Temporal and spatial changes of magmatism related to the Japan Sea opening in the Hokuriku region, central Japan: stratigraphy, chronology and petrology of Oligocene to middle Miocene volcanic rocks

The opening of the Japan Sea, one of the Oligocene to Miocene back-arc formations in the western Pacific, was the most significant Cenozoic event in the Japan arc. Extensive magmatism occurred during this event in the Japan Sea and the circum-Japan Sea province, including the Hokuriku region where volcanism was initiated with basalts and andesites (Konosuyama Formation) in the Noto peninsula in the early Oligocene (ca. 33 Ma?). In the Early Miocene, andesitic volcanism (Kamiwazumi and Matsunagi formations) followed the deposition of late Oligocene dacitic pyroclastic flows (Goroku Formation). Rhyolitic pyroclastic rocks including moonstone rhyolites were deposited widely as pyroclastic flows in the Hokuriku region from ca. 23-22 Ma. The moonstone rhyolites are alkalic in composition, suggesting a continental-arc rift origin. From ca. 21-17 Ma, basaltic to andesitic volcanic rocks (Iwaine and Ito-o formations and Besshodake andesites) of various geochemical compositions (tholeiitic and calc-alkalic basalts to andesites, adakite, and high-Mg andesite) were effused widely in the Hokuriku region. This basaltic to andesitic volcanism was followed from ca. 17-16 Ma by active dacitic to rhyolitic volcanism (Iozen and Fukuhira formations) with geochemical compositions indicating a continental-arc to island-arc origin. This rhyolitic volcanic episode, and minor basalts and andesites from 16 Ma, mark the end of the magmatism related to the opening of the Japan Sea, coinciding with the termination of the clockwise rotation of southwest Japan. Previous petrological and geochemical studies suggest that magmatism related to the opening of the Japan Sea in the Hokuriku region was caused by upwelling of the asthenosphere into the mantle wedge below the margin of the eastern Eurasian continent.