Greenstone belt
Greenstone belt ( en. greenstone belts ) are zones of different metamorphic mafic to ultramafic volcanic sequences that occur together with sedimentary rocks in Proterozoic cratons between archaic and granite and gneiss complexes.
The name is derived from the greenish color of the rocks, which is caused by the metamorphic minerals contained therein. Typical representatives of these minerals are chlorite, Aktinolithe and other green amphiboles.
Overview
A greenstone belt is usually a few dozen to several thousand kilometers long, and is regarded as a coherent stratigraphic group - at least on a continental scale - although it may consist of a large number of very different rock units. Because of this diversity of rock a greenstone belt forms a tectonic guideline in otherwise very extensive and homogeneous granites of the Archaic and Proterozoic areas. For the same reason, he provides a lot of information about tectonic and metamorphic events as well as about deformation history and paleogeography than the gneiss and granite areas surrounding it.
Greenstone belts consist of the majority of converted volcanic rocks as basalts ( diabase ) and sedimentary rocks, they are essentially metamorphic volcanic belt. Therefore, they offer a rewarding area of research for the study of the geological history of the Archean.
Design and Education
Greenstone belts are constructed primarily from today metamorphic volcanic rocks, most commonly made of basalt, sedimentary rocks form only minor intercalations in it. The proportion of sedimentary rocks is greater than in older, the proportion of ultramafic rocks in geologically younger greenstone belts - be it as ultramafic to mafic layered intrusions or as komatiites - has decreased about it. In addition to metamorphic basalts ( metabasalt ) numerous other types of metamorphic rocks have been developed in greenstone belts and described from there. Thus the concepts of green slate, white slate and blue slate come from the study of the greenstone belt.
In relation to the surrounding bedrock is a change of structure and relationship can be established. , Is in the archaic greenstone belts if at all, only a loose network of basalt - peridotite - rocks with granites recognizable, where they end up. In the Proterozoic belt located on a granitic, one consisting of gneiss bedrock or other greenstone belts. In the Phanerozoic, however, clear examples of island arc volcanism and sedimentation as well as typical ophiolite sequences are known. Due to these relations between archaic, Proterozoic and Phanerozoic greenstone belts a formation in the former mid-ocean ridges and island arc terranes, is not ruled out for the geologically old belt.
This development is seen as reflecting an increasingly more complete formation of the plate- tectonic processes in the course of history. Plate tectonics in the Archean probably did not take place on fully developed earth's crust, are so out of the allochthonous, tectonically connected into gneiss and granite greenstone belts to find little evidence today of magmatic and sedimentary processes. In Proterozoic larger cratons were formed, at their edges magmatism took place and in the form of barely existed by plate tectonic processes to be destroyed continental crust long time existing sediment sources so that more sediments were deposited and preserved. This trend continued during the Phanerozoic with the formation of large continental areas and the decrease of the heat flow from the mantle on, and more and more sediments to be preserved.
Occurrence
Greenstone belts can be found throughout the geological history through, from the Phanerozoic Franciscan belts of California, where blue, white and green shales are encountered on Paleozoic greenstone belt 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 the Yilgarn Craton in Western Australia or in the Gawler Craton in South Australia. Other examples are known from South and East Africa, such as the Barberton greenstone belt from the Kapvaal craton. Other deposits are located in the Innerern of Madagascar, West Africa and Brazil, in the northern Scandinavia and the Kola Peninsula ( Baltic Shield ). The Abitibi greenstone belt in Quebec / Ontario in Canada is one of the most archaic greenstone belt of the earth.
Proterozoic greenstone belts occur as narrow strips between the Yilgarn and Pilbara craton which in Western Australia, as well as adjacent to the Proterozoic Gawler Craton and within the vast West Australian belt rocks. Others are found in West Africa and in the metamorphic complexes that surround the archaic core of Madagascar. More examples can be found in the USA ( Blue Ridge Mountains ) and Canada as well as in northern Scandinavia.
Phanerozoic greenstone belts come about in the Franciscan complex in the southwestern United States, in the Lachlan Fold Belt and in Gympie Terran in eastern Australia, in the Ophiolithgürteln of Oman and around the Guyana craton.
Greenstone belts often contain ore deposits of gold, silver, copper, zinc and lead.
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 ( West Greenland )
- Nuvvuagittuq greenstone belt (Quebec / Ontario, Canada)
- Pieterberg greenstone belt (South Africa)
- Tai Shan greenstone belt (China)
- Temagami greenstone belt (Ontario, Canada)