Petrogenesis and LA-ICP-MS zircon U-Pb dating of Late Neoproterozoic granitic gneisses in western Qilian Mountain

Neoproterozoic granitic gneisses from the Diaodaban area of the western Qilian Mountain outcrop in two-mica gneiss of the Beidahei Group. The granitic gneisses are mainly alkaline and peraluminous, with A/CNK values of 1.04~1.12. Their low Rb values and high Sr and Ba values as well as K/Rb ratios of 179~225 show that the rocks are not highly fractionated granite; nevertheless, the rocks have obvious negative Eu anomaly (Eu/Eu?=0.25~0.35), indicating that the magma experienced weak fractionation crystallization. The diagram of (Al2O3+TFeO+MgO+TiO2)-Al2O3/(TFeO+MgO+TiO2) suggests that the protolith of the rocks was sourced from graywacke or mafic-intermediate igneous rocks. Samples are enriched in incompatible elements and have geochemical characteristics of the intraplate granite in Y-Nb and (Y+Nb)-Rb diagrams. LA-ICP-MS zircon U-Pb dating of the granitic rocks yielded a weighted mean age of 736±5Ma, suggesting that there is a magmatic record of 750~730Ma in the Qilian Mountain, probably resulting from intraplate extension in the late Neoproterozoic. The Neoproterozoic magmatic activities in the Qilian Mountain recorded a long-term evolution history from early Neoproterozoic convergence through Late Neoproterozoic extension to breakup.