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

An ultramafic lamprophyre dyke is described from the otherwise tholeiitic F errar 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. H owever, 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 pe rovskite and spinel microphenocrysts in a groundmass of amphibole, nephelin e and biotite. Carbonatitic globules contain calcite, dolomite, Fe-rich car bonate, 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 simila r initial Nd-143/Nd-144 to OIB, notably Bouvet, Crozet and Reunion, but sig nificantly higher initial Sr-87/Sr-86. Th, lamprophyre magma is interpreted as having been generated by low-degree partial fusion of metasomatized lit hospheric mantle as a result of heat conducted from an underlying Jurassic mantle plume. The same mantle plume was probably also responsible for gener ating one of the world's largest layered gabbro bodies, the Dufek-Forrestal intrusions.