滇西芒市江东花岗岩体的锆石U-Pb年龄、地球化学特征及构造意义

Zircon U-Pb Age and Geochemistry of Jiangdong Intrusion and Its Tectonic Implications in the Mangshi,Western Yunnan

腾冲地块江东花岗岩体分布于芒市江东镇—项丘一带,主要由黑云母二长花岗岩组成.该花岗岩锆石LA-ICP-MS U-Pb定年结果表明,锆石U-Pb年龄加权平均值为58.29±0.44 Ma,表明该花岗岩体的形成时代属于古新世.岩石地球化学研究表明,该花岗岩具有中等SiO2 （68.66％～73.50％）,高碱（K2O ＋Na2O =6.87％～9.04％）,高钾（K2O/Na2O=1.87～2.20）,铝饱和指数（A/CNK）为0.97～1.36的特征.岩石属钾玄岩—高钾钙碱性系列,为过铝质S型花岗岩.稀土元素总量（∑REE）为158.85×10-6～304.26×10-6,稀土元素配分曲线呈右倾型,具有弱到中等程度的负铕异常.根据岩石地球化学特征判别,该花岗岩的源岩可能是富含粘土的变质杂砂岩,在构造体制由挤压向拉张转变的后碰撞环境下部分熔融形成的.江东古新世后碰撞环境黑云母二长花岗岩的形成,预示着印度板块与冈底斯-波密-腾冲板块的主体碰撞在60 Ma之前（即古新世初期之前）已经发生.冈底斯-滕冲构造带古新世构造岩浆造山事件与新特提斯洋的闭合及印度板块与冈底斯-滕冲板块间的俯冲、碰撞造山作用密切相关.

The Jiangdong granites are located in the Jiangdong-Xiangqiu of Mangshi,the western yunnan Tengchong block.Field investigation has found that the Jiangdong intrusion consists of biotite monzo-granite.Most of the Jiangdong granites is from the original Gaoligong Group disintegration by the author through the 1 ∶ 50000 regional geological mapping.To limit the precise age of the Cenozoic magmatic mineralization in the Mangshi area,the Jiangdong granites have been analyzed using LA-ICP-MS zircon U-Pb techniques and geochemical methods.The zircon LA ICP-MS dating indicates that the biotite monzo-granites was emplaced at 58.29±0.44 Ma.The granite mass is formed in the Paleocene.Geochemical data show that the Jiangdong granites are potassic basalt to high K cale-alkaline series,characterized by medium content of SiO2 （68.66% ～73.50%）,high K（K2O/Na2O =1.87～2.20） and peraluminous ratio（A/CNK）=0.97 ～ 1.36）.The biotite monzo-granites is riched in Th,U,Rb and Ba,and depletion in Tb,Nb,Zr and Hf,belong to k black to high-K cale-alkaline peraluminous S-type granites.which are similar to calc-alkaline rocks in island arc or active continental margin.The ΣREE ranges from 158.85 × 10-6 ～304.26 × 10-6,REE diagram shows rightward incline and obvious negative Eu abnormity.Magma source is probably rising and emplacement of partially melt granitic magma dominated by greywacke,which is typical of crustal genesis.Magma source is probably rising and emplacement of partially melt granitic magma dominated by greywacke,which is typical of crustal genesis.The Jiangdong biotite monzo-granite was derived from partial melting ofthe Mid-Proterozoic metamorphosed basement （Gaoligong Group）. According to geochemistry and the trace element discrimination diagrams for tectonic interpretation,the the Cenozoic granitoids are considered to be of the post-collision type formed in a transitional tectonic setting from the compressional system to the tensional system.The Jiangdong granites probably formed in a post-collisional setting,which may imply that the Indian plate collided with the Gangdese-Bomi-Tengchong block before 60 Ma（prior to Cenozoic）.The Gangdese-Tengchong tectonic belt entered the tensional stage.The Gangdese-Tengchong block tectono-magmatic orogenesis was intimately related to the closure and subduction and collide orogenesis of the new-Tethys Sea.The determination of the Jiangdong granites＇s crystallization provides new evidence for the Cenozoic tectonic evolution of the Tengchong massif.