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[SCG48-P09] Zircon U-Pb ages of the Ryugamori granitic rocks, northern Akita Prefecture, Japan
Keywords:U-Pb dating of zircon, Ryugamori, Granite
Granitic rocks are distributed in the Ryugamori area in northernpart of Akita, Tohoku, Japan. Hujimoto (1970) divided the granitic rocks into two types based on the field survey; the older granitic rocks which have been considered as a member of granitic rocks of the Kitakami belt or granitic rocks of basement of green tuff region, and the younger granitic rocks which is the Tertiary granitoids of northeastern Japan.
The older granitic rocks have been dated and yeilded to 18 Ma (biotite K-Ar age, Kawano & Ueda, 1966), 38Ma (whole-rock K-Ar age, Natural Resources and Energy, 1977), and 19.6 ± 2.7 Ma (hornblende K-Ar age, Hujimoto, 1983). The younger granitic rocks show 8.9 Ma (whole-rock K-Ar age, Natural Resources and Energy, 1977).
In this study, we collected an older granitic rock (RGM2) from the middle stream of Sendoishizawa in the southwestern part of the area, and a younger granitic rock (RGM1) from the upper stream. We investigated U-Pb ages of zircons using LA-ICP-MS equipped at the Graduate Shool of Engineering Science, Akita University.
The ages of RGM2 and RGM1 are 21.3 ± 0.3 Ma and 21.7 ± 0.4 Ma, respectively. These are identical age of older and younger granitic rocks within analytical error. The ages in this study are consistent with the cooling ages indicated by amphibole and biotite K-Ar dating. The ages of the younger granitic rocks are different from the Tertiary granitoid in northeastern Japan and indicate igneous activity which formed granitic rocks in the early Miocene in northern Tohoku, Japan.
The older granitic rocks have been dated and yeilded to 18 Ma (biotite K-Ar age, Kawano & Ueda, 1966), 38Ma (whole-rock K-Ar age, Natural Resources and Energy, 1977), and 19.6 ± 2.7 Ma (hornblende K-Ar age, Hujimoto, 1983). The younger granitic rocks show 8.9 Ma (whole-rock K-Ar age, Natural Resources and Energy, 1977).
In this study, we collected an older granitic rock (RGM2) from the middle stream of Sendoishizawa in the southwestern part of the area, and a younger granitic rock (RGM1) from the upper stream. We investigated U-Pb ages of zircons using LA-ICP-MS equipped at the Graduate Shool of Engineering Science, Akita University.
The ages of RGM2 and RGM1 are 21.3 ± 0.3 Ma and 21.7 ± 0.4 Ma, respectively. These are identical age of older and younger granitic rocks within analytical error. The ages in this study are consistent with the cooling ages indicated by amphibole and biotite K-Ar dating. The ages of the younger granitic rocks are different from the Tertiary granitoid in northeastern Japan and indicate igneous activity which formed granitic rocks in the early Miocene in northern Tohoku, Japan.