恐山火山の岩石学的研究

Abstract

Osore-yama is a composite volcano formed in the latest Pleistocene age and resting on Neogene and Pleistocene sediments. It is 878.6m in height and situated at the northern end of Honshu Island. The history of volcanic activity is devided into the following three stages: 1) Kamabuse-yama stratovolcano stage consisting of lava flows and scoria flows of basaltic andesite (A) and lava flows of pyroxene dacite (B); 2) main body stage of pumice flows, intermediated pyroclastic flows (Aramaki, 1957) and lava flows of pyroxene andesite and dacite (C); and 3) post “caldera” stage of pumice flows, lava flows and lava domes of hornblende andesite and dacite (D). Basaltic andesite (A) and pyroxene dacite (B) belong to the pigeonitic rock series, while pyroxene andesite and dacite (C) and hornblende andesite and dacite (D) belong to the hyperthenic rock series (Kuno, 1950). This paper is based mainly on the bulk chemical composition and trace element contents of twenty-nine representative rock samples from the volcano; and partly on groundmass materials and phenocrystic minerals of some of the samples. The chemical data suggest that both andesite-dacite suites are produced by fractional crystallization of early precipitated olivine, orthopyroxene, augite, magnetite and plagioclase from basaltic andesite magma which is represented by the most mafic rocks in this volcano. It is concluded that this type of basaltic andesite magma is itself a parental magma, because it is impossible to derive such magma of extremely low K2O content from any other kind of basaltic magma. The variation of Ni, Co and Cr can be explained by the modified Rayleigh fractionation model. The difference in Ni and Cr contents between the pigeonitic rock series and the hyperthenic rock series of this volcano is considered to be due to the differences in the degree of fractionation of mafic minerals under varying condition. The origen of the basaltic andesite magma which is characterized by low Ni and Cr contents is probably neither derived by fractionation from olivine tholeiitic magma nor by partial melting of peridotite. It is probably more a product of partial melting of other rocks composed of minerals having low Ni and Cr contents.