Basalts from the Central Pacific Basin: Evidence for the origin of Cretaceous igneous complexes in the Jurassic western Pacific

Abstract

Studies of marine magnetic anomalies suggest that oceanic crust of Jurassic age underlies the Nauru, East Mariana and northwestern Central Pacific basins of the west central Pacific Ocean. However, the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program have only recovered 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 Late Cretaceous alkalic dolerite sills at Sites 170 and 169 above the extrusives. Early Cretaceous crust was recovered from Site 166. The Site 169 tholeiites are LREE depleted but slightly enriched in highly incompatible elements relative to normal mid‐ocean ridge basalt (MORB), giving them trace element ratios similar to MORB erupted near hot spots. The Sr, Nd, and Pb isotopic compositions of the tholeiites (87Sr/86Sri = 0.70341–0.70348; εNd(t) = +6.2–6.4; 206Pb/204Pbmeas = 18.63–18.68) overlap with MORB from the Indian Ocean, but fall outside of the Sr and Nd isotopic ranges for Pacific MORB. The Site 169 tholeiites are compositionally almost identical to basalts from the Nauru and East Mariana basins and are isotopically similar to some Ontong Java Plateau basalts. Site 166 crustal lavas are similar to normal‐MORB from the East Pacific Rise. Chemical and isotopic data for the Site 169 tholeiites are consistent with an origin at a spreading center contaminated with EM I‐type plume materials, probably from the Ontong Java plume head. Based on geochemical and geophysical data from the region, we propose that the Site 169 tholeiites, as well as basalts from the Nauru 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 Jurassic 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 producing young seamounts in the eastern Pacific today. However, the intrusion of these sills, as well as their HIMU (high μ) isotopic affinity, may have been influenced by nearby mantle plumes.