New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

The Cornudas Mountains are a group of Eocene-Oligocene alkaline laccoliths, plugs, sills, and dikes expressed as nine prominent mountains that straddle the New Mexico-Texas line approximately 60 miles east of El Paso. The intrusions are exposed on the Otero Mesa, situated between the Tularosa and Salt Basins, on the eastern edge of the Rio Grande Rift. The Cornudas Mountains have recently become of interest because of their economic potential for rare earth elements (REEs), as REE-bearing minerals such as eudialyte, bastnaesite and monazite are observed in some intrusions and altered zones. The petrology, mineralogy, and ages of these intrusions were described in detail in the 1970’s and 1990’s and summarized in McLemore (2018). The purpose of the work is to update these datasets to help guide exploration and understand the emplacement of intrusion.

The lithology of the Cornudas Mountains can be grouped into nepheline syenite to syenite laccoliths and plugs, phonolite sills, and phonolite dikes that are intruded into Permian sediments, which are largely limestone. Many dikes are altered, though pervasive alteration is not found in larger intrusions. The emplacement history of the Cornudas Mountains is not well understood, since only a few of the intrusions were dated using the K-Ar dating method. The previously reported age ranges are 36.8-31.6 Ma. McLemore (2018) provides a new ⁴⁰Ar/³⁹Ar age for Wind Mountain, which is being checked. New ⁴⁰Ar/³⁹Ar ages of the intrusions show an emplacement history of ~12 Ma, from 37.14 to 25.58 Ma, which is longer than those previously reported. These new ages suggest three periods of emplacement: an early laccolith and plug stage (37.14-35.77 Ma), a middle sill stage (32.78-31.07 Ma), and a younger dike stage (32.18-30.16 Ma). One sill (Flat Top, 25.58 Ma) does not follow this progression and is being reassessed.

We incorporate the whole rock and clinopyroxene chemistry of each intrusion into the clinopyroxene-liquid geothermobarometer (Putirka, 2008) to determine the temperatures and pressures of emplacement. Applying the thermometer shows that the syenite intrusions crystallize at higher temperatures (821-1019^oC) than the phonolite sills (763-797^oC); barometry yields similar pressures within error of the model (1.4-2.7 ± 2kb).

We use the crystallization pressures and a limestone density of 2730 kg/m³ to calculate emplacement depths (5-11 km). Pairing these depths with the new geochronology, we calculate minimum exhumation rates for intrusions in the Cornudas Mountains that range from 0.1-0.4 mm/y, with faster exhumation rates for younger intrusions. This is faster than the average rate of erosion for a drained part of continents, which could indicate a tectonic component to the exhumation of these intrusions. The timing of the emplacement and exhumation of the Cornudas Mountains could be valuable to understanding the eastern extent of Paleogene extension prior to Rio Grande rifting.

References:

1. McLemore, V. T. (2018). Rare Earth Elements (REE) Deposits Associated with Great Plain Margin Deposits (Alkaline-Related), Southwestern United States and Eastern Mexico. Resources, 7(1), 8. https://doi.org/10.3390/resources7010008
2. Putirka, K. D. (2008). Thermometers and Barometers for Volcanic Systems. Reviews in Mineralogy and Geochemistry, 69(1), 61–120. https://doi.org/10.2138/rmg.2008.69.3