| Popis: |
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. |
