The geochemistry of basalts from the Wallis Islands, northern Melanesian Borderland: Evidence for a lithospheric origin for Samoan-type basaltic magmas?

Abstract

The Wallis Islands are located on the western extension of the Samoan seamount chain and are composed largely of basaltic flows and pyroclastics with ages that are less than 0.5 Ma. The youngest eruptives are tholeiitic in composition and are relatively depleted in large ion lithophile and high field strength trace and minor elements. In contrast, the older basaltic flows and pyroclastics, which make up the bulk of exposed volcanic rocks, have transitional alkalic chemistry and are relatively enriched in incompatible trace and minor elements. The younger tholeiites have lower 87Sr/ 86Sr (0.7045–0.7048) and tend to have higher 143Nd/ 144Nd (0.512844–0.512910) than the older alkalic rocks ( 87Sr/ 86Sr: 0.7049–0.7052; 143Nd/ 144Nd: 0.512800–0.512895). Major- and trace-element modelling based on relationships inferred from the isotopic and trace-element compositions of stratigraphically related samples supports the hypothesis that some of the evolved basalts could have been generated by crystal fractionation from less evolved parental basalts. The Quaternary volcanism on the Wallis Islands is too young to be directly related to passage of the Pacific Plate over the Samoan hot spot, but isotopically Wallis basalts are very similar to shield lavas of the Samoan Islands. This suggests that Wallis basaltic magmas were generated in the lithosphere by a later thermal disturbance that was probably related to deformation along the transform boundary between the Pacific and Indo-Australian plates. A model is proposed whereby the basalts derived from a complex, heterogeneous, lithospheric mantle by progressive melting which initially extracted relatively alkalic melts from mantle domains dominated by an enriched component. With time, as melting proceeded, the enriched component was diluted by a more depleted, and more refractory component. The relationship of the mantle plume, that generated the Samoan seamount chain, to the volcanism on the chain may be largely thermal with very little geochemical input from the plume into the erupted magmas. The northern Melanesian Borderland may be a region where unusual lithospheric mantle has existed for some time and the shield lavas of the Samoan chain, along with the Wallis basalts, may have been extracted largely from this lithosphere.