Crystallization Sequence and Magma Chamber Processes in the Ferrobasaltic Sept Iles Layered Intrusion, Canada

The Sept Iles layered intrusion (Quebec, Canada; 564 Ma) is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro, and an upper border series with anorthosite, capped by cupolas of A-type granite. Chilled margin compositions suggest a ferrobasaltic parental magma close to that of the Skaergaard intrusion, but much richer in iron and titanium. Samples from drill-cores and surface sampling of the 4·7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 Fe–Ti oxide layers cm- to m-thick and many anorthositic autolithic blocks. The sequence of crystallization in the layered series is: plagioclase (An72–34) and olivine (Fo75–21) followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. An olivine gap is observed between Fo66 and Fo59. The saturation of Fe–Ti oxides before Ca-rich pyroxene is interpreted to be the result of the high FeOt and TiO2 contents and the low CaO content of the parental magma. Contamination by old continental crust has occurred during crystallization of the layered series, as indicated by Sr isotopic compositions (87Sr/86Sr564 = 0·70360–0·70497). The differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases, Cr content of magnetite and lower Sr isotope ratios. These reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma.

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