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

—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.