Xenoliths of Grenvillian granulite basement constrain models for the origin of voluminous Tertiary rhyolites, Davis Mountains, west Texas

Granulite xenoliths from two recently discovered localities in west Texas p rovide new information on the age and isotopic composition of the basement, and they furnish important constraints on petrogenetic models for the orig in of silicic rocks from a major Tertiary volcanic field. One xenolith loca lity lies within the Tertiary Davis Mountains volcanic field, and the other lies similar to 100 km to the south. The protolith crystallization age, de termined by U-Pb zircon geochronology, of a xenolith from the southern loca lity 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 syst ematics, Pb and Sr isotope compositions of the xenoliths indicate that the basement underwent a Proterozoic granulite-facies metamorphic event that co incided with or postdated the crystallization of the sample dated at 1090 M a, The Tertiary volcanic series of the Davis Mountains has almost constant Nd and Ph isotope compositions over the range basalt to high-silica rhyolit e. 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 epsilon(Nd) units. Furthermore, there is a substantial ga p in Pb-206/Pb-204 ratios between the Proterozoic basement samples (less th an or equal to 16.9) and the volcanic rocks (greater than or equal to 17.6) . The documentation of the strong isotopic contrast between the volcanic ro cks and the deep crust seriously weakens models advocating a crustal origin for the Tertiary silicic magmas.