Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35–3.15 Ga, minor 3.15–2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U–Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U–Pb and Lu–Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35–3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65–2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U–Pb ages of 3.35–3.15 Ga, and 176 Hf/ 177 Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general &z.epsiv;Hf t increase with decreasing age from &z.epsiv;Hf 3.9Ga = −6.3 to &z.epsiv;Hf 3.3–3.1Ga = −0.2, indicating that Hadean crust older than 4.0 Ga ( T DM = 4.45–4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo’s hinterland is reflected by the ∼3.6 Ga zircon population, which shows &z.epsiv;Hf 3.6Ga between −4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite–trondhjemite–granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps due to the onset of modern-style plate tectonics.
Geochimica et Cosmochimica Acta – Elsevier
Published: Nov 1, 2008
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