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Research Article Tectonochemistry of the Brooks Range Ophiolite, Alaska

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Research Article Tectonochemistry of the Brooks Range Ophiolite, Alaska GeoScienceWorld Lithosphere Volume 2020, Article ID 7866789, 17 pages https://doi.org/10.2113/2020/7866789 Research Article Tectonochemistry of the Brooks Range Ophiolite, Alaska 1 1 2 Joseph Biasi , Paul Asimow , and Ronald Harris 1Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA 2Department of Geological Sciences, Brigham Young University, S389 Eyring Science Center, Provo, Utah 84602, USA Correspondence should be addressed to Joseph Biasi; [email protected] Received 23 September 2020; Accepted 7 December 2020; Published 24 December 2020 Academic Editor: Sarah Brownlee Copyright © 2020 Joseph Biasi et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). We present new whole-rock geochemical data from the Brooks Range ophiolite (BRO) together with new mineral chemistry data from the BRO, South Sandwich forearc, Izu-Bonin forearc, and Hess Deep. The analyses reveal that the Brooks Range ophiolite (BRO) was most likely created in a forearc setting. We show that this tectonic classification requires the Brookian orogeny to begin at ~163-169 Ma. The middle-Jurassic BRO contains abundant gabbros and other intrusive rocks that are geochemically similar to lithologies found in other forearc settings. Based on major, minor, and trace element geochemistry, we conclude that the BRO has clear signals of a subduction-related origin. High-precision olivine data from the BRO have a forearc signature, with possible geochemical input from a nearby arc. The Koyukuk terrane lies to the south of the Brooks Range; previous studies have concluded that the BRO is the forearc remnant of this arc-related terrane. These studies also conclude that collision between the Koyukuk Arc and the Arctic Alaska continental margin marks the beginning of the Brookian orogeny. Since the BRO is a forearc ophiolite, the collision between the Koyukuk Arc and the continental margin must have coincided with obduction of the BRO. Previously determined 40Ar/39Ar ages from the BRO’s metamorphic sole yield an obduction age of 163-169 Ma. Since the same collisional event that obducts the BRO also is responsible for the Brookian orogeny, we conclude that the BRO’s obduction age of ~163-169 Ma marks the beginning of this orogenic event. 1. Introduction focused on the extrusive layer (pillow basalts and sheeted dikes), as the extrusive section is readily sampled by seafloor Subduction is one of the defining processes that allows plate drilling and features characteristic rock types such as boni- tectonics to operate and thereby determines the whole nites and forearc basalts (e.g., [6]). It therefore follows that character of our planet. It is now well established that the most tectonochemical indicators for ophiolites and seafloor majority of ophiolites are produced via subduction-related basalts have been developed with the extrusive layer in processes [1]. Although our understanding of suprasub- mind [5]. duction zone (SSZ) ophiolites has advanced greatly over In this study, we present novel geochemical data from the the last 30 years—due in part to ocean-drilling campaigns Brooks Range ophiolite, South Sandwich forearc, Izu-Bonin (e.g., [2, 3])—many aspects of these ophiolites are still forearc, and Hess Deep. With these data, we show that the poorly understood. This is especially true when it comes intrusive section of the Brooks Range ophiolite (BRO) to processes occurring below the extrusive layer. (Figure 1) in northern Alaska preserves a forearc geochemi- In the North American Cordillera, most ophiolites have cal signature. We then use the forearc model of the BRO to experienced complex syn- and postemplacement deforma- resolve several outstanding issues regarding the Mesozoic tion and alteration [4]. This greatly complicates their geo- tectonic history of northern Alaska. Notably, the tectonic chemical characterization, which is required to identify the history of the BRO requires that the Brookian orogeny begin tectonic setting in which they are generated [5]. Previous at ~163-169 Ma, when the BRO is obducted onto the Arctic studies of in situ forearcs, arcs, and back-arcs have mostly Alaska margin. Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/7866789/5292989/7866789.pdf by guest on 24 September 2021 2 Lithosphere 170 W 160 150 W 140 W (a) Chukchi Sea Canada basin Beaufort Sea Lisburne North slope 70 N Pen. Colville basin Canada U.S. U.S. Russia Brooks Range BRO & AM AM Figure 1(b) KB Seward KY RT Tintina fault Peninsula KB YTT? Kaltag fault YTT FW Contact Surfcial deposits (Quaternary) Basin axis Foreland basin (Cretaceous) Cenozoic fault Hinterland basin (Cretaceous) Mesozoic fault Koyukuk arc (Jurassic-Cretaceous) O´Brien et al. (2018) Brooks Range ophiolite (Jurassic) Study area Accreted terranes of Angayucham ocean (Dev.-Early Cret.) Low-grade thrust sheets (Devonian to Early Cretaceous) Other, Arctic Alaska (Proterozoic-Mesozoic) Other, North America (Proterozoic-Mesozoic) Semischists, phyllites (Proterozoic-Paleozoic) Penetratively deformed schists, gneisses (Prot.-Paleozoic) Accreted continental terranes (Proterozoic-Paleozoic) (b) Brooks Range ophiolite Angayucham terrane Colville basin Avan Siniktanneyak Iyokrok Misheguk Mayumerak Asik Brooks Range Kotzebue Sound Yukon-Koyukuk basin 100 km Figure 1: (a) Overview map showing major areas discussed in the text. Lightly modified from [42]. Abbreviations: AM—Angayucham terrane; BRO—Brooks Range ophiolite; KB—Yukon-Koyukuk basin; KY—Koyukuk Arc; FW—Farewell terrane; RT—Ruby terrane; YTT—Yukon-Tanana terrane. (b) Map of the western Brooks Range showing exposures of the Brooks Range ophiolite (green) and Angayucham terrane (purple). The dashed lines mark the approximate boundaries of the Brooks Range. Based on maps by Wilson et al. [66] and Harris [7]. Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/7866789/5292989/7866789.pdf by guest on 24 September 2021 Lithosphere 3 2. Geological Background today south of the Brooks Range (Figure 1) (e.g., [7, 21]). Arc-continent collision between the Koyukuk Arc and the 2.1. Brooks Range Ophiolite (BRO). The Brooks Range ophio- Arctic Alaska continental margin probably led to obduction lite (BRO) lies in northwestern Alaska (Figure 1), and is likely of the BRO [19, 21, 23]. middle-Jurassic in age [7]. As it is currently (and conserva- 40Ar/39Ar plateau ages are interpreted to give overlapping tively) mapped, the BRO covers 1800 km2. It is composed cooling ages of 163 − 169 ± 5 Ma for both the BRO and its of six klippe-like massifs. Most studies have focused on three metamorphic sole [7, 24, 25]. Such overlap is a common of these massifs (Misheguk, Avan, and Siniktanneyak) [7]. characteristic among SSZ ophiolites with a metamorphic sole Complete ophiolitic sections (from metamorphic sole to pil- [19]. U-Pb zircon data from late-stage BRO melts yield an low basalts/chert) can be found in these three massifs. Glacial age of 170 ± 3 Ma [26]. This age is within error of 40Ar/39Ar erosion of the ophiolite bodies provides nearly 100% expo- ages from BRO gabbro and the metamorphic sole [7]. These sure in many places. geochronological constraints indicate that the BRO was Only a handful of papers have been published on the obducted onto the Angayucham terrane shortly after forma- BRO, and most studies were published pre-2000. Early tion. The BRO’s quick obduction, combined with its struc- reconnaissance-level studies of the western Brooks Range turally highest position in the Brooks Range, has allowed produced various models for the tectonic evolution of the ophiolite to remain only lightly serpentinized and avoid the BRO [8–18]. The most recent and thorough work on overprinting by postobduction events that commonly affect the BRO was done in the late 1980s and 1990s and other Cordilleran ophiolites. focused mostly on the Siniktanneyak and Misheguk massifs [7, 19–22]. This work mainly involved structural mapping, 2.2. Other Oceanic Rocks. Typically, all Mesozoic oceanic petrology, and age analysis. Some major and trace element rocks within the Brooks Range are referred to as the geochemical data were also reported. Hornblende 40Ar/39Ar Angayucham terrane [27]. This terrane grouping, however, plateau ages were reported for gabbro and metamorphic sole is somewhat misleading. The BRO has a suprasubduction rocks from the Avan, Asik, Misheguk, and Siniktanneyak zone origin [20], while the structurally underlying basalts massifs. The conclusions most relevant to this study are as have an oceanic plateau/MORB affinity [17, 19, 28, 29]. Some follows: authors (e.g., [23]) have therefore divided the Angayucham terrane into the Misheguk Mountain Allochthon and the (1) The Brooks Range ophiolite contains a complete sec- structurally underlying Copter Peak Allochthon. However, tion of oceanic lithosphere composed, from bottom differences in age and geochemistry preclude any shared geo- to top, of a metamorphic sole with partial melts, logic history between these units prior to emplacement [21]. dunitic and harzburgitic peridotite (serpentinized Further complicating the issue, many oceanic rocks in inte- near the sole but otherwise pristine), transitional rior Alaska are also referred to as the Angayucham terrane ultramafic cumulates (petrologic Moho), thick sec- (Figure 1) [30]. Ideally, the Angayucham terrane would be tions (~4 km) of layered gabbro, massive gabbro,
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