Résumé : OCEANIC plateaus are large accumulations of volcanic and plutonic rocks on the ocean floor, whose detailed nature and origin are only poorly understood. Their immense volumes, frequent occurrence since the Cretaceous period1 and conspicuous presence in all major ocean basins point to their significance in understanding the Earth's thermal and chemical evolution. We report here the discovery of mafic granulite xenoliths from Kerguelen island, at the northern end of the Kerguelen plateau - one of the two largest oceanic plateaus. Granulites are normally found only in continental settings, where the crust is thick enough to experience the high temperatures and pressures required for granulite-facies minerals to form. We speculate that the Kerguelen xenoliths represent basaltic magmas underplated during the evolution of the plateau, and more generally that such mafic granulites are an important constituent of the lower parts of oceanic plateaus, which are inaccessible by drilling.