Petrogenesis and tectonic implications of Triassic high-K calc-alkaline granitoids and mafic microgranular enclaves from the western West Qinling Orogen, Central China

Abstract

The West Qinling Orogen (WQO) of the northeastern region of the Tibet Plateau occupies a key tectonic position at the junction between the Pan-Asian and the Tethyan tectonic domains. However, there are ongoing debates regarding the exact timing of final continental collision in the WQO and closure time for the Paleo-Tethyan ocean. Here, we present LA-ICP-MS zircon UPb dating results along with major-trace elemental and Sr-Nd-Hf isotopic data for four granitic plutons across the western WQO, from north to south including Xiangyu, Tongren, Heri, and Wenquan. Notably, both the Tongren and Wenquan plutons contain abundant mafic microgranular enclaves (MMEs). Our new data indicate that the Xiangyu granites were emplaced at 249 Ma in a continental arc setting. These rocks are high-K calc-alkaline, formed by partial melting of Precambrian basement within the normal lower crust (< 40 km) followed by fractional crystallization of primary crustal melts. The Heri granitoids were emplaced at 237 Ma in a continental arc setting as well. They are also high-K calc-alkaline, but generated by partial melting of Precambrian basement at depths of 40–50 km. The Wenquan granitoids along with MMEs were emplaced at 234–233 Ma in a continental arc setting. The enclave magmas were formed through orthopyroxene-dominant fractional crystallization of primary mantle-derived melts originating from a phlogopite-bearing lherzolitic lithosphere at depths no more than 60 km, while the host granitoids were formed through mixing of the crustal melts resembling the Heri granitoids with approximately 40 to 60% enclave magmas. The Tongren granitoids along with MMEs were emplaced at 231–228 Ma in a back-arc setting. The enclave magmas originated from a phlogopite-bearing lherzolitic lithosphere at depths of 60–70 km, while the host granitoids were formed through mixing of the crustal melts resembling the Xiangyu granites with approximately 18 to 40% enclave magmas. We suggest that the Paleo-Tethyan (Anemaqen) oceanic crust had subducted northward approaching the northernmost WQO by 249 Ma followed by the initiation and gradual southward migration of the oceanic slab rollback. The slab subduction did not cease at least before 228 Ma.