BASEMENT CONTROL OF ALKALIC FLOOD RHYOLITE MAGMATISM OF THE DAVIS MOUNTAINS VOLCANIC FIELD, TRANS-PECOS TEXAS

Silicic lava flows, erupted at ~37-36 Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000 km² with an initial volume possibly as great as 1,000 km³. These form the Star Mountain Rhyolite (~ 220 km³) of the eastern Davis Mountains, the Crossen Formation (~75 km³) of the southern Davis Mountains, the Bracks Rhyolite (~75 km³) of the Rim Rock region and the slightly younger Adobe Canyon Rhyolite ( ~125 km³, 36.51 Ma), and Sheep Pasture Formation (~125 km³, ~36 Ma) of the Davis Mountains. Individual flows extended as far as 50 km; some composite units form exposures as thick as 300 meters.

Flood rhyolite lavas are marginally peralkaline quartz trachyte to low-silica rhyolite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, apatite, and, rarely, fayalite, as well as zircon in a few rocks. Many Star Mountain flows may be assigned to four 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, C and D) have, in addition, well developed negative anomalies for Ba and Eu, suggesting alkali feldspar fractionation. The more strongly peralkaline samples (Group B) exhibit the lowest Sr and Ba. All flood rhyolite units show negative Nb anomalies. Temperatures ranged from ~900 to 860 ^oC in trachyte to low silica rhyolite. We suggest that flood rhyolite magma evolved from trachyte magma by filter pressing processes, and trachyte from mafic magma in deeper seated plutons.

Flood rhyolite magmatism was confined to the DMVF segment of the Trans-Pecos Magmatic Province, which overlies Grenville basement and is separated from the older Southern Granite and Rhyolite Province to the north and from the younger Coahuila terrane to the south. In the Big Bend region south of the DMVF, magmatism began ~10 Ma earlier and produced abundant mafic lava; north of the Grenville Front, magmatism was delayed several Ma, less voluminous, and dominated by shallow intrusions of nepheline trachyte. We suggest that basement structure strongly influenced the timing and nature of Trans-Pecos magmatism, probably in varying degrees of impeding the ascent of mantle-derived mafic magma.