Abstract

An exceptional river platform exposure on the farm Nooitgedacht, located on the northern half of the Johannesburg Dome, has been mapped and studied with the view to obtaining information that might assist in understanding the Archaean crustal evolution of the central Kaapvaal Craton. The investigation complements previous studies that provided a regional assessment of the geology of the Johannesburg Dome and which resulted in the recognition of several stages of granite-greenstone development. Successive events led to the emplacement of: (1) mafic and ultramafic volcanic rocks comprising komatiites, basaltic komatiites, high-magnesian basalts and tholeiites, as well as their plutonic equivalents consisting of serpentinized dunites, harzburgites, pyroxenites and gabbros; and (2) a suite of tonalite-trondhjemite-granodiorite (TTG) granitoid rocks that were intruded into the greenstones, thereby resulting in their fragmentation, metamorphism, assimilation and migmatization. The Nooitgedacht platform provides a glimpse of these latter processes that are believed to have ensued following the TTG emplacement. Characterization of the exposure is achieved with the aid of a detailed outcrop map and carefully selected sampling of the various lithological components on display. Petrological analysis and major-and trace-element geochemical data, including rare-earth-element (REE) abundances obtained from a range of rock types that include early-stage amphibolitic greenstones, trondhjemitic-tonalitic-dioritic gneisses, and amphibolitic dykes, together with later-stage homogeneous trondhjemitic-granodioritic-pegmatitic granitoids, are employed to support the view that processes of metamorphism, assimilation and metasomatism have been variously responsible for generating the mixed assemblages seen in outcrop. The chondrite-normalized REE data suggests that the Archaean Nooitgedacht amphibolites, with their moderately flat to slightly depleted HREE patterns, are similar to those reported elsewhere for oceanic island volcanic rocks. REE patterns for the range of dioritic-tonalitic granitoids are generally similar to those of the amphibolites and show moderate LREE enrichment and flattish to slight HREE depletion suggestive of some genetic link with the mafic rocks. Two mafic dyke varieties that post-date the trondhjemitic gneisses were identified on the basis of distinctive REE signatures and their outcropping structural disposition. The Nooitgedacht trondhjemitic granitoids consist of a low- and a high-Al 2 O 3 variety and display steep REE patterns with strong LREE. enrichment and moderate to strong HREE depletion, similar to granitoids described from Archaean terranes elsewhere in the world. Plots of the Nooitgedacht geochemical data in various discrimination diagrams suggest that the Archaean events displayed on the Johannesburg Dome probably occurred in a tectono-magmatic setting analogous to that found in modern-day, volcanic-arc environments.

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