Abstract
The Kokkaibashi plutonic rocks from the northeastern edge of the Akaishi Mountains, central Japan, were dated by the K-Ar method with two hornfels in the contact aureole. The ages of the intrusions of the Kokkaibashi plutonic masses and the Kaikomagatake granitic body are in the ranges of 22.4-12.6 Ma and 13-11 Ma, respectively. These two igneous activities might be strong-ly influenced by geotectonic movement of the Shimanto Belt within the Akaishi Mountains. The beginning of the igneous activity of the Kokkaibashi plutons predated the formation of the Itoigawa-Shizuoka Tectonic Line, and was contemporary with the development of the Japan Sea Basin.
