Abstract

Studies of plutonic rocks in Archean terranes such as the southern Abitibi subprovince have been an important component in the development of an accretionary tectonic model for the Superior province. In the southern Abitibi subprovince, pretectonic ( approximately 2.75-2.70 Ga), low K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., Bourlamaque batholith; Flavrian pluton) are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic intrusions (e.g., Dundonald sill, Kamiskotia gabbroic complex: 2.72-2.70 Ga) show in situ fractionation trends, and locally, have mixed magma textures with contemporaneous calc-alkaline granitoids. They are believed to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal component. Pre- to syntectonic ( approximately 2.72-2.69 Ga) porphyry and tonalite plutons (e.g., Timmins porphyries, Round Lake batholith) have geochemical characteristics typical of Archean tonalite-trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades. Late to post-tectonic (<2.69 Ga) diorite-monzonite and syenite plutons (e.g., Watabeag batholith, Lamaque stocks, Otto stock) have large ion lithophile element-enriched calc-alkaline to sho-shonitic compositions. These intrusions and associated Timiskaming-type volcanic rocks are attributed to melting of large ion lithophile element-enriched mantle in the late stages of subduction. Late to post-tectonic biotite-muscovite granite intrusions contain a significant contribution of evolved crust (e.g., S-type characteristics of the Preissac-Lacorne batholith).In the southern Abitibi subprovince the tectonic setting of mantle- and crust-derived intrusions is broadly comparable to magmatic processes in Phanerozoic arcs and accreted terranes. The plutonic rocks record a transition from pre- to synaccretionary subduction-related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources. This framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern Abitibi subprovince which range from Ni-Cu sulfides in layered synvolcanic tholeiitic intrusions (e.g., Kamiskotia gabbroic complex, Ontario) to volcanogenic sulfides in early tholeiitic and calc-alkaline sequences (e.g., Flavrian pluton, Noranda district, Quebec) to the spatial association of gold with late large ion lithophile element-enriched intrusions (e.g., Lamaque plugs, Val d'Or district, Quebec).

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