Petrological variation in the Banks Islands, New Hebrides

Abstract

The Banks Islands represent the most northern group of volcanic islands in the linear Central Chain of Pliocene - Recent age in the New Hebrides. A study o f the petrological variation in these northern islands provides a basis for the development of a petrogenetic-tectonic model which is applicable to most, if not all, the Central Chain islands. Integration of petrographic, mineralogical and geochemical data on each island in the Banks Group has revealed a systematic petrological variation across the islands, represented chemically by a decrease in K20 (and related incompatible elements) away from the New Hebrides trench. The correlation with depth to the Benioff zone is therefore the reverse of that typically found in island arcs. REE evidence and source modelling based on this data indicates that this variation represents a progressive depletion in LREE/HREE in the upper mantle, laterally away from the trench. The variation also represents a progressive increase in partial melting of the source, in the same direction. However partial melting differences alone could not produce the observed variation. The variation in source composition and degree of partial melting are blended in to a model in which the compositional variation is attributed to an earlier west-facing Miocene subduction event, and the variation in degree of partial melting is a function of the location of the islands with respect to the postulated heat source: the active back-arc basin. The enrichment of the upper mantle in the Miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements, from the west-dipping Benioff zone, and subsequent reaction with the overlying upper mantle. The model thus envisages approximately contemporaneous development of the Central Chain volcanoes and the back-arc basin in the late Pliocene, with partial melting of hydrous, laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back-arc basin. The role of the present subduction regime in magma generation is considered very limited. The mineralogical and geochemical data show that all compositions in the range from basaltic andesite to dacite (ie. > 53wt.% SiO2) in the Banks Islands can be derived from basalts by fractional crystallization of the observed phenocryst phases (including magnetite in the Gaua suite), after allowances have been made for inherited differences between some melt fractions. Together with the paucity of andesites in the Central Chain, this suggests that the primary melts of the Pliocene - Recent volcanism are basaltic. The generation of calc-alkaline magmas in the early Miocene are also believed to have involved basaltic primary melts as both the Miocene Eastern and Western belts are dominantly basaltic.