Lava Creek Tuff

Ash flows of the Lava Creek Tuff are divided among six members, informally named unit 1, unit 2, member A, and member B from bottom to top, with units 3 and 4 having unspecified stratigraphic positions. The emplacement of the Lava Creek Tuff was not instantaneous and continuous, but rather, there were multiple pauses, and the members were erupted at different times. To date the timings of their eruptions, two common methods of radiometric dating are employed: 40Ar/39Ar on sanidine and U–Pb on zircon. The interpretation of the two techniques differs in that zircon crystallization occurs early and progressively during magma evolution; therefore, U–Pb ages must predate the instantaneous age of volcanic eruption as recorded by sanidine. Two samples from ignimbrite visually closely similar to unit 1 or 2, the oldest ignimbrite units of the Lava Creek Tuff, have 40Ar/39Ar ages of kyr and kyr. 40Ar/39Ar dating experiments on sanidine from member B have yielded eruption ages of kyr, kyr, and . U–Pb dating for zircon crystals from both the member A and B yields an age of kyr, which is indistinguishable from the 40Ar/39Ar date of sanidine. Another team reported U–Pb ages of kyr and kyr for zircon from member A and member B, respectively. == Petrography ==

The ignimbrite sheet was formed from rhyolite magma and contains phenocrysts of quartz, sanidine, and subordinate sodic plagioclase, along with minor proportions of magnetite, ilmenite, ferroaugite, fayalite, iron-rich hornblende, zircon, chevkinite, and allanite. However, the abundance of phenocrysts differs between the members. Hornblende is relatively abundant in member A but rare in other members. Unit 3 is distinguished from unit 1 and 2 by higher crystal content and more plagioclase. Member A is distinguished from member B primarily by the presence of the mineral amphibole in the former. The zircon and phenocrysts rims recorded that the magma of the Lava Creek Tuff was generated from a mix of mantle, Archean crust, and shallow hydrothermally altered intra-caldera rocks. Member A and B were sourced from separate magma reservoirs prior to eruption, at a depth range of and a temperature of . The eruption of member B was probably triggered by a combination of an injection of new silicic magma into the reservoir and volatile exsolution from crystallizing magma. == Eruption ==

The eruption of the Lava Creek Tuff has been reconstructed through geological analysis of the deposits. Proximal ignimbrite units of member A and B have been studied in detail and correlated with distal air-fall. Meanwhile, the newly identified units 1, 2, 3, and 4 are only known at a few locales; nonetheless, they indicate that the Lava Creek eruption was much more complex than previously thought. Unit 1 and 2 These ignimbrite units represent the earliest known eruptive events of the Lava Creek episode. ==See also==