Volcanism associated with extension in an Oligocene—Miocene arc, southwestern Viti Levu, Fiji

The Wainimala Group rocks of southwestern Viti Levu, Fiji, represent part of an Oligocene-Miocene island arc. Some of the early volcanism in this arc took place on a substrate of Eocene-Oligocene frontal-arc crust. Mainly andesitic, transitional calcalkaline lavas were erupted from major subaerial volcanic edifices, while elsewhere tholeiitic magmas formed low-lying basaltic lava fields and small felsic volcanic centres on the seafloor. The intrusion of a dense bimodal basalt-dacite dyke swarm into the underlying frontal-arc crust indicates that significant crustal extension accompanied the eruption of the inter-edifice lavas. Geochemical variations within the Wainimala volcanic suite are thought to have been generated through fractional crystallisation processes. While relatively slow ascent and mixing of magmas took place beneath the major edifices, efficient shallow-level fractionation and rapid ascent of magmas occurred in the areas between them. The contrasting styles of volcanism may reflect the interaction of ascending magmas with pre-existing heterogeneities in the frontal-arc substrate.