Chemically sector-zoned garnet in Sanbagawa schists; its mode of occurrence and growth timing

Abstract

Garnet in the Sanbagawa belt commonly shows concentric compositional zoning of a normal type. However, chemically sector zoned garnet has also been reported. The additional discovery of new localities in the Besshi district of the Sanbagawa belt shows this is a more widespread phenomenon than has generally been realized. All the new localities are situated in the oligoclase-biotite zone on the north side of the Tonaru amphibolite mass. The sector-zoned garnet is generally fine-grained (ca. 0.1 mm in diameter) and occurs in pinkish quartz-rich bands with or without rare medium-grained (ca. 0.4 mm) garnet. The comparative study of compositional zoning of these garnets indicates that the sector-zoned garnet has grown in the latest stage of the Sanbagawa metamorphism. The spatial association of sector-zoned garnet with amphibolite masses (in particular the Tonaru mass) suggests the development of the sector zoning may be genetically linked to the emplacement of these bodies.