Hydrothermal reconstruction of the granitoids: As inferred from the mineralogy of the two contrasting Cretaceous granitoid plutons, west to Lake Biwa, Japan

Mafic and feldspar minerals in the two contrasting granitoid plutons of the Ohgi Granodiorite and O'hara Tonalite were examined chemically and texturally, using electron microprobe analyzers. The results show distinct differences between the two plutons. The chemical variation trends of amphiboles and biotites record the magmatic crystallization and succeeding hydrothermal reconstruction. The feldspars show some textural differences between the two plutons. Magmatic resorption of the primary plagioclase, associated with the later alkali feldspar crystallization, is likely to have occurred more intensely in the O'hara Pluton than in the Ohgi Pluton. Microperthitic textures common in both the plutons produced by subsolidus hydrothermal reactions are mainly of bead type, and they are different from patch microperthites characteristically to other granite plutons around Lake Biwa. The mineralogy of granitoid plutons provides important information on cooling conditions and processes different from those of more evolved granite plutons.