Isotopic and Trace Element Constraints on the Composition and Evolution of the Lithosphere Beneath the Southwestern United States

The effects of subduction and crustal recycling on the composition and evolution of the subcontinental lithospheric mantle beneath the southwestern United States are recorded in the isotopic and trace element compositions of Cenozoic (< 17 Ma) alkalic basalts from the Basin and Range and its tectonic boundaries to the west (Western Great Basin) and east (Colorado Plateau-Basin and Range transition zone). Basin and Range alkalic basalts are chemically similar to ocean island basalts, having high epsilon-Nd and TiO2, with low Ba, Ba/Nb, and Sr-87/Sr-86, which suggests an asthenospheric mantle origin. Lavas from the Western Great Basin and the Eastern Transition Zone have higher Ba contents coupled with higher Ba/Nb and lower TiO2 than Basin and Range basalts, characteristics typically associated with subduction zone magmas. Western Great Basin lavas typically have higher Ba/Nb, K/Ti, and Sr isotope ratios than basalts from the Eastern Transition Zone. Lavas from the northern portion of the Eastern Transition Zone (northern Arizona to central Utah) have unusually low Sr-87/Sr-86 combined with low Nd-143/Nd-144 and plot below and to the left of the mantle array, indicating low time-integrated Rb/Sr ratios in the source. Pb isotope ratios are distinct for each area. Basin and Range basalts exhibit only a small isotopic range, whereas transition zone basalts have overlapping to lower Pb-206/Pb-204 and higher Pb-207/Pb-204. Linear distributions on Pb-Pb plots for both the Western Great Basin and the Eastern Transition Zone basalts yield source ages of 1.8 Ga, but basalts from the southeastern margin of the Colorado Plateau are displaced to lower Pb-207/Pb-204, indicating a lower U/Pb ratio (mu-1). Interaction between asthenospheric basaltic magmas and overlying crust appears to be unable to explain the Sr, Nd, and Pb isotope trends since (1) correlations between Sr-87/Sr-86 and 1/Sr are not observed, and (2) analyzed lower to midcrustal xenoliths, inferred lower to midcrustal compositions and Proterozoic supracrustal rocks from the area do not form reasonable mixing end-members for both isotopes and trace elements. The high Ba/Nb, Rb/Sr, and K/Ti ratios in the transition zone basalts indicate that subduction processes have enriched the subcontinental lithosphere beneath the southwestern United States. The range in isotopic and trace element ratios suggest that enrichment processes ranged from fluid-dominated (Western Great Basin) to melt-dominated (Eastern Transition Zone). Although the effects of recent subduction cannot be dismissed, isotopic evidence suggests that these characteristics were largely imposed upon the lithosphere at about 1.8 Ga. Chemical differences in magmas that interacted with lithospheric mantle reflect ancient, rather than recent, subduction processes in the western United States. Geochemical boundaries in the lithosphere correspond to domains established during the Proterozoic which were ultimately assembled into western North America during Proterozoic and late Paleozoic time.