The Nhlangano gneiss dome in south-west Swaziland – A record of crustal destabilization of the eastern Kaapvaal craton in the Neoarchaean

Abstract

The Nhlangano granite-gneiss in south-west Swaziland, dated at 2.98Ga, 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. 2.96Ga, 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 3.07Ga and matching geochemical provenance characteristics. Ar-Ar analysis of hornblende separates dates amphibolite-facies metamorphism and associated dome emplacement at c. 2.72Ga. 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.