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

Abstract The basement in the Pointe Geologie Archipelago, around the French Dumont D'Urville station in Terre Adelie, comprises a metapelitic migmatitic complex with a 1.7 Ga metamorphic evolution differing from that generally found in other areas of the East Antarctic Shield. In Terre Adelie, although the oldest crustal precursors (Nd T DM model ages) are ca. 2.2–2.4 Ga old, inherited zircon SHRIMP ages are ca. 1.73–1.76, 2.6 and 2.8 Ga. The migmatitic evolution is restricted to a single event which is dated at 1.69 Ga by newly formed zircons (SHRIMP) and by U–Pb and 207 Pb/ 206 Pb evaporation TIMS ages 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, suggestive 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 migmatite complex is related to a major lithospheric thinning associated with a thermal anomaly coeval with syn-metamorphic mafic magmatism. Such thinning may have developed in a 2.8/2.4 Ga old basement, comparable with the Port Martin formations located 50 km further east. In the Gawler Craton (South Australia) similar units are found which could have formed, together with the Terre Adelie basement, as a single shield by a collage of various terrains that existed prior to ca. 1.5 to 1.6 Ga ago, comprising the `Mawson Continent'.