Petrographic characteristics and geological implications of the Cretaceous O'hara and Ohgi plutons, west of Lake Biwa, southwest Japan

Two adjacent but contrasting Cretaceous granitoids, the O'hara and Ohgi plutons, crop out west of Lake Biwa, southwest Japan. The O'hara pluton (4×1 km) consists mainly of a fine to medium-grained hornblende-biotite tonalite, with minor quartz diorite and granodiorite. The main facies of the pluton comprises magnetite-series grani-toids with adakitic compositions that yield a mineral K-Ar age of ca. 100 Ma. The O'hara pluton is located at the eastern end of a series of adakitic plutons, that are distributed throughout the central Tamba Belt. In contrast, the Ohgi pluton (5×0.5-1.0 km) consists of fine- to medium-grained hornblende-biotite granodiorite, containing conspicuous phenocrysts of hornblende. Most of the rocks are ilmenite-series granitoids that lack adakitic features and were emplaced at ca. 70 Ma. From the mode of occurrence, the Ohgi pluton might belong to the ring complex, the intrusion of which was related to formation of the cauldron, south of Lake Biwa.