贺根山缝合带南侧西乌旗哈尔敖包早石炭世二长花岗岩的发现及其构造意义OA北大核心CSTPCD

The newly recognized monzogranite distributes along the south of Hegenshan suture zone in Haer,Xiwuqi,Inner Mongolia,and its chronology,tectonic setting and genesis has not been studied yet,which is of great significance for studying the geological evolution of the Hegenshan ocean.The LA-ICP-MS zircon U-Pb dating shows that the age of the monzogranite is 327.7±2.6 Ma.The rock has high SiO2(the average value of SiO2 is 76.04％),alkali(the average value of K2O+Na2O is 8.82％),Al2O3(the average value of Al2O3 is 13.44％)and low MgO,TFeO,MnO,CaO and P2O5(the average value of P2O5 is 0.03％).The Rb,Pb,Th,U,Ta and LREE are relatively enriched,while Sr,Eu,Nb,Ti and HREE are relatively depleted,with obvious Eu anomaly(the average value of δEu is 0.30).The Zr/Hf,Nb/Ta and rock differentiation index(DI=94.93～96.81)indicate that the magma of Haer monzogranite might have been experienced strong crystallization differentiation during the emplacement,with the characteristics of Ⅰ-type granite.The lithogeochemistry indicates that the Haer monzogranite has the characteristics of Ⅰ-type granite,which is preliminarily believed that the monzogranite was formed in an initial extensional tectonic environment during the collisional post orogenic transition stage.Together with regional geologic research results,the identification and determination of the Early Carboniferous Haer monzogranite indicate that collision orogeny may have occurred between the Siberian plate and the Xilinhot microcontinental block during the Early Carboniferous.The Hegenshan ocean experienced brief closure until to the Late Early Carboniferous,and it opened quickly again entering a subduction and subduction stage.The Hegenshan ocean exhibited a feature of multi-cycle evolution.