Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars

Monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood. This paper explores these relationships, building upon earlier studies of exposed shallow plumbing (in the upper hundreds of meters of crust) beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes, such as scoria cones and maars, respectively. Eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the San Francisco Volcanic Field (Arizona, USA). Well-constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types. Sedimentary xenolith contents at scoria cones are <10(-3) and commonly <10(-4) (volume fraction of xenoliths) and are composed almost entirely of fragments from the uppermost sedimentary formation (Kaibab Formation, depth interval similar to 50-200 m). These xenolith contents are consistent with conduits or dikes that widen mainly in the uppermost tens of meters of the crust, as observed at exposed plumbing systems of eroded scoria cones. Sedimentary xenolith contents in tephra ring deposits at one of the studied maar volcanoes also are typically <10(-3) (volume fraction) and the remaining fraction is dominated by clasts of pre-maar volcanic rocks that formed an similar to 50 thick surface layer over the sedimentary formations; the second studied maar has much higher xenolith contents but this also appears to be dominated by the shallowest unit. The maars' tephra ring deposits contain xenoliths from all of the major sedimentary units beneath the volcanoes (to depths of similar to 1200 m) but the abundances and proportions of xenoliths are not consistent with the volumes of sub-volcanic units that would be disrupted assuming dimensions that are commonly observed in exposed maar plumbing systems (diatremes). These differences illustrate the different mechanisms for conduit/dike widening in magmatic versus phreatomagmatic eruptions. Namely, ascending eruptive mixtures driven by magmatic volatiles widen their relatively shallow conduits by erosion and mechanical failure of the walls and ejection of the resulting xenoliths. Maar-forming eruptions produce wide and deep diatremes mainly by mechanical disruption of country rock during many discrete magma-water explosions at varying depths in the subsurface. Deep explosions cause debris jets that may not erupt, and the disrupted country rock and juvenile material gradually churns and mixes within the diatreme. Only a small fraction of the debris is ejected from the crater by especially strong and/or shallow explosions, to form tephra ring deposits. (C) 2012 Elsevier B.V. All rights reserved.