The least metasomatized xenoliths of spinel lherzolite and gabbro recovered from alnöite intruding crust of the southern Ontong Java Plateau on the island of Malaita yield an Sm-Nd age of ca. 160 Ma and an initial εNd value of ∼+8. In contrast, the plateau basement is ca. 120 Ma with initial εNd between +3.7 and +6.5. The xenoliths appear to represent normal Pacific oceanic lithosphere (uppermost mantle and lower crust) formed ∼40 m.y. before the plateau, indicating that the southern part of the plateau was emplaced off axis on mature seafloor. The closest 160 Ma seafloor to Malaita is >1800 km to the north, off the northeastern margin of the plateau, implying the presence of either an intervening large-offset fracture zone or a triple junction. The presence of significantly older oceanic lithosphere beneath the plateau places restrictions on proposed origins for the plateau.