RbSr and SmNd isotope geochemistry and chronology of cherts from the Onverwacht Group (3.5 AE), South Africa

Abstract

We report Rb-Sr and Sm-Nd isotopic analyses of chert samples from the Geluk Subgroup (previously called the mafic-to-felsic unit (MFU)) of the Onverwacht Group in South Africa. These Archean cherts have the same characteristic Rb and Sr concentrations as Phanerozoic cherts with high Rb Sr ratios. The dense, massive, essentially low-carbonate cherts from the Onverwacht Group show a very large range of 87Rb 86Sr ratios (from 0.859 to 7.99). One sample of chert leached in acid yielded 87Rb 86Sr = 7.04. The data define a very good correlation line in the 87Rb- 87Sr evolution diagram which corresponds to an age of 2119 ± 44 m.y. with an initial 87Rb 86Sr ratio of 0.72246 ± 16. Exchange of Sr in the cherts with the associated carbonate appears very limited. The correlation line is interpreted as reflecting the age of rehomogenization of the Sr in the protolith and the recrystallization of these cherts due to circulating hydrothermal fluids during regional metamorphism about 1.4 AE after deposition of the Onverwacht Group. It may also correspond to the closure of the Rb-Sr system in the cherts when tectonic activity stopped in the region (~2200 m.y.). The initial 87Rb 86Sr ratio obtained for the cherts requires the protolith which provided the Sr to have Rb Sr ~ 1. The Sm-Nd systematics do not provide any strict chronologic information but appear to reflect an ancient source age ~3.5 AE. The ϵ Nd at 3.54 AE for the cherts are grouped around zero, i.e. close to the initial value observed for volcanics of the same age, and provide no evidence for the presence of abundant crustal material older than 3.5 AE.