RE-EVALUATING THE EMPLACEMENT HISTORY AND TECTONICS OF THE CORNUDAS MOUNTAINS, SOUTHERN NEW MEXICO

The Cornudas Mountains are a group of Eocene-Oligocene syenite and nepheline syenite laccoliths and plugs with phonolite and syenite sills and dikes that form nine mountains in southern New Mexico and western Texas, east of El Paso. These mountains intrude Permian limestones of the Otero Mesa, east of the Rio Grande River between the Tularosa and Salt Basins. These mountains are of economic interest due to their potential for critical minerals such as rare earth elements, beryllium, and niobium. Here, we produce a new emplacement model of the Cornudas Mountains to augment the existing petrology and geochronology studies of the area and guide exploration. This is part of Earth MRI Project UMRI10.

The history of the Cornudas Mountains is not well understood, due to low-precision dating. Five intrusions have been dated with the K-Ar dating method, and only one has been dated with ⁴⁰Ar/³⁹Ar geochronology. This study uses ⁴⁰Ar/³⁹Ar dating methods on 21 intrusions, including 11 plugs and laccoliths, five sills, and four dikes. The new ages show an emplacement history from 37.1-26.4 Ma, which is longer than previously reported ages (36.0-31.6 Ma). We find no trend in the timing of plugs and laccoliths; however, dike intrusion is constrained from 35.8-30.2 Ma, where ages show an ~2 Ma hiatus in intrusion between 34.5 and 32.5 Ma.

We applied a clinopyroxene-liquid thermobarometer to determine the emplacement temperatures and pressures of each intrusion, using only crystal compositions in equilibrium with the whole rock based on ^Fe-MgK_D values. The plugs and laccoliths contain clinopyroxenes that record hotter temperatures (830±2-917±1^oC) than the sills (760±3-869±2^oC). Pressures range from 0.3-3.3 (±0.2) kbar, and we observe no patterns between pressure and intrusion type. The crystallization pressures correspond to emplacement depths between 1-12 (±0.6) km. Pairing these depths with the new geochronology, we calculate minimum average exhumation rates for intrusions in the Cornudas Mountains of 0.03-0.34 (±0.1) mm/y. These fall between rates of erosion (0.05 mm/y) and Rio Grande exhumation (0.4 mm/y), indicating a possible tectonic component to exhumation. Despite the emplacement being pre-rifting in origin (~25 Ma), the speed of exhumation suggests that the Cornudas Mountains were exhumed during Rio Grande rifting.