Plagioclase lamellae in hypersthene,Tikkoatokhakh Bay,Labrador

Abundant lamellae of plagioclase are present in the (100) planes of hypersthene megacrysts in andesine anorthosite along Tikkoatokhakh Bay, northwest of Nain, Labrador. Spongy intergrowths of plagioclase in hypersthene also occur. Plagioclase lamellae have mean compositions ranging from An₄₃ to An₉₂, with extreme compositions from An₃₉ to An₉₇; the calcic compositions are the more abundant. Such lamellae are always accompanied in the hypersthene by grains or lamellar segments of magnetite, and rarely by lamellae of olivine, augite, magnetite, or ilmenite. Some calcic plagioclase lamellae contain antiperthitic spindles of orthoclase. The host rocks of the hypersthene megacrysts are layered leuconorites and anorthosites with mean plagioclase compositions ranging from An₄₁ to An₅₅. The plagioclase lamellae in hypersthene are characteristically much more calcic than the host-rock plagioclase. There is little doubt that the lamellae exsolved from a pyroxene host, dominantly by a coupled redox reaction which generated magnetite, thereby releasing silica to combine with the Ca-Tschermak and jadeite components of the precursor pyroxene. Rapid growth of megacrysts may account for their aluminous nature.