PETROLOGY AND GEOCHEMISTRY OF THE PRECAMBRIAN THUNDERBIRD FORMATION, FRANKLIN MOUNTAINS, EL PASO COUNTY, TEXAS.
The Thunderbird Formation in the Franklin Mountains of north of El Paso, Texas consists of three members composed of volcanic and sedimentary units. Conglomerate, consisting of sedimentary and volcanic cobbles and pebbles in a recrystallized arkosic matrix, forms the lowest member of the Thunderbird Formation. The middle member is a mixed unit consisting of a lower melanocratic (amphibole and/or chlorite rich) porphyritic trachyte, and intercalated tuffaceous sandstones and conglomerates, silicified ignimbrites, very thin ash fall deposits, and trachytes. The upper member is the thickest unit consisting predominantly of rhyolitic ignimbrites (crystal-vitric tuffs) with a few porphyritic rhyolite dikes.
Contact metamorphism due to intrusion of the Precambrian Red Bluff Granite recrystallized the entire formation to the albite-epidote hornfels facies. The recrystallized groundmass in most of the ignimbrites of the upper member makes it almost impossible to determine the original degree of welding. However, a few of the least altered ignimbrites exhibit devitrified, compact pumice and glass shards with eutaxitic texture characteristics of densely welded tuffs.
Radiometric ages for the volcanics of the Thunderbird Formation, Red Bluff Granite, and a micrographic granite porphyry are 953 (+OR-) 13 million years old. The micrographic granite porphyry may have been the plutonic source of the trachytes of the middle member of the Thunderbird Formation. The Red Bluff Granite appears to have been the plutonic source of the ignimbrites of the upper member of the Thunderbird Formation based upon chemical and mineral similarities.
The Precambrian silicic metavolcanics at Pump Station Hills, and the volcanic and granite cuttings recovered from deep wells in the Precambrian basement of the west Texas and southern New Mexico area are correlative in age, and similar in lithology to the Precambrian volcanics and granites in the Franklin Mountains. The late Precambrian volcanic-epizonal granite complex of the Franklin Mountains igneous province forms an association similar to the south-central United States. The radiometric age of 1240 m.y. for the Carrizo Mountain Rhyolite indicates that this unit is older and, hence, not related to the volcanics of the Franklin Mountains igneous province.
THOMANN, WILLIAM FREDERICK, "PETROLOGY AND GEOCHEMISTRY OF THE PRECAMBRIAN THUNDERBIRD FORMATION, FRANKLIN MOUNTAINS, EL PASO COUNTY, TEXAS." (1980). ETD Collection for University of Texas, El Paso. AAI8019774.