Zircon U-Pb Ages Of Miocene Granitic Rocks In The Koshikijima Islands: Implications For Neogene Tectonics In The Kyushu Region, Southwest Japan

The opening of the Japan Sea separated southwest Japan from the Eurasian continent during the Early to Middle Miocene. Since then, diverse igneous activities have occurred in relation to the subduction of the Philippine Sea Plate beneath southwest Japan. The Okinawa Trough formed in the back-arc region of the Ryukyu Arc since the Late Miocene. In the Koshikijima Islands, off the west coast of Kyushu and near the northern end of the Okinawa Trough, felsic to intermediate igneous rocks with Middle to Late Miocene radiometric ages occur as granitic intrusions and dikes. We obtained zircon U-Pb ages and whole-rock major- and trace-element compositions of Koshikijima granitic rocks to elucidate their magmagenesis. The U-Pb ages of granitic rocks in Kamikoshikijima and Shimokoshikijima and a dacite dike are about 10 Ma, suggesting that most magmatism on the Koshikijima Islands was coeval with early rifting in the Okinawa Trough. We infer that magmagenesis occurred via melting of lower crustal mafic rocks related to rifting in the Okinawa Trough based on the arc-like trace-element compositions of these I-type granites. Andesitic dikes preceded felsic igneous activity on the Koshikijima Islands, and their ages and petrochemistry will help elucidate the magmatism and tectonics in this area throughout the Miocene.