ABSTRACT

The volcanic history of the Shimabara Peninsula, Kyushu, Japan, is divided into two stages; Pre-Unzen volcano stage (4 Ma-500 ka) and Unzen volcano stage (500 ka-present). Pre-Unzen volcanic rocks comprise olivine basalt and two-pyroxene andesite lava flows and pyroclastics. The similarity of trace elements chemistry indicates that Pre-Unzen basalts evolved from different primary magmas originated in the same mantle source. They were differentiated by olivine-dominant fractional crystallization and crustal assimilation. This process produced parental magmas for Pre-Unzen andesite. The evolution of Pre-Unzen andesites can be explained by the combination of plagioclase + pyroxenes + magnetite fractional crystallization and crustal assimilation. Unzen volcanic rocks are composed of hornblende andesite to dacite lava domes, lava flows and pyroclastics. They have petrographical and geochemical features indicating that they were produced by magma mixing. Mafic inclusions are commonly included in Unzen volcanic rocks and show evidence of hybridization between aphyric basalt and phenocryst-rich dacite magma in various ratios. The existence of mafic inclusions with positive Nb anomalies in spidergram indicates an injection of ocean-island type basaltic magmas. The estimated mafic endmember for the dacite magma is an evolved basalt with MgO = 5.5 wt.%. This corresponds to the most evolved composition of the Pre-Unzen basalt and suggests continuous basaltic magma plumbing system throughout the eruptive history of the Shimabara peninsula.