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.

Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

“…Therefore, the much-publicised gradations from melilitites, through aln oites to aillikites (Rock, 1986(Rock, ,1991 may be due to progressively increasing role for carbonate-bearing veins. Further evidence for similarities in the evolution of the respective melts may be seen in the fractionation control by olivine and spinel in potassic lamprophyres of West Greenland (Scott, 1979), and similar disequilibrium olivine phenocrysts and Cr-spinels in the type aillikites (Malpas et al, 1986). Compositionally similar spinel grains occur as inclusions in olivine phenocrysts in melilitites of South Africa (Fig.…”

“…UML may be subdivided, following Rock (1986Rock ( , 1991, into two common types de®ned by the predominance of either melilite or carbonate in the groundmass as aln oite (v. Eckermann, 1966) and aillikite (Malpas et al, 1986), respectively. Rarer types include polzenite (melilite and feldspathoids in the groundmass), ouachitite (feldspathoids and carbonates) and damkjernite (feldspathoids, carbonates and rare alkali feldspars).…”

The Mlindi ring structure. An example of an ultrapotassic pyroxenite to syenite differentiated complex

“…Unlike the pyroxenite, this rock shows approximately equivalent K20 and Na20 values (respectively 2.0 and 2.25%) and thus cannot be classed as an ultrapotassic rock. It displays geochemical analogies with alkaline lamprophyre (ROCK, 1987) and has characteristics in common with the sannaite from Aillik Bay (MALPAS et al, 1986) -i.e. alkaline lamprophyre richer in normative orthoclase than in nepheline -associated, in this case, with olivine-mica lamprophyre.…”

Geochemistry and age of the Aillik Group and associated plutonic rocks, Makkovik Bay area, Labrador: implications for tectonic development of the Makkovik Province

“…The Aillik Group in the study area, as elsewhere in the Aillik domain, has been intruded by other granitoid bodies and a variety of dykes, including lamprophyre (Gower et al 1982;Malpas et al 1986). The clearly younger units were generally not included in the present study.…”