Geochemical and petrological investigations of the deeper portions of the Ontong Java Plateau : Malaita, Solomon Islands

The island of Malaita in the Solomons Islands represents a unique opportunity to study the deeper portions of the largest oceanic plateau discovered to date - the Ontong Java Plateau (OJP). Malaita exposes several kilometres of Cretaceous OJP lavas, stratigraphically named the Malaita Volcanic Group (MVG), within en-echelon inliers. Most of the lavas have suffered low-grade (brownstone facies) metamorphism. The MVG basalts are low-K tholeiites, displaying a limited range of major and trace element concentrations, consistent with the lavas being well mixed prior to eruption, probably in shallow level magma chambers. However, two distinct groups, Type A and Type C-G, can be identified from their rare earth elements (REEs), platinum group elements, Sr and Nd radiogenic isotopes, indicating that mixing has not effectively wiped out all source-related and melting signatures.;The MVG basalts were formed by extensive melting, mostly within the spinel Iherzolite field as indicated by their chrondritic REE patterns, suggesting a ridge-centred eruption. However, melting in the MVG's source started within the garnet Iherzolite field, consistent with a thermal anomaly such as a decompressing mantle plume. Paradoxically for a ridge centred plume, the MVG erupted in a deep submarine environment, when the elevation caused by a surfacing plume would surely imply a subaerial eruption environment, such as seen in Iceland. In addition, the MVG's overlying sedimentary sequence implies a history of uplift, rather than that of subsidence which would be expected as the plume wanes or crust is transported away.;The OJP plume was probably heterogeneous, consisting of enriched (HIMU) streaks in a depleted matrix. This depleted component is thought not to be entrained upper MORB material, but a depleted component intrinsic to the plume itself, possibly entrained as the plume ascended through the lower mantle.