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 emplacement occurred more than 50 m.y. after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust-mantle delamination. Volcanism and batholith formation had ceased by 2700 and 2680 Ma, respectively, while granulite formation occurred between 2660 and 2640 Ma and ductile deformation continued until 2585 Ma. Emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite, while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite. Widespread tectonic underplating is implied since all Superior Province granulites have similar young ages. Anomalous Ar ages that post-date accretion by about 150 m.y., and scrambled Pb-Pb and Rb-Sr whole-rock systems, also suggest that late low-temperature alteration accompanied this event. Temporal relationships and the presence of post-volcanic (2670 Ma) conglomerate cobbles at granulite grade support a model of ductile underplating 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 zones 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 indicate that shallow underplating, granulitization and devolatization was a widespread phenomenon at this time. Post-accretion ductile underplating must be considered as a primary cause of seismic reflectors in Archean terrains.