PETROLOGY, PETROGENESIS AND TECTONIC SETTING OF PLUTONIC ROCKS IN THENORTH MOUNTAIN AREA, WEST-CENTRAL CAPE-BRETON-ISLAND, NOVA-SCOTIA

The Marble Mountain area of west-central Cape Breton Island is underla in mainly by granitoid rocks of the Marble Mountain, Big Brook, and We st Bay plutons, migmatitic gneisses of the Lime Hill gneissic complex, and low-grade metasedimentary rocks of the Malagawatch Formation. The Late Precambrian Marble Mountain and Big Brook plutons are composed o f hornblende-biotite tonalite to granodiorite. These plutons underwent similar but separate evolutionary histories involving fractionation o f plagioclase and hornblende +/- biotite. They display petrological si milarities to Cordilleran I-type suites, as exemplified by the Peninsu lar Ranges Batholith, and are interpreted to have formed by partial me lting of a mainly basaltic source, and to represent the root zone of a primitive volcanic arc at a convergent plate margin. In contrast, the West Bay Pluton consists of megacrystic monzogranite and associated, probably co-genetic, granitic porphyritic dykes. The pluton is a felsi c granite with I-type mineralogy but displays evolutionary trends dist inct from typical felsic I-, S-, and A-type granites. The tectonic set ting is interpreted to be post-orogenic, and the magma may have formed by a high degree of partial melting of a mainly crustal source. The a ge of the West Bay Pluton is uncertain, but may be Early Ordovician, b ased on petrological similarity to other granitic plutons in the Bras d'Or terrane which have yielded U-Pb (zircon) ages of ca. 495 Ma.