Flood rhyolites are lavas that have areal extents similar to large ignimbrites. The Davis Mountains volcanic field differs from most other Cenozoic volcanic fields in the southwestern United States in that a major volume of the silicic rocks erupted there are flood rhyolites rather than ignimbrites. A comparative isotopic study was made of a major flood rhyolite unit, the Star Mountain, and mafic to silicic rocks from three other magma systems. The entire range in Nd isotopic compositions found in the volcanic field is relatively small, with &egr;Nd values ranging from about +2 in a basalt to between +1.0 and -1.5 in the silicic rocks including the Star Mountain rhyolite. Pb isotope ratios of the Star Mountain lavas (206Pb/204Pb≈17.9) are considerably higher than those of xenoliths of Proterozoic deep crustal granulites from the region (<17.4). Two types of crustal sources appear viable for the origin of the flood rhyolites. First, if the Star Mountain samples represent near primary melts from a crustal source, then that source must have been Cenozoic in age and silicic in composition. Second, if the Star Mountain samples represent highly fractionated magmas, then the crustal source could have been mafic, but it could not have experienced the Proterozoic granulite facies metamorphism known to have affected the basement. The overall isotopic trends of the volcanic rocks are qualitatively consistent with the hypothesis that all the silicic rocks including the flood rhyolites evolved from mantle-derived basalts with variable, but generally small, amounts of crustal contamination. ¿ American Geophysical Union 1995