[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-05] Occurrence of the Kerama Formation (Shimanto Belt) in the Kerama Islands, Central Ryukyu and detrital zircon U-Pb ages

*Ryuichi Shinjo^1,2, Ken Usami³, Hiroyuki Miyagi, Seiya Nakandakari², Ke-han Song⁴, Nagi Takahashi² (1.Research Institute for Humanity and Nature, 2.Dept Physics and Earth Sciences, University of the Ryukyus, 3.Okinawa Prefectural Museum and Art Museum, 4.Graduate School of Engineering and Science, University of the Ryukyus)

Keywords:Detrital zircon, Depositional age, LA-ICP-MS, Kerama Formation, Nago Formation, in-situ green rocks

The Kerama Islands are located approximately 40 km west of Naha City, Okinawa Prefecture. The geology of the Kerama Islands consists mainly of sandstone, green rocks, slate and phyllite of the Kerama Formation. Because of its lithology, the Kerama Formation is comparable to the Nago and Kayo Formations in northern Okinawa Island, and is considered to be the southernmost part of the Shimanto Belt, which extends from the Boso Peninsula in Chiba Prefecture to the Kerama Islands. Since no fossils have been found in the Kerama and Nago Formations, their ages are unknown. Fossils of nummulite have been reported from the Kayo Formation above the Nago Formation. Therefore, the age of the Kayo Formation is considered to be Eocene, and the Nago Formation is considered to be Cretaceous based on the stratigraphic relationship between the Kayo Formation and the Motobu Formation (below the Nago Formation). In order to clarify the depositional age, we attempted U-Pb dating of detrital zircons in the massive sandstones of the Kerama Formation and in vein-like sandstone lenses showing boudin structure in green rocks using LA-ICP-MS. Here, we report preliminary U-Pb dating results, focusing on descriptions of outcrops of the Kerama Formation, and discuss the environment and formation at the time of deposition.