The Rexiang hydrothermal Cu deposit, a newly discovered medium-sized deposit, is located in the central Yidun Island Arc (YIA) in northeastern Tibet. Zircon LA-ICP-MS U–Pb dating for a Rexiang copper-bearing monzogranite sample gives a crystallization age of 79.3 ± 0.7 Ma (MSWD = 3.8). Geochemical data indicate that Rexiang copperbearing monzogranite has high-K calc-alkaline to shoshonitic compositions, moderate A/CNK values (lying primarily within 1.02–1.06), high differentiation index (DI = 86–94), and moderate amounts of corundum (0.34–2.19%), indicating an affinity to weakly peraluminous and highly fractionated I-type granite. REE patterns show enrichment in LREE relative to HREE. The monzogranite is characterized by moderately negative Eu anomalies (δEu = 0.22–0.77), strong enrichment in Rb, Th, U and Ta, depletion in Ba, Sr and P, and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70957–0.7114). Combined with the analysis of its tectonic setting, the monzogranite is interpreted as resulting from partial melting of supracrustal rocks in the postorogenic stage after the collision orogenesis of the YIA in the Late Yanshanian. A Late Cretaceous porphyry (hydrothermal) Cu–Mo-polymetallic mineralization event not only exists in the southern segment of the YIA (the Zhongdian arc) but also in the central segment of the YIA.