Tertiary diatremes of the Navajo volcanic field brought a wide variety of Proterozoic xenoliths to the surface of the Colorado Plateau. Examination of crustal xenoliths from the Navajo volcanic field diatremes permits reconstruction of Proterozoic pressure-temperature (P-T) histories beneath the Colorado Plateau. Diatremes from the northwest part of the Navajo volcanic field carry the greatest variety of xenoliths, including metasedimentary rocks, amphibolites, felsic gneisses, mafic granulites, and crustally derived eclogites; these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories. In contrast, diatremes from the southeast part of the Navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration; metasedimentary and eclogitic xenoliths are absent in these diatremes. The P-T paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 °C and pressures equal to or greater than 10 kbar. Later hydrous alteration occurred around 500 °C and 8–12 kbar. Xenoliths from the southeast diatremes preserve little evidence of their P-T evolution, but paths involve heating and/or decompression. The differing rock types, P-T paths, and alteration histories of the northwest and southeast populations suggest that Proterozoic tectonism juxtaposed two distinct crustal blocks beneath the Colorado Plateau. Previous workers have postulated that the boundary between the Proterozoic Yavapai and Mazatzal provinces occurs in the region; our data support the existence of a northeast-trending boundary beneath the Four Corners area. If the eclogitic metamorphism and hydrous alteration are Proterozoic in age, their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during Proterozoic accretion of the Mazatzal province onto North America.