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 S-1 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 similar to2626 Ma cores and as anhedral grains, partly characterised by high Th/U ( >1.2), at similar to2450-2480 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 similar to2450-2480 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 similar to500 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. (C) 2002 Elsevier Science B.V. All rights reserved.