Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system

Abstract

Lawsonite, pumpellyite and epidote coexist in the chlorite zone pelitic schists of the Sanbagawa metamorphic belt. As these pelitic schists contain calcite ubiquitously, the phase relations of hydrous Ca-Al silicate minerals are examined by mixed volatile equilibria in the K2O-Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O system with excess quartz, calcite and a fluid phase. XCO2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T-XCO2 diagram including mixed volatile reactions shows that the stable hydrous Ca-Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone with progressive increase in both temperature and XCO2 of the fluid phase, though ferric iron expands the stability of epidote throughout the chlorite zone. The fluid phase released by the metamorphic reactions in the chlorite zone is high in H2O content even in the presence of CaCO3. The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO3-bearing Sanbagawa basic schists and to the interpretation of the mineral sequence of the hydrous Ca-Al silicates of other high P/T metamorphic belts.