K-Ar biotite age of hornfels near the Okushibetsu plutonic complex, northern Hidaka magmatic belt, Hokkaido, Japan

Abstract

We present K-Ar ages of biotite (20.2±0.5 Ma, 19.0±0.4 Ma, 17.8±0.4 Ma) from garnet-cordierite-bearing biotite hornfels adjacent to the Tadoshunai, Sakkuru Higashi, and Sakkuru Nishi masses of the Okushibetsu plutonic complex, Hidaka magmatic belt, central Hokkaido. The Sakkuru Higashi and Sakkuru Nishi masses are composed of troctolite, olivine gabbro, and diorite, and the Tadoshunai mass of quartz diorite. It is likely that some or all of the masses were the heat source for the thermal metamorphism, thereby indicating that mafic and/or intermediate magmatism in the northern half of the Hidaka magmatic belt occurred during the early Miocene.