Date of Award

Spring 1-1-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

First Advisor

Kevin H. Mahan

Second Advisor

Craig H. Jones

Third Advisor

G. Lang Farmer

Abstract

High-temperature, high-pressure mineral assemblages preserved in much of the North American lithosphere owe their origins to Archean and Proterozoic tectonic processes. Whether subsequent mechanical, thermal, or chemical modification of ancient lithosphere affects overlying crust and the extent to which such processes contribute to anomalous deformation and topography in the interior of continents is poorly understood. This study addresses the occurrence and effects of hydration on continental crust in producing regionally elevated topography in the Colorado Plateau since the Late Cretaceous.

Mineralogical characteristics of two deep crustal xenoliths (GR-11 and RM-21) from the Four Corners Volcanic field record varying degrees of hydrous alteration including extensive replacement of garnet by hornblende, secondary albite and phengite growth at the expense of primary plagioclase, and secondary monazite growth in association with fluid-related allanite and plagioclase breakdown. Results from forward petrological modeling for both deep crustal xenoliths are consistent with hydration at ≥20 km depth prior to exhumation in the ~20 Ma volcanic host. In situ Th/Pb dating provides evidence for a finite period of fluid-related monazite crystallization in xenolith RM-21 from 91 ± 2.8 Ma to 58 ± 4 Ma, concurrent with timing estimates of low-angle subduction of the Farallon slab.

Hydration-related reactions at depth lead to a net density decrease as low-density hydrous phases (hbl±ab±phg) grow at the expense of high-density, anhydrous minerals (gt±pl) 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 xenolith GR-11 for an ~25 km thick crustal layer yield uplift estimates on the order of hundreds of meters associated with phase changes at depth. The results of this study substantiate the hypothesis that chemical alteration of lower continental crust by slab-derived fluids played a role in producing Laramide-related surface uplift of the Colorado Plateau and establishes chemical modification of continental lithosphere as a credible possibility for producing elevated regional topography in continental interiors.

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