Juvenile crust formation in the northeastern Kaapvaal Craton at 2.97 Ga—Implications for Archean terrane accretion, and the source of the Pietersburg gold

Abstract

Combined U–Pb and Lu–Hf isotope data of detrital and magmatic zircon grains provide evidence that the Murchison Greenstone Belt, NE Kaapvaal Craton (South Africa), hosts an important Archean suture zone, which primarily formed by a collision 2.97Ga ago between a back arc system to the north and an evolved terrane, the proto-Kaapvaal Craton, to the south. This model is supported by the observations that igneous rocks of the northern terrane, comprising mafic rocks and tonalites of the Rooiwater Complex and felsic volcanics of the Rubbervale Formation, were both emplaced at about 2.97Ga and show highly superchondritic ɛHft of +4.4 to +5.1. In contrast, U–Pb–Hf isotope data of detrital zircon grains from quartzitic schists of the Murchison Unit and the La France Formation, both part of the southern terrane, provide evidence for a southern provenance, which was affected by granitoid intrusions accompanied by crust re-working at 3.53–3.42Ga (ɛHft=+1.8 to −4.8), 3.30–3.20Ga (ɛHft=+1.8 to −6.3), and 3.13–3.05Ga (ɛHft=+1.3 to −5.6); pointing to a connection with the Barberton Greenstone Belt and Swaziland. A spatial separation of the two terranes prior to 2.97Ga, is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane, having ages between 2.99 and 2.97Ga, show subchondritic ɛHft between −6.5 and −1.5, in contrast to the highly superchondritic ɛHft obtained from the contemporaneous magmatic rocks of the northern terrane. The juvenile character of the Rooiwater and Rubbervale magmatic rocks, along with the occurrence of VMS deposits, makes them a likely candidate for the source of the gold–pyrite bearing conglomerates exposed in the adjacent Pietersburg Greenstone Belt.