INSIGHT INTO SMALL-VOLUME VOLCANIC FIELDS IN CENTRAL RIO GRANDE RIFT—ALBUQUERQUE VOLCANOES, NEW MEXICO

The Albuquerque Volcanoes (ABQ) are located in one of the younger Pliocene-Pleistocene volcanic fields of the central Rio Grande rift. Better constraining the age and petrogenesis of the olivine tholeiites in this small-volume volcanic field is important for the broader understanding of Cenozoic volcanism in the Rio Grande rift because it provides (1) an opportunity to characterize the magma evolution for the central rift and (2) a comparison to the more compositionally diverse and larger-volume Servilleta basalts and Cerros del Rio volcanic field of the northern rift. With the addition of lidar imagery and the continued development of high-precision ⁴⁰Ar/³⁹Ar dating, we are revisiting detailed mapping of this region. We present a revised 1:24,000 map of the Albuquerque Volcanoes and Petroglyphs National Monument with approximately 13 flow units, new whole-rock and Sr-Nd-Pb isotopic data, and preliminary ⁴⁰Ar/³⁹Ar ages determined using the latest generation of noble gas mass spectrometers. The ABQ volcanic field consists of a chain of cinder and splatter cones previously interpreted to represent fissure eruptions, along with several smaller vents located in the western part of the mapping area and multiple lava flows that extend eastward towards Albuquerque, New Mexico.

The ABQ volcanoes are characterized by a narrow range of Mg# (53-61), and low TiO₂ and K₂O (avg: 1.46 wt. % and 0.50 wt. %, respectively). The basalts are enriched in Sr, Rb and Ba relative to MORB, but have low K/Ba, Nb/Ba, and La/Nb (avg: 23, 0.06, and 0.97, respectively). The ABQ are also defined by moderately radiogenic Pb (²⁰⁶Pb/²⁰⁴Pb=18.5-19.2) and depleted Sr and Nd isotopic signatures (⁸⁷Sr/⁸⁶Sr ~0.70390-0.70500 and εNd = 3.1-4.0). Initial results suggest a depleted subduction-modified lithospheric mantle source, which is similar to other young mafic rift basalts in the central and southern rift region. Preliminary dating of basaltic groundmass from the two oldest flows yielded ages of 131 ± 11 and 195 ± 15 ka. However, most dated samples yielded a discordant spectrum. Continued dating will focus on determining if the flows were erupted during multiple events or if the samples are affected by excess Ar, Ar-loss, or hydrocarbon contamination.