The Petrogenesis And Tectonic Environment Of The Eocene Granites From Zhongba Area,Western Gangdise

Zhongba area,located in the southwest of Tibet plateau-the roof of the world，isgeotectonically included to the south belt of Gangdise compound magmatic belts ofGangdise-Nyainqentanglha tectonic zone,where develops of multi-periods, large-scaleintermediate-acidic intrusive rocks and volcanic rocks since Mesozoic time.Based on the previous studies,this thesis takes the Cenozoic granites as researchobject,by means of petrology, geochemistry， Zircon SHRIMP U-Pb Geochronology,mineralogy， spectrum analysis,analyzing the petrogenesis and tectonicenvironment,evaluating the mineralization potentiality,discussing the evolution anddeep action processes of crust-mantle materials that they represent.The Cenozoic granites are mainly composed of medium-coarse grained quartzmonzonite, monzonitic granite, biotite monzogranite, quartz diorite and potassiumgranite.They contains varied fractures,which may related to multiple tectonicmovements occurring in this area.Through geochemical analysis, we find that the SiO₂content is64.76%-76.96%（average,69.82%）, the K₂O content is3.82%-5.83%（average,4.83%）,with highK₂O/Na₂O ratio（1.21².43） and low P₂O₅content（0.02%⁰.27%）;δ=1.79-4.00,calc-alkaline seriesï¼› A/CNK=0.86-1.03（average,0.95<1.1）, I type granite;CaO,Fe₂O₃,TiO₂,Al₂O₃,MgO,P₂O₅shows obvious negative correlation with SiO₂; thecontents of trace elements,whose standardized values are more often than1,aresignificantly higher,with normalized curves skewing to the right,presenting relativelyhigher U concertration,strongly lossing Ba,Nb,Sr,Ti;∑REE=84.94-304.51μg/g（average,211.91μg/g）; intensely enriched in LREE, LREE/HREE=13.95-20.00（average,17.32）; strong negative Eu anomalies.The refered datas reveal that thegranites are probably the mixtures of crust-mantle magmas and the results of theremelting of crust-derived rocks from different depths.The datas of SHRIMP U–Pb dating of zircon from Mesozoic Zhongba granitesindicate that this granites,which were formed at38.25Ma, were the consequences ofthe post-collisional stage of Tibet plateau.The mineralogy shows that some granites contain higher grade of Azurite,chalcopyrite, pyrite and sphalerite. According to spectroscopic datas,we find thatthere are higher grade of Ag,Cu,Sb in several collected samples, it indicates that theremay exist relative deposits.