Origin and evolution of magmas on the Ontong Java Plateau

The Early Cretaceous Ontong Java Plateau (OJP) represents by far the largest igneous event on Earth in the last 200 Ma and yet, despite its size, the OJP’s basaltic crust appears to be remarkably homogeneous in composition. The most abundant rock type is a uniform low-K tholeiite, represented by the Kwaimbaita Formation on Malaita and found at all but one of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites on the plateau and in the adjacent basins. This is capped by a thin and geographically restricted veneer of a slightly more incompatible-element-rich tholeiite (the Singgalo Formation on Malaita and the upper flow unit at ODP Site 807), distinguished from Kwaimbaita-type basalt by small but significant differences in Sr-, Nd- and Pb-isotope ratios. A third magma type is represented by high-Mg (Kroenke-type) basalt found in thick (> 100 m) successions of lava flows at two drill sites (ODP Sites 1185 and 1187) 146 km apart on the eastern flank of the plateau. The high-Mg basalt is isotopically indistinguishable from Kwaimbaita-type basalt and may therefore represent the parental magma for the bulk of the OJP. Low-pressure fractional crystallization of olivine followed by olivine+augite+plagioclase can explain the compositional range from high-Mg Kroenke-type to Kwaimbaita-type basalt. The Singgalo-type basalt probably represents slightly smaller-degree, late-stage melting of an isotopically distinct component in the mantle source. Primary magma compositions, calculated by incremental addition of equilibrium olivine to aphyric Kroenke-type basalt glass, contain between 15.6% (in equilibrium with Fo90) and 20.4% (Fo92) MgO. Incompatible-element abundances in the primary OJP magma can be modelled by around 30% melting of a peridotitic primitive-mantle source from which about 1% by mass of average continental crust had previously been extracted. This large degree of melting implies decompression of very hot (potential temperature >1500°C) mantle beneath very thin lithosphere. The initiation of an exceptionally large and hot plume head close to a mid-ocean ridge provides the best explanation for the size, homogeneity and composition of the OJP, but is difficult to reconcile with the submarine eruption of virtually all of the basalt so far sampled.

Accretionary-lapilli-bearing pyroclastic rocks at ODP Leg 192 Site 1184: a record of subaerial phreatomagmatic eruptions on the Ontong Java Plateau

Detailed analysis of lithologies and lithofacies associations within the 337.7 m thick basement volcaniclastic succession, recovered during Ocean Drilling Program (ODP) Leg 192 at Site 1184 on the Ontong Java Plateau, shows that in bulk it is made up of pyroclastic deposits of phreatomagmatic origin. The succession is essentially made up of two lithologies: lapilli tuff (59% of the total recovered core length) and tuff (34%), consisting almost entirely of juvenile clasts (>97%) and containing significant amounts of matrix-supported accretionary and/or armoured lapilli clasts. The evidence indicates that the succession was formed by at least six (and perhaps as many as 10) major phreatomagmatic eruptions that were subaerial and associated with the main phase of volcanism on the Ontong Java Plateau.

Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40Ar/39Ar age for volcaniclastic rocks at ODP Site 1184

The discovery of a thick (337.7 m drilled) succession of volcaniclastic sediments at Ocean Drilling Program (ODP) Site 1184, on the eastern salient of the Ontong Java Plateau (OJP), shows that at least part of the plateau formed at or near sea level. All the other parts of the OJP that have been drilled or are exposed on land are composed of basaltic lava flows erupted in a deep-marine environment. The composition of the volcaniclastic rocks at Site 1184 is essentially indistinguishable from that of basaltic lava flows recovered from the other drill sites on the OJP, suggesting that the eastern salient formed during the same magmatic event (at approximately 120 Ma) that formed the main part of the plateau. A steep (−54°) magnetic inclination preserved in the volcaniclastic succession is consistent with an Early Cretaceous age, but rare nannofossils recovered from the volcaniclastic rocks suggest a much younger Eocene age. In order to resolve this paradox, we attempted to date the succession by the 40Ar/39Ar technique, even though the rocks are highly altered. Two samples of feldspathic material separated from basaltic clasts give minimum age estimates of c. 74 Ma. The basaltic clasts have compositions similar to that of their host rocks and are therefore probably cognate. A weighted average of the results of total fusion experiments on four or five small euhedral plagioclase crystals separated from the matrix of the volcaniclastic rocks at each of four levels in the lower part of the succession gave a value of 123.5 ± 1.8 Ma, and this probably represents a reasonable estimate of the age of eruption. Although not very precise, the 40Ar/39Ar results are the best that presently can be obtained from such altered rocks. They rule out an Eocene age for the volcaniclastic succession at Site 1184, and we suggest that the Eocene nannofossils were introduced through contamination, probably along fractures.

Compositional variability in lavas from the Ontong Java Plateau: results from basalt clasts within the volcaniclastic succession at Ocean Drilling Program Site 1184

Tholeiitic basalts have been recovered from drill sites in different locations on the Ontong Java Plateau (OJP) and are remarkably homogeneous across this large igneous province. The most abundant basalt type is represented by the Kwaimbaita Formation on Malaita in the Solomon Islands, where it is capped by the isotopically distinct and slightly more incompatible-element-enriched basalt of the Singgalo Formation. Ocean Drilling Program (ODP) Leg 192 drilled five sites on the OJP, four of which penetrated basement lava successions. All basalt recovered during Leg 192 is chemically and isotopically indistinguishable from Kwaimbaita-type lavas.

Site 1184 of ODP Leg 192 is situated on the eastern salient of the OJP, and is unique because the recovered volcaniclastic succession contains the first conclusive evidence for emergence of part of the OJP above sea level. Within this succession are clasts of basaltic material. We report the major element-, trace-element and isotopic compositions of 14 moderately to highly altered basalt clasts. On the basis of incompatible-element concentrations, specifically high field strength elements (HFSE) and rare earth elements (REE), four groups of clasts are defined. Group 1 clasts are similar to basalt from the Kwaimbaita Formation. Group 2 clasts show variable composition, but the heavy rare earth element (HREE) concentrations are similar to those of basalts from the Kwaimbaita Formation. Group 3 clasts have compositions similar to the high-MgO Kroenke-type basalt recovered during ODP Leg 192. Group 4 clasts are more evolved than the Kwaimbaita or Singgalo lavas, and contain deep negative Eu and Sr anomalies on primitive-mantle (PM)-normalized diagrams, as well as high concentrations of Nb, Ta and Th. Group 4 clasts also show a large fractionation of Nb from La and have (Nb/La)PM ratios of approximately 2. Sr-, Nd- and Pb-isotope ratios were measured on five clasts covering all four groups. Although the Sr- and Pb-isotope ratios exhibit some variability, which we attribute to alteration, the Nd-isotope ratios are within the field defined for Kwaimbaita-type lavas.

We conclude that most of the compositional variability displayed by these clasts is a result of alteration and that Ta appears to be the most immobile incompatible trace element. All of the clasts were derived from the mantle source that produced the Kwaimbaita-type and Kroenke-type basalts. Our data emphasize the widespread nature of Kwaimbaita-type basalt and show that the source region was active under both the eastern salient and the high plateau of the OJP.