The Lac-St-Jean Anorthosite in the St-Henri-de-Taillon area (Grenville Province); a relic of a layered complex

The Lac-St-Jean anorthosite massif underlies 20 000 km (super 2) of the 'Allochthonous Polycyclic Belt' of the Grenville Province in Quebec. This heterogeneously deformed massif was emplaced during a major extensional episode that predated the Grenvillian orogeny. In the SW part of the massif, an almost undeformed subcircular layered complex approx 10 km in diameter was identified and subdivided into four major units. This complex, which shows strong similarities with the anorthosite massifs of the Nain and Churchill provinces, consists largely of variable amounts of anorthosite and leucotroctolite interlayered at a decimetric to decametric scale. The rocks consist predominantly of large homogeneous cumulus plagioclase (approx An (sub 60) ) and of six intercumulus minerals: reversely zoned plagioclase (An (sub 59- 83) ), olivine (approx Fo (sub 69) ), minor amounts of orthopyroxene (approx En (sub 72) ), clinopyroxene (approx Wo (sub 45) En (sub 44) ), Fe-Ti oxide minerals and apatite. No significant cryptic layering was observed from the bottom to the top of the suite, possibly implying that only a small part of the huge layered complex is exposed. The near-cotectic proportions of plagioclase and olivine suggest that the intercumulus material represents a trapped liquid composition that crystallized isobarically. The magma and its entrained plagioclase are thought to have originated at the crust-mantle interface. [Authors' abstract]