GEOCHEMICAL EVIDENCE FOR A MIDCRETACEOUS SUPERPLUME

Basalt lavas with a high Nb/Y ratio for a given Nb/Zr ratio occur in t he Polynesian ''superswell'' region of the South Pacific, which probab ly formed by upwelling of a deep-mantle superplume. The distinctive ge ochemical characteristics of the Polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic (anc ient mid-oceanic-ridge basalt) component, Basalts displaying such chem ical signatures have been found on Shatsky Rise, the Ontong Java Plate au, and greenstones from subduction-zone complexes of Sakhalin Island, The occurrence of Polynesian-type basalts, together with an estimate of their ages, suggests that the South Pacific superplume was active a s long ago as 90-150 Ma, The superplume activity preceded the onset of the superchron, supporting an idea that the superplume acted as a tri gger for such a global event.