NEODYMIUM ISOTOPIC EVIDENCE FOR DECREASING CRUSTAL CONTRIBUTIONS TO CENOZOIC IGNIMBRITES OF THE WESTERN UNITED-STATES - IMPLICATIONS FOR THE THERMAL EVOLUTION OF THE CORDILLERAN CRUST

Numerous studies of large-volume rhyolite systems from the western Uni ted States conclude that rhyolite is primarily generated by extensive fractional crystallization of basalt accompanied by assimilation of cr ustal wallrock. Relative crustal contributions to large-volume rhyolit e can be estimated by comparing the Nd isotopic composition of the rhy olite with the Nd isotopic characteristics of crust and mantle reservo irs associated with different continental basement age provinces. We h ave estimated the crustal contributions to 12 early Oligocene to Pleis tocene rhyolite systems located throughout the Cordillera. We have det ermined that (1) crustal contributions to large-volume rhyolite system s decrease from the Oligocene to the Miocene, and (2) rhyolite systems younger than 20 Ma are dominated by mantle components. The crustal co ntributions to rhyolite systems may be controlled by system size and d uration, crustal thickness, tectonic setting, crustal composition, cru stal density, and crustal temperature. We conclude that regional cooli ng of the lower crust, which progressively limited the amount of crust al wallrock assimilated by rhyolite systems, is the only parameter tha t is consistent with geologic and geochemical data for rhyolite system s and the geologic evolution of the Cordillera. A quantitative model t hat relates the amount of crustal contribution to assimilation/recharg e rates and the temperature of the crust indicates that lower-crustal temperatures would have to decrease about 300-degrees-C between early Oligocene and early Miocene time to account for the decrease in crusta l contributions.