FLOOD RHYOLITE MAGMATISM OF THE DAVIS MOUNTAINS, TRANS-PECOS TEXAS

Voluminous silicic lava flows, erupted at ~37 Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000 km² with an initial volume of roughly 1,000 km³. Exposed lava flows have been assigned to three major stratigraphic units: the Star Mountain Formation (minimum 220 km³) of the eastern Davis Mountains and adjacent Barilla Mountains, the Crossen Formation (~75 km³) of the southern Davis Mountains, and the Bracks Rhyolite (~75 km3) of the Rim Rock region west of the Davis Mountains proper. Stratigraphic sections of the Star Mountain Formation contain from one to more than three lava flows, with a maximum total thickness of ~300 m. The Bracks Rhyolite largely consists of one major flow with a length of about 50 km. The remainder of the above initial volume estimate is assumed to be present as unexposed lava beneath the central Davis Mountains and Valentine Graben.

Flood rhyolite lavas are marginally peralkaline quartz trachyte to low-silica rhyolite ( ~ 65-72 wt.% SiO₂). In the Al2O₃-FeO_T classification, they plot as comenditic trachyte to comendite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, and apatite, and, rarely, fayalite, as well as zircon in a few samples. Most Star Mountain units may be assigned to one of three geochemical groupings. The least evolved units (Group A) have mantle-normalized plots with well-developed negative anomalies for Sr, P, and Ti, suggesting fractionation of plagioclase feldspar, apatite and FeTi oxides. More evolved units (Groups B and C) have, in addition, well developed negative anomalies for Ba and Eu, the former suggesting alkali feldspar fractionation. The more strongly peralkaline samples (Group B) exhibit the greatest negative anomalies. Samples from all the flood rhyolite units show negative Nb anomalies.

Flood rhyolite magmatism was confined to the DMVF segment of the Trans Pecos Magmatic Province. The DMVF overlies metamorphic Grenville basement, separated from a younger, craton-margin sedimentary and volcanic terrain to the north by the Grenville Front, and from sedimentary rocks to the south in Big Bend by the Ouachita Front. This suggests that basement control might have been a significant factor in the generation of flood rhyolites.