Genesis of Late Carboniferous volcanic rocks in Kebai Fault zone, Western Junggar, Xinjiang: constraints from SHRIMP zircon U-Pb age, whole rock geochemistry and Sr-Nd-Pb isotopes

Due to the complex evolutionary history and the limited understanding of the western Junggar region, studies on the genesis and formation environment of volcanic rocks in the Kebai fault zone remain insufficient. This study employs SHRIMP zircon U-Pb dating, as well as geochemical analyses of elements and isotopes, to investigate the eruption age, petrogenesis, and tectonic setting of Carboniferous volcanic rocks in the Kebai fault zone. The U-Pb age of SHRIMP zircon from tuff samples is 316.8±1.7 Ma, while the U-Pb age of basalt LA-ICP-MS zircon is 321.7±1.8 Ma, both of which correspond to the early Late Carboniferous volcanic eruption. Stratigraphically, these volcanic rocks correlate with the Genghis Khan Formation in the region. The volcanic rocks are classified as calc-alkaline, with SiO2 content ranging from 53.46 wt% to 61.57 wt%, TiO2 content from 0.75 wt% to 1.20 wt%, and a K2O/Na2O ratio between 0.10 and 0.66, exhibiting a sodium-rich and potassium-poor signature. Light rare earth elements (LREE) are relatively enriched, while heavy rare earth elements (HREE) are relatively depleted, as evidenced by (La/Yb)N ratios ranging from 2.72 to 7.89. Large ion lithophile elements (LILEs) such as Ba, Th, U, and Sr are enriched, while high field strength elements (HFSEs) such as Nb, Ta, Zr, and Hf are depleted. The δEu values range from 0.17 to 0.35, displaying a weak negative Eu anomaly. The Zr/Nb (29.36–65.60) and Hf/Ta (12.82–30.16) ratios are significantly higher than those of ocean island basalts (Zr/Nb = 3.0–6.0, Hf/Ta = 10–20) and mid-ocean ridge basalts (Zr/Nb = 10–30, Hf/Ta = 8–15). The volcanic rocks exhibit low (87Sr/86Sr)i values (0.703941–0.705675) and positive εNd(t) values (7.5–8.0), indicating a mantle-like isotopic signature. The Zr-Nb, Th/Zr-U/Th, and Ce/Pb diagrams (values ranging from 2.52 to 13.38, mean 4.61) suggest the involvement of subduction-zone fluids during the volcanic formation process. Furthermore, the Hf/3-Th-Ta, Nb×2-Zr/4-Y, V-Ti/1000, and La/10-Y/15-Nb/8 identification diagrams support the conclusion that the volcanic rocks in the Kebai fault zone were primarily influenced by ridge extension and subduction processes, consistent with a backarc basin extensional tectonic environment.