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606 90 7 - 3 DIAS Atlas of metamorphic rocks and their textures Atlas of metamorphic rocks and their textures B.W.D. Yardley, W.S. MacKenzie and C. Guilford lii!|;ïrïrF#l Copublished in the United States wit l l John Wiley & Sons, Inc., New York , l Longman Scientilic & T€chnicâl' I-ongman Group Limited, LongEan House, Bumt Mill, HaÍlow' Essex CM20 2JE, England and Associated Conpalies throuqhoüt the v')ord. Copublìshed in the United States wíth Johrr Wiley & Sons, Inc., 605 ThiÍd Avenue, New Yorlç Nv 10t58 @ l-ongman Group UK Limited 1990 All rights reseÍved: no paÍ of this publication mây be reproduced, sÍored in a Íetrieval system, or tÍânsmitted in any foÍm or by aÍy means, electÍonic, mechanicâI, photocopying, Íecording, or oúelv,/ise withoui eiúer the prior written permission of the PubüsheÍs oÍ À licencè permitting restricted copying in úe United Kingdom issued by the Copyright Licensing Agency Ltd, 33-34 AlfÍed Place, Inndon, wClE 7DP FiÍst published 1990 RepÍinted 1995 BÍitish LibÍary Catsloguing in Publication Dalâ YâÍdley, B.W.D. (Bruce W.D.) Atlas of metamorphic rocks and their textures 1. Metamorphic rocks I.Title II. MacKenzie, W S. (William Scott) III Guilford, C. 552'.4 ISBN 0-582-30166-l Library of Congr€ss Câtâloging-iú-Publication Datâ Yardley, B.w.D. AtÌas of metamorphic rocks ând their textule / B.w.D. Yardley, w.S. MacKenzie, and C. Guilford p. cm. Includes bibliogÌaphical Ìeferences. ISBN 0-470-21677-8 (wileY) 1. Rocks, Metamorphic - Atlases 2 PetrofabÍic analysis - Atlases. Ì. MacKenzie, W.S. II. Guilford, C. oE47s.A2Y35 1990 552'.4-dcm Set in 9/10 Times Printed in Hong Kong GA02 IIL Title. 90-33566 CIP Contents Preface A&nowledgemenÍs bhoduclion PaÍ I Varieties of metamorphic rocks (Numbers in bold refer to photographs) Sciliogs of metamorphism Contaci metamorphism I Cordierite chloÍite biotite hornfels 2 Peridotite hornfels Regional/dynamic metamorphism 3 carnet biotite schist 4 Garnet staurolite schist/biotite schist 5 Peridotite mylonite Sea-fl oor and hydrothermal metamorphism ó Sea-floor amphibolite ? Epidosit€ Impact metamorphism E Impact metamorphic Íock líetâmorphism of scdimêntâÌy rocks Metamorphism of pelitic rocks M edìu m p re ssurc me tamo rp his m PÍehnite pumpellyite facies 9 Graphìtic slate Greenschist facies - chlorite zone l0 Chlori te muscoviÌe albite schist Greenschist facies - biotite zone ll Biotite chlorite rÍruscovite schist 12 Microcline epidote mica schist 13 Chloritoid slate Greenschist facies- garnet zone 14 GaÍnet chlorite biotite schist 15 Garnet chloritoid schist Amphibolitefacies staurolitezone 16 Staurolite schist vi i i , , Amphibolire facies - kyanite zone l7 Garnet stauÍolite kyanite gneiss IE Kyanite biotite grâphite schisl Amphibôlite facies - sillimanite zone . 19 Sillimanite stauÍolite schist H í8 h te Ínp e rat ure me lamo ry h is m Amphibol i te lacies-siÌ l imanite K-feldsparzone , . 20 Sillimanite K-feldspar biorire schist 2l ' GaÍnet cordieÍite plagioclase sillimânite gneiss Granulite facies 22 Garnet cordierite K-feldspar gneiss 23 Migmatitic gneiss ã Carnet cordier i te spinel quarrz gneiss 25 Sapphir inegranuli t€ 26 SapphiÍ ine coídìeri le biot ire gneiçs Effect ofpressuÍe variations on pelite assemblages Low pressure fietamorphísm Hornblende hornfels facies 27 Andalusite(chiasrol ire)hornfels 2E Cordierite ândalusite hornfels (spoÍted slate) 29 . Andalusite biotite schist 30 Andalusite staurolite schist Pyroxene hornfels f acies 3l Andalusìte coÍdieÍite K-feldspar hornfels Sanidinite facies 32 Cordierite plagioclase corundum spinel hornfels 33 Buchite 34 Buchite H igh p re s s ure me tamo r p h is m Blueschist facies 35 Carpholite chloritoid schist Ecologite facies 36 Talc kyanite schist (whiteschisr) 37 Pyrope kyaniÌe talc phengite schist with coesite GranÌrlite facies 38 K-feldspar kyânite granulite Metamorphism oftuffs, greywackes and cherts Tuffs and greywackes Zeolite facies 39 Laumontitemetagreywacke 40 Heulanditemeta-tuff Blueschist facies 4l JadeitegÌaucophanemetagreywacke Prehnite pumpellyite facies 42 Pumpellyite actìnolite schist 43 Stilpnomelânemetagreywacke 2l 22 23 24 26 27 28 29 29 6 7 9 l0 l0 t1 t2 13 t4 t4 t4 Ì6 t7 l8 l8 l9 20 30 3l 32 33 34 3ó 36 37 38 39 40 41 4t 42 44 45 Cherts and ironstones Greenschist facies 44 Sti lpnomelaneschist Blueschist facies 45 Riebeckiteaegirine-augitemetachert 46 Piemontitemetachert GÍeenschist facies 4? MinnesotaitegruneÍitemeta'ironstone Amphibolite facies rE Grunerit€ magnetite quartzite Metamorphism of maÍbles and calc-silicate rocks GÍeenschist facies 49 Talc marble Medium to high grade facies 50 Tremolite marble . 51 Diopside phlogopite marble 52 Clinohumite forsterite spinel maÍble 53 Scapolite marble 54 Wollastonite diopside grossulaÍ calc-silicate rock 55 Actinolite âídesine schist 5ó Clinozoisite schist MetamoÌphism of igneoua Ìocks Metamorphism ofbasic and intermediate igneous rocks M edium press ule me Íamorphis m Prehnite pumpellyite facies 57 Pumpellyitemetabasalt Greenschist facies 5t Creenschist with igneous relics 59 Epidote âctiiÌolite schist Amphibolite facies ó0 Epidote amphibolite ó1 Amphibolite 62 Cordieriteanthophyllitegneiss Granulite facies 63 Feldspathicgranulite 64 Garnet homblende pyroxene granulite H igh p resswe me Íamorp hism Blueschist facies 65 Crossite schisÍ 66 Lawsoniteblueschist 67 Garnet glaucophane schist Eclogite facies 68 Eclogite 69 Kyanite eclogite 70 EclogìtizeddoleÍite Metamorphism of ultrabasic rocks 7l . Serpentinite Amphibolite facies 72 Olivine talc carbonâte rock GranuÌite facies 73 Serpentinizedmeta-peridotite Metamorphism of acid plutonic rocks Greenschist facies 74 Metatonalite Amphibolite facies 75 Augen gneiss Granulite lacies 76 Charnockite Eclogite facies 7? Jâdeite metagranite 7E Jadeite gneiss 51 85 86 46 48 48 49 61 (A 66 67 68 69 70 71 72 73 Part 2 Textures of metamorphic rocks lntroduction Simple textuÌal teÌms Grain size and shape 79 Foliations (bedding and schistosity) 80 GranoblasÌic polygonal texluÍe 8l Acicular texture E2 Decussate texture 83 Porphyíoblasts and pressure shadows Plastic deformation and mylonitization E4 Strained quartz with sutured boundaries 85 Mylonite 86 GÌaniteultra-mylonite 87 Ultra-mylonite 88 MyÌonite with porphyroclasts and ribbon texture Time r€lâtions betw€€n defoÍmation aíd metâmoÌphism E9 PÍe-tectonicporphyroblasts 90 Probablesyntectonicporphyroblast 9l Late tectonic porphyroblasts 92 Post-tectonicporphyroblasts 93 Syntectonic(snowóal4porphyroblasts 94 Multiple porphyroblast growth 95 Complexmetamorphiddeformationhistory Reaction textuÌes 96 Armoured relics 97 Armoured relics 98 Zoned crystals 99 Atoll structure 100 Pseudomorphtexlures fof Reaction rims 102 Crrona texture I 103 Corona textuÍe II 104 Corona texture III 105 Diffusion metâsomatic zoning Textüres of polymelmorphism 106 Contact metamorphism after regional 107 Blueschist overprint on eclogìte 108 Greenschist facies overprint on blueschìst 109 Retrogrademetamorphism Polymorphic tÍansitions 110 Kyanite after chiastolite in graphite hornfels 111 Sillimanite after andalusite in sillimanite hornfels 112 Andalusite after kyanite in kyanite mica schist 113 Topotactic replacement ofaragonite by calcite Rekrent:es Index of fiíneral hames General index 87 88 88 89 90 90 91 92 93 93 94 53 54 55 56 5',1 58 59 60 94 96 96 97 98 98 99 100 100 101 102 102 103 104 105 10ó to1 107 108 108.109 110 l l l 1.12 t12 113 114 115 117 118 11.9 75 76 77 77 78 80 8l Preface The study of rocks in thin section using a petrographic microscope is an essential part of any undergÍaduate course in geology. This is the fourth volume in a series of photogÌaphic atlases of minerals and rocks in thin section. As in previous volumes the main purpose ofthe book is to pÍoyide the student with a handbook for use in practical classes to enable him or her to become familiar with the more common mineral associations and textures to be found in metamomhic rocks. In addition, some more unusual rocks which have given se to particuiar significant ideas about metamoryhism are also illustrated;the aim ofthis atlas is ho]vever to complement, not replace, a theoretical course in metamorphism. The book has been divided into two parts. Part 1 consists of descriptions of photographs of thin sections of a wide range of rocks of different chemistries metamorphosed under a variety of physical conditions. Part 2 deals with the textures characteristic of metamorDhic rocks. It is bevond the scoDe oï this atlas to consider the origins of the rocks òr to try to interyret the signìficance of their occu[ence or their texture in depth, but in the matteÍ of arrangement both ofthe rock t)?es and thefu texture we have had to make some assumptions about their origins. For example in considering the pélitic rocks, we have subdivided them under the following headings: (1) medium pressure (also known as Barrovian type), (2) high temperature at medium pressure, (3) low pressure and (4) high pressure. These headings are somewhat akin to Miyashiro's different facics senes. Early studies of metamoÍphism considered only two main tlpes viz ÍegioÍaI metamorphism and contact metamoryhism. As knowledge of the subject has increased it has become necessary to consider a greater vaÌiety of processes causing a change in mineralogy oÌ chemistry to pre-existing rocks and, in the fiÌst chapter, examples of rocks produced by different types of metamorphism are illustrated. In general, the wayin which material is presented corresponds to that adopted by Yardley (1989) An Introducíion to Metqmoryhic Petrology (Lo.'g- man). Some additional roçk types are however illustrated here. As in pÌevious atlases we have t ed to describe where the essential minerals appeaÌ in the photographs without the use of arrows or overprinting. We have tended to ignore individual minerals or textures which cannot be clearly seen on our original photographs because these vrill be even less visible in printed repro- ductions and there is nothing more frustrating than a photograph which does not show what it puryoÌts to shoìtr. One or two reviews of previous atlases have noted the lack of çomplete petrogÌaphic descriptions of any ofthe rocks. This omission is intentional since we have set outto describe only what caÍl be seen in the photographs ratheÌ than what could be seen if a thin section was available for study. This is one of the obvious limitations of a book of photomicrographs and in some cases Ìve have tried to lessen this drawback by illustratilg the rock at more thao one magnification. The number of minemls with which the student should be familiar in order to name a metamorphic ro€k is moÌe than is requiÌed to give a name to the avemge cÌystalline igneous rock, but it is still a relatively small numbeÌ of minerals. We have not commented on the optical propeÍties of these common minerals except whereitis useful toidentifythem inthe photomicrographs. The relative simplicity of the nomenclatuÌe of the metamorphic rocks compaÌed with igneous rocks is some compensation for the greater variety of minerals in the former. A question which has to be kept constantly in mind, particularly in the study of metamorphic rocks, is how representative of the rock is the thin section? A hand specimen of the type made famous by Krantz, and found in rock collections throughout the world, measures perhaps gcm x 6cm x 3cm and thus has a volume of about 162 cubic cm: a thin section has an area of about 7 sq cm and a thickness of 0.003 cm i.e. 0.021 cubic cm in volume, and so is thus approximately one eight thousandth paÍt of the hand specimen. In a fine-grained homogeneous rock this may be an acceptable sample but in a well foliated rock, particularly if 'coarse-grained, moÌe than one thin section, cut in different orientations would be necessary to begin to desc be the rock; we do not always Ìemember to do this. Finally we must emphasize that there is no substitute for the actual study of thin sections under the microscope. We hope however that such study can be made more rewarding for the student if he or she can, while using the microscope, compare mineraÌ assemblages and textures with those which we have illustrated here. Although no two rocks are identical it is surp singhow similartheycan be, both in mineralogy and in textuÍe, and the Ìecognition of the same mineral assemblages appeadng regularly is an indication that equilibrium is being ap- pÌoached. The study oftextures in metamorphic rocks has given valuabÌe insights into metamorphic processes. Acknowledgements We are again much indebted to colleagues and friends who. have looked out thin sections.for us aIId permitted us to take photographs of them. They include the following: S O Agrell, S Banno, K Brastad, P Bdmblecombe, K Brodie, WD Carlson, D A Carswell, C Chopin, R A Cliff, c T R Droop, B W Fvans, B R Frost, B Goffé, W L Griffi n, S L Harley, T Hirojima, R A Howie, C B Long, Ì R MacKenzie, M B M<irk, J L Rosenfeld, D C Rubie, W Schreyer, J Treagus. We are especially grateful to Dr Giles Droop of Manchester Unive$ity Geol- ogy DepaÌtment foÍ looking through all of our photographs and helping us to decide which to reject in oÌder to improve the balance between different íock types, as well as supplyiDg us with specimens; we alone are howeveÌ responsible for deficiencies in this resDect. The staff of the publishers have been very patient with us du ng the pÌepara- tion of this book, especially as several years have elapsed since the pÌevious atlas appeared, We hope that our experience over the intervening yeals in choosing suitable mateÌials and selecting the best photographs has resulted in a better product than it might otherwise have been. Finally we wish to express our appreciation of the help given to us by Miss Patricia Crook in her accuÌate production of a typesc pt fiom our manuscÍipt. Introduction The aim of this book is to illustrate a range of the most common and most significant metamorphic rock types, and to dãmonstrate the way in which deduc- tionscan be made about metamorphic conditions and the metamorphic history of a region, from observations in thin section. Metamorphism occuls as a response to changes in the physical or chemical environment of any pre-existing rock, such as variations in pressure or teÍ pcra- ture, stÍain, or the infiltration of fluids. It involves recrystallization of existing minerals into new grains anúor the appearance of new mineral phases and breakdown of others. Metamorphiç processes take place essentially in the solid state, in that the rock mass does not normally disaggregate and lose coherence entirely, however small amounts offluids are frequently present and may play an important cataÌytic role; at very high grades, mélts may be produced. Metamorphic settings In this book we have followed the classification of metamorphic seftings used by Yardley (1989): Contact metomorphism takes place as a result ofheating of the country rocks in the immediate vicinity of igneous intrusions or beneath thick flows. It is chaÌacteÌ- ized by the gÌowth of new metamorphic mineÍal grains in random orientations, sìnce any deformation is usuallytoo weak to produce marked mineral alignments. Contact metamoÌphismis also known as theÌmal metamorphism, and its typical products are rocks known as hornfels. Regional metamoryh ttm gives Ìise to laÌge tracts ofmetamorphic rocks charac- teristic of many mountain belts and ancient shield areas. Typically regional metamorphism involves beating, buÍial to produce elevated pressures controlled by the depth attained in the crust or mantle, and defoÍmation to Droduce tectonicÍabrics. Buríil metamorphísmis aïoÍm ot tegionalscale metamo;phism that takes plaçe at low temperatures (< circa 250'C) in the absence ofappreciable deforma- üon. Dynamic metamorp,rtnr occurs in response to intense strain and hence is usually of localized occurrence! notably in shear zones. Hyd.rothermal metamorphísm involves chemical Ìeactions brought about by circuÌaling fluids and is often accompanied by a change in the chemical composi- tion of the rock, known as metosomatism. The most widespread occurrence of hydÌothermal metamorphism is sea-floor metamoryhism t;king place ar active spreading centres. In contrast, much metamorphism involves little chemical change exçept loss of volatiles, and is telmed isochemical. Impact metamorphism has no genetic relation to the other types and is brought about by the impact of large, high-velocity meteorites on planetary surfaces. It resuÌts from extreme shock effects and can produce dense minerals, normally formed only at mantle depths, on the earth's suÌface. With the exception of the last, these categories aÍe not entirely distinct. Instead they grade into one another as a result of different processes acting together; for exampÌe, intense strain can occur locally within a region undergoing regional metamorphism. It is aÌso possible for rocks within a broadly regional metamor- phic belt to have been subjected to different types of metamorphism at different times in thei history. Metamorphic rock names The terminology of metamorphic rock names used here is that of Yardley (1989), from which the followìng passage is taken: There aÌe four main criteÍia foÌ naming metamorphic rocks: 1. the nature of the parent material; 2. the metamorphic mineralogy; 3. the rock's texture; 4. any appropriate special name. Names indicating ahe nature of the paÌent mateÌial These may be very general e.g. metasediment, ormore speçifice.g. marble. Such names maybe used asnouns with or without additional qualification e.g. diopside marble, or as adjectives qualifying a textural name e.g. pelitic schist. Some ofthe common names, and their adjectival forms are as follows: Original materíal Argillaceous or clay-rich sediment Arenaceous or sandy sediment Clay-sand mixture Quartz sand MaÌl Limestonc Basalt IÌonstone Metamorphic roSk type (nounladjective) Pelite/pelitic Psammite/psammitic oÍ quartzofeldspathic (if appropriate) Semi-pelite Quartzite Calc-silicate/calcareous Marble Metabasite/mafic Meta-ironstone/ ferruginous ' In addition, it is acceptâble to prefix any igneous or sedimentaly rock name by metd- to denote the metamorphic eqúvalent, as in the last tvr'o examples. Metamorphic mineraldgy The names of particularly significant metamorphic minemls that may be present are often used as qualifiers in the metamorphic rock names, e,g. garnet mica schist, forsterite marble. There aÍe two possibl€ conventions heÍe: the mineral names may be given in order ofabundance for the principal metamorphic miner- als, to denote the modal mineralogy, e.g. garnet sillimanite schist; or the names of particularly significant mineÌals can be given, which indicate specific conditions of metamorphism, irrespective of theiÌ abundance, e. g. sillimanite musçovite schist. The frIst convention rnight be more appropriate foÌ a field geologist Ìvho wishes to make stratigmphic correlations and can use the modal mineralogSr as a rough guide to rock composition. On the other hand a petrologist studying variations in metamorphic gÍade will specify only those minerals that indicate particular condi- tions of metamorphism, Some cssentially monomineralic rocks are named for theiÍdgminant mineÍaÌ e,g. quartzite, serpentinite or homblendite. A number of other names referÍing to particular mineral associations are described under Special nsmes beIow. The rock's texture TexturaÌ terms are very important for naming metamoÌphic rocks and indicate whether or not oriented fabric elemonts are pÍesent to dominate the rock's appearance, and the scale on which they aÍe developed. Although mineÍal pre- fered orientations are best developed in pelites and semi-pelites, they can form in a wide range of rock qpes if defomation is sutEciently intense. In many regionally metamorphosed rocks micas develop a preferred orientation, aügned perpen- diculaÍ to the marimum compression direction, giving rise to a planar fabric or foliation. The names used lor planar fabrics depend on the grain size and gross appearânce of the Íock. Deformation and metamorphism of clay-bearing clastic sediments give rise to the following sequence of rocks with chaÌacteristic fabrics, in order of increasing grade of metamorphism: Sr4te - a strongly cleaved rock in which the cleavage planes are pervasively developed thÌoughout the rock, due to orientation of very fine phyllosilicate grains. The indiüdual aligned grains aÍe too small to be seen with the naked eye, and the rock has a dull appearance on fresh surfaces. Phyllíte - similar to slate but the slightly coaÌser phyllosilicate grains aÌe sometirnes discernible in hand specimen and give a silky app.'arance to cleaved surfaces, Often, the cleavage surfaces are less perfectly planar than in slates. Sc[rJt- chaÍacterized by parallel alignment of moderately coarse gÌains usually clearty visible vdth the naked eye. This type of fabric is known as schistosity and where deformation is fairly intense it may be developed by other minerals, such as hornblende, as well as by phyllosilicates. Gze;rj - gneisses are coaNe, with a grain size of several millimetres, and foliated (i,e. with some sort ofplanar fabric, such as schistosity or compositional layering). English and North American usage emphasizes a tendency for diffeÍent mineÍals to segÌegate into layers parallel to the schistosity, known as gneissic layering; typically quartz- and feldspar-rich layers segregate out ftom more micac€ous oÍ mafic layers. European usage of gneiss is for coarse, mica-poor, high grade rocks, irrespective of fabric. Th e teÍÍÍr oúhogneiss ìs used for gneisses of igneous paÍentage, paragneiss for metasedimentary gneisses. 2 In practice the boundaries between all these types are gradational. Mylonite -is a terlrJ, used for fine-gmined rockJproducéd in zones of intense ductile deformation where pre-existing grains have been deformed and Íecrystal- lized as finer srains- Hornfeb - iontact metamorphism in the absence of deformation gives rise to a random fabric of interlocking gains which produces a tough roãk known as hornfels. _ Some metamorphic rocks. particuÌarly those relatively pooÌ iri sheet silicates, have textures that are not obviously schistose, even though the rocks are not hornfelses. Winkler (1976) has propósed the termfe/s for su-ch rocks, although it has not been universally adopted. I; the older liter;turdthe te rm granulite is \sed for some such rocks, particularly psammites with an equigranulaÌ iexture, but this term rs now reserved to denote particular physical conditions of metamorphism. TextuÌal names are usually uóed asnouns, qualified by adjectives indicating the parçnt material or the present mineralogy (e.g. gamet schist, pelitic hornfcls). Special names Special narnes are meÍcifully Ìare in metamoryhic petrology and most that a(e used are also descriptive. However, the mineral associatións indicated by the names carry implications for the conditions of metamoÌphism, Some of the commonest are the followins: ,Greerschíst - green foliaté-d metabasite, usually composed predominantly of chlodte, epidote and actinolite. Blues-ch,ist - dark,lilac-grey foliated metabasite, owing its colour to the pres- ence of abundant sodic amphibole, typically glaucophane or crossite: seÍdom tÍuly blue in hand specigren, . .Amphibolite - an essentially bimineralic dark green rock made up of horn-blende and plagioclase. A wide range of minerals may occur as accessóries. Most ampÌibolites aÌe metabasites (onho-amphibolite) but some may be metamoryh- osed calcareous sediments (para-amphibolites). . Seryentinite- gÍeen, black or reddiih rock coínposed predominantly of serpen- tine, Formed by hydration of igneous or metamorphic peridotites (olivine:rich ultrabasic rocks). Eclogíte - metabasite composed of garnet and omphacitic cÌinopyroxene with no. plagioclase feldspar. Common accessoÍies include quartz, kyanite, amphi- boles, zoisite, rutile or minor sulDhides. Granulite - rcck characterized by both a texture of moÌe or less eq úidimensron- al,-straight sided (polygonal) grains for alÌ mineral species, and ã mineralogy indicative of very high tempeÌature metamoryhism, clòsely akin to the minerãl- ogy of calc-alkaline basic to moderately acid plutonic rocks (feldspar, pyroxene, amphibole) , The craraocftie suite constitutes a distinct varietv of K-feldsDar and hypersthene bearing granulites. Migmatite - amited rock composed of a schistose oÌ gneissose Dortion intimate- ly mixed üth veins of apparently igneous quaÍtzo-feld;pathic màterial (known as leucosomesr. Textural terms The textural terms used in the desqiptions in this volume are introduced at the beginning of Part 2. Physical conditions of metamorphism: metamorphic fâcies One of the most important goals of metamorphic petrology is to determine the pressures (P) (and hence depths) and temperatures (I) at wtrich particulaÌ Íocks formed. An account ofthis is entirely beyond the scope of this book and we deal here only with those aspects which aÍe essential to understanding the layout of the main part of the book. The appeaÌance ofparticular metamorphic minerals depends on both the bulk composition of the rock and the P-? conditions which it exDeÌienced. With increased heating for examile, peliric schists develop a sequence ofprogressively hrgher temperature assemblages of minerals. The metamorphic teÍrane can theÌefore be divided into zoneseach characterizedbv a Darticulaì mineral or surte of minerals. Rocks subjected to higher temperaturés and pressure are said to be higher grade than those subjectéd to less extreme conditi,ons. Zone boundaries represent a constant gÌade and so are known as iíograds. Different original rock types respond in diffeÌent ways to the same conditions of metamorphism according to theiÌ bulk composition, and some show far fewer mineralogical changes than othe$. For this Ìeason it is not usually possibte to trace zones defined by assemblages in one rock type thÌough regions wheÌe that rock type is absent. To overcome this problem Eskola (1915) devised a scheme of more broadly defined metamorphic facies each corÌesponding to Ìegions of the P- Tdiagram which may be distinguished bythe assemblagesin anyone ofa range of rock types. The assemblages of metabasites are however the primary basis for the facies classification. The scheme of metamorDhic facies used here is illustrated in Fieure A 30e o Using this book We have divided this volume into two parts: in the first the p mary aim is to illustrate some important metamoÍphic mineral assemblages grouped according to parental rock composition and P-7 conditions of metamorphism; PaÍ 2 specifically illustrates textures. PaÌt 1is divided into sections açcording to paÌental rock type, correspondingin part to chapteÌs in Yardley (1989). Sequences of photomicrographs filst illustrate progressive metamorphic zones encountered in normal medium pressure meta- morphism, followed by examples of unusually high temperature metamorphism at intermediate pressures. After this, as approp ate, are examples of lower pressure metamorphic sequences aod higher pressure sequences. Part 2 illustÌates some basic textural terminology and then has sections illustrat- ing deformational textuÌes, Íeaction textures and timing Ìelationships of de- foÌmation and porphyroblast growth. Inevitably there is considerable overlap between the two paÍts and we have provided cross referencing as appropÍiate, Thus, additional examples ol many of the pelite assemblages ftom PaÌt 1 are found in Part 2. In writing the bÍief descÌiptions to accompany the photographs we have assumed that the reader has some general familia ty with optical mineralogy, but we have provided sufficient details of the prope ies of tlÌe more unusuaÌ minerals to enable them to be identified. rühere a mineral's colour is given, this refe^ to its colour in plane polarized light unless interference (or bireftingence) colour is specified. The abbreviations PPL and XPL are used for plane polarized light and qoss pola zed light respectively. Where it is necessary to indicate the location of particular featuÌes this is sometimes done using notional geographic coordinates: north (N) is the top of the photograph, etc. In addition, N-S features may be Ìefened to as vertical. and so on. Cross references in bold numerals ÌefeÌ to the rock number; more rarely, where cross references are to page numbers, this is specified. For some rocks, a specific literatuÍe reference to a papeÌ descÌibing them is given at the end ofthe caption. Geíéral references giveÍi elsewhere in the text aÍe listed at the end of the book. ' 1' ' : FIG. A Pressure temperature dìagram showìng the fields of the various metamoryhic facies, after Yardley (1989). Hfls : horn- fels, AE = albite-epídote, HBL= hornblende, PX: ptroxene, PR EH- P U M P : prchnìte-pumpellyite 408 ã. ã Temperatwe 0C) Part 1 Varieties of metamorphic rocks Contact metamorphism Contact of thermal metamorph rirz occurs in the country rocks surrounding an igneous intrusion as a result of magmatic heating. The resulting metamorphic rocks comprise a metamorphic aureole around the ìntrusion or group of intru- sions that provided the heat source, and often show concentdc metamorphic zones. Nlost typically, contact metamorphism results in the production of lornjfelses i.e. rocks whose metamorphic minerals have intergrown in a random interlocking pattern because of the absence of strain as they grew, however in some aureoÌei magma emplacement was accompanied by deformation of the country rocks, leading to the formation ofcontact metamorphic schists texturally similarto those produced in regional metamorphism. Contact metamorphism can aifect a wide range of rock types, but most aureoles are developed in metasedim€ntary rock with a previous history of regional metamorphism, usualÌy at low to moderate grades. However contact metamorphism of sediments is common at high, suo- volcanic levels, and aureole effects in pre-existing igneous rocks occur also. In this section we illustrate two classic examples of homfels. The cordierite chlorite biotite hornfels (l) is typical of spoïèd slates, pÍoduced by thermal metamoÍphism ofslates around granitic plutons, while the peridotite homfels (2) is a more unusual rock type which displays very graphically the way in which newly-formed metamorphic minerals can grow to produce an interlocking tex- ture. Cordierite chlorite biotite hornfels Contact metamorphism This rock shows elongate crystals of brown biotite and finer grained, gre€n, low birefringence chÌorite in ran- dom orientation typical of a hornfeÌs. The main colour- less mineral in the rock is poikiloblastic cordierite, recognized readily from the XPL view in rvhich cycÌic (sector) twinning is seen. The Ínatrix between the por- phyroblasts comprises a fine-grained intergrowthof muscovr le. opaque grains and qutr tz. Localíty: Skicldaw aurcole, England. Magnificatíon: x 52, PPL and XPL. Metamorphic seftings Peridotite hornfels CoíÍtâct melamorphism l-:: characteristichoÌnfelstextureof randomlvoriented -:: r.ocling crystals is particulaÍly well displayed by this ::cs- although its compositiol is unusual for a hornfels. -: :i .omposed predominantly of olivine and oÌthopy- ::r:re. \\ith the oÌthopyroxene forming randomly ::::rred prisms showing cleavage and reÌatively low ::::foneence, set in an olivine matrix. Highly birefrin- -:::: maÌerial replacing some orthopyroxene grains is .:: T'rs rock occurs in the aureole of a maior batholith. -:rrç lÌ culs a serpentinite body. The heat from the ::-r-rion has broken down serDentine and more or less '-.:..red the original igneous mineralogy ofthe ultrama- :: :.\k. albeit with a distinctive texture. -..-iii.Ì: Mount Stuart, Nofihem Cascades, Washing- . L S-1. Magnification: t 14, PPL and X PL. Regional metamorphism Regional metamorp&Àm is usuaÌly more extensive than contact metamoryhism and is not closely focused around a specific magmatic heat source; indeed no heat source may be apparent. Typically, the growth of new metamorphic minerals,in regional metamorphism is accompanied by deformation and by the production of tectonic mineral fabÍics in response to strain. Textural studies show that although metamorphic mineral growth broadly accompanies deformation in regioDal metamorphism, in detail different deforma- tion episodes may have occurred and mineral growth does not always correspond with periods of deformation (p. 94). Most metamorphic rocks have undergone predominantly regional metamorph- ism , and there is a very wide range of conditions of pressure and temperatuÌe over which regional metamorphism can occur. At the high temperature, low pressure end of the spectÌum of metamorphic facies, regional metamoÌphism is usually closely associated with the emplacement of magmas; there is no fundamental division between (i) regional metamoÍphism driven by magmatiç heating fÌom mulliple intrusions, so that there is no single focus, and (i, contact metamorph- ism at similar pressures and temperatures localized in an aureole to a specific intrusion. RegionaÌ metamorphism may also oveÍpúnt earlier hydrothermal metamorphism, notably in metavolcanic rocks. Regional metamorphic rocks often contain zones of high strain, especially along shear zones and faults, within lvhich the rock texture is dominated by deformational effects. In this case, regiooal metamorphism becomes transitional to dvnamic metamoÍDhism. Dynamic metamorphism Dynqmìc metamorphr,rm is dominated by deformation and recrystallization due to strain , and is usually accompanied by a reduclion in grain size. 'lhe rame my lonite is used foÌ rocks that have undergone dynamic metamorphism, and mylonites aÌe usually of restricted occurrence within fault zones, including thrusts and shears. However some shearzones can be several kilometÌes inwidth and extendfortens or even hundreds of kilometÍes. Dynamic metamorphism is a process that progressively affects pÌe-existing metamorphic origneous rocks, only destroying all trace of original fabrics if it is very intense. Since ductile deformation processes aÌe involved, temperatures are likely to be in excess of about 300"C, and so truly unmetamoÌphosed sediments are unlikely to be affected. DiffeÌent mineÍals respond in very diffeÍent ways to d€formation. In crustal rocks containing quartz. it is the quartz that deforms most readily. forming strained grains with undulose extinction, which then break down to a finer- grained matrú of undeformed gÌains through the process ofsyntectonic Íecrystal- lization. Minerals such as feldspar and garnet aÌe relatively strong, and often remain as relatively laÌge relic grains, somewhat rounded by bÌeaking-off oÌ recrystallization oftheir edges aud corners. These grains aÌe known as porphyro- clasts, Micas and other phyllosilicates readily recrystallize in mylonites and ÍÌìay also be produced by hydÌation reactions due to infiltÌation ofwater into the zone of deformation. A range of siliceous mylonites are illustrated later in the book (84-88, pp. 9G-94), the example shown here is more unusual, being a mylonite of ultÍabasic composition produced by deformation of peridotite undeÍ uppeÌ mantle condi- tions. At high temperatures in oliüne-rich rocks itisthe olivine that deforms most readily, while p].Íoxenes, garnet oÌ spinel form porphyroclasts. 8 Metamorphic settings Garnet biotite schist Regionâl and dynamic metamorphism , :.;.::\ : Franz Joseph Glacier, Westland, New Zealand. ' ' ' , :-.: ' icdtìot1: x 1A, PPL and XPL. Metamorphic settings Garnet staurolite schist/ biotite schist Regional metamorphism with weak deíormation only This low magnification view shows the contact between two contrasting original beds, one of clay (pelite), the other of sand (psammite). The pelite band is now com- posed of euhedral porphyroblasts of staurolite (high relief, pale yellow) and gamet (high relief, grey) in a matrix of biotite, muscovite and minor quartz and ilme- nite. The assemblage of the pelite layer indicates amphiboÌite facies metamorphism, but the psammite layeÌ nevedheless retains evidence of original sedimentary structure. The biotite-rich stdpes within ìt mimic original bedding and pick out cross-stratifiçation. Note that the grain size of the originally coarser psam- mite has probably changed relatively Ìittle, whereas the original clay band is now much coarser. Locality: Coos Canyon, Rangely Dístrict, Moine, USA. MaqniÍïcatíon: x 5, PPL. Peridotite mylonite Dynamic metamo.phism This rock is a protomyìonite of unusual composition. Porphyroclasts are of olivine and pyroxene (both clino- and ortho-), and may have been markedly bent during deformation, so that extinction varies considerably alongthe Ìength ofthe grains. Some porphyroçlasts have long, drawn out ,aiLç, which break up or recrystaÌlize to contribute to the finer-grained matix, itself made up of the same minerals as oçcur as porphyroclasts. An eÌon- gate, low birefringence orthopyroxene grain near the centre of the fieÌd of view contains fine pale E rW stÍipes that are lamellae of clinoenstatite. This mineral is ex- tremely mre, occurring only as the result of stress- induced polymorphic transition from enstatite. Small isotropic grains ofdark brown spinel are present and are also highly deformed. Locality: Premosello, Val d.'Ossola, Northern ltaly (Ivrea Zone). Magniïicatiòn: x 7, XPL. Metamorphic settings Sea-floor and hydrothermal metâmorphism Hydrothermal metamorphism can occtr in a úde range of settings but is character- ized by the involvement of hot aqueous fluid which passes through the metamor- phosing rock and leads to changes in its chemicd cómposition ol metasomatism. The extent of such changes can be rather minor, dominated by hydration, or may be extensive and result in the formation of a monomineralic metasomatic rock in which the abundances of many of the chemical elements in the rock have been cnangeo. Although Ìocalized hydrothermal metamorphism is common around igneous intrusions and in shear zones and faults. by far the most important o..ui."nces volumetrically are produced by interaction of heated sea-water with newly- created oceanic crust at mid-ocean ridges. This type ofhydrothermal metamorph- ism is known as sea-fio or metamorphíim and is únd in ophiolites on land as well as on the present ocean floor. Rocks with an early hisiory of sea-floor meta- morphism may subsequently undergo regional metamorphism, and the second of th-e examples iÌ lustrated has probably undergone a complex metamorphic history of this type (7). Sea-floor amphibolite S€+.flooÌ metamoÌphism --r:::ample was collected in a dredgehaul from the :.-.". Ìloor. It is a very fine-grained rock with a random ::r.:rre rhat may mimic an original fine-grained ophitic ::.:-: ÌexÌure. The dominant constituents are pale green .:--::,rìiÌe and plagioclase, with appreciable opaque ox- :: Small amounts of calÇite are also present. -: :;!!n: Peake Deep Area, Atlantic Ocean. Magnifica- .: < -38, PPL and XPL. Metamorphic settings Epidosite HydrotheÌmal metamorphism This rock is a product of hydÍothermal metamorphism accompanied by extensive metasomatism. It is com- posed predominantly of epidote, but minor green chlo- rite and clear quartz crystals are visible in the PPL view. The XPL view is an enlaryement of part of the aÌea shown in PPL. and also contains minor ouartz. The texture seen in PPL appears to be a typical ophitic texture of basalt. However both the clear yellow laths, corresponding to original plagioclase, and the intervening brown areas (originally pyroxene or glass) are now epidote. Careful inspection of the arças of uniform bireftingence colour in the XPL view shows that single epidote crystals pre- sent noÌv are coarser than the original grain size and have a granoblastic texture. Hence clusters of adjacent laths are now ghosts within a single epidote gÌain. Loca.lity: Clqggan Bay, Achill Island, Irelond. Magni- fication: x 22, PPL and x 45, XPL. t2 Metamorphic settings Impact metamorphism Impact metamorphisn has no gen€tic link with the other categories of meta- morphism, and affects rocks on the earth's surface close to the site of impact of large high-velocity meteorites. Such events are very rare on Earth and ancient meteorite impact sites have usually been extensively reworked by erosion and other geological processes. On tectonically inert planets like the Moon however, meteorite impact may be the dominant geological process reworking the planet- ary surface. The shock wave that passes out from the point of impact subjects rocks to pressures normaliy only expe enced at mantle depths for extremely short periods of time, whiÌe the subsequent stress relaxation Ieads to temperatures that may be sufficiently hot to melt or even vapourize the rock. Shock effects dissipate outwards from the site of impact and mnge from fracturing of rock and internal defoÌmation of grains to the production of high pressure mineral polymorphs (such as the dense forms of SiO2, coesite and stishovite) or melting. lmpact metamorphic rock This sâmple illustrates a range of features typical of rocks produced by intense shock metamorphism at a meteorite impact site. The rock contains angular frag- ments of quartz, feldspar and biotite set in a fine matrix(hat is in Ìarge part glass produced by impact meÌting. The colour ofthe glass is variable because ofits extreme chemical heterogeneity, whilst the crystalline materiaÌ comprises angular ftagments of original coane-grained granite basement mateÌial, and so is distinct fÌom volca- aic phenocrysts. Biotite in the lower right corner has tteen distinctly bent by the impact event. Locality: Ríes Crater, Germany. MegniÍicatíon: x 43, PPL and XPL. Metamorphism of pelitic rocks Medlum pressure metamorphism 'fhe term Balrovian metamorphism is one that is widely used in the English- speaking world to describe medium grade metamorphism that has taken place at moderate pressures i.e..over a range of P-I conditions corresponding approx- imately to a normal geothermal gradient in conünental crust, The name derives from the woÌk of G M Barow, in the late 19th century, on metamorphic zones in the southeÌn Highlands of Scotland (Barrow, 1893). BaÍovian metamorphism spans the temperature range ofthe greenschist and amphibolite facies (FiguÌe A), at sufficiently high pressuÌes, such that kyanite ratheÌ than andalusite is the first Al2SiOs polymorph to appeaÌ on heating. A similar pattertr ofmetamoÌphism to that found by Barrowhas been reported from many parts of the world and the pelite zones are illustrated in the folloúng section in ordeÍ of in€Ìeasing metamoÌphic grade. They include examples of both lower and higheÍ grade medium pressure metamorphism than those occuring in Barrorü's tlDe area. Graphitic slate Prehnito pumpellyite Íacies This very fine-grained rock represents the lowest gmde of metamorphism. At an advanced stage of diagenesis, the clay minerals are dominantly chlorite and illito, and $rith further metamorphism the illite itself becomes coa$er-grained and recrystallizes to a phengitic mica which is riçher in Si and poorer in Al than pure musco- üte and contains some Me and Fe. This rock contains detritãl grains of quartz and minor alkali feldspaÍ with a fine matdx of phengitic mica, graphite and some chlorite. The Ìock has been intensely deformed pro{ucing a pervasive slaty cleavage and at the same time, odginal fine scale bedding has been disrupted by folding. Frag- mented silty layels rich in detrital quartz appear as light- er regions in a darker pelitic matrix. Note that úe slaty cleavage cuts across contacts between the bed types and is itself cut by two late-stage veinlets. Locolity: Routebum Track, South Island, New Zealand, Magnification: x 12, PPL. Metamorphosed sedimentary rocks 10 Chlorite muscovite albite schist G.eenschist Íaciês- chlorite zone Addit ioôal examole: 83) This rock is from the chlorite zone oÌ the Dalradian of :h3 British IsÌes. The chlorite muscovite intergrowths :.:r be rvell seen in the higher magnification photo- J3phs: the pale green colouration of the muscovite is :h. result of an appreciable phengite content. The col- :urless minerals are quartz and albite, the latterforming Ì:tinct porphyroblasts sometimes simply twinned as -ul.ated here. Accessory minerals visible in the high ::STÌification view include apatite, occu[ing as colour- .:::. near-isotropic high reliefgrains enclosed in musco- '.::i and albite, opaque oxides, and a small zircon (within ::: r lbite at its upper edge). FIaws in the section appear -. -ircular high reliefareas in the upperdght quadrant of --:: r -j0 views. This rock displays a pronounced crenulation fabric, ;.:h an earÌier pervasive phylìosilicat€ fabric folded to ::ìuce a new spaced cleavage. The higher magnifica- -- r:l !ie\v shows that the albite poryhyroblasts overgrow ]r:h the fab cs, and therefore grew post-tectonically. -::alín: Cloghmore, southeast Achill Island, Ireland.trl:ini.fication: x 14, XPL; and. x 30, PPL and XPL. Metamorphosed sedimentary rocks 11 Biotite chlorite muscovite schist Greenschist íacies - biotite zone(Addit ional example: 92) The bright interference colours in this rock are mainÌy due to the high proportion of muscovite which is pre- sent. The biotite and chlorite can be seen readily in the PPL view. Other minerals present are dominated by quaÍz; albite (now partly sedcitized) has a patchy pale brown appearance in PPL and there is a smalÌ percen- tage of opaque grains. The small scale folding of the original schistosity in which the platy minerals were aligned has produced a crenulation cleavage (J€e texture section). This has been accompanied by some segregation of the quartz into horizontal layers corresponding to crenulation hinges and separated by layers that are nearly pure phyllosili- caÌe. Locality: northwest Mayo, IreLand. Magnification: x 27, PPL and XPL. Ió MetamorDhosed sedimen!ãr. . :ir- Microcline epidote mica schist Greenschist Íacies - biotite zone This is a seni-pelit ic rock consisting of green biotite, muscovite. epidote, microcline and quartz. Reaction between chlorite and microcline produces biotite at a slightly Ìower grade than in pelit ic rocks lacking K- feldspar, and this reaction accounts for the absenõe of chlorite in this rock. The high RI mineral showing bright inrederence colours is epidote; a small grain is presentnext to the upper edge. half-way along it. Locality: norÍhwest Mayo, Ireland. Magnificatíon: x 20, PPL antl XPL. Ìt l t , + Metamorphosed sedimentary rocks 13 Ghloritoid slate Greenschist Íacies- biotite zone A fine-grained slate containing randomly oriented chloritoid poryhyroblasts, which in this specrmen are manganese-rich and have the variety name ottrelite. The fine-grained groundmass ofthe rock consists ofchlorite, muscovite, quartz and hematite. Note that fine scale sedimentaÌy layerilg is well preserved, cut by an oblique slaty cleavage, despite the fact that the chloÍitoid is of comparable size to the spacing of the original lamina- tions. Some of the chloritoid crystals are so full of incÌu- sions that they appeaÍ almost opaque. The hour glass structure is not uncommon in chloritoid. Localíty: South of Vielsahn station, Ardennes, Belgium. Magnifícation: x 20, XPL. 14 . Garnet chlorite biotite schist Greenschist Íacies - gamêt zone{Additional examoles: 82, 89,91, 99} The diagnostic garnet zone pelite assemblage of ganet + biotite + chlorite + muscovite + quartz is well dis- played in this sample. The rock has a markedly porphy- roblastic t€xtuÌe with very large (<1 cm) euhedral or subhedml gamet gÌains in a fine matrix. Biotite is also porphyroblastic though much finer-grained than garnet. Chlorite and muscovite occur with quartz in the mat- Ìix and define a complex fabric formed over at least two stages of deformation which apparentÌy pre-dated the peak metamoryhic tempemtures at which biotite and garnet glew. Locality: Bfidgewater Corners, Vermon4 USA. MaSni- fication: x 18. PPL and XPL. Metamorphosed sedimentary rocks Garnet chloritoid sch ist Greenschist íacies - gaÌnet zone The mineral assemblage seen here is typical of highly aluminous pelites in the garnet zone of Éarrovian úpã metamorphism. Note that bioti le is absent in most such chloritoid schists. Chlorjtoid is recognized by irs green absorprion col- ours and high relief. DitfeÍent grains show three distinct colours, and the palest, stmw colour is very similar toÌhat ol garneÌ. This is i l lustrated by the two ppl views, tâken with.the polarizer at right angles. The much lower K r or rne chtonte makes it possibìe Ìo distinguish readily between. the two green minerals. Only oie gamet i's shown, just below the centre of the field. TLe other mineraÌs present are rnuscovite, quaÌtz and albite. Localíty: Ebeneck,6 km northwestof Mallnü2, Kiiruten, ,Austia. Magnification: x 22, ppL', ppL and XpL. ' Metamorphosed sedimentâry rocks Staurolite schist Amphibolite íacies - staurolite zone(Addit ional examples: 90, 94, 95,97) This rock contains poikiloblasts of high relief staurolite which display more marked yellow pleochroism than is usuaÌ. The Ìargest poikiloblasts are of plagioclase (e.g. near mid-point of lower edge); other minerals present are muscovite, quartz, green biotite and an opaque mineral . The sect ion is s l ight l ] th ic l and in consequence quartz crystals have a lellowish tinge to therÍ inierfer- ence colour. The schistosity in this rock ìs defined by both musco- vite and the opaque grains, and is seen in the lower right corner lo pas\ cont inuou. l ) into the inclusion trai ls wi th- in plagioclase, without disruption. Note that the quartz inclusions within staurolite are very fine, whereas the matnx quartz is very coarse, evidently there was exten- slve recrystaÌlization of quartz after staurolite grew. Locality: Connectícut, USA (precìse localìty unknown). Magnificatíon: x 7, PPL and XPL. I - , - : : ) ' " . ' ' ' t'y'^) ...a, 4::" 'ì:. ë :; , - , *e , '1, ' , ./: .; 2 ..- MetâmoÌphosed sedimentâr-r rorl \ 17 Garnet staurolite kyanitegnetss Amphibolite Íaciês - kyanite zoíe ]l^Ì'-.fBlI part of the ìarger field of view is maintyoccupred by porphyroblasts consisting of epitaxial inter'-growths of,staurolite and kyanite, *úite tt e to*e. fuit .omprrses.larger garnet cryçtals in a muscovite mairix. r ne rock 15 somewhaÌ al tered and ha. !e ins ol chlorr te associated with the girnet crvstals. _-Tl: ui:Ìt ar hìghèr magniÍication snor\ a composrrep-orphyÌobtasl of epiraxially inrergroun kyanite and\raurotlÍe rn greater detail. It i . cut to rhe righr by a vein oI chlorite. perhaps resulting from retrograde alteration along a crack. Al though much ol the matf lx is coaÍseqecussate muscovite, there is some retrograde chlorite and the porphyroblast is flanked on its upper margin byplagioclase. The occurrence ot piìrâllel inÍergro\rths ol stautoliÌe ano kyanrte is noÌed in most mineralogical te.\ts and ,esul ts. f rom lhe simi lâÍ i t ) o l parts of their i t ructure\ .\evennetes\ t l ts not verJ frequentlv observed. Locali.ry; Zion Hìll. Or lllounrain:, Co Slìgo. treLtnd. .vtlgnilícaion; ^ 7. XpL: and , 20 ppL àn,l XpL. i ' Metamorphosed sedimentar! rocks Kyanite biotite graphite schist Amphibolits Íaciês - kyanite 2one Two porphyroblasts of kyanite are shown in this view, one of them is simply twinned and both of them are surrounded by a retÍograde shimmer aggregate of fine muscovite. The groundmassofthe rock is mainly biotite, muscovite, graphite and qua z with rare crystals of touÌ- mallne. The dominant, diagonal mica foliation may itselfhave been produced by crenuÌation of an earlier fab c. Graphite in the lower left quadrant picks out numerous microfolds to which the dominant foliation is axiaÌ DlanaÌ. Localíty: Chiwaukum Cascades, Washington, and. XPL. schist, Stevens Pass, Noúhern USA. Magnifícation: x 9, PPL Metamorphosed sedimentary rocks Sill imanite staurolite schist Amphibolite tacies - sillimanite zone(Addit ional examples: 96, 100) The field of view shown in the lower masnification rhotographs reveals that the main minerals in this rock Je staurolite, biotite, plagioclase and quartz. Several conspicuously zoned grains of tourmaline are also visible rear the middle of the upper and right-hand edges , green cores, yeÌlow rims), and there are some corroded remnants of an original gamet porphyroblast which are :o heaviÌy clouded as to appear nearly opaque in PPL. The gamet Íemnants are mantled by biotite which is uìtergrown with, and partiaÌly replaced by, fibÌolitic sillimanite. This can be seen more clearly in the detailed high power view (some small air bubbles in the lower ngfit part ofthe slide should not be confused with miner- .lls). The replacement of garnet in this way teads ulti- maÌelv to the development of sillimanite pseudomorphs úer gamet (see 100) as a Ìesult of a complex ionic reaction cycle. Localit),: Cw Hill, Connemara, Ireland. Magnification: . :0. PPL and XPL: and x 56. PPL. High temperature metamorphism In some parts of the world, for example the Appalachian belt in northeast USA, the Barrovian sillimanite zone is succeeded by progressiveÌy higher grade zones. The first evidence for this is the breakdown of muscovite + quartz + K-feldspar + sil l imanite + fluid, and the appearance of migmatite leucosomes of broadly granitic compositions. The transition from the upper amphibolite facies to the granulite facies is marked by the coexistence of the four phases garnet + cordier- ite + K-feldspar + sillimanit€. While in some areas extensive migmatites are developed under amphibolite facies conditions, elsewhere significant melting is restricted to the gÍanulite facies. These contrasting styles of high temperature metamorphism are probably controlÌed by the availabiÌity ofwater. In a few parts of the world extremely high temperature metamorphism ofpelites hastaken place resulting in the formation ofexotic mineral assemblages such as the coexistence of sapphiÌine + quatz. Sil l iman ite K-feldspar biotite schist Amphibolite Íacies - sillimanite K-Íeldspar zonelAddìtional example: 84) This assemblage is rather typical of high grade schist in which the temperature has been sufficiently high for the muscovite to react with quartz to pÍoduce K-feldspar and an aluminium sil icate mineraÌ. in this case sil l ima- nite. To make it possible to distinguish the K-feldspar easi l l f rom un(wrnned plagiocla5e or quar l / the .ect ion has been stained with sodium cobaltinitrite solution after being etched with hydrofluoric acid vapour. In the lower part of the PPL view the pale yellow. stained K-feÌdspar can be readily distinguished from yeÌÌow-brown biotite and quartz and plagioclase. ln the upper part of the field of view fine needles of f ibrolit ic silÌ imanite are easily seen intergrown with quartz and biotite. A few muscovite cíystals are probably of retro- grade origin rather than relics from lower grade. The segregation of the fibrolitic sillimanite and K-feÌdspar into separate domains, although a common phenom- enon at this grade, is not weÌl understood. In marr l terrane.. melt ing precedes or Jccumpânic. muscovite breakdown. The absence of migmatit ic tèa- tures in this rock reflects Ìather low pressures oï meta- morphism. Localìty: Maumeen, Connemara, lrclan(l. Magnifica' tìon: x 27. PPL and XPL. MetamorDhosed sedimentar\ rocks Garnet cordierite plagioclase si l l imanite gnetss I - phibol i te Íacies - si l l imanite K-feldsoaÍ zone --. ::r. ' in minerals in this rock aÍe l isted in the narÌe : .rnd we cân also jnclude biotite and quartz. . saÍnet and biotite aÌe Ìeadilv identif ied. while the ' - :: i i form of sil l imarÌite is developing mainly at the : :. :: !ìf biotite. notably in the centre of the field of : .r L:rge crvstals in the top left quadrant are pseudo- ::::.1tter cordierite which has been almost entiÌely .r .::J b\ lìne sericite due to a retrograde reaction. : .:: :.asc shows incipient alteration in PPL (lower :- : :u i ìdrant) . but st i Ì l d isplays mult ip le rwinning. - -,::r t: clear and unaÌtered. --: ' .,..(ìciation of garnet. cordierite and sil l imanite r: K'fe ldspar is diagnostic of low to medjutn press- ', ...:-rl:,eÌitesirltÌìeuppermostpaÍtof t lÌeamphibolite :. This rock is the schistose portion of ir migmatite : . :nrichccl in aluntinous minerals due to melting . : :. i relt ite). It is typical of arcas where there has : :\ ien\i\e rreìting under upper amphibolite. as - -:,1 1(ì sranulite. facies conditions. ' LtLe Ntthaslean, Connentora, Irelatd. Magni . 1-1. PPL otd XPL. Metamorphosed sedimentary rocks .. Garnet cordierite K-feldspar gneiss Granuli tefacies(Addit ional example: 101) This rock consists mainly of microcline-perthite and quartz with concentrations of garnet, cordierite and a small amount ofbiotite and iron ore. The cordie tecan be recognized by the very characteristic alteration to yellowish pinite seen in the upper part of the PPL view. The XPL view at higher magnification (a detailfrom the upper Ìeft quadrant of the lower power view) displays the isotropic veins and crack fillings replacing low bt- refringence cordierite which are very characte stic of this aìteration. The clear mineraÌ speckled with inclusions in this rock is quartz, whereas the microperthite is free from tiny inclusions. The assembÌage of this rock is typical of lower granu- lite facies pelitic migmatites. Locality: Kakola, Turku, Finland. Magnifícatton: x 9, PPL and XPL; awi x 25, XPL. Metamorphosed sedimeÌrrarl N{iii l l !íYilgmattttc gneiss 3í-ânulite Íacies Metamorphosed sedimentary rocks ./ Garnet cordierite spinel quartz gneiss Granuli teÍacies These photographs show a fairly coarse-grained rock in which garnet and a dark green (nearly opaque) spinel can be readiÌy distinguished. The coÌourless mineÍals are microperthitic K-feÌdspaÌ, plagioclase. cordierite and quartz. The cordierite is weakly c/oady in this rock due to myriads of small inclusions. A number of quartz crys- tals are in the extinction position and these have cracks filled with a micaceous mineral. Plagioclase displays multiple twinning, but no good examples of K-feldspar are visible in this view. In the high magnification photograph a Íim ofcordier- ite can be seen armouring the spinel from contact with the quartz. This enlarged Íegion is just to the right-of- centre of the lower power photograph in which the cor- dierite rimshowsupin the XPLview as a white border to the spineÌ crystals. Two biotite crystals can be seen within the galnet. these are the only hydrous minerals present, and have perhaps only been preserved under extremely hiSh temperâture conditions because they were armoured by the garnet. Localíty:5 kmwest of Fo Dauphin, South Madagascar' Mognífication: x 16, PPL and XPL; (ud x 13, PPL. óapphir ine granul i te :-."ìulite Íacies - ': ::: in minerals present in this rock are antiperthit ic : t.:- ir (not seen here), quiÌrtz and skeletal high relieí . : : - . : : rne. Near the top of the f ie ld of v iew some - - .\ r.r\ene crystals showing first- to second,ordeÍ ' :r::rance coÌours. occur as rims around sapphirine .- .:,.: The assemblage sapphirìne + quartz is stable .: ren high temperatures. At lower temperature -. ::urralent assemblage is orthopyroxene + sil l ima- - - .r lhat the orthopyroxene may have formed by a : _- ì :e\Sl \e react lOn. : ' .,::emblage is pÍobably the highesr temperature :-r.Jse formed on a regional scaie in metasedi- : :. lr requires temperatures in excess of850"C. and ' .- -.. uround 10i)0"C (r-ee aho l0l). .:. Entlerby Land, Antarctica. Magnificatíon: -- PPL md XPL. '. ' :-t. Harkl S L 1983 In Oliver R L, James P R, .. .- B ruls.) Antarctìc Eqrth Sciences, Cambridge : '. i1 P,.?.r.r, pp. 25 3A Sapphir ine cordier i te biot i te gneiss i ;_'- :e facies - -! .!ìnsists mainly of these three minerals. The ,-. .:rphirine is intergrown with cordierite. which , : ... .. L,e ntisidentif iedasplagioclasesìnceitshows - : : : . ( : : Ìn ingandlackssomeof i tsdist inct ivechar- : .: ::. .uch as pÌeochroic haloes or alteration to -::: ,-rf moderate relief grains are of apatite. !:,.ope clain, Beitbridge, Zimbabwe. Magní- . :t t. PPL. . D,oop GTR 1989 Journal of Metamorphic - : \-ì .103 MetamorDhosed sedimentar\ rr]{ks , : . ' : } | ' . - r ' . ï . " , ' Effect of pressure variation on pelite assemblages Low p ressu Íe metamorp hism In the lower pressure parts of the greenschist and amphibolite facies, pelitic schists and homfelses develop andalusite rather than kyanite, while garnet be- comes rare or absent and cordiedte appears at progressively lower temperatures with a drop in pressure. At the lowest pressures, biotite hornfelses are succeeded by spotted homfelses containing poikiloblastic cordierite (see l) while andalusite appeaN subsequently at higher grade. A number of examples of very high tempeÍature metamorphism of peÌite at near-surface pressures have been de- scribed from the vicinity ofbasaltic complexes. Here, wholesale melting ofpelite, especially in xenoliths, may take pÌace. Andalusite (ch iastolite) hornfels Hornblende hornÍelsÍacies(Additìonal examples: 1,'106i Two porphyroblasts of andalusite (variety chiastolite) are shown in this view and each has a rim of shimmer aggregate (probably muscovite). The andaÌusites âre characterized by a pattern of graphite inclusions which has been compared to a Maltese cÍoss. In some examples we find that although the original crystals of andalusite may have been compÌetely replaced by fine micas, the pattern of inclusions remains. Generally the centres of the crystals are full of inclusionsi ìn som€ cases, how- ever, the centre of the cross may be free of them. Despite the intense hornfelsing, the original slaty fab- ric and grainsize is still apparent in the groundmass, which contains quartz, chlorite, biotite, muscovite and graphite. Localíty: Evans Lake aureole, Okanogon Co, Washing' ton, LJSA. Magnification: x 11, PPL and XPL. Metamorphosed sedimentary rocks Gordierite andalusite hornfels (spotted slate) flornblêndê hoÌnÍels f acies ---. rock displays the çharacte stic appearance of a .:\:::3J slate (although this view has a greater density of .n: --r :han in many other exampÌes). The spots are com- r---i of two minerals riz andalusite and cordierite and : -; PPL view they can be distinguished fairly easily .i-:,.. :he andalusit€ crystals have higher relief than the .:::::nte: here they are also relatively ftee from inclu- -,.. t Three andalusite crystals are in the centre of the :ç : ãhile the other spots are ofcordieÌite. Some of the :::l:.ite crvstals show sector twinning the cÌystal -:,:-. : rhe centre ofthe field shows two sectors which are -: -s: :lack and two are dark grey. --:. !.esence of andalusite and absence of chlorite i:ìr -,r :hat this rock represents a higher metarnorphic ri-ié :han sample 1, from the same aureole, but what -:-::i:s of the muscovite-rich matÌix is noneth-eless still :-. 1e-grained. -,::-::,: Skiddaw aureole, England, Magniïícatíon: I PPL and. XPL. 7',.,1':' ,.1 Ì { :i; r . : , L; ' Metamorphosed sedimentary rocks Andalusite biotite schist Hornblende hoÌníels íacies Large poikiloblasts ofandaÌusite are set in a groundmass composed mainly of greenish-brown biotite, muscovite and quartz in this low pressure regionaÌÌy metamor- phosed rock. The inçlusions within the andaÌusites are distinctly finer-grained than the rock matrix and in some cases (e.g. in the lower rightcorner) define aN-S fabric thatis at a high angle to the dominant E-W schistosity. In detail, it can be seen in the central and upper parts ofthe field of view that the E-W fabric is a crenulation schistosity produced by Ìefolding of the early N S foliation. This process has been accompanied by segregation into phyllosilicate-rich and quartz-rich bands. A finaÌ phase ofdeformation has oroduced kinks in the E-\V schistos- ity near the upper èdge of the field of view. Locality: Black Water River, 1.5 km southwest of Brìdgend, Grampian Region, Scotland. Magnücation: x L PPL and. XPL. Metamorphosed sedimentarj- rocks 30 Andalusite staurolite schist Hornblende hoÌnÍels Íacies This rock consists of large poikilobÌasts of andalusite r\lth staurolite ìn a groundmass of biotite, with finer- rlained muscovite and quartz. There is no clear evi- Jence of any feldspar. The poikiloblasts ol stauroÌite are much smaÌÌer than :hose of andalusite and appear very dark in the PPL rhotograph. Just to the left-of-centre of the field ofview :s a poikiloblast that has been almost compÌetely re- rlaced by pale yellow pinite and may have been of .oÍdierite oÌiginally. AÌthough described as a schist because the general :rass ofthe rock has a schistose structure, the schistosity :r not well defined in thin section. Locality: Whítehílls, near Banff, Scotland. MagnifÌca- ::on. x 8, PPL and XPL. Metamorphosed sedimentary rocks Andalusite cordierite K-feldspar hornfels PvÌoxene horníels Íacies This is a fine-grained rock consisting mainly of cordier- ite, andalusite, alkali feldspar and quartz. Cordierite is very difficult to distinguish, but sometimes, as around the central skeletal andalusite crystal in the high pow- ered view, it shows lamellar twinning. Yellow pleochroic haloes are also seen in some cordierites. The aÌkaÌi feld- spar has a microperthitic texture and this aids in identify- ing it by giving a distinctive pattern of fine parallel lines or simply a patchy appearance in CPL. Biotite and magnetite are present in smaÌl amounts. Minor muscovite is probably of retrograde origin. The association of andalusite and K-feldspar results from breakdown of muscovite with quartz at very low pressures where andalusite, rather than sil l imanite, is stable. Locali4t: aureole of Ben Nevís granite, Scotland. Magní- fìcation: x 26, PPL and XPL; and x 52, XPL. ti ' l i l ' -r ì í i . ' - i . ' j MetamorDhosed sedimentarl rocks Cordierite pla gioclase corundum spinel hornfels Sanidinite Íacies l : : i is a very fine-grained metasediment in which the -:.,::e small high relief grains with first-order yellow a:::tnngence colouÍs are corundum and the crystals i.:.h 3ppear opaque in the low power view are in fact :.:, i sreen spinels. The original Ìayering is visible in the ?L photograph and in the XPL view large poikiloblas- : - a,)Ídierites are seen to occur throughout. Between :-: paÌches of cordierite in the upper left of the Ìow : .rered view there are veins of what is probably aÌkali : l .par. Some plagioclase feldspar is intergrown with . : .LìÍdierite but it is diff icuÌt to estimate the relative a: nrìrt ions of these two minerais. :i i rock comes from an ultrametamorphosed xeno- :- The high content of Al-rich mineraÌs shows that it ,. iormerly of pelit ic composition. but high tempera- -.: i have destroyed all hydrous phases and it has been r::leÌed in sil ica and alkalis by nelting. , tlirt: Invergeldie Burn, Glen Lednock, Comrie, ' . :lLtntl. MagníJícation: x I, PPL and XPL; and x 34, Metamorphosed sedimentary rocks Buchite Sanidinitê Íaciês This name originalÌy denoted a glassy rock formed by fusion of sandstone by an igneous rock but was later extended to include fused aÌuminous clays. This speci- men contains rectangular. newly-formed crystals of cor- dierite and smaller laths of plagioclase. One partially resorbed relic crystal ofquartz remains at the top right of the field oï view. Other minerals present .Ìre oÍthopy- roxene and possibly needÌes of mull i lc. Locality: Cushendall, Co Ant m, Northern lreland. Magnifícation: x 52, PPL and XPL. 34 Buchite Sanidinile facies This is another exampÌe of a glassy rock formed by melting of sediment at a lava contact. The mineraÌs present in this view all crystalÌ ized from the melt and are dominated by plagiocÌase, cordierite, orthopyÍoxene and mullite. Both plagioclase feldspar and cordierite have moderate relief and are crowded with inclusions making them diff icult to distinguish. The high relief crystals are of orthopyroxene and the minute needles in the colourless areas of glass are mullite. The opaque phases are iÌmeniti j and magnetite. Localíty: near Cushendall, Co Antrin, Northern lte' ldnd. Magnìfic.ttion: x 34, PPL. Hi gh p ressure metamoÍph ism The.effects of high pressures on peljte assemblages have been less rvell kno*n untrl recent.yeiìrs. because Ìhe pelites of most high pressure metamorphic belts come ffom less mature sedimentary environments than most Barrovian pelites. Kecent work. notably in the European Alps, has however identif ied a number of otshnct hrgh pressure minerals and mineral assemblages. These include tne occu rrence of carpholite (JsJ and the coexistence of tulc with phensire muscor ire or. at h igher remperarure. kyani te (Jó). FuÍther deÌails of the metamorphic assemblage and reactions of pelit ic schrsts are Biven in Yardley ( lqRq. Chaprer 3). Carpholite chloritoid schist glueschist Íacies --::: is a rather fine-grained metamorphosed argil- .:;.rus rock within a Triassic limestone series. fhe - -.:ral. ir conlains âÍe Mg-rich carpholite (about 70"; : ' :, ' \19 end-member) chlorirojd. phengiric mrca. cal_ :::: and smalÌ amounts of chlofite and quartz. llre distìnctive mineral indicative of unusuallv low -:-:krdrure and high pressure metamorphism is the\1: Fe carpholite. This occurs as bundles of near- --:1ìeì prisms with only moderate reÌief (similar to that: :ìuscovite). Where the prisms are cut paralÌel to their :r=h Ìhev dispìay low first-ordergrey birefringence ::.r:rrs (as at the centre ofthe fieÌd ofview). Obìioue. . - j :a:dl sections tend to be lozenge shaped anrJ show --:- ' f inr-order birefringence colours ianging up ro -:- :'Ìd-order blue. The smaller radiating aggregatés of ::. '::ls of much higher RI than the carpholité are of - - .-::!ìid. The rock matrix is predominantly of phengite ' :a lsser amounts of calcite and quartz. Western 29't 302. Metamorphosed sedimentary rocks Talc kyanite schist(whiteschist) Eclogitefacies This view shows an elongated crystal of kyanite and at the bottom left of the field of view are two other kyanite crystals. The low birefringence mineral surrounding each of the kyanite crystals is cordierite produced by retrograde depressurization. The rnicaceous mineral occupying a large part of the field of view, and showing bright second-order interfer- ence colours, is talc; this cannot easily be distinguished from muscovite in thin section. The rest of the field of view is made up mainly of quartz. The assemblage of talc kyanite is an indicator of high pressure and in the presence ofexcess quartz it reverts to cordierite at high€r temperatures and loweÍ pressure. The name nhllesclÌi.ç/ was adopted by W Schreyer to describe the ïacies of rocks formed under conditions where talc + kyanite are stable. Locality: Sar e Sang, Afghanìstan. Magni[icdtion: x 20, PPL ancl XPL. Reference: Kulke H, Schreyer W 1973 Earth and Planet- ary Science Letters 18].824 8 Metâmorphosed sedimentary rocks Pyrope kyanite talc phengite schist with coesite EclogiteÍacies This is a metasedimentary rock from a high grade eclo- giÌe facies region. It is characterized by having pale sarnets varying in size from 0.2 to 25 cm in diameter. These photogmphs are of one of the garnets surrounded by talc, kyanite, phengite and quartz. Inclusions in the sarnet aÌe mostly kyanite and quartz, but the laÌge low relief quartzinclusions contain high relief remnants of its denser polymorph coesite. The quartz in these inclu- :ions also displays a curious texture that is characteristiç of pseudomorphs after coesite. Radial cracks in the gar- net around these inclusions have been caused by the arge voÌume increase on inversion of the coesite to quartz and this may have happened at relatively low temperature during uplift. Locality: Dora Maira massif, Western AIps. Magnifica- :.an: x 25, PPL and XPL. Re.ference: Chopìn C 1984 Contributions to Mineralogyjtld Petrolosy 86: 107-18 Metamorphosed sedimentary rocks K-feldspar kyanite granulite Granuli teÍacies This rock is a mylonitized and recrystallized granulite. The thin section is slightly rhick so that kyânite cryslals have somewhat high intederence colours. In addition to kyanite, the main minerals present in this rock are gar- net, biotite, perthitic K-feldspar and quartz. To the left- of-centre of the field we can see a K-feldspar porphyro- clast containing exsolved blebs of plagiocÌase in its core while its margins have recrystallized to polygonal, non- perthitic K-feldspar. The quartz is largely fine-grained. The presence of K-feldspar togetherwith kyanite is an indication ofthe breakdown of muscovite with quartz in the kyanite stability field (cf. 20, 3l). The fact that the deformation rounded and corroded kyanite and garnet and caused perthite to break down as it recrystallized demonstrates that the myÌonitization is a later, lower temperature event, after the peak of metamoÍphism Locality: Slishwood, Co Sligo, Ireland. Magnificatìon: x 12. PPL and XPL. Metamorphism of tuffs, greywackes and cherts The lithologies illustrated in this chapter are Ìargely absent from the metamorphic rocks of the Caledonian-Appalachian beÌt in which many early classic studies were carried out, but prove to be valuable metamorphic indicators in very low grade and high pressure metamorphic belts. Indeed the zeolite facies was first erected by D S Coombs (1954) on the basis ofthe assemblages in metagreywackes from New Zealand. Volcanogenic greywackes develop metamorphic assemblages even at very low temperatures because they contain highly reactive fragments ofglass and igneous minerals whilst retaining, at least initially, the porosity of a sandstone. Hence the igneous materials break down very soon after burial to produce low temperature zeolite minerals. At higher temperatures, assemblages are probably very simiÌar to those of other metamorphosed igneous rocks of comparable composition; it is their unique reactivity that makes greywackes valuable low grade indicators. Cherts (,í4 46) and ironstones (47-48) display an even greater diversity of compositio. and assemblages than greywackes. While all cherts are, by defini- tion, rich in silica, some have high levels of Fe (44, 45) while others aÍe Mn-rich (4ó) and develop minerals close to the Mn end-members of Fe Mn solid soÌu- tlons. Laumontite metagreywacke ZeoliteÍaciês This rock has been subjected to rather Ìow grade meta- morphism and most of its characteristic sedimentary Íèatures are still visible. It must have originally consisted of a poorly sorted collection of angular voÌcanogenic fragments of feldspar and quartz together with ferro- magnesian minerals which have since been replaced by secondary material rich in chlorite but stained by ferric iron. These ftagments are embedded in a groundmass too fine-grained for optical determination. In PPL the clearest mineÌal fragments are of quartz, whereas the teìdspar has been partly replaced by laumontite. Ìn the lower left quadrant one nearly rectangular fragment of clouded feldspar is partÌy replaced by clear polycrystal- line laumontite at its lower left corner. Laumontite is distinguished ftom other zeolites by beingbiaxiaÌ negative and having a low optic axial angle. Locality: Jurassic sed.íments, near Ship Cove, Hokonui Hìlls, New Zealand. Magntfícation: x 72, PPL and XPL. Reference: Boles I R, Coombs D S 1975 Geological Society of Ameica BulleÍin 86: 163-73 Metâmorphosed sedimentary rocks Heulandite meta-tuff Zeolite Íacies In addition to crystalline particÌes, this tuff originalÌy contained abundant glass shards. Angular ftagments of both individual feldspar crystals and fine-grained vol- canic rock (seen at the left hand edge) are almost un- altered. However the fine-grained groundmass is partly replaced by green chlorite and individual elongate glassy shards are outlined by rims of chlorite. The interior portions of the shards are replaced by fine-grained aggregates ofthe zeoÌite heulandite. Secondary caÌcite is also present. The absence of deformation is typical of rocks sub- iected to burial metamorphism. Range, South Island, New Zealand. 53, PPL and XPL. Locality: North Magnification: x Metamorphosed sedimentary rocks 11 Jadeite glaucophane metagreywacke Bl u eschist Íacies This rock, originaÌly a greywacke, was metamorphosed to produce the jadeite-glaucophane assemblage and is now weakly foliated. One gÌaucophane crystal, readily 3pparentftom its blue colour, occurs above the centre of the field of view, but most ofthe other high relief mater- ial is jadeite, forming 20-30% of the rock. It occurs l ike Slaucophane as bundles of radiating crystals, and has lorv birefringence. Much ofthe rest ofthe rock is quartz, rncÌuding relic detrital grains, and there is minor phen- glre. Locality: Panoche Pass, Califomía, USA. Magnífica- ríon: x 20, PPL and. XPL. Reference: Emst W G 1965 Geological Society of AmeL íca Bulletin 76: 879 914 Metamorphosed sedimentary rocks Pumpellyite actinolite schist Prehnitg pumpellyite íacies This is an extremely fine-grained rock and mineraÌ iden- tification is not easy. The bulk of the rock is composed of fine-gÌained quaÌtz with chlorite and minor epidote. Sporadic bands parallel to the schistosity
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