Petrology of high Al hornblende- and magmatic epidote-bearing plutons in the southeastern Cape Breton Highlands, Nova Scotia

Six large late Precambrian plutons in the southeastern Cape Breton Highlands of Nova Scotia are composed dominantly of diorite, quartz diorite, tonalite, and granodiorite. Their petrochemical characteristics indicate that they are the plutonic equivalents of moderate- to high-K orogenic andesites, formed in a continental margin subduction zone. The occurrence of high-Al hornblende and magmatic epidote in the plutons in the northwest suggests that crusal levels of ca. 20 km are exposed in that area. Calculated pressures of hornblende crystallization decrease systematically to the southeast, indicating that granodiorites exposed in that sector crystallized in the epizone. The magmas may have formed by variable amounts of partial melting of mafic granulitic source-rocks, followed by fractionation of mafic minerals and some plagioclase, to produce the internal variation within each pluton.

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