A lamproite stock with ellipsoidal phlogopite nodules at Oqaitsúnguit, Disko Bugt, central West Greenland

Abstract

A small stock containing a medium- to coarse-grained phlogopite-rich lamproite occurs in the Archaean Ata granitoid in West Greenland. It was emplaced by a combination of forceful intrusion and assimilation. Metasomatic alteration has taken place in the country rock near the stock and in xenoliths. Although there are similarities with other lamproites, the Oqaitsunguit lamproite differs with respect to (1) mineralogy. The main mineral phases are phlogopite, microcline, K-richterite, K-Mg-arfvedsonite, aegirine-augite and diopside; (2) coarse grain size; (3) low whole-rock TiO₂, FeO and SiO₂ contents; (4) low TiO₂ in phlogopites and amphiboles; (5) the presence of 0.5–6 cm long ellipsoidal phlogopite nodules, which occur scattered throughout the rock. Each phlogopite nodule is composed of a central phlogopite grain, which is enveloped by a mantle or shell of another phlogopite grain.