Abstract

Rocks belonging to the Lower Oligocene to Middle Miocene Wainimala Group in southwestern Viti Levu can be divided into an are assemblage, representing the axial part of a volcanic are, and a basinal assemblage, representing the trenchward flank of the are and the inner part of the fore-are basin. The are assemblage consists mainly of volcaniclastic rudites and lava flows, with minor shallow-water limestones, and is intruded by Mid- to Late Miocene gabbro and tonalite stocks. The basinal assemblage to the north is floored by part of an earlier, Eocene to Lower Oligocene are. The lower part of this assemblage includes volcaniclastic rudites representing the dispersal aprons of major volcanic edifices to the trenchward of the are axis, and basaltic lavas and fissure-fed rhyolite lava domes and breceia ridges erupted in deeper water away from these centres. The rhyolites are directly related to the acidic members of a dense, compositionally bimodal dyke swarm that intrudes the underlying earlier are rocks, recording substantial crustal extension. The basinal volcanic rocks are interbedded with and overlain by a sequence of pelagic carbonates, thin-bedded epiclastic turbidites, and acidic tuffs. The basinal succession evolved from an intra-are to an inner fore-are setting as volcanism within the assemblage died away in the Early Miocene. In the late Early Miocene increased uplift and emergence of the are massif increased the sediment yield to the fore-are basin, resulting in the deposition of thicker-bedded, coarser-grained turbidites.

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