ORIGIN OF THE JOYA-HONDA MAAR, SAN-LUIS-POTOSI, MEXICO

Joya Honda is a Quaternary maar of unusual type from the Mexican Basin and Range Province. Its similar to 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 pyroclasti c-surge and -fall layers that we interpret as dry-surge deposits. Thes e are overlain by the main sequence of strongly indurated, massive tuf f 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 t he nature of the deposits. Groundwater 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, wh ich are typical of deposits found at many maars and tuff rings. As the eruption continued, the crater grew and the hydromagmatic blasts frac tured the limestones around the explosion foci. A marked increase in t he water/magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was int ersected by the expanding crater. Subsequent phreatomagmatic explosion s occurred in a system with groundwater flow rates several orders of m agnitude larger than in the initial dry-surge stage. At the maar rim t hese wet eruptions led to the emplacement of massive tuff breccias thr ough a combination of fallout, steeply dipping tuff breccias are simil ar to deposits found at many tuff cones. Juvenile clasts in the near-v ent deposits show marked upward increases in both hydration (palagonit ization) and vesicularity. The increased palagonitization with height in the section appears to be a consequence of the overall increased we tness of the eruption with time, correlating with greater carbonate ce mentation and lithification in the upper part of the deposit. The tran sition toward higher vesicularity is interpreted as evidence of a grad ual reduction in the confining pressure for the ascending magma prior to explosive fragmentation, perhaps related to unroofing during progre ssive excavation of the overlying maar crater. Thus, Joya Honda does n ot support maar-formation models that invoke downward displacement of explosion foci, caused by formation of a cone of depression in the aqu ifer, in order to maintain the confining pressure for the hydromagmati c blasts.