The Geochemistry and Geochronology of the Eocene Absaroka Volcanic Province, Northern Wyoming and Southwest Montana, USA by Margaret M. Hiza A THESIS submitted to Oregon StateUniversity in partialfulfullment of the requirements for the degree of Doctor of Philosophy Presented March 30, 1999 Commencement June 2000 a AN ABSTRACT OF THETHESIS OF Margaret M. Hiza, for the degree of Doctor of Philosophy in Geology presentedon March 30, 1999. Title: The Geochemistry and Geochronology of the Eocene Absaroka Volcanic Province, Northern Wyoming and Southwest Montana, USA. Abstract approved: AnitaL. Grunder The Absaroka volcanic province is the largest of Eocene volcanic fields in the northern Cordillera of the western U.S., and consists of 25,000 km2 of lava flows, shallow intrusions, ash-flow tuffs and volcaniclastic deposits.It is aligned with northwest-trending Precambrian lineaments, and includes the remains ofat least ten volcanic centers. This study presents 40Ar/39Ar geochronological data, and major, trace element and isotopic compositional data which are the result of detailed mapping and field sampling of four representative volcanic centers, peripheral lava flows, intrusions, and ash-flow tuffs.Age data show that volcanism occurred between 53 and 43 Ma in a general northwest to southeastage progression, and have allowed significant revisions in regional correlationsacross the volcanic province. Local dike orientations from one volcanic center suggest that volcanism occurred during extensional faulting. Geochemical and Nd, Sr and Pb isotopic data show that mafic lavas are enriched in incompatible elements derived froman ancient source. Mafic rocks (<53% SiO2, >5% MgO) are characteristically potassic, andare typical products of early eruptions at each of the Hyalite, Crandall, Ishawooa, and Rampart volcanic centers, and not spatially restricted toany one region. Although products of volcanism are broadly similar,volcanic centers exhibit distinct mineralogic, compositional and isotopic characteristics.Least squares calculations based on mineralogical data indicate that shoshonites can be produced from mafic samples by fractionation of olivine and augite t plagioclase.More silicic samples have petrographic features and compositions which indicate theyare derived from a mixture of sources including continental crust.Changing chemical and isotopic compositions suggest that early eruptions contain a lithospheric mantle component, with an increasing melt contribution from crustalsources with time. The last eruptions include rhyolite from the southern Absaroka volcanic province and basanite with an asthenospheric isotopic composition sampled froma peripheral lava flow in the northern Absaroka volcanic province. The Geochemistry and Geochronology of the Eocene Absaroka Volcanic Province, Northern Wyoming and Southwest Montana, USA by Margaret M. Hiza A THESIS submitted to Oregon StateUniversity in partial fulfullment of the requirements for the degree of Doctor of Philosophy Presented March 30, 1999 Commencement June 2000 I I Doctor of Philosophy thesis of Margaret M. Hiza presented on March 30, 1999 I APPROVED: I Major professor, representing Geology I I Chair ofDepartment of Geosciences I i Deanof Graduate S I I I I understand that my thesis will become part of the permanant collection of Oregon State University libraries. My signature below authorizes release ofmy thesis to any I reader upon request. I I N \L,I Margaret M. Hiza, Author I I I ACKNOWLEDGMENT I wish to express my sincere gratitude to several people, all of whom have played a major role in the completion of this thesis, to my thesis advisor, Anita Grunder, who has been both supportive and inspirational, and gave me the opportunity to feel likea member of the human race, to Lawrence Snee, who taughtme that I was capable, to John David Love, who always believed in my abilities, to Dave Graham, for his sincere interest inmy work and insightful discussions, and to Dan Unruh, who has always treatedme as a member of the geological community. A substantial part of my success,however, is also due to my three children, Aidan, Robert, and Shondiin Redsteer, to whom I dedicate this thesis. They not only understood, and letme follow a difficult educational path, but also tolerated my working long nights and days, and became proficient at holdingour small family together through it all. CONTRIBUTION OF AUTHORS Dr AnitaL. Grunder was involved in the organization and editing of each manuscript.Dr Lawmece Snee assisted in interpretationof 40ArP9Ardata collected in his laboratory, and read and assisted in manuscript editing. Dr Daniel Unruh analyzed samples for Nd Sr and Pb isotopic compositions, and provided valuable discussions on the interpretation of the isotopic data and assited in data interpretation.Dr James Budahn analyzed samples for trace element composition by neutron activation, and assited in geochemical modeling. TABLE OF CONTENTS INTRODUCTION The Geochemistry and Geochronology of the Eocene Absaroka Volcanic Province Northern Wyoming and SouthwestMontana, USA....................1 CHAPTER 1 Temporal and GeochemicalVariabilityin Eocene Alkaline Volcanism, Absaroka VolcanicProvince,Greater Yellowstone: southwest Montana and northwestern Wyoming.......................4 INTRODUCTION..........................................................................5 PREVIOUS WORK......................................................................... 7 METHODS...................................................................................9 AGE RESULTS.............................................................................11 ASH-FLOW TUFFS....................................................................... 17 SloughCreek Tuff..................................................................17 Lost Crrek Tuff...................................................................... 20 Blue PointAsh-Fall................................................................. 22 RegionalCorrelations..................................... .............23 MAJOR ELEMENT CHEVIISTRYOF VOLCANICCENTERS.....................25 DISCUSSION............................................................................. 28 The Sequence of Eruptions........................................................ 28 Eruptive Volume.................................................................... 31 Geographic Distribution of Eocene Volcanism................................... 31 CONCLUSIONS............................................................................. 37 REFERENCES CITED-1.................................................................... 39 CHAPTER 2 Mineralogic and GeochemicalVariabilityin Shoshonitic Magmatism from the Type Locality: Absaroka Volcanic Province, SouthwestMontana andNorthern Wyoming, USA....................48 INTRODUCTION......................................... :............................ 49 ANALYTICAL METHODS............................................................ 52 TABLE OF CONTENTS(Continued) Pie GEOLOGY AND MINERALOGY OF VOLCANIC CENTERS 53 The Crandall Volcanic Center.......................................................54 The Ishawooa Volcanic Center.....................................................55 The Rampart Volcanic Center .......................................................56 The Hyalite Volcanic Center........................................................57 Peripheral L a v a s......................................................................58 COMPOSITIONOF VOLCANICCENTERS............................................63 Sample compositions from the Crandall Volcanic Center.......................70 Sample compositions from the Ishawooa Volcanic Center....................78 Sample compositions from the Rampart Volcanic Center......................80 Sample compositions from the Hyalite Volcanic Center........................82 Samples Peripheral to Volcanic Centers...........................................85 MINERAL COMPOSITIONS 87 Olivine.............................................................................. 87 Pyroxene.............................................................................87 DISCUSSION.............................................................................. 92 Crandall Volcanic Center......................................................... 94 Ishawooa Volcanic Center........................................................ 102 Hyalite Volcanic Center......................................................... 105 Comparison of Fractionation Paths.............................................. 105 Source Enrichment................................................................. 108 Source Mineralogy..................................................................108 CONCLUSIONS...........................................................................114 REFERENCES CITED-2 115 CHAPTER 3 The Petrogenesis of Mafic Potassic Magmas, Absaroka VolcanicProvince,Wyoming-Montana...................... 120 INTRODUCTION............................................................................ 121 GEOLOGIC SETTING...................................................................... 124 Volcanic Centers..................................................................... 124 Samples............................................................................... 125 TABLE OF CONTENTS(Continued) Page MAJOR AND TRACE ELEMENT COMPOSTIONS ....................................126 Major Elements......................................................................