The Geochemistry And Geochronology Comprehensive Study Of Precambrian-Cambrian Strata In Hunan And Guangxi Province

The study area is located in the central of South China, and is the boundary transition zone of the Yangtze Block and Cathaysia Block in the southwestern region. Northwest of the study area as the Jiangnan orogen is belong to the Yangtze Block, and southeast of the study area as the Caledon orogen is belong to the Cathaysia Block. Here is an ideal place to study the basis geological problems of South China.Geochemistry and detrital zircon U-Pb geochronology and Lu-Hf isotope study on the Precambrian-Cambrian metasedimentary rocks from the eastern of Hunan, southern of Hunan, southwestern of Hunan-southeastern of Guangxi, northern of Guangxi. The combination of petrographic and geochemical analysis results show that these low metamorphic rocks’protolith are terrigenous clastic sedimentary rocks. And the metamorphic rocks show a moderate degree of weathering, most of them formed on the passive continental margin, a small amount of them formed on the active continental margin.Geochemistry and detrital zircon U-Pb age characteristics show that the detritus of the Cambrian sedimentary rocks of Miaoershan in southwestern of Hunan-southeastern of Guangxi, the Sinian and Cambrian sedimentary rocks of Jinjiling in southern of Hunan and the Sinian sedimentary rocks of Dayishan are similar. And the sedimentary rocks characterized by containing a lot of detrital zircons of Grenville age, and the lack of800-700Ma zircon peak, showing the obvious affinity with Cathaysia Block. However, the Banxi group and Sinian sedimentary rocks of Miaoershan, the Sibao group sedimentary rocks of Motianling and the Banxi group sedimentary rocks of Xiema are rich of800-700Ma detrital zircon and different in the geochemistry with the above mentioned sedimentary rocks. The detrital zircon ages and geochemical characteristics consistent, showing that the source area of Miaoershan sedimentary basins changes from Yangtze Block to Cathaysia Block before the middle-late Cambrian(>505Ma). This shows that a tectonic movemet between the Sinian and Cambrian made the Cathaysia Block further uplift or the basin of Miaoershan further subsidence(the migration of sedimentary basin center). It also shows that the Yangtze Block and Cathaysia Block have aggregated, and their boundary of southwestern region should be through the Miaoershan area and Jinjiling area, and the Motianling area, Xiema area are located in the western part of the boundary.The detrital zircon Hf model ages show that the Yangtze Block and Cathaysia Block have Archean basement.Detrital zircons of the Yangtze Block record the magmatic events of2600-2500Ma,1800Ma,1500Ma,800-700Ma. There are juvenile crust generated in the eastern Hunan at2500Ma, while the same period in the northern Guangxi is mainly old crustal melting. The crust is remelting at the1800Ma, and there are juvenile crust generated or mantle material to join at the1500Ma of eastern Hunan. At the800Ma of Neoproterozoic, the eastern Hunan crust is remelting, and the southwestern Hunan-northern Guangxi and northern GUngxi is manifested in the growth of crustal and the remalting of old crust. Detrital zircon record the magmatic events of the Cathaysia Block at2461Ma,1585Ma,1300-1000Ma. In the Archean and Paleoproterozoic, the Cathaysia Block is manifested in the old crust remelting. In the1300-1000Ma of Neoproterozoic, the Cathaysia Block is manifested both in the old crust remelting and the adding of mantle material.