Geochronology, geochemistry and geodynamic significance of Awengcuo composite pluton and its dark microgranular enclaves in the north of Tibet

The petrology, whole rock geochemistry and U-Pb isotopic chronology of the Awengcuo composite pluton and its dark microgranular enclaves in the western part of Bangong Co-Nujiang suture zone were studied in this paper. The results show that Awengcuo composite pluton contains abundant mafic magmatic enclaves (MMEs) comprising mainly biotitic diorite. The MMEs display igneous texture and long prisms of apatite. Host rock and MMEs are transitional rock belonging to quasi-aluminum, highcalcium alkaline-potassium kaolin series, and the main oxides in the Harker diagram show good linear relationship. Rare earth element distribution curve and normalized spider web for trace element primitive mantle exhibit a high degree of consistency. It shows that there is a strong geophysical relationship between the two groups, which experienced similar magmatic evolution processes. A good linear relationship is shown between the host rock and the inclusions in the denominator covariant map for the major and trace elements. The original magma of the inclusions might have been derived from the mantle wedge by fluid of the subduction zone, whereas the host rock is a product of mantle-derived magma mixed with the molten lower crust magma, mainly molten lower crust magma. The LA-ICP-MS single-zircon U-Pb dating demonstrates that the magmatic age is 109.1±1.0Ma for the host rock and 107.4±0.7Ma for the enclave. The two ages are basically the same, suggesting that the mixing event took place during the period of Early Cretaceous. Combined with the evolution and tectonic discrimination of Bangong Co-Nujiang suture zone, it is concluded that the Awengcuo composite pluton was formed at the soft-collision stage in the Bangong Co-Nujiang Tethyan Ocean, being a product of mantle wedge ascending with the derived mixture of subduction zone fluid and magma.