A Petrological Model for the Minamigassan Volcano, Nasu Volcanoes, Northeast Japan Arc(<Special Section>Models of Volcano (II))

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  • BAN Masao
    Department of Earth Sciences, Faculty of Science, Yamagata University

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  • 東北日本弧,那須火山群,南月山火山の岩石学的モデル(<特集>火山のモデル(II))
  • 東北日本弧,那須火山群,南月山火山の岩石学的モデル
  • トウホク ニホンコ ナス カザングン ミナミ ガッサン カザン ノ ガンセキガ

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Abstract

Nasu volcanoes, the southern part of the volcanic front of Northeast Japan arc, comprises 10 small volcanic centers. Minamigassan is one of these centers and is located at the southern end of this volcanoes. A petrological model of magmatic proccess of Minamigassan volcano is presented based on the mineralogy and whole-rock chemistry. The eruption products of Minamigassan volcano can be divided into five units ; E-1, E-2, L-1, L-2, and L-3 units from lower to upper. The caldera collapse occured at the beginning of L-1 unit. The E-1 unit belongs to medium-K tholeiite, E-2 to low-K tholeiite, L-1 to medium-K calc-alkaline, L-2 to medium-K tholeiite, and L-3 to medium-K calc-alkaline categories, respectively. L-1 unit is petrologically similar to E-1 unit. Least squares subtraction calculation for major elements and Rayliegh fractionation model calculation for trace elements indicate that the chemical variation within E-1 and E-2 unit can be interpreted by fractional crystallization of phenocrystic minerals. On the other hand calc-alkaline suites, L-1 and L-3 units suffered magma mixing. These units formed by mixing between basic magma which resembles E-1 unit magma and felsic magma which is derived from early stage magma through crustal contamination or crustal melt. Distinct differences in LIL/HFS ratio among E-1 and E-2 units cannot be explained either by the difference in degrees of partial melting of a common source or by any fractional crystallization process. The difference between E-1 and E-2 units have been originated in chemical difference of source material.

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