Lunar Crater Volcanic Field (Reveille and Pancake Ranges, Basin and Range Province

The Lunar Crater volcanic field (LCVF) in central Nevada (USA) is domi­nated by monogenetic mafic volcanoes spanning the late Miocene to Pleisto­cene. There are as many as 161 volcanoes (there is some uncertainty due to erosion and burial of older centers); the volumes of individual eruptions were typically ~0.1 km 3 and smaller. The volcanic field is underlain by a seismically slow asthenospheric domain that likely reflects compositional variability rela­tive to surrounding material, such as relatively higher abundances of hydrous phases. Although we do not speculate about why the domain is in its cur­rent location, its presence likely explains the unusual location of the LCVF within the interior of the Basin and Range Province. Volcanism in the LCVF occurred in 4 magmatic episodes, based upon geochemistry and ages of 35 eruptive units: episode 1 between ca. 6 and 5 Ma, episode 2 from ca. 4.7 to 3 Ma, episode 3 between ca. 1.1 and 0.4 Ma, and episode 4, ca. 300 to 35 ka. Each successive episode shifted northward but partly overlapped the area of its predecessor. Compositions of the eruptive products include basalts, teph­rites, basanites, and trachybasalts, with very minor volumes of trachyandesite and trachyte (episode 2 only). Geochemical and petrologic data indicate that magmas originated in asthenospheric mantle throughout the lifetime of the volcanic field, but that the products of the episodes were derived from unique source types and therefore reflect upper mantle compositional variability on spatial scales of tens of kilometers. All analyzed products of the volcanic field have characteristics consistent with small degrees of partial melting of ocean island basalt sources, with additional variability related to subduction­-related enrichment processes in the mantle, including contributions from recycled ocean crust (HIMU source; high­-µ, where µ = 238 U/ 204 Pb) and from hydrous flu­ids derived from subducted oceanic crust (enriched mantle, EM source). Geo­chemical evidence indicates subtle source heterogeneity at scales of hundreds of meters to kilometers within each episode­ scale area of activity, and tempo­rary ponding of magmas near the crust­-mantle boundary. Episode 1 magmas may have assimilated Paleozoic carbonate rocks, but the other episodes had little if any chemical interaction with the crust. Thermodynamic modeling and the presence of amphibole support dissolved water contents to ~5–7 wt% in some of the erupted magmas. The LCVF exhibits clustering in the form of overlapping and colocated monogenetic volcanoes that were separated by variable amounts of time to as much as several hundred thousand years, but without sustained crustal reservoirs between the episodes. The persistence of clusters through different episodes and their association with fault zones are consistent with shear­ assisted mobilization of magmas ponded near the crust­-mantle boundary, as crustal faults and underlying ductile deformation persist for hundreds of thousands of years or more (longer than individual episodes). Volcanoes were fed at depth by dikes that occur in en echelon sets and that preserve evidence of multiple pulses of magma. The dikes locally flared in the upper ~10 m of the crust to form shallow conduits that fed erup­tions. The most common volcanic landforms are scoria cones, agglomerate ramparts, and ‘a‘ ā lava fields. Eruptive styles were dominantly Strombolian to Hawaiian; the latter produced tephra fallout blankets, along with effusive activity, although many lavas were likely clastogenic and associated with lava fountains. Eroded scoria cones reveal complex plumbing structures, includ­ing radial dikes that fed magma to bocas and lava flows on the cone flanks. Phreatomagmatic maar volcanoes compose a small percentage of the land­ form types. We are unable to identify any clear hydrologic or climatic drivers for the phreatomagmatic activity; this suggests that intrinsic factors such as magma flux played an important role. Eruptive styles and volumes appear to have been similar throughout the 6 m.y. history of the volcanic field and across all 4 magmatic episodes. The total volume and time­ volume behavior of the LCVF cannot be precisely determined by surface observations due to erosion and burial by basin­fill sediments and subsequent eruptive products. However, previous estimates of a total volume of 100 km3 are likely too high by a factor of ~5, suggesting an average long­term eruptive flux of ~3–5 km 3 /m.y.