MONOGENETIC VOLCANISM OF PETROGLYPH NATIONAL MONUMENT, NORTHERN RIO GRANDE RIFT, USA

Monogenetic basaltic volcanism is characteristic of waning stages of Rio Grande rift magmatism. Albuquerque Volcanoes of Petroglyph National Monument is the largest of these centers in the northern rift preserving approximately 3.5 km³ of erupted magma distributed across an area of approximately 45-50 km² along the eastern margin of the Llano de Albuquerque. Cinder and spatter deposits mark five, north-south aligned, vent locations. Mapped lava flows coalesce to form an asymmetric basaltic shield suggesting 14-15 discrete eruptive events. A thin, discontinuous, stratigraphic unconformity marked by coarse fluvial gravel is preserved locally delineating a single significant temporal break in eruptive activity of unknown duration. Previously determined ⁴⁰Ar/³⁹Ar ages on multiple lava flows are Middle Pleistocene (mean avg. 218±.014 ka). North-trending, syn- and post-eruption faulting is well preserved in the field and reflected in subsurface models of high-resolution aeromagnetic data. Faults are dominated by dip-slip displacement and are interpreted as extensional faults of the central Albuquerque Basin of the Rio Grande rift. Elongate distribution of vents for most of the volcanic deposits are spatially associated with the westernmost of these faults and are interpreted to reflect eruptions from fissures paralleling regional extensional fault trends.

Lava flows of the Albuquerque Volcanoes are olivine tholeiites, 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). These lavas 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). Isotopic data suggests a depleted subduction-modified lithospheric mantle source, although the relatively high εNd values and elevated ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb for some samples may support an asthenospheric melt component as well. All lavas are interpreted to have been erupted over a very short time span, with no more than two temporally distinct eruptive pulses with low effusion rates, and possibly reflecting a single mantle melting event.