Page 288 - Global Tectonics

SUBDUCTION ZONES 273

perature of the mantle is reduced (Stolper & Newman, material within the mantle wedge (Sisson & Bronto,
1994). Thus, water acts as a primary agent that drives 1998). If hot material rises quickly enough so that little
partial melting beneath arcs. heat is lost, the reduction in pressure may cause pres-
It is thought that as much as half of the water carried sure release or decompression partial melting (see also
down into a subduction zone is released below the Section 7.4.2).
forearc region, partly into the crust, and also into the A detailed study of the depth to the zone of seismic-
mantle producing serpentinite (Fig. 9.19) (Bostock et al., ity and, hence, to the lithospheric slab directly beneath
2002). Most of the water carried to great depths is arc volcanoes has shown that, although these depths are
sequestered in hydrous minerals in altered and meta- consistent within a particular arc, they vary signifi cantly
morphosed crust, including serpentinite. With in- from arc to arc within a range of 65–130 km (England
creasing pressure, hydrous basalt and gabbro are et al., 2004). Surprisingly these depths correlate not with
metamorphosed progressively to blueschist, then the age or rate of underthrusting of the subducting
amphibolite, and then eclogite (Section 9.9). At each lithosphere but inversely with the vertical rate of
transformation water is released. Serpentinite is par- descent of the slab. England & Wilkins (2004) suggest
ticularly effective in transporting water to great depth, that a high rate of descent increases the rate of fl ow in
but the extent to which the subducting lithosphere is the mantle wedge and hence the rate at which hot
serpentinized is unclear. Fast spread oceanic crust is mantle is drawn towards the corner of the wedge. This
thought to contain little or no serpentinite, but slow would produce higher temperatures, and hence melting,
spread crust is known to contain some, perhaps as much at a shallower depth than in the case of slow rates of
as 10–20% (Carlson, 2001). However, as described above descent.

(Section 9.4), the lithospheric mantle in the downgoing Where sufficient partial melting occurs, probably 10
slab may be hydrated to a depth of tens of kilometers ± 5% (Pearce & Peate, 1995), the melt aggregates and
as a result of the normal faulting associated with the begins to rise toward the base of the crust. As the
bending of the plate as it approaches the subduction magma moves into the crust it differentiates and may
zone. mix with either new, crust-derived melts or older melts,
A generalized model of arc magmatism begins with eventually forming the magmas that result in the calc-
the subduction of hydrated basalts beneath continental alkaline and alkaline series (Fig. 9.25). In the context of
or oceanic lithosphere (Fig. 9.3). As the slab sinks continental arcs, the generation of crust-derived melts
through the mantle, heat is transferred to it from the appears to be common because the melting point of
surrounding asthenosphere and the basalt in the upper continental crust may be low enough to result in partial
part of the slab begins to dehydrate through a series of melting. Many of the Mesozoic Cordilleran-type batho-
metamorphic mineral reactions (Sections 9.4, 9.9). Sed- liths of western North America (Tepper et al., 1993), the
iments that have been subducted along with the basalt Andes (Petford & Atherton, 1996), West Antarctica
also dehydrate and may melt due to their low melting (Wareham et al., 1997), and New Zealand (Tulloch &
temperatures. The release of metamorphic fl uids from Kimbrough, 2003) contain chemically distinctive plutons
the slab appears to be quite rapid, possibly occurring in that are thought to have originated from the partial
as little as several tens of thousands of years (Turner & melting of the lower continental crust. Tonalites, which
Hawkesworth, 1997). By contrast, the recycling of sub- are varieties of quartz diorite (see also Section 11.3.2),
ducted sediment into the upper mantle may be slow may be produced if the melting occurs at relatively high
(2–4 Ma). As heat is transferred to the slab, temperature temperatures (∼1100°C). Granodiorite may be pro-
gradients are established such that the asthenosphere in duced if the melting occurs at cooler temperatures
the vicinity of the slab becomes cooler and more viscous (∼1000°C) and in the presence of suffi cient quantities
than surrounding areas, particularly near the upper part of water. Melts that move through a thick layer of con-
of the slab. This more viscous asthenosphere is then tinental crust may become enriched in incompatible
dragged down with the slab causing less viscous mantle elements before reaching the surface. These magmas
to flow in behind it, as indicated in Fig. 9.3. It is the also may lose some of their water content and begin to

interaction of this downwelling mantle with aqueous crystallize, with or without cooling. This latter process
fluids rising from the sinking slab that is thought to results in volcanic rocks that are characteristically frac-

produce partial melting of the mantle. In addition, tionated, porphyritic, and wet. With time, the crust is
some melts may result from the upwelling of hot mantle thickened by overplating and underplating (Fig. 9.25).