Late Jurassic—Early Cretaceous Magmatism And Its Tectonic Background In Yihe Nor Area,Inner Mongolia

On the basis of the"Three 1:5 0000 regional geological survey reports of the Xilin gol League,Inner Mongolia autonomous region"project,through field geological exploration system analysis and comprehensive information,this paper selects the late Mesozoic igneous rocks in the Yihe Nor region as the research object,focusing on the issues of the characteristics of rock assemblage,geochronology,geochemistry and tectonic significance.Zircon U-Pb age of granite porphyry is 133±1 Ma,indicating that they formed at early Cretaceous.Combining with the previous chronology studies,we identified two phases of magmatism in the study area,namely,the late Jurassic and early Cretaceous.Late Mesozoic volcanic rocks in the work area comprise Manketouebo group and Manitu group.The rock assemblage is mainly composed of andesite,dacite,rhyolite,which belong to alkaline and high-K calc-alkaline series.The chondrite-normalized curve of rare earth elements shows weak right deviation.The light REE fractionation is weak,and the Eu and Sr anomalies of volcanic rocks gradually increases from basic to felsic;the REE distribution curves of different rock types are similar,indicating that they share close genetic relationship.The Mg#value of andesite is 47,Cr and Ni concentrations are 143 ppm and 76.3 ppm,respectively,indicating that they were derived from a mantle source.In addition,this andesite sample show high Al₂O₃ contents and Nb/La rations,suggesting that they were affected by crustal contamination.Whereas the dacites and rhyolites have relatively high SiO₂ contents and relatively low transitional elements?e.g.,MgO?Cr?Ni?contents,combined with the good correlation between major elements,we consider that they were generated by fractional crystallization of coeval andesite.The late Mesozoic intrusive rocks can be divided into two stages,the late Jurassic quartz syenite-syenogranite and the early Cretaceous granodiorite porphyry-granite porphyry.They generally exhibit high K₂O contents and alkali contents with low MgO,Cr and Ni contents,and depletion in Eu,Sr,Ba,Ti,P,showing strong affinity with the alkaline rocks?A-type granite?.Petrogenetic studies indicate that they originated from the partial melting of the crustal rocks at low pressure,high temperature and no water conditions with the plagioclase as residual phase.Based on the rock assemblages,geochemical characteristics and rock genesis of late Mesozoic igneous rocks in Yihe Nor area,and compared with the previous studies and the geochemical characteristics of igneous rocks formed in typical tectonic settings.We proposed that they were formed in an extensional environment.The late Jurassic magmatism were formed in an extensional environment after the closure of the Mongol-Okhotsk Ocean,whereas the early Cretaceous magmatism were formed in an extensional environment related to either?i?the closure of the Mongol-Okhotsk Ocean;?ii?back-arc extension related to the subduction of the Paleo-Pacific Plate;or?iii?the combined influence of these two tectonic regimes.