Archean Greenstone Belts Are Not Oceanic Crust

Archean greenstone belts with a preponderance of mafic volcanic rocks, often preserved in tectonically complex sequences, are obvious candidates in the search for remnants of Archean ocean crust. We review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as Archean ophiolites and conclude, on the basis of basal unconformities, presence of xenocryst zircons, geochemical and isotopic evidence for crustal contamination, intrusive relationships with older basement and their internal stratigraphy, that none of these examples is derived from Archean oceanic crust. Thermal, tectonic, and isostatic constraints imply that Archean oceanic basins did exist and were covered with several kilometers of water. We consider it possible, indeed probable, that relict Archean oceanic crust is preserved in granite-greenstone terrains or other Archean tectonic settings but that an unequivocal Archean ophiolite has not yet been recognized.