• Archean terranes are affected by significant preservation biases. • Archean detrital zircons are often derived from local sources. • Zircon sources are tracked through Hf, O isotopes and trace elements. • K 2 O-rich granites were already present in the Paleo-Archean crust. • Compositionally diverse micro-continents in the Paleo-Archean. The formation of the Earth’s earliest silicic crust has been largely addressed through the study of the dominant Archean felsic lithology, i.e. tonalite – trondhjemite – granodiorite (TTG) rocks. However, Archean terranes may have been affected by significant preservation biases, which have been overlooked so far. To address this issue, we have conducted a comprehensive study of U-Pb ages, Hf and O isotopic and trace element compositions of detrital zircons deposited over the entire sequence of clastic sediments of the Barberton Greenstone Belt (South Africa). Our study first reveals the importance of local sourcing of sedimentary successions at all stratigraphic levels, which highlights the importance of investigating supracrustal zircons from a large sample set, representative of the lithological diversity and stratigraphy in a given area. Second, our results point to significant biases in the preservation of the Paleoarchean crustal record. First, zircons deposited in both the ca. 3.43 Ga Hooggenoeg Formation and the ca. 3.25 Ga Schapenburg Schist Belt show age populations and/or trace element compositions that do not have plutonic equivalents in the plausible source terranes and suggest erosion of felsic rocks different from the exposed TTGs. Second, compiling our results with datasets from previous work, 40% of all detrital zircons of the ca. 3.22 Ga Moodies Group, while having similar age distribution as that of igneous zircon from regionally exposed TTGs/gneisses, show distinct εHf (t) values (dominantly sub-chondritic) and trace element contents from those of igneous zircons (notably showing dominantly supra-chondritic εHf (t) ). This also applies to ca. 3.53-3.45 Ga, negative-εHf (t) zircons from two samples of felsic schists of the Theespruit Formation. We therefore interpret these non-radiogenic zircons to have been sourced from an unpreserved, isotopically evolved and non-TTG magma source, most likely similar to granitic/rhyolitic clasts of the Moodies Basal Conglomerate and the felsic volcanic sequences of the Theespruit Formation. The high proportion of zircons pointing to non-TTG sources and their presence at all stratigraphic levels of the Barberton Greenstone Belt suggest that these zircons are the relics of hitherto much more voluminous components of the Paleoarchean felsic crust than presently exposed. This result calls for a reassessment of the lithological diversity of the Earth’s first silicic crust.
Combaz et al. (Thu,) studied this question.