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.

中文翻译：

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越纪岛中新世花岗岩的锆石U–Pb年龄：对日本西南部九州地区新近纪构造的启示

在中新世早期至中期，日本海的开放将日本西南部与欧亚大陆分开。从那时起，与日本西南部下方的菲律宾海板块俯冲有关的各种火成活动已经发生。自中新世晚期以来，冲绳海槽形成在琉球弧的弧后区域。在九州西海岸和冲绳海槽北端附近的越志岛群岛中，中新世至中晚期辐射年龄的长石质至中层火成岩发生花岗岩侵入和堤防。我们获得了越石岛花岗岩岩的锆石U–Pb年龄以及全岩主要和微量元素组成，以阐明其成岩作用。Kamikoshikijima和Shimokoshikijima中的花岗岩的U–Pb年龄和达西特堤防大约为10 Ma，这表明，越志岛群岛上的岩浆活动最多，与冲绳海槽中的早裂有关。我们推断，这些I型花岗岩的弧状痕量元素组成是通过与冲绳海槽裂谷有关的下地壳镁铁质岩石融化而发生的。在Koshikijima群岛上的长石质火成活动之前，安第斯山脉的堤坝，它们的年龄和石化作用将有助于阐明整个中新世该地区的岩浆作用和构造。