Mafic granulite facies xenoliths found in the Pleistocene-Recent alkali basalts of southern Patagonia, South America, have an unfoliated granoblastic matrix with the mineral assemblage clinopyroxene + orthopyroxene + plagioclase + or - olivine + or - titanomagnetite. Wherever olivine and plagioclase are in close proximity to one another they are separated by symplectic intergrowths of pyroxenes and spinel. Two-pyroxene geothermometry indicates that the granoblastic matrix phases equilibrated at approximately 900 degrees C. The pyroxene-spinel symplectites formed at 860 degrees C, at which temperature the reaction boundary of olivine + plagioclase to produce pyroxenes + spinel occurs at 5-7 kbar. The xenoliths are interpreted as fragments of a gabbroic pluton which exists at a depth of approximately 20 km in the crust. This is consistent with suggestions that mafic igneous rocks comprise a large portion of the lower continental crust. Capture of the xenoliths in the host basalt must have resulted in extensive heating, but the petrologic evidence of this event is restricted to heterogeneously distributed high temperature oxidation effects.--Modified journal abstract.