Kaapvaal Craton - Barberton Greenstone Belt

Barberton Greenstone Belt

The Barberton greenstone belt is situated in the eastern edge of Kaapvaal Craton. It is well known for its gold mineralisation and for its komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth (greater than 3.6 Ga) are located in the Barberton greenstone belt of the Swaziland–Barberton areas and these contain some of the oldest traces of life on earth. Only the rocks found in the Isua greenstone belt of Western Greenland are older.

The Barberton greenstone belt consists of a sequence of mafic to ultramafic lavas and metasedimentary rocks emplaced and deposited between 3.5 and 3.2 Ga. The granitoid rocks were emplaced over a 500-million-year time span and can be divided into two suites: The tonalite-trondhjemite-granodiorite (TTG) suite (emplaced approximately 3.5–3.2 Ga), and the granite-monzogranite-syenitegranite (GMS) suite (emplaced approximately 3.2–3.1 Ga). The GMS suite are found over large parts of the Kaapvaal Craton and their emplacement coincides with the first stabilization of the central parts of the craton. "The GMS suite in the Barberton granite-greenstone terrane shows very different internal and external characteristics from the earlier TTG suite. Individual plutons may cover several thousand square kilometres and these composite granitoid bodies have traditionally been referred to as batholiths, alluding to their compositionally and texturally heterogeneous nature and enormous areal extent. For the most part, the plutons appear undeformed." (Westraat et al., 2005).

The Barberton area underwent two tectonic episodes of terrane accretion at about 3.5 and 3.2 Ga. Early stages of shield development are exposed in the Barberton Mountains where the continent formation first took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks. Several small diachronous blocks (3.6-3.2 Ga) have been found in the area. Apparently each block represents a cycle of arc-related magmatism and sedimentation. The Hooggenoeg Formation of the Barberton greenstone belt is dated at 3.45 Ga. and evolved through magmatism. This crustal development phase was followed by a period of Mesoarchaean cratonic magmatism (3.1–3.0 Ga) and is marked by the formation of a large crescent-shaped, juvenile arc that was accreted onto the northern and western margins of the evolving Kaapvaal shield. Archaean greenstone belts are hypothesized to have been formed from passive margin oceanic crust that became part of an extensive subduction-undercut margin. The TTG intrusions are thought to have been formed by post-subduction magmatism when subduction was halted, perhaps by arrival of a micro-craton.

The 3.1 Ga Mpuluzi batholith in the Barberton granite–gneiss terrane is made up of granite sheets. The structurally higher parts are underlain by an anastomosing network of steeply dipping, variably deformed dikes and sheets. According to a study done by Westraat et al. (2005): "Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 million years. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets."