Evidence for Late Permian-Triassic volcanism in the Hida Gaien Belt, Southwest Japan: New U-Pb ages from the Motodo, Ashidani, and Otani formations

We carried out a U-Pb geochronological study of the Motodo, Ashidani, and Otani formations in the Hida Gaien Belt of the Kuzuryu area, Fukui Prefecture, in the northeastern part of Southwest Japan, to constrain their age of deposition. The LA-ICPMS U-Pb zircon ages we newly obtained are as follows. Andesitic volcaniclastic conglomerate and tuff breccia of the Lower Member of the Motodo Formation were dated at 254.2 ± 2.5 Ma (2σ) and 253.0 ± 3.6 Ma, respectively. A granodiorite clast of the Middle Member of the formation was dated at 252.6 ± 3.6 Ma. The detrital zircons from the sandstone of the Middle Member of the Motodo Formation and the Ashidani Formation both showed a unimodal age distribution centered at 272-271 Ma. Volcaniclastic sandstone of the Otani Formation showed a multimodal age distribution with peaks at 241 Ma, 433 Ma, 493 Ma, 1720 Ma, and 2409 Ma. Considering the age of tuff breccia and the conformable or interfingering relationship between the members of the Motodo Formation, we interpret that the formation is of Late Permian to Early Triassic age. The youngest age-peak from the volcaniclastic sandstone of the Otani Formation, 243 Ma, presumably indicate the age of syn-sedimentary volcanism, suggesting that the formation is of Middle Triassic age. The Paleoproterozoic zircons in the Otani Formation, on the other hand, indicate the sediment supply from a continent in the Middle Triassic. Combining the new data and the results of previous studies, it is concluded that the Hida Gaien Belt holds evidence for continual volcanism from the Ordovician to the Late Triassic.