Petrology of a mantle-derived rhyolite, Hokkaido, Japan

The Miocene Kitami rhyolite, consisting of orthopyroxene and plagioclase-ph yric lavas and dikes, occurs on the back-are side of the Kuril are with coe val basalts and Fe-rich andesites. Temperatures estimated from orthopyroxen e-ilmenite pairs exceed 900 degrees C. Although the whole rock compositions of the Kitami rhyolite correspond to S-type granites (i.e., high K, Al, la rge ion lithophile elements, and low Ca and Sr), Sr-Nd isotope compositions are remarkably primitive, and similar to those of the coeval basalts and a ndesites. They are distinct from those of lower crustal metamorphic rocks e xposed in the area. Comparison of chondrite-normalized rare earth element ( REE) patterns between the rhyolite and the basalts and andesites show that the rhyolite is more Light REE enriched, but has similar heavy REE contents than the basalts. All rhyolites show negative Eu anomalies. The geochemica l data suggest that did not formed by simple dehydration melting of basalti c rocks or fractional crystallization of basaltic magmas. The features of s lab-derived fluids expected from recent high pressure experimental studies indicates that mantle wedge is partly metasomatized with "rhyolitic" materi als from subducted slabs; it is more likely that very low degree partial me lting of the metasomatized mantle wedge formed the rhyolite magma. (C) 1999 Elsevier Science B.V. All rights reserved.