Petrogenesis of Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc; Temporal change of arc volcanism during the back-arc spreading in the Japan Sea

Yamada, Raiki; Takahashi, Toshiro*; Ogita, Yasuhiro  

Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc, that were formed during back-arc spreading in the Japan Sea, were examined to reveal their petrogeneses and temporal change of arc volcanism during the Japan Sea opening. We reported whole-rock major and trace element and Sr-Nd isotopic compositions of andesites and rhyolites (23-16 Ma) and quantitatively discussed their petrogenesis, based on comparison with the results of model calculations, in this paper. Main concluding remarks are as follows: (1) Rhyolite magmas were generated by crustal assimilation of basaltic to andesitic magmas. (2) Andesites consist of high-Mg andesite, high-Sr, and tholeiitic andesite, generated by interaction among slab fluid, slab melt, and crustal material. (3) Upwelling of the asthenospheric mantle into the mantle wedge caused melting of the subducting Pacific plate, and andesitic magmatism induced rhyolitic magmatism.