Greenstone belt
Greenstone belt ( Engl. Greenstone belts ) are different zones metamorphic mafic and ultramafic volcanic sequences which together with sedimentary rocks in archaic and Proterozoic cratons between granite - and gneiss occur complexes.
The name is derived from the greenish color of the rocks , which is caused by the metamorphic minerals they contain . Typical representatives of these minerals are chlorites , actinolites and green amphiboles .
overview
A greenstone belt is usually a few tens to several thousand kilometers long, and is considered a coherent stratigraphic group - at least on a continental scale - although it can consist of a large number of very different rock units. Due to this variety of rocks, a greenstone belt forms a tectonic guideline in otherwise very extensive and homogeneous granites of the archaic and Proterozoic areas. For the same reason, it offers a lot of information about tectonic and metamorphic events as well as deformation history and paleogeography as the gneiss and granite areas surrounding it.
Greenstone belts consist mostly of converted volcanic rocks such as basalts ( diabase ) and sedimentary rocks , they are essentially metamorphic volcanic belts. Therefore, they offer a worthwhile area of research for studying the geological history of the Archean.
Construction and education
Greenstone belts are mainly made up of volcanic rocks that are metamorphic today, most often basalt, sedimentary rocks only form small inclusions. The proportion of sedimentary rocks is greater in geologically younger greenstone belts than in older ones, while the proportion of ultramafic rocks - be it as ultramafic to mafic layered intrusions or as komatiites - has decreased. In addition to metamorphic basalts (metabasalt), numerous other types of metamorphic rocks have emerged in greenstone belts and have been described from there. The terms green slate , white slate and blue slate come from the study of the green stone belt.
In relation to the surrounding basement , a change in structure and relationship can be observed. In the archaic greenstone belts, if at all, only a loose connection between the basalt peridotite rocks and the granites at which they end can be recognized. In the Proterozoic, the belts lie on a granitic, gneiss basement or other greenstone belts. In the Phanerozoic , however, clear examples of island arc volcanism and sedimentation as well as typical ophiolite sequences are known. Because of these relationships between archaic, Proterozoic and Phanerozoic greenstone belts, an emergence in former mid-ocean ridges and island arch terranes cannot be ruled out for the geological old belts .
This development is seen as the expression of an increasingly complete formation of the plate tectonic processes in the course of the earth's history. The plate tectonics in the Archean period probably did not take place on a fully developed earth crust , so that apart from the allochthonous greenstone belts tectonically interposed in gneiss and granite, there are hardly any signs of today's magmatic and sedimentary processes. In the Proterozoic, larger cratons were formed, at the edges of which magmatism took place and sediment sources existed for a long time in the form of continental crust that could hardly be destroyed by plate tectonic processes , so that more sediments could be deposited and were preserved. This development continued in the Phanerozoic with the formation of large continental areas and the decrease in the flow of heat from the Earth's mantle , and more and more sediments could be preserved.
Occurrence
Greenstone belts can be found throughout geological history, from the Phanerozoic Franciscan Belts of California, where blue, white and green slate are found, to Paleozoic greenstone belts such as the Lachlan Fold Belt in eastern Australia, to a large number of examples from the Proterozoic and Archean.
Archaic greenstone belts can be found in the Slave Craton in Northern Canada , in the Pilbara Craton and in the Yilgarn Craton in Western Australia or in the Gawler Craton in South Australia. Further examples are known from South and East Africa , such as the Barberton greenstone belt from the Kaapvaal craton . Other occurrences are in the interior of Madagascar , in West Africa and Brazil , in northern Scandinavia or on the Kola peninsula ( Baltic shield ). The Abitibi greenstone belt in Québec / Ontario in Canada is one of the largest archaic greenstone belts on earth.
Proterozoic greenstone belts occur as narrow strips between the Yilgarn and Pilbara Cratons in Western Australia, also adjacent to the Gawler Craton and within the extensive Western Australian belt of Proterozoic rocks. Others can be found in West Africa and in the metamorphic complexes that surround the archaic core of Madagascar. Further examples can be found in the USA ( Blue Ridge Mountains ) and Canada, as well as in northern Scandinavia.
Phanerozoic greenstone belts occur in the Franciscan Complex in the southwestern United States, in the Lachlan Fold Belt and Gympie Terran in eastern Australia, in the ophiolite belts of Oman and around the Guyana Craton .
Greenstone Belt often contain mineral deposits with gold , silver , copper , zinc and lead .
List of greenstone belts
- Abitibi greenstone belt (Québec / Ontario, Canada)
- Barberton Greenstone Belt (South Africa)
- Bird River Greenstone Belt ( Manitoba , Canada)
- Flin-Flon greenstone belt (Manitoba / Saskatchewan , Canada)
- Isua Greenstone Belt (Southwest Greenland )
- Nuvvuagittuq greenstone belt (Québec / Ontario, Canada)
- Pietersberg Greenstone Belt (South Africa)
- Tai Shan Greenstone Belt ( China )
- Temagami Greenstone Belt (Ontario, Canada)
literature
- J. de Wit Maarten and Lewis D Ashwal: Greenstone Belts. Clarendon Press, 1997, ISBN 0-19-854056-6 Excerpts from text and maps
- DR Lowe: Accretionary history of the Archean Barberton Greenstone Belt (3.55-3.22 Ga), southern Africa. Geology, 22, 12, pp. 1099-102, 1994 PMID 11539408