New Mexico Geological Society Annual Spring Meeting — Abstracts

This study focused on igneous intrusions of the Turkey Mountains, Mora County, New Mexico, to relate the igneous structures and age of crystallization to the regional tectonomagmatic setting. The Turkey Mountains are a 365 km² wooded massif situated on the Southern High Plains physiographic province 5 km northeast of Fort Union National Monument. The main mass is a laccolith while subsidiary dikes, stocks, and plugs intrude the sedimentary rock cover. Because the laccolith intrusion is not exposed at the surface, well log and well cuttings from Union Land and Grazing #1 Fort Union were studied to characterize the igneous rocks that encompass the laccolith. Igneous rock was encountered at six discrete intervals within the Upper Pennsylvanian Madera Group at depths of 843-856 m, 930-980 m, 994-1012 m, 1078-1103 m, 1185-1197 m, and 1225-1239 m. The igneous rock cuttings were white colored, homogeneous, very fine grained, concoidally fractured, and dull lustered. They classified as porphyritic rhyolite based on the presence of sanidine and quartz microlites in a highly siliceous matrix. We report two new ⁴⁰Ar/³⁹Ar cooling age spectra from a middle-level (1078-1103 m) and lowest-level bulk well cutting samples. Both have classic saddle shapes that indicate excess argon. Taking the youngest part of the youngest age spectra, we interpret the sills as having a maximum crystallization age of 30. 5± 0.2 Ma. This age is significantly older than the Middle Miocene lamprophyre dikes, stocks, and plugs that intrude the Turkey Mountains as well as Late Miocene to Pleistocene extrusions of the Mora-Ocate volcanic field. We submit that the Turkey Mountain granitic laccolith was emplaced within a compressive stress regime related to the waning stages of the Laramide orogeny.