Age Determination of a Granite Gneiss from the Precambrian Basement of Scioto County, Ohio

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Biotite and potassium feldspar from a specimen of granite gneiss from the Precambrian basement of Scioto County (Permit No. 212, Greenup Quadrangle of Green Township) have been dated by the Rb-Sr method. The model dates, calculated relative to an assumed initial 87Sr/86Sr ratio of 0.7040, are 898±40 million years (biotite) and 1242±46 million years (potassium feldspar). The dates are discordant, which suggests that the minerals gained or lost rubidium and/or strontium during an episode of metamorphism. The age of the metamorphism is estimated as 840=t42 million years, assuming that both minerals were isotopically re-equilibrated and had the same 87Sr/86Sr ratio of 0.7178± 0.0026. The model date of the biotite agrees with previous age determinations of basement rocks from Ohio and confirms the interpretation that the Grenville Province of the Canadian Precambrian Shield extends southward into Ohio.


Author Institution: Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 and Ohio Division of Geological Survey, Columbus, Ohio 43212



The Ohio Journal of Science. v72 n1 (January, 1972), 49-53