Nature and formation[ edit ] Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes. Greenstone belts are primarily formed of volcanic rocks, dominated by basalt , with minor sedimentary rocks inter-leaving the volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock either as layered intrusions or as volcanic komatiite has decreased. Archaean plate tectonics did not take place on mature crust and as such the presence of thrust-in allochthonous greenstone belts is expected. By the Proterozoic, magmatism was occurring around cratons and with established sedimentary sources, with little recycling of the crust , allowing preservation of more sediments.
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To the south and west, the Kaapvaal Craton is flanked by Proterozoic orogens , and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the break-up of Gondwana. The craton is a mixture of early Archean 3. Subsequent evolution of the Kaapvaal Craton 3. This was then followed by episodic extension and rifting 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. The belt is an east-northeast trending zone of granulite facies tectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons. Limpopo Central Zone[ edit ] The crustal evolution of the Limpopo Central Zone can be summarised into three main periods: 3.
Early Proterozoic high-grade metamorphic conditions produced partial melting that formed large amounts of granitic melt. This implies that the Limpopo Complex together with the Zimbabwe Craton only became attached to the Kaapvaal Craton at approximately 2. Barberton greenstone belt[ edit ] Main article: Barberton Greenstone Belt The Barberton greenstone belt, also known as the Makhonjwa Mountains , is situated on 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.
Johannesburg Dome[ edit ] 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. 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 — 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 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.
The greenstone belt consists of a sequence of mafic to ultramafic lavas and metasedimentary rocks emplaced and deposited between 3. The granitoid rocks were emplaced over a million year time span and can be divided into two suites. The TTG suite emplaced approximately 3. According to a study by Yearron et al. Nd isotope analyses show that the 3. In contrast, the 3. Extensive granite plutons of a subsequent magmatic episode are associated with the intrusion of vast amounts of granodiorite-monzogranite-syenite GMS suites.
Barberton Geology Greenstone Belts occur as belts of deformed volcanic and sedimentary strata and get their name from the presence of green minerals Actinolite, amphiboles,Chromium muscovite "fuchsite", epidote, green chlorite and serpentine These ancient rocks are among the oldest in the World. The Barberton Greenstone belt is one of the oldest and best exposed Archaean greenstone belts on Earth and is almost 3. It is known world wide in geological circles as a treasure house of other significant geological phenomenae. These ancient rocks are 3. If you were able to travel back in time to visit Earth during the Archaean period you would not recognize it as the same planet we inhabit today. The first forms of life appeared on Earth during the early Archaean period.
Some features of this site are not compatible with your browser. Install Opera Mini to better experience this site. At the time of publication, it represented the best available science. Later examinations of the area revealed that the rocks date back some 3. The bare rocks of mountain peaks appear as a pale gray-green, accentuated by the sharp relief of sunlit slopes and their shadows. Deeper shades of green indicate vegetation, including some vegetation on the lower slopes.