Tectonic history of the Barberton greenstone belt, South Africa: 490 million years of Archean crustal evolution

Rocks of the 3570–3080 Ma Barberton greenstone belt record a geological history of at least 490 million years (m.y.), and were affected by regional thermal perturbations for over 1500 m.y. Small tectonic wedges of 3570–3511 Ma schistose tonalite are the oldest rocks yet recognized, but little is known about their tectonothermal history. The earliest, well-recognized tectonothermal events (3490–3450 Ma) represent mid-ocean ridge-like processes. These were followed by two periods of arc-related and trench-related processes, separated by ∼160 m.y.; the first (3445–3416 Ma) recording an intraoceanic suprasubduction-like environment and the second, (3260–3225 Ma) intra-arc and interarc-like processes, culminating in arc amalgamation. Accretion-like convergent processes dominated between ∼3230 Ma and 3080 Ma, overlapping with the assembly of the Kaapvaal craton. At 3080 Ma the Barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system. Soon afterward, transtension became distributed across the entire Kaapvaal craton, heralding the onset of formation of the Witwatersrand basin by ∼3070 Ma.