Page 290 - Global Tectonics

SUBDUCTION ZONES 275

across the central Andes (Fig. 10.7) suggests the pres- release water into the upper mantle wedge (Section 9.8)

ence of fluids, including partial melt, at 20–30 km depth and increase the density of the subducting slab. These
beneath the volcanic arc (ANCORP Working Group, reactions involve specifi c metamorphic transformations

2003). that reflect the abnormally low geothermal gradients
−1
Measurements of disequilibria between short-lived (10°C km ) and the high pressures associated with the
uranium series isotopes in island arc lavas have sug- subduction zone environment (Section 9.5).
gested that melt ascent velocities from source to surface Prior to its subduction, oceanic basalt may exhibit
3
−1
can be extremely rapid (10 m a ) (Turner et al., 2001). low pressure (<0.6 GPa)/low temperature (<350°C)
Such rates are much too fast for ascent to occur by metamorphic mineral assemblages of the zeolite and
grain-scale percolation mechanisms. Instead, melts prehnite-pumpellyite facies (Fig. 9.26). In some places
probably separate into diapirs or form networks of low greenschist facies minerals also may be present. In

density conduits through which the flow occurs, either basalt, this latter facies typically includes chlorite,
as dikes or as ductile shear zones. There is general epidote and actinolite, which impart a greenish color to
agreement that deformation greatly enhances the rate the rock (see also Section 11.3.2). This type of alteration
of magma ascent. Laboratory experiments conducted of basalt results from the circulation of hot seawater in
by Hall & Kincaid (2001) suggest that buoyantly upwell- hydrothermal systems that develop near ocean ridges
ing diapirs of melt combined with subduction-induced (Section 6.5).
deformation in the mantle may create a type of chan- As the altered basalt descends into a subduction
nelized flow. Predicted transport times from source zone, it passes through the pressure–temperature fi eld

regions to the surface by channel flow range from tens of the blueschist facies (Fig. 9.26), which is characterized

of thousands to millions of years. It seems probable that by the presence of the pressure-sensitive minerals
a range of mechanisms is involved in the transport of glaucophane (a sodic blue amphibole) and jadeite (a
magma from its various sources to the surface. pyroxene). A transitional zone, characterized by the
The emplacement of plutons and volcanic rock presence of lawsonite, also may occur prior to the trans-
within or on top of the crust represents the fi nal stage formation to blueschist facies. Lawsonite is produced at
of magma transport. Most models of magma emplace- temperatures below 400°C and at pressures of 0.3–
ment have emphasized various types of deformation, 0.6 GPa (Winter, 2001), conditions that are not yet high
either in shear zones (Collins & Sawyer, 1996; Saint enough to produce glaucophane and jadeite. Lawson-
Blanquat et al., 1998; Brown & Solar, 1999; Marcotte ite, along with glaucophane and other amphibole min-
et al., 2005) or in faults (e.g. Section 10.4.2), fractures erals, is an important host for water in subducting ocean
and propagating dikes (Clemens & Mawer, 1992; Daczko crust.
et al., 2001). Some type of buoyant flow in diapirs also One of the most important metamorphic reactions

may apply in certain settings (e.g. Section 11.3.5). resulting in the dehydration and densification of sub-
Various mechanisms for constructing plutons and ducting oceanic crust involves the transformation from
batholiths are discussed by Crawford et al. (1999), the blueschist facies to the eclogite facies (Fig. 9.26).
Petford et al. (2000), Brown & McClelland (2000), Miller Eclogite is a dense, dry rock consisting mostly of garnet
& Paterson (2001b), and Gerbi et al. (2002), among and omphacite (i.e. a variety of clinopyroxene rich in
many others. sodium and calcium). The exact depth at which eclogite
facies reactions occur depends upon the pressures and
temperatures in the subducting oceanic crust (Peacock,
9.9 METAMORPHISM 2003). In relatively cool subduction zones, such as in
northeast Japan (Plate 9.3a between pp. 244 and 245),
AT CONVERGENT the transformation may occur at depths of >100 km
(Fig. 9.31). In relatively warm subduction zones, such
MARGINS as in southwest Japan (Plate 9.3b between pp. 244 and
245), the transformation may occur at depths as shallow
as 50 km (Fig. 9.27). This transformation to eclogite
enhances the negative buoyancy of the descending
As oceanic basalt is subducted at convergent margins, lithosphere and contributes to the slab-pull force acting
it undergoes a series of chemical reactions that both on the subducting plate (Section 12.6).