Eruption rates and compositional trends at Los Humeros Volcanic Center, Puebla, Mexico

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Abstract
The Los Humeros volcanic center, located 180 km east of Mexico City, is one of several Pleistocene silicic centers in the “back arc” portion of the Mexican Neovolcanic Belt. The eruptive history at Los Humeros suggests that average eruptive rates increased with time and exceeded rates of regeneration of differentiated magma. Silicic volcanism began ∼0.47 Ma ago with extrusion of high‐silica rhyolite domes. Shortly thereafter, at 0.46 Ma ago, 115 km3of magma zoned from high‐silica rhyolite to rhyodacite were erupted, resulting in formation of the Xáltipan Ignimbrite and collapse of the 21×15 km Los Humeros caldera. High‐silica rhyolite domes then erupted along the northwestern ring‐fracture zone of the caldera. They are covered by the Faby Tuff, a dominantly rhyodacitic sequence of plinian fall deposits erupted from the southeast quadrant of the Los Humeros caldera, dated at 0.24 Ma. A second major caldera‐forming event occurred ∼0.1 Ma ago with the eruption of the Zaragoza Tuff, a nonwelded ignimbrite zoned from rhyodacite to andesite. Eruption of these 12 km3of magma led to collapse of the 10‐km‐diameter Los Potreros caldera, which is nested within the older Los Humeros caldera. Between 0.04 and 0.03 Ma ago, an arc of andesitic scoria cones, concentrated along the southern ring‐fracture zone of the Los Humeros caldera, fed lavas that flowed southward from the volcanic center, and andesite lavas built two small shields between the eastern rims of the two calderas. Approximately 6 km of andesitic magma were extruded during this stage. Activity continued up to 0.02 Ma ago with eruption of 10 km3of rhyodacitic to andesitic lava flows from centers near the northern margin of the Los Potreros caldera, in the area between the eastern rims of the two calderas, and within a broad band where the southern segments of the inferred Los Humeros and Los Potreros ring‐fracture zones nearly coincide. Simultaneous venting of rhyodacitic and andesitic tephra within this band led to formation of the 1.7‐km‐diameter El Xalapazco caldera. Minor fault‐bounded uplift of the southeastern quadrant of the Los Potreros caldera followed. The latest stage of volcanic activity is represented by the eruption of ∼0.25 km3of olivine basalt lavas along the southern ring‐fracture zone of the Los Humeros caldera and on the floors of the Los Potreros and El Xalapazco calderas. Erupted magmas show an overall trend with time toward more mafic compositions. Volumetric eruptive rates increased from ∼0.06 km3per thousand years 0.25 Ma ago to ∼0.2 km3per thousand years in the last 0.1 Ma. The increase in eruptive rate may have been the result of a progressive decrease in the structural integrity of the roof zone of the system as successive caldera‐forming eruptions reactivated old zones of weakness and created new ones. An increasingly disrupted roof allowed mafic and intermediate magmas to reach the surface relatively rapidly, decreasing their residence time in a high‐level chamber and thus the time available for their differentiation
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