Abstract

Rb–Sr analyses have been carried out on samples of lamprophyre dikes and of granophyric phases of the Mongowin Pluton, two units that intrude Huronian metasediments in the Espanola–Massey area. Data on whole rocks and separated biotites from the lamprophyres yield an isochron age of 1415 ± 40 m.y., which is interpreted as the age of intrusion of the lamprophyre dikes. Whole rock granophyric samples from the Mongowin Pluton do not define a good isochron, but an "age" of 1770 ± 75 m.y. may be inferred from the data; the possibility exists, however, that the true age may be younger or older. The lamprophyre dikes and the Mongowin Pluton are apparently unrelated to each other; furthermore, neither unit appears to be the same age as the 1510 ± 50 m.y. old Croker Island Complex. Thus, there is little evidence to suggest that there was a major intrusive 'event' approximately 1500 m.y. ago in the north shore area; instead, it appears that diverse igneous and metamorphic activity occurred at several scattered times during the interval 1200 m.y. to 1800 m.y. ago.

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