A Reinterpretation of Some Correlations of Chisos Group Mafic Lavas with New Geochemistry and Geochronology, Big Bend National Park, Texas

New geochemical data and ages from Oligocene Chisos Group mafic lavas allow reinterpretation of the Bee Mountain Basalt (BMB), unnamed lava (UL), and the Tule Mountain Trachyandesite (TMT), oldest to youngest, respectively. Two UL subunits, each consisting of multiple flows, were previously mapped in areas extending from Tule Mountain to Sierra del Chino. The lowest UL subunit is chemically similar to the underlying 33.8 Ma BMB. Chemical analysis from the lower UL and BMB plot in close proximity on a total alkali silica diagram and UL data are on the same Hf vs Ta trend as BMB data. These results suggest the lower UL may represent later, more evolved magmas from sources of the BMB. Conversely, upper UL lavas are chemically similar to rocks mapped as TMT dated at 30.3 Ma. The upper UL lavas have lower SiO₂ content and plot separately from TMT rocks although their Hf vs Ta and Hf/Ta vs La/Yb plots are nearly identical. Chemical analysis of TMT rocks from the type-section at Tule Mountain and areas to the southeast show confusing results. Samples from the type-section are chemically distinct from other TMT to the southeast. The lavas at the type-section have slightly lower alkali content than TMT to the southeast and plots of Hf vs Ta and Hf/Ta vs La/Yb further demonstrate compositional differences. A preliminary ⁴⁰Ar/³⁹Ar age of 33-34 Ma for the type TMT indicates it is significantly older than other rocks mapped as TMT. Our study indicates rocks at the type-section are not correlative with TMT to the southeast and highlights the need for future investigation into a new type-section for the 30.3 Ma rocks previously mapped as TMT.