Résumé
The Grunehogna Craton (East Antarctica) was a part of the Archean Kalahari Craton of Southern Africa prior to Gondwana breakup. Granite from the basement of the craton has been dated by U-Pb zircon dating to 3,067 Ma with inherited grains showing ages of up to 3,433 Ma [1]. At the eastern margin of the craton, the Ahlmannryggen nunataks comprise an approximately 2000 m thick pile of clastic and volcanic sediments of the Ritscherflya Supergroup. These were sourced from eroding a proximal active continental arc as demonstrated through the age distribution and internal zoning of detrital zircon [2]. Detrital zircon grains from the Ritscherflya Supergroup show an age distribution with a dominant age peak at approximately 1,130 Ma, i.e., close to the sedimentation age. Older age peaks include those at 1370 Ma, 1725 Ma, 1880 Ma, 2050 Ma, and 2700 Ma. Palaeo- and Mesoarchaean zircon grains (2800-3445 Ma) were also discovered, corresponding to the age of the Kalahari-Grunehogna Craton basement. Most significantly we found a number of inherited Archaean cores in approximately 1130 Ma zircons. They demonstrate that the volcanic arc was indeed located on Archaean continental crust, rather than in Mesoproterozoic, intra-oceanic island arcs. The age spectrum of the zircons bears strong evidence for (i) derivation of the entire Ritscherflya sediment sequence from an active continental convergent margin; (ii) a cratonic provenance of part of the sediments from population peaks coinciding with major tectono-thermal events in the Kalahari Craton; (iii) at least some of the active volcanism being located on cratonic basement rather than a juvenile island arc. Detrital zircons in the approximately 1130 Ma age group show several distinct populations in their Hf isotopic compositions. The dominant group shows negative epsilon Hf values of -11.5 corresponding to a model age (T (sub DM) ) of approximately 2700 Ma (average crustal (super 176) Lu/ (super 177) Hf = 0.015). A smaller group shows epsilon Hf values of +2 to +6, which may represent mantle-derived subduction-zone volcanism at approximately 1130 Ma. The lowest epsilon Hf group with approximately -21 corresponds to Mesoarchaean model ages, consistent with the oldest U-Pb ages found in the detrital record. The 1725 Ma zircon age group shows relatively juvenile epsilon Hf values ranging from -2 to +6. The oldest Hf model ages were found in Archaean grains and point to Eoarchaean and even Hadean DM model ages of up to 4.2 Ga. Hence, the Ritscherflya sediments were sourced from contemporaneous volcanic activity hosted by several continental sections with contrasting crustal ages. We conclude that the proposed active continental margin hosted volcanism on old cratonic sections, but also on approximately 1725 Ma crust, which may represent Paleoproterozoic island arcs accreted to the Kalahari-Grunehogna Craton during the Mesoproterozoic. [1] Marschall et al. (2010) J. Petrol., 51: 2277-2301. [2] Marschall et al. (2013) Precam. Res., DOI: 10.1016/j.precamres.2013.07.006.