Abstract
The Pounamu Ultramafic Belt is an elongate zone of serpentinites and associated rocks within the Alpine Schist of north Westland, New Zealand. It includes metamorphosed pyroxenite-peridotite or boninite, gabbro and a breccia dominated by clasts of basalt and gabbro, overlain by pelagic sediments, a mid-Cretaceous tuff (108-105 Ma), and quartzofeldspathic turbidites with Cretaceous (< 105 Ma) detrital zircon minimum age populations. Rare earth element geochemistry indicates metabasites are similar to fractionated normal mid-ocean ridge basalt (N-MORBs), with minor enriched mid-ocean ridge basalt (E-MORB). However, because high field strength element concentrations of these metabasites are depleted relative to N-MORB, they have affinities to fore-arc magmas. The Pounamu ophiolite is likely, therefore, to have formed in a supra-subduction zone setting. Formation of oceanic crust and sedimentation during the Cretaceous is most likely in the re-entrant plate boundary zone between northwest and southeast Zealandia. Subsequent (c. 70 Ma) metamorphism occurred during accretion, subduction and interfolding of this exotic Pounamu Terrane with Rakaia Terrane-derived Otago Schist beneath western southeast Zealandia.