PLUTONISM IN THE SOUTHERN ABITIBI SUBPROVINCE - A TECTONIC AND PETROGENETIC FRAMEWORK

Studies of plutonic rocks in Archean terranes such as the southern Abi tibi subprovince have been an important component in the development o f an accretionary tectonic model for the Superior province. In the sou thern Abitibi subprovince, pretectonic (approximately 2.75-2.70 Ga), l ow K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., B ourlamaque batholith; Flavrian pluton) are associated with volcanism a nd are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic t o calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic i ntrusions (e.g., Dun-donald sill, Kamiskotia gabbroic complex: 2.72-2. 70 Ga) show in situ fractionation trends, and locally, have mixed magm a textures with contemporaneous calc-alkaline granitoids. They are bel ieved to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal com ponent. 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-monzonit e 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-typ e volcanic rocks are attributed to melting of large ion lithophile ele ment-enriched mantle in the late stages of subduction. Late to post-te ctonic biotite-muscovite granite intrusions contain a significant cont ribution of evolved crust (e.g., S-type characteristics of the Preissa c-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 subduc tion-related plutonic suites to late to postaccretionary suites derive d from enriched mantle and crustal sources. This framework provides a basis for understanding mineral deposits associated with intrusive roc ks of the southern Abitibi subprovince which range from Ni-Cu sulfides in layered synvolcanic tholeiitic intrusions (e.g., Kamiskotia gabbro ic complex, Ontario) to volcanogenic sulfides in early tholeiitic and calc-alkaline sequences (e.g., Flavrian pluton, Noranda district, Queb ec) to the spatial association of gold with late large ion lithophile element-enriched intrusions (e.g., Lamaque plugs, Val d'Or district, Q uebec).