Granulite xenoliths from two recently discovered localities in west Texas provide new information on the age and isotopic composition of the basement, and they furnish important constraints on petrogenetic models for the origin of silicic rocks from a major Tertiary volcanic field. One xenolith locality lies within the Tertiary Davis Mountains volcanic field, and the other lies ∼100 km to the south. The protolith crystallization age, determined by U-Pb zircon geochronology, of a xenolith from the southern locality is 1090 ± 4 Ma, and a Grenvillian age for most other xenoliths from this locality and some from the Davis Mountains was confirmed by Sm-Nd systematics. Pb and Sr isotope compositions of the xenoliths indicate that the basement underwent a Proterozoic granulite-facies metamorphic event that coincided with or postdated the crystallization of the sample dated at 1090 Ma. The Tertiary volcanic series of the Davis Mountains has almost constant Nd and Pb isotope compositions over the range basalt to high-silica rhyolite. The difference in Nd isotopes between the silicic volcanic rocks and the most fusible component of the basement, the intermediate to silicic rocks, was at least six ϵNd units. Furthermore, there is a substantial gap in 206Pb/204Pb ratios between the Proterozoic basement samples (≤16.9) and the volcanic rocks (≥17.6). The documentation of the strong isotopic contrast between the volcanic rocks and the deep crust seriously weakens models advocating a crustal origin for the Tertiary silicic magmas.