Abstract:
Geochemical data for basalts from the island of San Cristobal (Makira) demonstrate the presence of OJP basement: isotopic, major and trace element data show significant overlap with the OJP drill sites and the OJP basalts on Malaita and Santa Isabel. OJP-type basalts from Makira also cluster into A- and C-G-type groups as defined by basalts from ODP Site 807 and the island of Malaita. Despite all of these similarities, it is the differences exhibited by some of the basalts on Makira that could prove to produce the most significant information. Major elements and primitive mantle-normalized diagrams display an additional group of basalts, termed H-type, possessing high-Ti and slightly higher abundances of incompatible elements than A-type basalts. Isotopically, these basalts are similar to the Site 803/Units C-G basalts. Ar-Ar age dating of the OJP-type basalts yields a bimodal distribution of ages ( approximately 90 Ma and approximately 60 Ma), with the exception of one sample dating at approximately 34 Ma. Previous dating of OJP basalts revealed ages of approximately 122 Ma and approximately 90 Ma. Significantly all of the geochemical data indicates that ocean-island (OIB) and mid-ocean ridge (MORB) basalts are also present, and appear to be intercollated with the OJP-type basalts. The OIB-type samples have isotopic compositions similar to the Samoan Shield field, and the MORB-type sample is isotopically similar to the Pacific MORB field. Curiously, Pb and Nd isotope data of one OJP-type basalt (MTG F1) are similar to the Louisville Ridge and Manihiki Plateau. Further study of this sample may coincide with previous hypotheses concerning the relationship between the Louisville Ridge (Mahoney and Spencer, 1991, Earth Planet. Sci. Lett., 104:196; Yan & Kroenke, 1993, Proc. ODP Sci. Results 130:697), the Manihiki Plateau (Tejada et al., 1996, J. Petrol. 37:361), and the OJP. The presence of MORB could support a near-ridge origin for the plume head/s which created the OJP, while the OIB may represent a corresponding plume tail.