Late Neoproterozoic/Early Palaeozoic events in central Dronning Maud Land and significance for the southern extension of the East African Orogen into East Antarctica

Abstract

Abstract New SHRIMP zircon data from Gjelsvikfjella and Muhlig-Hofmann-Gebirge indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. High-grade overprinting occurred between c. 560 and 490 Ma. This period is considered to bracket the collision of E- and W-Gondwana along the East African/Antarctic Orogen (EAAO) and also the subsequent collapse of the orogen. The onset of collision probably predates our oldest dates. We can clearly differentiate between an early compressional stage (Pan-African I) and a later dilational episode (Pan-African II). The Pan-African I event is characterised by the intrusion of leucogranites, and the formation of medium-U metamorphic zircon overgrowth. A sample of a syn-tectonic leucogranite gave a well-constrained crystallisation age of 558.4±5.6 Ma. These rocks are intensely deformed and have formed a second gt-bearing leucosome after their emplacement. Identical ages come from low-U metamorphic zircons and low-U rims from another leucosome at Festninga (557±13 Ma). The Pan-African II event (c. 530 and 490 Ma) is characterised by widespread melting and only little deformation. It is associated with charnockite magmatism, charnockitisation and high-temperature metamorphism. Wide, very high-U zircon rims form during this period. The early Pan-African II overprint between c. 530 and 520 Ma is recorded in almost all samples. An undeformed mafic dyke with a zircon crystallisation age of 523.2±4.8 indicates that there is no or minor deformation after its intrusion. This strongly metasomatised dyke, however, also indicates that although deformation ceased, intense fluid circulation continued. The youngest crystallisation ages come from a post-tectonic granite sheet which yields a crystallisation age of 486.9±3.8 Ma. Final cooling to temperatures below c. 600 °C is recorded by a titanite age of c. 483±11 Ma from the Stabben gabbro. Late to post-tectonic granitoids are voluminous and intrude over a period of c. 30 Ma, between c. 520 and 490 Ma. They are alkaline in composition and can be classified as A2-type. This indicates that they are probably formed by melting of a tonalitic to granodioritic lower crust in the cause of crustal thickening, or from underplated lower crust.