Geochronology of the Lumby Lake greenstone belt: A 3 Ga complex within the Wabigoon subprovince, northwest Ontario
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.