Zircon oxygen-hafnium and biotite magnesium isotopic compositions of Late Cretaceous to Oligocene (88–27 Ma) granitoids from the Gyeongsang continental arc, southeastern Korea, collectively provide a new insight into shallow magma petrogenesis. Zircon crystals extracted from eight calc-alkaline or alkaline plutons commonly contain xenocrystic cores and typically show magmatic cathodoluminescence zonings and sharp Raman spectra. Most zircons have positive εHf values plotting below the mid-ocean-ridge basalt evolution path, indicating that the arc magmatism was dominated by the recycling of juvenile crustal materials. Zircon δ18O values shifted upward or downward from the mantle range attest to the O isotopic exchange of crustal protoliths with surface waters at various temperature conditions. Small (<1‰) but discernable core-to-rim decreases of δ18O values are recognized selectively in magmatic zircon grains from four plutons possessing biotite δ26Mg values (–0.07‰ to +2.32‰) distinctly higher than the mantle range. Such a concomitant O-Mg isotopic variation provides compelling evidence for a series of self-induced hydrothermal alteration and assimilation processes (i.e., “crustal self-cannibalization”). This study shows that the supracrustal input to magmas in a young and juvenile orogen can be traced effectively by the combination of stable isotope records from the plutonic root.