Trace Element Evidence for Hydrous Metasomatism at the Base of the North American Lithosphere and Possible Association with Laramide Low‐Angle Subduction

Trace element signatures of mantle xenoliths along an east‐west transect extending from the Sierra Nevada to the Colorado Plateau were investigated in this study. Those beneath the Sierra Nevada are highly enriched in fluid‐mobile elements (Cs, Pb, U, and Sr) and, in particular, are characterized by low U/Pb and high Sr/Nd. These signatures imply that aqueous fluids derived by dehydration of seawater‐altered lithologies have been introduced into the Sierran xenoliths. Peridotite xenoliths from beneath the Colorado Plateau, lying ∼1000 km inboard, are also enriched in fluid‐mobile elements, but the extent is lower, the U/Pb ratios are high, the Sr/Nd ratios are low, and there is a strong enrichment in the light rare earth elements but negligible enrichment in the heavy rare earths. These features require the introduction of an aqueous component and a silicate melt component into the plateau xenoliths, with the latter, in particular, derived from a garnet‐rich source such as an eclogite that has already been stripped of Pb and Sr relative to U and Nd by earlier dehydration. This hypothesized eclogite source may represent subducted Farallon oceanic crust or preexisting Proterozoic oceanic crust. In both cases, a source of aqueous fluids from greater depths is required to explain the enrichment in fluid‐mobile elements and to sufficiently depress the eclogite solidus for melting to take place. Such fluids are suggested here to come from dehydration of serpentinite, whose most probable origin is the colder core of the shallowly subducting Farallon plate. The distinctive signatures of the Sierran xenoliths, by contrast, probably derive from dehydration of seawater‐altered oceanic crust earlier in the subduction sequence. These observations corroborate suggestions that low‐angle subduction during the Laramide orogeny may have hydrated a laterally extensive region of western North America. However, the vertical extent of hydration remains an open question.