北八ヶ岳火山新期の岩石学的モデル(<特集>火山のモデル(I))

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書誌事項

タイトル別名
  • A Petrologic Model for the Younger Period of Northern Yatsugatake Volcanoes, Central Japan(<Special Section>Models of Volcano (I))
  • 北八ケ岳火山新期の岩石学的モデル
  • キタ ヤツガタケ カザン シン キ ノ ガンセキガクテキ モデル

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説明

Northern-Yatsugatake volcanoes in Central Japan form a Quaternary volcanic chain linearly distributed for over 14 km from north to south. The eruption history is divided into two periods : the Younger period and the Older period. Detailed mineralogical and geochemical studies of volcanic rocks of Northern-Yatsugatake revealed that the calc-alkaline series of the Younger period were produced by magma mixing between basaltic and dacitic magma, and that two basaltic end-members (BE-I and BE-II) can be recognized. BE-I contains phenocrysts of calcic plagioclase (An = 92-75 mol%) and less magnesian olivine and clinopyroxene ±orthopyroxene (Mg-value = 81-75), whereas BE-II contains less calcic plagioclase (An = 85-70) and magnesian olivine (Fo=91-81) and clinopyroxene (Mg-value = 87-81). Because the liquid compositions and incompatible element characteristics of these basic end-members are similar, they were probably derived from a common parental magma. Therefore the mineralogical contrasts are due to the difference in fractionation pressure ; BE-II fractionated olivine and Ca-rich pyroxene at high pressure (>7-10 kb) whereas BE-I fractionated olivine at low pressure. Hornblende phenocrysts are often included in the mixed andesite lavas. Microscopical observations and chemical compositions of the hornblende indicate that the hornblende phenocrysts were derived from the dacitic end-member magma. The experimentally obtained phase relations for the St. Helens dacite pumice (Rutherford et al., 1985), which is similar to the dacitic end-member magma of Northern-Yatsugatake in major bulk compositions, suggests that the hornblende crystallized at the pressure range of 1-4 kb, in upper crustal ondition. Therefore the mixing between basaltic and dacitic magma occurred in the magma chamber or the conduit in the upper crust. The mineralogical evidences indicate only one mixing event between basaltic and dacitic magma for a mixed andesite magma. Dacitic magma may not have been derived from fractional crystallization in a single steady-state open-system magma chamber. The absence of intermediate magma without evidence of magma mixing favors the model of partial melting of lower crust for the origin of the acidic end-member magma.

収録刊行物

  • 火山

    火山 36 (1), 93-112, 1991

    特定非営利活動法人 日本火山学会

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