BASALTS FROM THE CENTRAL PACIFIC BASIN - EVIDENCE FOR THE ORIGIN OF CRETACEOUS IGNEOUS COMPLEXES IN THE JURASSIC WESTERN PACIFIC

Studies of marine magnetic anomalies suggest that oceanic crust of Jur assic age underlies the Nauru, East Mariana and northwestern Central P acific basins of the west central Pacific Ocean. However, the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program have only recovere d basalts of Cretaceous age from these basins, indicating either that large areas of the Jurassic western Pacific are covered by Cretaceous intraplate igneous complexes or that Cretaceous ocean crust is present in these areas. We present chemical and isotopic data on basalts and dolerites recovered by DSDP Leg 17 from the Central Pacific Basin(CPB) . Drilling in the predicted Jurassic-age portion of the CPB recovered Late Albian (100-105 Ma) tholeiitic pillow basalts at Site 169 and Lat e Cretaceous alkalic dolerite sills at Sites 170 and 169 above the ext rusives. Early Cretaceous crust was recovered from Site 166. The Site 169 tholeiites are LREE depleted but slightly enriched in highly incom patible elements relative to normal mid-ocean ridge basalt (MORE), giv ing them trace element ratios similar to MORE erupted near hot spots. The Sr, Nd, and Pb isotopic compositions of the tholeiites (Sr-87/Sr-8 6(i)=0.70341-0.70348; epsilon(Nd)(t) = +6.2-6.4; Pb-206/(204)pb(meas)= 18.63-18.68) overlap with MORB from the Indian Ocean, but fall outside of the Sr and Nd isotopic ranges for Pacific MORE. The Site 169 thole iites are compositionally almost identical to basalts from the Nauru a nd East Mariana basins and are isotopically similar to some Ontong Jav a Plateau basalts. Site 166 crustal lavas are similar to normal-MORE f rom the East Pacific Rise. Chemical and isotopic data for the Site 169 tholeiites are consistent with an origin at a spreading center contam inated with EM I-type plume materials, probably from the Ontong Java p lume head. Based on geochemical and geophysical data from the region, we propose that the Site 169 tholeiites, as well as basalts from the N auru and East Mariana basins, were created at a system of short-lived mid-Cretaceous spreading centers extending from the East Mariana Basin into the northwestern Central Pacific Basin, and that rifting of Jura ssic crust was initiated as a result of the rapid formation of Ontong Java Plateau. The Sites 169 and 170 sills appear to have been intruded as a result of near-ridge, non-hot spot volcanism similar to that pro ducing young seamounts in the eastern Pacific today. However, the intr usion of these sills, as well as their HIMU (high mu) isotopic affinit y, may have been influenced by nearby mantle plumes.