Abstract

Age study of 12 Nova Scotia granites by the Rb-Sr whole-rock method shows a number of possible isochrons of which three, at 373 million years, 318 m.y., and 268 m.y., are preferred. The ages agree in a general way with the time scale for Acadian and Appalachian orogenies.

The isochron envelope converges at zero Rb87/Sr86 to a Sr87/Sr86 zone between 0.705 and 0.711. As this range of values is consistently higher and more variable than that of oceanic and non-orogenic continental basalt, contamination by crustal material of a granitic magma generated in the deep crust or upper mantle is postulated.

Mica ages from whole rocks of the 373-m.y. group are in general accord with the whole-rock age. The majority of mica ages from rocks of the other two isochrons are anomalously 10–25 per cent higher. This problem requires more detailed study.

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