New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Cenozoic and Mid-Tertiary basalts throughout the western United States have distinct differences in major element, trace element, and isotopic composition. Mid-Tertiary basalts, with E_Nd ~0 to +1, ate often ascribed to a subcontinental lithospheric source, in contrast to Upper Cenozoic basalts with E_Nd > +4, which most authors agree have a source in convecting asthenosphere. This study describes specific differences between the Mid-Tertiary basalts of the Uvas volcanic field (28.5 to 26 Ma) and Upper Cenozoic basalts (10 to 0.02 Ma) of south-central New Mexico.

Upper Cenozoic basalts are oceanic in Sr-, Nd-, and Pb-isotopic composition. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb compositions fall along the Northern Hemisphere Regression Line (NHRL) between the oceanic depleted mantle (DM) and high ²⁰⁶Pb/²⁰⁴Pb (HIMU) sources (Hart, 1988, EPSL, 90:273-296). Sr and Nd isotopic compositions fall between DM, HIMU, and an enriched mantle (EM) source. These basalts are similar to Upper Cenozoic alkalic basalts throughout the western U.S. in trace element geochemistry as well as isotopic composition (Fitton et al., 1991, JGR, 96:13693-13711; Kempton et al., 1991, JGR, 96:13713-13735). Incompatible trace element patterns are OIB-like, with high Nb (32-72 ppm), Ta (1.9-5.1 ppm), La (55-113 X chondrite), La/Lu_N (6.6-12.5), and low Ba/Ta (121-226). Although these basalts do not contain mantle xenoliths, there is scarce evidence for crustal contamination; isotopic variations are not correlated with any differentiation index.

Mid-Tertiary Uvas basalts have slightly higher ⁸⁷Sr/⁸⁶Sr, lower E_Nd, and lower Pb ratios than the Upper Cenozoic suite, coupled with low Nb (~8 ppm), Ta (0.20-0.23 ppm), La (27.6-28.5 X chondrite), and La/Lu_N (2.7), and higher BalTa (1243-1251). These tholeiitic basalts fall along the Sr-Nd mantle array towards EMI, but have less radiogenic lead than most oceanic basalts. The Uvas volcanic field is dominated by basaltic andesites, with lesser amounts of basalts and andesites. Isotopic and trace element trends indicate contamination by a non-rediogenic lower crustal source. However, Uvas basalts cannot be produced by crustal contamination of partial melts of asthenosphere; mass balance of Nb and Ta requires > 50% crust in simple contamination models, inconsistent the basaltic compositions (SiO₂ = 49.4-50.0%; MgO = Differences in degree of melting of a heterogeneous mantle is precluded because the Upper Cenozoic basalts, representing less amounts of melting, have higher incompatible trace element concentrations but are isotopically depleted relative to Mid-Tertiary basalts.

Two distinct mantle sources are required: a depleted asthenospheric source active after 10 Ma, and an isotopically enriched, low Nb, low Ta source active in the mid-Tertiary. This enriched (hydrated?) mantle source probably resides in the subcontinental lithosphere and was melted during extension (Gallagher and Hawkesworth, 1992, Nature, 358:57-59). Perhaps the best evidence for the location of this source is its occurrence in time and space. In southern New Mexico, the EM source was active from 36 Ma to 26 Ma and replaced by asthenosphere at 10 Ma. In the Socorro area, the transition occurred at approximately the same time (Heatherington, 1988, Ph.D., Washington Univ.). However, in the Lucero region near Albuquerque, the transition occurred between 8 and 4 Ma (Perry et al, Nature, 332: 432-434), and even the youngest basalts in the Taos Plateau volcanic field in northern New Mexico are of the EM type (Dungan et al., 1986,
JGR, 86:5999-6028). One possibility is that this trasgression indicates that the EM source is plume-related and now resides under northern New Mexico, a highly unlikely scenario that requires southward motion of the North American plate at 22 km/Ma. Alternatively, the EM source could reside in the subcontinental lithosphere and is replaced by asthenosphere as extension reaches some critical dimension, as suggested by Perry et al. (1987, JGR, 92:9193-9213).