K-Ar age of hornblende from a dioritic rock in the Tochibora ore deposit of the kamioka mine, in the Hida metamorphic region, central Japan.

A mass composed mainly of coarse-grained heterogeneous dioritic rocks is exposed between the Tochibora and Maruyama ore deposits in the Kamioka mining area. The rocks have gneissic texture and have been called "metabasite" in this mining area . The mass is emplaced in the Hida gneisses and thermally metamorphosed by the intrusion of granitic dykes. The dioritic rocks are made up mainly of plagioclase and greenish brown hornblende. Any of plagioclase ranges 31-34 in the coarse-grained diorite and 35-38 in the fine-grained part included in the coarse-grained one. Hornblende is mostly ferroan pargastic with ferro-edenitic margin. K-Ar age of hornblende separated from coarsegrained diorite is 192•}6 Ma, which corresponds to K-Ar ages of hornblende from the Hida gneisses and Funatsu granites. This age represents the time of re-heating of the diorite by the intrusion of the Funatsu granites.