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The Nhlangano granite-gneiss in south-west Swaziland, dated at 2.98 Ga, forms an elliptical dome of remobilised, migmatitic crust. Strongly deformed supracrustal rocks wrap around or form synformal keels infolded with subdomes of Nhlangano Gneiss and represent Mesoarchaean Pongola Supergroup strata at amphibolite- to granulite-facies metamorphic grade. Amphibolites represent the metamorphosed equivalents of Nsuze Group basalts and andesites on the basis of similar trace element and Nd isotope data. Garnetiferous Mahamba Gneiss, dated at c. 2.96 Ga, represents the metamorphosed equivalents of felsic to intermediate volcanic rocks and minor pelitic interflow sedimentary units of the Nsuze Group. Quartzites and associated metamorphosed iron formations are part of the lower Mozaan Group (Sinceni Formation) as indicated by the presence of detrital zircons as young as 3.07 Ga and matching geochemical provenance characteristics. Ar-Ar analysis of hornblende separates dates amphibolite-facies metamorphism and associated dome emplacement at c. 2.72 Ga. Exhumation took place during NW-SE crustal extension and was facilitated by partial melting of the mid-crustal rocks. Exhumation took place below a southeastward-dipping detachment fault, giving rise to NW-SE oriented elliptical gneiss domes due to vertical and lateral flow of partially molten rocks. The Nhlangano gneiss dome has features in common with migmatite-cored metamorphic core complexes. Its emplacement points to tectonic instability of the eastern part of the Kaapvaal Craton in the Neoarchaean. [ABSTRACT FROM AUTHOR]

Abstract: High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative -values, whereas whole-rock - and -data are slightly negative to slightly positive. Similar results were obtained for ca. 3.45 Ga felsic rocks in the Hoeggenoog and Noisy Formations higher up in the greenstone stratigraphy. These data rule out derivation of the felsic units from melting of basaltic precursors and favor a crustal source, most likely of TTG composition. The isotopic data are not compatible with an entirely oceanic origin of the Barberton greenstone sequences and favor a pre-greenstone basement beneath the volcanic rocks. Crustal melting of Eo- to Paleoarchaean lower crust probably generated the felsic volcanic rocks and is likely to have been responsible for gradual stabilization of the Kaapvaal craton. [Copyright &y& Elsevier]