Research Paper GEOSPHERE Lunar Crater volcanic field (Reveille and Pancake Ranges, Basin and Range Province, Nevada, USA) 1 2,3 4 5 4 5 1 GEOSPHERE; v. 13, no. 2 Greg A. Valentine , Joaquín A. Cortés , Elisabeth Widom , Eugene I. Smith , Christine Rasoazanamparany , Racheal Johnsen , Jason P. Briner , Andrew G. Harp1, and Brent Turrin6 doi:10.1130/GES01428.1 1Department of Geology, 126 Cooke Hall, University at Buffalo, Buffalo, New York 14260, USA 2School of Geosciences, The Grant Institute, The Kings Buildings, James Hutton Road, University of Edinburgh, Edinburgh, EH 3FE, UK 3School of Civil Engineering and Geosciences, Newcastle University, Newcastle, NE1 7RU, UK 31 figures; 3 tables; 3 supplemental files 4Department of Geology and Environmental Earth Science, Shideler Hall, Miami University, Oxford, Ohio 45056, USA 5Department of Geoscience, 4505 S. Maryland Parkway, University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA CORRESPONDENCE: gav4@ buffalo .edu 6Department of Earth and Planetary Sciences, 610 Taylor Road, Rutgers University, Piscataway, New Jersey 08854-8066, USA CITATION: Valentine, G.A., Cortés, J.A., Widom, ABSTRACT some of the erupted magmas. The LCVF exhibits clustering in the form of E., Smith, E.I., Rasoazanamparany, C., Johnsen, R., Briner, J.P., Harp, A.G., and Turrin, B., 2017, overlapping and colocated monogenetic volcanoes that were separated by Lunar Crater volcanic field (Reveille and Pancake The Lunar Crater volcanic field (LCVF) in central Nevada (USA) is domi­ variable amounts of time to as much as several hundred thousand years, but Ranges, Basin and Range Province, Nevada, USA): nated by monogenetic mafic volcanoes spanning the late Miocene to Pleisto­ without sustained crustal reservoirs between the episodes. The persistence Geosphere, v. 13, no. 2, p. 391–438, doi:10.1130 /GES01428.1. cene. There are as many as 161 volcanoes (there is some uncertainty due to of clusters through different episodes and their association with fault zones erosion and burial of older centers); the volumes of individual eruptions were are consistent with shear­assisted mobilization of magmas ponded near the 3 Received 12 September 2016 typically ~0.1 km and smaller. The volcanic field is underlain by a seismically crust­mantle boundary, as crustal faults and underlying ductile deformation Revision received 15 November 2016 slow asthenospheric domain that likely reflects compositional variability rela­ persist for hundreds of thousands of years or more (longer than individual Accepted 3 January 2017 tive to surrounding material, such as relatively higher abundances of hydrous episodes). Volcanoes were fed at depth by dikes that occur in en echelon sets Published online 24 February 2017 phases. Although we do not speculate about why the domain is in its cur­ and that preserve evidence of multiple pulses of magma. The dikes locally rent location, its presence likely explains the unusual location of the LCVF flared in the upper ~10 m of the crust to form shallow conduits that fed erup­ within the interior of the Basin and Range Province. Volcanism in the LCVF tions. The most common volcanic landforms are scoria cones, agglomerate occurred in 4 magmatic episodes, based upon geochemistry and ages of 35 ramparts, and ‘a‘ā lava fields. Eruptive styles were dominantly Strombolian eruptive units: episode 1 between ca. 6 and 5 Ma, episode 2 from ca. 4.7 to to Hawaiian; the latter produced tephra fallout blankets, along with effusive 3 Ma, episode 3 between ca. 1.1 and 0.4 Ma, and episode 4, ca. 300 to 35 ka. activity, although many lavas were likely clastogenic and associated with lava Each successive episode shifted northward but partly overlapped the area of fountains. Eroded scoria cones reveal complex plumbing structures, includ­ its predecessor. Compositions of the eruptive products include basalts, teph­ ing radial dikes that fed magma to bocas and lava flows on the cone flanks. rites, basanites, and trachybasalts, with very minor volumes of trachyandesite Phreato magmatic maar volcanoes compose a small percentage of the land­ and trachyte (episode 2 only). Geochemical and petrologic data indicate that form types. We are unable to identify any clear hydrologic or climatic drivers magmas originated in asthenospheric mantle throughout the lifetime of the for the phreatomagmatic activity; this suggests that intrinsic factors such as volcanic field, but that the products of the episodes were derived from unique magma flux played an important role. Eruptive styles and volumes appear to source types and therefore reflect upper mantle compositional variability on have been similar throughout the 6 m.y. history of the volcanic field and across OLD G spatial scales of tens of kilometers. All analyzed products of the volcanic field all 4 magmatic episodes. The total volume and time­volume behavior of the have characteristics consistent with small degrees of partial melting of ocean LCVF cannot be precisely determined by surface observations due to erosion island basalt sources, with additional variability related to subduction­related and burial by basin­fill sediments and subsequent eruptive products. However, enrichment processes in the mantle, including contributions from recycled previous estimates of a total volume of 100 km3 are likely too high by a factor OPEN ACCESS ocean crust (HIMU source; high­µ, where µ = 238U/204Pb) and from hydrous flu­ of ~5, suggesting an average long­term eruptive flux of ~3–5 km3/m.y. ids derived from subducted oceanic crust (enriched mantle, EM source). Geo­ chemical evidence indicates subtle source heterogeneity at scales of hundreds INTRODUCTION of meters to kilometers within each episode­scale area of activity, and tempo­ rary ponding of magmas near the crust­mantle boundary. Episode 1 magmas Intraplate volcanism has long been a topic of research focused on basalt may have assimilated Paleozoic carbonate rocks, but the other episodes had petrogenesis and upper mantle composition and structure, due to relatively This paper is published under the terms of the little if any chemical interaction with the crust. Thermodynamic modeling and low degrees of crustal overprinting on geochemical signatures and, in many CC‑BY license. the presence of amphibole support dissolved water contents to ~5–7 wt% in cases, abundant mantle xenoliths (e.g., Wörner et al., 1986; Farmer et al., 1989; © 2017 The Authors GEOSPHERE | Volume 13 | Number 2 Valentine et al. | Lunar Crater volcanic field 391 Research Paper Lum et al., 1989; Menzies, 1989; Luhr et al., 1995; Witt-Eickschen and Kramm, episode. An additional issue that sometimes arises in the literature is how to 1998; Gazel et al., 2012; Cousens et al., 2013; McGee and Smith, 2016). A grow- distinguish monogenetic volcanoes from polygenetic volcanoes that undergo ing body of literature in the past decade has also focused on physical pro- multiple eruptive episodes separated by dormant periods (e.g., Németh, 2010). cesses of intraplate volcanoes, including magma ascent, relationships with This distinction can be ambiguous because in a volcanic field there is always crustal structure, and eruption (see reviews by Valentine and Gregg, 2008; a chance that a new monogenetic volcano will form within the footprint of a Németh, 2010; Le Corvec et al., 2013a, 2013b; integrated case studies by Le previous one; as described herein, this is especially common where the vol- Corvec et al., 2013c; Murcia et al., 2016). Recent work has been partly aimed canoes form dense, long-lived clusters. In our view, polygenetic volcanism at assessing the potential hazards associated with eruptions in monogenetic typically requires a single sustained magma plumbing system in the crust that volcanic fields (e.g., Ort et al., 2008; Valentine and Perry, 2009; Houghton et al., produces multiple eruptive episodes. Thus in the absence of petro logic evi- 2006; Németh et al., 2012), but also has been driven by the increasing rec- dence for sustained magma reservoirs shared by colocated centers of differ- ognition of the complexity of these small volcanoes, which were traditionally ent ages, we assume that colocated volcanoes are simply overlapping mono- thought to be simple and well understood (e.g., Houghton and Schmincke, genetic volcanoes. We understand that at some point the distinction between 1986; Ort and Carrasco-Nuñez, 2009; Genareau et al., 2010; Martí et al., 2011; monogenetic and polygenetic volcanism is rather subjective, but this issue has Bolós et al., 2012; van Otterloo et al., 2013; Pedrazzi et al., 2016). no impact on the work presented here. We also distinguish eruptive episodes Several fundamental questions arise in the study of intraplate, dominantly at individual volcanoes from magmatic episodes on the scale of the volcanic monogenetic, volcanic fields. (1) Are the mantle magma sources homo- field. In the case of the LCVF we have identified four major magmatic episodes, geneous or are they heterogeneous in time and space for a given field? (2) Are each of which includes many monogenetic volcanoes. These large-scale epi- there relationships between magma sources, clusters, and preexisting crustal sodes are each characterized by a specific range of volcano ages and by similar structures, and why? (3) What are the characteristics of the transition from magma compositions. crustal magma transport, mainly via dikes, to eruptions at the surface? (4) How do individual volcanoes grow and feed various eruptive processes? (5) Are there relationships between source