AN ABSTRACT OF THE THESIS OF Todd C. Feeley for the degree of Master of Science in Geology presented onJune 2. 1989. Title: The Egan Range Volcanic Complex: Implications for the Evolution of a Mid-Tertiary Synextensional Volcanic System. Abstract approved: Redacted for Privacy Dr. Anita L. *Under The Egan Range volcanic complex (ERVC),located approximately 30 km northwest of Ely, Nevada,lies astride a boundary separating a zone of high crustal extension (approximately 300% extension) to the east,including the Snake Range metamorphic core complex, anda relatively unextended domain (approximately 15% extension)to the west, including the Butte Mountains. The volcanic and minor sedimentary rocks of the ERVChave been divided into three stratigraphic groups: the early, middle, and late groups. Two new 40Ar-39Ar age determinations indicate that the volcanic rocks were erupted approximately35 Ma ago and were coeval with other mid-Tertiary volcanicrocks in east-central Nevada. Volcanic rocks of the earlygroup range from basaltic andesite (54% Si02) to rhyodacite (70% Si02),although the bulk of the lavas are two-pyroxene andesitesand dacites. Apparent compositional gaps are present at56 to 58% Si02 and 66 to 68% Si02, although thesemay be an artifact of incomplete sampling. Textural evidence, including plagioclase with sieved cores mantled by normally and reversely zoned rims, olivine coexisting with quartz surrounded by augite coronas, and quenched inclusions of basaltic andesite, suggest that thesemagmas represent incomplete mixtures of mafic and silicicmagmas. Volcanic rocks of the middle group are exposed ina northwest trending belt of outcrops in the north-eastern to central-western region of the studyarea. In contrast to the large compositional diversity of the earlygroup, rocks of the middle group are relatively homogeneous biotite, hornblende, pyroxene dacites. With respect to bulk composition, rocks of the middlegroup range from 65.7 to 68.5 wt.% SiO2 and fall in theupper gap in SiO2 content defined by the early group. During the late cycle of magmatisma bimodal suite of fine-grained andesite and petrographicallyand compositionally homogeneous biotite rhyodaciteswas erupted. The total volume of volcanic rocks inthe ERVC is approximately 40 km3. Dacites and rhyodacites comprise greater then 95% of the volume of eruptedproducts. Volumes of mafic lavas (basaltic andesitesand andesites) are greatest in the early group, absent in the middle group, and minor in the late group. The volume of silicic rocks dramatically increases upsection. Two generations of high-angle normal faultingare observed in the ERVC; an older set of N30-60°E, N30-40°W, and roughly east-west trending faults,and a younger set of approximately north-south trendingfaults. At the present level of exposure, faults of the olderset appear to have small displacements, althoughsome may have vertical offsets as greatas 600 m. Apparent vertical separation on faults of theyounger set are as great as 500 m. An angular unconformity developed between unit of the middle group and N40°E alignment of middle group vent loci suggest that the older set was most active during eruption of the early and middle groups. The age of initiation of movement on the younger faults is not known, although they probably postdate the volcanic rocks. Early and middle group andesites and dacites have elevated Cr (280-10 ppm), Sc (26-7 ppm), and Rb (60-180 ppm) concentrations and define linear trends indicating mixing between contaminated mantle-derived magmas and crustal melts. Isotope data are consistent with intermediate lavas containing a large crustal component. Chemical models require fractionation of minor amounts of cpx (< 3.5%) and accessory phases (<1%) prior to or during mixing. Cross- over patterns on REE plots (La/Lui, = 9.9fic to26.7,ilicic)preclude significant fractional crystallization or assimilation-fractional crystallization to produce the compositional spectrum. Low Ni concentrations (<25 ppm) and Mg# (<45) in basaltic andesites indicate fractionation of olivine prior to mixing. Major- and trace-element variations within the late group may be explained by up to 90% crystal fractionation of the observed phenocryst assemblage. Slightly depleted HREE concentrations and constant LREE and Zr concentrations in dacites relative to andesites suggest that amphibole fractionated in proportions greater than in the mode. Crystal fractionation cannot, however, account for elevated Rb, Th, and U in the dacites and thus, moderate amounts of assimilation are required. The sequence of magma types in the ERVC are explained by a model where mantle-derived magmas are repeatedly injected into the crust. The early group records the initial injections of basaltic magma, production of silicic crustal melts, and wholescale mixing of these magmas. The middle group records further heating and melting of the crust and, thus, an increase in the volume of silicic magma in the system. The volume of silicic magma in the system during the middle group was large enough to prevent wholescale mixing of basalt and silicic crustal melt, although newly generated crustal melts mixed with resident intermediate hybrid magmas. The switch to crystal fractionation in the late group indicates a decrease in the rate of supply of new basaltic magma to the system. As a result, thermal gradients and, therefore, convective velocities began to decline and crystallization and fractionation followed. The Egan Range Volcanic Complex: Implications For the Evolution of a Mid-Tertiary Synextensional Volcanic System by Todd C. Feeley A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed June 2, 1989 Commencement June 1990 APPROVED: Redacted for Privacy Assistant Professor of a4ology in charge of major Redacted forPrivacy/ ---e Head27epartment of Geology L Redacted for Privacy Dean of School Date thesis is presented June 2, 1989 Acknowledgements I feel fortunate to have been associated withso many helpful and inspiring people during the last threeyears. Their encouragement and support not only enabledme to complete this thesis, but also begin my study of Geology. I owe the bulk of my thanks to Anita Grunder. Her guidance on matters both scientific and personalwere invaluable. I thank my committee members John Dilles and Roman Schmitt. Their thorough reviews significantly improved the thesis and my ability to think critically. Roger Nielsen offered helpful advice on petrologic modeling. Although I thank everyone in the Geology Department, I want to particularly acknowledge Jeff Marso for stimulating petrologic discussions, computer help,and friendship. I thank all the members of the Journal Club, including Reed Lewis, Gary Davidson, Britt Hill, and George Moore for provocative Friday afternoon discussions. Zhang Ming, cofounder of the Expanding Earth Society, deserves special credit for his rational, collected thinking in the face of adversary. Office mates Phyllis Camilleri and Bill Gallahan displayed admirable patience during often difficult times. I am indebted to Therese Belden for her generosity and help in administrative matters. I am particularly grateful to Phil Gans for very helpful discussions on volcanic stratigraphy and structure during a visit to the field, and for age determinations. Thanks go to Bob Walker for analytical support and enlightening discussions on Tertiary magmatism. Mike Conrady supplied his expertise on matters pertaining to reduction of the INAA data. The unsung hero award goes to the Albuquerque Dukes for providing many exciting moments during two, often lonely field seasons. Financial support was generously supplied by the Geological Society of America, Sigma Xi, the National Science Foundation, and Chevron. I thank Kimie Fukuda for moral support during my last, most difficult year at OSU. I appreciate deeply the support of Ellen Morris Bishop. Thank you Ellen for everything; I will never forget it. Most of all, I thank my parents, Bill and Joan Feeley. The unyielding support they have given me throughout my life is my greatest inspiration. TABLE OF CONTENTS CHAPTER ONE: INTRODUCTION 1 Location and Physiography 4 Previous Work and Geologic Setting 6 CHAPTER TWO: STRATIGRAPHY AND LITHOLOGIC CHARACTER OF MAP UNITS 12 Early Group 19 Lower Clastic and Pyroclastic Unit (Tees) 19 Lavas Bearing Magmatic Inclusions (Tv') 30 Discussion of the Early Group 37 Middle Group 39 Kalamazoo Tuff (Tkt) 40 Autoclastic Dacites (Tvml) 42 Smokey Quartz-Bearing Rhyodacite (Tvm2) 45 Glomeroporphyritic Dacites (Tvm3) 47 Discussion of the Middle Group 51 Late Group 53 Dacite Lavas (Tvll) 55 Aphyric Andesite Lava (Tv12) 58 Rhyodacite Lavas (Tv13) 59 Disscussion of the Late Group 63 Summary of Chapter Two 65 CHAPTER THREE: STRUCTURE OF THE EGAN RANGE VOLCANIC COMPLEX 67 The Pre-volcanic Unconformity and the Timing of Extension 68 Faulting 74 Summary and Discussion 80 CHAPTER FOUR: PETROGENESIS OF METALUMINOUS VOLCANIC ROCKS OF THE EGAN RANGE VOLCANIC COMPLEX 83 Analytical Techniques 84 Mineral Chemistry 87 Feldspar 87 Pyroxenes 95 Amphibole and Biotite 101 Whole-Rock Chemistry 105 Early Group 110 Middle Group 115 Late Group 116 Isotopes 119 Petrogenesis of Magmas of the Egan Range Volcanic Complex 128 Methods 129 Early Group 130 Middle Group 144 Late Group 148 Summary and Discussion 164 Physical Considerations 165 Mantle Versus Crustal Sources 170 Composition of the Primary Basalt 173 Bibliography 184 APPENDIX I: Sample Locations and Petrographic