MAGMATIC EVOLUTION OF THE EARLY TERTIARY RHYOLITE AND ASSOCIATED INTRUSIVE ROCKS OF SIERRA BLANCA AND ROUND TOP IN THE TRANS-PECOS REGION OF TEXAS

The Trans-Pecos region of Texas hosts extensive Paleogene magmatic activity linked to subduction-related processes, caldera-sourced felsic pyroclastics, and bi-modal magmatism associated with subsequent regional extension. This magmatic province also includes numerous plutons (more than 100 in the region), many of which are shallowly emplaced felsic laccoliths. Five laccoliths, including Sierra Blanca, and Round Top peak, intrude the limestone just to the northwest of the town of Sierra Blanca, Texas. These laccoliths are chemically and mineralogically anomalous when compared to other laccoliths in the area, although not many have been studied in detail.

The magmatic rocks at Sierra Blanca and Round Top are enriched in Li, Be, F, Rb, Y, Zr, Nb, Sn, REEs, and Th. Several uncommon minerals host these elements, including HREE-rich xenotime, zircon, tantalite, columbite, cassiterite, yttro-cerite and yttro-fluorite. More than half of the rhyolite is composed of alkali feldspar (48-52%), followed by quartz (28-30%), plagioclase feldspar (8-14%), biotite (4-5%), and opaque oxides (2-3%). The rhyolites are peraluminous, low- to high-silica, peralkaline, with elevated HREE concentrations and a significantly negative Eu-anomaly. The rhyolites have high concentrations of Zr (800-1400 ppm), but very little zircon is visible in petrographic and SEM observations, consistent with a highly soluble, alkali, F-rich, magmatic system.

The variability in mineralogy, textural and compositional differences in at least four discrete cogenetic phases of rhyolite suggest fractional crystallization processes played an important role in phase differentiation prior to, and during, emplacement. REEs and Zr are found throughout the rhyolite in sub-micron groundmass. This ubiquitous dissemination coupled with HREE-rich mineral phases present as secondary, interstitial phases is indicative of late- to post-magmatic hydrothermal alteration. Textural overprint suggests the alteration is thorough and pervasive, mobilizing normally immobile elements from the igneous source rocks.