Isotopic ages of the Usukigawa quartzdiorite along the Usuki-Yatsushiro tectonic line in eastern Kyushu and their geological significance

K-Ar and Rb-Sr age determinations were carried out on the Usukigawa quartzdiorite and associated metamorphic rocks along the Usuki-Yatsushiro Tectonic Line in eastern Kyushu, Southwest Japan. The Usukigawa quartzdiorite is composed of hornblende-biotite quartzdiorite to tonalite, and those in the western area of the body are strongly mylonitized. K-Ar hornblende ages of four samples of the Usukigawa quartzdiorite range from 252 to 272 Ma. Rb-Sr whole-rock isochron for four quartzdioritic rocks and three aplitic rocks gives an age of 275 ± 56 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7040. On the other hand, K-Ar hornblende age of amphibolite which is intercalated as a small lenticular body along the northern boundary fault of the quartzdiorite is 124 Ma. The results of age and initial Sr isotope ratio of the Usukigawa quartzdiorite suggest that this Permian granitoid is correlative to the Kinshozan quartzdiorite which forms a nappe structure on the Mikabu green rocks in the Kanto Range, and is also correlative to the granitic clasts in the Usuginu-type conglomerate in South Kitakami belt. On the other hand, the K-Ar hornblende age of amphibolite and lithology of pelitic gneiss which is intercalated along the southern boundary fault of the quartzdiorite suggest that these high-grade metamorphic rocks are correlative to the Oshima metamorphic rocks in the Maana belt in western Shikoku and the Yorii metamorphic rocks in Kanto Range, and also correlative to the Takanuki metamorphic rocks in the Abukuma belt. The correlation of rocks mentioned above and close association and occurrence of three rock types; Permian granitoid Early Cretaceous high-grade metamorphic rocks and Late Cretaceous terrigenous sediments strongly suggest that the rocks of South Kitakami and Abukuma belts remain as a nappe on the Sambagawa metamorphic rocks in both Kanto and eastern Kyushu.