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

The Late Paleozoic tectonic evolution of the Junggar Basin played a critical role in the formation of its oil and gas resources; however, as most of the basin is covered by desert, the evolutionary process remains controversial. In this study, we present a comprehensive analysis of the rock associations, petrography, ages, and systematic elemental and isotopic compositions of Carboniferous volcanic rocks obtained from drilling holes along the western margin of the basin, aiming to better constrain the late Paleozoic tectonic evolution of this region. Two zircon samples, from tuff and andesite layers, yielded concordant ages of 316.8 ± 1.7 and 321.7 ± 1.8 Ma, respectively, indicating intense volcanic eruptions during the Late Carboniferous. The analyzed volcanic rocks exhibited sodic calc-alkaline affinities, with SiO 2 content ranging from 53.46 to 61.57 wt%, TiO 2 content from 0.75 to 1.20 wt%, and a significantly higher Na 2 O content than that of K 2 O. In terms of trace elements, they showed variable enrichment in light rare earth elements ((La/Yb) N = 2.72 to 7.89) and large ion lithophile elements such as Ba, Th, U, and Sr. Low Cr (0.67–65.72) and Ni (0.86–34.90) contents, together with significant negative Eu anomalies ( δ Eu = 0.17 to 0.35), suggest variable fractionation of the primitive magma prior to eruption. All these features, combined with prominent Nb-Ta depletions in PM-normalized incompatible element spider diagrams, unambiguously indicate their genetic relationship with a subduction environment. However, the volcanic rocks showed low initial ( 87 Sr/ 86 Sr) i values (0.7039–0.7057) and positive ε Nd ( t ) values (7.5–8.0). Moreover, the combined ( 208 Pb/ 204 Pb) i ranged from 37.7295 to 37.7851, ( 207 Pb/ 204 Pb) i ranged from 15.4935 to 15.5037, and ( 206 Pb/ 204 Pb) i ranged from 17.8910 to 17.9254, indicating that their primitive magma originated from an extremely depleted mantle source with crustal contamination. Integrating this study with regional geology, we propose that the volcanic rocks in the Kebai Fault Zone were most likely formed in a back-arc basin setting induced by ridge subduction.