RAILROAD MOUNTAIN, CHAVES COUNTY, NEW MEXICO: A GEOCHEMICALLY UNIFORM, SINGLE-PHASE MAFIC DIKE EMPLACED AT THE CRATONIC MARGIN

Railroad Mountain is a single-phase, ENE to WSW-trending mafic dike which forms a resistant ridge approximately 50 km long, 30-45 m wide and up to 25 m high which intruded the margin of the stable craton. Host rocks include the Upper Chinle Group, Santa Rosa Sandstone and the Artesia Group. Contact metamorphic effects are minimal. Railroad Mountain represents the eastern-most intrusion in the Lincoln County Porphyry Belt (LCPB), part of the post-Laramide Rocky Mountain alkalic province. The dike is aligned along the WNW-trending Capitan lineament, a leaky transverse fracture of the Rio Grande rift. Igneous activity in the LCPB began in the mid-Tertiary when rocks were emplaced along northeast-trending faults during transition from Laramide compression to extension. The second period was late Tertiary when mafic alkalic dikes and granite plutons were emplaced along the Capitan lineament during the early stages of Rio Grande rifting. The dike has a K-Ar age of 27.9 ± 1.4 Ma.

Major oxides are remarkably consistent: SiO₂, Al₂O₃ and FeO* vary by less than 0.5 wt% and the other major oxides vary by less than 0.17 wt%. The mean Mg# is 41.84. Chondrite normalized REE patterns show moderate LREE enrichment (La/Yb_N = 12.72-13.26), minimal HREE enrichment (Tb/Yb_N = 2.37-2.48) and negligible Eu anomalies (Eu/Eu* = 0.94-0.98). The La/Yb_N ratios suggest garnet was not important in the source region. The spidergram is relatively smooth with moderately to highly incompatible trace elements enriched relative to primitive mantle. The rocks have an alkali-lime index of 47-48 and plot in the trachybasalt field on the TAS diagram. Most of the rocks exhibit a fine-grained diabasic texture dominated by lath-shaped subhedral plagioclase and scattered larger anhedral plagioclase. The rocks are holocrystalline intergranular with anhedral interstitial augite, anhedral olivine and subhedral opaques. Samples display a moderate to strong flow foliation.

REEs and Sr and Nd isotopes suggest a low degree of partial melting of a primitive mantle source at depth with LREE enrichment and alkali metasomatism of the mantle possibly caused by devolatilization of minerals in the subducted Farallon plate.