Lithofacies, geological and geochemical characteristics and tectonic setting of Late Carboniferous volcanic-sedimentary rocks in the Kumtag-Shaquanzi area, East Tianshan

Abstract：Ten lithofacies and their sublithofacies, including piedmont diluvial facies (fluvial facies), subaqueous delta facies, tidal flat facies, neritic facies, volcanic-sedimentary facies, volcanic outpouring facies, volcanic eruption facies, hydrothermal deposition facies, volcanic hydrothermal alteration facies and skarn facies, are recognized based on the lithofacies types and characteristics of Late Carboniferous sedimentary and volcanic rocks in the East Tianshan, Xinjiang. These characteristics indicate that the association of volcanic-sedimentary lithofacies formed on active continental margins in a continental compressional regime. The structure of vertical facies sequence indicates the strong volcanism occurred above and below the hydrothermal depositional facies, which provided heat needed for mineralization. In the Late Carboniferous Kumtag compressional basin, the vertical lithofacies sequence structure of the hydrothermal depositional facies is hydrothermal breccia subfacies→siderite rock/gypsum rock subfacies→ankerite rock subfacies→jasper rock subfacies→ankerite rock subfacies. This association is the lithofacies indicator for looking for large Kumtag type gypsum-siderite deposits in the East Tianshan. The general depositional sequence of the volcanic-sedimentary facies in the study area shows the trend of upward deepening facies sequence and then upward shallowing facies sequence. The provenance of clastic rocks was mainly the erosion area of the Variscan island arc zone in the north and the Central Tianshan basement uplift in the south. However, subaqueous volcanic eruption still took place in a neritic environment, showing the sedimentary characteristics of a hinterland basin in a continental compressional regime (Central Tianshan orogenic belt). The Late Carboniferous prototype sedimentary basin is likely to be a hinterland basin, which is favourable for preservation of large gypsum-siderite deposits.