The timing of successive fluid events in the Southern Marginal Zone of the Limpopo Complex, South Africa: Constraints from 40Ar–39Ar geochronology

Abstract

We have carried out a geochronological study in the high grade metamorphic Southern Marginal Zone of the Limpopo Complex, South Africa, aimed at providing time constraints on episodes of fluid activity. This work chiefly comprises 40Ar/39Ar dating, with accompanying petrography, of amphibole and mica, and further incudes zircon U–Pb geochronology on a post peak metamorphic trondhjemite intrusion. The so-called Zone of Rehydration within 30km N of the Hout River Shear Zone (where the Southern Marginal Zone is thrust over the Kaapvaal Craton) was studied in particular, but sampling also included areas of patchy metasomatism in the Granulite Zone further north, where no post-peak metamorphic rehydration on a regional scale occurred.Amphibole in a mafic granulite and an older generation of zircons in the Petronella trondhjemite yielded dates in the 2700–2730Ma range, corresponding to the published age of peak granulite metamorphism in the Southern Marginal Zone. A cluster of 40Ar/39Ar dates on amphiboles from the Zone of Rehydration indicate an age between 2660 and 2670Ma for this event, in accord with results from previous work on the Hout River Shear Zone. The main population of U–Pb zircon dates from the trondhjemite, dating its intrusion, also falls in this age range, as does the published age of the Matok pluton. These data suggest that rehydration at 2660–2670Ma was a discrete episode. A further 40Ar/39Ar age cluster, defined by dates on both amphiboles and micas, is seen at 2010–2040Ma, which is the age of a major tectonometamorphic event in the Central Zone of the Limpopo Complex, and previously reported from the Southern Marginal Zone as Rb–Sr biotite dates. Events we found in the 2010–2040Ma bracket are strike-slip shearing (dated by fabric forming amphibole), as well as localized potassic metasomatism (dated by both amphiboles and micas). The latter was not confined to the Zone of Rehydration, and appears in one case to be associated with gold mineralization. A number of 40Ar/39Ar dates intermediate between 2600 and 2000Ma were obtained, but these are diverse and can be accounted for by mixing, incomplete resetting and/or excess 40Ar. They do not indicate events in this intermediate age bracket.