Paper No. 2
Presentation Time: 8:30 AM


BUTCHER, Lesley Ann, Dept. of Geological Sciences, University of Colorado - Boulder, CB 399, Boulder, CO 80309, MAHAN, Kevin, Dept Geol. Sciences, University of Colorado, Boulder, CO 80309, JONES, Craig H., Dept. of Geological Sciences & CIRES, University of Colorado - Boulder, CB 399, Boulder, CO 80309-0399 and FARMER, G. Lang, Department of Geological Sciences, University of Colorado, Boulder, CO 80309,

High-temperature, high-pressure mineral assemblages preserved in much of the North American lithosphere owe their origins to Archean and Proterozoic tectonic processes. The timing and extent to which subsequent mechanical, thermal, or chemical modification of ancient lithosphere may have occurred and how such processes contribute to anomalous deformation and topography in continental interiors is, however, poorly understood. Here we report new petrological and in situ geochronological data for a hydrated mid-crustal xenolith from the Four Corners region of the Colorado Plateau and investigate the effects of hydration in producing regionally elevated topography. A garnet biotite schist xenolith from the Navajo Volcanic field exhibits two distinct mineral assemblages: an early metamorphic assemblage (Gt + Bt + Ms + Pl + Kfs + Qtz) and a secondary assemblage (Ab + Ph + Cc + Rt + Ilm). The secondary assemblage variably replaces the peak assemblage, primarily at the expense of Kfs and Pl, and reflects late-stage fluid-alteration at depth; results from forward petrological modeling are consistent with hydration at ~25 km (0.65 GPa, 450°C) prior to exhumation. Th/Pb dating of monazite grains associated with fluid-related breakdown of Aln and Pl yields a wide range of dates, from the Paleoproterozoic to Paleocene, with a significant majority falling between 91 and 58 Ma. Late Cretaceous dates are interpreted to represent a finite period of monazite crystallization associated with hydrous alteration of the deep crust, possibly by fluids sourced from a shallowly subducting Farallon slab. These data also support crustal hydration as a mechanism for producing regionally elevated topography. Fluid-related reactions at depth can lead to a net density decrease as low-density hydrous phases (e.g. Ms + Amp + Cc) replace high-density, anhydrous minerals (e.g. Gt + Fsp + Opx + Cpx) abundant in unaltered Proterozoic crust. If these reactions are sufficiently pervasive and widespread, reductions in lower crustal density would provide a significant and quantifiable source of lithospheric buoyancy; calculations for density decreases associated with extensive hydration recorded in a garnet amphibolite xenolith from the Colorado Plateau yield uplift estimates on the order of hundreds of meters.