U-Pb geochronology of middle�late Eocene intermediate volcanic rocks of the Palm Park Formation and Orejon Andesite in south-central New Mexico

A bstrAct —U-Pb zircon ages from the middle–late Eocene intermediate volcanic rocks of the Palm Park Formation and Orejon Andesite provide new geochronologic constraint on the duration of volcanism that took place just after the Laramide orogeny and prior to the onset of latest Eocene bimodal volcanism and initiation of the Rio Grande rift in south-central New Mexico at ~36 Ma. Presented here are nine new U-Pb zircon ages (n=247 analyses) from the Organ, Doña Ana, Robledo, and Sierra de las Uvas Mountains that fall within a range of ~45–40 Ma. The oldest age determined from this study was collected from an ash-fall tuff exposed in the lower part of the Palm Park Formation in the Robledo Mountains (Apache canyon) and yields an age of 45.0±0.7 Ma. Three samples from intermediate volcanic flows in the Orejon Andesite of the Organ Mountains (Fillmore canyon) yield ages of 44.0±1.5, 43.8±0.4, and 42.8±1.5 Ma. Four samples were collected from a series of intermediate composition (andesite to dacite) volcanic flows in the Doña Ana Mountains (Cleofas canyon) and Robledo Mountains (Faulkner canyon) and yield ages of 41.6±0.7, 41.3±0.7, and two nearly-identical ages of 41.0±0.6 Ma. The youngest age reported from this study was determined from an ash-fall tuff exposed near the top of the Palm Park Formation in the eastern part of the Sierra de las Uvas fault block and yields an age of 39.6±0.5 Ma. New data are presented here, together with previously reported ages and fossil occurrences from the Palm Park Formation, Orejon Andesite, and equivalent units throughout south-central New Mexico, including the Cleofas Andesite and Rubio Peak Formation. Based on new ages and fossil age ranges, we favor a model where these rocks were erupted and deposited over a ~10 my period between ~46–36 Ma during which mantle wedge material was re-introduced throughout south-central New Mexico just prior to onset of the Rio Grande rift.