Paleogeography and tectonic evolution of a late Paleozoic to earliest Mesozoic magmatic arc in East Asia based on U-Pb ages of detrital zircons from the Early Triassic Shingai Unit, Kurosegawa Belt, Southwest Japan

The Shingai Unit in the Kurosegawa Belt is subdivided into sedimentary zone and melange zone. Sandstones of the sedimentary zone vary from lithic wackes to feldspathic arenites in ascending stratigraphic order. However, the timing of this change and the depositional ages are not constrained. U-Pb age spectra for detrital zircons from sandstone samples have main clusters of late Paleozoic-earliest Mesozoic ages and minor Proterozoic-early Paleozoic age components. Detrital zircon ages show that the depositional age of the Shingai Unit is Early Triassic and that these grains were derived mainly from igneous rocks and hornfels of the Paleo-Ryoke Belt and from rocks of the Kurosegawa Belt. These age data combined with sandstone compositions and stratigraphic relationship suggest that basement rocks in the Kurosegawa Belt constituted the provenance area for the lower Shingai Unit before uplift of the volcanic arc, the provenance subsequently changed to a volcanic arc either because these rocks had been completely eroded or the supply of sediment derived from the volcanic arc increased. This provenance change supports progressive denudation of the volcanic arc. By comparison with the detrital age spectra, upper Permian-Lower Triassic strata in the Kurosegawa and South Kitakami belts are considered to have the same provenance. In addition, the sedimentary environment of these strata is inferred to have been a trench and shelf adjacent to an island arc that was isolated from the Sino-Korean and South China blocks by a back-arc basin.