THE PETROGENESIS OF TURKEY MOUNTAIN DIKES, MORA COUNTY, NEW MEXICO, AND THEIR RELATIONSHIP TO REGIONAL MAGMATISM

Abundant NNW to NNE trending mafic dikes intrude the Tertiary Turkey Mountain (TM) laccolith, a structural dome located 11 km west of Wagon Mound in northeastern NM. We studied the mineralogy and chemistry of these dikes as well as two volcanoes from the adjacent Late Cenozoic Ocate Volcanic Field (OVF) and a dike set near Las Vegas, NM, to characterize the magma(s) that gave rise to the TM dikes and assess their relationship to regional igneous activity. The TM dikes and the OVF samples are alkaline basalts and display vesicular or amygdaloidal textures. They contain olivine ± clinopyroxene ± plagioclase phenocrysts in a microcrystalline plagioclase-mafic mineral intergranular groundmass with minor post-magmatic carbonate alteration. Pearce element ratio (PER) analysis indicates that olivine, clinopyroxene, and plagioclase were major fractionating phases. The rocks display a narrow range of the compatible elements Co, Cr, Ni, and Sc and overlapping MORB-normalized incompatible trace element patterns. The basalts show no enrichment in the least incompatible elements Y and Yb relative to MORB but strong enrichment in the most incompatible elements Ba, Th, Ta and Nb which is characteristic of within plate basalts. The rocks also show parallel rare earth element (REE) patterns with strongly fractionated light REE (La/Sm = 5.35-8.89) and slightly to moderately fractionated heavy REE (D/Yb = 1.95-3.93). All samples show a small positive Eu anomaly. These data suggest that the rocks have a common or similar magma source. The Las Vegas dike geochemistry consistently plots outside of the TM dike group on compatible element variation diagrams. The Las Vegas dikes also show a different structural signature than the TM dikes. They display a strong NNE trend (mean strike N20°E; N=13) and are interpreted to have formed in response to extension perpendicular to the swarm whereas the TM dikes show a modest radial pattern and likely exploited fractures that formed above an expanding pluton. We also compared the TM dike chemistry to the 37-20 Ma Chico Sill Complex, a broad domal structure in the nearby Raton-Clayton Volcanic Field. PER analysis illustrates that these suites are not comagmatic. Dissimilarities in incompatible trace element and REE concentrations further demonstrate that these intrusions are not consanguineous.