Provenance Analysis and Tectonic Significance of Paleozoic Strata in the Boluokenu Area,West Tianshan

Paleozoic strata are widespread throughout the Boluokenu area,West Tianshan.We here present the results of geochemistry of the Cambrian to the Lower Permian Formation strata in this area.The contents of SiO2 are in the range of 66%-73%for the majority of the Boluokunu Paleozoic strata.Each formation has similar geochemical parameters,such as TFeO+MgO,Al2O3/SiO2,Rb,Ba,Sr,Th/U,La/Sc,La/Th and REEs compositions,which are similar to those average values of the upper continental crust.The Cambrian strata are composed of mudstones and marl rocks,while the Middle Ordovician Kekesaleixi Formation is made up of mudstones.Geochemical classification of other formations show that the strata are dominated by greywackes intercalated with litharenite and arkose.Provenance analysis indicate that the Middle Ordovician and the Lower Permian samples contain a certain proportion of sedimentary recycling composition,while the source area of other strata experienced weak chemical weathering.The provenances mainly were felsic igneous rocks.Tectonic discrimination allow us to develop the following model for plate tectonics in the Boluokenu area.During the Cambrian to the Early Ordovician,the Boluokenu area was in a passive continental margin tectonic environment,then the tectonic environment changed to an active continental margin in the middle Ordovician.The subduction process intermitted during the Middle Devonian,astronger round of subduction started in the Late Devonian.During the Late Carboniferous,the Yili plate collided with the Zhungaer plate and formed an unified continent.In the Early Permian,the region entered into the intracontinental evolution stage.