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Possible Origins of Dikes Exposed in Northeastern New Mexico and Implications for Mid-Tertiary Alkalic Magmatism in the Region

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. 161 New Mexico Geological Society Guidebook, 70th Field Conference, Geology of the Raton-Clayton Area, 2019, p. 161-168. INTRODUCTION The Eagle Rock dike, or sometimes referred to as the Tinaja dike, is well exposed along I–25 near the Tinaja exit approximately 26 km (16 mi) south of Raton, New Mexico (Fig. 1). The dike is one of a more expansive swarm of little-studied dikes exposed regionally in northeastern New Mexico. These dikes are alkalic (Scott et al., 1990), generally strike in a west to east direction, and can be traced continuously for locally short distances. Some of the larger dikes extend for over 10 km and can be connected by tracing discontinuous exposures across regional low-lying hills and local drainages (Fig. 2A, B). Individual dikes display different compositions and textures (Fig. 2C). Many of the dikes appear composite in that a single dike has multiple internal contacts suggesting pulsed magma flow during emplacement. The Eagle Rock dike, and the nearby Little Eagle Rock dike exposed ~1 km south along I–25, are thicker and more prominent than most of the dikes that are exposed in this region. The magmatic, tectonic, and geochemical significance of the Eagle Rock and Little Eagle Rock dikes in relation to similar surrounding alkalic dikes and larger magmatic centers of northern New Mexico is not well constrained. Regionally, these dikes may be related to alkalic magmatism that generated mid-Tertiary alkalic rocks in the mountains to the east that are loosely referred to as the Chico Sill complex. Alternatively, the Eagle Rock dike and other dikes in the region may result from distal emplacement of magmas associated with the 25.4 Ma Questa caldera (Zimmerer and McIntosh, 2012) located within the Latir volcanic field along the eastern edge of the Rio Grande rift in the southern Rocky Mountains south of the Colorado-New Mexico border. The Eagle Rock and Little Eagle Rock dikes, along with other regional dikes, are radial to both the Chico Sill complex (located ~10 to 30 km away) and the Questa caldera (located ~60 to 90 km away). We briefly discuss preliminary geochronology and geochemistry of the Eagle Rock and Little Eagle Rock dikes in assessing their origins and potential connections, if any, to these two different alkalic magmatic provinces.