Fe and Ti in plagioclase from the Lac Saint–Jean anorthosite: Significance of the ⟨Si₄O₈ and AlAl₃SiO₈ end-members in plagioclase

Abstract

Phenocrysts of black plagioclase from the Lac Saint-Jean Proterozoic massif type anorthosite in Quebec, Canada were studied to explain the simultaneous existence of ilmenite, magnetite, hematite, hemo-ilmenite and fine unidentified Fe-Ti dust, and the unusual presence of the ⟨Si₄O₈ and AlAl₃SiO₈ feldspar end-members in the plagioclase. Our result indicate that significant amounts of Fe2+ were originally present in the tetrahedral and cavity sites of the plagioclase and that Ti⁴⁺ was also present in the tetrahedral site. We conclude that plagioclase in the Lac Saint-Jean anorthosite crystallized under fairly reducing conditions and later underwent postmagmatic oxidation when the plutonic mass was uplifted and came into contact with crustally-derived