Geochoronology, Geochemistry And Origin Of Carboniferous-permian Alkali Granites Ineastern Tianshan, Hami, NW China

Central Asian Orgenic Belt (CAOB) is the world’s largest Phanerozoic accretionorogenic belt and develops massive granite,and late Paleozoic granites which arewidely developed, concerned.Eastern Junggar-Eastern Tianshan area is located in thesouthern edge of CAOB, and aslo in the joint of the Siberian plate,Kazakhstan-Junggar plate and Tarim plate, which is a area with complicated tectonicsetting and violent magmatic action. For a long time, it is controversial about its latePaleozoic tectonic environment,and including arcs, post-orogenic, intracontinental riftand so on.This area with a characteristic of a large volumes ofCarboniferous-Permian A-type granitesto solve this problemprovides someusefulinformation.In this study, we made detailed field investigating, and study on petrology,mineralogy, zircon U-Pb geochronology, geochemistry, zircon Hf isotope of thefouralkali granites in Hami. And comprehensive analysis of timing and spacingdistribution characteristics of the alkali (A-type) granites in Altay-EasternJunggar-Eastern Tianshan area in northern Xijiang,the following achievements andconclusions havebeen made.1. Four alkali granite plutons were confirmed as Shichengzi, Qincheng, Balikunand Xiamaya by zircon U-Pb dating in Hami. Their LA-ICP-MS zircon U-Pb agesare310±1Ma,320±1Ma,289±1Ma and312±1Ma. They belong to lateCarboniferous to early Permian.2.These four alkali granite plutons containingalkaline mineralsin varying degrees,and appear the same characteristics of highSi (SiO2=74.90%-77.98%), high alkali(Na2O+K2O=7.91%-8.74%, AR=4.51-5.27, δ=1.8-2.4), low Al(Al2O3=11.74%-13.04%), low Ca (CaO=0.3%-0.58%), low Mg(MgO=0.02%-0.12%), low Fe (FeOT=0.85%-1.71%). Meanwhile, these alkaligranites show rich of large iron lithophile elements (Rb, K), and iniense depletion inBa, Sr, P, Ti and Eu, meanwhile, to comparewith Rb, K and Th, it is a low deplrtion inNb and Ta. All these features show that these four alkali granites are A-type (alkali) granites.3. Zircon in situ Hf isotope research show the value of176Hf/177Hf is0.282746to0.283045, εHf(t) is+5.36to+15.34, all value are positive,and two-stage Hf modelages (TDM2) of343to985Ma. It is showing young mantle source material,and alsoproviding an important contribution to these alkali (A-type) granites’ formation.4. Combined with regional geological, geochemical and Hf isotopiccharacteristics, determine these alkali (A-type) granites formed the stretchbackground of post-orogenic (post-collision), becauseof mantle-derived magmaunderplate, resulting in a bottom pad earlier dive young mantle partial melting, and asmall amount of crustal contamination, through the fractional crystallization process,eventually forming A-type granites.5. It preliminarily summarizes thatAltay-Eastern Junggar-Eastern Tianshan areaA-type granitesfrom north to south,in addition to the presence of the Altai region lateDevonian-early Carboniferous A-type granites, all the rest are concentrated in the ageof the late Carboniferous-early Permian, andthey do not show migration features inspace,namely it doesnot exist a trend on north old and south new. Comprehensiveanalysis shows that the entire region mass synchronization stretch wasoccurredin thelate Carboniferous-early Permian, revealing the area nearly intopost-orogenicextensional environment beforethe late Carboniferousperiod.