Garnet, owing to its exceptional physicochemical stability and strong capacity to incorporate trace elements, is widely employed to constrain metamorphic conditions and interpret geological processes. However, during continental deep subduction and exhumation, garnet often exhibits complex compositional zoning and multi-stage growth or modification, complicating its geological interpretation. This study investigates phengite-rich gneisses and leucogranites from the Yangkou area of the Sulu orogenic belt, integrating inclusion petrography with major- and trace-element analyses of garnet. Four types of garnet with distinct genetic origins are identified: Grt I (gneiss core) formed during prograde to peak high-pressure–ultrahigh-pressure metamorphism; Grt II (gneiss mantle–rim) represents dissolution–reprecipitation induced by external melt infiltration during exhumation; Grt III (leucogranite core) records diffusive modification of captured metamorphic garnet by anatectic melt; and Grt IV (leucogranite mantle–rim) crystallized directly from the evolved granitic melt at a late magmatic stage. This sequence of four garnet generations systematically documents the evolution of the continental crust from subduction through exhumation to anatexis, highlighting the significant influence of melt–rock interaction on garnet chemistry. Combined zircon geochronology and calculated rare earth element (REE) partition coefficients reveal that peak-stage Grt I is significantly enriched in HREE relative to coexisting zircon, indicating that garnet may serve as the principal HREE reservoir under “dry” metamorphic conditions. In contrast, Grt IV and associated zircon display near-equilibrium partitioning or slight HREE enrichment in zircon, consistent with experimental observations from melt-bearing systems. These results elucidate the multi-stage evolution of garnet in the Sulu UHP rocks and provide new insights into melt activity, mineral competition, and element redistribution during deep continental subduction.