Cretaceous volcaniclastic sediments sampled at the Nauru and East Mariana basins were chemically and isotopically analysed in order to learn more about the formation and mantle source of the Ontong Java Plateau, an oceanic large igneous province. Despite their variably altered state, the volcanogenic sedimentary components still contain petrogenetic tracers (e.g. high field strength elements, and Nd-and Pb-isotopes) that can be used to constrain the composition of their respective mantle sources. The Nauru volcaniclastics have incompatible trace-element and Nd-and Pb-isotope compositions typical of the Kwaimbaita-type tholeiitic lavas of the Ontong Java Plateau. Combined with the results of recent investigations, the presence of Kwaimbaita-type volcaniclastics in the Nauru Basin reinforces the proposal that the Kwaimbaita-type lavas comprise the bulk of the giant plateau. On the other hand, the East Mariana volcaniclastics have high incompatible trace-element concentrations and Nd- and Pb-isotope ratios typical of alkalic ocean island basalts. Their source is either the Limalok Guyot in the Ratak seamount chain of the Marshall-Gilbert Islands or the ancestral Manihiki-Hikurangi Plateau. Other geological data argue for the Manihiki-Hikurangi Plateau as the source. This implies that the mid-Cretaceous Pacific upper mantle was dominated by the sources of lavas that formed giant oceanic plateaus.