Early Trans-Pecos magmatism: petrology and geochemistry of Eocene intrusive rocks in the El Paso area
— James D. Hoover, Susan E. Ensenat, Calvin G. Barnes, and R. Dyer

Hypabyssal dikes, sills and plugs that crop out in the vicinity of El Paso, Texas are one of the earliest manifestations of Trans-Pecos magmatism. These porphyritic rocks of trachyandesite composition intrude Lower Cretaceous rocks and have been dated at 47-48 my. They are a comagmatic series that possibly represent part of the roof zone of a caldera-sized pluton. The trachytic liquid component of these rocks evolved from a more basic parent magma by fractional crystallization involving amphibole and plagioclase at deeper levels in the crust. Ubiquitous enclaves of diorite, monzonite and quartz diorite appear to represent cognate xenoliths associated with this fractionation. Enclaves of deformed anorthosite may be xenoliths of lower crustal material. Similarities in the texture, mineralogy, enclaves and major and trace element compositions of these rocks  indicate that they are co-magmatic. Though contamination by Cretaceous country rock material is minimal, phenocryst zoning patterns reflect changes in liquid composition by crustal assimilation or magma mixing. Distinctive incompatible trace element compositions are also indicative of an origin in an orogenic environment. The elements Rh, Ba, Cs, Sr and LREE are especially enriched in these rocks relative to MORB's, and suggest source material with a MORB-like composition enriched in these hygromagmatic components. These rocks locally intrude post-Laramide high-angle faults, indicating a coincidence between this early phase of Trans- Pecos magmatism and a change in the local stress regime.