Alkaline mafic and ultramafic lamprophyres from the Aillik Bay area, Labrador

Abstract

The Aillik Bay lamprophyric dyke swarm comprises abundant sannaites, plus rarer olivine sannaites, aillikites and carbonatites. Sannaites are characterized by phenocrysts of Ti–Al titansalite plus rarer olivine and phiogopite in a groundmass dominated by pyroxene, biotite, titanomagnetite, and K-feldspar. Minor mineral phases may include apatite, nepheline, analcime, carbonate, rutile, and pyrite. Most sannaites contain well-developed leucocratic ocelli, which are commonly zoned. Olivine sannaites are similar, but olivine (Fo77–85) is more abundant, and ocelli are rarer. Both types occur together as banded dykes, and calculations support the suggestion of a common parental melt.Aillikites have zoned phenocrysts of olivine (Fo71–87) and mica in a groundmass of carbonate, apatite, mica, titanomagnetite, and perovskite. Carbonatite dykes usually exhibit textures with features akin to those of the aillikites. The mineraiogical and geochemical characteristics of the aillikites are distinct from those of kimberlites.It is the intent of this paper to provide new data on these rocks. These data are used to support one of several viable models for the generation of all dyke-rock types by partial melting of LREE-enriched mantle at depths of 90–110 km. Carbonatite dykes are believed to represent the smallest melt fraction but are the latest emplaced dykes. These small melt fractions may have followed structural weaknesses created by earlier lamprophyre magmatism.