Geochemistry, Geochronology And Space-time Evolvement Of Granites In Nujiang Area Of Northwestern Yunnan

The Nujiang zone of northwest Yunnan is located in the southeast of Eastern Himalayan Syntaxis. It’s an important part of the Sanjiang Tethys. Previous studies of the granites in this region has more focused on Gaoligong tectonic belt and Tengchong-Lianghe region, but in Gongshan block, the study is not enough. Based on Field investigation, the granite in both sides of Nujiang is systematically studied in this thesis, in the context of petrology, elemental geochemistry, U-Pb isotope chronology, and zircon Hf isotopic geochemistry. Correlation with the known U-Pb dating, the purpose of this study endeavors to bring up the geological constraints for regional magmatic evolution under the tectonic setting.The Gongshan granite is mostly biotitic granite and gneissic granite in terms of lithology, and there is another sample of gabbro. The zircon U-Pb age of the3samples of biotitic granite is112Ma、116Ma、119Ma,2samples of gneissic granite is55Ma、246Ma,1sample of gabbro is294Ma. The Gongshan representative granite samples are high kalium calc-alkaline and strongly peraluminous series, with SiO2and A12O3range67.47-75.59%and11.57～15.18%, the K2O and Na2O of the granitoid range2.99%～5.81%and3.00%～3.99%. K2O/Na2O ratio is high, ranging from1.0to1.7, A/CNK varies from1.26to1.56. Whereas the gabbro has shown that SiO2and MgO48.05%and7.88%, it belongs to basite. A/CNK is1.14, so the gabbro is mid kalium calc-alkaline and peraluminous serie. Gongshan granite samples are enriched in lithophile element and relatively depleted of high appearance elements.3biotitic granite samples from Gaoligong tectonic belt have negative εHf(t) values(-2.1～28.8) and old Hf-isotope crust model ages (1.3～2.9Ga), suggesting that biotitic granite was formed by partial melting of old crust; the measurement of zircon Hf isotope in the gneissic granite exhibits wide range of εHf(t) values (-5.19～+7.14), and the relatively old crustal model ages (0.7-1.6Ga) suggesting that biotitic granite was formed by partial melting of old crust. The Gaoligong early Cretaceous granites (112～119Ma) may have stemmed from the collision between the Lhasa Block and the Qiangtang Block in late Jurassic and Early Cretaceous. The early Cenozoic granite (55Ma) in Gongshan block may have stemmed from the collision between the India plate and the Eurasian plate after Neo-Tethys Sea closed. The late Permian granite (246Ma) in Gongshan block was intimately related to the closure of the Paleo-Tethys Sea.