PETROGENESIS AND GEOCHEMICAL CHARACTERIZATION OF THE BLACK MOUNTAINS: INSIGHTS INTO REE-ENRICHED ALKALINE MAGMATISM IN THE TRANS-PECOS REGION OF TEXAS

The Trans-Pecos magmatic province of Texas is characterized by significant Paleogene magmatic activity, driven by subduction-related processes, caldera-derived felsic pyroclastics, and bimodal magmatism associated with subsequent regional extension. The magmatic province contains over 100 plutons, many of which were shallowly emplaced, felsic to intermediate, alkaline intrusions. The Black Mountains represent one such occurrence and are located 8 kilometers northwest of the Sierra Diablo Range and approximately 65 kilometers southeast of alkaline intrusions of the 37 – 27 Ma Cornudas Mountains. Similar to the Cornudas Mountains, the Black Mountains samples are alkaline, ferroan, peralkaline to metaluminous, and display within-plate- granite geochemical affinity. They are intermediate in composition, containing predominantly k-feldspar and nepheline phenocrysts within a sodium plagioclase groundmass. Mafic minerals include pyroxenes, such as aegirine-augite and amphiboles, such as riebeckite.

Whole-rock geochemistry shows an enrichment in LREEs at Black Mountains, with average REE + Y concentrations of 848 ppm (n=22). These results are more elevated than the average REE + Y values of the Cornudas Mountains which were 770 ppm, 543 ppm, and 589 ppm for Chattfield, Washburn, and San Antonio, respectively. Fluorine enrichment in the Black Mountains is higher than the Cornudas, with an average value of 2065 ppm in Black Mountains compared to an average value of 539 across the Cornudas samples. Petrogenetic modeling of intrusive rocks of Black Mountains and Cornudas Mountains provides insights into the nature of REE + Y enriched alkaline systems across Trans-Pecos Texas.