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greenstone belt A greenstone belt is an elongate accumulation of generally mafic volcanic and plutonic rocks, typically associated with assemblages of sedimentary rocks that include sandstones, mud-stones, banded iron formations, and, less commonly, carbonates and mature sedimentary rocks. Most greenstone belts are Archean or at least Precam-brian in age, although similar sequences are known from orogenic belts of all ages. Nearly all are metamorphosed to greenschist through amphibolite facies and intruded by a variety of granitoid rocks. older gneissic rocks are associated with some greenstone belts, although most of these are in fault contact with the greenstone belts.
Greenstone belts display a wide variety of shapes and sizes and are distributed asymmetrically across Archean cratons in a manner reminiscent of tectonic zonations in Phanerozoic orogens. For instance, the Yilgarn craton in Australia has mostly granitic gneisses in the southwest, mostly 2.9 billion-year-old greenstones throughout the central craton, and 2.7 billion-year-old greenstones in the east. The Slave Province in Canada contains remnants of a circa 4.2-2.9 billion-year-old gneissic terrain in the western part of the province, dominantly mafic greenstone belts in the center, and 2.68 billion-year-old mixed mafic, and intermediate and felsic calc-alka-line volcanic rocks in the eastern part of the province. Other cratons are also asymmetric in this respect; for example, the Zimbabwe craton, in Africa, has mostly granitic rocks in the east and more greenstones in the west. The Superior Province contains numerous subparallel belts, up to thousands of miles long, that are distinct from each other but similar in scale and rock type to Phanerozoic orogens. These distributions of rock types are analogous to asymmetric tectonic zonations, which are products of plate tectonics in younger orogenic belts, and emphasize that greenstone belts are perhaps only parts of once larger orogenic systems.
There are three significantly different end-member regional outcrop patterns of greenstone belts reflecting the distribution of these belts within cra-tons. These include broad domal granitoids with interdomal greenstones; broad greenstone terrains with internally branching lithological domains and irregular granitoid contacts; and long, narrow, and straight greenstone belts. The first pattern includes mostly granitoid domes with synformal greenstone belts, which result from either interference folding or dome-shaped granitoids. The second pattern includes many of the terranes with thrust belt patterns, including much of the Yilgarn Province in western Australia and the Slave Province of Canada. Contacts with granitoids are typically intrusive. The third pattern includes composite thrust/strike slip belts dominated by late strike-slip shear zones along one or more sides of the belt. Granite-greenstone contacts are typically a fault or shear zone.
Until recently few complete ophiolite-like sequences were recognized in Archean greenstone belts, leading some workers to the conclusion that no Archean ophiolites or oceanic crustal fragments are preserved. Research documenting partial dismembered ophiolites in several greenstone belts and a complete ophiolite sequence in the North China cra-ton recently challenged these ideas. Archean oceanic crust was possibly thicker than Proterozoic and Pha-nerozoic counterparts, and this resulted in accretion predominantly of the upper basaltic section of oceanic crust. The crustal thickness of Archean oceanic crust may have resembled modern oceanic plateaus. If this were the case, complete Phanerozoic-like ophiol-ite sequences would have been very unlikely to be preserved from Archean orogenies. In contrast, only the upper, pillow lava-dominated sections would likely be accreted. Archean greenstone belts have an abundance of accreted ophiolitic fragments compared with Phanerozoic orogens, suggesting that thick, relatively buoyant, young Archean oceanic lithosphere may have had a structure favoring separation of the uppermost parts during subduction and collisional events.
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