Geochemical Characteristics And Tectonic Implication Of Carboniferous Volcanic Rocks In Atengtao Mountain,Yining Block

According to field geological investigation,it has identified the sequence,rock assemblage,and occurrence pattern of two sets of Carboniferous volcanic rocks,Early Carboniferous Dahalajunshan Formation and Late Carboniferous Yishijilike Formation,in Atengtao Mountain of Yining Block.On this basis,the methods of petrology,petrography,chronology,geochemistry of major,trace elements,and Lu-Hf isotopes have been used to systematically research,and the geologic age,petrogenesis and tectonic setting of the volcanic rocks have been discussed.The main understanding is as follows.1.The volcanic rocks of the Early Carboniferous Dahalajunshan Formation in Atengtao Mountain of Yining Block are mainly intermediate-felsic volcanic rocks,with explosive facies dominating.Contacted with Early Carboniferous Akeshake Formation,which consists of sedimentary rocks of marine and delta facies,by transformation interface,Dahalajunshan Formation is stronger in deformation.There are only a few outcrops of the Late Carboniferous Yishijilike Formation in Atengtao Mountain.It is mainly composed of rhyolites,with weaker deformation,overlying the Akeshake Formation with angular unconformity.2.Zircon U-Pb dating results indicate that the volcanic age of Dahalajunshan Formation in Atengtao Mountain ranges from 338 Ma to 356 Ma,while that of Yishijilike Formation is 314 Ma.3.The bubble rhyolites and no-bubble rhyolites of the Dahalajunshan Formation in the study area are characterized by A-type granite and are derived from the partial melting of continental crust under the tensioning background.Hf isotopic composition indicates that the original rocks of bubble rhyolite are juvenile continental crust,and the original rocks of no-bubble rhyolite are Precambrian basement continental crust,among which the rhyolites in the bimodal volcanic rock are added with mantle source materials.Marine volcanic rocks are characterized by the structure of bubble rhyolites,which indicates the tectonic environment of the back-arc basin.The basaltic andesites of the bimodal volcanic rocks are alkaline series of sodium-rich Atlantic suite,which shows the geochemical characteristics of the arc volcanic rocks due to the mixing of continental crust.High Mg# andesites are formed by the interaction between eclogite melts of subduction slab and mantle peridotites,and the melting of the subduction slab also coincides with the thermal anomaly of back-arc extension.The Yishijilike Formation rhyolites in the Late Carboniferous have more obvious characteristics of rift volcanic rocks than the Dahalajunshan Formation.4.Based on previous studies,the southern part of the Yining Block began to be subducted by the oceanic crust of South Tianshan Ocean at ?430Ma,and the back-arc area began to extend at ? 366 Ma,forming a back-arc basin in Atentaishan area.The closure of the South Tianshan Ocean marked by Shanshan movement between Early and Late Carboniferous,and the southern part of the Yining Block came into the post-collision extension regime in the Late Carboniferous.