Page 287 - Global Tectonics

272 CHAPTER 9

5
Basalt Basaltic Andesite Dacite and Rhyolite
andesite Shoshonite series
4 Calc - alkaline and High-K
tholeiitic series
K 2 O (wt %) 3 2 Med-K

1
Low-K
0
48 50 52 54 56 58 60 62 64 66 68 70 72 74 76
SiO (wt %)
2
Izu - Bonin Shoshonitic province Marianas
Figure 9.24 Potassium-silica diagram for the mean composition of 62 volcanoes collected along the Izu–Bonin–

Mariana arc system (modified from Stern et al., 2003, by permission of the American Geophysical Union. Copyright ©
2003 American Geophysical Union).

to describe large composite bodies of plutonic rock that (1981), Davidson (1983), and Hilton & Craig (1989),
were created above ocean–continent subduction zones. among others, demonstrated that certain isotopic ratios
The majority of these batholiths are composed of hun- require a large contribution from continent-derived
dreds to thousands of individual intrusions that range sediments. Consequently, sediments from the trench
in composition from gabbro, tonalite and diorite to must be carried down the subduction zone and incor-
granodiorite and granite. Compositional similarities porated into the asthenospheric melt (Plank & Lang-
among many plutonic and some nearby volcanic rocks muir, 1993). Most authors have concluded that the
suggest that the former represent the crystallized igneous crust of the subducting lithosphere contributes
residua of deep magma chambers that once fed shallow only very small amounts of melt, except, possibly, in
parts of the system. Their exposure in mature arcs special circumstances where young, hot lithosphere is

results from prolonged periods of uplift and erosion. subducted or warmed by mantle flow (Plate 9.3 between
One important, and highly controversial, area of pp. 244 and 245). In this latter case, distinctive
research centers on the origin of the magmas supplying melt compositions such as adakites may be produced
volcanic and plutonic complexes. Certainly the genera- (Johnson & Plank, 1999; Yogodzinski, 2001; Kelemen
tion of the magmas must be linked in some way to the et al., 2003).
Benioff zone, as there is a very strong correlation A major problem of arc magmatism is the source of
between its depth and the systematic variation in the heat required for melting the asthenosphere above
volcanic rock composition and elemental abundances. the descending slab. It was originally believed that this
Early models (e.g. Ringwood, 1975) suggested that the was derived solely by shear heating at the top of the
magmas were derived from melting of the top of the slab. However, this is unlikely because the viscosity of
descending oceanic slab. However, this idea subse- the asthenosphere decreases with increasing tempera-
quently was rejected as a general model, in part because ture, and at the temperatures required for partial fusion
thermal models indicate that subducted lithosphere the asthenosphere would have such a low viscosity that
rarely becomes hot enough to melt (Peacock, 1991). In shear melting could not occur. Ringwood (1974, 1977)
addition, petrologic and mineralogic evidence (Arculus suggested that partial melting takes place at a relatively
& Curran, 1972) and helium isotope ratios (Hilton & low temperature because of the high water vapor pres-
Craig, 1989) indicate that the parental magmas origi- sure resulting from the dehydration of various mineral
nate by partial melting of asthenospheric mantle imme- phases in the downgoing slab. Indeed, the greater the
diately overlying the descending plate. Karig & Kay amount of water present, the more the melting tem-