PAN-AFRICAN GRANITOID MAGMATISM IN CENTRAL DRONNING MAUD LAND CAUSED BY DELAMINATION?

Central Dronning Maud Land represents the southern end of the East African-Antarctic orogen (EAAO) that originated from the collision of various parts of East- and West- Protogondwana during Neoproterozoic until early Paleozoic times. This megaorogen is more than 8000 km long and stretches from northern Egypt-Arabia through Mozambique into East Antarctica. It shows a strong lateral variation in orogenic styles, separated by the E-W-trending Lurio belt in Mozambique with accretion in its northern part and continent-continent collision in the southern part. In addition, numerous Pan-African granitoids are observed in the southern part that are virtually absent in the northern part. Two contrasting models try to explain the existence of these granitoids: (i) SE-directed thrusting of nappes from the combined Damara-Zambesi mobile belt towards Dronning Maud Land [2] thereby explaining the granitoid magmatism by crustal thickening; (ii) extensional tectonics with delamination of the orogenic root [1] thereby explaining the granitoid magmatism by upwelling of hot asthenosphere under a thinned continental crust. Major and trace element signatures of these granitoids show a subalkaline ferroan character with the exception of the granitoids of the Petermann-Ketten that are alkaline. In general, all granitoids show an A₂-type signature. This points towards an extensional regime for the evolution of granitoid melts. Intrusion of the granitoids seems to shift from ~510 Ma in the Nampula province (Mozambique) to ~500 Ma in central Dronning Maud land (Antarctica) [1]. No older components in these rocks have been found via U/Pb zircon age determination sofar, i.e. no older Mesoproterozoic zircon cores have been observed yet. Sm-Nd isotope characteristics of Antarctic granitoids reveal consistently older TDM around 1.5 Ga and no correlation with their respective SiO₂-content is observed. In our opinion that indicates the older Sm-Nd signature to be derived from the mantle and not via assimilation during ascent, thus favouring the second proposed model.

[1] Jacobs et al. (2008) In: Satish-Kumar et al. Geol. Society, London, Spec. Pub. 308, 69-90. [2] Grantham et al. (2008) In: Satish-Kumar et al. Geol. Society, London, Spec. Pub. 308, 91-119.