Geochronology,Geochemistry And Geological Significance Of The Xiaodaqingshan Granites In Jining,Inner Mongolia

The study area is located in the Jining area of Inner Mongolia,within the middle domain of the northern margin of the North China Craton.Mesozoic magmatism is frequent in the area and closely related with the regional mineralization.In this study,based on the analysis of petrographic and field observation,we present the LA-ICP-MS U-Pb zircon dating and geochemical results for granites that exposed in the Xiaodaqingshan granites,to constrain its diagenetic age,rock type and magma source area characteristics,discuss the tectonic environment and the dynamic background.The Xiaodaqingshan granite invades amphibole plagioclase gneiss and feldspar quartz gneiss of Jining Group.According to the petrographic characteristics,it can be divided into three different phases.Center-phase granite contains large and abundant phenocrysts,transition-phase granite contains small and few phenocrysts,edged-phase is fine medium granite.In addition,there is granite vein in the edge.The U-Pb zircon results show that edged phase?Fine medium granite?is formed at 135.4±1.2 Ma?MSWD=1.8?,and the central phase granite?contains large and abundant phenocrysts?age is 133.3±1.3 Ma?MSWD=2.8?and 130.8±1.4 Ma?MSWD=3.4?,the granite vein age of 122.9±1.7 Ma?MSWD=0.92?,reflects the granite invaded in the Early Cretaceous and was the product of the Yanshanian magmatism.The geochemical characteristics of the samples indicate that these granites are rich silica?73.37%⁷7.21%?,rich alkali?Na2O+K2O=5.93%⁸.77%?,low magnesium iron?MgO range from 0.11%to 0.27%,TFe₂O average 0.83%?,belong to the aluminum-rich high-potassium-calcium-alkaline series and highly-differentiated I-type granite.The difference of trace element content and the"four-group"effect of rare earth elements show that a large of fluids interact with the melt during the early stage of the magma embedding process,and the fine-grained granite has a higher differentiation index?96.09?and more intense negative Eu anomaly,the fluid gradually depleted during the late stage of crystallization,the central phase granite has a lower crystalline differentiation index,while the contents of Cr,Ni,Co,Sr and Ba are higher.The Xiaodaqingshan granites have the same material source as the Caosiyao granites,which have high whole-rock initial ⁸⁷Sr/⁸⁶Sr?0.7391,0.7094 and0.7170?,low whole-rock?_Nd?t??-16.19 to-15.37?,and old two-stage model ages for whole-rock Nd?2233²180 Ma?.As well as whole-rock Pb further revealed that the Xiaodaqingshan granites originated from the partial melting of the Paleoproterozoic lower crust.The formation of the Xiaodaqingshan granites in the post-orogenic stretching environment may resulting in the closure process of the Mongol-Okhotsk Ocean.