ABSTRACT

Major and trace elements have been determined in spinel lherzolite, spinel pyroxenite and garnet websterite xenoliths from the Hoggar and in their host basanites and nephelinites. Transition metal and REE abundances do not support a genetic link between these ultramafic rocks and the lavas erupted at the surface. Mineral chemistry and textures suggest a complex evolution and subsolidus re-equilibration in the range 800–1, 000°C. High-density CO₂ fluid-inclusions define minimum pressures of 10–12kb. Thus the ultramafic xenoliths came from a relatively low pressure regime within an inhomogeneous lithospheric mantle segment characterized by a disturbed geotherm.