Archean crustal growth and reworking revealed by combined U-Pb-Hf-O isotope and trace element data of detrital zircons from ancient and modern river sediments of the eastern Kaapvaal Craton

Abstract

It is intriguing to speculate on how continental crust grew and differentiated during the Archean and which tectonic regimes and magmatic mechanisms were responsible for such a process. The continental nucleus of the Kaapvaal Craton, which includes the Barberton granitoid-greenstone terrane (BGGT) in South Africa and the Ancient Gneiss Complex (AGC) in Swaziland, exhibits the best preserved geological record from 3.6 to 3.0 Ga and thus provides the optimal location to address this issue. In this study, combined U-Pb, O and Lu-Hf isotope and trace element analyses were carried out on detrital zircon grains from two Moodies Group sandstones and three modern river sand samples in the Barberton and Swaziland regions, southern Africa. The results, in conjunction with previously published data, show that the detrital zircons in the Moodies sandstones on both sides of the Inyoka Fault, had two similar U-Pb age clusters of 3.25–3.30 Ga and 3.40–3.47 Ga and the oldest age up to 3.57 Ga. This suggests that they were derived from older felsic volcanic rocks of the greenstone belt and the granitoid gneisses in the BGGT and AGC. Using the approach created for Earth’s oldest zircons (i.e., Jack Hills detrital zircons), we inferred that the source magmas of the studied detrital zircons had average arc-like andesitic compositions (SiO2 = 58.0 ± 5.6 wt.%; Th/Nb = 5.0 ± 3.2), similar to those of the Jack Hills detrital zircons. However, these inferred compositions conflict with the source rocks of the detrital zircons, namely, the felsic volcanic rocks and the granitoid gneisses in the BGGT and AGC, which have high SiO2 contents (average = 70 ± 4.3 wt.%) and moderate Th/Nb ratios (average = 0.76 ± 0.44). These findings suggest that the inferred average andesitic compositions for the Earth’s oldest zircons should be reevaluated. The U-Pb age spectra of detrital zircons in modern river sands suggest that the exposed rocks in southwestern Swaziland are dominantly ca. 3.45 Ga tonalitic, trondhjemitic and granodioritic (TTG) gneisses without Eoarchean crust. The concordant zircon U-Pb age data reveal four major magmatic episodes for the eastern Kaapvaal Craton at 3.52, 3.46, 3.26 and 3.10 Ga. The first magmatic episode is suggested to dictate crustal growth, whereas the other three magmatic episodes are predominated by crustal reworking. This process is manifested by major positive changes in εHf(t) and εNd(t) values starting at ∼3.5 Ga, gradual decreases in εHf(t) and εNd(t) values afterwards, and increases in δ18O values at ∼3.25 Ga. Combined with other geological observations, we suggest that modern-style plate tectonic processes, such as subduction, may not have been a requirement for crustal growth and reworking in the eastern Kaapvaal Craton during the Paleoarchean (3.6–3.2 Ga). Instead, they could be ascribed to episodic partial convective overturns at different crustal depths.