Permalink : http://hdl.handle.net/2115/36738

Dredged andesites and dacites from the Ogasawara forearc contain high abundances of MgO at high SiO2, high K2O, low rare earth element abundances and flat patterns, and very low TiO2 contents. The chemical character and geologic setting support the interpretation that these lavas are evolved members of the boninite series formed by high degrees of partial melting of a previously depleted arc source, followed by enrichment in K, Th, and large-ion lithophile elements, and finally differentiated to higher SiO2 contents than are usually found in boninites. This is consistent with existing tectonic models of the Izu-Mariana arc system in which young, hot Philippine Sea crust produced high geothermal gradients in this region from the Tertiary.