Two types of garnet in Sanbagawa pelitic schists along Asemigawa River, central Shikoku

Abstract

The mode of occurrence of garnet in pelitic schists of the Sanbagawa metamorphic belt along the Asemigara River in central Shikoku is examined. Two different types of garnets, A and B, are identified from the albite-biotite and the oligoclase-biotite zones of the Sanbagawa belt. Type-A garnets are larger than type-B garnets and show concentric normal chemical zoning. There is no evidence of disequilibrium crystal growth in type-A garnets. On the contrary, type-B garnets are small, and some of them occasionally show sector zoning, which is a strong evidence for a disequilibrium crystal growth. The chemical trend observed between the core and the rim of type-B garnets is different from that observed in the case of type-A garnets, although only some of the type-B garnets show chemical sector zoning. Type-B garnets are typically found in clusters in muscovite-rich layers or are included in albite porphyroblasts. These two types of garnets can be distinguished from each other on the basis of grain size, cluster formation, and chemical composition. Type-B grains are found only in the structurally lower part of the oligoclase-biotite zone and in the lower albite-biotite zone. These two types of garnets are thought to represent two stages of garnet growth. Further, in the case of type-A garnets (early garnet), chemical equilibrium is maintained during prograde metamorphism, and hence, these garnets are suitable for the use in thermobarometric studies.