Greenstone belts are geographical zones that are made up of metamorphosed sedimentary and volcanic rocks, which are commonly found in continental shields. Greenstone belts are characterized by varying sequences of volcanic and sedimentary rocks. Greenstone belts are considered to be Archaean in age, with some Proterozoic occurrences, and are long and narrow in shape.
The name of these belts is derived from the green color of the rocks. The mafic minerals, such as chlorite, actinolite, etc. impart color to these rocks. Basalt is the dominant constituent of these rocks. Some forms of greenstones containing other metamorphic rocks are greenchist, whitechist, and bluechist.
Formation of Greenstone Belts
The history of the occurrence of greenstone belts has been associated with ancient oceanic spreading centers and island arc terranes. Greenstone belts are typically composed of volcanic rocks - basalt being the dominant rock - with minor sedimentary rocks present in an inter-leaved pattern. The structure of these rocks is synclinoriums, having steep foliations. These belts typically span over an area ranging from 20 to 100km in width, and many hundred kilometres in length.
Distribution of Greenstone Belts
Greenstone belts have been found in North American, Australian, Indian, and South African cratons.
- Asia –from the Taishan Belt, South East Asia and Dharwar Craton, India
- Africa – from the Barberton greenstone belt and Pietersberg greenstone belt in South Africa
- North America – from Abitibi greenstone belt, Elmers rock greenstone, and Temagami greenstone belt
- South America – from the Faina greenstone belt, Guarinos greenstone belt, and Crixas greenstone belt in Brazil