Geochemical Characteristics and Tectonic Significance of the Early Carboniferous Nb‐Enriched Arc Basalts in Wusun Mountain, Yili Block (Chinese Western Tianshan)

ABSTRACT Nb‐enriched basalt is a distinct type of island arc basalt characterised by unique geochemical signatures. The basalts and basaltic andesites of the Dahalajunshan Formation, primarily composed of plagioclase and olivine phenocrysts, constitute the dominant lithology of the Early Carboniferous marine strata in the Wusun Mountain region of the Yili Block. Thirty LA‐ICP‐MS zircon U–Pb measurements yielded a concordant 206 Pb/ 238 U age of 343.8 ± 5.7 Ma (MSWD = 1.3), confirming an Early Carboniferous age, which is temporally consistent with spatially associated adakites. Geochemically, the basalts exhibit arc‐like signatures (LREE enrichment, HREE depletion, pronounced Nb, Ta and Ti negative anomalies) but are distinguished by elevated TiO 2 (1.66–3.04 wt%), P 2 O 5 (0.47 – 1.74 wt%) and significant enrichments in Nb (> 7 ppm), Sr (> 500 ppm) and Zr (> 170 ppm; Zr/Y > 4). Their low (La/Nb) PM (avg. 2.59) and high (Nb/Th) PM (avg. 0.64), coupled with uniform Sr–Nd isotopes ( 87 Sr/ 86 Srᵢ = 0.70440–0.70494; ε Nd ( t ) = +1.58 to +3.08), confirm affinities to Nb‐enriched basalts rather than normal island arc basalts, implying a unique subduction‐modified mantle source. Comprehensive analysis suggests that the Nb‐enriched basaltic magma of Wusun Mountain was derived from a spinel‐phase lherzolite source. It is proposed that adakitic melts, generated by the partial melting of subducting slabs, metasomatically interact with the mantle peridotite, inducing partial melting and the formation of Nb‐enriched basaltic magma. The discovery of Nb‐enriched island arc basalts in the Wusun Mountain region provides new evidence for the subduction‐related tectonic setting of the Yili Block during the Early Carboniferous and temporally constrains the closure of the Tianshan palaeo‐ocean basin to post‐date this period. These findings offer valuable insights into the mechanisms of Nb enrichment in arc magmas and the prolonged subduction evolution of the Chinese Western Tianshan orogenic belt.