Tracking the Tectonic Evolution of the Junggar-Balkhash Ocean: A Case Study from the Post-Collisional Takergan Pluton in the West Junggar, Xinjiang

Abstract

The Late Carboniferous and Early Permian igneous rocks are widely developed in the West Junggar, Xinjiang, which are considered to be related to the evolution of the Junggar-Balkhash Ocean. However, their tectonic settings have been controversial for a long time. With the aim of providing new evidence for the Late Paleozoic tectonic evolution of the West Junggar, we present petrology, zircon U-Pb chronology, whole-rock major and trace elemental and Sr-Nd isotopic data, to discuss the petrogenesis and tectonic setting of Takergan pluton from the Barleik Mountains in the West Junggar. The Takergan pluton is mainly composed of quartz diorite porphyry and quartz monzonite. The quartz diorite porphyry has low SiO2 (57.76 wt.%–57.81 wt.%), high total alkali contents (Na2O+K2O=6.29 wt.%–6.56 wt.%), and high Mg# values (45–46), with a zircon U-Pb age of 304±5 Ma. The quartz monzonite shows relatively high SiO2 (58.71 wt.%–64.71 wt.%), total alkali contents (7.73 wt.%–9.70 wt.%), and Mg# values (34–47), with the A/CNK values of 0.91–0.98, which belongs to shoshonitic and metaluminous I-type granite series. The quartz monzonite yields zircon U-Pb ages of 302±2 and 296±3 Ma, and is characterized by low initial Sr ratios of 0.703 97–0.704 09, high eNd(t) values of +6.8–+7.0, and young Nd model ages of 551–587 Ma. Both the quartz diorite porphyry and quartz monzonite are enriched in light rare earth elements and Rb, Th, U, K, and depleted in Nb, Ta, Ti, with different degrees of negative Eu anomalies. These features indicate that the Takergan pluton was most likely formed in a post-collisional setting by partial melting of a depleted mantle source that had been metasomatized by subduction-related fluids, with significant fractional crystallization and slightly contaminated by crustal materials. Combined with the widespread distribution of the coeval stitching plutons, the occurrences of terrestrial Late Carboniferous to Permian volcano-sedimentary formations, and the absence of subduction-related rocks later than Early Carboniferous, it is believed that the Junggar-Balkhash Ocean was closed at about 320 Ma, and the central West Junggar has transformed to a post-collisional environment during the Late Carboniferous and Early Permian.