GEOCHRONOLOGY AND GEOCHEMISTRY OF THE MESOZOIC GRANITOIDS IN THE SOUTH SECTION OF GREAT XING'AN RANGE, INNER MONGOLIA AND ITS TECTONIC IMPLICATIONS

U-Pb dating and geochemical data are reported for three granitoids in Duerji areas in Ulanhot, Inner Mongolia, China,in order to constrain its formation time,petrogenesis and the regional tectonic setting. Located in the south section of the Great Xing'an Range, which is superimposed on the location of ancient Asian and the west coast of the Pacific tectonic domain, Duerji intrusive bodys occur as batholith-like, and consist of monzogranite, granite, alkalic feldspar granite, granite-porphyry, plagioclase-granite , biotite-bearing monzogranites,and biotite-bearing granite, whose main rock type is flesh pink monzogranite. The zircons from the intrusive rocks were dated by the LA-MC-ICP-MS. Three reliable weighted mean ages of ²⁰⁶Pb/²³⁸U (131.3±0.25 Ma,MSWD=0.3; 130.5±0.45 Ma,MSWD=3.3 and 213.0±0.7 Ma,MSWD=0.4) respectively were obtained for monzogranite, alkalic feldspar granite and alkalic feldspar granite in various parts of Duerji pluton. Furthermore,two older age information in the two latter samples were also gained, there are (150.2±0.37) Ma,MSWD=0.96 and (238.1±1.8)Ma,MSWD=1.7. Combining with the data of biotite K-feldspar granites(154.5±0.5Ma,U-Pb method,Sihong Jiang et al.,2011), we speculate that Duerji granitoid intrusive body is a composite batholith formed by polyphasic intrusion and three times of magmatic activities at least might have occurred during the Mesozoic era in the district. The Duerji granitoids are characterized by high silic, high alkali, peraluminous and low calcium with SiO₂ of 73.09%~77.26%, Na₂O+ K₂O of 7.78%~8.74%, CaO of 0.37%~0.89%. They are K-rich calc alkaline series and peraluminous, enriched in light rare earth elements ( LREEs) and large ion lithophile elements ( LILEs),but depleted in heavy rare earth elements ( HREEs) and high field strength elements ( HFSEs) . Their values of ε_Hf (t) from zircon of the Duerji lithological units are about +7.8 for Early Cretaceous rocks and +10 for Triassic rocks,indicating that that their primary magmas were derived from partial melting of the juvenile lower crust magma and a diminished fraction of mantle over time.

Acknowledgements: Research supported by ISTCP (2011DFA22460)and China Geological Survey 1212010811066,12120113086400).