244-11 Hydration of lower crust and its contribution to uplift of the Colorado Plateau

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

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The cause and timing of Colorado Plateau uplift are widely debated, with proposed mechanisms ranging from mantle-driven dynamic uplift in response to lithosphere delamination, lithospheric thickening by mid-crustal flow, and isostatic response to dedensification by thermal or compositional changes to the lower lithosphere. The Navajo Volcanic Field (NVF) in the central Colorado Plateau hosts a variety of lower crustal and lithospheric mantle xenoliths. Detailed petrologic and geochemical analysis of NVF xenoliths yields insights into the evolution of continental lithosphere and possible mechanisms of plateau uplift. A large portion of NVF xenoliths exhibit secondary, hydrous alteration, with water contents exceeding those of typical high-grade metamorphic rocks of the lower lithosphere (i.e., up to 7 wt % H₂O in mantle xenoliths and 4 wt % H₂O in lower crustal xenoliths). Density reduction, calculated from published modal mineral estimates of altered xenoliths, ranges from ~50 to 300 kg/m³ and could be responsible for up to 1.5 km of Colorado Plateau uplift. Integrating published geochronologic data of NVF xenoliths with isostatic uplift models suggests hydration of lower lithosphere occurred from late-Cretaceous through early-Oligocene, and uplift associated with hydrous alteration likely continued through the middle-Miocene.

Secondary hydrous minerals in felsic to intermediate lower crustal xenoliths include sericite after plagioclase and garnet, and chlorite after biotite and hornblende. Mafic granulite xenoliths contain abundant retrograde actinolite, which is largely a product of pyroxene and hornblende alteration in the presence of aqueous fluids. The source of metasomatic fluids in Colorado Plateau lower crust has been constrained using H stable isotope data (δ²H values) of hydrous minerals from the alteration assemblage. Secondary biotite, chlorite, and actinolite from lower crustal xenoliths reveal δ²H values of -67 ‰ to -41 ‰, compared to δ²H values of primary biotite of ~-77 ‰. Calculated δ²H values of aqueous fluids in equilibrium with secondary hydrous phases range from -28 ‰ to -13 ‰, consistent with estimates of δ²H values for slab-derived fluids in modern subduction settings. These data support previous interpretations of seismic, geochronologic, and petrologic data, which suggest that hydration of the lower crust and lithospheric mantle beneath the Colorado Plateau was the result of slab dehydration during Laramide flat-subduction, and that dedensification resulting from hydrous metasomatism was at least partially responsible for uplift of the Colorado Plateau.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8261

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