THE LATE ARCHEAN SCHREIBER-HEMLO AND WHITE RIVER DAYOHESSARAH GREENSTONE BELTS, SUPERIOR PROVINCE - COLLAGES OF OCEANIC PLATEAUS, OCEANIC ARCS, AND SUBDUCTION-ACCRETION COMPLEXES

The late Archean (ca. 2.80-2.68 Ga) Schreiber-Hemlo and White River-Da yohessarah greenstone belts of the Superior Province, Canada, are supr acrustal lithotectonic assemblages of ultramafic to tholeiitic basalt ocean plateau sequences, and tholeiitic to calc-alkaline volcanic are sequences, and siliciclastic turbidites, collectively intruded by are granitoids. The belts have undergone three major phases of deformation ; two probably prior to, and one during the assembly of the southern S uperior Province. Imbricated lithotectonic assemblages are often disru pted by syn-accretion strike-slip faults, suggesting that strike-slip faulting was an important aspect of greenstone belt evolution. Field r elations, structural characteristics, and high-precision ICP-MS trace- element data obtained for representative lithologies of the Schreiber- Hemlo and White River-Dayohessarah greenstone belts suggest that they represent collages of oceanic plateaus, juvenile oceanic island arcs, in subduction-accretion complexes. Stratigraphic relationships, struct ural, and geochemical data from these Archean greenstone belts are con sistent with a geodynamic evolution commencing with the initiation of a subduction zone at the margins of an oceanic plateau, similar to the modern Caribbean oceanic plateau and surrounding subduction-accretion complexes. All supracrustal assemblages include both ocean plateau an d island-arc geochemical characteristics. The structural and geochemic al characteristics of vertically and laterally dismembered supracrusta l units of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts cannot be explained either by a simple tectonic juxtaposition o f lithotectonic assemblages with stratified volcanic and sedimentary u nits, or cyclic mafic to felsic bimodal volcanism models. A combinatio n of out-of-sequence thrusting, and orogen-parallel strike-slip faulti ng of accreted ocean plateaus, oceanic arcs, and trench turbidites can account for the geological and geochemical characteristics of these g reenstone belts. Following accretion, all supracrustal assemblages wer e multiply intruded by syn-to post-tectonic high-Al, and high-La/Yb-n slab-derived trondhjemite-tonalite-granodiorite (TTG) plutons. The ama lgamation processes of these lithotectonic assemblages are comparable to those of Phanerozoic subduction-accretion complexes, such as the Ci rcum-Pacific, the western North American Cordilleran, and the Altaid o rogenic belts, suggesting that subduction-accretion processes signific antly contributed to the growth of the continental crust in the late A rchean. The absence of blueschist and eclogite facies metamorphic rock s in Archean subduction-accretion complexes may be attributed to eleva ted thermal gradients and shallow-angle subduction. The melting of a h otter Archean mantle at ridges and in plumes would generate relatively small, hot, and hence shallowly subducting oceanic plates, promoting high-temperature metamorphism, migmatization, and slab melting. Larger , colder, Phanerozoic plates typically subduct at a steeper angle, gen erating high-pressure low-temperature conditions for blueschists and e clogites in the subduction zones, and low-La/Yb-n granitoids from slab dehydration, and wedge melting. Metasedimentary subprovinces in the S uperior Province, such as the Quetico and English River Subprovinces, have formerly been interpreted as accretionary complexes, outboard of the greenstone belt magmatic arcs. Here the greenstone-granitoid subpr ovinces are interpreted as collages of subduction-accretion complexes, island arcs and oceanic plateaus amalgamated at convergent plate marg ins. and thr neighbouring metasedimentary subprovinces as foreland bas ins. (C) 1998 Elsevier Science B.V. All rights reserved.