GENESIS OF ULTRAMAFIC LAMPROPHYRES AND CARBONATITES AT AILLIK BAY, LABRADOR: A CONSEQUENCE OF INCIPIENT LITHOSPHERIC THINNING BENEATH THE NORTH ATLANTIC CRATON

Abstract

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite, damtjernite) and subordinate dolomite-bearing carbonatite with U-Pb perovskite emplacement ages of ~590-555 Ma occur in the vicinity of Aillik Bay, coastal Labrador. The ultramafic lamprophyres principally consist of olivine and phlogopite phenocrysts in a carbonate- or clinopyroxene-dominated groundmass. Ti-rich primary garnet (kimzeyite and Ti-andradite) typically occurs at the aillikite type locality and is considered diagnostic for ultramafic lamprophyre-carbonatite suites. Titanian aluminous phlogopite and clinopyroxene, as well as comparatively Al-enriched but Cr-Mg-poor spinel (Cr-number < 0.85), are compositionally distinct from analogous minerals in kimberlites, orangeites and olivine lamproites, indicating different magma geneses. The Aillik Bay ultramafic lamprophyres and carbonatites have variable but overlapping 87 Sr/ 86 Sr i ratios (0.70369-0.70662) and show a narrow range in initial εNd (+0.1 to +1.9) implying that they are related to a common type of parental magma with variable isotopic characteristics. Aillikite is closest to this primary magma composition in terms of MgO (~15-20 wt %) and Ni (~200-574 ppm) content; the abundant groundmass carbonate has δ 13 C PDB between -5.7 and -5‰, similar to primary mantle-derived carbonates, and δ 18 OSMOW from 9.4 to 11.6‰. Extensive melting of a garnet peridotite source region containing carbonate- and phlogopite-rich veins at ~4-7 GPa triggered by enhanced lithospheric extension can account for the volatile-bearing, potassic, incompatible element enriched and MgO-rich nature of the proto-aillikite magma. It is argued that low-degree potassic silicate to carbonatitic melts from upwelling asthenosphere infiltrated the cold base of the stretched lithosphere and solidified as veins, thereby crystallizing calcite and phlogopite that were not in equilibrium with peridotite. Continued Late Neoproterozoic lithospheric thinning, with progressive upwelling of the asthenosphere beneath a developing rift branch in this part of the North Atlantic craton, caused further veining and successive remelting of veins plus volatile-fluxed melting of the host fertile garnet peridotite, giving rise to long-lasting hybrid ultramafic lamprophyre magma production in conjunction with the break-up of the Rodinia supercontinent. Proto-aillikite magma reached the surface only after coating the uppermost mantle conduits with glimmeritic material, which caused minor alkali loss. At intrusion level, carbonate separation from this aillikite magma resulted in fractionated dolomite-bearing carbonatites (δ 13 C PDB -3.7 to -2.7‰) and carbonate-poor mela-aillikite residues. Damtjernites may be explained by liquid exsolution from alkali-rich proto-aillikite magma batches that moved through previously reaction-lined conduits at uppermost mantle depths. © 2006 Oxford University Press.