MINERAL EQUILIBRIA CONSTRAINTS ON THE TEMPERATURES OF METALUMINOUS TO PERALKALINE "FLOOD" RHYOLITE IN THE DAVIS MOUNTAINS VOLCANIC FIELD, TRANS-PECOS TEXAS

The Davis Mountains are located between the Rio Grande River and the Pecos River in the Trans-Pecos Magmatic Province in west Texas. They are comprised of metaluminous to peralkaline quartz trachyte and rhyolite lavas and tuffs, including several "flood rhyolites". These erupted between 39 Ma and 35 Ma. "Flood rhyolite" units include the ~36.8 Ma Star Mountain Rhyolite, the ~36.5 Ma Adobe Canyon Formation, and the ~35.9 Ma Sleeping Lion Formation; identified rheomorphic tuffs include the ~36.7 Ma pantelleritic Gomez Tuff and the ~35.6 Ma Barrel Springs. The samples to be studied are two samples of the Gomez Tuff, two samples of the Star Mountain Rhyolite, one sample of the Adobe Canyon Formation, one sample of the Barrel Springs Formation, one sample of the Sleeping Lion Formation, and one sample of the Weston Laccolith. Mafic minerals in each sample were analyzed by electron microprobe and phase equilibria was used to constrain intensive crystallization parameters. These results are used to calculate viscosities to help determine if these volcanic units are true flood rhyolite lavas or rheomorphic tuffs.