Petrology and Mineralogy of the Campus Andesite Pluton, El Paso, Texas

Abstract

The Campus Andesite represents a small igneous pluton located in the Rio Grande valley and crops out on the campus of The University of Texas at El Paso. The igneous mass is post-Cretaceous in age, showing intrusive contacts with shale and marl of the Boquillas Formation and is surrounded by Quaternary lake or alluvial deposits (or both). Absolute age of the pluton, based on a K-Ar date, is 47.1 ± 2.3 m.y. Texturally, the intrusive is porphyritic with phenocrysts averaging 2 to 3 mm in length and set in an aphanitic groundmass. The phenocrysts, composed predominantly of plagioclase (andesine), comprise approximately 40 percent of the rock. The mineralogy of the andesite consists of phenocrysts of plagioclase, biotite, and hornblende with groundmass constituents of plagioclase, K-feldspar, and minor quartz and magnetite. Two chemical analyses of the Campus Andesite indicate a chemical composition intermediate between an andesite and a dacite. Mineralogically, the rock is classified as a porphyritic andesite.