Genesis of the post-caldera eastern Upper Basin Member rhyolites, Yellowstone, WY: from volcanic stratigraphy, geochemistry, and radiogenic isotope modeling

Abstract

An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low d 18 O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom's Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., 206 Pb/ 204 Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic 87 Sr/ 86 Sr in plagioclase phenocrysts (0.7134-0.7185) than groundmass and whole-rock ratios (0.7099-0.7161), and d 18 O depletions on the order of 5% found in the Tuff of Sulphur Creek and Canyon flow indicate at least a twostage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes, which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low d 18 O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow.

A field trip guide to the petrology of Quaternary volcanism on the Yellowstone Plateau

“…Compared to intracaldera rhyolites, extracaldera rhyolites have lower 143 Nd/ 144 Nd, which suggests a stronger crustal influence during their formation (fig. 6;Hildreth and others, 1991;Pritchard and Larson, 2012;Pritchard and others, 2013). Modeling by Pritchard and Larson (2012) suggests that the isotopic composition of the extracaldera rhyolites can be explained by injection of basaltic magmas into the crust, followed by assimilation of Archean crust and extensive fractional crystallization.…”

“…6;Hildreth and others, 1991;Pritchard and Larson, 2012;Pritchard and others, 2013). Modeling by Pritchard and Larson (2012) suggests that the isotopic composition of the extracaldera rhyolites can be explained by injection of basaltic magmas into the crust, followed by assimilation of Archean crust and extensive fractional crystallization. In addition to lower 143 Nd/ 144 Nd, the extracaldera rhyolites have higher δ 18 O relative to intracaldera eruptions at Yellowstone and are similar to the earliest Huckleberry Ridge Tuff magmas (Hildreth and others, 1984).…”

“…The heat source driving this remelting has been attributed to mafic magmas ponding beneath the silicic magma reservoir and potentially silicic recharge magmas injected into the reservoir (Bindeman and others, 2008;Girard and Stix, 2009 dome. Similar to the early Upper Basin Member rhyolites, the late Upper Basin Member rhyolites have less chemically evolved mineral and whole-rock compositions relative to the younger Central Plateau Member (CPM) rhyolites (Vazquez and others, 2009;Stix, 2009, 2010 (Bindeman and others, 2008;Pritchard and Larson, 2012;Till and others, 2013;Vazquez and Reid, 2002;Vazquez and others, 2009 Stix, 2009, 2010;Vazquez and Reid, 2002;Watts and others, 2012). In fact, Loewen and Bindeman (2015) refer to the late Upper Basin Member rhyolites as the beginning of the Central Plateau Member eruptive episode rather than the end of the Upper Basin Member eruptive episode.…”

“…The Upper Basin Member rhyolites can be divided into the early Upper Basin Member rhyolites and the late Upper Basin Member rhyolites. The early Upper Basin Member rhyolites, which erupted between 516±14 ka and 479±20 ka (Gansecki and others, 1996) Hildreth and others, 1984;Bindeman and Valley, 2001;Bindeman and others, 2008;Pritchard and Larson, 2012) and zircon populations with heterogeneous δ 18 O values (Bindeman and Valley, 2001;Bindeman and others, 2008). Additionally, each Upper Basin Member rhyolite has a distinct trace-element composition relative to the other Upper Basin Member rhyolites (Girard and Stix, 2009).…”

“…The field labeled "All pre-CPM Yellowstone rhyolites*" includes the range of data for all Yellowstone rhyolites except for Huckleberry Ridge Tuff member C, which shows evidence for large amounts of crustal assimilation and is plotted separately (see Hildreth and others, 1991). Neodymium isotopic data are from Hildreth and others (1991), Vazquez and Reid (2002), Bennett (2006), Nash and others (2006), and Pritchard and Larson (2012). …”