The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa = 0.10-0.28)-bearing assemblages in pelitic schists, and represents the highestgrade 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 P H ,~ = Psol,d suggests that metamorphic conditions of this zone are about 610+2S"C and 10 +_ 1 kbar.Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclasebiotite 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: omphacite zone garnet + glaucophane + omphacite +muscovite = oligoclase-biotite zone biotite + hornblende + plagioclase +quartz + H20 oligoclase-biotite zone garnet + hornblende + paragonite + muscovite = upper staurolite zone staurolite + biotite + plagioclase -I-quartz + H 2 0 0263-4929/83/0/0600-0141%02.00 0 1983 Blackwell Scientific Publications dP/dT values of these equations calculated using natural compositions are 3-6 bar/deg. and 16-19 bar/deg., respectively, and their gradients are gentle. This implies that the contrast in mineral parageneses of pelitic schists related by the above equations result mainly from the difference in pressure during progressive metamorphism.The mineral assemblages observed in the Sanbagawa oligoclase-biotite zone are characteristic of pelitic schists in the high-grade part of a high-pressure intermediate type of metamorphism.
