[M-IS23] New frontiers in geology

[MIS23-P10] Geochemistry and zircon U–Pb geochronology of the Oura plutonic complex and Northern zone of the Maizuru Belt, southwest Japan

*Shohei Hashimoto¹, Yuji Ichiyama¹, Hisatoshi Ito², Akihiro Tamura³, Tomoaki Morishita³ (1.Chiba University, 2.Central Research Institute of Electric Power Industry, 3.Kanazawa University)

Keywords:Maizuru Belt, Yakuno Ophiolite, Oura plutonic complex, zircon U-Pb geochronology, South China Craton

The Japanese islands have ca 700 million years of history as an orogenic belt after the breakup of the Rodinia supercontinent (Isozaki and Maruyama, 1997). However, most Paleozoic or older basement rocks have been lost due to tectonic erosion (Suzuki et al., 2010). The Early Paleozoic Oeyama Ophiolite and Late Paleozoic Yakuno Ophiolite distributed in the Inner Zone of southwest Japan are most likely to contain crucial information for reconstructing the Paleozoic structure and evolutionary processes of the eastern margin of the proto-Asian continent. The Yakuno Ophiolite dominates the southern zone of the Maizuru Belt, whereas the northern zone of the Maizuru Belt is composed of a large amount of granitic rocks and a gabbroic massif with granitic and metamorphic blocks. Recently, Early Paleozoic and Paleoproterozoic granitoids have been discovered in the northern zone and the Tsuwano area of the Maizuru Belt, and it has been proposed that the Maizuru Belt was fragments of the eastern margin of the proto-Asian continent ranging from the Central Asia Orogenic Belt to North China Craton (Kimura et al., 2021). In the present study, we carried out the zircon U-Pb dating and petrological characterization of metaplutonic rocks and pelitic schist in the Oura complex and northern zone of the Maizuru Belt to reconstruct the tectonics in the eastern margin of the Paleozoic proto-East Asia. The Oura plutonic complex, located in the Oura Peninsula, northern Kyoto Prefecture, is composed of metagabbro, metadiorite, and ultramafic cumulate (Agata, 1974). The chemical composition of metagabbro in the Oura complex indicates the formation from MORB-like magma, whereas that of metadiorite shows island arc affinities. The zircon U-Pb ages of the metagabbro and metadiorite range from 261.0 ± 2.1 to 292.1 ± 3.9 Ma, which are consistent with Permian ages reported from the Yakuno Ophiolite (around 290 Ma; Suda et al., 2014). Nakae et al. (2022) regarded the Oura complex as a member of the Oeyama Ophiolite, but the radiometric data presented in this study indicate that the Oura complex is part of the Yakuno Ophiolite as proposed by the previous study (Kano et al., 1959). Metadiorite and metagranite in the northern zone show island arc affinities. Two gneissic metagranite samples show almost the same zircon U-Pb age as those of the Yakuno Ophiolite (290.4 ± 1.9 Ma and 313.3 ± 1.9 Ma), but the other two samples contain Early Paleozoic (421-510 Ma) and Proterozoic (583-1780 Ma) zircons in addition to Permian zircons (269-298 Ma). Detrital zircons from the pelitic schist have the main U-Pb age clusters of Early Paleozoic (422-487 Ma) and Neoproterozoic (553-920 Ma) with Meso-Paleoproterozoic ages (1114-2474 Ma). The presence of Early Paleozoic and Neoproterozoic zircons in the rocks of the northern zone implies that they were formed in the marginal areas where the Oeyama Ophiolite was distributed on the South China Craton. Ishiwatari et al. (1990) and Ichiyama and Ishiwatari (2004) suggested that the Yakuno Ophiolite is originated from fragments of a Permian back-arc basin. The Maizuru Belt might have been developed as a Permian arc-backarc basin system formed by the breakup of the continental crust at the eastern margin of the South China Craton, and the pre-Permian rocks reported from the northern zone might have been continental fragments left behind in the Maizuru arc-backarc system.