SYN-LARAMIDE UHP METAMORPHISM UNDER THE COLORADO PLATEAU: XENOLITH EVIDENCE FOR PROTEROZOIC ACCRETIONARY TECTONICS OR FLAT SUBDUCTION OF THE FARALLON PLATE?

Syn-Laramide flat subduction of the Farallon plate was inferred in the 1970’s from (1)low-temperature eclogite xenoliths in Najavo diatremes of the Colorado Plateau, (2)spatial and temporal migration patterns of Late Cretaceous and Early Tertiary arc magmatism, and (3) plate tectonic reconstructions of the western margin of North America. Although much geological evidence has been cited in support of the flat subduction model, the origin of the eclogite xenoliths remains controversial, with some authors suggesting that they are samples of a Precambrian accretionary boundary rather than of the Mesozoic subducted Farallon plate. In order to constrain the timing of eclogite formation, we obtained zircon ages ranging from 81 to 33 Ma from coesite- and lawsonite-bearing eclogite xenoliths from the Moses Rock and Garnet Ridge diatremes and interpreted the two extremes as indicating the onset of eclogite crystallization during subduction-related UHP metamorphism and the age of recrystallization by the host magmatic event, respectively (Usui et al., 2003, Geology, vol. 31, 589-592). Smith et al. (2004, G3, vol 5, No. 4)found similarly young zircons (70 to 35 Ma) in three eclogite xenoliths also containing discordant zircon fractions with 207Pb/206Pb ages greater than 1000 Ma and concluded that all Navajo eclogites were derived from Proterozoic protoliths affected by eclogite-facies recrystallization catalyzed by fluids derived from a more than 200 km deep Farallon slab. While textural and metamorphic histories of certain Navajo eclogite types are compatible with derivation from a subcontinental mantle wedge,the lower-temperature, progressively metamorphosed, coesite and lawsonite-bearing varieties dated by us are better envisaged as fragments of lenses or imbricated slices of oceanic crust near the top of a shallower subducted Farallon plate (~ 100 km). Considering that the eclogites traveled eastward for at least 800 km between ca. 80 and 30 Ma, the minimum rate for the horizontal component of Farallon plate subduction is ~ 1.6 cm/y. Laramide tectonic models should thus take into account that the lower parts of the subcontinental Cordilleran lithosphere were replaced by Mesozoic oceanic lithosphere at least as far to the east as the Four Corners region of the Colorado Plateau, while the mantle wedge remaining above the slab was massively hydrated.