Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Abstract

Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.