Palaeogene K-Ar ages from the Kamuikotan metamorphic rocks, southern area of the Kamuikotan Gorge, central Hokkaido, northern Japan

Mica separates from 44 metamorphic rocks (mostly pelitic) from the southern area of Kamuikotan Gorge, central block of the Kamuikotan zone were dated using the K-Ar method. The ages of white micas, separated from pelitic schists, range from 51 to 74 Ma. The Palaeogene K-Ar ages are supported by 40Ar-39Ar dating. Judging from mineral paragenesis and X-ray diffraction studies (RM values) of white micas, most of the pelitic schists belong to the intermediate pressure type. In the Orowen River district, the northern part of the study area, biotite from metavolcanic rocks of a lower pressure assemblage yields the youngest K-Ar ages (45-50 Ma). Although the Kamuikotan zone was considered to have been a subduction-related metamorphic belt along a Mesozoic consuming margin, the time span of the metamorphism and the lowering of metamorphic pressure in younger rocks suggest that: (1) exhumation of the younger Kamuikotan rocks accompanied the erosion of forearc basin sediments (the Yezo Supergroup); (2) the increase in the thermal gradient was probably related to the approach of the Kula-Pacific ridge to the subduction zone; and (3) the main stage of exhumation of the Kamuikotan rocks ceased before westward movement of the Pacific plate, which induced the collision of the Hidaka-Nemuro belts (palaeo-Kurile are-trench system) with the Kamuikotan zone.