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

Archean granite-greenstone terranes represent juvenile continental crust fo rmed in a variety of plate tectonic settings and metamorphosed through a co mplex series of structural and magmatic events. Most Archean granite greens tone terranes appear to have acquired their first-order structural and meta morphic characteristics at convergent plate margins, where large accretiona ry wedges similar in aspect to the Chugach, Makran, and Altaids grew throug h offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigen ous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may ha ve been underplated beneath these orogens, forming thick crustal roots char acterized by interleaving between the depleted slabs and undepleted astheno sphere. Back-stepping of the subduction zones after accretion of plateaux a nd arcs caused the arcs magmatic fronts to migrate trenchward through the a ccretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the m antle 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 abili ty to support its own mass, which initiated gravitational collapse and deco mpressional release of syn- to late-tectonic granitoids from wedges of fert ile mantle trapped between underplated oceanic lithospheric slabs, and aide d in the cratonization of the granite-greenstone terranes. Deeply penetrati ng structural discontinuities such as shear zones and sutures provided path ways for fluids and granitoids to migrate into the mid- and upper-crust, fo rming ore deposits and plutons. Most preserved granite-greenstone terranes have been tectonically stable since the Archean, and form the cratonic inte riors of many continents. (C) 1999 Elsevier Science B.V. All rights reserve d.