A high-resolution aeromagnetic survey flown in March of 2001 (http://pubs.usgs.gov/of/2002/ofr-02-0049/) detected over 200 shallow, igneous intrusive bodies whereas fewer than 20 had previously been reported. The volcanic field surveyed, sometimes called the Uvalde igneous field, is centered in Uvalde County and extends both west into Kinney County, south into Zavala County, and to the east as far as Bexar County.
The aeromagnetic map shown above reveals numerous bump-like magnetic anomalies in Medina and Uvalde Counties. These anomalies are set against broader regional magnetic highs (red) and lows (blue) and are caused by contrasts in magnetic susceptabilities because the igneous rocks contain much more magnetite than limestone. These anonmalies effectively distinguish the intrusive rocks from the surrounding country rock. Using matched-filtering techniques on the aeromagnetic data, the shallow (0-500 m) anomalies were separated from the moderately-deep plutons and crystalline basement. This processing, together with 3-D modeling of the data, shows that the isolated anomalies are most likely caused by nearly-vertical volcanic pipes. Clustered and overlapping anomalies probably represent a number of pipes and associated volcanic flows.
The apparent random distribution of these igneous bodies, and the sparse number of dikes and dike-like structures found, raises questions as to whether they represent a single intrusive episode or possibly several. This question can best be answered by isotopic, geochemical, and geochronological analysis.
A preliminary 40Ar/39Ar geochronologic analysis of various mineral separates and groundmass concentrates from a variety of rock types exposed in Uvalde County (http://pubs.usgs.gov/of/2004/1031/) revealed two distinct age groups, one at approximately 82 to 80 Ma (million years ago) and the other at 74 to 72 Ma. Previous K/Ar ages indicate that igneous activity spanned a range in age from 90 to 60 Ma. Dominant rock types are alkalai basalt, melilite-olivine nephelinite, olivine nephelinite, nepheline basinite, and phonolite.
Fractions of K-feldspar were obtained from east of Uvalde along U.S. Highway 90 and at Fort Inge. Plagioclase and groundmass concentrates were obtained from an intrusive exposure north of Cline. Future studies for the Uvalde rocks will include the age dating of nepheline. Alunite from the hydrothermal mineralization associated with the intrusions, will be analyzed to determine if the formation of alunite occurred during or after the emplacement of the intrusions. Also, samples will be collected from the southernmost, westernmost, and northernmost exposures to determine if there is a geographic distribution in the ages of the intrusions based on geochronological and geochemical data.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/edwards/geochron.html
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Page Last Modified: Wed 12-Oct-2005 10:03:45 MDT
