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
2002 年 111 巻 5 号 p. 640-660
詳細
2002 年 111 巻 5 号 p. 640-660
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 XCO2 because titanite was stable in pelitic schists from the chlorite zone. With the formation of garnet, XCO2> increases. XCO2 released by the garnet formation at XCa = 0.35 is 0.240 and led the pelitic system to the XCO2-buffered system defined by the reaction of Titanite + CO2 = Calcite + Rutile + Quartz. In the XCO2-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.
