Petrogenesis of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: implication on Miocene volcanism related to back-arc rifting in San’in region

抄録

Shimane Peninsula in the San’in region, SW Japan, has preserved a wide distribution of igneous rocks related to back-arc rifting in the Miocene. We investigated the petrography, geochemistry, and Sr-Nd isotope systematics of rhyolite lavas (Josoji rhyolite: 18-15 Ma) and basaltic to dacitic intrusions of Stage I (∼ 14 Ma) and II (∼ 13 Ma) intrusive rocks. They are classified as medium-K magma series. The Josoji rhyolite and Stage I rocks show chemical compositions of arc-type signatures, whereas the Stage II rocks show elevated Nb and Ta abundances suggesting weaker arc signatures. The geochemical characteristics indicate that the Josoji rhyolite was produced by partial melting of arc-type basalt under lower to middle crustal conditions. The arc signatures of the Stage I rocks were inherited from a remnant metasomatized lithosphere formed by the subduction of the Pacific Plate before the opening of the Japan Sea. The Stage II rocks were formed from a fertile magma that might be produced by the melting of upwelling asthenospheric mantle. We conclude that various magmatic processes took place during and just after the back-arc rifting development in the San’in region.