The Lumby Lake greenstone belt lies within the central, predominantly metaplutonic terrane of the western Wabigoon subprovince. It is conformable with gneiss of the Marmion Lake tonalite batholith. U-Pb analyses of zircons from two felsic tuffs and a quartz porphyry within the greenstone belt give a scattered lead-loss pattern indicative of early disturbance. The ages of the oldest, concordant fractions agree, however, and define an age of 2999 ± 1 m.y. for emplacement of these felsic units. Zircons from a tonalite gneiss within the Marmion Lake batholith are also somewhat disturbed. The oldest abraded fraction is concordant and has an age of 3003 ± 5 m.y., indicating that the gneiss is probably close in age to the supracrustal rocks.
U-Pb analyses of rutile from one of the felsic units give an age of 2690 ± 5 m.y. and record the effect of Late Archean metamorphism. Three titanite fractions from the tonalite gneiss agree within error and have an age of 2809 ± 1 m.y. The age and consistency of the titanite fractions indicate that titanite in this rock was formed as a result of a metamorphic event and was never subsequently disturbed. This age is therefore a minimum estimate for high-grade metamorphism of the gneiss. The presence of a pre-2809 Ma fabric in a gneiss of regional extent suggests that an early metamorphic-deformational event affected this area.
These data constitute the first firm evidence for extensive 3.0 Ga crust in the southern Superior province and give the first evidence of a regional deformational event which predates cratonization of the Superior province.