Geochemical Characteristics And Petrogenesis Of Muscovite Granites From Cuogang In Eastern Manzhouli,Inner Mongolia

Magnanimous researches show that there are special petrogenesis and tectonic significances of muscovite granites.Compared with Southern China,where muscovite granites are widely distributed and have been full researched,muscovite granites expose seldom in Da and Xiao Hinggan Mountains known as “Granite Sea”,so the muscovite granites in this area are short of detailed study and analysis.Fortunately,we found garnet muscovite granites in eastern Manzhouli of central Da Hinggan Mountains,and through detailed petrography,zircon U-Pb geochronology,geochemistry and zircon Hf isotopes researching to these muscovite granites,we would discuss their petrogenesis and tectonic background.There are two types of garnet muscovite granites exposed in northern Cuogang,eastern Manzhouli: medium-grained garnet two-mica granites(MTG)and fine-grained garnet muscovite granites(FMG),and the latter intruded into the former with vein type.Zircon U-Pb dating results indicate MTG and FMG were formed in the middle Triassic(236±3Ma)and the early Cretaceous(130±2Ma)respectively.MTG and FMG are peraluminous calc-alkaline series with characteristics of high silica(SiO2=74.65%-78.12%),alkaline-rich(Na2O+K2O=7.74%-8.35%),poor magnesium(Mg#=0.14-0.35)and low iron(TFe2O3=0.34%-1.03%).They have similar characteristics of low total rare earth elements contents(?REE=17.73×10-6-73.52×10-6),relative enrichment in light rare earth elements and depletion in high field strength elements,with obvious Eu negative abnormality(?Eu=0.19~0.51).The trace elements are characterized by relative enrichment in large-ion lithophile elements such as Rb,Th,K,Rb and depletion in high field strength elements such as Nb,Zr,P and Ti,while enriching in Ta.The zircon ?Hf(t)of MTG and FMG values of 10.34-16.63 and 12.07-16.17,with the two-phase model age(TDM2)between 606-203 Ma and 413-138 Ma,respectively.Zr content(15.5×10-6-99.7×10-6)and Zr+Nb+Ce+Y content(78.4×10-6-166.9×10-6)of the two types of granites are significantly lower than the minimum values of typical A-type granites(250×10-6 and 350×10-6),and the A/NK ratio(1.17-1.24)is greater than the average value of A-type granites(1.05).Combining with the differentiation index(DI=92.60-95.75)and trace elements discrimination diagrams,these features indicate that they are highly fractionated I-type granites.MTG have the geochemical characteristics of low Sr and high Yb,belonging to the Nanling-type granites,while FMG have the geochemical characteristics of low Sr and low Yb,belonging to the Himalaya-type granites.We believe that the two types of granites are both originated by partial melting of a depleted basal lower crust,but the magmatic source of FMG is deeper than MTG.The middle Triassic MTG formed in the active continental margin environment related to the Mongol-Okhotsk tectonic domain,and the early Cretaceous FMG formed in extensional environment caused by thickened continental crust.