Components And Tectonic Implication Of Neoproterozoic-Cambrian Sedimentary Rocks In The Northern And Eastern Guangxi,South China

The western boundary between the Cathaysia and the Yangtze block has not been well constrained due to lacking in the exposed Precambrian basement rocks.The study presents a suite of whole-rock geochemical and detrital zircon U-Pb-Hf isotopic compositions for Neoproterozoic to Cambrian sedimentary rocks in the northern and eastern Guangxi Province,South China,in order to provide new petrologic evidence to constrain this boundary,as well as basement components in both areas.The sedimentary rocks in eastern and northern Guangxi have high SiO2,low CaO,MgO,with variable K2O and Al2O3.They have REE patterns with high LREE,low HREE,and medium negative Eu anomalies.Compared with the PAAS(Post-Archean Australia shale),the sedimentary rocks in eastern Guangxi show lower compatible elements(Ni,Cr,Sc)and higher incompatible elements(Ba,Th,U,Zr,Hf)with obvious negative Sr and Nb anomalies.The Nanhua-Sinian sedimentary rocks in the Dayaoshan area,eastern Guangxi,have been dated by zircon U-Pb geochronology to deposit later than 741?622Ma.These sedimentary rocks have detrital zircon U-Pb age spectra with significant?1.0Ga peak and weak?2.5Ga peak,similar to those basement rocks in the Nanling-Yunkai terrane,western Cathaysia Block.Neoproterozoic sedimentary rocks in the Longsheng area,northern Guangxi,contain abundant Neoproterozoic detritus with a main peak of?0.80Ga and two weak peaks at?2.0Ga and?2.5Ga,similar to those Neoproterozoic sedimentary rocks in the Jiangnan Orogen,southern Yangtze Block.Neoproterozoic sedimentary rocks in the Longsheng area generally have low K2O/Na2O,Al2O3/(Na2O+CaO)and CIA values,suggesting low maturity and weathering extent in the provenance,whereas Nanhua-Sinian to Cambrian sedimentary rocks in the Dayaoshan area have contrast geochemical features,indicating higher maturity and weathering extent.Both major and trace element characteristics suggest that the provenance of Neoproterozoic sedimentary rocks in the Longsheng area have more mafic components,similar to other Neoproterozoic sedimentary rocks in the Jiangnan Orogen,whereas the provenance of NanhuaSinian to Cambrian sedimentary rocks in the Dayaoshan area consists mainly of felsic components,similar to those basement rocks in the Nanling-Yunkai terrane.Combined with the similarity of Hf-isotope compositions of detrital zircons from a Sinian sedimentary rock in the Dayaoshan with those basement rocks in the Nanling-Yunkai terrane and other available observations,it is suggested that the Dayaoshan area of the eastern Guangxi belongs to the Cathaysia Block,and the Longsheng area of the northern Guangxi is a part of the Jiangnan Orogen.Consequently,the western extension of the boundary between the Yangtze and Cathaysia blocks must pass between these two areas.Available data(including in this study)show that the provenance of sedimentary rocks in northern Guangxi has changed several times from Neoproterozoic to Late Paleozoic.The change between the Sibao Group and Danzhou Group corresponds to the Jinning Movement,and the change between Silurian and Devonian successions corresponds to the Caledonian Movement.The change from the late neoproterozoic(Sinian)to early Paleozoic(Cambrian)identified in this study is likely to result from another tectonic movement.This tectonic movement can be ascribed to Pan-African event,which affected most areas of the Gondwana.However,this effect is relatively weak in South China,which did not cause strong magmatism,metamorphism and deformation,but only caused uplifting of southern provenance and the sinking of sedimentation basin in the northern Guangxi,leading to the provenance change of Sinian to early Cambiran sedimentary rocks in northern Guangxi from the Yangtze block itself to the Cathaysian block.