The mechanisms driving the elevated oxidation state of postsubduction porphyry magmas and porphyry copper mineralization are still not fully understood. We present an integrated study of trace elemental and Mg-Hg isotopic compositions of pre-, syn-, and postcollisional magmatic rocks from the eastern Gangdese belt in southern Tibet. Our findings show increased magmatic oxidation states linked to the enrichment of terrigenous sediments from Neo-Tethys subduction to postcollision, as evidenced by distinctive mass-independent Hg isotope fractionation in postcollisional porphyries (Δ¹⁹⁹ Hg = -0.25 to 0.22‰) and mantle-derived ultrapotassic rocks (Δ¹⁹⁹ Hg = -0.54 to 0.25‰) and negative δ²⁶ Mg values in ultrapotassic rocks (δ²⁶ Mg = -0.51 to -0.04‰). These results suggest that oxidation of postcollisional magmas was driven by the subduction of carbonates and/or sulfates from the Indian plate, enhancing metal enrichment. This finding challenges the traditional view that oceanic subduction primarily drives porphyry copper formation, highlighting the significance of continental subduction processes in metallogenic models.