Provenance Tracing of Detrital Zircons from Nanhua System in Bikou Microblock, Northwestern Margin of Yangtze and Its Geological Significance

A set of gravel-bearing sedimentary strata developed above the Bikou Group in the Huayansi area of the Bikou microblock on the northwestern margin of the Yangtze Plate. The exact time limit of its deposition is of great significance for studying the depositional characteristics and tectonic evolution of this area. In this paper, the detrital zircon LA-ICP-MS U-Pb dating analysis of the two sandstone samples in this set of formations shows that the two samples of detrital zircon have good average crystal form, colorless and transparent, and relatively roundness. Poor, with typical magma growth oscillation zone and rhythm structure inside, Th/U ratio is 0.28~3.6, zircon U-Pb age is between 706~2489 Ma. There are three main age groups: Neoproterozoic age group (706~951 Ma)(92.9 percent), with significant peaks of 850 Ma and 843 Ma; Mesoproterozoic age group (1017~1080 Ma) (2.6 percent ) and Paleoproterozoic age group (1628~2489 Ma) (4.5 percent ), the smallest age group 706~715 Ma (peak value is 711 Ma), combined with the regional geology and research dates, the sedimentary stratigraphic age of the Huayansi area should belong to the Nanhua Period, and the sedimentary time limit is about 720~635 Ma. The source area includes the Bikou micro-block on the northwestern margin of the Yangtze Plate, the Houlongmenshan tectonic belt and the Hannan-Micangshan microblock on the southeast side. In the late Neoproterozoic Bikou microblock and the post collision cracking stage of the northwest margin of the Yangtze plate, the Nanhua sedimentation in Huayansi area was formed by the rapid accumulation of Bikou microblock and Neoproterozoic magmatic rocks in the adjacent area in the marginal rift environment.