Abstract
The petrologically renowned Haast Schist, previously acknowledged as having formed by Jurassic collision and amalgamation of the exotic Rakaia and indigenous Caples terranes during late Mesozoic orogenesis, contains a component of the Alpine Schist whose protolith was deposited in the mid-Cretaceous. An air-fall tuff deposited on the ophiolitic Pounamu Ultramafic Belt (PUB) has an eruption age of 108 Ma. The PUB is in tectonic contact to the east with Rakaia terrane-derived Alpine Schist of Triassic age and is overlain by turbiditic metasediments that have Cretaceous (109-140 Ma) detrital zircon populations. The dominant zircon component of these sediments is Permo-Triassic, compatible with autocannibalistic reworking from the Torlesse Composite terrane. Reconnaissance analysis of the Alpine Schist indicates that Cretaceous protoliths extend at east 200 km along strike, while possible correlatives elsewhere in New Zealand suggest that the original basin may have been thousands of kilometres in extent. Models for the emplacement of these exotic Cretaceous sediments, the proposed Pounamu terrane, are explored. Our favoured interpretation is that the Cretaceous Alpine Schist protoliths were deposited in an ensimatic sedimentary basin north of earlier Mesozoic subduction zones. Multi-phase deformation and metamorphism occurred during collision of the LIP Hikurangi Plateau with Zealandia, resulting in the emplacement of an ophiolite (PUB)-soled allochthonous thrust sheet of the Pounamu terrane onto the previously metamorphosed Rakaia-Otago Schist-Caples massif. Based on new and regional evidence, this collision and the climactic metamorphism of the Pounamu terrane occurred at similar to 72 Ma. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.