Remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs: An example from the Izu-Bonin arc

The Izu-Bonin volcanic arc is an excellent example of an intra-oceanic convergent margin. A total of 1011 chemical analyses of 17 Quaternary volcanoes of the arc are reviewed to estimate relative proportions of magmas erupted. Basalt and basic andesite (SiO2 < 57 wt %) are the predominant eruptive products of the Izu-Bonin arc, and rhyolite (SiO2 > 70 wt %) forms another peak in volume. Such rhyolites possess compositions identical to those of partial melts produced by dehydration-melting of calc-alkaline andesites at low pressure (<7 kbar). Meanwhile, the major element variation of the Shirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula, completely overlaps that of the Quaternary Izu-Bonin arc volcanoes, and groundmasses of Shirahama Group calc-alkaline andesites have compositions similar to those of Izu-Bonin rhyolites. Moreover, phenocryst assemblages of calc-alkaline andesites of the Shirahama Group resemble restite phase assemblages of dehydration-melting of calc-alkaline andesite. These lines of evidence suggest that the rhyolite magmas may have been produced by dehydration-melting of calc-alkaline andesite in the upper to middle crust. If so, then the presence of large amounts of calc-alkaline andesite (3-5 times more abundant than the rhyolites) within the oceanic arc crust would be expected, which is consistent with a recently proposed structural model across the Izu-Bonin arc. The calc-alkaline andesite magmas may be water saturated, and would crystallize extensively and solidify within the crust. The model proposed here suggests that rhyolite eruptions could be triggered by an influx of hot basalt magma from depth, reheating and partially melting the calc-alkaline andesite component of the crust.