LEAD ISOTOPIC EVIDENCE FOR SYNEXTENSIONAL LITHOSPHERIC DUCTILE FLOW IN THE COLORADO RIVER EXTENSIONAL CORRIDOR, WESTERN UNITED-STATES

Temporal changes in the Pb isotopic compositions of Miocene lavas erup ted in the northern Colorado River extensional corridor suggest that l ithospheric mantle and middle to deep crust migrated from beneath the Colorado Plateau into the corridor during extension. Basaltic to rhyol itic lavas erupted in the extensional corridor prior to 12.2 Ma have P b isotopic values that are similar to those of Tertiary to Quaternary lavas erupted through Proterozoic Mojave crust, which comprises surfac e exposures of basement in the corridor and much of the extended terri tory to the west. In contrast, most post-12.2 Ma lavas from the same r egion have Pb isotopic compositions similar to those of lavas erupted through Arizona crust, which forms the basement of the Colorado Platea u. The changes in isotopic compositions of the basaltic lavas, and per haps a portion of the changes in isotopic compositions of silicic lava s, are attributed to a change in the composition of the mantle source. However, the (206)pb/(204)pb ratios for lavas erupted before and afte r 12.2 Ma in the corridor decrease with decreasing MgO concentrations, suggesting that the Pb isotopic compositions of crustal assimilants c hanged at about the same time as the composition of the mantle. In the area of the Black Mountains accommodation zone, the surface boundary between the Arizona and Mojave crustal provinces lies a minimum of 60- 80 km to the east of the westernmost lava with an Arizona Pb isotopic signature. This distance cannot be accounted for by displacements alon g nearby major faults, suggesting that middle to deep Arizona crust fl owed a significant distance to the west during extension.