Geology and Petrology at Shimabara Peninsula, Kyushu, SW Japan

From recent results

Abstract

This paper summarizes the recent results of geological and petrological research at Shimabara peninsula. The Shimabara peninsula is located at about 100 km behind the volcanic front of the Kyushu island. No subduction related seismicity is observed beneath the peninsula. There are a number of E-W trending normal faults in the peninsula due to the N-S extension. This resulted in the formation of the Unzen graben. Volcanic succession in the Shimabara peninsula is divided into two stages; Pre-Unzen volcano (4 Ma - 500 ka) and Unzen volcano (500 ka - present). The nature of volcanism of Pre-Unzen and Unzen volcano differs clearly; the former represents a monogenetic volcano group which is exposed in the southern Shimabara peninsula, whereas the latter is a composite volcano which is distributed east to west along the Unzen graben. The eruptive products of Pre-Unzen volcano have also been recognized beneath the the volcanic succession in central and northern parts of the peninsula by borehole surveys. Pre-Unzen volcanic rocks are composed of olivine basalt and two-pyroxene andesite lava flows and pyroclastics. The evolution of Pre-Unzen volcanic rocks can be basically explained by olivine-dominant fractional crystallization for basalts, and the combination of plagioclase + pyroxenes + magnetite fractional crystallization for andesites. Unzen volcano has been divided into three substages; the older Unzen (500-300 ka), the middle Unzen (300- 150 ka) and the younger Unzen (150 ka - present). Unzen volcanic rocks are composed of hornblende andesite to dacite lava domes, lava flows and pyroclastics. The evolution of Unzen volcanic rocks can be explained by magma mixing between aphyric basalt and phenocryst-rich dacite magma in various ratios. The existence of mafic inclusions with positive Nb anomalies indicates an injection of ocean-island type basaltic magmas. This suggests continuous basaltic magma plumbing system throughout the eruptive history of the Shimabara peninsula.