Rubidium-strontium age determinations from the Churchill Province of northern Manitoba

Abstract

The ages of three rock units from Churchill Structural Province of Manitoba were determined by the rubidium-strontium, whole-rock method. A minimum age of 2636 +/- 163 million years, with an initial ratio of 0.7025 +/- 0.0034 was obtained for a foliated quartz monzonite from the Kasmere Lake area in Northern Manitoba (87Rb decay constant of 1.39 X 10-11yr-1). This unit probably represents an Archean basement for the metasedimentary rocks of the Wollaston Lake Belt in Manitoba. Also from the Kasmere Lake area, a grey quartz dioritic to grandodioritic gneiss gave a minimum age of 1941 +/- 25 million years and an initial 87Sr/86Sr ratio of 0.7038 +/- 0.0003. The isotopic data implies that this unit was formed from primary igneous material which was emplaced early in the Hudsonian Orogeny and underwent alkali metasomatism during a subsequent event. A minimum age of 1818 +/- 199 million years was obtained for a ferrohypersthene metatonalite, metadiorite and anorthositic gabbro from the Burntwood Lake area in central Manitoba. This age and the initial 87Sr/86Sr ratio of 0.7032 +/- 0.0013 suggest that these ferrohypersthene rocks were derived from primary magma intruded during the Hudsonian Orogeny.