New Mexico Geological Society Annual Spring Meeting — Abstracts

During middle–late Eocene time, southern New Mexico was marked by a resurgence of intermediate volcanism that took place just after Laramide deformation and prior to the late Eocene (~36 Ma) ignimbrite flareup and initiation of the Rio Grande rift. Volcanism and volcaniclastic sedimentation during this time are recorded in south-central New Mexico by rocks of the Palm Park Formation, Orejon Andesite, and equivalent units (i.e., Cleofas Andesite). Although recent efforts have been made to constrain the age and duration of middle–late Eocene volcanism in southern New Mexico, very little is known about the geochemistry and magmatic sources of these rocks. We present Sr and Pb isotope ratios of single plagioclase crystals from eight sample localities in the Doña Ana, Organ, Robledo, and Sierra de Las Uvas Mountains in south-central New Mexico. Plagioclase crystals were extracted from volcanic flows and ash-fall tuffs from the Palm Park Formation and Orejon Andesite. Results are summarized from stratigraphic oldest to youngest.

Some of the oldest parts of the Palm Park Formation crop out in the Robledo Mountains (Apache Canyon; 45.0±0.7 Ma) and are characterized by unradiogenic age-corrected ⁸⁷Sr/⁸⁶Sr signatures (0.70498–0.70519) and Pb isotopes (e.g., ²⁰⁶Pb/²⁰⁴Pb = 17.20–17.22). Compared to Apache Canyon, the slightly younger Orejon Andesite in the Organ Mountains (Fillmore Canyon; 43.8±0.4 and 42.8±0.5 Ma) have variable Sr and Pb isotopes. The first Orejon Andesite lava flow is more radiogenic than the later. ⁸⁷Sr/⁸⁶Sr signatures (0.70617–0.70644) are more radiogenic than the second lava flow (0.70470–0.70480). Pb isotopes are similar in that the first flow is more radiogenic (e.g., ²⁰⁶Pb/²⁰⁴Pb = 17.65–17.96) than the second (17.24–17.26). Samples from some of the younger parts of the Palm Park Formation in the Doña Ana Mountains (Cleofas Canyon; 41.6±0.7 and 41.3±0.7 Ma) have unradiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.70450–0.70468) and less radiogenic Pb isotopes (e.g., ²⁰⁶Pb/²⁰⁴Pb = 17.23–17.53). Palm Park Formation volcanic rocks in the Robledo Mountains (Faulkner Canyon; 41.0±0.6 Ma) have unradiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.70438–0.70471) and Pb isotopes (e.g., ²⁰⁶Pb/²⁰⁴Pb = 17.18–17.33). Finally, the youngest parts of the Palm Park Formation crop out in the Sierra de Las Uvas Mountains (Bell Top Mountain; 39.6±0.5 Ma) have radiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.70572–0.70688) and unradiogenic Pb isotopes (e.g., ²⁰⁶Pb/²⁰⁴Pb = 17.27–17.39).

Overall, feldspars from some of the early-erupted lavas (~45–44 Ma) have more radiogenic Sr and Pb isotope ratios that those in late-erupted lavas (~41–39 Ma). These variations likely resulted from involvement of different, discrete magmatic sources or magmatic systems that were incorporating more crust. The two ash-fall tuffs with large variation in Sr and Pb values from the oldest (~45 Ma) and youngest (~39 Ma) parts of the Palm Park Formation were likely far-traveled and derived from a distal magmatic source (vent).