K-Ar ages of the Ryoke and related plutonic rocks in the Akechi area, Gifu-Aichi prefectures, central Japan

Abstract

The Akechi area in the western Mikawa-Tono region extends from Aichi to Gifu prefectures, central Japan. Here, four K-Ar biotite ages are reported from plutonic rocks in this area: (1) Inagawa Granite, Obara Mass (67.7±1.5 Ma); (2) Inagawa Granite, Gneissose facies (66.0±1.4 Ma); (3) Inagawa Granite, Massive facies in the Ryoke belt (67.0±1.5 Ma); and (4) Naegi-Agematsu-Toki Granite proximal to the Ryoke plutonic rocks (64.1±1.4 Ma). The first three ages are within error of K-Ar biotite ages from a corresponding lithology in the southern Asuke area. The age for (4) is a reliable estimate of the cooling age of the Naegi-Agematsu-Toki Granite in this region.