ND ISOTOPIC COMPOSITIONS OF FELSIC IGNEOUS ROCKS IN CAPE-BRETON ISLAND, NOVA-SCOTIA

Nd isotopic data from 18 felsic plutonic and volcanic units in Cape Breton Island show variations consistent with other geological and geophysical evidence for at least three distinct terranes. A ca. 1.2 Ga syenite considered to be part of Grenvilleage basement exposed in the northwestern part of the island yields an initial epsilon(Nd) value of +0.4 and a depleted-mantle model age (T(DM)) of 1.66 Ga, suggesting substantial involvement of older (Archean or Early Proterozoic) crust in its petrogenesis. A T(DM) of 1.38 Ga indicated by Nd isotopic data for a Devonian granite spatially associated with the syenite is also consistent with the presence of older crust in that part of Cape Breton Island. In contrast, Silurian rhyolite and Devonian granites from the Aspy terrane have T(DM) ranging from 0.7 to 1.1 Ga and initial epsilon(Nd) between +2.8 and -1.2, and overlap in isotopic characteristics with late Precambrian and Early Ordovician plutons in the Bras d'Or terrane that yield T(DM) of 0.9-1.1 Ga and initial epsilon(Nd) of +1.4 to -1.8. The relatively small variation in epsilon(Nd) and T(DM) in these terranes suggests that old crust like that under the Blair River Complex may not be present. Granitic plutons and rhyolite in the Mira terrane of southeastern Cape Breton Island have a range in T(DM) (0.8-1.2 Ga) similar to that of the Aspy and Bras d'Or terranes, but initial epsilon(ND) values ranging from +0.8 to +5.0 indicate more juvenile crust. This juvenile crust also appears to be present under southern New Brunswick and eastern Newfoundland and may be characteristic of the Avalon terrane.