Medium-high-pressure metamorphic rocks of the Tuhua Orogen, western New Zealand, as lower crustal analogues of the Lachlan Fold Belt, SE Australia

Abstract

Lower Palaeozoic rocks are widely distributed in the western province of New Zealand. They include: (1) Ordovician greywackes, slates and minor paragneiss in the Buller terrane; (2) weakly metamorphosed (subgreenschist- to greenschistfacies) Cambrian mafic volcanics, Ordovician limestones and Silurian siliciclastic rocks in the Takaka terrane; and (3) higher-grade rocks mainly in Fiordland, which include 350–380 Ma amphibolite-facies metasediments held to be more highly metamorphosed equivalents of the Buller and Takaka terranes. Metamorphic rocks derived from late Precambrian protoliths may also be present. Together with the Buller and Takaka terranes, these Fiordland rocks were regionally metamorphosed and deformed during the mid-Palaeozoic Tuhua Orogeny. Before late Mesozoic continental rifting and breakup of the Pacific margin of Gondwana, they were part of a much more extensive Palaeozoic orogen which included the Lachlan Fold Belt and extended from southern New South Wales through Victoria and Tasmania into New Zealand and Antarctica (Victoria Land). In common with the Lachlan Fold Belt, regional metamorphism in the western province occurred largely under low-pressure-high-temperature conditions (andalusite-sillimanite-facies series) such that in Fiordland and the higher-grade parts of the Western Province low pressure (0.3–0.5 GPa) sillimanite and sillimanite + potash feldspar ± cordierite assemblages developed in rocks of appropriate bulk chemical composition. Higher-pressure (0.7–0.9 GPa) rocks in which kyanite is the dominant aluminosilicate polymorph are almost entirely confined to Fiordland and represent the deepest exposed crustal levels within the western province. At these levels, migmatized pelitic schists containing kyanite and garnet are interstratified with calc-silicate and quartzofeldspathic rocks, tonalitic gneisses, metamorphosed anorthosites and granite orthogneisses. These rocks were exhumed from depths of 25–30 km and offer useful insights into the nature and composition of the continental crust beneath what was once part of eastern Gondwana. Exhumation itself was brought about by late Mesozoic crustal extension combined with more recent transpression along the mid-Tertiary Alpine Fault. Assuming plate reconstructions for Gondwana during the Palaeozoic are valid, the lower to middle crustal rocks of Fiordland may give some indication as to what lies at depth beneath the Lachlan Fold Belt.