Probable Neoproterozoic retro-arc basins on the southern Kalahari craton: The search for an active margin bordering southern Gondwana
|Location||International Geological Congress,oslo 2008|
|Author||Zimmermann, Udo۱; Tait, Jennifer۲; Miyazaki, Takashi۳; Naidoo, Thanusha۴|
|Holding Date||05 October 2008|
The Neoproterozoic remains one of the most interesting and controversial periods of Earth history. It marks the disintegration of Rodinia, global glaciations are thought to have occurred, and major reorganisation of cratons resulted in formation of Gondwana. Palaeogeography and interaction of the cratons is critical to our understanding of the Neoproterozoic. The Kalahari craton plays a key role and its margins remain controversial, in particular the development of the Saldania Belt of southern South Africa. An active margin during the Upper Neoproterozoic seems to be likely (Naidoo et al., 2006). Recently, Rapela et al. (2007) interpreted that the southern South African basins were related to Neoproterozoic rocks of western South America (Puncoviscana Formation) and both areas deposited in a fore-arc or retro-arc basin. Geochemical data for the Kaimaans and Gamtoos Inlier revealed three formations in the outcrop areas characterized by the input of less fractionated, most probably andesitic material. Ratios of Th/Sc, La/Sc, Zr/Sc and Ti/Zr are well below typical upper continental crustal values as are absolute Nb and Ta concentrations in the studied metagreywackes and pelites. This is in contrast to the finding of more than 100 geochemical analyses of the Puncoviscana Formation (Zimmermann, 2005). Pb-Pb isotope data clearly similarities between all outcrops and tend to have similar characteristics as rocks of the Antarctic Peninsula and point to a relation to these successions.
Therefore, until our age dating of detrital zircons is not interpreted and the Pb-Nd analysis are not finalized, we assume a basin evolution in the Kaimaans and Gamtoos area comparable to those of the Kango cave Inlier: small extensional basins were formed during the activation of an active margin around 600 Ma, which evolved to larger entities, where significant recycling was possible. During the uppermost Neoproterozoic the influence of active continental margin material is the highest and disappears at latest during the Lower Cambrian. The definite active volcanic arc could not be identified in the supracrustal successions, as volcanic rocks do not crop out, despite conspicuous finely laminated pelitic layers. Hence, we interpret the specific formations of the Gamtoos basin and the Kaimaans basin as being deposited in a retro-arc basin, although the andesitic component is more significant than in the northern Neoproterozoic basin, the Cango Cave basin. The actual continental arc was probably located further south on a continental fragment, and might have drifted away during the Cambrian.