泥質片岩のザクロ石のカルシウム累帯構造を用いた三波川変成作用の温度圧力変化と流体相の化学組成変化の推定

  • 後藤 篤
    姫路工業大学理学部生命科学科地球テクトニクス講座

書誌事項

タイトル別名
  • Changes in Pressure, Temperature and Fluid Composition during the Sanbagawa Metamorphism Estimated by Grossular Zoning of Garnet in Pelitic Schists.
  • デイシツヘンガン ノ ザクロイシ ノ カルシウムルイタイ コウゾウ オ モチイタ サンバガワ ヘンセイ サヨウ ノ オンド アツリョク ヘンカ ト リュウタイソウ ノ カガク ソセイ ヘンカ ノ スイテイ

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説明

The Ca-zoning of garnet in Sanbagawa pelitic schist shows its maxima at the intermediate region. The compositional trend of the grossular component, which is controlled by a dehydration-decarbonation reaction of 2Clinozoisite + 5Calcite + 3Quartz = 3Grossular + 5CO2 + H2O, is examined thermodynamically in the system of CaO-TiO2-Al2O3-SiO2-C-H2-O2 in the presence of calcite, clinozoisite, quartz, graphite, and fluid phase to estimate changes in pressure, temperature, and fluid composition during Sanbagawa metamorphism. At the onset of garnet formation, the chemical composition of a metamorphic fluid was low in XCO<SUB>2</SUB> because titanite was stable in pelitic schists from the chlorite zone. With the formation of garnet, XCO<SUB>2</SUB>> increases. XCO<SUB>2</SUB> released by the garnet formation at XCa = 0.35 is 0.240 and led the pelitic system to the XCO<SUB>2</SUB>-buffered system defined by the reaction of Titanite + CO2 = Calcite + Rutile + Quartz. In the XCO<SUB>2</SUB>-buffered system, the grossular content of garnet is determined as a function of pressure and temperature. The isopleth of the grossular content of garnet on the pressure-temperature diagram depends slightly on pressure and distinctly on temperature. In carbonate-bearing metamorphic rocks, mixed volatile reactions take place at much lower temperatures than dehydration reactions for major mineral formations.

収録刊行物

  • 地学雑誌

    地学雑誌 111 (5), 640-660, 2002

    公益社団法人 東京地学協会

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