Petrogenesis And Implications Of The Neoproterozoic Basic Rocks In South Anhui Province,jiangnan Orogen

The Jiangnan Orogen,located between the Yangtze Block and Cathaysia Block,the key area for understanding the crustal evolution?magmatism and metallogenesis of South China Block.However,the tectonic evolution of the Jiangnan Orogen still remains controversial.An integrated study of petrology,zircon U-Pb geochronology,geochemistry and Sr-Nd isotope geochemistry was carried out on the basalts from South Anhui Province,eastern Jiangnan orogen.LA-ICP-MS zircon U-Pb dating show that the basalts formed at 817±13Ma.They are enriched in LILE(Rb,Th,Pb),and also have high Nb and Ti O2 contents suggesting they have been metasomatized by both melts and fluids.They have a slightly positive?_Nd(t)values(+1.9?+2.1),indicating a relative enriched mantle source than MORB.These geochemical and isotopic signatures suggest that they are derived from lithospheric mantle metasomatized by both melts and fluids released from the subducted slab.Their REE and trace elements patterns are similar to the Okinawa Trough basalts.On the tectonic discrimination,Yingchuan basalts exhibit similar characteristics to the adjacent back-arc basins,implying that they were formed in a back-arc basin setting.Their formation age and tectonic setting suggest that the Yingchuan Formation is corresponding to the underlying basement of Jiangnan orogen,and is inappropriately incorporated into Jingtan Formation.The basalts of Xikou Group were approximately formed at 840 Ma.They were enriched in LILE?LREE,and depleted in HFSE and HREE,resembled that the basalts formed in island arc region.Their trace elements and Sr-Nd isotopic characteristics suggest they were formed in enriched mantle wedge which previously metasomatized by fluids.On the tectonic discrimination diagrams,they show both the characteristics of IAB and MORB,indicating they are formed in a back-arc basin.The occurrence of the ca.840 Ma-820 Ma back-arc basin basalts in eastern Jiangnan Orogen shows that there was back-arc basin in the eastern Jiangnan Orogen,indicating that the subduction in the eastern Jiangnan Orogen lasted at least to ca.820 Ma.The final amalgamation of Yangtze Block and Cathaysia Block probably happened in 810-800 Ma.