TheKaapvaal Craton (centred on theLimpopo Province inSouth Africa), along with thePilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5Ga (billion years ago) crust on Earth. Similarities of rock records from both thesecratons, especially of the overlying lateArchean sequences, suggest that they were once part of theVaalbarasupercontinent.[1]
The Kaapvaal Craton covers an area of approximately 1,200,000 km2 (460,000 sq mi) and is joined to theZimbabwe Craton to the north by theLimpopo Belt. To the south and west, the Kaapvaal Craton is flanked byProterozoicorogens, and to the east by theLebombomonocline that containsJurassicigneous rocks associated with the break-up ofGondwana.[2]
The Kaapvaal Craton formed and stabilised between 3.7 and 2.6 Ga[2] by the emplacement of majorgranitoidbatholiths that thickened and stabilised the continentalcrust during the early stages of anarc-relatedmagmatism andsedimentation cycle. The craton is a mixture of early Archean (3.0–3.5 Ga) granitegreenstoneterranes and oldertonaliticgneisses (ca. 3.6–3.7 Ga), intruded by a variety of graniticplutons (3.3–3.0 Ga). Subsequent evolution of the Kaapvaal Craton (3.0–2.7 Ga) is thought to be associated with continent–arc collision that caused an overlayingsuccession of basins filled with thick sequences of bothvolcanic and sedimentary rocks. This was then followed by episodic extension andrifting when the Gaborone–Kanye and Ventersdorp sequences were developed. Early Archean crust is well exposed only on the east side of the craton and comprises a collage of subdomains and crustal blocks characterised by distinctive igneous rocks and deformations.
Late Archeanmetamorphism joined the Southern Marginal Zone of the Kaapvaal Craton to the Northern Marginal Zone of theZimbabwe Craton approximately 2.8–2.5 Ga by the 250 kilometres (160 mi) wideorogenic Limpopo Belt. The belt is an east-northeast trending zone ofgranulitefaciestectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons.
The crustal evolution of the Limpopo Central Zone can be summarised into three main periods: 3.2–2.9 Ga, 2.6 Ga, and 2.0 Ga. The first two periods are characterised by magmatic activity leading to the formation of ArchaeanTonalite-Trondhjemite-Granodiorite (TTG) such as the Sand River Gneisses and the Bulai Granite intrusion. EarlyProterozoic high-grade metamorphic conditions produced partial melting that formed large amounts of granitic melt.[3]
There is no indication that theNeoarchean to earlyPaleoproterozoic succession on the craton were sourced from the 2.65–2.70 Ga orogenic event preserved in the Limpopo Metamorphic Complex. However, younger late-Paleoproterozoicred bedsuccessions containzircons of this time interval as well as many ~2.0 Gadetrital zircons. This implies that the Limpopo Complex together with the Zimbabwe Craton only became attached to the Kaapvaal Craton at approximately 2.0 Ga during formation of the Magondi Mobile Belt which in turn sourced the voluminous latePaleoproterozoic red beds of southern Africa.[4] Evidence of the horizontal layering and riverine erosion can be found throughout theWaterberg Massif within the Limpopo Central Zone.
The Barberton greenstone belt, also known as theMakhonjwa Mountains, is situated on the eastern edge of Kaapvaal Craton. It is well known for its gold mineralisation and for itsKomatiites, 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 theBarberton greenstone belt of the Eswatini–Barberton areas and these contain some of the oldest traces of life on earth.
The Archaean Johannesburg Dome is located in the central part of the Kaapvaal Craton and consists of trondhjemitic and tonalitic granitic rocks intruded into mafic-ultramafic greenstone.[2] Studies using U-Pb single zircon dating for granitoid samples yield an age of 3340 ± 3 Ma and represents the oldest granitoid phase recognised so far. "Following the trondhjemite-tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later, crosscutting, potassic granitoids.
These rocks consisting mainly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121 ± 5 Ma....The data, combined with that from other parts of the Kaapvaal craton, further supports the view that the evolution of the craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the ca. 3.5 Ga Barberton- Eswatini granite-greenstone terrane situated in the southeastern part of the craton."[5]