| Summary: | Mapping and petrologic studies of the Creignish Hills Pluton in the Bras d'Or Terrane of central Cape Breton Island have shown that the pluton consists mainly of tonalite-diorite and coarse-grained monzogranite units, with smaller areas of granodiorite-tonalite, granodiorite-monzogranite, and fine-grained monzogranite. Petrographic and chemical characteristics of the tonalite-diorite suggest that hornblende fractionation (± biotite accumulation) may have produced much of the observed variation in the unit, whereas feldspar fractionation was the dominant process producing variation within each of the other units. The tonalite-diorite unit is interpreted to be late Precambrian (ca. 560 Ma) in age like most other dioritic and tonalitic plutons in the Bras d'Or Terrane. It is probably not co-genetic with the intermediate and felsic units of the pluton. The latter may be of Early Ordovician age (ca. 495 Ma) on the basis of penological similarities of the coarse-gTained monzogranite unit to the Cape Smoky and Kellys Mountain monzogranites. Like other Bras d'Or Terrane plutons, the Creignish Hills Pluton probably formed in a continental margin volcanic arc setting during and after late Precambrian to Early Cambrian subduction. RÉSUMÉ Des travaux de cartographie et de pétrologie ont permis d'établir que le pluton de Creignish Hills (Lanière de Bras d'Or, centre de l'ile du Cap Breton) se compose surtout de tonalite-diorite et d'unités de monzogranite à grain grossier, avec des llots de granodiorite-tonalite, granodiorite-monzogranite et monzogranite à grain fin. Les caractéres de sa pérographie et de son chimisme suggèrent que le variation prisente au sein de la tonalite-diorite émane pour une grande part du fractionnement de la hornblende (± accumulation de biotite). En revanche, la variation observée dans chacune des autres unités provient surtout du fractionnement du feldspath. On ...
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