Sedimentary provenance of the Neoarchean Ventersdorp Supergroup, southern Africa: Shedding light on the evolution of the Kaapvaal craton during the Neoarchean

Abstract

The Ventersdorp Supergroup, an undeformed, subgreenschist-to-greenschist facies volcanosedimentary succession deposited on the Kaapvaal Craton between 2729 and 2665 Ma, represents an exceptional opportunity to study early cratonic evolution on Earth. Provenance studies of siliciclastic and volcaniclastic successions of the supergroup show that the Ventersdorp basin was sourced from two different regions, one that had a Mesoarchean history and another that maintained its juvenile character at the time of deposition. The volcaniclastic wackes of the Ventersdorp Supergroup's Kameeldoorns Formation are mainly composed of Mesoarchean detrital material with Nd isotope model ages (TDM) between 3.3 and 3.1 Ga and detrital zircon ages with a dominant peak between 3.0 and 3.3 Ga. Geochemical proxies (rare earth element [REE] pattern, LaN/YbN, Th/Sc, Th/U), however, yield typical post-Archean signatures. Thus, material that sourced the Kameeldoorns Formation was stored in the crust of the Kaapvaal Craton for nearly 600 m.yr. and reflects the result of crustal differentiation processes. The younger Bothaville Formation, in contrast, displays geochemical proxies similar to Archean trondhjemite-tonalite-granodiorite suites (TTGs). TDM ages are younger than those for the Kameeldoorns Formation, straddling 3.0-2.9 Ga and corresponding well with the dominant population of detrital zircons (2.9-2.8 Ga). The dominant detrital component in the Bothaville Formation thus suggests an event of crustal addition in the so-called Archean style, as Archean TTGs contain a less differentiated signature than the crustal sources of the Kameeldoorns Formation. Therefore, the crust of the Kaapvaal Craton was evidently thick enough during the Mesoarchean that long-term crustal recycling could take place. During the Neoarchean, the emplacement of Archean-style TTGs contributed to crustal thickening along the margins of the craton, which implies that Archean and post-Archean heat flow regimes existed during the Neoarchean and that their tectonomagmatic processes contributed to crustal thickening. Thus, modern plate tectonic processes could have operated earlier in the Ventersdorp Supergroup than elsewhere. It can therefore be speculated that Archean and post-Archean processes were not exclusive of one another but may have coexisted and that modern plate tectonic processes may have operated during the Archean.