Geochemical characteristics and tectonic setting of Carboniferous sandstone in the eastern part of north Qiangtang depression

In order to explore the evolution history of Carboniferous Qiangtang basin, samples were collected from Early Carboniferous Zaduo Group sandstone in the Kemo area, the east of north depression of Qiangtang basin, to carry out the geochemical studies.The results indicate that the sandstones mainly consist of K-feldspar, calcite, magnesium biotitea and quartz etc.The chemical indexes of weathering (CIW), alteration (CIA) parameters and the A-CN-K plot of the sandstone suggest that the clastic constituents were moderately weathered in the area, and the clastic constituents underwent K-metasomatisim and replacement of feldspar by illite during weathering processes.The chemical composition index (ICV) indicates that some clastics represent the first cycle sediment, which is not affected by sedimentation sorting and recycling.The A-CN-K diagram also shows that the content of plagioclase in sandstone source rocks is higher than that of potash feldspar, and the variation is mainly between granite and granodiorite.The characteristics of major, trace and rare earth elements indicate that the sedimentary tectonic background of sandstone represents active continental margin and continental island arc, which suggests that the ancient Tethys Ocean began to enter the stage of subduction extinction and shrinkage in the Early Carboniferous.