HIGH-PRECISION U-PB AGES FOR GRANULITE METAMORPHISM AND DEFORMATION IN THE ARCHEAN KAPUSKASING STRUCTURAL ZONE, ONTARIO - IMPLICATIONS FOR STRUCTURE AND DEVELOPMENT OF THE LOWER CRUST

Precise U-Pb isotopic ages for two subhorizontal granulite gneisses in the Kapuskasing structural zone indicate that formation and emplaceme nt occurred more than 50 m.y. after cessation of volcanism in the stee ply folded nearby greenstone belts by successive stages of ductile und erplating across a zone of crust-mantle delamination. Volcanism and ba tholith formation had ceased by 2700 and 2680 Ma, respectively, while granulite formation occurred between 2660 and 2640 Ma and ductile defo rmation continued until 2585 Ma. Emplacement of the oldest granulite c aused the development of shear planes and melting in the older overlyi ng tonalite, while emplacement of the younger granulite caused rupturi ng and rehydration of the overlying older granulite. Widespread tecton ic underplating is implied since all Superior Province granulites have similar young ages. Anomalous Ar ages that post-date accretion by abo ut 150 m.y., and scrambled Pb-Pb and Rb-Sr whole-rock systems, also su ggest that late low-temperature alteration accompanied this event. Tem poral relationships and the presence of post-volcanic (2670 Ma) conglo merate cobbles at granulite grade support a model of ductile underplat ing of a post-accretion, thermally softened back arc-fore arc complex, with components welded onto an overlying volcanic-plutonic complex at successively deeper levels. Multistage deformation in active fault zo nes and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 Ma. Anomalously abundant gold production from Archean rocks may i ndicate that shallow underplating, granulitization and devolatization was a widespread phenomenon at this time. Post-accretion ductile under plating must be considered as a primary cause of seismic reflectors in Archean terrains.