ISOTOPIC COMPOSITION OF OLIGOCENE MAFIC VOLCANIC-ROCKS IN THE NORTHERN RIO-GRANDE RIFT - EVIDENCE FOR CONTRIBUTIONS OF ANCIENT INTRAPLATE AND SUBDUCTION MAGMATISM TO EVOLUTION OF THE LITHOSPHERE

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 Sr-87/Sr-86 = 0.70495, epsilon-Nd = -4, Pb-206/Pb-204 = 18.2, and the other by Sr-87/Sr-86 = 0.7044, epsilon-Nd = 0, Pb-206/Pb-204 = 18.2. The low-epsilon-Nd 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 epsilon-Nd values of the MANTLE 1 component were probably produced by evolution of a light rate earth element-enriched upper mantle or input of low-epsilon-Nd material during development of the lithosphere in the Early Proterozoic. The epsilon-Nd values and Pb-206/Pb-204 ratios in evolved rocks are as low as -7 and 17.3, respectively, indicating interaction with lower crust that had epsilon-Nd less-than-or-equal-to 12 and Pb-206/Pb-204 less-than-or-equal-to 17.0. Initial Sr-87/Sr-86 ratios both increase and decrease slightly in evolved rocks, indicating interaction with lower crust that had Sr isotope ratios that were generally similar to those of the mantle. The epsilon-Nd value of the modern lithospheric mantle beneath the northern Rio Grande rift is > 18 units lower than the projected modern values of the asthenospheric mantle from which the Proterozoic crust was originally derived. The higher-epsilon-Nd 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 Pb-207/Pb-204 ratios that are significantly higher than the Pb-206/Pb-204 - Pb-207/Pb-204 array defined by crust in the region. Although the crustal array is also displaced to higher Pb-207/Pb-204 ratios as compared to the northern hemisphere oceanic mantle array, indicating incorporation of Archean Pb during crust formation, the still higher Pb-207/Pb-204 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.