EARLY CARBONIFEROUS GABBRO AND BASALT IN THE ST-PETERS AREA, SOUTHERNCAPE-BRETON ISLAND, NOVA-SCOTIA

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 o f 339 +/- 2 Ma, similar to the age of the surrounding sedimentary unit s of the Horton and Windsor groups. The gabbros were intruded at shall ow depth, consistent with their association with minor mafic volcanic rocks. Petrological studies suggest that evolution of the gabbros invo lved fractionation of Mg-rich olivine and pyroxene, but not significan t plagioclase. The chemical characteristics of the suite have been par tly modified by alteration, evidenced in albitization of plagioclase a nd zones of carbonate alteration, but generally indicate a continental within-plate tectonic setting and tholeiitic affinity. The parent mag ma may have been depleted in heavy rare-earth elements, and was possib ly derived from a garnet-bearing mantle source. The St. Peters gabbros and basalts are part of widespread mid-Devonian to Carboniferous igne ous activity in Atlantic Canada, apparently related to extension durin g the development of the Maritimes Basin.