Petrography and Petrology of Big Aguja Sill, Northwest Davis Mountains, Jeff Davis and Reeves Counties, Texas

The Big Aguja Sill is located in the northeast Davis Mountains in Jeff Davis County, Texas. The purpose of investigating this unit is to delineate the horizontal extent of the Big Aguja Sill, to determine if the unit was intrusive, to determine its relationship to the Huelster and Washington Tank Formations and to determine its relationship to the Star Mountain Rhyolite. Field work indicates that the Big Aguja Sill, if continuous and exposed, would have covered 100 square miles. The Big Aguja Sill was intruded as a finely crystalline, soda-rich, peralkaline traechyle porphyry along the contact of the Huelster and Washington Tank Formations. The Big Aguja Sill and the Star Mountain Rhyolite shared a common origin and possibly a common vent.

Field work, petrographic and chemical analyses indicate that the Big Aguja Sill is a massive, homogeneous unit. The predominant phenocrysts are anorthoclase, but sanidine and mafic phenocrysts also occur. Most phenocrysts show resorption and exsolution. The ground mass is composed of feldspar with minor amounts of mafic minerals, glass and opaques. Quartz grains are small and rare. Pilotaxitic texture is the most striking textural aspect.