U/PB GEOCHRONOLOGY OF VOLCANIC ROCKS FROM THE INDIO MOUNTAIN RESEARCH STATION, SW TEXAS: EVIDENCE OF LARAMIDE SUBDUCTION-RELATED VOLCANISM

Tertiary volcanism in the Trans-Pecos region of West Texas (WT) is related to the transition from compression associated with the Laramide Orogeny to later extension associated with the Basin and Range. It is observed in the compositional variation from highly varied large-scale calc-alkaline ignimbritic to subsidiary alkaline mafic volcanics during Laramide deformation to later smaller-scale, almost exclusively mafic intrusives during Basin and Range extension (Barker, 1987). Fourteen representative samples of large-scale ignimbritic volcanism and small-scale effusive volcanism were sampled from different locations on the volcaniclastic fan, likely surrounding the Eagle Mountain Caldera, located within a small region of the Indio Mountain Research Station in Hudspeth County, about 26 miles southwest of Van Horn, TX.

This study uses U/Pb zircon geochronology collected by LA-ICP-MS methods to constrain the age of volcanic tuffs, rhyolites, and basalts in the region. Here we report our preliminary zircon ages obtained from three samples that help to refine the ages of volcanic episodes in this area and better constrain the end of subduction-related volcanism and the initiation of Basin and Range related volcanism in the region that is generally regarded to occur between 32-30 Ma.

One rhyolite and two rhyolitic tuffs derived from the Van Horn or Eagle Mountain Calderas (30-44 Ma; Henry and Price, 1986) collected from the region have been analyzed and yielded ages of 38.02±0.99, 37.65±0.75, and 37.34±0.48 Ma (all errors 2σ). These ages correlate with the main pulse of volcanism associated with Laramide compression in WT, which occurred between 39-32 Ma (Price and Henry, 1984). Previous studies have provided isotopic ages of 24-17 Ma for mafic dikes believed to be associated with Basin and Range extension. The volcanic rocks dated here are displaced by the Indio Fault, a major NW trending normal fault associated with Basin and Range faulting in the area (Smith and Julian, 1991), supporting their pre-Basin and Range origin. Further analysis of the remaining mafic to rhyolitic igneous samples collected and their structural relationships should better constrain the transition from subduction-related Laramide contractional deformation to the initiation of Basin and Range extension in the Indio Mountains.