Greenstone Belts

Definition

A greenstone belt is a usually an elongate structure composed dominantly of metamorphosed volcanic and sedimentary rocks that, together with granitoids and gneiss, are the constituents of Archean and Proterozoic cratons. The volcanic rocks range in composition from komatiite to rhyolite but are dominated by basalt; sedimentary rock types include shale, quartzite, sandstone, and chert. Geochemical compositions of the basaltic rocks show that some formed as oceanic crust, perhaps as part of oceanic plateaux and others formed in island arcs. Although the metamorphic grade ranges from sub-greenschist to amphibolite facies, greenschist facies dominates. Abundant chlorite and actinolite in greenschist-faciesbasalts imparts a green color that gives rise to the name greenstone belt. Most units are steeply dipping, tightly folded, and cut in places by large shear zones, which develop as the belts are accreted to the continent following their generation in oceanic crustal or arc...