Characteristics and Geological Significance of Chronology and Geochemistry of Bayandulan Monzonitic Granites in Dong Ujimqin Qi,Inner Mongolia

In order to learn the characteristics of magma activity along the southern plate of Siberianlate during Late Palaeozoic Eta.,petrography,U-Pb chronology of the zircon,zircon Hf-isotopeand geochemistry of the Bayandulan monzonitic granite are studied to constrain the genesis and tectonic setting of the rock in Dong Ujimqin Qi,Inner Mongolia.Zircon U-Pb dating obtained by LA-ICP-MS analyses from the monzonitic granite is(277.2±0.7) Ma,suggesting that it is the production of magmatic activities in later period of Early Permian.The ε_Hf(t) values and the crust model ages(t_DMc) of Bayandulan monzonitic granite,are 6.8-14.1and 865-655 Ma respectively.The Bayandulan monzonitic granite is enriched in K(K=3.63%-4.95%,K₂O/Na₂O=0.91-1.47)and alkali(K₂O+Na₂O=7.52%-9.16%),suggesting the feature of metaluminous to weak peraluminous(A/CNK =0.96-1.15).It is apparently different in LREE and HREE,with most rocks having weakly negative Eu anomalies(δEu=0.34-1.17).The rock is depleted in rare earth elements(∑REE=38.82×10^-6-193.32×10^-6),and Sr,Nb,Ta,P,Ti enriched in partial LILE(LREE,Rb,K and so on)with weakly negative Zr anomaly and weakly positive Hf anomaly,showing characteristics of post-orogenic granites.The monzonitic granite produced in extent environment,and was the product of post-orogenic evolution after closing of the Hegenshan Ocean.And the formation of the rock is partial melting of juvenile continental crust resulted from subducted oceanic crust exised during 865-655 Ma.