Relationship between the Aspy and Bras d'Or "terranes" in the northeastern. Cape Breton Highlands, Nova Scotia: Discussion

Abstract

The author, S. Lin, has made major contributions to the understanding of the complex geology in the Cape Breton Highlands through his detailed mapping in the vicinity of the Eastern Highlands shear zone (see also Lin 1992). However, some generalizations made in his paper do not accurately portray the geology, and imply an unrealistic degree of certainty about some interpretations. The purpose of this discussion is to emphasize that many problems remain unresolved in the vicinity of the Eastern Highlands shear zone, contrary to the impression given by him. The author asserted that the late Precambrian rocks of the Bras d'Or terrane are overlain by the Clyburn Brook formation, a volcanic sedimentary sequence of Silurian age. However, the age of the Clyburn Brook formation is uncertain because it is based on a Rb-Sr isochron; such ages have been shown to be unreliable in the Cape Breton Highlands and elsewhere (e.g., Dunning et al. 1990). In addition, the reported isochron (Keppie and Halliday 1986) is based (with one exception) on Rb Sr data for samples from rhyolite on Ingonish Island, not on samples from the Clyburn Brook formation. Unlike rhyolite in the Clyburn Brook formation, the Ingonish Island rhyolite is unmetamorphosed and lacks associated basaltic, volcaniclastic, and sedimentary units (Raeside and Barr 1992); hence, correlation between the Ingonish Island rhyolite and the Clybum Brook formation is speculative. The statement in the abstract of the paper that the Clyburn Brook formation is Silurian is premature. It may be, but that has yet to be demonstrated. The author (p. 1773) implied that amphibolite-facies rocks occur only in the part of the McMillan Flowage Formation within the Eastern Highlands shear zone, with the further implication that there is a connection between the amphibolitefacies metamorphism and the shear zone. Amphibolite-facies metamorphism in the McMiiian Flowage Formation persists for 16 km southeast of (away from) the shear zone. Furthermore, titanite in the McMillan Flowage Formation has yielded ages of Ca. 540 Ma (Raeside and Barr 1992; G.R. Dunning, personal communication, 1990), showing that peak metamorphism was late Precambrian to Early Cambrian, much older than the deformation associated with the Eastern Highlands shear zone as documented by the author. In describing the Aspy terrane, the author (p. 1774) implied that the Cheticamp Lake Gneiss is the only supracrustal rock unit in the Aspy terrane. As documented by Raeside and Barr (1992), as well as numerous other workers (e.g., Currie 1982; Jarnieson et al. 1987, 1990), the Aspy terrane includes various