Genesis of Intrusive Rocks in the Bairiqili Area: Constraints on the Closure Time of the Paleo-Tethys Ocean in the East Kunlun Orogen

Abstract

To explore the tectonic evolution of the Bairiqili area in the East Kunlun Orogen, we conducted petrological, geochemical, and geochronological analyses of fine-grained quartz diorite, coarse-grained quartz diorite, and granite porphyry from the region. The results show that the weighted average ages of these rocks are 251.6 ± 1.2 Ma for fine-grained quartz diorite, 250.2 ± 0.6 Ma for coarse-grained quartz diorite, and 245.8 ± 0.9 Ma for granite porphyry, indicating formation during the Early and Middle Triassic periods. The coarse-grained quartz diorite is peraluminous, low in silicon, and high in iron. It belongs to the calc-alkaline series of magmatic rocks, enriched in large ion lithophile elements (Rb, K, Ba), and relatively depleted in high-field strength elements (P, Nb, Ti). The zircon ε_Hf(T) values for fine-grained quartz diorite range from –8.68 to –3.39, with two-stage model ages (T_DM2) between 1493 and 1828 Ma. The zircon ε_Hf(T) values for coarse-grained quartz diorite range from –9.46 to –2.88, with T_DM2 ages from 1459 to 1875 Ma, and for granite porphyry, the zircon ε_Hf(T) values range from –3.37 to –1.56, with T_DM2 ages from 1374 to 1488 Ma. These geochemical data suggest that the three plutons primarily derive from crust-mantle mixed sources. All plutons formed in a volcanic arc environment, related to arc magmatism driven by the northward subduction of the Paleo-Tethys oceanic crust. Regional data indicate that the Paleo-Tethys Ocean closed likely in the Middle Triassic.