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SUBDUCTION ZONES 277

Samples of blueschist and eclogite obtained from orogens (Section 10.2.5). Both associations affect large
convergent margins (Section 9.7) provide important areas of the crust at convergent margins and thus refl ect
information on the physical and chemical conditions the large-scale thermal and tectonic disturbance associ-
that occur within and above subducting lithosphere. ated with subduction and orogeny.

Some of the first direct evidence of the conditions in The most common groups of rocks associated with
the vicinity of the subduction zone décollement beneath regional metamorphism belong to the greenschist,
a forearc has been provided by observations in the amphibolite and the granulite facies (Fig. 9.26). The
Mariana forearc. In this setting, large serpentine mud transition from greenschist to amphibolite facies, like
volcanoes up to 30 km in diameter and 2 km high occur all metamorphic reactions, is dependent on the initial
in the forearc slope above an erosive margin (Fig. 9.19). composition of the crust as well as the ambient

In addition to serpentine, the volcanoes erupt slab- pressure, temperature and fluid conditions. In meta-
derived fluids and blueschist facies clasts that record the morphosed basalt this transition may be marked by the

relatively cool temperatures of 150–250°C and pres- change from actinolite to hornblende as amphibole is
sures of 0.5–0.6 GPa (Maekawa et al., 1993). These able to accept increasing amounts of aluminum and
determinations are consistent with thermal models of alkalis at high temperatures (>500°C) (Winter, 2001). At
the slab–mantle interface where abnormally low geo- temperatures greater than 650°C amphibolite trans-
thermal gradients result from the rapid descent of cool forms into granulite. Granulites are highly diverse and
oceanic lithosphere at trenches (Section 9.5) and from may be of a low, medium or high pressure variety
low to moderate levels of friction (Peacock, 1992). (Harley, 1989). In general, granulite facies rocks are
Samples of material obtained by drilling the Mariana characterized by the presence of anhydrous mineral
mud volcanoes also provide evidence of the interac- assemblages such as orthopyroxene, clinopyroxene, and

tions among pore fluids, sediment and metamorphic plagioclase.
rock that occur in an accretionary prism (Fryer et al., If conditions at high temperatures are hydrous, then
1999). Similar material, known as sedimentary serpenti- migmatite may form. Migmatite is a textural term that
nite, occurs in blueschist facies metamorphic belts pre- describes a mixed rock composed of both metamorphic
served within continental crust. These belts commonly and apparently igneous material. Proposed mechanisms
are interpreted to represent the suturing of ancient con- for migmatite formation have included the partial
tinental margins following the consumption of an inter- melting of a rock, the injection of igneous (granitic or
vening ocean (Sections 10.4.2, 11.4.3). Blueschist also is tonalitic) material into a rock, and the segregation of
associated with ophiolitic suites (Ernst, 1973), lending silicate material from a host during metamorphic rather
support to the interpretation that some ophiolites than igneous activity. Migmatites are best developed in
formed in the forearc region of incipient subduction pelitic metasedimentary rocks, but also may occur in

zones (Section 2.5). mafic rocks and granitoids. Brown et al. (1999) describe
In addition to the low temperature/high pressure the structural and petrologic characteristics of migma-
type of metamorphism associated with subduction tite derived from pelitic and basaltic rocks. Suda (2004)

zones, some convergent margins also exhibit a type of summarizes the formation and significance of migma-
regional metamorphism characterized by high tem- tite in an intra-oceanic island arc setting. Klepeis et al.
peratures (>500°C) and low to moderate pressures. (2003) and Clarke et al. (2005) provide summaries of the
This type of metamorphism commonly is associated tectonic setting and possible interpretations of a high-
with the high geothermal gradients that characterize pressure (1.2–1.4 GPa) mafic granulite and associated

magmatic arcs. Index minerals in metamorphosed migmatite belt located in Fiordland, New Zealand.
sedimentary rocks, such as andalusite and sillimanite These latter rocks represent the hot, lower crustal root
(Fig. 9.26), provide evidence of high temperatures in of thick Cretaceous continental arc crust that has been
these regions. Temperature gradients of more than exhumed during subsequent tectonic activity.
−1
−1
25°C km up to about 50°C km result from the ascent Attempts to place the evolution of the high pres-
of magmas generated where aqueous fluids from the sure/low temperature and the high temperature/low

subducted slab infiltrate the mantle wedge (Section 9.8). pressure varieties of metamorphic rocks in the context

This type of metamorphism also is associated with the of subduction zone processes are common in the
high differential stresses, deformation, and crustal thick- scientific literature. One important early effort by

ening that accompany the formation of Andean-type Miyashiro (1961, 1972, 1973) led to the concept of