IMPACTS OF LARAMIDE CRUSTAL HYDRATION ON COLORADO PLATEAU ELEVATIONS: UPDATES FROM CRUSTAL XENOLITH STUDIES FROM THE NAVAJO VOLCANIC FIELD (COLORADO, UTAH, ARIZONA, NEW MEXICO) AND THE HENRY MOUNTAINS, UTAH, USA

Laramide-aged metasomatic effects on the composition and physical properties of western North America lithosphere are still poorly known. One proposed mechanism for producing some of the modern 2 km elevation of the Colorado Plateau is lithospheric hydration, where mantle and crustal density reduction induced isostatic elevation gain. This is an update on xenolith studies of these processes in central Colorado Plateau crust. Xenoliths from the Oligocene Navajo Volcanic Field (NVF) from the Four Corners region have a range of hydration textures but commonly retain enough unaltered portions to quantify both pre-hydration (M₁) and syn-hydration (M₂) P-T conditions and densities. M₁ conditions (Proterozoic) range from 0.5 to 1.2 GPa and 650 to 1050 °C and those for M₂ are 400-500 °C at similar pressures. This is consistent with a Proterozoic Moho T near 1000 °C and near 600 °C in the late Cretaceous, the latter being closer to modern estimates for the interior of the plateau derived from P_n velocities. Hydrogen isotope data from secondary phases (e.g., actinolite, zoisite, muscovite) in >20 samples, and the calculated composition of the water that was in equilibrium with these minerals (d²H -50 to +5 ‰ vs. VSMOW), along with late Cretaceous Th-Pb dates from M₂ monazite in one xenolith, are consistent with a fluid source from a shallowly subducting Farallon slab. Our xenolith-based estimates of the hydration-induced surface elevation change suggest that felsic lithologies have minimal impact (less than +100 m if 20-25 km of hydrated lower crust were that composition) whereas intermediate and mafic lithologies would induce higher values of up to about +1000 m. The latter compositions are probably more important since published seismic studies and model crustal columns suggest relatively mafic plateau deep crust. This range includes prior estimates of 290 m and 900 m from studies that are based on model bulk crustal compositions or geophysical density models, respectively. The Oligocene Henry Mountains laccolith complex is ~150 km NW of the NVF and provides a new opportunity to evaluate the spatial extent of crustal hydration. The xenoliths are dominantly mafic and textures indicate that some have early pyroxene and/or garnet with amphibole reaction rims suggesting a history involving hydration prior to upper crustal emplacement.