Majorelement chemistry of arc magmatic rocks

As could be expected from the heterogeneity of the matter in the descending slab and surrounding mantle region and the complexity of the processes occurring there, subduction-related magmatism generates a great variety of different rock types, including basalts (some of which are indistinguishable from MORBs), rocks of broadly granitic composition (typical of continental crust) and a variety of rocks of intermediate compositions. Volumetrically, basalts (with SiO2 < 54%) are the most abundant rocks, followed by andesites (with SiO2 > 54%). Below only primitive basalts and andesites are discussed, i.e. those with magnesium index mg# > 60. The magnesium index generally decreases as a melt evolves, and the most primitive rock types come closest to the primary-melt composition (Kelemen et al., 2003).

Table 24.1 shows that the SiO2 content of arc basalts is indistinguishable from that typical of MORBs, and other oxide contents are similar, except for a somewhat higher K2O content.

Primitive andesites are found almost exclusively in arcs (both continental and oceanic), and they are particularly frequent in arcs situated above young, slow-subducting, plates. Therefore they are indicative of "fossil arcs" in older geological provinces (Gill, 1981). Primitive andesites, besides having (by definition) an enhanced SiO2 content relative to basalts, also show substantially higher concentrations of the alkali metals K and Na but are underabundant in CaO, MgO and FeO (Table 24.1). Therefore in K2O-Na2O, FeO-Na2O and CaO-Na2O concentration plots, the fields of basalts and andesites overlap only slightly (see the corresponding figures in Kelemen et al., 2003).

The concentrations of major elements in andesites and in the mean continental crust are similar, implying that potentially these rocks could be important as crustal precursors. Even though they appear not to be abundant enough at the surface (at least at present) to represent a principal component of crustal growth, a considerable portion of andesitic magma could have solidified deeper in the continental crust. These magmas are more viscous than basaltic magmas; they are also less hot and can "freeze" in the crust as hydrous fluid exsolves and escapes, increasing the solidus temperature (Kelemen et al., 2003).

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