Magmatic intrusions and extrusions significantly influence the architecture of sedimentary basins, yet their genetic mechanisms in hyper-extended rifted margins remain debated. Using basin-wide 3D seismic data, we investigated magmatism in the central depression belt of the Qiongdongnan Basin (QDNB), northern South China Sea. We identified a complex igneous province comprising diverse intrusive bodies (sills, dykes, laccoliths) and 24 volcanic edifices, defined 13 volcanic architectural elements, and classified five conduit types. Detachment faulting thinned the crust to 5–10 km, and the resulting weak zones facilitated melt ascent. We propose a “Lithospheric Decompression Melting–Mantle Plume Upwelling Dual Dynamic Control Model,” in which Cenozoic magmatism was governed by three coupled factors: (1) hyper-extension weak zones that provided inherited pathways for melt ascent; (2) thermal anomalies from the Hainan Mantle Plume; and (3) southward–southeastward migration of the South China Block relative to the plume axis. This interaction produced a “tectonic conveyor belt” effect, explaining two magmatic pulses: a late Oligocene–Pliocene pulse centered in the QDNB and a Pliocene–Quaternary pulse in the Leiqiong area. This study provides a typical case of lithosphere–mantle interaction, with implications for hydrocarbon exploration and geothermal assessment in igneous-rich deep-water basins.