A study of detrital zircons of Late Carboniferous-Middle Triassic strata in the northern margin of North China block and its geological implication

In order to interpret the uplift history of Inner Mongolia Paleo-uplift and understand the tectonic evolution of Paleo-Asian Ocean, the authors conducted U-Pb dating and Hf isotope analysis of detrital zircons from distinct levels of Late Carboniferous-Middle Triassic strata in Pingquan. U-Pb ages of detrital zircons from five samples fall into five groups, i.e., Group 1 of Middle-Late Permian (270~250Ma), Group 2 of Late Carboniferous-Early Permian (325~290Ma), Group 3 of Devonian (400~360Ma), Group 4 of Paleoproterozoic (1900~1700Ma) and Group 5 of Neoarchean-Paleoproterozoic (2600~2400Ma). Among these five samples, age spectra of Benxi Formation are composed of Group 3, Group 4 and Group 5, age spectra of Shanxi Formation, Sunjiagou Formation and Liujiagou Formation mainly consist of Group 2 and Group 4, and age spectra of Lower Shihezi Formation are composed of Group 1, Group 4 and Group 5. In addition, zircons from the five samples have the same Hf isotope characteristics. εHf(T) values of Phanerozoic zircons range from -3.2 to -25.5, consistent with the values of Late Paleozoic plutons in Inner Mongolia Paleo-uplift. The zircon age constituents and Hf isotopes suggest a provenance from Inner Mongolia Paleo-uplift, and the zircon age variation through different levels implies two critical uplifting events, which occurred during 360~312Ma and 276~258Ma respectively. 360~312Ma uplift built up the tectonic framework of the North China basin and provided sediments for the basin. After 276~258Ma uplift event, Inner Mongolia Paleo-uplift was steadily denuded and thus supplied a great amount of coarse-grained sediments to the sedimentary basin which had unchanged U-Pb age spectra of detrital zircons, suggesting that the Paleo-Asian Ocean might have been closed before 276Ma.