Understanding the composition, origin, and evolution of the continental crust: Case studies in the southern Rio Grande rift, New Mexico, United States of Americaand the Coast Plutonic Complex, British Columbia, Canada

The work in this thesis examines the origin and evolution of the continental crust in two localities within the North American Cordillera: the Potrillo volcanic field, located in the southern Rio Grande rift in New Mexico, and the Coast Mountains of northwestern British Columbia. A reconstruction of the geologic and P-wave velocity profile through the entire crust and upper mantle beneath the Potrillo volcanic field suggests that supracrustal rocks are present throughout much of the lower crust. This attests to the important role of crustal shortening during crustal formation in this region. P-wave velocities calculated using data from garnet granulite xenoliths range from 6.9--8.0 km/s and are, in general, positively correlated with garnet content. P-wave velocities calculated using data from spinel peridotite xenoliths are between 7.75 and 7.89 km/s. These velocities are consistent with geophysical measurements, suggesting that the low velocities can be explained by the pressure, temperature, and composition of the xenoliths. The calculations suggest that the fertile composition of the upper mantle may have played an important role in controlling extension in the southern Rio Grande rift. Additionally, there is no evidence for a thick mafic underplate currently beneath the Potrillo volcanic field. Studies of Mt. Gamsby, located at the southern end of the Central Gneiss Complex in the Coast Mountains, indicate that this region was affected by at least three phases of deformation (D1-3). During D2, plutons were emplaced as sill-like bodies at high pressures during transpression involving NE-SW directed shortening. Estimated temperatures and pressures are 600-750°C and 0.8-1.0 GPa. Lower temperatures, cooling at depth, and the absence of a major low-angle detachment indicate that Mt. Gamsby followed a distinct exhumation and cooling history from other parts of the Central Gneiss Complex. The presence of magmatic epidote in the plutons at Mt. Gamsby suggests affinities to mid-Cretaceous plutons of the Insular terrane to the west. Both regions record evidence for crustal thickening followed by magmatism and extension. These processes are likely to be key to the formation of continental crust.