Record of Neoarchean Continent Collision and Exhumation GEOSPHERE; V

Research Paper THEMED ISSUE: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

GEOSPHERE 2.7 Ga high-pressure granulites of the Teton Range: Record of Neoarchean continent collision and exhumation GEOSPHERE; v. 14, no. 3 Susan M. Swapp1, Carol D. Frost1, B. Ronald Frost1, and D. Braden Fitz-Gerald2 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA https://doi.org/10.1130/GES01607.1 2Exxon Mobil Corporation, 13401 N Freeway, Houston, Texas 77060, USA

14 figures; 4 tables; 1 set of supplemental files ABSTRACT However, high-pressure granulites are uncommon in the Archean. A compila- CORRESPONDENCE: swapp@uwyo.edu tion by Brown and Johnson (2018) of more than 450 localities with well-deter Continent-continent collisional orogens are the hallmark of modern plate mined metamorphic conditions and ages identifies very few occurrences of CITATION: Swapp, S.M., Frost, C.D., Frost, B.R., and tectonics. The scarcity of well-preserved high-pressure granulite facies ter- Archean rocks that record pressures of 10 kbar or greater (Table 1). Most are Fitz-Gerald, D.B., 2018, 2.7 Ga high-pressure granulites of the Teton Range: Record of Neoarchean conti- ranes minimally obscured by later tectonic events has limited our ability to from relatively small outcrops and commonly they have been overprinted by nent collision and exhumation: Geosphere, v. 14, no. 3, understand how closely Archean tectonic processes may have resembled subsequent metamorphic events, making their tectonic significance difficult to p. 1031–1050, https://doi.org/10.1130/GES01607.1. better-understood modern processes. Here we describe Neoarchean gneisses establish. All but two are younger than 3 Ga; in one of these two, the Barberton in the Teton Range of Wyoming, USA, that record 2.70 Ga high-pressure gran- terrane, temperatures do not reach granulite facies. Science Editor: Shanaka de Silva ulite facies metamorphism, followed by juxtaposition of gneisses with dif- In this contribution we evaluate the geological record of an Archean Guest Associate Editor: Allen J. McGrew ferent protoliths, and then by intrusion of leucogranites generated through high-pressure granulite terrain in the Wyoming Province, North America. No

Received 24 August 2017 decompression melting in response to post-collisional uplift. This evidence is Proterozoic or Phanerozoic metamorphism or deformation has affected these Revision received 12 February 2018 best explained as the result of a 2.70–2.68 Ga Himalayan-style orogeny, and rocks, making the Teton Range a suitable place to identify Archean tectonic pro- Accepted 9 April 2018 suggests that, although subduction may have been occurring earlier in the cesses. Archean rocks of the northern Teton Range include metapelitic gneisses Published online 7 May 2018 Archean, doubling of continental thickness by continent-continent collisions that record 2.70 Ga high-pressure granulite facies metamorphism followed by may date back to at least 2.7 Ga. tectonic juxtaposition with gneisses of different protoliths and geologic histo- ries, and then by intrusion of leucogranites. This evidence is best explained by INTRODUCTION doubling of crustal thickness through continent-continent collision followed by exhumation, decompression melting, and intrusion of 2.68 Ga leucogranitic sills. The nature of tectonic processes affecting the early Earth is an enduring Field and structural relations, petrology and geochemistry, Nd isotopic and controversial topic (Van Kranendonk, 2010; Hamilton, 2011; O’Neill et al., compositions, and U-Pb zircon geochronology allow us to describe the ori- 2016; Roberts et al., 2015; Smart et al., 2016; Tang et al., 2016). One of the most gin and evolution of this well-preserved assemblage. This terrain preserves significant obstacles to unraveling the early history of tectonic processes on evidence of both crustal thickening (based on high-pressure granulite facies Earth is the scarcity in both time and space of representative rock samples; the metamorphic conditions) and exhumation as evidenced by later, amphibolite situation becomes more limiting as time before present increases. Given this facies metamorphism. Zircon geochronology brackets the time for the whole fundamental fact, studies of Archean rocks produced by tectonic processes of cycle to occur. Although the terrain is only 50 × 15 km in size, it is an excellent OLD G that era and minimally affected by later tectonism and metamorphism are place to study Archean tectonic processes because there are rocks that are of primary interest in the study of these phenomena. sensitive recorders of metamorphic conditions and there are no significant Because tonalite-trondhjemite-granodiorite (TTG) and granite-greenstone younger tectonic overprints to complicate the interpretations. terrains are distinctive Archean assemblages, they have been the subject OPEN ACCESS of considerable study. However, their tectonic interpretation remains un- GEOLOGIC BACKGROUND settled, with some workers concluding that they may form in a static lid or plume-dominated regime whereas others call on plate tectonics to supply the The Teton Range, located in northwestern Wyoming, USA, exposes water needed for their formation (i.e., Condie, 2016). In this study, we look some of the westernmost outcrops of the Archean Wyoming Province instead at Archean high-pressure granulites as indicators of tectonic regime. (Fig. 1). The northern portion of the Teton Range is underlain by a series In the Phanerozoic, granulite-facies crustal rocks that preserve a record of of variably deformed paragneisses and orthogneisses (Reed, 1973) that This paper is published under the terms of the transport from the surface to depth along a clockwise P-T-t path are diagnostic are intruded by dikes of undeformed 2547 ± 3 Ma peraluminous leuco CC‑BY-NC license. of collisional plate tectonic processes that double the thickness of the crust. granite known as the Mt. Owen Quartz Monzonite (Zartman and Reed, 1998).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1031 Research Paper

TABLE 1. SUMMARY OF HIGH-PRESSURE ARCHEAN GRANULITE TERRAINS (AFTER BROWN AND JOHNSON, 2018) Age of metamorphism Location T and P maxRock types Inferred tectonic process Primary references 3658 Ma Itsaq Gneiss Complex, 870 °C, 1.25 Gpa Mafic garnet granulite Tectonic crustal thickening in collisional orogeny, followed by Nutman et al. (2013, 2015); West Greenland with associated felsic extension (Nutman et al., 2013, 2015) Brown and Johnson (2018) leucosome 3.23 GaInyoni shear zone, Barberton 600–650 °C, Garnet amphibolitesSubduction and burial of continental terrane (Moyen et al., 2006) Moyen et al. (2006); Van granite-greenstone terrain 1.2–1.5 Gpa OR foundering of greenstones into granitic gneisses during Kranendonk et al. (2014) convective overturn (Van Kranendonk et al., 2014) 2.819 GaVoronezh Crystalline Massif, 1000 °C, Banded iron formationCollisional orogeny and burial under thickened continental crust Fonarev et al. (2006); Savko et al. Eastern European Platform 1.0–1.1 Gpa (Fonarev et al., 2006) (2010) >2.70 GaAssynt block, Lewisian Gneiss 900–950 °C, Garnet granulitesSubduction OR intraplate sagduction (Johnson et. al. 2016) Crowley et al. (2015); Zirkler et al. Complex, Scotland 1.05–1.45 Gpa (2012); Johnson et al. (2016) 2.70 GaGridino area, Belomorian 710 °C, 1.85 Gpa Ecologites in mélange Subduction during accretionary-collisional orogeny on the southern Mints et al. (2010); Li et al. (2015) complex, Kola Peninsula complex margin of the Kola continent (Mints et al., 2010) 2.7 GaSouthern Marginal Zone, 850 °C, 1.1 Gpa Granulite facies Continental collision of Zimbabwe and Kaapvaal cratons Nicoli et al. (2015) Limpopo Belt, South Africa metapelites 2622 MaUweinat-Kamil basement 1050 °C, 1.0 GpaSapphirine-garnet-quartz Thickening and partial melting of continental crustKarmakar and Schenk (2015) complex, East Sahara metapelites Ghost Craton 2.56 GaMajorqaq Belt, West Greenland 760 °C, 1.15 Gpa Garnet-bearing migmatitic Collision of continental block with North American craton and Dyck et al. (2015) metapelites crustal thickening 2.55 GaSnowbird tectonic zone, east 800–850 °C, Mafic garnet granulites Granulite facies metamorphism followed widespread magmatism, Flowers et al. (2008); Dumond Lake Athabasca region, 1.3 to >1.4 Gpa overprinted at 1.9 Ga by second high-P metamorphism (Flowers et al. (2015) western Canadian shield et al., 2008)

W111° W104°

Beartooth Mtns. N. Dakota

N45° Montana Wyoming Bighorn Mtns.Black Hills Idaho Wyoming Province Figure 1. Precambrian geology of Wyoming Teton Range and adjacent states showing the location of the Teton Range and its relation to other Archean exposures in the Wyoming province. Precambrian rocks are exposed in S. Dakota the cores of Laramide uplifts (patterned). Nebraska The southwestern extent of the province is 1 100 uncertain. The northeastern boundary of Km the Wyoming province is after Worthington Wind River Range et al. (2015). Granite Mtns. Laramie Range

Cheyenne Belt

N41° Wyoming Proterozoic Utah Colorado Rocks Archean rocks Proterozoic rocks Wyoming province

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1032 Research Paper

The metamorphism and deformation described in this contribution are thus ure 2. Highly retrograded pelitic gneisses occur along the Teton divide at the required to be older than the 2547 Ma Mt. Owen Quartz Monzonite. The geo- head of Leigh Creek, ~5 km south of the southern margin of Figure 2. These logic map of the northern Teton Range (Fig. 2) is derived from the map of occurrences also may be part of the Moose Basin gneiss. Isolated, highly retro Reed (1973). Reed recognized three gneiss units: the Layered Gneiss, the leuco graded pelites occur in Osprey Canyon, on the eastern slope of the range (sam- granitic Webb Canyon Gneiss, and Augen Gneiss. A portion of the Layered ple 07T32). The eastern margin of the Moose Basin gneiss is a complex contact Gneiss of Reed consists of mafic gneiss and pelitic gneiss, both of which show zone ~100 m wide consisting of interlayered amphibolite and Webb Canyon evidence of high-pressure granulite metamorphism. Because these rocks are Gneiss. Following Reed (1973), we include this contact zone within the Moose so distinctive, we separate them out as the Moose Basin gneiss. (We do not Basin gneiss on Figure 2. capitalize “gneiss” here because this unit has not been formally recognized on The Layered Gneiss consists of pelitic and psammitic paragneisses that are U.S. Geological Survey maps [Frost et al., 2006]). interlayered with quartzofeldspathic orthogneiss, amphibolite, metagabbro, A characteristic feature of the Moose Basin gneiss is large kyanite blades and metaperidotite (Reed, 1963; Miller et al., 1986); pelitic gneisses are rare. in the pelitic gneiss, which are commonly more than a cm in length (Fig. 3A). Locally the paragneiss is migmatitic (Fig. 3D). Structurally beneath the pelitic gneiss is a mafic gneiss that locally retains Intrusive into the Moose Basin and Layered gneisses is foliated leuco granulite assemblages (Fig. 3B). In a few places, the mafic gneiss is cut by granitic Webb Canyon Gneiss of Reed (1973). On the basis of major element garnet-bearing leucosomes (Fig. 3C). The areal extent of the Moose Basin geochemistry, Frost et al. (2016) subdivided this unit into the Webb Canyon gneiss is reasonably well constrained. Kyanite is found as far south as where Gneiss and the Bitch Creek gneiss. The Webb Canyon Gneiss is the major upper Bitch Creek crosses the Teton gneisses near the southern margin of Fig- leucogranite gneiss in the area. It forms the large tabular exposures shown

location

Precambrian Quaternary Paleozoic Webb Canyon Gneiss Amphibolite USA Layered Gneiss Ultramafic rocks Moose Basin gneiss Mount Owen Quartz Monzonite

W110°55′ W110°43′

N43°57′

T77E a

t i o 06T29, 30, 31 n a l P a r k 08T31 B 03T5 o u n

d 04T33,42

a 07T8,9,11 r T2-31B,34A,G,H y 04T30 07T27 07T32 05T4 06T16 Doane 07T13,T35E,46A,46B Peak 07T6,T57 T32A

Rammel Jackson Mountain Lake N43°54′ 06T8,9,10

Figure 2. Geologic map across the northern Teton Range showing the relations between the high-pressure granulites (Moose Basin gneiss), leucogranites (undifferentiated Webb Canyon and Bitch Creek gneisses of Frost et al., 2016) and Layered Gneiss. Modified after Love et al. (1992). Numbers identify the samples cited in this paper. Two samples of Moose Basin gneiss analyzed for Sm-Nd isotopic compositions are located south of the map area (08T10 and 08T25).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1033 Research Paper

A B

Figure 3. Photos of field relations in the Moose Basin and Layered gneisses. Moose Basin gneiss: (A) kyanite-bearing pelitic gneiss with leucosome interpreted as the result of partial melting during granulite-facies metamorphism; (B) granulite-fa- C D cies assemblages surrounded by amphibolite in mafic gneiss; (C) partially melted mafic gneiss that contains garnet-bearing leucosomes. Layered Gneiss: (D) folded paragneiss with layers of leucosome.

on Fig. 2. In contrast, the Bitch Creek gneiss occurs in small sills, dikes, and METHODS plutons that cannot be delineated on Figure 2. Reed (1973) mapped Augen Gneiss near the western limits of Precambrian Structural measurements made in the field were plotted and interpreted outcrop. Because Miller et al. (1986) and Frost et al. (2016) have interpreted using Stereonet 9 software described by Cardozo and Allmendinger (2013). A this unit as a porphyritic portion of the Webb Canyon Gneiss, we have shown compilation of the structural measurements is available as Table S4 (footnote 1). it as part of the Webb Canyon Gneiss on Figure 2. The major ferromagnesian U-Pb zircon geochronology was undertaken using the sensitive high- mineral is biotite with accessory hornblende. It is moderately well-foliated and resolution ion microprobe with reverse geometry (SHRIMP-RG) at Stanford lineated, with little or no compositional layering. University. Zircon grains were mounted in epoxy. After polishing, cathodo- Titanite-bearing amphibolites with well-developed fabric occur as gener- luminescence (CL) scanning electron microscope (SEM) images were taken ally concordant but locally cross-cutting bodies within both the Moose Basin for all zircon grains. Isotopic ratios and U, Th, and Pb concentrations were gneiss and the Layered Gneiss. These rocks are described in more detail below standardized against VP-10 zircons. The data were reduced following the meth- under “Thermobarometry.” ods of Williams (1998) and using the SQUID Excel macro of Ludwig (2000).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1034 Research Paper

TableS1. Zircon analyses from leucosomes in partially melted mafic granulite (0 6T16) Radiogenic Ratios Age (Ma) Uncertainties given for individual analyses (ratios and ages) are at the 1σ level, rors). Uncertainties on Sm and Nd concentrations are ± 2% of the measured 204 206 207 207 206 207 U Th 206Pb* Pb Pb Pb Pb Pb Pb % 206 238 235 206 238 206 Disc Grain spot (ppm) (ppm) Th/U (ppm) Pb f206% U % err U % err Pb % err r+U + Pb 204 Rims and homogeneous cores (Fig. 7B) with correction for common lead made using the Pb lead and the model values. Sm-Nd isotopic data are given in Table 2. 06T16r-5 160360.2 70 1.18E-04 0.16 0.5069 0.8 12.71.0 0.1829 0.50.793 2640 18 2666 10 1 06T16E-3.1R71290.4 31 1.26E-031.720.51231.2 12.7 2.00.1978 0.80.642 2629 27 2675 25 2 06T16R-3 156500.3 69 3.15E-05 0.04 0.5159 0.913.01.0 0.1826 0.60.824 2681 19 2673 10 0 06T16E-13258 56 0.2110 2.30E-040.310.4962 0.7 12.50.9 0.1856 0.50.776 2591 14 2679 93 Pb evolution curve of Stacey and Kramers (1975). Concordia plots and linear Mineral compositional data were used in conjunction with whole-rock 06T16r-8 119290.2 53 3.46E-05 0.05 0.5206 1.013.11.2 0.1834 0.60.827 2701 22 2680 11 -1 06T16E-2 200660.3 88 3.22E-050.040.5121 0.812.90.9 0.1835 0.50.819 2665 16 2682 91 06T16E-6.1R149 50 0.3663.22E-040.440.5120 0.8 12.91.2 0.1874 0.60.700 2656 18 2684 14 1 06T16E-7 454179 0.4198 2.04E-05 0.03 0.5086 0.512.90.6 0.18410.3 0.81326501126886 1 discordia regression fits were carried out using Isoplot (Ludwig, 2003) and un- chemical composition data for thermobarometric analysis and for construc- 06T16E-5 196550.3 87 2.31E-050.030.5154 0.8 13.10.9 0.1841 0.50.830 2679 17 2688 90 06T16R-2 277148 0.5123 4.51E-050.060.5167 0.6 13.10.8 0.1844 0.40.818 2684 14 2688 70 06T16r-6 182450.2 79 4.06E-04 0.55 0.50470.8 12.71.2 0.18910.8 0.6372622162691153 06T16r-7 238440.2 1053.61E-040.490.5139 0.713.01.0 0.1889 0.40.659 2663 15 2694 13 1 certainties are reported at the 95% confidence level. U-Pb data are reported in tion of pseudosections. Whole-rock geochemical analyses used for pseudo- 06T16r-11286 99 0.3126 1.00E-320.000.5111 0.6 13.00.8 0.1845 0.40.827 2661 14 2694 71 06T16r-1057190.3 26 8.67E-05 0.12 0.5292 1.3 13.51.6 0.1857 0.90.810 2735 30 2695 16 -1 06T16E-8.2R214 54 0.3962.69E-050.040.52050.7 13.3 0.90.1851 0.50.825 2701 16 2696 80 1 06T16R-1 129400.3 56 1.00E-32 0.00 0.5054 0.912.91.1 0.1851 0.60.829 2637 20 2699 10 2 Supplemental Tables S1 and S2 . section calculations were obtained either from Frost et al. (2018) or by X-ray 06T16E-1 229660.3 1006.59E-050.090.5081 0.7 13.00.9 0.1862 0.50.813 2646 15 2702 82 06T16E-10.2R 197500.3 89 1.24E-050.020.5249 0.7 13.40.9 0.1860 0.50.829 2719 16 2706 80 06T16r-9 173520.3 79 1.43E-050.020.52920.8 13.6 1.00.1863 0.50.830 2738 18 2708 9-1 Cores (Fig. 7A) Sm-Nd isotopic data were obtained at the University of Wyoming on a VG fluorescence (XRF) using a Panalytical Axios 4 KW wavelength dispersive XRF 06T16r-12235 32 0.1943.09E-040.410.4644 0.7 12.60.9 0.2006 0.50.724 2451 14 2801 10 14 06T16E-6.2C201 71 0.4891.01E-040.130.51510.7 14.3 1.10.2031 0.80.672 2675 16 2842 13 6 06T16E-8.1C140 69 0.5653.67E-050.050.5404 1.016.02.0 0.2155 1.70.521 2784 23 2944 27 6 06T16E-10.1C 244156 0.6113 3.61E-050.050.53990.7 16.4 0.80.2208 0.40.843 2782 15 2984 77 Sector 54 thermal ionization multiple collector mass spectrometer. Samples instrument at the University of Wyoming. Whole-rock compositional data used 06T16E-1272430.6 36 2.99E-040.380.5810 1.218.21.5 0.2311 0.70.818 2944 28 3036 13 3 06T16E-4 220229 1.0115 7.68E-040.980.6054 0.719.02.8 0.2388 2.50.265 3028 18 3053 43 0.8 06T16E-9 254157 0.6129 8.79E-06 0.01 0.5918 0.7 19.11.2 0.2348 1.00.552 2996 16 3084 16 3 206 were dissolved in HF and HNO , then converted to chlorides. One aliquot of in calculation of pseudosections are given in Table 3. Notes: Uncertainties given at the 1 σ level. f206%, percentage of Pb that is common Pb (correction for common Pb made using the measured 3 204Pb/206 Pb ratio); % Disc., discordance, where 0% denotes a concordant analysis; r, correlation coefficien;t strike-through font indicates data excluded because of either high discordance or high U or Th. Stanford Univ. SHRIMP-RG, July, 2007. the sample was spiked with 149Sm and 146Nd. Rare-earth elements (REE) were Electron probe microanalysis of rutile, garnet, pyroxene, and hornblende

1Supplemental Tables. Table S1: Zircon analyses separated using conventional cation exchange chromatography, and Sm and were conducted at the University of Wyoming using the JEOL 8900 micro- from leucosomes in partially melted mafic granulite Nd were further separated by eluting HCl through di(2-ethylhexyl) orthophos- probe (Tables 4A–4C). (06T16). Table S2: Zircon analyses from leucosomes in phoric acid columns. An average 143Nd/144Nd ratio of 0.511846 ± 11 (2σ) was Pseudosections were constructed using Perple_X_6.7.7 (Connolly 2009) partially melted layered gneiss (06T9) Table S3: Loca- measured for the La Jolla Nd standard, normalized to 146Nd/144Nd = 0.7219. and fluid equation of state for H O and thermodynamic data set from Holland tions and brief descriptions for all samples referenced 2 in this contribution. Table S4: Compilation of the struc- Uncertainties in Nd isotopic ratio measurements are ± 0.00001 (2 standard er- and Powell (1991, 1998, 2011). In all samples, it was assumed that there was a tural measurements made in the field, plotted and interpreted using Stereonet 9 software described by Cardozo and Allmendinger (2013). Please visit https:// TABLE 2. Sm-Nd ISOTOPIC DATA FOR LAYERED AND MOOSE BASIN GNEISSES OF THE NORTHERN TETON RANGE doi.org /10 .1130/GES01607 .S1 or the full-text article on 143 144 Sm Nd Initial Nd/ Nd Initial εNd www.gsapubs.org to view the Supplemental Tables. Sample Location (ppm) (ppm) 147Sm/144Nd 143Nd/144Nd (2685 Ma) (2685 Ma) Mafic rocks within Layered Gneiss 07T9 Doane Peak 4.15 13.55 0.1851 0.512602 0.509323 3.3 07T13# Waterfalls Canyon0.924 3.539 0.1579 0.511941 0.509144 –0.2 07T27 Osprey Canyon 16.05 54.59 0.1778 0.512416 0.509267 2.2 T35E*Waterfalls Canyon4.52 24.81 0.1102 0.511063 0.509112 –0.8 Layered Gneiss 06T10 Talus Lake 23.099 89.73 0.1556 0.511828 0.509071 –1.6 06T8 Talus Lake 13.607 49.82 0.1651 0.512146 0.509221 1.3 07T6 Waterfalls Canyon 13.71 66.91 0.1239 0.511367 0.509172 0.4 07T11 Doane Peak 17.95 72.45 0.1498 0.511880 0.509226 1.4 07T8 Doane Peak 13.08 49.83 0.1587 0.511984 0.509173 0.4 T32A*Waterfalls Canyon 21.20 85.26 0.1504 0.511802 0.509139 –0.3 T46A*Waterfalls Canyon3.87 22.71 0.1031 0.511074 0.509248 1.9 T46B*Waterfalls Canyon4.36 12.13 0.2176 0.512988 0.509133 –0.4 T57* Waterfalls Canyon4.04 24.05 0.1015 0.511055 0.509257 2.0 Moose Basin gneiss 06T29# Bitch Creek Narrows4.53 22.64 0.1210 0.511200 0.509057 –1.9 06T30# Bitch Creek Narrows 30.93 184.62 0.1013 0.510811 0.509017 –2.7 06T31 Bitch Creek Narrows 21.74 81.62 0.1611 0.511875 0.509022 –2.6 07T32 Osprey Canyon2.70 14.18 0.11516 0.511007 0.508967 –3.7 08T10# Head of Moran Canyon4.17 16.22 0.1556 0.511852 0.509096 –1.1 08T25# Head of Leigh Creek 3.63 23.03 0.0954 0.510634 0.508944 –4.1 08T31# East of Bitch Creek Narrows2.48 14.21 0.1056 0.510929 0.509059 –1.9 T2-31B*Moose Basin1.848.940.1246 0.511146 0.508939 –4.2 T2-34A*Moose Basin2.44 10.54 0.1398 0.511499 0.509023 –2.6 T2-34G*Moose Basin2.73 13.77 0.1196 0.511234 0.509115 –0.8 T2-34H*Moose Basin2.00 10.54 0.1146 0.511037 0.509007 –2.9 T77E*Bitch Creek Narrows2.71 16.98 0.0964 0.510915 0.509207 1.1 #Finley-Blasi (2009). *Samples from Miller et al. (1986) analyzed at University of Wyoming and reported in Frost et al. (2006).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1035 Research Paper

TABLE 3. WHOLE-ROCK GEOCHEMICAL DATA FOR PELITES FROM TABLE 4A. GARNET ANALYSES, NORTHERN TETON PELITES MOOSE BASIN GNEISS ANALYZED WITH PSEUDOSECTIONS MBG* LG† 04T30* 06T10† 04T30 06T10 wt%mol% wt% mol% Avg. core Avg. rimAvg. core Avg. rim

SiO2 56.31 64.64 75.61 82.79 SiO2 38.00 37.80 37.79 37.69

Al2O3 19.48 13.18 11.42 7.37 Al2O3 21.41 21.17 20.89 20.78

TiO2 0.93 0.80 0.41 0.34 MgO6.003.783.412.57

Cr2O3 0.10 0.04 0.01 0.00 FeO 33.05 34.95 35.89 37.15 MgO 7.24 12.40 1.62 2.64 MnO0.761.700.370.45

Fe2O3 10.45 4.51 6.77 2.79 CaO0.971.010.530.52 MnO 0.06 0.06 0.05 0.05 S 100.19 100.41 98.88 99.16 CaO 0.55 0.68 0.72 0.84 Cations/12 Oxygens Na2O 1.45 1.61 2.43 2.58

K2O 2.83 2.07 0.85 0.59 Si 2.998 3.016 3.054 3.056 S 99.40 100.00 99.89 100.00 Al 1.99 1.991 1.99 1.987 *Moose Basin gneiss; data from University of Wyoming X-ray fluorescence laboratory; Mg 0.706 0.449 0.41 0.311 pseudosection in Figure 10. Fe 2.181 2.332 2.426 2.52 †Layered Gneiss; data from ALS Analytical Service, pseudosection in Figure 11. Mn 0.051 0.115 0.025 0.031 Ca 0.082 0.086 0.046 0.045 S8.008 7.989 7.951 7.95 *MBG—Moose Basin gneiss. saturated component (SiO2). The thermodynamic components selected were †LG—Layered Gneiss. t Na2O, MgO, Al2O3, K2O, CaO, TiO2, FeO and H2O. Solution models employed in the calculation of both pseudosections include: Gt(W), Chl(W), St(W), Crd(W), TABLE 4B. RUTILE IN MOOSE BASIN PELITES Ctd(W), Bi(W), Mica(W), Ilm(W), and melt(W) (White et al., 2014) and feldspar 04T30 05T4 (Fuhrman and Lindsley, 1988). Assumptions for fluid composition are given in Garnet In kyanite and Garnet In kyanite and the text for each pseudosection. inclusion matrix inclusion matrix Mineral abbreviations used in the text are after Whitney and Evans (2010). n6566 Mineral abbreviations used in the pseudosection plots are those generated by SiO n.d. n.d. 0.14 0.59 the Perplex program, correlate with the solution models listed above, and ad- 2 Nb2O5 0.46 0.25 0.11 0.18 here to the following conventions. If an abbreviation begins with an upper case ZrO2 0.50 0.06 0.31 0.05 letter, a solution model was used for that mineral in generating the pseudo Zr (ppm) 3700 444 2295 370 section. If an abbreviation begins with a lower case letter, a pure endmember FeO0.230.010.330.14 was used in generating the pseudosection. Mineral abbreviations beginning Ta 2O5 0.02 0.01 n.d. 0.01 TiO † 98.79 99.68 99.11 99.03 with upper case letters and shown in italics are subsets of the solution model 2 S 100.00 100.00 100.00 100.00 used in the calculation of the pseudosection. For example, Pl refers to plagio clase and Afs refers to alkali feldspar produced when the feldspar (ternary feld- Note: 04T30 and 05T4 are GRAIL (garnet–rutile–Al2SiO5 polymorph–ilmenite–quartz) pelites. n.d.—below detection limit. spars) solution model was employed. † TiO2 by difference. Locations and brief descriptions for all samples referenced in this contribution are given in Table S3 (footnote 1).

Layered Gneiss (Fig. 4). The second, F2, produced decameter-scale isoclinal RESULTS folds in the Moose Basin gneiss, the limbs of which define the major foliation in the region (A in Fig. 4). In Moose Basin the mafic gneiss forms the cores of

Structure the F2 antiforms. Adjacent to the pelitic gneiss the mafic gneiss is amphibolite, locally with garnet. Distal from the contact with the pelitic gneiss the mafic The older gneisses in the northern Teton Range have been affected by three gneiss contains granulite facies assemblages. The contact between the Moose

phases of folding. The first, F1, is manifested by rarely preserved refolded iso- Basin gneiss and the leucogranites is conformable to the limbs of the F2 folds clinal fold hinges within the pelitic gneiss in the Moose Basin gneiss and in the (B in Fig. 4).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1036 Research Paper

TABLE 4C. PYROXENE ANALYSES FROM MOOSE BASIN GNEISS MAFIC GRANULITES N Cpx Opx Moose Basin gneiss 20° 04T33 04T42 04T33 04T42 Pelitic schist

SiO2 50.00 50.20 48.85 49.75 F2 Webb Canyon Gneiss TiO2 0.28 0.20 0.10 0.09 Mafic gneiss Al2O3 2.57 1.62 0.95 0.83 C 30° F1 Cr2O3 0.01 0.01 0.02 0.01 Amphibolite Granulite 30° FeO 15.96 15.93 34.43 36.19 A MnO 0.22 0.27 0.60 0.59 MgO 9.64 9.84 12.01 12.39 CaO 20.23 19.81 0.88 0.81

Na2O 0.31 0.24 0.01 0.02 K2O 0.09 0.01 n.d. 0.01 D S 99.31 98.13 97.85 100.69

Cations/6 Oxygens Si 1.936 1.965 1.984 1.974 B B Ti 0.008 0.006 0.003 0.003 F2 Al 0.118 0.075 0.046 0.039 Layered Gneiss Cr 0.000 0.000 0.000 0.000 Fe 0.517 0.522 1.169 1.201 F1 Mn 0.007 0.009 0.021 0.020 boudinaged mafic dike Mg 0.557 0.575 0.727 0.733 Ca 0.839 0.831 0.038 0.035 Na 0.023 0.018 0.001 0.002 Figure 4. Block diagram showing relations among the major units in the northern Teton Range. K 0.004 0.000 0.000 0.000 Key features: (A) The Moose Basin gneiss consists of pelitic schist and mafic gneiss with the S 4.009 4.001 3.989 4.007 mafic gneiss forming the cores of the F2 antiforms. The margins of the mafic gneiss bodies are n.d.—not detected; cpx—clinopyroxene; opx—orthopyroxene. always foliated amphibolites but the core of the antiform may contain granulite assemblages. The F1 folds occur only as isolated fold axes in the pelitic schist and in the Layered Gneiss. Major

foliation in the area is parallel to the limbs of the F2 folds. (B) Contacts between the Moose Basin gneiss, Webb Canyon Gneiss, and Layered Gneiss lie parallel to the major foliation in the area.

The Webb Canyon and Bitch Creek gneisses do not contain F1 elements, (D) In most areas, the Webb Canyon Gneiss shows no folding; F2 folds are found only locally. Late

amphibolite dikes have locally been boudinaged. All units have been folded into large F3 folds. but they have a lineation that is parallel to the trend and plunge of the F2

fold axes of the Webb Canyon Gneiss (C in Fig. 4); F2 fold hinges are seen only locally in the leucogranites (D in Fig. 4), and F2 folds occur in some of

the migmatites formed during peak metamorphism. The leucogranites are as the axial plunge of the F3 fold. The fact that the lineations in the Moose Basin intruded by amphibolite dikes that locally show extensional sense-of-shear area (Fig. 5B) have the same trend as all the lineations across the northern part

(Frost et al., 2016). of the range (Fig. 5D) indicates that the F2 and F3 folds were nearly coaxial and

The F3 event is seen as a broad open fold that exhibits no axial planar suggests that they formed in the same deformation event. schistosity and involves all metamorphic rocks in the area (Fig. 4). The Moose

Basin gneiss occurs in the core of a major F3 synform and hence lies structurally above the Layered Gneiss, although the direct contact is nowhere preserved. GEOCHRONOLOGY

Poles to the F2 foliations measured in a small region around Moose Basin show a strong maximum in the southeast quadrant (Fig. 5A). Lineations in the same Two leucosome rocks contain zircon that formed during crystallization of area lie ~90° away in the northeast quadrant (Fig. 5B). The poles of the foliation the partial melts, and U-Pb dating of zircon from these rocks constrains the for all the gneisses across the northern Teton Range define a fold, the axis of timing of metamorphism in the northern Teton Range (Tables S1 and S2 which trends 20° and plunges 24° (Fig. 5C). We interpret this as the axial orien- [footnote 1]). One is a garnet-bearing leucosome that formed from its host

tation of the F3 folding. Lineations across the northern part of the range form a mafic granulite in the Moose Basin gneiss (06T16, Fig. 3B). The zircon grains strong maximum that lies in the northeast quadrant, with the same orientation in this sample range in shape from rounded to euhedral and contain inherited

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1037 Research Paper

A B cathodoluminescence, yield a U-Pb age of 2694.8 ± 6.7 Ma (MSWD = 1.0) (Fig. 7B; one highly discordant analysis was excluded from the calculation). We interpret these data as indicating the age of peak metamorphism and the partial melting of the mafic rocks. The zircon from 06T9 yields U-Pb ages of 2684.8 ± 4.9 Ma (MSWD = 0.3), which we interpret to represent the age of zircon growth

from partial melts associated with the F2 deformation event and the M2 metamorphic event (Fig. 7C). The formation of leucosome in 06T9 was coeval with the intrusion of Webb Canyon and Bitch Creek gneisses between 2686 and 2675 Ma (Frost et al., 2016).

ISOTOPE GEOCHEMISTRY Poles to foliations Lineations N = 61 N = 72 Sm-Nd isotopic data were obtained to help constrain the protoliths of C D the Moose Basin and Layered gneisses. We report data for 20 felsic samples (8 samples of the Moose Basin gneiss, 12 samples of the Layered Gneiss), and 4 samples of mafic rocks within the Layered Gneiss (Table 2; Fig. 8). In general, Sm and Nd contents and Sm/Nd ratios are higher in the Layered Gneiss than in the Moose Basin gneiss. These characteristics reflect the high REE contents and lack of light REE (LREE)-enrichment typical of the Layered Gneiss compared to the generally lower REE contents and LREE enrichment of the Moose Basin gneiss (Frost et al., 2018). Mafic rocks interlayered within the Layered Gneiss have variable Sm and Nd contents and Sm/Nd that reflect flat to LREE-enriched REE patterns (Frost et al., 2018). Amphibolite

dike samples (07T9 and 07T27) have higher Sm/Nd and positive initial εNd, whereas gabbro samples (07T13 and T35E) have lower Sm/Nd and slightly Poles to foliations Lineations negative initial ε . N = 140 Nd N = 216 Both the Moose Basin and Layered gneisses are characterized by varia- Contour interval = 2σ tion in initial εNd, but there are important differences between the two units. With one exception, the Moose Basin gneisses have initial less than –0.8, Figure 5. Structural data for the northern Teton Range. (A) Poles to the F2 εNd

foliations in the Moose Basin area. (B) Lineations (mostly L2 lineations) in whereas with one exception the felsic and mafic Layered Gneiss samples all the Moose Basin area. (C) Poles to foliations across the whole northern Teton have initial εNd –0.8 or greater (Fig. 8). These differences in initial Nd isotopic Range. Cylindrical fit to the foliations defines the F fold, which has a trend of 3 composition of the Moose Basin and Layered gneisses indicate that the two 20° and a plunge of 24°. (D) Lineations (mostly L2) from across the northern Teton Range. The lineations have a strong maximum near the axial trend of paragneiss units were composed of clastic detritus that was derived from dif- the F fold axes. Diagrams were calculated from program Stereonet (Cardozo 3 ferent sources: those of the Moose Basin gneiss are on average older crustal and Allmendinger, 2013). Contour intervals are 2σ above standard deviation (Kamb, 1959). sources, and those of the Layered Gneiss are more juvenile. This conclusion is also supported by the presence of >3 Ga zircon cores in the Moose Basin gneiss and the absence of these ancient cores in all zircons analyzed from the cores that are resorbed, zoned, and relatively bright in cathodoluminescence Layered Gneiss. (Fig. 6A). The second sample is a leucosome from partially melted Layered Gneiss (06T9, Fig. 3C). The zircon grains from this sample are euhedral, show primarily oscillatory growth zoning under cathodoluminescence, and lack THERMOBAROMETRY recognizable older cores (Fig. 6B). There are two age populations of zircon from 06T16 (Fig. 7). The cores of Rocks with low variance assemblages were selected for thermobarometric the zircon grains, some of which are relatively bright under cathodolumines- analysis. Mineral and whole-rock chemical data were evaluated in the con- cence, give ages around 3.1 Ga (Fig. 7A). We interpret this as reflecting the text of petrographic observations described above to deduce the P-T-t path age of the protolith. The rims of the grains, which are generally dark under inferred below.

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1038 Research Paper

06T16 2686±5 06T9

2706 ± 8 3084 ± 16 2944 ± 27 2682±7 2678±10

2689±9

2984 ± 7 2683±5

2696 ± 8

2686±8

100 microns 100 microns

Figure 6. Cathodoluminescence image of some of the zircon grains from 06T16 and 06T9. Grains from 06T16 show bright low-U cores that are considerably older than the margins. Grains from 06T9 lack recognizable older cores and yield concordant ages from all positions in the grains. Ages are 204Pb corrected 207Pb/206Pb ages with 1 σ errors.

Moose Basin Gneiss Pelitic Rocks 9C). Staurolite is locally reacting to cordierite + sillimanite (Fig. 9D). Ilmenite occurs as mantles on rutile within the matrix (Fig. 9E), but is absent on rutile The pelitic rocks in the Moose Basin gneiss are interlayered with leuco- inclusions in unfractured garnet (Fig. 9B). Textural relations clearly indicate somes, indicating significant melting at peak metamorphic conditions (Fig. that the Gt–St–Crd–Bt–Sil–Pl–Ilm–Zrn–Qz assemblage post-dates the Gt–Ky– 3A). The least retrograded Moose Basin gneiss pelites preserve evidence of Sil–Zrn–Qz assemblage, and the delicate reaction textures lack evidence of an early, high-pressure granulite facies metamorphic event predating or co- significant deforma tion after reaction. Finally, alkali feldspar is restricted to the eval with penetrative deformation and a later amphibolite facies event that leucosomes and is completely absent in the pelitic rocks. is largely unaffected by penetrative deformation. Garnets with rare kyanite, Temperatures for both assemblages are constrained by the Zr-in-rutile sillimanite, zircon, pyrite, and rutile inclusions record the peak metamorphic thermometer (Tomkins et al., 2007). Rutile occurring as isolated inclusions in conditions experienced by these rocks. While sparse biotite, plagioclase, and garnet has Zr as high as 3800 ppm, whereas rutile in the matrix and near frac- ilmenite after rutile occur along fractures within the garnets and along the gar- tures in the garnet ranges from 260 to 400 ppm Zr (Table 4B). The Zr-in-rutile net margins, none of these phases was observed to occur as inclusions within thermometer yields temperatures of 893 °C (8 kbar) to 920 °C (12 kbar) for the homogeneous portions of the garnets suggesting they were not stable during rutile inclusions in garnet, and temperatures of 630 °C to 690 °C (depending

early garnet growth at peak metamorphic conditions (Figs. 9A and 9B). on the SiO2 polymorph and the pressure) for the rutile in the matrix. Because The later amphibolite facies reaction assemblage (in the same rocks) is rutile loses Zr during re-equilibration at lower temperature, the maximum tem- Gt–St–Crd–Bt–Sil–Pl–Ilm–Zrn–Qz. Retrograde chlorite after biotite and garnet perature was at least 900 °C.

and intergranular sericite are also present. The garnets are rimmed by cor A pseudosection was calculated for one sample (04T30) assuming SiO2

dierite, are typically irregular in shape and have likely experienced significant saturation, fixed 4% 2H O by weight, and components Na2O, MgO, Al2O3, K2O,

resorption, consistent with enrichment of Mn and Fe around current grain CaO, TiO2, and FeO (Fig. 10). Details for solution models used in the calculation boundaries (Table 4A). Because of extensive resorption and generally anhedral are given in the “Methods” section. We did not include MnO in this calcula- shapes of garnets, relative timing of crystallization of sillimanite and kyanite tion because of its low concentration in the whole-rock chemical analysis. We

is not constrained by spatial distribution of the aluminosilicate inclusions in did not include Fe2O3 because the ilmenite is very nearly stoichiometric and garnets, but cm-sized kyanite blades in the matrix are partially replaced by sil- pyrite is present in both the matrix and as inclusions in the garnets in this

limanite, indicating that the kyanite predates sillimanite in these samples (Fig. sample. We based our estimate of H2O on loss on ignition from preparation of

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1039 Research Paper

A 3100 B 0.62 0.55 06T16 06T16 Rims 2760 0.58 Cores 0.53 U

U 2800

38 0.54 238 2 / / Pb Pb 6 6 0.

0 51 20 2

0.50 2600 2600

0.49 Intercepts at Intercepts at 0.46 1766±380 &3095±81 Ma 865±470&2694.8±6.7Ma MSWD =5.8 MSWD =1.00

0.42 0.47 10 12 14 16 18 20 22 11.8 12.2 12.613.013.413.814.2 207 235 207Pb/235U Pb/ U

0.56 C

2800 0.54 06T9 All Points

0.52 2700

U Figure 7. U-Pb concordia diagrams for 06T16 and 06T9. The low-uranium relict 8 3

2 0.50 2600

/ cores in 06T16 yield Mesoarchean ages (A), while the higher-uranium rims and unzoned crystals yield younger ages (B). Zircon from 06T9 lacks recognizable Pb 6

0 older cores and yields Neoarchean ages from all locations in the crystals (C). 2 0.48 2500 Complete analytical data are included in Tables S1 and S2 (see footnote 1). MSWD—mean square of weighted deviates.

0.46 Intercepts at

0.44 85±200 &2684.8±4.9Ma MSWD =0.31

0.42 10.5 11.5 12.5 13.5 14.5

207 235 Pb/ U

the XRF borosilicate beads; this estimate is supported by the fact that signifi- results are independent of assumptions about H2O, MnO and Fe2O3 abun-

cantly lower estimates for H2O (2.5%) yield large alkali feldspar fields near the dances or activities. These conditions are well above the wet melting curve

independently estimated T and P, and significantly higher estimates for 2H O for biotite, consistent with the presence of significant amounts of leucocratic

(6% and H2O saturation) truncate the kyanite/sillimanite reaction curve at a material in the rocks. Biotite, plagioclase, and alkali feldspar are not stable in melt field above 700 °C. this sample at these conditions, consistent with their absence as inclusions in The 900 °C Zr-in-rutile temperature intersects the kyanite/sillimanite phase the garnet. Interestingly, garnet core compositions (Table 4A) combined with boundary at 12 kbar, constraining the peak granulite facies conditions. These 900 °C estimate for temperature also yield a minimum GRAIL (garnet–rutile–

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1040 Research Paper

Moose Basin the range of 816 °C to 832 °C. The presence of garnet-bearing leucosomes 4 gneiss in the mafic granulites suggests temperatures closer to 900 °C (Fig. 3C; Patti- Layered Gneiss son et al., 2003). Without preserved garnet these rocks provide no constraints Figure 8. Diagram comparing initial 3 on pressure of equilibration. However, they yield temperatures comparable to εNd calculated at 2685 Ma for the Mafic rocks Moose Basin gneiss with that for N the more reliable pelite-based temperatures described above, and their granu- the Layered Gneiss. Data including 2 lite-facies assemblage and close proximity to those high-pressure pelites sug- sample numbers and sources, Sm gests that they probably equilibrated at pressures comparable to those above. and Nd abundances, and all ratios 1 are included in Table 2. Layered Gneiss Migmatitic Pelites -4 -2 02 εNd The Layered Gneiss includes interlayered quartzofeldspathic, migmatitic, and amphibolitic rocks. The most common assemblage in the Layered Gneiss is Qz–Bt–Pl±Afs±Gt, and rocks with assemblages suitable for thermobarome-

Al2SiO5 polymorph–ilmenite–quartz) pressure of equilibration for these rocks try are rare. We did not recognize high-pressure granulite facies assemblages of 12 kbar (Bohlen et al., 1983). The Gt–St–Crd–Bt–Sil–Pl–Ilm–Zrn–Qz amphibo- in the Layered Gneiss, but rare migmatitic pelitic samples yield amphibolite lite facies assemblage occurs at 675 °C < T < 710 °C and 6.4 kbar < P < 7.5 kbar. facies metamorphic conditions (Fig. 3D). Sample 06T10 is a pelitic enclave in This result is completely independent of but consistent with the Zr-in-rutile a leucosome (06T9) from which the zircons yielded an age of 2685 Ma (Fig. 7). temperature based on rutile occurring outside of garnets (and rimmed by il- This rock contains Gt–St–Bt–Sil–Pl–Rt–Zrn–Qz. The biotite is partially replaced menite) in this sample. by chlorite and the plagioclase is partially replaced by sericite. Plagioclase laths >5 cm in length also occur, suggesting the presence of melt zones. The Moose Basin Gneiss Mafic Granulites garnets are subhedral, fractured, and contain abundant inclusions of quartz, plagioclase, biotite, rutile, zircon, and rare, minute grains of ilmenite (Fig. 9F). Like the pelites, the least retrograded Moose Basin gneiss mafic granulites Patches of sericite and quartz with myrmekitic texture are common (Fig.9G), preserve evidence of an early, granulite facies metamorphic event and a later indicating significant hydrothermal alteration and possibly potassium meta amphibolite facies event. Mafic rocks in the cores of F2 folds preserve the gran- somatism. The rutile invariably occurs intergrown with chlorite and Zr concen- ulite facies assemblage Opx–Cpx–Pl–Ilm–(Gt)±Qz, and garnet-bearing leuco tration in rutile occurring within the matrix and as inclusions in garnets is gen- somes from partial melting of these rocks occur locally (Fig 3B, C). The pres- erally below detection limits (Figs. 9H and 9I). For these reasons, we interpret ence of garnet in the pre-amphibolite facies mafic granulites is inferred based the rutile as a retrograde product forming after ilmenite, and we believe the on occurrence of likely pseudomorphs of cummingtonite/gedrite/plagioclase principal Ti phase at peak metamorphic conditions was ilmenite. after garnet in the rocks and on the presence of garnet in the associated Because of evidence for late re-equilibration of mineral compositions, we leucosomes; no garnet has been identified in the mafic granulites. The later restrict our analysis of thermobarometric conditions of equilibration for this amphibolite-facies assemblage (in the same rocks) is Hbl–Pl–Ilm–Qz, where sample to inferences based on assemblage alone (and not on individual min- hornblende occurs exclusively as mantles on ilmenite and relict orthopyrox- eral compositions). Garnet and staurolite are texturally early, suggesting that ene and clinopyroxene formed during the earlier granulite facies metamor- 06T10 records a melting event in which the pelitic rocks contained staurolite.

phism. These rocks are generally massive with only weakly developed fabric. We calculated a pseudosection for this sample assuming 4% H2O, SiO2 satu-

The absence of surviving garnet and the strong dependence of P, T estimates ration, and no Fe2O3 (Fig.11). This pseudosection requires the Gt–St–Bt–Sil–Pl–

for these rocks on PH2O, however, limits their value in establishing quantitative Ilm–Zrn–Qz assemblage to coexist along a reaction curve from 665 °C, 6 kbar estimates of metamorphic conditions for either the granulite facies or the am- to 690 °C and 7.3 kbar; the absence of kyanite and primary rutile restricts this

phibolite facies metamorphic events. assemblage to P < 7.3 kbar (Fig. 11). Allowing some Fe2O3 would lower the The two-pyroxene mafic granulite assemblage does provide temperature activity of ilmenite and extend its stability to higher pressure. Based on this constraints for these rocks but not pressure constraints. Both pyroxenes are calculation, coexistence of this assemblage and melt restricts the assemblage moderately exsolved and have outer rims depleted in exsolution lamellae, to P > ~7.2 kbar. The assemblage Gt–St–Sil–Pl–Qz has been shown experimen- indicating that some re-equilibration of peak compositions has occurred. An tally to coexist with a water-saturated melt in the limited P-T range 665 °C < estimate of maximum T can be obtained using cation exchange thermometry T < 680 °C and 6.6 kbar < P < 7.5 kbar (shaded area in Fig. 11; Garcia-Casco

for the two-pyroxene assemblage, which is not dependent on PH2O. Using in- et al., 2003). The results of the pseudosection calculation are within 10 °C and tegrated compositions from cores of orthopyroxene and clinopyroxene grains 100 bars of the experimental results for the same assemblage. These relations (Table 4C), the QUILF program of Andersen et al. (1993) yields temperatures in suggest that the melting of 06T10 occurred at ~680–700 °C and 6.5–7.5 kbar.

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1041 Research Paper

A B C

Sil Gt Py

Figure 9. Backscattered electron and visi- Crd ble light micrographs illustrating textures 1 mm Crd in Moose Basin gneiss pelites and in Lay- 500 microns ered Gneiss pelites. Moose Basin gneiss 500 microns images from sample 04T30: (A) Large, partially resorbed anhedral to subhedral D E F garnets with very few inclusions, Fe and Mn-enriched margins, and cordierite (Crd) coronas. Py—pyrite. (B) Anhedral garnet (Gt) with a single, pristine rutile (Rt) inclu- Ilm sion that is not associated with ilmenite. (C) Minimally deformed partial replace- ments of cm-scale kyanite crystals by sillimanite. (D) Staurolite (St) being replaced by cordierite and sillimanite (Sil). (E) Rutile in matrix partially replaced by ilmenite (Ilm). Layered Gneiss images from sample 06T10: (F) Large subhedral garnet with Crd abundant inclusions (quartz, plagioclase, biotite, rutile + chlorite, chlorite, zircon and monazite). Detail for red box is shown St in H. (G) Typical myrmekitic intergrowth Rt of sericite (Ser) and quartz (Qz). (H) Rutile 400 microns and chlorite (Chl) inferred to have formed 200 microns Sil 1 mm from oxidation of primary ilmenite. (I) Typ- ical intergrowth of rutile and chlorite asso G H I ciated with matrix biotite; the rutile + chlorite is inferred to have formed from Ser oxidation of primary ilmenite. Gt

Chl Qz Rt

300 microns 50 microns 200 microns

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1042 Research Paper

Solidus Plagioclase Out Cordierite Staurolite Sillimanite Sillimanite + Cordierite Staurolite + Cordierite H2O Out Biotite Out Chl Gt WM Bi Gt WM Bi 30 31 q Pl ru q H O 18 Gt melt ky ru 2 Pl ru q H2O 12 16 17 M1 8 15 ky ru q Gt melt Bi q 14 22 Gt melt Bi Pl ky ru q Chl Gt WM Bi Gt melt sil ru

Pl ru q H O Ky 2 13 O

10 2 Sil Gt melt ru q 24

sill ru q

Gt melt Bi

H q ru Pl

Gt St Bi

Pl ru q H2O P (kbar) 7 Gt melt Bi Pl sill ru q Gt melt ru 8 6 23 Chl Gt St Bi 29 Gt melt Pl ru q H O Bi St melt Gt 2 M2 21 Pl sill ru q Crd ru q 5 Chl Gt St Bi Gt melt Bi Crd Pl ilm q 20 Gt melt Bi Crd Pl ru q Gt melt Bi Gt melt Crd ru Chl St Bi Pl 11 ilm q 19 Crd ru H 6 2 O 12 H 9 10 Gt melt Bi Cr 28

O Pl ilm q H m Gt melt Bi Crd Pl ilm q 27 Gt melt Crd Il Gt Bi Crd Pl ilm q H O 2 2 O d 4 26 Ilm Crd 2 3 25 Gt melt Bi melt Crd Ilm 4 1 600 700 800 900 1000 T (oC)

1 Chl Bi Crd Pl ilm H2O9Gt St Bi Crd Pl ilm H2O17Gt melt Bi Pl ky ru H2O25 Gt melt Bi Crd Pl ilm [q]

2 Chl Gt Bi Crd Pl ilm H2O10Gt Bi Crd Pl ru H2O18Gt melt WM Bi Pl ru H2O26Gt melt Bi Crd ilm [q]

3 Chl Gt Bi Crd Pl ilm H2O11Gt St Bi Crd Pl ru H2O19Gt melt Bi Crd Pl ru H2O27Gt melt Bi Crd ilm

4 Chl St Bi Crd Pl ilm H2O12Gt Bi Crd Pl sill ru H2O20Gt melt Bi Crd Pl sill ru H2O28Gt melt Bi Crd ru [q]

5 Chl WM St Bi Pl ilm H2O 13 Gt WM St Bi Pl ru H2O21Gt melt St Bi Crd Pl ru H2O29Gt melt Bi Crd sill ru

6 Chl Gt WM St Bi Pl ilm H2O14Gt St Bi Pl ky ru H2O22Gt melt St Bi Pl ky ru H2O30 Gt melt Bi Afs ky ru

7 Chl Gt WM St Bi Pl ru H2O15Gt WM St Bi Pl ky ru H2O23 Gt melt St Bi Pl ru q 31 Gt melt Afs ky ru

8 Chl Gt WM Bi Pl ru H2O16Gt WM Bi Pl ky ru H2O24 Gt melt St Bi Pl ky ru

Figure 10. Pseudosection for migmatitic pelite from Moose Basin gneiss (sample 04T30) showing coexisting high-pressure granulite facies assemblage (M1, 900 °C and 12 kbar) and younger amphibolite facies assemblage (M2, 690 °C and 7 kbar). The T/P conditions for the granulite facies assemblage are constrained by very high Zr-in-rutile occurring as inclusions in undamaged garnet, the presence of both sillimanite and kyanite and the absence of ilmenite as inclusions in the garnets, the general scarcity of inclusions and absence of feldspar and biotite inclusions in the garnet, the evidence for pervasive melting, and the absence of alkali feldspar in the samples. The T/P conditions for the amphibolite facies assemblage are constrained by the St–Crd–Sil–Rt/Ilm–Zrn–Qz assemblage and are consistent with the temperature independently predicted by the much lower Zr-in-rutile occurring in the matrix and near fractures in the garnets. Mineral abbreviations used in the pseudosection plots are those generated by the Perplex program (see text for details).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1043 Research Paper

(All assemblages + Quartz) 8.0 06T10 5 7 2 4

6 Gt melt Bi

7.5 3

Gt melt Bi

sill ru

Chl Gt WM Gt melt

Bi Pl ilm H O Pl 2 Figure 11. Pseudosection for a migmatitic

sill r pelitic sample from the Layered Gneiss Gt St Bi Pl ilm H2O M2 Pl (sample 06T10) showing the amphibo-

u sill ru lite facies assemblage (M2, ~7.3 kbar and

7.0 H ~690 °C). This sample includes no evi-

O dence for an earlier, granulite-facies meta-

P (kbar) morphic event. The T/P conditions for the

Gt melt Bi amphibolite facies assemblage are constrained by the assemblage Gt–Bt–Pl–St– Sil–(Ilm)–Zrn–Qz and by the petrograph- ically indicated coexistence of melt with

that assemblage (darkly shaded triangle,

sill ilm ilm sill Pl Gt Bi Bi Gt Garcia-Casco et al., 2003). Ilm is inferred

sill ilm to have been a part of the assemblage 6.5 1 12 16 before hydration and retrograde processes 13 oxidized it to produce Rt based on the fact that Rt is everywhere intimately inter-

2 15 grown with chlorite, the Zr concentrations O in the Rt are everywhere near or below de- O

2 tection limits in this sample, and there is H

H 14 2 no stability field with coexisting Rt + Sil + O St for this sample. Mineral abbreviations 11 used in the pseudosection plots are those

Pl ilm 8 Chl Gt St Bi 9 10 generated by the Perplex program (see 6.0 text for details). WM—white mica. 5500 600 650700 750 T (oC) Solidus Garcia-Casco Expt. Staurolite Sillimanite H2O Out St+Sil+melt Ilmenite Out

1Chl Gt WM St Bi Pl ilm H2O7Gt melt Bi Pl ky ru H2O13 Gt melt Bi Crd Pl sill ru

2Gt WM Bi Pl ilm H2O8Gt melt Bi Crd Pl sill ilm H2O14Gt melt Crd Pl sill ilm H2O

3Gt WM St Bi Pl ilm H2O9Gt melt Bi Crd ilm H2O15 Gt melt Crd Pl sill ilm

4Gt St Bi Pl ru ilm H2O10Gt melt Crd Pl ilm H2O16 Gt melt Crd Pl sill ru

5Gt St Bi Pl ru H2O 11 Gt melt Crd Pl ilm

6Gt melt St Bi Pl ru H2O12Gt melt Bi Crd Pl sill ru H2O

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1044 Research Paper

Garnet Amphibolites, Moose Basin Gneiss, and Layered Gneiss erally concordant but locally cut the leucogranites and migmatites, suggesting that they were emplaced either during or after the em- Two compositionally distinct groups of amphibolites occur in the north- placement of the leucogranites. Titanite-bearing garnet amphibolites ern Teton Range with the assemblage Hbl–Pl–Bt–Ilm–Qz±Gt±Ttn±Cum. One in both gneisses record pressures and temperatures of 4 ± 1 kbar and set has high Fe/(Fe+Mg) whole-rock and mineral compositions, comparatively 540 °C to 600 °C. sodic plagioclase, and garnet is present in virtually all of these samples. The 2. These units are intruded by the Webb Canyon and Bitch Creek leuco second set has lower Fe/(Fe+Mg) ratios and more calcic plagioclase and only a granites. These leucogranites did not experience the earliest folding

few samples in this category contain garnet (Fitz-Gerald, 2008). Both varieties event, F1, which is preserved in both the Moose Basin gneiss and Lay- occur in both the Moose Basin gneiss and in the Layered Gneiss. The am- ered Gneiss. phibolites form generally concordant but locally cross-cutting bodies within 3. Peak granulite facies metamorphism is dated at 2695 ± 7 Ma by zircon the host gneisses and leucogranites; they all have well-developed fabrics. growth in leucosome associated with mafic granulite in the Moose Basin Fitz-Gerald evaluated pressures and temperatures for these rocks using the gneiss. The date of partial melting of the Layered Gneiss is 2685 ± 5 Ma, barometer of Kohn and Spear (1990) and the thermometer of Dale et al. (2000). within the range of crystallization ages for the Webb Canyon and Bitch Mineral chemistry appropriate for this barometer occurred only in the lower Creek leucogranites (2675–2686 Ma). Fe/(Fe+Mg) amphibolites. Two garnet-bearing samples within the prescribed composition range for the barometer yield 540 °C to 600 °C and 4.0 ± 1 kbar and there is no systematic difference between amphibolites from the Layered P-T-t Path of the Northern Teton Range Gneiss and the Moose Basin gneiss (Fitz-Gerald, 2008). These results are consistent with the presence of titanite in the assemblage and the absence of gar- The thermobarometry shows that the rocks from the northern Teton net in all but the most iron-rich amphibolites in the region. Range record three distinct thermal regimes (Fig. 12). The Moose Basin gneiss records a high-pressure granulite-facies event of T ~ 900 °C and P > 12 kbar (M1) with a cooling trend to 690 °C and P ~7 kbar (M2). The DISCUSSION Layered Gneiss records peak conditions of only amphibolite-facies, with a pressure of ~7.3 kbar and temperature of ~690 °C. Significant melting accom- Key Features of the Northern Teton Range panied both metamorphic events, and some unknown amount of melt was probably lost from the affected units. Garnet amphibolites that occur within Based on the information presented above, the following observations must the Moose Basin gneiss and the Layered Gneiss record pressures lower than be taken into account in a tectonic interpretation of the Archean rocks of the 5 kbar and temperatures less than 600 °C (M3, significantly lower grade con- northern Teton Range. ditions than M2). From the P-T relations and the geochronology we infer that the Moose Ba- 1. Two contrasting gneiss units are present in the northern Teton Range: sin gneiss underwent high-pressure granulite metamorphism and melting at a. Moose Basin gneiss, composed of metapelitic and mafic rocks. These 2694.8 ± 6.7 Ma; these rocks were then tectonically juxtaposed with the Lay- have reached granulite facies and have partially melted. Peak pres- ered Gneiss at conditions near those of Barrovian metamorphism at 2684.8 ± sures and temperatures are in excess of 12 kbar and 900 °C. The pelitic 4.9 Ma (Fig. 12). A final lower grade amphibolite facies event is recorded in rocks contain zircon cores ca. 3.1 Ga and their Nd isotopic compo- both the Moose Basin gneiss and the Layered Gneiss garnet amphibolites at sitions are consistent with derivation from older continental crustal 540 °C < T < 600 °C and P 4 ± 1 kbar (M3). This event also produced titanite in sources. The Moose Basin gneiss preserves evidence of both F1 and amphibolite and gabbro throughout the Teton Range. This event is interpreted F2 deformation. as the time of accretion of crustal blocks, including the northern Teton Range, b. The Layered Gneiss, composed of quartzofeldspathic paragneiss, to the Wyoming craton (Frost et al., 2018) and is unrelated to the 2.70–2.68 Ga interlayered amphibolite and areas of peridotite and gabbro. These collisional orogeny described in this paper. This event may also be responsible quartzofeldspathic gneisses are migmatitic but apparently did not for some of the extensive retrogression of the higher grade assemblages ob- exceed amphibolite facies conditions of ~7.5 kbar and 700 °C. The served in the Layered Gneiss and in the Moose Basin gneiss. Layered Gneiss has relatively radiogenic Nd isotopic compositions These observations require a clockwise P-T path for the Teton Range (Fig. consistent with derivation from juvenile sources. The Layered Gneiss 12). In Moose Basin gneiss pelites, both kyanite and sillimanite occur as preserves evidence of both F1 and F2 deformation. inclusions in garnets, but sillimanite clearly replaced kyanite as metamor- c. Amphibolites, some of which contain garnet, occur in both the Moose phism and deformation progressed. Rutile inclusions in garnet record tem- Basin gneiss and in the Layered Gneiss. Amphibolite dikes are gen- peratures of 900 °C and garnet-bearing leucosomes formed in mafic gran-

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1045 Research Paper

13 emplacement of leucogranites M1 Moose Basin gneiss: 2695 Ma Zr in Rutile Inclusions in Garnet 07T16 Bitch Creek Gt - Ky/Sil - Rt Assemblage 04T17 gneiss Melting of Mafic Rocks to Produce Garnet 06T33 11 06T39 Webb Canyon 06T35 Gneiss Moose Basin gneiss: 06T12 high-P Zr in Rutile Inclusions in Kyanite 07T4 granulite St - Crd - Sill - Ilm Assemblage 03T8 9 metamorphism Layered Gneiss Migmatites: 07T30 Gt - St - Sil - Pl - Melt Assemblage tectonic 06T9 Likely P-T assembly 06T16 Path P (kbar ) 2690 2700 7 04T30 2670 2680 ? Age (Ma) 06T10 M2 Figure 13. Summary of geochronology of the northern Teton 2685 Ma 03T5 Range. The age of high-pressure metamorphism (M1) is interpreted from U-Pb zircon ages in the leucosome of partially 5 Moose Basin gneiss: melted mafic granulite (sample 06T16). The timing of tectonic Garnet Amphibolites assembly and M2 amphibolite grade metamorphism is pro- M3 Cum - Hbl - Gt Assemblage vided by the age of leucosome in partially melted Layered Relationship to M1/M2 path Gneiss (06T9). Leucogranitic gneisses were intruded coincident is unclear. with tectonic assembly; samples of two distinct leucogranitic suites, the Bitch Creek gneiss and the Webb Canyon Gneiss, 2 are described in detail by Frost et al. (2016) and are arranged in 400 500 600 700 800 900 1000 order of their location from east to west. T (oC)

Figure 12. P-T-t path inferred for the northern Teton Range. show F2 folding and some do not, Frost et al. (2016) proposed that the emplacement must have occurred in at least two stages. The Webb Canyon and Bitch Creek leucogranites, although both trondhje- ulites (M1). Approximately 10 m.y. later, amphibolite facies assemblages mitic, have distinct geochemical characteristics. The Webb Canyon Gneiss is replace the granulite facies assemblages in Moose Basin gneiss pelites strongly ferroan, comparatively low in alumina, and is characterized by high and mafic granulites, and rutile everywhere except as inclusions in garnet Zr and Y, low Sr, and high REE contents that define “seagull” shaped patterns. lost most of its Zr in response to falling temperatures. Migmatitic Layered The Bitch Creek gneiss is lower in Zr, Y, and REE and alumina and Sr are higher Gneiss lacks evidence of the early granulite facies metamorphic event (M1), than in the Webb Canyon Gneiss. Frost et al. (2016) concluded that these dif- but clearly records evidence of the later amphibolite facies event (M2). This ferences reflect two different petrogenetic processes, both of which are typi- clockwise P-T path is likely the product of rapid burial by thrusting, heat- cal of collisional environments. The geochemical characteristics of the Bitch ing, uplift, and exhumation, all characteristic of continent-continent colli- Creek gneiss are consistent with water-excess melting in a collision-related sions in Phanerozoic time (England and Thompson, 1984; Thompson and overthrust, where a relatively cool, hydrous lower plate releases water into a England, 1984). hotter upper plate. The Webb Canyon magmas formed by dehydration melting The boundary between the Moose Basin and Layered Gneiss was intruded caused when dramatically thickened crust undergoes gravitational collapse by leucogranites between 2674 ± 5 Ma and 2686 ± 5 Ma; the dates for the and tectonic extension. Because the slope of the dehydration reaction is posi- Webb Canyon and Bitch Creek leucogranites are indistinguishable (Fig. 13; tive, melts formed by this process can migrate to shallower levels without in-

Frost et al., 2016). These dates indicate that the high-pressure metamorphism tersecting their solidus. The sheet-like form and F2 fabrics of the Webb Canyon occurred around 10 m.y. before tectonic assembly, although within error it Gneiss are consistent with layer-parallel magma migration during orogenic could have been nearly coeval. Extensive emplacement of leucogranites could collapse. Both water-excess and dehydration melting have been called upon to have followed the 2685 Ma tectonic assembly for up to 10 m.y. The ages of the explain leucogranite of the Himalaya (Le Fort et al., 1987; Reichardt and Wein- leucogranites fall within error of each other, but because some leucogranites berg, 2012; Searle et al., 2009; Visona’ et al., 2012).

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1046 Research Paper

Similarity between Northern Tetons and melting also may have taken place at greater depths. Because water in the Phanerozoic Continental Collisions source mantle is transferred almost completely into the melt phase, a thicker dehydrated lithosphere is formed, one which is stiffer and stronger than hy- Five features in the gneisses of the northern Teton Range are characteristic drated mantle. As these thick plates become more dense with age they will of modern continent-continent collisional orogens. reach neutral buoyancy, and then subduct. If Archean lithosphere is strong enough to subduct, why are Archean (1) High-pressure granulite-facies metamorphism is followed by clock- high-pressure granulites so rare? Korenaga speculates that the paucity of wise cooling. high-pressure rocks early in Earth’s history may reflect some difficulty in the (2) High-pressure metamorphism precedes the tectonic assembly of the exhumation stage. Thermomechanical numerical models indicate that the ulti gneisses by ~10 m.y. This is typical of high-pressure rocks from the Hima mate driver is the ratio of the intrinsic buoyancy of the subducted continen- laya and Alps, where rocks subducted to great depths are exhumed as tal crust to side traction forces in the conduit (Butler et al., 2014). Models by the subduction zone migrates prior to the final collision (Rubatto et al., Husson et al. (2009) show that slab rollback within subduction zones induces 1999; Babist et al., 2006). conditions where material in the subduction channel flows upwards. More- (3) Leucogranites are emplaced in an extensional environment immedi- over, when rollback is associated with a decrease in slab dip, the exhumation ately following tectonic assembly (Frost et al., 2016). Such a time scale process becomes more efficient. These studies suggest that exhumation is not is typical of the Himalayan orogeny (Searle et al., 2003). Our results sug- expected to accompany vertical tectonic processes that have been called on gest that leucogranite emplacement accompanied tectonic assembly at to explain many Archean terrains. High-pressure granulites may instead be 2685 Ma, and continued for as long as 10 m.y. restricted to those areas where subduction enables exhumation rather than

(4) Initial εNd of the metasedimentary rocks in Moose Basin gneiss and where sagduction or crustal overturn processes occur. those in the Layered Gneiss are distinct, which suggests the two sedi We have argued above that the Archean rocks of the northern Teton Range mentary packages were derived from different sources and were de- are best interpreted in terms of a collisional orogeny in which fine-grained posited in separate basins. The Nd isotopic evidence thus supports sediments and ultramafic rocks are transported along a clockwise P-T path to the interpretation that the Moose Basin gneiss represents detritus de- depths of 35 km or greater, then exhumed during orogenic collapse and in- rived from a different, more ancient continental block than the Layered trusion of leucogranites. This distinctly plate tectonic interpretation has not Gneiss, which is derived from relatively juvenile crustal sources, and been universally invoked for the other Archean high-pressure granulite ter- that these were juxtaposed during collision at high-pressure granulite rains shown on Table 1 and Figure 14. In many cases, incomplete preservation facies conditions. Ultramafic rocks within the Moose Basin and Layered or a subsequent metamorphic overprint precludes definitive identification of gneisses may represent remnants of oceanic crust caught up with the tectonic process. For the oldest high-pressure rocks, both plate tectonic and two sedimentary packages during collision. pre-plate vertical tectonic interpretations have been proposed. The oldest (5) The association of metaperidotite, metagabbro, and juvenile metasedi high-pressure granulites occur within the 3.8 Ga Itsaq gneiss. Brown and John- mentary rocks in the Layered Gneiss is an assemblage that is found in Phanerozoic mountain belts where an ocean basin has been consumed during continental collision. An example is the Tectonic Accretion Chan- 25 nel in the central Alps (Engi et al., 2001). Scourie 20 Kola Snowbird Tetons 15 Barberton Archean High-P Granulites Kbar Limpopo P, 10 Itsaq Majorqaq VCM E Sahara Korenaga (2013) observes that for high-pressure granulites to be observed 5 at the Earth’s surface, two processes must occur. First, crustal rocks must be 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 taken down to depth where they are metamorphosed under high pressure and Time of metamorphism, Ga temperatures conditions. Second, they must then be exhumed rather rapidly in order to preserve peak metamorphic conditions. One might predict that in Figure 14. Diagram showing pressure conditions for Precambrian high-pressure granulites as a a hotter Archean Earth, hotter slabs would be weaker and less likely to sub- function of age. Plotted are localities that record pressures in excess of 10 kbar from Brown and duct. Korenaga’s analysis of the strength of Archean slabs suggests that to the Johnson (2018). Kola and Barberton did not reach granulite facies temperatures. High-pressure granulites are rare prior to the Neoarchean. High-pressure granulites of the Teton Range record contrary, they are likely to be stronger. A hotter mantle early in Earth history a 20 million year cycle of subduction and exhumation, a history that is not overprinted by post- is likely to have experienced a greater degree of partial melting and partial Archean metamorphism and deformation. VCM—Voronezh Crystalline Massif.

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1047 Research Paper

son (2018) estimate that the assemblage Gt–Cpx–Hbl–Qz formed at ~12 kbar by water excess melting and dehydration melting were intruded. This history and 870 °C. Some authors have interpreted these rocks to record a collisional is analogous to Cenozoic continental collisions such as the Alps (Engi et al., orogeny (Nutman et al., 2013, 2015), others propose that stagnant-lid tectonics 2001; Babist et al., 2006; Searle et al., 2009). were dominant at this time and that the Itsaq rock record reflects local, epi- A number of authors, including Voice et al. (2011), Condie and Aster (2010), sodic subduction (e.g., Brown, 2016). Likewise, the high-pressure amphibolites and Condie et al. (2011) have identified major zircon U-Pb age peaks at 2.7– of the Inyoni shear zone, Barberton granite-greenstone terrane have yielded 2.5 Ga, 2.0–1.7 Ga, 1.6 Ga, 1.2–1.0 Ga, and 0.7–0.5 Ga. These age peaks correlate multiple tectonic interpretations. Moyen et al. (2006) suggested that the high- with times of supercontinent formation. Brown and Johnson (2018) noted grade rocks were subducted to depths of at least 35 km before being exhumed that ages of peak metamorphism also cluster at these times, and suggest along the shear zone in a cycle that took place between 3.23 and 3.22 Ga. They that metamorphism records the amalgamation of continental fragments into interpreted synkinematic trondhjemites as the result of decompression melt- supercontinents. Neoarchean continent assembly includes the construction of ing during return flow. Alternatively, Van Kranendonk et al. (2014) propose that Superior, Sclavia, and Vaalbara (Bleeker, 2003). Collisional orogeny recorded the greenstones foundered to depths of 12–15 kbar during partial convective in the Teton Range supports the hypothesis that plate tectonic processes were overturn at around 3.23 Ga. They suggest that the greenstones were exhumed involved in the amagalmation of crustal blocks into larger continental masses along a mylonite zone associated with an intrusive event at 3.11 Ga (Van by the Neoarchean. Kranendonk et al., 2014). If so, then exhumation was much later than expected by analogy with modern continent-continent collisions. Most Archean high-pressure granulites are Neoarchean, and one of the ACKNOWLEDGMENTS best studied also has disparate tectonic interpretations. The granulites of The authors acknowledge financial support provided by National Science Foundation grant EAR the Assynt block, Lewisian Gneiss Complex, record multiple periods of defor- 0537670 to B.R. Frost, C.D. Frost, and S.M. Swapp and by University of Wyoming/Grand Teton National Park grant 49182 to B.R. Frost and C.D. Frost. We extend sincere thanks to Dave Mogk and mation and metamorphism, making the Archean high-pressure event difficult to one anonymous reviewer for extremely thorough and very helpful reviews; the paper is greatly to interpret. Johnson et al. (2016) conclude that a subduction-related origin improved thanks to their comments and suggestions. We also thank Al McGrew for his help as is possible in part because many of the rocks have trace element signatures the managing editor for this submission. Dr. Joe Wooden is thanked for hosting our analytical ses- sions on the SHRIMP-RG instruments at Stanford. This manuscript was completed while C.D. Frost characteristics of modern arcs. However, the authors propose that a process was serving at the National Science Foundation. of sagduction is also possible, in which mafic and ultramafic rocks sank into the deep crust due to their greater density compared to underlying, partially molten felsic orthogneisses. The authors suggest that downward flow would REFERENCES CITED be arrested by increasing stiffness of the orthogneiss residua as partial melt Andersen, D.J., Lindsley, D.H., and Davidson, P.M., 1993, QUILF: A PASCAL program to assess was extracted. equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine, and quartz: Computers & Geosci- In contrast to Archean terranes that have been affected by Proterozoic ences, v. 19, p. 1333–1350, https://doi.org/10.1016/0098-3004(93)90033-2. Babist, J., Handy, M.R., Konrad-Schmolke, M., and Hammerschmidt, K., 2006, Precollisional, metamorphism and deformation, the last such events in the Wyoming prov- multistage exhumation of subducted continental crust: The Sesia Zone, western Alps: Tec- ince were Neoarchean. The lack of subsequent tectonism greatly simplifies tonics, v. 25, TC6008, https://doi.org/10.1029/2005TC001927. the interpretation of Archean events in the northern Teton Range, allowing the Bleeker, W., 2003, The late Archean record: a puzzle in ca. 35 pieces: Lithos, v. 71, p. 99–134, https://doi.org/10.1016/j.lithos.2003.07.003. peak conditions and clockwise P-T-t path to be identified and the 20 million year Bohlen, S.R., Wall, V.J., and Boettcher, A.L., 1983, Experimental investigations and geological

cycle of subduction and exhumation to be quantified. applications of equilibria in the system FeO-TiO2-Al2O3-SiO2-H2O: The American Mineralo- gist, v. 68, p. 1049–1058. Brown, M., 2016, Archean metamorphism and geodynamics: from episodic and local to continu- ous and global subduction and mobile-lid plate tectonics: International Geological Congress CONCLUSIONS Abstracts, v. 35, abstract 3493. Brown, M., and Johnson, T., 2018, Secular change in metamorphism and the onset of global We suggest that the Archean rocks of the northern Teton Range formed by plate tectonics: The American Mineralogist, v. 103, p. 181–196, https://doi .org/10.2138/am -2018-6166. a process involving lateral, collisional orogeny. The metapelitic rocks of the Butler, J.P., Beaumont, C., and Jamieson, R.A., 2014, The Alps 2: Controls on crustal subduction Moose Basin gneiss derived from a ca. 3.1 Ga continental source were tecton- and (ultra)high-pressure rock exhumation in Alpine-type orogens: Journal of Geophysical ically buried to depths corresponding to >12 kbar at 2695 Ma. This material Research, v. 119, p. 5987–6022. Cardozo, N., and Allmendinger, R.W., 2013, Spherical projections with OSXStereonet: Com was exhumed and juxtaposed with the Layered Gneiss, composed of juve- puters & Geosciences, v. 51, p. 193–205, https://doi.org/10.1016/j.cageo.2012.07.021. nile metasediments and interlayered amphibolites, gabbros, and peridotite, at Condie, K.C., 2016, Crust and mantle evolution, in Condie, K.C., Earth as an Evolving Planetary depths equivalent to ~7 kbar at 2685 Ma. The mafic and ultramafic rocks are System (third edition): London, Academic Press, p. 147–199. Condie, K.C., and Aster, R.D., 2010, Episodic zircon age spectra of orogenic granitoids: The super interpreted as the remnants of ocean crust consumed prior to collision. Shortly continent connection and continental growth: Precambrian Research, v. 180, p. 227–236, thereafter and for as long as 10 million years following, leucogranites formed https://doi.org/10.1016/j.precamres.2010.03.008.

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1048 Research Paper

Condie, K.C., Bickford, M.E., Aster, R.C., Belousova, E., and Scholl, D.W., 2011, Episodic zircon Husson, L., Brun, J.-P., Yamato, P., and Faccenna, C., 2009, Episodic slab rollback fosters exhu- ages, Hf isotopic composition, and the preservation rate of continental crust: Geological mation of HP-UHP rocks: Geophysical Journal International, v. 179, p. 1292–1300, https://doi Society of America Bulletin, v. 123, p. 951–957, https://doi.org/10.1130/B30344.1. .org/10.1111/j.1365-246X.2009.04372.x. Connolly, J.A.D., 2009, The geodynamic equation of state: What and how: Geochemistry, Geo- Johnson, T.E., Brown, M., Goodenough, K.M., Clark, C., Kinny, P.D., and White, R.W., 2016, Sub- physics, Geosystems, v. 10, Q10014, https://doi.org/10.1029/2009GC002540. duction or sagduction? Ambiguity in constraining the origin of ultramafic-mafic bodies in Crowley, Q.C., Key, R., and Noble, S.R., 2015, High-precision U-Pb dating of complex zircon the Archean crust of NW Scotland: Precambrian Research, v. 283, p. 89–105, https://doi .org from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach: /10.1016/j.precamres.2016.07.013. Gondwana Research, v. 27, p. 1381–1391, https://doi.org/10.1016/j.gr.2014.04.001. Kamb, W.B., 1959, Ice petrofabric observations from Blue Glacier, Washington in relation to Dale, J., Holland, T.J.B., and Powell, R., 2000, Hornblende-garnet-plagioclase thermobarometry; theory and experiment: Journal of Geophysical Research, v. 64, p. 1891–1909, https://doi.org a natural assemblage calibration of the thermodynamics of hornblende: Contributions to /10.1029/JZ064i011p01891. Mineralogy and Petrology, v. 140, p. 353–362, https://doi.org/10.1007/s004100000187. Karmakar, S., and Schenk, V., 2015, Neoarchean UHT metamorphism and Paleoproterozoic UHT Dumond, G., Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2015, Monazite as a monitor reworking at Uweinat in the East Sahara Ghost Craton, SW Egypt: evidence from petrology of melting, garnet growth and feldspar recrystallization in continental lower crust: Journal of and texturally controlled in situ monazite dating: Journal of Petrology, v. 56, p. 1703–1742, Metamorphic Geology, v. 33, p. 735–762, https://doi.org/10.1111/jmg.12150. https://doi.org/10.1093/petrology/egv051. Dyck, B., Reno, B.L., and Kokfelt, T.F., 2015, The Majorqaq Belt: A record of Neoarchaean orogen- Kohn, M.J., and Spear, F.S., 1990, Two new geobarometers for garnet amphibolites, with applica- esis during final assembly of the North Atlantic Craton, southern West Greenland: Lithos, tions to southeastern Vermont: The American Mineralogist, v. 75, p. 89–96. v. 220–223, p. 253–271, https://doi.org/10.1016/j.lithos.2015.01.024. Korenaga, J., 2013, Initiation and evolution of plate tectonics on Earth: theories and observa- Engi, M., Berger, A., and Roselle, G.T., 2001, Role of the tectonic accretion channel in collisional tions: Annual Review of Earth and Planetary Sciences, v. 41, p. 117–151, https://doi .org/10 orogeny: Geology, v. 29, p. 1143–1146, https://doi.org/10.1130/0091-7613(2001)029<1143: .1146/annurev-earth-050212-124208. ROTTAC>2.0.CO;2. Le Fort, P., Cuney, M., Deniel, C., France-Lanord, C., Sheppard, S.M.F., Upreti, B.N., and Vidal, P., England, P.C., and Thompson, A.B., 1984, Pressure-temperature-time paths of regional metamor- 1987, Crustal generation of Himalayan leucogranites: Tectonophysics, v. 134, p. 39–57, https:// phism I. Heat transfer during the evolution of regions of thickened continental crust: Journal doi.org/10.1016/0040-1951(87)90248-4. of Petrology, v. 25, p. 894–928, https://doi.org/10.1093/petrology/25.4.894. Li, X., Zhang, L., Wei, C., and Slabunov, A.I., 2015, Metamorphic PT path and zircon U-Pb dating Finley-Blasi, L., 2009, The significance of the Webb Canyon Gneiss in the Archean rocks of the of Archean eclogite association in Gridino complex, Belomorian province, Russia: Precam- northern Teton Range, Wyoming [M.Sc. thesis]: University of Wyoming, Laramie, Wyoming. brian Research, v. 268, p. 74–96, https://doi.org/10.1016/j.precamres.2015.07.009. Fitz-Gerald, D.B., 2008, Evidence for an Archean Himalayan-style orogenic event in the northern Love, J.D., Reed, J.C., Jr., and Christiansen, A.C., 1992, Geologic map of Grand Teton National Teton Range, Wyoming [M.Sc. thesis]: University of Wyoming, Laramie, Wyoming. Park, Teton County, Wyoming: U.S. Geological Survey Miscellaneous Investigations Series Flowers, R.M., Bowring, S.M., Mahan, K.H., Williams, M.L., and Williams, I.S., 2008, Stabilization Map I-2031, scale 1:62,500, 17 p. text. and reactivation of cratonic lithosphere from the lower crustal record in the western Cana- Ludwig, K.R., 2000, SQUID 1.02, A User’s Manual: Berkeley, California, USA, Berkeley Geochro- dian shield: Contributions to Mineralogy and Petrology, v. 156, p. 529–549, https://doi .org/10 nology Center Special Publication. .1007/s00410-008-0301-5. Ludwig, K.R., 2003, ISOPLOT: A geochronological toolkit for Microsoft EXCEL: Berkeley, Califor- Fonarev, V.I., Pilugin, S.M., Savko, K.A., and Novikova, M.A., 2006, Exsolution textures of ortho nia, USA, Berkeley Geochronology Center Special Publication No. 4, 70 p. pyroxene and clinopyroxene in high-grade BIF of the Voronezh Crystalline Massif: evidence Miller, S.H., Hildebrandt, P.K., Erslev, E.A., and Reed, J.C., Jr., 1986, Metamorphic and deforma- of ultrahigh-temperature metamorphism: Journal of Metamorphic Geology, v. 24, p. 135– tion history of the gneiss complex in the northern Teton Range, Wyoming, in Garrison, P.B., 151, https://doi.org/10.1111/j.1525-1314.2006.00630.x. ed., Geology of the Beartooth Uplift and Adjacent Basins, United States (USA): Montana Frost, B.R., Frost, C.D., Cornia, M., Chamberlain, K.R., and Kirkwood, R., 2006, The Teton-Wind Geological Society and Yellowstone Bighorn Research Association Joint Field Conference River domain, a 2.68–2.67 Ga active margin in the western Wyoming Province: Canadian and Symposium, p. 91–110. Journal of Earth Sciences, v. 43, p. 1489–1510, https://doi.org/10.1139/e06-102. Mints, M.V., Belousova, E.A., Konilov, A.N., Natapov, L.M., Shchipansky, A.A., Griffin, W.L., Frost, B.R., Swapp, S.M., Frost, C.D., Bagdonas, D.A., and Chamberlain, K.R., 2018, Neoarchean O’Reilly, S.Y., Dokukina, K.A., and Kaulina, T.V., 2010, Meosarchean subduction processes: tectonic history of the Teton Range: Record of accretion against the present-day western 2.87 eclogites from the Kola Peninsula, Russia: Geology, v. 38, p. 739–742, https://doi .org margin of the Wyoming Province: Geosphere, v. 14, 10.1130/GES01559.1. /10.1130/G31219.1. Frost, C.D., Swapp, S.M., Frost, B.R., Finley-Blasi, L., and Fitz-Gerald, D.B., 2016, Leucogranites of Moyen, J.-F., Stevens, G., and Kisters, A., 2006, Record of mid-Archaean subduction from meta- the Teton Range, Wyoming: A record of Archean collisional orogeny: Geochimica et Cosmo- morphism in the Barberton terrain, South Africa: Nature, v. 442, p. 559–562, https://doi .org chimica Acta, v. 185, p. 528–549, https://doi.org/10.1016/j.gca.2015.12.015. /10.1038/nature04972. Fuhrman, M.L., and Lindsley, D.H., 1988, Ternary-feldspar modeling and thermometry: The Nicoli, G., Stevens, G., Moyen, J.-F., and Frei, D., 2015, Rapid evolution from sediment to ana- American Mineralogist, v. 73, p. 201–215. tectic granulite in an Archean continental collision zone: The example of the Bandelierkop Garcia-Casco, A., Haissen, F., Castro, A., El-Hmidi, H., Torres-Roldan, R.L., and Millan, G., 2003, Formation metapelites, South Marginal Zone, Limpopo Belt, South Africa: Journal of Meta- Synthesis of staurolite in melting experiments of natural metapelite: Consequences for the morphic Geology, v. 33, p. 177–202, https://doi.org/10.1111/jmg.12116. phase relations in low-temperature politic migmatites: Journal of Petrology, v. 44, p. 1727– Nutman, A.P., Bennett, V.C., Friend, C.R.L., Hidaka, H., Yi, K., Lee, S.R., and Kamiichi, T., 2013, The 1757, https://doi.org/10.1093/petrology/egg056. Itsaq Gneiss Complex of Greenland: Episodic 3900 to 3660 Ma juvenile crust formation and Hamilton, W.B., 2011, Plate tectonics began in Neoproterozoic time, and plumes from deep recycling in the 3660 to 3600 Ma Isukasian orogeny: American Journal of Science, v. 313, mantle never operated: Lithos, v. 123, p. 1–20, https://doi.org/10.1016/j.lithos.2010.12.007. p. 877–911, https://doi.org/10.2475/09.2013.03. Holland, T.J.B., and Powell, R., 1991, A compensated-Redlich-Kwong (CORK) equation for vol- Nutman, A.P., Bennett, V.C., and Friend, C.R.L., 2015, Proposal for a continent ‘Itsaqia’ amalga

umes and fugacities of CO2 and H2O in the range 1 bar to 50 kbar and 100–1600 C: Contribu- mated at 3.66 Ga and rifted apart from 3.53 Ga: initiation of a Wilson cycle near the start of tions to Mineralogy and Petrology, v. 109, p. 265–273, https://doi.org/10.1007/BF00306484. the rock record: American Journal of Science, v. 315, p. 509–536, https://doi .org/10.2475/06 Holland, T.J.B., and Powell, R., 1998, An internally consistent thermodynamic data set for phases .2015.01. of petrological interest: Journal of Metamorphic Geology, v. 16, p. 309–343, https://doi .org O’Neill, C., Lenardic, A., Weller, M., Moresi, L., Quenette, S., and Zhang, S., 2016, A window for /10.1111/j.1525-1314.1998.00140.x. plate tectonics in terrestrial planet evolution?: Physics of the Earth and Planetary Interiors, Holland, T.J.B., and Powell, R., 2011, An improved and extended internally consistent thermodynamic v. 255, p. 80–92, https://doi.org/10.1016/j.pepi.2016.04.002. dataset for phases of petrological interest, involving a new equation of state for solids: Journal Pattison, D.R.M., Chacko, T., Farquhar, J., and McFarlane, C.R.M., 2003, Temperatures of gran- of Metamorphic Geology, v. 29, p. 333–383, https://doi.org/10.1111/j.1525-1314.2010.00923.x. ulite-facies metamorphism: Constraints from experimental phase equilibria and thermo

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1049 Research Paper

barometry corrected for retrograde exchange: Journal of Petrology, v. 44, p. 867–900, https:// Thompson, A.B., and England, P.C., 1984, Pressure-temperature time paths of regional meta- doi.org/10.1093/petrology/44.5.867. morphism II: Their inference and interpretation using mineral assemblages in metamorphic Reed, J.C., Jr., 1963, Structure of Precambrian crystalline rocks in the northern part of Grand rocks: Journal of Petrology, v. 25, p. 929–955, https://doi.org/10.1093/petrology/25.4.929. Teton National Park, Wyoming: U.S. Geological Survey Professional Paper, v. 475-C, Tomkins, H.S., Powell, R., and Ellis, D.J., 2007, The pressure dependence of the zirconium-in- p. C1–C6. rutile thermometer: Journal of Metamorphic Geology, v. 25, p. 703–713, https://doi .org/10 Reed, J.C., 1973, Geologic map of the Teton Range, Wyoming: U.S. Geological Survey Open-File .1111/j.1525-1314.2007.00724.x. Map, scale 1:62,600. Van Kranendonk, M.J., 2010, Two types of Archean continental crust: Plume and plate tectonics Reichardt, H., and Weinberg, R.F., 2012, The dike swarm of the Karakoram shear zone, Ladakh, on early Earth: American Journal of Science, v. 310, p. 1187–1209, https://doi .org/10.2475/10 NW India: linking granite source to batholith: Geological Society of America Bulletin, v. 124, .2010.01. p. 89–103, https://doi.org/10.1130/B30394.1. Van Kranendonk, M.J., Kroner, A., Hoffmann, J.E., Nagel, T., and Anhaeusser, C.R., 2014, Just Roberts, N.M.W., van Kranendonk, M.J., Parman, S., and Clift, P.D., 2015, Continent formation another drip: re-analysis of a proposed Mesoarchean suture from the Barberton Moun- through time, in Roberts, N.M.W., et al., eds., Continent Formation Through Time: Geo- tain Land, South Africa: Precambrian Research, v. 254, p. 19–35, https://doi.org/10.1016/j logical Society of London Special Publication 389, p. 1–16, https://doi.org/10.1144/SP389.13. .precamres.2014.07.022. Rubatto, D., Gebauer, D., and Conpagnoni, R., 1999, Dating of eclogite-facies zircons: the age of Visona’, D., Carosi, R., Montomoli, C., Tiepolo, M., and Peruzzo, L., 2012, Miocene andalusite Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps): Earth and Planetary Science leucogranite in central-east Himalaya (Everest-Masang Kang area): Low-pressure melting Letters, v. 167, p. 141–158, https://doi.org/10.1016/S0012-821X(99)00031-X. during heating: Lithos, v. 144–145, p. 194–208, https://doi.org/10.1016/j.lithos.2012.04.012. Savko, K.A., Kotov, A.B., Sal’nikova, E.B., Korish, E.Kh., Pilyugin, S.M., Artemenko, G.V., and Voice, P.J., Kowalewski, M., and Eriksson, K.A., 2011, Quantifying the timing and rate of crustal Korikovskii, S.P., 2010, The age of metamorphism of granulite complexes of the Voronezh evolution: global compilation of radiometrically dated detrital zircon grains: The Journal of Crystalline Massif: the monazite U-Pb geochronology: Doklady Earth Sciences, v. 435, Geology, v. 119, p. 109–126, https://doi.org/10.1086/658295. p. 1575–1580, https://doi.org/10.1134/S1028334X10120056. White, R.W., Powell, R., Holland, T.J.B., Johnson, T.E., and Green, E.C.R., 2014, New mineral Searle, M.P., Simpson, R.L., Law, R.D., Parrish, R.R., and Waters, D.J., 2003, Structural geom activity-composition relations for thermodynamic calculations in metapelitic systems: Jour- etry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal— nal of Metamorphic Geology, v. 32, p. 261–286, https://doi.org/10.1111/jmg.12071. South Tibet: Journal of the Geological Society, v. 160, p. 345–366, https://doi.org/10.1144 Whitney, D.L., and Evans, B.W., 2010, Abbreviations for names of rock-forming minerals: The /0016-764902-126. American Mineralogist, v. 95, p. 185–187, https://doi.org/10.2138/am.2010.3371. Searle, M.P., Cottle, J.M., Streule, M.J., and Waters, D.J., 2009, Crustal melt granites and migma- Williams, I.S., 1998, U-Th-Pb geochronology by ion microprobe: Reviews in Economic Geology, tites along the Himalaya: melt source, segregation, transport and granite emplacement v. 7, p. 1–35. mechanisms: Earth and Environmental Science Transactions of the Royal Society of Edin- Worthington, L.L., Miller, K.C., Chamberlain, K.R., Erslev, E., Sheehan, A., et al., 2015, Build- burgh, v. 100, p. 219–233, https://doi.org/10.1017/S175569100901617X. ing on the legacy of deep probe and CD-ROM: Crustal structure of the Bighorn Mountains, Smart, K.A., Tappe, S., Stern, R.A., Webb, S.J., and Ashwal, L.D., 2016, Early Archaean tectonics Wyoming and its relationship to Precambrian and Laramide tectonics: Geological Society of and mantle redox recorded in Witwatersrand diamonds: Nature Geoscience, v. 9, p. 255– America Abstracts with Programs, v. 47, no. 7, p. 463. 259, https://doi.org/10.1038/ngeo2628. Zartman, R.E., and Reed, J.C., 1998, Zircon geochronology of the Webb Canyon Gneiss and the Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a Mount Owen Quartz Monzonite, Teton Range Wyoming: significance to dating late Archean two-stage model: Earth and Planetary Science Letters, v. 26, p. 207–221, https://doi .org/10 metamorphism in the Wyoming craton: The Mountain Geologist, v. 35, p. 71–77. .1016/0012-821X(75)90088-6. Zirkler, A., Johnson, T.E., White, R.W., and Zach, T., 2012, Polymetamorphism in the mainland Tang, M., Chen, K., and Rudnick, R., 2016, Archean upper crust transition from mafic to felsic Lewisian complex, NW Scotland—phase equilibria and geochronological constraints from marks the onset of plate tectonics: Science, v. 351, p. 372–375, https://doi.org/10.1126/science the Cnoc an t’Sidhean suite: Journal of Metamorphic Geology, v. 30, p. 865–885, https://doi .aad5513. .org/10.1111/j.1525-1314.2012.01003.x.

GEOSPHERE | Volume 14 | Number 3 Swapp et al. | Record of Neoarchean continent collision and exhumation 1050