Granitoids and Greenstone Belts of the Pietersburg Block—Witnesses of an Archaean Accretionary Orogen Along the Northern Edge of the Kaapvaal Craton

Abstract

The Pietersburg Block (PB), the northernmost terrane of the Kaapvaal Craton, can be subdivided into four Archaean grey gneiss units: Makhutswi-Murchison South (MMS), Groot Letaba-Duivelskloof (GDL), Goudplaats-Hout River (GHR) and Southern Marginal Zone (SMZ). These domains are limited by greenstone belts (Pietersburg, Murchison, Rhenosterkoppies and Giyani) and major shear zones. These lithologies were intruded by a mafic-layered intrusion (the Rooiwater Complex) and voluminous syn- to post-tectonic granitoids forming either small bodies in grey gneisses or voluminous plutons/batholiths. The grey gneisses are made up of Mesoarchaean (3.12–2.85 Ga) and seldom Palaeoarchaean (3.46–3.17 Ga) TTGs formed by melting of metabasaltic rocks at >45 km depth. The greenstone belts consist of Palaeoarchaean (3.46–3.20 Ga) mafic metabasalts, metakomatiites (and intrusive counterparts) and rare Mesoarchaean (2.95–2.84 Ga) felsic volcanics, intercalated with BIF, chert and siliciclastic rocks deposited at 3.05–2.92 Ga in the Murchison belt; 2.88–2.80 Ga in the Pietersburg and Giyani belts; and 2.76–2.71 Ga in the Rhenosterkoppies belt and SMZ. All lithologies were affected by greenschist—to upper amphibolite-facies metamorphism, mainly during Mesoarchaean N–S-directed compression. The 2.97–2.96 Ga Rooiwater Complex and coeval felsic volcanics of the Murchison belt formed through fractionation of mantle-derived magma. Granitoids of the PB include Bt-(Ms-) granites formed at 2.84–2.75 Ga by melting of TTGs (± sediments), and Hbl-Bt-(Cpx-)sanukitoids and Bt-Ep-granitoids at 2.70–2.67 Ga by melting of enriched lithospheric mantle and interactions with the crust during magma ascent. After formation of a Palaeoarchaean (3.46–3.18 Ga) micro-terrane(s) in an island arc and/or oceanic plateau setting, the geodynamic evolution of the PB features a Meso- to Neoarchaean (3.15–2.65 Ga) accretionary orogen formed during protracted southward subduction. The Mesoarchaean evolution began at 3.15–2.97 Ga with the formation of an arc/back-arc system along the northern margin of the proto-Kaapvaal Craton. Subsequently, it was controlled by the docking of the Palaeoarchaean micro-terrane at 2.97–2.84 Ga, causing intense crustal reworking in an Andean-type continental margin setting (2.84–2.73 Ga) and by Neoarchaean (2.73–2.65 Ga) continent-continent collision with a crustal block located farther North, possibly the precursor (>2.7 Ga) rocks of the Central Zone of the Limpopo Belt.