A set of metamorphic mineral assemblages produced by metamorphism of basic igneous rocks under high-pressure, moderate-temperature conditions and typically characterized by the development of the mineral assemblage pyrope garnet and omphacite (an Na-rich, high-pressure pyroxene). Other rocks of contrasting composition, e.g. shales or limestones, are not known to show mineral assemblages characteristic of the high pressures and moderate temperatures involved. Eclogite facies can form when a slab of oceanic crust is subducted into the mantle. The basic igneous rocks of the slab can then be converted to a typical eclogite mineral assemblage.