Petrogenesis of Murgab gabrro-diorite from Pamir:Evidence from zircon U-Pb dating, Hf isotopes and lithogeochemistry

Located in the central part of Pamir, Tajikistan, near the Rushan-Pshart suture zone, the Murgab pluton separates the middle Pamir from southern Pamir. Petrologically, the pluton consists of gabrro and diorite which intruded into the epimetamorphic detrital rocks of late Proterozoic (?). LA-ICP-MS zircon U-Pb dating shows that the age of gabbro and that of diorite is 232±1.5 Ma and 231.5±1.9 Ma respectively, which represents the formation age of this pluton. The ε_Hf (t) values of the zircon are in the range of 4.8-12.1 and 6.4-10, with a weighted average of 8.1±1.5 (MSWD=6.5) and 7.9±0.8(MSWD=2.4), suggesting that its original rock was derived from mantle materials. The one-stage Hf model ages (T_DM1) are 477-621 Ma, 391-672 Ma respectively indicating an original rock of Cambrian-Precambrian basement. The lithogeochemical data shows that gabbros are characterized by rich Mg and, poor Al and alkali, thus belonging to low-potassium rocks, whereas the diorites are rich in Si, Al and poor in Mg, Ti, hence belonging to calc-alkaline to high K calc-alkaline rocks. The gabbros and diorites share similar REE and trace elements features, which are characterized by high total REE content and rich LREE with slight normal Eu anomaly. The trace elements are characterized by rich LILEs and poor HFES. Based on geochemical characteristics, the authors infer that the pluton formed in an island-arc environment. The subduction was continuing during late Triassic, which indicates that the Rushan Ocean basin was closed at least later than 232 Ma.