Petrography and chemistry of Hime-shima volcanic rocks in Oita, southwest Japan

Abstract

Hime-shima volcanic rocks in the the Hime-shima Island, southwest Japan are composed of five bodies: Daruma-yama lava (garnet-bearing biotite-hornblende dacite), Shiro-yama lava (garnet-bearing rhyolite), Yahazu-dake lava (biotite-bearing hornblende dacite-rhyolite), Morose lava (biotite-bearing hornblende dacite) and Hashiraga-dake lava (garnet-bearing biotite-hornblende rhyolite). These rocks can be divided into two types: garnet-bearing and garnet-free types. The lavas of garnet-bearing type are subdivided into hornblende-bearing and hornblende-free subtypes. The lavas of garnet-free type are distributed in cenrtal parts of the Hime-shima Island, while those of garnet-bearing type are in the eastern and western ends of the Hime-shima Island. Hime-shima volcanic rocks range in composition from dacite (morose lava; SiO2=68.81%) to rhyolite (Shiro-yama lava; SiO2=74.88%). The lavas of garnet bearing type tend to be more felsic than those of garnet free type. The Hime-shima volcanic rocks may represent a later stage of magmatic differentiation, but it seems difficult to decide whether these volcanic rocks are derived from an original basaltic magma or they are remelting products of deeper part of the crust.