Xenolithic evidence for Proterozoic crustal evolution beneath the ColoradoPlateau

Tertiary diatremes of the Navajo volcanic field brought a wide variety of P roterozoic xenoliths to the surface of the Colorado Plateau, Examination of crustal xenoliths from the Navajo volcanic held diatremes permits reconstr uction of Proterozoic pressure-temperature (P-T) histories beneath the Colo rado Plateau, Diatremes from the northwest part of the Navajo volcanic fiel d carry the greatest variety of xenoliths, including metasedimentary rocks, amphibolites, felsic gneisses, mafic granulites, and crustally derived eclo gites; these rock types show variable degrees of hydrous alteration and evi dence for complex reaction histories. In contrast, diatremes from the south east part of the Navajo volcanic held contain primarily mafic and felsic gr anulites that show fewer reaction textures and less alteration; metasedimen tary and eclogitic xenoliths are absent in these diatremes, The P-T paths f rom the northwest xenoliths are counterclockwise and reach temperatures as high as 850 degrees C and pressures greater than or equal to 10 kbar, Later hydrous alteration occurred around 500 degrees C and 8-12 kbar, Xenoliths from the southeast diatremes preserve little evidence of their P-T evolutio n, but paths involve heating and/or decompression. The differing rock types , P-T paths, and alteration histories of the northwest and southeast popula tions suggest that Proterozoic tectonism juxtaposed two distinct crustal bl ocks beneath the Colorado Plateau, Previous workers have postulated that th e boundary between the Proterozoic Yavapai and Mazatzal provinces occurs in the region; our data support the existence of a northeast-trending boundar y 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 dippin g during Proterozoic accretion of the Mazatzal province onto North America.