Abstract
The Ancient Gneiss Complex (AGO) in Swaziland1–5 is a complexly deformed suite of gneisses that may be divided into three major units5: (1) the Biomodal Suite (BMS) of inter-layered siliceous low-K leucocratic orthogneisses and amphibolites; (2) homogeneous tonalitic gneiss (the ∼ 3,320 Myr old Tsawela gneiss)6,7; and (3) supracrustal rocks constituting two main lithologic sequences—the Mkhondo Valley1,5 and Dwalile (ref. 8 and M.P.A.J. and A.C.W. unpublished data) metamorphic suites. The Tsawela gneiss is intrusive into both the BMS and the Dwalile metamorphic suite8. Relating the BMS to the greenstone sequence of the Barberton Mountain Land (the Onverwacht Group of the Swaziland Supergroup) has caused much controversy. It has been thought1 that the BMS predates the Onverwacht Group and may represent a basement to the latter. Another view9,10 is that all of the leucocratic gneisses of the AGC are younger than the Onverwacht Group and were derived from an unrelated mantle source. The amphibolite and metasedimentary rocks in the AGC are xenoliths of the Swaziland Supergroup. This controversy has remained unsolved due to the lack both of unambiguous contact relationships between the AGC and the Swaziland Supergroup, and of reliable radiometric data on the emplacement age of either unit. At the only known locality where these two units are juxtaposed, the contact is technically deformed. Early radiometric age data6,11–20 have given results that are either too imprecise or reflect subsequent metamorphism7. Recently, however, a precise age of 3,510±60 Myr (2σ) has been obtained by the Sm–Nd technique for extrusion of the volcanic rocks of the Komati Formation of the lower Onverwacht Group21. The results are reported here of Rb–Sr whole rock and biotite–whole rock isotopic analyses of samples of the leucocratic orthogneisses of the BMS. Results are also presented of initial 87Sr/86Sr ratio studies of samples of amphibolite from the Komati Formation. The relationship of the AGC to the Swaziland Supergroup is re-examined. All Rb–Sr ages mentioned are calculated using 1.42×10−11 yr−1 as the decay constant of 87Rb.
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Barton, J., Hunter, D., Jackson, M. et al. Rb–Sr age and source of the Bimodal Suite of the Ancient Gneiss Complex, Swaziland. Nature 283, 756–758 (1980). https://doi.org/10.1038/283756a0
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DOI: https://doi.org/10.1038/283756a0
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