The origin of metavolcanic and associated argillaceous rocks at Island Bay, Wellington, New Zealand

Abstract

Abstract The alternating greywacke-argillite turbidite beds exposed on the eastern side of Island Bay, Wellington, New Zealand, belong to the Torlessia fossil zone of the Torlesse Supergroup. They were deposited in a deep-water marine environment. Subsequent reworking by bottom currents has in places formed contourites. The volcanic sequence, comprising altered and sheared doleritic and variolitic metabasite and associated red and yellow argillite, is concordant with the surrounding sedimentary rocks. Major element content and elemental ratios suggest that the metabasites are tholeiitic basalts. Discriminant diagrams using immobile elements show that the data are consistent with either ocean floor or withinplate settings. Field relationships and geochemistry of the volcanic sequence, and modem seamount analogues, suggest that an inactive talus-flanked seamount was carried by sea-floor spreading towards the subduction zone and became progressively tilted as it approached the trench axis, thus enabling the lava flow debris together with pelagic clays to slide into the zone of turbidite deposition. Any soft-sediment deformation which resulted from the sliding event was largely overprinted by tectonic deformation which occurred when the trench deposit was incorporated into the accretionary prism. Contemporaneous metamorphism raised the rocks to prehnite-pumpellyite grade.