A Discussion On Sedimentary Provenance And Tectonic Setting Of Sibao Group In Northern Guangxi Zhuang Autonomous Region,South China

The Sibao Group distributed in northern Guangxi Zhuang Autonomous Region and southeastern Guizhou province,it is composed of low metamorphic terrigenous clastic rocks sedimentation inter-bedded with igneous rocks.Sibao Group is an important sedimentary record of tectonic evolution of Jiangnan Orogenic Belt in the early Proterozoic.There are different viewpoints about its tectonic setting,such as island arc,continental margin rift,arc-related basin.Combined with information from previous authors,the material composition and sources of clastic rocks in Sibao Group are analyzed,basin type and tectonic setting are discussed by using the paleocurrent analysis,sandstone detrital clasts component analysis,U-Pb detrital zircon ages and Hf isotopic compositions test.We have made the following progress:1.Sandstone detrital clasts component analysis indicates that the clastic rocks are mainly composed of quartz,feldspar,volcanic debris and a small number of biotite,pyroxene.Volcanic debris and pyroxene may come from contemporaneous volcanic activities,quartz and feldspar may come from basement granites and quartzite.And the carcase minerals Dickinson diagram shows that the tectonic feature of the provenance of Sibao Group is transition area of Uplifted Basement and Dissected Arc.2.U-Pb detrital zircon age spectrum consists of a 903Ma primary age peak and three minor age peaks:2504Ma,1788Ma,952Ma.The average age of 18 grains of the youngest detrital zircon is 866±7Ma,which limits the lower bound of stratigraphic age of the Sibao Group.The young zircons?1457-808Ma?are euhedral or subhedral and exhibit oscillatory zones,their Th/U value is greater than 0.2,and they have the characteristics of typical magmatic zircons.The shape of vast majority zircons that own older age peak?2504Ma,1788Ma?is nearly round,and have features of recycled zircons,indicating that zircons are from recycled sediments?3.U-Pb detrital zircon ages and Hf isotope show that the ?_Hf?t?value of old detrital zircons?>1.2Ga?in Sibao Group range from-14.3 to 20.3,almost the same as the Kunyang Group's and Huili Group's in western margin of the Yangtze Block,all these detrital zircon are from crystalline basement of Yangtze Block and recycled sedimentary rocks.The ?_Hf?t?value of zircons that own 902⁸30Ma age ranges from-25.4 to 4.8.it is consistent with contemporaneous magmatic rocks in northern Guangxi.All their zircons are from the contemporaneous magmatism in the basin.Based on the above findings,detrital compositiont and zircon isotope chronology show that sedimentary provenance of basin come from crystalline basement of Yangtze Block,recycled sedimentary rocks and contemporaneous magmatism.The source region has the characteristics of Uplifted Basement and Dissected Arc.Meanwhile,the Sibao Group in northern Guangxi decelopment bidirectional paleocurrent pointing to East and West,contemporaneous underwater volcanic eruption and deep-water density current sedimentary,all these indicate that the Sibao Group has the sedimentary characteristics of the back-arc basin.Combined with results of eastern Jiangnan Orogenic Belt from previous authors,it is inferred that the southeastern Yangtze Block is an active continental marginal trench-arc-basin system in the early Neoproterozoic?920⁸30ma?,which is similar to the current western margin of the Pacific Ocean.The Sibao Group is the product of the back-arc basin adjacent to continent.