The regional significance of Cretaceous magmatism and metamorphism in Fiordland, New Zealand, from U-Pb zircon geochronology

The western Fiordland Orthogneiss (WFO) is an extensive composite metagabbroic to dioritic arc batholith that was emplaced at c. 20-25 km crustal depth into Palaeozoic and Mesozoic gneiss during collision and accretion of the arc with the Mesozoic Pacific Gondwana margin. Sensitive high-resolution ion microprobe U-Pb zircon data from central and northern Fiordland indicate that WFO plutons were emplaced throughout the early Cretaceous (123.6 +/- 3.0, 121.8 +/- 1.7, 120.0 +/- 2.6 and 115.6 +/- 2.4 Ma). Emplacement of the WFO synchronous with regional deformation and collisional-style orogenesis is illustrated by (i) coeval ages of a post-D1 dyke (123.6 +/- 3.0 Ma) and its host pluton (121.8 +/- 1.7 Ma) at Mt Daniel and (ii) coeval ages of pluton emplacement and metamorphism/deformation of proximal paragneiss in George and Doubtful Sounds. The coincidence emplacement and metamorphic ages indicate that the WFO was regionally significant as a heat source for amphibolite to granulite facies metamorphism. The age spectra of detrital zircon populations were characterized for four paragneiss samples. A paragneiss from Doubtful Sound shows a similar age spectrum to other central Fiordland and Westland paragneiss and SE Australian Ordovician sedimentary rocks, with age peaks at 600-500 and 1100-900 Ma, a smaller peak at c. 1400 Ma, and a minor Archean component. Similarly, one sample of the George Sound paragneiss has a significant Palaeozoic to Archean age spectrum, however zircon populations from the George Sound paragneiss are dominated by Permo-Triassic components and thus are markedly different from any of those previously studied in Fiordland.