A METASOMATIC ORIGIN FOR LARAMIDE TECTONISM AND MAGMATISM

Tomographic studies image a 200-km thickness of very slow uppermost mantle beneath the Colorado Rocky Mountains compared to the adjacent Great Plains and Colorado Plateau, indicating that Rocky Mountain upper mantle is relatively hot and partially molten. The spatial correlation between seismically slow mantle and the Rocky Mountain province suggests a Laramide origin for this anomalous upper mantle. This mantle is thought to be modified North America lithosphere because correlation would not be preserved within mechanically weak asthenosphere. In support, isotopic studies indicate that Laramide and post-Laramide mantle-derived magmas originated from North America lithosphere. Furthermore, hydrated mantle was involved with the generation of many of these magmas. We suggest that Laramide tectonism and magmatism were related to metasomatization of a ~200-km-thick North America lithosphere. Such metasomatism would increase the buoyancy and fertility of Rocky Mountain lithosphere, causing uplift and triggering melting of the lithosphere. Lithospheric metasomatism and warming due to magmatic ascent would decrease lithospheric strength, allowing shortening in the Rocky Mountains and accounting for the low seismic velocities. We attribute Rocky Mountain metasomatism to de-volatilization of the subducting Farallon slab during the Laramide orogeny, which we think was in contact with the base of North America lithosphere during this time. The average orientation of Laramide shortening across the Southern Rocky Mountains is ENE. This is consistent with Laramide-age weakening of lithosphere beneath the Rockies and an application of tractions to the base of North America west of the Rockies by the ENE-moving Farallon plate. The ENE direction of shortening differs from the easterly direction expected from gravitational collapse of the north-trending Sevier welt. Much of the Great Plains and Colorado Plateau uplift also is thought to result from similar mantle metasomatism, but apparently not to the point of triggering significant magmatism.