ARCHEAN ROCKS

Image courtesy of Encyclopedia Brittanica

 

ARCHEAN ROCK ASSOCIATIONS:

Granulites and Greenstones are two very important rock assemblages characteristic of the Archean Eon. Granulites are high grade metamorphic and igneous rocks that are thought to be derived from the plutons associated with volcanic arcs. Granites and gneisses are the dominant rock types within granulites. Greenstones are an assemblage of volcanic and volcanosedimentary units containing, from the base upwards: ultramafic volcanic rocks (komatiites), mafic, intermediate and finally felsic volcanic rocks, overlain by tuffaceous rocks and sediments. Greenstone belts are thought to be derived from back arc and/or forearc basins adjacent to volcanic arcs. Greenstone belts include massive volcanogenic sulfide deposits (MVS) which represent old mid ocean ridge or volcanic arc sites enriched in copper and lead sulfides. The Crandon deposit formed in such a region. Together these Precambrian assemblages record early tectonic activity and have preserved valuable metal deposits on virtually all of the continents.

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