The genesis of the quartz-sericite schists of the Toggekry Formation, Nondweni Greenstone Belt, South Africa.

Abstract

The aim of this study was to investigate the origin of sulphide-bearing quartz sericite-schists of the Toggekry Formation in the Archaean Nondweni greenstone belt (NGB) in the SE Kaapvaal craton, and to compare them with similar units in the Barberton greenstone belt (BGB). Geochemical studies reveal that the quartz-sericite schists had a rhyolite protolith. These rocks were subjected to two major deformation phases. D1 involved thrusting and stacking of the greenstone stratigraphy while D2 formed the large syncline structure of the greenstone belt. Within the study area, the F2 buckling mechanism seems to have been tangential longitudinal strain. Peak metamorphism occurred after D2 at 3230 Ma when the Mvunyana granodiorite intruded. The extensive alteration of the schists is interpreted as being partially due to the deformation and metamorphism but mainly because of the position of the schists in the contact metamorphic aureole of the Mvunyana granodiorite. The tholeiitic and calc-alkaline signatures of both the mafic and felsic rocks of the Toggekry Formation indicate that they formed in a back-arc setting. The enrichment in LREE relative to HREE that the rocks display is characteristic of crustal contamination and/or subduction zone magmas, enriched mantle source or small degrees of melting. The positive Pb anomalies and negative Nb-Ta are characteristic of subduction zone processes and indicate crustal involvement in the magma process. Epsilon Hf data suggests derivation either from a depleted mantle source with contamination by older continental crust or from a depleted mantle at an earlier age followed by re-melting. The Toggekry Formation has an age of 3.54 Ga and is indistinguishable in age to the Theespruit Complex of BGB. Both sequences are lithologically similar and interpreted as forming in backarc settings. However significant geochemical differences indicate that they formed in coeval but separate basins. Four models are proposed to account for this. The peak metamorphic event at ca. 3.2 Ga in both areas is considered to reflect an accretionary event during the formation of the Kaapvaal craton.