Accretionary complex records cold subduction initiation: Insights from the Early Carboniferous Anqi Complex in southern West Junggar, Central Asian Orogenic Belt

Subduction initiation in hot subduction zones leaves a distinct geological record characterized by forearc basalt−boninite−arc tholeiite assemblages and supra-subduction zone ophiolites. However, cold subduction initiation usually lacks these diagnostic features, complicating its identification in the orogenic record. The Central Asian Orogenic Belt has experienced multiple subduction initiation events, making it a natural laboratory for studying such cases. This study presents a new method for identifying cold subduction initiation in the Anqi accretionary complexes in southern West Junggar, Northwest China. By employing field investigation, large-scale mapping, geochemistry, and geochronology, we divided the target area into five tectonic units. Unit 1 predominantly comprises ca. 410 Ma turbidites that deformed before 314 Ma, indicating Devonian ocean basin rifting and the generation of the newly formed ocean. Unit 2 consists of coherent trench turbidites deposited after 358 Ma and deformed before 342 Ma. Unit 3 is an ocean plate stratigraphy mélange formed after ca. 347 Ma. Together, units 2 and 3 indicate an Early Carboniferous accretionary complex distributed along the Anqi fault. Unit 4 contains ca. 314 Ma coherent turbidites, representing a Late Carboniferous mature accretionary complex. Unit 5 comprises post−327 Ma coarse sediments overlying other units, representing a trench-slope basin. Significantly, the detrital zircon provenance of the Early Carboniferous accretionary complex (units 2 and 3) shows a peak predating any intrusive plutons in the area, indicating an immature initial arc stage with mainly eruptive lavas but lacking intrusive rocks. The Early Carboniferous accretionary complex is the only geological record of this initial subduction in the region, suggesting that it preserves evidence of cold subduction initiation.