The Sedimentary Environments And Tectonic Setting Of The Mesoproterozoic Bingmagou Formation In The Mianchi, Western Henan Province

As Precambrian supercontinent assembly,accretion and breakup,potential source of clastic sediments in the same time would have changed over time,which could record the tectonic evolution of basin to some degree.While the sedimentary records between the Precambrian and the Phanerozoic-Modern are different,the range of basin types and their fills in both time periods show non-meaningful deviations,and the present sedimentary records are sufficient to reflect the sedimentary processes and products in the early earth history.The early Mesoproterozoic volcanic-sedimentary successions crop out in the southern of the North China Craton(NCC),as a sedimentary stratum to known the evolution of the early Mesoproterozoic,the Bingmagou Formation records how the NCC changes from active to stable state,and it could constraint the evolution of the NCC.This Formation is considered to be the earliest clastic deposits and records the earliest information of tectonic evolution after the Xiong'er volcanism.In this contribution,we present petrological,sedimentological and whole-rock geochemical data for the Mesoproterozoic Bingmagou Formation in the west Henan province of the southest margin of NCC.Based on the comprehensive analysis of Bingmagou Formation in sedimentary facies,depositional environment,as well as paleogeography,the filling sequences in this early Mesoproterozoic basin have been identified.(1)The results suggest that is characterized by pebbly quartzite sandstone and coarse quartzites sandstone at the base,overlain by siltstone and argillaceous rocks,pebbly quartzites at the top.The Bingmagou Formation makes a set of transgression-regression,including delta plain,delta front,pro-delta subfacies,delta front,and the coastal facies from bottom to up.(2)Bingmagou Formation is consists of a series of pebbly sandstones,coarse-to-fine-coarse tograined sandstones and siltstones,and the average particle size ranged from 1.62-4.02 ?.The standard deviation was 0.45-1.03,the degree of sorting is general to well.The probability accumulation curve are mostly single-peak type,and the partly are double peaks.The particle size characteristics reflect the complicated changes of hydrodynamic,which is characterized by the fact that the hydrodynamic force is stronger to weaker to stronger.(3)The difference of light and heavy rare earth is obvious in Mianchi area,light rare earth elements was more contented and heavy rare earth elements was inadequate amounts.Intense chemical weathering of the Bingmagou Formation samples is indicated by their high plagioclase index of alteration values.The Bingmagou Formation was derived mainly from an old upper continental crust,and some recycle detritals add to the Bingmagou Formation sediments.The sedimentary rates of the Bingmagou Formation in Mianchi maybe slower than the sediments of the Jiyuan and Yichuan.(4)Five filling sequences have been identified in this early Mesoproterozoic basin,including(1)Sequence 1: alluvial fan with coarse clastic sediments recording the initial subsidence of the basin;(2)Sequence 2:(fan)delta subaerial facies during the initial transgression;(3)Sequence 3: transitional(fan)delta facies corresponding to the further transgression;(4)Sequence 4:(fan)delta marine deposits representing the maximum flooding surface;(5)Sequence 5: delta front-littoral deposits during the following regressive interval.All the sequences indicate that it should be formed in the extensive setting basin after the Xiong'er volcanism.Both at the top and the base,a regional unconformity developed significantly.(5)At the southern margin of the NCC,a series of terrestrial to marine sedimentary rocks deposited after ~1.78 Ga Xiong'er volcanism.The Bingmagou Formation and the early Mesoproterozoic sediments can be interpreted as a successor sedimentary sequence deposited in an extensional basin.After the end of Bingmagou Formation sediments,the southern margin of NCC experienced a brief uplift with a short period of erosion within an overall regressive setting,on which the middle and late Mesoproterozoic terrestrial and marine succession has been superimposed gradually.