Early Carboniferous gabbro and basalt in the St. Peters area, southern Cape Breton Island, Nova Scotia
DOI:
https://doi.org/10.4138/2132Abstract
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. RÉSUMÉ De petites intrusions ignées gabbrolques et de faibles quantités de roches volcano-mafiques sont présentes dans la région de St. Peters, dans le sud-ouest de File du Cap-Breton. Une datation au U-Pb a partir de zircon et de baddeleyite révèle que le gabbro est êge de 339 ± 2 Ma et qu'il remonte done au Carbonifère inférieur, similairement aux unités setlimentaires des groupes de Horton et de Windsor. Les gabbros ont été penetnis a une faible profondeur, conformement à leur association avec les roches volcano-mafiques. Des études pétrologiques permettent de supposer que l'évolution des gabbros a comporté un fractionnement d'olivine et de pyroxene riches en Mg, mais pas de fractionnement de plagioclase important. Les caractéristiques chimiques de la sirie ont été particllcment modifiers par alteration, comme le demontrent l'albitisation du plagioclase et les zones d'alteration du carbonate, mais elles ré vèlent generalement une compaction tectonique intra-plaque continentale et une affinité thoteiitique. Le magma parental a pu être appauvri d'éléments 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'activité ignée generate du milieu du Dévonien au Carbonifère dans les provinces de l'Atlantique, laquelle est apparemment reliée a l'extension survenue pendant le développement du bassin des Maritimes. [Traduit par la rédaction]Downloads
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