Petrology of the reversely zoned Mickey Pass Tuff, west-central Nevada

The 27.0-Ma Mickey Pass Tuff (MPT) is a voluminous (>600 km3) ash-flow deposit at the base of a thick sequence of Oligocene to early Miocene ash-flow tuffs exposed in west-central Nevada. The Guild Mine and Weed Heights Members of the MPT exhibit reverse compositional zoning, that is from more mafic magmatic ejecta at the base of the deposits to more silicic material at the top. The compositional zoning is the stratigraphic reverse of nearly all zoned tuffs. Bulk-tuff compositions from the lower Guild Mine Member grade from dacite to high-silica rhyolite (65.7–78.1 wt. % SiO₂), with a gap between 70 and 73 wt. % SiO₂. The upper Weed Heights Member displays a narrower compositional range from rhyodacite to high-silica rhyolite (71.5–75.5 wt. % SiO₂).; The range of major and trace element data from bulk tuff can be modelled by a compositionally zoned magma chamber for the Guild Mine Member that was derived primarily by crystal fractionation of an assemblage consisting of plagioclase, biotite, and magnetite with clinopyroxene, sanidine, and/or quartz. The compositional gap, bimodal glass analyses, multiple populations of biotite analyses, the range of estimated Rayleigh numbers, and a pronounced vertical gradient in calculated magma densities support a model that the pre-eruptive MPT chamber consisted of several, stacked compositional layers that each were internally convecting. The compositional zoning in the MPT is consistent with eruption of a normally zoned chamber with a non-flat roof geometry that was tapped from the deeper, more mafic parts first where a silicic cap was less extensive. During eruption, the less evolved and more evolved compositions in the chamber were mixed to yield compositions in the successively erupted units.; The MPT was erupted at the onset of extensional and transform tectonics in the central Walker Lane, which is a zone of right-lateral, strike-slip faulting in west-central Nevada. The eruption of the MPT and other comparable reversely zoned tuffs occurred in the incipient stages of extension and may be a precursor to rapid rates of extension.