ABSTRACT

Sr isotopic ratios and Rb, Sr and REE abundances are determined for granitic clasts from the middle Permian Kozaki Formation in the Kurosegawa Belt, Kumamoto, Southwest Japan, in order to elucidate their source and formation. Garnet-biotite granodiorite clasts, uniquely found in the Kozaki Formation, show M-type REE tetrad patterns and wide variation of Rb/Sr ratios, which enable their petrogenesis and age determination. The REE patterns indicate shallow formation of the garnet-bearing granodiorite. The Rb-Sr isotopic data yield an age of 251 ± 46 Ma with an initial ratio of 0.7055 ± 0.0006. The Rb-Sr age is close to the sedimentation age of the Kozaki Formation defined by paleontological data and K-Ar ages of the corresponding granitic clasts in the Permian conglomerate (“Usuginu conglomerate”) of the Kitakami area, Northeast Japan. The initial Sr isotopic ratio of 0.7055 is slightly higher than the ratios around 0.7040 reported for the Permian granitic rocks from the Akaishi Range and the eastern Kyushu and the corresponding Permian granitic clasts in the Kitakami area. The initial ratio for the garnet-biotite granodiorite clasts is also slightly higher than the ratios (0.7037 and 0.7032) at 345 and 400 Ma reported for Yatsushiro granite of the Kurosegawa Belt (Nohda, 1973; Yanagi, 1975). These Sr isotopic data suggest that the garnet-biotite granodiorite clasts correspond to rocks differentiated from the Kurosegawa granitic rocks. The Sr initial ratio of 0.7055 at 251 Ma for the garnet-bearing granodiorite clasts is slightly enriched but still primitive, and the Sr initial ratio clearly indicates that the clasts were not derived from typical-continental crust. The Rb-Sr isotopic data and our preliminary Sm-Nd isotopic data for the garnet-biotite granodiorite clasts yield ε_Sr (251 Ma) = +18 and ε_Nd (251 Ma) = +6, suggesting an intra-oceanic arc provenance for the garnet-biotite granodiorite body from which the clasts in the Kozaki Formation had been derived.