Impact of Metasomatism on Colorado Plateau Lower Crustal density: Insights from Xenoliths from the Moses Rock and Mule Ear Diatremes, Navajo Volcanic Field

The Colorado Plateau has abnormally high topography, with 2 km of post-Cretaceous surface uplift that cannot be explained from shortening during the Laramide orogeny alone. Among several previously proposed mechanisms is a syn-Laramide deep crustal hydration event with fluids potentially sourced from the subducted Farallon slab. The resulting metasomatic alteration could have decreased lower crustal density, which would result in isostatic uplift and contribute to the apparent excess increase in elevation. Lower crustal xenoliths provide important constraints on modern lower crustal conditions in combination with geophysical data of bulk lithology and lower crustal density models. Xenoliths from the Navajo Volcanic Field (central Colorado Plateau) contain a petrologic record of lower crustal hydration and associated retrograde metamorphism. Four metasomatized xenoliths from the Moses Rock and Mule Ear diatremes were used to construct pre- and post- hydration density models for discrete lithologies at lower crustal P and T conditions. Density is estimated to have changed by ca. +0.3% to -6.9% with hydration, dependent on lithology and estimated retrograde reactions during hydration. Lithology-dependent changes in density with hydration have consequences for estimating related uplift, and imply better constraints are needed on the lithological makeup of the Colorado Plateau lower crust.