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

Abstract

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.

Oligocene–Miocene tectonic evolution of the South Fiji Basin and Northland Plateau, SW Pacific Ocean: Evidence from petrology and dating of dredged rocks

“…Ballance et al (1999) from fossil evidence show that the Kermadec and Colville Ridges were sites of volcaniclastic deposition in the Early Miocene. Fiji also has an Early Miocene arc volcanic record (Wharton et al, 1995).…”

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance

“…Such an origin could imply that the felsic rocks were erupted from the continentward fl ank of a volcanic arc and emplaced as ash fl ows within a back-arc basin, which was simultaneously erupting tholeiitic basalt. Coexisting basalts and dacites in apparent bimodal suites have been reported (e.g., Wharton et al, 1995), but unlike the Hillabee and Pumpkinvine Creek, they are generally not tholeiitic and calc-alkaline, respectively. For example, bimodal suites in modern continental rifts like East Africa usually involve basaltic and felsic alkaline rocks (e.g., Davies and MacDonald, 1987), but most dacites and rhyodacites (like the Hillabee and Pumpkinvine) are calc-alkaline rocks that are associated with suprasubduction rather than intraplate rift tectonics.…”

Lode Gold Deposits and Archean Mantle Plume–Island Arc Interaction, Abitibi Subprovince, Canada

“…As is typical in the Pacific (Wharton et al 1994), the young arc underwent extension that may have generated the Malartic Block crustal melt rhyolites and resulted in the eruption of Heva Formation tholeiites. Further extension resulted in back arc development via ∼2701 Ma eruption of the Mg and Fe-tholeiite Kinojevis Formation along a propagating east-west-oriented spreading center (present coordinates).…”