Analysis Of Sedimentary Age,provenance And Tectonic Background Of Fengyang Group And Bagongshan Group Of Precambrian In Fengyang Area

The Neoarchean-Paleoproterozoic Wuhe Group and the Fengyang Group crystalline basement and the Neoproterozoic-Paleozoic marine sedimentary caprocks in the Fengyang area on the southeastern margin of the North China Craton developed well,with clear sequence,it has long attracted the attention of geologists.However,the basic geological problems such as the precise age attribution,provenance and sedimentary tectonic background of the Precambrian strata are still controversial.Two youngest age of detrital zircons,2292±44Ma and 230 ±45Ma,are obtained from quartzite at the bottom of Fengyang group.Combined with regional metamorphism age of 1865±23Ma,the sedimentary age of Fengyang group should be between 2.3 and 1.86 Ga.The detrital zircons age peak from Fengyang group are dominated by ~2.5Ga,2.7~2.9Ga and 3.3~3.5Ga.The most likely provenance was the Huoqiu/Wuhe complex in Xuhuai area,and the sedimentary tectonic environment was the passive continental margin rift.The weighted average age of Neoproterozoic detrital zircons in the Liulaobei formation and Silichangshan formation of the Bagongshan group is 986±21Ma(MSDW = 0.106,n=6),which restricted that the sedimentary age of the Bagongshan group should be less than 986 Ma and be in Qingbaikou period.The peak of detrital zircons age spectra from the Wushan formation in lower Bagongshan group are dominated by ~1.85 Ga,~2.1Ga,and ~2.35 Ga.The most likely provenance was the central and southern part of the Central Orogenic Belt(especially in the western Henan region),and the sedimentary tectonic environment was the passive continental margin.However,the age of detrital zircons from the Silichangshan and Liulaobei formations in upper Bagongshan group are dominated in mesoproterozoic,which are similar to those of detrital zircons from the qingbaikou system around the north China Craton.The provenance was transformed into the Meso-and Neoproterozoic tectonic belt similar to the North Qinling area,and the sedimentary tectonic environment was transformed into the collision orogenic environment,which indicates that the east magrin of the North China Craton transformed from the passive continental margin into the active continental margin in the early neoproterozoic.The age spectra of detrital zircons from the Fengyang group,Wushan formation,Liulaobei formation and Silichangshan formation record the different stages of the Precambrian crustal evolution of the North China Craton.Therein,those of the Fengyang group record the evolution of crustal growth of craton in the archean,the Wushan formation record the evolution of final formation period of craton in the paleoproterozoic,and the Silichangshan formation and the Liulaobei formation record that the margin of the North China Craton was strongly involved in the process of Columbia supercontinent breakup and Rodinia supercontinent assembly in the Mesoproterozoic,which indicated the inside-out lateral crustal growth process of the North China Craton.