Rb/Sr chronology of the Rainy Lake Archean granitoid batholith, Wabigoon Belt, northwestern Ontario

Abstract

In the Lake Despair area, the Rainy Lake batholithic complex subdivides into three syntectonic, foliated to migmatized granitoids (Jackfish Lake Complex, Northwest Bay Complex, Footprint Lake Gneiss). Field relationships predate the high level, late tectonic, granitoid stocks lying within adjacent supracrustal rocks. The three units give a 10 point York I Rb/Sr whole-rock isochron age of 2616 ± 66 Ma (R0 = 0.7012 ± 0.0005).This age is statistically indistinct from a pooled isochron for seven late tectonic Wabigoon stocks. One geochemical outlier, from Footprint, lies on a 3241 Ma reference line, interpreted as a pseudoisochron.The Wabigoon Belt formed at about 2.6 Ga as one geotectonic cycle of supracrustal accumulation, plutonism, and metamorphism. The hiatus between syntectonic and late-tectonic activity is less than 60 Ma and is indistinguishable radiometrically from multiple intrusion.R0 locates on a single stage, linear mantle Sr evolution model, indicating that the granitoids originated directly from the upper mantle or as low Rb/Sr material from the lower crust, in residence for less than about 50 Ma.