New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/69 Sr and Pb isotope variations of feldspars in the middle to late Eocene Palm Park Formation and Orejon Andesite: Implications for regional variability and magmatic source characteristics F.C. Ramos, M. Jacobs, and B.A. Hampton, 2018, pp. 181-188 in: Las Cruces Country III, Mack, Greg H.; Hampton, Brian A.; Ramos, Frank C.; Witcher, James C.; Ulmer-Scholle, Dana S., New Mexico Geological Society 69th Annual Fall Field Conference Guidebook, 218 p. This is one of many related papers that were included in the 2018 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. 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New Mexico Geological Society Guidebook, 69th Field Conference, Las Cruces Country III, 2018, p. 181-188. 181 SR AND PB ISOTOPE VARIATIONS OF FELDSPARS IN THE MIDDLE TO LATE EOCENE PALM PARK FORMATION AND OREJON ANDESITE: IMPLICATIONS FOR REGIONAL VARIABILITY AND MAGMATIC SOURCE CHARACTERISTICS FRANK C. RAMOS, MAKAYLA JACOBS, AND BRIAN HAMPTON Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003 USA, [email protected] Abstract—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. Plagioclase crystals from Palm Park volcanic rocks in these different exposures have variable Sr and Pb isotope characteristics that broadly reflect an origin from either multiple magmas and/or a changing magmatic system that becomes less radiogenic over time (i.e., from ~45 to ~39 Ma). These isotope variations constrain the evolution and sources of Palm Park volcanic rocks when magmatism related to Laramide orogenesis transitioned to magmatism associated with the ignimbrite flare-up, a poorly studied portion of the middle to late Tertiary magmatic history of southern New Mexico. INTRODUCTION METHODS Although the ages and sources of late Eocene, ignimbrite Whole rocks were collected from eight individual sites (Fig. flare-up-related magmatism have been evaluated in south-cen- 1) from five regional Palm Park exposures including those in tral New Mexico (e.g., Zimmerer and McIntosh, 2013; Ver- the Sierra de Las Uvas (Bell Top Mountain), Robledo (Apache planck et al., 1999; Ramos et al., this volume), little is known and Faulkner Canyons), Doña Ana (Cleofas Canyon), and Or- about the age, duration, and sources of middle to late Eocene gan Mountains (Fillmore Canyon). Two additional samples volcanism and volcaniclastic sedimentation that occurred just from the Doña Ana Mountains (Ramos et al., this volume) prior to the flare-up. The period marks the transition between will also be discussed. Whole rocks were crushed and sieved. Laramide orogenesis and ignimbrite flare-up magmatism at Individual feldspars, typically plagioclase crystals, were hand ~36 Ma prior to initiation of Rio Grande rifting and is reflect- picked from sieve fractions. Feldspars were then etched in ed in the volcanic and volcaniclastic rocks of the Palm Park 10% hydrofluoric acid for 5–20 minute intervals, rinsed in dis- Formation. tilled water, and sonicated to obtain crystals free of adhering The Palm Park Formation in south-central New Mexico is materials. Feldspars chosen for single crystal analyses ranged composed of a suite of compositionally variable volcanic rocks from 0.1 to 3 mg. and volcanoclastic sedimentary rocks scattered throughout the Individual clean plagioclase crystals were dissolved using region. The rocks are primarily andesitic and dacitic in com- hydrofluoric, nitric, and hydrochloric acids. Strontium was position, are commonly variably altered, and are difficult to purified using cation exchange chromatography and 2.5N hy- stratigraphically correlate across localities. Here, Sr and Pb drochloric acid. Lead was purified using 1N hydrobromic acid isotope characteristics of feldspars hosted in Palm Park lavas and anion exchange chromatography. Lead was analyzed using and tuffs are used to evaluate the magmatic sources and com- multi-collector inductively coupled plasma mass spectrometry positional diversity of Palm Park volcanic rocks regionally. with Pb isotopes normalized to NBS997 205Tl/203Tl=0.41892 These include ash-fall tuffs and andesitic and dacitic lavas ex- and Sr was analyzed using thermal ionization mass spectrome- posed in the Organ Mountains, Doña Ana Mountains, Robledo try with Sr normalized to 86Sr/88Sr=0.1194 at the Johnson Mass Mountains, and Sierra de Las Uvas Mountains (Fig. 1). The Spectrometry Laboratory at NMSU. isotopic characteristics of feldspars in these poorly-studied Age corrections for Sr and Pb isotopes used measured volcanic rocks constrain the nature of pre-rift volcanism (46– 87Rb/86Sr ratios and assumed U/Pb and Th/Pb ratios. Age cor- 36 Ma, Clemons, 1979; Hawley et al., 1975) occurring during rections for Pb isotopes assumed U/Pb and Th/Pb ratios for the waning stages of Laramide orogenesis and offer insights plagioclase of 0.02 and 0.08, which are significantly higher regarding other age-equivalent, pre-rift volcanic and volcani- than ratios determined by Wolff and Ramos (2003) for potassi- clastic strata exposed in other parts of south-central New Mex- um feldspars. Actual U/Pb and Th/Pb ratios were not measured ico (e.g., in the Caballo, Potrillo, San Diego, and southern San but even these higher ratios result in age corrections that gener- Andres Mountains). ate only minor variations in ratios compared to overall Pb iso- 182 RamosRAMOS ,et JACOBS al._FIGURE_01, AND HAMPTON B A 108°W 106°W Caballo Mtns. Black Point of Range Socorro Rocks S O U T H E R N Carrizozo N E W M E X I C O Palm 33°N Alamagordo Valley Silver City Rincon Deming Carlsbad Las Cruces Hills 32°N 32°40'N 25 Major towns Study area San Diego 0 100 Km Mtns. Middle-late Eocene igneous rocks (~46–36 Ma) Late Cret.-mid. Eocene igneous rocks (~80–46 Ma) Selden San Andres Goodsight Mtns. Sierra de las Uvas Hills Doña Ana Uvas Valley Mtns. Mtns. BT CC Hardscrabble Hill Cooke’s FC Range 32°20'N Robledo Mtns. FL AC Sleeping Organ Lady Hills LAS Mtns. CRUCES Potrillo 0 10 Km 107°30'W Mtns. 106°40'W Pliocene–Present alluvium/colluvium and sedimentary and volcanic rocks (includes Camp Rice and Paomas Fms.) - Latest stage of Rio Grande Rift Neogene normal faults Late Eocene–Pleistocene sedimentary and volcanic rocks (includes Bell Top Fm. and equivalent caldera complexes, Modern Rio Grande river Thurman/Uvas Fms., as well as Hayner Ranch and Rincon Valley Fms.) - Early/middle stages of Rio Grande Rift Major interstates/highways Middle–late Eocene (~46–36 Ma) volcanic and volcaniclastic rocks (includes Palm Park and Rubio Peak Fms. as well Major towns as Orejon Andesite and Cleofas Andesite) - Pre Rio Grande Rift/post-Laramide deformation Field localities: Paleocene–early Eocene sedimentary rocks (includes the Love Ranch Fm.) BT - Bell Top Mountain - Final deformational stage of the Laramide Orogeny FC - Faulkner Canyon Paleozoic sedimentary rocks (includes Bliss Sandstone, Montoya and Fusselman Fms., as well as Hueco, Abo, and AC - Apache Canyon Yeso Fms.) - Passive margin (Camb–Miss) and Ancestral Rocky Mtns (Penn–Perm) CC - Cleofas Canyon Precambrian metamorphic and igneous rocks - Mazatzal basement province (Paleoproterozoic) and Mesoproterozoic FL - Fillmore Canyon granitoids FIGURE 1. Regional map of south-central New Mexico showing sample locations of whole rocks collected for this study. Palm Park Formation tuffs were collected from Apache Canyon (Robledo Mountains) and Bell Top Mountain (Sierra de Las Uvas Mountains). Andesitic and dacitic lavas were collected from Fillmore Can- yon (Organ Mountains), Cleofas Canyon (Doña Ana Mountains), and the Robledo Mountains as indicated (map modified from Creitz et al., this volume). tope variations and do not substantially impact interpretations in the Robledo Mountains (Seager et al., 2008) that contains regarding feldspar origins. 45.0±0.7 Ma zircons (Creitz et al., this volume). This ash-fall tuff is present in Palm Park strata but may originate from a RESULTS more distal source not reflected by the same local magmatic sources as most Palm Park volcanic rocks (Fig.