Correlation and emplacement of a large, zoned, discontinuously exposed ash flow sheet: The 40Ar/39Ar chronology, paleomagnetism, and petrology of the Pahranagat Formation, Nevada

Abstract

Many single‐crystal 40Ar/39Ar ages and thermoremanent magnetization directions have resolved the problematic stratigraphie correlation of the laterally and vertically zoned rhyolite ash flow sheet of the Pahranagat Formation in the southern Great Basin. This outflow sheet was previously designated by four different stratigraphie names in different locations over its highly discontinuous exposure area of 33,000 km2. We show that it is a single cooling unit emplaced at 22.639+0.009 Ma around its source, the Kawich caldera. The volume of the outflow sheet was about 1600 km3 after compensation for 50% post volcanic east‐west extension. A comparable volume of tuff likely accumulated inside the Kawich caldera. Modal and chemical compositions of bulk tuff and cognate pumice fragments, together with compositions of phenocrysts, show the preemption magma body was zoned from high‐silica rhyolite (two feldspars, quartz, biotite, and titanomagnetite) to underlying, silica‐poor, more mafic rhyolite and trachydacite (plagioclase, minor biotite, titanomagnetite, amphibole, and clinopyroxene). Initial evacuation of the uppermost evolved zone produced proximal outflow hundreds of meters thick of relatively densely welded, pumice‐poor, high‐silica rhyolite tuff. As eruption progressed, tens of meters of more mafic ejecta were deposited in distal areas and locally near the caldera and consist of less welded, pumice‐rich ash flow tuff derived by physical mixing of pyroclasts from all zones of the magma chamber. This mixing during eruption invalidates direct comparison of the composition of tuff and a particular part of the magma chamber. The Pahranagat ash flow sheet provides a rigorous test case for application of high‐precision correlation tools because of the zonal emplacement of ejecta from the compositionally stratified magma chamber together with the subsequent tectonic dismemberment and erosion of the sheet that created widely scattered exposures.