TheBarberton Greenstone Belt is a geologic formation situated on the eastern edge of theKaapvaal craton inSouth Africa. It is known for its gold mineralisation and for itskomatiites, an unusual type ofultramafic 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 Eswatini–Barberton areas and these contain some of the oldest traces of life on Earth, second only to theIsua Greenstone Belt of WesternGreenland. TheMakhonjwa Mountains make up 40% of the Baberton belt.[1] It is named after the townBarberton, Mpumalanga.
The Barberton Greenstone Belt consists of a sequence ofmafic 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–syenite granite (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 stabilisation of the central parts of the craton. "The GMS suite in the Barberton granite-greenstoneterrane shows very different internal and external characteristics from the earlier TTG suite. Individualplutons 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."[2]
TheBarberton area underwent two tectonic episodes of terraneaccretion at about 3.5 and 3.2 Ga. Early stages of shield development are exposed in theBarberton Mountains where the continent formation first took place by magmatic accretion and tectonicamalgamation of smallprotocontinental blocks. Several smalldiachronous 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.Archaeangreenstone 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 Mpuluzibatholith in the Barberton granite–gneiss terrane is made up of granite sheets. The structurally higher parts are underlain by ananastomosing network of steeply dipping, variably deformeddikes and sheets. According to a study done by Westraatet al. (2005): "Multiple intrusive relationships andgeochronological 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."[2]
The Barberton Greenstone Belt records impact events in 8 layers containing spherules (tiny spheres which form from condensed vapour created by the impact) dubbed S1 through S8, spanning from about 3.5 to 3.2 billion years ago,[3] which likely represent at least 4 but perhaps as many as 8 or more impact events.[4] The impactors that generated these events are thought to have been much larger than those that created the largest known still existing craters/impact structures on Earth (Vredefort andChicxulub), with the impactors having an estimated diameter of ~20–50 kilometres (12–31 mi), with the craters generated by these impacts having an estimated diameter of 400–1,000 kilometres (250–620 mi). The Barberton deposits are thought to have been distant from the impacts.[5]
The most powerful of the impacts such as the S2 impact,[6] which dates to 3.26 billion years ago,[6] probably caused extremely strong earthquakes worldwide,megatsunamis thousands of meters high,[7][6] as well as boiling the surface of the oceans, resulting in tens of meters of global ocean evaporation. Earth would have also been plunged into darkness lasting years to decades.[6]
The Barberton Mountain is a well preserved pre-3.0 Ga granite-greenstone terrane. The 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 TTG suite (emplaced approximately 3.5–3.2 Ga) containstonalites,trondhjemites andgranodiorites; and the GMS suite (emplaced approximately 3.2–3.1 Ga) includesgranites,monzogranites and a smallsyenite–granite complex.
According to a study by Yearronet al. (2003):
Some controversy exists pertaining to the origin and emplacement of Archaeanfelsic suites. According to a dissertation by Louzada (2003): "The upper part of the Hooggenoeg Formation[9] is characterized by ultramafic massive andpillow lavas, atrondhjemitic suite of silicified felsic intrusive and flow banded rocks, and sedimentarychert beds. Veins of felsic, chert and ultramafic material intrude the belt. Thedepositional environment is thought to be a shoaling shallow sea in which the Hooggenoeg Formation has been deposited in a west-block down, listric faulted, synsedimentary setting."[10][better source needed]
The Hooggenoeg Formation felsic rocks can be divided into two groups: an intrusive group of interlocking and shallow intrusive rocks, and aporphyritic group of rocks from the veins. Lavas from the upper part of the felsic unit are too altered to be assigned to one of these groups. The intrusive group is related to thetonalite-trondhjemite-granodiorite TTG-suiteStolzburg Pluton, which intruded along the southern margin of the Barberton Greenstone Belt. Melting of anamphibolitequartzeclogite has been suggested as a probable origin for these high-Al2O3 felsic magmas.Ultramafic rocks of the Hooggenoeg Formation were most likely not parental for the felsic rocks.Subduction processes may have played a role in the generation of the felsic rocks, but atectonic setting for the ultramafic rocks remains uncertain. The felsic units of the Hooggenoeg Formation are very similar to those of the Panorama Formation[11] of the Early Archaean Coppin Gap greenstone belt of Western Australia (SeeYilgarn craton). Similarities in geological setting,petrography, and geochemical (trace elements in particular) characteristics suggest a possible genetic relation between the two formations and support the theory that a combined continentVaalbara existed ~3.45 Ga.
In respect of the research carried out on this 'unique remnant of ancient Earth's crust', the 'Archaean Barberton Greenstone Belt' was included by theInternational Union of Geological Sciences (IUGS) in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an 'IUGS Geological Heritage Site' as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.' The outcrops of the Barberton Greenstone Belt had previously been inscribed onUNESCO's list ofWorld Heritage Sites in 2008 as 'Barbeton Makhonjwa Mountains.[12]
Although not astate ornational park, it is patrolled byrangers armed withmachine guns who accompany expeditions for protection againstrhinos andelephants.[13]
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