U-PB GEOCHRONOLOGY OF THE CENTRAL UCHI SUBPROVINCE, SUPERIOR PROVINCE

U-Pb zircon and titanite ages for rocks of the central Uchi Subprovince in northwestern Ontario indicate a late Archean magmatic and tectonic development spanning over 200 Ma. An early period at 2900-2800 Ma formed volcano-plutonic complexes, presumably linked to 3.1 -2.8 Ga terrains of the northwestern Superior Province. A later period of southward growth by magmatic and tectonic accretion occurred at 2750-2710 Ma and was concluded by large scale compression and plutonism at 2700 Ma. The oldest 2890-2860 and 2840-2820 Ma components occur in the Pickle Lake and Meen-Dempster greenstone belts and as gneisses in the Seach-Achapi and the Lake St. Joseph batholiths in northern and central sectors of the region. Together with distinct 2750-2740 Ma volcano-plutonic complexes they form a collage assembled by multiple episodes of tectonic juxtaposition and magmatic accretion. Plutons of 2730-2710 Ma age are intrusive into these older, northern domains, whereas their volcanic counterparts compose the Lake St. Joseph and Miminiska-Fort Hope greenstone belts to the south. Late-tectonic to posttectonic granitoid rocks intruded a region extending from the northern Berens River Subprovince to the southern English River Subprovince at 2700 Ma. These plutons were cut by regional scale faults formed by residual north-northwest directed shortening. The timing of this movement seems to be recorded by titanite ages of 2690-2670 Ma. Reactivation of the same faults may account for Proterozoic Pb loss observed in some of the zircon populations. The age patterns are consistent with crustal growth along a continental margin in a north-dipping subduction environment.