Detrital anisotropic grandite garnet as an indicator of denudation level of Permian volcanic arc in the provenance of the South Kitakami Belt, Northeast Japan

摘要

In this study, the chemical and optical features of detrital garnets from the Middle Permian to Upper Triassic sandstones in the South Kitakami Belt, Northeast Japan, were examined to reveal the tectonic movement in the provenance. The sandstones contain a large amount of detrital grandite garnet grains with a wide range of andradite content. Among them, some grandite garnet grains show optically anisotropic features and rarely oscillatory zoning and sector twinning. The proportion of the detrital anisotropic grandite garnet increases from the Permian to the Middle Triassic and decreases in the Late Triassic. Such grandite garnets with various andradite contents occur in skarn deposits. Isotropic grandite garnets in the early stage of skarn evolution are distributed widely around a pluton, which affects the thermal metamorphism of the surrounding strata. However, anisotropic grandite garnets are formed along veinlets and fractures in the middle to late hydrothermal stage as the pluton cools, and their distribution is limited to a narrower area near the pluton compared to the metamorphic aureoles formed in the early stage. Changes in the chemical and optical features of the detrital garnets indicate a progressive denudation of the plutonic body accompanying skarn deposits in the provenance. The proportion of detrital anisotropic grandite garnet grains among all of the detrital grandite garnet is considered to be a sensitive indicator of the denudation level in a deeper part of the volcanic arc in association with skarn deposits, together with traditional sandstone composition datasets. This newly proposed method should be useful for clarifying the paleogeography during the Permian to Triassic in the East Asian continental margin, associated with uplift and denudation of the Permian volcanic arc, which seems to have been induced by the collision of the North China and South China Blocks.