Petrogenesis And Tectonic Implications Of The Triassic Granites In Suyouqi&Xiwuqi Areas Of Inner Mongolia

The Xing'an-Mongolia orogenic belt?XMOB?,located in the eastern segment of the Central Asian Orogenic Belt?CAOB?,has a complex tectonic history.Granites are widely distributed in the middle segment of XMOB.Detailed geochronological and geochemical studies of these granites are conducive to better understanding the tectonic evolution of XMOB.Previous studies mostly focused on the Hercynian granites formed in the late Paleozoic,with less attention paid to the Meosozoic Indosinian-Yanshanian granites,especially for the Triassic.Therefore,understanding the tectonic history of XMOB requires a systematic investigation of Triassic granites distributed in the study area.This paper presents integrated studies of lithology,geochronology and geochemistry on granite samples collected from Suyouqi&Xiwuqi areas,Inner Mongolia,with the aim to determine the intrusive ages,petrogenesis and tectonic background where they were formed.Samples from the Baoraoleaobao pluton in Suyouqi and the Wulannaoer pluton in Xiwuqi yielded zircon U-Pb ages of 235.0±1.7Ma and 237.1±2.1 Ma,respectively,indicating that both were formed in the middle Triassic.Geochemical analyses show high SiO₂?72.20⁷6.81%?,high K₂O?3.39⁵.21%?,high Na₂O?2.92³.85%?,but low Al₂O₃?12.19¹4.16%?in studied samples,characteristic of A-type monzonitic granite.The average values of K₂O/Na₂O and A/CNK are 1.125 and 1.045,respectively,suggesting they belong to weak peraluminous granites.Meanwhile,the A/CNK values of the studied samples?<1.1?are inconsistent with the S-type granite.Caculation shows that the zircon saturation temperature ranges from 785.81 to 823.70°C,with an average of 815.63°C,suggesting that they are high-K calc-alkaline A-type granite.Further,these granitoids have low total REE content??REE79.58³48.87?g/g?;the average?La/Yb?_N is 22.47,the low average?Eu value?⁰.20?reveals remarkably negative Eu anomalies.Trace elements analyses show high enrichment of large ion lithophile elements Rb?118.00²41.00?g/g?and Th?15.80³1.20?g/g?but strong depletion of high field strength elements Nb?6.47¹7.78?g/g?and Ta?0.79¹.52?g/g?.The research shows that the formation mechanism of Baoraoleaobao pluton and Wulannaoer pluton is the mixing of crustal acidic magma and mantle basic magma.Specifically,under the circumstance of crust thinning,the rising magma from the mantal resulted in patially melting of crust and mingling of magmas of different sources;As temperature decreased,the mixed magmas became lithified through crystallization.Geochemcial data project showed that they were emplaced in post-orogenic extensional environment,marking the end of massive collisional orogeny and the beginning of new intraplate tectonic evolution stage.