Abstract

Uranium mineralization in pegmatites and granites occurs widely in the Grenville Province but is especially important near Bancroft, Ontario and north of Mont Laurier and Johan Beetz in Quebec. Most investigators have concluded that these rocks were produced by anatexis of a uranium-enriched sedimentary protolith. We have used Rb–Sr isotopic methods to test this and other hypotheses and have reexamined the petrologic and structural features that led to these hypotheses.Rb–Sr whole-rock isochron ages from all three areas range from 937 to 980 Ma, some 150 Ma younger than the age of the peak of Grenville metamorphism as inferred from numerous ages from syntectonic intrusive rocks and high-grade metasediments. The ages of the pegmatites are similar to or just slightly greater than K–Ar ages from this region. The initial 87Sr/86Sr ratios range from 0.7028 to 0.7054, plus one poorly defined value of 0.709. Excluding this last one, these values are only slightly greater than mantle ratios, and calculations show that these rocks could not have been derived from the quartzofeldspafhic gneisses of these areas and certainly not from Grenville Supergroup sediments.The late timing of the emplacement of these rocks and the popularity of a continental collision model for the Grenville Province producing a very thick crust lead us to suggest an origin by deep melting due to isothermal decompression during rapid uplift. This suggestion is supported by the location of all the occurrences near zones interpreted to be paleorift systems.

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