The geology of northern Cape Breton Island is characterized by two structurally and compositionally distinct metamorphic units and an extended history of igneous activity ranging in age from Precambrian to Devonian. The older metamorphic unit (George River Group(?)) is composed of metamorphosed psammitic and semipelitic rocks with some distinctive quartzite and coarse-grained marble layers. It was deformed and probably metamorphosed, prior to emplacement of Precambrian diorites. A younger metamorphic unit (Fourchu Group(?)) consists mostly of metamorphosed intermediate to acidic volcanic and volcaniclastic rocks and rests with angular unconformity on the George River Group (?) and the diorites. Shallow-level plutonic activity was contemporaneous with the Fourchu Group(?). Younger leucocratic granodiorites (410 m.y.) were emplaced at a depth of about 15 km and require deposition of a thick surficial sequence during the early Paleozoic (between 560 and 410 m.y.). Mississippian sedimentary rocks rest unconformably on all of the older rocks.Igneous rocks of the Precambrian are dominantly andesitic and are similar to present-day suites in island arcs associated with subduction zones. Their composition requires that they be generated in the mantle. The average compositions of igneous rocks emplaced from late Precambrian to Devonian time become more granitic and probably reflect increasing generation of magma from crustal sources.

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