Abundant garnet-clinopyroxene-plagioclase-hornblende granulite xenoliths in minette and serpentinite diatremes of the Colorado Plateau are mostly of olivine tholeiitic composition and display slight LREE enrichment and moderate Eu anomalies. Although mineral assemblages and equilibration temperatures (∼600 °C) are compatible with a lower-crustal origin, the very low jadeite contents of the clinopyroxenes (0% to 2%) suggest equilibration at relatively shallow depth (possibly 9 to 15 km). Xenoliths of sillimanite-garnet-biotite schist, felsic granulite, and potassic granite may represent subordinate components of the granulite terrane. Extensive alteration under greenschist-facies conditions affected the granulites in the serpentinite diatremes but not those in the minettes. This alteration is suggested to be a result of “stewing” of the xenoliths in the serpentinite host, possibly during slow diapiric emplacement of the serpentinite along deep fault zones prior to the final stage of fluidized eruption that formed the diatremes.