Petrogenesis of the Railroad Mountain Alkali Olivine Diabase Dike: Intrusion at the Edge of the Stable Craton in Eastern New Mexico

The Railroad Mountain dike is a single-phase alkali olivine diabase intrusion located in east-central New Mexico that represents the easternmost igneous body in the Lincoln County porphyry belt, which is part of the post-Laramide Rocky Mountain alkalic province. The dike forms a resistant ridge, Railroad Mountain, approximately 50 km long, 30–45 m wide, and up to 25 m high. Two new groundmass 40 Ar/ 39 Ar ages are nearly identical, averaging 27.66 ± 0.03 Ma. The dike rocks are uniformly fine-grained and unaltered except for minor local effects, with textures in thin sections ranging from fine-grained trachytic to diabasic. Modal mineralogy averages 60% plagioclase, 20% augite, 10% olivine, 10% opaques (magnetite and ilmenite), and accessory apatite. Major element oxide concentrations from 37 samples are remarkably consistent, with differences between minimum and maximum concentrations of less than 1 wt% for all oxides. The mean silica concentration is 47.51 ± 0.18 wt%, the MgO concentration is 5.05 ± 0.071 wt%, and Na 2 O is greater than K 2 O. Mg#s average 41.84 ± 0.43, and the concentrations of TiO 2 , FeO*, CaO, and Sc increase with decreasing Mg#, suggesting clinopyroxene fractionation. The decrease in Cr and Ni with decreasing Mg# likely reflects changes in olivine composition during differentiation. The silica and alkali values plot in a tight cluster