No 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 K 2 O -Na 2 O -CaO -MgO -Al 2 O 3 -SiO 2 -CO 2 -H 2 O system with excess quartz, calcite and a fluid phase. XCO 2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T -XCO 2 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 XCO 2 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 H 2 O content even in the presence of CaCO 3 . The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO 3 -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.

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