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

Abstract

The Aillik Bay lamprophyre dyke swarm comprises abundant sannaites, plus rarer olivine sannaites, aillikites and carbonatites. Sannaites are characterized by phenocrysts of Ti-Al titansalite plus rare olivine and phlogopite in a groundmass dominated by pyroxene, biotite, titanomagnetite and K-feldspar; minor mineral phases may include apatite, nepheline, analcite, 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 rare. 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 mineralogical and geochemical characteristics of the aillikites are distinct from those of kimberlites. New 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. -J.M.H.