The 55 km3 mid-Tertiary Taylor Creek Rhyolite in southwestern New Mexico consists of 20 lava domes and flows. This rhyolite is metaluminous to weakly peraluminous and contains an average of 77.6 ± 0.4% SiO2; Al2O3, Na2O, and K2O, the other major constituents, also vary little throughout the lava field. The Taylor Creek Rhyolite contains about 15–35 vol% phenocrysts, which are principally subequal amounts of quartz and sanidine accompanied by minor oligoclase and altered biotite or hornblende or both.

Compositional zonation in feldspar phenocrysts analyzed by electron microprobe is very minor and nonsystematic; most grains show less than 1 mol% Ab variation. In addition, the compositions of each feldspar species vary little throughout the suite of analyzed samples. This chemical homogeneity of phenocrysts reflects similar whole-rock homogeneity and suggests that the lavas were tapped from a single large reservoir of magma. Ages of sanidine phenocrysts determined using 40Ar/39Ar indicate that the Taylor Creek Rhyolite lavas were emplaced during a period of less than 0.42 m.y. and possibly less than 0.13 m.y., which is consistent with the single-reservoir scenario. Based on the distribution of vents for the lavas, it is inferred that the single reservoir was at least several hundreds of square kilometers in plan view.

Two-feldspar geo thermometry suggests that Taylor Creek Rhyolite phenocrysts crystallized at about 775 °C, at an assumed pressure of 2 kbar. Fe-Ti-oxide geothermometry suggests phenocryst growth at about 800 °C. These temperatures are equal, within a conservative estimate of the uncertainties associated with the methods of determination, and they are consistent with fluid-inclusion-homogenization temperatures and 18O-fractionation temperatures between about 700 °C and 800 °C for vapor-phase minerals deposited in the outer rinds of the lavas as they cooled, immediately following emplacement. A temperature of 800 °C for phenocryst growth in high-silica rhyolite magma suggests a volatile-poor (H2O, F, Cl) system. Experimental studies suggest that quartz and potassium-feldspar crystals that grow from H2O-undersaturated granitic magmas should exhibit resorption texture, a texture ubiquitous to Taylor Creek Rhyolite quartz and sanidine phenocrysts.

Insofar as experimental results in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O are applicable to the Taylor Creek Rhyolite, a comparison between the natural and experimental systems indicates that Taylor Creek Rhyolite magma at 800 °C could have been saturated with H2O and at or near the liquidus at a pressure of about 0.5 kbar. We tentatively conclude that the Taylor Creek Rhyolite magma was H2O undersaturated and subliquidus at an unspecified pressure greater than 0.5 kbar during phenocryst growth and that Taylor Creek Rhyolite pyroclastic deposits formed because volatile saturation developed during the ascent of magma to sites of eruption.

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