Evidence for early Mesoproterozoic (ca. 1590 Ma) ultrahigh-temperature metamorphism in southern Australia

The correspondence between granulite ultrahigh-temperature (UHT) events with the record of supercontinent/supercraton formation defines distinct gaps in the geological record where UHT events are rare. One gap occurs from the late Palaeoproterozoic to the late Mesoproterozoic (1.85-1.1 Ga). This time interval is currently represented by only two HT to UHT events and corresponds to the interpreted amalgamation and breakup of the supercontinent Columbia. The proposed tectonic setting of UHT events associated with the younger Gondwana supercontinent is the closure of back-arc basins. However, it is unclear whether older UHT metamorphic belts developed in a similar tectonic setting. In situ LA-ICPMS monazite U-Pb geochronology coupled with metamorphic phase equilibria modelling provide evidence for regional-scale high temperature to UHT early Mesoproterozoic (ca. 1590 Ma) metamorphism in the Gawler Craton in southern Australia. Metapelitic granulites intersected from the Coober Pedy Ridge region contain spinel + quartz + cordierite + garnet-bearing assemblages that formed at around 925 degrees C and 6.5 kbar on a clockwise-style pressure-temperature (P-T) evolution at approximately 1585 Ma. In the adjacent Mabel Creek Ridge region, garnet + sillimanite + biotite + rutile-bearing metapelites record slightly lower temperature conditions of around 850 degrees C and 9 kbar at ca. 1600 Ma, and underwent near isothermal syn-deformational decompression to similar to 6 kbar by 1580 Ma, producing cordierite-bearing assemblages. The minimum duration of granulite facies metamorphism in these two regions is on the order of 20 M.y. The age of this high-temperature metamorphism falls within the late Palaeoproterozoic to late Mesoproterozoic 'UHT gap' and represents a rare opportunity to investigate the potential role of Mesoproterozoic Australia in the context of the evolution of the Columbia supercontinent. (C) 2010 Elsevier B.V. All rights reserved.