Abstract

Numerous studies of large-volume rhyolite systems from the western United States conclude that rhyolite is primarily generated by extensive fractional crystallization of basalt accompanied by assimilation of crustal wallrock. Relative crustal contributions to large-volume rhyolite can be estimated by comparing the Nd isotopic composition of the rhyolite with the Nd isotopic characteristics of crust and mantle reservoirs associated with different continental basement age provinces. We have estimated the crustal contributions to 12 early Oligocene to Pleistocene rhyolite systems located throughout the Cordillera. We have determined that (1) crustal contributions to large-volume rhyolite systems decrease from the Oligocene to the Miocene, and (2) rhyolite systems younger than 20 Ma are dominated by mantle components. The crustal contributions to rhyolite systems may be controlled by system size and duration, crustal thickness, tectonic setting, crustal composition, crustal density, and crustal temperature. We conclude that regional cooling of the lower crust, which progressively limited the amount of crustal wallrock assimilated by rhyolite systems, is the only parameter that is consistent with geologic and geochemical data for rhyolite systems and the geologic evolution of the Cordillera. A quantitative model that relates the amount of crustal contribution to assimilation/recharge rates and the temperature of the crust indicates that lower-crustal temperatures would have to decrease about 300 °C between early Oligocene and early Miocene time to account for the decrease in crustal contributions.

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