Tectonic setting and development of the Upper Paleozoic volcano-sedimentary sequence in the Hida-gaien belt (Inner Zone of Southwest Japan) based on zircon U–Pb ages and geochemistry

Abstract:

Late Paleozoic magmatic activity of the Paleozoic terranes in Japan is a key for an understanding of the “proto-Japan arc”; however, its detail has not been studied since the distribution of volcanic/pyroclastic rocks is very limited. In the present study, U–Pb dating and geochemistry for a Carboniferous–Lower Permian volcano-sedimentary sequence of the Hida-gaien belt have been conducted.

In the Hida-gaien belt, the Arakigawa Formation consists mainly of volcanic/pyroclastic rocks and contains partly Early–Late Carboniferous brachiopods and fusulinids. The lower part of this formation is subdivided into lower mafic and felsic volcanic rocks and upper mafic and felsic tuff with limestone. Zircon U–Pb age from volcaniclastic sandstone of the upper unit indicates a clear peak of 334 Ma. Based on SiO₂-Na₂O+K₂O diagram, mafic volcanic rocks and felsic volcanic rocks of the lower unit plot in the “basalt to basaltic andesite” fields and the “rhyolite” filed, respectively. After forming of the lower part of the Arakigawa Formation, shallow marine shelf carbonates (Ichinotani Formation) were accumulated. The Lower Permian Mizuyagadani Formation is the strata above the stratigraphic level of the upper part of the Arakigawa Formation and the Ichinotani Formation. The lower part of the Mizuyagadani Formation is characterized by radiolarian-bearing tuff and mudstone. The middle to upper parts of this formation are composed mainly of sandstone, mudstone, and conglomerate. Zircon U–Pb ages from felsic tuff of the lower part and volcaniclastic sandstone of the middle part show the youngest peaks of 284 Ma and 268 Ma, respectively. These data indicate that the magmatic activity of the Hida-gaien belt occurred intermittently during a Carboniferous–early Middle Permian time. In addition, the Late Carboniferous carbonate platform was formed broadly at basins of the proto-Japan arc after Early Carboniferous bimodal volcanism, based on this study and the geochemical result by Kawamura and Kawamura (1989) in the South Kitakami belt of Northeast Japan.