Basement control of alkalic flood rhyolite magmatism of the Davis Mountains volcanic field, Trans-Pecos Texas, U.S.A.

Abstract

Voluminous silicic lava flows, erupted ~37.4Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000km2 with an initial volume as great as 1000km3. Lava flows form three major stratigraphic units: the Star Mountain Rhyolite (minimum 220km3) of the eastern Davis Mountains and adjacent Barilla Mountains, the Crossen Formation (~75km3) of the southern Davis Mountains, and the Bracks Rhyolite (~75km3) of the Rim Rock region west of the Davis Mountains proper. Similar extensive rhyolite lava also occurs in slightly younger units (Adobe Canyon Rhyolite, ~125km3, 37.1Ma), Sheep Pasture Formation (~125km3, ~36Ma) and, less voluminously, in the Paisano central volcano (~36.9Ma) and younger units in the Davis Mountains. Individual lava flows from these units formed fields as extensive as 55km and 300-m-thick.Flood rhyolite lavas of the Davis Mountains are marginally peralkaline quartz trachyte to low-silica rhyolite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, and apatite, and, rarely, fayalite, as well as zircon in less peralkaline units. Many Star Mountain flows may be assigned to one of four geochemical groupings. Temperatures were moderately high, ranging from ~911 to 860°C in quartz trachyte and 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.The Davis Mountains segment of Trans-Pecos Texas overlies Grenville basement and is separated from the older Southern Granite and Rhyolite Province to the north by the Grenville Front, and from the younger Coahuila terrane to the south by the Ouachita Front. 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 magmas, which were produced by upwelling of asthenospheric mantle above the foundered Farallon slab. Basalt was able to penetrate Coahuila crust in the Big Bend region. Thicker Grenville crust under the Davis Mountains retarded ascent of mafic magmas, allowing mafic plutons to differentiate into silicic magma that was eventually erupted as flood lava. North of the Grenville Front, magmatism was further delayed and thicker, older crust there may have helped concentrate magmatism under the Davis Mountain region. Only after the onset of Basin and Range faulting was true basalt erupted over much of the Trans Pecos.