Geochronology,Lithogeochemistry And Tectonic Background Of Early Cretaceous Intrusive Rocks In Western Zhalantun,Inner Mongolia

This paper takes Early Cretaceous intrusive rocks in Qianpuzi,Dujiajie and Jiajiagou areas?Western Zhalantun,Inner Mongolia?as the research object.The goal is to redefine the formation age,and recognize the petrogenesis and associated tectonic background of Early Cretaceous intrusive rocks in Western Zhalantun by means of field route survey,profile measurement,sample collection and test analysis for studies on petrology,geochemistry and geochronology.The research results provide new geological data about this region,meanwhile,found up a scientific basis for Early Cretaceous magmatic evolution in the region of Western Zhalantun.In this research,tonalite mainly distributes in Qianpuzi area,granodiorite in Dujiajie area,and alkali-feldspar granite in Jiajiagou area.The samples were analyzed by LA-ICP-MS test,and rock mass ages are: granodiorite 124.9±2.4Ma,alkali-feldspar granite 121.0±2.4Ma,tonalite 117.8±2.2Ma.The data suggest that granite in this region was formed in the Early Cretaceous.Rock masses in Dujiajie and Qianpuzi areas have a characteristic of high Sr and low Yb,they are quasi-aluminum high potassium calcium alkaline rocks.The rare earth elements in the sample characterize the enrichment of LREE and relative loss of HREE,and samples have an apparently positive anomaly of Eu.Besides,the trace elements in the sample show the enrichment of large ionic lithophile elements?LILE?and relative loss of high field strength elements.Lithogeochemical characteristics suggest rock masses in Dujiajie,Qianpuzi are Type I granitic rock.Rock mass in Jiajiagou area has an enrichment of silicon,low strontian,they are high alkali peraluminous calc-alkaline granitic rocks.The REE in the sample suggest LREE enrichment;meanwhile the HREE is relatively deficient.Samples have an apparently negative anomaly of Eu.In addition,large ionic lithophile elements?LILE?are relatively enriched,and high field strength elements are relatively deficient.The content of Zr element is 18.95×10^-6-167.14×10^-6,which is less than the lowest value of A type granite(250×10^-6),lower limit value of Zr+Nb+Ce+Y=68×10^-6-262.28×10^-6 lower than that of A type granite(350×10^-6).Lithogeochemical characteristics show the rock mass in Jiajiagou is Type I granitic rock.Combined with previous research results,granodiorite in Dujiajie and tonalite in Qianpuzi were formed in the lower crust with higher pressure,they are associated with the interaction of the mantle-derived melts after the end of the orogeny and the Mongolian-Okhotsk oceanic crust fragments and its subduction or the paleo-Pacific plate.In the extensional environment,alkali-feldspar granite in Jiajiagou was formed due to the upwelling of upper-mantle substances,resulting from delamination of thickened lithosphere.