Potassium and incompatible-element enrichment in shoshonitic lavas from the Tavua volcano, Fiji

Abstract

Shoshonitic lavas from the Tavua volcano on the island of Viti Levu, Fiji, display a range of compositions from mafic absarokite through shoshonite to evolved banakite. They were erupted during the early stages of the breakup of the once-continuous Vanuatu-Fiji-Tonga arc, yet their geochemical characteristics are similar to those of subduction-related volcanism. They have low TiO2 (0.5–0.7%) and HFSE abundances (e.g., 0.03–0.05 ppm Ta in the absarokites), very high LILE/HFSE ratios and slight LREE enrichment (CenYbn 2.7–4.5). Unusually for potassic rocks, they do not have anomalous radiogenic isotopes, Sr, Nd and Pb isotope ratios being similar to other circum-Pacific arcs, and Pb in particular being similar to MORB (143Nd144Nd ≈ 0.51300, 87Sr86Sr ≈ 0.7035–0.7040, 206Pb204Pb = 18.5–19.0). Simple mass-balance calculations restrict the amount of subducted sediment in the shoshonite source region to <4%.Variations in HFSE abundances and ratios are attributable to variable degrees of melting of a depleted mantle source region whereas major-element abundances indicate that the absarokites result from high degrees of melting. This is consistent with the slight enrichment of the LREE relative to the HREE. Incompatible-element enrichment is selective, the shoshonites being particularly enriched in Rb, Ba, K, Sr, Pb and P relative to island arc tholeiites, and this is reflected in, for example, anomalously high P/Nd ratios of between 120 and 180. These features are interpreted to be the result of source enrichment by hydrous fluids released from subducted oceanic lithosphere that scavenged material from both the mantle wedge and possibly subducted altered oceanic crust. High oxygen fugacities are indicated by small but significant negative Ce anomalies, the lack of Eu anomalies even in the most evolved samples despite plagioclase dominated fractionation, and the early appearance of titanomagnetite in the fractionating assemblage.Comparison with more typical island arc tholeiites indicates that the source enrichment of the Tavua shoshonites was an extreme example of that operating beneath arcs in general. Two alternatives are presented to explain the unusual composition of the shoshonites. The first implies an unusually large fluid input from the slab to the magma source region although the cause of this is unclear. The second appeals to the effects of tectonics on the convective regime in the mantle wedge. A change in the ratio of the fluid velocity to that of the convecting mantle modifies the range of elements that are transported by the fluid into the magma source region. This type of model is encouraged by the relationship between element enrichment and ionic radius and the association of shoshonites with changes in arc tectonics both in Fiji and other localities.