The structural metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

This paper presents the results of a detailed and fully integrated pressure-temperature-time-deformation (P-T-t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D-1-D-4). Relict S-1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S-2, defined by biotite in biotite schist, gneissic banding in biotite gneiss, long ayes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F-2. F-3, occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W- to ENE-trending, shallowly plunging folds. D-4, which was constrictional-prolate in nature, refolded Si, S-2, F-2 and F-3. F-4 folds and an L-4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S-2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L-4. Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SW-wards. The moderately NE- to NNE-plunging F-4 sheath folds and L. lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D-2 and D-4 structures respectively to c. 2037-2040 +/- 22 Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of similar to 6.5 kbar and 680 degrees C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from similar to 5 kbar at 600 degrees C to similar to 6-6.5 kbar at 650-680 degrees C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P-T-t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural-metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton. (C) 2011 Elsevier B.V. All rights reserved.