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

Mafic intrusions are distributed widely on the northern margin of South Pamir and intruded into the Early Permian strata. The LA-ICP-MS zircon U-Pb dating of the Qieshijiebie gabbro yielded an age of 269.5±1.3Ma, revealing that the mid-Permian mafic magmatic activity existed in this area. The intrusions belong to alkaline rock series, and their SiO2 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 island-arc setting during Permian, as shown by the comparison of the Qieshijiebie gabbro with the mafic volcanics in adjacent regions. The conclusion reached by the authors provides a basis of magmatism for dividing Carboniferous-Permian tectonic units in Pamir and Tibetan Plateau.