Metamorphic conditions of staurolite schists of the Ryoke metamorphic belt in the Hazu-Hongusan area, central Japan

Abstract

Among the Ryoke metamorphic terrane, staurolite are well exposed in the Hazu-Hongusan area. Metamorphic condirions of the staurolite-bearing schists in the Hazu-Hongusan area are discussed in this paper. Three regional metamorphic zones, each characterized by the presence of andalusite, sillimanite and sillimanite-K-feldspar respectively, are mapped, and an additional contact metamorphic zone is also developed around the granitic intrusives in this area. Thirty three outcrops of the staurolite schists have been found in all the zones except the sillimanite-K-feldspar zone. The chemistry of staurolite schists in this area is similar to that of ordinary pelitic schists in other areas of the Ryoke metamorphic belt. Staurolite occurs in the following four modes. A. Relic-like inclusions (porphiroblasts and small grains) in andalusite porphyroblasts. B.Porphyroblasts and grains coexisting in equilibrium with the other constituent minerals. C. Relic-like inclusions (small grains) in plagioclase and quartz. D. Relic-like inclusions (small grains) in muscovite plates. Despite such various modes of occurrence, staurolite-bearing assemblages with quartz are of two kinds, one associated with muscovite and the other free from muscovite, both of which contain staurolite, andalusite (+sillimanite in the sillimanite zone), biotite, plagioclase, quartz, ilmenite, graphite and sometimes garnet. Staurolites in the former assemblage with muscovite have higher Zn/(Fe+Mg+Zn) than those in the latter assemblage without muscovite. By consideration of textural relations and mineral parageneses, this difference of Zn content could be ascribed to the difference in advancing degree of the continuous staurolite-breakdown reaction during the progressive metamorphism. Thus, it is suggested that staurolite persisted stably through both the andalusite and athe sillimanite zones. Compared with other areas of the Ryoke metamorohic belt, the Hazu-Hongusan area is characterized by (1) the regional presence of staurolite, (2) appearance of K-feldspar in sillimanite-bearing schists on the higher grade side of the sillimanite zone, and (3) absence of cordierite in both the andalusite and the sillimanite zones, and appearance of cordierite in the sillimanite-K-feldspar zone. It is probable from the above evidence that higher pressure (at least, higher PH2O) conditions prevailed during the progressive metamorphism in this area compared with other areas of the Ryoke terrane.