Origin of the trachyte–quartz trachyte–peralkalic rhyolite suite of the Oligocene Paisano volcano, Trans-Pecos Texas

Abstract

Volcanic rocks of the Paisano volcano include trachyte, quartz trachyte, and peralkalic rhyolite. Mafic rocks, hawaiite and mugearite, occur within units that stratigraphically underlie and overlie rocks of the volcano. Quartz trachyte, trachyte, and nephe-line trachyte occur as discordant plugs and dikes intruded into strata of the volcano. Central eruptions from dike swarms led to the formation of the shield complex of the volcano ∼ 35 m.y. ago. The central dike complex, a generalized eruptive sequence of rhyolite–quartz trachyte–trachyte within the eruptive products of the volcano, and the development of a 5-km-diameter caldera suggest the presence beneath the volcano of one or more shallow plutonic bodies in which differentiation may have occurred. Fractionation calculations, using whole-rock analyses to represent liquid compositions and electron-probe microanalyses of phenocryst minerals to represent compositions of fractionating phases, indicate plagioclase-plus-olivine control in the evolution of trachyte from mugearite, and anorthoclase control in the evolution of rhyolite from trachyte. The feldspar fractionation model is supported by strong enrichment of Rb and Zr, and strong depletion of Sr in the series, and by a striking Eu anomaly in REE plots. A compositional gap between mugearite and trachyte, analogous to the “Daly Gap” of oceanic islands, disappears when oxides of major elements are plotted versus Zr. Zr, however, cannot be used as a strict index of fractionation because Zr concentrations were buffered by the crystallization of zircon with feldspar in the crucial trachyte stage of magmatic evolution. In more advanced quartz trachyte and rhyolite stages, the melts greatly increased in peralkalinity and zircon did not crystallize. This effect produced Zr concentrations as high as 2,500 ppm in highly fractionated rhyolite. Glomeroporphyritic clusters of feldspar, zoned from andesine to calcic anorthoclase, along with augite, opaques, and, in some samples, olivine, are ubiquitous in mafic trachyte. These clusters have textures indicative of crystallization in intrusions. They suggest a genetic relationship between mugearite and trachyte where residual trachytic liquids are segregated by filter pressing from crystallizing magma of over-all mugearitic composition in subvolcanic chambers. Paisano igneous rocks closely resemble suites of volcanic rocks from Afro-Arabian central volcanoes associated with intracontinental rifting and suites from some oceanic islands. Alkalic rocks of the Paisano volcano may be related to mantle diapirism triggered by subduction processes.