THE LATE TRIASSIC-EARLY JURASSIC SEDIMENTARY PROVENANCE CHANGE IN ZIGUI BASIN: IMPLICATION FOR QINLING OROGENESIS

The propagation of orogen and migration of foreland basin could make a significantly large-scale change to the regional topography. Qinling-Dabie orogen marks the Triassic collisional suture between the North China and South China cratons. Through the analysis of the Zigui Basin in the southern margin of the orogenic belt, the evolution of the orogenic belt can be reconstructed.

The Late Triassic-Early Jurassic in the Zigui Basin is mainly composed of Shazhenxi and Tongzhuyuan Formations. The sandstones from the Shazhenxi Formation are deposited in delta environment and rich in monocrystalline quartz grains with very minor felspar grains and lithic fragments. The sandy matrixes from the braided channel conglomerate in the lowermost Tongzhuyuan Formation have high contents of polycrystalline quartz grains and contain sedimentary rock and low-grade metamorphic rock fragments. The fluvial medium grained sandstones from the Tongzhuyuan Formation contain more contents of lithic fragments mainly of volcanic origin. The sandstones of the Shazhenxi Formation and the conglomerates of the lowermost Tongzhuyuan Formation display a comparable pattern with two major age groups of 2100-1700 Ma and 900-700 Ma. Some grains of 240-200 Ma and 500-400 Ma are also present but quite few. For the lithic sandstones of the Tongzhuyuan. The main age component of detrital zircon is 250-200 Ma with very few grains of 900-700 Ma and 500-400 Ma.

Based on comparison of detrital zircon U-Pb age spectra, it is suggested that the detritus of 2100-1700 Ma and 900-700 Ma are derived directly or recycled from the Paleoproterozoic and Neoproterozoic basement of South China with the latter also possibly being sourced from the Qinling orogen. Integrated the sandstone modes and measured paleo-currents, the volcanic source of 250-200 Ma is likely situated within the Qingling Orogen although there are no reports of such rocks at present-day. Through the debris composition and zircon U-Pb geochronology analysis, it is speculated that in the Late Triassic to Early Jurassic, there are two significant changes in the sediment source in the basin. Considering the north-to-south paleo-currents, the siliceous gravels and polycrystalline quartz grains of the Early Jurassic conglomerates are rare in the underlying Shazhenxi Formaiton and indicate a recycled sedimentary source within Qinling Orogen. They are likely derived from the massive erosion of the southward foreland fold and thrust belts newly incorporated into the southern margin of the growing orogen.