U-Pb geochronology from Tonagh Island, East Antarctica: Implications for the timing of ultra-high temperature metamorphism of the Napier Complex

Ion microprobe U-Pb zircon geochronology of an orthopyroxene-bearing felsic orthogneiss from central Tonagh Island, Enderby Land, East Antarctica provides insight into the chronological-metamorphic evolution of the Archaean Napier Complex, the details of which have been the source of debate for over two decades. The orthogneiss crystallised at 2626 ± 28 Ma, predating peak, ultra-high temperature (UHT) metamorphism and development of an intense regional S₁ gneissosity. Two subsequent episodes of zircon growth/resetting can be identified. A minor period of zircon growth occurred at 2546 ± 13 Ma, the regional significance and geological nature of which is unclear. This was followed by an episode of abundant zircon growth, as mantles on ∼ 2626 Ma cores and as anhedral grains, partly characterised by high Th/U (> 1.2), at ∼ 2450-2480 Ma. This age coincides with both lower and upper concordia intercept ages from other U-Pb zircon studies, and several Rb-Sr and Sm-Nd whole-rock isochron ages from the Napier Complex. We conclude that UHT metamorphism occurred at ∼ 2450-2480 Ma, and find no compelling evidence that UHT occurred much earlier as has been postulated. The zircon U-Pb data from this study also indicates a lower intercept age of ∼ 500 Ma, which coincides with the emplacement of Early Palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the Napier Complex.