Tectonic Setting And Petrogenesis Analysis Of Xiaorequanzi Formation In Shanshan Xinjiang

Xiaorequanzi Formation lies in Dananhu island-arc belt in Junggar plate. It is located in the north of Kanggul Fault belt in the investigation area. Although predecessors have in-depth research for Dananhu island-arc belt,to Xiaorequanzi Formation,it still in relatively low limitation.So,this paper scientifically demonstrates the nature,process and cause of formation of Xiaorequanzi Formation igneous rocks according to systematically research in petrography, geochronology,rare earth elements and trace elements geochemistry. Through the integration of wild investigation and study-in-door,we get some initial understanding as follows:1 .The main volcanic rock types of Xiaorequanzi Formation are basalt, basaltic andesite, trachyte, trachytic andesite, trachytic dacite, dacite, rhyolite;andesitic, dacitic, rhyolitic volcanic breccia and tuff; dark gray alteration in gravel - coarse-grained lithic sandstone, dark gray sandy breccia.2.Rhylite samples are collected from Xiaorequanzi Formation volcanic rocks,its formation age is 325±3.2Ma by SHRIMP U-Pb zircon determination in the centre of Beijing Ion probe,belonging to Germany-Docking stage of Early Carboniferous.3. Xiaorequanzi Formation volcanic rocks have characters of high volume of REE, LREE enrichment, LREE enrichment , HREE loss, and week degree of REE fractionation, strong degree of LREE fractionation. It was LILE enrichment and correspondingly HFSE depletion,especially Nb,Ta. Compatible elements such as V,Co,Ni are strongly depleted. It has the characteristics of volcanic rocks calc-alkaline volcanic rocks in the island arc tectonic environment.4. Xiaorequanzi Formation volcanic rocks vary from mafic volcanic rocks to acid volcanic rocks. Mafic volcanic rocks are typical calc-alkaline basalts in island arc environment. They are formed by partially melting from the mantle wedge peridotites which had metasomatism with LILE and LREE-rich fluid released by underlying subduction slab. Intermediate volcanic rocks of Xiaorequanzi Formation are formed by fraction and crystallization evolvement of its own basic volcanic rocks. Acid volcanic rocks are formed by partially melting from earth's crust rocks near the Moho which were heated by underplating of mantle magma melt by mantle wedge overlying the subduction slab. The main fractional and crystallizational mineral are clinopyroxene, the fraction and crysrallization of plagioclase get minor effection in the process of magmatic evolution. Intermediate and mafic volcanic rocks are contaminated and assimilated at certain degree in the process of magma rise emplacement.