Abstract

Early Palaeoarchaean granite clasts in the basal conglomerate of the Moodies Group, Barberton Greenstone Belt, South Africa, belong to the oldest true granites on Earth and document that intracrustal melting occurred early during generation and stabilization of continental crust. Zircons from granite-gneiss clast MD6 of the Moodies conglomerate analyzed on SHRIMP II yielded a magmatic crystallization age of 3564.5 ± 2.2 Ma and contained two older xenocrysts with 207Pb/206Pb ages of 3602 ± 3 and 3634 ± 3 Ma, respectively. These xenocryst ages are almost identical to zircon ages for the oldest tonalite-trondhjemite components of the Ancient Gneiss Complex (AGC) in Swaziland. Hf-in-zircon isotopic analyses on the dated zircons yielded predominantly negative ɛHf(t) values (-2.3 to +0.7), supporting an origin through melting of older felsic crust for the MD6 granite, as predicted by earlier melting experiments. We suggest that the oldest granite clasts in the Moodies conglomerate were derived from crustal-melt granites in the AGC or in the southern Barberton greenstone terrane that rose to high crustal levels and were eroded during an unroofing event that affected large areas of the southeastern Kaapvaal craton during a major, ca. 3.2 Ga tectono-metamorphic event. This is why the source rocks of these granite clasts can no longer be found.

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