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Middle Jurassic ultramafic lamprophyre dyke within the Ferrar magmatic province, Pensacola Mountains, Antarctica

¹ British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
² Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
³ British Antarctic Survey, c/o NERC Isotope Geoscience Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

^* E-mail: p.leat{at}bas.ac.uk

An ultramafic lamprophyre dyke is described from the otherwise tholeiitic Ferrar magmatic province of Antarctica. We report an Ar-Ar age of 183 ± 2.2 Ma for the dyke, indistinguishable from those of the Ferrar tholeiites. However, the dyke has mineralogical and major and trace element compositions, and radiogenic isotopes ratios, very different from the Ferrar tholeiites. The sample consists of olivine and rare clinopyroxene phenocrysts with perovskite and spinel microphenocrysts in a groundmass of amphibole, nepheline and biotite. Carbonatitic globules contain calcite, dolomite, Fe-rich carbonate, nepheline, biotite, orthoclase, pyrite, clinopyroxene, apatite and silicate glass, and were formed by liquid immiscibility. The rock is mildly potassic and classifies as an ouachitite. It is strongly enriched in both moderately and highly incompatible trace elements and is the first high-Ti rock to be described from the Ferrar magmatic province. The rock has similar initial ¹⁴³Nd/¹⁴⁴Nd to OIB, notably Bouvet, Crozet and Réunion, but significantly higher initial ⁸⁷Sr/⁸⁶Sr. The lamprophyre magma is interpreted as having been generated by low-degree partial fusion of metasomatized lithospheric mantle as a result of heat conducted from an underlying Jurassic mantle plume. The same mantle plume was probably also responsible for generating one of the world’s largest layered gabbro bodies, the Dufek-Forrestal intrusions.

KEYWORDS: ultramafic, lamprophyre, dyke, Ferrar, Pensacola Mountains, Antarctica

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