Geochemistry and geochronology of the Mkhondo suite, Swaziland: evidence for passive-margin deposition and granulite facies metamorphism in the Late Archean of Southern Africa

Abstract

The Archean Mkhondo suite in southern Swaziland is a multiply deformed succession of metasediments intruded with amphibolite dykes and sills and granitoid gneisses. Mineral and textural relationships indicate an early period of granulite facies metamorphism, followed later by amphibolite facies metamorphism. Geothermobarometry indicates maximum temperatures of 700–900°C and burial depths of 25–3 km. Paragneisses and biotite quartzites have LREE enriched patterns with small negative Eu anomalies, whereas white quartzites show variable REE patterns and low REE concentrations. BIF has slight LREE enrichment and Eu anomalies. Amphibolites have moderate LREE enrichment and depletions in TaNb and P. Unlike many Archean granitoids, the Mkhondo granitoid gneisses are high in K and other LILE, have large negative Eu anomalies and are not depleted in HREE. SHRIMP U Pb isotopic analyses of detrital zircons from a biotite quartzite define a source age of ∼3600–3460 Ma. A deformed granitoid in tectonic contact with the Mkhondo suite yields a zircon evaporation 207 Pb 206 Pb mean age of 3192±5 Ma, which is interpreted as the age of emplacement. A zircon evaporation age of a granitic melt patch in paragneiss, as well as whole-rock and garnet SmNd isotopic ages, suggest that the peak of high-grade metamorphism in the Mkhondo suite occurred at about 2750 Ma. This is the first evidence for Late Archean high-grade metamorphism in the southeastern Kaapvaal craton. The age data of this study restrict deposition of the Mkhondo suite to between ∼3.2 and ∼2.75 Ga. Mkhondo paragneisses are interpreted as shales with biotite quartzites as iron- and quartz-rich detrital sediments. Geochemical mixing calculations indicate that the sediment sources were composed of basalt (±komatiite), TTG and Eu-depleted granitoids. The Mkhondo assemblage may have been deposited along a passive continental margin or in a continental interior basin. The presence of minor BIF with positive Eu anomalies suggests minor hydrothermal input into the sedimentary basin. Intense chemical weathering was probably most important in production of the relatively pure quartz sands.