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.

First Page Preview

First page PDF preview
You do not currently have access to this article.