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Volcanology and Petrology of the Rattlesnake Ash-Flow Tuff, Eastern Oregon

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Volcanology and Petrology of the Rattlesnake Ash-Flow Tuff, Eastern Oregon AN ABSTRACTOF THE THESIS OF Martin J. Streck for the degree of Doctor of Philosophy in Geology presented on April 27, 1994. Title: Volcanology and Petrology of the Rattlesnake Ash-Flow Tuff, Eastern Oregon. Abstractapproved: Anita L. Grunr The RattlesnakeAsh-Flow Tufferupted 7.05±0.01 Ma from the western HarneyBasin,southeasternOregon.The location of the vent area is inferred based on vent-ward increases in size of pumices, in degree ofwelding,and in degree of post- emplacementcrystallization.Today's outcrops cover 9000 km2 and estimated original outcrop coverage wasca.35000km2.Tuff thickness is uniform ranging mainly between 5 and 30 in with maxima up to 70in.Estimated eruption volume is 280 km3 dense rock equivalent (DRE). Lithological variations include vitric non- to densely weldedtuff, vapor-phase, devitrified, spherulitic, lithophysal,and rheomorphic tuff. Lithological zoning characteristics of the tuff change locally at nearly constant tuff thickness over distancesof 1 to 3 kmgrading from incipiently welded tuff to highly zonedsections.Regional variations become apparent by integrating many sections from onearea.A three-dimensional facies model is developed describing the local and regional facies variations. The RattlesnakeTuffconsists of high-silica rhyolite (HSR) erupted as pumices and glass shards. Dacite pumices make up less than1%of the total volume and quenched basalt and basaltic andesite inclusions inside dacite pumices constitute << 0.1 volume %. HSR pumices cluster in 4 to 5 compositional groups which are discerned best by La,Eu,Ba, Hf(Zr), Ta(Nb).Major element variations are minor but consistent between groupswith Si02increasing andFeO*, MgO, Ti02,and CaO decreasing with differentiation.Modalmineralogy,mineralchemistry,and partition coefficients also changeprogressively. The diversityof HSR is likely the product of crystal fractionation processes. A model isproposedby which astratified magma chamber is generated from the roof of the chamber downwardby progressivelymore evolved HSR. Least evolved HSRis likely the product ofdehydration melting of high-grade intermediate to mafic protoliths. High Ba/Rb ratios of 30 in the least evolved HSR limit the potential protoliths to lithologies with equal or higher Ba/Rb. Dacite pumices and mafic inclusions reveal the nature of the mafic root zone to the Rattlesnake Tuff magma chamber. Quenching and mingling textureswith the host pumice indicate that basaltic andesite and basalt inclusions are cognate. Daciteformed at the interface between HSR and underlying enriched basaltic andesite and wasgenerated by mixing of these two components. Basaltic andesite is fractionated andenriched in trace elements compared to regional primitive tholeiite (HAOT).Basaltic andesite has evolved from HAOT's mainly through fractionation and recharge whilebeing stalled underneath a silicic cap. Volcanology and Petrology of the Rattlesnake Ash-Flow Tuff, Eastern Oregon by Martin J. Streck A THESIS submitted to Oregon StateUniversity in partial fulfillment of the requirements for the degree of Doctor of Philosophy CompletedApril 27, 1994 Commencement June 1995 APPROVED: at,-"k / Associate Professor of Geology, charge of major Date thesis is presented April 27. 1994 Typed by theauthor Martin J. Streck Acknowledgements I would like to thank Anita Grunder for her continuing support and advice whenever needed. Her constant striving and abilities to improve my ideas and text kept me focused on clarity and significance. Many thanks go also to the team at the OSU Radiation Center, especially to Bob Walker (deceased), Art Johnson, Jack Higginbotham, Roman Schmitt, and Mike Conrady. Without their scientific and financial support, this thesis would be a lot thinner! The time and efforts of my graduate committee, Gordon Goles, Ed Taylor, Bob Duncan, Roger Nielsen, and John Dilles, are much appreciated. Among many people who contributed with discussions, friendship, hospitality, and others (e.g. 4WD vehicles) are: Jim MacLean, Anita Grunder, Jenda Johnson, Dave Sherrod, John Dilles, the Basalt Bunch (also known as Chemically Dependent Petrologists at OSU), and Hernan Garcia. Finally and foremost, I would like to thank Soraya Simons for her love and support. Also, I owe much gratitude to my parents for their love, trust, andfor the principles they taught me. An honest tale speeds best being plainly told. King Richard III ACT IV Table of Contents Chapter I: Facies variations in a thin, widespread ignimbrite sheet: The RattlesnakeTuff,EasternOregon....................................... 1 Abstract ....................................................................................1 Introduction............................................................................... 2 Tectonicsetting........................................................................... 3 Name and age............................................................................. 6 General aspect............................................................................ 6 Distribution,thickness and volume.................................................... 7 Today's outcrops................................................................ 7 Original areal coverage.......................................................... 16 Volume........................................................................... 17 Depositionalfacies....................................................................... 22 Pumiceclasts..................................................................... 22 Lithic fragments................................................................. 23 Speculations on pumice and lithics variations................................24 Source area................................................................................ 25 Welding andcrystallization facies...................................................... 29 Terminology,appearance and occurrence.................................... 29 Welding facies................................................................... 29 Crystallizationfacies............................................................ 36 Rheomorphictuff................................................................ 41 Transitions between facies.....................................................42 Facies model..............................................................................42 Regional facies variations...................................................... 43 LocalFacies variations.......................................................... 50 Discussion of welding and crystallization facies......................................51 Comparison to existing facies models........................................ 51 Timing............................................................................ 51 Causes of facies variations..................................................... 52 Conclusions.............................................................................. .54 References................................................................................. 55 Table of Contents, Continued: ChapterII:The RattlesnakeTuff,part I: Relationships between high-silica rhyolites, dacites and mafic inclusions................................ 60 Abstract.................................................................................... 60 Introduction............................................................................... 61 Geologic and tectonic setting............................................................ 64 Analyticalmethods....................................................................... 64 Eruptive productsof theRattlesnakeTuff............................................. 65 High-silicarhyolites............................................................. 65 Dacites............................................................................ 83 Basalticinclusions............................................................... 84 Discussion.................................................................................85 Rhyolite genesis..........................................................................85 Fractionation models............................................................ 86 Partial melting models.......................................................... 87 Mafic root zone.......................................................................... .92 Contemporaneous and regional basalts....................................... 92 Mafic inclusion- HAOT connection .......................................... 93 Modeling......................................................................... 96 Dacites:mafic-silicic interface........................................................... 103 Mixing - mingling and hybridization.......................................... 103 Compositionalgap.............................................................. 107 Magma chamber reconstruction.........................................................108 Conclusions.............................................................................. .111 References................................................................................. 113 Chapter III: The Rattlesnake Tuff, part II: Differentiation of the high-silica rhyolites ..........................................................................118 Abstract................................................................................... .118 Introduction..............................................................................
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