Title: Two modes of occurrence of staurolite from the Ryoke metamorphic rocks in the Hazu area, central Japan.
Abstract:Many localities of staurolite-bearing schists have been found in a lower grade part of the sillimanite zone in the Hazu area, the Ryoke metamorphic belt in central Japan. Two modes of occurrence of st...Many localities of staurolite-bearing schists have been found in a lower grade part of the sillimanite zone in the Hazu area, the Ryoke metamorphic belt in central Japan. Two modes of occurrence of staurolite have been recognized in this area. (A) Staurolite in a relic form armored with porphyroblastic andalusite, which is found in mica schists and quartz schists. The andalusite occurs in a matrix composed of quartz, biotite, muscovite, sillimanite, garnet, plagioclase, ilmenite and graphite. This is the prevailing mode of occurrence in this area. (B) Staurolite is coexisting with quartz, biotite, garnet, sillimanite, plagioclase, andalusite, ilmenite and graphite, but with no muscovite. This is rarely found only in quartz schists. It is notable that the mineral assemblage observed in porphyroblastic andalusite in which staurolite of the mode (A) occurs is similar to that of the quartz schist with staurolite of the mode (B). Three staurolite-bearing schists and three sillimanite-andalusite schists with no staurolite were chemically analyzed. There have not been found any significant difference in chemical composition among them, except for the K-value in the AKF ratio. On the basis of petrography and chemistry of the staurolite-bearing schists, it is concluded that staurolite of the mode (A) may be an armored relic after the metamorphic reaction of staurolite with muscovite, and that it as well as staurolite of the mode (B) seem to be stable under physical conditions in the lower grade part of the sillimanite zone in this area. The Ryoke metamorphism in the Hazu area is considered to be of higher PH2O conditions than those in the other areas in the Ryoke belt.Read More