Mount Early and Sheridan Bluff (87 degrees south) are the above-ice expression of Earth’s southernmost volcanic field and are isolated by more than 1000 km from any other exposed Cenozoic volcano in Antarctica. These monogenetic, Early Miocene volcanoes are constructed mostly of olivine-phyric basaltic pillow lavas and breccias (Mount Early) and pāhoehoe lavas (Sheridan Bluff) whose differentiation is controlled by the fractional crystallization of olivine with lesser quantities of clinopyroxene, plagioclase and magnetite. Fractional crystallization or contamination by crust cannot account for the coexistence of olivine tholeiite and alkaline compositions but their relationship can be explained by change from higher (5-6%) to lower (1.5-2%) degrees of mantle partial melting, respectively, of a source that is heterogeneous on a small-scale. Both alkaline and subalkaline magma types have geochemical and isotopic signatures that differentiate them from volcanism of the West Antarctic rift system. Data trends in Sr-Nd-Pb isotope space along with major and trace element characteristics indicate mixing of at least two-distinct mantle sources; 1) a relatively depleted component similar to sources for mid-ocean ridge basalt from the extinct Antarctic-Phoenix spreading center, and 2) an enriched component similar to sources for mafic magmas of the Jurassic Karoo‒Ferrar large igneous provinces. The availability of these two mantle source types was facilitated by the detachment, sinking and heating of metasomatized continental lithosphere (enriched source) that released volatiles into the surrounding asthenosphere (depleted source) to promote flux melting. Volcanism triggered by lithospheric detachment is therefore uniquely applied to Mount Early and Sheridan Bluff to explain their isolation and enigmatic tectonic setting but can also account for source heterogeneity and the ephemeral change in degree of mantle partial melting recorded in their mafic compositions.