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