A palaeomagnetic study of 3.5 to 3.2 billion year old rocks from the Barberton Greenstone Belt, South Africa

A palaeomagnetic study of some of the best preserved Palaeoarchaean (3.2 -3.6 Ga) rock successions in the world – the Barberton Greenstone Belt, South Africa – has been carried out in order to shed light on Early Earth processes. The aim is to improve the understanding of the long term evolution of the Earth and the surface conditions under which the first forms of life originated through using palaeomagnetic records. The study follows on from work by Layer et al., (1998), Yoshihara & Hamano (2004), Usui et al., (2009), Tarduno et al., (2010) and Biggin et al., (2011) that indicate that rocks from the Barberton Greenstone Belt have the potential to record a near-primary direction of remanence and a reversing geomagnetic field at ca. 3.5 Ga. The rocks of the Barberton Greenstone Belt are excellently preserved and have only been subjected to low grade metamorphism (greenschist facies), making them good candidates for palaeomagnetic studies. Here, new data obtained from three Onverwacht Group Formations and from the Nelshoogte Pluton are presented. The reliability of the new palaeomagnetic data is affected by the complex history and the age of the rocks but no more so than any other published study of rocks of this age. The Noisy Complex and the Nelshoogte Pluton are affected by lightning induced isothermal remanent magnetisations (IRMs) which result in poor quality results. Nevertheless, two new poles, produced by combining the new findings with previously published data, are calculated for the Komati and Hooggenoeg Formations. The new Komati pole shows improved clustering when compared to previous studies. Whilst the data of the Hooggenoeg Formation are encouraging, the results are ambiguous and open to interpretation. When combined with the results of Biggin et al., (2011) they exhibit considerably improved clustering when the directions are corrected for the tectonic deformation resulting in the formation of the Onverwacht Fold, dated at 3.23 Ga. The new results presented here support the findings of previous palaeomagnetic studies of the Barberton Greenstone Belt and support the existence of a stable geomagnetic field at ca. 3.5 Ga. The results presented here support the findings of Biggin et al., (2011) of moderate latitudinal plate motion during this time and do not rule out the hypothesis that the Pilbara Craton (Western Australia) and the Kaapval Craton (South Africa) were conjoined in the Palaeoarchaean.