PRELIMINARY 40AR/39AR AGE CONSTRAINTS FOR THE UVALDE IGNEOUS INTRUSIONS WEST OF SAN ANTONIO, TEXAS

The igneous rocks within Uvalde County, West of San Antonio, Texas represent the southwestern margin of the Balcones igneous province and consist of fine to course-grained ultramafic and hypabyssal rocks. These rocks economically are important, forming natural gas traps as they intrude the surrounding Late Cretaceous sedimentary rocks. Olivine and clinopyroxene constitute 25-60 % of the exposed rocks and are a unique group of igneous rocks that represent dikes, plugs, and shallow intrusions. They may influence the flow paths of the Edwards aquifer. A high-resolution aeromagnetic survey flown in 2001 detected over 200 shallow igneous intrusive bodies and fewer than 30 have surface expression. The igneous Uvalde field is intensely faulted and the igneous rocks could also act as barriers to ground-water flow by filling primary voids with secondary minerals, decreasing permeability. The Uvalde igneous field also may influence the major Edwards aquifer flow paths as they reportedly change direction just east of Uvalde (Maclay and Land, 1988).

Studies of these volcanic rocks have been reported since the early 1900’s. Previous K/Ar age constraints reported by Baldwin and Adams (1971) indicate that igneous activity spanned a range in age from 90 to 61 Ma. Preliminary 40Ar/39Ar dating of 4 samples from the Uvalde igneous rocks has yielded a more precise volcanic episode ranging from 78 to 71 Ma. One sample yielded an age of 71.51±1.15 Ma, whereas disturbed age spectra for the other 3 samples only indicate a Late Cretaceous age.