Myron G. Best. Igneous and metamorphic 2003 Blackwell Science
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the lower crust in the region of granitic magma gener-
ation. Many granulite-facies terranes (Section 19.6) are
characterized by subhorizontal planar fabrics that are
consistent with extreme extension during melt extrac-
tion and migration into adjacent ascending diapirs.
Moreover, the P–T path in many granulite terranes pre-
serves a nearly isobaric cooling path resembling that
expected (Figure 18.20d).
18.3.4 Mineral Assemblages in Mafic Protoliths:
A Brief Overview
Basaltic rocks are widespread in a variety of tectonic
environments (Chapter 13) and consequently are com-
mon in most metamorphic terranes. Although such
protoliths might seem to have the potential of provid-
ing important insights into metamorphism, two factors
limit their usefulness. First, at low grades, relict prim-
ary minerals from basaltic protoliths are preserved
because of the restricted access to volatile fluids, chiefly
water, that allows equilibration of new, more stable
hydrous minerals. Second, compared to, say, pelites,
metabasite assemblages consist of fewer minerals, solid
solution is extensive, and mineral reactions tend to be
of the continuous type, involving progressive changes
in chemical composition over a range of P and T
(Yardley, 1989, p. 91). These factors limit the develop-
ment of distinct, sharp isograds in metabasites in the
field. Because narrowly defined P–T conditions cannot
be recognized, the broader facies designation is used
instead of the zonal one for pelites.
Plagioclases are stable in most grades, except in the
eclogite facies, and tend to be more calcic at higher
temperatures and less calcic at higher pressures. At
lower grades, Ca and Al are sequestered in hydrous
Ca–Al silicates such as zeolites, prehnite, and epidote
so that plagioclase, if present, is albitic.
Amphiboles are widespread in hydrated metabasites
and their composition corresponds broadly with P–T
conditions of metamorphism. Tremolite and actinolite
that contain little or no Al characterize low-grade
metabasites, whereas Al-bearing hornblendes are sta-
ble at higher temperatures and glaucophane at higher
pressures. Before the advent of the electron micro-
probe allowing direct analysis in thin sections, charac-
terization of amphibole compositions depended on
tedious extraction of grains from a rock and equally
tedious wet chemical analyses. One result of microprobe
analyses of amphiboles is the demonstration of a mis-
cibility gap, or solvus, between two calcic amphiboles:
pale green actinolite and darker green (in thin section)
hornblende coexist in greenschist-facies metabasites
(Bégin and Carmichael, 1992).
18.3.5 Metabasites at Intermediate Pressures
Most prograde changes in mineral assemblages in
metabasites are accomplished by continuous reactions;
consequently they are relatively subtle as viewed in
thin sections (contrast Figures 14.6 and 18.14) and in
composition-paragenesis diagrams (Figure 18.21).
Greenschist Facies. This facies comprises the chlorite,
biotite, and garnet zones in spatially associated meta-
morphosed pelitic rocks (Figure 18.13). Metabasites
equilibrated under these conditions begin to be of
sufficient grain size for standard optical microscopy to
be used for investigation. Relict protolith minerals tend
to have been consumed, even though some relict fab-
rics may be preserved, such as in the greenstones in
Figure 14.6a and b. At lowest grades, calcite may be
stabilized if the activity of CO
2
is sufficient. At increas-
ing T, a discontinuous tie-line switching reaction may
occur (Figure 18.21a) that can be generalized as
18.13 calcite chlorite →
epidote actinolite H
2
O-CO
2
fluid
Hence, the assemblage epidote chlorite actinolite
albite quartz calcite titanite Fe–Ti oxides
characterizes metabasites of the middle greenschist
facies. In relatively potassic metabasites, biotite replaces
stilpnomelane that is stable at lower temperatures.
In the higher T part of the greenschist facies in
metabasites that corresponds approximately to the
garnet zone in pelites, aluminous hornblende replaces
actinolite and garnet may also be stabilized. The exact
continuous reactions responsible for this gradual
change in mineral assemblage are impossible to write
because of the complexity of the involved solid solu-
tions. Breakdown of Fe–Al-rich chlorite might produce
almandine garnet according to
18.14 Fe-rich chlorite albite epidote
quartz → almandine Mg-rich chlorite
hornblende H
2
O
Yardley (1989, p. 101) suggests the following two
model reactions might be responsible for production of
end-member hornblendes
18.15 NaAlSi
3
O
8
Ca
2
(Mg,Fe)
5
Si
8
O
22
(OH)
2
albite actinolite
→ NaCa
2
(Mg,Fe)
5
AlSi
7
O
22
(OH)
2
4SiO
2
hornblende quartz
18.16 Ca
2
(Mg,Fe)
5
Si
8
O
22
(OH)
2
actinolite
7(Mg,Fe)
3
Al
4
Si
6
O
20
(OH)
16
28SiO
2
chlorite quartz
4Ca
2
Al
3
Si
3
O
12
(OH)
2
→
epidote
25Ca
2
(Mg,Fe)
3
Al
4
Si
6
O
22
(OH)
2
44H
2
O
hornblende
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones
587
588 Igneous and Metamorphic Petrology
Calcite Actinolite
Calcite chlorite →
epidote actinolite H
2
O-CO
2
fluid
Albite actinolite → hornblende quartz
Actinolite chlorite epidote quartz →
hornblende H
2
O
Chlorite albite epidote quartz →
almandine hornblende H
2
O
Calcite Hornblende
Chlorite
Chlorite
Garnet
FC FC
Epidote
Epidote
Albite quartz
Fe-Ti oxides
MID-AMPHIBOLITE
FACIES
AA
GREENSCHIST FACIES
(b)
(a)
Calcite Clino-
pyroxene
Clino-
pyroxene
Horn-
blende
Antho-
phyllite
Hornblende quartz calcite →
diopside albite H
2
O CO
2
Anthophyllite → orthopyroxene quartz H
2
O
Hornblende quartz → albite diopside hypersthene
Hornblende → garnet anorthite hypersthene H
2
O
Ortho-
pyroxene
Garnet
Garnet
Hbl
FFCC
Plagioclase
Plagioclase
Quartz Fe-Ti oxides LOWER
GRANULITE
FACIES
AA
UPPER AMPHIBOLITE
FACIES
(d)
(c)
18.21 ACF diagrams for metabasites in intermediate-P facies associated with Barrovian pelite zones. See also Figures 14.6 and 18.13. Dark
shaded area represents most basaltic rock compositions; lighter shaded area more mafic basaltic rocks, such as picrites. (a) Greenschist
facies. Note tie-line switching between calcite chlorite compatibility at lower grade (dashed line) and epidote actinolite at higher
grade. (b) Middle amphibolite facies. Compatibility of hornblende with epidote and possibly either calcite or chlorite (or perhaps
almandine garnet). (c) Upper amphibolite facies. (d) Lower granulite facies. Stability of hornblende shrinks in this facies until, in
the upper higher grade part, it disappears completely, allowing compatibility of orthopyroxene plagioclase (dashed line). Antho-
phyllite is replaced by orthopyroxene, so that the mineral assemblages of the upper granulite facies are entirely anhydrous. See also
Figure 16.9a.
Amphibolite Facies. With increasing T into amphi-
bolite facies conditions in metabasites, epidote and
chlorite decrease in abundance and eventually dis-
appear, while hornblende, almandine-rich garnet,
and plagioclase increase (Figures 18.21b, c). All these
changes result from continuous reactions involving solid
solutions, possibly of the form (Yardley, 1989, p. 101)
18.17 3(Mg,Fe)
10
Al
4
Si
6
O
20
(OH)
16
chlorite
12Ca
2
Al
3
Si
3
O
12
(OH)
2
4SiO
2
→
epidote quartz
10Ca
2
(Mg,Fe)
3
Al
4
Si
6
O
22
(OH)
2
hornblende
4CaAl
2
Si
2
O
8
2H
2
O
anorthite
The anorthite produced in this reaction combines with
existing albite to yield oligoclase. Hence, most meta-
basites will consist of hornblende plagioclase; with
isotropic fabric, this is an amphibolite rock type (Fig-
ure 14.6c). Minor amounts of almandine-rich garnet
may appear in some amphibolites (Figure 14.9) and,
transitional into the granulate facies, relatively calcic
basaltic protoliths will contain a minor amount of
clinopyroxene formed by initial breakdown of horn-
blende. A possible model reaction for production of
diopside albite (the latter entering plagioclase solid
solutions) involves quartz calcite (or wollastonite) as
reactants with Al-poor hornblende
18.18 NaCa
2
(Mg,Fe)
5
AlSi
7
O
22
(OH)
2
6SiO
2
hornblende quartz
3CaCO
3
→ 5Ca(Mg,Fe)Si
2
O
6
calcite diopside
NaAlSi
3
O
8
H
2
O 5CO
2
albite
Granulite Facies. The transition from the amphibolite
to granulite facies in metabasites (Figure 18.21c, d) is
gradational and results from continuous reactions, as in
lower-grade transitions. Hornblende and locally biotite
persist into the lower part of the granulite facies,
whereas only anhydrous mafic phases are stable in
the upper, highest T part of the facies (Figure 14.6d)
in plagioclase–pyroxene granofels. Low- to middle-
granulite-facies hornblendes in thin section tend to be
pleochroic brown in plane-polarized light, presumably
because of higher concentrations of Ti, rather than the
usual bluish-green of amphibolite-facies hornblendes.
Two reactions that could possibly consume horn-
blende, both Al-poor and Al-rich, producing plagio-
clase pyroxene Ca–Mg–Fe garnet, are
18.19 NaCa
2
(Mg,Fe)
5
AlSi
7
O
22
(OH)
2
3SiO
2
hornblende quartz
→ NaAlSi
3
O
8
2Ca(Mg,Fe)Si
2
O
6
albite diopside
3(Mg,Fe)SiO
3
H
2
O
hypersthene
18.20 Ca
2
(Mg,Fe)
3
Al
4
Si
6
O
22
(OH)
2
→
hornblende
Ca(Mg,Fe)
2
Al
2
Si
3
O
12
CaAl
2
Si
2
O
8
garnet anorthite
(Mg,Fe)SiO
3
H
2
O
hypersthene
In more mafic rocks, anthophyllite breaks down to
yield orthopyroxene quartz water according to
18.21 (Mg,Fe)
7
Si
8
O
22
(OH)
2
→ 7(Mg,Fe)SiO
3
SiO
2
H
2
O
Orthopyroxene is stable with clinopyroxene and, with
the disappearance of hornblende near the middle of
the facies, orthopyroxene is compatible with andesine
or perhaps labradorite plagioclase (Figure 18.21d). At
higher pressures in the granulite facies, this compat-
ibility is broken and is replaced by garnet clinopyro-
xene (Figure 16.9).
Granulite-facies rocks are more widespread in
Precambrian cratons (Section 19.6) than in the highest
T part of Phanerozoic Barrovian terranes. Locally,
especially in shear zones where fluids have infiltrated,
granulite-facies rocks are variably retrograded to
amphibolite- and greenschist-facies assemblages. Many
granulite-facies metabasites and those in the higher T
amphibolite facies will have migmatitic leucosomes of
sodic plagioclase and quartz produced by dehydration
partial melting (Figure 11.21).
18.4 OCEAN-RIDGE METAMORPHISM
Before proceeding onto high-P metamorphism in near-
trench terranes we make a brief digression to describe
the nature and products of the metamorphism that oc-
curs at ocean ridges. Interaction between cold seawater
and hot mafic–ultramafic rocks in the 65 000-km-long
oceanic spreading-ridge system is likely the largest
active metamorphic system on the planet (Humphris
et al., 1995). It is metasomatism on a grand scale! The
entire mass of the world’s oceans is circulated through
the seafloor about once every 5 My (Special Interest
Box 11.3). Because of seafloor spreading, the metaso-
matic products cover a major part of the planet and,
therefore, in terms of area, are the most extensive
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones
589
of any metamorphic terrane. Initial studies in the
1950s and 1960s obtained random rock samples by
dredging of the seafloor from ships. With later devel-
opment of shipboard drilling to depths as much as
2 km into the oceanic crust (in 1992) and by use of
manned and remotely controlled submersibles, strati-
graphically and structurally controlled sampling be-
came possible. With submersibles, visual observations
of “field” relations also became a reality. Thick sections
of oceanic crust (Figure 13.1) and the underlying upper-
most mantle are exposed in fault scarps in the axial
regions of spreading ridge systems and transform-fault
scarps on ridge flanks. Recrystallized gabbroic and
variably serpentinized peridotitic rocks, some myloni-
tized, are found at slow-spreading ridges such as the
Mid-Atlantic.
The most accessible products of ocean-ridge meta-
morphism are exposed as ophiolite (Section 13.6) on
land along margins of overriding plates in subduction
zones. Although the nature of ophiolites provides
important insights into the geology of the oceanic
lithosphere, particularly of the deeper crust and mantle
interface inaccessible by drilling on the seafloor, most
ophiolites have experienced subsequent overprinting
metamorphism and tectonic dismemberment after leav-
ing their ocean source during emplacement. In addi-
tion to these processes that obscure primary features, it
appears that most ophiolites are created in arc settings
at spreading back-arc basins (Figure 11.16) rather than
at divergent junctures of major plates in the open ocean.
Thus, caution must be exercised in drawing conclu-
sions about processes on the seafloor from ophiolite.
18.4.1 Petrology of Metamorphosed Seafloor Rocks
Variable degrees of recrystallization and metasomatism
at spreading ridges convert oceanic basaltic rocks
(MORB) into low-grade greenstones, spilites, and less
common amphibolites (Sections 14.2.3 and 15.2.8).
Disequilibrium mineral associations are typical. Recon-
stitution is patchy, even on the scale of a thin section,
because of focusing along fractures. Veins up to several
centimeters in width are widespread and consist of
combinations of quartz, chlorite, epidote, and calcite.
The mineralogically simple, even monomineralic, vein
assemblages reflect development by metasomatic
processes and transport of dissolved ions in hydrother-
mal fluids. Locally, vein assemblages are out of equilib-
rium with the mineralogical composition of the host,
indicating continuing percolation of hydrothermal
solutions of different temperatures and compositions
after reconstitution of the host rock. Vesicles in sub-
marine basaltic lavas are filled with carbonates and
zeolites, producing amygdules. Other relict magmatic
fabrics are commonly preserved, including pillow rinds,
interpillow clastic fabrics, and variolitic, intergranular,
ophitic, and porphyritic textures. Rocks with discern-
ible primary fabrics can be classed as metabasalt, meta-
gabbro, and so on where appropriate. Although most
newly imposed fabrics are isotropic, rare mylonitic
samples reflect high-strain-rate deformation in fault
and shear zones.
Mineral assemblages in metasomatized oceanic
crust are like those of on-land zeolite, greenschist, and
possibly prehnite–pumpellyite facies. Assemblages that
are transitional or that rightfully belong to the amphi-
bolite facies are less common and tend to occur in
metamorphosed diabases and gabbros rather than
basalts, as might be anticipated because of higher
temperatures in the deeper oceanic crust. In a spatial
context (Figure 18.22), mineral zonation is telescoped
together as a result of steep geothermal gradients. The
inferred gradient in the relatively thin crustal segment
represented in the East Liguria, Italy, ophiolite se-
quence was 900–1300°C/km, 5–8 times the highest
gradients (170°C/km) measured at active oceanic rifts.
A more modest gradient of 150°C/km is inferred for
the Troodos, Cyprus, ophiolite.
Strictly speaking, it is inappropriate to refer to the
mineral assemblages formed just beneath the seafloor
as belonging to the same facies as in continental ter-
ranes because of the lower pressures and the strongly
telescoped and overlapping index assemblages. Together
with the lack of pervasive reconstitution, some petro-
logists feel justified in referring to the seafloor pro-
cesses as alteration rather than metamorphism.
Similar tightly telescoped mineral zonations associ-
ated with steep geothermal gradients in shallow con-
tinental settings occur in active geothermal areas
such as in Iceland, New Zealand, and the Salton Sea
area of California and adjacent Mexico (Special Inter-
est Box 18.1).
Exchange of chemical constituents in ocean-ridge
hydrothermal systems clearly justifies the metasomatic
designation. Seawater is highly oxygenated relative to
ocean-ridge basalts and contains appreciable concen-
trations of Cl
, Br
, HCO
3
, CO
3
2
, and SO
4
2
. As
seawater advects (Figure 18.23) into the relatively re-
duced magmatic rocks, chemical reactions such as the
following oxidize the rock:
18.22 Fe
2
SiO
4
1
/
2
O
2
→ Fe
2
O
3
SiO
2
in olivine in seawater hematite silica
18.23 19Fe
2
SiO
4
4SO
4
2–
→ 18Fe
2
O
3
in olivine in seawater hematite
2FeS
2
19SiO
2
pyrite silica
Reactions such as these that form Fe-oxides and sulfides
also release large amounts of silica, contributing to the
formation of deep marine cherts so typically associated
590 Igneous and Metamorphic Petrology
with ophiolite sequences. Farther along its path of cir-
culation through hot crustal rocks, seawater becomes
more reduced and large amounts of metals such as
Mn, Fe, Co, Ni, Cu, Zn, Ag, and Au are leached from
the mafic rocks and pass into solution as chloride
complexes. Depletion of these elements in spilites has
already been noted (Section 15.2.8). These hot (as much
as 350°C) chloride brines are discharged into the over-
lying cold seawater as “black smokers” (Figure 11.17),
the dissolved metals precipitating as black sulfides and
hydrated ferric and manganese oxides. Precipitates
coat lavas and sediment and form chimneys as much
as 40 m high immediately over hydrothermal vents.
Hundreds of fossil vents have been found in the ac-
creted Mesozoic Franciscan terrane in northern coastal
California.
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones
591
Red metapillow
lavas
Blue-green
metapillows
Yellow-green
metapillows
Metadiabase
feeder dikes
Metapillow
lavas
Metadiabase
feeder dikes
E. LIGURIA
TROODOS
Analcime
Heulandite
Stilbite
Mordenite
Laumontite
Smectite
Celadonite
Albite
Chalcedony
Sphene
Quartz
Chlorite
Actinolite
Epidote
Hornblende
Pumpellyite
Calcite
Hematite
Magnetite
Thickness (m)
300
Thickness (m)
700 1000
0 200
Temperature of
formation (°C)
400
18.22 Mineral assemblages and inferred metamorphic temperatures in the pillow lava and sheeted feeder dike members of the Troodos
and East Liguria ophiolites (thicknesses not to scale). Note the greater thickness of the observed metamorphic effects in the Troodos
ophiolite. Compare Figure 13.1. Redrawn from Gass and Smewing (1973) and Spooner and Fyfe (1973).
Sediment
Submarine
pillow
lavas
Sheeted
feeder
dikes
Gabbroic
rocks
Zeolite
facies
Greenschist
facies
(Rare
amphibolite
facies)
Fresh magmatic
rock
Chert, Mn–Fe oxides,
sulfides
Ocean
Cold oxygenated sea water
Hot reducing brine
containing Si, Fe,
Mn, Ni, Z, etc
containing Cl
−
,
CO
3
2−
, SO
4
2−
, etc.
Perturbed
isotherm
18.23 Conceptual model of convective circulation of seawater through hot oceanic crust. Influx of cold water depresses isotherms, whereas
outflow of hot brines elevates isotherms. The T of alteration increases rapidly with depth into the highly fractured pillow lavas and
sheeted dikes, but limited access of seawater below the dikes essentially precludes reconstitution of underlying hotter gabbroic and
ultramafic rocks. Compare Figure 13.1.
18.5 INTACT SLABS OF OPHIOLITE
At convergent plate junctures, segments of oceanic
lithosphere are emplaced on land in essentially two
contrasting manners—as large, relatively intact slabs
and as much smaller dismembered scraps tectonically
mingled with other rocks in accretionary wedges.
Intact slabs of ophiolite are described here and the
mélange association in Section 18.6.2.
Unlike widespread alpine peridotites that were
emplaced tectonically as typically cold bodies, some
large, relatively intact slabs of ophiolite were hot when
emplaced on land and produced significant dynamo-
thermal metamorphism in the immediately underlying
metamorphic sole. For example, mineral thermo-
barometers in the early Paleozoic White Hills peridotite
at the northern tip of Newfoundland (Figure 13.26)
indicate equilibration at 950°C and 7–10 kbar. The
metamorphic sole beneath it is as much as several hun-
dred meters thick and is composed of tightly folded
and locally mylonitic mafic and ultramafic tectonites
(Jamieson, 1980). The intensity of metamorphism
declines progressively below the peridotite slab and
temperatures decrease from 860°C immediately under
the sheared peridotite to about 360°C 800 m below the
contact. (A similar inverted metamorphic zonation
occurs in the Mount Everest region of the Himalaya
Orogen beneath the Main Central Thrust (Figure 13.33).
England and Molnar (1993) have investigated the ther-
mal aspects of this inverted metamorphic regime.)
Semail Ophiolite, Oman. At 550-km-long by 150-km-
wide (Figures 18.24 and 18.25), this is the largest, and
best preserved, thrust sheet of oceanic lithosphere ex-
posed on Earth (Searle and Cox, 1999). Volcanic rocks
overlying the ophiolite have an arc chemical signature,
indicating its origin in a spreading back arc (Figure
11.16). Beneath the 4–7 km of oceanic crust and under-
lying 8–12 km of harzburgite tectonites that repre-
sent residual mantle is an unusually well developed
metamorphic sole. Where exposed in one window
eroded through the ophiolite, oceanic protoliths in the
sole are basalt and minor manganiferous chert and
limestone that are believed to have been heated as they
descended to a depth of about 20 km on a northeast-
dipping subducting slab. As these oceanic protoliths
were tectonically emplaced during the Cretaceous onto
the continental margin with the attached overlying
ophiolite, a tightly telescoped, inverted dynamothermal
metamorphic zonation was overprinted during intense
ductile shearing. The resulting tectonites consist of
80 m of garnet–clinopyroxene-bearing amphibolite
underlain across a sheared contact by a variety of
greenschist-facies rocks that in turn are underlain
below a brittle thrust fault by unmetamorphosed rocks.
Elsewhere, the sole consists of granulite-facies rocks
that include continental quartz–sandstone and lime-
stone protoliths. P–T paths for sole rocks are tight
counter-clockwise loops (Figure 18.26) consistent with
rapid heating by the overlying 1100–1200°C mantle
harzburgite followed by cooling and decompression
592 Igneous and Metamorphic Petrology
AFRICAN
PLATE
Red Sea
Sea
Crete
Cyprus
Mediterranean
Alps
Black
Sea
Caspian
Sea
PLATE
Oman
Arabian
Sea
EURASIAN
Atlantic
Ocean
Ophiolite
Thrust-fault zone
0 800km
0 500miles
18.24 Generalized map of the Mediterranean area showing major thrust-fault zones (teeth on overriding plate) and ophiolite belts in the
complex suture zone between the mostly continental Eurasian and African Plates. Redrawn from Michard et al. (1995) and Twiss and
Moores (1992).
during emplacement. Local dikes intruded into the
ophiolite are of peraluminous granite that contains two
micas, garnet, andalusite, and cordierite; they likely
originated as partial melts of pelitic continental rocks
in the underlying sole. In eastern Oman, the upwarped
Saih Hatat Nappe beneath the Semail ophiolite (Figure
18.25a) exposes a sequence of high P/T metasediments
and blueschist- and eclogite-facies metabasalts and
pelites. These represent deeply subducted (70–80 km)
and subsequently exhumed continental-margin rocks
that define clockwise P–T paths (Figure 18.26).
The Semail ophiolite provides important insights
regarding what can happen—at least in one locale—
in a subduction zone where a continental plate is
underthrust beneath a spreading back-arc. The leading
edge of the ophiolite now lies 120 km southwest of
the known northeast limit of the Arabian continental
crust. Reconstruction of former positions of thrust
sheets exposed beneath the ophiolite lead Searle and
Cox (1999) to believe that the ophiolite “realistically”
moved more than 400 km horizontally onto the con-
tinental margin from its place of origin in a spreading
back-arc basin. This astonishing obduction of the
oceanic lithosphere occurred as the thin Arabian con-
tinental margin followed its attached northeastward
subducting oceanic plate to at least 70–80 km depth
beneath the overriding back arc basin. Subsequent
rapid, nearly isothermal exhumation of the subducted
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones
593
(a)
(b)
km
0
10
20
Hawasina nappes;
mélange in Eurasia
Blueschist and
eclogite facies
Ophiolite
Thrust fault; teeth on
upper plate
Arabian
platform
56°E57° 58°
Saih
Hatat
Gulf of
Oman
Eurasia
55°E
26°N
24°
22°
62°E
58°
0 150miles
0 200km
S
E
M
A
I
L
O
P
H
I
O
L
I
T
E
N
S
Tertiary
rocks
Hawasina
nappes
SEMAIL OPHIOLITE Metamorphic sole
Saih Hatat
Gulf of
Oman
N
Arabian
Craton
Mesozoic
Permian
0 100km
050miles
18.25 Geologic map and cross-section of the Semail Ophiolite in the Sultanate of Oman. Redrawn from Michard et al. (1995). (a) Generalized
geologic map of Oman in the Arabian Peninsula and southeastern Iran in Eurasia showing ophiolites and related near-trench rock
associations. (b) North–south cross-section across the eastern end of the Semail Ophiolite; section line (]) shown in (a). Metamorphic
sole (black) beneath the ophiolite is schematic. The ophiolite slab is large remnant of a back-arc oceanic basin that was underthrust by
the subducting margin of the Arabian continental.
facies) metasedimentary rocks; a central belt of tecton-
ically mixed rocks of diverse origins and histories; and
an eastern belt of variably deformed but coherent
sheets of metasedimentary rock of the blueschist facies
several kilometers thick and tens of kilometers along
strike.
Bedded to massive nonbedded sandstones, silt-
stones, and mudrocks and their variably metamor-
phosed equivalents constitute most of the Franciscan
terrane. Immature (feldspathic and lithic) terrigenous
sandstones, commonly called graywackes, and finer
clastic rocks are composed of erosional debris shed off
a neighboring volcanic arc and deposited into the
trench; some limestone, chert, and mudstone were
scraped off the seafloor into the trench during subduc-
tion. Clay-rich masses are largely represented by tec-
tonically mixed mélange (see below). Although some
graywackes are thoroughly recrystallized schists, most
metagraywackes reveal little or no pervasive defor-
mation and have well preserved relict clastic fabric
(Figures 14.17a and 18.27) despite locally containing
metamorphic mineral assemblages. Both sheared and
undeformed massive metagraywackes with obvious
clastic fabric contain zeolite veins and patchy replace-
ments of detrital feldspar grains in lowest-grade rocks
of the zeolite facies. Prehnite, pumpellyite, epidote,
and albite appear in higher grade metagraywackes of
the prehnite–pumpellyite facies. Lawsonite, jadeitic py-
roxene with quartz, and glaucophane appear in
blueschist-facies rocks. Many of the critical minerals
in all metamorphic grades are easily overlooked even
in thin section because of their fine grain size.
Other protoliths include locally thick sequences of
marine chert and basaltic rocks. Metamorphosed
cherts are thinly bedded and made of granoblastic
aggregates of quartz hematite Fe–Mn garnet
(spessartine) riebeckite piemontite (Mn-epidote).
Basaltic rocks make up approximately 10% of the
Franciscan Complex. Most are weakly metamorphosed
greenstones. Glaucophane-bearing rocks constitute
1% of the Franciscan. In addition to its presence as
disseminated grains in metagraywackes, glaucophane
also occurs in schists, some of which are metabasalts,
with combinations of lawsonite, quartz, pumpellyite,
epidote, almandiferous garnet, jadeitic pyroxene, and
aragonite; quartz veins are common. Coarse-grained
glaucophane occurs in rounded blocks a meter or so in
diameter with sheared margins locally of actinolite and
talc lying in a matrix of serpentinite.
18.6.2 Mélange
The central belt of the Franciscan Complex that ex-
tends for hundreds of kilometers along strike and as
much as tens of kilometers wide is a chaotic terrane
of a type of tectonic breccia called mélange (French
for mixture). It is a collage of blocks of diverse size,
594 Igneous and Metamorphic Petrology
P (kbar)
Depth (km)
30
25
20
15
10
5
0
90
80
70
60
50
40
30
20
10
0
0 200 400 600 800 1000
T (°C)
HIGH
P/T
ROCKS
Eclogite-
facies rocks
S
O
L
E
R
O
C
K
S
A
m
p
h
i
b
o
l
i
t
e
facies
G
r
a
n
u
l
i
t
e
18.26 P–T paths of rocks associated with the Semail ophiolite,
Oman. Dashed line paths are for rocks in the thin metamor-
phic sole immediately below the ophiolite. Full line paths are
for rocks in thick thrust sheets below the ophiolite in eastern
Oman in the Saih Hatat nappe. Redrawn from Searle and Cox
(1999).
slab (Figure 18.26) likely resulted from buoyancy-
driven rebound.
The extent to which the obduction of the Oman
ophiolite provides insights into the emplacement of
other large, relatively intact ophiolite sheets, such as
the Josephine in northern California and the Brooks
Range in Alaska (Harris, 1998), remains to be seen.
These also appear to have formed near the continental
margins that they now overlie because the crystalliza-
tion ages of the ophiolite and metamorphic sole are not
very different.
18.6 NEAR-TRENCH METAMORPHIC
ASSOCIATIONS
Near-trench rock associations crop out between the
deep oceanic trench and the magmatic arc in the
accretionary prism or wedge (Figure 14.24). Here,
accreted segments of island arcs and seafloor rocks,
including seamounts and plateaus (Section 13.3.1), are
intimately mingled with terrigenous (land derived) sedi-
mentary rocks and metamorphic rocks of the high P/T
zeolite, prehnite–pumpellyite, blueschist, and eclogite
facies series. These strikingly contrasting rocks typify
young Mesozoic and Cenozoic orogens around the
margin of the Pacific Ocean and in the Mediterranean
region.
18.6.1 The Franciscan Complex, California
The classic accretionary-wedge terrane is the late
Mesozoic Franciscan Complex in northern coastal Cali-
fornia (Figure 18.5). It can be conveniently divided
into three belts: a western belt along the coast that con-
sists of coherent and weakly metamorphosed (zeolite-
composition, metamorphic character, and source—all
tectonically mingled together by subduction processes
in the accretionary wedge. A significant component is
highly dismembered scraps of oceanic lithosphere—
the second, and more common, occurrence of ophio-
lite. Blocks range in size downward from kilometers
and lie within a pervasively deformed matrix. Illustra-
tions of mélange are shown in Figures 18.28 and 18.29;
further examples and description can be found in
Evans and Brown (1986).
Mélange obviously involves pervasive brecciation
of large volumes of different sorts of rock that is
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies, and Zones
595
18.27 Relict bedding in jadeitic metagraywacke exposed in roadcut near Pacheco Pass, 130 km southeast of San Francisco, California. US
Geological Survey photograph courtesy of E. H. Bailey.
Glaucophane
lawsonite
quartz schist
Serpentinite
Eclogite and
retrograded
eclogite
Greenstone
(a)
1000m
3000ft
60m
200ft
(b)
18.28 Schematic map views of mélange in the Franciscan Complex, northern California Coast Ranges. (a) Distribution of rock types in a small
area 3 km southwest of Healdsburg. Redrawn from Borg (1956). Each symbol represents an outcrop, usually several meters across. Other
than for serpentinite, it is not possible to draw contacts between rock types because of their chaotic distribution. The terrane is inter-
preted to be a mélange that, if wholly exposed, might appear something like the sketch in (b), showing exotic heterogeneous blocks in a
sheared matrix. Usually, only the more erosionally resistant blocks—locally called “knockers”—are exposed, whereas the softer matrix
in which they are embedded is covered by soil and vegetation. Compare with Figure 18.29.
chaotically mixed in some way. Most blocks are
lenticular (Figure 18.28b) and many resemble brittle
boudins in a ductilely deformed matrix whose pelitic
mineral assemblages suggest equilibration at 250°C
and 8 kbar. Diverse blocks include chert, graywacke,
dismembered and variably metamorphosed ophiolite
(serpentinite, peridotite, metagabbro, metabasalt,
amphibolite), eclogite, and rocks of the blueschist
facies. More erosionally resistant blocks, locally called
“knockers,” typically constitute the only outcropping
rock.
Cloos (1982) believes the properties of mélange re-
sult from large-scale flow where subducting oceanic
slabs enter the mantle. Down-dragged rocks and
accreted sediment can only go so far into the dense
mantle before buoyancy forces a return flow. During
downward and return flow, a variety of rocks plucked
from different depths are incorporated into the moving
mass. Mixing is enhanced because of differences in
block size and density in the nonuniform velocity field
(compare the dynamically similar grain dispersive
pressure in Section 8.2.3). Typical oblique convergence
of lithospheric plates causes blocks to take a helical
trajectory along the strike of their juncture.
18.6.3 Serpentinite
Serpentinite (Section 15.2.7) exposed in near-trench
accretionary prisms has formed by hydration of ultra-
mafic rocks from different sources in one or more
environments. Multiple metamorphic overprinting is
common. At oceanic spreading ridges and cross-
cutting transforms, serpentinite is exposed in sub-
marine fault scarps. Protoliths can be ultramafic
cumulates formed in crustal magma chambers and
underlying tectonized mantle peridotite (Figure 13.1)
hydrated by seawater that gained access along frac-
tures. Serpentinites that have
18
O values consistent
with hydration by seawater can be emplaced on
land with less hydrated ultramafic lithosphere. Serpen-
tinization and deformation can also occur during and
after emplacement of this ophiolite. Higher grade ultra-
mafic assemblages of anthophyllite, talc, tremolite,
and chlorite are commonly partially retrograded to
greenschist-facies assemblages, including serpentine.
These rocks are believed to represent hydrated peri-
dotite in the mantle wedge overlying subducting
oceanic lithosphere that was retrograded during uplift
and cooling (Figure 11.16).
596 Igneous and Metamorphic Petrology
18.29 Franciscan mélange exposed in wall of quarry near Greenbrae, 16 km north of San Francisco, California. Blocks of graywacke, green-
stone, tuff, and chert lie in a matrix of sheared shale. Exposure partly covered by regolith and disturbed by local minor landslides. US
Geological Survey photograph courtesy of E. H. Bailey.