Possible Origins of Dikes Exposed in Northeastern New Mexico and Implications for Mid-Tertiary Alkalic Magmatism in the Region
— Zimmerer, Matthew J., Ramos, Frank C. and Sebastian Orozco

Numerous, little-studied dikes are exposed in northeastern New Mexico, particularly in the area south of Raton along the eastern margin of the Raton basin. We describe preliminary geochronology, geochemistry, and geographic relationships of a small suite of these dikes and discuss their relationships to two large magmatic centers of northern New Mexico, the distal 28.5-15.5 Ma Questa caldera and the proximal 36.7-20.3 Ma Chico Sill complex. Some of the dikes are radial to both the Questa caldera and Chico Sill complex. In contrast, some of the dikes are only radial to the Questa caldera. Two of the more topographically prominent dikes are the Eagle Rock and Little Eagle Rock dikes, which yield new ⁴⁰Ar/³⁹Ar ages of 23.58±0.06 and 21.36±0.06 Ma, respectively. Despite their close proximity to each other (~1 km) and some similarities in composition, the new ages indicate the dikes were emplaced during two separate episodes of magmatism. Although these two dikes are radial to both of the larger potential magmatic sources, the new ages closely match the youngest pulses of postcaldera magmatism at Questa. ⁸⁷Sr/⁸⁶Sr ratios are similar between some Questa caldera mafic rocks and the Eagle Rock dike, but there are prominent differences in ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, suggesting that if the dikes are related to the Questa caldera magmatic system, the dikes tapped isotopically distinct reservoirs or acquired different isotopic signatures during magmatic transport. No published isotopic data exists for the Chico Sill complex, hindering the establishment of potential links between the dikes and sill complex. Published ages for a dike west of Raton and along the New Mexico-Colorado border, both of which are only radial to the Questa caldera, are 15.5 and 19.5 Ma, respectively, closely correlating to the timing of Questa magmatism. Thus, preliminary data suggest that some dikes exposed in northeastern New Mexico are possibly distal radial dikes associated with the youngest pulses of magmatism at the Questa caldera. However, additional geochronology and geochemistry, together with field assessment to identify the total number of dikes and anisotropy of magnetic susceptibility to determine flow directions, will be necessary to fully resolve potential connections between the dikes of northeastern New Mexico and their relationships to regional tectonics and large mid-Tertiary magmatic systems of the area.