秋田焼山火山の地質学的・岩石学的研究 ２ マグマ組成の変化

Abstract

Magmatic evolution of Akita-yakeyama volcano, northeastern Japan, has been investigated based on bulk chemistry (major and trace), Sr isotopic features and mineral chemistry in combination with stratigraphical study. The main body of the volcano was built by eruptions of a large amount of hypersthene-augite andesite magma and a minor amount of olivine bearing hypersthene-augite andesite magma as lava flows and pyroclastic falls, and an edifice of stratovolcano was built. No evidence of magma mixing or crustal assimilation can be recognized in the olivine free andesite, but olivine-pyroxene disequilibrium in the olivine bearing andesite indicates magma mixing. After a repose period, a series of eruptions began from the central vent and erupted hypersthene-augite andesite magma as fine ash fall and hypersthene-augite-olivine andesite magma as lithic-rich pyroclastic falls. As stated above the olivine-free andesite has no evidence of magma mixing, but the olivine bearing andesite has the mineralogical and chemical evidence of magma mixing, such as disequiliblium phenocryst assemblage, reverse zoning of pyroxene and dusty plagioclase. Flank eruptions followed and erupted quartz bearing olivine-hypersthene-augite andesite magma, quartz bearing augite-hypersthene andesite magma and augite-hypersthene dacite magma as lithic-rich pyroclastic falls, lava flows and pumice fall. Among them quartz bearing andesites have high 87Sr/86Sr ratios, suggesting the involvement of crustal assimilation. At the end of these eruptions, augitehypersthene dacitic lava dome and olivine bearing augite-hypersthene dacitic lava dome intruded into central crater. Dacite magma of the lava domes has no or little evidence of magma mixing or crustal assimilation. Chemical variation of trace elements (Zr, Rb, Y) from the hypersthene-augite andesite of the main body to the augite-hypersthene dacite of the domes cannot be accounted for by fractional crystallization, and difference of degree of partial melting of the source material is required.