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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