Abstract

U-Pb ages have been measured for six lithologies that are relevant to the Proterozoic igneous activity around Lake Nipigon and the northern part of Lake Superior. The data show the presence of previously unrecognized pre-Keweenawan felsic magmatic events and provide a framework for relating events in the Nipigon plate to the Keweenawan rift.

Tonalite gneiss from the Archean basement underlying the Nipigon plate has a minimum age of 2716 Ma. An anorogenic granite pluton at English Bay, Lake Nipigon, that is observed to grade into rhyolite is dated at 1536.7 + 10/−2.3 Ma. The rhyolites are intercalated with quartz arenites, and this may date the initiation of Sibley Group sedimentation. Samples (two) of the Logan diabase sills from different parts of Lake Nipigon have been dated using zircon and baddeleyite. The zircon fractions define an age of 1108.8 + 4/−2 Ma.

A rhyolite from the Osier volcanics at Agate Point on the north shore of Lake Superior gives an age of 1097.6 ± 3.7 Ma. This is from the magnetically reversed sequence and provides a maximum age for the magnetic reversal. The zircons show evidence of inheritance and define a mixing line with an upper intercept of 2635 +143/−125 Ma. A porphyry, probably a flow, from the base of the Osier Group, dated at 1107.5 + 4/−2 Ma, is similar in age to the Logan sills. The presence of inherited zircons in the felsic rocks of the Osier Group indicates partial melting of Aichean crust during emplacement of basaltic magma. A variety of zircon ages from a conglomerate containing felsic porphyry clasts at the base of the Osler Group suggests that pre-Keweenawan felsic magmatism took place at about 1730 Ma and 1600 Ma in the Black Bay Peninsula area.

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