滇东北东川下田坝A型花岗岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其构造意义

通过对滇东北东川下田坝地区黑云母二长花岗岩和似斑状钾长花岗岩2种岩体的地球化学分析及对黑云母二长花岗岩的U-Pb同位素测试，得到如下认识：黑云母二长花岗岩和似斑状钾长花岗岩总体地球化学特征相似，主量元素表现出高SiO2（平均71.7%）、过铝质（A/CNK指数在1.03～1.52）的特征，过碱指数在0.95～1.37之间，总体属于钙碱系列；稀土元素总量较高（ΣREE平均313×10-6），（Ce/Yb）N均值为8.85，（La/Yb）N均值为9.54，呈现轻稀土元素（LREE）富集，仅Eu亏损（δEu均值0.31）的海鸥型右倾模式；大离子亲石元素Rb、Th、U、K等相对富集，高场强元素Nb、Ta、Sr、Ti等呈明显负异常，说明岩浆源岩以陆壳成分为主；Ga/Al均值为2.69，(Zr+Nb+Ce+Y)均值为383；微量元素Sr、Eu低，富集Nb、Zr等元素，反映其源区存在斜长石的残留；锆石饱和温度为724～786℃，表明初始岩浆温度较高。上述特征说明，下田坝花岗岩为典型的A型花岗岩，形成于中上地壳的板内伸展背景。黑云母二长花岗岩LA-ICP-MS锆石U-Pb年龄为801.1±6.6Ma，同时受到762.1±6.2Ma热事件的影响，与新元古代Rodinia超大陆裂解事件在时间上一致，亦与东川铜矿成矿时代相符，说明新元古代裂谷环境的岩浆事件范围可扩大至滇东北东川，并可能引发热液成矿作用。

LA-ICP-MS zircon U-Pb age and geochemistry of Xiatianba A-type granites in Dongchuan,Northeast Yunnan,and their tectonic significance

Geochemical analysis and LA-ICP-MS zircon U-Pb dating of Xiatianba granites of two granitic bodies in Dongchuan of northeast Yuannan were conducted in this study. The general geochemical characteristics of biotite adamellite and porphyraceous moyite are similar to each other, characterized by high SiO2（71.7% on average）and belonging to the peraluminous (A/CNK value between 1.03～1.52) and alkaline series (AI value between 0.95～1.37). They have high ΣREE content with an average ΣREE value of 313×10-6, an average（Ce/Yb）N value of 8.85, an average（La/Yb）N value of 9.54 and an average δEu value of 0.31. The chondirte-normalized REE patterns display right-oblique curves. The large ion lithophile elements, such as Rb, Th, U and K, are relatively enriched, whereas high HFSE such as Nb, Ta, Sr and Ti are relatively depleted, which indicates the existence of residual plagioclase in the granite source area. Calculation also shows that the melt of Xiatianba granites was formed in an intraplate extension environment with temperatures between 724℃ and 786℃. These characteristics show that the Xiatianba granites are typical A-type granite. LA-ICP-MS U-Pb dating of zircon from biotite adamellite yielded a granite age of 801.1±6.6Ma (MSWD=0.84, n=14) and a thermal event age of 762.1±6.2Ma (MSWD=0.72, n=14), which are highly in accordance with the time of breakup of Rodinia supercontinent in the western Yangtze block, and are also consistent with the metallogenic epoch of the Dongchuan copper deposit, suggesting that the magmatism of the Neoproterozoic rift environment affected Dongchuan area and probably triggered hydatogenesis mineralization.