Geochemistry And Petrogenesis Of Gudong And Menglian Granite Plutons In Tengchong Block,Southeastern China

The Gudong pluton is located in the north of the Tengchong block,with a exposed area less than 10 km2.And their lithology are respectively medium-coarse-grained granite,fine-medium-grained granite.The zircon U-Pb ages of the two intrusive stages suggest that Gudong pluton was formed in early Cretaceous.The zircon U-Pb dating of medium-coarse-grained granite yield a weighted mean age of 126 ±2Ma,and the zircon U-Pb dating of fine-medium-grained granite yield a weighted mean age of 124 ± 3Ma.The rocks are high K calc-alkaline,slight peraluminosity granite,which is characterized by high silica,high aluminum,high alkali and low MgO,low FeO?The Menglian pluton is exposed in the east of the TengChong county and it is one of the largest rock in the region with a exposed area about 450 km2 and lots of dark enclaves developed in this pluton.Zircon U-Pb dating results of Menglian pluton yield age of 127±2Ma,consistent with the formation age of dark enclaves(125 ±2Ma).According to its geochemical characteristics,Menglian pluton are composed of quartz monzonite,granodiorite and granite,with the characteristics of high K calc-alkalinity,aluminous-weak peraluminous.Gudong and Menglian plutons are enriched in Rb,K,and Pb,and depleted in Nb,Ta,Ti,P and Eu.In the P2O5?Pb vs.SiO2 diagrams,all samples show typical trend for ?-type granites.Although the formation age of dark enclaves and the host rock are consistent,the dark enclaves display a smaller grain size and it has higher REE content(305.2×10-6-322.8×10-6)and much higher Nb/Ta ratios than the host rock,indicating that the dark enclaves should be an incomplete mixture residue.The initial 87Sr/86Sr(0.708014 to 0.709772)and negative?Nd values(-3.23 to-4.88)of Gudong pluton,and the initial 87Sr/86Sr ratios(0.708287 to 0.712001)and initial?Nd(t)(-2.06 to-4.57)of Menglian pluton are similar to those of Lhasa northern magmatic belt.In the tectonic distinguishing diagrams,Gudong pluton and Menglian pluton show the geochemical properties of island arc granite,suggesting a geodynamical setting related to the westward subducted Bangong-Nujing Tethys Ocean,and then the collision between Tengchong and Baoshan block.