Open system magmatic evolution of the Taos Plateau volcanic field, northern New Mexico: 1. The petrology and geochemistry of the Servilleta Basalt
Authors: Dungan M. A. , Lindstrom M. M. , McMillan N. J. , Moorbath S. , Hoefs J. , Haskin L. A. 
Published: February 2008

The Servilleta Basalt is a sequence of voluminous (> 200 km3), Pliocene, low‐ to medium‐K2O tholeiitic lavas that are the dominant eruptive lithology of the Taos Plateau volcanic field (TPVF). Subordinate coeval rock types are olivine andesite, pyroxene dacite, and minor rhyolite. The Servilleta lavas formed widespread floods of multiple thin flows, which comprise three main members of comparable areal extent and volume, lower (LSB), middle (MSB), and upper (USB). Most flows are olivine‐phyric pahoehoe characterized by diktytaxitic‐ophitic groundmass textures and vesicular segregations formed during postemplacement migration of residual liquids. Servilleta basalts are moderately evolved (Mg# = 61–54) compared with magmas that could be in equilibrium with mantle peridotite. No progressive temporal evolution trend is present. Compositional variations defined by all Servilleta analyses include modest decreases in FeO*, CaO, MgO, and TiO2 with increasing SiO2, K2O, and the incompatible trace elements Rb, Sr, Ba, light rare earth elements, Th, Hf, Zr, and Nb. Indices of olivine fractionation (Mg# and Ni content) and the transition elements Sc, Cr, and V exhibit very weak correlations with SiO2, K2O, and incompatible trace elements. Reasonable amounts of crystal fractionation (maximum of 15–20% constrained by Mg# variations) of liquidus phases olivine + plagioclase are inadequate to explain the overall trends even if multiple parent magmas are invoked; e.g., the calculated depletion of Ni is far greater than observed, and enrichments of K2O and incompatible elements are less than observed. A more plausible mechanism for deriving the range of basalt compositions is mixing of Servilleta parent magma with a variety of coeval andesite and dacite compositions. Mixing models are consistent with all the observed elemental correlations and the anomalous enrichments in SiO2, K2O, and incompatible trace elements compared with values predicted by fractionation calculations. The erupted basaltic end‐member has low SiO2, K2O, and incompatible trace elements (LSKI). The LSKI lavas may contain up to 10% admixed andesite. The maximum amount of additional mixing appears to be about 25–35%, depending on the composition of the andesite or dacite end‐member involved. The three major eruptive episodes are inferred to reflect three successive mafic magma replenishment events, each of which led to variably mixed (and fractionated) lavas being erupted to form heterogeneous units. Mixing of basalt with andesite or dacite magmas characterized by comparatively high Mg#'s (59–45) and low crystal contents resulted in mafic hybrids with limited textural and mineralogical indicators of mixing. Chemical heterogeneities in some hybrid basalts probably reflect incomplete homogenization subsequent to mixing. Pb, Sr, and O isotopic variations for TPVF basalts, andesites, and dacites correlate with major and trace element compositions. 87Sr/86Sr and δ18O generally increase with increasing SiO2 (0.70417–0.70520, 5.8–9.5‰), although values markedly higher than the basaltic range are found primarily in high‐SiO2 rocks (≧59%). LSKI basalts have the most radiogenic Pb isotopic compositions (e.g., 206Pb/204Pb ∼ 18.0–18.2). With increasing SiO2, K2O, and incompatible elements, the Pb isotope ratios decrease (e.g., 206Pb/204+Pb ∼ 17.2–17.0 in dacites). Half the range in Pb isotope ratios occurs among basalts. Combined isotopic and elemental trends indicate incorporation of an ancient crustal component of low Rb/Sr, U/Pb, and Th/Pb, such as granulite facies lower crust. The isotopic imprint of crustal assimilation was probably imparted to the basalts largely by mixing with previously contaminated hybrid andesite and dacite magmas, although the isotope data do not allow discrimination between direct assimilation of crust by the basalts and complex multistage mixing in an open magmatic system in which fractionation, assimilation, and mafic magma recharge all operated.