瀬戸川層群中の斑状組織をもつ超苦鉄質岩類について

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  • 高沢 耕一
    東京支部:東京教育大学理学部地質学鉱物学教室

書誌事項

タイトル別名
  • On the Ultramafic Rocks with Porphyritic Texture in the Setogawa Group
  • セトガワソウグンチュウ ノ ハンジョウ ソシキ オ モツ チョウ クテツシツガ

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抄録

Several small ultramafites are distributed in the Setogawa zone. Some of them, particularly Ooka peridotites and Aobane olivine-clinopyroxenite, are characterized by the prophyritic texture. These ultramafites consist of large orthopyroxene and clinopyroxene containing opposite pyroxene lamellae and groundmass-like part composed of relatively small grains of which pyroxenes do not show exsolution phenomena. Additionally the chemical compositions of two types of orthopyroxene which are morphologically distinctive, large grain with lamellae and small grain without exsolution lamellae, are plotted on the individual field of pyroxene quadrilateral, and frequency diagrams of various oxides percent such as Al2O3, CaO and Cr2O3 of the two types of orthopyroxenes display distinctive distribution patterns. But clinopyroxene does not show such clear difference between large and small grains. The texture of these rocks, the intricated boundaries between small grains and of large pyroxenes, occurrence of large pyroxenes especially clinopyroxene showing interstitial occurrence and in a few cases corroded form of large clinopyroxene, suggests that this texture is igneous origin and large crystal is to be a kind of phenocryst and small one is to form matrix. If it is correct, it is inferred that these ultramafites crystallized at relatively shallow depth in the crust and chemical compositions of matrix of such rocks inherited that of liquid which formed matrix of peridotites and olivine-clinopyroxenite. Former is able to be confirmed from the estimation of pressure condition of matrix. Pressure condition obtained is about less than 5 kb at 1030-1040℃ and 1000-1010℃ for peridotites and olivineclinopyroxenite respectively. These P. T. conditions also considered it to be unprobable to accept the metamorphic origin on the ultramafites in the Setogawa zone. Later possibility has come arise from the texture of ultramafites and as first approximate estimation, liquid composition is obtained from modal analyses of matrix, although large crystals more or less enlarged own margin with small grain simultaneously formed at the final stage of magma solidification. Results display the plagioclase-bearing Iherzolitic composition.

収録刊行物

  • 地球科学

    地球科学 30 (6), 317-330b, 1976

    地学団体研究会

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