Deborah M. Conrad

B. Sc. Honours Thesis

The Question of Relationship Between Low and High Grade Metamorphic Rocks in the French Mountain Area, of the Cape Breton Highlands, Nova Scotia>/A>

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French Mountain in the Cape Breton Highlands of Nova Scotia is underlain by two broad metamorphic complexes: a metasedimentary complex and a composite gneiss complex. The metasedimentary complex, located on the southwestern slopes of the mountain, consist of interlayered phyllite, semi-pelite and psammite. The gneiss, located on the top of the mountain, consists of mainly a biotite-quartz-feldspar gneiss interlayered with minor amphibolite gneiss and feldspar augen gneiss. Over a kilometer of land devoid of outcrop separates the two complexes. Other mappable lithologies within the area include various granitoid bodies, amphibolite, diorite and granitic dykes.

The metasedimentary complex shows a progressive metamorphic sequence indicated by the presence of the key metamorphic index minerals; chlorite, biotite, garnet and staurolite. Mineral-fabric relations indicate that the metamorphism began during the early stages of the deformation which produced the pervasive schistosity, and continued until the later stages or even after deformation had ceased. The gneiss locally contains kyanite which developed before the deformation that produced the schistosity within the gneiss.

The orientation of structural elements differs considerably between the two complexes. The gneiss has undergone at least three phases of deformation while the metasedimentary complex has undergone at least two phases. Both complexes contain a pervasive schistosity, minor folds, crenulations and rare kink folds and the metasedimentary complex also contains a biotite mineral lineation and a poorly developed second crenulation. It is unknown whether the schistosity within the gneiss was developed during the same "event" that produced the schistosity within the metasedimentary rocks, and thus it is unknown whether the kyanite within the gneiss belongs to the progressive sequence found within the metasedimentary complex. Schistosity within the metasedimentary complex strikes northeast-southwest in the west, dipping moderately to the northwest, and swings northwest-southeast in the east dipping moderately to the northeast, defining a large fold which folds the metasedimentary complex. Schistosity within the gneiss strikes consistently north-south dipping steeply to the east. No unconformity was observed between the two complexes, however the distinctly different orientations of the structural elements suggest that a structural break of some sort exists. The steeper dip of the gneiss suggests that the gneiss may have been thrust from east to west over the metasedimentary complex.

It is unresolved whether the gneisses are basement rock to the metasedimentary complex or whether the two complexes represent one protolithic package with the gneiss a higher metamorphic grade equivalent of the metasedimentary rocks.

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Pages: 230
Supervisor:  Rebecca Jamieson