40Ar/39Ar analysis of a Monteregian intrusion: isotopic dating of Mount Megantic, Quebec
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Date
1983-12
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The Ohio State University
Abstract
The easternmost igneous complex of the Monteregian Hills Province, Mount Megantic, was studied. This alkaline complex, Mesozoic in age, was emplaced in Paleozoic country rock. The pluton is comprised of three major units; the order of emplacement was gabbro-diorite, syenite, and then granite. The age of Mt. Megantic and the time span of the three rock units were studied by the 40Ar/39Ar technique of age determination. Six mineral separates, three each of biotite and amphibole, two from granite, syenite and gabbro-diorite were studied. 40Ar/39Ar analysis of the complex gives an age of 123.6±1.9m.y. Time span of emplacement was very short, less than a few hundred thousand years, and could not be analytically resolved. Essentially the same age was found for all three intrusive units. The age spectra for biotite samples analyzed are concordant and yield conclusive evidence. Amphibole spectra reveal complexities in discordant age spectra which are related to recoil of 39Ar and impurities in samples.
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Research Award 1983