Pb-isotope ratios of high-grade metamorphic minerals such as K-feldspar, biotite, magnetite, apatite, sillimanite, and garnet define collinear arrays on 207Pb/204Pb vs. 206Pb/204Pb diagrams. The samples are from a zone extending 35 km southeast of the Grenville front; the dates are interpreted to reflect peak granulite conditions during an Archean metamorphic event having a minimum age of 2.65 Ga. The gneiss was derived from preexisting crustal precursors and was barely affected by the Grenvillian event, a milder perturbation followed by rapid uplift.

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