The Geochemical Char-acteristics And Comparision Of The Carboniferous Volcanic Rocks In The Area Of Keguqin And Wulasitai,Yining Block

The area of Keguqin and Wulasitai has complex histroy about geological tectonic and igneous activity. It lies in the northwest of Yining Block. The volcanic rocks of Carboniferous are closely related with mineralization, also record the important information about the transition of tectionic system on Yining Block and the acient Tianshan ocean. Therefore some meticulous reserch on the Carboniferous volcanic rocks has important scientific meaning for better understanding the two sets of Carboniferous volcanic formation, volcnic combination, tectionic environment and the transition of regional tectionic system. On the basis of previous work, this paper take the two sets of volcanic rocks exposed in Early Carboniferous and Late Carboniferous in the whole region as an object of study, by systematically studying the aspects of petrography, geochemical and geochronology, we summarized the rock association, eruption sequences and the difference between the two sets of volcanic rocks. We also discussed their lithogenesis, tectonic setting, ralationship each other and provided new evidence on closing of the Tianshan ocean basin and the transition of tectionic system in Yining Block, Made the following understanding:1.The Carboniferous volcanic rocks in area of Keguqin and Wulasitai consist of the Early Carboniferous Dahalajunshan Formation and the Late Carboniferous Yishijilike Formation volcanic rocks, the Akeshake Formation sedimentary rocks has the relationship of unconformity between them. We take the Akeshake Formation as a distinction for the two suits volcanic rocks, the lower volcanic rocks is the Dahalajunshan Formation rocks, and the upper is the Yishijilike Formation.2.The Early Carboniferous Dahalajunshan Formation volcanic rocks consist of basic, neutral, and acidic lava and pyroclastic, among them the andreessen mass rock are mianly. The Formation occurred drape and the rock deformed. They were formed 331~359Ma. Their geochemical character shows they were from continental marginal arc with high LILE, Low HFSE and calc-alkalinic suites.3.The acidic and basic bimodal rock association are the mianly part of the Late Carboniferous Yishijilike Formation, the intermediate-basic rock missed. The Formation is simple monoclinic and the rock undeformed. The results of 302 Ma were got. Their geochemical character shows they were from postorogenic extension with high LILE, selective Low HFSE, tholeiite and alkali basalt suites.4. The petrogeochemistry comparison shows the Dahalajunshan Formation volcanic rocks is high some LILE elements which enriched in upper crust and compatibility elements; the Yishijilike Formation volcanic rocks with high some LILE elements which enriched in med-depth crust and low compatibility elements, the HFSE are same between them. The genesis study indicated the Dahalajunshan Formation volcanic rocks were effected by subduction of ocean plate, pelagic sediment and subduction fluid in becoming the rocks, however the Yishijilike Formation volcanic rocks are influenced by med-depth rhyolitic magma quality.5. The area of Keguqin and Wulasitai of the Yining Block may experienced the period that island arc dived in early Early Carboniferous, back-arc basin extended localtensionally in lately Early Carboniferous, the north ancient Tianshan oceanic merged after Shanshan Movement, the system of post-collision rifted in Late Carboniferous, last it turned into intraplate rift valley in late Late Carboniferous or later.