Abstract To constrain the source and tectonomagmatic processes that gave rise to the Andong Ultramafic Complex (AUC) in South Korea, we determined the clinopyroxene Sr–Nd–Hf–Pb isotope and trace element compositions as well as the whole-rock and mineral compositions for the Late Triassic (ca. 222 Ma) ultramafic rocks from the complex. They are composed of dunites, wehrlites, pyroxene/hornblende peridotites, and pyroxenites. The constituent minerals are olivines, diopsides/augites, bronzites, calcic-amphiboles, and spinels. Clinopyroxenes exhibit a convex-upward rare earth element (REE) pattern, with an apex at Sm. The whole-rock compositions plot away from the residual mantle peridotite trends, with variable but lower Al 2 O 3 and SiO 2 contents, and higher CaO, FeO*, and TiO 2 contents at a given value of MgO. Estimated equilibrium temperatures for the AUC rocks range from 420 to 780 °C. These observations, together with the absence of reaction or melt impregnation textures, indicate that the AUC ultramafic rocks are magmatic cumulates emplaced within the crust rather than residual mantle or mantle-melt reaction products. The AUC clinopyroxenes have compositions intermediate between the oceanic island basalt- and arc basalt-related cumulate clinopyroxenes. The AUC spinels have lower Cr#s than the arc-related magmatic cumulate spinels. They plot within the field for spinels from mid-ocean ridge basalts (MORB) on a TiO 2 vs. Cr# diagram. However, the AUC clinopyroxenes have much more radiogenic Sr ([ 87 Sr/ 86 Sr] i = 0.70554 to 0.70596), unradiogenic Nd ([e Nd ] i = − 1.0 to − 0.3), and Hf ([e Hf ] i = + 4.4 to + 6.6) isotopic compositions than those of the MORB or fore-arc basalts (FAB). In the Sr–Nd isotopic correlation diagram, the AUC clinopyroxenes plot in the enriched extension of the “mantle array”. They also have more elevated 207 Pb/ 204 Pb ratios at a given 206 Pb/ 204 Pb than those of the MORB or FAB. In the Nd-Hf isotope space, the AUC clinopyroxenes have somewhat elevated 176 Hf/ 177 Hf ratios at a given 143 Nd/ 144 Nd compared to the “mantle–crust” array. These observations indicate that the sub-continental lithospheric mantle (SCLM) overprinted by secondary volatile-rich silicate melts might be the principal source of the AUC magmatism. Heat from the upwelling asthenosphere, through the slab window produced by detachment of the oceanic slab from the buoyant continental lithosphere during continental collision between the North and South China Cratons, might lead to partial melting of the overlying metasomatized SCLM, resulting in the post-collisional Triassic magmatism in South Korea.