[S-GL27] Geochronology and Isotope Geology

[SGL27-05] Result of age determination of samples from Hokuriku Group

*Noriko Hasebe¹, Ryo Kamiguchi² (1.Institute of Nature and Environmental Technology, Kanazawa University, 2.Department of Earth Sciences, Kanazawa University)

Keywords:Hokuriku Group, TR1 tephra, Tng tephra

The Hokuriku Group is distributed from the southern part of Ishikawa Prefecture to the eastern part of Toyama prefecture and deposited in the Neogene period (about 25 Ma to 2.5 Ma). This study reports zircon fission track (ZFT) and U-Pb ages from several volcanic rocks distributed in the Tsurugi district, the southern part of Ishikawa Prefecture, Kanazawa University district, and Uozu area in Toyama prefecture. The Iozen Formation in Tsurugi district show ZFT ages between 12.2 ± 1.9 to 15.7 ± 2.8 Ma (error, 2 sigma) and U-Pb ages between 17.2 ± 0.5 to 21.0 ± 0.9 Ma. U-Pb ages coincide with the Japan Sea opening event, and eruption age by ZFT is about 5 mys after zircon crystallization. One sample from Otogawa Formation shows an equivalent age with the Iozen Formation, suggesting a rapid volcanic sedimentation for both formations. The sample from a quarry, which we thought is from TR1 (Ohkubo, 1999), therefore estimated age is about 4.4 Ma (Tamura and Yamazaki, 2004) by the correlation, shows much older age of ZFT 33.2 ± 4.6 Ma and U-Pb 65.3 ± 2.0 Ma, and further investigation, especially on the sample identification, is necessary. For Uozu area, ZFT ages of 3.4 ± 0.7 to 9.7 ± 1.4 Ma and U-Pb ages of 2.9 ± 0.8 to 71.7 ± 2.8 Ma are obtained, and eruption ages are equivalent to tephra found in Yatsuo district (Sumii, 1986)