The Geochemistry And Geochronology Of Neoproterozoic-Devonian Sedimentary Rocks In Guangxi Province,South China

In order to better constrain the age of Neoproterozoic-Devonian metasedimentary rocks from the northern Guangxi and determine the provenance and their tectonic significance,these rocks has been studied for their geochemistry and detrital zircon U-Pb geochronology.Comparing to PAAS,these sedimentary rocks in northern Guangxi area are characterized by low CaO,Cr,Nb and high Ba.The REE patterns show they have intermediate enriched in LREE with strong LREE fractionation and weak HREE fractionation.They have moderate negative Eu anomalies similar to PAAS.The mean CIA value of these sedimentary rocks is 63.9,suggesting a medium weathering of source area.Some samples shifted away from the predicted weathering trend,indicating they experienced some extent of post-depositional K metasomatism.Six sedimentary rocks from different strata are chosen for detrital zircon U-Pb dating,to constrain their depositional time.Dating results show that the maximum depositional age of Danzhou Groups in northern Guangxi Province is 716Ma,and that of a Sinian sedimentary rock is ca.650Ma,while the other 4 samples are probably deposited in Early Cambrian(<523Ma),Early Ordovician(<483Ma),Late Silurian(<422Ma)and Late Devonian(<379Ma).The U-Pb age spectra of the detrital zircons from these six samples have showed that the sandstones from Early Paleozoic(Cambrian,Ordovician,Silurian)sedimentary rocks have a large number of Grenville-age zircons with another subordinate age peak at 2.5-2.4 Ga,showing a close affinity with the Cathaysia block.It is suggested that the provenance of these rocks is the Cathaysia block.The zircons from the Danzhou Groups,Sinian and Devonian all have a age peak of 800 Ma and weak 2.0 Ga peak without Grenville-age zircons,similar to those in the Yangtze Block,showing a close affinity with the Yangtze Block.From the Neoproterozoic to late Paleozoic,the provenance of the sedimentary rocks in northern Guangxi Province has changed for three times.The change between Sibao Groups and Danzhou Groups responds to the Jinning movement(Jiangnan Orogeny),and the provenance change between late Silurian and late Devonian is related to late Paleozoic(Caledonian)orogeny.However,the change between the late Neoproterozoic(Sinian)and Early Paleozoic probably implies another tectonic movement unidentified before.On the other hand,the multi-change of provenance suggests that the study area is close to the boundary of Cathaysia and Yangtze blocks.