Depositional Age,provenance And Tectonic Background Of Neoproterozoic Strata In Western Henan Province

Since the new Chinese late Precambrian chronology was introduced by the National Commission on Stratigraphy in 2009,new chronostratigraphic framework of late Precambrian strata in each rift systems of North China Craton(NCC)have basically determined.However,the chronostratigraphic framework in Xiong'er aulacogen still was debatable,which locates in western Henan area and is one of the best exposed regions of late Precambrian strata of NCC because of relatively continuous outcrops and marked boundaries in the field.Based on zircon U-Pb-Hf isotope analysis,the depositional age,sedimentary provenance and tectonic environment of Neoproterozoic Luanchuan group and Hejiazhai formation in Wufoshan group are determined in this thesis.The detrital zircon U-Pb age and Hf isotope characteristics of Luanchuan group are similar to that of Hejiazhai formation in Wufoshan group.Their main peaks of detrital zircon U-Pb age are of 1.3 to 1.0 Ga,1.6 to 1.3 Ga with secondary peak of 1.85 to1.65 Ga.Since majority of detrital zircons have positive ?Hf(t)value,and are relatively concentrated near the neo-crustal evolution lines of 1.8Ga,2.1Ga and 2.5Ga,the provenance region experienced three important stages of Proterozoic continental crust growth,which are correspond to Proterozoic magmatic-metamorphic thermal events of the early Precambrian metamorphic basement in southern NCC.The youngest detrial zircon U-Pb ages of three metamorphic sedimentary rocks from Baishugou formation,Meijiaogou formation and Yuku formation in Luanchuan group are 991±72 Ma,884±20 Ma and 882±18 Ma,respectively,zircon U-Pb age of the metamorphic andesite in Dahongkou formation is 854 ± 8Ma,and the ages of~830 Ma and ~844 Ma for correlative intrusive rocks have been reported.The youngest detrial zircon U-Pb ages of two sandstones from Hejiazhai formation in Wufoshan group are 1057±47 Ma and 991±21 Ma.These age data suggested that depositional ages of Luanchuan group and Hejiazhai formation are in Qingbaikou Period of Neoproterozoic era,which is equivalent to that of Neoproterozoic strata in eastern marginal rift of NCC.Based on provenance analysis by detrial zircon U-Pb age and Hf isotope,it is suggested that the main provenance of Luanchuan group and Hejiazhai formation were Mesoproterozoic and early Neoproterozoic geologic bodies in the northern Qinling area,followed by Palaeoproterozoic geological body of Xiong'er groupvolcano rock and non-orogenic magmatite in southern NCC.Their sedimentary tectonic setting was passive continental marginal rifting environment.The eastern and southern margins of NCC constitute a tridential rift basin in Qingbaikou Period,whose rift center should be located to the south of Luanchuan area.It is suggested by regional comparison analysis that the Mesoproterozoic to Neoproterozoic tectonic belt,which was similar to North Qinling area,developed in eastern to southern margins of NCC.The tectonic belt widely recorded that the peripheral regions of NCC were strongly involved in the process of breakup of Columbia or Nuna supercontinent and convergence of Rodinia supercontinent,and were converted to the main provenance supply area of the sedimentary basin on the periphery of NCC in the Qingbaikoukou period of early Neoproterozoic.