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Small gabbroic plutons and minor mafic volcanic rocks occur in the St Peters area of southwestern Cape Breton Island. U-Pb dating of zircon and baddeleyite show that the gabbro has an Early Carboniferous age of 339 ± 2 Ma, similar to the age of the surrounding sedimentary units of the Horton and Windsor groups. The gabbros were intruded at shallow depth, consistent with their association with minor mafic volcanic rocks. Penological studies suggest that evolution of the gabbros involved fractionation of Mg-rich olivine and pyroxene, but not significant plagioclase. The chemical characteristics of the suite have been partly modified by alteration, evidenced in albitization of plagioclase and zones of carbonate alteration, but generally indicate a continental within-plato tectonic setting and tholeiitic affinity. The parent magma may have been depleted in heavy rare-earth elements, and was possibly derived from a garnet-bearing mantle source. The St Peters gabbros and basalts are part of widespread mid-Devonian to Carboniferous igneous activity in Atlantic Canada, apparently related to extension during the development of the Maritimes Basin.  RESUME  De petites intrusions ignees gabbrolques et de faibles quantites de roches volcano-mafiques sont presentes dans la region de St. Peters, dans le sud-ouest de File du Cap-Breton. Une datation au U-Pb a partir de zircon et de baddeleyite revele que le gabbro est ege de 339 ± 2 Ma et qu'il remonte done au Carbonifere inferieur, similairement aux unites setlimentaires des groupes de Horton et de Windsor. Les gabbros ont ete penetnis a une faible profondeur, conformement a leur association avec les roches volcano-mafiques. Des etudes petrologiques permettent de supposer que l'evolution des gabbros a comporte un fractionnement d'olivine et de pyroxene riches en Mg, mais pas de fractionnement de plagioclase important. Les caracteristiques chimiques de la sirie ont ete particllcment modifiers par alteration, comme le demontrent l'albitisation du plagioclase et les zones d'alteration du carbonate, mais elles re velent generalement une compaction tectonique intra-plaque continentale et une affinite thoteiitique. Le magma parental a pu etre appauvri d'elements lourds des terres rares et il provient possiblement d'une source mantetique grenatifere. Les gabbros et les basaltes de St. Peters font partie de l'activite ignee generate du milieu du Devonien au Carbonifere dans les provinces de l'Atlantique, laquelle est apparemment reliee a l'extension survenue pendant le developpement du bassin des Maritimes.  [Traduit par la redaction]

Early Carboniferous gabbro and basalt in the St. Peters area, southern Cape Breton Island, Nova Scotia