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
Kl. Cameron et al., 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, J GEO R-SOL, 101(B9), 1996, pp. 20407-20422
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.