Alkali picrites formed by melting of old metasomatized lithospheric mantle: Manîtdlat member, vaigat formation, Palaeocene of West Greenland

Abstract

Alkaline picrites and basalts constitute 20-200 m of lava flows and hyaloclastites in the middle part of an ∼ 2 km thick succession of tholeiitic picrites and basalts formed during continental rifting of West Greenland around 60 Ma. The alkaline rocks, found only in northern Disko, have phenocrysts of olivine + chromite ± clinopyroxene; lava flows contain abundant groundmass clinopyroxene and plagioclase, whereas pillow breccias contain abundant fresh alkali basaltic glass. Six compositional types are present; all are strongly but variably enriched in incompatible trace elements [Ba, U, Nb, Ta, light rare earth elements (LREE)], yet their major elements, with relatively high SiO2 and Al2O3 and low Na2O, do not suggest an origin by small degrees of mantle melting. The isotope compositions are unusual, with negative εNd and mostly negative εSr (below the mantle array), high 206Pb/204Ph (below the Northern Hemisphere Reference Line), and mostly negative γOs. The most likely source for the alkaline magmas is old metasomatized lithospheric mantle in which melting was induced by the passing hot, asthenosphere-derived, tholeiitic magmas. Simple mass-balance calculations suggest that the melting assemblages consisted of ∼ 60% pargasitic amphibole, 26-30% clinopyroxene, ∼ 9% olivine and ∼ 1% apatite. Mica in the source is required for only the least enriched magma type. For the most enriched magmas small amounts of Ba-U-Nb-Sr-LREE-rich oxides (lindsleyite and hawthorneite) are required in the melting assemblage and dominate the Pb isotope compositions. The various magma types and the partly complementary relation between them suggest that the lithospheric mantle had an ordered structure, possibly with old metasomatic zones formed by successive trapping of elements in migrating fluids.