Petrology of the Seven Springs Formation, Davis Mountains, Texas
McKay, Gordon Alan
Rogers, John J. W.
Master of Arts
A petrological study was made of the Seven Springs Formation in the northeastern Davis Mountains, West Texas. This formation consists of a basalt and three ignimbrites, interbedded with air-fall tuffs. The basalt is very poor in phenocrysts. The ignimbrites contain only one abundant phenocryst type, anorthoclase of composition 0r35-0r40. Systematic variations are present in the phenocryst content and average chemical composition of the ignimbrites. In each successively younger ignimbrite the phenocryst content increases, Si02 and total Fe decrease, and Na20, K20, Al203, Th, and U increase. These features can best be explained by fractionation of alkali feldspar and post-eruptive loss of alkalis. It is suggested that a magma of the pantelleritic composition of the underlying formation, the Star Mountain Rhyolite, differentiated by gravitational settling and resorption of alkali feldspar to form the ignimbrites of the Seven Springs Formation. The igneous rocks of the northeast Davis Mountains are similar in composition to the igneous rocks of the area 100 miles to the south, and seem to be part of a large igneous province characterized by alkaline rocks poor in Ca0 and MgO.