Chemical evolution of a magmatic system: The Paintbrush Tuff, Southwest Nevada Volcanic Field

Abstract

The chemical and thermal evolution of a single magmatic system is recorded in the Paintbrush Tuff, a series of four ash flow sheets, that were erupted from the same caldera within 0.6 Ma. Glassy pumice samples were collected from the tops and bottoms of the ash flow sheets. Because these samples closely approximate magma compositions, their chemical and mineralogical compositions were used to quantitatively evaluate magma mixing and fractional crystallization as potential differentiation mechanisms. Prior to eruption of the first ash flow sheet, the Topopah Spring Member (>1200 km3), an abrupt compositional interface existed in the magma chamber between high‐silica rhyolite and quartz latite magmas. The subsequent Pah Canyon Member (<40 km3) records mixing of the contrasting magma types represented in the Topopah Spring Member. The magma mixing was most likely due to disruption of the compositional interface during eruption of the Topopah Spring Member. The next unit, the Yucca Mountain Member (<20 km3), represents the reestablishment of high‐silica rhyolite magma in the subcaldera chamber. The origin of this high‐silica rhyolite is best modeled by 15% to 24% fractional crystallization from the Pah Canyon Member. The Tiva Canyon Member is the fourth ash flow sheet (>1000 km3) to be erupted, and it consists of three compositional modes, a higher‐silica rhyolite, rhyolite, and quartz latite. The high‐silica rhyolite of the Yucca Mountain Member is an early eruptive phase of the higher‐silica rhyolite of the Tiva Canyon Member, and their origins are the same. The rhyolite most likely formed by a combination of fractional crystallization and magma mixing of the Pah Canyon magma and the quartz latite of the Tiva Canyon Member. Alternatively, the rhyolite may have formed by fractional crystallization and magma mixing of higher‐silica rhyolite and quartz latite of the Tiva Canyon Member, or by fractional crystallization from the Tiva Canyon Member. The origin of the quartz latite has not been determined. Quantitative modeling indicates that all of the chemical variation within the Pah Canyon, Yucca Mountain, and Tiva Canyon members can be accounted for by fractional crystallization or magma mixing operating alone or in conjunction; no other processes need be invoked. Volume estimates of the magma bodies based on these quantitative models predict much larger volumes than the sizes of the ash flow sheets would indicate. A recurring theme in the evolution of the Paintbrush Tuff is the interaction of quartz latitic magmas with high‐silica magmas in conjunction with crystal fractionation.