Geochemistry and origin of a Mesozoic ophiolite : the Pounamu Ultramafics, Westland, New Zealand

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. (author).