PETROGRAPHY AND PETROLOGY OF THE ALKALINE AND THOLEIITIC LAVAS FROM THE LA TETRA AREA, ZUNI-BANDERA VOLCANIC FIELD, NEW MEXICO

This project presents a focused geochemical study of Holocene and upper Pleistocene volcanic rocks from the Zuni-Bandera volcanic field (ZBVF) and specifically, the McCartys, Bandera, Twin Craters and Hoya de Cibola flows. Each erupted from a distinct volcanic vent over ~20,000 years. All flows <52% SiO₂ with MgO between 5.5 and 10 wt%. ⁸⁷Sr/⁸⁶Sr range from 0.703625-0.70299 and ¹⁴³Nd/¹⁴⁴Nd= 0.512597-0.51297. Evolution of the magmas in the volcanic field is suggested to be the result of fractional crystallization of olivine, cpx and spinel in the upper crust with little modification by assimilation. The Twin Craters flow is a 15.8 ka series of cinder cones and tholeiitic lava flows. It is mostly covered by younger lava flows Outcrops only exist east of Bandera crater. It is the stratigraphically oldest flow included in this study. The stratigraphically younger Hoya de Cibola flow is a tholeiitic lava flow erupted of unknown age. It is typically buried by the McCartys and Bandera flows or concealed by eolian material. The Bandera flow is the youngest alkali basalt in the ZBVF at 9.17-12.5 ka. It is porphyritic with phenocrysts of olivine, cpx and rare plagioclase. Previous studies have concluded that the flow is heterogeneous in composition. The youngest lava flow is the tholeiitic McCartys flow at 2.4-3.9 ka. Petrologicially, the flow is heterogeneous along strike and similar in modal composition to the Bandera flow.

Here, we determine if small-scale compositional features are preserved in each lava flow or is the source homogeneous? In other words, is the primary enriched mantle compositions, ocean island basalts (OIB), just simply a larger version of the individual volcanoes, or is the magma source different between volcanoes. We present new major and trace element compositions and Sr-Nd-Pb isotopic ratios of each lava flow samples in order to constrain variations in the regional geochemistry and petrogenesis. Data is combined with additional published datasets to assess variation between and within flows. Whole-rock major and trace element geochemistry was obtained by XRF and additional trace element contents including the rare earth elements by ICP-MS. Radiogenic isotope analysis is currently in progress.