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GEOSPHERE Petrogenesis of the 91-Mile Peridotite in the Grand Canyon: Ophiolite Or Deep-Arc Fragment? GEOSPHERE, V

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GEOSPHERE Petrogenesis of the 91-Mile Peridotite in the Grand Canyon: Ophiolite Or Deep-Arc Fragment? GEOSPHERE, V Research Paper GEOSPHERE Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc fragment? GEOSPHERE, v. 17, no. 3 S.J. Seaman1, M.L. Williams1, K.E. Karlstrom2, and P.C. Low1 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA https://doi.org/10.1130/GES02302.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 10 figures; 5 tables DEDICATION u This manuscript represents one of the last projects that Dr. Sheila J. Seaman was working on before her untimely passing. It is completed in CORRESPONDENCE: [email protected] her honor as a beloved researcher, teacher, and colleague. CITATION: Seaman, S.J., Williams, M.L., Karlstrom, K.E., and Low, P.C., 2021, Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc ABSTRACT ■ INTRODUCTION fragment?: Geosphere, v. 17, no. 3, p. 786–803, https:// doi .org /10.1130 /GES02302.1. Recognition of fundamental tectonic boundaries has been extremely diffi- The broad Proterozoic orogenic belt of southwestern North America has cult in the (>1000-km- wide) Protero zoic accretionary orogen of southwestern been interpreted in terms of the assembly and accretion of both continental Science Editor: Shanaka de Silva North America, where the main rock types are similar over large areas, and and oceanic terranes, blocks, and/or provinces between ca. 1.8 and 1.0 Ga (Ben- Received 16 June 2020 where the region has experienced multiple postaccretionary deformation nett and DePaolo, 1987; Karlstrom and Bowring, 1988; Bowring and Karlstrom, Revision received 22 November 2020 events. Discrete ultramafic bodies are present in a number of areas that may 1990; Whitmeyer and Karlstrom, 2007). Geologic mapping, isotopic analyses, Accepted 3 February 2021 mark important boundaries, especially if they can be shown to represent and geochemical investigations have been carried out for several decades, but tectonic fragments of ophiolite complexes. However, most ultramafic bodies suture zones between tectonic blocks (or provinces) are still extremely difficult Published online 21 April 2021 are small and intensely altered, precluding petrogenetic analysis. The 91-Mile to identify. This is probably due, at least in part, to the intensity of multiple peridotite in the Grand Canyon is the largest and best preserved ultramafic syn- and postassembly deformational and metamorphic events, but the lack body known in the southwest United States. It presents a special opportu- of clear suture boundaries has led to questions about the tectonic significance nity for tectonic analysis that may illuminate the significance of ultramafic of the blocks and provinces themselves. Specifically, do the tectonic blocks rocks in other parts of the orogen. The 91-Mile peridotite exhibits spectacular represent microplates that were assembled into Proterozoic continental crust, cumulate layering. Contacts with the surrounding Vishnu Schist are inter- or, alternatively, do they represent tectonically rearranged but not exotic com- preted to be tectonic, except along one margin, where intrusive relations have ponents of a single crustal province? Ultramafic rocks occur as tectonic lenses been interpreted. Assemblages include olivine, clinopyroxene, orthopyrox- (meters to rarely hundreds of meters in diameter) throughout the Proterozoic ene, magnetite, and phlogopite, with very rare plagioclase. Textures suggest orogen, and many occur in or near suspected tectonic boundary regions (Fig. 1). that phlogopite is the result of late intercumulus crystallization. Whole-rock This has led to speculation that these exotic rocks may represent fragments compositions and especially mineral modes and compositions support deri- of ophiolites (back-arc or oceanic crust) that mark sutures, or they may serve vation from an arc-related mafic magma. K-enriched subduction-related fluid as markers of other types of significant tectonic boundaries within the orogen. in the mantle wedge is interpreted to have given rise to a K-rich, hydrous, Field relations and geochemical constraints are required to illuminate the high-pressure partial melt that produced early magnetite, Al-rich diopside, significance of the ultramafic bodies. However, most of the ultramafic occur- and primary phlogopite. The modes of silicate minerals, all with high Mg#, the rences consist of relatively small and isolated blocks, and most are strongly sequence of crystallization, and the lack of early plagioclase are all consistent altered and/or metamorphosed. Primary structures and textures are generally with crystallization at relatively high pressures. Thus, the 91-Mile peridotite not preserved, and geochemical data can be suspect because of alteration. body is not an ophiolite fragment that represents the closure of a former The 91-Mile peridotite, exposed in the Upper Gorge of the Grand Canyon, is ocean basin. It does, however, mark a significant tectonic boundary where a significant anomaly in terms of its size and the degree of preservation. The lower-crustal arc cumulates have been juxtaposed against middle-crustal ultramafic body is ~1 km in diameter, and primary assemblages and textures schists and granitoids. are superbly preserved. This body is of particular interest because it occurs within several kilometers of the proposed Crystal suture zone between the This paper is published under the terms of the isotopically distinct Mojave and Yavapai crustal provinces (Fig. 1; Ilg et al., CC-BY-NC license. Michael Williams https://orcid.org/0000-0003-0901-3396 1996; Karlstrom and Williams, 2006; Holland et al., 2015). © 2021 The Authors GEOSPHERE | Volume 17 | Number 3 Seaman et al. | 91-Mile peridotite in the Grand Canyon Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/786/5319423/786.pdf 786 by guest on 02 October 2021 Research Paper The goal of this study was to characterize the 91-Mile peridotite exposure and to use field, petrographic, and compositional information derived from 42o the peridotite to constrain its provenance and emplacement history, and spe- E WYOMING PROVINCE MCC Green Mountain EMG Magmatic Arc cifically to determine whether the peridotite is a fragment of an ophiolite LOC (i.e., a tectonic sliver from a mid-ocean ridge or back-arc/intra-arc basin) or FMB BGsz BCsz a cumulate that developed in an arc-associated magma chamber. The mini- o CHEYENNE BELT 40 FCSCsz MMsz mally altered, metamorphosed, or deformed nature of the 91-Mile peridotite provides a special opportunity to interpret field relations, primary textural Hsz features, mineral assemblages, and compositional characteristics, and to eval- uate the tectonic setting of formation of these basement rocks. Interpretations o PPZ 38 derived from this well-preserved exposure can provide insight into the petro- MOJAVE PROVINCE Salida-Gunnison genesis of other ultramafic occurrences within the Grand Canyon and across Magmatic Arc YAVAPAI Dubois Terrane the orogenic belt and may ultimately help to answer the question of whether PROVINCE Virgin Mtns MOJ-YAV TRANSITION the ultramafic bodies are associated with, and can be used to identify, major MGG 91-Mile Peridotite o Gold Butte (UGG) NM tectonic boundaries within the orogen. 36 LGG Pecos Terrane YAV-MAZ TRANSITION ■ REGIONAL GEOLOGY Zuni Mtns o PO 34 The lithosphere of the Grand Canyon region of southwestern Laurentia was assembled between 1800 Ma and 1400 Ma (Condie, 1992; Whitmeyer and Karl- MAZATZAL PROVINCE strom, 2007) through the accretion of a 1000- km- wide accretionary orogenic complex interpreted to be composed of island-arc terranes, continental fragments, 32o and their syntectonic cover (e.g., Karlstrom and Bowring, 1988; Bowring and Karlstrom, 1990; Whitmeyer and Karlstrom, 2007). This 1000-km-wide orogen has 0 100 been divided into three northeast-southwest– trending Proterozoic crustal prov- km inces, the Mojave, Yavapai, and Mazatzal Provinces (Fig. 1; Karlstrom and Bowring, 30o 1988; Whitmeyer and Karlstrom, 2007). The Mojave and Yavapai Provinces are o o o both made up of 1840–1700 Ma rocks that were deformed and metamorphosed 114 110 106 W during the ca. 1700 Ma Yavapai orogeny (Holland et al., 2015). However, the Figure 1. Map of southwestern North America showing locations of Proterozoic provinces Mojave Province has a distinctive isotopic signature indicating the cryptic pres- and province boundaries and the locations of Proterozoic ultramafic exposures (yellow ence of Archean crustal material (Wooden and DeWitt, 1991; Holland et al., 2018). dots), from Low (2009); adapted from Karlstrom and Bowring (1988), Bowring and Karl- strom (1990), Condie (1992), Aleinikoff et al. (1993), Premo and Fanning (2000), Premo The boundary between the Mojave and the Yavapai Provinces is shown in and Loucks (2000), Bryant et al. (2001), Tyson et al. (2002), Strickland et al. (2003), and Figure 1 as a broad (75-km-wide) northeast-trending “Moj-Yav transition zone” Cavosie and Selverstone (2003). UGG, MGG, LGG—Upper, Middle, Lower Granite Gorges, (Fig. 1), defined mainly on the basis of Pb, Nd, and Hf isotopes (Bennett and Grand Canyon. Dashed black lines—block and terrane boundaries proposed by Condie DePaolo, 1987; Wooden and DeWitt, 1991; Duebendorfer et al., 2006; Holland (1992). BCsz—Big Creek Gneiss shear zone; BGsz—Buckskin Gulch shear zone; EMG— Elkhorn Mountain Gabbro; FMB—Farwell Mountain belt; LOC—Lake Owen Complex; et al., 2015, 2018). The Mojave Province to the west is made up of
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