Generation of alkalic flood rhyolite: Insights from evolution of the Paisano volcano, Davis mountains, Trans-Pecos Texas, U.S.A.

Abstract

Extensive silicic lava (flood rhyolite) constitutes part of the mid –Tertiary Paisano Volcano shield. This lava, borderline peralkaline quartz trachyte/rhyolite, forms part of a sequence of peralkaline rhyolite – quartz trachyte – trachyte produced as eruptions tapped deeper portions of underlying plutons. Flood rhyolite lava extended as far as 20 km from dikes of the eruptive center of the volcano, creating flows as thick as 100 m.Flood rhyolite was nearly aphyric to sparsely porphyritic, relatively hot (~911 °C) and erupted over a topographically subdued region, forming two lobes that extended north-northeast and southeast from a highland of previously-erupted rhyolite domes. Flood rhyolite extrusion followed eruption of a local ignimbrite, which filled valleys, flattening topography. Successive silicic flows off-lapped each other away from the volcano. Flood rhyolite eruption was followed by a second eruptive sequence that contained coarsely-porphyritic mafic trachyte lava.Silicic lava was most-likely generated by filter pressing from mush zones of mafic plutons. Glomeroporphyritic trachytes of the volcano are mixed rocks, silicic lava incompletely segregated from entrained fragments of mafic plutons. Continued evolution of quartz trachyte/low-silica rhyolite to peralkaline rhyolite was dominated by fractionation of alkali feldspar.