Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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UNDAMENTAL

UESTIONS

ONSIDERED IN

HIS

HAPTER

1. What speciﬁc fabrics are found in metamorphic

rocks?

2. How are metamorphic rocks classiﬁed?

3. What are the attributes of the major types of

metamorphic rocks?

4. What graphical devices are available to represent

the stable mineral assemblage of a rock relative to

its bulk chemical composition?

INTRODUCTION

It will come as welcome news that the descriptive petro-

graphic terminology of metamorphic rocks is simpler

than that of magmatic rocks. Fewer basic fabrics are

recognized and named and classiﬁcation is much sim-

pler. Although the chemical compositions of metamor-

phic rocks range over a broad spectrum of magmatic,

sedimentary, and metasomatic protoliths, some chem-

ical groups are lacking or, at least, very rare; these

include sedimentary evaporites, such as gypsum and

halite, and highly alkaline magmatic rocks, such as

kamafugites and kimberlites. The intricate chemical

classiﬁcation of magmatic rocks has no counterpart in

metamorphic classiﬁcations. Thus, widespread tholeiitic

and alkaline basalts as well as less common basanite,

nephelinite, hawaiite, and so on that are narrowly

deﬁned on the basis of silica and alkali contents can all,

upon metamorphism, be composed essentially of the

same simple assemblage of plagioclase  hornblende in

the rock type called amphibolite. The chemical variations

in these several maﬁc protoliths are accommodated and

concealed by the extensive solid solution in the two

constituent minerals. Metamorphic rock classiﬁcation

is both simple and ﬂexible. To a relatively few basic

names can be appended descriptive modiﬁers that fea-

ture particular rock properties or mode of origin.

This chapter is a petrographic catalog. It deals

mostly with the descriptive aspects of metamorphic

rocks based on observations of outcrops, hand sam-

ples, and thin sections viewed under the microscope. It

is designed to be used as an adjunct to study of rocks

in the ﬁeld and laboratory.

Careful observation and documentation of petro-

graphic attributes is always a fundamental step in

petrologic investigations as they provide a lasting foun-

dation for subsequent interpretations, some of which

are presented in following chapters. Interpretations

may be modiﬁed or abandoned as new observations,

paradigms, and methods of study emerge in the course

of time, but accurately documented petrographic attrib-

utes do not change.

15.1 METAMORPHIC FABRICS

Relict fabrics preserved in low-grade rocks that have

experienced limited recrystallization can be described

Petrography of

Metamorphic

Rocks: Fabric,

Composition, and

Classiﬁcation

CHAPTER

in terms of the protolith fabric, such as the relict por-

phyritic texture in a meta-andesite (Figure 14.3).

Although the imposed fabric of metamorphic rocks

created under subsolidus conditions shares some sim-

ilarities with the fabric of magmatic rocks, there are

distinct differences. Metamorphic rocks are aphanitic

and phaneritic like magmatic rocks; they are never

glassy because metamorphic systems, by deﬁnition,

never contain a melt that can quench to glass. Nor do

conﬁned metamorphic rock systems in the deep crust

have a separate expanding volatile ﬂuid phase that is

involved in volcanic systems. Where present, a ﬂuid

is limited to intergranular spaces and local fractures.

There are only a limited number of ways by which

basic grain shapes can be stably arrayed in a recrystal-

lizing solid. Subsolidus equilibration of grain shapes in

metamorphic aggregates tends to converge toward a

few basic patterns and mutual relations, thus yielding

fewer fabrics than are created in crystallizing magmas.

Despite these simplifying factors, whose expression

is obvious in many isotropic fabrics (Section 14.1.2),

complex interactions between solid state recrystalliza-

tion and ductile deformation that results from imposed

nonhydrostatic stresses are widespread in metamor-

phic systems. These interactions are expressed in the

penetrative anisotropic fabrics of tectonites that typify

regional terranes in orogens. Tectonites commonly

evolve through multiple overprints of deformation and

recrystallization during polymetamorphism. Complex

tectonite fabrics are, at once, both beautiful to behold

and challenging to unravel and interpret. This com-

pound attribute has resulted in a number of richly

illustrated compendia with interpretive discussions;

these include Weiss (1972), Borradaile et al. (1982), Fry

(1984), Barker (1990), Yardley et al. (1990), Twiss and

Moores (1992, Chapter 13), Passchier and Trouw

(1996), and Snoke et al. (1998).

15.1.1 Anisotropic Fabrics of Tectonites

The penetrative aspect of tectonite fabrics implies

particular properties are present throughout every

part of the rock body. In the ideal case of a uniform,

or homogeneous, deformation, properties observable

on the microscopic scale integrate and build into the

increasing larger scale attributes possessing similar

form and geometry that can be seen in hand samples,

outcrops, and even entire terranes (Figure 14.14). This

fractal-like character of tectonites means that there

can be little real distinction between the endeavors of

the structural geologist and the metamorphic petrolo-

gist investigating tectonite fabrics; they can, in fact, be

quite complementary.

It must be pointed out, however, that many meta-

morphic bodies have been inhomogeneously deformed

because of heterogeneities in rheologic response in

different parts (Figure 14.38). In addition, overprinted

processes, such as localized high-strain-rate shear in

mylonite zones and partial melting, can also create

heterogeneous fabric on the outcrop scale (see Figure

15.5).

Imposed penetrative tectonite fabrics can be ex-

pressed in many different ways. Some depend on di-

mensional aspects of individual mineral grains in the

rock aggregate. Thus, platy phyllosilicate and graphite

grains may have a preferred orientation so that their

longest dimensions are more or less parallel in a rock;

the fabric is a planar foliation. In other rocks, columnar

or needle-like amphibole grains may have a preferred

orientation so that their one longest dimension is more

or less parallel in a rock; the fabric is a lineation. Felsic

and carbonate mineral grains that are ordinarily equant

may be ﬂattened or elongate in some rocks so as to ex-

press a foliation or lineation or both. Other anisotropic

fabrics depend on compositional layering and inequant

aggregates of minerals; still others on structures, such

as folds. In some tectonites, the anisotropy is a cryptic

preferred orientation of crystal lattices in the mineral

grains that is not visible to the unaided eye. For ex-

ample, the c-axes of quartz grains in a mylonite can be

uniformly oriented at a large angle to the foliation.

The most common anisotropic fabric is foliation,

sometimes designated as an S-surface from the German

word Schiefer, meaning schist. Multiple foliations in a

rock may be desigated, from oldest to youngest, as S

, S

, and so on. A tectonite that possesses one or more

foliations is said to be an S-tectonite. Rarely are tec-

tonites only lineated; one such L-tectonite would be a

nonfoliated rock that contains subparallel columnar

amphiboles. Many S-tectonites also have a penetrative

lineation that is visible on or within foliation surfaces;

such rocks are L–S-tectonites. It should be remem-

bered that line traces of a foliation exposed on random

oblique surfaces do not constitute a true lineation

(Figure 14.12).

In tectonites, there is never a perfect preferred

orientation of inequant grains, aggregates of grains, or

lattices of grains that deﬁne the anisotropic fabric.

Not every one of the columnar amphiboles in the

L-tectonite just cited is exactly parallel. The degree

to which the preferred orientation is developed varies

between the ideal end-member extremes of perfectly

oriented (never attained in nature) and random

(isotropic).

Under the same metamorphic conditions (non-

hydrostatic state of stress, P, and T), the character

of anisotropy in a rock depends on its mineralogical

and modal composition. Understandably, abundant

oriented platy micas and chlorites in low-grade pelites

create a strong foliation so that the rock readily splits

into parallel plates or slabs when struck by a hammer.

This physically or mechanically signiﬁcant foliation in

aphanitic rocks has been utilized for centuries in the

448 Igneous and Metamorphic Petrology

production of rooﬁng slates. A high-grade rock of

similar pelitic composition made of typically equant

grains of quartz, feldspar, garnet, and pyroxene will

likely not be capable of splitting into slabs; any folia-

tion (perhaps a segregation layering) tends to be mech-

anically passive and insigniﬁcant. Interlayers of thick

quartzite and mica schist metamorphosed under the

same conditions can display conspicuously different

development of foliation. In contrast to the strongly

foliated schist, the quartzite may be only weakly foli-

ated, if at all, because of discoidal quartz grains and,

when struck by a hammer, will likely break across any

weak foliation; it is mechanically insigniﬁcant. These

contrasting expressions of foliation are one basis for

classiﬁcation of metamorphic rocks considered below.

Fabric attributes depend, of course, on the scale of

observation. For example, the interlayered quartzite

and mica schist in outcrop would be obviously foliated,

whereas a hand sample of the quartzite might reveal no

penetrative S-surface.

It may be noted in passing that the mineralogical

composition of a rock can also control its rheologic

behavior. In granitic protoliths, for example, because

of the greater ductile strength of plagioclase than

quartz (Figure 8.8), grains of the former are less de-

formed porphyroclasts surrounded by more strongly

deformed, ﬁner quartz (Figure 14.16c). Rocks that

contain more quartz might experience more ductile

deformation than neighboring ones that contain more

feldspar. In such inhomogeneous rocks, deformation

can be inhomogeneous (Figure 14.15).

Some low-grade metasedimentary rocks have a

preserved relict bedding consisting of compositionally

and/or texturally contrasting layers. Where a planar

fabric inherited from the protolith is the only anisotropic

attribute, as in some hornfelses in contact aureoles, it

does not qualify the rock as a tectonite. Nonetheless,

other low-grade metasedimentary rocks are tectonites,

such as the slates in Figures 14.18 and 14.25b, because

of their imposed metamorphic foliations.

Foliations. The two most common expressions of foli-

ation are compositional layering and a preferred orien-

tation of inequant mineral grains (Figures 15.1 and

7.36). Aligned platy grains, such as micas and chlorite

in low-grade slates, phyllites (deﬁned below), and

schists, deﬁne a lepidoblastic texture. Layering is more

common in higher grade rocks and is typical of many

gneisses. Where both types of foliation occur in the

same rock, they are usually parallel. But this parallelism

is not universal (Figure 15.2, lower left).

In aphanitic slates and also phyllites, a lepidoblastic

array, which may locally include ﬂakes of graphite,

deﬁnes a slaty cleavage. Careful examination of the

slaty cleavage in Figure 14.25b reveals very thin, darker

laminae made of opaque material that might be an

insoluble residue of Fe-oxides remaining after more

soluble quartz has been dissolved and carried away

in solution. In other aphanitic rocks, there are more

conspicuous alternating laminae or lenses, of contrast-

ing attributes that can be compositional, textural, or

both. These contrasting planar domains are called

microlithons and the rocks in which they occur are

said to have a spaced cleavage. (In older geologic

literature, some spaced cleavage was called fracture

cleavage, but this appelation is no longer recommended

because the implied origin by fracturing generally does

not apply.) There are two categories of spaced cleavage

depending on the presence or absence of a pre-existing

S-surface in the rock (see also Twiss and Moores, 1992,

Chapter 13):

1. Crenulation cleavage cuts across pre-existing S-

surfaces in the rock preserved in microlithons

Petrography of Metamorphic Rocks: Fabric, Composition, and Classiﬁcation

449

Planar preferred orientation

of inequant grains

Compositional

layering

Discoidal

segregations

Planar fabric

domains

Combined compositional

layering and preferred

grain orientation

15.1 Expressions of foliation in metamorphic rocks (S-tectonites).

Schistosity of schists and slaty cleavage of slates are chieﬂy

represented by the upper left-hand block. Middle blocks typ-

ify some granoblastic gneisses. Lower left block is a stack of

microlithons containing a crenulated older cleavage (compare

Figure 14.18). Lower right-hand block is common expression

of gneissic layering in gneisses.

wherein the old foliation is regularly crenulated

or wrinkled; crenulations can be asymmetric or

fairly symmetric. In zonal crenulation cleavage,

the pre-existing platy grains in the microlithon are

warped at a small angle, forming a continuously

varying orientation relative to the new foliation

(Figure 14.18b). In discrete crenulation cleavage,

platy minerals are aligned in the new cleavage dir-

ection and are sharply discordant to the old folia-

tion in the microlithons (Figure 15.3).

2. Disjunctive cleavage occurs in rocks lacking pre-

existing foliation. The new cleavage is marked by

seams of concentrated Fe–Ti oxides, accessory min-

erals, or micas, possibly residues from differential

solution, that may be aligned parallel to the seam

(Figure 15.4).

Where microlithons are subtle or lacking, the cleavage

is said to be continuous, as in Figure 14.25b.

In phaneritic schists, generally but not always of

higher grade than slates and phyllites, lepidoblastic

arrays deﬁne a schistosity. In some schists, this planar

fabric is augmented by parallel compositional layers or

lenses (Figure 14.17c, d).

In generally still coarser grained gneisses, again

commonly but not necessarily of still higher grade,

compositionally expressed gneissic layering is prom-

inent (see Figure 19.24a). It is locally folded (Figure

15.5). Alternating or segregated felsic and maﬁc layers

may owe their origin to metamorphic differentiation or

to local partial melting. Foliation can have a compound

expression where lepidoblastic biotites and/or aligned

prismatic amphiboles tend to be more modally concen-

trated in maﬁc layers that alternate with felsic layers;

layering and preferred mineral orientations are parallel

(Figure 15.1, lower right). But many gneisses have little

or no mica and amphibole and their foliation depends

solely on compositional layering and/or on lenticular

mineral grains or aggregates of grains (Figures 15.1,

center right, and 15.6, and 15.13b). Lenticular, eye-shaped

megacrysts, commonly of feldspar (Figure 15.7), are

called augen (German for eyes) and the rock contain-

ing them is an augen gneiss.

Passchier et al. (1990) provide a well illustrated

discussion of gneiss fabric.

Lineations. Linear fabric is expressed in different ways

(Figure 15.2):

1. Mineral lineation, or nematoblastic fabric (Figure

15.8). This can consist of aligned acicular, colum-

nar, and prismatic grains of amphibole, sillimanite,

and kyanite.

450 Igneous and Metamorphic Petrology

Preferred orientation

of bladelike grains

Folded foliation;

hinge lines deﬁne

a lineation

Oblique foliations with a lineation

at their intersection

15.2 Combined expressions of foliation (S ) and lineation (L) in

metamorphic rocks (L–S tectonites). Preferred orientation of

bladelike grains is geometrically similar to the aligned blade-

like deformed cobbles in the metaconglomerate shown in

Figure 14.13.

P domain Q domain

0 0.5mm

15.3 Crenulation foliation in a quartz-mica schist, Seve complex,

Marsfjällen, Sweden. Q-domains are mostly quartz but sparse

micas deﬁne a relict early S

that is now crenulated. P-

domains are mostly subparallel micas deﬁning S

. Photomi-

crograph courtesy of Paul F. Williams.

2. Stretching lineation. In many metamorphic rocks,

especially schists, the foliation surface has a streaked

appearance expressed by elongate aggregates of

contrasting minerals (Figure 15.9b). These thin,

blade-like clusters commonly stand out in relief on

exposed foliation surfaces. Aggregates can be of mica

in a quartz-rich schist but many other contrasting

minerals occur. The mineral streaks superﬁcially

resemble slickenlines produced by granulation and

slip focused onto slickenside surfaces of localized

faults. However, unlike brittle fault-surface features,

stretching lineations lie on foliation surfaces that

are pervasive throughout the metamorphic body

and result from stretching during penetrative duc-

tile ﬂow that affects the whole rock mass.

3. Elongate deformed quartz cobbles in metacon-

glomerate (Figure 14.13) and deformed oolites in

limestone. In L–S-tectonites, the deformed feature

can be described as a triaxial ellipsoid (Figure

14.12), whereas rare biaxial (dirigible-like) features

occur in L-tectonites.

4. Boudins. These are segments of a once intact

layer of relatively brittle rock that have been

separated and pulled apart during extensional ﬂow

of the more ductile surroundings (Figures 14.38c

and 15.10). In cross-section, the boudins resemble

a string of linked sausages. (Boudin means sausage

in French.) Boudinage is the process of their

formation. Because boudins are generally less

deformed than their enclosing surroundings they

can provide important insight into the pattern of

strain.

5. Two oblique foliations. Their intersection is a

lineation (Figures 14.18a, top, and 15.2, bottom).

Where both foliations are well developed in aphanitic

tectonites, the rock breaks into “pencils” upon

weathering. Pervasive crenulations and hinges of

small-scale folds constitute a lineation (Figure 15.2,

center).

Petrography of Metamorphic Rocks: Fabric, Composition, and Classiﬁcation

451

36% volume loss after

pressure solution

Flattened

Undeformed

(a)

(b)

(c)

15.4 Fabric resulting from localized pressure solution. (a) Initial undeformed block that contains oblique marker veins. (b) Ductile ﬂattening

of the block and folding of the veins. Volume of rock to be subsequently removed by differential pressure solution (see Section 17.2.1) lies

between thin dashed lines. (c) Localized dissolution and removal of rock along discrete surfaces produce spaced cleavages on which less

soluble residues (e.g. Fe–Ti oxides, phyllosilicates) are concentrated. These cleavages have the appearance of shear planes because the

marker veins are offset across them, but the sense of offset is inconsistent and, hence, the cleavages cannot have resulted from shear.

15.5 Deformed quartzo-feldspathic (felsic) gneiss. Felsic layers that

are possibly of migmatitic origin have been folded and trans-

posed during ductile ﬂow. Proterozoic Farmington Canyon

complex, Weber Canyon, Utah.

15.1.2 Summary List of Metamorphic Textures

To facilitate the identiﬁcation and description of meta-

morphic rock textures in hand sample, the following

comprehensive list is presented. Some have already

been discussed. If a grain-scale texture observed in a

rock does not ﬁt any of the following, it may simply be

described.

Augen (singular, auge). Refers to ovoidal or eye-shaped

crystals, typically of feldspar, in a ﬁner, foliated matrix

(Figure 15.7) and to the texture that contains them,

regardless of origin. Compare with the genetic terms

ﬂaser and porphyroclastic described later.

Blasto-. A preﬁx sometimes appended before a pro-

tolith fabric, such as blastoporphyritic. But this creates

confusion with fabrics resulting wholly from solid state

recrystallization, such as porphyroblastic, where the

root term blastos is appended as a sufﬁx. Hence, use of

the preﬁx is not recommended and, for example,

“relict porphyritic” is preferred (Figure 14.3).

Cataclastic. A structureless (isotropic) mass of angular

rock and mineral grain fragments with through-going

cracks that is formed by brittle mechanical breaking,

crushing, and pulverization in high-strain-rate, shallow

crustal faults (Figure 8.7).

Corona. A mantle of mineral grains surrounding

an anhedral core of another partially consumed min-

eral. The rim is the product of an incomplete reaction

between the core and neighboring phases (see Fig-

ures 16.6 and 16.8).

Decussate. An aggregate of interpenetrating platy

or columnar grains; may be isotropic or weakly

anisotropic (Figure 15.1, top right).

Epitaxial. Refers to an oriented overgrowth of a min-

eral onto another substrate mineral that has similar

atomic structure (Figure 14.7).

452 Igneous and Metamorphic Petrology

02cm

15.7 Augen gneiss. Ovoidal or eye-shaped megacrystals of alkali

feldspar in biotite-rich lepidoblastic matrix. Rock breaks

across foliation.

(a) (b)

02cm

15.6 Lineated and weakly foliated feldspar–quartz–biotite gneiss. Photos are parallel (a) and perpendicular (b) to lineation deﬁned by blade-

like aggregates of biotite. Note that the foliation and lineation are mechanically insigniﬁcant so that the rock breaks across them.

Petrography of Metamorphic Rocks: Fabric, Composition, and Classiﬁcation

453

Plagioclase

(c)

Hornblende

Epidote

02mm

02cm

Quartz vein

(a) (b)

15.8 Lineated (nematoblastic) and weakly foliated hornblende–plagioclase–epidote schist. Photographs are perpendicular (a) and parallel (b)

to lineation. Microscopic scale drawings (c) are similarly oriented. Note vein of quartz in (a).

(a) (b)

15.9 Outcrop of quartz–feldspar–mica schist. (a) View almost parallel to strong foliation showing slabby weathering typical of bodies of schist.

(b) View of the foliation surface on which is a well developed stretching lineation (parallel to hammer handle) expressed by streaks of

alternating darker biotitic and lighter felsic material. Note three orientations of joints subperpendicular to foliation, two of which are at

about 90° to each other with the lineation bisecting that angle and the poorly deﬁned third joint set roughly perpendicular to the

lineation. Such jointing is common in foliated metamorphic rocks. Precambrian terrane in the Colorado Front Range.

Flaser. Texture of mylonites where large ovoidal crys-

tals called porphyroclasts that have survived ductile

deformation lie in a ﬁner foliated matrix (Figure

14.16c). Synonymous with porphyroclastic.

Granoblastic. A mosaic of more or less uniform sized,

equidimensional, anhedral grains that are essentially

free of strain as revealed by uniform extinction under

cross-polarized light in thin section (Figures 6.24,

14.6c, d, 14.25c, d). Grain boundaries are straight or

only slightly curved. If present, inequant grains are

randomly oriented. Typical of hornfelses formed in

contact aureoles and of many high-grade rocks in

regional terranes.

Lepidoblastic. Abundant platy mineral grains—micas,

talc, chlorite, graphite—with strong planar preferred

orientation that impart a schistosity to schists (Fig-

ure 14.17c, d) and cleavage to slates (Figure 14.25b).

Megacrystic. Texture in which large crystals of any

origin, called megacrysts, lie in a ﬁner matrix. They

can originate during solid-state recrystallization as

porphyroblasts (Figures 14.8–14.10, 14.17d), during

high-strain-rate ductile deformation as porphyroclasts

(Figure 14.16c), or as relict phenocrysts (Figure 14.3).

Metacrystic. Contains metacrysts. Synonymous with

more widely used terms porphyroblastic and porphy-

roblasts.

Nematoblastic. Abundant aligned acicular or columnar

grains that impart a lineation to the rock (Figure 15.8).

Poikiloblastic. A texture in which porphyroblasts con-

tain numerous small, randomly oriented inclusions of

other minerals (Figures 14.10, 14.25c). The inclusion-

ﬁlled porphyroblasts are called poikiloblasts. In thin

sections, a high-relief poikiloblast of garnet and stauro-

lite that contains abundant inclusions of low-relief

quartz resembles a sponge or a sieve, hence the alter-

nate designation sieve texture (see Figure 18.14d).

Porphyroblastic. Large subhedral to euhedral grains,

called porphyroblasts, surrounded by a ﬁner grained

matrix of other phases, all of which grew during solid-

state recrystallization (Figures 14.8, 14.9, 14.17d).

Porphyroclastic. See ﬂaser.

Strain shadow. Roughly cone-shaped domains adja-

cent to and on opposite sides of a relatively rigid

object, such as a boudin (Figure 15.10b) or megacryst,

that are ﬁlled with a mineral aggregate differing from

the object and the surrounding matrix. Called a strain

fringe if the aggregate is of ﬁbrous minerals. Aggregates

lie in the direction of the matrix foliation and may ex-

press a lineation in three dimensions. See also Passchier

and Trouw (1996, Chapter 6).

Symplectite. Intimate, vermicular (wormy) intergrowth

of two minerals that nucleated and grew together by

454 Igneous and Metamorphic Petrology

(a) (b)

Boudin

Neck

15.10 Boudins. (a) Schematic diagram of brittle layer segmented into linear boudins due to lateral extensional ﬂow (arrows) of more ductile

surrounding rock. Boudins show a variety of shapes in cross-section; some equant ones can be separated and rotated by shear stress

(inset lower right). S, foliation; L, lineation. (b) Strain, or pressure, shadow in neck space between angular, brittly fractured boudins of

hornblende–plagioclase schist that has been ﬁlled with coarse-grained white quartz and an infold of relatively more ductile mica schist

adjacent to the boudin. Pencil for scale. Damara belt, Namibia. Photograph courtesy of Cees Passchier. Reproduced with permission

1990 by Springer-Verlag.

breakdown of an earlier unstable phase (Figure 15.11)

or by reaction between two adjacent unstable phases

(see Figures 16.6 and 16.8). Commonly is a corona-like

shell or rim around an anhedral core; if the rimming

material is very ﬁne micro- to cryptocrystalline it is

called a kelyphytic rim. Compare myrmekite, which is

a sodic plagioclase-quartz intergrowth of controversial

origin in high-grade quartzo-feldspathic rocks and

granitoids (Section 7.4 and Figure 5.18b).

15.2 CLASSIFICATION AND

DESCRIPTION OF METAMORPHIC

ROCKS

Compared to the intricate, generally quantitative classi-

ﬁcation of magmatic rocks that is encumbered by many

hundreds of names, metamorphic rocks have a simple

ﬂexible classiﬁcation. Basic names are relatively gener-

alized. There are several bases of classiﬁcation that can

be used for different purposes and in different contexts

but metamorphic facies is not a basis for classiﬁcation

of an individual rock. The most important feature

of classiﬁcation of metamorphic rocks to keep in mind

is ﬂexibility. To a basic name may be appended de-

scriptive or genetic modiﬁers that are deemed import-

ant and suit the purposes of the petrologist. For

example, to emphasize the contrasting composition of

two schists, one might be designated as a maﬁc schist

and the other as a pelitic schist. Or, if fabric is the con-

text of discussion, lineated schists can be distinguished

from porphyroblastic schists.

The several bases of classiﬁcation are as follows:

1. Fabric. This is the principal basis for metamorphic

rock classiﬁcation as detailed below.

2. Protolith, where recognizable. Metamorphic min-

eral and modal composition, which together mani-

fest the bulk chemical composition, and, especially,

surviving relict fabric can justify appending the

preﬁx meta- to the name of the original rock.

Examples include the broad metasedimentary and

metavolcanic rock categories and more speciﬁc

metatuff (Figure 14.2), meta-andesite (Figure 14.3b),

metadiabase (Figure 14.6a and possibly 14.6b),

foliated and lineated metaconglomerate (Figure

14.13), and foliated metagraywacke (Figure 14.17b).

Note how the appended fabric preﬁxes on the last

two rock names convey important information. The

preﬁxes para- and ortho- are used speciﬁcally to

denote sedimentary and igneous protoliths, respect-

ively, where these can be ascertained. Examples in-

clude para-amphibolite (metamorphosed dolomitic

shale) and orthogneiss (metamorphosed granitic

rock). Distinguishing between para- and ortho-

amphibolite is a common dilemma but Leake

(1964) has shown that chemical analyses of a suite

of samples from an amphibolite body can be help-

ful in choosing between sedimentary and magmatic

protoliths.

3. Special mineralogical names. This category in-

cludes marble (a granoblastic rock made of calcite),

dolomarble (dolomite), quartzite (quartz), and am-

phibolite (plagioclase  hornblende). Serpentinite

is made mostly of serpentine minerals, regardless

of fabric.

4. Geologic setting or nature of metamorphism. For

example, skarn is a metasomatic calc-silicate rock,

such as at Crestmore, California (Section 14.2.5)

and ultrahigh-P rocks in continent collision zones.

5. Grade. Examples include a low-grade rock and a

partially retrograded eclogite (Figure 14.27).

6. Chemical composition, such as pelitic and maﬁc

rocks.

15.2.1 Metamorphic Rock Names Based on Fabric

The classiﬁcation most commonly used for hand sam-

ples and outcrops is based on how well a foliation is

developed; for example, strongly foliated schist versus

a nonfoliated marble (Table 15.1). Fabric reﬂects the

state of stress during metamorphism but mineralogical

and modal composition and scale of observation are

also factors, as discussed above. Because the mineral

Petrography of Metamorphic Rocks: Fabric, Composition, and Classiﬁcation

455

100 m

15.11 Symplectite of plagioclase and diopside partially replacing

omphacite in eclogite from North-East Greenland (Gilotti,

1993). The back-scatter electron image, produced in an elec-

tron microprobe analyzer, shows the “wormy” textured sym-

plectites developed along former omphacite grain boundaries

and partially replacing them. The symplectite consists of black

plagioclase and light gray diopside. The omphacite is slightly

darker gray. Image courtesy of Jane A. Gilotti.

assemblage in a rock is dictated by intensive para-

meters prevailing during equilibrating recrystallization,

as well as its bulk chemical composition, citing mineral

constituents as preﬁxing qualiﬁers on the base fabric

name makes a metamorphic label more meaningful be-

cause it connotes grade as well as chemical category

(Table 14.2 and Section 14.2.1). Hence, for example,

two contrasting schists might be named quartz–biotite–

garnet–sillimanite schist and chlorite–actinolite–epidote–

albite schist. The ﬁrst is an intermediate- to high-grade

rock derived from a pelitic protolith whereas the latter

is a low-grade metabasite. There is no standard way of

listing mineral constituents in terms of their modal

abundance, whether most abundant ﬁrst or last.

The three fabric subdivisions used in classiﬁcation

of metamorphic rocks are as follows:

1. Strongly foliated rocks that break readily along the

foliation into slabs or plates, usually but not neces-

sarily because of abundant oriented mica or other

phyllosilicate ﬂakes (lepidoblastic fabric). Foliation

is mechanically signiﬁcant.

2. Weakly foliated rocks, in which the foliation,

though perceptible, is mechanically rather passive

or insigniﬁcant; the rock tends to break across the

foliation rather than along it.

3. Nonfoliated rocks have essentially an isotropic fabric.

Obviously, these fabric categories grade into one an-

other without natural perceptible breaks or disconti-

nuities. Hence, it is unavoidable that one geologist’s

schist (fabric subdivision 1) will be another’s gneiss (2).

The following list of hypothetical names illustrates

how compositional and other modiﬁers can be at-

tached to basic fabric names to make them more com-

plete and informative.

Quartz–biotite–garnet schist. A strongly foliated

phaneritic rock whose major constituents are quartz,

biotite, and garnet.

Porphyroblastic garnet schist. Conspicuous porphyro-

blasts of garnet in a strongly foliated phaneritic rock.

Lineated hornblende–plagioclase schist. Strongly aligned

acicular hornblendes in an L–S-tectonite made of horn-

blende and plagioclase.

Crenulated phyllite. An aphanitic, strongly foliated

rock that has an older foliation intricately and perva-

sively folded on a small scale, creating a superposed

spaced cleavage.

Quartzitic or chloritic schist. The most conspicuous

mineral is quartz or chlorite in a schist.

Lineated low-grade pelitic schists. Name for a group of

L–S-tectonites derived from shale protoliths and whose

mineral assemblages are of low grade.

Blue schist or green schist. Strongly foliated rock that is

blue by virtue of the abundance of blue glaucophane,

or of green chlorite, epidote, and actinolite, respect-

ively. Not recommended because of possible confu-

sion with the metamorphic facies of the same names.

Calc–silicate gneiss. Compositionally layered, weakly

foliated phaneritic hand sample made of calc–silicate

minerals.

Schistose amphibolite. An amphibolite whose foliation

is too weak for it to be a schist; alternatively could be

called a weakly foliated amphibolite.

Semihornfels. A rock with a nearly isotropic fabric cre-

ated by thermal metamorphism in a contact aureole.

In the following descriptions of metamorphic rock

types listed in Table 15.1, migmatites, eclogites, and

serpentinites are considered in more detail because of

their controversial origin.

15.2.2 Strongly Foliated Rocks

Rocks in this category are the classic tectonites of re-

gional metamorphic terranes. They readily break along

a foliation that is usually expressed by lepidoblastic

phyllosilicate mineral grains, less commonly by ﬂat-

tened grains of felsic and carbonate minerals. Thin

layers of contrasting mineralogical composition in

schists may locally augment the foliation. In addition

to the strong foliation, many phyllites and schists are

lineated. The increase in grain size from slate through

phyllite to schist usually but not invariably reﬂects an

increasing T of higher grade metamorphism.

Slate is aphanitic and has a dull luster on well

developed pervasive slaty cleavages. Most, but by no

means all, is derived from shale, from which it is

distinguished by a “tougher” or “harder” character.

Rarely, in metasedimentary rocks, thin relict silty or

sandy beds may lie parallel or oblique to the cleavage

and these can show relict grading and other primary

sedimentary features (Figure 14.18). Even though clay

minerals in shale protoliths have recrystallized to chlo-

rites and micas, the angular outlines of the original

456 Igneous and Metamorphic Petrology

Table 15.1. Classiﬁcation of Metamorphic Rocks

According to Expression of Foliation

TRONGLY

EAKLY

ONFOLIATED

FOLIATED FOLIATED

Slate Gneiss 1. Greenstone

Phyllite Migmatite 2. Amphibolite

Schist Mylonite 3. Eclogite

4. Granofels

5. Charnockite

6. Quartzite

7. Marble

8. Hornfels

9. Serpentinite

*Listed in order of their description in Sections 15.2.4 through

15.2.6.