A reevaluation of the origin of late Cretaceous and younger magmatism in the southern Rocky Mountain region using space-time-composition patterns in volcanic rocks and geochemical studies of mantle xenoliths

Intraplate, continental interior magmatism remains one of the most enigmatic forms of terrestrial igneous activity. Little consensus exists regarding what sources of magmatism are involved (continental "lithosphere" or sublithospheric, convecting mantle, mafic to felsic composition continental crust), or the trigger mechanism(s) responsible for initiating melting. The western U.S. is ideal for study of continental interior magmatism where the source and trigger mechanism(s) for Phanerozoic magmatism are particularly enigmatic. Here Late Cretaceous to Early Tertiary and mid-Tertiary magmatism occurs as much as 1000 km inboard of the plate margin and in a region with thick continental lithosphere.;This dissertation presents a reevaluation of these two episodes of Phanerozoic continental magmatism in western North America: the Late Cretaceous to Early Tertiary magmatism of the Colorado Mineral Belt, and the mid-Tertiary ignimbrite flare-up of the southern Rocky Mountains. Isotopic and geochemical studies of basaltic volcanic rocks were determined to identify the potential sources of magmatism of the southern Rocky Mountains, demonstrating that a mantle with relatively low &egr;Nd (+1 to -7) and high 87Sr/ 86Sr (∼0.705) values was likely the source of Late Cretaceous and younger magmatism in the southern Rockies. Presented here are new isotopic and geochemical studies of mantle xenoliths entrained in Miocene basalts, with &egr;Nd (∼-2 to -5) and 87Sr/ 86Sr (∼0.705) values that confirm this low &egr;Nd, high 87Sr/86Sr reservoir exists in the continental lithospheric mantle (CLM), that it was long lived, records a history of melt extraction plus enrichment, and could be the source of Late Cretaceous and younger magmatism in the southern Rockies. Numerical modeling and evaluations of time-space-composition relationships of volcanic rocks were also conducted to evaluate potential mechanisms responsible for generating melts in the Rocky Mountain CLM. Results of these studies demonstrate that simple inland migration of a subduction-related volcanic arc is unlikely to be responsible for Late Cretaceous and younger magmatism of the southern Rockies. This dissertation further outlines the conditions that must be satisfied for any hypothesis regarding generation of Rocky Mountain magmatism.