The Terre Adelie basement in the East-Antarctica Shield: geological and isotopic evidence for a major 1.7 Ga thermal event; comparison with the Gawler Craton in South Australia

The basement in the Pointe Geologie Archipelago, around the French Dumont D 'Urville station in Terre Adelie, comprises a metapelitic migmatitic comple x with a 1.7 Ga metamorphic evolution differing from that generally found i n other areas of the East Antarctic Shield. In Terre Adelie, although the o ldest crustal precursors (Nd T-DM model ages) are ca. 2.2-2.4 Ga old, inher ited zircon SHRIMP ages are ca. 1.73-1.76, 2.6 and 2.8 Ga. The migmatitic e volution is restricted to a single event which is dated at 1.69 Ga by newly formed zircons (SHRIMP) and by U-Pb and Pb-207/Pb-206 evaporation TIMS age s of monazite. When interpreted as cooling ages, Sm-Nd garnet (1.60 Ga) and Rb-Sr micas (1.50 Ga) ages would be indicative of a slow cooling rate, sug gestive of a long-lived major thermal anomaly. Geological processes such as sediment deposition, HT-LP metamorphism and anatexis and coeval intrusion of mafic magmas occurred during a very short time. This suggests that the m igmatite complex is related to a major lithospheric thinning associated wit h a thermal anomaly coeval with syn-metamorphic mafic magmatism. Such thinn ing may have developed in a 2.8/2.4 Ga old basement, comparable with the Po rt Martin formations located 50 km further east. In the Gawler Craton (Sout h Australia) similar units are found which could have formed, together with the Terre Adelie basement, as a single shield by a collage of various terr ains that existed prior to ca. 1.5 to 1.6 Ga ago, comprising the 'Mawson Co ntinent'. (C) 1999 Elsevier Science B.V. All rights reserved.