Formation processes of the fine-grained mafic rocks of the Ryoke metamorphic belt in the Asuka area, central Kinki district, SW Japan.

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  • Nishiwaki Hitoshi
    Department of Geosciences, Graduate School of Science, Osaka City University
  • Okudaira Takamoto
    Department of Geosciences, Graduate School of Science, Osaka City University

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  • 近畿中央部，飛鳥地域の領家変成帯に産する細粒苦鉄質岩の形成過程
  • キンキ チュウオウブ アスカ チイキ ノ リョウケ ヘンセイタイ ニ サンスル サイリュウクテツシツガン ノ ケイセイ カテイ

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In the Ryoke metamorphic belt in the Asuka area, Kinki Province, SW Japan, many mafic rocks are found as sheet-shaped xenoliths in the Older Ryoke granitoids. They show a strong to weak foliation defined by preferential alignment of long axis of plagioclase and amphibole grains. The grains of plagioclase and amphibole exhibit hypidiomorphic to xenomorphic shapes, and show no chemical zonings. Almost all plagioclases and amphiboles have unique chemical compositions of An40-45 and magnesiohornblende-ferrotschermakite, respectively. Using a conventional thermometer, equilibrium temperatures of plagioclase and amphibole rims are estimated to be 500-600°C that is too low to obliterate the pre-existing zoning profiles due to low diffusivity in plagioclase and amphibole under such mid-crustal conditions. These observations suggest that the mafic rocks were amphibolite-facies metamorphic rocks. In some outcrops, there are obliquities between the trend of foliation of the mafic rocks and that of their contact planes to the Older granitoids. In many cases, the trend of foliation of the mafic rocks is oblique to that of the Older granitoids. The foliation of the Older granitoids is defined by preferential alignment of long axis of euhedral plagioclases and biotites, and may result from the magmatic-submagmatic deformation during their ascending and emplacement. Because there is a discordant relationship between foliation of the mafic rocks and that of the Older granitoids, the formation of foliation of the mafic rocks may predate the magmatic-submagmatic deformation of the Older granitoids. Consequently, the amphibolite-facies metamorphism of the mafic rocks occurred before their involving to the Older granitoids ascending/emplacing to the upper crust.<br>

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