Zircon ages and metamorphic evolution of the Archean Assegaai-De Kraalen granitoid-greenstone terrain, southeastern Kaapvaal Craton

Abstract

The Assegaai, De Kraalen and Witrivier greenstone belts in the southeast part of the Kaapvaal craton contain strongly deformed volcano-sedimentary successions affected by amphibolite- to granulite-facies metamorphism at ∼3.22 to 3.20 Ga. These greenstone belts consist predominantly of mafic-ultramafic volcanic successions intercalated with chert and BIF. Single zircon SHRIMP ages of 3222±8 and 3193±5 Ma for two granitoids intrusive into the Assegaai greenstone belt provide a minimum age for the succession. Calc-silicates and amphibolites of the De Kraalen and Witrivier greenstone belts record a high-pressure metamorphic event (M₁) at ∼12 to 15 kbar and ∼600 °C and 800 °C, respectively. This high-pressure event was overprinted by a medium-pressure (7-8 kbar) amphibolite-facies event (M_2A) that was recorded in all three greenstone belts, suggesting that the De Kraalen and Witrivier supracrustal rocks were buried to a deeper crustal level than the Assegaai rocks during crustal thickening. Decompression of the rocks along clockwise P-T paths possibly occurred during subsequent crustal extension at ∼3.22 to 3.20 Ga. The low geothermal gradient of this area (15-20 °C/km) contrasts with higher geothermal gradients reported for other Archean terrains worldwide and may support the view that crustal thickening processes related to convergence in a setting similar to subduction zones were operating in Paleoarchean times.