CORRELATION AND EMPLACEMENT OF A LARGE, ZONED, DISCONTINUOUSLY EXPOSED ASH-FLOW SHEET - THE AR-40 AR-39 CHRONOLOGY, PALEOMAGNETISM, AND PETROLOGY OF THE PAHRANAGAT FORMATION, NEVADA/

Many single-crystal Ar-40/Ar-39 ages and thermoremanent magnetization directions have resolved the problematic stratigraphic correlation of the laterally and vertically zoned rhyolite ash flow sheet of the Pahr anagat Formation in the southern Great Basin. This outflow sheet was p reviously designated by four different stratigraphic names in differen t locations over its highly discontinuous exposure area of 33,000 km(2 ). We show that it is a single cooling unit emplaced at 22.639 +/- 0.0 09 Ma around its source, the Kawich caldera. The volume of the outflow sheet was about 1600 km(3) after compensation for 50% post volcanic e ast-west extension. A comparable volume of tuff likely accumulated ins ide the Kawich caldera. Modal and chemical compositions of bulk tuff a nd cognate pumice fragments, together with compositions of phenocrysts , show the preeruption magma body was zoned from high-silica rhyolite (two feldspars, quartz, biotite, and titanomagnetite) to underlying, s ilica-poor, more mafic rhyolite and trachydacite (plagioclase, minor b iotite, titanomagnetite, amphibole, and clinopyroxene). Initial evacua tion 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 e jecta 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 tu ff 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 s cattered exposures.