Zircon U-Pb Geochronological, Elemental and Sr-Nd-Hf Isotopic Constraints on Petrogenesis of Late Triassic Quartz Diorite in Balong Region, East Kunlun Orogen

The study of granite is of great significance to the inversion of the material composition of the lower crust and the deep dynamic process of orogeny. The quartz diorite in Balong region is located in the east of the East Kunlun orogen. In this paper, it presents LA-ICP-MS zircon U-Pb age for the Balong quartz diorite to determine precisely the time of the magmatism, and also presents geochemical, Sr-Nd-Hf isotope data for the Balong quartz diorite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon yielded a weighted mean age of 229.5±1.4 Ma, indicating that it was emplaced in the Late Triassic. The quartz diorites have contents of SiO2 (59.86%-61.83%), Na2O (3.38%-3.55%), Al2O3 (16.38%-17.03%) with Na2O/K2O ratios ranging from 1.25 to 1.39 and Mg# values ranging from 50.1 to 51.2. They are characterized by high silicon and belonging to the high-potassium-calcium-alkaline rock. Meanwhile, they are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements(HFSEs). In addition, the quartz diorite shows characteristics of high Sr/Y (32.31-40.86) and (La/Yb)N (13.34-15.32) and low contents of Yb (1.34×10-6-1.75×10-6) and Y (13.40×10-6-15.60×10-6). These features indicate that the quartz diorite is similar to adakite. All rock samples are enriched in large ion lithophile elements and light rare earth elements, but depleted in high field strength elements. The (87Sr/86Sr)i ratios range from 0.708 186 to 0.708 428, εNd(t) values range from -5.75 to -5.27 with corresponding two-stage Nd model ages ranging from 1 432 to 1 471 Ma. The εHf(t) values are from -5.2 to -3.2 and two-stage Hf model ages rang from 1 305 to 1 420 Ma. Integrated geological, geochemical and isotopic data suggest that the quartz diorite from Balong region is most likely generated via partial melting of thickened mafic lower continental crust and with subordinate mantle-derived basic magma. In combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the quartz diorite from Balong region was formed in post-collisional extensional tectonic environment. Slab break-off, triggered by continuous collision between the Bayanhar block and EKOB, led to underplating of basic magma formed by partial melting of enriched mantle. The quartz diorite from Balong region was formed by partial melting of lower crust. © 2021, Editorial Department of Earth Science. All right reserved.