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Greenstone belt

Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.
The name comes from the green hue imparted by the colour of the metamorphic minerals within the mafic rocks: the typical green minerals are chlorite, actinolite, and other green amphiboles.
Typically, a greenstone belt within the greater volume of otherwise homogeneous granite-gneiss within a craton contains a significantly larger degree of heterogeneity and complications and forms a tectonic marker far more distinct than the much more voluminous and homogeneous granites. Additionally, a greenstone belt contains far more information on tectonic and metamorphic events, deformations and palaeogeologic conditions than the granite and gneiss events, because the vast majority of greenstones are interpreted as altered basalts and other volcanic or sedimentary rocks. As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean geological history.
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.
This change in nature is interpreted as a response to the maturity of the plate tectonics processes throughout the Earth's geological history. 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. By the Phanerozoic, extensive continental cover and lower heat flow from the mantle has seen greater preservation of sediments and greater influence of continental masses.
The West African Lower Proterozoic greenstone belts are similar to the Archean greenstone belts. These similarities include a decrease in the amount of ultramafic and mafic rocks as you move up the stratigraphic column, in addition to an increase in pyroclastics, felsic and/or andesite rocks. Also, the rock successions tend to have clastics in the upper portion and tholeiitic suites in the lower. Calc-alkaline dikes are common in these suites.
Archaean greenstones are found in the Slave craton, northern Canada, Pilbara craton and Yilgarn Craton, Western Australia, Gawler Craton in South Australia, and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar, as well as West Africa and Brazil, northern Scandinavia and the Kola Peninsula (see Baltic Shield).
One of the best known greenstone belts in the world is the South African Barberton greenstone belt, where gold was first discovered in South Africa. The Barberton Greenstone belt was first uniquely identified by Prof Annhauser at the University of the Witwatersrand, Johannesburg. His work in mapping and detailing the characteristics of the Barberton Greenstone belt has been used as a primer for other greenstone belts around the world.