- Greenstone belt
Greenstone belts are zones of variably metamorphosed
maficto ultramafic volcanicsequences with associated sedimentary rocksthat occur within Archaeanand Proterozoic cratons between graniteand gneissbodies.
The name comes from the green hue imparted by the colour of the metamorphic
minerals within the mafic rocks. Chlorite, actinoliteand other green amphibolesare the typical green minerals.
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- gneisswithin a cratoncontains 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 tectonicand metamorphic events, deformations and palaeogeologic conditions than the granite and gneiss events, because the vast majority of greenstones are interpreted or provably basalts and other volcanicor sedimentary rocks. As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaeangeological history. Greenstone belts are basically metamorphosed volcanic belts.
Nature and formation
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 intrusive ultramafic to mafic layered intrusionsor 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-
peridotitesheets of a greenstone belt and the granites they abut, and the Proterozoicwhere greenstone belts sit upon granite-gneiss basements and/or other greenstone belts, and the Phanerozoicwhere clear examples of island arcvolcanism, arc sedimentation and ophiolitesequences become more dominant.
This change in nature is interpreted as a response to the maturity of the
plate tectonicsprocesses throughout the Earth's geological history. Archaean plate tectonics, if it occurred at all (as is debated), 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' etcetera;
greenschist, whiteschistand blueschistare all terms spawned from study of greenstone belts.
"Greenstone belts" are distributed throughout geological history from the Phanerozoic
Franciscan Beltsof Californiawhere blueschist, whiteschistand greenschist facies are recognised, through to the Palaeozoicgreenstone 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 cratonand Yilgarn Craton, Western Australia, Gawler Cratonin South Australia. Examples are found in South and Eastern Africa, namely the Kaapvaal cratonand also in the cratonic core of Madagascar, as well as West Africaand Brazil, northern Scandinaviaand 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 beltin Ontarioand Quebecis one of the largest Archean greenstone belts in the world.
Phanerozoic ophiolite belts and greenstone belts occur in the
Franciscan Complexof south-western North America, within the Lachlan Fold Belt, the Gympie Terraneof Eastern Australia, the ophiolite belts of Omanand around the Guiana Shield.
The belts often contain
oredeposits of gold, silver, copper, zincand lead.
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.
He noted the existence of
pillow lavas, indicating a lava being rapidly cooled in water, as well as the spinifextextures created by crystals formed under rapidly cooling environments, namely water.
List of greenstone belts
Abitibi greenstone belt(Quebec/Ontario, Canada)
Barberton greenstone belt(South Africa)
Bird River greenstone belt(Manitoba, Canada)
Flin Flon greenstone belt(Manitoba/Saskatchewan, Canada)
Isua greenstone belt(Southwestern Greenland)
Pietersberg greenstone belt(South Africa)
Taishan greenstone belt(Southeast Asia)
Temagami greenstone belt(Ontario, Canada)
*Maarten J de Wit and Lewis D Ashwal (1997) "Greenstone Belts", Clarendon Press ISBN 0-19-854056-6 [http://web.uct.ac.za/depts/cigces/gsbelts.htm Excerpts and maps]
*Lowe, D. R. (1994) "Accretionary history of the Archean Barberton Greenstone Belt (3.55-3.22 Ga), southern Africa" Geology. 1994 Dec;22(12):1099-102. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11539408&dopt=Abstract Abstract]
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