Mafic lavas erupted during initiation of regional extension at 26 Ma in the northern Rio Grande rift were derived from at least two isotopically distinct mantle sources. One is characterized by 87Sr/86Sr=0.70495, ϵNd=-4, 206Pb/204Pb=18.2, and the other by 87Sr/86Sr=0.7044, ϵNd=0, 206Pb/204Pb=18.2. The low-ϵNd value source (MANTLE 1) is interpreted to largely reflect the isotopic compositions of the lithospheric mantle in the region. Isotopic compositions of Cenozoic mafic lavas and Proterozoic rocks are used to constrain models for evolution of the lithosphere. The low ϵNd values of the MANTLE 1 component were probably produced by evolution of a light rare earth elment-enriched upper mantle or input of low-ϵNd material during development of the lightosphere in the Early Proterozoic. The ϵNd values and 206Pb/204Pb ratios in evolved rocks are as low as -7 and 17.3, respectively, indicating interaction with lower crust that had ϵNd≤-12 and 206Pb/204Pb≤17.0. Initial 87Sr/86Sr ratios both increase and decrease slightly in evolved rocks, indicating interaction with lower curst that had Sr isotope ratios that were generally similar to those of the mantle.

The ϵNd value of the modern lithospheric mantle beneath the northern Rio Grande rift is >18 units lower than the projected modern values of the asthenoispheric mantle from which the Proterozoic crust was originally derived. The higher-ϵNd value source (MANTLE 2) may reflect mixing of asthenosphere and lithosphere components. The mantle source for most late Cenozoic mafic lavas in the northern Rio Grande rift region lies at 207Pb/204Pb ratios that are significantly higher than the 206Pb/204Pb-207Pb/204Pb array define by Proterozoic crust in the region. Although the crustal array is also displaced to higher 207Pb/204Pb ratios as compared to the northern hemisphere oceanic mantle array, indicating incorporation of Archean Pb during crust formation, the still higher 207Pb/204Pb ratios in the Rio Grande rift mantle source is interpreted to reflect continued input of Archean Pb into the developing Early Proterozoic lithospheric mantle after crust formation. These relations are strong evidence that the lithosphere became stabilized shortly after the major crust formation events in the Early Proterozoic. ¿American Geophysical Union 1991