|This article needs additional citations for verification. (April 2013)|
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.
A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a 'stratigraphic grouping' in its own right, at least on continental scales.
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 basically metamorphosed volcanic belts.
Nature and formation
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.
There is also a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt-peridotite sheets of a greenstone belt and the granites they abut, and the Proterozoic where greenstone belts sit upon granite-gneiss basements and/or other greenstone belts, and the Phanerozoic where clear examples of island arc volcanism, arc sedimentation and ophiolite sequences become more dominant.
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.
Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with 'metabasalt' et cetera; greenschist, whiteschist and blueschist are all terms spawned from the study of greenstone belts.
"Greenstone belts" are distributed throughout geological history from the Phanerozoic Franciscan belts of California where blueschist, whiteschist and greenschist facies are recognised, through to the Palaeozoic greenstone belts of the Lachlan Fold Belt, Eastern Australia, and a multitude of Proterozoic and Archaean examples.
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).
Proterozoic greenstones occur sandwiched between the Pilbara and Yilgarn cratons in Australia, and adjoining the Gawler Craton and within the extensive Proterozoic mobile belts of Australia, within West Africa, throughout the metamorphic complexes surrounding the Archaean core of Madagascar; the eastern United States, northern Canada and northern Scandinavia. The Abitibi greenstone belt in Ontario and Quebec is one of the largest Archean greenstone belts in the world.
Phanerozoic ophiolite belts and greenstone belts occur in the Franciscan Complex of south-western North America, within the Lachlan Fold Belt, the Gympie Terrane of Eastern Australia, the ophiolite belts of Oman and around the Guiana 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 a primer for other greenstone belts around the world.
List of greenstone belts
- Barberton greenstone belt (South Africa)
- Pietersberg greenstone belt (South Africa)
- Gwanda Greenstone Belt (Zimbabwe)
- Lake Victoria Greenstone Belt (East Africa)
- Taishan greenstone belt (Southeast Asia)
- Dharwar Craton, India
- Kostomuksha greenstone belt (Russia)
- Central Lapland Greenstone Belt (Lapland, Finland)
- Mauken greenstone belt (Norway)
- Abitibi greenstone belt (Quebec/Ontario, Canada)
- Bird River greenstone belt (Manitoba, Canada)
- Elmers Rock greenstone belt (Wyoming, USA)
- Flin Flon greenstone belt (Manitoba/Saskatchewan, Canada)
- Hunt River greenstone belt (Newfoundland and Labrador, Canada)
- Isua greenstone belt (Southwestern Greenland)
- Rattlesnake Hills greenstone belt (Wyoming, USA)
- Seminoe Mountains greenstone belt (Wyoming, USA)
- South Pass greenstone belt (Wyoming, USA)
- Temagami greenstone belt (Ontario, Canada)
- Rio-das-Velhas greenstone belt (Minas Gerais, Brazil)
- Rio-Itapicuru greenstone belt (Bahia, Brazil)
- Crixás greenstone belt (Goiás, Brazil)
- Faina greenstone belt (Goiás, Brazil)
- Guarinos greenstone belt (Goiás, Brazil)
- Pilar-de-Goiás greenstone belt (Goiás, Brazil)
- Harris greenstone belt (Australia)
- Jack Hills greenstone belt (Australia)
- Norseman-Wiluna greenstone belt (Australia)
- Southern Cross greenstone belt (Australia)
- Yandal Greenstone Belt (Australia)
- Yalgoo-Singleton greenstone belt (Australia)
- Maarten J de Wit and Lewis D Ashwal (1997) Greenstone Belts, Clarendon Press ISBN 0-19-854056-6 Excerpts and maps
- Lowe DR (December 1994). "Accretionary history of the Archean Barberton Greenstone Belt (3.55–3.22 Ga), southern Africa". Geology 22 (12): 1099–102. Bibcode:1994Geo....22.1099L. doi:10.1130/0091-7613(1994)022<1099:AHOTAB>2.3.CO;2. PMID 11539408.
- Map of the Norseman-Wiluna greenstone belt accessed: 6 September 2009