Geochemistry of Ontong Java Plateau Basalt and Gabbro Sequences, Santa Isabel, Solomon Islands.

Ian J. Parkinson, and R. J. Arculus (Department of Geology, Australian National University, Canberra, A.C.T. 0200, Australia; e-mail: iparkins@geology.anu.edu.au). R. A. Duncan (College of Oceanography, Oregon State University, Corvallis, OR 97331, USA.)

The Ontong Java Plateau (OJP) is the world's most voluminous large igneous province. However, only a total of 200 m of igneous basement from the plateau has been recovered by the Ocean Drilling Program (ODP). With an aim to gaining further insights into the formation of the OJP we have being studying slices of OJP igneous basement which are exposed in the Solomon Islands. Here we present new geochemical, isotopic and radiometric data for basalts and gabbros from the island of Santa Isabel.

Two distinct sequences of basalts are exposed on the northeastern flank of Santa Isabel; a sequence of altered pillow lavas which have 39Ar/40Ar ages of 90 Ma and a sequence of fresh pillow lavas, sills and flows with 39Ar/40Ar ages of 122 Ma. The two ages are the same as those found for basalts recovered by ODP Leg 130 providing further evidence for at least two major pulses of magmatism associated with the formation of the OJP. Geochemically the 90 Ma basalts are very similar to basalts from ODP Hole 803D with flat to convex-upward REE patterns but with lower Ti contents. Within the 122 Ma sequence three geochemical groups, which occur as discrete stratigraphic units in the field, have been recognised. Two of the groups are similar to basalts from ODP Hole 807C, whereas a high-Ti group has not been previously recognised from ODP drilling. Calculations using the thermodynamic program MELTS indicate that the high-Ti group is consistent with anhydrous low-pressure fractionation from a primitive melt whereas the other groups are consistent with fractionation at 0.3 GPa with 0.5-1.0 % H2O at relatively oxidising conditions (FMQ+1).

Troctolites and gabbros are spatially associated with the basalts. Sr and Nd isotopes indicate that the gabbros are clearly related to the OJP basalts with 87Sr/86Sr values of 0.7032-0.7044 and eNd values of 5.5-7.5. Moreover, calculated melts in equilibrium with the gabbros have REE patterns indistinguishable from the OJP basalts, except that melts that are even more fractionated than the high-Ti basalts must exist within the OJP, but have yet to be discovered. It is clear that further deep sea drilling is needed to elucidate more about of the origins of the OJP.