Journal of Petrology | Volume 44 | Number 1 | Pages 3-38 | 2003
© Oxford University Press 2003
Alkali Picrites Formed by Melting of Old Metasomatized Lithospheric Mantle: Manîtdlat Member, Vaigat Formation, Palaeocene of West Greenland
1GEOLOGICAL SURVEY OF DENMARK AND GREENLAND, ØSTER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK
2GEOLOGICAL MUSEUM, ØSTER VOLDGADE 5–7, DK-1350 COPENHAGEN K, DENMARK
3MINERALOGISK–GEOLOGISK MUSEUM, SARS GATE 1, N-0562 OSLO, NORWAY
4GEOLOGICAL INSTITUTE, UNIVERSITY OF COPENHAGEN, ØSTER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK
5DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK
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 
Nd and mostly negative Sr (below the mantle array), high 206Pb/204Pb (below the Northern Hemisphere Reference Line), and mostly negative 
Os. 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.
KEY WORDS: alkali picrite; amphibole melting; Greenland; lithosphere melting; metasomatism
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