Abstract

A dozen ultramafic lamprophyre dykes have been identified in the eastern part of Ungava Bay, Québec. The dark grey dykes are composed of macrocrysts of olivine and phlogopite featuring tetraferriphlogopite rims. These minerals are included in a matrix consisting of fine-grained phlogopite, olivine, spinel, and interstitial carbonate. The geochemical composition, mineral assemblage, chemical analyses of the xenocrysts and phenocrysts and Sm–Nd isotopic signature indicate that these rocks are carbonated ultramafic lamprophyre dykes. The chemical zonation of the micas and the presence of andradite suggest a complex sequence of crystallization. An Ar isotope correlation analysis indicates an age of approximately 550 Ma. The dykes have intruded the Tasiuyak gneiss, a suite of Paleoproterozoic metasedimentary rocks. This unit occupies the Torngat Orogen collision zone between the Rae Province and the Nain Province. The dykes are considered to have been emplaced within tension gashes during reactivation of major Paleoproterozoic structures and fractures associated with the opening of the Iapetus Ocean. They are possibly related to a mantle plume that simultaneously caused both the rupture of the Laurentia and the formation of the ultramafic magma. The Abloviak lamprophyres have quite similar analogues in southwest Greenland. The age, geochemical characteristics, isotopic signature common to all these dykes, the geological environment of the southwest Greenland are all comparable with the data presented here for the Abloviak lamprophyres. Furthermore their pre-drift geographical proximity, prior to the opening of the Labrador Sea, suggests that they are related to a common magmatic event which constitutes a diamondiferous alkaline province located in northeastern North America.

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