PETROGENESIS OF THE RAILROAD MOUNTAIN OLIVINE DIABASE DIKE, AN ALKLINE INTRUSION ON THE HIGH PLAINS OF EASTERN NEW MEXICO

The Railroad Mountain dike is an alkali olivine diabase intrusion located in east-central New Mexico that represents the easternmost body of the Lincoln County Porphyry Belt, part of the post-Laramide Rocky Mountain alkalic province. The dike forms a resistant ridge approximately 50 km long, 30-45 m wide, and up to 25 m high. Forty samples were collected for analysis along 14 km of the exposed position of the dike. Two new groundmass ⁴⁰Ar/³⁹Ar ages yielded nearly identical results averaging 27.66 ± 0.03 Ma. The rocks are uniformly fine-grained, holocrystalline intergranular, and very fine-grained at the contacts, grading into fine-grained trachytic close to the contacts and diabasic in the core. Mineralogy averages 60% plagioclase, 20% augite, 10% olivine, 10% Fe-Ti oxides, and accessory apatite. Major element oxide concentrations show only slight variation with a mean silica concentration of 47.51 wt.% and Na₂O > K₂O. Mg#s range from 40.93 to 43.30. Concentrations of TiO₂, FeO*, CaO, and Sc increase with decreasing Mg#, suggesting clinopyroxene fractionation. The rocks are nepheline normative and are classified as sodic alkali trachybasalts. LREEs are moderately enriched (La/Yb_N = 12.72-13.26), and HREEs exhibit minimal enrichment (Tb/Yb_N = 2.37-2.48). The lack of HREE fractionation suggests a mantle source with no residual garnet. The LILEs are enriched 26 to 160 times primitive mantle values, and the HFSEs are enriched 14 to 98 times primitive mantle values. All samples plot in the within plate field on the Zr-Zr/Y tectonic discrimination diagram.

The proposed source rock is a spinel lherzolite. Melt modeling was done using published spinel lherzolite chemistry and the mineralogy of Kilbourne Hole spinel lherzolite nodules. The results of nonmodal equilibrium and nonmodal fractional batch melting models are similar and indicate a small degree (∼5%) of partial melting. The dike was intruded at the edge of the stable North American craton following the foundering and rollback of the Farallon slab as asthenospheric upwelling induced melting in the previously metasomatized lithosphere. The timing of intrusion corresponds with the early stages of Rio Grande rifting.