Oregon State University, United States

Analysis of basalts from Ocean Drilling Program drill cores and subaerial outcrops on the Solomon Islands have delineated a bimodal age distribution for the world's largest LIP, the Ontong Java Plateau (OJP), of approximately 122Ma and approximately 90 Ma. This bimodality of ages is challenged by the analysis of basalts from Makira, Solomon Islands. The majority of basalts from this island are geochemically and isotopically similar to previously analyzed OJP basalts falling into two groups (Kwaimbaita and Singgalo basalts). A third group of OJP basalts is present on Makira. This group is isotopically identical to the Kwaimbaita basalts, but contains higher Ti and incompatible trace element abundances. We have named this the Wairahito basalt group. This group appears to be a fractionated variant of the Kwaimbaita basalts. One sample from Malaita (ML 476) falls in this group and this is the fractionated top of a thick flow with ML475 being the cumulate base. Ages of the basalts on Makira are approximately 90 Ma, approximately 62 Ma, and possibly approximately 34 Ma. A general younging to the north of the island is consistent with the regional dip of the basaltic basement. Stratigraphically intercalated with the OJP basalts are MORBs and rare OIBs. We present the following working hypothesis. The approximately 90 Ma basalts approximately 62 Ma and approximately 34 Ma basalts represent the products of the Samoan plume (assuming a double plume head after Bercovicci & Mahoney, 1994, Science 266, 1367) near which the OJP passed around approximately 62 Ma. The presence of MORBs indicates an "on ridge" location for the surfacing plume head. We take the OIBs to represent the plume tail component. Such episodic growth has implications for the magnitude of volcanism required for the formation of the OJP, but the extent of the approximately 62 and approximately 34 Ma eruptions remains to be defined.