Cooling history of the Cretaceous Toki granite in the eastern Sanyo Belt, Central Japan

Yamasaki, Seiko; Umeda, Koji

The thermal history of the Toki granite body exposed in the eastern Sanyo Belt, Central Japan, was evaluated from various chronological data by K-Ar, fission track methods and Rb-Sr. The K-Ar ages obtained from biotite ranged from 72 to 74 Ma, which were concordant with the K-Ar ages of hornblende and the reported Rb-Sr whole-rock isochron ages. Fission-track ages for zircon and apatite in the Toki granite were 64 to 73 Ma and 37 to 40 Ma, respectively. These thermochronological data revealed two distinctive cooling stages for the Cretaceous Toki granite; (1) a rapid cooling stage which the granitic magma was cooled to the temperature of host rock soon after intrusion at depths of 5 - 6 km in the upper crust, and (2) the subsequent slow cooling stage (7 - 10 C/m.y.) associated with the exhumation of the granite body after ca. 60 Ma.