Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones

567

P–T conditions resulting from lack of attainment of

equilibrium in the mineral assemblage, difﬁculties in

interpreting relict mineral assemblages by the petrolo-

gist, inaccuracies in thermobarometric analyses accru-

ing from poor thermodynamic data and calibration or

improper methodology, and other factors.

Geochronology and Time-T Paths. To place a P–T

path in an absolute time frame there must be age de-

terminations at distinct points on the path using iso-

topic systems. Several isotopic systems and minerals

are potentially useful. However, unlike quickly cooled

volcanic rocks, metamorphic rocks typically experience

(a) 42 My elapsed

100

(b) 75 My elapsed

100

4003002001000

200

400

600

800

1000°C

Greenschist

Granulite

Blueschist

Amphibolite

Granulite

Eclogite

200

400

600

800

1000°C

Greenschist

Amphibolite

Granulite

Eclogite

18.2 Finite-element tectonothermal model of a continent–continent collision zone, courtesy of Gregory T. Roselle. The model is appro-

priate for the late Cretaceous–Tertiary Alpine Orogen. Upper, middle, and lower layers of continental crust have thicknesses of 5, 11,

and 24 km and their radioactive heat generation is 1.8, 1.0, and 0.2 microwatts/m

, respectively. Heat generation in the mantle is

0.02 microwatts/m

. (a) After 42 My of convergence at an average rate of 0.8 cm/y and no erosion. In top panel, isotherms are deﬂected

downward in the overriding plate as a result of the heat-sink effect of the cooler upper layer of the subducting slab. Bottom panel shows

distribution of metamorphic facies. (b) After 75 My of convergence at an average rate of 0.17 cm/y and an average erosion rate of

0.27 cm/y after 60 My. Isotherms are warped upwards in overriding plate where slices of radioactively enriched upper crust from the

subducting slab (triangles) have been tectonically inserted and almost exhumed by erosional unrooﬁng. In the distribution of meta-

morphic facies in the bottom panel it can be noted that no blueschist facies is present because of the increased heating that has brought

amphibolite- and granulite-facies conditions closer towards the surface. Compare Figures 14.33 and 18.1.

slow decreases in T from peak conditions, with con-

sequent ample opportunity for differential loss or gain

of both parent and daughter isotopes (Section 2.6)

because of their typically dissimilar chemical behavior.

This open-system behavior is extreme in the case of

decay of radiogenic

K to

Ar, an inert gas which has

no chemical bonding to any crystal structure and,

therefore, can readily diffuse out of the host potassic

mineral. But daughter atoms in any isotope system are

subject to a degree of diffusion that decreases continu-

ously and exponentially with decreasing T. The T at

which the rate of diffusion is effectively nil where the

isotope system becomes closed is called the closure

temperature, T

. Some of these are shown in Figure 18.3

where it can be noted that a useful low T

chronologic

system involves ﬁssion tracks in apatite and zircon.

During ﬁssion of radioactive U atoms incorporated

into a growing crystal, outward ﬂying decay particles

damage the surrounding atomic structure in the host

crystal grain. These damage tracks can be enhanced

chemically so as to be made visible under high-power

optical magniﬁcation; the proportion of countable

tracks to the determinable U-concentration in the

crystal allows an age to be obtained. Fission tracks

are annealed by internal recrystallization and hence

eliminated above the respective T

of the mineral.

Because different isotopic systems in different min-

erals have different closure temperatures, a T–t path

can be constructed for a rock, or set of coeval rocks,

that contain the different systems. As the rock cools be-

low the T

of a particular isotope–mineral system, the

time of closure—when the “clock” is started—can be

determined. Two or more such determinations give a

cooling rate for the rock.

Numerous examples of P–T–t paths have been

described and representative ones are shown in later

sections. Paths can be relatively open loops that corre-

spond to continuous changes in both P and T or tight

“paper-clip” loops where the retrograde path closely

568 Igneous and Metamorphic Petrology

Special Interest Box 18.1 Low-grade

active metasomatism in a continental

geothermal ﬁeld

Synextensional magmatism related to spreading in

the Gulf of California has created the Cerro Prieta

geothermal ﬁeld in the Salton Sea area just south of

the California–Mexico border (Figure 13.29; see

also Schiffman et al., 1984). Carbonate-cemented

quartzo-feldspathic sediments are being actively

metasomatized into calc–silicate mineral assemblages

by reaction with alkali–chloride-rich hydrothermal

solutions, or “brines,” at depths 2.5 km and tem-

peratures (determined from oxygen isotope ratios

in calcite) between 200 and 370°C. Advection of

the solutions is presumed to be driven by heat from

buried intrusions. Drill cores and cuttings from

50 wells constrain the conditions of metasoma-

tism and equilibrated mineral assemblages (Figure

18.18). X in the ﬂuid declines below about 0.5

with increasing T. Because of buffering by car-

bonaceous material in the sediments, f is slightly

more reducing than QFM (Figure 3.14). Time-

integrated ﬂuid–rock ratios are about 3. The lowest

T decarbonation reaction just below 200°C yields

the Ca–zeolite wairakite from the assemblage of

calcite (cement) and (detrital) illite, quartz, feldspar,

and chlorite. At about 230°C, epidote forms from

calcite  wairakite followed by prehnite  actino-

lite at about 300°C by decomposition of calcite 

chlorite. The highest T minerals, clinopyroxene and

biotite, appear at 320°C either by the dehydration

of actinolite  prehnite (yielding also epidote) or

by the mixed ﬂuid reaction actinolite  calcite →

clinopyroxene  H

O  CO

. The distinctive

low-P assemblage of clinopyroxene  epidote 

prehnite has been found in a wide range of pro-

toliths in other geothermal systems. Signiﬁcantly in

this commercial geothermal ﬁeld, devolatilization

reactions create secondary porosity.

T (°C)

1000

800

600

400

200

Zircon

Monazite Grossular

Allanite

Almandine garnet

Muscovite

Pyrope

Rutile

Apatite

Biotite

K-feldspar

Hornblende

Muscovite

Biotite

K-feldspar Titanite

Zircon

Apatite

U-Pb Sm-Nd Rb-Sr Ar

Fission

track

18.3 Closure temperature, T

, for some common minerals used in

geochronology according to the isotopic systems indicated

across the top of diagram. Data from Hodges (1991), Thom-

son et al. (1998), and personal communication from Stephen

Nelson and Bart Kowallis. For a given mineral and isotopic

system, T

depends on cooling rate and effective distance of

diffusion to a free surface in the mineral grain.

follows the prograde. Like phase equilibria, published

P–T paths have differing degrees of accuracy and

reliability. They must be viewed with caution and a

bit of skepticism. Spear (1993, p. 766) concluded that

“It is the obligation of every petrologist to apply

rigorous methods to the analysis of petrogenesis and

to try not just to obtain a P–T path, but to analyze

its certainty.” His treatise explains how this can be

accomplished.

18.2 A BRIEF ANATOMICAL OVERVIEW

OF METAMORPHISM IN OROGENS

No two orogens are exactly alike and neither are the

metamorphic terranes in them, making it difﬁcult to

create a brief but accurate generalized summary.

As early as 1961, before plate tectonic concepts

were fully established and accepted, Miyashiro (see

1994) recognized that metamorphic mineral assem-

blages in Japan deﬁne two juxtaposed, side-by-side

metamorphic terranes, or paired metamorphic belts

(Figure 18.4). South of a major fault zone—the Median

Tectonic Line—and adjacent to the trench bordering

the Paciﬁc are rocks of the Shimanto–Chichibu–

Sanbagawa Belts metamorphosed to the zeolite and

prehnite–pumpellyite facies and culminating in higher

P rocks transitional to the blueschist facies. Farther in-

land, north of the Median Tectonic Line, are rocks of

the Ryoke Belt metamorphosed to lower P facies. On

the opposite side of the Paciﬁc Ocean, in California

alongside the now active right-lateral San Andreas con-

tinental transform fault (Figure 18.5), rocks of the high

Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones

569

Ryoke Belt;

intermediate P/T

Sanbagawa Belt;

high-P preh.-pump.

Chichibu Belt;

prehnite-pumpellite

Jurassic

0 miles 150

km 200

Kyoto

Jurassic

JAPAN

SEA

Pre-

MTL

PACIFIC OCEAN

Tokyo

Shimanto Belt; zeolite

facies; Cret.–Tertiary

18.4 Accreted terranes and their metamorphism in Japan. Terranes were accreted at progressively later times from north to south. The strike-

slip Median Tectonic Line (MTL) separates the higher T intermediate-P Ryoke belt from the high P/T prehnite–pumpellyite-facies

Sanbagawa–Chichibu Belts. Redrawn from Miyashiro (1994).

42°N

40°N

38°N

36°N

124°W 122°W 120°W 118°W

0 100mi

0 100km

Ocean

Paciﬁc

Great

Sierra

Nevada

Valley

San

Francisco

Laumontite (zeolite)

Metamorphic facies

Pumpellyite

Glaucophane–

lawsonite (blueschist)

Klamath

Mountains

18.5 Simpliﬁed geologic map showing high P/T metamorphic

zonation in the Franciscan Complex in the California Coast

Ranges. Heavy lines with teeth are traces of the Coast Range

Thrust; teeth are on upper plate rocks of the Great Valley

Sequence (unpatterned). Postmetamorphic, mid- to late-

Cenozoic faults of the right-lateral San Andreas transform sys-

tem are unpatterned heavy lines. Note Sierra Nevada farther

inland where calc–alkaline arc magmatic rocks are associated

with intermediate-P metamorphic rocks. Redrawn from Ernst

(1975).

P/T facies series (zeolite, prehnite–pumpellyite, and

blueschist) are found in the Coast Ranges. Farther in-

land, rocks of the intermediate P/T facies series (green-

schist and amphibolite) are spatially associated with

granitic rocks of the Mesozoic Sierra Nevada Batholith.

In the plate tectonic model in Figure 14.24, the polar-

ity of these two paired Mesozoic–Cenozoic metamor-

phic belts results from tectonothermal phenomena

associated with subducting oceanic lithosphere. The

higher P/T rocks in the accretionary wedge next to the

trench lie immediately above the cool subducting plate

that acts as a heat sink, maintaining temperatures well

below the granite solidus, thus essentially precluding

any migmatization and magmatism, and creating steep

metamorphic ﬁeld gradients. Rocks metamorphosed

along lower P/T ﬁeld gradients lie farther inland from

the trench in thickened and heated continental crust

where calc–alkaline arc magmatism and deep crustal

migmatites testify of elevated temperatures. Ernst

(1973) points out that the polarity of ﬁeld gradients

and facies series can be applied to ancient subduction

zones where active seismicity and plate motions no

longer exist to indicate the direction of dip of the

descending lithospheric slab.

However, it is difﬁcult to invoke this simple plate

tectonic model for every orogen. Even in Japan, the

Median Tectonic Line has experienced signiﬁcant left-

lateral strike-slip displacement accompanying oblique

convergence of the paired terranes; this opens the pos-

sibility that the Ryoke and Sanbagawa–Chichibu Belts

were not created side by side as adjacent paired trench

and inland terranes but were formed contemporane-

ously along strike (Brown, 2000).

A major complication in the evolution of orogens

arises where, as commonly happens during the course

of plate motions, an oceanic island arc or plateau clogs

a subduction zone or where two continental plates of

any size converge on one another in a collision zone. In

the Cordilleran Orogen of Alaska, high P/T rocks are

associated with accretionary wedges and oceanic plate

convergence as well as with collisional continental

plates. These oceanic and continental terranes were su-

tured onto the North American Plate as early as the

Permian and continued episodically and diachron-

ously through the Cretaceous (Patrick and Day, 1995).

Because high P/T rocks occur far inland, the simple

metamorphic-belt polarity recognized to the south in

California does not apply. As oceans close during the

evolution of convergent plate systems, thicker contin-

ental lithosphere can collide with the overriding con-

tinental plate. This happened during the Paleozoic in

the Appalachians in eastern North America and the

Urals in central Russia, during the late Cretaceous in

the Alps in Europe, and the Cenozoic in the Himalayas

in southern Asia. Hence, metamorphic terranes formed

in different settings along different P–T paths and at

different times can be closely juxtaposed and sutured

together into a thickened collage that undergoes fur-

ther deformation, metamorphism, and possible mag-

matism, typically over hundreds of millions of years.

In the Alps, metamorphic terranes have evolved in a

complex manner as northern and southern continental

plates collided (see Tauern Window below).

The Appalachian Orogen (Figures 13.26 and 18.7;

see also Blatt and Tracy, 1996, p. 390) from Alabama to

Newfoundland contains rocks metamorphosed and

deformed as early as 525 Ma during the Taconic

Orogeny when the ancient Iapetus Ocean closed and a

converging continental mass trapped trench, accre-

tionary wedge, and island arc rocks in the broad suture;

ocean-derived ophiolites are particularly evident in

Newfoundland. Metamorphism in the thickened crust

along intermediate-P ﬁeld gradients and clockwise

P–T paths yielded Barrovian zones from greenschist

through granulite facies. Relict blueschist- and eclogite-

facies rocks have been found in only a very few

locales in the Appalachians, probably because of

widespread overprinting by subsequent higher T equi-

libration but possibly also because the seaward high

P/T rocks were rifted away during the Mesozoic open-

ing of the Atlantic Ocean. Subsequent orogenic pulses

resulting from colliding continental masses produced

metamorphism and deformation at 420–370 Ma and

320–270 Ma. The earlier of these two pulses—the

Acadian—was especially signiﬁcant in the northern

Appalachians. Barrovian metamorphism in the western

part of New England partly overprinted earlier Taconic

assemblages, whereas in east-central New England and

adjacent Canada lower P Buchan zones developed.

Later Alleghanian metamorphism (320–270 Ma) was

felt throughout the length of the orogen but is mostly

of greenschist- or lowest amphibolite facies.

18.2.1 Speciﬁc Regional Metamorphic Terranes

Terranes of mineralogically and texturally zoned tec-

tonites typify orogenic belts. Some of these terranes are

of such distinct character and evolution as to warrant

speciﬁc names. Those occurring in the intermediate- to

low-P Barrovian and Buchan terranes (see Section

18.3) are described next.

Slate belts are prominent features of most orogens.

For example, the Appalachian Mountains of eastern

North America harbor multiple belts that individually

are as much as 50 km wide and hundreds of kms long

(Figure 18.8). Although dominantly metamorphosed

shales, other aphanitic tectonites derived from sedi-

mentary and volcanic protoliths are interspersed in

slate belts. The typifying slaty cleavage is generally

steeply inclined and subparallel to the belt; local over-

printing spaced cleavage (Figure 14.18) is common.

Mineral assemblages are generally of the subgreen-

schist and lower greenschist facies.

570 Igneous and Metamorphic Petrology

Amphibolite facies

GENEVE

LYON

Zeolite and prehnite–

pumpellyite facies

Greenschist facies

0 km 150

0 miles 200

MÜNCHEN

ZÜRICH

BERN

MONACO

LIGORIAN

SEA

GENOVA

HP and UHP assemblages

overprinted by greenschist facies

Alpine

granitoid

Folded rocks; not

metamorphosed

Ultramaﬁc

rocks

MILANO

VENEZIA

ADRIATIC

SEA

Line

INNSBRUCK

Insubric

GRENOBLE

18.6 Distribution of late Cretaceous–early Tertiary Alpine metamorphic zones and facies in the western Alps (see also Figure 18.24). Note the

scarcity of ultramaﬁc rocks and virtual absence of Alpine-age granitoids in this continent–continent collision zone. The Insubric Line is

a late, mostly post-metamorphic, steeply dipping fault that has considerable right-lateral strike-slip displacement. The Tauern Window

(TW) in Austria and the Dora Maira Massif (D) in northwestern Italy near the border of France are important metamorphic terranes

exposed by erosion of domical Oligo-Miocene upwarps. Arrow shows location of UHP rocks in the southern part of the Dora Maira

Massif. Redrawn from Frey et al. (1974).

Granitic intrusions

(chieﬂy postmetamorphic)

Greenschist facies

with biotite (b) isograd

Amphibolite facies

with sillimanite (s) isograd

Granulite facies

Precambrian

metamorphic rocks

0 150miles

0 160km

Atlantic

Ocean

42°N

74°W

46°N

70°W

Cretaceous

and

Tertiary

rocks

36°N

40°N

84°W

80°W

76°W

Brevard

fault mylonite

zone

Trace of Al

SiO

triple point

18.7 Generalized map of regional metamorphism in the Appalachian Orogen of the eastern USA. The New England segment (top) joins onto

the central and southern segment (bottom) along the Pennsylvania–New Jersey state line (heavy dashed line). Redrawn from Morgan (1972).

Regional Metamorphism in Subduction Zones: P–T–t Paths, Facies, and Zones 571

Mantled gneiss domes (Eskola, 1949) found in the

deeper, ductile, higher grade metamorphic cores of

orogens are overlain by essentially concordant but

locally folded layers of schist, amphibolite, and other

metamorphic rocks. Belts of such domes occur in

many deeply eroded orogens, such as the northern

Appalachians (Figure 18.9). Buoyant upward mush-

rooming of core granitic orthogneisses in the con-

tractional orogenic environment deforms covering

rocks into large-scale recumbently folded nappes.

(Recumbent folds are strongly overturned so that their

axial surfaces are nearly horizontal. Nappe is French

for “coversheet” and was coined for fold and

thrust-sheet structures in the Alps.) In mushrooming

migmatite domes, buoyancy is enhanced by partial

melting of the high-grade metamorphic rock. Spec-

tacular examples occur in the rugged mountainous

terrane of the East Greenland Caledonide Orogen

(Figure 18.10).

The astute reader will realize that there is little

difference in the creation of mantled gneiss domes,

migmatite domes, and granitic diapirs (Section 9.3;

Figures 9.15 and 9.21). All are buoyant masses that

dome overlying roof rocks during their ascent into

or through the low-ductile-strength lower continental

crust. Gneiss and migmatite domes can be viewed as

a continuum depending on the degree of partial

melting, but whether the more melt-rich granitic

diapirs can be derived from migmatite by more

advanced melting is controversial. Many petrologists

believe migmatites do not evolve into granite plutons.

Instead, they view migmatites as “failed” granites, in

part because the small volumes of hydrous partial melt

are likely to crystallize across a negatively sloping

solidus (in P–T space) before rising very far from the

source.

Metamorphic core complexes. Controversial domal

structures along the eastern side of the Mesozoic–

early Cenozoic Cordilleran Orogen of western North

America (Figure 18.11) are possible shallow crustal

expressions of gneiss domes. These metamorphic core

complexes are composed of Precambrian to Cenozoic

metamorphic and magmatic rocks overprinted by com-

plex, polyepisodic magmatic intrusion, metamorphism,

and ductile to brittle deformation. A tectonically

detached and distended cover of unmetamorphosed

rocks is exposed on the ﬂanks of the uplift but mostly

eroded from off the uparched core. Features com-

mon to most or all include (Crittenden et al., 1980;

Armstrong, 1982):

1. Tectonic denudation (unrooﬁng) from the Pale-

ocene to mid-Miocene accomplished by low-dip-

angle extensional faulting that began in the ductile

regime, producing mylonite, but culminated in the

brittle regime, producing cataclastic fabric. The

actual timing and development of the ductile versus

brittle shearing depended on local variations in

thermal regime and strain rate.

2. Unmetamorphosed rocks tectonically overlie a

domed core of metamorphosed rocks intruded by

granitoids—hence, metamorphic core complex.

The grade of metamorphism is variable in the core.

3. Prominent low-dip, ductile to brittle, foliation sub-

parallel to the denudation faults. Axes of local, re-

cumbent, isoclinal (limb-parallel) folds are parallel

to a strong pervasive stretching lineation on folia-

tion surfaces. Flattening above an upwelling gneiss

572 Igneous and Metamorphic Petrology

36°N

34°N

82°W78°W

0 miles 100

0 km 200

Virginia

Kentucky

Tennessee

Georgia

INNER

BLUE

PIEDMONT

SLATE

BELT

RIDGE

CAROLINA

(

)

PROVINCE

South

Carolina

North

Carolina

Ocean

Atlantic

18.8 Generalized geologic map of the southern Appalachian Mountains showing slate belts (heavy stippling) interspersed with higher grade

metamorphic rocks, such as in the Inner Piedmont, and sedimentary rocks to the northwest. Major thrust faults shown by lines with teeth

on upper plate. Redrawn from Weaver (1984).

6

s.l.

6

s.l.

6

s.l.

Mass.

Vt.

42°30N

BB

B

CC

C

A

43°00N

72°30W

05mi

08km

N H

18.9 Generalized geologic map and cross sections showing mantled gneiss domes (shaded) and overlying fold nappes in the New England

Appalachian Orogen. See Figure 18.7. Dashed lines show foliation in margins of domes. Longer lines and dotted lines in cross-sections

show complex recumbent fold nappes in metamorphic rocks mantling domes. Heavy lines are postorogenic (Triassic) faults. Redrawn

from Thompson et al. (1968).

s.l.

2

s.l.

2

05mi

010km

Sedimentary

rocks

Metamorphic rocks

Chlorite

Biotite

 garnet

Biotite  garnet

 plagioclase

Staurolite

 kyanite

Migmatites

18.10 Migmatite domes and nappes in the Caledonide Orogen in east Greenland. (a) Geologic section across the head of Kejser Franz Joseph

Fjord. Note that horizontal and vertical scales are the same: s.l., sea level. (b) Interpretive model of formation. Redrawn from Haller (1971).

dome or simple shear (Figure 17.21) or both may

have created the metamorphic foliation and lin-

eation. Strain in these L-S tectonites is extreme

(Figure 14.13 and Problem 14.1).

4. Broadly syntectonic, Cretaceous to early-Cenozoic

granitic intrusions in the metamorphic core that are

variably overprinted by ductile–brittle shearing.

Many granitoids are peraluminous and have rela-

tively radiogenic Sr-isotope ratios; both of these

properties are consistent with magma generation

from, or at least assimilation of, metasedimentary

rock.

The timing of development of the core complexes

following Mesozoic–earliest Cenozoic thrusting and

their spatial distribution in the belt of overthrusts onto

the Precambrian cratonic margin of the continent (Fig-

ure 18.11) are probably no coincidence. All thermal

models of thickened crust indicate signiﬁcant heating

resulting from thermal relaxation that culminates tens

of millions of years following thrusting. It is not

unreasonabe to suppose that heating may have been

sufﬁcient to generate crustal magmas whose buoyant

ascent created mantled gneiss domes in the deep crust

and core complexes in the shallower crust.

Tauern Window. This classic domical structure in the

eastern Alps (Figure 18.12) exposes high-grade rocks

of the Penninic nappes beneath a low dipping tectonic

carapace of lower grade Austroalpine nappe rocks.

As will be seen, in some respects, the Tauern Window

resembles Cordilleran metamorphic core complexes.

The Alpine Orogen is a result of convergent tecton-

ism. Southward subduction of a European (Tethys)

oceanic basin beneath the southern (Adriatic) conti-

nental plate during the late Cretaceous was followed

by continent–continent collision in the early Tertiary.

Metamorphosed and deformed European plate oceanic

rocks are represented in the Penninic nappes, while

the Austroalpine rocks represent the southern con-

tinent. Thickening of the continental crust during

collision created the regional Alpine metamorphism

(Figure 18.6).

In this tectonic scenario, it is not surprising that the

tectonic contact between the two nappe sequences in

the Tauern Window has been conventionally interpreted

to be a thrust. However, some geologists are beginning

to recognize evidence for late Cenozoic tectonic thinning

and unrooﬁng along low-dip-angle extensional (nor-

mal) faults. If this extensional activity is sigiﬁcant, it

impacts the tectonic and metamorphic evolution of the

Alps. Do the metamorphic rocks in the window furnish

any insight into these conﬂicting interpretations?

Selverstone (1988) has documented consistent

shear-sense indicators in strongly lineated tectonites in

the western part of the window that indicate ductile

thinning and extensional shearing of an overriding

carapace moving westward. The fabric indicators in-

clude rotated garnet porphyroblasts, asymmetric augen

and strain shadows, and S–C fabric (Section 17.3.5;

Figures 17.30 and 17.35).

574 Igneous and Metamorphic Petrology

Paciﬁc

Ocean

Arizona

Colorado

New

Mexico

Wyoming

Idaho

Utah

Nevada

California

Washington

Oregon

Alberta

Montana

SHUSWAP

Mexico

0 km 400

0 miles 250

British

Columbia

18.11 Metamorphic core complexes in western North America.

Core complexes (heavy stippling) lie within the belt of Meso-

zoic overthrusts (eastern limit marked by line with teeth on

upper plate) on buried margin of continental craton (western

limit shown where documented by open circles). Line seg-

ments in core complexes represent prominent lineations that

are oriented approximately perpendicular to belt. Major

Mesozoic granitic batholiths (Figure 9.16) shown by diagonal

ruling. Large areas of mostly late Cenozoic volcanic rocks

omitted for clarity (but see Figures 10.13 and 13.29). Redrawn

from Armstrong (1982).

Innsbruck

schist

Lower

Central gneiss

Upper

schist

Eclogite

0km 30

0 miles 20

(a)

(b)

80–70 Ma

62 Ma

30 Ma

20 Ma

LEU

18.12 Generalized geologic map, interpretive cross-sections, and clockwise P–T paths for the Tauern Window, eastern Alps (Figure 18.6).

Redrawn from Spear (1993) and Selverstone (1985). (a) Map showing lower and upper schist units overlying central core gneiss and a

sliver of eclogite (black) between schist units. (b) Sequence of interpretive cross sections and P–T paths for lower (L) and upper (U) schist

units and eclogite (E). Positions of units in cross-sections are schematic. Although now juxtaposed in the window, these three units

experienced different P–T histories during collision of continental plates; note leading, deeper parts of subducting plates are of oceanic

lithosphere (heavy stippling). Note substantial uplift and ﬁnal exhumation of rocks since about 20 Ma as a result of underthrusting of

a second continental plate (plus pattern).

The earlier tectono-metamorphic history and P–T

path of Penninic rocks is preserved in mineral assem-

blages in three rock units: the lower and upper parts of

the schist cover (Schieferhülle; overlies the central

gneiss, Zentralgneis) and a sliver of eclogite between

them (Figure 18.12a). All three experienced nearly

isothermal decompression following attainment of

peak pressures of about 7 kbar for the upper schist,

9 kbar for the lower schist, and 16–20 kbar for the

eclogite. The large differences in these pressures indi-

cate correspondingly large differences in the depths to

which they were buried, about 25 to 58–65 km, before

rapid exhumation (Figure 18.12b).

Rb–Sr dating of zoned garnet porphyroblasts pro-

vides quantitative chronologic data on the tectono-

metamorphic history of the two schist units in the

western window area (Christensen et al., 1994). Both

units were ﬁrst metamorphosed under blueschist-facies

conditions beginning about 62 Ma (near the Creta-

ceous–Tertiary time boundary) before being subjected

to overprinting greenschist- and amphibolite-facies re-

equilibration. Growth of garnet ceased at about 30 Ma

in the Oligocene near the peak T of metamorphism.

Measured growth rates for two garnets in the upper

schist were 0.7–0.9 mm/My, or an average growth du-

ration of roughly 5 My. The growth rate for a lower

schist garnet was 0.2 mm/My for a growth duration of

about 32 My. The amount of shear deformation

recorded by inclusion trails in apparently rotated gar-

net poikiloblasts indicates an average shear-strain rate

of 3  10

14

/s.

18.3 INTERMEDIATE- TO LOW-P

METAMORPHIC ZONES

AND FACIES

This section deals with regional metamorphic terranes

in thickened continental crust that is spatially associ-

ated with the magmatic arc inland from the trench

(Figure 14.24). In such orogenic terranes, metamor-

phic ﬁeld gradients and facies series—most commonly

greenschist to amphibolite–range upwards from about

300°C at intermediate-P and low-P; these two P paths

lead to development of Barrovian and Buchan meta-

morphic zones, respectively. Locally, temperatures ex-

ceed the granitic solidus so that a culminating zone of

deep crustal anatectic migmatites develops. Barrovian

and Buchan terranes are the typical home of tectonites

whose pervasive anisotropic fabric reﬂects crystalliza-

tion under nonhydrostatic stresses that create folds of

all scales and local high-strain ductile shear zones.

Generally, multiple episodes of heating and deforma-

tion are recorded in polymetamorphic rocks. Granitic

arc magmatism is broadly associated in space and time

but typically culminates late in the orogenic episode,

post-dating peak regional metamorphism. But indi-

vidual intrusions may pre-date a particular regional

metamorphic event and post-date another.

18.3.1 Pelitic Rocks in Typical Barrovian Zones at

Intermediate Pressures

The pelitic zones in Scotland (Figure 14.30) ﬁrst stud-

ied by George Barrow a century ago that now bear his

name have served as a standard reference for compari-

son with mineral assemblages in other chemical rock

groups and with other P–T types of metamorphism.

Pelites offer considerable advantages in elucidating

variations in P–T conditions of metamorphism com-

pared with other widespread chemical groups, such as

maﬁc rocks. Pelitic mineral assemblages are relatively

sensitive to changing P–T conditions and many reac-

tions are of the discontinuous type, allowing demarca-

tion of isograds and zones in the ﬁeld. Shales and

mudrocks that constitute pelitic protoliths are the most

abundant class of sedimentary rocks. They, together

with “immature” feldspathic and lithic sandstones into

which they grade, make up most of the clastic material

shed off topographically high land masses that is de-

posited along passive continental slopes and active

continental margins next to magmatic arcs (Figure

14.24). Whether in basin, slope, or trench, these clastic

sediments can accumulate into thick prisms, become

deeply buried, and be caught up into subsequent

orogenic activity and metamorphosed.

A schematic overview of the pelitic mineral assem-

blages through the succession of Barrovian zones is laid

out in Figure 18.13. Accompanying changes in petro-

graphic aspects and mineral assemblages and the

reactions creating them are presented in Figure 18.14.

The sequence of AFM diagrams in this ﬁgure depicts

changes in mineral assemblages with respect to in-

creasing P and especially T during metamorphism.

Continuous variations in the orientations of tie lines

associated with contracting and expanding stability

ﬁelds result from continuous reactions between solid-

solution product and reactant phases. Discontinuities

in the basic pattern, or topology, of connecting tie-lines

that deﬁne two- and three-phase regions result from

discontinuous reactions that involve the disappearance

of a stable phase, emergence of a new one, or change in

phase compatibilities. In metamorphic terranes, con-

tinuous reactions occur within mappable metamorphic

zones separated by continuous and discontinuous reac-

tions marking isograds. Not every mineral in every rock

participates in every reaction.

Although there is considerable chemical variation in

shales and mudrocks, they are typically high in Al and

alkalies, have high K/Na, but low Ca (Section 14.2.1).

As a result of the high Al and alkalis, pelitic metamor-

phic rocks contain abundant biotite and white micas

along with quartz at low to intermediate grades but

576 Igneous and Metamorphic Petrology