Sandstone petrography and whole rock geochemistry of the Ashio terrane, a Jurassic accretionary complex in Japan: Implications for provenance and tectonic setting

Turbiditic sediments found in accretionary complexes (ACs), originally deposited in a trench environment, document processes and environmental conditions governing sediment production and deposition in a subducting continental margin, including tectonic setting and paleogeographic configuration. Along the East Asian continent, subduction of the Paleo-Pacific plate has continued since the late Paleozoic, resulting in the development of numerous ACs containing Paleozoic and Mesozoic sediments that now make up a large part of the Japanese Islands. The Ashio terrane, distributed in central Japan, consists of Upper Paleozoic-Jurassic pelagic and turbiditic sediments and is considered equivalent to the Mino-Tamba terrane, the AC covering a large area of the main island. This study examines the sandstone petrography and whole-rock geochemistry of turbiditic sandstone and shale of the Ashio terrane to interpret their provenance and tectonic history. The sandstones of the Ashio terrane, classified as lithic arkose to feldspathic litharenite, are sub-angular to sub-rounded and poorly sorted. The major framework grains are quartz, plagioclase, K-feldspar, and lithic fragments (metamorphic and sedimentary, including chert), with minor amounts of mica, garnet, and heavy minerals. In the Qt-F-L diagram of provenance tectonic setting, they are all interpreted to have been derived from a recycled orogen provenance. Based on the presence of chert grains, we hypothesize that at least part of the pre-Jurassic ACs was probably uplifted and exposed in the hinterland during deposition of the Ashio terrane sediment. Furthermore, the presence of easily weatherable feldspar and metamorphic lithic fragments suggests that not only the ACs sediment but also metamorphic and/or igneous rocks were exposed in the source area. The detrital garnet assemblages in the Ashio terrane sandstones are characterized by a pyrope-rich almandine garnet with low grossular content, reflecting their origin in granulite-facies metamorphic rocks. Such detrital garnets have been previously reported from Jurassic to Cretaceous sandstones of Japan with rare occurrences in Permian sandstones, suggesting that the Ashio terrane garnets were likely first-cycle detritus from a nearby high-grade metamorphic basement. Based on their rare earth element patterns, the source rock composition of the Ashio terrane sediments seems felsic and more fractionated than the upper continental crust (UCC), which is known to be similar to granodiorite. In the A-CN-K compositional space, the Ashio terrane sediments suggest weathering from granodioritic source rocks, and the scatter of the samples along the weathering trend indicates that non-steady-state weathering conditions in the source area. In a tectonic discrimination diagram based on chemistry, their composition resembles sediments derived from a passive continental margin, which is consistent with the sandstone modal composition. Previous studies of Cretaceous Japanese ACs suggested sediment recycling and supply from uplifted pre-existing ACs, supporting our interpretation of the Ashio terrane provenance. Additionally, the compositional difference between the sandstones of the Mino terrane (arkosic) and Ashio terrane (lithic) infer a heterogenous nature of depositional conditions in a subduction zone. These findings provide paleogeographic information and more comprehensive interpretations of the environmental conditions surrounding the subduction zone along the East Asian continental margin during the Mesozoic.