Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XX, No.4


Petrology of Historic Rhyolite and Dacite from Usu Volcano, North Japan

Ōba, Yoshio;Katsui, Yoshio;Kurasawa, Hajime;Ikeda, Yasuo;Uda, Tsuyoshi

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After several thousand years of dormancy, Usu Volcano renewed its activity in 1663, and seven eruptions occurred up to the present. The essential products are typical of low-K rhyolite and dacite, and varied with time from rhyolite to dacite, decreasing in silica and increasing in Al2O3, CaO, MgO, and FeO + Fe2O3 contents, with a slight variation in trace-element abundances, but their Sr isotope ratios remains almost constant (0.70396 - 0.70399). In consistent with the variation in chemistry of the rocks, most of the plagioclase phenocrysts become more calcic and orthopyroxene more magnesian. The sequence of historic eruptions and the variation in composition of the products can be interpreted in terms of a compositionally zoned magma chamber which has formed before the first historic eruption. The presence of disequilibrated phenocrysts in the historic felsic rocks, i.e. calcic plagioclase, magnesian orthopyroxene, clinopyroxene, and pargasitic hornblende, indicates a complex magma process. The process of incorporation of these crystals into the felsic magma and the formation of the zoned magma chamber are discussed.