Rubidium-strontium age determinations in the Bird River area, southeastern Manitoba

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Penner, Alvin Paul
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Rubidium - strontium whole - rock ages have been obtained from greenstones and adjacent granitic intrusions in the Bird River-Winnipeg River area of southeastern Manitoba. A grey quartz diorite unit occurring north of the greenstone belt and intrusive into it gave an age of 2640 +- 135 m.y. (~Rb87 = 1.39 x 10-11 yr.-1). The Lac du Bonnet quartz monzonite lies south of the greenstone belt and is intrusive into the quartz diorite and the rocks of the greenstone belt. This unit gave an isochron age of 2495 +- 130 m.y. The volcanic rocks of the greenstone belt were dated at 2650 +- 35 m.y. The initial Sr87/Sr86 ratios for the quartz diorite, the Lac du Bonnet quartz monzonite, and the volcanics are 0.7014 +- 0.0021, 0.7088 +- 0.0068, and 0.7015 +- 0.0014. respectively. These results are compatible with geologic interpretations of the area. Initial results on feldspars from the Bernic Lake pegmatite suggest extensive post-intrusive redistribution of radiogenic strontium. A lithium muscovite from this pegmatite gives an age of 2760 m.y.