Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications

Abstract

The Akiyoshi Belt in the Inner Zone of Southwest Japan is made up of a Permian accretionary complex, the main component of which is the Late Permian Nishiki Group that mainly consists of sandstone, mudstone, felsic tuff and a minor amount of chert and conglomerate. We employ multiple methods, which includes sandstone petrography, detrital garnet composition and detrital zircon U-Pb dating to investigate the likely sources of these terrigenous deposits and to reconstruct the paleogeographic link between the proto-Japan and the East Asian continent. The highly immature Late Permian sandstones are interpreted to derive from multi-type source rocks that include felsic igneous rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks in proximal locations. The detrital zircon U-Pb results show that all samples contain a dominant Early to Late Permian zircon population (294–254 Ma) and these zircons are interpreted to be derived from an active volcanic arc, which was most likely caused by subduction of the Paleo-Pacific plate beneath the South China Block. We found that spessartine-rich almandine garnets and almandine garnets dominated assemblages in the lower unit changed to grossular-andradite garnets dominated assemblage in the upper unit, which was caused by a progressive uplifting and denudation of the Permian volcanic arc.