TESTING CRUSTAL MELTING MODELS FOR THE ORIGIN OF FLOOD RHYOLITES - A ND-PB-SR ISOTOPIC STUDY OF THE TERTIARY DAVIS MOUNTAINS VOLCANIC FIELD, WEST TEXAS

Flood rhyolites are lavas that have areal extents similar to large ign imbrites. The Davis Mountains volcanic field differs from most other C enozoic volcanic fields in the southwestern United States in that a ma jor volume of the silicic rocks erupted there are flood rhyolites rath er 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 compositio ns found in the volcanic field is relatively small, with epsilon(Nd) v alues ranging from about +2 in a basalt to between +1.0 and -1.5 in th e silicic rocks including the Star Mountain rhyolite. Pb isotope ratio s of the Star Mountain lavas ((206)pb/(204)pb approximate to 17.9) are considerably higher than those of xenoliths of Proterozoic deep crust al granulites from the region (<17.4). Two types of crustal sources ap pear viable for the origin of the flood rhyolites. First, if the Star Mountain samples represent near primary melts from a crustal source, t hen that source must have been Cenozoic in age and silicic in composit ion Second, if the Star Mountain samples represent highly fractionated magmas, then the crustal source could have been mafic, but it could n ot have experienced the Proterozoic granulite facies metamorphism know n to have affected the basement. The overall isotopic trends of the vo lcanic rocks are qualitatively consistent with the hypothesis that all the silicic rocks including the flood rhyolites evolved from mantle-d erived basalts with variable, but generally small, amounts of crustal contamination.