Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic belt

The oligoclase‐biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (X Ca = 0.10–0.28)‐bearing assemblages in pelitic schists, and represents the highest‐grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH 2 O = P solid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar. Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high‐pressure type of metamorphic belt and with those in a generalized‘upper staurolite zone’as an example of a medium‐pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as: