Research Paper THEMED ISSUE: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke GEOSPHERE Neoarchean tectonic history of the Teton Range: Record of accretion GEOSPHERE; v. 14, no. 3 against the present-day western margin of the Wyoming Province doi:10.1130/GES01559.1 B. Ronald Frost1, Susan M. Swapp1, Carol D. Frost1, Davin A. Bagdonas1,2, and Kevin R. Chamberlain1,3 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA 18 figures; 1 set of supplemental files 2Carbon Management Institute, University of Wyoming, Laramie, Wyoming 82071, USA 3Faculty of Geology and Geography, Tomsk State University, Tomsk 634050, Russia CORRESPONDENCE: [email protected] ABSTRACT INTRODUCTION CITATION: Frost, B.R., Swapp, S.M., Frost, C.D., Bag- donas, D.A., and Chamberlain, K.R., 2018, Neoar- chean tectonic history of the Teton Range: Record Although Archean gneisses of the Teton Range crop out over an The Wyoming Province is an Archean craton that occupies most of of accretion against the present-day western margin area of only 50 × 15 km, they provide an important record of the Ar- Wyoming and portions of Montana, and adjacent states. The Archean rocks of the Wyoming Province: Geosphere, v. 14, no. 3, chean history of the Wyoming Province. The northern and southern are exposed in the cores of basement-involved Laramide uplifts. The early p. 1008–1030, doi:10.1130/GES01559.1. parts of the Teton Range record different Archean histories. The north- mafic crust appears to have been Hadean (Frost et al., 2017), though most of ern Teton Range preserves evidence of 2.69–2.68 Ga high-pressure the exposed area consists of Paleoarchean to Neoarchean quartzofeldspathic Science Editor: Shanaka de Silva Guest Associate Editor: Joshua Schwartz granulite metamorphism (>12 kbar, ~900 °C) followed by tectonic as- orthogneisses that retain an isotopic signature of that ancient crust (Frost, 1993). sembly with isotopically juvenile quartzofeldspathic metasedimen- The Wyoming craton is subdivided into three main provinces (Fig. 1; Mueller Received 16 May 2017 tary rocks under high-pressure amphibolite-facies conditions (~7 kbar, and Frost, 2006). The northwestern province is the Montana metasedimentary Revision received 18 December 2017 675 °C) and intrusion of extensive leucogranites. Together, these events province, which is an area composed of quartzite, pelitic schist, and carbonate Accepted 1 March 2018 record one of the oldest continent-continent collisional orogenies on rock associations that are structurally interleaved with quartzofeldspathic gneiss, Published online 11 April 2018 Earth. Geochemical, thermobarometric, and geochronological data from all of which were accreted at ca. 2.55 Ga. The Beartooth-Bighorn magmatic zone, the gneisses of the southern Teton Range show that this part of the up- which occupies the core of the craton, is dominated by orthogneisses. Most of lift records a geologic history that is distinct from the northern part. the Beartooth-Bighorn magmatic zone contains rocks that were last deformed It contains a variety of quartzofeldspathic gneisses, including a between 2.86 Ga and 2.71 Ga. On the southern and western margins of the 2.80 Ga granodioritic orthogneiss and the 2.69–268 Ga Rendezvous Beartooth-Bighorn magmatic zone, these older gneisses were overprinted by Gabbro. None of these preserves evidence of the granulite metamor- deformation that is as young as 2.63 Ga. The southern margin of the craton con- phism seen in the northern Teton Range. Instead, they have affinities tains the southern accreted terranes, consisting of various fragments of arcs and with rocks elsewhere in the Wyoming Province. The boundary between continents that were accreted to the Wyoming Province at ca. 2.63 Ga. the northern and southern areas is occupied by the Moran deformation The Teton Range, a small range of spectacular mountains in northwestern zone, a broad zone of high strain along which the northern and southern Wyoming, exposes some of the westernmost outcrops of the Archean Wyo- areas were assembled at ca. 2.62 Ga under moderate pressures and tem- ming Province (Fig. 2). The northern portion of the range, described by B. Frost OLD G peratures (T = 540–600 °C and P < 5.0 kbar). The final Archean event of the et al. (2006), Frost et al. (2016a), and Swapp et. al. (2018), contains some of the Teton Range was the emplacement at 2.55 Ga of the Mount Owen batholith, oldest high-pressure granulites in the world. In this paper, we summarize the a peraluminous leucogranite that intrudes the Moran deformation zone. past work on the northern Teton Range, identify a deformation zone that marks The rocks of the northern Teton Range record events that are not the contact between gneisses of the northern and southern Teton Ranges, and OPEN ACCESS present elsewhere in the Wyoming Province. We propose that they discuss how this structural belt relates to the final Neoarchean assembly of the formed at 2.70–2.67 Ga some place distal to the Wyoming Province Wyoming Province. and that they were accreted from the west against the Wyoming Province along the Moran deformation zone at ca. 2.62 Ga. This date GEOLOGIC BACKGROUND is coeval with deformation and metamorphism in the southern accreted terranes and indicates that at this time, accretion was taking place Preliminary geologic mapping of the range was conducted by John C. Reed This paper is published under the terms of the along both the southern margin and western margins of the Wyoming Jr. from 1962 to 1970 (Fig. 2; Reed, 2014). Subsequent studies (Miller et al., CC-BY-NC license. Province. 1986; B. Frost et al., 2006; Frost et al., 2016a; Swapp et al., 2018) concentrated © 2018 The Authors GEOSPHERE | Volume 14 | Number 3 Frost et al. | Neoarchean tectonic history of the Teton Range 1008 Research Paper Figure 1. Map of the Wyoming Province, showing the mountain ranges exposing Archean rocks and the location of the three major subprovinces: the Montana metasedimentary province, the Beartooth-Bighorn magmatic zone, and the southern accreted terranes. The study area in the Teton Range is indicated by the rectangle and shown in Figure 2. Colors identify time of final Archean deformation and magmatism; see text for discussion. TTG—tonalite-trondhjemite-gra- nodiorite. Figure is adapted from Frost et al. (2016a). on the Archean history of the northern part of the range, roughly north of Leigh Canyon, where evidence of high-pressure (high-P) granulite facies is pre- served. These studies show that the high-P granulites were metamorphosed at 2695 Ma and were tectonically assembled with layered gneisses at 2685 Ga (Swapp et al., 2018). These rocks were intruded by trondhjemitic leucogranites between 2685 and 2675 Ma (Frost et al., 2016a). The basement rocks of the southern Teton Range include quartzofelds- pathic gneisses and hornblende gabbro (Reed, 1973; Love et al., 1992). They lack the 2675–2685 Ma leucogranites and all traces of the 2695 Ma granulite metamorphism. Intruding both domains is the Mount Owen batholith, an un- deformed, peraluminous leucogranitic batholith that was emplaced at 2547 ± 3 Figure 2. Geologic map of the Archean rocks of the Teton Range, modified after Love et al. Ma (Zartman and Reed, 1998). The Mount Owen batholith underlies the rugged (1992), showing locations of samples included in this study. high peaks in the central part of the range and forms granitic and pegmatitic dikes throughout the uplift. GEOSPHERE | Volume 14 | Number 3 Frost et al. | Neoarchean tectonic history of the Teton Range 1009 Research Paper Archean Geology of the Northern Teton Range Basin gneiss and the Layered Gneiss. No older zircon areas were identified, consistent with mainly positive initial ƐNd values for the Layered Gneiss The Archean rocks of the northern Teton Range consist of three major rock (Swapp et al., 2018). types. Most distinctive of these is the Moose Basin gneiss1, a suite of mafic Reed (1973) mapped large panels of leucogranitic gneisses that intruded and pelitic rocks that were metamorphosed into granulite facies (Swapp et al., the Moose Basin and the northern Layered Gneiss, which he named the Webb 2018). The Layered Gneiss, a sequence of heterogeneous gneisses, consists Canyon Gneiss. Based on the geochemistry of these leucogranitic gneisses, dominantly of psammitic to pelitic paragneiss with minor amounts of quartz- Frost et al. (2016a) recognized two compositionally distinct units, both of ofeldspathic orthogneiss, and mafic and ultramafic rocks. The Layered Gneiss which are calcic and silica-rich. The dominant unit is the Webb Canyon Gneiss, was mapped by Reed (1973) as a single unit that extended the whole length which is ferroan and, based on the classification of Barker (1979), is low-Al. of the range. Frost et al. (2016a) and Swapp et al. (2018) separated the Moose The less-voluminous unit, the Bitch Creek gneiss, forms small dikes and plu- Basin gneiss from the Layered Gneiss. In the northern Teton Range, the Lay- tons within the Moose Basin and northern Layered gneisses. It is distinguished ered Gneiss and the Moose Basin gneiss are intruded by two compositionally from the Webb Canyon Gneiss because it is magnesian and shows high-Al distinct leucogranitic gneisses, the Webb Canyon Gneiss and the Bitch Creek values, based on the classification of Barker (1979). Frost et al. (2016a) postu- gneiss (Frost et al., 2016a). lated that these compositional differences are produced by different modes The Moose Basin gneiss extends from Moose Basin at the northern end of of origin: water-excess melting for the Bitch Creek gneiss, and dehydration the Teton Range southwards along the crest of the range for ~10 km (Fig. 2). melting for the Webb Canyon Gneiss. Because the Webb Canyon Gneiss is by It is composed of kyanite-bearing metapelitic rocks with leucogranitic patches far the most voluminous, both types of leucogranitic gneiss are included in the interpreted as leucosomes formed by partial melting (Swapp et al., 2018).