Abstract: Abstract: [Objective] The East Kunlun Orogenic Belt (EKOB) is a fundamental component of the Tethyan tectonic domain and preserves crucial records of the orogen’s evolution. Although remarkable progress has been achieved in tectonic evolution of the EKOB, systematic studies on its western segment remains insufficient. In particular, no unified consensus has been reached on the closure timing of the Proto-Tethyan Ocean, shift of tectonic regimes, provenance of crustal material, and associated metallogenic processes. [Methods] To better constrain the timing of tectonic transition and assess its metallogenic significance for regional polymetallic mineralization, this study conducts integrated petrological, geochemical and zircon geochronological analyses on three representative plutons (North Heihai, Jiadong, and Suhaitu) in the Kunlun River area, with comparisons to coeval granitoids in the eastern EKOB. [Results] Zircon U–Pb dating yields formation ages of 437.7–428.8 Ma for the monzogranites. Geochemically, these rocks belong to calc-alkaline series with weak peraluminosity, and exhibit higher A/NK ratios than coeval monzogranites of the Baitonggou pluton in eastern EKOB. The monzogranites have total REE contents of 212–256 ppm, with (La/Yb)_N = 8.78–21.05 and LREE/HREE = 8.15–14.13, reflecting LREE enrichment, flat HREE patterns, and pronounced negative Eu anomalies (0.23–0.73). Primitive mantle-normalized trace element patterns reveal enrichment in large-ion lithophile elements (Rb, K) and depletion in high field strength elements (Nb, Ta, Ti, P). [Conclusions]These characteristics indicate that the monzogranites are highly fractionated I-type granites derived from partial melting of ancient mid–upper crustal basement, and emplaced during a post-collisional extensional transition following Proto-Tethyan Ocean closure. This magmatic event records the tectonic switch from compression to extension and provides potential thermal and material conditions for polymetallic mineralization. The eastern and western EKOB experienced this tectonic transition nearly synchronously, demonstrating synchronous and unified evolutionary processes of the Proto-Tethyan Ocean across the whole EKOB.