40Ar/39Ar data for biotite and amphibole from six plutonic complexes of the Monteregian petrographic province in Quebec, Canada, are presented. Generally uncomplicated and concordant incremental heating spectra are observed, although there is evidence for minor excess 40Ar at several localities. Biotites and amphiboles are concordant, consistent with rapid cooling of the high-level complexes. Ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic. The 40Ar/39Ar ages of six Monteregian complexes (St. Hilaire, Rougemont, Johnson, Shefford, Brome, Megantic) all fall within the very restricted range of 124 ± 1 m.y. It is suggested that all of the Monteregian plutons, with the exception of Oka, were formed within a short interval (1–2 m.y.) during the Cretaceous, consistent with paleomagmetic results. The new ages, along with geochemical characteristics, support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in New England and the chain of New England Seamounts.

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