THE EVOLUTION OF THE SOUTHERN ABITIBI GREENSTONE-BELT IN LIGHT OF PRECISE U-PB GEOCHRONOLOGY

This paper summarizes U-Pb zircon ages for the Archean southern Abitib i greenstone belt and integrates them, together with zircon and baddel eyite data for three new samples, into a general framework for the evo lution of the region. New ages are reported for the Ghost Range gabbro ic complex (2713(+7/-5) Ma) and for a pyroclastic volcanic rock of the Gauthier Group (2700 +/- 3 Ma). A previously determined age of 2701(3/-2) Ma for the felsic volcanic Skead Group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic componen t (greater-than-or-equal-to 2720 Ma) indicating that the Skead volcano was built on top of an older volcanic sequence. The compiled ages sho w that the southern Abitibi greenstone belt developed between 2750 and 2670 Ma. The earliest ages date remnants of volcanic complexes that o ccur at the periphery of the belt. Major preorogenic magmatism was con fined mainly to the period 2720 to 2700 Ma. In general, each episode w as characterized by the essentially coeval formation of komatiitic, th oleiitic, and calc-alkalic igneous rocks. Taking into consideration th e known geologic, geochemical, and isotopic data, this association is consistent with evolution in paired, active are and back-arc systems. Older crust was absent from the region, but it may have interacted wit h these arcs and rift systems farther to the west. The extensional reg ime was followed by a period of compression that caused folding and th rusting, emplacement of calc-alkalic plutons, and deposition of turbid ites between 2700 and 2688 Ma. The magmatism became predominantly alka lic during the main Timiskaming period between 2681 and 2676 Ma. This event was accompanied by the deposition of alluvial-fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting, probably between approximately 2676 and 2670 Ma. Lampro phyre dikes were associated with these final stages of deformation. Po storogenic activity includes faulting and episodes of hydrothermal act ivity, locally associated with gold mineralization. Ages in the range 2630 to 2580 Ma for hydrothermal minerals (e.g., titanite and rutile; Camflo) suggest a correlation of these events with lower crustal magma tism and metamorphism, as recorded, for example, in the adjacent Kapus kasing zone.