem- Mineralogical Association of Canada find it very challenging blages of residual minerals. Furthermore, contact-related to undertake such a publication, owing to the high costs of anatexis usually is not discussed. On the other hand, stu- production. In this instance, the Association is most fortu- dents in an upper-level course in igneous petrology do deal nate to have coproduced the volume with NRC Research with products of partial melting in the crust, and can speak Press. On behalf of the MAC, I thank Suzanne Kettley, Mark at length about the physical properties of a silicate magma Boileau, and Diane Candler for their valuable input and and the results of its fractional crystallization. But they deal their key roles in creating this book. In addition, I acknowl- with ready-made plutons. Such students do not develop a edge the involvement of Pierrette Tremblay, Director of good understanding of the steps that precede the forma- Publications of the Association, in ensuring close com- tion of a pluton, where small batches of leucosome coalesce munications with NRC Research Press throughout the and rise through a deforming mass of neosome and resister preparation of this book. Also, Pierrette applied for and lithologies. obtained funds from the Canadian Geological Foundation and Natural Resources Canada, part of which paid for the As is made clear below, Professor Sawyer has remedied printing of a poster to publicize this book. I thank Vicki the situation by writing the first authoritative treatise about Loschiavo, who entered the editorial corrections on the the petrology of migmatites since the work of Karl Richard master files of text and captions as I progressed through Mehnert (1913–1996), of Berlin, published 40 years ago. He the book. I am grateful to have had this opportunity to has abandoned the purely descriptive approach of Mehnert help bring this major contribution in the area of migmatite in favor of an openly genetic approach: a migmatite is a research to fruition. rock that is the product of partial melting. If melting can be shown not to have taken place, the rock is simply not a mig- Robert F. Martin matite. Furthermore, he has recognized the importance of Editor, The Canadian Mineralogist

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Preface

Migmatites are some of the most confusing-looking, yet The arrival of this Atlas of Migmatites is very timely. It is the aesthetically pleasing rocks. The most striking consist of first book devoted to migmatites since Mehnert’s iconic light-colored quartzofeldspathic segregations in a darker 1968 text. In the intervening 40 years, dramatic strides have host, with the light segregations showing a diverse and been made in the understanding of these complex rocks. often spectacular range of appearances, in some cases in swirls, in some others cross-cutting, and in yet others Ed Sawyer is without question the world’s leading expert discrete, or diffuse, or folded, or containing high-grade met- on migmatitic textures and structures. In this atlas, he amorphic minerals like garnet and pyroxene. Migmatites provides a wide range of superb field and thin-section justify their age-old name; they are “mixed rocks.” photographs of migmatites from around the world, both his own and those of others, each carefully described and Migmatites commonly look “squishy.” This is no coincidence, interpreted. The photographic part of the atlas is preceded for they are interpreted to be rocks frozen in the act of by a substantial introductory section in which he clarifies partially melting. A mixture of melt, disaggregated minerals, the dizzying range of descriptive terms for migmatites, both and unmelted rock is mechanically highly heterogeneous. It new and old. is no wonder that migmatites show such a wide range of textures and structures. Although migmatites will remain among the most complex of rocks to understand, this atlas goes a long way to making Migmatites are of crucial importance in understanding the them more understandable. genesis of the huge volumes of granitic magma that are found in batholiths and that make up much of the continen- David R.M. Pattison tal crust. Migmatites are widely considered to be examples University of Calgary of assemblages where such magma was caught in the act of being generated and escaping. Migmatites thus furnish a unique perspective on a fundamental process in the evolu- tion of the Earth.

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Acknowledgements

I would like to start by expressing my gratitude to Mike Negra), Olivier Vanderhaeghe (Massif Central), and Dirk Brown and Richard White for their encouragement and van Reenen (Limpopo Belt). There has always been an their reviews of the text. They pointed out my errors, easy exchange of thoughts and views within the migmatite omissions, and tracts of muddled writing; the changes community, and next I would like to thank Marian Holness, that they suggested have resulted in a far better text. The Tracy Rushmer, Bernardo Cesare, Roger Powell, Alfons remaining faults are entirely mine. Many people took the Berger, and Claudio Rosenberg for illuminating some of time to send me their photographs of migmatites for this the connections between what one can see in migmatites atlas. The choice of which ones to use was far from easy, and the various processes that have happened to them. but each photograph, whether included or not, has had an influence. I thank everyone who sent me a photograph or In most endeavors, progress comes through sustained electronic image for his/her contribution. and focused effort, together with some serendipity, of course. My work on migmatites, and ultimately this atlas, My perception of migmatites and what happened within have been no different and would not have been possi- them has been influenced by seeing and discussing mig- ble without continuity of funding. Therefore, I am grateful matites with many colleagues from around the world. for the philosophy behind the discovery grant program of To have seen such a wide range of migmatites has been the Natural Sciences and Engineering Research Council of invaluable, and I am grateful for the hospitality and gener- Canada, which funds long-term, curiosity-driven research osity of all those who have shown me their field areas. In programs. particular, and more or less in chronological order: Ned Chown (Grenville Front), Ron Vernon and Bill Collins Finally, I thank Pierrette Tremblay and Robert F. Martin for (Wuluma and Mount Hay), Daniel Lamothe and Alain the invitation to write a book on migmatites and for their Leclair (Ashuanipi Subprovince), Mark Severson and Steve encouragement throughout, and to Robert for all his hard Hauck (aureole of the Duluth Igneous Complex), Mike work in reading and editing. Brown (Saint-Malo), Gary Solar (Maine), Abdelali Moukhsil Edward W. Sawyer (Eastmain area), Geoff Clarke and Richard White (Mount Université du Québec à Chicoutimi Stafford, Broken Hill and Wuluma), Fernando Bea (Peña