Field, geochemical, and isotopic constraints on the genesis of rhyolite ash-flow tuffs and related rocks of the Tomochic volcanic center, Sierra Madre Occidental, Mexico

Field, chronologic, geochemical, and isotopic data constrain the genesis of rhyolite ash flows and related magmas at a volcanic center that was active during the final ;Geochemical and isotopic data are consistent with rhyolite genesis by fractional crystallization of two-pyroxene andesite. The least-evolved vista sample represents no more than 40% the original mass of andesite; an additional ;The evolution of volcanism at the Tomochic volcanic center, from andesitic to rhyolite-dominated with late extrusion of basaltic andesite, occurred elsewhere in the volcanic field as well, and roughly coincided with a sharp decrease in the rate of Farallon plate subduction. It is proposed that this decrease contributed to (a) the onset of brittle crustal extension and shorter residence times for mafic melts formerly ponded in the deep crust, and (b) a decreased flux of mafic melts into the crust from below, leading to (i) the stalled ascent and subsequent differentiation of earlier-formed andesitic magmas, (ii) eruption of derivative rhyolite ash flows, and (iii) the termination of calcalkalic volcanism throughout the Sierra Madre Occidental.