云南永平县水泄辉长岩锆石U-Pb年代学、地球化学及地质意义

Zircon U-Pb dating,geochemistry and geological significance of the Shuixie gabbros in Yongping County,Yunnan Province

滇西地区二叠纪—三叠纪岩浆岩展布还不清楚,导致相关古特提斯构造演化模型还存在争议。本文对兰坪-思茅盆地西缘水泄地区的辉长岩开展了LA-ICP-MS锆石U-Pb定年、锆石Hf同位素、全岩主微量元素及Sr-Nd同位素分析,探讨了岩石成因及地质意义。水泄辉长岩2件样品的锆石U-Pb年龄分别为237.8±3.4 Ma和240.8±3.3 Ma,代表其结晶年龄,属于中三叠世。样品具有较高的Na2O/K2O(3.81~49.14),明显富集Ba、Rb、Th、U等大离子亲石元素,相对亏损Nb、Ta等高场强元素,稀土元素弱分馏,轻稀土弱富集,δEu值为0.87~1.07,无明显的Eu异常。锆石εHf(t)值全为正值,主要介于+5.67~+15.75,全岩初始87Sr/86Sr比值为0.70453~0.70776,ε_(Nd)(t)值全为正值,介于+4.9~+5.2。地球化学特征显示水泄辉长岩来源于亏损地幔的部分熔融,在岩浆演化过程中发生了镁铁质矿物和铁钛矿物的分离结晶作用。水泄辉长岩位于江达-维西弧和云县弧衔接部位,产出于陆缘弧环境。该岩脉与杂多弧、江达-维西弧和云县弧同属古特提斯主洋盆俯冲的产物,由此构成了一条上万千米长的弧岩浆岩带。

The distribution of Permian-Triassic magmatic rocks in western Yunnan Province is still unclear,resulting in controversy regarding the Paleo-Tethyan tectonic evolution model.Here,we present new LA-ICP-MS zircon U-Pb,zircon Hf isotopes whole-rock major and trace element,and Sr-Nd isotopic data for the Shuixie grabbos of the western Langping-Simao basin in the Sanjiang Orogen,SW China.The zircon U-Pb ages of two samples from Shuixie gabbros are 237.8±3.4 Ma and 240.8±3.3 Ma respectively,representing their crystallization age and belonging to the Middle Triassic.The samples have high Na_2O/K_2O(3.81~49.14),and are enriched in LILEs(such as Ba,Rb,Th,U),depleted in HFSEs(such as Nb,Ta).The REEs are weakly fractionated,and the LREEs are weakly enriched.TheδEu values ranges from 0.87 to 1.07,without obvious Eu anomaly.Zirconε_(Hf)(t)values are all positive,mainly ranging from+5.67 to+15.75.The initial~(87)Sr/~(86)Sr ratio of the whole rock ranges from 0.70453~0.70776,andε_(Nd)(t)values are all positive,ranging from+4.9 to+5.2.Geochemical characteristics show that Shuixie gabbros originated from the partial melting of the depleted mantle,and the fractional crystallization of mafic minerals and Fe-Ti minerals occurred during the magmatic evolution.Shuixie gabbros are located at the junction of the Jiangda-Weixi arc and the Yunxian arc,and occurs in the continental margin arc environment.These dykes,together with the Zadoi arc,the Jiangda-Weixi arc and the Yunxian arc,are the products of the subduction of the Paleo-Tethys main ocean basin,thus forming an arc magmatic belt with a length of tens of thousands of kilometers.