Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar/39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca/K and Cl/K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T0 (original blocking age) = 2673 +/- 3 Ma (1-sigma) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 +/- 3 Ma (1-sigma). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T0 ages of approximately 2350-2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025-2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of approximately 60,000 km2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar/39Ar ratios < 295.5) signatures. Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar/39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca/K and Cl/K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T0 (original blocking age) = 2673 +/- 3 Ma (1-sigma) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 +/- 3 Ma (1-sigma). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T0 ages of approximately 2350-2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025-2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of approximately 60,000 km2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar/39Ar ratios < 295.5) signatures. Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar/39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca/K and Cl/K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T0 (original blocking age) = 2673 +/- 3 Ma (1-sigma) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 +/- 3 Ma (1-sigma). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T0 ages of approximately 2350-2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025-2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of approximately 60,000 km2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar/39Ar ratios < 295.5) signatures.
