CENOZOIC EVOLUTION OF MANTLE LITHOSPHERE IN WESTERN U.S. FROM MAGMATIC SPACE-TIME-COMPOSITION PATTERNS

Changes in the physical and chemical composition of the continental lithospheric mantle (CLM) can influence both the timing and style of continental deformation events, including continental rifting events. But because of a lack of direct observational information on the CLM through time, it is difficult to define the exact role modifications in the CLM may have in influencing the overall tectonic modification of continental lithosphere. One proxy record of the modification in the CLM lies in the chemical compositions of mantle-derived continental igneous rocks. In the western U.S., the North American Volcanic and Intrusive Igneous Rock database (NAVDAT) allows a detailed assessment of space-time-composition patterns in igneous activity in this region over the past ~ 80 m.y. and of how this magmatism reflects changes in the composition of the CLM through time. During Late Cretaceous-Early Tertiary Laramide orogeny, arc related magmatism clearly migrated inboard in western Canada and northern Mexico, but in much of western U.S. was restricted to relatively narrow belts (e.g. Colorado Mineral Belt) oriented parallel to relative motions of North American and Farallon plates. Existing chemical data indicate that parental magmas were mantle derived, potentially derived from secondary asthenospheric convection cells induced by the shallowing of the Farallon plate. If so, then modification of the CLM was likely spatially restricted to these NE-SW oriented zones. In contrast, the mid-Tertiary ignimbrite flareup affected virtually all of SW North America and the calc-alkaline magmatism associated with this event likely originated in melting of CLM that had been hydrated and refrigerated by the Farallon plate during early Tertiary low angle subduction. Alkalic magmatism, however, marked the easternmost extent of mid-Tertiary magmatism in the western U.S and has higher eNd(0) and lower ⁸⁷Sr/⁸⁶Sr than their calc-alkaline counterparts. These magmas were likely derived from dry, upwelling asthenosphere and so delineates the eastern edge of hydrated CLM. The region affected by ignimbrite flareup and subsequent Basin and Range extension corresponds to where the CLM was hydrated and weakened by the addition of slab fluids. Stronger, dry CLM to the east remained intact through the Late Cenozoic.