松山道後の高マグネシア安山岩 : 生安山岩マグマの一例

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タイトル別名
  • High magnesian andesite from Dogo of Matsuyama, Ehime Prefecture
  • 松山道後の高マグネシア安山岩--初生安山岩マグマの一例
  • マツサンドウゴ ノ コウ マグネシア アンザンガン ショセイ アンザンガン マ

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High magnesian andesite was found at Dogo, Matsuyama city, Ehime Prefecture, Southwest Japan. The rock is nearly non-porphyritic one consisting of several percent of phenocryst of olivine (Fo_91-87) and orthopyroxene (En_88-85), and a small amount of chromite microphenocryst. The bulk chemical composition exhibits significantly high Mg/Mg+Fe^2+ up to 0. 80. The variation of Ni0 content within a olivine phenocryst is very small, ranging from 0. 44 in the core to 0. 30 wt% at the rim. Equilibrium relation as to Fe- Mg partitioning is attained between the olivine phenocryst and the liquid. Groundmass pyroxene have a less-fractionated character with high Cr_2O_3 content. From these facts this high magnesian andesite is concluded to represents a primary andesite magma derived from the upper mantle. No remarkable compositional modification by crystal fractionation occurred during the ascent. The temperature of the high magnesian andesite magma estimated from the pyroxene geothermometry for the groundmass bronzite and augite pair give high temperature of about 1200℃.On the other hand, the solidus temperature of hydrous peridotites under upper mantle conditions experimentally determined were about 900-1000℃ (MYSEN and BOETTCHER, 1975 a ; GREEN, 1976). This temperature discrepancy, therefore, suggests that this primary andesite magma was equilibrated with the upper mantle peridotite at much higher temperature than the solidus one determined experimentally. This andesite magma may be formed by relatively high degree of melting of the upper mantle peridotite.

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