Detrital garnet and chromian spinel chemistry of Permian clastics in the Renge area, central Japan: Implications for the paleogeography of the East Asian continental margin

The Japanese Islands comprise a complex assembly of allochthonous continental shelf deposits and subduction-related accretionary complexes. The paleogeography of Permian clastic strata was restored based on chemical analysis of detrital garnets and chromian spinels, which are useful for provenance discrimination.Detrital garnet assemblages from the Permian clastic rocks in Japan are characterized by a grandite-rich assemblage and a grandite-poor assemblage. Most trench-fill deposits in Permian accretionary complexes contain the grandite-poor assemblage, which is composed of spessartine-rich almandine, grossular-rich almandine, and rarely pyrope-rich almandine, and is commonly associated with chromian spinels and volcanic rock fragments. It is suggested that the sediments were derived from a volcanic arc and associated non-calcareous contact-metamorphic rocks along the North China Block, where pelitic granulites including pyrope-rich almandine garnets are extensively exposed in the northern margin. Ophiolite and high-P/T schist also crop out along the continental margin.Most Permian shelf deposits and the trench-fill deposits of the central part of the accretionary complexes in Japan contain the grandite-rich assemblage. Analyses suggest provenance from a calc-alkaline volcanic arc and association with calcareous contact-metamorphic rocks including skarn deposits. Sediments lack ophiolite detritus. A part of the grandite-poor trench-fill deposits of the accretionary complexes is overlain by grandite-rich clastic rocks. This indicates that the grandite-rich detritus increased and the supply extended through the fore-arc basin to the trench.The trench-fill deposits in the northernmost part of the Permian accretionary complex in the Renge area yield a grandite-poor assemblage, particularly uvarovite-rich andradite garnets and chromian spinels derived from a back-arc basin basalt, which is very similar to the clastic composition of the Permian back-arc basin strata of Japan. Hence, we conclude that a back-arc basin existed in the northernmost region of the Permian subduction zone along the North China Block.