Components And Tectonic Attribution Of Sinian-cambrian Sedimentary Rocks In The Pinbian Area,Southeastern Yunnan

The basement component of the Pingbian area,SE Yunnan,has not been well studied before.Tectonic attribution of this area is very important for constraining the western boundary between the Yangtze and Cathaysia blocks.Whole rock genochemistry and U-Pb-Hf isotopic compositions of detrital zircons from the Sinian-Cambrian sedimentary rocks in the Pingbian area have been detailedly studied to provide new evidence for determining this boundary.The major element compositions show that these Sinian-Cambrian sedimentary rocks have high SiO2(66.3%?77.6%)and low CaO,MgO,Na2O and P2O5 with variable K2O and Al2O3 contents.Relative to the PAAS,these meta-sedimentary rocks are weakly enriched in Zr,Hf,Sm,Nd,Y,Yb,slightly depleted in Cs,Rb,Ba,Cr,Ni.They show REE patterns with high LREE,low HREE and weakly negative Eu anomalies.They also have strong depletion in Sr except one sample.Significantly varied chemical compositions and low SiO2/Al2O3 reflect the complex provenance components,low composition maturity,and close sediment source.Zircon U-Pb dating results indicate that the formation age of the Pingbian Group sedimentary rocks probably formed in the Late Neoproterozoic,not earlier than 710 Ma.Sinian and Cambrian sedimentary rocks share the similar detritus components with abundant?820 Ma detritus and a small amount of?1835 Ma,2300?2400 Ma and?2630 Ma ones.The sedimentary rock shows the unimodal age spectrum with dominant Neoproterozoic detrital zircons,similar to those sedimentary rocks in the Yangtze Block,but different from those in the Cathaysia and Indochina blocks.Geochemistry and petrographic characteristics of sedimentary rocks are similar to those of the Neoproterozoic tillites,which is widely developed in the Yangtze Block.The discrimination diagram of provenance,the zircons appear idiomorphic,the unimodal age spectrum and Hf isotopic characteristics of detrital zircons all suggest the affinity of the Pingbian Group with the Yangtze Block.We believe that the Pingbian area in SE Yunnan Province belongs to the Yangtze Block.Thus,the western boundary between the Yangtze and Cathaysia blocks should be located at south or southeast of the study area,rather than the Shizong-Mile-Luodian Fault to north.Tectonic setting discrimination diagrams,the characteristics of detrital zircon age distribution,the provenance analysis and lithologic characteristics all indicate that the Sinian Pingbian Group was deposited in the back-arc basin,implying that Neoproterozoic subduction along the southwestern margin of the Yangtze Block lasted until-710 Ma.After that,the sedimentary basin in the area gradually turns to the passive continental margin.Neoarchean,Paleoproterozoic and Neoproterozoic crustal evolution of the Yangtze Block involve both the recycled old crust and the growth of juvenile crust.However,the proportion of involved mantle materials is different in different periods and different regions.Neoarchean-Paleoproterozoic magmatism in the area mainly involved the reworking of older crust,similar to the western Yangtze Block,while most intense juvenile crust generation was brought about in Meso-Neoproterozoic,similar to the southern margin of the Yangtze Block.