Hafnium isotope record of the Ancient Gneiss Complex, Swaziland, southern Africa: evidence for Archaean crust–mantle formation and crust reworking between 3.66 and 2.73 Ga Armin Zeh Axel Gerdes Leo Millonig 10.6084/m9.figshare.3452852 https://geolsoc.figshare.com/articles/dataset/Hafnium_isotope_record_of_the_Ancient_Gneiss_Complex_Swaziland_southern_Africa_evidence_for_Archaean_crust_mantle_formation___and_crust_reworking_between_3_66_and_2_73_Ga/3452852 <p>Combined U–Pb and Lu–Hf isotope analyses of zircons from 16 tonalite–trondjemite–granodiorite (TTG) gneiss and granite samples from Swaziland reveal that the oldest rocks of the Ancient Gneiss Complex in southern Africa formed by reworking of Early Archaean or perhaps Late Hadean crust at 3.66 Ga, and that new crust was extracted from a depleted mantle source during Palaeoarchaean events between 3.54 and 3.32 Ga. This interpretation is supported by εHf<sub>t</sub> of −1.6 ± 2.0 obtained from 3.66 Ga TTG gneisses, corresponding to hafnium model ages between 3.77 ± 0.18 Ga, for a presumed Hadean–Early Archaean chondritic mantle, and 4.08 ± 0.18 Ga, for a presumed Hadean depleted mantle reservoir, with the first model age being the most likely in the light of recent data from worldwide sources. Furthermore, it is reflected by superchondritic εHf<sub>t</sub> up to +2.2 ± 2.0 for TTGs formed at 3.54, 3.45 and 3.32 Ga. The new datasets additionally show that the Palaeoarchaean crust formed between 3.54 and 3.32 Ga was intensely reworked afterwards, without significant addition of depleted mantle derived material, during orogenic and intracratonic melting processes at 3.23, 3.1 and 2.7 Ga. This is well reflected by an array of decreasing εHf<sub>t</sub> from +2.2 to −7.2 between 3.3 and 2.7 Ga, which can be forced by <sup>176</sup>Lu/<sup>177</sup>Hf of 0.0113, which is similar to that of present-day average continental crust, and might result from lower crust zircon fractionation during Archaean crust reworking. </p> 2016-06-21 10:21:56 Hafnium isotope record ε Hf t 3.66 Ga TTG gneisses hafnium model ages crust zircon fractionation Ancient Gneiss Complex 2.7 Ga mantle 3.32 Ga Archaean crust reworking Geology