Preliminary report on the significance of magmatic enclaves in adakite genesis at Hakusan volcano, central Japan

Abstract

Hakusan volcanic rocks younger than the major dormant period ~0.3-0.1 Ma are adakitic, while those older than the dormant period have geochemical affinities with common island arc andesite, dacite, and rhyolite that are characterized by low Sr/Y ratios. The younger lavas, not necessarily adakitic in origin, became adakitic by magmatic mixing with magmatic enclaves (MEs), which are richer in Sr to varying extents and less silicic than the host lavas. We suggest that the younger MEs were derived from adakite magmas that interacted with the upper mantle material during their ascent to the surface. The Sr content in MEs in the older lavas is as low as that in the host lavas. The process of magma generation beneath the Hakusan volcano is likely to have changed between 0.3 Ma and 0.1 Ma, from one triggered by the dehydration of the subducted Pacific plate to one caused by the partial melting of the Philippine Sea plate.