南帕米尔北缘切实界别辉长岩LA-ICP-MS锆石U-Pb定年、地球化学特征及其地质意义

LA-ICP-MS zircon U-Pb dating, geochemistry and tectonic implications of the Qieshijiebie gabbro on the northern margin of South Pamir

南帕米尔北缘切实界别地区广泛分布的基性岩体（岩脉）侵位于下二叠统中，LA-ICP-MS锆石U-Pb同位素定年获得其成岩年龄为269.5±1.3Ma，反映该地区存在中二叠世基性岩浆活动。岩体中SiO2含量为46.68%~49.20%，TiO2平均值为1.40%，（K2O＋Na2O）在3.94%~5.83%之间，属于碱性系列；稀土元素配分模式呈LREE富集型，δEu值为0.92~1.31，无明显Eu异常；明显富集K、Rb、Sr、Ba等大离子亲石元素，相对亏损Nb、Ta、Ti、P等高场强元素及Th、U放射性生热元素，类似于板内玄武岩；微量元素构造环境判别图亦显示其形成于板内构造环境。与邻区二叠纪基性火山岩地球化学特征对比，结果表明，南帕米尔和南羌塘在二叠纪同处于板内伸展环境，而冈底斯带属于岛弧背景，这为帕米尔和青藏高原石炭纪-二叠纪构造单元的划分与对比提供了岩浆作用方面的依据。

Mafic intrusions are distributed widely on the northern margin of South Pamir and intruded into the Early Permian stra-ta. The LA-ICP-MS zircon U-Pb dating of the Qieshijiebie gabbro yielded an age of 269.5±1.3Ma, revealing that the mid-Perm-ian mafic magmatic activity existed in this area. The intrusions belong to alkaline rock series, and their SiO 2 values are 46.68%~49.20%, TiO2 value is 1.40% on average, and K2O＋Na2O=3.94%~5.83%. The REE patterns of the samples exhibit enrichment of LREE, with δEu equal to 0.92~1.31, and have no obvious Eu anomalies. The characteristics of trace elements are similar to those of intraplate basalts, showing relative enrichment of LILE such as K, Rb, Sr and Ba and depletion of HFSE （Nb, Ta, Ti, P） and Th, U. The trace element identification diagrams also show that they were developed in an intraplate setting, suggesting that South Pamir and South Qiangtang were both in a tectonic environment of intraplate extension, whereas the Gangdise belt was in an is-land-arc setting during Permian, as shown by the comparison of the Qieshijiebie gabbro with the mafic volcanics in adjacent re-gions. The conclusion reached by the authors provides a basis of magmatism for dividing Carboniferous-Permian tectonic units in Pamir and Tibetan Plateau.