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

[SGL23-03] New insights from unique clasts in the Misaki Formation, Jogashima, Kanagawa Prefecture

*Mitsuhiro Nagata¹, Ouhiko Miyawaki¹, Tatsuo Kanamaru¹, Sota Niki², Takafumi Hirata³, Shinji Takeuchi¹ (1.Department of Earth and Environmental Sciences, College of Humanities and Sciences, Nihon University, 2.Division for Chronological Research, Institute for Space-Earth Environmental Research, Nagoya University, 3.Geochemical Research Center, The University of Tokyo)

Keywords:Sambagawa Belt, zircon, Kanagawa Prefecture, Miocene, accretionary prism

The Misaki Formation, a Miocene to Pliocene (ca. 10-4.7 Ma) accretionary complex, and the Hatsuse Formation of the Miura Group, a trench slope deposit, are known to occur on Jōgashima Island, southern Miura Peninsula, Kanagawa Prefecture, Japan. The Misaki Formation consists of clastic rocks with scoria clasts and is unconformably overlain by the Hatsuse Formation. The clasts in the Misaki Formation are mainly scoria derived from the northern extension of the Izu-Ogasawara Arc. Otherwise, accidental clasts such as volcanic and plutonic rocks have been reported (e.g. Suzuki and Hirano, 2016). The origin of the accidental clasts has been interpreted as xenoliths from the Paleo-Izu Arc volcano (e.g. Suzuki and Hirano, 2016).
The authors discovered a unique clast of high-P/T metamorphic rocks in the Misaki Formation for the first time, and describe its significance below. The clast was found in the lower part of the Misaki Formation, and no sedimentary structures were observed around host rock (sandstone). The clast has clear schistosity, but no porphyroblasts are observed. The mineral assemblage of the clast suggests that it is a low-grade (chlorite zone) pelitic schist. Zircon grains were separated from the clast and U-Pb dating was carried out using LA-MC-ICPMS at the Geochemical Research Center, The University of Tokyo. The dates obtained are ca. 2371-92 Ma, which is similar to the age spectra reported from the Southern Unit of the Sambagawa Belt in the Kwanto Mountains (Tsutsumi et al., 2009). The dating results, together with the fact that the Southern Unit of the Sambagawa Belt is classified as the chlorite zone, suggest that this metamorphic clast is derived from the Southern Unit of the Sambagawa Belt in the Kwanto Mountains. A Pterotorigonia (trigonia fossil) has been reported from the Misaki Formation in Misaki-Machi (Matoba, 1964), on the opposite shore of Jōgashima. In the Kwanto region, the fossils have been reported from the Sanchu-Choshi groups. This report supports the interpretation that the metamorphic clast originate from the Sambagawa Belt. Although the main provenance of the Misaki Formation is considered to mainly an extension of the Izu-Ogasawara Arc, this suggests that the Misaki Formation was also supplied with clastic material from the north (around the Kwanto Mountains; Figure). The authors are searching for further unique clasts from the Misaki Formation, and the stratigraphy and geochronology of the Misaki Formation will also be discussed.