Origin of the Joya Honda maar, San Luis Potosي, México

Joya Honda is a Quaternary maar of unusual type from the Mexican Basin and Range Province. Its ~ 300-m-deep crater is excavated in Cretaceous limestones. The surrounding tephra deposit, which in places is > 100 m thick, begins with a series of weakly indurated pyroclastic-surge and -fall layers that we interpret as dry-surge deposits. These are overlain by the main sequence of strongly indurated, massive tuff breccias that we interpret as wet-surge deposits. Joya Honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate-type basanitic magma carrying peridotitic mantle xenoliths. Local aquifer characteristics controlled the style of eruption and the nature of the deposits. water in the limestone-hosted aquifer beneath Joya Honda was apparently contained within solution-enhanced fractures. At the onset of the eruption, magma began to interact with a moderate amount of groundwater, producing the dry-surge deposits, which are typical of deposits found at many maars and tuff rings. As the eruption continued, the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci. A marked increase in the water/magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater. Subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry-surge stage. At the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout, surges and mudflows. These steeply dipping tuff breccias are similar to deposits found at many tuff cones. le clasts in the near-vent deposits show marked upward increases in both hydration (palagonitization) and vesicularity. The increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time, correlating with greater carbonate cementation and lithification in the upper part of the deposit. The transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation, perhaps related to unroofing during progressive excavation of the overlying maar crater. Thus, Joya Honda does not support maar-formation models that invoke downward displacement of explosion foci, caused by formation of a cone of depression in the aquifer, in order to maintain the confining pressure for the hydromagmatic blasts.