Restoration of the Paleo Ryoke Terrane - Geotectonic Evolwtion of Kyshu

点击翻译按钮获取中文摘要

K-Ar age determinations were carried out for granitic clasts from the Cretaceous Onogawa Group, Yamanaka Granodiorite distributed to the north of Onogawa Group in eastern Kyushu, and granitic clast from Permian Mizukoshi Formation in the Higo Terrane, western Kyushu. CHIME monazite and zircon age determina-tions were also carried out for Higo metamorphic rocks. The major results are as follows :1. K-Ar hornblende and biotite ages for four granitic clasts from Onogawa Group are concentrated in 103-108 Ma.K-Ar hornblende, biotite and K-feldspar ages for Yamanaka granodiorite are 103, 104 and 75.1Ma, respectively. These results suggest that the granitic clasts in the Onogawa Group were supplied from the Asaji metamorphic terrane on the north. However, the origin of Asaji metamorphic rocks remains unsolved.2. K-Ar hornblende age for tonalite clast from the Mizukoshi Formation is 260 ± 13 Ma. The initial isotopic Sr ratio ranges from 0.7037 to 0.7053. The XRF whole rock analyses show that the granitic clasts in the Mizukoshi Formation has characteristics of M-type volcanic arc granite. These results are identical to the characteristics of the granitic clasts in the Usuginu-type conglomerate in the South Kitakami and Kurosegawa Terranes, and also as those of the Permian granitic rocks constituting Paleo-Ryoke Terrane. Accordingly, the terrane from where the granitic clasts were supplied in Permian time must be immature island arc extending an order of 1000 km where M-type granitic rocks were exposed.3. CHIME monazite ages of the Higo metamorphic rocks concentrate about 120 Ma, whereas CHIME zircon ages have a peak of 250 Ma. The younger age shows peak metamorphic age, whereas the older age is considered to be inherited age retained in the detrital zircon.