Sandstone petrography and geochemistry of the Icheonri Formation in southeast Korea: Implications for provenance and tectonic setting

 The Gyeongsang Basin, situated in the southeastern part of the Korean Peninsula, is a non-marine sedimentary basin that formed as a result of the subduction of the Paleo-Pacific Plate beneath the East Asian continent during the Cretaceous. While numerous studies of the basin fill (Gyeongsang Supergroup) of the Gyeongsang Basin have revealed the tectonic history of East Asia, most of them have focused on the western part of the basin, with less attention paid to the strata in the eastern part of the basin. However, the geological units currently consisting the Japanese islands originally developed along the eastern margin of the Korean Peninsula during the Cretaceous. The Icheonri Formation, distributed in the southeastern part of the Gyeongsang Basin, probably records the composition and tectonic settings of the provenance along the continental margin. This study investigates the provenance and tectonic history of the Icheonri Formation based on sandstone petrography, detrital mineral chemistry, and whole-rock geochemistry of sandstones and mudrocks. The analyzed sandstone samples of the Icheonri Formation are very fine- to coarse-grained, subangular, and poorly sorted. They are mainly composed of quartz, plagioclase, and rock fragments, with minor grains including chromian spinel, pyroxene, and epidote. Based on the modal composition, the sandstones are classified as feldspathic litharenite, derived from a transitional arc provenance. In addition, volcanic rock fragments are nearly absent in the studied samples, indicating weak volcanic activity in the southeastern part of the Gyeongsang Basin during deposition of the Icheonri Formation, contrary to previous interpretations from the western part of the basin. Whole-rock geochemistry suggests that the source rock composition is felsic to intermediate, derived from a continental arc. Furthermore, the occurrence of detrital Cr-spinels suggests the presence of ultramafic bodies exposed in the provenance. The composition of Cr-spinel grains from the Icheonri Formation indicates that they were originated from mantle-wedge peridotites. Together with the occurrence of highly weatherable pyroxene, this suggests that forearc peridotite containing Cr-spinels and pyroxene-bearing mafic rocks were exposed near the Icheonri depocenter. Previous studies have reported Cr-spinels from forearc peridotite in sandstones from the western Gyeongsang Basin and interpreted that the peridotite was exposed along at least two fault zones that developed in the west of the basin as a result of oblique convergence. In conclusion, while subduction zones and volcanic arcs developed around the basin, the Icheonri sediments were derived from eroded volcanic arc or basement during the cessation of volcanism rather than from active volcanoes. Moreover, the forearc peridotite is interpreted to have been exposed by protrusion along fault zones that probably formed around the eastern part of the basin as a result of subduction tectonics. Our result contributes to a better understanding not only of the Cretaceous tectonic developments along the East Asian continental margin, but also of the nature of arc-related sedimentary basins and basin fills.