University of Dayton eCommons Geology Faculty Publications Department of Geology 1997 The Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1-Introduction & Road Log Arthur W. Snoke University of Wyoming Keith A. Howard U.S. Geological Survey Allen J. McGrew University of Dayton, [email protected] Bradford R. Burton University of Wyoming Calvin G. Barnes Texas Tech University SeFoe nelloxtw pa thige fors aaddndition addal aitutionhorsal works at: https://ecommons.udayton.edu/geo_fac_pub Part of the Geology Commons, Geomorphology Commons, Geophysics and Seismology Commons, Glaciology Commons, Hydrology Commons, Other Environmental Sciences Commons, Paleontology Commons, Sedimentology Commons, Soil Science Commons, Stratigraphy Commons, and the Tectonics and Structure Commons eCommons Citation Snoke, Arthur W.; Howard, Keith A.; McGrew, Allen J.; Burton, Bradford R.; Barnes, Calvin G.; Peters, Mark T.; and Wright, James E., "The Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1-Introduction & Road Log" (1997). Geology Faculty Publications. 39. https://ecommons.udayton.edu/geo_fac_pub/39 This Article is brought to you for free and open access by the Department of Geology at eCommons. It has been accepted for inclusion in Geology Faculty Publications by an authorized administrator of eCommons. For more information, please contact [email protected], [email protected]. Author(s) Arthur W. Snoke, Keith A. Howard, Allen J. McGrew, Bradford R. Burton, Calvin G. Barnes, Mark T. Peters, and James E. Wright This article is available at eCommons: https://ecommons.udayton.edu/geo_fac_pub/39 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