Geological Society of America

BRIGHAM YOUNG UNIVERSITY

GEOLOGICAL SOCIETY OF AMERICA

FIELD TRIP GUIDE BOOK

1997 ANNUAL MEETING SALT LAKE CITY, UTAH

PAR'

EDITED BY PAUL KARL LINK AND BART J. KOWALLIS

VOIUME 42 I997 PROTEROZOIC TO RECENT STRATIGRAPHY, TECTONICS, AND VOLCANOLOGY, UTAH, NEVADA, SOUTHERN IDAHO AND CENTRAL MEXICO Edited by Paul Karl Link and Bart J. Kowallis

BRIGHAM YOUNG UNIVERSITY GEOLOGY STUDIES Volume 42, Part I, 1997

CONTENTS

Neoproterozoic Sedimentation and Tectonics in West-Central Utah ...... Nicholas Christie-Blick 1

Proterozoic Tidal, Glacial, and Fluvial Sedimentation in Big Cottonwood Canyon, Utah ...... Todd A. Ehlers, Marjorie A. Chan, and Paul Karl Link 31

Sequence Stratigraphy and Paleoecology of the Middle Cambrian Spence Shale in Northern Utah and Southern Idaho ...... W. David Liddell, Scott H. Wright, and Carlton E. Brett 59

Late Ordovician Mass Extinction: Sedimentologic, Cyclostratigraphic, and Biostratigraphic Records from Platform and Basin Successions, Central Nevada ...... Stan C. Finney, John D. Cooper, and William B. N. Beny 79

Carbonate Sequences and Fossil Communities from the Upper Ordovician-Lower Silurian of the Eastern Great Basin ...... Mark T. Harris and Peter M. Sheehan 105

Late Devonian Alamo Impact Event, Global Kellwasser Events, and Major Eustatic Events, Eastern Great Basin, Nevada and Utah ...... Charles A. Sandberg, Jared R. Morrow and John E. Warme 129

Overview of Mississippian Depositional and Paleotectonic History of the Antler Foreland, Eastern Nevada and Western Utah ...... N. J. Silberling, K. M. Nichols, J. H. Trexler, Jr., E W. Jewel1 and R. A. Crosbie 161

Triassic-Jurassic Tectonism and Magmatism in the Mesozoic Continental Arc of Nevada: Classic Relations and New Developments ...... S. J. Wyld, and J. E. Wright 197

Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1- Introduction & Road Log ...... Arthur W. Snoke, Keith A. Howard, Allen J. McGrew, Bradford R. Burton, Calvin G. Barnes, Mark T. Peters, and James E. Wright 225

Part 2: Petrogenesis and thermal evolution of deep continental crust: the record from the East Humboldt Range, Nevada ...... Allen J. McGrew and Mark T. Peters 270 Part 3: Geology and petrology of Cretaceous and Tertiary granitic rocks, Lamoille Canyon, Ruby Mountains, Nevada ...... Sang-yun Lee and Calvin G. Barnes

Part 4: Geology and geochemistry of the Hanison Pass pluton, central Ruby Mountains, Nevada ...... Bradford R. Burton, Calvin G. Barnes, Trina Burling and James E. Wright

Hinterland to Foreland Transect through the Sevier Orogen, Northeast Nevada to North Central Utah: Structural Style, Metamorphism, and Kinematic History of a Large Contractional Orogenic Wedge ...... Phyllis Camilleri, W. Adolph Yonkee, Jim Coogan, Peter DeCelles, Allen McGrew, Michael Wells

Part 2: The Architecture of the Sevier Hinterland: A Crustal Transect through the Pequop Mountains, Wood Hills, and East Humboldt Range, Nevada ...... Phyllis Camilleri and Allen McGrew

Part 3: Large-Magnitude Crustal Thickening and Repeated Extensional Exhumation in the Raft River, Grouse Creek and Albion Mountains ...... Michael L. Wells, Thomas D. Hoisch, Lori M. Hanson, Evan D. Wolff, and James R. Struthers

Part 4: Kinematics and Mechanics of the Willard Thrust Sheet, Central Part of the Sevier Orogenic Wedge, North-central Utah ...... W. A. Yonkee

Part 5: Kinematics and Synorogenic Sedimentation of the Eastern Frontal Part of the Sevier Orogenic Wedge, Northern Utah ...... W. A. Yonkee, F! G. DeCelles and J. Coogan

Bimodal Basalt-Rhyolite Magmatism in the Central and Western Snake River Plain, Idaho and Oregon ...... Mike McCurry, Bill Bonnichsen, Craig White, Martha M. Godchaux, and Scott S. Hughes

Bimodal, Magmatism, Basaltic Volcanic Styles, Tectonics, and Geomorphic Processes of the Eastern Snake River Plain, Idaho ...... Scott S. Hughes, Richard l? Smith, William R. Hackett, Michael McCuny, Steve R. Anderson, and Gregory C. Ferdock

High, Old, Pluvial Lakes of Western Nevada ...... Marith Reheis, and Roger Momson

Late Pleistocene-Holocene Cataclysmic Eruptions at Nevado de Toluca and Jocotitlan Volcanoes, Central Mexico ...... J. L. Macias, F! A. Garcia, J. L. Arce, C. Siebe, J. M. Espindola, J. C. Komorowski, and K. Scott A Publication of the Department of Geology Brigham Young University Provo, Utah 84602

Editor

Bart J. Kowallis

Brigham Young University Geology Studies is published by the Department of Geology. This publication consists of graduate student and faculty research within the department as well as papers submitted by outside contributors. Each article submitted is externally reviewed by at least two qualified persons.

Cover photos taken by Paul Karl Link. Top: Upheaval Dome, southeastern Utah. Middle: Luke Bonneville shorelines west of Brigham City, Utah. Bottom: Bryce Canyon National Park, Utah.

ISSN 0068-1016 9-97 700 23348124218 Preface

Guidebooks have been part of the exploration of the American West since Oregon Trail days. Geologic guidebooks with maps and photographs are an especially graphic tool for school teachers, University classes, and visiting geologists to become familiar with the temtory, the geologic issues and the available references. It was in this spirit that we set out to compile this two-volume set of field trip descriptions for the Annual Meeting of the Geological Society of America in Salt Lake City in October 1997. We were seeking to produce a quality product, with fully peer-reviewed papers, and user-fnendly field trip logs. We found we were buck- ing a tide in our profession which de-emphasizes guidebooks and paper products. If this tide continues we wish to be on record as producing "The Last Best Geologic Guidebook." We thank all the authors who met our strict deadlines and contributed this outstanding set of papers. We hope this work will stand for years to come as a lasting introduction to the complex geology of the Colorado Plateau, Basin and Range, Wasatch Front, and Snake River Plain in the vicinity of Salt Lake City. Index maps to the field trips contained in each volume are on the back covers. Part 1 "Proterozoic to Recent Stratigraphy, Tectonics and Volcanology: Utah, Nevada, Southern Idaho and Central Mexico" contains a number of papers of exceptional interest for their geologic synthesis. Part 2 "Mesozoic to Recent Geology of Utah" concentrates on the Colorado Plateau and the Wasatch Front. Paul Link read all the papers and coordinated the review process. Bart Kowallis copy edited the manu- scripts and coordinated the publication via Brigham Young University Geology Studies. We would like to thank all the reviewers, who were generally prompt and helpful in meeting our tight schedule. These included: Lee Allison, Genevieve Atwood, Gary Axen, Jim Beget, Myron Best, David Bice, Phyllis Camillen, Marjorie Chan, Nick Christie-Blick, Gary Christenson, Dan Chure, Mary Droser, Ernie Duebendorfer, Tony Ekdale, Todd Ehlers, Ben Everitt, Geoff Freethey, Hugh Hurlow, Jim Garrison, Denny Geist, Jeff Geslin, Ron Greeley, Gus Gustason, Bill Hackett, Kimm Harty, Grant Heiken, Lehi Hintze, Peter Huntoon, Peter Isaacson, Jeff Keaton, Keith Ketner, Guy King, Me1 Kuntz, Tim Lawton, Spencer Lucas, Lon McCarley, Meghan Miller, Gautarn Mitra, Kathy Nichols, Robert Q. Oaks, Susan Olig, Jack Oviatt, Bill Peny, Andy Pulham, Dick Robison, Rube Ross, Rich Schweickert, Peter Sheehan, Norm Silberling, Dick Smith, Bany Solomon, K.O. Stanley, Kevin Stewart, Wanda Taylor, Glenn Thackray and Adolph Yonkee. In addition, we wish to thank all the dedi- cated workers at Brigham Young University Print Services and in the Department of Geology who contributed many long hours of work to these volumes.

Paul Karl Link and Bart J. Kowallis, Editors The Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1-Introduction & Road Log

ARTHUR W. SNOKE Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

KEITH A. HOWARD US. Geological Survey, MS 975, 345 Middlefield Road, Menlo Park, Cal$ornia 94025

ALLEN J. MCGREW Department of Geology, The University of Dayton, Dayton, Ohio 45469-2364

BRADFORD R. BURTON Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

CALVIN G. BARNES Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053

MARK T. PETERS Woodward-Clyde Federal Services, 11 80 Town Center Drive, Las Vegas, Nevada 89134

JAMES E. WRIGHT Department of Geology and Geophysics, Rice University, Houston, Texas 77251

ABSTRACT The purpose of this geological excursion is to provide an overview of the multiphase developmental histo- ry of the Ruby Mountains and East Humboldt Range, northeastern Nevada. Although these mountain ranges are commonly cited as a classic example of a Cordilleran metamorphic core complex developed through large-magnitude, mid-Tertiary crustal extension, a preceding polyphase Mesozoic contractional his- tory is also well preserved in the ranges. An early phase of this history involved Late Jurassic two-mica granitic magmatism, high-temperature but relatively low-pressure metamorphism, and polyphase deforma- tion in the central Ruby Mountains. In the northern Ruby Mountains and East Humboldt Range, a Late Cretaceous history of crustal shortening, metamorphism, and magmatism is manifested by fold-nappes (involving Archean basement rocks in the northern East Humboldt Range), widespread migmatization, injection of monzogranitic and leucogranitic magmas, all coupled with sillimanite-grade metamorphism. Following Late Cretaceous contraction, a protracted extensional deformation partially overprinted these areas during the Cenozoic. This extensional history may have begun as early as the Late Cretaceous or as late as the mid-Eocene. Late Eocene and Oligocene magmatism occurred at various levels in the crust yield- ing mafic to felsic orthogneisses in the deep crust, a composite granitic pluton in the upper crust, and vol- canic rocks at the surface. Movement along a west-rooted, extensional shear zone in the Oligocene and early Miocene led to core-complex exhumation. The shear zone produced mylonitic rocks about 1 km thick at deep crustal levels, and an overprint of brittle detachment faulting at shallower levels as unroofing proceed- ed. Megabreccias and other s~nextensionalsedimentary deposits are locally preserved in a tilted, upper Eocene through Miocene stratigraphic sequence. Neogene magmatism included the emplacement of basalt dikes and eruption of rhyolitic rocks. Subsequent Basin and Range normal faulting, as young as Holocene, records continued tectonic extension. 226 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I INTRODUCTION AND ORGANIZATION (Burton, Barnes, Burling, and Wright). For convenience, OF THE FIELD GUIDE guidebook article figures are numbered sequentially, and the "References cited section is cumulative. The Ruby Mountains and East Humboldt Range form two adjoining ranges in northeastern Nevada, each reach- ACKNOWLEDGMENTS ing approximately a mile above surrounding lowlands, and together over 80 mi (-130 km) long (Fig. 1). They lie Recent field studies in the Ruby Mountains by Snoke within the late Cenozoic Basin and Range province, in a and Howard were supported by NSF grant EAR-9627958, region that was also the site of part of the CordilIeran mio- and the geochemical studies on the Harrison Pass pluton geocline during the Neoproterozoic to early Mesozoic. This and granitic rocks of upper Lamoille Canyon were sup- area was subjected to multiple episodes of plutonism, meta- ported by NSF grant EAR-9627814. Additional support morphism, and deformation in the Mesozoic, and was in for geochemical studies on the Harrison Pass pluton were the hinterland of the Sevier orogenic belt during the late provided by the Department of Geosciences, Texas Tech Mesozoic and early Tertiary. Finally, it was subjected to a University, and INAA data were obtained at the Lunar complex history of volcanism, sedimentation, and crustal and Planetary Institute through the courtesy of Graham extension during the Cenozoic. For many years the Ruby Ryder. Ken Johnson is thanked for his assistance in the Mountains and East Humboldt Range has been recognized field. Some geochemical data on granitic rocks of the upper as a classic example of a Cordilleran metamorphic core Lamoille Canyon area were obtained through a DOE reac- complex (e.g., Crittenden et al., 1980). The large magnitude tor sharing grant from Oregon State University. McGrew Cenozoic crustal extension associated with core complex and Peters thank Steve Wickham and many co-workers for development has exposed an important window into mid- their collaborative efforts in this region over the past several dle crustal rocks of the Sevier hinterland and provides an years. In addition, Peters acknowledges the support of opportunity to study the Mesozoic root zone of the Cor- Woodward-Clyde Federal Services and grants from the dilleran fold-and-thrust belt. Because of the complex over- NSF and Geological Society of America. McGrew acknowl- printing of multiple episodes of deformation, magmatism, edges early support from NSF EAR-87-07435 awarded to and metamorphism in this area-some during contraction A.W. Snoke and most recently a University of Dayton and some during extension-a major problem in inter- Research Council Seed Grant. The field studies of Burton preting the geology of the Ruby Mountains and East Hum- were partially supported by a Geological Society of America boldt Range has always been deciphering to which event research grant. Burton thanks Barbara E. John of the a particular fabric, structural feature, or plutonic body was University of Wyoming for many instructive discussions associated. Although significant progress has been made about the mechanics of pluton emplacement and petroge- in recent years toward sorting out the deformational, mag- nesis of granitic rocks. The authors thank Phyllis A. matic, and metamorphic chronology of the Ruby Mountains Camilleri, W Adolph Yonkee, and Paul K. Link for their and East Humboldt Range, the complexity and overprint- critical review comments on an earlier version of the man- ing of events must always be considered whenever any uscript. Their many useful suggestions greatly improved rock body in the Ruby Mountains and East Humboldt the clarity and overall presentation of this field trip guide. Range is examined. In part, this guidebook article has been modified from PREVIOUS GEOLOGIC STUDIES Snoke and Howard (1984). However, during the past thir- Geologists of the Fortieth Parallel Survey first recog- teen years and continuing to the present, there have been nized the anomalously high metamorphic grade of rocks substantial new geologic studies in the Ruby-East Humboldt within the Ruby Mountains and East Humboldt Range (at area and thus a prime purpose of this trip is to highlight that time both ranges were referred to as the Humboldt results from these new studies. In that light, this article is Range) and consequently inferred a Precambrian age for divided into the following sections: (1)an introductory sec- these rocks (King, 1878). Robert E Sharp was the first tion written by Snoke and Howard that summarizes previ- geologist to study the ranges comprehensively and report- ous work and provides a brief geologic framework for the ed on the geomorphology (1940), glacial deposits (1938), trip; (2) a detailed roadlog prepared by Snoke with contri- flanking Tertiary deposits (1939a), Paleozoic stratigraphy butions from McGrew and Peters, Howard, and Burton; and and structure (1942), and basin-and-range faulting (1939b). (3) a set of three short reports on new results of ongoing Among his many perceptive observations, Sharp established studies on metamorphic petrology in the East Humboldt that the ranges are horsts bounded by normal fault systems Range (McGrew and Peters), geochemistry of granites in and reported that granitic material is intermingled with the upper Lamoille Canyon area (Lee and Barnes), and metasedimentary rocks in the metamorphic core of the geology and geochemistry of the Harrison Pass pluton ranges with an overall increase in the amount of granite SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA

Inferred mo~mboundary of Archmn boment -,

LITHOLOGIC UNITS @ Sumcial deposits (Quaternary) Jasperold brecda (Tertiary) a Rhydltlc rocks (Miocene)-Dated about 15 Ma a Sedimentary rocks (Oligocene and Mlocene) @@ Basaltic andesite and associated sedlmentary rocks (late Eocene) a Granitlc rocks (late Eocene) @ Ruby Mountains-East Humboldt Range igneous and metamorphic complex a Nonrnetarorphosed to low-grade sedimentary rocks (Triassic and Paleozoic) SYMBOLS Contact

'" Normal fault--ball-bar symbol on downthrown slde, dotted where inferred or concealed Low-angle normal fault (detachment fault)--dotted where Inferred or concealed 1''"Fault (non-specific)--dotted where Inferred or concealed Trend and direction of plunge of major fold--from left to rlpht: anticline, syncline, overtumed anticline, overtumed synd~ne MyloniUc zone-approximate trend of elongation lineations shown by dashes

Figure 1. Generalized geologic map of the Ruby Mountains-East Hurnboldt Range, Nevada. C.H. = Clover Hill, S. V = Secret Valley, L. C. = Lamoille Canyon. 228 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I southward toward the Harrison Pass area (Fig. 1).There, a involved in the low-angle faulting that characterized the composite pluton, now known to be late Eocene (-36 Ma; upper levels of the structural pile (Snoke, 1975). During see Swisher and Prothero [1990] for determination of the summer of 1981, mylonitic rocks were discovered in Eocene-Oligocene boundary), separates a migmatitic ter- the mid-Tertiary Harrison Pass pluton (Snoke et al., 1982) rane from a homocline of Cambrian to Mississippian car- and the concept evolved that much of the mylonitization bonate, shale, and quartzite strata (Sharp, 1942). He also was Tertiary in age rather than Mesozoic as had been proposed that similar Paleozoic strata were the protoliths assumed since Snelson's (1957) descriptions of the mylo- for marble and metaquartzite in the northern part of the nitic rocks of the East Humboldt Range and northernmost range (Sharp, 1939b, 1942). Many of the major geologic Ruby Mountains. U-Pb geochronology coordinated with elements of the ranges were recognized as a result of the geologic mapping established that many of the mylonitic studies of Sharp and other early workers: (1)unmetamor- plutonic bodies in the Ruby Mountains and East Hum- phosed lower and middle Paleozoic strata in the southern boldt Range were Tertiary intrusive bodies subsequently Ruby Mountains, (2) a migmatitic core of granitic and deformed in a ductile shear zone (Wright and Snoke, 1986, upper amphibolite facies metamorphic rocks exposed 1993) and consequently the recognition of a Tertiary shear throughout most of the remainder of the ranges, (3) outliers zone became a fundamental element in the structural chro- of upper Paleozoic strata on the range flanks and in the nology. The cooling history of the mylonitic shear zone Secret Creek gorge area between the ranges, and (4) Ter- and migmatitic infrastructure has been explored by vari- tiary continental strata. ous thermochronometric studies (Armstrong and Hansen, Snelson's (1957) mapping in the East Humboldt Range 1966; Kistler et al., 1981; Dallmeyer et al., 1986; Dokka et and northernmost Ruby Mountains added another impor- al., 1986; McGrew and Snee, 1994). While geochronological tant element, a thick mylonitic zone developed in the studies indicated the importance of plastic Tertiary deforma- metamorphic and igneous rocks, structurally separated tion in the Ruby Mountains and East Humboldt Range, from overlying upper Paleozoic strata by low-angle faults. kinematic studies of the mylonitic rocks provided impor- Snelson (1957) inferred that this mylonite zone, as well as tant clues to the overall geometry of the mylonitic shear associated low-angle faults that typically place unmetarnor- zone including the recognition of a west-rooted, plastic-to- phosed sedimentary rocks onto a highly metamorphosed brittle fault system with top to the west-northwest motion footwall, were manifestations of a Mesozoic dkcollement along the west flank of both the Ruby Mountains and East zone related to regional contraction. Armstrong and Hansen Humboldt Range (Snoke and Lush, 1984; Mueller and (1966) inferred that a regional, mobile metamorphic infra- Snoke, 1993a). The mylonitic zone has been studied with structure developed in the Phanerozoic, based in part on thermobarometric techniques which indicated that the Tertiary K-Ar ages obtained from the Ruby Mountains and deeper parts of the mylonitic shear zone were deformed other metamorphic rocks in the eastern Great Basin. under amphibolite-facies conditions (Hurlow et al., 1991). Willden and Kistler (1969, 1979) further documented Recrystallization grain-size studies also provided informa- the Paleozoic strata in the southern Ruby Mountains. tion on paleostress during mylonitization (Hacker et al., Detailed geologic mapping and structural studies in the 1990). These studies coupled with geologic mapping indi- migmatitic rocks of the northern Ruby Mountains by cate that the mylonitic shear zone is about 1 kilometer Howard (1966, 1971, 1980) established the presence of a thick, extensional in character and rooted to the west, coherent metasedimentary stratigraphy metamorphosed exhibits a plastic-to-brittle history during exhumation, in- to amphibolite facies (Fig. 2). This stratigraphy outlines volved plutonic rocks as young as 29 Ma, and is cut by large-scale recumbent folds and persists even where com- younger normal faults related to late Cenozoic Basin and mingled granite constitutes half or even 90 per cent of the Range extension. exposures. Howard (1966, 1980) also divided the igneous The fold-nappes originally recognized by Howard (1966, and metamorphic complex of the northern Ruby Mountains 1980) in the northern Ruby Mountains have always been a into a migmatitic metamorphic infrastructure and a struc- major geologic curiosity in this part of the Sevier hinter- turally higher mylonitic zone. This subdivision has proven land. Studies in the East Humboldt Range have now to be useful throughout the Ruby Mountains and East demonstrated that a basement-cored (Archean) fold-nappe Humboldt Range (Snoke et al. 1990b), and we use this is exposed in the northern part of that range (Lush et al., nomenclature in this article. 1988; McGrew, 1992). The age of this fold-nappe is still In 1971 Snoke began a study of the transition from the controversial, but Allen McGrew has made a strong case migmatitic infrastructure into the mylonitic zone and for Late Cretaceous development based on detailed geo- included klippen of unmetamorphosed rocks within his logic mapping and some sparse U-Pb dating of associated initial map area in the northernmost Ruby Mountains. As granitic rocks. McGrew (1992) concluded that the fold- this study progressed, he realized that Tertiary rocks were nappe formed during crustal contraction associated with SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 229

LITHOLOGY CORRELATION Nara~hiticcalcite marble commonlv Guilmette Formation, Devonian

dolomite marble with subordinate midPaleozoic dolomite sequence, tremolite, diopside, graphite; Ordovician to Devonhn diopside-quartz rock; scarce metaquartzite lenses Eureka Quartzite, ... ..: ..p..:'.:..: -:..m)m.. white vitreous metaquartzite with Middle Ordovician mqtracesof muscovite andor graphite graphitic, white calcite marble locally siliceous and locally Pogonip Group (?), Lower Ordovician dolomitic I

cab-silicate gneiss and marble, calc-schists / tannish-brown dolomite marble, forsterite (tchondrodite) calcite marble Middle to ~6perCambrian 1calc-silicate gneiss and marble impure cattmnate rocks and associated terrigenous rocks raphitic cab-silicate paragneiss '4 rusty weathering zone) ......

feldspathic micaceous metaquartzite with interlayers of pelitic schist and Lower Cambrian Prospect Mountain paragneiss; subordinate calc-sili,cate Quartzite and Neoproterozoic metaquartzite McCoy Creek Group

Figure 2. Schematic stratigraphic column of mtasedirnentary rock units (afer Howard, 1971; Snoke, 1980). the Sevier orogeny and was subsequently strongly over- (MacCready et al., 1993, in press). The development of this printed by a large-scale Tertiary shear zone during core infrastructure lineation may in part overlap the develop- complex development. The magnitude of Late Cretaceous ment of the west-northwest lineation of the mylonitic shear deformation, magmatism, and metamorphism in the Ruby zone. The key evidence for this conclusion is that selected Mountains and East Humboldt Range is still just being Tertiary plutonic orthogneisses of the infrastructure, dated explored via geologic mapping and coordinated radiomet- by U-Pb radiometric techniques, contain this fabric. ric dating, but ongoing studies on the relationship between granite emplacement and deformation in the GEOLOGIC FRAMEWORK Lamoille Canyon area may provide an important key to The Ruby Mountains-East Humboldt Range records a this history. We will study some of these important rela- polyphase Mesozoic-Cenozoic history of deformation (Fig. tionships in upper Lamoille Canyon on DAY THREE of 3). Although a complex Mesozoic history is inferred to the field trip. have affected the entire Ruby Mountains-East Humboldt An important recent discovery is that a pervasive elon- Range prior to core complex development, this early history gation lineation (sometimes defined by sillimanite), wide- is best understood in the Ruby Mountains. In the central spread in parts of the migmatitic infrastructure of the Ruby Mountains, Hudec (1990, 1992) identified three northern Ruby Mountains, developed during the Tertiary phases of penetrative deformation and two amphibolite- 230 BYU CEOLOCY STUDIES 1997, VOL. 42, PART I

( Sedimentation 1 Magmatism I Deformation I Time scale I SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 231 facies metamorphic events, all of which he interpreted as (Snoke and Lush, 1984). The physical conditions of mylo- broadly synchronous with the emplacement of Late Jur- nitization include a broad range of P-T conditions, as dif- assic two-mica granite (Hudec and Wright, 1990). Metarnor- ferent levels of the shear zone are exposed along > 100 km phic monazite from a pelitic schist in the northern Ruby of strike (Snoke et al., 1990a; Hacker et al., 1990; Hurlow Mountains yielded a Late Cretaceous U-Pb date suggest- et al., 1991). As the shear zone evolved and uplift and tec- ing a younger high-temperature metamorphic event in tonic denudation progressed, the character of extensional that area (Snoke et al., 1979). Sillimanite-bearing, peg- deformation changed from plastic to brittle, with low-angle matitic leucogranite intimately interlayered with upper normal faults localized in the mylonitic shear zone. These amphibolite-facies metamorphic rocks in the northern Ruby low-angle faults dismembered macroscopic folds related Mountains (Lamoille Canyon area) has also yielded a Late to the earlier periods of deformation, and they juxtaposed Cretaceous monazite date (Wright, Snoke, Howard, and upper crustal, unmetamorphosed to low-grade metamor- Barnes, unpublished data; Snoke et al., 1992). These data phic rocks against rocks originally resident in the middle suggest the importance of Late Cretaceous granitic mag- crust. The mylonitic deformation was thus part of a long- matism and metamorphism during the evolution of the lived, extensional fault system (e.g., Mueller and Snoke, high-grade, migrnatitic core of the Ruby Mountains (Kistler 1993a). Activity on this fault system began in the Eocene et al., 1981; Snoke et al., 1992). In summary, the Mesozoic and has continued into the Holocene, thereby indicating a history of the Ruby Mountains (and perhaps also the East protracted history of exhumation for the Ruby Mountains- Humboldt Range) involved at least two thermal pulses- East Humboldt core complex. one in the Late Jurassic clearly associated with granitic pluton emplacement, and one in the Late Cretaceous ROAD LOG associated with tectonic thickening and the production of DAY ONE leucogranites (Snoke, 1994). Tertiary intrusion in the Ruby Mountains-East Hum- Salt Lake City International Airport to Wells, Nevada via boldt Range was areally widespread, occurred over an ex- Interstate 80 West (Fig. 4). tensive time period (40-29 Ma), and is well-documented There will be no formal geologic stops along our route by U-Pb geochronology (Wright and Snoke, 1993; Fig. 3). from the Salt Lake City International Airport to Wells, Tertiary intrusions exhibit a broad range in composition Nevada (Fig. 4). However, a brief summary of the physiog- from mafic quartz diorite to leucogranite; the intrusions raphy and geology that we traverse along this segment of occur as dikes, sills, and large sheet-like bodies. Wall-rock the field trip is included in this road log as well as some contacts range from sharp to diffuse. Some bodies clearly references. cross-cut wall-rock structures (e.g., foliation), whereas other After leaving the Salt Lake City International Airport, bodies are subparallel to wall-rock structures or appear to we drive west toward the Great Salt Lake Desert. Our route share the same fabric elements. takes us around the north end of the Oquirrh Mountains Superposed on this polyphase magmatic, metamorphic, near Magna, where complex folding of the Pennsylvanian- and deformational history is the top-to-the-west-north- Permian Oquirrh Group is well exposed in the North west, normal-sense mylonitic shear zone of Tertiary age Oquirrh thrust plate (Tooker and Roberts, 1970). Shore-

Figure 3. Geologic history of the Ruby Mountains and East Humboldt Range, Nevada, including sedimentation, qmutisnz, and defw- mation. Only selected stratigraphic units are delineated in the sedimentation column. Logarithmic time scale in Ma; geologic time scale after Palmer (1983). This diagram was designed afer Dickinson (1991,figure 5). The age of the Archean orthogneiss of Angel Lake is from Lush et al. (1988), the age range for the mid-Tertiary orthogneisses is from Wright and Snoke (1993), and the age range of the Wilh Creek rhyolite complex is $-om E. H. McKee (unpublished data). The age range of the Miocene basalt dikes is based on data reported in Snoke (1980) and Hudec (1990). The age range of the leucogranites (90 to 70 Ma) is based on data sumdzed in Snoke et al. (1992) as well as the unpublished U-Pb radiometric ages of J. E. Wright. The age of the two-mica granite of Dawley Canyon is from Hudec and Wright (1990). The age range for the exhumation of the core complex is based on data reported in Dallmeyer et al. (1986), Dokka et al. (1986),and Wright and Snoke (1993). The stratigraphic age range for the "lower Humboldt Formation" (Zate Eocene to Oligocene) is based on paleontological data reported in Good et al. (1995). The age range of the late Eocene volcanic and associated rocks in the East Humboldt Range is based on data summarized in Brooks et al. (1995). The presence of ash deposits eruptedfiom the Yellowstone volcanic plateau is based on a preliminary identajcation made by A. M. Sam-Wojcicki (personal. comm., 1992) of a sample of vitric ash collected by Snoke from near the center of section 7, T 34 N, R. 59 E (John Day Creek, Soldier Peak 7 112 minute quadrangle). 232 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

Figure 4. Map showing route (along 1-80 West)from Salt Lake City International Airport to Wells, Nevada (see geologic description of the route on DAY ONE). line features related to Pleistocene Lake Bonneville, studied Warnaars et al., 1978; Moore and McKee, 1983). Extensive by G. K. Gilbert (1890), are also well preserved along the copper mineralization is associated with the epizonal gra- margin of the Oquirrh Mountains. Further south on the nitic intrusions and adjacent sedimentary country rocks. east side of the range, in the Bingham Canyon area, units The Bingham copper deposit is one of the largest in the of the Oquirrh Group are complexly intruded by late world with pre-mining reserves estimated at almost 3 bil- Eocene granitic stocks, dikes, and sills (Lanier et al., 1978; lion tons of 0.67 per cent copper ore (Babcock et al., 1995). Warnaars, et al., 1978). Possible cogenetic volcanic rocks The large, barren island within the Great Salt Lake to occur on the east side of the range and have yielded late the north is Antelope Island, which has been preserved as Eocene and Oligocene radiometric ages (Moore, 1973; a state park and is known for the largest buffalo herd in SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 233

------= ,.\' ? 5 Newfoundland - --

GREAT SALT LAKE

the State of Utah (Doelling, 1989). The island is 15 miles After passing across Tooele Valley, we will skirt the north long and about 5 112 miles wide and exposes a wide range end of the Stansbury Mountains where excellent shore of rocks including extensive exposures of Paleoproterozoic features of Lake Bonneville are also well-preserved. The and Archean(?) rocks of the Farmington Canyon complex structural geology of this range is controversial. Rigby (Doelling et al., 1990). These Precambrian rocks are in the (1958) mapped several large-scale folds in the range and footwall of the Willard thrust, a regional thrust fault well argued that they were cut by high-angle faults. Tooker and exposed in the Wasatch Mountains to the east (Camilleri Roberts (1971) and Tooker (1983) postulated that imbri- et al., this volume). Lake Bonneville deposits are also cate, east-directed thrusting was the dominant structural well-preserved on the island, and shoreline levels are pattern of the range and related this deformation to the clearly notched into the ridges of .the island (Doelling, Sevier orogeny (Armstrong, 1968). Cashman (1992) found 1989; Doelling et al., 1990). that some of the folding in the range is related to blind 234 BYU GEOLOGY STUDIE S 1997, VOL. 42, PART I faulting in the subsurface, and interpreted the whole range indicating that ductile deformation had ceased by the late as in the hanging wall of the Tintic Valley thrust, an unex- Eocene (Miller et al., 1987). posed regional thrust postulated to exist east of the range. At Silver Zone Pass in the Toano Range, we pass through Deseret Peak (11,031 ft [3,364 m]), the highest peak in the a granitic pluton (chiefly hornblende-biotite granodiorite) range, is underlain by massive, light-colored Cambrian dated as Jurassic (-160 Ma, J. E. Wright, personal comm., Tintic Quartzite (Rigby, 1958). 1997). Glick (1987) most recently mapped this area and West of the Stansbury Mountains, we cross the northern proposed that the metasedimentary wall rocks experi- end of Skull Valley and pass immediately south of the enced Jurassic regional metamorphism and deformation Lakeside Mountains (Young, 1955). Near the northern end prior to emplacement of the pluton. Miller and Camilleri of the Cedar Mountains, we cross low hills composed of (1992) confirmed that the pluton intruded and contact Pennsylvanian-Permian rocks prior to entering the Great metamorphosed previously regionally metamorphosed Salt Lake Desert. Near Knolls, the Newfoundland Moun- Cambrian strata. tains are visible north of 1-80, there Allmendinger and West of the Toano Range, we cross the broad Goshute Jordan (1984) documented an older-over-younger thrust Valley. Interpretations of east-west-striking, migrated seis- (Desert Peak thrust) that is intruded by a Late Jurassic mic reflection profiles across the basin (Effimoff and (-160 Ma) granitic stock. They also mapped low-angle Pinezich, 1981; Anderson et al., 1983) indicated an asym- normal faults, some of which are intruded by Jurassic metric basin bounded on the east flank by a major low- granitic dikes thus indicating a mid-Mesozoic history of angle fault. Strecker et al. (1996) recognized two syntec- contraction, extension, and magmatism. tonic Tertiary seismic sequences-ne recording an early As we approach the Nevada State Line, the Silver Island sag stage and a younger sequence recording an expansion Mountains, a northeast-trending range that begins near of the stratigraphic section across an intrabasin graben. Wendover, Utah-Nevada forms the skyline. The north- Strecker et al. (1996) argued that the early sequence formed eastern end of the Silver Island Mountains is called Crater in response to a planar normal fault, whereas the younger Island and this area was recently studied in detail by sequence formed in response to listric growth faulting. Miller and Allmendinger (1991) who mapped a set of north- After we pass Oasis, we begin a long climb to Pequop and northeast-striking normal faults and an associated Summit. During this climb up the east flank of the northern north-northwest dextral strike-slip fault system which pre- Pequop Mountains, we begin our traverse in upper Eocene date a -160-Ma pluton. These authors argued that these volcanic and sedimentary rocks that rest unconformably relationships, coupled with other examples of coeval Jur- on folded and faulted Devonian through Permian strata assic extension and magmatism, suggest a Jurassic region- (Brooks et al., 1995, their figure 4). The highway then tra- al extensional event in northwest Utah, whereas Jurassic verses through various upper and middle Paleozoic sedi- contraction is characteristic of several localities in north- mentary units. Past Pequop Summit, the Upper Devonian east Nevada. Guilmette Formation forms spectacular gray carbonate cliffs Near Wendover, Utah-Nevada, several large roadcuts as the highway descends down Maverick Canyon. expose Upper Devonian Guilmette Formation (Schaeffer, After crossing Independence Valley, we pass the north- 1960). West of Wendover, tilted 12-Ma rhyolite uncon- ern end of the Wood Hills at Moor Summit where east- formably overlies Permian rocks (Miller and Camilleri, dipping Miocene Humboldt Formation(?) is exposed in low 1992). Beyond Wendover, Pilot Peak (named by J. C. Fre- roadcuts. The Wood Hills and adjacent Pequop Mountains mont; elevation = 10,704 ft [3,265 m]) in the Pilot Range were studied by C. H. Thorman for his Ph. D. dissertation to the north forms a prominent landmark that has guided research project at the University of Washington (1962), many pioneers, including the ill-fated Donner-Reed party supervised by late Professor Peter Misch. Thorman (1970) of 1846. The geology of the Pilot Range and environs has recognized a mappable, metamorphosed lower Paleozoic been studied by Miller et al. (1987), Miller and Hoisch stratigraphy as well as several low-angle faults. This area (1992), and Miller and Lush (1981). The basic structural was subsequently studied by E A. Camilleri for her Ph. D. feature of the range is the Pilot Peak dkcollement, a low- dissertation (1994) at the University of Wyoming, and a angle normal fault that separates a footwall of metamor- recent published paper by Camilleri and Chamberlain phosed Cambrian and Neoproterozoic clastic rocks from a (1997) provides new structural, stratigraphic, metamor- hanging-wall plate consisting of unmetamorphosed mio- phic, and geochronologic data on the area. An important geoclinal rocks ranging in age from Permian to Cambrian. conclusion of Camilleri and Chamberlain (1997) is that A small, -39-Ma pluton (Bettridge Creek Granodiorite) the peak of regional, Barrovian metamorphism was during and associated dikes cut all rocks and structures below the Late Cretaceous time and subsequently the area experi- dkcollement, as well as intrude the dkcollement, thereby enced a complex extensional history. SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 235

After crossing Moor Summit, the highway grades grad- Pull off road to the right into large ually down hill into the town of Wells, originally named parking space. Humboldt Wells in reference to the creeks and springs From this locality there is an excellent that mark the origin of the Humboldt River. This ends the view of the high country of the northern roadlog for DAY ONE; we stay in Wells overnight, and on East Humboldt Range as well as the tree- DAY TWO we examine parts of the East Humboldt Range, covered forerange and low-lying foothills. the high mountain range southwest of the town. The high country is underlain by amphi- bolite-facies metamorphic rocks, and is DAY TWO part of the high-grade footwall of the Summary of route: Wells to Clover Hill to the northern East Humboldt metamorphic core com- East Humboldt Range (Angel Lake area) to the southeast- plex (McGrew, 1992).The forerange, how- ern East Humboldt Range to Ruby Valley to Elko (Fig. 5). ever, is underlain by unmetamorphosed upper Paleozoic sedimentary rocks, and the low foothills expose Tertiary sedi- mentary and volcanic rocks (Fig. 7). These The roadlog starts at the Super 8 Motel, Paleozoic and Tertiary rocks constitute Wells, Nevada. Turn left onto Sixth Street part of the hanging-wall sequence of the and proceed west. East Humboldt Range core complex. Turn left onto Humboldt Avenue. The East Humboldt plastic-to-brittle Intersection of Humboldt Avenue and shear zoneldetachment fault system orig- Angel Lake Highway. Turn right and head inally separated these disparate crustal toward Angel Lake. levels before being offset by late, high- Large roadcut (on the right) of conglom- angle normal faults (Mueller and Snoke, 1993a). erates of the lower part of the Tertiary A principal feature of the metamorphic Humboldt Formation, informally called terrane is the basement-cored, south- here the "lower Humboldt Formation" closing, Winchell Lake fold-nappe (Lush (Thl) (Fig. 6). From this vantage point et al., 1988; McGrew, 1992). Chimney you can begin to see ahead the north- Rock, a prominent isolated peak (at about western corner of Clover Hill dome, azimuth 235O), is composed of orthog- mantled by Tertiary rocks which are in neiss and paragneiss that forms the core low-angle, normal fault contact onto sub- of this large-scale recumbent fold. The jacent metasedimentary rocks. Metamor- tree-covered forerange consists chiefly phic rocks form the dip-slope surface of of Pennsylvanian and Permian carbonate Clover Hill and are variably mylonitic rocks (Ely Limestone, Pequop Formation, Ordovician and Cambrian calcite marbles Murdock Mountain Formation), but also that tectonically overlie mylonitic Cam- includes Pennsylvanian and Mississip- brian and Neoproterozoic quartzite and pian Diamond Peak Formation. A promi- schist (Snoke, 1992). nent high-angle normal fault separates To the right are massive exposures of the Paleozoic rocks from the Tertiary Miocene (about 13.8 Ma) quartz + feld- strata. Reddish-brown-weathering sili- spar-phyric rhyolite porphyry. This litho- ceous breccia (so-called jasperoid) crops logic unit forms a laccolithic mass that out locally along the contact (e.g., at intruded along the contact between the about 240"). Prominent outcroppings in "Th12" unit of the lower Humboldt For- the low-lying Tertiary terrane are lens- mation and overlying rhyolite lava flows like masses of megabreccia derived from dated at about 14.8 Ma (Figs. 6 and 7). middle Paleozoic carbonate rocks; we STOP 2-1-Overview of the northern examine some of these deposits in detail East Humboldt Range and a well-ex- at STOP 2-2. Finally, rhyolite porphyry posed, low-angle normal fault contact in forms an isolated body along the range roadcut. The following description is front at about azimuth 205O, and a modified from STOP 7 of Mueller and prominent mass of rhyolite is at about Snoke (199313). 300". 236 BYU GEOLOGY STUDIES 1997, VOL,. 42, PART I

Figure. 5. Mrrp of the R14hy Morrntains ant1 East Hurrtholrit Range, Neoada, and szrrrot~nc-lingareas sumn9narizing the locations of the field tr

A 237

Flat-lying, postdetachment boulder conglomerate Including cobbles ,,,,,,,,.,,,,,. of Willow Creek rhyolite complex and scarce vesicular basalt. upper Miocene(?) " or Pliocene(?)

Ash-fall tuff and reworked sandstone and siltstone, pebbly conglomerate, platy lacustrine siltstone and minor algal limestone. Lower part of section records initial unroofing of non-mylonitic metamorphic complex.

Extrusive rhyolitic lava flows (14.8fl.5 Ma., K-Ar-sanidine) and scarce silicified siltsone.

Chiefly Intrusive quartz-feldspar rhyolite porphyry (13.8k0.5 Ma., K-Ar-sanidine).

Volcaniclastic siltstone, sandstone and conglomerate, scarce limestone (commonly fossiliferous).

Conglomerate and sandstone (red to blue-grey) with locat limestone intercalations.

Conglomerate and sedimentary breccia with lenses of megabreccia; local limestone intercalations.

Figure 6. Generalizecl strrttigr-rrphic column of Tertiary rocks irz the nnriheasterr~E~ast NtcirrJ?ol~lt Rarzge (rrlo(liJiedfi.on~ Afueller awl Smoke, 19931,).

A low-angle norrnal fatilt exposed in fault-hounded slice of metadolomite (Si- the west roadcut immediately north of luriari and/or Ilevonian protolith) is pres- the pull-out separates stccply dipping ent along this tectonic boundary (Fig. 7). Mioceile fimglomerate of the hanging Rctrirn to vehicles and continue west- wall from a footwall of platy marble rep- ward along the Artgel Lake Highway. resenting Ordovician and Canlbrian strata 1.8 8.1 Roadcut exposure of lacustrine lime- intrudcd by peg~natiticleticogranite. Red stone with interl~ecldedconglomerate on gouge separates thcse twi) units, and the right (unit Thll of Fig. 6). Trlrn left into top of the tnarble is brecciated. As the Rat area aihrossborn the \vest end of the fault is tvaced ~rorthwardalong strike, a exposure.

SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA

GEOLOG lC MAP OF THE NORTHEASTERN EAST HUMBOLDT RANGE, NEVADA Lithologic units El Surficial deposits (Quaternary) El Boulder conglomerate (Pliocene or late Miocene) El Upper Humboldt Formation ( Miocene) ElRhyolite (-13.4 Ma ) @El Volcan iclastic breccia ( Miocene) IEl Rhyolite quartz porphyry (43.8 ~a)Yy=dikes EEl Rhyolite (44.8 Ma) late Eocene ? El Lower Humboldt Fm.--Thl2 BThll ( Oligocene ) El Volcaniclastic conglomerate, sandstone, andesite - rhyolite ( late Eocene) tEiX Permian undivided (Murdock Mtn.8 Pequop formations) ESI Ely Limestone 8 Diamond Peak Formation (Carboniferous) El Guilmette Formation (Devonian) ElMetadolomite ( Devonian ,,,,etasedimentary 8 Si.lurian) rocks undivided Impure calcite marble 81 (Devonian to calc-silicate rock (Ordo- Cambrian ) vician 8 Cambrian) Impure quartzite 8 schist1 (Cambrian 8 Proterozoic Z) W Orthogneiss & paragneiss (Archean) 8 quartzite. schist, 8 amphi bolite (Proterozoic ?) Symbols

J Contact 2 Low-angle fault, Pre-folding low- brittle /d angle fault; over- &.-.- Normal fault, high-angle; turned on right dotted where covered DAY 2 / Low-angle fault, Field trip stop plastic-to-brittle

Geology by A. W Snoke, A. J. McGrew, and A. P Lush. BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

STOP 2-2-Megabreccia deposits and 0.4 10.8 View of Chimney Rock straight ahead. surrounding Tertiary strata. Please watch 0.6 11.4 View (to the south-southwest) of rugged out for rattlesnakes, especially around exposures of hornblende-biotite quartz the megabreccia outcrop. The following dioritic orthogneiss, dated at -40 Ma by description is modified from STOP 8 of the U-Pb zircon technique (Wright and Mueller and Snoke (1993b). Snoke, 1993, sample RM-19). The roadcut immediately across from 0.1 11.5 Trailhead to Winchell Lake on left. the parking place and those to the west 1.1 12.6 Sharp curve with excellent view of the provide a useful starting place to exam- southwestern flank of Clover Hill and ine the megabreccia deposits and encas- adjacent expanse of Tertiary rocks. ing beds that are all within the lower Humboldt Formation, (Thl) (Fig. 6). After 0.8 13.4 STOP 2-3-Angel Lake. briefly examining the roadcut exposing This stop affords an opportunity to lacustrine limestone and conglomerate, observe the style of metamorphism and walk west up the road (stratigraphically intrusive relationships as well as defor- downward) toward the dark gray road- mational character of the high-grade, cuts of megabreccia derived from Upper migmatitic infrastructure of the East Devonian Guilmette Formation. This Humboldt metamorphic core complex. megabreccia is one of a group of lens- Angel Lake cirque exposes both limbs of like masses within the Tertiary lower the Winchell Lake fold-nappe; although Humboldt Formation and it both over- the fold structure is not well displayed lies and underlies lacustrine limestone in this area because the axial trace of and conglomerate. Therefore, this this large-scale fold trends through the megabreccia lens, as well as the others, middle of a massive Archean orthogneiss are encased in Tertiary deposits. After sequence. However, the Greys Peak fold, examining these roadcut exposures, walk a map-scale parasitic fold on the upper north along the megabreccia lens exam- limb of the Winchell Lake fold-nappe, is ining the brecciated internal structure of visible on the east cliff face directly be- the deposit. Despite pervasive breccia- neath Greys Peak (Fig. 8). Rocks in the tion, the megabreccia deposits locally upper part of the cirque are overprinted display vestiges of relict bedding. If time by mylonitic fabrics related to Tertiary is available, walk northeastward to extensional deformation. These mylonites another megabreccia lens composed of gradually give way to coarse-grained metadolomite and quartzite derived from gneisses with increasing structural depth. Devonian and/or Silurian protoliths. Our traverse will cross the lower limb of Similar metadolomite and quartzite are the Winchell Lake fold-nappe to the base exposed in place on Clover Hill to the of the Archean orthogneiss sequence, east. and will be located entirely within the Return to vehicles and continue west- infrastructure beneath the mylonitic ward up the Angel Lake Highway. shear zone. This will entail a steep climb 0.9 9.0 Continue straight on the Angel Lake with an elevation gain >600 ft over a Highway, past road to Angel Creek carnp- distance of less than 0.5 mi (0.8 km). ground (on left). In order from bottom to top, the char- 1.3 10.3 Exposure of Pleistocene glacial deposits acteristic rock sequence on the lower on right. limb of the fold-nappe is: (1) a quartzite 0.1 10.4 On right, the beginning of numerous ex- and schist sequence of probable Early posures of Permian sedimentary rocks Cambrian to Neoproterozoic age, (2) a (chiefly Pequop Formation and Park City thin sequence of calcite marble and calc- Group). These unmetamorphosed upper silicate paragneiss that is probably Cam- Paleozoic rocks form a high-angle, fault- brian and Ordovician in age, (3) a dis- bounded belt between the high-grade continuous, thin orthoquartzite layer metamorphic rocks of the high country (<2 m thick) here correlated with the and Tertiary rocks of the foothills. Ordovician Eureka Quartzite, (4) dolo- SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 241 mitic marble presumed to correlate with creasingly prominent in the well-devel- Ordovician to Devonian dolomite, (5) cal- oped mylonitic rocks at higher structural cite marbles which locally include isolated levels. Thermochronologic constraints enclaves of intensely migrnatized, rusty- imply that these assemblages equili- weathering graphitic schist (upper Paleo- brated between Late Cretaceous and zoic?), (6) severely migrnatized quartzitic Oligocene time (McGrew and Snee, and quartzo-feldspathic paragneiss in- 1994). 40Ar/39Ar muscovite and biotite ferred to be Paleoproterozoic, and (7) a cooling ages indicate that the range was thick sequence of Archean monzogranite mostly exhumed by approximately 21.5 orthogneiss (Lush et al., 1988). The con- Ma. tact between the Neoproterozoic-Paleo- Stable isotope studies in the northern zoic miogeoclinal sequence (1-5) and East Humboldt Range (e.g., Peters and Paleoproterozoic(?) paragneiss (unit 6) is Wickham, 1995) concentrated on Angel inferred to be a pre-folding, premeta- Lake cirque and Lizzies Basin to the morphic fault. The nature of the contact south. Detailed outcrop and cirque-scale between units (6) and (7) is probably an mapping and sampling constrain the iso- inverted unconformity. topic evolution of this core complex over Small-scale folds occur throughout the a wide range of structural levels. The transect. Fold hinge lines and stretching degree of depletion and lBO/160 lineations show an average orientation of homogenization in the metamorphic rocks approximately 295O, 5'; and the axial correlate strongly with the proportion of surfaces of small-scale folds are subpar- intrusive rocks. The stable-isotope sys- allel to foliation, with an average strike tematics at deep structural levels are and dip of 270°, 15". Three-dimensional probably the ultimate product of fluid- constraints from mapping indicate that rock interaction associated with exten- the Greys Peak fold and Winchell Lake sion-related magmatism, dominated by fold-nappe show the same west-north- leucogranites and monzogranites (Peters west trend as the smaller scale structures. and Wickham, 1995). However, there is considerable disper- Locality A. Follow the well-developed sion in fabric orientations at deep struc- path around the south side of the lake to tural levels, and an approximately north- the low bedrock hill labeled "A" on the trending lineation of uncertain age occurs geologic map (Fig. 8). This locality pro- locally. vides an opportunity to inspect the in- An outcrop of schist near the base of ferred Cambrian-Neoproterozoic quart- the cirque on the south side yielded an zite and schist sequence exposed at the internally consistent P-T estimate of 4.5 deepest structural levels on the lower kb, 657"C, but this probably reflects rel- limb of the fold-nappe. atively late-stage equilibration because The East Humboldt Range is perme- localities in the upper part of the cirque ated with intrusive rocks of various ages yield P-T estimates as high as 8.7 kb, and compositions. In particular, note the 790°C that probably record an earlier abundance of leucogranite at this deep stage in the metamorphic history (see structural level. Here and throughout section by McGrew and Peters, this field the traverse, how many distinct genera- guide). The most characteristic pelitic tions of leucogranitic rocks can you dis- mineral assemblage at all structural levels tinguish? We believe we can distinguish in the northern East Humboldt Range is at least four generations of leucogranitic biotite + sillimanite + garnet + quartz intrusion based on cross-cutting relation- + plagioclase f K-feldspar f chlorite f ships. In addition to the leucogranites, a muscovite f rutile + ilmenite. However, sheet of biotite monzogranitic orthogneiss scarce relict subassemblages of kyanite occurs near this locality. These small mon- + staurolite survive on the upper limb zogranitic bodies are abundant through- of the Winchell Lake fold-nappe. Late- out the Ruby Mountains and East Hum- stage muscovite and chlorite become in- boldt Range and have yielded U-Pb zircon 242 BYU GEOL,OGY STUI>XES 1997, VOI,. 42, PART I

Figurr. 8. Geologic t~zapc?f'ArtgeE Lake cirque u?rd surrotmtling crreas, zuith field -trip trunsect (STOP 2-3) 1al)elr~d(Sites A-F). SNOKE, ET AI,.: RUBY-EAST liiUMl3OLDT CORE COEtlPLES, NEVADA

Legend Cenozoic and Mesozoic units Paleozoic and Precambrian units Marble, undifferentiated Colluvium (Quaternary) (Devonian to Cambrian) Rusty-weathering graphitic schist Talus (Quaternary) (Mississippian?) Dolomitic marble, undifferentiated Glacial deposits (Quaternary) (Devonian to Ordovician) 1-1 1-1 Basalt dikes (14-17 Ma) Eureka quartzite (Ordovician) quartzite and schist, undifferentiated Biotite monzogranite (-29-Ma) (Cambrian and Neoproterozoic) Hornblende biotite quartz diorite Paragneiss sequence of Angel Lake (-43-Ma) (Paleoproterozoic) Leucogranite (Cretaceous and Orthogneiss sequence of Angel Lake Tertiary) (-2.5 Ga, Archean ) Ma.Symbols

Strike and dip of grains-shape and/or compositional foliation with trend and plunge of grain-shape lineation. 91Strike and dip of C'-planes with trend and plunge of slickenlineation, strike and dip of axial surface of small-scale fold with trend and plunge %30 of hinge line and vergence indicated where known.

/C Surface trace of hinge surface of map-scale recumbent fold (Winchell Lake fold-nappe and Grey's Peak fold) \#'

Outcrop trace of premetamorphic tectonic contact, dotted where covered, broken where inferred.

Geology by A. J. AkGrecfi. BYU GEOLOGY STUDIES 1997, VOL. 42, PART I ages of -29 Ma in a variety of locations, quence. Can you find any amphibolite including one at the northwest comer of boudins near here? Normally, amphibo- Angel Lake (see Locality F) (Wright and lite bodies are restricted to the inferred Snoke, 1993, sample RM-5). Consequently, Paleoproterozoic and Archean gneiss se- these monzogranitic sheets offer a useful quences, providing a useful guide for gauge for deciphering the structural chro- correlation. In most cases, they probably nology of the range. The biotite monzo- represent metamorphosed mafic intru- granitic rocks both cut and are cut by sions that were emplaced during Paleo- various generations of leucogranites, and proterozoic rifting along the southern some leucogranitic rocks appear to inter- margin of the Archean Wyoming province. mingle with the monzogranites. At higher However, here one or two amphibolite structural levels the monzogranites are bodies occur in the inferred Cambrian clearly overprinted by mylonitic fabrics, and Neoproterozoic quartzite and schist providing a crucial constraint on the age sequence, raising the problem of how of mylonitic deformation. The final phase these rocks should be correlated. The in the intrusive history of the East Hum- key to correlation is provided by anom- boldt Range is represented by a series of alously high 613C values in the marble steeply dipping, amygdaloidal basalt dikes units. As discussed in Peters et al. (1992) which form three, deeply eroded fissures and Wickham and Peters (1993), the slicing steeply through the south wall of high 613C values probably represent pro- the cirque. The end of one of these amyg- tolith values and allow the assignment of daloidal basalt dikes can be observed a Neoproterozoic age to these rocks. Thus, near this locality. Similar dikes yield ages carbon isotope analyses in this region of 14 to 17 Ma elsewhere in the Ruby provide a strong tool for stratigraphic Mountains and East Humboldt Range, correlation. providing a younger limit on the age of This locality also exposes the second plastic deformation (Snoke, 1980; Hudec, of the three outcrops near the lake that 1990). were the subject of detailed mapping, Locality A exposes one of three out- sampling, and stable isotopic analyses. crops near the lake which were the sub- This particular outcrop involves a mar- ject of detailed mapping, sampling, and ble-calc-silicate gneiss-paragneiss+rtho- stable isotope analyses (Peters and Wick- gneiss sequence discussed in detail by ham, 1995). We will contrast this locality Peters and Wickham (1995). Figure 9 pre- with one of the other two outcrops ex- sents a schematic summary of the iso- posed at Locality B. At Locality A the topic profile for this outcrop, revealing a sample transect crossed a monzogran- higher degree of heterogeneity of 18O/ ite-orthogneiss-leucogranite-metaquart 160 on the scale of meters and a smaller zite sequence. Figure 9 presents a sche- degree of isotope depletion as compared matic summary of the isotopic results for with the outcrop visited at Locality A these outcrops. The rocks at Locality A (see discussion in McGrew and Peters, were characterized by significant 180/160 this field guide). We attribute these dif- homogenization on the scale of meters ferences to the lower abundance of and substantial I80 depletion of meta- intrusive rocks and the higher propor- quartzite relative to likely protolith val- tion of marble in this sequence. The ues (Peters and Wickham, 1995; and dis- marble acted as a flow barrier and a rela- cussion by McGrew and Peters, this tively high reservoir. field guide). Locality C. Contour along the base of Locality B. Continue over and around the slope for a short distance, and then the small hill on the southwest side of proceed northward up the hill to point the lake and cross the stream to point "C." As you walk, notice the interesting "B" on the map. The rocks near the base small-scale fold and fault relationships in of the slope form a particularly diverse the quartzite and schist sequence, in- and structurally complex paragneiss se- cluding a rare minor thrust fault. Most SNOKE, 1';T Al,.: RUIJY-EAST FIIJMBOIJDT CORE CORLP1,EX. ?JE;\irZL?A

o quartz (int) Angel Lake Upper Sequence 1500 p * 25% leucogranit@/ meter-scale profile shapes t .. 10%monzogranite Angel Lake Lower Sequence 40% Ieucogmnite/ + 10% monzogranite 15% monzogranite - 8 9 10 It 12 13 14

Figurc~9. Su?nnlu~ydicgrurn (I~O*.!/~BPEisotup~~ datc~jn)~n th~ r~nrih~ni Eu,rt lft~m?~okfltHung?> pbttcd nr*cufdirtgto rr,latii-i>S~~ICTII(-CI~~C~~ 11~rl(cIatizfr(~irz 1Vit;l.l~atrz (azrl Pctcr.~,1,990, 19,92, WIickhorn et 01, 1,991, cinrl P~fijrsand b%'~elzhan~.1995) 8180 r.cllrrm arc cltmrtz VJKI- rntrs in trtetnttiorphic @llc~cEcircles) and irriri~5sze ((>pendrcles) rorls, and qucu~-tzrte(open rqrca1t.s) tuhole-rock samples itchidl c lowly npproxirnclte tl~eqtdudz @'O oalilcsj. Alsu shot~lzrA tlzr prq~ortionof lemcogratzife and biotite i~zor~zogrrmit~ir~fitrlr diJf~rt,nt zonc~ in Angel Lctkt, cirque (which is risift~clill8 thc&t&l trip STOP 3-3)and Iizzies Bcsitr, !he sftr4cftdmlly J~?v(B~L.s~part ~f the Ewt fil~)iboI(lt Hung?, locciteti approlarrtufelfj9 km to the sotitlt (fAngc.1 Lnk Arzg(.l Lrrke ctrilup zs dz~zd~u!nnto a bw~rsiJ(!twnCC t~l~t~r~rlrltnalerl trti- verses tcew coniple*fcd,and on rippel rcqumnce (>8400f~etel~c~ution) tt\hure lrrrge-sccrkt, sc~tnplington? done 6180 ta111esfor qu(~rtz scr7npks collected leas t!lczri 1 nt azi)oyfr orrt rriurble lrryr~ure intlrcciter! by the $ekk lak~eted"ntrnrble hounrior;/ laljrr cluurtr. '2130 showit are the typical shapes of ~tinrl~lc-silicutt.1avid sibcafp-stlieate profiles at z)ariow strz~cfurnlbrjels isil-si1ic;czte. nu~rl7-rtuzrhle rrs-rrzetcrsedirne~~taryrock, crnd gr-gt-(tnifoirO

chedr surf:xces exprrsed in the East Flutn- tion is corrthct,here it is possblr to walk boIdt Range En contrast show normal- thrvrtgh the ei~tlrescct~on In a vert~cal sense di5placenrent reltited to Tertiary tlistance of just 25 m! O~rerlyingthe extetlrional tectonisrr~.Where prcwnt, ortlxoquartzite ic a few rt~ctersof dolo- Iipproxirrmtcl?;70% of the ktncmat~cin- rnitic miirhle proltably cquivalcnt to dicators ohscrved at deep structural lev- Upper Orclov~ciianto Ilevonia~lclolornites el$ are directed top tonard t11c cast- expuscd in rleal-t)y ratlges. Marldc and soutlreast, antithetie to the 13tllk \hear calc-silicbate nssen.lblages typically con- sense in the overlying nlylonitic zone sist af calcitcb t diopside t cjtlartz i (MeGrew; 1992).This s~tggestsa deep- dolorritte + plrlogopite + p1:~gioclasei crustal \etting in which extellsional de- grossul~t scCipohte t Ei-feldspar t formation was diffitscly partitioned into sphenu F anlphihole +- epiclote. Peters weakly developed, mastornosing antithet- and Wickhan~(1994) report that anlptti- ic hear systerns, lrole t grossular -I-epidote fonn a sec- After climbing acbross the 1mzrer mar- ondary su~~~aseerrtl~lagt~that records infil- ble secluctrcc, pause at an approxirnzitely tration of water-rich fluids r~ndcra meta- 1-rn thick layer of white c~t~artzitethat morphic rtaglme that pn~reededfrom probably correlates with the Ordocician high temperature ((iOO°C-'i=iO°Cj to lo^ er Elireka Qaart~ite.In nearby ~nountatn ternperature (<52Fi°C) conditions. This rangrs, the Upper Cambria~xto hliddle event wat probably relatrd to Tcrtia~?; Clrdovician limestone sequence is at least extension dnd associated magmatisrn. 1 lcrn thick. Assuming that tbi\ correla- The ewlier; ~rrirniwyassrrrtl~lages prol),~- BYU GEOLOGY STUDIES 1997, VOL. 42, PART I bly equilibrated at 26 kb, 550°C-750°C wish to inspect the shapes of deformed and likely record conditions during Late feldspars in the paragneiss immediately Cretaceous or mid-Tertiary time. above the contact. Could the rocks near Locality D. Proceed westward, gaining this contact be annealed mylonites? Look- elevation gradually to point D. At this ing toward the south, notice the thin, point, we can see a thin raft of distinc- white ribs of aplitic leucogranite cutting tive, rusty-weathering graphitic para- at high angle through the gneisses. gneiss seemingly suspended in a mass of Though clearly quite late, these rocks pegmatitic leucogranite. On the upper contain a weak grain-shape foliation sug- limb of the fold-nappe, this same rock gesting that they intruded during the type forms a continuous layer approxi- last gasps of plastic deformation. mately 25 m thick that in general is only Continue across the Paleoprotero- slightly migmatitic. Remarkably, the tran- zoic(?) paragneiss sequence to the expo- sition between these two contrasting out- sures of Archean orthogneiss at Locality crop styles occurs over a distance of <2 E. These rocks have a biotite monzo- km as this unit is traced from the upper granitic composition and are generally limb of the fold-nappe (where it typically gray in color with a distinctive banded contains <25% leucogranite) into the appearance due to segregation of biotite. hinge zone of the fold-nappe above Win- In addition, they commonly contain large chell Lake (where it contains >60% leu- augen of alkali feldspar. U-Pb zircon dat- cogranite). These meter-scale leucogra- ing qf a sample collected on the north nitic bodies are clearly folded around side of the cirque yielded a minimum the nose of the fold-nappe, but the fact age of 2520 f 110 Ma (Lush et a]., 1988, that isopleths of leucogranite concentra- sample RM-9). tion cut the fold-nappe implies that nappe Locality E Retrace your steps to the emplacement and leucogranite segrega- ledge above Locality B, and then con- tion must have occurred at the same time tour around to the point labeled "F" at the (McGrew, 1992). The variations in leuco- northwest corner of Angel Lake. As you granite abundance are also borne out in walk, try to decipher the relative age cirque-scale estimates summarized in relationships between leucogranitic intru- Peters and Wickham (1995). In addition, sions. Some leucogranites are fully in- relict kyanite also disappears over this volved in folding, whereas others cut same interval, being completely replaced folds. Outcrops at Locality F consist pre- by sillimanite on the lower limb of the dominantly of biotite monzogranitic and fold-nappe. Consequently, a preliminary leucogranitic orthogneiss. The biotite 70-90 Ma U-Pb zircon age (J. E. Wright, monzogranite at this locality has yielded unpublished data) on these leucogranitic an U-Pb zircon age of 29 + 0.5 Ma rocks probably dates both fold-nappe em- (Wright and Snoke, 1993, sample RM-5). placement and a major phase of migma- Is this sheet of monzogranite folded? tization and sillimanite zone metamor- Walk around the corner of the outcrop phism. before you decide. Some monzogranitic Locality E. Climb upward and then sheets are clearly involved in folding, proceed westward toward point E. This but others cut folds, and at map scale a part of the transect will take us across number of monzogranitic bodies cut the additional marbles overlying the gra- Winchell Lake fold-nappe itself, lending phitic schist unit and into and through credence to the interpretation that the the overlying Paleoproterozoic(?) para- Winchell Lake fold-nappe is Late Cre- gneiss sequence. Pause momentarily at taceous in age. However, the monzo- the contact between the marble and the granitic orthogneisses bear the same overlying paragneiss. By inference, this west-northwest-trending stretching lin- contact is a premetamorphic, prefolding eations as the country rock, and at high- thrust(?) fault of unconstrained, but er structural levels they are overprinted probably large displacement. You may by mylonitic microstructures, thus docu- SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 247 menting a Tertiary age for extensional ------* z -- deformation. It seems highly likely that older structures were profoundly trans- - ? .- ,<: ry ,%:>;? :-*, posed during Tertiary deformation, includ- *"-- ing the Winchell Lake fold-nappe itself. .I Return to vehicles and retrace the route .. to the interesection of the Angel Lake u -4.' Highway and Humboldt Avenue in Wells (i.e., -11.7 miles). Turn left onto Hum- , 'i ? J boldt Avenue and drive north toward Wells. Turn right onto Sixth Street. Super 8 Motel on right. Intersection with U. S. Highway 93 (Great Basin Highway), turn right (south) onto u. S. 93. Wood Hills on the left and Clover Hill at about 2 o'clock on the right. See com- ments about the Wood Hills in the road- log for DAY ONE. Clover Hill exposes a structurally complex, metamorphosed stratigraphy of Paleozoic and Proterozoic strata intruded by various granitic rocks (Snoke,1992). A low-angle fault system separates the metamorphic and igneous rocks from weakly to unmetamorphosed middle and upper Paleozoic rocks. The mine on the Figure 10. Hinge zone of the Winchell Lake fold-nappe as east face of Clover Hill is chiefly a tung- exposed along the back wall of Winchell Lake cirque on the east- sten prospect (Lipten, 1984). ern face of the northern East Humboldt Range. The core of the signal ill, a small conical hill compose^ fold at this locality consists of dark outcrops of rusty-weathering, of Upper Devonian Guilmette Forma- graphitic paragneiss surrounded by white-weathering Cambrian and Ordovician marble and Ordovician metaquartzite. Envelop- tion, on the right at about 2 o'clock. ing the fold is a thick sequence of Neoproterozoic and Lower Cam- The range at l1 is Spruce Moun- brian jlaggy quartzite and schist. These metasedimentaq rocks, tain7 studied In this particularly the metaclastic units, all contain abundant sheet-like range, the low-angle Spruce Spring fault bodies of leucogranite orthogneiss. The hinge line of this major separates a footwall of metamorphosed structure trends west-northwest, approximately parallel to mineral Ordovician through Upper Devonian elongation lineations in the deformed rocks. Closure is to the south. strata (chiefly low-grade rnetacarbonate TO the north, the fold is cored by Archean and Paleoproterozoic rocks) from a faulted hanging wall gneisses, and a pre,folding low-angle fault is inferred to separate consisting of Mississippian through Per- the metasedimentary rocks shown in this photograph from the basement complex enclosed in the core of the fold. CZqs = mian sedimentary rocks. The metamor- Cambrian and Neoproterozoic quartzite ancl schist, rgs = rusty- phosed stratigraphy of the footwall at weathering graphitic schist, DCmq = Cambrian to Deoonian meta- Spruce stratigraphic carbonate rocks and quartzite. DCmu = Cambrian to Dezjonian units equivalent to the higher grade metacarbonaterocks metasedimentary rocks exposed in the Photograph by A. W Snoke, sketch by Phyllis A. Ranz. southeastern East Humboldt Range (e.g., STOP 2-4). Road to Clover Valley on right. nappe is visible at about 280". See Fig- Railroad crossing. ure 10 and the accompanying caption for Road to Tobar on left. From this locality, more explanation. an excellent view of the east face of the 2.7 37.4 The northern end of the Cherry Creek East Humboldt Range is afforded in Range on the horizon to the left of the morning light. The Winchell Lake fold- road. Pequop Mountains to the east (left). 248 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

Generalized longitudinal cross-section of the East Humboldt Range, Nevada

@@7g B METERS FEET 4Wo Mylonitic sheor zone 1*WO IO.WO 3000 14000 -0 WOO 6000 IWO UMO 2WO SEh LEVEL SEA LEVEL

543210- - - 5 MILES 543210 5 10 KILOMETERS C - - -

Lithologic unlts

Volcanic rock8 (bte Eocene) 10. Ordovician Eureka metoquortzite

HMnblende-biotite quortl dnoritic uthogneims (late Eocene1 Ordoviclon 8 Cambrian impure marble and CaIC-lil~coteDaragneisa

Lausocrotic pegmotitic granite (Late Cretaceous) Combrion 8 Naoproteroloic impure quar~ziteand 8chi.l

Trlomenc lo Penn~rlvonionredimestar). rocks Ne~pr~ler~z~icimpure quartzite and Ishiat

hvonian to Ordovician mctosedimentory rocks PoIe0~r0ler020i~(~)paragn~lms

Oevonlon to Combrion rnetoscdimentary rocks Archean orthogneimm (-2.5 Go)

Figure 11. Generalized north-south cross-section ofthe East Humboldt Range, Nevada. Derivedfrom geologic studies by A. J. McGrew (in the north) and A. U! Snoke (in the south).

Road to Clover Valley on right. lowing description is modijied from STOP Note the overall southward dipping foli- 19 of Snoke and Howard (1984). ation in the southern East Humboldt Metamorphosed Guilmette Formation Range. This prominent foliation is prin- here is part of a non-migmatitic meta- cipally defined by the Tertiary mylonitic sedimentary sequence that ranges from shear zone which forms a structural cara- Ordovician (Pogonip Group) to Missis- pace above a migmatitic infrastructure sissippian (upper part of Pilot Shale) and (Fig. 11).Road to ranch on right. is structurally above the main mylonitic Road to ranch on right. shear zone but in turn is structurally Entrance to the Warm Creek Ranch on overlain by allochthonous upper Paleo- left. zoic, lower Mesozoic, and Tertiary rocks Spruce Mountain ridge at 10 o'clock. (Fig. 12). The rock is a dark to light-gray Turn right onto Nevada Highway 229 to white, calcite + dolomite marble; toward Ruby Valley. color variation is apparently chiefly re- At 1 o'clock on the skyline is pyramidal lated to the amount of graphite in the Snow Lake Peak in the northern Ruby marble. Abundant evidence of ductile Mountains. flowage is evident in many exposures of Turn right onto dirt road semicircle and the marble including a penetrative folia- take the far right turn out of the semicir- tion, small-scale isoclinal folds, and de- cle to proceed northeast to STOP 2-4. formed fossils such as brachiopods, gas- Turn left onto the road that passes under tropods, and stromatoporoids. As you walk the electric lines. up the hill to the south, you will note that the foliation becomes very steep and Park at outcrop of gray metalimestone eventually you cross a contact with topo- adjacent to hills. graphically overlying metadolomite. You STOP 2-4. Fossiliferous, metamorphosed have actually walked down section into Guilmette Formation (Fig. 12). The fol- Devonian metadolomite (Simonson Dolo- SNOKE, ET AL.: RUBY-EAST HUM [BOLDT CORE COMPLEX, NEVADA 249 mite, part of DOd on map), and we are, More exposures of Miocene Humboldt therefore, on the steep limb of an asym- Formation on left. metric fold. Finally, float fragments of Cross the trace of the approximately hypabyssal intermediate igneous rocks north-south-striking Poison Canyon nor- are fairly common and are perhaps intru- mal fault. sive equivalents to a widespread upper Large roadcut of chiefly mylonitic mig- Eocene "basaltic andesite unit." We will matitic impure quartzite and pelitic schist. drive pass poor exposures of this vol- This locality is the beginning of STOP 3- canic unit along Nevada 229 as we head 1. See log of DAY THREE for details. west toward Ruby Valley (see note on this Roadcut on the right exposes a low- locality at cumulative mileage of 59.4). angle normal fault (part of an extensional Turn around retrace route back to duplex-see STOP 3-1) that separates Nevada 229. dipping Miocene Humboldt Formation Intersection with Nevada 229, turn right from a footwall slice of "broken forma- onto blacktop road proceed westward tion" derived from the Chainman-Dia- toward Ruby Valley. mond Peak formations. Poor exposures of upper Eocene basaltic Roadcut exposure of westward-dipping andesite, flow-brecciated lava (see Brooks normal fault that has Chainman and et al., 1995). Diamond Peak formations in the foot- Road to North Ruby Valley on right. Stay wall, and Miocene Humboldt Formation straight. in the hanging wall. At 10 o'clock, the low area in the Ruby Secret Peak (9,167 ft [2,796 m]) on the Mountains is Harrison Pass underlain by left. Roadcuts expose reddish to laven- granitic rocks of the Harrson Pass plu- der fluvial deposits of the Miocene Hum- ton. We will visit this area on the morn- boldt Formation. Note that these sedi- ing of DAY FOUR. mentary rocks are tilted toward the range. Turn right onto Ruby Valley road and Holocene fault exposed in creek bank proceed northward. Elko is aproximately (Secret Creek) on the left. Thls fault which 57 miles from our present location. cuts Quaternary alluvium was originally Humboldt Peak (11,020 ft [3,361 m]) is recognized and described by Sharp on the skyline to the north. Humboldt (1939b, figure 7) in his classic paper on Peak consists chiefly of mylonitic impure Basin-Range structure of the Ruby Moun- quartzite and pelitic schist correlative tains-East Humboldt Range. with the Neoproterozoic McCoy Group (of Misch and Hazzard [1962]) and per- Quaternary stream gravels on right. haps the Cambrian Prospect Mountain More Quaternary stream gravels on Quartzite. Intercalated with these upper right. amphibolite-facies metasedimentary rocks Road to the village of Lamoille on left. are variably deformed granitic rocks including both probable Mesozoic as This ends the road log for DAY TWO. For a continuation well as Tertiary intrusive rocks. of the log to Elko, we refer you to the road log for DAY North Ruby Valley road on right. THREE (specifically starting at mileage point 29.2 and Secret Pass (6,457 ft [1,969 m]). following the route in reverse order to Elko). At the turn- Secret Valley on the right. Secret Valley off to Lamoille, we thus are 29.2 miles from Elko. We will is a structurally controlled feature, bound- spend the next two nights in Elko. ed on the west by an east-dipping nor- mal fault (Poison Canyon fault, also see DAY THREE below at cumulative mileage point 84.5). Summary of route: Elko to Secret Creek gorge (northern- Mylonitic impure quartzite and pelitic most Ruby Mountains) to Lamoille Canyon to Elko (Fig. 5). schist with intercalated granitic rocks Incre. Cum. form the foothills of the southwestern -- Miles Miles flank of the East Humboldt Range. -- Exposure of Miocene Humboldt Forma- 0.0 0.0 Start at Red Lion Inn & Casino, Elko, tion on the left. Nevada. 250 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I 0.1 0.1 Turn left onto northwest trending road dated the thrust emplacement of the that leads to 1-80 entrances. western assemblage rocks. 0.1 0.2 Turn right into the entrance to 1-80 E An excellent view of the East Humboldt (i.e., toward Wells and Salt Lake City). Range straight ahead. 2.0 2.2 View of the Elko Hills on the right. This View to right into northern end of Osino small range consists of thrusted upper Canyon (cut by the Humboldt River). Paleozoic sedimentary rocks overlain and One of the most controversial contacts intruded by late Eocene(?) silicic igne- regarding the developmental history of ous rocks. The range was mapped and the Elko Hills is exposed in a set of rail- studied by K.B. Jaeger (1987) for his road cuts along the northwest wall of M.S. thesis research project at the Uni- this canyon. Here Jaeger (1987) inter- versity of Wyoming, and K.B. Ketner preted the contact between Tertiary rhy- (1990) also mapped this range. Although olite and overlying Pennsylvanian-Mis- both Jaeger and Ketner agree on many sissippian Diamond Peak Formation as a aspects of the geology of the range, they deformed contact (locally faulted) along disagree about the amount of intrusive the top of a shallow intrusive body. In versus extrusive silicic rock exposed in contrast Ketner (1990) mapped this con- the range. Jaeger (1987) concluded that tact as a thrust fault related to late much of the rhyolitic rocks were part of Eocene(?) contraction. For further dis- a shallow intrusive body, whereas Ketner cussion of this controversy see Thorman (1990) mapped these rocks as part of a et al. (1991, STOP 1-3 , p. 874-876). sequence of rhyolite lava flows and Exit #314 to Ryndon. pyroclastic tuffs. Exit #317 to Elburz. 1.9 4.1 The East Humboldt Range is on the sky- line. View of the northern Ruby Mountains; 0.5 4.6 The mountain range on the left is the the prominent peaks include Secret Peak Adobe Range consisting chiefly of Paleo- (9,167 ft 12,796 m]) and Soldier Peak zoic through Triassic sedimentary rocks (10,089 ft [3,077 m]). including allochthonous, western assem- The topographic low at 2 o'clock is Secret blage (eugeoclinal) rocks. The stratigra- Creek gorge. Note the range-bounding phy and structure of this area was de- normal fault system prominent along the scribed by Ketner and Ross (1990). A western flank of the northern Ruby particularly important structural feature Mountains. of this range is a large-scale Mesozoic(?) Take exit #321 to Halleck and Ruby syncline (Ketner and Smith, 1974). Valley. According to these authors, the Adobe Cross railroad tracks; Halleck on the syncline can be traced to the northern right. The Halleck Station was established Cortez Mountains where rocks as young in 1869 by the Central Pacific Railroad, as Late Jurassic (Pony Trail Group) are it once served as a major shipping point folded. Furthermore, Ketner and Ross for livestock from the local ranches (1990) concluded that this folding pre- (Patterson, 1977).

Figure 12. Geologic map of part of the southeastern East Humboldt Range, Nevada. Qa = Alluvium (Quaternary); Ta = basaltic andesite (lavas and breccias) (upper Eocene),T]gp = granite porphyry (age uncertain, Tertiary to Jurassic[?]), TRt = Thaynes Formation (Lower Triassic), Pg = Gerster Limestone (Permian), Ppl = Plympton Formation (Permian); Pph = phosphorite-bearing strata (Permian), Pk = Kaibab Limestone (Permian); Pp = Pequop Formation (Permian); Mp = Pilot Shale (phyllite and metalimestone) (Mississippian and Devonian[?]); Dg = Guilmette Formation (metalimestone and metadolomite) (Devonian), DOd = Metadolomite (Devonian to Ordovician); Oe = Eureka metaquartzite (Ordouician). Standard symbols for attitudes of bedding and foliation, geologic contacts, and normal faults (except double tick marks on upper plate of brittle, low-angle normal fault [detachment faultl). Faults are dashed where approximate and dotted where concealed. Geology by G. K. Taylor (1984). SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 251

0 1I2 1 Mile Geology by G.K. Taylor (1984) ----I 0 .5 1 Kilometer I---/ BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

Cross railroad tracks; Secret Peak at 1 location of each locality is shown on the o'clock. Note triangular facets related to accompanying simplified geologic map Holocene normal faulting developed in Figure 13. along the west flank of the northern Locality A (roadcut exposure along Ruby Mountains. Nevada 229)-Mylonitic, interlayered Soldier Creek canyon at 2 o'clock with migmatitic schist and impure quartzite Old Man in the Mountain peak in the with subordinate orthogneiss, cut by nu- background. merous westward-dipping normal faults. Main entrance to the 71 Ranch on the Many of the normal faults are planar, but right. a few are clearly curviplanar (Fig. 14). Road to Starr Valley on the left. Exposures Associated with the westward-dipping of Miocene Humboldt Formation form normal faults are spectacular drag fea- the hills to the left. tures as well as crushed zones and thin Road to the village of Lamoille on the ultramylonitic to cataclastic layers dong right. After STOP 3-1, we will take this the fault planes. In addition, flaggy mica- road to get to Lamoille Canyon (STOP ceous quartzites with conspicuous mica 3-2). For the present, continue straight porphyroclasts ("mica fish) are useful on Nevada 229. indicators of the sense-of-shear in the Old stream gravels of Secret Creek on mylonite zone. Other mesoscopic crite- left. ria useful in the determination of sense- More exposures of stream gravels. of-shear include asymmetric feldspar Beginning of exposures of Miocene Hum- porphyroclasts, composite planar sur- boldt Formation; south of the road, faces in pelitic schists (S-C-C' fabric), Secret Creek cuts through an excellent and mesoscopic folds that deform the exposure of a Holocene fault (see com- mylonitic foliation. Well-developed micro- ment on DAY TWO, mileage point 88.0). structural criteria are also common in More exposures of Miocene Humboldt these mylonitic rocks (e.g., the mylonitic Formation. impure quartzites are classic examples of High-angle normal fault contact between Type I1 S-C mylonites of Lister and the Miocene Humboldt Formation and Snoke, 1984). All these criteria taken Mississippian part of the Diamond Peak together indicate a west-northwest sense- Formation (here, interlayered sandstone, of-shear (top to the west-northwest) siltstone, and black mudstone). throughout the quartzite and schist unit Pull right into large parking space. From in the northern Ruby Mountains and here we will walk up the road to STOP southwestern East Humboldt Range. 3-1. The following description is modi- The mylonitic quartzites at this locali- fied from STOP 12 of Snoke and Howard ty as well as other rock types in the (1984). Please watch out for rattlesnakes Secret Creek gorge area were the sub- in this area. ject of a stable isotope study by Fricke et This stop will consist of a guided tra- al. (1992) which demonstrated the im- verse through an anastomosing system portance of meteoric water infiltration of distinctive lithologic slices bounded during mylonitization. by low-angle normal faults (perhaps best Locality B (roadcut exposure along referred to as an extensional duplex Nevada 229-west of Locality A)-We structure). The purpose of this traverse have crossed a low-angle normal fault is to demonstrate the complex structural that separates the overlying Horse Creek style characteristic of the low-angle fault assemblage from the underlying quart- complex, but also to develop the struc- zite and schist unit. The Horse Creek tural chronology between mylonitic de- assemblage is &verse and includes impure formation, low-angle normal faulting, and calcite marble and calc-silicate gneiss high-angle normal faulting. To facilitate and schist (inferred Ordovician and the use of this guide, specific localities Cambrian protoliths), and mafic to felsic have been designated (A-H), and the orthogneisses including a distinctive de- SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 253

Figure 13. Geologic map of the Secret Creek gorge area, northern Ruby Mountains, Nevada. Modijed afer Snoke (1980,figure 5). Qa = Alluvium (Quaternary); Qoa = Older alluvium (Quaternary); Qh = Landslide deposits (Quaternary); Ts = Sedimentary rocks (Miocene); Tr = Rhyolite (Miocene); Tgn = Granitic orthogneiss (Tertiary); Pp = Pequop Formation (Permian); Plc = Limestone and conglomerate (Permian); Pu = Permian rocks undivided; Pe = Ely Limestone (Pennsylvanian); PMdp = Diamond Peak Formation (Pennsylvanian and Mississippian); Dg = Guilmette Formation (Devonian); DOd = Metadolomite (Devonian to Ordovician); Oe = Eureka metaquartzite (Ordovician); hc = Horse Creek assemblage (metasedimentary and granitic rocks); CZqs = Impure metaquartzite and schist (Cambrian and Neoproterozoic). Standard symbols for attitude of bedding w foliation, trend and plunge of lineation, hinge line of mesoscopic fold, geologic contacts, and nmlfaults (except that double tick marks on upper plate of brittle low-angle normal fault [detachment fault] and filled squares on upper plate of plastic-to-brittle low-angle normal fault [detachment fault]). Faults are dotted where concealed. The approximate location of localities A-H are also plotted on the map. Geology by A. W Snoke. BYU GEOLOGY STUDIES 1997, VOL. 42, PART I formed biotite-hornblende mafic quartz contact is probably composite in charac- diorite. Mylonitic rocks are ubiquitous ter; parts of the contact are a low-angle, and include spectacular calc-mylonite ductile-brittle fault and parts are younger, containing competent mineral grains and low-angle brittle normal faults that have rock clasts. Many of the obvious folds soled into the older ductile-brittle fault. deform the mylonitic foliation and dis- These relations suggest a continuum play westward vergence (Fig. 15A). Folded from the mylonitic shear zone deforma- boudins and dismembered folds are tion to brittle low-angle normal faulting. other manifestations of a complex strain Locality F-As we climb the slope history (inferred as progressive, non- above the old road we will cross two coaxial deformation). Scarce sheath folds low-angle normal faults. The lower fault also occur in the Horse Creek assem- (the main break in metamorphic grade blage. and here the top of the mylonitic zone) Locality C-Folded, mylonitic white separates the Horse Creek assemblage quartzite in the Horse Creek assem- from a slice of low-grade metadolomite; blage. This westward vergent fold dis- the upper fault separates the metadolo- plays rotation of an earlier rnylonitic lin- mite from a structurally higher slice of eation during late folding in the mylo- low-grade but fossiliferous Upper Devo- nitic shear zone. nian Guilmette Formation (metalime- Locality D (along the old road)- stone). Remember that low-angle nor- Disharmonic westward-vergent folds in mal faults are structurally below, and as the Horse Creek assemblage (Fig. 15B). you will soon realize, there are more low-angle normal faults structurally above Lithologies include impure calcite mar- involving younger rocks. ble with mafic pelitic layers, granodiorit- When you reach the top of the hill ic augen gneiss, and white quartzite. (i.e., the resistant gray exposures of Guil- Note how layer thickness controlled the mette Formation), if you look approxi- amplitude and wavelength of the folds. mately east-southeast (i.e., toward the As we walk from locality D to E along paved road), you will see a portion of a the old road, we will cross the low-angle roadcut (locality H). This roadcut is topo- fault contact between the overlying Horse graphically and structurally above ydur Creek assemblage and the underlying present position. In the roadcut, Missis- quartzite and schist unit several times. sippian and Pennsylvanian Diamond Peak Note that when the mylonitic foliation in Formation is structurally overlain by Mio- both the upper and lower plates is cene Humboldt Formation. Therefore, roughly subparallel, a secondary black another low-angle normal fault must ultramylonite is common along this con- separate the Guilmette Formation from tact (i.e., a ductile-brittle low-angle fault). the Diamond Peak Formation and Hum- In other cases, where foliation in the boldt Formation. If you include the duc- upper plate is highly discordant to folia- tile-brittle low-angle fault that separates tion in the lower plate, the contact the Horse Creek assemblage from the appears to be a brittle low-angle normal structurally lower quartzite and schist unit, fault that has perhaps soled into (i.e., there are at least five low-angle normal reworked) the earlier ductile-brittle low- faults in the immediate area that attenu- angle fault. Furthermore, there are also ate a stratigraphic section that includes steeper normal faults that cut the Horse rocks ranging in age from Neoprotero- Creek plate. zoic to Miocene. In this area, the order Locality E-We are presently situated of units is such that younger rocks are on the Horse Creek assemblage, however, faulted onto older rocks, so that the sec- as you look up the canyon you can see tion is attenuated. These characteristics the contact between the overlying Horse closely resemble so-called "chaos struc- Creek assemblage and underlying quart- ture" originally described by Noble (1941) zite and schist unit in fine detail. Again from the Death Valley region (Wernicke note that the mapped low-angle fault and Burchfiel, 1982). SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 255

Figure 14. Composite photograph of roadcut (along north side of Nevada 229) in mylonitic, interlayered migmatitic schist and impure quartzite cut by westward-dipping normal faults (hoth planar and listric), Secret Creek gorge area. Thus roadcut is near locality A on Figure 13 and is 4 to 5 m high. Photograph by A. W Snoke.

As we walk from locality F to G, we zontal west-northwest-trending slicken- are traversing through poorly exposed side lineation. The overlying Guilmette slices of chiefly low-grade, middle Paleo- Formation is a medium gray, highly cal- zoic metacarbonate rocks (Devonian Guil- cite-veined, low-grade metalimestone. A mette Formation and Ordovician to Devo- similar tectonic slice of Guilmette Forma- nian metadolomite but also sparse meta- tion exposed south of Secret Creek gorge morphosed Ordovician Eureka Quart- yielded Late Devonian conodonts with zite) structurally above the top of the CAI = 5 112 suggesting that the host mylonitic Horse Creek assemblage. In a rock reached 300°C to 350°C (A. Harris, few places along this portion of the tra- written communication, 1982). verse, the exhumed structural top of the Locality H (along Nevada 229, at park- Horse Creek assemblage is exposed; note ing area)-Tuffaceous siltstone and sand- that the orthogneissic rocks are com- stone of the Humboldt Formation in monly retrograded (chloritic alteration) low-angle normal fault contact with Mis- at these locations. The role of abundant sissippian and Pennsylvanian Diamond fluids along this important low-angle nor- Peak Formation. The Diamond Peak For- mal fault is clearly indicated. Further- mation is a thin tectonic slice structurally more, the retrogression indicates that above the mylonitic Horse Creek assem- the brittle low-angle normal faulting, at blage and the low-grade Guilmette For- least in part, evolved under lower grade mation metalimestone. Note that bedding conditions than the mylonitic rocks (bio- in the Diamond Peak Formation has been tite in the mylonitic orthogneisses is ret- dismembered; and the formation, which rograded to chlorite at the structural top here consists of pebble conglomerate, grit, of the Horse Creek assemblage). siltstone, sandstone, and black mudstone, Locality G--Slice of mylonitic Ordov- takes on the overall appearance of "bro- ician Eureka Quartzite structurally be- ken formation." neath non-mylonitic but brecciated, low- Upon completion of the traverse, please grade Guilmette Formation and structural- congregate near the vehicles in the park- ly above mylonitic Horse Creek assem- ing area (adjacent to locality H). blage. The Eureka Quartzite at this local- We will now turn the vehicles around ity is 'a somewhat fractured quartz mylo- and retrace our route to the road to nite. The surface of the exposure appears Lamoille (-4.7 miles to the east-south- tectonically polished with a near hori- east). BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

Turn left onto the road that goes to Quartzite which overlies dolomite mar- Larnoille. ble (Devonian to Ordovician) to the left Secret Peak at 10 o'clock. Just to the right but is below calcite marble (Ordovician of Secret Peak you can see a landslide to Cambrian) to the right. The calcite scar that developed on May 30, 1983, marble, in turn, is overlain to the right after intense Spring rains coupled with a by Eureka Quartzite on the right-side- rapid snow melt. Note triangular facets up limb of the anticline. This fold has a along the west flank of the northern Ruby curvilinear hinge line that appears to trace Mountains; a sketch of these facets were an arc through 180"; it is thus a large- included in Sharp (193913, figure 6). scale sheath fold. Road to Starvation Canyon on the left Hidden Lakes uplift, a north-south (private property, permission required to trending fold with a core of upper am- drive on this road). Note small offset of phibolite facies Neoproterozoic and Quaternary fan deposits indicating recent Lower Cambrian, metamorphosed Pros- Holocene faulting (fault scarp is accen- pect Mountain Quartzite, is at 9 o'clock. tuated by vegetation). Bear left onto Clubline Road. Ninety degree turn in road. To the left is Turn left. Soldier Peak at 9 o'clock. Enter the village of Lamoille (elevation Fort Halleck historical marker. Fort = 5,890 ft [1,796 m]). Turn right onto Halleck (1867-1886) was established to Nevada 227. protect the California Emigrant trail and Turn left onto the road that leads into the construction work on the Central Pacific Lamoille Canyon Recreation Area. Railroad. The fort was named for Henry Road climbs onto a piedmont moraine of Wagner Halleck who was Commander- the Lamoille substage of glaciation. The in-Chief of the United States Army in Lamoille Canyon glacier that deposited 1867. this moraine was -12 miles long (Sharp, Cross Soldier Creek. 1938). Road up Soldier Creek Canyon on left. Watch ahead for fault scarp cutting the Cross John Day Creek. moraine. A recumbent anticline is exposed in the Entrance to Ruby Dome Ranch on right. foothills between Cold Creek and Soldier At ditch, the road climbs the late Qua- Creek on your left. This fold is best ternary fault scarp that cuts the moraine. defined by the bouldery white slopes Continue past right turnoff to Power- that expose metamorphosed Ordovician house picnic area. Eureka Quartzite. The bouldery white Roadcut in mylonitic orthogneiss of slope on the left as you face the range Thorpe Creek, an extensive sill of gar- (i.e., look eastward) is inverted Eureka net-two-mica leucogranite gneiss. This

Figure 15. Defomtional features in the Tertiary mylonitic shear zone as exposed in the Secret Creek gorge area, northern Ruby Mountains (STOP 3-1). A, Fold in mylonitic orthogneiss with thickened hinge zone and attenuated limbs (Locality B on Figure 13). Note that the mylonitic foliation (i.e., gneissic layering) of the shear zone is deformed by this fold. B, Late, post-mylonitic folds in the mylonitic shear zone (Locality D on Figure 13). The main rock types are m, impure calcite marble with pelitic interlayers; gn, biotite granodiorite orthogneiss; and qtz, flaggy white quartzite. This exposure is part of a cascade of asymmetric, west-verging folds within the mylonitic shear zone. These folds illustrate the non-coaxial character of defomtion in the shear zone, wherein layers were folded in the shortening field despite an overall extensional defomtion. Note the disharmonic nature of the folding, controlled by contrasting layer thickness and competency. Hammer used for scale is lying on the outcrop above small bush lower right. C,Large clast of massive and competent quartz-bearing hornblende gabbradiorite encased in mylonitic calcite marble. The area of photograph D is outlined. D, Rotated clast of competent paragneiss (indicated by arrow and label) in mylonitic calcite marble. Note that a mylonitic fluxion struc- ture is deflected around the clast which itself contains a mylonitic foliation. Both foliations developed during a continuous, non-coaxial extensional defmation. Photographs by A. W Snoke.

BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

intrusive unit has yielded U-Pb mon- gneiss. This dike is penetratively de- azite ages of -36-39 Ma (Wright and formed and yielded an 40~r/39~rhorn- Snoke, 1993; MacCready et al., in press). blende age of 25.6 +- 0.8 Ma (Dallmeyer 60.1 Lamoille Canyon sign and pullout on et al., 1986, their sample #2). right. We are presently in the mylonitic 0.4 63.5 Lower cliffs above (north of) the road are zone that forms a carapace above the gneissic pegmatitic leucogranite con- migmatitic metamorphic infrastructure, taining pods of calc-silicate rock and exposed at structural deeper levels in impure calcite marble inverted on the Lamoille Canyon. lower limb of the Lamoille Canyon fold- 60.5 Roadcut exposes pegmatitic leucogranite nappe. with enclaves of calc-silicate rock and 0.4 63.9 Wall of quartzite across the canyon con- sillimanite-bearing pelitic schist of the tains a deep vertical cleft that probably Ordovician and Cambrian impure mar- hides a Miocene basalt dike. Such dikes blelcalc-silicate unit (Fig. 2). At this loca- typically weather recessively in clefts. tion, the unit is in an inverted position Some of the basalt dikes are amygda- on the lower limb of the Lamoille Can- loidal and were evidently emplaced at yon fold-nappe. We are passing into the relatively shallow depths. infrastructure which is an upper amphi- 0.9 64.8 STOP 3-2-Thomas Canyon camp- bolite facies, granite-rich zone structural- ground. The following description is ly beneath the Tertiary mylonitic shear mod$ed from STOP 8 of Snoke and zone. Howard (1984) and Howard (1987). See 60.7 Roadcuts along lower Lamoille Canyon Figure 16 for a geologic map of the are chiefly mixtures of pegrnatitic leuco- northern Ruby Mountains indicating the granite gneiss and darker gneisses of location of this STOP and others in uncertain parentage. The pegrnatitic leu- Lamoille Canyon. Representative cross cogranite gneiss predominates in this sections for the northern Ruby Moun- relatively deep part of the infrastructure. tains are shown in Figure 17. 61.9 The large talus blocks above and below This locality is deep within the mig- the road include: (1) the granodiorite matitic, metamorphic infrastructure of gneiss of Seitz Canyon (medium-grained the Ruby Mountains core complex. The biotite granodiorite gneiss) and (2) peg- high cliff to the northwest, across matitic leucogranite gneiss. Lamoille Canyon, is composed of Cam- 62.4 Road to Camp Larnoille on the right. Con- brian-Neoproterozoic Prospect Mountain tinue on the main road up the canyon. Quartzite above intensely folded Ordovi- 62.85 High cliff to the northwest (uphill to the cian-Cambrian calc-silicate paragneisses. left) exposes a nose of the granodiorite We are therefore looking at the inverted gneiss of Seitz Canyon in the core of the limb of the Lamoille Canyon fold-nappe. Lamoille Canyon fold-nappe. The gray Note the contrasting styles of pegmatitic gneiss is enveloped by brown Prospect leucogranite in the two stratigraphic Mountain Quartzite. units: sills in the calc-silicate paragneiss 63.1 This large roadcut includes a mixture of unit and irregular dikes in the quartzite. granitic rocks of probable Cretaceous Furthermore, especially note the two and Tertiary age. A strongly foliated, gray curved, rainbow-shaped dikes of biotite granitic orthogneiss (Cretaceous?) is in- monzogranite in the left part of the cliff; truded by pegmatitic leucogranite gneiss each are about 6-7 m thick. The dikes (Late Cretaceous?). Both of these units truncate compositional layering and foli- are intruded by a 29-Ma biotite monzo- ation in the wall rocks (that is, sillimanite- granite dike (RM-15 in Wright and Snoke, bearing, micaceous feldspathic quartzite). 1993). Angular fragments of pegmatitic Four zircon and two monazite fractions leucogranite gneiss occur in the biotite were analyzed with U-Pb techniques from monzogranite dike. At the western end the dike on the right (Wright and Snoke, of this roadcut is a dark quartz dioritic 1993). The two monazite fractions yield dike that intrudes pegmatitic leucogranite a crystallization age of 29 + 0.5 Ma for SNOKE. E'r Al,.. RUBY-EAST IIli4IBOLU'T CORE COXfPLEX, NEVADA 259

EXPLANATION

L-J L-J Sedimentary deposits (Cenozoic)

-,+ n... Granitic rocks (Mesozoic; shown L-j - .- where no metasedimentary relics)

Limestone (Pennsylvanian)

Marble of Snell Creek (Devonian)

Dolomite (Devonian to Ordovician)

[-'I Eureka Quartzite (Ordovician)

Marble of Verdi Peak (Ordovician and =Cambrian) and granitic rocks Prospect Mountain Quartzite (Lower Cambrian and Precambrian?) and granitic rocks Fault-dotted where concealed; ball "....ZII,on downthrown side; sawteeth on upper plate of thrust A-----A' Line of section shown in figure 17

E'igt11.e 16. Geologic ttlap of pt't Of th~norflb~~rn Rtibcy Mourttaiizs (r,totl$eclf,.orrz Efou'ctrr( [198Oj, (igr11.i~$3) ssltou:iizg STOPS 3-2-3-4 alortg fhe Lcrrrtoille C(~tr!yonRoad.

this sample. "fie zircon analyses yield a marttle/oalc-silicate rocks, capped by less precise, lower concordia intercept brown cliffs of cl~tartzite.The stratigraphic of 26 rt 8 $fa, ~vhicltis analytically indis- section is i~~vertt~d;the overlying browrl tinguishable from t1.w monazite analyses. cliffs are the Carnltrian and Neoprotero- The analyzed sample is cqtiigranular, zoic Prospect Mountain Quartzite, Thc ~llediunigrainecl, and apxlrently uncle- stivrr of cytartzite, inteq~rc.tedas an in- formed; the other dike is weakly foliated verted thrust slice of the Prospect Moun- p;trnllol to thth dike walls. tain Quartzite (F.Tow~arc1,1987), can I)e The southwest wall of Larnoille Canyon traced for miles. When this inferred thrust above tlrc Thonras Canyon carnpgror~nd \liver is rustorcd to a pre-folding posi- exposes thick \ill\ of peglnatitic leuco- tion it ancl others pinch out to tlie west, granite (Late Cretaceous?, see S'TOP 3-3 suggesting west-directc-.cl thrusting (Snokc for radiorrretric age data 012 po\sibly sirn- and Ilow:trd, 1984, figure 10). ilar intrusive rocks) intrr~dedinto calcite 1.7 66.5 Contintie past roadside pullout for valley n~arl~leand cdc-silicate p~tragneiss(Ordo- overlook and view of hanging valleys. vician and C;unbrian protoliths) (Fig. 18A). 0.6 (57.1 Outcrops of buff rnar1,le ant3 white peg- Above the niarl)Ie/calc-silicate rock unit matitic leucogranite arc. altove the road is a thin sliver of cl~iart~itc,then more on the left (cast). NAPPE

EXPLANATION

o-mica granite and adamell Marble of Snell Greek (Devonian) olomite and Eureka Quartzite Devonian to Ordovician) arble of Verdi Peak (Ordovician and nodiorite gneiss of Seitz Canyon ambrian) and granitic rocks rown dolomite (Cambrian) anite gneiss of Thorpe Creek rospect Mountain Quartzite {Lower ambrian and Precambrian?)

AAAAAA I -x-_ " I X ^ Premetamorphic thrust fault Approx~matebase of mylonitic zone

Figtire IT Cross sections rtlortg Lontoillu C[tr~yc>rl(R13 '1 trrrrf ofller iocttflorts rt~ilii~ ~iorfht~r $1 Rrll)r/ Jforciitiirrlc ir~~crtl$r*t/ftot~i Ifowirt ri [l98fj, figrnc. 4) ('I-ow-sccktzort 1lnc.c lor~~ft~lon Eigzinj 10 SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 261 0.4 67.5 Roadcuts are pegmatitic leucogranite and Cambrian protoliths) are intermixed gneiss and enclaves of folded calc-sili- with the granitic rocks (Fig. 18C). The cate paragneiss, part of STOP 3-3. prominent rugged, light-colored peak to 0.2 67.7 STOP 3-%Turn left into Terraces day- the north down the valley is Verdi Peak use area and drive to parking area (0.1 (11,074 ft [3,378 m]). The saddle in the mi). There is a charge (presently $2) to ridge along the skyline to the south is park here. Liberty Pass (-10,500 ft [3,202 m]); the View stop of hinge of Lamoille Can- peak (11,032 ft [3,365 m]) to right of the yon fold-nappe at Terraces day-use area. pass is an unnamed feature that rises The following description is mod$ed above Lamoille Lake. On another visit, from STOP 6 of Snoke and Howard when you have more time, we recom- (1 984) and Howard (1987). mend a hike up to Lamoille Lake or be- Bring binoculars, if available, and yond-this high country is spectacular assemble on the large flat outcrop of for hiking, especially from mid-July gneissic pegmatitic leucogranite in the through early September. picnic grounds. From this locality we The Ruby Crest Trail begins at the end can view the hinge of the recumbent of the road (-8,800 ft at trailhead sign) Lamoille Canyon fold-nappe (Fig. 18B). and continues south for about 40 mi (64 The view northward down the canyon km) along the crest of the Ruby Moun- shows brown cliffs of Prospect Mountain tains to Harrison Pass. We will walk south Quartzite (Neoproterozoic and Cambrian) a short distance along the trail with the forming an eastward closing fold nose chief objective being to examine some of with metacarbonate rocks (Ordovician the intrusive granitic rocks and metased- and Cambrian) wrapping around it. imentary enclaves. A new, collaborative Within the quartzite nose is a repetition research project (University of Wyoming, of the stratigraphically higher unit of Texas Tech University, U.S. Geological marble and calc-silicate rocks, separated Survey, and Rice University and funded from the quartzite unit by an inferred by the National Science Foundation) is pre-folding thrust fault (Ogilvie thrust). in progress to study the petrogenesis, The outcrops of pegmatitic leucogran- age, emplacement mechanisms, and defor- ite gneiss that we are standing on, and mational history of the granitic rocks in along the Lamoille Canyon road just this area. Evidence indicates that most, below, contain sillimanite and monazite; if not all, of these granitic rocks are either enclaves of calc-silicate paragneiss are Late Cretaceous or Tertiary in age; look common. Monazite from the leucogran- for evidence for consistent cross-cutting ite yielded a U-Pb date of -83 Ma (J. E. relationships between the various types Wright, unpublished data, cited in Wright of granitic rocks (Fig. 19). and Snoke, 1993), suggesting a Late Cre- The trailhead begins at the sign near taceous emplacement age. the parking lot. After crossing the bridge 0.2 67.9 Return to main road and turn left (i.e., with railing, the first outcrop along the continue up the canyon). trail is a banded, foliated, and lineated 0.8 68.7 'hvalanche chute" parking area on right. two-mica granitic orthogneiss with sparse Continue to the end of the canyon. garnet and sillimanite (Cretaceous or Jurassic?). This granitic gneiss contains 1.3 70.0 STOP 3-&End of the road, parking lot, an enclave of calc-silicate paragneiss and and trailhead at elevation -8,800 ft is intruded by pegmatitic, two-mica leu- (2,684 m). cogranite gneiss (Late Cretaceous?). Con- The walls of upper Lamoille Canyon tinue along the trail until you reach an expose a variety of granitic rocks occur- exposure of foliated biotite monzogranite ing as dikes, sills, and sheet-like intru- (Oligocene?) cut by several pegmatite sive bodies; metasedimentary enclaves dikes (note: this outcrop is located just of impure calcite marble, calc-silicate before you cross a second bridge). From paragneiss, and pelitic schist (Ordovician this locaIity verge off the trail to the 262 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 263

Figure 18. Views of the Larnoille Canyon area showing relationships between intrusioe granitic rocks, upper amplzibolite-facies metaseditnentary wall rocks, and large-scale structural features. A, Southwest wall of Lamoille Can!yon, ahove the mouth of Thornas Canyon, showing a complex network of sills and (likes of granitic rocks intruderl into calcite innrlde and calc-silicate paragneiss (Ordovician and Cambrian protoliths) (near STOP 3-2). B, The Lamoille Canyon fold-noppe as exposetl on the north wall of Lamoille Canyon. The hinge of the nappe is outlined by brown-weathering cliffs of upper arnplzil3olite facies Prospect Mountain Quartzite (Cambrian and Neoproterozoic). The quartzite nose is surrounded by calcite inarlde nnd calc-silicate parclgneiss (Orrlovician and Cambrian protolith) pervasively intruded b~ynu7nerou-s sills of white-weathering pegrnatitic leucogrunite. Calcite marlde cmd calc-sili- cute paragneiss also occur in the core of the fold and have been interpreted by Howard (1966) as the Cambrian and Orrlovician unit in the originally lower plate of the pre-inetainorphic and pre-foMing Ogilvie thrust fault. C,East wall of upper Larnoille Canyon exposing a granitic dike-sill complex intruder1 into upper amplzibolite-facies crrlc-silicate parugneiss and impure calcite inarlde (Orrlovician and Cambrian protolitlzs) (near STOP 3-4). Photographs hy A. U! Snoke

right following an azimuth of -280". We will return to the parking lot via This trend will lead you to an excellent the stock trail. If you haven't already exposure of foliated, and locally banded, noticed, the steep east wall of upper biotite > muscovite granitic orthogneiss Lamoille Canyon exposes a super11 gra- cut by dikes and irregular bodies of peg- nitic dike and sill complex intruded into matitic leucogranite. At this exposure impure calcite marble and calc-silicate there is also a "Z-shaped" medium- paragneiss (Fig. 18C), and the view is grained biotite leucogranite body. Is it excellent from the stock trail in the after- part of the Tertiary biotite ~nonzogranite noon. suite? From this locality, we will traverse to a large exposure that has been studied This is the last stop for DAY THREE. We will drive in detail by Sang-yun Lee (summarized back down the canyon (approximately 12.6 mi) and turn in section by Lee and Barnes, this field left at the intersection with Lamoille Highway and thus guide). return to Elko. 264 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I DAY FOUR Exposures of the Eocene cherty lime- Elk0 to Hamson Pass to Ruby Valley (Road Canyon area) stone unit on left. to Salt Lake City. Lamoille summit and exposures of Elko Formation on the left. Cum. View of the west flank of the northern Miles Ruby Mountains. The large canyon to 0.0 Start at Red Lion Inn & Casino, Elko, the southeast is Lamoille Canyon, the Nevada. focus of several stops on DAY THREE. Northeast Nevada Museum. The smooth, west-dipping flank of the Turn left (south) onto 12th Street. range is principally controlled by folia- Cross the Humboldt River. tion in -1-km thick mylonitic shear Turn left at "T" onto Lamoille Highway zone. (Nevada 228). Stop light. Intersection of Lamoille High- For information about the Humboldt way and Spring Valley Parkway on left. National Forest turn left and proceed to Turn right onto Nevada Highway 228 Headquarters but for our trip stay and proceed toward Jiggs. straight. Note dip slope formed on flaggy rocks of Road to City of Elko muncipal landfill. the mylonitic shear zone on the west The roadcut exposures on the left expose flank of the Ruby Mountains. Entering tribal lands. some of the oldest Tertiary rocks in Mountain range at 2 o'clock is the Piiion Nevada (Solomon et al., 1979; Solomon Range. This area has been studied by and Moore, 1982). These rocks referred Smith and Ketner (1978). to as the "conglomerate, sandstone, and Leaving tribal lands. shale unit" by Solomon et al. (1979), South Fork Recreation Area road on the include a tuff layer dated by the K-Ar right. method (biotite) at 43.3 + 0.4 Ma. These Twin Bridges on the right. The red soils rocks underlie fossiliferous Eocene are developed on the "conglomerate, rocks (cherty limestone unit of Smith sandstone, siltstone, and limestone" unit and Ketner, 1976) as well as the upper (Eocene?) of Smith and Howard (1977). Eocene and Oligocene(?)Elko Formation Cherty limestone (Eocene), however, and, therefore, must be Eocene or older. forms the bulk of the hill immediately The dated tuff is apparently one of the west of the Twin Bridges area. earliest manifestations of Tertiary vol- Road to Lee on the left. Note light-col- canism in Nevada (Solomon et al., 1979). ored exposures of Miocene Humboldt These Paleogene rocks form the core Formation in roadcut on left. Pebble con- of a normal-faulted, northeast-plunging glomerate in this section of Humboldt anticline. The folding and normal fault- Formation contains abundant lithic clasts ing must pre-date 35 Ma, because rela- presumably derived from the Ruby Moun- tively flat-lying and unfaulted Oligocene tain igneous-metamorphic complex. Mylo- andesite (exposed about 2 km to the nitic clasts are not present, and the dom- southwest) unconformably overlies the inant clast-types are leucocratic granitic deformed older Tertiary rocks (Solomon rocks and various impure calcite mar- and Moore, 1982). bles and calc-schists.

Figure 19. Intrusive relationships among granitoids exposed in upper Lamoille Canyon area. A, irregular dark dike of biotite tonalite (Tertiary?) intruded into two-mica pegmatitic granite gneiss (Late Cretaceous?). Note head of hammer near upper edge of photograph. B, Equigranular biotite monzogranite, correlated with the 29-Ma suite, intruded by three generations of pegmatite-aplite dikes. The youngest dike exhibits a distinctive texture characterized by coarse sheaves of muscovite. C, Composite enclave of dark-colored calc- silicate paragneiss (lower center) and concordant, equigranular, two-mica granitic gneiss (upper center) within two-mica pegmatitic granite gneiss. The pegmatitic leucogranite gneiss is correlated lithologically with the rock dated near the Terraces day-use area as Late Cretaceous by U-Pb isotopic analysis on monazite. Photographs by A. W Snoke. SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 265 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I Cross the South Fork of the Humboldt Jiggs Fire Station and school. The village River. is named for the comic strip character of View up Rattlesnake Canyon on left. the same name; it was originally called Pearl Peak at 11 o'clock. North of Pearl Skelton, then Hylton, and finally Jiggs in Peak is Harrison Pass. 1918 (Patterson, 1977). During the 1970s Road to Lee on left. and early 1980s, the Huntington Valley Tree-covered hill north-northwest of area, southwest of Jiggs, was the site of Pearl Peak is Cedar Mountain which is an exploratory drilling program for hydro- chiefly composed of allochthonous, un- carbons (Schalla, 1992). Five deep wild- metamorphosed Devonian sedimentary cat wells were drilled and substantial oil rocks (Devils Gate Limestone and and gas shows were reported. Pan Ameri- Nevada Formation) that lie structurally can #1 Jiggs-USA, drilled in 1971, above granitic rocks of the Harrison Pass reached a total depth of 13,600 ft (4,145 pluton. Willden et al. (1967) recognized m), making it the deepest well drilled in cataclastic textures in the underlying Huntington Valley. All five wells pene- granitic rocks as well as the overall trated Cenozoic sedimentary and volcanic allochthonous character of the Devonian rocks as much as 11,040 ft (3,367 m) rocks at this locality. These authors thick, including the Elko, Indian Well, argued that the klippe was a manifesta- Humboldt, and Hay Ranch formations tion of west-to-east Tertiary thrusting which lie unconformably on Pennsyl- (i.e., contractional deformation). Snoke vanian and Mississippian rocks. Sub- and Howard (1984, their Stop 16) sug- surface maps based on drilling and seis- gested that this klippe as well as several mic data indicate a pronounced north- westward-dipping, siliceous breccia sheets ward dip for the Paleozoic-Cenozoic north of the klippe along the west flank unconformity as well as a horst block of the Ruby Mountains are part of an bounded by generally north-south-strik- extensional allochthon related to post- ing normal faults (Schalla, 1992). emplacement normal or detachment Excellent view of Cedar Mountain at 11 faulting. Blackwell et al. (1984, 1985), o'clock with Pearl Peak in the back- Reese (1986), Hudec (1990), and Burton (in preparation) indicate that a low-angle ground. normal fault forms the base of the klippe Dip slopes of siliceous breccia ("jasper- and breccia sheets and that this fault (an oid") sheet at 10 o'clock. originally steeper, west-dipping normal Continue straight toward the Ruby Lake fault) as well as the Hamson Pass pluton National Wildlife Refuge. and environs have been tilted eastward, Another excellent view of tree-covered perhaps as much as 30°, in the footwall Cedar Mountain. View of dip slope of of a west-rooted normal fault system. siliceous breccia sheet at 9 o'clock. Excellent view of Cedar Mountain klippe, Pavement ends. west of Harrison Pass. View of the sprawling ridge north of Robinson Mountain at 2 o'clock is part Toyn Creek. This area is an especially of a widespread rhyolitic to dacitic ash- important locality to study age relation- flow tuff unit that forms a major part of ships between intrusive phases of the the Oligocene Indian Well Formation Harrison Pass pluton. Here, numerous (Smith and Ketner, 1976). A detailed biotite f muscovite monzogranite sills paleomagnetic-structural study of these and dikes as well as pegmatite, alaskite, rocks was reported on by Palmer et al. and aplite dikes intrude the megacrystic (1991). biotite f hornblende granodiorite phase Village of Jiggs (elevation = 5,476 ft of the Harrison Pass pluton. [1,670 m]) and Jiggs Bar. Jiggs serves as Road on the left goes up Green Moun- an important center of communication tain Creek. in this ranching community and is the Another view at 10 o'clock of the ridge site of many local activities as well as the north of Toyn Creek. SNOKE, ET AL.: RUBY-EAST HUMBOLDT (:ORE COMPLEX, NEVADA 267 0.2 43.3 Outcrop of composite mafic dike with granitic suite can be seen in the distance commingled(?)felsic component on north to the northwest. These rocks weather a side of the road. distinctive orange color and are exposed 1.3 44.6 The road crosses a prominent outcrop of on the flank of two prominent knobs in vein quartz in granitic rocks of the Har- the distance. rison Pass pluton. This northeast-strik- Turn around and retrace our route back ing vein is shown on the geologic map of to the main road. Willden and Kistler (1969). Intersection with Hamson Pass road, turn 1.5 46.1 Beginning of numerous rounded expo- right. sures of megacrystic granodiorite on Exposure of closely spaced joints in gra- both the right and left sides of the road. nodiorite phase of the Harrison Pass 1.8 47.9 Turn right onto dirt track at Harrison pluton. Pass (elevation = 7,248 ft 12,211 m]), Fresh, coarse-grained, megacrystic bio- once known as Fremont Pass. tite monzogranite of the older granodi- 0.3 48.2 STOP 4-1. Harrison Pass view stop. orite unit of the Harrison Pass pluton in This locality affords an excellent view roadcut on left. of the late Eocene (-36 Ma) Harrison Buildings on right are remains of the Pass pluton and environs (see Burton et Star Tungsten mine, discovered in 1916 al., this field trip guide including a geo- or 1917. The mine chiefly produced logic map and cross section of the plu- scheelite from skarn deposits along the ton). The extensive set of outcrops to the contact between the Harrison Pass plu- north are chiefly comprised of mega- ton and metacarbonate country rocks. crystic granodiorite. A good view into Additional remains of Star Tungsten mine Ruby Valley as well as parts of various on left. Approximate contact between ranges (including Spruce Mountain and granitic rocks of the Harrison Pass plu- Medicine Range) is visible to the east. ton and metacarbonate rocks of the The roof zone of the pluton is also ex- Paleozoic country-rock sequence. posed on the east side of the range. The Exposure of porphyritic dikes from the contact is located at the change in vege- Hamson Pass pluton (immediately before tation cover on several ridges that can crossing cattle guard). Metacarbonate be seen to the east. The southern con- rocks of the Ordovician Pogonip Group tact is along the near-side of a long ridge that extends west from Pearl Peak, direct- underlie the adjacent hills, and these ly to the south. Paleozoic sedimentary rocks are pervasively intruded by dikes rocks on this ridge are folded in the con- and apophyses of the Harrison Pass tact aureole of the pluton, whereas south pluton. of this point the Paleozoic rocks form an Contact metamorphosed metacarbonate eastward dipping homocline (Sharp, 1942; rocks in the aureole of the Hanison Pass Willden and Kistler, 1979). The northern pluton. Locally the metacarbonate rocks contact of the pluton cannot be seen from are characterized by large porphyrob- this locality. To the west is the Cedar lasts of vesuvianite. Mountain klippe (discussed previously at More exposures of metacarbonate rocks mileage point 29.1). The view west down in the aureole of the Harrison Pass plu- Toyn Creek affords a look into Hunting- ton. ton Valley with the Piiion Range in the Interesection of the Harrison Pass road distance on the horizon. with the Ruby Valley road, turn left (i.e., Rocks at this locality are part of the proceed northward). The prominent hill older granodioritic suite of the Harrison directly south is composed of west-dip- Pass pluton. Biotite granodiorite is ex- ping Pennsylvanian Ely Limestone, down- posed here and is cut by aplite and faulted from the main part of the range alaskite dikes. Small bodies of associated (Hudec, 1990). biotite monzogranite are exposed -100 m South end of Franklin Lake on the right to the east. Rocks of the younger monzo- at about 2 o'clock. 268 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

1.3 54.3 Turn left onto the dirt road up Road thus exhibits an overall stair-step geom- Canyon. etry. Along the contact, blocks of wall 0.3 54.6 Cross creek. rock are separated by dikes and sills of 0.55 55.15 Head frame for mine worhngs on right. leucocratic granitic rock. Brittle fracture 0.05 55.2 Park near remains of buildings. These played a key role in the emplacement of buildings and several small winzes at the the dikes and sills as well as in the de- crest of the ridge to the north comprise velopment of the stair-step contact. How- the Eddie claims of the Valley View min- ever, folding of the wall rocks also ing district (Lapointe et d., 1991). These occurred during the emplacement of the prospects were developed on tungsten- Harrison Pass pluton. At approximately manganese skarn deposits in the contact the 7300-foot elevation is a prominent aureole of the Harrison Pass pluton; how- outcrop of folded siliceous wall rock with ever, the claims were not productive. a folded sill of leucogranite in its core. Turn vehicles around and retrace our STOP 4-2. Contact zone between Harri- route back to the Ruby Valley road (-0.9 son Pass pluton and metamorphosed Pale- mi). ozoic wall rocks. Intersection with Ruby Valley road, turn Proceed northward by ascending a left (i.e., proceed northward). north-south-trending ridge through com- Exposures of Jurassic two-mica granite plexly folded roof rocks. Small ampli- of Dawley Canyon on the left at about tude folds lunge to the southeast and 10 o'clock. define a conical distribution on a stere- Stone buildings on right. Exposure of onet. The stratigraphic sequence here is two-mica granite of Dawley Canyon on attenuated by as much as 70% in com- left is the site where a sample for U-Pb parison to the inferred equivalent se- dating was collected in 1980 from which quence exposed to the south near Pearl J. E. Wright found an age on monazite of Peak. Continue your ascent to about the 153 + 1 Ma (Hudec, 1990; Hudec and 7000-foot elevation where the contact Wright, 1990). between the Harrison Pass pluton and Rock House on left. Cambrian impure metacarbonate rocks Road to Overland Lake trailhead on left. is exposed. In this area and along the Exposures of two-mica granite of Dawley west-facing slope of this ridge to about Canyon to the west. the 7300-foot elevation, slabs of contact- Cross Overland Creek. metamorphosed Cambrian wall rocks Humboldt Peak (11,020 ft [3,361 m]) on are exposed within a sheeted zone of the the skyline at 11 o'clock. Southeast East pluton margin. Wall-rock slabs are sepa- Humboldt Range at 10 o'clock. rated by dikes and sills of leucocratic Road to the abandoned Battle Creek phases considered part of the granodior- mines on left. This road also serves as itic suite of the Harrison Pass. The wall- a public access road to the Humboldt rock slabs consist of carbonaceous argillite, National Forest. ph~llite,impure quartzite, and calc-sili- Ruby Valley Fire Department Station #2 cate rocks. At this locality, the pluton- on left. Lake Peak (10,922 ft [3,331 m]) at wall-rock contact is discordant, whereas 10 o'clock. by looking south across Road Canyon Ruby Valley school on left, built in 1962. you can see the transition to a concor- Another view of Lake Peak northwest of dant roof zone. the school. The contact between the Harrison Pass Pavement begins. pluton and metasedimentary wall rocks Massive exposures of sillimanite-grade is irregular, but is approximately coinci- metaquartzite at 11 o'clock. Joe Billy Basin dent with the edge of the coniferous for- at 10 o'clock. est on this ridge as well as on the ridge End of Nevada 767, turn right onto to the south. The contact consists of both Nevada 229 and proceed eastward. This steep- and shallow-dipping segments and is the end of the road log for DAY FOUR. SNOKE, ET AL.: RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA 269 Our present location is the same as mile- ceded mileage point 70.2 on DAY TWO age point 70.2 on DAY TWO of the trip as we now drive toward U. S. 93, Wells, but today we approached this locality and eventually Salt Lake City, the site of from the south rather than from the east the 1997 Annual Meeting of The Geo- as we did on DAY TWO. Thus, you can logical Society of America. refer to the road log material that pre- Grand Tour-Part 2: Petrogenesis and thermal evolution of deep continental-crus t: the record from the East Humboldt Range, Nevada

ALLEN J. MCGREW Department of Geology, The University of Dayton, Dayton, Ohio 45469-2364

MARK T. PETERS Woodward-Clyde Federal Services, 11 80 Town Center Drive, Las Vegas, Nevada 89134

INTRODUCTION crustal basement provinces of Archean and Proterozoic age, respectively (Fig. 1). The northern part of the East Humboldt Range, Nevada, The structural architecture of the northern East Hum- provides a rare opportunity to explore the petrogenetic boldt Range is dominated by two kilometer-scale features: environment of deep levels in the middle crust during a large, southward-closing, basement-cored recumbent both large-scale Mesozoic contraction and Cenozoic re- fold known as the Winchell Lake fold-nappe, and the > 1- gional extension. On this segment of the field trip, we will kilometer thick mylonitic shear zone referred to above explore evidence bearing on the character of the meta- (Figs. 10 and 11) (Snoke and Lush, 1984; McGrew, 1992). morphic and magmatic history of this terrane, and attempt We present evidence that the Winchell Lake fold-nappe to link these constraints to the rheology and tectonic evo- was emplaced originally during the Late Cretaceous but lution of the middle crust during the Mesozoic and was later transposed during Tertiary mylonitic shearing. Cenozoic. An Archean orthogneiss occupies the core of the Winchell Lake fold-nappe and is surrounded by probable Paleopro- TECTONIC SETTING terozoic paragneiss (Lush et al., 1988). Folded around this The northern East Humboldt Range forms the north- Precambrian gneiss complex, and separated from it by an ernmost and perhaps the deepest seated part of the foot- inferred prefolding, premetamorphic fault, is a metasedi- wall of the Ruby Mountains-East Humboldt Range meta- mentary sequence of quartzite, schist, and marble that morphic core complex (Fig. l).The western flank of the probably correlates with the Neoproterozoic to Mississip- range forms a dip-slope on a gently inclined, normal- pian (?) miogeoclinal sequence of the eastern Great Basin. sense mylonitic shear zone at least 1 km thick, whereas The inferred Paleozoic marble sequence repeats twice the eastern flank of the range is bounded by a younger, more at depth beneath the Winchell Lake fold-nappe, with steep east-dipping range-front normal fault that cuts through a thick sequence of probable Neoproterozoic paragneiss the metamorphic core complex to expose a spectacular intervening (see fig. 7B in Camilleri et al., this volume) natural cross-section (Figs. 10 and 11).Like other parts of (McGrew, 1992; Peters et al., 1992; Wickham and Peters, the Ruby Mountains-East Humboldt Range, the northern 1993). Inferred premetamorphic tectonic contacts bound East Humboldt Range records a complicated, polyphase each major package of rocks, and a thick sheet of horn- history beginning with tectonic deep burial before the blende-biotite quartz dioritic orthogneiss dated at 40 f 3 Late Cretaceous followed by deep-crustal tectonism dur- Ma (U-Pb zircon, Wright and Snoke, 1993, RM-19) cuts ing the Late Cretaceous to Oligocene and tectonic exhuma- gently across the various allochthons, including the tec- tion during Oligocene to early Miocene time. An impor- tonic slide at the base of the Winchell Lake fold-nappe. tant contrast between the northern East Humboldt Range Numerous intrusions of biotite monzogranitic orthogneiss and other parts of the Ruby Mountains-East Humboldt which are part of a widespread 29-Ma suite of dikes and Range is that it exposes the only known outcrops of sheets also cut the various allochthons, but are partially in- Archean rocks in the State of Nevada. Based on U-Pb geo- volved in folding and extensively overprinted by the mylo- chronology and Sr, Nd, and Pb isotopic geochemistry, nitic deformation (McGrew, 1992; Wright and Snoke, 1993). Wright and Snoke (1993) argue that the East Humboldt Taken together, the East Humboldt Range forms a stacked Range and Ruby Mountains are underlain by contrasting sequence of allochthons assembled during a complex, MCGREW AND PETERS: EAST HUMBOLDT RANGE, NEVADA-PART 2 271 polyphase tectonic history. A portion of the same stacked ably record tectonic burial in the Late Jurassic and/or structural sequence is exposed east of the northern East Early Cretaceous (Hodges et al., 1992; Snoke, 1992; Humboldt Range in the "Clover Hill" area (Snoke, 1992). Camilleri and Chamberlain, 1997). The characteristic peak metamorphic assemblage in

PRESSURE-TEMPERATURE HISTORY pelitic schists consists of biotite + sillimanite + garnet- + quartz plagioclase f K-feldspar f muscovite f chlorite Developing a pressure-temperature-time history (PTt + f rutile f ilmenite. In a few samples from deep structural path) for the East Humboldt Range depends critically on levels, sillimanite and metamorphic potassium feldspar integrating quantitative thermobarometry with available coexist in rocks that are nearly devoid of muscovite, sug- cooling age constraints from radiometric dating and with gesting that peak metamorphism may have reached the relative age constraints developed from interpretive pet- second sillimanite isograd. As noted above, much of the rography and careful field observation. Below we first dis- sillimanite shows textural relationships that are pre- or cuss constraints on the pressure-temperature path based early synkinematic relative to emplacement of the Winchell on traditional petrography, field relationships, and quanti- Lake fold-nappe, and therefore pre-lunematic with respect tative thermobarometry, and then we integrate time-tem- to the later extensional shearing. For example, mats of silli- perature constraints developed from radiometric dating in manite are folded in the hinge zone of the Winchell Lake order to synthesize a PTt-path for the northern East Hum- fold-nappe, and in the mylonitic zone bundles of silliman- boldt Range from the Late Cretaceous to early Miocene. ite are commonly boudinaged, deflected by extensional crenulation cleavage, or cut by microfractures oriented Petrographic and thermobarometric constraints perpendicular to Tertiary stretching lineation. Below we The northern East Humboldt Range exposes a diverse argue that this initial phase of sillimanite growth occurred suite of metamorphic rocks, including leucogranitic to synchronously with fold-nappe emplacement (M,) and dioritic orthogneiss, amphibolite, calcite and dolomite mar- was Late Cretaceous. Nevertheless, textural relationships ble, calc-silicate paragneiss, pelitic schist, and quartzite. commonly record more than one phase of sillimanite The strongest constraints on the PTt history of the north- growth, and the final phase of sillimanite growth (M,+l) ern East Humboldt Range come from pelitic schists and probably occurred during Oligocene extensional deforma- to a lesser extent from amphibolites and calc-silicate tion. For example, fine-grained intergrowths of dynamically paragneisses. For convenience, in the discussion below recrystallized quartz, fibrolite, and biotite occur along we will adopt emplacement of the Winchell Lake fold- some extensional shear bands. nappe as a benchmark for notation of relative age relation- Many shear band fabrics and extensional microfrac- ships, designated by the subscript "n." Thus, M, refer to tures also contain relatively late-stage minerals (Mn+2) metamorphism inferred to be synchronous with fold- such as chlorite and muscovite, demonstrating that mylo- nappe emplacement whereas MnP1 refers to metamorphic nitic deformation continued through a range of progres- mineral growth predating fold-nappe emplacement, and sively lower grade conditions during extensional exhuma- Mn+l refers to metamorphic mineral growth postdating tion. Although muscovite occurs at all structural levels, fold-nappe emplacement. overprinting of earlier fabrics and assemblages by coarse, In general, metamorphic mineral assemblages show lit- patchy muscovite becomes increasingly conspicuous at tle geographic variation throughout the northern East higher levels in the mylonitic shear zone. Humboldt Range. However, the oldest phase in the meta- The assemblages described above are amenable to quan- morphic history of the northern East Humboldt Range is titative thermobarometry based on electron microprobe preserved only on the upper limb of the Winchell Lake analysis using the Thermocalc computer program of fold-nappe where scarce subassemblages of kyanite f Powell and Holland (1988) and the internally consistent staurolite + biotite persist locally as inclusion suites in thermodynamic data set of Holland and Powell (1990). P-T garnet. Kyanite also survives locally in the matrix of some estimates obtained by this method define a linear trend pelitic schists where it is mantled or pseudomorphosed by from approximately 8.7 kb, 790°C to 2 kb, 540°C (Fig. 20) later sillimanite. At present, the only constraint on the age (Hurlow et al., 1991; Hodges et al., 1992; McGrew and of this kyanite-bearing subassemblage is that it must pre- Peters, unpublished data). We believe that this steeply date formation of the Winchell Lake fold-nappe because inclined P-T trend records decompressional metamor- the sillimanite that replaces it is itself pre- to early synkine- phism that accompanied exhumation of the East Humboldt matic with fold-nappe emplacement. However, by analogy Range from the Late Cretaceous to late Oligocene (see with similar assemblages in the Wood Hills and Clover below). The samples yielding the lowest P-T estimates Hill to the east, we suggest that the kyanite-bearing assem- (24.2 kb, 540°C-650°C) include minerals such as mus- blages (M,l) in the northern East Humboldt Range prob- covite that grew ~ynkinematicall~with extensional shear- 272 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I ing as part of the equilibrium assemblage (Hurlow et al., cally with the fold-nappe. It belongs to a suite of leuco- 1991; Hodges et al., 1992; McGrew and Peters, unpub- granitic bodies that are strongly associated with a rusty- lished data). Accordingly, these results represent the best weathering, graphite-rich pelitic schist and paragneiss estimate for P-T conditions accompanying mylonitic unit that forms a continuous, distinctive layer within the deformation. In addition, because the closure tempera- inferred Paleozoic miogeoclinal sequence on the upper ture range for 40~r/39Ardating of hornblende (480°C- limb of the Winchell Lake fold-nappe (McGrew, 1992). As 580°C) overlaps these P-T estimates, cooling ages of this unit is traced into the hinge zone of the Winchell 26-32 Ma in the northern Ruby Mountains (Dallmeyer et Lake fold-nappe, the volume percent of leucogranite al., 1986) and 2936 Ma at deep structural levels in the increases gradationally but dramatically, from <25% to East Humboldt Range (McGrew and Snee, 1994) proba- >65% over a distance of -2 km. In addition, relict kyan- bly record the approximate timing of this final stage of ite on the upper limb of the Winchell Lake fold-nappe metamorphic equilibration (see below). occurs most commonly in this same unit, but it is com- Metacarbonate and metabasite mineral parageneses are pletely replaced by sillimanite over this same interval. largely compatible with the results from pelitic schists re- The volume percent of leucogranite in the weakly migrna- ported above. For a discussion of calc-silicate phase equi- titic marbles that surround these rocks show no systemat- libria see the section on metamorphic fluids below. Meta- ic variations over this same interval. On the lower limb of basite mineral assemblages most commonly occur in small the fold-nappe this same unit never exceeds 5 m in thick- amphibolite bodies in the Paleoproterozoic and Archean ness, and in many localities it is completely absent or gneiss sequences of Angel Lake near the northern end of exists only as isolated rafts of graphitic, biotite-rich melano- the range. Characteristic assemblages commonly include some suspended in small bodies of leucogranite. Commonly, amphibole + plagioclase f biotite f garnet f quartz f the leucogranite can be traced into the country rock clinopyroxene f ilmenite f sphene f magnetite f rutile f where it forms small pods and seams intricately interdigi- apatite f chlorite. Many amphibolites contain moderately tated with selvages of melanosome enriched in biotite, to severely resorbed garnet porphyroblasts surrounded by iron oxide, and graphite, suggesting an origin by localized a symplectite of plagioclase f hornblende f biotite. This partial melting. In any case, the observation that migrna- classic decompression texture lends additional support to titic layering clearly wraps around the fold hinge in com- the decompressional PTt-path outlined above. Depending bination with the fact that leucogranite isopleths cut the on fluid activities, P-T results for the above assemblage Winchell Lake fold-nappe, implies that leucogranite gen- range from 6.1 kb, 470°C for a~20= 0.1 to 8.4 kb, 600°C eration and emplacement was synkinematic to fold-nappe for aHzo = 1.0 (Fig. 20) (McGrew and Peters, unpublished emplacement. Therefore, the Late Cretaceous U-Pb zir- data). The temperature range for this sample corresponds con age of 70-90 Ma referred to above probably dates quite closely with the closure temperature range for both fold-nappe emplacement and a major syntectonic 40Ar/39Ar dating of hornblende, so an 40Ar/39Ar horn- magmatic-metamorphic event (M,). We suggest that the blende cooling age of 51.0 f 2 Ma on a nearby amphibo- highest P-T results shown on Figure 20 (approximately lite provides a plausible estimate for the timing of equili- 8.7 kb, 790°C) probably date from this time. bration. The lower temperature of this P-T estimate as com- The thermal history of the East Humboldt Range fol- pared with pelitic assemblages raises the possibility that lowing peak metamorphism in the Late Cretaceous depends the amphibolite sample records a cooling event at this mostly on 40~r/39Arand fission-track cooling age con- time that is not seen in the results from the metapelites. straints summarized in Figure 21. Temperature estimates for pelitic assemblages in the northern half of the East Radiometric age constraints and an integrated PTt-path Humboldt Range are all well above the nominal closure temperature range for 40Ar/39Ar dating of hornblende A variety of radiometric age data including U-Pb, (480°C-580°C), and thus 40Ar/39Ar hornblende cooling 40Ar/39Ar,and fission-track data provide critical constraints ages bracket the age of metamorphism. For samples col- on the timing of metamorphism in the East Humboldt lected above 9600 ft (2930 m) this age limit is defined by Range. The key date constraining the timing of peak meta- hornblende cooling ages between 50 and 63 Ma (McGrew morphism (M,) comes from a preliminary U-Pb, zircon and Snee, 1994). However, at lower elevations final cool- age of 70-90 Ma (J. E. Wright, personal comm., 1992) on ing through 40Ar/39Ar hornblende closure temperatures a small body of pegmatitic leucogranite collected in the did not occur until the early Oligocene (30-36 Ma), result- hinge zone of the Winchell Lake fold-nappe. The leuco- ing in a much broader age bracket. Samples from higher granite in question is clearly folded around the nose of the structural levels yield P-T estimates in the range 6.4 to Winchell Lake fold-nappe, but larger-scale field relation- 8.7 kb, 635°C to 800°C (for aH2~= 1.0), whereas those ships strongly suggest that it was emplaced synkinemati- from deeper structural levels range from 4.5 to 9.1 kb and MCGREW AND PETERS: EAST HUMBOLDT RANGE, NEVADA-PART 2 PT Results, East Humboldt Range

Metapelites (McGrew and Peters, unpublished) A Metabasite (Peters, unpublished)

a Clover Hill (Hodges et al., 1992)

o N. Ruby Mts. (Hodges et al., 1992)

SW EHR (Hurlow et al.. 1991)

0.0 0 100 200 300 400 500 600 700 800 900 lo00 Temperature (OC)

Figure 20. Synoptic diagram of P-T estimates from the central and northern East Humboldt Range determined by the authors using the updated, internally consistent thermodynamic data set of Holland and Powell (1990) and the Thermocalc computer program of Powell and Holland (1988) combined with previously published P-T results from elsewhere in the Ruby-East Humboldt metamorphic complex (Hurlow et al., 1991; Hodges et al., 1992). The large shaded arrow represents the general PTt-path inferred for the East Humboldt Range for late Mesozoic time to -20 Ma. The light arrows represent P-T-paths based on Gibbs Method modeling of garnet zoning profiles reported in Hodges et al. (1992). Shaded lines provide reference geotherms of lS°C/km, 25OC/km, 40°C/km, and 75OClkrn. The modern day geothermal gradient in the Basin and Range province is approximately 25OC/km, whereas the Battle Mountain heat flow high is characterized by geothermal gradients on the order of 40-75OClkm (Lachenbruch and Sass, 1978). Stability fields for the A12Si05 polymorphs are also provided for reference (also calculated using the data of Holland and Powell [1990] and Powell and Holland [1988]).

640°C to 765°C (McGrew and Peters, unpublished data). this history are that the northern East Humboldt Range The broader range in pressures recorded at deep levels must have experienced 220 km of unroofing before it suggests that the lower pressure samples were probably experienced dramatic cooling, and that over 7 km (>2 kb) reset during reheating accompanying Oligocene exten- of this unroofing may have occurred before middle Eocene sional unroofing. In addition, one sample from the central time. This lends credence to previously published sugges- part of the range yields a much lower pressure estimate of tions that extensional unroofing in northeastern Nevada 2 kb, 540°C, and exhibits textural relationships that began as early as Eocene or even Late Cretaceous time strongly imply that it reequilibrated during extensional (Hodges et al., 1992; McGrew and Snee, 1994; Camilleri shearing. This sample was collected at approximately the and Chamberlain, 1997). level of the transition between the younger and older Following cooling through 40Ar/3g~rhornblende clo- hornblende cooling ages, so an Oligocene age for final sure temperatures (48O0C-580"C), the cooling history of equilibration is plausible (Hurlow et al., 1991; McGrew the northern East Humboldt Range is constrained mostly and Peters, unpublished data). Important implications of by 40~r/3~Armica and fission-track apatite cooling ages 274 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I NEHR, T-t curve show how these cooling age relationships could be ex- plained by progressive rolling-hinge style unroofing of the footwall of a west-northwest-directed normal fault system.

METAMORPHIC FLUIDS: GEOCHEMISTRY, EVOLUTION, AND RELATIONSHIP TO Srrucwal Levels LEUCOGRANITE PETROGENESIS In this section we outline the relationship between fluid geochemistry and the metamorphic and magmatic evolution of the East Humboldt Range, with particular attention to the origin and significance of the abundant leucogranitic bodies that occur throughout the range. At Yeanr Before F'resent (Ma) relatively high structural levels throughout the northern half of the range, variations in leucogranite percentage are Figure 21. A proposed time-temperature history for the northern primarily controlled by the host lithology. Marbles and East Humboldt Range from the Late Cretaceous to Present, inte- pure quartzites commonly contain < 10% leucogranite, grating the 40Ar/39Arresults of McGrew and Snee (1994) with whereas pelitic or semi-pelitic units, impure metapsam- older thennochronometric results (Dallrneyer et al., 1986; Dokka mitic rocks, and quartzo-feldspathic gneiss contain vari- et al., 1986). Pressure estimates are also included, based on inte- able amounts of leucogranite, ranging from < 15% to > 75% gration of P-T results reported by McGrew (1992), Peters (19921, by volume (McGrew, 1992). and Hurlow et al. (1991) (see also Fig. 20). The solid curve repre- sents relatively shallow structural levels (and thus also a lower At deeper structural levels, beneath the hornblende- temperature bound for relatively deep structural levels), whereas biotite quartz diorite sill, leucogranite proportions in the the dotted curve represents an upper temperature bound for rela- paragneiss sequence of Lizzies Basin increase systemati- tively deep structural levels. Heavy arrows denote important cally and dramatically, from values generally 65% (locally >go%) at eleva- dioritic orthogneiss sheet at 40 _+ 3 Ma and eruption of andesitic tions below 9000 ft. (2745 m) (see Fig. 7B in Camilleri et to rhyolitic tuffs; and ro) intrusion of biotite monzogranitic dikes al., this volume) (McGrew, 1992; Peters and Wickham, and sheets at 29 _+ 0.5 Ma (Wright and Snoke, 1993). We favor a 1995). Textural relationships, such as fine-scale interdigi- distinct thermal pulse at the time of quartz dioritic intrusion and tations of leucosome and melanosome, suggest that some widespread ignimbrite eruption (-40 Ma), but we cannot rule out rocks represent in situ partial melts. However, the sheer the alternative interpretation of gradual slow cooling from -50 Ma to -30 Ma. volume of leucogranite suggests that a sizable fraction must have been externally-derived (Peters and Wickham, 1995). We refer to this sequence of extremely migmatitic rocks informally as the migrnatite complex of Lizzies Basin, (Dallmeyer et al., 1986; Dokka et al., 1986; McGrew and and we take the 65% leucogranite isopleth as its upper Snee, 1994). In any given area 40Ar/39Armuscovite and bio- boundary, although it should be remembered that this tite cooling ages are identical to each other and to fission- boundary is actually gradational across a narrow interval. track apatite cooling ages within analytical uncertainties, In addition, large, finger-like bodies of leucogranite origi- thus implying cooling rates on the order of 100°C/m.y. in nating from the Lizzies Basin migmatite complex locally the northern part of the range during the late Oligocene penetrate upward into the overlying rocks. No age dates and early Miocene (Fig. 21) (Dokka et al., 1986; McGrew currently exist for the Lizzies Basin leucogranite suite, and Snee, 1994). In addition, 40Ar/39Ar mica cooling ages but we note that some of the leucogranitic bodies cut the document diachronous cooling of the East Humboldt Range 40-Ma quartz dioritic sill and 29-Ma monzogranitic sheets. through 40Ar/3gAr biotite closure temperatures (250°C- The roof of the migmatite complex of Lizzies Basin also 300°C), from -32 Ma in the south to -21 Ma in the north coincides with an important oxygen isotope discontinuity (Dallmeyer et al., 1986; McGrew and Snee, 1994). More- (the contact between the lower zone and the upper zone over, geographic comparison of fission-track and mica in Figure 9) and with the nearly complete replacement of cooling ages indicates that the southeastern part of the marble by calc-silicate rock below (Wickham and Peters, range had cooled through fission-track apatite closure 1990; McGrew, 1992; Peters and Wickham, 1995). The temperatures (-110°C) while areas as little as 7 km to the metasedimentary rocks of the lower zone (i.e., the mig- west had not yet cooled through 40Ar/39Ar muscovite clo- matite complex) exhibit uniformly low 6l8O values imply- sure temperatures (300°C350"C). McGrew and Snee (1994) ing equilibration with a large, isotopically light fluid reser- MCGREW AND PETERS: EAST HUMBOLDT RANGE, NEVADA-PART 2 275 voir (see also Wickham and Peters, 1992; Wickham et al., structural levels cooled through the closure temperature 1993; Bickle et al., 1995). By contrast, the rocks of the range for 40Ar/3gAr dating of hornblende (480°C-580"C) upper zone show higher and generally more variable 6180 by -50 Ma, but rocks at deeper structural levels did not values (Wickham and Peters, 1990; Peters and Wickham, finally close to argon diffusion in hornblende until early 1995). Furthermore, all calc-silicate samples in the Lizzies Oligocene time (30-36 Ma). During this time period, Basin migmatite complex show extensive late-stage over- metamorphism at conditions of at least 2.0-4.2 kb, 540°C- growths of amphibole & epidote f garnet, suggesting a 650°C developed synkinematically with the early stages of late-stage, high-temperature, H20-rich fluid infiltration west-northwest-directed extensional mylonitic deforma- event (Peters and Wickham, 1994). In addition, where this tion. However, fault rocks in the northern East Humboldt late-stage assemblage is well-developed at shallower struc- Range show a continuous ductile through brittle deforma- tural levels it is generally associated with nearby leuco- tional evolution. In addition, 40Ar/39Ar mica and fission- granites, further documenting the relationship between track cooling age constraints indicate that the East Hum- metasomatism and magmatism (Peters and Wickharn, 1994; boldt Range was ultimately exhumed by diachronous 1995). Finally, this late-stage assemblage occurs locally in unroofing in the footwall of an evolving, west-rooted nor- extensional shear bands, pull-apart zones, or veins orthog- mal fault system from the late early Oligocene to early onal to the extensional stretching lineation, suggesting a Miocene (Mueller and Snoke 1993a). synextensional origin for both the migmatite complex of Secondary subassemblages of amphibole + epidote + Lizzies Basin and the final episode of high-grade meta- garnet in calc-silicate rocks probably also grew during morphism. synextensional Oligocene metamorphism, recording inten- sive metasomatism by water-rich brines that probably SUMMARY AND CONCLUSIONS came from crystallization of abundant leucogranitic bodies at deep structural levels. During this late stage, H20-rich The northern East Humboldt Range preserves a com- fluid infiltration event metamorphic conditions probably plex, evolving record of deep-crustal metamorphism, fluid evolved from early high temperature (600°C-750°C) to infiltration, and anatexis during large-scale contractional later, lower temperature conditions (~525°C).Pervasive and extensional tectonism from the late Mesozoic to early fluid infiltration at deep structural levels produced whole- Miocene. Relict kyanite staurolite assemblages record + sale reequilibration and homogenization of the oxygen tectonic deep burial of the northern East Humboldt Range isotope systematics to uniformly low values of 6180. We during the earliest stages of this history before the Late suggest that the leucogranitic melts acted as a proxy, or Cretaceous. Peak metamorphism occurred in the Late Cre- vector of transport for fluids that ultimately derived from taceous synkinematically with emplacement of a large, injection of mantle-derived basaltic magma at lower southward-closing recumbent fold-nappe, the Winchell crustal levels. In contrast, rocks at higher structural levels Lake nappe. Peak metamorphism attained P-T conditions show higher and more variable values of 6180 that reflect as high as 8.7 kb, 790°C, approaching or exceeding the the original sedimentary isotope systematics. If these con- second sillimanite isograd, and resulting in the generation clusions are true, then much of the deep crust of north- and emplacement of abundant leucogranitic bodies. Primary eastern Nevada may have been buffered at melting condi- assemblages in calc-silicate rocks may date from this time tions during the primary phases of extensional tectonism and indicate equilibration with an internally buffered(?), and metamorphic core complex development. Consequently, C02-rich pore fluid. we envision a deep-crustal environment that was rheolog- Following the Late Cretaceous the rocks experienced ically weak and subject to asthenosphere-like flow on geo- decompressional metamorphism through much of the logically reasonable time-scales. Tertiaw,.. with decomvression of at least 2 kb (>7 km of unroofing) before approximately 50 Ma. ~ocksat high Grand Tour-Part 3: Geology and petrology of Cretaceous and Tertiary granitic rocks, Lamoille Canyon, Ruby Mountains, Nevada

SANG-YUN LEE CALVIN G. BARNES Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053

INTRODUCTION banding is common. This rock unit is composed of medi- um- to coarse-grained anhedral quartz, alkali feldspar, and Deep structural levels of the Ruby Mountains core plagioclase. Biotite and muscovite occur as subhedral complex are exposed in the Lamoille Canyon area (Figs. 1 grains in a groundmass of alkali feldspar, quartz, and pla- and 16). Howard (1980) showed that the proportion of gioclase. Anhedral garnet is locally present, as are rare sil- granitic rocks increases structurally downward, and cross- limanite grains. The gneissic texture that is apparent in sections by MacCready et al. (in press) show that several the field is less clear in most thin sections. granitic- units in the Ruby Mountains were deformed dur- ing development of the core complex. Therefore, Lamoille Canyon is an excellent locality to study the structural and Sillimanite-bearing two-mica granitic gneiss petrogenetic relationships between core complex forma- (Cretaceous) tion and granitic magmatism. This unit, qualitatively the most abundant in the area, Here, we report the results of field studies and prelimi- is coarse- to very coarse-grained (commonly pegmatitic) nary laboratory analysis of granitic rocks collected pre- sillimanite-bearing two-mica granitic gneiss. It grades dominantly from upper Lamoille Canyon. The field data into muscovite-bearing pegmatitic granitic gneiss which show that consistent cross-cuting relationships are pre- locally has the appearance of leucosomes in host two-mica sent, which permits identification of distinct, mappable pegmatitic gneiss. On outcrop scales, the unit forms net- granitic rock units. The geochemical data indicate that works of sills and dikes, or irregular bodies. Metasedi- many of these units are compositionally distinct and that mentary enclaves are common. more than a single petrogenetic mechanism is necessary Biotite is more abundant than muscovite, except in the to explain their origins. pegmatitic rocks. Accessory garnet is sparse. Where pres- ent, sillimanite most commonly occurs as needles and IGNEOUS ROCKS OF THE fibrolitic mats in biotite. Perthitic alkali feldspar crystals LAMOILLE CANYON AREA form large anhedral plates. Plagioclase is commonly serici- The intrusive sequence was determined on the basis of tized and myrmekite is also common. cross-cutting relationships, an example of which is shown in Figure 22. From oldest to youngest, the sequence is Age of the gneissic units equigranular muscovite-biotite granitic gneiss (Kgn), silli- Monazite from a sillimanite-bearing, pegmatitic mus- manite-bearing pegmatitic two-mica granitic gneiss (Kt), a covite leucogranite collected near the Terraces day-use suite of biotite monzogranites (Tbm), fine-grained biotite area in upper Lamoille Canyon (STOP 3-3) yielded a Late tonalite dikes (Tdd), and undeformed biotite-muscovite Cretaceous age (data cited in Wright and Snoke, 1993). granitic pegmatite (Tp). We view this rock as correlative with the sillimanite-bear- ing two-mica granitic gneiss. At all locations where cross- Equigranular muscovite-biotite granitic gneiss cutting relations were observed, this unit cuts the equi- (Cretaceous or Jurassic) granular muscovite-biotite granitic gneiss (e.g., Fig. 19C). This unit consists of medium- to coarse-grained, equi- Therefore, the latter unit is Late Cretaceous or older in granular muscovite-biotite monzogranitic and leucogranitic age. The close association of these two units in the field, gneiss. Foliation is typically well developed and gneissic and locally indistinct contacts, suggest that the two share

278 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

1.35 similar ages. As shown below, they also share similar geo- 0 K gneiss T bio ton I chemical features, so both are thought to be Late Creta- K peg gn 0 other (Tertiary) P T bio mzgr + T pegs ceous in age. 1.25 I To + ; v Biotite monzogranite suite (Oligocene) . B PT A This unit consists of massive to foliated, mehum-grained, v 4~ A equigranular biotite monzogranitic dikes and sheets, some of which form interconnected networks. These intrusions A ~~~~ cut earlier granitic gneisses as well as foliation, folds, and . , v lithologic layering in the host rocks (Howard, 1980; Wright 'I and Snoke, 1993). Biotite-rich schlieren were observed locally, particularly near the base of a shallowly-dipping sheet in upper Lamoille Canyon. Figure 23. AICNK (molar A1203 divided by the sum of CaO, In thin section, quartz, alkali feldspar, and plagioclase Na20, and K20) plotted against Si02for Cretaceous and Tertiary are subhedral to anhedral. Poikilitic alkali feldspar crystals granitic rocks from the Lamoille Canyon area. enclose sericitized plagioclase and irregularly-shaped, fine-grained quartz. Biotite is the predominant varietal mineral, but blue-green hornblende is sparsely present. alkali feldspar, zircon, and minute prismatic apatite are Accessory minerals are apatite, allanite, zircon, and accessory minerals. opaque oxides. Muscovite locally replaces plagioclase and sericite is common, as is myrmekite. The biotite monzo- Pegmatitic dikes (Oligocene?) granite intrusions are cut by leucocratic muscovite-biotite monzogranitic dikes. Many of these dikes are composite, The youngest granitic intrusions in Lamoille Canyon are undeformed muscovite- and biotite-muscovite peg- with rims of pegmatite and cores of medium-grained leu- matitic granite. They form parallel-walled dikes that range cocratic monzogranite. The leucocratic dikes are petro- up to at least 5 m wide and are locally associated with graphically similar to the biotite monzogranite, but mag- aplitic garnet-bearing muscovite leucogranite. Alkali feld- matic muscovite and sparse garnet are present. A com- spar crystals range in diameter to at least 30 cm and plete compositional gradation exists between the biotite graphic texture is common. These Tertiary pegmatitic monzogranite and leucocratic monzogranite (see below); dikes are distinct from the Cretaceous ones in their lack of therefore, we treat them as the "biotite monzogranite deformation, predominance of muscovite over biotite, lack suite." of sillimanite, and more common occurrence of garnet. Zircon fractions from biotite monzogranite suite dikes Where these pegmatites cut calc-silicate wall rocks, gar- yielded discordant U-Pb ages with lower intercepts of net-idocrase-diopside skarns are common. -29 Ma (Wright and Snoke, 1993; MacCready et al., in press). These ages are similar to ages of biotite monzo- Other analyzed granitic rocks: gneiss of Seitz Canyon granitic dikes elsewhere in the Ruby Mountains and East (Tertiary, Cretaceous, or Jurassic?) and gneiss of Thorpe Humboldt Range and were interpreted to be crystalliza- Creek (Eocene) tion ages (Wright and Snoke, 1993). The gneiss of Seitz Canyon crops out in the core of the Tonalitic dikes (Oligocene?) Lamoille Canyon fold-nappe (Howard, 1980; see also mile- age point 62.85 on DAY THREE). It is a medium- to Fine-grained biotite tonalitic to quartz dioritic dikes coarse-grained biotite granodioritic orthogneiss with intrude both of the gneissic units. These dark-colored traces of garnet and muscovite. A cross-cutting medium- dikes also locally intrude rocks of the Oligocene biotite grained biotite monzogranitic gneiss is tentatively consid- monzogranite suite. However, leucogranitic two-mica mon- ered to be part of the Seitz gneiss. Muscovite and euhe- zogranites of this suite cut the tonalitic dikes. Nevertheless, dral garnet are varietal minerals; accessory minerals are the intrusive relationships of this unit are commonly apatite and zircon. Mymekitic intergrowths occur next to unclear because it crops out as irregular, discontinuous large poikilitic alkali feldspar. dikes (Fig. 19A). Most samples are fine- to medium-grained, The orthogneiss of Thorpe Creek is a medium-grained, weakly foliated, assemblages of anhedral to subhedral pla- garnet-biotite-muscovite leucogranitic gneiss that is local- gioclase, quartz, and biotite. Blue-green hornblende is ly sillimanite-bearing (see mileage point 59.9 on DAY present in some of the most mafic samples, and muscovite, THREE). Wright and Snoke (1993) reported a U-Pb (mona- LEE AND BARNES: LAMOILLE CANYON, RUBY MOUNTAINS, NEVADA-PART 3 279 500 I GEOCHEMICAL DATA Most of the data used in this report were obtained by ICP-AES analysis for major and trace elements at Texas Tech University. Rb was determined by flame emission and the rare-earth elements (REE), Th, U, Ta, and Hf by INAA at Oregon State University as part of the DOE Re- actor Sharing Program. Additional data are from the U.S. Geological Survey (K. A. Howard, written comm., 1996). Granitic rocks from the Lamoille Canyon area are pre- dominantly peraluminous. That is, A/CNK, molar A1203/ (CaO + NazO + K20)>l (Fig. 23). Most samples are sili- ca rich, in accord with their granitic nature, but it is note- worthy that less evolved compositions are present, partic- Kgnelss ularly among the biotite monzogranite suite. Kpeggn 120 A Tblomzgr Cretaceous(?) granitic and pegmatitic gneisses ldO r Tbloton t A These gneisses occupy a rather narrow range of major A + Tpegs element contents. Although their compositions tend to overlap, the muscovite-biotite granitic gneiss has higher 60 contents of total iron (Fe203*) and MgO, and Na20/K20 ratios >1, compared to the pegmatitic gneiss. The latter unit is also characterized by higher A1203, K20, and AICNK>1.12, and by its low Na20/K20 ratios (

Tertiary(?) tonalitic, granitic, and pegmatitic rocks The tonalitic samples contain relatively low silica abun- dances (<68 wt% Si02; Fig. 23) and are characterized by relatively high A1203 (most >17 wt.%), Fe203 (>5.2 wt.%), MgO (>1.1 wt.%) and CaO (>2.7 wt.%) contents. In spite of their relatively mafic nature, their AICNK values are Figure 24. Zr, (Nb+Y), and Sr abundances plotted against SiOz larger than 1.1. These values reflect the high proportions for Cretaceous and Tertiary granitic rocks in the Larnoille Can- of biotite in the tonalitic rocks. The biotite monzogranite yon area. Data points for Thorpe Creek and Seitz Creek gneisses suite displays the widest range of elemental contents (Fig. labelled with "T"and "S, " respectively. 26). Most elemental abundances decrease with increasing SO2, but K20, Na20, and Rb are exceptions. The lowest zite) age of -39 Ma. MacCready et al. (in press) have ana- silica samples (~70wt.% Si02) are broadly similar in ele- lyzed additional monazite samples of the orthogneiss of mental contents to the most siliceous tonalites. Leuco- Thorpe Creek that yield dates that range from 36.1 to 37.9 cratic members of this suite are all high silica (>74 wt.% Ma. Both Wright and Snoke (1993) and MacCready et al. SO2). (in press) conclude that these dates indicate an Eocene In comparison with the Cretaceous(?) gneisses, the crystallization age. tonalites and biotite monzogranite suite are characterized 280 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

monzogranite suite

K gneisses

Figure 25. Plots of rare earth element abundances. Thefield with horizontal ruling is for Cretaceous(?) monzogranitic and peg- matitic gneisses. The field with vertical ruling is for the biotite monzogranite suite. - by higher, and relatively variable, contents of Zr, and especially Y and Nb (Fig. 24). Zr abundances tend to decrease with increasing Si02, but Y and Nb behave incompatibly. This relationship is best seen when Rb is used as a differentiation index (Fig. 27) because Si02 val- ues vary only slightly among the leucocratic granites. REE patterns for samples of the biotite monzogranite suite (Fig. 25) are characterized by a "dog-leg," with near- ly flat HREE patterns and HREE abundances -7-15X chondrites. All of these samples have negative Eu anom- alies. Resultant LaN/LuNratios are in the range of 6 to 36. In general, light REE abundances decrease with differen- tiation, whereas heavy REE abundances increase. This type of variation suggests that REE abundances are con- trolled by light REE-rich, accessory phases such as allan- ite and/or monazite. The late pegmatitic dikes exhibit the lowest Fe203*, MgO, CaO, TiOz contents. The Na20/K20 ratios are vari- able for this rock unit. All are alumina oversaturated; four of the five analyzed samples have A/CNK>1.15. Abun- dances of Sr and Ba are quite low (<59 ppm and <52 ppm, respectively), as are Zr contents (<50 ppm). Samples of the gneiss of Thorpe Creek are strongly peraluminous (Fig. 23) and the single sample analyzed for trace elements is broadly similar to the Cretaceous gneiss- Figure 26. Major oxides and trace element concentrations plotted es in terms of Sr and Nb + Y contents (Fig. 24). Except against SiOz for the Tertiary tonalitic dikes and granitic rocks for high Sr contents, the few analyses of the gneiss of from Lamoille Canyon. "S" and "T" label Seitz Creek and Thorpe Seitz Creek are similar to the biotite monzogranites. Creek gneisses, respecti~el~. LEE AND BARNES: LAMOILLE CANYON, RUBY MOUNTAINS, NEVADA-PART 3 281

PETROGENETIC CONSIDERATIONS 00 A T bio mzgr 1 Cretaceous(?) granitic and pegmatitic gneisses V T bio ton Limited isotopic data for the Cretaceous pegmatitic gneisses (87Srl86Sr = 0.72609, &Nd = -15.1,Wright and 60 Snoke, 1993; 6180(qtz) = + 13.0, H. Karlsson, unpublished data) combined with their high K20/Na20, and strongly peraluminous nature suggests crustal anatexis of metased- imentary rocks as a likely origin. The markedly depleted heavy REE patterns and low Y contents of these samples indicate that garnet was residual in the source region. Furthermore, Eu anomalies are small or absent among these samples, and Sr contents are relatively high. These features suggest that feldspars were not important resid- ual minerals during the melting event. If one assumes a Figure 27. Nb content plotted against Rb for tonalitic dikes and metasedimentary source, as indicated by the isotopic data, biotite monzogranitic suite. Note the positive correlation between such features are consistent with one of two conditions in the two elements which is an indication that both elements are the source region: partial melting under H20-saturated incompatible in the biotite monzogranite suite. conditions with feldspars melting nearly entirely in early (low-T) melt fractions, or partial melting of an eclogitic assemblage in which feldspars were entirely absent (e.g., the biotite monzogranite suite would represent crustal Martin, 1987). In either case, garnet was residual, which melts. However, our preliminary work suggests that the is indicative of relatively high-pressure anatexis. trend from tonalitic to biotite monzogranitic rocks could Zircon saturation temperatures for these rocks are low be a fractional crystallization trend. Furthermore, trace (639 "C to 749 "C), which is also consistent with relatively element compositions suggest that two distinct composi- high H20 contents. Inasmuch as these temperatures rep- tional populations might exist among the monzogranite resent near-solidus conditions, comparison with experi- suite. Thus, a thorough understanding of the origin of the mental melting studies of haplogranite is appropriate Tertiary granitic and tonalitic rocks will require more (Ebadi and Johannes, 1991) and suggests H20 activity detailed geochemical and isotopic data and modeling. between 0.5 and 1.0. At the present time, we can say that the rock composi- Finally, Late Cretaceous (-83-75 Ma) magmatism is tions do not plot near minimum-melt compositions for broadly associated with crustal thickening during the water-saturated granites in the normative Ab-Or-Qz sys- Sevier orogeny (e.g., Wright and Wooden, 1991; Camilleri tem, but instead cluster near an anhydrous 4 kb, 1000°C and Chamberlain, 1997). The relatively high pressures minimum determined by Steiner et al. (1975). Higher mag- implied by residual garnet in the source region is consis- matic temperatures are consistent with zircon saturation tent with an origin of the Cretaceous(?) granitic gneisses temperatures, which range from 807°C to 867°C. By com- by anatexis of deep-seated crustal rocks during the peak parison to experimentally determined solidus conditions, of Sevier-age metamorphism. zircon saturation temperatures indicate activities of H20 less than 0.35 (Ebadi and Johannes, 1991), consistent with Tertiary tonalitic and monzogranitic rocks the low H20 contents implied by normative compositions. No matter what the origin of the Tertiary granites, their Initial 87Sr/86Sr and &Ndvalues for the biotite monzo- prominent Eu anomalies and compatible behavior of Sr granite suite belong to an array of isotopic data that and Ba (but not Rb) indicate that residual plagioclase and Wright and Snoke (1993) and Wright and Wooden (1991) alkali feldspar played a significant role in differentiation. interpreted as a mixing line between mantle-derived mag- The lack of heavy REE depletion shows that garnet did not. mas and partial melts of Proterozoic basement rocks. In addition, whether the granites formed by fractional crys- Similar explanations were proposed for Tertiary volcanic tallization, partial melting, or mixing, the anticorrelation rocks in the Snake and Egan ranges to the south (Gans et of light and heavy REE contents indicates that one or al., 1989; Feeley and Grunder, 1991; Grunder, 1992). If more accessory minerals controlled REE concentrations. the tonalitic rocks are assumed to be associated with the biotite monzogranite suite, the geochemical data are con- Tectonic discrimination diagrams sistent with a mixed source. In such a case. the tonalitic rocks would represent magmas with higher proportions of For those who appreciate discrimination diagrams for mantle component, whereas the leucogranitic members of granitic rocks, we provide the following. Diagrams that 282 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

1 Nb, Y, etc.), combined with high Fe203*/Mg0 and low ? K gneiss / transition metal concentrations, are consistent with a clas- o K peg gneiss / / sification as A-type granite (Loiselle and Wones, 1979).

+ / 19 Aa However, although A-type granites are commonly thought to be fluorine-rich (e.g., Eby, 1990), normative Ab-Or-Qz compositions of the biotite monzogranite suite lack evi- / dence of fluorine control of melt compositions. 0 / 0 I 0 / 0 "*G I 0 A JA I / Conclusions / I / Granitic rocks in the Lamoille Canyon area can be broad- a T bio mzgr I 1' ORG ly divided into at least two groups: Cretaceous(?) granitic r T bio ton I / and pegmatitic gneisses and Tertiary granitic gneisses and 0 other (Tertiary) I I / monzogranites, tonalites, and related rocks. The youngest + Tpegs I / granitic magmatism in the area is represented by unde- 10 4 :: r formed pegmatitic dikes. The older gneissic rocks are dis- 1 1000 loppm (Y + ~b)loo tinct in their higher concentrations of Sr, their lower con- centrations of Nb, Y, and Zr, their steep REE patterns Figure 28. Discrimination diagram (Pearce et al., 1984). Note the with strongly depleted heavy REE, and their lack of appre- clear distinction between "K" (Cretaceous[?]) granitic gneisses ciable Eu anomalies. These features suggest an origin by and the "T" (Tertiary) biotite monzogranite suite on the basis of crustal melting at conditions where garnet was a residual Y+Nb contents. VAG = oolcanic-arc granites, ORG = ocean- mineral but feldspars were not. Such an origin is consis- ridge granites, WPG = within-plate granites, syn-COLG = syn- collisional granites. tent with deep-seated melting of crust thickened during the Sevier orogeny. Tertiary tonalitic dikes and the biotite monzogranite suite may represent a compositionally ex- utilize Rb, Y, and Nb contents (Fig. 28; Pearce et al., 1984) tended suite of related magmas. Their rather complex imply a volcanic ,arc setting for the Cretaceous(?) mus- trace and major element behaviors can presently be ex- covite-biotite granitic gneiss. However, compositions of plained in several ways, including anatexis of widely vari- the Cretaceous pegmatitic gneiss, which show a geochem- able Proterozoic crust, source mixing among crustal and ical relationship to the older gneissic unit, plot in a scat- mantle reservoirs, and mixing of mantle- and crustally- tered zone among the syn-collisional and volcanic arc derived magmas. In all cases, fractional crystallization is fields. likely to have played a role as well. Therefore, a thorough The Oligocene biotite monzogranite suite plots in the understanding of these granites requires further detailed within-plate granitic field. Its geochemical characteristics, analytical work and modeling. high concentrations of high field strength elements (Zr, Grand Tour-Part 4: Geology and geochemistry of the Harrison Pass pluton, central Ruby Mountains, Nevada

BRADFORD R. BURTON Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

CALVIN G. BARNES TRINA BURLING Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053

JAMES E. WRIGHT Department of Geology and Geophysics, Rice University, Houston, Texas 77005

ABSTRACT Harrison Pass pluton is a late Eocene, composite granitoid body with related mafic and hypabyssal rocks. The pluton was emplaced at upper-crustal levels probably during the initiation of a phase of large-magni- tude crustal extension that resulted in the eventual exhumation of the Ruby Mountains core complex. Post- emplacement tilting facilitated the exposure of a partial cross section of the pluton from a roof zone exposed on the east side of the range to granitic rocks on the west side of the range that were originally about 5 km deeper than the roof zone. The pluton is composed of two distinct suites of granitic rocks. The oldest suite consists of granodiorite and minor, related rock types that occupy the structurally higher eastern half of the pluton. Compositional variation within the weakly peraluminous to metaluminous granodioritic suite can be explained by crystal fractionation and magma-mixing processes. This plutonic suite was the magmatic source of hypabyssal dikes and sills that cut country rocks in the roof zone. Emplacement of the granodioritic suite was accommodated by stoping, wall-rock shortening, and uplift. Slightly younger granitic rocks comprise the monzogranitic suite which underlies much of the western half of the pluton. Three distinct rock types are recognized: two-mica monzogranite which forms a body informally called the "granite of Green Mountain Creek," other two-mica monzogranitic rocks, and biotite monzogranite. These granitoids are peraluminous and chemically, but not isotopically, similar to Jurassic monzogranite exposed in the central Ruby Mountains. Except for the granite of Green Mountain Creek, much of the monzogranitic suite consists of layered, tabular units that were emplaced as composite sheets intruded into rocks of the granodioritic suite.

INTRODUCTION zircon geochronology indicates that the two major intru- The central Ruby Mountains are a zone of transition sive units of the pluton were emplaced -36 Ma (Wright between the metamorphic and igneous infrastructure of and Snoke, 1993). The Harrison Pass pluton and its wall the Ruby Mountains core complex to the north, and low- rocks were then cut by a large-magnitude, normal-sense, grade to non-metamorphosed rocks to the south (Fig. 1) plastic-to-brittle fault zone called the Ruby Mountains shear (Hudec, 1990, 1992; Snoke et al., 1990b). During the late zone. Discontinuities in mid- to lower-crustal density and Eocene, this transition zone was intruded by the compos- seismic velocity indicate that the Harrison Pass pluton ite Hamson Pass pluton (Fig. I), which underlies an area of also lies above a major, northward-dipping crustal struc- subdued topography on either side of Hamson Pass. U-Pb ture (Stoerzel, 1996). Eastward tilting of the footwall of 284 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I the Ruby Mountains shear zone has exposed a partial cross- This systematic progression in biotite K-Ar cooling ages section through the Harrison Pass pluton (Kistler et al., thus suggests eastward tilting of the range subsequent to 1981). the emplacement of the -36-Ma pluton. Fission-track data Harrison Pass pluton is particularly interesting in regard on apatite and zircon as summarized in Blackwell et al. to core complex development, because magmatism occurred (1984, 1985) and Reese (1986) also support eastward tilt- just prior to, or perhaps concurrent with, large-magnitude ing of the range, although additional thermochronologic crustal extension and exhumation of the Ruby Mountains studies are in progress to further explore the post-emplace- core complex. Cooling of the pluton through the closure ment kinematic history. temperature for 40Ar in biotite occurred by 35 Ma in parts of the eastern roof zone, and by 25 Ma in the western part PLUTONIC UNITS of the pluton (Kistler et d., 1981). Thus, the pluton repre- Harrison Pass pluton is a composite granitic body con- sents a short-duration, high-intensity thermal-magmatic sisting of two major intrusive suites: the granodioritic and episode in the development of a core complex during monzogranitic suites, each characterized by a variety of rock which at least four distinct granitoid magmas were gener- types (Snoke and Lush, 1984; Wright and Snoke, 1993). The ated and emplaced. suites differ in relative age, mechanisms of emplacement, geochemistry, and petrogenesis. GEOLOGY OF THE HARRISON PASS PLUTON Harrison Pass pluton is exposed over an area of more Granodioritic suite than 110 km2 (43 sq mi) in the central Ruby Mountains Granodioritic rocks comprise the bulk of the oldest in- (Figs. 1 and 29). A thickness of more than 1,200 m (4,000 trusive suite (Fig. 29). These rocks are present at all levels ft) of plutonic rocks is exposed, but the true overall thick- of the pluton and are the principal rock type in the struc- ness of the pluton is unknown. Wide-angle seismic reflec- turally highest part of the pluton which includes part of tion studies conducted along Ruby Valley road, east of the the original roof zone. Small bodies of granodiorite also pluton, and a short seismic line recorded between Ruby occur within wall rocks along the western extent of the Valley and Harrison Pass do not image the base of the in- southern margin of the pluton; it is apparent that granodi- trusion (Stoerzel, 1996), nor do gravity and aeromagnetic oritic magma occupied deeper structural levels of the plu- data from the area define this subsurface boundary. Field ton. This intrusive suite consists mainly of granodiorite, relations indicate that most of the monzogranite intrusive but includes related tonalite, quartz monzonite, and biotite units are tabular in shape and dip gently to moderately monzogranite (Fig. 31). Several mela- and leucocratic vari- eastward, whereas the granodiorite unit, and the "granite eties are also present. Contacts between these small-vol- of Green Mountain Creek (petrographic description pro- ume units are only locally observed, and the granodiorite vided later in this article) are internally massive to foliat- suite is mapped as a single unit (Fig. 29). Within the grano- ed. Fabric elements are roughly parallel to adjacent wall- diorite unit is a distinctive biotite monzogranite, here in- rock contacts. formally called the "granite of Corral Creek for outcrops The exposed part of the Hanison Pass pluton is inter- near the head of Corral Creek; the unit is also exposed preted to represent an upper-crustal cross section from along Hanison Pass Creek (Fig. 29). External contacts of deeper structural levels in the west, to shallower levels in the granite of Corral Creek cut a fabric in the granodiorite the east (Fig. 30). This interpretation is supported by the defined by aligned feldspar megacrysts. This relationship observation that the Paleozoic sedimentary rocks of the suggests that the granite of Corral Creek is younger than southern Ruby Mountains, which are intruded by the Harri- the granodiorite; however, it is geochemically gradational son Pass pluton, comprise a monotonous eastward-dipping with the granodiorite. Conversely, the granite of Corral homocline (Sharp, 1942; Willden et al., 1967; Willden and Creek is geochemically distinct from rocks of the monzo- Kistler, 1967, 1969, 1979). In the pluton, hypabyssal dikes, granitic suite. stoped blocks, and porphyries are more common along Rocks of the granodioritic suite are coarse- to very the eastern margin of the pluton, suggesting that this area coarse-grained, and contain megacrysts of alkali feldspar. was structurally higher than the western part of the body. Biotite is the principal mafic mineral; hornblende is most Perhaps the most convincing evidence of variation in common as a constituent mineral in the northeastern expo- structural level across the exposed part of the Harrison sures of the unit. Sphene is a prominent accessory mineral Pass pluton is the biotite K-Ar age contours delineated by and is visible in hand sample as isolated crystals that may Kistler et al. (1981, their figure 1). These data indicate a reach 12 mm long. Allanite, apatite, opaque minerals, and biotite K-Ar age progression from <25 Ma on the west side zircon are microscopic accessory minerals. Mafic enclaves of the pluton to 235 Ma on the east side of the pluton. are ubiquitous but are generally sparse, except in several ULrH'TON, ET ,\IA.. tiAKKISON t),-ZSS PLUTOK, RUBY klOUNTriINS. NE\.'tiI)A-I"-ZRT 4 285

intrusive rocks of

Pe Pennsylvanian Ely Limestone Dn Devonian Nevada Formation Slrn Silurian Lone Mountain Dolomite

Figtrrt. 29. SZmyl$t.~l geologic: ntup of the I5arrkon. R~ssph~ior~ arzcl en~it-ons.\Vc~t:!l lines indicatg fnnzylanific r-ocks. Geology by B. R. ~urf&z. F~gttr(710 Sltrlpllfi~d~ro~r-s(~cf~ori (I('I-~)SP fli~Il~rrzso~~ fkcs jtlrrfon drrzwtl ~tnrrillc~llo tht' (Iit-t~(-lt~~iofdil;>lizf IW~CIE~ of flit li(i)tgr~~gii (I/! of the Ruby Jfottntc~ins~hrtrr zorw c~nd~~~rpcricl~c~rrlor to fl~crinfcned crzir of njtafion of rh~fr)t)tu~rlll 7'rtr = r~rnfrt\it(,( t, Inrt = jt~iiftc enclrrr r.9. othc~rsy.mhols \crtnp ric fig 29 I:t~r~logt/Irrj R N Blrrton

localities in tlic northrastc~ritpart of the pl~rtoii\vlrt.r~ tlrey gerlrborrs, rnai\i\ c bod! of ti3 o-rllrcb,t ~rrorr/ogra~r~t(~liirlitic tincti\ r coarse-gr'iined, tttrc,t-ncit cncla~ec ,111ii cln-~C~I~C arid a1,iskitic dikes, ant! a srnall numher of i~~ic*rogratnllarclots ofmd.is~\t, qn'rr t~. r. dacitic dike\ prior to rmplacctrrcnt of the nronzogntnitic Iwo-nrica granltcs arc ,il~rrntl,~ntm the 1% cb\t-c.t\ntralp,rt.t stitte. In tlrc hordcr hcics, whert= granoclioritio rocks are of tlit plrrt.orr. 1)nt 'Jro rrltnlc-its tlrc gr,t~rotlro~~trc* ttrirt Itr 111c in cont,tct with carbonate \vall and roof rocks, tllese intru- cactcrn jr*u.t of tllc pluto~r 'L'hc\ are dr\t~~lc*tfrorn the si\c rock\ ,ire rncdium- to coar\e-grained and 1,ir.k inega- grinritch of (ireen Lloiirrtain (:I wl\ rn ttxxtirre ,111cl ho~rlo- erj sts Alortg the e,istcarrr coritac8tof the pltltoi-i, in tlir roof gencsity, 'l'hty range frorr~mcd~rirtr- to cturct~-gr,trxtctt,u~d zone, po~phyriticdikrs of g1';1iiocliolitic conipo\ition inttlrde ttic carl,or-ratc wall rocks. 'l'l~esedikes arc clraractt.ri~cdby pl~erioc~r)rst\of pl:~rr;iocl;tsr,clrr,irtz, ,111~1relict rtr,lfic miner- ,ils trr ,i fine-grairrecl gro~indti.rdss. mon, ancl gcrrerdll> inero,iier 112 gram srze .itrd ~l,iirrtl,t~~cc~ from MVI~to e'ist 111 tfrc centr,~lpart of tlttb Irlt~to~lClafic enc1,lves hr~crt fourltl witlirrl tl~c~iclock\, liut 11it.i ,ire scarce 'The trion~ogl-anit~c\~i~tc (Fig. 29) inclrtd(~sxrtinlcroiis Biotite nu)n~oqr,u~ltt>tlikt%\ rtrop out a\ \nl,ilI ril,cistbr, distinct intruhi\rc urrits. Tlltsr' arc predorr~irratttl\1)iotite prcctotrir1~~11rl1)111 the 5otltll-ct.rltral p'irt of tl.rc. pltrto~t,iiltl ~~toir~ogranitearid hvo-mica mon~ogranitc,l~ut include a rlncorrl- nlafic rock\ '~lsoocc~tc 'T'il~ular l~octies of 1)iotitc. nrorrm- nion caortipitric~nt\of' I,oilr thc gs,lnodrt)l-ifc ,ir~ctrtti)n/o- gr'ir~itciritrnde the granodiorrtic unit protl~tci~lgai1)tnldant gran~ttsrittcl\ of tllc' plnton 111 si-lc~a1 c>\pasrtrtss, th('4c scrreirr of granodronte \v~thrr~nionzograrirtie rock5. These rock\ clip ~rit)t!uratt~l\czn\t concoirf,ttlt:\11tl1 'r\\ocldtcd CAI)- rclation\h~psclearly inclicatc that the nron~ogr~ltutlcsuite niar r1nii.5 of nrorrtograrirte At \ortic, 10e~ilrt1c.snlr)ltti)- is ?orr~rgcrthan the gmiioclioritc strite. Petrograpltic and gr,iti~trcrocskil cut 111,rfrc. I ocks geoctrcmital data \ho\\ that ttlv rnonzogr;initc~rriiit cur 1)r. dividtld ~ntoat least thrctb stibr~nits:t\\o-inica rxiotizogr,itl- ite of t:rcct~ hlountain Creek, other two-rnica moilzogr,ul- itcs, and biotite itrc>rr~ograriitc. Clort. rlr'nz 60 \'tirtplt~i frorix tI1(, IIarnsorr F5\5 plrrton In an area north of Crecrr hfo~mtainCreek (i,rx., in the were ~I~~~\LCCIb) ICI'S anci l\,I,l for nlaltrl- ,trlt! tl~ice- riortlzcxrn part of the pltrton), a distincti\t i~itrnsii(~I)odv (~lrt~it~rit~~~IIIIC~'IIIC~\ Aitiicl~~gl~ t elcitri ~h fctn \~trt> Iwre rnfi)r~uallynarnc~l tl-ic 'kranite of Crccn Clountain c~\:ail~tltl,lcfroln rock\ of olt1c.t or i~cpr~\,rlt~~itagc iii tit(, rf,grori. Creek" crops out (Fig. 29). Thi\ unit is a re1;itivrl~lro~rro- the dcttciXC' ~o~~ll>ri~t'dto tt\,~llr~t>l~ COIT~J)~)\~~IOII~~~ tlcit,l tor BURTON, E'F AIL.. fllARNISON PASS Pl,UTON, RUBY MOUNTAINS, NEVAIIA-PART 4 287

AJCN K en .. 1.2 e A monzogranite of Corral Creek

.. e 1.0 "r P e 0 granodiorite unit e iwall zone dikes .. e e, d mafic enclaves, diE 0.8 X bio granite d $l d A 2 mica dikes d X 7 Green Mtn Ck I. 9 Jurassic gran 0.6 J I 50 60 70 80

10 35 65 90 SiO, n=48 0 Granodrortte l?om eastern part of pluton Figure 32. AICNK (rnolnrA120i dioiile?tl$iyC(IO, h7a2O, ur~tl KzO) Granodror~tefrom western part of pluton plotted clgcrinst SiOd fi,r gronitic rocks c$ the EIatrisotr Puss plu- <::::.':::: Gran~teof Corral Creek ton. &Ioi~~ngrcrniticrocks rf the grcrilodiorite unit (tvtonzogrnnite a Monzogran~te of Corral Ci-(>ek)errloscrl /XJ (L ikislzerl litw, 'i>nnt-q>rc~scnts c~rzluces Mafic d~kesand srlls in the two-rnica grtrnite of Grivtl M~untairzCreek. "T" I-epres~nts (I) Jurass~cgranod~or~tc syriplutotiic tonillitic tlik~ill the gt-arkoclioritc uilit.

E'igurc 31. Suminclr-y c$ rnotlal thtiz from gr~rniticrocks of the Harrison Pa5 s plrlton. Mafic dikes and mafic ~~~claves&on1 the granodioritic unit are typically metaluminous. hlicaceous enclaves front the two-mica granite of Grrt~tiMourrtairi Creek are strong- the granite of llawley Canyon, a Late J~~rttssici~itr~tsive ly peraluminous a~tclhave low Mg/(hlg+Fe) values nearly unit (Hudec, 1990; Hudec and Wright, 1990) that crops identical to that of their host rocks. out just north ofthe EZarrison Pass pluton (Figs. 32-37). 'ho geochemical groups can be distingrtished on the Major elelrtent data show that the bttlk of the rocks of the basis of Y altm~da~tccs(Fig. 34). Samples of dle granodio~itic Harrison Pass pluton are nrildly peralrtminous (alumina suite, its enclaves, and ~riaficdikes, are typically low in Y sah~rationindex [AS1 = ~rtolarAI2O3/(CaO + NqO + K20)] (most <40 ppn) over a large rarrge of Si02 values; Y and less tl1a1-i I .lbut greater than 1.0) (Fig. 32). Tlte ganodioritic SiOe display a weak negative correlatiorr. Sart~plesof the unit, roof dikes, and a synplutoiiic tonalitic dike sliow dis- ~-t~onzograniticsuite show a range of Y contents fronr 10 to tinct Milgl(Mg+Fe) whtes (Fig. 33), wltich, with hvo excep- >90 ppm. Mernbers of this group ir~clltdeboth groups of tions, range fron10.33 to 0.39. The hlg/(Mg+Fe) values of two-mica granite, biotite morlzogranite dikes, and Jurassic etrclaves are sirriilar to tl-rose of the 11ost granodiorite, but two-mica granite of Dawley Canyon. At these high SiOz Mg/(Mg+Fe) values of mafie (monzonitic) dikes are signif- values, no correlation between Y and Si02 is present. icantly higher (0.46 to 0.59). Iti arktition to their hlg/(Mg Most samples frorn tlre granortio~iticunit of the Han-i- + Fe) valtles, t11e roof [likes from tile eastern side of tlte 5011 Pass plutou display subparallel rare cart11 element pluton are compositionally quite similar to what coulcl be (BEE) patterns characterized by negative slope, a slight considered at1 "average" value for the graxrodioritic unit. negative Eu at~ontalpand :I \ve& dog-leg with a lower slope The granite of Cree11 Moul~tailiCreek, most other two- among the heavy REE (Fig. 35). Tlrc pattents of two roof mica granites, arid the Jurassic granite of I3awley Canyon clike sarrlples are identical to those of the granodioi-itic unit, are strongly peralr~nliiious(MI> 1.1). The two-mica gran- and two cpartz mor-r~onitedikes are also similar but have ites of Green Mouittai~iCrcek artd 1)iotite monzogranite slightly higher REE iikturrdar.~ce.Biotite monzogranites fro111 dikes have ?c.lg/(Mg+Fe)values less than 0.28. 'livo samples the granite of Corral Crcck (part of the granotlioritic suite) of two-mica granitic dikes display low Mg/(Mg+Fc) values, show distinct REE piitterns in that tlrey contain lower total whereas three salnples have values between 0.40 and 0.52, REE concentrations ant! lack Etl anomidies. Mafic enclaves the I-iighest observed among felsic rocks in the area. In the granoclioritic unit range widely in REE al~r~ndance 288 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I Mg/(Mg + Fe) pprn Y en 0.6 en

e

n X e

d d$gdzG; e

T

Figure 34. Plot of Y against Si02 for the granitic rocks of the Hawison Pass plutov. Symbols as in Fig. 32. Line separates high- Figure 33. Plot of MglMg+Fe against Si02 for the granitic rocks Y, high-Si02 samples from other samples of the pluton. of the Harrison Pass pluton. Symbols as in Fig. 32.

DISCUSSION OF GEOCHEMICAL DATA but generally display patterns similar to those of their host Granodioritic suite rocks, but with more pronounced negative Eu anomalies. Biotite monzogranites of the younger intrusive suite have For the granodiorite unit and roof dikes, when Si02, relatively low REE abundances, the most prominent neg- Mg/(Mg+ Fe), and CaO/A1203 are used as differentiation ative Eu anomalies, and flat to slightly positive slopes. The indices, other elemental concentrations and ratios tend to REE pattern of the granite of Green Mountain Creek is plot in linear arrays with variable scatter (Fig. 37). In vir- similar to rocks of the granodioritic suite except for its tually all plots, the mafic end of these arrays is a horn- large Eu anomaly. The REE pattern of a micaceous enclave blende-biotite tonalite dike (sample BB-70-94), which is a from the granite of Green Mountain Creek is similar to that large mafic enclave that is part of an aligned, disarticulated of its host, but the enclave contains higher REE contents, mafic dike -5 m thick. The dike cuts porphyritic granodi- with La at lOOOX chondrites, consistent with an origin by orite but is also cut by the host granodiorite and by leuco- accumulation of accessory minerals. Other two-mica granites granitic pegmatite. Petrographically similar hornblende- of the pluton (sheets and dikes) are characterized by vari- bearing rocks of the granodioritic unit are slightly more able, shallow negative slopes, negative Eu anomalies, and evolved than this sample. In binary elemental plots, biotite variable REE abundances. monzogranites from the granodioritic unit plot at the felsic Most of the isotopic data for the Harrison Pass pluton end of the data array. cluster in a narrow range of initial 87Sr/86Srfrom 0.7097 This linear relationship can result from magma mixing to 0.7116 and &Ndfrom -10.2 to -15.9 (Fig. 36). These data or from fractional crystallization. Fractional crystallization are distinct from those for two samples from the two-mica models (Burling, 1996) show that major- and trace-ele- granite of Green Mountain Creek (monzogranite and a ment concentrations can be explained by removal of pla- granitic enclave) which have initial 87Sr/86Sr about 0.725 gioclase from a hornblende-bearing granodioritic parent. and &Ndof -21.3. However, textural evidence indicates that the granodiorit- Quartz from rocks of the granodioritic unit has 6180 ic magma was saturated with plagioclase, quartz, biotite, (SMOW) values of about + 10%o (H. Karlsson, unpublished sphene, and allanite f hornblende f alkali-feldspar f zir- data). In contrast, the two-mica granite of Green Mountain con f opaque minerals. Thus, models in which plagioclase Creek has 6180 of about +8.5%0. The fact that the two- is the only fractionated phase are unlikely. mica granites have lower 6180 values is of note, because Major element data are also consistent with a simple in many granitic systems, initial 87Sr/86Sr and 6180 are mixing model between tonalitic and monzogranitic end positively correlated. members. A mixing model would also explain linear varia- BUWI'ON, ET AL . HANKISON PASS PLUTON, RUBY MOUNTAINS, NEVADA-PAR'L'4 289

0

-5

-E-++ roof dikes -10 -qtr rnonzonite dikes -1 5

-20

-25

-30 0 initial "SrPSr

Figure 36. Plot of &'d against initial "7SrIMSrfor sel~cterlgrilt~itic rocks of the Rzrbty hloncritains inr3h1cling ~omplesfrotrzthr I.10r~i5ctr1 Puss plutotz. ?i,rtiary granitic rocks 1rrI)~lledwith (l~z ''L' or "L'," zohiclt ciettok possible Archeun or Proterozoic sottrce (or contarn- inantj 1-egions, rc~specticely(Wright clnd Snoke, 1993).

Cbernical similarities among tlie granodioritic unit itnd roof dikes stlggest that the roof dikes represent "leaks"fiotn the grariociiorite magma chamber. This is supported 1)y the observation that most roof dike compositions lie on a inajor and trace element mixing line defined by the grano- dioritic rocks (Fig. 37).

bfonzogranitic suitc

Late cltage biotite monzogranitic rocks arc distinct frmri the biotite motizogranites of the granodioritic group (e.g., granite of Corral Creek) in a number of ways. The yotlngcr rocks have lower hfg/(Mg+Fe), A1203, Ba, Sr, Zr, Hf, and Th conteilts arid generally higher concentrations of Nit, Ta, ancl U. In addition, they display positive REE slopes and large negative Eu anomalies. This cornbinzktion of elernen- tal data iridicate that this unit is unrelated to the granodi- ctritic unit. Mass balance calcttlations show that it is also an unsuital>le felsic end membcr in the granodiorite mix- ing models. The granite of Green Mountaiti Creek contains higher contents of Rb, Nb, Zr, and Zn than other monzogranites t;igurttrr. 35 Plots ofrcire ecrrtlz clrwzef~tsnlrundances in the pluton, and Iiigher Y contents than any sample from the granodioritic unit. Its BEE patterns are distinct fro~ri those of the gratlodioritic unit hetailst' of their pronol~~lced tion among the trace elements. Scatter aniolig Ba concen- negative Eu atlornaly. trationb is probably due to local flow sorting of alkali- Other two-mica granites are clisti~ictfrom the graliodi- feldspar, whereas local erlricl~rne~ltsin Rb and K20 suggest oritic unit in their strongly peraluminotrs nature, their wide late-stage, fluid-enhanced enricl~mentof these eleitients. variation in Mg/(Mg+ Fe), their range of REE patterns 290 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

CaO EMPLACEMENTHISTORY Emplacement of the granodioritic and monzogranitic 0 granodiorite unit d suites was accommodated by distinctly different mecha- I wall zone dikes d d nisms which may have been controlled by the tempera- e, d mafic enclaves, dikes d d - X bio granite e ture and viscosity of the different magmas, the difference - A 2 mica dikes e e in depth of emplacement between the two intrusive suites, r Green Mtn Ck and the rheology of the rocks into which they were em- 0 Jurassic granite 07 placed. Emplacement of the granodioritic suite was accom- modated by a combination of "passive" and "forceful" intru- sive mechanisms as reflected by structural studies of the contact metamorphic aureole and pluton. These mechanisms I 0 z are summarized in Figure 38. The monzogranitic suite was subsequently emplaced into a thermally and struc- turally prepared zone which was occupied by rocks of the A A earlier granodioritic suite. . rn A 0 'I Granodioritic suite 0.I 0.2 0.3 0.4 0.5 0.6 MgI(Mg + Fe) Stoping Emplacement of the granodiorite suite was accommo- Figure 37. Plot of CaO against MglMg+Fe for granitic rocks of dated in part by stoping of existing wall and roof rocks. the Harrison Pass pluton. Symbols as in Fig. 32. Long-dashed line Numerous large pendants are present within the granodi- indicates possible mixing line between mafic tonalitic and monzo- oritic unit, especially adjacent to the southem margin of granitic end members. the pluton and eastern roof zone where the pluton-wall rock contacts are discordant (zone 1 on Fig.-. 38). These and La/Lu values, and their pronounced negative Eu blocks consist of impure calcite marble and calc-silicate rocks that have irregular, angular contacts with the sur- anomalies. In addition, when total Fe contents as Fe203 are rounding host granodiorite. Leucocratic granitoid dikes used as a differentiation index, most samples of this group form a marginal facies to most of these blocks and also do not plot on an extension of the granodioritic trend. These occur within the stoped blocks. Tungsten skarn mineral- features are not readily explained by fractional crystalliza- ization is present along many of these granodiorite-calc- tion from an granodioritic composition; therefore, the late silicate rock contacts (Tingley, 1992). two-mica monzogranites are thought to represent distinct Excellent exposures in the Road Canyon area (STOP 4-2) batches of magma unrelated to earlier phases of the pluton. illustrate a mechanism by which stoping occurred. Here, In summary, geochemical data indicate the following: stoped blocks (1-10 m thick) are separated from the adja- (1) the granodiorite unit is compositionally distinct from cent wall rocks by a network of dikes and sills. The stoped all younger intrusive units in the pluton; (2) compositional blocks have been translated westward and rotated clock- variation within the granodioritic suite is predominantly wise about a steeply plunging pole of rotation. The magni- the result of magma mixing between tonalitic and monzo- tude of rotation and translation is roughly equivalent to the granitic end members, with lesser crystal-liquid fractiona- thickness of the overlying network of dikes and sills. Intru- tion processes; (3) the low &Nd, high 87Sr/86Sr, and rela- sive units generally follow the compositional layering in the tively low 6180 of the two-mica granite of Green Mountain greenschist-facies wall rocks, but step upward to higher suggest source rocks that are quite different from those of stratigraphic levels via thin dikes. Thus, an overall "stair other monzogranites in the pluton; (4) dikes and sills of step geometry characterizes much of the granodiorite- biotite monzogranite and two-mica granite cannot be read- wall rock contact along the eastern margin of the pluton. ily related to either the granodioritic- unit or the granite- of Green Mountain Creek. Therefore, the Harrison Pass plu- Wall-rock strain ton consists of at least one intermediate magma composi- tion (tonalitic end member of the granodiorite unit) plus at Penetrative deformation of wall rocks has occurred ad- least four distinct monzogranitic magmas: felsic end mem- jacent to the Hanison Pass pluton and has accommodated ber of the granodioritic unit, granite of Green Mountain emplacement space. Small amplitude folds (5-20 cm) are Creek, late biotite monzogranite, and late two-mica mon- present in a contact-metamorphic aureole, about 500 to zogranite. 800 m thick, adjacent to the granodiorite suite. An axial BURTON, ET AL.. HARRISON PASS PLUTON, RUBY MOCN'rA'IiVS, NEV4DA-PAKI'-i 29 1 planar ftilrric of cotrtact-n~etarnorphiccalc-silicate miner- als suggests that the folds formed during pluton emplace- ment. Hinge lines of the folds trerid approxirrrately paral- lel to the pluton-wall rock corrtact and plunge eastward. 'The folds are cctrritllonlp overturnecl, arid yorne are reciinr- bent and isoclinal. hleasurcments of these folds suggest drat alrol~tSO% shortening of tlw wall rocks occurrecl tlrrougltout much of thi\ aureole, uhich accounts for a h%7o-tlirncnsiondshortening of' -20 km2. I,;irge-anlplitude folds (0.3-1 km) also are present in wall rocks adjacent to the sol~tllernmargin of the pluton (Fig. 29). These folds have accommodated 30% shortening o\cr an area of -40 km2. \VCtll-rock\tratigraphic units are iittc~xluatcclin the 1iml)s of the folds, and tlie hinge zones are thickened.

South of the IIarrison Pass pluton, the southern Ruby Xlo~tntaiilsare a sirzlple I~onlocli~~eof eastward tilted Paleo- zoic rocks which strike -NlOE. Adjacent to the southern ntargiri of the Elnrrison Pas\ pltiton, thp strike is deflected sharply cashvard and dips increase to -60". The deformed Ired\ de\crihe a southeast-pltinging synfonn. Adjacent to the northeastern margin of the pluton, a tight srnfonn exists in which tlie lirnlrs are highly attenuated. These synforms are separated by a broad, open antiform which forms thc roof ~oncof tlie plutorr, and which is structurally displacecl

Figrm) 38. hfotl~lof the emplac.rtncnt fijr the Hamison Pass plu- ion. A, Rcirly etrrplr~ce~taetitof gmnoclioritic. stcite was c~ccorratno- htud by hrittb ernplacetrzent of dikes cirarl sheets t?zat proclztced ~tof~iragof the tczll rocks and ~riinortiplgt. R, Eny>lacerrtetatofthe rrarnciindcr of the gmnoclioritic sirite proclrrcecl substa~atic~lstrain itt thenrtcilly caizrl inr>chanicall!jroc>crkei~cri zc;rtll rocks, z~pl'lifiof the roof zonp. rt~itror.stoping in thc roof anif plriton margins, and irrternul .strcritr. C, S~I)seqtreraternpl(~cerrz~nt of the trtonzograraitic mite (Trrrg) (1s layc*redtuhrtlar rirtits tliut percu,sirely infrutlerl the grclnodioritic suite (~tt(1z~crll rocks, cind tlzp eirz/~lncemeiztof the g~ariitc' o$ Grrw hfozlritcrin Crrvk (Tgtrt) as cr lto~raog~rtcous~nciss u~hicitsizorten~d clrtsl clttenucit~dJurassic igneous (and .rnctanzor- phic rocks (Jgi (i.c., tccill rocks to the north). 0blicp.e line rhotos tht. firtiire j>ositiotz of the Rzrb~y Ifoztntairu shear zone. I), Ohticlue-nltgle cmss sccfiorz tl~t-otighthe I-lmrrison Puss pluton Ji-om trc~ttsposetlclnd t.otatcd rtmp t ieto shocirrg final state of c~tn~)lac~~~nertt-reltztrtlstrain nizrl stoping. Zone I is the concordant roof'zcrnc cf the pltlton that shot~xsericlencc of stoping crrztl uplifi. %ort~11 is the discor(1rit~tLCNZI ZOTL~t~-h~rg stopitig ant1 wall-rock strain wcre irnportatzt etnpluc~inentmcchunistns. Structz~rally rlrer)r.r Zotzt. 111 is charclct~rizrr1b!y truttsposecl nnrl highly s trciinerl, concor(lr~rzftr(zl1-rock ~otat~rcts. 292 BYU GEOLOGY STUDIE S 1997, VOL. 42, PART I from the projected trajectory of strata south of the pluton. in contact with Jurassic monzogranite. A thin screen of Uplift of these rocks during emplacement of the Hanison hornfels along this zone marks the contact between the Pass pluton is considered to be an important mechanism two rock types. This screen represents a highly attenuated of emplacement. The structural thickness of the pluton is portion of the Paleozoic wall rocks that are found south -5 km, and it was emplaced into a wall-rock succession and northeast of the pluton. -2.7 krn thick. Other bodies of monzogranite are dikes and sills that are emplaced into older plutonic rocks or into the adjacent Internal Strain wall rocks. The southern margin of the pluton is pervasively intruded by small monzogranite dikes, sills, and lensoidal Mafic enclaves within the granodiorite suite are ellip- intrusive bodies. Emplacement of these units is thought soids of oblate spheroidal shape. Measurement of a-c axes to have been accommodated by brittle dike and sill propa- of these bodies and quantitative strain analysis using the gation and localized wall-rock shortening. Rf-$ method (Ramsey and Huber, 1983) show that the en- claves were subject to 30% flattening. Shortening is oriented CONCLUDING REMARKS perpendicular to the pluton wall-rock contact. Foliation in the granodiorite also describes a great circle with P point The Harrison Pass pluton was emplaced in the footwall to the southeast, consistent with a southeastern orientation of a major core complex during an early stage of regional of the direction of maximum shortening. The direction of crustal extension. This tectonic event was accommodated maximum elongation described by the 2-dimensional strain by mid-crustal flow which has been recognized in the ellipses of the mafic enclaves defines a gently east-dip- northern Ruby Mountains (MacCready et al., in press). ping great circle which is coincident with the shortening Crustal extension in the Ruby Mountains and East Hum- direction reflected by wall-rock folds. boldt Range may have initiated in the Eocene along a west- rooted, plastic-to-brittle shear zone-detachment fault sys- Monzogranitic suite tem (Mueller and Snoke, 1993a). Cooling data suggest that a significant component of tectonic denudation occurred The monzogranitic suite of the pluton was emplaced within one million years of the emplacement of the Harrison into a complex zone that was previously occupied by grano- Pass pluton. However, a direct relationship between tec- diorite. In the central part of the pluton, emplacement of tonism and magmatism is not reflected in the mechanisms tabular intrusions of monzogranite into a granodioritic host of emplacement of the pluton. Instead, Harrison Pass pluton was accommodated by stoping. Here, the monzogranitic was emplaced by the combination of mechanisms includ- rocks are layered line rock. Dilation by the monzogranitic ing: stoping, wall-rock strain, and localized uplift. sheets produced translation of the granodiorite. Brittle The presence of intermediate magma compositions propagation of the monzogranitic sheets is indicated by (mafic tonalite as an end member of the granodioritic unit truncation of alkali feldspar megacrysts and mafic enclaves and monzodioritic dikes among the late-stage intrusions) in the host granodiorite. The sheets strike northeastward suggests the likelihood of a mantle-derived component in and dip moderately eastward, consistent with the tilt of the pluton. Similar conclusions were reached for coeval, the mountain range, suggesting that the sheets were initially extension-related volcanic rocks in the Snake and Egan subhorizontal. ranges (e.g., Cans et al., 1989; Grunder, 1992). A direct The granite of Green Mountain Creek lacks the internal relationship between granitoid plutonism and crustal exten- segregation and layering of the monzogranites described sion is thus reflected in the presence of these intermedi- above and was apparently emplaced as a homogeneous ate magmas in that advected heat from parental mafic mass. Crystal-plastic strain produced a foliation in these magmas is probably necessary to explain the abundance of rocks which is parallel to the orientation of the Ruby Moun- crustally-derived monzogranitic magmas. In addition, the tains shear zone, so that any magmatic foliation that may compositional variety of the monzogranitic rocks suggests have been present is no longer identifiable. Along its that crustal source region(s) were of diverse rock type. northern margin, the granite of Green Mountain Creek is SNOKE, ET AL.: REFERENCES CITED 293 REFERENCES CITED Dickinson, W.R., 1991, Tectonic setting of faulted Tertiary strata associated (for all 4 parts) with the Catalina core complex in southern Arizona: Boulder, Colo- rado, Geological Society of America Special Paper 264,106 p. Allmendinger, R.W., and Jordan, T.E., 1984, Mesozoic structure of the Doelling, H., 1989, Antelope Island State Park, the history, the geology, Newfoundland Mountains, Utah: Horizontal shortening and subse- and wise planning for future development: Survey Notes, Utah Geo- quent extension in the hinterland of the Sevier belt: Geological logical & Mineral Survey, v. 23, no. 1, p.2-14. Society of America Bulletin, v. 95, p. 1280-1292. Doelling, H.H., Willis, G.C., Jensen, M.E., Hecker, S., Case, W.E, and Anderson, R.E., Zoback, M.L., and Thompson, G.A., 1983, Implications Hand, J.S., 1990, Geologic map of Antelope Island, Davis County, of selected subsurface data on the structural form and evolution of Utah: Utah Geological and Mineral Survey Map 127, scale 1:24,000, some basins in the northern Basin and Range province, Nevada and accompanying booklet 27 p. Utah: Geological Society of America Bulletin, v. 94, p. 1055-1072. Dokka, R.K., Mahaffie, M.J., and Snoke, A.W., 1986, Thermochronologic Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: Geo- evidence of major tectonic denudation associated with detachment faulting, northern Ruby Mountains-East Humboldt Range, Nevada: logical Society of America Bulletin, v. 79, p. 42M58. Tectonics, v. 5, p. 995-1006. Armstrong, R.L., and Hansen, E., 1966, Cordilleran infrastructure in the Ebadi, A., and Jobannes, W., 1991, Beginning of melting and composition eastern Great Basin: American Journal of Science, v. 264, p. 112-127. of first melts in the system Qz-Ab-Or-HzO-COz: Contributions to Babcock, R.C., Jr., Ballantyne, G.H., and Phillips, C.H., 1995, Summary Mineralogy and Petrology, v. 106, p. 286-295. of the geology of the Bingham district, Utah, in Pierce, EW., and Eby, G.N.,1990, The A-type granitoids: A review of their occurrence and Bolm, J.G., eds., Porphyry copper deposits of the America Cordillera: chemical characteristics and speculations on their petrogenesis: Lithos, Tucson, Arizona Geological Society Digest 20, p. 316-335. v. 26, p. 115-134. Bickle, M.J., Chapman, H.J., Wickham, S.M., and Peters, M.T., 1995, Effimoff, I., and Pinezich, A.R., 1981, Tertiary structural development of Strontium and oxygen isotope profiles across marble-silicate con- selected valleys based on seismic data: Basin and Range province, tacts, Lizzies Basin, East Humboldt Range, Nevada: constraints on northeastern Nevada. Royal Society of London, Philosophical Trans- metamorphic permeability contrasts and fluid flow: Contributions to actions, Series A, v. 300, p. 435-442. Mineralogy and Petrology, v. 121, p. 400-413. Feeley, TC., and Grunder, A.L., 1991, Mantle contribution to the evolution Blackwell, D.D., Kelley, S.A., and Reese, M., 1984, Fission track evidence of middle Tertiary silicic magmatism during early stages of extension: on the Late Cenozoic deformation of the Ruby Mountains, Nevada: the Egan Range volcanic complex, east-central Nevada: Contributions Geological Society of America Abstracts wth Programs, v. 16, p. 446. to Mineralogy and Petrology, v. 106, p. 154-169. Blackwell, D.D., Reese, N.M., and Kelley, S.A., 1985, Evolution of Basin Fricke, H.C., Wickham, S.M., and O'Neil, J.R., 1992, Oxygen and hydro- and Range structure in the Ruby Mountains and vicinity, Nevada, in gen isotope evidence for meteoritic water infiltration during myloni- Lucchitta, I., Morgan, E, and Soderblom, L.A., eds., Proceedings vol- tization and uplift in the Ruby Mountains-East Humboldt Range ume for the conference on heat and detachment in crustal extension core complex, Nevada: Contributions to Mineralogy and Petrology, v. on continents and planets: Houston, Texas, Lunar and Planetary Insti- 111, p. 203-221. tute Contribution 575, p. 12-14. Gans, EB., Mahood, G.A., and Schemer, E., 1989, Synextensional mag- Brooks, W.E., Thorman, C.H., Snee, L.W., 1995, The 40Ar139Ar ages and matism in the Basin and Range province: A case study from the east- tectonic setting of the middle Eocene northeast Nevada volcanic field: em Great Basin: Boulder, Colorado, Geological Society of America Journal of Geophysical Research, v. 100, p. 10,403-10,416. Special Paper 233,53 p. Burling, T.C., 1996, Magmatism associated with extension, Harrison Pass Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph pluton, Ruby Mountains, Nevada [M.S. thesis]: Lubbock, Texas, Texas 1,438 p. Tech University, 136 p. Glick, L.L., 1987, Structural geology of the northern Toano Range, Elko County, Nevada [M.S. thesis]. San Jose, California, San Jose State Burton, B.R., in preparation, Structural geology and emplacement history University, 141 p. of the Hamson Pass pluton, central Ruby Mountains, Elko County, Good, S.C., Nester, EL., and Snoke, A.W., 1995, Lacustrine molluscs from Nevada 1Ph.D. dissertation]: Laramie, University of Wyoming. the lower Humboldt formation, northeastern Nevada, and possible Camilleri, PA., 1994, Mesozoic and Cenozoic tectonic and metamorphic late Eocene to Oligocene age: Geological Society of America Abstracts evolution of the Wood Hills and Pequop Mountains, Elko County, with Programs, v. 27, no. 1, p. 49. Nevada [Ph.D. dissertation]: Laramie, University of Wyoming, 196 p. Grunder, A.L., 1992, Two-stage contamination during crustal assimilation: Camilleri, EA., and Chamberlain, K.R., Mesozoic tectonics and 1997, isotopic evidence from volcanic rocks in eastern Nevada: Contributions metamorphism in the Pequop Mountains and Wood Hills region, to Mineralogy and Petrology, v. 112, p. 219-229. northeast Nevada: implications for the architecture and evolution of Grunder, A.L., 1995, Material and thermal roles of basalt in crustal mag- the Sevier orogen: Geological Society of America Bulletin, v. 109, p. matism: case study from eastern Nevada: Geology, v. 23, p. 952-956. 74-94. Hacker, B.R., Yin, A., Christie, J.M., and Snoke, A.W., 1990, Differential Cashman, EH., 1992, Structural geology of the northeastern Stansbury stress, strain rate, and temperature of mylonitization in the Ruby Mountains, in Wilson, J.R., ed., Field guide to geologic excursions in Mountains, Nevada: implications for the rate and duration of uplift: Utah and adjacent areas of Nevada, Idaho, and Wyoming: Utah Geo- Journal of Geophysical Research, v. 95, p. 8569-8580. logical Survey Miscellaneous Publication 92-3, p. 171-178. Hodges, K.V., Snoke, A.W., and Hurlow, H.A., 1992, Thermal evolution Crittenden, M.D., Jr., Coney, EJ., and Davis, G.H., eds., 1980, Cordilleran of a portion of the Sevier hinterland: the northern Ruby Mountains- metamorphic core complexes: Boulder, Colorado, Geological Society East Humboldt Range and Wood Hills, northeastern Nevada: Tec- of America Memoir 153,490 p. tonics, v. 11, p. 154-164. Dallmeyer, R.D., Snoke, A.W., and McKee, E.H., 1986, The Mesozoic- Holland, T.J.B., and Powell, R., 1990, An enlarged and updated internally Cenozoic tectonothermal evolution of the Ruby Mountains-East consistent thermodynamic dataset with uncertainties and correlations: Humboldt Range, Nevada: A Cordilleran metamorphic core complex. the system Kz0-Naz0-Ca0-Mg0-Mn0-Fe0-Fez03-A1203-Ti02- Tectonics, v. 5, p. 931-954. Si02-C-Hz-02:Journal of Metamorphic Geology, v. 8, p. 89-124. 294 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I

Hope, R.A., 1972, Geologc map of the Spruce Mountain quadrangle, Elko Lipten, E.J.H., 1984, The geology of Clover Hill and classification of the County, Nevada U.S. Geological Survey Quadrangle Map GQ-942,3 Wells tungsten prospect, Elko County, Nevada [M.S thesis] West p., scale 1.62,500 Lafayette, Ind~ana,Purdue Univers~ty,239 p Howard, K.A., 1966, Structure of the metamorphic rocks of the northern Lister, G.S., and Snoke, A W, 1984, S-C mylonites Journal of Structural Ruby Mountains, Nevada [Ph.D. d~ssertation]New Haven, Yale Uni- Geology, v. 6, p 617-638. versity, 170 p. Loiselle, M.C , and Wones, D.R., 1979, Characteristics and origin of Howard, K.A, 1968, Flow direction m triclinic folded rocks: American anorogenlc granites. Geolog~calSociety of Amer~caAbstracts w~th Journal of Sc~ence,v 266, p. 75&765. Programs, v 11, no. 7, p. 468. Howard, K.A., 1971, Paleozoic metasedlments In the northern Ruby Lush, A I?, McGrew, A J., Snoke, A W, and Wright, J E , 1988, Alloch- Mountains, Nevada. Geological Society of America Bulletin, v. 82, p. thonous Archean basement In the northern East Humboldt Range, 259-264. Nevada Geology, v. 16, p. 349353. Howard, K A , 1980, Metamorphic infrastructure In the northern Ruby MacCready, T., Snoke, A.W., and Wright, J E , 1993, Evldence for nearly Mountains, Nevada, in Cnttenden, M D , Jr., Coney, EJ , and Davis, orthogonal Oligocene crustal flow beneath the coeval mylonitlc shear G.H., eds., Cordilleran metamorph~ccore complexes Boulder, Colo- zone of the Ruby Mountans core complex, Nevada. Geologcal Soc~ety rado, Geolog~calSociety of America Memoir 153, p. 335-347. of Amer~caAbstracts w~thPrograms (Comb~nedCorddleran-Rocky Howard, K.A., 1987, Lamoille Canyon nappe in the Ruby Mountains Mountam Sect~onalMeeting), v 25, no. 5, p. 112. metamorphic core complex, Nevada, in Hill, M.L., ed , Geological MacCready, T., Snoke, A.W., Wnght, J.E., and Howard, K.A, m press, Society of America Centenn~alField Guide-Cord~lleran Section Mid-crustal flow dur~ngTert~ary extension In the Ruby Mounta~ns Boulder, Colorado, Geolog~calSociety of America, p. 9.5100. core complex, Nevada. Geolog~calSoc~ety of America Bulletln. Hudec, M.R., 1990, The structural and thermal evolution of the central Martin, H., 1987, Petrogenesis of Archean trondhjemites, tonalites, and Ruby Mountains, Elko County, Nevada [Ph.D. dissertation]: Laram~e, granodlorltes from eastern F~nland.major and trace element geo- University of Wyoming, 272 p. chem~stry.Journal of Petrology, v. 28, p. 921-953. Hudec, M.R , 1992, Mesozoic structural and metamorphic h~storyof the McGrew, A.J., 1992, Tecton~cevolut~on of the northern East Hurnboldt central Ruby Mountains metamorphic complex, Nevada. Geological Range, Elko County, Nevada [Ph D dissertat~on].Laram~e, Univers~ty Soc~etyof America Bulletin, v. 104, p. 10861100 of Wyommg, 191 p Hudec, M R, and Wright, J.E , 1990, Mesozoic history of the central McGrew, A.J., and Snee, L W, 1994, 40Ar/39Arthermochronometr~c con- part of the Ruby Mounta~ns-EastHumboldt Range metamorphic straints on the tectonothermal evolut~onof the northern East Hum- core complex, Nevada. Geolog~calSociety of Amenca Abstracts with boldt Range metamorph~ccore complex, Nevada. Tectonophysics, v. Programs, v. 22, no 3, p. 30. 238, p. 42-50, Hurlow, H.A., Snoke, A.W., and Hodges, K.V., 1991, Temperature and Miller, D.M., and Allmendinger, R.W, 1991, Jurass~cnormal and str~ke- pressure of mylonit~zationin a Tert~aryextensiond shear zone, Ruby slip faults at Crater Island, northwestern Utah. Geolog~calSoc~ety of Mountains-East Humboldt Range, Nevada. tectonic implicat~ons. Amer~caBulletin, v 103, p 1239-1251 Geology, v. 19, p. 82-86. M~ller,D.M., and Cam~llerl,PA, 1992, Day 2. Road log from the P~lot Jaeger, K.B., 1987, Structural geology and stratigraphy of the Elko Hills, Range to Wells, Nevada, in Wllson, J.E., ed, Field guide to geolog~c Elko County, Nevada [M.S. thesls] Laramie, Univers~tyof Wyoming, excursions m Utah and adjacent areas of Nevada, Idaho, and Wyom~ng: 70 p. Utah Geolog~calSurvey M~scellaneousPublication 92-3, p. 40-46. Ketner, K.B., 1990, Geologic map of the Elko Hills, Elko County, Nevada: Miller, D.M., and Hoisch, TD., 1992, Mesozo~cstructure, metamorphism, U.S. Geological Survey Miscellaneous Investigations Senes Map I- and magmatism In the Pllot and Toano Ranges, tn Wilson, J E , ed , 2082, scale 1.24,000. Ketner, K.B., and Ross, R.J., Jr., 1990, Geologc map of the northern Adobe F~eldguide to geologic excursions In Utah and adjacent areas of Range, Elko County, Nevada. U.S. Geological Survey M~scellaneous Nevada, Idaho, and Wyoming. Utah Geolog~calSurvey Mlscellane- Investigations Series Map 1-2081, scale 1.24,000 ous Publication 92-3, p. 77-92 Ketner, K.B., and Sm~th,J.E, Jr., 1974, Folds and overthrusts of Late M~ller,D.M., and Lush, A P, 1981, Prelim~narygeolog~c map of the Pllot Jurassic or Early Cretaceous age in northern Nevada. Journal of Peak quadrangle and adjacent quadrangles, Elko County, Nevada, and Research U S Geological Survey, v 2, p. 417419. Box Elder County, Utah. U.S. Geolog~calSurvey Open-F~leReport King, C., 1878, Systematic Geology-geolog~cal exploration of the Forbeth 81-658,21 p ,2sheets, scale 1.24,000. Parallel, v 1. Washington, D C., U.S. Government Pr~ntingOfice, Miller, D.M., H~llhouse,W.C., Zartman, R.E , and Lanphere, M.A., 1987, 803 p Geochronology of ~ntrus~veand metamorphic rocks In the Pllot Range, Kistler, R.W., Ghent, E.D., and O'Neil, J R , 1981, Petrogenesis of garnet Utah and Nevada, and comparison wlth reg~onalpatterns. Geological two-m~cagranites in the Ruby Mountains, Nevada. Journal of Geo- Society of Amenca Bulletm, v. 99, p. 886-879 physical Research, v. 86, p. 10,591-10,606. Misch, E, and Hazzard, J.C., 1962, Strat~graphyand metamorphism of Lachenbruch, A.H., and Sass, J.H., 1978, Models of an extend~nglithos- late Precambr~anrocks In central northeastern Nevada and adjacent phere and heat flow in the Basln and Range province, in Smlth, R.B., Utah. Amencan Assoclat~onof Petroleum Geolog~stsBulletln, v 46, and Eaton, G.E, eds., Cenozo~ctecton~cs and regional geophysics of the p 289-343. western Cord~lleraDenver, Colorado, Geolog~calSoc~ety of America Moore, W.J., 1973, A summary of rad~ometr~cages of Igneous rocks in Memolr 152, p. 20s250. the Oquirrh Mounta~ns,north-central Utah Economlc Geology, v. Lanier, G, John, E.C., Swensen, A J., Reid, J., Bard, C.E., Caddey, S.W., 68, p. 97-101 and W~lson,J.C., 1978, General geology of the B~nghammine, Bing- Moore, W.J. and McKee, E.H., 1983, Phanerozo~cmagmatlsm and mm- ham Canyon, Utah. Economic Geology, v. 73, p. 1228-1241. eralization in the Tooele 1" x 2" quadrangle, Utah, in Miller, D.M., Lapointe, D.D., T~ngley,J.V, and Jones, R.R., 1991, Mineral resources of Todd, VR., and Howard, K.A., eds., Tectonlc and stratigraphic stud- Elko County, Nevada Reno, Nevada Bureau of Mines and Geology ies in the eastern Great Basln. Boulder, Colorado, Geolog~calSoc~ety Bulletin 106,236 p. of Amer~caMemoir 157, p. 18S190. SNOKE, ET AL.: REFERENCES CITED 295

Mueller, K.J., and Snoke, A.W., 1993a, Progressive overprinting of nor- Sharp, R.P, 1938, Pleistocene glaciation in the Ruby-East Humboldt mal fault systems and their role in Tertiary exhumation of the East Range, northeastern Nevada: Journal of Geomorphology, v. 1, no. 4, Humboldt-Wood Hills metamorphic complex, northeast Nevada: p. 296-323. Tectonics, v. 12, p. 361-371. Sharp, R.P, 1939a, The Miocene Humboldt Formation in northeastern Mueller, K.J., and Snoke, A.W., 1993b, Cenozoic basin development and Nevada: Journal of Geology, v. 47, p. 133-160. normal fault systems associated with the exhumation of metamorphic Sharp, R.E, 1939b, Basin-Range structure of the Ruby-East Humboldt complexes in northeast Nevada, in Lahren, M.M., Trexler, J.H., Jr., Range, northeastern Nevada: Geological Society of America Bulletin, and Spinosa, C., editors, Crustal evolution of the Great Basin and v. 50, p. 881-920. Sierra Nevada: Cordilleran/Rocky Mountain Section, Geological Sharp, R.P, 1940, Geomorphology of the Ruby-East Humboldt Range, Society of America Guidebook, Department of Geological Sciences, Nevada: Geological Society of America Bulletin, v. 51, p. 337-372. University of Nevada, Reno, p. 134. Sharp, R.E, 1942, Stratigraphy and structure of the southern Ruby Moun- Noble, L.E, 1941, Structural features of the Virgin Spring area, Death tains, Nevada: Geological Society of America Bulletin, v. 53, p. Valley, California. Geological Society of America Bulletin, v. 52, p. 647-690. 941-1000. Smith, J.E, Jr., and Howard, K.A., 1977, Geologic map of the Lee 15- Palmer, A.R., 1983, Decade of North American Geology 1983 geologic minute quadrangle, Elko County, Nevada: U.S. Geological Survey time scale: Geology, v. 11, p. 503-504. Geologic Quadrangle Map GQ-1393, scale 1.62,500. Palmer, H.C., MacDonald, W.D., and Hayatsu, A,, 1991, Magnetic, struc- Smith, J.E, Jr., and Ketner, K.B., 1976, Stratigraphy of post-Paleozoic tural and geochronologic evidence bearing on volcanic sources and rocks and summary of resources in the Carlin-Pinon Range area, Oligocene deformation of ash flow tuffs, northeast Nevada. Journal of Nevada with a section on Aeromagnetic Survey by D.R. Mabey. U.S. Geophysical Research, v. 96, p. 2185-2202. Geological Survey Professional Paper 867-B, 48 p. Patterson, E.B., 1977, What's in a name? A history of Elko County place Smith, J.E, Jr., and Ketner, K.B., 1978, Geologic map of the Carlin-Pifion names: Elko, Nevada, Northeastern Nevada Historical Society Quar- Range area, Elko and Eureka counties, Nevada. U.S. Geological Survey terly, Spring and Summer 1977, p. 3-47, Miscellaneous Investigations Series Map 1-1028, scale 1:62,500. Snelson, S., 1957, The geology of the northern Ruby Mountains and the Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace element dis- East Humboldt Range, Elko County, Nevada [Ph.D. dissertation]: crimination diagrams for the tectonic interpretation of granitic rocks: Seattle, University of Washington, 214 p. Journal of Petrology, v. 25, p. 956-983. Snoke, A.W., 1975, A structural and geochronological puzzle: Secret Peters, M.T., 1992, A petrologic and stable isotope study of the role of Creek gorge area, northern Ruby Mountains, Nevada. Geological fluids in high-grade metamorphism: the East Humboldt Range, Society of America Abstracts with Programs, v. 7, no. 7, p. 1278-1279. Nevada, USA [Ph.D, dissertation]: Chicago, Illinois, The University Snoke, A.W., 1980, Transition from infrastructure to suprastructure in of Chicago, 279 p. the northern Ruby ~duntains,Nevada, in Crittenden, M.D., Jr., Coney, Peters, M.T., and Wickham, S.M., 1994, Petrology of upper amphibolite EJ., and Davis, G.H., eds., Cordilleran metamorphic core complexes: facies marbles from the East Humboldt Range, Nevada, USA; evidence Boulder, Colorado, Geological Society of America Memoir 153, p. for high-temperature, retrograde, hydrous votalite fluxes at mid-crustal 287333. levels: Journal of Petrology, v. 35, p. 205-238. Snoke, A.W., 1992, Clover Hill, Nevada: Structural link between the Peters, M.T., and Wickham, S.M., 1995, On the causes of 180-depletion Wood Hills and East Humboldt Range, in Wilson, J.R., ed., field guide and 1801160 homogenization during regional metamorphism; the to geologic excursions in Utah and adjacent areas of Nevada, Idaho, E?st Humboldt Range core complex, Nevada: Contributions to Miner- and Wyoming: Utah Geological Survey Miscellaneous Publication alogy and Petrology, v. 119, p. 68-82. 92-3, p. 107-122. Peters, M.T., Wickham, S.M., and Miller, D.M., 1992, High 613C Late Snoke, A.W., 1994, Oblique views of the crust in the Ruby Mountains- Proterozoic carbonates of the North American Cordillera: Geological East Humboldt Range, northeastern Nevada: implications for super- Society of America Abstracts with Programs, v. 24, no. 7, p. A114. posed Mesozoic and Cenozoic metamorphism. Geological Society of Powell, R., and Holland, T.J.B., 1988, An internally consistent dataset with America Abstracts with Programs, v. 26, no. 2, p. 94. uncertainties and correlations: 3. Applications to geobarometry, worked Snoke, A.W., and Howard, K.A., 1984, Geology of the Ruby Mountains- examples and a computer program: Journal of Metamorphic Geology, East Humboldt Range, Nevada: A Cordilleran metamorphic core com- v. 6, p. 17S204. ~lex(includes road log), in Lintz, J., Jr., editor, Western Geological Ramsey, J.G., and Huber, M.I., 1983, The techniques of modem structural Excursions, v. 4: Reno, Nevada, Department of Geological Sciences geology, volume 1: Strain analysis: New York, Academic Press, 307 p. of the Mackay School of Mines, p. 260403. Reese, N.M., 1986, Cenozoic tectonic history of the Ruby Mountains and Snoke, A.W., and Lush, A.P, 1984, Polyphase Mesozoic-Cenozoic defor- adjacent areas, northeastern Nevada: constraints from radiometric dat- mational history of the northern Ruby Mountains-East Humboldt ing and seismic reflection profiles [M.S. thesis]: Dallas, Texas, Southern Range, in Lintz, J., ed., Western Geological Excursions, v. 4: Reno, Methodist University, 88 p. Nevada, Department of Geological Sciences of the Mackay School of Rigby, J.K., 1958, Geology of the Stansbury Mountains, eastern Tooele Mines, p. 232-260. County, Utah: Utah Geological Society Guidebook to the geology of Snoke, A.W., McKee, E.H., and Stem, T.W., 1979, Plutonic, metamorphic, Utah, no. 13, p. 1-134. and structural chronology in the northern Ruby Mountains, Nevada: Schaeffer, EE., editor, 1960, Geology of the Silver Island Mountains, Box A preliminary report: Geological Society of America Abstracts with Elder and Tooele counties, Utah and Elko County, Nevada: Utah Programs, v. 11, p. 520521. Geological Society Guidebook to the geology of Utah, no. 15, 185 p. Snoke, A.W., Durgin, S.L., and Lush, A.P, 1982, Structural variations in Schalla, R.A., 1992, Results of exploratory drilling in Hunginton Valley the northern Ruby Mountains-East Humboldt Range, Nevada. Geo- near Jiggs, Elko County, Nevada, in Trexler, J.H., Jr., and others, eds., logical Society of America Abstracts with Programs 1982, v. 14, no. 4, Structural geology and petroleum potential of southwest Elko County, p. 235. Nevada: Reno, Nevada, 1992 Fieldtrip Guidebook, Nevada Petro- Snoke, A.W., Hudec, M.R., Hurlow, H.A., and McGrew, A.J., 1990a, The leum Society, Inc., p. 71-85. anatomy of a Tertiary extensional shear zone, Ruby Mountains-East 296 BYU GEOLOGY STUDIE S 1997, VOL. 42, PART I

Humboldt Range, Nevada. Geological Soc~etyof America Abstracts in M~ller,D.M., Todd, V.R , and Howard, K.A., eds., Tectonic and with Programs, v. 22, no. 3, p. 66. stratigraphic stud~esm the eastern Great Basin. Boulder, Colorado, Snoke, A.W., McGrew, A.J, Valasek, PA., and Smithson, S.B., 1990b, A Geological Soc~etyof America Memoir 157 , p. 61-73. crustal cross-sectlon for a terrain of superimposed shorten~ngand Tooker, E.W., and Roberts, R.J., 1970, Upper Paleozoic rocks ~n-the extension. Ruby Mounta~ns-East Humboldt Range metamorphic Oqu~rrhMountains and B~nghamM~ning D~stnct, Utah U.S. Geolog- core complex, Nevada, in Salisbury, M.H., and Fountain, D.M , eds., cal Survey Professional Paper 629-A, 76 p. Exposed cross-sections of the continental crust, NATO Advanced Tooker, E.W., and Roberts, R.J., 1971, Structures related to thrust faults Studies Institute. Dordrecht, The Netherlands, Kluwer Academic In the Stansbury Mountains, Utah: US Geological Survey Profes- Publishers, p. 103-135. s~onalPaper 750-B, B1-B12. Snoke, A.W., Wright, J.E., Hudec, M.R., and McGrew, A.J., 1992, Meso- Warnaars, EW., Sm~th,WH., Bray, R E., Lanier, G, and Shafiqullah, M , zoic magmatic-metamorphic-deformationalhistory of the Ruby Moun- 1978, Geochronology of ~gneousintrus~ons and porphyry copper mmer- tains-East Humboldt Range, Nevada: Geological Society of America allzation at Bingham, Utah Econornlc Geology, v. 73, p. 1242-1249. Abstracts with Programs, v. 24, no. 6, p. 63. Wernicke, B., and Burchfiel, B.C., 1982, Modes of extens~onaltecton~cs Solomon, B.J., McKee, E H , and Anderson, D.W, 1979, Strat~grapbyand Journal of Structural Geology, v. 4, p. 105-115 depositional environments of Paleogene rocks near Elko, Nevada, in Wickham, S.M., and Peters, M.T., 1990, An oxygen Isotope discontinuity Armentrout, J.M., Cole, M.R., and TerBest, H., Jr., eds., Cenozo~c in high-grade rocks of the East Humboldt Range, Nevada Nature, v. paleoge~graph~of the western United States, Pacific Coast Paleo- 345, p. 150-153 geography Symposium 3: Los Angeles, Californla,The Pacific Section, Wickham, S.M , and Peters, M.T., 1992, Oxygen and carbon soto ope pro- files in metased~mentsfrom L~zz~esBasin, East Humboldt Range, Society of Economic Paleontoloa~stsand Mineralorrists,- -p. 75-88. Solomon, B.J., and Moore, S.W., 1982, Geologic map and oil shale deposits Nevada. constrants on mid-crustal metamorphic and magmahc volatile of the Elko East quadrangle, Elko County, Nevada: U.S. Geolog~cal fluxes: Contribut~onsto Mineralogy and Petrology, v 112, p 4665 Survey Map MF-1421, scale 1:24,000. W~ckham,S M., and Peters, M.T., 1993, High S'3C Neoproterozo~ccar- Steiner, J.C., Jahns, R.H., and Luth, W.C., 1975, Crystallization of alkali bonate rocks in western North Amenca Geology, v 21, p 165-168. feldspar and quartz in the haplogranite system N~AISI~O,-KAIS~~O,-W~ckham, S.M., Taylor, H P, Jr., Snoke, A W., and O'Neil, J R., 1991, An Si02-H20 at 4 kb: Geological Society of Amer~caBullet~n, v. 86, p. oxygen and hydrogen Isotope study of high-grade metarnorph~smand 83-98. anatexis In the Ruby Mounta~ns-East Humboldt Range core com- Stoerzel, A,, 1996, Impl~cationson the nature of extended crust from multi- plex, Nevada, in Taylor, H P, Jr, O'Neil, J.R., and Kaplan, I.R., eds., component seismic studies of the Ruby Mountans metamorph~ccore Stable isotope geochem~stry.A tribute to Samuel Epstein The Geo- complex, northeastern Nevada [Ph.D. d~ssertabon]:Laramie, University chemical Soc~ety,Specla1 Publication No. 3, p. 37K390. of Wyoming, 281 p. Willden, R., and Kistler, R W., 1967, Ordov~ciantectonism in the Ruby Strecker, U., Smithson, S.B., and Steldtmann, J.R., 1996, Cenozo~cbasin Mountains, Elko County, Nevada. U.S. Geologcal Survey Professional Paper 575-D, p. D6PD75 extension beneath Goshute Valley, Nevada, in Beratan, K.K., ed., Reconstructing the history of Basin and Range extension uslng sedi- Willden, R., and K~stler,R.W., 1969, Geologic map of the J~ggsquadran- mentology and stratigraphy: Boulder, Colorado, Geological Soclety of gle, Elko County, Nevada. U.S Geologcal Survey Map 64-859, scale America Special Paper 303, p. 15-26. 1.62,500. Swisber, C.C., 111, and Prothero, D.R, 1990, Single-crystal 40Ar139Ar Willden, R., and K~stler,R.W., 1979, Precambrian and Paleozo~cstratig- dating of the Eocene-Oligocene transibon In North America: Science, raphy in central Ruby Mountains, Elko County, Nevada, in Newman, v. 249, p 760-762. G.W., and Goode, H.D., eds., Bas~nand Range symposium and Great Taylor, G.K., 1984, Stratigraphy, metamorphism, and structure of the Basin field conference: Denver, Colorado, Rocky Mountam Assocla- southeastern East Humboldt Range, Elko County, Nevada [M.S. the- tion of Geolog~sts,p. 221-243. sis]: Columb~a,University of South Carolina, 148 p. Willden, R., Thomas, H.H., and Stern TW., 1967, Oligocene or younger Thorman, C.H., 1962, Structure and strat~graphyof the Wood H~llsand thrust faulting in the Ruby Mounbns, northeastern Nevada Geo- a portion of the northern Pequop Mountains, Elko County, Nevada log~calSociety of America Bulletin, v 78, p 1345-1358 [Ph D. dissertation]: Seattle, University of Washmgton, 218 p. Wright, J.E., and Snoke, A.W., 1986, Mid-Tertiary mylonit~zationm the Thoman, C.H., 1970, Metamorphosed and nonmetamorphosed Paleozoic Ruby Mountan-East Humboldt metamorphic core complex, Nevada. rocks in the Wood Hills and Pequop Mountains, northeast Nevada. Geolog~calSoc~ety of America Programs wth Abstracts, v 18, no. 6, Geologcal Society of America Bulletin, v. 81, p. 2417-2448. p. 795. Thorman, C.H., Ketner, K.B., Snoke, A.W., Brooks, W.E., and Mueller, K.J., Wright, J.E., and Snoke, A.W, 1993, Tertiary magmat~smand mylonltlza- 1991, Ev~dencefor the involvement of the Roberts Mounta~nsalloch- tion in the Ruby-East Humboldt metamorphic core complex, north- thon in Mesozoic tecton~csand its effect on m~neraldeposits and eastern Nevada. U-Pb geochronology and Sr, Nd, and Pb soto ope geo- petroleum accumulation models in northeast Nevada, in Buffa, R.H., chemistry: Geological Society of Arnenca Bulletin, v. 105, p. 93.5952 and Coyner, A R., eds., Geology and ore deposits of the Great Basin, Wright, J.E , and Wooden, J.L., 1991, New ST, Nd, and Pb isotopic data Field Tnp Guidebook Compendium, volume 2: Reno, Nevada, Geo- from plutons in the northern Great Basm. Irnpl~cat~onsfor crustal log~calSociety of Nevada, p. 869-905. structure and granite petrogenesis in the h~nterlandof the Sev~er Tingley, J.V., 1992, Min~ngd~str~cts of Nevada Reno, Nevada Bureau of thrust belt Geology, v. 19, p. 457460 M~nesand Geology Report 47,80 p. Young, J.C , 1955, Geology of the southern Lakes~deMountams, Utah Tooker, E.W., 1983, Var~at~onsm structural style and correlat~onof thrust Utah Geolog~caland Mineralog~calSurvey Bullet~n56, 110 p plates in the Sevier foreland thrust belt, Great Salt Lake area, Utah,