Rb-Sr Age of Middle Keweenawan Rocks, Mamainse Point and Vicinity, Ontario, Canada

Abstract

Rb-Sr analyses have been carried out on felsites that are interlayered with middle Keweenawan mafic volcanic rocks and clastic sedimentary rocks in the Mamainse Point area, on the east shore of Lake Superior. With both analytical and geological uncertainties taken into consideration, an overall age of 1,070 ± 50 m.y. is assigned to these rocks, with the possibility that some, if not all, of them may be as old as 1,100 ± 30 m.y. In addition, three samples of a quartz-feldspar porphyry from the Jogran Mine, 10 km east of Mamainse Point, yield an age of 1,070 ± 50 m.y., and one sample of felsic volcanic rock from Gargantua Point yields a calculated age of 1,090 ± 40 m.y. There is no strong evidence at this time to suggest that more than one period of igneous activity is represented, but the data do not rule out this possibility. In any case, these rocks are comparable in age (~ 1,100 m.y.) to other intrusive and extrusive middle Keweenawan rocks of the Lake Superior region.