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Geological mapping (1:10,000 scale) and sampling of the Belle C么te Road orthogneiss and First Fork Brook gneiss, Cape Breton Highlands, in the summer of 1996 has revealed that the Belle C么te Road orthogneiss (previously dated at 442 + 3 Ma; U-Pb zircon) intruded the First Fork Brook gneiss and that both of these units are intruded by the Taylors Barren Pluton (previously dated at 430 + 2 Ma; U-Pb zircon). The Belle C么te Road orthogneiss is a medium- to coarse-grained, well foliated composite granitoid intrusion. The orthogneiss consists of at least two varieties (variety 1 - granodiorite to tonalite and variety 2 - tonalite), both having formed from different magmas. Variety 3 consists of megacrystic orthogneiss which has a geochemical signature similar to variety 1 and therefore may have come from the same magma. All varieties of orthogneiss are peraluminous having A/CNK ratios of 1.44-1.78 that decrease with the increase in silica content, typical of S-type granitoid bodies. Rb/(Y+Nb) and Nb/Y plots suggest that the orthogneiss was formed in a volcanic arc setting.

The First Fork Brook gneiss consists of mainly heterogeneous, strongly foliated, coarse to fine grained amphibolite and minor paragneiss. It may represent mafic flows interbedded with sedimentary rocks much like the rocks observed in the Jumping Brook metamorphic suite elsewhere in the Cape Breton Highlands. Geochemical characteristics suggest that the protoliths of the amphibolite were tholeiitic and were formed in a volcanic arc setting.

The First Fork Brook gneiss has been affected by at least 4 deformation events (D1-D4). The Belle C么te Road orthogneiss was intruded during the formation of the pervasive foliation (D2) that is shared by all the metavolcanic, metasedimentary and metaplutonic units in the Aspy terrane. Metamorphism of the First Fork Brook gneiss reached amphibolite facies with peak metamorphic conditions of 7.2 + 1.5 kbars and 650-700oC. Minimum peak metamorphic conditions of 5 kbars and 600oC are recorded for the Belle C么te Road orthogneiss. Peak metamorphic conditions were reached before 411 Ma and during collision between the Aspy and Bras d'Or terranes. An average 40Ar/39Ar age of 372 Ma obtained from hornblende in samples of the Belle C么te Road orthogneiss and First Fork Brook gneiss indicates that cooling in the central part of the Aspy terrane occurred approximately 10 m y. later than in the rest of the Aspy terrane, thus suggesting that the central part of the Aspy terrane reflects a higher metamorphic grade and greater depth of burial relative to the rest of the metavolcanic, metasedimentary and metaplutonic rocks in the Aspy terrane.

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Supervisor: S. Barr / R. Raeside / P. Reynolds

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