Abstract

This study represents the first detailed field, geochemical and geochronological study of Archaean rock units that crop out along the Ncotshane River in the southern part of Swaziland. These rock units were mapped as Mahamba Gneiss on the geological map of Swaziland (Wilson, 1982). However, field examination indicated that the area consists of a heterogeneous assemblage of ductilely deformed supracrustal rocks (amphibolite and quartzite) and granitoid gneisses intruded by diorite, granite and dolerite. Amphibolites were derived from the metamorphism of tholeiitic basalts characterized by geochemical signatures similar to those of volcanic rocks of the nearby Pongola Supergroup. Quartzites are compositionally similar to sandstones from the Mozaan Group. LA-ICP-MS U-Pb analysis of zircon grains from grey quartzite define source ages of circa 3.35 Ga, 3.16 Ga, 3.04 Ga, 2.98 Ga, 2.91 Ga and 2.83 Ga, similar to, albeit ~40 Ma younger than previous detrital zircon studies of the Mozaan Group. The age of 2.83 Ga is regarded to represent the maximum age of deposition of the grey quartzite precursor, indicating an overlap in time between the deposition of the Mozaan Group and the Central Rand Group of the Witwatersrand Supergroup. A SHRIMP U-Pb age of 2810 ± 5 Ma was obtained for granitic augen gneiss, interpreted as the age of protolith emplacement. Zircons from a granite were dated at 2808 ± 10 and are regarded as xenocrysts sourced from the augen gneiss during partial melting. The chemistry and U-Pb zircon ages suggest that augen gneiss and granite are part of the Mswati granite suite, which consists of post-tectonic plutons that cross-cut all Archaean rock types in Swaziland. All rock units have been subjected to three deformational events (D1, D2 and D3) at amphibolite and retrograde greenschist facies grade. The upper limit for the age of deformation and metamorphism is provided by a U-Pb age of titanite at 2.73 Ga, reflecting crustal reworking of the eastern Kaapvaal craton in the Neoarchaean.

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