Mid-Tertiary ultramafic microbreccia diatremes from the Navajo volcanic field in the Four Corners area of the Colorado Plateau contain an ultramafic xenoliths suite in which lower-temperature, prograde-metamorphic eclogites are associated with higher-temperature garnet clinopyroxenites that were retrograded and hydrated to chlorite eclogite. Assemblages and directions of metamorphic reactions are consistent with the model of a Laramide age inverted metamorphic gradient established as prograde reactions in a dehydrating oceanic slab caused hydration and cooling in the overlying, subcontinental upper mantle wedge. Comparison with an inverse metamorphic gradient in the Pelona schists, inferred to represent an exposed part of the same subduction system in southwestern Arizona and southern California, confirms the shallow dip of the Laramide subduction zone predicted by the regional distribution of subduction-related volcanic rocks. ¿ American Geophysical Union 1991