Formation Age and Cooling History of Kojaku Granite

Thermochronometric analyses are applied to Kojaku and Tsuruga bodies of Kojaku granite, southwest Japan, to reconstruct detailed cooling histories and to reveal geo- and thermo-chrono-logic relationships of the two bodies. Zircon U ⊖ Pb ages are estimated to be 69.2 ⊖ 68.0 Ma, sug-gesting coeval emplacement of the two bodies. These results support the previous petrographic classification of Kojaku granite in terms of geochronology. Fission-track （ FT ） ages obtained are 59.6 ⊖ 53.0 Ma and 44.8 ⊖ 20.9 Ma for zircon and apatite, respectively. Zircon FT lengths imply rapid cooling, so the heterogeneous zircon FT ages are attributable to differences in the timing of cooling just after intrusion related to the distance from the margin of the granite body. Apatite FT lengths suggest more complicated cooling histories; samples of the Tsuruga body reflect long-term denudation whereas those of the Kojaku body might have been reheated by volcanic activity prior to ～ 20 Ma. Plagioclase K ⊖ Ar dating is also performed on different lithofacies of a basaltic dike intruding into the Tsuruga body. The ages are inferred to be 20.1 ± 0.5 Ma for both lithofacies. Even if the two lithofacies were formed by multiple intrusions, the time gap must have been short, probably less than 10 5 years.