New Mexico Geological Society

Volcanic rocks of the Palm Park Formation, exposed in multiple locations throughout south-central New Mexico, are composed of volcanic and volcanoclastic rocks that are lithologically and compositionally variable at scales within individual exposures and between exposures at different localities. 40Ar/39Ar ages of plagioclase and biotite from Palm Park volcanic rocks range from ~43.5 to ~39.5 Ma. These eruption ages can be used to correlate exposures throughout south-central New Mexico and are generally consistent with zircon ages from the same units. Overall, these ages constrain the timing of magmatism related to the Laramide Orogeny as it transitioned to magmatism associated with the ignimbrite flare-up. This is a poorly studied portion of the middle Tertiary magmatic history of southern New Mexico and Palm Park Formation volcanic units provide an excellent opportunity to refine our knowledge of the geologic history of this time interval. 165 INTRODUCTION Although the ages of late-Eocene, ignimbrite flare-up-related magmatism have been well documented in south-central New Mexico (e.g., Zimmerer and McIntosh, 2013; Verplanck et al., 1999; Ramos et al., this volume), little is known about the specific ages of middleto late-Eocene volcanism and volcaniclastic sedimentation that occurred just prior to this time period. Tectonically, the period encompasses the transition between Laramide orogenesis and the onset of late Eocene/ early Oligocene bimodal volcanism (prior to initiation of Rio Grande rifting in this region at ~36 Ma) and is reflected in the volcanic and volcaniclastic rocks of the Palm Park Formation. The Palm Park Formation in south-central New Mexico is composed of an agglomeration of lithologically variable volcanic rocks and volcanoclastic sedimentary rocks, mostly andesitic or dacitic in composition, that are scattered throughout the region. Although lavas within regional stratigraphic sections are local in nature, accompanying ash-fall deposits likely originate from more distal volcanic sources. Overall, these rocks are primarily andesites or dacites and are difficult to stratigraphically correlate across localities. The oldest exposed section of the Palm Park Formation is marked by a light-colored ash-fall tuff (PALMP(RB/AC)-02) in the Robledo Mountains (Seager et al., 2008) that yields a weighted-mean zircon age of 45.0±0.8 Ma (Creitz et al., this volume). This ash-fall tuff is present in Palm Park strata but may originate from a more distal source and not reflect the same local magmatic sources as most Palm Park Formation volcanic rocks (Ramos et al., this volume). Similarly, the youngest section of the Palm Park Formation is defined by an ash fall tuff (PALMP(SU/BT)-01) in the uppermost Palm Park Formation in the Sierra de las Uvas Mountains that yields a weighted-mean zircon age of 39.6±0.5 Ma (Creitz et al., this volume). This tuff may also originate from a distal magmatic source (Ramos et al., this volume). Intervening strata are, howNew Mexico Geological Society Guidebook, 69th Field Conference, Las Cruces Country III, 2018, p. 165-171. ever, related to Palm Park Formation volcanic sources that are local in nature (Ramos et al., this volume). Here, 40Ar/39Ar ages are determined for a range of Palm Park lithologies that include ash-fall tuffs and andesitic and dacitic lavas exposed in the Doña Ana Mountains, Robledo Mountains, and Sierra de las Uvas Mountains (Fig. 1). These 40Ar/39Ar ages are compared to the zircon ages of Creitz et al. (this volume) that were obtained from the same or similar rocks. In general, Palm Park Formation rocks can be difficult to date because they are commonly altered and thus new ages determined here will ultimately provide a critical temporal foundation in which to correlate and understand the origins of highly disparate and poorly understood exposures of Palm Park Formation rocks scattered throughout south-central New Mexico (e.g., in the Organ, Caballo, Potrillo, and southern San Andres Mountains). METHODS Whole rock samples were collected from eight individual sites (Fig. 1, Table 1) encompassing four regional Palm Park exposures including those in the Sierra de las Uvas (Bell Top Mountain), Robledo (Apache and Faulkner Canyons), Doña Ana (Cleofas Canyon), and Organ Mountains (Fillmore Canyon). Two additional samples, one from the basal portion and one from the upper portion of the Palm Park Formation in Cleofas Canyon in the Dona Ana Mountains, collected by Ramos et al. (this volume), will also be discussed. Whole rocks were crushed and sieved to obtain the largest grains available (typically >50 to <300 um). Individual plagioclase feldspars and biotite (from a single dacite from Cleofas Canyon) were hand picked from sieve fractions. Plagioclase crystals were etched in 10% hydrofluoric acid for 5-20 minute intervals, rinsed in distilled water, and sonicated to obtain crystals free of adhering materials. Plagioclases chosen for single crystal analyses ranged from 0.1 to 3 mg. Biotite from the Cleofas Canyon dacite was gently cleaned in water. Ramos, HeizleR, and Hampton 166 U va s V al le y Black Range Point of Rocks G oo ds ig ht M tn s. Robledo Mtns. Doña Ana Mtns. Caballo Mtns.