滇东北下田坝花岗岩年代学、地球化学及其构造意义

Geochronology,Geochemistry and Tectonic Significance of the Xiatianba Granite in Northeastern Yunnan

滇东北下田坝花岗岩的SiO2质量分数在71.1%～78.15%之间,Al2O3值在11.55%～14.90%之间,A/CNK=0.94～1.25,属于弱过铝质花岗岩。稀土元素总量(∑REE)为259.71×10-6～804.60×10-6,∑LREE/∑HREE为4.12～10.28,说明轻稀土较重稀土富集。岩石具有明显的负Eu异常(δEu=0.04～0.07),大离子亲石元素Rb、Ba、Pb、U等相对富集,高场强元素Nb、Ta、Zr、Ti等明显亏损,这些特征说明下田坝花岗岩是由壳源物质部分熔融形成的,具有A型花岗岩的特征。对下田坝花岗岩进行的激光探针等离子体质谱(LA-ICP-MS)锆石U-Pb同位素年代学研究获得了206Pb/238 U加权年龄年龄769±4.4Ma(MSWD=0.45),可代表花岗岩的形成年龄,下田坝花岗岩并非原来认为的早古生代花岗岩。结合对区域地质资料的分析认为,下田坝花岗岩的形成年龄与扬子地块西部Rodinia超大陆裂解事件在时间上一致,该花岗岩是在Rodinia超大陆裂解过程中由地幔柱活动引起的大陆裂谷环境中形成的,这也证实了扬子地块西南缘新元古代的裂谷环境已影响到滇东北的东川地区。

Xiatianba granite,which is located in the northeast of Yunnan Province,are characterized by the mass fraction of SiO3（71.1%～78.15%）,Al3O3 of 11.55%～14.90%,the value of A/CNK is 0.94～1.25 （1.15 on average）.These characteristics indicate that Xiatianba granite belongs to peraluminous S-type granite.The total REE contents of the rocks range from 259.71× 10-6～804.60× 10-6 （402.54× 10-6 on average）,and the ratio of LREE/HREE is between 4.12 and 10.28,which suggest that LREE is richer than HREE.Strong negative Eu anomaly （δEu=0.04～0.07）,the enrichment of LILE （Rb,Ba,Pb,U）and depletion of HFSE （Nb,Ta,Sr,Ti） show that the granite derived from partial melting of the crust.LA-ICP-MS U-Pb dating for zircon from Xiatianba granite yielded a weighted mean 306 pb/338U age of 769＋ 4.4 Ma,which represents the formation age of the granite but not Early Paleozoic as recognized before.Combined with regional geological settings,the formation age of Xiatianba granite is consistent with the time of breakup of Rodinia supercontinent in the western Yangtze block.The Xiatianba granite may form in such an extensional setting caused by the mantle plume activity in the breakup of Rodinia supercontinent during the continental rift environment.This also confirms that the southwestern margin of Yangtze Block in the Neoproterozoic rift environment affected the Dongchuan area of northeastern Yunnan province.