Zircon U–Pb Ages of Granitoids in Northern Kyushu, Southwest Japan: Implications for the Transition of Cretaceous Igneous Activity Along the Eastern Margin of the Asian Continent

Age determination of the Cretaceous granitoids in Japan is important for understanding igneous activities caused by subduction of the Izanagi–Kula–Pacific plates beneath the eastern margin of the Asian continent. New zircon U–Pb age data for 51 rock samples from 49 localities were obtained from northern Kyushu to build chronological evidence for further discussion of East Asian tectonics. The granitoids are divided into two parts across the Kokura–Tagawa Tectonic Line (KTTL); the Western Granitoids are classified as magnetite series, whereas the Eastern Granitoids are classified as ilmenite series. The Western Granitoids become younger toward the south, that is, toward the trench, from ca. 112 to 95 Ma, while the Eastern Granitoids become younger toward the north, that is, toward the continent, from ca. 107 to 99 Ma. Their age trends and overlapping age ranges demonstrate that two igneous zones existed in parallel and simultaneously during the Cretaceous and became closer to each other with time. The traces of the igneous zones on the continental and oceanic sides would correspond to the Korean Peninsula and the San-yo Province of Southwest Japan, respectively, and Kyushu, which had features of both, was situated at the junction of the two igneous zones. Considering only the ages of granitoids, the original position of Southwest Japan before the formation of the Sea of Japan would have been located more to the southwest than previously thought.