A comparative study of the Mesoproterozoic late orogenic porphyritic granitoids of southwest Namaqualand and Natal, South Africa

The voluminous Mesoproterozoic late orogenic porphyritic granitoids of the Spektakel and Oribi Gorge Suites from the Bitterfontein area (southwest Namaqualand) and Natal (Mzumbe and Margate Terranes), respectively, are comparable in terms of field relationships, structure, petrography, geochemistry and age, despite being situated over 1200 km apart within the Namaqua-Natal Metamorphic Province. Within the Spektakel Suite, three distinct porphyritic granitoid types have been identified, namely the Jakkalshoek, Strandfontein and Bloukop granites. Charnockite development appears to be more widespread in the Oribi Gorge Suite than in the Spektakel Suite of the study area, though the latter becomes more charnockitic in the granulite grade terrain to the north. The chemical composition of the three Spektakel Suite granite types is similar to that of the Oribi Gorge Suite. Both suites are typified by tholeiitic trends and broadly A-type/within-plate granite characteristics on discriminant diagrams. U-Pb zircon data for the Bloukop granite suggest an emplacement age of 1065 +/- 2 Ma, showing it to be coeval with the Oribi Gorge Suite. The observed similarities imply that the Oribi Gorge and Spektakel Suites was emplaced under comparable tectonothermal conditions at similar to 1050 Ma, after the main collisional orogenic event had taken place in both areas at similar to 1100 Ma. In Natal, it has been proposed that the Oribi Gorge Suite was emplaced during a major late (D-3) oblique transcurrent shearing event, a situation mirrored in southwest Namaqualand, where the Spektakel Suite were also emplaced during the D-3 event, which is characterised in the study area by upright folds with highly sheared limbs, formed during regional strike-slip tectonism. The Namaqualand rocks are more pervasively altered than those of the Oribi Gorge Suite, suggesting that they suffered a late retrogression event not seen in Natal, possibly as a result of the partial inclusion of the area within the Neoproterozoic (Pan African) West Coast Belt. Copyright (C) 1997 Elsevier Science Ltd.