Petrology and geochemistry of Pleistocene dacitic and rhyolitic pyroclastic flows from southern part of northeast Honshu, Japan.

Abstract

Large volumes of dacitic and rhyolitic pyroclastic flows erupted approx 4-1 m.y. ago along the volcanic front in NE Honshu prior to the formation of the basaltic and andesitic volcanoes. Major and trace element analyses of the rocks and compositions of plagioclases, clinopyroxenes, orthopyroxenes, hornblendes, biotites, magnetites and ilmenites are given. The rocks are highly porphyritic, containing 20-47 volume % of phenocrysts (quartz, plagioclase, pyroxenes, amphiboles and iron oxides). Biotite rhyolites lack pyroxenes and contain sanidine, cummingtonite and biotite besides the minerals mentioned above. T of the magma that formed these rocks are estimated from mineral thermometries to be 910 approx 980 degrees C, similar to those of Quaternary dacitic rocks in the same region. The major and incompatible trace elements vary smoothly with increasing SiO2, following a typical calc-alkaline salic andesite and dacite trend. The authors suggest that the dacitic and rhyolitic magmas are later stage differentiation products formed by fractional crystallization of the typical island-arc calc-alkaline mafic magma derived from the upper mantle, but not the products of partial melting of the gabbroic lower crust.