Low-Pressure and High-Temperature Type Metamorphism on the Suo Belt From Ozushima Island, Seto Inland Sea Area, Yamaguchi Prefecture, Southwest Japan: Evidence From Detrital Zircon U–Pb Dating and Mineral Paragenesis

Abstract

Metamorphosed pelitic and psammitic rocks with small amounts of granitic stock are found on Ozushima Island, in the Seto Inland Sea area of Yamaguchi Prefecture, Japan. The metamorphosed pelitic rocks exhibit pervasive foliation defined by the preferred orientation of muscovite, biotite, andalusite, and sillimanite. General foliation shows an E–W trend and dips to the north and south, forming upright folds. The mineral assemblages and microstructures reveal that the metamorphic rocks of Ozushima Island can be divided into the Bt zone, And zone, and Sil zone, representing three metamorphic zones and corresponding to three deformational stages. Based on field surveys, the following deformational events were identified. Stage 1 deformation resulted in S1 foliation parallel to the bedding plane (S0). The critical minerals in each metamorphic zone formed the S1 foliation. In Stage 2, the S2 foliation developed along the axial planes of the microfolding of the S1 foliation. A NW-SE striking, NE-dipping thrust passed through the southern part of the island. In Stage 3, S3 foliation was locally present along the thrust plane. The metamorphic conditions of the And and Sil zones were estimated using conventional geothermobarometers at 530°C, 60 MPa and 600°C–710°C, 330–400 MPa, respectively. In addition, these metamorphic conditions plot within the metamorphic field gradient of the Cretaceous Ryoke metamorphic rocks from the Yanai-Iwakuni and Omuta areas. The detrital zircon grains separated from the psammitic rocks exhibited two age peaks at c. 1800 Ma and c. 250 Ma. A younger age indicates an older limit on the depositional age of the protoliths. These findings suggest that the metamorphic rocks of Ozushima Island originated from the Suo Belt and underwent low-pressure and high-temperature metamorphism, probably caused by heat supplied from the Cretaceous intrusive rocks, similar to the Ryoke metamorphic rocks.