Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Metamorphic Rocks and Metamorphism: An Overview

417

(a) (b)

14.13 Metaconglomerate produced by recrystallization and ductile ﬂow under nonhydrostatic stress. Compare with Figure 14.12. Outcrop

in regionally metamorphosed late Proterozoic–early Paleozoic rocks, Raft River Mountains metamorphic core complex, northwestern

Utah (see Figure 18.11). (a) Hammer rests on a nearly horizontal relict bed of conglomerate that lies between beds of metamorphosed

quartzo-feldspathic sandstone, now schist. Entire cliff exposure is a strongly foliated and lineated tectonite. (b) View looking down

onto foliation deﬁned by similar orientation of ﬂattened relict cobbles; their common direction of elongation deﬁnes a lineation. Note

pocket knife for scale.

Mountain range

Outcrop

Thin section

Hand sample

14.14 Many terranes of regionally metamorphosed rock have been homogeneously deformed on all scales of observation—from mountain

range down to thin section. This penetrative anisotropic tectonite fabric is, therefore, scale invariant and fractal-like. In this hypothetical

diagram, looking down the axis of folds, their orientation and style are the same regardless of the size of the area observed.

418 Igneous and Metamorphic Petrology

14.16 Progressive mylonitization and development of tectonite fabric in granitic rock, southeastern San Gabriel Mountains, California.

Hand-sample photographs on left are about natural size. Photomicrographs on right are enlarged about 10; thin sections are cut

perpendicular to foliation where present. (a) Original granitic rock. Photomicrograph under cross-polarized light. (b) Hand sample of

slightly deformed rock has a subtle streaked appearance on a faint foliation. Photomicrograph under cross-polarized light discloses many

ﬁner grains. (c) More advanced deformation has produced a conspicuously foliated and ﬁne-grained mylonite with numerous larger relict

feldspars. These feldspars, called porphyroclasts, have greater ductile strength and are less susceptible to deformation than quartz

(Figure 8.8), which has been preferentially deformed to make the ﬁne-grained matrix. Well deﬁned lineation is obvious on foliation in

hand sample. (d) Uniformly very ﬁne-grained, strongly foliated mylonite that also qualiﬁes, in a descriptive nongenetic sense, as slate. No

lineation is visible on foliation in hand sample. Samples collected by Raymond M. Alf. From Shelton JS. Geology Illustrated. New York,

W. H. Freeman, 1966. Reproduced with permission of John S. Shelton, who holds the copyright.

deformation process. Mylonites typically develop in

ductile shear zones (Figure 14.15) where high rates of

strain are focused into relatively thin layers; such zones

are the deep crustal counterparts of more shallow

brittle faults where cataclasis breaks rocks into pieces

(Figure 8.9). Shear zones can evolve over long periods

of time and there is ample opportunity for complex

overprinting of brittle and ductile fabrics. Interpreta-

tions must, therefore, be made cautiously.

Figure 14.16 shows the effects of progressive

mylonitization of a phaneritic granitoid protolith. The

ﬁnal end-product is an exceedingly ﬁne grained

felsic mylonite is ﬂint-like. Although early investigators

believed mylonites originated by brittle pulverization

(mylonite is from the Greek mylon, to mill), it is now

generally agreed that they are a result of ductile defor-

mation. The typifying grain-size reduction in mylonites

originates from a dynamic interplay between destruc-

tive grain deformation under nonhydrostatic stress and

constructive dynamic recrystallization. This is a ductile

process where higher energy strained grains, whose

stored work of distorting their atomic structures makes

them unstable, are continuously replaced by lower

energy strain-free grains of smaller size during the

Strain rate

Shear zone

14.15 Schematic proﬁle of a ductile shear zone (shaded) in a protolith that has isotropic fabric outside zone. Reference strain ellipses (black)

show partitioning (focusing) of strain into a narrow zone where the strain rate is exceptionally high.

(a)

14.16 (Continued).

(b)

(c)

(d)

Metamorphic Rocks and Metamorphism: An Overview

419

420 Igneous and Metamorphic Petrology

(a)

Lithic

fragment

Feldspar

(b)

Slip surfaces

Quartz

0.5 mm 0.5 mm

(c)

Muscovite

Quartz 

albite

(d)

Muscovite

Quartz

Chlorite

Epidote

0.5 mm 0.5 mm

Oligoclase

Garnet

Biotite

14.17 Progressive metamorphism and development of tectonite fabric in lithic wacke (graywacke), South Island, New Zealand. Within the

thick, areally extensive sequence of low-grade rocks, Turner (1981) distinguished four textural zones that can be distinguished in these

thin section views. (a) Original rock in textural zone 1 is composed of clasts of quartz, feldspar, ferromagnesian minerals, and volcanic

rock (lithic fragment) in a clay matrix. Fabric is isotropic. Plagioclase and alkali feldspar are extensively altered to secondary Ca–Al

silicates and sericite, respectively. (b) Foliated metagraywacke or phyllite of textural zone 2. Relict clasts, especially of quartz, are still evi-

dent even though the rock has been pervasively deformed, reducing the size of coarser grains and promoting development of through-

going cleavage surfaces, commonly marked by ﬁlms of iron oxide that might be a residue after pressure solution (see Section 17.2.1).

Recrystallization of some of the original clasts and clay matrix has produced new but very small grains of quartz  epidote  albite 

chlorite  white mica, the latter two forming platy grains, which, because of a strong preferred orientation, enhance the foliation. (c)

Aphanitic phyllite of textural zone 3. Continued growth of new mineral grains under nonhydrostatic stress has produced a well foliated

rock in which all vestiges of the original sandstone fabric and mineral composition have been erased. Note subtle segregation of miner-

als into alternating layers rich in quartz  albite and in chlorite  epidote. (d) Fine-grained schist of textural zone 4. Continued crystal

growth under nonhydrostatic stress has produced still coarser grains, platy ones possessing a strong preferred orientation which makes

a foliated fabric. This schistosity is enhanced by segregation layers of quartz  oligoclase and of muscovite  biotite. Oligoclase grains

are distinguished from quartz by cleavage and small mineral inclusions. Assemblage in (c) of chlorite  epidote  albite has reacted in

prograde manner to an assemblage in this rock of biotite  garnet  oligoclase.

(aphanitic) and foliated mylonite. In a metamorphic

rock classiﬁcation based solely on fabric, this rock

qualiﬁes as a slate, despite its granitoid protolith! The

name “slate” carries no connotation as to origin or

protolith, although to most readers slates are derived

from shales.

Transformation of Graywacke into Schist. A thick se-

quence of rocks in the South Island of New Zealand

(see Figure 18.30) shows progressive metamorphism

and development of tectonite fabric from an initially

isotropic protolith of feldspathic and lithic sandstones,

sometimes called graywackes (Figure 14.17). The

changes in these rocks are a result not simply of deep

burial at increasing P and T but also of increasing

degree of recrystallization under nonhydrostatic stress.

Widespread folds attest to the deformation in these

tectonites. Although the feldspar and quartz grains in

the ﬁnal end result—a schist—have 120° triple junc-

tions resembling granoblastic texture, the aligned platy

minerals and slightly ﬂattened felsic minerals deﬁne an

anisotropic tectonite fabric. Signiﬁcantly, in the more

recrystallized, coarser rocks, the foliation expressed

by the aligned platy minerals is augmented by parallel

layers of contrasting composition that are another ex-

pression of metamorphic differentiation. Like growth

of porphyroblasts, mineral components moved dif-

ferentially during recrystallization but rather than to

isolated crystal nuclei they somehow segregated into

planar layers. This ordering seems to defy the law of

spontaneously increasing entropy. Whatever its origin,

metamorphic differentiation can create compositional

layers where none previously existed. Hence, com-

positional layers in a metasedimentary rock do not

necessarily represent relict bedding. Careful attention

must be paid to the degree of recrystallization, grain

size, and possible preservation of a relict fabric, as well

as ﬁeld relations, to accurately interpret the origin of

compositional layering.

Slaty and Crenulation Cleavage. Slates possess a slaty

cleavage deﬁned by aligned aphanitic platy phyllosilic-

ates (micas and chlorite) and commonly minute ﬂakes

of graphite. Thin, quartz-rich layers and less continu-

ous lenses may also occur and these can be locally, but

not necessarily, subparallel to the slaty cleavage. Subtle

gradational changes in grain size within the quartz-rich

layers, together with the very ﬁne overall grain size of

the rock, suggest little recrystallization has occurred.

Hence, it is reasonable to interpret the compositional-

textural layering as relict sedimentary interbeds of

siltstone and sandstone in shale. Low-grade meta-

morphism created bedding-parallel ﬂakes of micas

and chlorites that nucleated and grew in the pro-

tolith, mimicking the orientation of the original ﬂat

clay particles. In a later deformation event this slaty

cleavage can be crenulated so that limbs of the over-

printing crenulations deﬁne a second, oblique foliation

—a crenulation cleavage (Figure 14.18). This is

another example of polymetamorphism.

Transposition. Existing compositional layers or other

types of foliation in a metamorphic body can be

sheared or smeared out into a new and different orien-

tation during ductile deformation and recrystallization.

An example of this transposition related to ductile

folding of layers is displayed in Figure 14.19b. Note the

ﬂattening perpendicular to the new metamorphic foli-

ation and corresponding extension parallel to it. Trans-

position occurs on a wide range of scales, from thin

section to mountain range. Figure 14.20 shows that

pillows of lava can also be transposed into thin lenses;

with continued ductile deformation under nonhydro-

static stress these could become more ﬂattened. Once

again, thin lenses and even layers can be created in a

tectonite where none previously existed.

14.1.4 Crystalloblastic Series

In the tectonite in Figure 14.17d, grain boundaries

appear to have equilibrium shapes resembling those in

granoblastic texture, except for two attributes. First,

inequant platy micas have a strong preferred shape

orientation that precludes use of the term granoblastic

as conventionally deﬁned. Second, careful examination

reveals that micas are relatively euhedral in contact

with anhedral grains of quartz and oligoclase but in

contact with euhedral garnet are decidedly anhedral.

Hence, these four minerals manifest a hierarchy of

development of characteristic crystal forms during

solid-state growth. The mutual competition for com-

mon space during such growth is distinctly different

from crystallization in melts where each growing grain

—at least near the liquidus—is not affected in its shape

by growing neighbors. This hierarchy of euhedralism in

metamorphic rocks was ﬁrst recognized by F. Becke in

1909 and called a crystalloblastic series. It can be seen

in all sorts of metamorphic fabrics—anisotropic as well

as isotropic. In the following list of metamorphic rock-

forming minerals, denser ones having tighter packing

of ions tend to be more euhedral as they compete for

space during recrystallization, as follows:

MOST EUHEDRAL

Titanite, rutile, pyrite

Garnet, sillimanite, staurolite, tourmaline

Epidote, magnetite, ilmenite

Andalusite, pyroxene, amphibole

Micas, chlorite, dolomite, kyanite

Calcite, vesuvianite, scapolite

Feldspar, quartz, cordierite

LEAST EUHEDRAL

Metamorphic Rocks and Metamorphism: An Overview

421

422 Igneous and Metamorphic Petrology

expelled from the solidifying carbonatite magma; equi-

libration required its dissolution in these fenitizing

ﬂuids coupled with simultaneous precipitation of

more stable amphibole. (The alkali feldspar in the

granite appears to have been stable in the presence

of the fenitizing solutions.) However, throughout the

replacement process, the characteristic graphic out-

lines of the original quartz grains were faithfully pre-

served. Preservation of the relict protolith fabric,

coupled with the observation in the ﬁeld of unreplaced

quartz farther from the carbonatite dike, beyond the

inﬂuence of the fenitizing solutions, is a crucial line

of evidence indicating the nature of the metasomatic

process.

Metasomatism is a signiﬁcant change in the chemical

composition of a rock resulting from the replacement

14.18 Polymetamorphic slate with thin relict interbeds of weakly metamorphosed sandstone and siltstone. (a) Gently arched slaty cleavage in

slate that is subparallel with relict bedding is overprinted by a nearly perpendicular crenulation cleavage. Redrawn from Gilluly et al.

(1975). (b) Photomicrograph in plane-polarized light of a similar fabric except the crenulation cleavage has a smaller wavelength and

intersects the pre-existing slaty cleavage and paralleling relict beds at about 45°. Crenulations are not expressed in quartz-rich relict beds.

Relict silty

laminae

(b)

10mm

sandy

bed

Imposed

metamorphic

foliation

Relict

Trace of relict bed on foliation

02mm

20cm

(a)

14.1.5 Metasomatism

In the examples of metamorphism described thus far

there was little signiﬁcant change in the chemical com-

position of the rock, except for the amount of water.

The nonvolatile chemical composition remained essen-

tially constant in closed rock systems; metamorphism

was isochemical.

Figure 14.21 illustrates another type of metamor-

phism in which a marked change in composition has

taken place. Quartz grains in graphic granite (Figure

7.20) wallrock next to a hot carbonatite dike (Figure

13.40) have been replaced by ﬁne-grained aggregates of

sodic amphibole, resulting in an alkali-rich amphibole–

feldspar rock called fenite. The quartz was unstable

in the presence of the highly alkaline volatile ﬂuid

of original unstable mineral grains by others of dif-

ferent composition. In some cases, but not all, this is

a constant volume replacement which preserves the

form and the size of the original grains, as just

described. Metasomatism is usually made possible by

advective ﬂow of large quantities of a ﬂuid solution

through an open-system, permeable rock body. Integ-

rated over the time of the metasomatism, ﬂuid/rock

ratios can be relatively large. Energy driving the advec-

tive ﬂow can be thermal, as from a nearby cooling

magma, or gravitational, or both.

In contrast to the local metasomatism next to the

thin dike just described, global-scale metasomatic sys-

tems operate at oceanic ridges where seawater advects

through highly fractured hot basaltic rocks emplaced

along the rift (Special Interest Box 11.3), transforming

them, in part, into metasomatic rocks called spilite (see

Section 14.2.3).

Metamorphic Rocks and Metamorphism: An Overview

423

Initial foliation

(a)

New foliation

14.19 Transposed foliation. (a) Purely mechanical rotation of rigid slices of an initial layering produces a new foliation orientation. However,

this model is unrealistic because of the assumed rigidity of the slices and the voids created at the ends of the slices. (b) Progressively

tighter ductile folding transposes layer into a new foliation oblique to the initial one. (c) Hand sample showing transposition of a folded

layering into an oblique, planar, new layering. Note that this new foliation, as well as the one in (b), is parallel to the axial plane of

related folds.

Initial foliation

(b)

New foliation

Initial foliation

New foliation

(c)

14.2 THE NATURE OF METAMORPHISM

The purpose in describing the foregoing examples of

metamorphic equilibration has been to create some

notion of the nature of metamorphic rocks and meta-

morphism. Obviously, the response of solid rocks to

changing conditions (changes in intensive variables

and state of stress) along metamorphic paths is inﬁn-

itely variable. Not only do these changes range widely

but the character of protoliths is also highly variable.

Interestingly, however, contrasting protoliths and con-

trasting metamorphic paths can converge to a similar

metamorphic rock product. For example, a metamor-

phic path that involves extreme reduction in grain size

can produce a mylonite from a granitoid (Figure 14.16)

that resembles in terms of fabric a slate created by

slight metamorphism of a shale. Large volumes of a

particular advecting solution are potentially capable of

transforming different protoliths into virtually the same

metasomatic rock.

In the remainder of the chapter, signiﬁcant facets of

metamorphic rocks and metamorphism—based in part

on observations of thin sections, hand samples, and

outcrops discussed above—are organized into a pre-

lude to more detailed discussions in following chapters.

14.2.1 The Nature of the Protolith

Protoliths of metamorphic rocks include virtually the

entire spectrum of magmatic and sedimentary rocks as

well as previously metamorphosed rocks, including

diverse metasomatic ones. Only sedimentary evaporites

and rare highly alkaline magmatic rocks are seemingly

not represented.

Sedimentary Differentiation. Sedimentary rocks have

formed by weathering and erosion of pre-existing

primary magmatic rocks since the earliest stages of

cooling of the infant Earth. Because rock-forming min-

erals in magmatic rocks respond in different ways to

weathering processes on the surface of the Earth their

decomposition produces compositionally diverse sedi-

ment. During this sedimentary differentiation, magmatic

424 Igneous and Metamorphic Petrology

(a) (b)

14.20 Metamorphosed Archean pillow basalts, Yellow Knife, Northwest Territories. Canada. (a) Undeformed but recrystallized pillows. (b)

Highly deformed lighter-colored pillows have been folded and transposed into elongate shapes. Brunton compass for scale. Geological

Survey of Canada photographs courtesy of Maurice Lambert and W. R. A. Baragar.

14.21 Metasomatized graphic granite in wallrock adjacent to a car-

bonatite dike (Figure 13.40). Quartz grains have been replaced

by ﬁne-grained aggregates of sodic amphibole produced by

reaction with highly alkaline solutions expelled from the

carbonatite magma.

alumino-silicate minerals, especially widespread feld-

spars, decompose and hydrate into clay minerals (illite,

kaolinite, and montmorillonite in Tables 14.1 and A.2

in Appendix A). Clays are Al-rich minerals that contain

abundant (OH) and highly variable amounts of H



and H



. In widespread smectites, such as montmo-

rillonite, Mg, Fe, and K are also quite variable whereas

Ca and Na are minor constituents. Clays are the major

constituent of shales (pelitic rocks) together with lesser

quartz and carbonate minerals. Shales are more alum-

inous than most common magmatic rocks, as can be

seen in Figure 14.22 and Table 15.3. Ca and Na are

relatively soluble in surface waters and are removed

from decomposing magmatic minerals, eventually

being deposited as calcium carbonate (calcite and

metastable aragonite) and lesser gypsum and halite in

lakes and oceans. Quartz is relatively inert and is con-

centrated in sandstones along with residual feldspars

and other resistant minerals (e.g. zircon) plus lithic

fragments.

Because clays are derived from most of the rock-

forming alumino-silicate minerals in magmatic rocks,

shales are the most abundant sedimentary differentiate;

they actually constitute about half of all sedimentary

rocks. Sandstones make up about one-quarter, and cal-

careous limestones and dolostones (made of dolomite)

most of the remainder. Because the degree of weather-

ing and the extent of resulting sedimentary differenti-

ation vary, the compositional ranges of magmatic and

sedimentary rocks are not wholly distinct; they over-

lap to some extent. Thus, some lithic sandstones (e.g.

Figure 14.17a) in forearc–trench environments (see

Figure 14.24) may not differ much from the nearby

andesitic rocks from which they were derived.

Types of Protoliths. The following rock properties

allow recognition of protoliths:

1. Relict fabrics. In low-grade metamorphic rocks

where recrystallization has not created large new

grains or substantial deformation has not taken

place, boundaries of original grains (Figures 14.2

and 14.3) and outlines of fabric features such as pil-

lows in maﬁc subaqueous lava ﬂows (Figure 14.20a)

have not been erased.

2. Field relations. In some instances, metamorphism

has progressed from an unmodiﬁed protolith into

rocks of increasing grade; this transition can be

observed in the ﬁeld, such as in a contact meta-

morphic zone around a magmatic intrusion.

3. Bulk chemical composition manifested by the min-

eralogical and modal composition of the rock. This

may be the only clue to the protolith where critical

informative ﬁeld relations are lacking and relict fab-

rics have been erased. Even if there has been sub-

stantial metasomatism, some constraint is possible,

perhaps in ratios of less mobile elements (Figure

2.22). Keeping in mind the continuous spectrum of

rock compositions, several discrete, “end-member”

protolith categories, or bulk chemical rock groups,

can be recognized, as follows:

(a) Ultramaﬁc. Derived from high-Mg–Fe mag-

matic rocks—including peridotite, pyroxenite,

and dunite—composed of pyroxene and olivine

with limited or no feldspar. High-T metamor-

phic rocks are composed of these same min-

erals plus possible spinel and Mg–Fe garnet,

while lower T rocks contain combinations of

serpentine, talc, brucite, chlorite, tremolite, mag-

netite, and, where CO

fugacities are adequate

for stabilization, dolomite or magnesite.

(b) Maﬁc. Silicate rocks that have relatively high

concentrations of Mg, Fe, and Ca as well as

appreciable Al correspond to a basaltic (gab-

broic) protolith or, in some instances, a clay-

bearing dolostone or marl (mixed clay-carbonate

rock). Mg–Fe–Ca-rich minerals such as actino-

lite, hornblende, pyroxene, garnet, epidote,

plagioclase, chlorite, and pumpellyite are

abundant; Fe–Ti oxides may also be abundant.

Some petrologists refer to these rocks as

metabasites. Rare rocks of cordierite  antho-

phyllite are believed to be metamorphosed

spilites that are created by metasomatism of

basalt at oceanic ridges.

Metamorphic Rocks and Metamorphism: An Overview

425

CaO + Na

Granite

Grano

diorite

K-feldspar

Average

shale

Hundreds of

analyses of

shale

Plagioclase

Andesite

Basalt

Calcareous shales

Limestone

14.22 Chemical composition in wt.% of shales compared to average

common magmatic rocks (Table 2.2) and feldspars (Table A.1

in Appendix A). Shaded area encloses hundreds of analyses

of shale (from Hutcheon et al., 1998) that are mostly more

aluminous than common magmatic rocks and feldspars whose

decomposition yields clay minerals; relatively soluble K, Na,

and Ca are removed. Average shale from Table 15.3. Com-

positions of calcareous shales (dots) trend toward limestone

(calcite).

feldspar-bearing metamorphic rocks derived

from felsic magmatic rocks as well as felds-

pathic and lithic sandstones. Relatively minor

maﬁc minerals include biotite, chlorite, amphi-

bole, pumpellyite, stilpnomelane, and Fe–Ti

oxides. It should be realized that, without relict

fabrics and critical ﬁeld relations, it may be

impossible to discriminate between protoliths

which include rhyolite lava, tuff, obsidian,

granite, porphyry, and different kinds of sand-

stones. Sandstone protoliths composed of quartz

and minor clay are sometimes called psammites.

(d) Pelitic. Rocks that have clay-rich shale (mud-

stone) protoliths and are characterized by

Al-rich silicates, such as Al

SiO

polymorphs,

cordierite, almandine garnet, and staurolite.

Quartz is almost always present with abundant

micas—both biotite and white mica—which

are only absent in rocks recrystallized at high-

est T, where combinations of alkali feldspar,

SiO

polymorphs, Fe–Ti oxides, and pyr-

oxenes take their place. Graphite is conspicuous

in many pelites.

(e) Calcareous. Rocks derived from more or less

pure limestones and dolostones. Calcite and

dolomite in the absence of quartz and other

silicates are stable over a broad range of meta-

morphic conditions but as silicates and quartz

increase in modal abundance, calcareous rocks

grade into those of the next category.

(f) Calc-silicate. Rocks derived from impure

carbonate protoliths that contain substantial

proportions of clays and quartz in addition to

calcite and dolomite, or from more pure car-

bonate rocks metasomatized in contact aure-

oles (see skarns below). Metamorphic minerals

include carbonates of Ca, Mg, Fe, and locally

Mn plus numerous possible Ca-rich silicates

such as grossular–andradite garnet, vesuvianite,

epidote-group minerals, diopside–hedenbergite

solid solutions, wollastonite, and tremolite.

(g) Ferruginous. Rocks include enigmatic Pre-

cambrian banded iron formations and meta-

morphosed marine cherts. The former are

thinly bedded and composed chieﬂy of

quartz with subordinate hematite, mag-

netite, greenalite (Fe,Mg)

(OH)

, min-

nesotaite Fe

(OH)

, stilpnomelane,

riebeckite, Fe-chlorite, siderite, and ankerite

Ca(Mg,Fe

2

,Mn)(CO

)

. Metamorphosed cherts

are composed of quartz and accessory Fe–

Mn–silicates such as spessartine (Mn–Al gar-

net), piemontite (an epidote-like mineral), and

riebeckite.

14.2.2 Types of Metamorphism Based on

Metamorphic Conditions

A wide spectrum of changes in P, T, composition, and

state of stress cause protoliths to equilibrate to new

mineral assemblages and fabrics that constitute the

metamorphic rock. Although most metamorphism in-

volves changes in more than one of these parameters

along the metamorphic path, speciﬁc types correspond

to each parameter (Figure 14.23). Increase in T causes

thermal metamorphism. Inﬁltration into open rock

systems of chemically active ﬂuids that create new

mineral assemblages is metasomatism, or, because

the ﬂuids are hydrothermal solutions (Section 4.3.3),

hydrothermal metamorphism. Deformation resulting

from nonhydrostatic stresses can be focused in high-

strain-rate shear zones, causing dynamic metamor-

phism. Compounding the effects of heat and nonhy-

drostatic stress with subordinate changes in P and ﬂuid

composition leads to dynamothermal metamorphism.

14.2.3 Geologic Field Settings: Metamorphic Terranes

Identiﬁcation of the geologic settings where metamor-

phism occurred is a meaningful exercise as it allows us

to see what transfers of mass and heat were involved

and how they drove solid-state changes. Identifying

sites of metamorphism has a parallel in ﬁnding geologic

realms where partial melting can occur to localize

magmatism (Section 11.1). Metamorphic settings are

described in terms of the large-scale ﬁeld relations of

metamorphic terranes, which are tracts of exposed

rock distinct in some way from adjacent rock masses.

Terranes vary in areal extent and the conditions and

426 Igneous and Metamorphic Petrology

NON-

HYDRO-

STATIC

STRESS

Dynamic

Thermal

Impact

FLUID ACTIVITY

Dynamo-thermal

Burial

Contact

Ocean ridge

Regional

14.23 Tetrahedron representing the controlling factors in metamor-

phism (capital letters at four apices), types of metamorphism

(lower case letters), and environments, or ﬁeld settings, of

metamorphism (italicized lower case letters). Note that re-

gional metamorphism lies within tetrahedron because all four

factors are involved. Metamorphic equilibration processes are

also controlled by kinetic factors, which are not represented.