Mantle-Derived Dunite and Lherzolite Nodules from Ubekendt Ejland, West Greenland Tertiary Province

Jørgen Gutzon Larsen
Geologisk Centralinstitut, Øster Voidgade 10, 1350 Copenhagen, Denmark*
*Present address: Danmarks Geologiske Undersøgelse, Thoravej 31, 2400 Copenhagen NV.

Abstract: Scattered dunite and lherzolite nodules occur in one of the youngest basanitoid lavas on Ubekendt Ejland. They have protogranular to porphyroclastic textures. The dunites are composed of olivine (Fo93.2−91.9b), enstatite (En93.4−92.8) low in Al2O3 and CaO, and Cr-spinel (61-13% Cr2O3 and 3–55% Al2O3). A solitary lherzolite module has olivine (Fo94.7–94.1), enstatite (En94.7–94.2), Cr-rich spinel, Ti-phlogopite (11% TiO2), and hyalophane. Petrographic evidence suggests that the two latter minerals have not been introduced by magmatic injection from the host in spite of the refractory nature of the coexisting phases, and metasomatic processes prior to the last deformation are therefore indicated. Partial melting of such mantle material would presumably produce small amounts of alkaline liquids even at very high temperatures. Another lherzolite nodule from a lamprophyre dyke has minerals with lower Mg/(Mg + Fe) ratios which, together with its preserved igneous textures, suggest a high-pressure precipitate. The lowest well-esablished equilibrium temperatures of 700–830°C for both dunites and lherzolites indicate a pressure regime of 12·17 kbar, according to the oceanic geotherm, whereas unrealistically high pressure (20–5 kbar) are suggested using the continental shield geotherm.

Mineralogical Magazine; September 1982 v. 46; no. 340; p. 329-336; DOI: 10.1180/minmag.1982.046.340.05
© 1982, The Mineralogical Society
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