Greenstone belts at the northernmost edge of the Kaapvaal Craton: Timing of tectonic events and a possible crustal fluid source

Abstract

The Hout River shear zone, a complex thrust system, marks the boundary between the low- to medium grade metamorphic granitoid-greenstone Northern Kaapvaal Craton province and the high grade Southern Marginal Zone of the Limpopo Complex to the north of it. The Giyani greenstone belt, located on the Northern Kaapvaal Craton immediately adjacent to the Hout River shear zone, is in a key position for studying the tectonics of this shear zone. We have mapped two central transects through the greenstone belt, dated metamorphic minerals by 207Pb/206Pb stepwise leaching and 40Ar/36Ar, and compared our results with data from the Pietersburg/Polokwane and Rhenosterkoppies greenstone belts, which are close to the Hout River shear zone further west.The SE domain of the NE–SW striking Giyani greenstone belt appears to constitute a NW-vergent nappe. Movement along the Hout River shear zone is consistently SW-vergent, varying in character between frontal and lateral ramps, and nappe. The NW domain of the Giyani greenstone belt is dominated by ultramafic schists with mafic and banded iron formation (BIF) intercalations; it is considered a separate tectonic unit from the SE domain in which mafic (intrusive and pillow-volcanic) rocks are prevalent, with intercalated clastic sediments. Correlations between this SE domain and the lower mafic unit of the Pietersburg greenstone belt and with the mafic-BIF association of the Rhenosterkoppies greenstone belt are possible.Ages for peak metamorphism in the immediate footwall of the Hout River shear zone from this and previous work yield a weighted mean age of 2717±28Ma (95% confidence), documenting a “hot-iron” metamorphism. A garnet age of 2833±38Ma in the SE domain of the Giyani greenstone belt overlaps with ages from 2854 to 2870Ma for youngest zircons in syntectonic intrusions within the Giyani and the Pietersburg greenstone belts, as well as youngest detrital zircons in the (probably syntectonic) clastic sedimentary Uitkyk Formation of the latter. These ages constrain a tectonometamorphic event in both belts, termed here the PGB-Lwaji orogeny. Amphibole 40Ar/39Ar dates from the Hout River shear zone (from this and previous work) and shear zones within the Giyani greenstone belt yield disturbed age spectra with step ages mainly in the 2550–2670Ma range. The amphiboles are chemically heterogeneous and the ages can be shown to reflect mixtures of older and younger amphibole generations, due to reactivation of the shear zones and fluid activity after the main event in the Hout River shear zone.Thermal modelling shows that, in spite of the very low content of radioactive heat producing elements in the Southern Marginal Zone, its lower crust, after moderate crustal thickening, could be heated to granulite facies conditions in 30–60Ma, a much shorter time than the 120Ma age difference between the PGB-Lwaji orogeny and the Hout River shear event. A causal link between the two tectonic events is thus precluded. On the other hand, previously low-grade supracrustal rocks from the Kaapvaal Craton buried at lower crustal levels during Hout River shear event could provide a source for fluids up to about 80Ma after the event.