東北日本南部における中期中新世の酸性火山岩類

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  • Middle Miocene Acidic Volcanic Rocks in the Southern Area of North-East Japan

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The Middle Miocene rhyolite-dacite lava flows and associated acidic pyroclastic rocks are widely distributed in the southern area of North-East Japan. This volumes are about 3500 km^3. The acidic rocks are classified into two series, based on thier phenocryst composition, namely, the Pyroxene and the Amphibole Series. Thephenocrysts of the Pyroxene Series are of clinopyroxene (Mg/Fe+Mg=0.60-0.05), orthopyroxene (Mg/Fe+Mg=0.52-0.26), plagioclase (An 30-85) and quartz. Iron-rich pyroxene are the most characteristic feature of the Pyroxene Series. The phenocrysts of the Amphibole Series are of hornblend (Mg/Fe+Mg=0.70-0.50), cummingtonite (Mg/Fe+Mg=0.65-0.61 or 0.37-0.35), orthopyroxene (Mg/Fe+Mg=0.62-0.47), clinopyroxene (Mg/Fe+Mg=0.75-0.65) biotite, plagioclase (An 17-63) and quartz. Rhyolite-dacite and perlite have various K_20 content. In comparison with rhyolite-dacite, perliteis low in normative Q+Ab+Or, K_20, CaO and FeO, remarkably depletes in Sr. In addition, the perlite, especially that of the Amphibole Series, is characterized by low (EU)_A(=Eu^<2+>/Eu^<3+>). Geochemical and petrographic studies indicate that the differentiation process of the acidic magma is interpreted as the fractional crystalization of plagioclase and mafic mineral in the proportion of several percents. In addition, volcanism zone in the Tsugawa-Aizu Province consists of the Bimodal Volcanism Zone and the Amphibole Zone. The Bimodal Volcanism Zone is characterized by the acidic rocks of the Pyroxene Series and basaltic rocks.

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