More Evidence of EoArchean Crust in South Africa

Zircon ages defining deposition of the Palaeoproterozoic Soutpansberg Group and further evidence for Eoarchaean crust in South Africa

Authors:

Geng et al

Abstract:

The precise age of the volcano-sedimentary Soutpansberg Group, which was deposited upon the Palala shear belt separating the Kaapvaal Craton from the Central Zone of the Limpopo Belt, has long been debated. The Soutpansberg Group is subdivided into a lower and upper succession, which are separated from each other by a prominent regional unconformity. Zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the Soutpansberg Group rocks.

The zircon grains of the investigated samples from both successions yield a wide range of ages, spanning from 1831 to 3937 Ma. Most of the zircon grains have rounded shapes, however it is not clear whether they are mainly xenocrystic or detrital, or have been rounded by resorption in a silicic magma chamber. The youngest zircon grain ages obtained come from the lower succession and are 1832 ± 9 and 1831 ± 15 Ma. We interpret these youngest zircon grains as magmatic grains that have been rounded by resorption. This view is corroborated by the fact that no magmatic rocks of this particular age have been observed in the Kaapvaal Craton or the Central Zone of the Limpopo Belt, and that no apparent sedimentary admixtures are present in the well exposed pyroclastic rocks. We therefore conclude that deposition of the Soutpansberg volcano-sedimentary succession commenced around 1830 Ma.

The Soutpansberg rocks were deposited apparently over a lengthy period of time (ca. 230 Ma), as provided by the published age of 1604 Ma for pyroclastic rocks of the upper succession in Botswana. Zircon grain age spectra of our Soutpansberg samples show prominent peaks at 2.0, 2.6 and 3.2 Ga, indicating the Central Zone of the Limpopo Belt as the source area, but excludes the adjacent northern part of the Kaapvaal Craton. The oldest zircon grain identified in the Soutpansberg samples has an age of 3937 ± 4 Ma, one of the oldest zircon grain ages yet reported from the African continent.