U-Pb circon ages from the Milford Orthogneiss, Milford Sound, northern Fiordland : Paleozoic igneous emplacement and Early Cretaceous metamorphism

Milford Orthogneiss is an extensive hornblende-metadiorite/gabbro unit centred on Milford Sound, northern Fiordland. U-Pb TIMS data from zircon indicate both Paleozoic and Early Cretaceous age components, but cannot distinguish between a Palozoic or Early Cretaceous age for the igneous protolith. U-Pb zircon ion probe analyses, guided by cathodoluminescence images of the internal structure of the zircons, indicate that individual grains are either essentially Paleozoic (355 +/-10 Ma oscillaroty zoned igneous grains) or Early Cretaceous (134 +/-2 Ma patchy/sector zoned grains) in age. Rare grains show patchy/'sector' zones apparently overprinting and replacing oscillatory zonation. In most cases both types of grain are also rimmed by low-U zircon (120 +/-2 Ma) that may date the breakdown of a Zr-containing mafic phase during a second phase of prograde metamorphism. Other than the low U-rims, a (igneous) Cretaceous overgrowth over a Paleozoic core is not observed. We interpret these data to indicate that the protolith of the Milford Orthogneiss was emplaced at ∼360 Ma and subsequently metamorphosed in two distinct events in the Early Cretaceous (∼134 and ∼120 Ma. Thus 1) the Milford Orthogneiss cannot represent the metamorphic equivalent of the high-level Darran Complex (138 Ma) to the east, despite lithological and chemical similarities, and a major shear zone is confirmed between these two units; 2) Paleozoic 'cover' rocks in Fiordland may have suffered high-pressure metamorphism in the Cretaceous; 3) annealing or recrystallisation of zircon at ∼134 Ma is believed to record a high-pressure metamorphic event possibly related to subduction of the continental margin; and 4) Milford Orthogneiss may represent McCulloch et al.'s (1987) hypothesised source from which the Western Fiordland Orthogneiss was derived. (author). 36 refs., 9 figs., 2 tabs