Calcic myrmekite: possible evidence for the involvement of water during the evolution of andesine anorthosite from St-Urbain, Quebec.

Vermicular intergrowths of quartz + plagioclase feldspar are found in andesine anorthosite from the St-Urbain massif, Quebec, Canada, where they comprise = or < approx 1 vol.% of individual samples. Such 'myrmekite' occurs along plagioclase grain- boundaries of the host anorthosite as bulbous aggregates (= or < approx 1 mm across) to thin films (approx 10-100 mu m wide). Plagioclase in the myrmekites ranges from An₆₂ to An₉₂, with most compositions near An₈₀. The host plagioclase is an antiperthite with composition An_{40 + or - 3}. Quartz contents of the myrmekites average about 23% by volume. Boundaries between myrmekite and host plagioclase are sharp. These textural and chemical features suggest that the boundaries represent an arrested reaction-front, where andesine was replaced by calcic plagioclase accompanied by precipitation of quartz. Calculation shows that the conversion of An₄₀ to An₈₀ (conserving Al) yields almost exactly the amount of quartz observed to occur in the myrmekites. The calcic myrmekites are interpreted to be the products of corrosive interaction between cumulate plagioclase crystals and a magmatically derived, high-T aqueous fluid. -L.T.T.