Clay mineral composition and K-Ar ages from fault gouges in the Kojaku Granite, Tsuruga Peninsula

Niwa, Masakazu   ; Shimada, Koji   ; Sueoka, Shigeru   ; Tamura, Hajimu; Shibata, Kenji

X-ray diffraction analysis and K-Ar dating were applied for fault gouges in the late Cretaceous Kojaku Granite, Tsuruga Peninsula. The fault gouge includes smectite, 1M type illite, and kaolinite, based on the X-ray diffraction analysis. The K-Ar dating shows approximately 39 to 45 Ma for the 0.1 m size fractions separated from the gouges. The K-Ar ages and the reconstructed cooling history of the granite suggest that the 1M illite was formed at 60 to 110 C (approximately 2 to 4 km in depth for 30 C/km of geothermal gradient).