Neoproterozoic Rock Assemblage Of Likou Group From Eastern Part Of Jiangnan Orogen,South China:Implications For The Tectonic Framework

Neoproterozoic epimetamorphic basement exposures in the eastern part of Jiangnan Orogen?EJO?,which recorded assembly and breakup of the EJO,and revealed the EJO is formed by multi-island arc aggregation.Present studies indicate that Neoproterozoic orogenesis finished at about 825 Ma,post-orogenic collapse shortly after it,but had distinct ideas about the regime of collapse.Likou group is a succession of volcanic-sedimentary stratum,which distributes to the north of Qimen-Qiankou-Shexian fault zone in the adjacent region of Anhui and Jiangxi,South China.It is the upper part of Neoproterozoic epimetamorphic basement in the eastern part of Jiangnan orogen.In this paper,in order to analyze the characteristics of Neoproterozoic rift basin in the EJO,we observe five sections of Likou Group and carry out zircon U-Pb dating,zircon Lu-Hf isotope analysis and whole rock geochemical analysis to basalts from the upper part of Likou Group.Then,we conduct a discussion of the basin type represented by Likou Group.Last but not least,we provides evidence for the post-orogenic collapse evolution of the EJO.Interbedded basalt from Dengjia formation and basalt from Pulin formation have CAB-like REE distribution??La_N/Yb_N?=1.46².89?and trace-element patterns.Interbedded basalt from Dengjia formation and basalt from Pulin formation have Rittmann Index of 0.029².203 and 2.433⁷.064,respectively,suggesting they have been generated coevally but at different stages.Pulin formation serve as the upper Dengjia formation and Xiao'anli formation serves as the top Dengjia formation.Interbedded basalt from Dengjia formation is dated at?804.2±7.4?Ma by high-precision LA-ICP-MS U-Pb zircon dating.The zircons from interbedded basalt have initial ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.281837 to 0.282076,?_Hf?t?range from-15.9 to-7.0 and two stage Hf model ages range from 2.16 to 2.68 Ga,indicating the magmatic source had been strongly contaminated by ancient crust,which represents the middle stage of collapse had finished,and magmation evolved to the earth surface.Dengjia formation,Pulin formation and Xiao'anli formation of Likou group have clear boundary surfaces and successive forming process,therefore they should be merged into one formation which named Dengjia formation as before.The sedimentary characteristic of Likou group indicates that Neoproterozoic rift basin in the EJO is a passive rift basin,which was controlled mainly by rollback of subducting plate,whereas Rodinia plume made significant influence to the tectonic evolution.The collapse is resulted from interaction between plate subduction and Rodinia superplume.