Cooling history of the Kojaku granite pluton constrained from multi-system thermochronology and its geologic implications

Sueoka, Shigeru   ; Yasue, Kenichi; Niwa, Masakazu   ; Hanamuro, Takahiro  ; Shimada, Koji   ; Ishimaru, Tsuneari ; Umeda, Koji; Danhara, Toru*; Iwano, Hideki*; Gozu, Chitaro*

Seven major granitic plutons are distributed around the Lake Biwa. On the basis of their lithofacies, chemical compositions, and radiometric ages, four of the seven plutons to the south of the Lake Biwa are thought to be related to a huge cauldron formed at ca 70 Ma, while the Otsukimiyama pluton to the east and Hiei pluton to the west are derived from volcanic front of ca 100 Ma. On the other hand, the geologic implications of the Kojaku pluton to the north have not been discussed due to luck of the radiometric ages. We are planning to report zircon fission-track age, biotite K-Ar age, and zircon U-Pb age of the Kojaku pluton, to estimate its formation age and cooling history, and to discuss their geological implications.