RESEARCH

Deformation and extensional exhumation of 1.9 Ga high‑pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Eric J. Thiessen1, H. Daniel Gibson1, Daniele Regis2, and Sally J. Pehrsson2 1DEPARTMENT OF EARTH SCIENCES, SIMON FRASER UNIVERSITY, 8888 UNIVERSITY DRIVE, BURNABY, BRITISH COLUMBIA, V5A 1S6, CANADA 2GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONTARIO, K1A 0E8, CANADA

ABSTRACT

The origin of high-pressure granulites in the south Rae craton and Snowbird tectonic zone (STZ) is highly enigmatic. Current models for their formation and exhumation envisage continental collision at 2.55 Ga and intracratonic orogenesis at 1.9 Ga, or collision and exhu- mation at ca. 1.9 Ga. As an attempt to reconcile these disparate models, we conducted a regional and detailed mapping program along a geophysical discontinuity 100 km west of the STZ within the south Rae craton of the Northwest Territories, Canada. This work presents the discovery of a new crustal-scale shear zone, the Wholdaia Lake shear zone (WLsz), which deformed and transposed host rocks into a 20-km-wide and 300-km-long ductile high-strain zone. U-Pb zircon geochronology was utilized to establish host-rock crystallization ages, timing of deposition of metasedimentary rocks, and age constraints of metamorphism and ductile shearing. Hanging-wall meta­ sedimentary rocks have a new depositional range of 1.98–1.93 Ga and contain abundant metamorphic zircon at 1.91 Ga. The protoliths of the footwall mafic granulite orthogneisses crystallized at 2.6 Ga and were metamorphosed at 1.9 Ga, which extends the known footprint of 1.9 Ga metamorphism 100 km west of the STZ. During and after 1.9 Ga metamorphism, the WLsz began progressively exhuming footwall rocks in three distinct stages, associated with (1) normal-oblique shearing at high-pressure granulite-facies conditions, (2) normal-oblique shearing accompanied by mylonitization at amphibolite-facies conditions, and (3) normal-oblique shearing with ultramylonite develop- ment at amphibolite- to greenschist-facies conditions. Ductile shearing was waning by 1.86 Ga, based on ages obtained from late syn- to postkinematic crosscutting dikes. Collectively, the WLsz in concert with other regional structures aided both extensional and thrust-sense exhumation of a large high-grade terrane at 1.9 Ga in the south Rae craton.

LITHOSPHERE; v. 10; no. 5; p. 641–661; GSA Data Repository Item 2018268 | Published online 31 August 2018 https://doi​ .org​ /10​ .1130​ /L704​ .1​

INTRODUCTION al., 2014). A lack of overall understanding of the nature and significance of 2.55 Ga and 1.9 Ga high-pressure metamorphism in the southeastern Deeply eroded ancient orogens provide insights into the formation Rae craton results in competing tectonic models that appear incompat- and exhumation of stable continental masses involved in supercontinent ible. Much of the high-pressure granulite-facies metamorphism along amalgamation. The Rae craton of the western Churchill Province (Fig. 1), the STZ occurred at 1.9 Ga (Fig. 1) and is thought to relate to collision northern Canada, is thought to be the locus of amalgamation for the Paleo- with the Hearne craton (Berman et al., 2007), which produced eclogites proterozoic supercontinent Nuna (Hoffman, 2014), yet the current paucity (Baldwin et al., 2004, 2007) and buried Paleoproterozoic metasedimentary of regional data means its Proterozoic assembly is poorly understood. rocks (Martel et al., 2008; Bethune et al., 2013). Alternative models sug- Several large high-pressure granulite domains occur along the Rae craton’s gest that local occurrences of high-pressure granulite-facies rocks along eastern margin (e.g., Sanborn-Barrie et al., 2001; Baldwin et al., 2007; the STZ that have been dated at 2.55 Ga (Davis et al., 2006; Baldwin et Berman et al., 2007; Martel et al., 2008) within the Snowbird tectonic al., 2006; Mahan et al., 2006a; Mills et al., 2007; Flowers et al., 2008; zone (STZ), which separates these rocks from the lower-grade Archean Dumond et al., 2015, 2017) indicate a Neoarchean collision between the Hearne craton. In southeasternmost Northwest Territories (NWT) and Rae and Hearne cratons. In this scenario, high-pressure rocks deeply northern Saskatchewan, high-grade rocks of the Rae craton are particularly buried in Neoarchean time resided in the deep crust until a thermal pulse well exposed and preserve tectonothermal events at 2.55 Ga (Baldwin et accompanied by shearing resulted in high-pressure metamorphism and al., 2006; Mahan et al., 2006a), 2.5–2.3 Ga (Berman et al., 2013), 1.92 exhumation at 1.9 Ga (Flowers et al., 2006a, 2008; Dumond et al., 2015). Ga (Martel et al., 2008; Bethune et al., 2013), and 1.90 Ga (Baldwin et In order to better understand the nature of these high-grade meta- al., 2004; Berman et al., 2007), followed by exhumation and rheological morphic rocks and their tectonometamorphic evolution, we undertook stabilization of the crust by 1.75–1.70 Ga (Rainbird et al., 2005; Petts et a combined study of the crustal-scale shear zones, deformed supra- crustal assemblages, and deeply exhumed crust within the southeastern Eric Thiessen http://orcid.org/0000​ -0002​ -3204​ -525X​ Rae craton. Herein, we document detailed structural, metamorphic, and

LITHOSPHERE© 2018 The Authors. | Volume Gold 10Open | Number Access: 5 This | www.gsapubs.org paper is published under the terms of the CC-BY-NC license. 641

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

variably deformed and intruded by 1.8–1.75 Ga magmatic suites (Peter- A P K son et al., 2002). 65 °N Slave TTZ Thelon Bounding the south Rae craton to the west, there is the 1.99–1.91 Ga basin Taltson magmatic zone (Fig. 1), an orogen that contains high-grade Rae D Cb craton basement gneisses intruded by voluminous I-type (1.99–1.95 Ga) and S-type (1.94–1.92 Ga) plutonic rocks. Peak metamorphism in the An R Rae Taltson magmatic zone occurred at 1.94–1.93 Ga (Henderson et al., 1990; BH H McDonough et al., 2000; McNicoll et al., 2000). Recent work has also NWT Sn Hudson W STZ recognized that the western margin of the south Rae craton was affected G Hearne Bay Mu by a significant pre-Taltson orogenic event, the 2.5–2.3 Ga Arrowsmith A NU MB orogeny (Berman et al., 2013; Bethune et al., 2013). Following Arrow- TMZ 60˚ N Athabasca <1.83 & <1.75 smith and Taltson magmatic zone orogenesis, the south Rae craton was Ga basins basin 102˚ W affected by 1.9 Ga Snowbird (e.g., Berman et al., 2007) and ca. 1.85–1.79 1.9 Ga mod. & high-P Ga Trans-Hudson orogenesis (Corrigan, 2012). The latter overlaps in age AB SK <2.07 Ga with deposition of unmetamorphosed volcano-sedimentary rocks of the basins Baker Lake Group in a series of transtensional basins (Fig. 1; Rainbird Proterozoic THO orogens et al., 2006; Hadlari and Rainbird, 2011). Archean Separating the south Rae craton from the Hearne craton to the east, 200 km Su cratons there is the STZ, a >2000-km-long major crustal structure in the Canadian Figure 1. Regional context for the south Rae craton, highlighting 1.9 Ga Shield that contains a network of northeast-trending shear zones, trans- moderate- to high-pressure rocks from Berman et al. (2007). STZ—Snow- posed host-rock panels, and late faults active between 1.9 and 1.75 Ga bird tectonic zone, TMZ—Taltson magmatic zone, TTZ—Thelon tectonic (Mahan et al., 2003; Mahan and Williams, 2005; Flowers et al., 2006b; zone, THO—Internides of Trans-Hudson orogen (Sask craton not shown for Berman et al., 2007; Martel et al., 2008; Card, 2016). The latest ductile clarity), BH—Buffalo Head craton, Su—Superior craton, Cb—Chesterfield deformation on the STZ is constrained to be 1.85 Ga (Mahan et al., 2006a), block, A—Amer Group, P—Penrhyn and Piling Groups, K—Ketyet River Group, Sn—Snowbird domain metasedimentary rocks, G—Grollier Lake although brittle-ductile faulting occurred as late as ca. 1.75 Ga (Rainbird metasedimentary rocks, Mu—Murmac Bay Group, H—Hill Island Lake et al., 2005; Davis et al., 2015). assemblage, W—Waugh Lake Group, R—Rutledge River basin, A—Lake Athabasca, An—Angikuni Lake, D—Dubawnt Lake. Canadian Provinces: South Rae Craton Lithotectonic Domains AB—Alberta, SK—Saskatchewan, NWT—Northwest Territories, NU—Nun- avut, and MB—Manitoba. Dashed line is westernmost exposure of the A recent geochronological reconnaissance transect, which was aided Canadian Shield; dotted box is the outline for Figure 2. by a high-resolution aeromagnetic survey (Kiss and Coyle, 2012), was undertaken across the largely unmapped south Rae craton in NWT. The transect resulted in the delineation of northeast-trending lithotectonic geochronological analyses of the Wholdaia Lake shear zone (WLsz), domains (Davis et al., 2015; Pehrsson et al., 2015; Percival et al., 2016; a newly discovered, ~300-km-long, crustal-scale shear zone that runs Regis et al., 2017b) that correlate well with similar domains recognized roughly parallel to the STZ in the southern Rae craton. Constraining its in Saskatchewan (e.g., Ashton et al., 2007). geometry, timing, and kinematic evolution is critical to characterizing The Tantato domain of northern Saskatchewan (Fig. 2) is underlain the crustal architecture of the eastern flank of the south Rae craton and by abundant 2.6–2.55 Ga mafic to felsic orthogneiss (e.g., Hanmer, 1997; understanding the regional high-pressure tectonometamorphic events and Baldwin et al., 2003, 2006) and paragneiss (Hanmer, 1997; Dumond et exhumation that affected this area. We present new geochronologic data al., 2015). Along its eastern margin adjacent to the Hearne craton, the from within the WLsz and adjacent wall rocks, including new depositional shear-bounded Chipman subdomain (Hanmer et al., 1994; Mahan et al., and metamorphic ages of Paleoproterozoic metasedimentary rocks, and 2006b) is composed of a distinct ca. 3.4 Ga tonalite batholith intruded related timing constraints of displacement on the WLsz. Through detailed by the 2.1 Ga Chipman dike swarm (Regan et al., 2016). A separate geological and structural mapping, and in situ U-Pb zircon analyses using shear-bounded block of 3.4 Ga tonalite in NWT (Martel et al., 2008) is sensitive high-resolution ion microprobe (SHRIMP) and laser ablation– correlative with the Chipman subdomain in Saskatchewan. Their offset inductively coupled plasma–mass spectrometry (LA-ICP-MS), we unrav- is due to dextral strike-slip displacement along the Grease River shear eled the tectonometamorphic evolution of this shear zone and present its zone (Fig. 2), which was active between 1.9 and 1.8 Ga (Lafrance and implications with respect to the Paleoproterozoic architecture and tectonic Sibbald, 1997; Mahan and Williams, 2005; Dumond et al., 2008). To the evolution of the western Churchill Province. west and east of the Chipman domain in NWT, there are two high-strain zones (Fig. 2), called the Striding mylonite zone and the Chipman shear GEOLOGICAL SETTING AND PREVIOUS WORK zone (Martel et al., 2008), which correlate with the Cora Lake shear zone (Regan et al., 2014) and Legs Lake shear zone in Saskatchewan The south Rae craton consists of 3.4–2.5 Ga gneisses and 2.74–2.63 (Mahan et al., 2003). The Chipman and Legs Lake shear zones form the Ga volcano-sedimentary assemblages intruded by 2.68–2.32 Ga felsic surface trace of the STZ in this area (Fig. 2) and are interpreted to have to mafic plutons (Pehrsson et al., 2013; Davis et al., 2015; Regis et al., accommodated exhumation of the Chipman panel to midcrustal levels 2017b). Various younger ca. 2.17–1.95 Ga Paleoproterozoic metasedi- via east-vergent thrusting over the Hearne by ca. 1.85 Ga (Mahan et al., mentary packages (Bostock and van Breeman, 1994; McDonough and 2006b). Similar to the strain documented on the Striding mylonite zone, McNicoll, 1997; Martel et al., 2008; Ashton et al., 2013, 2017a; Shiels the Cora Lake shear zone exhibits normal-oblique sinistral shear sense et al., 2016; Ply, 2016; Thiessen et al., 2017) overlie the basement rocks. (Chipman domain up to the northeast) at ca. 1.89–1.87 Ga (Regan et al., The Paleoproterozoic cover and Archean substrate were subsequently 2014). North of Kasba Lake, an undeformed Nueltin intrusion cuts the

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 642

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

6800000 N N RBsz Rae craton Train Firedrake 600000 E EB WL

Dodge WLsz EB

Tantato GRsz Snowbird SL CLsz Chipman LLsz SbL d SMz BL Snowbir Tectonic Chipman Rae domains Athabasca Csz Hearne craton Basin Zone Shear zones NWT Thrust fault <1.75 Ga SK KL Normal fault 50 km Hearne craton

Figure 2. Geology of the Rae-Hearne boundary along the central Snowbird tectonic zone. The Wholdaia Lake shear zone (WLsz) separates the Firedrake/Train domain from the Snowbird/Dodge domain. “EB” refers to easterly bends of the WLsz as discussed in text. Major lakes for reference are: WL—Wholdaia Lake, SbL—Snowbird Lake, KL—Kasba Lake, SL—Selwyn Lake, and BL—Black Lake. Other shear zones include: RBsz—Ryckman Bay shear zone, GRsz—Grease River shear zone, CLsz—Cora Lake shear zone, LLsz—Legs Lake shear zone, SMz—Striding mylonite zone, and Csz—Chipman shear zone. Coordinates are in Universal Transverse Mercator (UTM) North American Datum 1983 (NAD 83). SK—Saskatchewan, NWT—Northwest Territories.

STZ at the Rae-Hearne boundary and provides a minimum age of ductile horizontal gravity gradient anomaly is also apparent along the bound- shearing of 1.74 Ga (Davis et al., 2015). ary (Sharpton et al., 1987), and high-strain zones were recognized, they West of the Grease River shear zone and the Striding mylonite zone, concluded the structure to be discontinuous and to have accommodated there are the Dodge (Saskatchewan) and Snowbird (NWT) domains (Fig. discrete shearing within a singular larger tectonic block. Ages for late- 2), which are correlated based on magnetic character and geological simi- kinematic granites and metamorphic monazite suggested ca. 1.9 Ga high- larities (Ashton et al., 1999; Martel et al., 2008; Knox and Biss, 2010; grade metamorphism was associated with this deformation (Hanmer, Knox et al., 2011). The Snowbird domain contains rocks with a charac- 1997; Krikorian, 2002). teristic magnetic pattern of high and low magnetic anomalies (Fig. 3) When analyzed more regionally, a broad zone of rectilinear, northeast- that highlight kilometer-scale type-2 fold interference patterns (Fig. 3; trending, low aeromagnetic character (Fig. 3; Kiss and Coyle, 2012) occurs e.g., Ramsay, 1967; Martel et al., 2008). Orthogneissic and metaplutonic for 300 km from southwest of Angikuni Lake to the Saskatchewan border rocks (clinopyroxene [Cpx] ± orthopyroxene [Opx]–bearing; mineral (Fig. 1). Reconnaissance mapping by helicopter focused along this discon- abbreviations according to Whitney and Evans, 2010) that underlie the tinuity (Davis et al., 2015) found that it separates the more highly magnetic high-intensity magnetic patterns (red colors in Fig. 3B) have ca. 2.73–2.70, and geologically distinct Snowbird and Firedrake domains; is coincident 2.66, 2.63, and 2.54 Ga crystallization ages (Martel et al., 2008; Regis et with transposed, tectonized, and mylonitized units along its length; and al., 2017b). The low-intensity magnetic patterns (blue colors) are typically importantly contains scattered high-pressure assemblages that are also underlain by pelitic-psammitic to calc-silicate metasedimentary rocks noted west of the break. Our research into the nature of this structure was with a maximum depositional age of 2.07 Ga (Martel et al., 2008). In the focused on its best exposures along an ~100 km segment in the vicin- western Snowbird domain, metasedimentary rocks originally inferred to ity of Wholdaia Lake and is discussed in detail in the following section. be Archean in age (Martel and Pierce, 2006) were recently determined to West of the Snowbird-Dodge domains, separated by the Ryckman Bay have Paleoproterozoic detrital zircon ages (Thiessen et al., 2017). These shear zone (Ashton and Card, 1998) and the WLsz, there are the correla- units are tentatively correlated with the Rae cover sequence (Martel et al., tive Train (Saskatchewan) and Firedrake (NWT) domains (Fig. 2). These 2008; Rainbird et al., 2010) and the upper Murmac Bay Group (Fig. 1) domains contain mafic-intermediate orthogneiss and minor paragneiss that to the southwest (Ashton et al., 2013). The Snowbird domain metasedi- are 2.7–2.57 Ga and abundant migmatitic granite that is ca. 1.85–1.81 Ga mentary rocks and basement gneisses are unconformably overlain by (Ashton et al., 1999, 2009; Davis et al., 2015; Regis et al., 2017b). The undeformed, unmetamorphosed ca. 1.83 Ga Christopher Island Formation Firedrake domain is also characterized by magnetic anomalies that are volcaniclastic rocks (Figs. 1 and 3) of the Baker Lake Group on Snowbird relatively high and tens-of-kilometer-scale curvilinear refolded patterns Lake (e.g., Roscoe and Miller, 1986; Peterson et al., 2002), indicating and kilometer-scale sheath folds (Fig. 3). final exhumation of the Snowbird domain by this time. At the latitude of Wholdaia Lake, high-pressure rocks were originally Previous Pressure-Temperature-Time Work in the Southeastern thought to be confined to an oblong domain of low-resolution magnetic Rae Craton anomalies (Dods et al., 1987) termed the Selwyn lozenge of the STZ (Hanmer et al., 1994). The western margin of the lozenge was investi- The bulk of the research on the pressure-temperature-time (P-T-t) con- gated by Hanmer (1997) and Krikorian (2002), and although a distinct ditions of south Rae craton metamorphism has come from the subdomains

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 643

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

New samples AB Mafic granulite 1.9 Ga metam. SMz Metasediment <1.98 Ga Dikes 1.86 Ga 5cde SbL EB Firedrake Snowbird

N 6780000N

560000

WLsz E 15ET260d

4bcd 15ET249 15ET253a 4ef 5ab WL 15ET273b 4a 5f

ST1 Grt-Cpx gneiss Monzogranite, tonalite and mafic gneiss

ST2 Hbl-Bt mylonite Tonalite gneiss 15EM68b Felsic-Intermediate Ultramylonite sy Wallrocks SL CIF volcanic rocks Para- and Ortho- gneiss Grt±Cpx±Opx gneiss Normal fault 20 km 20 km EB

Figure 3. (A) Wholdaia Lake shear zone (WLsz) geology highlighting high-grade (ST1; Grt + Cpx) and lower-grade (ST2; Hbl + Bt) domains, with mineral abbreviations according to Whitney and Evans (2010). Yellow circles are newly dated mafic granulites, the dark-blue circle is a newly dated metasedimentary sample, and the white circles are newly dated felsic dike samples. Pink polygon within the WLsz represents a weakly deformed 1.9 Ga syenite (sy) body. Reference lakes are: SbL—Snowbird Lake, WL—Wholdaia Lake, and SL—Selwyn Lake. Numbered annotations (e.g., 4bcd) refer to corresponding photographs in Figures 4 and 5. (B) Total field magnetic map (Kiss and Coyle, 2012) of the WLsz highlighting geological boundaries (black lines) and samples from Figure 3A. Higher-intensity magnetic signatures are red; lower-intensity signatures are blue. “EB” refers to easterly bends of the WLsz anc CIF refers to the Cristopher Island Formation, as discussed in text.

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 644

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

and crustal-scale shear zones within the Tantato domain, where thermo- the WLsz (Fig. 3), as do late east-west–trending weakly deformed lam- barometric and pseudosection analyses have yielded pressures of 0.9–1.6 prophyre dikes. GPa at both 2.55 Ga and 1.9 Ga (Baldwin et al., 2003, 2006, 2007; Flow-

ers et al., 2006a, 2008; Mahan et al., 2006a, 2006b, 2008; Dumond et al., D1–ST1 2010, 2015; Regan et al., 2014). The Chipman subdomain in Saskatch- ewan and NWT also shows pressures of 1.1 GPa and 795 °C at ca. 1.9 Ga The central WLsz is a composite structure with two distinct lithotec- (Mahan and Williams, 2005; Martel et al., 2008), but it is interpreted to tonic and structural domains, both of which trend northeast and dip

have reached 1.3 GPa at 2.55 Ga (Flowers et al., 2008; Mahan et al., 2008). southeast (Fig. 3). The earlier, high-grade fabric, ST1, consists of (Grt + The Neoarchean south Rae basement was exhumed to the surface prior Cpx + Opx)–bearing mafic and felsic granulite gneiss and mylonite that to deposition of its Paleoproterozoic cover sequence. These latter units, typically display centimeter-scale annealed gneissic banding. In con- widely dispersed across the region, record ca 1.9–1.85 Ga tectonometa- trast to the polydeformed, variably oriented structures that characterize

morphism at conditions ranging from amphibolite to granulite facies. the Firedrake and Snowbird domains, ST1 is consistently aligned into a The younger than 2.07 Ga Snowbird domain metasedimentary rocks northeast-striking, southeast-dipping (~60°) transposition foliation with yield peak conditions of 0.76–0.9 GPa and 800–840 °C at 1.92–1.90 Ga a rarely preserved shallowly southwest-plunging lineation (10°–30°). (U-Pb monazite) and record subsequent metamorphic zircon and monazite A gneissic monzogranite and associated mylonite (red unit in Fig. 3A) growth at 1.91–1.90 Ga (Krikorian, 2002; Martel et al., 2008; Thiessen occur as a distinct ~800-m-wide band of highly magnetic, highly sheared et al., 2017). The younger than 2.03 Ga Grollier Lake metasedimentary rocks within the easternmost margin of the central WLsz (Fig. 3B). To rocks (Ashton et al., 2017a) and upper Murmac Bay Group (Fig. 1) also the south, the magnetic signature of this unit is traceable for over 50 km, exhibit significant 1.94–1.90 Ga metamorphism at grades up to granulite but farther to the north and south, its signature becomes more cryptic. facies (Knox et al., 2011; Bethune et al., 2013; Ply, 2016), and in the far Additionally, the gneiss outlines a kilometer-scale z-fold indicating dextral west, the potentially correlative Rutledge River basin units were deformed folding of these gneissic and mylonitic units. The gneiss contains banded and metamorphosed to lower-granulite-facies conditions during foreland leucocratic layers (Hbl + Pl + Qz + Opx + Cpx + Grt) and mesocratic basin closure at ca. 2.10 Ga prior to 1.99–1.91 Ga plutonism within the layers (Hbl + Qz + Pl + Bt).

Taltson magmatic zone (Bostock and van Breeman, 1994). Mafic units of ST1 that occur west of the monzogranite gneiss have The Train domain, which lacks a younger sedimentary record, con- mottled to weakly linear magnetic patterns (Fig. 3) that commonly dis- tains widespread (Grt + Hbl)–bearing orthogneiss with relict Cpx + Opx play higher magnetic signatures. Although these units occur predomi- assemblages that have been heavily retrogressed and migmatized by injec- nantly along the easternmost WLsz, such occurrences are also found on tion of anatectic melt (Ashton et al., 1999; Heaman et al., 1999; Davis et the western margin adjacent to the Firedrake domain. Rocks in this unit al., 2015). Estimates for the timing of high-grade (Grt + Cpx, Grt-Hbl) consist of meter-scale amphibolite to granulite pods (Fig. 4A) injected metamorphism are ca. 1.81 Ga (U-Pb zircon; Heaman et al., 1999). by centimeter-scale, northeast-trending anastomosing tonalite to granite The tectonic significance of the 2.55 and 1.9 Ga events has been veins. These pods are strained and well foliated, yet they do not display extensively debated (Baldwin et al., 2004, 2006; Berman et al., 2007; gneissic texture. The tonalite to granite migmatite display southwest- Flowers et al., 2006a, 2008; Mahan et al., 2006a, 2006b, 2008; Mar- plunging stretching lineations (10°–30°). Metabasites and mafic gneisses tel et al., 2008; Dumond et al., 2010, 2015; Regan et al., 2014). While also occur with assemblages of Grt + Cpx + Opx + Pl + Qz + Hbl + the P-T conditions of the Paleoproterozoic metasedimentary sequences Bt. On southern Wholdaia Lake, these gneisses contain poorly defined clearly require renewed crustal loading in the Proterozoic (e.g., Martel compositional layering striking northeast and dipping moderately (~60°) et al., 2008; Bethune et al., 2013), the extent to which the high-pressure to the southeast. Their peak assemblage contains coarse-grained (>2 events signify Proterozoic or Archean collisional orogeny is controversial. mm) Grt + Cpx + Opx + Pl + Qz overprinted and replaced by Opx + Eclogite-facies basaltic sills mapped across the southern Tantato domain Pl + Hbl + Bt. (Knox and Lamming, 2015) record 1.9 Ga high-pressure (1.6 GPa, 750 Along strike, on the northern Wholdaia Lake, mafic gneiss units have °C) tectonometamorphism (Baldwin et al., 2004, 2007), but they have discrete layers of Grt + Cpx + Pl + Qz (Fig. 4B) and Grt + Opx + Pl + been also argued to have formed at ca. 2.55 Ga, based on the metamorphic Hbl + Qz (Fig. 4C). Layers with Grt + Cpx + Pl + Qz contain two textur- record of their host metasedimentary rocks (Dumond et al., 2015, 2017). ally distinct assemblages. The first assemblage (M1) occurs as 1–2 cm

elongate pods containing 0.5–1.0-cm-sized, partially disaggregated Grt1 GEOLOGY OF THE WHOLDAIA LAKE SHEAR ZONE crystals and coarse Cpx + Pl with minor quartz, ilmenite, and pyrite (Figs. 4C–4D). These large garnet crystals host small (<10 µm) ilmenite inclu- New Mapping sions, but otherwise they appear mostly inclusion free. A minor amount of quartz occurs as small, rounded crystals interstitial to Cpx. Plagioclase The following description of the WLsz and its adjacent wall rocks forms coronas around coarse garnet grains and occurs within late frac- comes from boat, helicopter, and foot traverses conducted during this tures. Based on the observed microtextures, we suggest that plagioclase study. Regional geological and predictive mapping (magnetic character) is a retrograde phase overprinting a coarse-grained, high-pressure, granu- suggests the WLsz is at least 300 km long (Fig. 2), although it is best lite- verging on eclogite-facies assemblage; however, we cannot rule out constrained with good exposures for ~100 km along strike in the vicinity entirely that a small portion of plagioclase in the fractures originated as

of Wholdaia Lake. At Wholdaia Lake, the WLsz is roughly 20 km wide inclusions in Grt1.

and has a foliation that strikes consistently at 030°. South and north of A second texturally distinct assemblage (M2) of Grt2 + Cpx + Pl + Qz this central segment, the foliation changes to 070°, and the width of pen- + Ilm (Figs. 4C–4D) has a finer granoblastic texture (<1 mm) and envelops

etrative deformation is restricted to <10 km (easterly bends [EB] in Fig. the older M1 domains. Garnet grains (Grt2) in this assemblage contain 2). The WLsz is composed of amphibolite- to granulite-facies, mafic to small round inclusions of quartz, plagioclase, and apatite. felsic orthogneiss and mylonite to ultramylonite, metagabbro, and local Additional centimeter-scale layers and pods of Grt + Opx + Pl + Hbl

mylonitic paragneiss. Abundant folded and sheared granitic dikes crosscut + Qz, which occur adjacent to the Grt2 + Cpx + Pl + Qz + Ilm layers

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 645

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

AB SW 60˚ NE

Cpx-Pl

Grt-Cpx-Pl-Qz

20 cm 1 cm C D Grt-Cpx-Pl-Qz-Ilm M2

Grt-Opx-Pl-Hbl-Qz

Grt-Cpx-Pl-Qz-Ilm Cpx M2 M1 Pl Grt Grt

Cpx M1 Pl 1 cm 5 mm EF NE

85˚ SW Hbl

annealed Pl

Qz Bt

50 cm 0.5 mm

Figure 4. (A) Northeast-trending amphibolite pods and tonalitic veins of ST1 with late east-west–oriented fracturing. (B) (Grt + Cpx)–bearing mafic

gneiss of ST1. (C) Lenses of Grt + Opx + Pl + Hbl + Qz and Grt1 + Cpx + Pl + Qz (M1) enveloped by a fine-grained Grt2 + Cpx + Pl + Qz + Ilm (M2)

assemblage. (D) Cross-polarized photomicrograph of the texturally distinct M1 Grt1 + Cpx + Pl + Qz assemblage and M2 Grt2 + Cpx + Pl + Qz +

Ilm assemblage in sample 15ET249. (E) Highly sheared mylonites of ST2 dipping steeply to the southeast with a shallowly southwest-plunging

lineation. (F) Photomicrograph of ST2 tonalite mylonite showing granoblastic textures in plagioclase consistent with late thermal annealing. Mineral abbreviations are according to Whitney and Evans (2010).

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 646

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

(Fig. 4C), may represent a retrograde assemblage or an early assemblage Snowbird Domain Hanging Wall that did not equilibrate at higher pressures. Garnet in the (Opx + Hbl)– bearing layers contains many rounded Qz + Pl inclusions. This (Opx + Rocks of the Snowbird domain that underlie the arcuate low-magnetic-

Hbl)–bearing assemblage does not display granoblastic textures, yet it anomaly patterns immediately east of the WLsz ST1 (Fig. 3) contain a has straight grain boundaries and appears to be in textural equilibrium. migmatitic metasedimentary gneiss with the assemblage Grt + Sil + Qz The boundary between compositional layers is sharp with no overlap of + Kfs + Bt + Rt. The sampled unit (15ET273b) is relatively homoge- assemblages. East-west–oriented hairline fractures associated with late neous in texture, with millimeter-scale compositional bands of Grt + Sil overprinting green-amphibole occur within these gneissic units, suggest- + Bt and Qz + Kfs, is strongly foliated, and displays ribbons of quartz ing later retrogression. and foliation-parallel prismatic sillimanite (Fig. 5F). The high amount of garnet suggests this unit is a highly residual rock that has lost a significant

D2–ST2 amount of melt.

A package of penetratively transposed (Hbl + Bt)–bearing mylonites U-Pb GEOCHRONOLOGY

(ST2) defined by a northeast-trending, low-magnetic linear pattern over-

prints the higher-grade gneissic rocks of ST1 (Fig. 3). These rocks are In order to constrain the protolith ages and timing of metamorphism typically dioritic to tonalitic in composition, with minor occurrences of in the WLsz and its hanging wall, five samples were selected (Fig. 3) for

paragneiss. The ST2 rocks include millimeter- to centimeter-scale, rectilin- zircon U-Pb isotopic analysis. Samples 15ET249 and 15EM68b are mafic

ear banded, highly sheared mylonites to ultramylonites striking northeast granulites from within the ST1 portion of the WLsz in which zircon grains and dipping ~85° to the southeast (Fig. 4E). Although these rocks are preserve both igneous and metamorphic crystallization ages. Samples

highly sheared, they do show signs of grain coarsening and develop- 15ET253a and 15ET260d are felsic dikes that crosscut the ST2 portion ment of granoblastic textures indicative of thermal overprinting (Fig. of the WLsz, and their igneous crystallization ages provide minimum 4F). Abundant dextral shear sense indicators are present in the horizontal ages for ductile shearing. The fifth sample, 15ET273b, is a metasedimen- plane, including winged porphyroclasts, z-folds in crosscutting dikes, and tary gneiss from the Snowbird domain previously analyzed by SHRIMP

C- and C′-type shear bands (Fig. 5A). On the ST2 foliation, a 10° to 60° (Thiessen et al., 2017). During this initial analysis (Thiessen et al., 2017), southwest-plunging stretching lineation occurs, and oriented thin sections metamorphic zircon crystals were targeted; however, five detrital zircon in the x-z plane indicate displacement of the Snowbird domain down to the cores were identified and analyzed, with resulting ages as young as 1.96 southwest. In southeast Wholdaia Lake, progressively overprinted mineral Ga. Follow-up LA-ICP-MS work (presented herein) was conducted to assemblages and structural fabrics are observed, and a continuum exists better constrain the maximum depositional age through dating additional between higher-grade, (Grt + Cpx + Opx)–bearing gneiss tectonites of detrital zircon and to characterize the metamorphic and detrital zircon

ST1 and lower-grade, (Hbl + Bt)–bearing mylonites of ST2. populations using trace-element analyses. A distinctive (Hbl + Bt) ultramylonite-mylonite (Figs. 5C–5E) occurs within the northern mapped reaches of the WLsz, where foliations domi- Analytical Methods nantly strike more easterly and the magnetic signature is very low (refer to easterly bends [EB] in Figs. 2–3). This domain has a dip of ~85° (Fig. U-Pb SHRIMP geochronology of zircon was conducted at the Geologi- 5C), contains a southwest-plunging stretching lineation and dextral sense cal Survey of Canada’s J.C. Roddick Ion Microprobe Facility in Ottawa, of shear, and has a minimum mapped width of 1 km, although the geo- Ontario, while U-Pb LA-ICP-MS geochronology of zircon was conducted physical signature is ~5 km wide (Fig. 3). This unit has relatively similar at the Isotope Geology Laboratory at Boise State University, Boise, Idaho. 1 structures and metamorphic grade compared to the ST2 mylonites of central Refer to the Data Repository supplemental material for detailed descrip- Wholdaia Lake; however, it preserves very fine-grained ultramylonites tions of sample preparations, analytical techniques, and the treatment of (Fig. 5D) with a lower degree of high-temperature static overprinting geochronological data, including common-Pb corrections and results of textures (Fig. 5E). For example, recrystallized quartz crystals show recov- primary and secondary reference materials. Zircon separates were mounted ery textures akin to low-temperature mylonites (200–500 °C; Trouw et in epoxy grain mounts, polished to expose grain centers, and imaged using

al., 2009), whereas mylonites elsewhere in ST2 typically contain static either backscattered electron (BSE) or cathodoluminescence (CL) imaging recrystallization textures and chessboard-quartz subgrains indicative of for targeting of spot analyses. U-Pb data are presented in Tables 1–2 and temperatures consistently above 550 °C (Stipp et al., 2002). Although Figures 6–9. Interpreted ages are reported using the calculated weighted these rocks preserve lower-temperature microstructures than the bulk of mean 207Pb/206Pb date at a 95% confidence level (2σ) unless otherwise noted.

ST2, we still consider them part of the ST2 structural domain.

Minimum age constraints on development of ST2 come from variably U-Pb GEOCHRONOLOGY RESULTS crosscutting mafic and felsic dikes. North of Wholdaia Lake, lampro- phyre dikes cut the mylonitic fabric in an east-west orientation and are Sample 15ET249: Mafic Granulite (SHRIMP) considered part of a regional swarm dated at ca. 1.83–1.81 Ga (Rainbird et al., 2006; Ashton et al., 2009). Numerous felsic dikes obliquely cut This sample was collected from northern Wholdaia Lake (Fig. 3),

ST2 and are themselves weakly folded, foliated, and locally sheared (Fig. where gneissic rocks exhibit a northeast-striking ST1 foliation. This mafic 5B). Enveloping surfaces for these folded dikes are generally oriented granulite gneiss contains Grt + Cpx + Opx and occurs adjacent to (Grt

northwest-southeast (perpendicular to ST2 foliations; Fig. 5B) and have subvertical dips, whereas axial planar foliations in the dikes are parallel 1GSA Data Repository Item 2018268, which includes (1) a text document of U-Pb analytical methods, trace-element analyses, and detrital zircon probability density to the ST2 foliations. The dikes display the penetrative southwest-plunging lineation associated with S , indicating late-kinematic emplacement. diagrams of 2.0 Ga south Rae craton basins discussed in the text, (2) Table S1 of T2 trace-element data, and (3) Excel versions of Tables 1 and 2 from the main text, Crystallization ages for two of these felsic dikes are presented in the providing SHRIMP and LA-ICP-MS U-Pb data, is available at http://www.geosociety​ results section. .org​/datarepository​/2018, or on request from [email protected].

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 647

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

AB C

C’

NE NE 2 cm 5 m CD

NE

85˚ SW

30 cm 2 cm EFleucosome NE

Grt-Sil-Kfs-Qz-Rt 2 mm 2 cm

Figure 5. (A) C- and C′-type dextral shear bands developed in ST2 tonalite mylonites from an island shown in B. (B) Aerial view of deformed dikes

within ST2 tonalite mylonite. (C) Ultramylonite developed along the northern easterly bend (EB of Fig. 3) of the Wholdaia Lake shear zone. (D) Highly deformed interlayered mafic and felsic components of an ultramylonite close to area in C. (E) Photomicrograph of an ultramylonite (from part C) that preserves fine-grain sizes and lacks substantial thermal annealing textures. (F) Outcrop photo of Snowbird domain migmatitic metasedimentary gneiss 15ET273b sampled for detrital zircon geochronology. Mineral abbreviations are according to Whitney and Evans (2010).

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 648

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 TABLE 1. U-Pb SENSITIVE HIGH-RESOLUTION ION MICROPROBE (SHRIMP) ZIRCON DATA

Spot Zircon U Th Err Yb 1s err Hf 1s err 204Pb Err f(206)204 206Pb* Isotopic ratiosApparent ages (Ma) Th/U Hf/Yb 206 Disc (%) name domain (ppm) (ppm) (abs) (ppm) (abs) (ppm) (abs) Pb (%) (%) (ppm) 208Pb*/206Pb* Err (%) 207Pb*/235U Err (%) 206Pb*/238U Err (%)Corr. coeff. 207Pb*/206Pb* Err (%) 206Pb/238U Err (abs) 207Pb/206Pb Err 1s (abs) Sample 15ET249 mafic granulite, NAD83 UTM zone 13, 6732291N, 553577E 80.1 CL bright 36 6 0.17 338 11.54301 10019 126 2.7E-4 45 0.46 10 0.0456 15.9 5.2542.8 0.3354 1.40.5103 0.1136 2.4186523 1858 44 0 10.1 CL bright 46 8 0.17 287 10.07 28 18186 104 1.9E-4 50 0.33 13 0.0431 14.8 5.3202.1 0.3344 1.40.6538 0.1154 1.6186022 1886 29 2 44.2 CL bright 28 4 0.16 393 13.66 26 110045 126 2.1E-4 58 0.36 8 0.0359 20.4 5.2822.5 0.3317 1.60.6222 0.1155 2.0184625 1888 36 3 34.2 CL bright 37 6 0.17 353 11.9630110616 133 1.7E-4 58 0.29 10 0.0490 14.3 5.3822.2 0.3336 1.40.6527 0.1170 1.7185623 1911 30 3 21.2 CL bright 62 9 0.15 256 8.59 37 1 9498 120 7.8E-5 71 0.13 16 0.0383 12.8 5.0181.7 0.3105 1.30.7217 0.1172 1.217431919142210 115.1 oz core 122 23 0.20 203 6.72 39 1 784099 3.4E-5 71 -0.06 35 0.0599 5.65.503 1.30.3392 1.10.8262 0.1176 0.71883171921132 22.1 CL bright 41 7 0.17 336 11.31331 11018 139 9.7E-5 71 -0.17 12 0.0620 10.1 5.4931.9 0.3316 1.40.7149 0.1201 1.41846221958247 72.2 CL bright 46 7 0.16 327 11.1433110828 137 4.6E-5 100 -0.08 14 0.0514 10.9 5.7561.8 0.3420 1.40.7491 0.1221 1.21896221987215 98.2 CL bright 59 11 0.20 218 6.96 56 212144 152 1.4E-4 50 0.24 17 0.0558 10.2 5.8181.7 0.3345 1.20.7166 0.1261 1.218602020452110 26.2 CL bright 34 5 0.16 299 9.95 35 110564 133 1.7E-4 58 0.29 10 0.0384 16.7 6.4672.1 0.3606 1.50.6935 0.1301 1.51985252099276 102.1 CL bright 34 5 0.16 260 8.50 42 110921 137 1.1E-4 71 0.19 10 0.0407 14.3 6.6042.0 0.3607 1.50.7228 0.1328 1.41985252135248 72.1 oz core 120 43 0.37 40 1.19 238 6 9478 119 8.8E-5 41 0.15 38 0.1046 4.26.830 1.30.3708 1.10.8328 0.1336 0.72033182146126 88.2 CL bright 58 8 0.14 231 7.46 51 211687 147 9.7E-5 58 0.17 18 0.0324 12.7 6.7831.6 0.3643 1.30.76800.1350 1.02003222164189 94.1 pz core6917 0.25 92 2.85 92 38511 107 4.9E-5 71 0.09 23 0.0709 6.67.366 1.40.3838 1.20.8153 0.1392 0.82094212217146 98.1 pz core 134 42 0.32 44 1.34 231 610078 140 1.1E-5 100 0.02 48 0.0827 3.98.717 1.10.4182 1.00.8952 0.1512 0.522521923599 5 20.1 oz core9935 0.36 36 1.13 235 7 8417 106 3.2E-5 71 0.05 36 0.0950 4.48.731 1.30.4182 1.10.8678 0.1514 0.62252212362115 95.2 CL bright 51 9 0.18 161 5.05 70 211282 142 3.2E-5 100 0.06 18 0.0447 9.88.840 1.60.4221 1.30.8257 0.1519 0.92270252367155 92.1 CL bright 37 7 0.20 118 3.64 98 311577 145 2.4E-4 41 0.42 14 0.0431 20.7 9.1382.9 0.4351 1.40.4804 0.1523 2.62329282372442 26.1 oz core5721 0.38 39 1.21 212 6 8357 116 1.3E-4 45 0.23 22 0.1027 9.99.744 1.50.4401 1.30.8143 0.1606 0.92351252462155 88.1 pz core 127 30 0.24 97 3.00 116 311287 142 2.5E-5 71 0.04 47 0.0728 4.49.470 1.40.4272 1.00.7646 0.1608 0.92293202464158 86.2 oz core 259 81 0.32 66 2.00 183 512143 153 5.0E-5 35 0.09 97 0.0915 2.89.633 1.00.4338 0.90.9243 0.1611 0.423231824677 7 95.1 pz core 162 54 0.35 42 1.26 259 710877 137 ——0.00 62 0.0940 3.410.065 1.10.4455 1.00.9128 0.1639 0.423752024968 6 17.1 pz core 252 52 0.21 63 2.32 114 47199 112 8.8E-5 25 0.15 98 0.0596 3.610.235 1.00.4527 0.90.9251 0.1640 0.424071924977 4 44.1 oz core 213 48 0.23 62 1.90 133 4 8292 105 ——0.00 83 0.0659 3.410.401 1.30.4559 1.30.9567 0.1655 0.424212525126 4 21.1 pz core 115 26 0.23 57 1.77 118 3 668994 6.6E-5 45 0.11 46 0.0553 5.510.860 1.20.4685 1.10.8824 0.1681 0.62477222539103 86.1 pz core8310 0.12 120 3.69 108 313009 164 7.0E-5 50 0.12 34 0.0307 9.410.9561.3 0.4710 1.10.8660 0.1687 0.72488242545113 68.1 oz core 444 104 0.24 46 1.38 194 5 8825111 3.3E-6 100 -0.01 177 0.0722 2.310.873 1.10.46481.0 0.9697 0.1697 0.324612125544 4 79.1 pz core8627 0.33 36 1.07 253 7 9026 113 1.5E-5 100 -0.03 36 0.0915 4.311. 3541.2 0.4832 1.10.8893 0.1704 0.62541232562101 19.1 CL bright 24 6 0.26 60 1.85 131 4 7894 100 3.6E-4 41 0.63 10 0.0550 15.7 11.381 2.30.4824 1.70.7338 0.1711 1.62538362569271 48.1 oz core 109 32 0.30 35 1.06 285 810081 127 ——0.00 45 0.0819 4.211. 4751.2 0.4849 1.10.9013 0.1716 0.5254923 2573 91 50.1 pz core 138 46 0.34 27 0.82 338 9 9282 117 1.1E-5 100 0.02 58 0.0938 3.511. 7061.1 0.4903 1.00.9102 0.1731 0.5257222 2588 81 34.1 pz core 102 27 0.28 68 2.07 158 410774 136 5.5E-5 50 -0.10 41 0.0774 4.611. 2911.2 0.4709 1.10.8867 0.1739 0.6248822 2595 95 109.1 pz core7214 0.20 91 2.82 117 310625 145 3.7E-5 71 0.06 30 0.0543 6.411. 7661.3 0.4898 1.20.8762 0.1742 0.6257025 2598 11 1 12.1 oz core 129 16 0.13 83 2.57 97 3 8078 102 9.9E-6 100 0.02 55 0.0364 5.611. 8911.1 0.4924 1.00.8983 0.1752 0.5258122 2608 81 Sample 15EM68b mafic granulite, NAD83 UTM zone 13, 6694927N, 520115E 18.1 c l 489 147 0.31 303 7.14 36 110796 100 1.5E-05 100 -0.00 140 0.0896 1.45.329 1.10.3335 1.00.9744 0.1159 0.2185517 1894 42 91.2 r d 180 102 0.59 191 4.57 53 110206 95 7.4E-05 41 0.05 51 0.1738 1.75.314 1.20.3325 1.10.9302 0.1159 0.4185118 1894 83 66.2 r d 284 74 0.27 144 3.33 73 110492 97 4.5E-05 35 0.04 81 0.0785 2.05.290 1.20.3309 1.20.9634 0.1160 0.3184319 1895 63 5.2 r d 647 131 0.21 136 2.90 86 111655 108 ——0.00 186 0.0624 1.55.338 1.10.3339 1.10.9810 0.1160 0.2185718 1895 42 6.2 r d 142 85 0.61 165 3.95 61 110043 93 6.8E-05 45 0.05 42 0.1785 1.95.478 1.20.3410 1.10.92000.11650.5 1891 18 1904 91 27.1 c l 1081 210 0.20 83 1.74 140 211671 108 5.4E-06 100 -0.00 319 0.0604 1.15.532 1.00.3432 1.00.98800.1169 0.2190217 1909 30 35.2 c d 109 57 0.54 206 4.82 51 110539 97 1.3E-04 29 0.15 32 0.1681 2.35.675 1.30.3435 1.20.8926 0.1198 0.61903191954103 47.1 c l 668 347 0.54 40 0.88 266 310583 120 1.8E-05 100 -0.00 194 0.1587 0.95.633 1.20.3384 1.20.9663 0.1207 0.318791919676 5 36.1 c l 764 369 0.50 59 1.22 198 211689 108 1.3E-05 58 -0.00 232 0.1497 0.85.898 1.10.3530 1.00.9585 0.1212 0.319491719745 1 14.1 c l 671 223 0.34 57 1.24 190 210754 100 8.6E-06 32 -0.02 214 0.1124 1.06.997 1.00.3717 1.00.9832 0.1365 0.220371821843 8 6.1 c l 605 396 0.68 36 0.86 292 41065 99 1.3E-05 100 -0.00 192 0.1981 0.96.948 1.00.3691 1.00.9805 0.1365 0.220251821844 8 16.1 c l 314 216 0.71 40 0.90 260 310437 97 3.6E-05 71 0.01 111 0.2039 1.18.449 1.10.4098 1.10.9725 0.1495 0.322142023404 6 66.1 c l 584 365 0.65 38 0.89 280 410555 98 1.1E-05 58 0.01 207 0.1892 0.88.624 1.10.4134 1.00.9422 0.1513 0.422311923616 7 13.1 c l 873 399 0.47 70 2.12 168 411743 108 8.7E-06 58 0.00 319 0.1479 0.78.931 1.10.4246 1.00.8926 0.1526 0.522811923759 5 43.1 c l 535 217 0.42 67 1.44 181 312124 124 1.8E-05 58 0.01 195 0.1217 1.09.224 1.10.4249 1.00.9472 0.1574 0.322832024286 7 39.1 c l 526 354 0.69 37 0.80 295 310829 101 1.8E-05 100 0.00 198 0.2024 0.89.530 1.10.4386 1.00.9442 0.1576 0.423452024306 4 35.1 r l 803 485 0.62 41 0.83 291 311809 110 1.0E-05 50 0.01 308 0.1802 0.79.895 1.00.4469 1.00.9742 0.1606 0.223812024624 4 5.1 c l 1897 1097 0.60 32 0.65 375 412142 114 2.5E-06 41 -0.00 751 0.1722 0.410.257 1.00.4609 1.00.9954 0.1614 0.124442024702 1 11.1 c l 668 436 0.67 36 0.79 295 310632 98 1.2E-05 100 0.00 262 0.1948 0.710.2371.1 0.4571 1.10.9882 0.1624 0.224272224813 3 9.1 c l 791 521 0.68 37 0.80 296 310999 102 4.2E-06 27 -0.02 309 0.1991 0.610.206 1.10.45421.0 0.9630 0.1630 0.324142024875 4 91.1 c l 476 324 0.70 36 0.79 296 310780 100 1.3E-05 100 -0.00 184 0.2033 0.810.130 1.50.4507 1.40.9913 0.1630 0.223992924873 4 44.1 c l 2589 1122 0.45 33 0.64 410 513530 125 3.2E-06 38 0.00 1047 0.1294 0.410.643 1.10.4710 1.10.9972 0.1639 0.124882224961 0 15.1 c l 766 429 0.58 45 1.00 256 411404 114 8.0E-06 100 0.00 308 0.1667 1.110.691 1.10.4682 1.10.9913 0.1656 0.124762225142 2 75.1 c l 228 104 0.47 72 1.59 150 210804 100 30.0E-0538 -0.03 90 0.1380 1.410.512 1.50.4591 1.20.7983 0.1661 0.92435252518154 52.1 c l 706 380 0.56 49 1.07 221 310722 100 7.4E-06 45 -0.01 292 0.1631 0.711. 1411.1 0.4813 1.10.9918 0.1679 0.125332425362 0 21.1 c l 640 340 0.55 42 0.96 242 310240 95 1.1E-05 71 0.00 258 0.1599 0.811. 1131.1 0.4696 1.00.9377 0.1716 0.424822125746 4 72.1 c l 723 490 0.70 35 0.74 323 411287 105 1.6E-05 38 0.01 300 0.1990 0.611. 5351.0 0.4840 1.00.9900 0.1728 0.125452125852 2 (continued)

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 TABLE 1. U-Pb SENSITIVE HIGH-RESOLUTION ION MICROPROBE (SHRIMP) ZIRCON DATA (continued)

Spot Zircon U Th Err Yb 1s err Hf 1s err 204Pb Err f(206)204 206Pb* Isotopic ratiosApparent ages (Ma) Th/U Hf/Yb 206 Disc (%) name domain (ppm) (ppm) (abs) (ppm) (abs) (ppm) (abs) Pb (%) (%) (ppm) 208Pb*/206Pb* Err (%) 207Pb*/235U Err (%) 206Pb*/238U Err (%)Corr. coeff. 207Pb*/206Pb* Err (%) 206Pb/238U Err (abs) 207Pb/206Pb Err 1s (abs) 30.1 c l 1178 678 0.59 34 0.70 343 411529 107 2.2E-06 41 -0.00 493 0.1710 0.511. 6721.0 0.4876 1.00.9938 0.1736 0.125602125932 2 48.1 c l 1312 803 0.63 32 0.64 374 411916 120 4.7E-06 58 -0.00 554 0.1809 0.511. 7851.0 0.4913 1.00.9821 0.1740 0.2257621 2596 31 10.1 c l 1352 687 0.52 44 0.90 270 311915 110 3.1E-06 71 -0.00 580 0.1498 0.512.016 1.00.4991 1.00.9934 0.1746 0.1261022 2602 2-0 8.1 c l 629 441 0.72 36 0.78 301 310860 101 3.5E-06 71 -0.00 264 0.2072 0.711. 7671.0 0.4883 1.00.9860 0.1748 0.2256322 2604 32 Sample 15ET253a granite dike, NAD83 UTM zone 13, 6732614N, 544563E 38.1 c oz 156048 0.03 28 1.73 376 19 10699 364 3.5E-05 29 0.06 457 0.009 7.35.337 1.00.3407 1.00.9494 0.1136 0.3189016 1858 6-2 10.2 c 819 39 0.05 47 1.98 239 8 11143 279 5.4E-05 33 0.09 227 0.013 9.45.045 1.00.3220 0.90.8885 0.1136 0.5180015 1858 94 59.1 c oz 3562 189 0.05 18 1.02 620 24 11444 457 10.0E-0635 0.02 1068 0.016 3.05.475 1.20.3489 1.20.9860 0.1138 0.2192919 1861 4-4 51.1 c oz 2339 108 0.05 22 1.12 520 21 11587 347 2.8E-05 26 0.05 698 0.014 4.45.454 1.10.3475 1.00.9720 0.1138 0.2192317 1862 5-4 58.1 c oz 2735 166 0.06 19 1.20 627 29 11742 516 1.0E-05 41 0.02 814 0.018 3.35.439 0.90.3463 0.90.9647 0.1139 0.2191714 1863 4-3 10.1† c 3488 139 0.04 17 0.69 659 22 11266 247 -1.4E-0641 -0.00 979 0.012 1.55.139 1.40.3269 1.40.9976 0.1140 0.1182322 1865 23 60.1 c oz 4228 273 0.07 26 1.48 519 22 13571 508 9.3E-06 33 0.02 1285 0.020 2.55.565 0.90.3538 0.80.9773 0.1141 0.2195314 1866 3-5 18.1† c oz 2480 128 0.05 23 0.88 492 16 11175 237 2.4E-06 41 0.00 716 0.016 1.75.294 1.20.3364 1.20.9839 0.1142 0.2186920 1867 4-0 42.2 c oz 121340 0.03 35 1.79 320 13 11278 363 2.3E-05 41 0.04 343 0.010 13.3 5.1751.3 0.3288 1.10.8565 0.1142 0.6183217 1867 12 2 21.1† c 453 230 0.52 62 2.42 185 611510 238 2.4E-05 29 0.04 186 0.150 1.211. 4502.0 0.4775 1.90.9925 0.1739 0.225174125954 4 9.1† c 464 236 0.52 59 2.30 188 6 11127 234 1.3E-05 35 0.02 194 0.149 1.111. 7391.2 0.4870 1.20.9851 0.1748 0.225582626044 2 49.1† c 626 421 0.69 36 1.38 317 10 11363 235 2.7E-05 22 0.05 265 0.194 0.911. 8701.5 0.4923 1.50.9914 0.1749 0.225813126053 1 54.1† c 405 286 0.73 34 1.30 312 10 10465 217 1.0E-04 17 0.18 162 0.218 1.111. 2062.3 0.4644 2.30.9927 0.1750 0.324594626065 7 14.1† c 818 527 0.67 36 1.40 350 12 12434 258 1.5E-05 27 0.03 342 0.185 0.911. 7661.3 0.4866 1.30.9917 0.1754 0.225562826093 2 33.1† c 420 304 0.75 37 1.46 284 10 10494 216 6.4E-06 50 0.01 176 0.211 0.911. 7911.3 0.4874 1.30.9871 0.1755 0.225592726103 2 64.1† c 581 288 0.51 62 2.42 191 611919 246 2.5E-05 23 0.04 250 0.144 1.712.149 1.30.5005 1.30.9894 0.1760 0.226162826163-0 24.1† c 421 202 0.49 63 2.44 187 611773 243 3.2E-05 24 0.06 174 0.139 1.211. 6421.5 0.4794 1.40.9256 0.1761 0.625252826179 4 36.1† c 626 434 0.72 36 1.38 328 11 11700 243 3.3E-05 20 0.06 263 0.204 0.911. 8842.0 0.4890 1.90.9651 0.1763 0.525664026189 2 32.1† c 331 183 0.57 51 1.98 209 710741 221 1.1E-14 --- 0.00 139 0.160 1.211. 9201.2 0.4902 1.20.9816 0.1764 0.225712626194 2 43.1† c 353 248 0.73 36 1.39 278 9 9978 207 3.5E-05 26 0.06 148 0.207 1.111. 8561.4 0.4874 1.40.9831 0.1764 0.325593026204 3 42.1† c 214 98 0.47 128 4.98 102 313122 270 1.2E-05 50 0.02 90 0.137 1.611. 9621.5 0.4893 1.50.9814 0.1773 0.325673126285 3 20.1† c 491 181 0.38 79 3.07 165 513085 270 1.7E-05 30 0.03 207 0.107 2.011. 9761.7 0.4897 1.50.9279 0.1774 0.62570332628103 Sample 15ET260d alkali feldspar granite dike, NAD83 UTM zone 13, 6783659N, 551196E 35.1 c oz 831 88 0.11 145 3.23 111216151 156 2.6E-05 27 0.05 227 0.031 2.74.989 1.60.3177 1.60.9830 0.1139 0.3177925 1862 55 66.1 r oz 588 79 0.14 179 5.44 81 214452 231 8.1E-05 18 0.14 170 0.041 4.15.286 1.60.3360 1.60.9789 0.1141 0.3186725 1866 6-0 72.2 c 945 201 0.22 140 3.59 101 214079 175 1.1E-05 33 0.02 267 0.065 1.45.181 1.80.32931.8 0.9919 0.1141 0.2183528 1866 42 72.1 c 709 142 0.21 153 3.38 92 214069 136 5.0E-06 58 0.01 205 0.059 1.75.312 1.70.3370 1.70.9880 0.1143 0.3187227 1869 5-0 126.1 c 106991 0.09 134 2.88 115 215337 147 3.9E-06 50 0.01 304 0.026 3.45.220 1.70.3311 1.70.9932 0.1143 0.2184428 1869 42 35.2 c oz 696 133 0.20 139 3.16 102 214257 172 1.7E-05 33 0.03 183 0.052 3.44.825 1.60.3054 1.50.9793 0.1146 0.3171823 1873 69 90.1 r oz 284 67 0.24 163 3.70 78 212706 138 1.7E-04 20 0.29 81 0.055 3.95.222 1.90.3302 1.80.9517 0.1147 0.6183929 1875 11 2 32.1 r 904 83 0.09 180 5.58 91 216381 256 8.9E-06 38 0.02 259 0.028 2.35.273 1.70.3333 1.70.9905 0.1147 0.2185427 1876 41 126.2 c 894 98 0.11 113 2.74 130 314669 172 4.0E-06 58 0.01 242 0.033 3.75.001 1.70.3148 1.70.99040.1152 0.2176426 1883 47 60.1 c 849 18 0.02 192 4.24 73 114003 133 -10.0E-06 38 -0.02 259 0.007 4.95.751 2.00.3549 1.90.9614 0.1175 0.5195832191910-2 115.2 r 579 43 0.08 142 4.09 106 315084 182 3.4E-05 24 0.06 172 0.024 3.45.629 1.70.3467 1.70.9845 0.1178 0.319192819235 0 60.2 c 569 27 0.05 377 9.20 34 112815 149 6.4E-06 58 0.01 159 0.014 4.25.270 2.90.3245 2.70.9463 0.1178 0.91811431923177 114.1 r oz 128291 0.07 141 3.47 101 214252 137 -8.9E-07 100 -0.00 394 0.021 2.15.822 1.80.3577 1.70.9560 0.1181 0.5197130192710-3 115.1 r 850 117 0.14 121 3.15 123 314867 143 1.5E-05 29 0.03 257 0.043 1.85.752 1.80.3521 1.80.9923 0.1185 0.219443019344-1 120.1 r 973 68 0.07 151 3.53 98 214757 197 1.2E-06 100 0.00 293 0.021 2.55.744 1.50.3510 1.50.9898 0.1187 0.219392519374-0 57.1 c 734 29 0.04 343 8.12 38 112970 124 -3.2E-0671 -0.01 220 0.012 3.95.702 1.60.3483 1.50.9865 0.1187 0.319262619375 1 19.1 c oz 152449 0.03 125 2.66 126 215712 150 1.0E-05 28 0.02 479 0.011 3.06.185 2.00.3659 1.80.93070.1226 0.7201031199413-1 102.1 c oz 1893 101 0.05 36 1.15 420 12 15075 242 1.7E-05 20 0.03 638 0.014 2.37.372 1.70.3927 1.60.9696 0.1362 0.421352921797 2 40.1 c oz 112584 0.08 145 3.77 92 213443 219 2.2E-05 22 0.04 371 0.021 2.57.560 1.60.3841 1.50.9469 0.1427 0.520952822619 9 22.1 c 127998 0.08 152 8.37 93 414161 522 9.8E-05 16 0.17 457 0.023 3.28.217 2.20.4156 2.00.9073 0.1434 0.92240392269161 22.2 r 1312 172 0.14 154 3.37 103 215872 153 4.1E-06 45 0.01 441 0.039 2.97.918 2.60.3910 2.00.7988 0.1469 1.52127372310269 37.1 c 2334 160 0.07 32 1.13 473 11 14990 409 2.1E-05 13 0.04 900 0.022 4.19.312 2.00.4490 1.90.9568 0.1504 0.6239137235110-2 18.1 c 2544 138 0.06 34 0.92 429 814579 292 -3.2E-07 100 -0.00 982 0.016 1.49.327 1.70.4494 1.60.9668 0.1505 0.423923223527-2 121.1 c 1095 226 0.21 108 2.72 129 313963 180 -2.4E-0658 -0.00 426 0.063 1.29.460 2.00.4524 1.60.7908 0.1516 1.2240631236421-2 42.1 c 1343 150 0.12 231 6.17 69 215946 211 6.7E-06 32 0.01 513 0.033 1.59.342 1.90.4444 1.90.9652 0.1525 0.523703723749 0 129.1 c 1137 138 0.13 155 4.54 99 215407 236 7.7E-06 32 0.01 441 0.036 2.49.513 2.60.4510 2.20.8390 0.1530 1.4240043238024-1 34.1 c 1476 155 0.11 204 5.87 77 215774 319 5.6E-07 100 0.00 614 0.030 1.410.273 2.80.4847 2.50.89220.1537 1.3254853238822-8 90.2 c 1247 155 0.13 188 4.87 78 214606 245 2.0E-05 18 0.04 497 0.037 3.19.889 2.30.4638 1.80.8078 0.1546 1.3245737239823-3 113.1 c oz 528 99 0.19 111 3.39 111312356 120 1.8E-06 100 0.00 201 0.058 2.09.513 2.80.4440 2.30.8238 0.1554 1.62369462406272 79.1 c oz 346 78 0.23 162 5.31 71 211499 183 1.1E-04 16 0.19 130 0.069 5.59.481 4.00.4352 1.70.4244 0.1580 3.62329332435625 114.2 c 258 38 0.15 54 3.16 211 12 11339 128 3.0E-05 33 0.05 101 0.042 3.110.243 2.80.45721.8 0.6648 0.1625 2.12427372482353 Note: Spot name follows the convention y.z, where y—grain number and z—spot number. Uncertainties are reported at 1s (%) and were calculated by numerical propagation of all known sources of error using Squid version 2.22. Errors in ages are 1s absolute in Ma. f(206)204 refers to mole percent of total 206Pb that is due to common Pb, calculated using the 204Pb-method; common Pb composition used is the surface blank (4/6: 0.05770; 7/6: 0.89500; 8/6: 2.13840). Discordance relative to origin = 100 × (1–[206Pb/238U age]/[207Pb/206Pb age]). Calibration standard 6266; U = 910 ppm; Age = 559 Ma; 206Pb/238U = 0.09060. Bold dates were used for weighted mean calculations. Abbreviations: NAD83 UTM—Universal Transverse Mercator North American Datum 1983, oz —oscillatory zoned, pz—patchy zoned, CL—cathodoluminescence, c—core, r—rim, l—light, d —dark. *Refers to radiogenic Pb (corrected for common Pb).

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 TABLE 2. LASER ABLATION–INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (LA-ICP-MS) U-Pb ZIRCON DATA

U Th Pb* Corrected isotope ratios Apparent ages (Ma) Spot name Th/U 206Pb/ 204Pb (ppm) (ppm) (ppm) 208Pb*/ 232Th 2s err (%) 206Pb*/ 207Pb* 2s err (%) 207Pb*/ 235U* 2s err (%) 206Pb*/ 238U 2s err (%) Error corr. 238U/ 206Pb*2s err (%) 207Pb*/ 206Pb*2s err (%) Error corr. 207Pb*/ 206Pb*2s err (abs) 206Pb*/ 238U* 2s err (abs) Disc. (%)

Sample 15ET273b migmatitic metasedimentary gneiss, NAD83 UTM zone 13, 6722130N, 546859E Cores L 240 352 186 218 0.53 1274 0.1711 6.6 6.131 3.9 9.574 5.6 0.4257 4.00.6976 2.3494.0 0.1631 3.91.23E-162488652287778 S 227 414 406 261 0.98 6465 0.1347 3.3 6.666 1.5 8.413 4.7 0.4067 4.40.9273 2.4594.4 0.1500 1.55.77E-162346252200836 M1 27 257 134 140 0.52 1571 0.1384 5.0 6.666 2.2 8.529 4.6 0.4124 4.00.8656 2.4254.0 0.1500 2.2-3.99E-162346382226765 M1 17 351 127 188 0.36 1864 0.1252 4.1 6.706 2.5 8.749 3.5 0.4255 2.40.6721 2.3502.4 0.1491 2.50.00E+002336422286462 S 218 507 90 272 0.18 12166 0.1613 10.8 6.827 8.2 8.164 9.5 0.4043 4.90.5040 2.4744.9 0.1465 8.2-1.95E-162305140 2189 91 5 M1 35 592 423 350 0.71 8094 0.1322 6.7 7.039 1.7 8.516 6.6 0.4348 6.40.9607 2.3006.4 0.1421 1.76.40E-162253302327125 -3 M1 15 886 220 453 0.25 3776 0.1262 4.2 7.050 1.9 8.241 5.0 0.4214 4.60.9117 2.3734.6 0.1418 1.90.00E+00225034226787-1 L 239 514 61 252 0.12 1672 0.2364 12.9 7.078 1.0 7.257 4.5 0.3725 4.40.9522 2.6844.4 0.1413 1.04.35E-162243172041779 M1 36 152 39 74 0.26 929 0.1554 10.5 7.278 2.7 7.412 4.3 0.3913 3.30.7585 2.5563.3 0.1374 2.70.00E+002195472129603 M1 19 969 306 536 0.32 5616 0.1358 5.8 7.291 1.6 8.508 7.7 0.4499 7.60.9759 2.2237.6 0.1372 1.6-2.97E-162192272395152 -9 L 244 722 365 371 0.51 13175 0.1486 13.1 7.700 3.4 6.139 4.8 0.3429 3.30.6647 2.9173.3 0.1299 3.4-3.57E-162096601900549 M1 21 830 217 411 0.26 5155 0.1092 3.8 7.781 1.8 7.333 3.3 0.4138 2.80.8267 2.4172.8 0.1285 1.81.81E-16207831223253-7 S 222 458 161 224 0.35 16096 0.1351 13.4 7.971 1.6 6.335 3.9 0.3663 3.60.8781 2.7303.6 0.1254 1.63.32E-162035292012611 M1 18 950 199 429 0.21 3332 0.1201 9.1 8.134 1.4 6.456 4.8 0.3808 4.60.9512 2.6264.6 0.1229 1.4-2.81E-16199924208082-4 M1 26 610 55 266 0.09 6282 0.1272 7.3 8.152 1.4 6.391 4.4 0.3779 4.20.9368 2.6464.2 0.1227 1.40.00E+00199525206674-4 M1 11 1467 127 604 0.09 4709 0.1519 10.2 8.250 1.1 5.960 5.0 0.3566 4.80.9663 2.8044.8 0.1212 1.16.57E-16197420196682 0 S 223 455 58 196 0.13 3997 0.1173 6.6 8.255 2.2 5.813 3.7 0.3480 3.10.7791 2.8733.1 0.1211 2.2-1.44E-161973381925512 M1 33 619 46 250 0.07 3385 0.1326 13.7 8.307 2.0 5.829 4.2 0.3512 3.80.8750 2.8483.8 0.1204 2.00.00E+001962351940631 Rims and fir-tree zoned zircon S 233 362 17 149 0.05 5753 0.0923 10.9 8.323 1.8 5.755 4.2 0.3474 3.80.8797 2.8793.8 0.1202 1.8-5.49E-161958321922642 M1 2 707 9 277 0.01 2674 0.1013 13.6 8.344 1.1 5.763 5.0 0.3487 4.80.9661 2.8674.8 0.1198 1.13.25E-161954201929811 S 231 461 21 189 0.04 2707 0.1404 13.6 8.345 1.3 5.600 2.3 0.3389 1.90.7296 2.9501.9 0.1198 1.31.94E-161954231882314 L 247 714 10 294 0.01 13729 0.1077 17.3 8.346 1.5 5.419 3.4 0.3280 3.10.8558 3.0493.1 0.1198 1.52.17E-161954271829496 S 221 387 18 156 0.05 2436 0.1095 14.7 8.349 1.9 5.596 3.7 0.3389 3.20.8165 2.9513.2 0.1198 1.91.51E-161953351881524 S 226 418 20 173 0.05 2565 0.1088 10.2 8.353 1.7 5.680 3.6 0.3441 3.20.8398 2.9063.2 0.1197 1.70.00E+001952311906522 L 238 739 29 301 0.04 3348 0.1091 7.7 8.369 1.5 5.255 5.1 0.3190 4.90.9397 3.1354.9 0.1195 1.5-2.75E-161949271785778 L 248 601 10 259 0.02 3753 0.1201 17.0 8.383 2.1 5.732 3.8 0.3485 3.20.7925 2.8703.2 0.1193 2.1-2.91E-161946381927531 S 215 590 36 238 0.06 4707 0.1061 8.9 8.411 1.6 5.349 3.3 0.3263 2.90.8325 3.0652.9 0.1189 1.64.10E-161940291820476 M1 12 624 33 247 0.05 2719 0.1005 10.6 8.415 1.7 5.705 4.8 0.3482 4.40.9217 2.8724.4 0.1188 1.74.59E-161939311926741 S 229 655 29 275 0.04 5051 0.1116 8.3 8.416 1.2 5.478 4.0 0.3343 3.80.9227 2.9913.8 0.1188 1.2-4.18E-161939221859624 S 230 461 34 199 0.07 3757 0.1070 8.7 8.421 1.7 5.858 4.7 0.3578 4.30.9067 2.7954.3 0.1188 1.70.00E+00193831197274-2 M1 4 685 26 271 0.04 86126 0.1059 8.4 8.442 1.5 5.720 5.3 0.3502 5.10.9500 2.8555.1 0.1185 1.56.83E-161933281936850 L 245 335 11 132 0.03 1295 0.1087 17.6 8.448 1.9 5.444 4.7 0.3336 4.30.8960 2.9984.3 0.1184 1.92.28E-161932341856704 M1 5 930 43 366 0.05 5610 0.1092 7.5 8.454 1.2 5.684 4.9 0.3485 4.70.9598 2.8694.7 0.1183 1.20.00E+001931221927780 M1 25 619 37 252 0.06 2542 0.0981 8.7 8.456 1.1 5.855 3.8 0.3591 3.60.9452 2.7853.6 0.1183 1.1-6.78E-16193019197862 -2 M1 29 946 25 380 0.03 12862 0.1203 11.5 8.469 1.4 5.807 3.8 0.3567 3.60.9188 2.8043.6 0.1181 1.4-1.81E-16192725196660-2 M1 28 112634 453 0.03 6978 0.1354 20.5 8.475 1.1 5.806 4.8 0.3569 4.60.9640 2.8024.6 0.1180 1.1-3.43E-16192620196779-2 S 217 824 29 337 0.03 9084 0.0984 10.2 8.486 1.1 5.241 4.2 0.3226 4.10.9421 3.1004.1 0.1178 1.10.00E+001924191802646 M1 10 679 37 266 0.05 68746 0.1139 8.6 8.497 1.8 5.594 4.7 0.3447 4.40.9169 2.9014.4 0.1177 1.80.00E+001921321909721 S 232 358 23 143 0.06 2757 0.1097 12.4 8.499 1.6 5.435 4.7 0.3350 4.40.9195 2.9854.4 0.1177 1.62.62E-161921291863723 L 242 370 20 144 0.05 1971 0.1022 13.5 8.504 1.5 5.277 5.0 0.3255 4.80.9373 3.0734.8 0.1176 1.50.00E+001920261816765 M1 30 562 23 217 0.04 8770 0.1125 10.6 8.510 1.4 5.525 5.0 0.3410 4.80.9553 2.9324.8 0.1175 1.4-2.72E-161919241892791 M1 3 629 24 246 0.04 319192 0.1013 10.8 8.511 1.5 5.612 4.7 0.3464 4.40.9357 2.8874.4 0.1175 1.52.65E-161918271917730 S 228 654 26 273 0.04 49914 0.1086 9.6 8.513 1.3 5.446 4.0 0.3362 3.70.9099 2.9743.7 0.1175 1.3-1.95E-161918241868603 S 225 559 30 232 0.05 3496 0.1109 12.6 8.513 1.3 5.494 3.5 0.3392 3.20.8842 2.9483.2 0.1175 1.32.22E-161918241883532 M1 20 571 26 226 0.04 7245 0.1145 10.2 8.513 1.6 5.667 4.9 0.3499 4.60.9334 2.8584.6 0.1175 1.6-2.36E-16191829193477-1 M1 24 524 26 207 0.05 4965 0.0932 9.4 8.520 1.7 5.663 4.1 0.3499 3.80.9004 2.8583.8 0.1174 1.70.00E+00191730193463-1 L 235 657 35 275 0.05 11013 0.1092 8.2 8.523 1.4 5.421 3.6 0.3350 3.40.8903 2.9853.4 0.1173 1.4-2.14E-161916241863543 S 224 472 22 200 0.05 11236 0.0920 13.2 8.531 1.6 5.701 4.6 0.3528 4.30.9139 2.8354.3 0.1172 1.62.78E-16191429194872-2 M1 1 711 8 276 0.01 14783 0.0984 16.3 8.532 1.5 5.610 4.2 0.3471 3.90.9181 2.8813.9 0.1172 1.50.00E+001914281921650 S 216 457 26 189 0.06 116496 0.1065 13.3 8.540 1.8 5.559 3.4 0.3443 2.90.8012 2.9052.9 0.1171 1.8-1.78E-16191233190748 0 S 213 608 40 258 0.07 3517 0.1223 9.2 8.553 1.7 5.498 4.5 0.3410 4.20.9044 2.9324.2 0.1169 1.70.00E+001910311892691 L 236 466 26 191 0.06 39941 0.1029 10.0 8.555 1.7 5.463 4.0 0.3390 3.50.8659 2.9503.5 0.1169 1.70.00E+001909311882581 M1 34 720 36 286 0.05 4986 0.1136 7.6 8.562 2.1 5.636 4.3 0.3500 3.70.8599 2.8573.7 0.1168 2.10.00E+00190837193562-1 L 237 437 23 189 0.05 1451 0.1121 11.3 8.571 1.6 5.750 3.4 0.3575 3.00.8399 2.7983.0 0.1167 1.6-1.93E-16190629197052-3 M1 16 685 31 274 0.04 32059 0.1090 11.0 8.584 1.1 5.698 4.3 0.3547 4.10.9545 2.8194.1 0.1165 1.13.81E-16190320195770-3 M1 13 664 34 263 0.05 30440 0.1012 6.8 8.591 1.3 5.609 4.5 0.3495 4.40.9499 2.8614.4 0.1164 1.33.19E-16190223193273-2 M1 14 578 24 230 0.04 2379 0.1062 11.4 8.592 1.8 5.666 4.8 0.3531 4.50.9210 2.8324.5 0.1164 1.80.00E+00190132194976-3 Note: Isotope ratios and ages are NOT corrected for initial common Pb. Isotope ratio and apparent age errors do NOT include systematic calibration errors of 0.309398558758649% (207Pb/206Pb), 0.619567622190339% (206Pb/238U) (all 1 sigma). Isotope ratios and ages were corrected using a measured linear secondary standard age bias–206Pb count rate relationship. Trace-element concentrations are in ppm, calculated using mean count rate method. Sweep-by-sweep downhole fractionation of U/Pb ratios was NOT corrected via the Si/Zr fractionation factor. Backgrounds were monitored between sweeps 12 and 24. Sample counts were integrated from sweeps 35 to 60. Ablation used a laser spot size of 25 microns, and a laser firing repetition rate of 10 Hz. NAD83 UTM—Universal Transverse Mercator North American Datum 1983.

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

207 206 A 115 48 109 0.54 B 15ET249 weighted mean Pb/ Pb Mafic Granulite igneous crystallization age 2592 ± 16 Ma 1921 ± 13 Ma 0.50 2600

2573 ± 9 Ma 0.46 2400 2598 ± 11 Ma 207 206 12 weighted mean Pb/ Pb metamorphic age 50 34 U 0.42 23 8 1890 ± 33 Ma / 2200 Pb 2588 ± 8 Ma 2595 ± 9 Ma 206 0.38 0.38 2000 C 2608 ± 8 Ma 0.34 2700 Ma 1911 ± 30 Ma 0.34 0.30 1800 22 0.26 10 /U

0.30 Th 44 0.22 1888 ± 36 Ma 1886 ± 29 Ma 1800 Ma 0.18 0.26 2512 ± 6 Ma 1958 ± 24 Ma 357 0.14 91113 207Pb/ 235 U 0.10 0100 200300 400 Hf/Yb Figure 6. (A) Cathodoluminescence (CL) images of representative zircon analyzed in mafic granulite sample 15ET249 (spots are 20 µm for scale) that show igneous-zoned cores and bright-CL recrystallized rims. (B) Concordia plot with calculated igneous (filled red) and metamorphic (filled blue) crys- tallization populations highlighted. Open red ellipses correspond with bright-CL zircon analyses that are discordant or old. (C) Th/U vs. Hf/Yb ratios of all analyses color-coded by 207Pb/206Pb age. Trend of decreasing Th/U and increasing Hf/Yb with decreasing age is apparent.

0.56 91 6 weighted mean 207Pb/206Pb A B 15EM68b igneous crystallization age 0.52 Mafic granulite 2602 ± 8 Ma 1894 ± 8 Ma 2184 ± 4 Ma

0.48 2500 2487 ± 3 Ma

U 0.44 2300 weighted mean 207Pb/206Pb 23 8

1904 ± 9 Ma / metamorphic age 1901 ± 8 Ma Pb 0.40 2100 0.75 20 6 C 8 10 0.65 2700 Ma 0.36 1900 0.55

/U 0.45

0.32 Th 1700 2604 ± 3 Ma 0.35 1800 Ma 2602 ± 2 Ma 0.28 0.25 357 91113 207 235 Pb/ U 0.15 20 60 100140 180220 260300 Hf/Yb

Figure 7. (A) Zircon core and rim analyses from mafic granulite sample 15EM68b (backscattered electron [BSE] images, spots are 20 µm for scale). (B) Concordia plot showing igneous (2.6 Ga) and metamorphic (1.9 Ga) populations (filled red ellipses were used for weighted mean calculation). (C) Th/U vs. Hf/Yb ratios of all analyses color-coded by 207Pb/206Pb age. Younger analyses have relatively high Hf/Yb and generally lower Th/U values.

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 652

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

Felsic dike - 15ET253a A B 49 9 0.54 C

0.50 2600

2605 ± 3 Ma 0.46 2400 Inherited zircons

2604 ± 4 Ma U

238 0.42 / 2200 10 59 60 Pb 20 6 0.38 1865 ± 2 Ma 2000 1866 ± 3 Ma 0.34 1800 1858 ± 9 Ma weighted mean 207Pb/206Pb 1861 ± 4 Ma 0.30 igneous crystallization age 1864 ± 2 Ma

0.26 35791113 207Pb/235U D E 42 0.56 F Felsic dike - 15ET260d 0.52 2374 ± 9 Ma 2600 35 0.48 2400 0.44 U 2200 238 1873 ± 6 Ma / 0.40 Inherited Pb zircons

20 6 2000 1862 ± 5 Ma 0.36

66 1800 0.32 weighted mean 207Pb/206Pb 1866 ± 6 Ma 0.28 igneous crystallization age 1871 ± 5 Ma 0.24 357911 13 207Pb/235U

Figure 8. (A) Outcrop relationship of felsic dike (sample 15ET253a) that crosscuts ST2 mylonites (1 m hammer for scale). (B) Representative backscat- tered electron (BSE) images of 2.6 Ga inherited and 1.86 Ga igneous zircon crystals (large spots are 20 µm; small spots are 10 µm). (C) Concordia plot showing the 2.6 Ga inherited population (dashed ellipses), which is representative of host-rock ages, and the 1.86 Ga population, which is interpreted

to represent the igneous crystallization age of the dike. (D) Outcrop relationship of felsic dike (sample 15ET260d) that crosscuts ST2 mylonites at a low angle (15 cm pencil pointing north/left for scale). (E) BSE images of representative zircon crystals with large black circles representing 20 µm diameter spots for scale. (F) Concordia plot showing the 1.87 Ga igneous population and older inherited analyses (dashed ellipses).

+ Cpx)–bearing intermediate to felsic gneiss. Zircon crystals in sample Thirty-four analyses were conducted on 25 individual zircon grains 15ET249 were irregular to subrounded, and stubby to elongate (2:1–3:1 yielding 207Pb/206Pb dates from 2608 to 1858 Ma (Fig. 6B; Table 1), with aspect ratio), with few crystal facets, and varied from light brown to pale two prominent age populations at ca. 2.6 Ga and at ca. 1.9 Ga. Analyses pink to clear in color. CL imaging revealed three internal zonation pat- of 20 oscillatory and sector/patchy zoned zircon span dates of 2608–1921 terns (Fig. 6A) that were targeted for SHRIMP analysis: (1) moderate to Ma and lie along a discordia line anchored at ca. 2.6 Ga, with a lower widely spaced oscillatory zoning (e.g., Fig. 6A, #115, #48, #109, #12) that intercept at ca. 1.9 Ga (black unfilled ellipses, Fig. 6B). The youngest was relatively darker under CL, (2) dark- to moderately bright-CL sector oscillatory zoned zircon analysis (#115) was concordant and yielded a and patchy zoning (e.g., Fig. 6A, #50, #34, #44), and (3) bright-CL over- 207Pb/206Pb date of 1921 ± 13 Ma (1σ). The Th/U values for this group growths/rims on oscillatory and sector/patchy zoned zircon cores (e.g., Fig. were all between 0.2 and 0.38, and Hf/Yb values were between 27 and 6A, #34, #44, #10, #22). Oscillatory and sector/patchy patterns in zircon 120 (Fig. 6C). The youngest 1921 Ma analysis had a Hf/Yb value of 202 commonly exhibited both faintly blurred or faded zoning (e.g., Fig. 6A, owing to its low Yb concentration of 39 ppm. No duplicate analyses were #34) as well as crisp well-defined zoning (e.g., Fig. 6A, #12). Bright-CL performed, so Pb-loss within individual grains could not be assessed, zones and rims varied from homogeneous zones that truncated primary yet the discordia array implies that Pb loss affected this population. The zoning to gradational boundaries within zoned zircon. These rims were gen- five oldest analyses yielded a 207Pb/206Pb date of 2592 ± 16 Ma (mean erally 10–20 µm wide and exhibited sinuous transgressive inner margins, square of weighted deviates [MSWD] = 2.2). Analyses younger than the whereas their outer margins usually had rounded to irregular morphologies. oldest five were excluded due to greater discordance and the presence

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 653

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

33 223 11 26 18 es A B Migmatitic metasedimentary gneiss 1974 ± 20 Ma 0.50 15ET273b 1962 ± 35 Ma 1973 ± 38 Ma 1995 ± 25 Ma 1999 ± 24 Ma

oung Cor 0.46 Old cores 2400 Y 218 17 27 227 36 U

es 2346 ± 38 Ma 0.42 2305 ± 140 Ma 2200

2195 ± 47 Ma 238

/ Young cores

2346 ± 25 Ma Pb 0.38 Old Cor 2336 ± 42 Ma 20 6 2000 C 1924 ± 19 Ma 0.9 16 34 24 20 25 cores 2500 Ma 1908 ± 37 Ma 1918 ± 29 Ma 0.34 0.7 fir-tee zoned 1800 Th/U 0.5 spheres 1930 ± 19 Ma 0.3 0.30 1900 Ma 460.181012 Metamorphic 1917 ± 30 Ma 1903 ± 20 Ma 207Pb/235U 1000 3000 5000 7000 Hf/Yb Figure 9. (A) Representative zircon cathodoluminescence (CL) images for distinct morphological groups and spot locations (25 µm) for Snowbird domain migmatitic metasedimentary gneiss sample (15ET273b). (B) Concordia plot showing distribution of detrital “old cores” and “young cores”; “fir-tree” spherical zircon analyses are not plotted. (C) Plot highlighting the low Hf/Yb and variable Th/U for zircon cores and the high Hf/Yb and low Th/U for fir-tree zoned crystals (ellipses are not representative of errors).

of homogeneous low-U rims, suggesting these younger analyses were Sample 15EM68b: Mafic Granulite (SHRIMP) more significantly affected by Pb loss. The single oscillatory zoned zircon (Fig. 6A, #115) with a 1.9 Ga concordant date is interpreted to have been This sample was collected 80 km southwest of Wholdaia Lake within completely isotopically reset by 1.9 Ga metamorphism, but it preserves the WLsz (Fig. 3) and contained (Grt + Cpx + Opx + Hbl)–bearing relict oscillatory zoning or “ghost texture” (Hoskin and Black, 2000) from metagabbro and metadiorite gneiss (Regis et al., 2017b). These gneissic

igneous crystallization. rocks display strong overprinting by the ST1 foliation. Zircon crystals The second group consists of analyses within 14 bright-CL domains in sample 15EM68b were subrounded, and stubby to elongate (2:1–3:1 with 207Pb/206Pb dates ranging from 2569 to 1858 Ma, with a cluster at ca. aspect ratio), with few crystal facets, and were generally light brown in 1.9 Ga and two concordant single analyses at 2569 ± 27 Ma and 2372 ± color. BSE imaging revealed two primary internal textures (Fig. 7A) that 44 Ma (1σ; red-rimmed ellipses in Fig. 6B). This group had Th/U values were targeted for SHRIMP analysis: (1) homogeneous to oscillatory zoned between 0.14 and 0.26 and Hf/Yb values between 118 and 393, with the cores and whole zircon crystals, and (2) 5–50 µm homogeneous darker oldest 2569 Ma analysis having a Hf/Yb value of 60 (Fig. 6C). These rims. Rims exhibited sharp truncations with zircon cores and had sinuous higher Th/U values are very similar to the values of the older 2592 Ma transgressive inner margins overprinting cores. Thirty-one analyses were population. The Hf/Yb values, however, are much higher than the older conducted on 26 individual zircon grains with 207Pb/206Pb dates that ranged population, except for the oldest 2569 Ma analysis, which has a similar from 2604 to 1894 Ma (Table 1). Like sample 15ET249, two promi- value. These older dates (2.57 Ga, 2.37 Ga) in bright-CL domains exhibit nent age populations can be observed in the data, ca. 2.6 Ga and 1.9 Ga large errors and have higher Hf/Yb values, possibly indicating growth (Fig. 7B). Twenty-two oscillatory zoned and homogeneous zircons ranged during a separate event, after Yb depletion of the host rock. We interpret between 2604 and 2184 Ma, which consisted of a concordant grouping at these older, bright-CL zircon domains to be younger metamorphic recrys- ca. 2.6 Ga and clear discordia toward ca. 1.9 Ga. The Th/U values for this tallization domains overgrowing 2.6 Ga zircon that underwent incomplete older group decreased with age from 0.5 to 0.34, and Hf/Yb values were isotopic resetting at 1.90 Ga. No clear discordia array exists within this consistently <100 (Fig. 7C). No duplicate analyses were performed, so population, nor are there any significant differences in chemistry or zoning Pb loss within individual grains was not assessed; however, the discordia characteristics. A cluster of the youngest eight analyses with comparable array does imply that Pb loss affected this population. The three oldest chemistry yielded a 207Pb/206Pb weighted mean date of 1951 ± 52 Ma concordant analyses yielded a 207Pb/206Pb date of 2602 ± 8 Ma (MSWD (MSWD = 5.6) with excess scatter. By only including concordant analy- = 1.8). Younger analyses were excluded due to increasing discordance ses that also clustered with respect to age, four zircon crystals yielded a and presumed Pb loss. 207Pb/206Pb date of 1890 ± 33 Ma (MSWD = 0.35). Older CL-bright analy- The second group consisted of five homogeneous rim domains and ses were excluded due to their lack of concordance and age clustering. four homogeneous cores that spanned dates from 1974 to 1894 Ma, with Owing to the low U (34–62 ppm) of these bright-CL domains, their dates a cluster at ca. 1.9 Ga. This group had Th/U values between 0.20 and 0.61 are less precise relative to the dark-CL domains with higher U (57–444 and Hf/Yb values mostly >100. The three oldest homogeneous zircon ppm). Nevertheless, these data show two distinct zircon populations at cores had Yb values of 140–266 ppm, whereas the remaining six grains ca. 2.6 Ga (igneous crystallization) and 1.9 Ga (metamorphic recrystal- yielded Yb values of 36–86 ppm. The cluster of six youngest concor- lization), distinguished by date, internal zoning patterns, and chemistry. dant analyses yielded a weighted mean 207Pb/206Pb date of 1901 ± 8 Ma Additionally, a regression anchored at 2.6 Ga has a lower intercept of 1.9 (MSWD = 3.2). A regression anchored at 2.6 Ga had a lower intercept Ga, within error of the four youngest analyses. of 1.9 Ga, within error of the six youngest analyses. These two zircon

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 654

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

populations are interpreted to represent igneous crystallization at 2.6 Ga from this sample are considered inherited. Interestingly, no 2.6 Ga zircon and metamorphic (re)crystallization at 1.9 Ga. crystals were analyzed despite the ca. 2.6 Ga nature of the local host rocks.

Sample 15ET253a: Felsic Dike (SHRIMP) Sample 15ET273b: Migmatitic Metasedimentary Gneiss (LA‑ICP‑MS) This sample represents a felsic dike of granitic to granodioritic com-

position that crosscuts mylonites of ST2 (Fig. 8A) in the central WLsz. Sample 15ET273b was collected to determine a maximum depositional The sampled portion was 50 cm wide, was locally folded, and contained age for this metasedimentary rock and to relate its metamorphic history

a weak axial planar foliation parallel to ST2. Only 64 zircon crystals were to the WLsz. This migmatite-bearing Grt + Sil + Kfs + Bt + Qz metasedi- recovered from a 5 kg sample, which were poor in quality, irregular to mentary gneiss (Fig. 5F) was collected from southeastern Wholdaia Lake subhedral, and brown. BSE imaging revealed mostly altered, yet prismatic (Fig. 3) within the Snowbird domain adjacent to the WLsz. These rocks and oscillatory zoned crystals, as well as larger less-altered homogeneous have the same low magnetic response as the metasedimentary rocks 50 km blocky subhedral crystals (Fig. 8B). Twenty-two analyses were conducted to the east described in Martel et al. (2008). Zircon crystals from sample on 20 separate zircon crystals. The results fall into two age clusters, one 15ET273b were largely spherical and rounded with minor elongate 3:1 at ca. 2.60 Ga and one at ca. 1.86 Ga (Fig. 8C; Table 1). Zircon crystals crystals and varied from brown to almost clear. BSE imaging showed that form the older cluster were blocky to prismatic and homogeneous to two distinct zircon zonation textures (Fig. 9A), (1) mostly homogeneous weakly oscillatory zoned. This group of 13 crystals yielded 207Pb/206Pb spherical zircon (Fig. 9A, #16, #34, #24, #20), and (2) rare oscillatory dates between 2628 and 2595 Ma, Th/U values between 0.1 and 1.0, and zoned cores (Fig. 9A, #17, #27, #36). CL imaging of the homogeneous Hf/Yb values between 34 and 128. Zircon crystals that form the younger spherical zircon crystals revealed bright and dark, well-developed, fir-tree population were prismatic (4:1 aspect ratio) with wide oscillatory zones (e.g., Rubatto, 2017) and sector zoning (Fig. 9A, #16, #34, #24, #20, #25) (Fig. 8B) and Th/U values between 0.01 and 0.1, Hf/Yb values between with inclusions of sillimanite. Zircon cores were resorbed but retained 10 and 100, and U values of 819–4228 ppm. This group of nine grains oscillatory zoning in both BSE and CL and luminesced both bright and had a range of 207Pb/206Pb dates between 1867 and 1858 Ma. These nine dark. The few elongate rounded crystals displayed faint thin oscillatory prismatic oscillatory zoned zircon yielded a 207Pb/206Pb weighted mean zoning in BSE and CL and were commonly rimmed by fir-tree zoned date of 1864 ± 2 Ma (MSWD = 0.39). The low Th/U values associated zircon (e.g., Fig. 9A, #218). with the young population are due to anomalously high U concentrations, Due to the paucity of preserved zircon cores, only 18 were observed which are not typical of metamorphic zircon (Hoskin and Schaltegger, that were amenable for analysis. These analyses are mostly concordant 2003), and which result in slightly reversely discordant (~1.5%) analyses. and yielded 207Pb/206Pb dates between 2.48 and 1.96 Ga (Table 2). In gen- Clustering of analyses at 1.86 Ga and the well-defined internal oscilla- eral, the data have large errors and display more discordance among older tory zones are indicative of magmatic crystallization and, therefore, we dates; however, two distinct populations are evident—an older population interpret ca. 1864 Ma to record the igneous crystallization age of the dike with 207Pb/206Pb ages between 2.50 and 2.20 Ga and a younger popula- and the ca. 2.6 Ga zircon population to represent an inherited population tion with ages between 2.10 and 1.96 Ga (Fig. 9B). Broadly, the older that matches the host-rock ages (Thiessen et al., 2017). population exhibited oscillatory zoning and both dark and light CL. The younger population commonly exhibited dark and homogeneous CL with Sample 15ET260d: Felsic Dike (SHRIMP) rare bright-CL cores. Due to the high metamorphic grade and paucity of zircon core domains in the sample, a conservative approach was used for Northeast of Wholdaia Lake, this 30-cm-wide alkali-feldspar granite assessing the maximum depositional age (YC2σ3+ method, which uses

dike intruded obliquely across the ST2 mylonitic host rocks at a low angle the youngest three [or more] grains overlapping within 2σ uncertainty; (Fig. 8D). Zircon crystals were spherical to prismatic and mostly brown in Dickinson and Gehrels, 2009). The mean age of the youngest five concor- color. BSE imaging displayed altered prismatic to spherical zircon crystals dant zircon analyses that overlapped with 2σ uncertainty yielded a date (Fig. 8E, #42, #35) that showed homogeneous to faint oscillatory zoning of 1983 ± 19 Ma (MSWD = 1.3), and this is considered a conservative and rare core-rim relationships (Fig. 8E, #66). Zircon rims were thin (10–20 estimate for the maximum depositional age. m) and had homogeneous to oscillatory zoning. Thirty-one analyses on 23 The fir-tree and sector zoned spherical zircon population was also ana- individual zircon grains yielded 207Pb/206Pb dates between 2482 and 1862 lyzed (Table 2). This population yielded 207Pb/206Pb ages between 1.96 and Ma (Fig. 8F; Table 1). The results fall into three age clusters at 2.4 Ga, 1.90 Ga with 2σ errors of ~20–30 m.y. per analysis. This population was 1.93 Ga, and 1.87 Ga. The oldest age cluster is between 2482 and 2350 previously well characterized by U-Pb SHRIMP analyses and discussed Ma and contains 10 mostly concordant analyses with large errors of ~20 by Thiessen et al. (2017), who obtained an age range of 1.93–1.88 Ga and m.y. (1σ). These zircon crystals are oscillatory zoned or homogeneous a 207Pb/206Pb weighted mean of 1915 ± 4 Ma (MSWD = 2.7). whole grains. The age cluster at 1.93 Ga consists of eight analyses of Zircon crystals were also analyzed for trace elements (see Data Reposi- homogeneous whole zircon, homogeneous rims, and one oscillatory zoned tory material and Table S1 [footnote 1]). A clear distinction existed in zircon. These analyses ranged from 1994 to 1919 Ma; all but one had Th/U the Th/U values (Fig. 9C), total rare earth element (ΣREE) values, and below 0.1; and all had Hf/Yb values above 100. The youngest age cluster chondrite-normalized REE values, and by observing the covariance of at 1.87 Ga consists of nine mostly concordant analyses of homogeneous U versus Th between older core domains and younger fir-tree zoned to oscillatory zoned whole zircon and rims with 207Pb/206Pb dates ranging overgrowths. Zircon cores are interpreted to be detrital and had Th/U and from 1883 to 1862 Ma (Fig. 8F). The Th/U values were above 0.1 (one ΣREE above 0.1 and 100 ppm, respectively, whereas metamorphic fir-tree analysis was 0.09), and the Hf/Yb values were consistently above 100. zoned zircon domains consistently showed opposite trends. Zircon core These nine youngest analyses yielded a weighted mean 207Pb/206Pb date domains had chondrite-normalized REE values that are typical for nor- of 1871 ± 5 Ma (MSWD = 1.9). The oscillatory nature of the 1871 Ma mal igneous zircons, displaying a moderately positive slope, positive Ce* population, coupled with their igneous chemistry, leads us to interpret anomaly, and a negative Eu* anomaly (Hoskin and Schaltegger, 2003). 1871 Ma as the igneous crystallization age of the dike. All older results Metamorphic zircon domains differed from core analyses by having lower

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 655

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

total concentrations of REEs, a steeper positive slope among the light (L) 100000 REEs, a slightly larger negative Eu* anomaly, and a shallow negative slope Migmatitic metasedimentary gneiss within the heavy (H) REEs (Fig. 10). The zircon cores lacked a distinct 10000 15ET273b Detrital Zircon age population, were commonly oscillatory zoned, and had variable U >1.96 Ga versus Th, further supporting a detrital origin. 1000 malized Metamorphic Zircon 100 DISCUSSION AND INTERPRETATION 1932-1888 Ma 10 Neoarchean Magmatism 1 Our study and previous work (Davis et al., 2015; Thiessen et al., C1 Chondrite nor 2017) on WLsz host rocks show that these rocks formed predominantly 0.1 in the Neoarchean, at ca. 2.6 Ga. As noted herein, the distinct discordia 0.01 arrays, along with the well-clustered oscillatory and sector zoned zircon La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu populations (with high Th/U values) in mafic granulite samples 15ET249 and 15EM68b, are interpreted to represent igneous crystallization at 2.6 Figure 10. C1 chondrite–normalized (Sun and McDonough, 1989) rare earth element (REE) spider plot for metamorphic zircon (light-gray field, Ga for the related protoliths. This Archean magmatism, termed the Snow n = 39) and detrital zircon cores (gray field, n = 18) in sample 15ET273b. Island Suite regionally, is well documented throughout the Rae craton as Metamorphic zircon crystals have a distinct steepening and shallowing occurring during a major crust-forming event (e.g., Davis et al., 2015; of the light REEs and heavy REEs, respectively, relative to the trend for Peterson et al., 2015; Regis et al., 2017a). Previously, mappable expo- the detrital zircon. sures of the mafic components of this event were primarily identified northeast of the Thelon Basin (Peterson et al., 2015); however, the units in the WLsz and adjacent Firedrake domain (Davis et al., 2015) expand farther north in the central Rae craton with ages of ca. 2.0 Ga. Detrital the distribution of their setting. samples from the south Rae craton have interpreted maximum deposi- tional ages of ca. 2.0 Ga, with minimum depositional ages of 1.97 Ga Ma Snowbird Domain Metasedimentary Rocks (Knox and Ashton, 2016; Ashton et al., 2017a), 1.99 Ma (Ply, 2016), and 1.93 Ga (Martel et al., 2008), and all have abundant metamorphic zircon Constraints on Timing of Deposition at 1.91 Ga. Additionally, the zircon data of Martel et al. (2008), Shiels et The migmatitic metasedimentary gneiss (15ET273b) collected in the al. (2016), and Ashton et al. (2017a) are remarkably similar to our detrital hanging wall of the WLsz has a restricted range of detrital and metamor- data, in that the majority of data plot between 2.4 and 2.0 Ga, with an phic grains. A conservative estimate for maximum depositional age, from exceedingly small Neoarchean signature. Collectively, these metasedimen- the five youngest concordant analyses that overlap within 2σ uncertainty, tary rocks have similar depositional time ranges to sequence 3 of the Rae is 1983 ± 19 Ma. The oldest metamorphic U-Pb zircon analysis from cover sequence, as documented by Rainbird et al. (2010). These similar the same sample is 1932 ± 5 Ma (1σ; Thiessen et al., 2017), and so the assemblages of shale, carbonate, banded-iron formation, and sandstones maximum depositional range of ages for this package is constrained to are interpreted to have been deposited on a thinning lithosphere developing 22–80 m.y. between 1983 ± 19 Ma and 1932 ± 5 Ma. on a passive margin related to a rifting event, with detrital sources from the west-central Rae craton (Rainbird et al., 2010). Regional Correlation and Provenance The presence of major sedimentary basins deposited between ca. 2.0 The 1.98–1.93 Ga depositional age, the range of ages between 2.4 and 1.95 Ga, the voluminous anorthosite intrusions in the southeast Rae and 1.96 Ga, and the absence of Neoarchean zircon in sample 15ET273b craton (Card et al., 2014), and the 2.1 Ga age recently acquired for the restrict the possible sources of detritus. A viable potential source includes Chipman mafic dikes along the margin of STZ in NWT (Regan et al., the 1.99–1.95 Ga I-type intrusions of the Taltson magmatic zone, which 2016) together suggest a rifted margin likely was situated along the south- bound the south Rae craton to the west and south. Rocks of this age east Rae craton. The rift margin developed prior to sedimentation and to the east of the present-day Hearne craton within the THO were not subsequent tectonometamorphism related to the Taltson magmatic zone, accreted to its southeast margin until ca. 1.87 (Corrigan, 2012), which or collision of the Hearne craton along the STZ (Berman et al., 2007; was well after basin inversion, tectonic thickening, and metamorphism Martel et al., 2008; Bethune et al., 2010, 2013). West of our study area, of the Snowbird domain metasedimentary rocks. Rocks of the ca. 2.1 Ga the southwest Rae craton has a similar, but more poorly constrained rifting Clearwater anorthosite (Card et al., 2014) and Chipman dikes (Regan et event between 2.34 and 2.13 Ga, prior to closure of the Rutledge River al., 2016), west of the STZ, constitute another potential proximal source basin and accretion of the Buffalo Head and Slave cratons at ca. 2.0–1.93 of detritus; however, these mafic units contain very few zircon of this age, Ga (Bostock and van Breeman, 1994; McDonough et al., 2000). Taken and therefore they probably did not contribute significant 2.1 Ga zircon together, these results support a model of widespread sedimentation along to nearby sedimentary basins. the southeastern and southwestern margins of the south Rae craton that The lack of Archean dates and zircon as young as 1.96 Ga lead us followed a rifting event at ca. 2.2–2.1 Ga (Fig. 11A). to suggest that a large proportion of the detritus came from the Taltson magmatic zone between 1.98 and 1.93 Ga. This is consistent with sur- WLsz Tectonometamorphic History from 1.93 to 1.86 Ga face uplift and erosion of the Taltson magmatic zone (Fig. 11A) coeval with presumed crustal thickening during metamorphism at ca. 1.93 Ga Our data show that metamorphism of WLsz units and hanging-wall (McDonough et al., 2000). units of the Snowbird domain occurred at 1.9 Ga, allowing us to compare Our detrital zircon results have similar depositional and metamorphic the tectonometamorphic history associated with the WLsz at middle- ages compared to other south Rae craton metasedimentary rocks and rocks to lower-crustal levels. The primary metamorphic assemblages (M2) in

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 656

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

AB1.93-1.90 Ga burial metamorphism <1.98 Ga N deposition sourced from TMZ

Basement involved Rifted margin deformation

Syn- to post-1.9 Ga 1.90-1.86 Ga normal-oblique CDST1 normal-oblique granulite-facies ST2 amphibolite-facies shearing on WLsz shearing on WLsz

edrake domain Fir

Present day d domain erosional level

Snowbir

Foliations Snowbird Basement Lower Metasediments erosional metasediments gneisses crust? Mafic granulites surface

Figure 11. Block schematic of Paleoproterozoic events in the Wholdaia Lake region. (A) Younger than 1.98 Ga westerly sourced deposi- tion (sourced from exhumation of the Taltson magmatic zone [TMZ]) of protoliths to the Snowbird domain metasedimentary rocks on a possible rifted margin. (B) 1.93–1.90 Ga thrusting, burial metamorphism, and thick-skinned deformation involving basement gneisses.

(C) Syn– to post–1.9 Ga development of the northeast-trending first transposition foliation (ST1) and normal-oblique granulite-facies

shearing along the Wholdaia Lake shear zone (WLsz). (D) 1.90–1.86 Ga normal-oblique amphibolite-facies shearing on the WLsz (ST2) juxtaposing footwall mafic granulites and hanging-wall (Snowbird domain) metasedimentary rocks.

WLsz mafic granulites contain Grt2 + Cpx + Pl + Qz + Ilm ± Opx, which The low-U, bright-CL zircon overgrowths and rims in the mafic granulite are commonly ascribed to the mid- to high-pressure granulite field (Pat- samples (Figs. 6–7) are interpreted to represent high-grade metamorphic tison, 2003), and are consistent with data from Krikorian (2002), who recrystallization at ca. 1.9 Ga that overprinted ca. 2.6 Ga igneous zircon. reported a peak P-T estimate of ~1.1 GPa and 900 °C for the northern The metamorphic zircon growth was likely related to shearing on the

Wholdaia Lake granulites. Since these assemblages define the T1S folia- WLsz and attendant cooling and possible decompression, which have tion, pervasive shearing must have occurred at granulite-facies conditions been shown to be a viable mechanism for zircon growth in high-pressure prior to static recrystallization and development of granoblastic textures mafic rocks (e.g., Kohn et al., 2015; Yakymchuk et al., 2017). We there-

(Fig. 4D). Within sample 15ET249, the coarse-grained Grt1 + Cpx + Qz fore interpret the ST1-defining M2 assemblage to have formed at 1.9 Ga, ± Pl assemblage provides evidence for an earlier high-pressure event which was also coeval with movement on other northeast-trending struc- (M1) not presently associated with a structural fabric. If all plagioclase tures along the STZ. The age of the earlier M1 high-pressure assemblage in this assemblage is retrograde, as suggested here, then M1 may have remains unconstrained. equilibrated at pressures above ~1.2 GPa, which are characteristic of the Within the hanging wall of the WLsz, sample 15ET273b contains a eclogite facies (e.g., Pattison, 2003). These two assemblages may have (Grt + Sil + Kfs)–bearing assemblage that is consistent with P-T condi- developed by (1) protracted growth of the coarse M1 assemblage at an tions of ~800 °C and <0.9 GPa (Bucher and Grapes, 2011). Metamorphic earlier time (e.g., 2.55 Ga), followed by shearing and decompression zircon is characterized by abundant fir-tree and sector zones with low to form the enveloping M2 assemblage (e.g., at 1.9 Ga), or (2) during a Th/U values (Fig. 9) and shallow HREE profiles (Fig. 10), indicative of continuous process where high-pressure coarse M1 growth was followed growth in equilibrium with stable garnet. The 1915 ± 4 Ma (2σ) mean immediately by shearing and decompression, forming the M2 assemblage. age (Thiessen et al., 2017) for these zircon domains likely corresponds

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 657

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

to cooling and melt crystallization following the thermal peak of meta- Regional Exhumation Systems morphism (Kelsey et al., 2008; Yakymchuk and Brown, 2014). Similar metamorphic conditions (840 °C and 0.76 GPa) were attained 50 km to High-pressure granulites of the south Rae craton have been previously the east in the younger than 2.07 Ga metapelite that crystallized meta- documented to have been exhumed in multiple phases at ca. 1.9–1.85 morphic zircon at 1910 ± 7 Ma (2σ; Martel et al., 2008), suggesting Ga along major STZ–related structures (Mahan et al., 2006b; Regan et that a regional moderate-pressure thickening event at 1.9 Ga affected the al., 2014), with the 1.85 Ga component involving east-directed Rae over Snowbird domain. Hearne craton displacement on the Legs Lake shear zone and Chipman The 1.9 Ga metamorphic ages obtained from mafic granulites within shear zone. Other structures farther west from the Rae-Hearne bound- the WLsz overlap with the age of zircon crystallization of the hanging- ary include the Striding mylonite zone, Cora Lake shear zone, and the wall metasedimentary gneisses, indicating probable coeval metamorphism WLsz, which record slightly older ca. 1.87 Ga normal-oblique displace- of deeper (1.1 GPa; Krikorian, 2002) and shallower (0.76 GPa; Martel ment and are responsible for exhumation of the Snowbird, western Tan- et al., 2008) crustal levels prior to subsequent juxtaposition (Fig. 11B). tato, and Firedrake domains. The overall style and nature of the 1.9 Ga Pervasive ductile shearing and retrograde metamorphic recrystallization tectonism remain unclear; however, it appears that syn– to post–1.9 Ga during progressive exhumation of the WLsz produced 5–20-km-wide belts normal-oblique sense displacement on multiple crustal-scale structures

of (Hbl + Bt)–grade mylonites (ST2) that overprinted the earlier fabric and within the south Rae craton was actively exhuming high-pressure rocks

higher-grade assemblages of ST1 (Fig. 11D). The 85° southeast dip of ST2 to 0.8–1.0 GPa conditions. coupled with the dextral horizontal and x-z plane shear-sense indicators Continued exhumation along the STZ (Mahan et al., 2006b) brought all suggest normal-oblique, top-down-to-the-southwest displacement, southeastern Rae craton rocks to ~0.5 GPa at 1.85 Ga. This must have which is compatible with the break in paleopressures across the WLsz. been followed by rapid normal faulting on the Striding mylonite zone to Consistent dextral shear sense and a southwest-plunging lineation explain the removal of 15–20 km of crust before brittle-ductile faulting and imply continued normal-oblique top-down-to-the-southwest displacement deposition of the ca. 1.83 Ga subaerial Baker Lake Group volcanic rocks of the hanging wall. This study established a minimum age of shearing for on granulite basement at Snowbird (Fig. 3) and Kamilukjuaq Lakes (e.g.,

the ST2 mylonitic units to be 1.87–1.86 Ga (Fig. 8) from late-kinematic Roscoe and Miller, 1986; Rainbird et al., 2005; Flowers et al., 2006b). crosscutting dikes (samples 15ET253a, 15ET260d). Taken together, dis- Late extension, final erosion, and uplift were complete by ca 1.74 Ga, with placement and exhumation along the WLsz accompanied by develop- brittle faulting on the STZ and deposition of the younger Wharton Group in

ment of ST1 and ST2 tectonic fabrics operated during 1.90–1.86 Ga in an restricted subbasins (Hadlari and Rainbird, 2011). Detritus removed during oblique-extensional environment. the uplift events likely ultimately found its way into the alluvial-fluvial An important question is: Do the distinctly zoned structural and meta- Nonacho-Thluicho (Aspler and Donaldson, 1985; Bethune et al., 2010) or

morphic facies within the WLsz (ST1, ST2-mylonites, ST2-ultramylonites) Kiyuk Groups (Aspler et al., 2002; Davis et al., 2005) and intracontinental preserve a record of punctuated or continuous deformation at progressively basins of the Dubawnt Supergroup (Rainbird et al., 2006). shallower crustal levels during exhumation? Field relations and timing constraints bracket formation of the first two events between ca. 1.90 and Comparative Zircon Ages in the Southeastern Rae Craton 1.86 Ga. The nature and age of the ultramylonites are more elusive; they are relatively thin (~1–5 km) and are observed north of Wholdaia Lake, One of the puzzles from the regional studies of Martel et al. (2008), where the WLsz bends to a more easterly strike (Figs. 2 and 3). South Davis et al. (2015), and our results herein is the lack of evidence for along the WLsz, the structure again deflects to more easterly strikes, and 2.55 Ga metamorphism, which is prevalent in the Tantato domain (e.g., although not mapped directly, magnetic patterns indicate a <1-km-wide Baldwin et al., 2006; Mahan et al., 2006a; Flowers et al., 2008; Dumond linear pattern separating the Snowbird and Firedrake domains. South of et al., 2015). Precise U-Pb thermal-ionization mass spectrometry (TIMS) the NWT-Saskatchewan border, the WLsz may deflect into the north- zircon data from Tantato domain mafic granulites highlight clear discordia south–oriented Ryckman Bay shear zone, which separates the Train and arrays with upper intercepts at ca. 2.55 Ga (Th/U > 0.2) corresponding Dodge domains (Ashton and Card, 1998). Additionally, these easterly to dark-CL oscillatory and sector zoned zircon and lower intercepts at bends are roughly parallel to the Grease River shear zone (Fig. 2) and ca. 1.9 Ga corresponding to bright-CL overgrowths and rims. Flowers could relate to similar deformation that occurred in several phases at ca. et al. (2008) interpreted these upper and lower intercepts of 2.55 Ga 1.9, 1.85, and 1.80 Ga (Dumond et al., 2008). Like the Grease River shear and 1.9 Ga (for sample 02M133a) to correspond to two metamorphic zone, the easterly bends in the WLsz may therefore correspond to late events, as previously interpreted by Mahan et al. (2008). Other mafic dextral strike-slip strain that accommodated the offset or partial offset of granulites from the Tantato domain also show similar U-Pb zircon arrays crustal domains (e.g., Mahan and Williams, 2005) in the south Rae craton, (e.g., Baldwin et al., 2003), where it is suggested that ca. 2.55 Ga upper resulting in their current configuration. intercepts may represent metamorphic recrystallization of the protolith. Within the Firedrake domain west of the WLsz, pervasive migma- In contrast, mafic granulite 15ET249 contains two texturally distinct tization and metamorphism between 1.85 and 1.81 Ga (Davis et al., high-grade assemblages that are remarkably similar to those of sample 2015; Regis et al., 2017b) imply that the Firedrake domain underwent 02M133a in Mahan et al. (2008); however, it only contains one well- a younger exhumation history. Basement gneisses and mylonite within constrained metamorphic zircon growth episode at 1.9 Ga. Oscillatory the Firedrake domain are associated with refolded magnetic anomalies zoned zircon cores, high Th/U values, and the presence of 2.6–2.55 Ga and are dextrally transposed into the WLsz at the kilometer scale (Fig. granitoid and mafic magmatism throughout the south Rae craton (e.g., 3). These relationships suggest that younger than ca. 1.85 Ga and more Hanmer, 1997; Martel et al., 2008; Davis et al., 2015; Regis et al., 2017b)

focused deformation within the WLsz postdated the ST1 and ST2 defor- lead us to suggest that the upper intercepts of Baldwin et al. (2003) and mation dated herein. Therefore, field relationships and metamorphic Flowers et al. (2008) may be better interpreted as igneous crystallization ages across the WLsz suggest that amphibolite- to greenschist-facies rather than metamorphic ages.

ST2-ultramylonites may have accommodated post–1.85 Ga exhumation Although zircon does not record widespread 2.55 Ga regional meta- of the Firedrake domain. morphism in the Snowbird domain (obtained by SHRIMP; Martel et al.,

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 658

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

2008; Regis et al., 2017a) and is questionable in many Tantato domain post–1.86 Ga dextral offsets associated with the Grease River shear zone rocks (obtained by TIMS; Baldwin et al., 2003; Flowers et al., 2008), a and other congruent structures. few Tantato domain units have been interpreted to have 2.57–2.55 Ga Our results document a significant 1.93–1.90 Ga tectonometamorphic metamorphic zircon obtained by LA-ICP-MS (Regan et al., 2017). One burial event, but evidence for metamorphism at 2.55 Ga is lacking, and so explanation for the disparate zircon record between the studies may lie the extent and significance of this older event remain elusive within the in the method used. The LA-ICP-MS process may yield apparently con- Snowbird domain. Furthermore, although the Taltson magmatic zone (1.93 cordant results for short mixing lines between 2.6 and 1.9 Ga, whereas Ga), which flanks the west and south side of the south Rae craton, and more precise methods yield predominantly clear two-point isochrons. For the Snowbird orogen (ca. 1.90 Ga), which lies along its eastern margin, example, an Archean garnetiferous orthogneiss from the Tantato domain appear closely linked in time, the relationship and possible interaction recently analyzed by U-Pb zircon SHRIMP (Ashton et al., 2017b) yielded between these Paleoproterozoic orogenic systems remain to be resolved. metamorphic zircon populations at 2.58 and 1.9 Ga; however, only the 1.9 Ga population contained high Hf/Yb values. This suggests that the ACKNOWLEDGMENTS younger zircon growth was associated with Yb depletion and probably Funding was provided by a Natural Sciences and Engineering Research Council of Canada grant to Gibson and by a Geo-mapping for Energy and Minerals 2 Natural Resources Canada garnet growth during high-grade metamorphism, whereas the older popu- Research Affiliate Program Bursary to Thiessen. Robyn Vezina helped prepare zircon grain lation, which overlaps in age with crystallization of basement protolith mounts, and Pat Hunt oversaw zircon imaging for sensitive high-resolution ion microprobe gneisses, may represent zircon growth during a thermal pulse related (SHRIMP) analyses at the Geological Survey of Canada. We thank Tom Pestaj and Bill Davis for operating and aiding the acquisition of SHRIMP analyses. Jim Crowley provided attentive over- to documented magmatism. Regional 2.55 Ga metamorphism does not sight of zircon preparation and laser ablation–inductively coupled plasma–mass spectrometry appear to be well documented by zircon ages; however, metamorphic analyses. Comments by Rob Berman and detailed reviews by Chris Yakymchuk and Kathryn monazite in the Tantato domain (e.g., Baldwin et al., 2006; Mahan et al., Bethune improved the clarity of this manuscript. We thank Laurent Godin for editorial handling. 2006a) does yield concordant ages at ca. 2.55 Ga. This phenomenon can REFERENCES CITED be explained by the fact that monazite is a sensitive recorder of meta- Ashton, K.E., and Card, C.D., 1998, Rae northeast: A reconnaissance of the Rae Province north- morphic reactions (e.g., Dumond et al., 2015) and is more likely to grow east of Lake Athabasca, in Summary of Investigations 1998: Saskatchewan Geological episodically throughout the P-T history of a rock compared to zircon Survey, Saskatchewan Energy and Mines Miscellaneous Report 98–4, p. 3–16. Ashton, K.E., Card, C.D., Heaman, L.M., and Coolican, J., 1999, Rae northeast: Analytical growth (e.g., Taylor et al., 2016). update and new field results from the north-central Train Lake domain,in Summary of The general model for deep-crustal residence between 2.55 and 1.9 Ga Investigations 1999, Volume 2: Saskatchewan Geological Survey, Saskatchewan Energy for the Tantato domain granulites and 1.9 Ga high-pressure metamorphism and Mines Miscellaneous Report 99-4.2, p. 3–16. Ashton, K.E., Card, C.D., Davis, W., and Heaman, L.M., 2007, New U-Pb zircon age dates from due to igneous intrusion (e.g., Flowers et al., 2008; Mahan et al., 2008) the Tazin Lake map area (NTS 74N), in Summary of Investigations 2007, Volume 2: Sas- is inconsistent with the evolution of the Snowbird-Dodge domains and katchewan Geological Survey, Saskatchewan Ministry of Energy and Resources Miscel- upper Murmac Bay Group metasedimentary rocks; the latter two were laneous Report 2007–4.2, CD-ROM, Paper A-11, 8 p. Ashton, K.E., Hartlaub, R.P., Heaman, L.M., Morelli, R.M., Card, C.D., Bethune, K., and Hunter, unconformably deposited on high-grade basement gneisses that were R.C., 2009, Post-Taltson sedimentary and intrusive history of the southern Rae Province exhumed to the surface before to ca. 1.98 Ga and subsequently reburied to along the northern margin of the Athabasca Basin, western Canadian Shield: Precambrian Research, v. 175, p. 16–34, https://​doi​.org​/10​.1016​/j​.precamres​.2009​.09​.004​. granulite-facies conditions at ca. 1.93 Ga (Bethune et al., 2013). Based on Ashton, K.E., Hartlaub, R.P., Bethune, K.M., Heaman, L.M., and Niebergall, G., 2013, New depo- these results, regions north and west of the Tantato domain appear to have sitional age constraints for the Murmac Bay Group of the southern Rae Province, Canada: been affected by a widespread 1.93–1.90 Ga loading event, and it stands Precambrian Research, v. 232, p. 70–88, https://​doi​.org​/10​.1016​/j​.precamres​.2012​.05​.008​. Ashton, K., Knox, B., Card, C., Rayner, N., Davis, B., Heaman, L., and Creaser, R., 2017a, Synthesis to reason that the Tantato domain also underwent this history. However, of U-Pb and Sm-Nd geochronological results from the Dodge domain, southeastern Rae until younger than 2.0 Ga metasedimentary rocks are documented in the Province, in Summary of Investigations 2017, Volume 2: Saskatchewan Geological Survey, Tantato domain, this inferred history remains speculative. Saskatchewan Ministry of the Economy Miscellaneous Report 2017–4.2, Paper A-12, 31 p. Ashton, K., Knox, B., Rayner, N., Creaser, R., and Bethune, K., 2017b, New U-Pb and Sm-Nd results from the Tantato domain, southeastern Rae Province, Saskatchewan, in Summary SUMMARY AND CONCLUSIONS of Investigations 2017, Volume 2: Saskatchewan Geological Survey, Saskatchewan Min- istry of the Economy Miscellaneous Report 2017–4.2, Paper A-14, 27 p. Aspler, L.B., and Donaldson, J.A., 1985, The Nonacho Basin (early Proterozoic), Northwest We have refined the timing of deposition of Paleoproterozoic metasedi- Territories, Canada: Sedimentation and deformation in a strike-slip setting, in Biddle, K.T., mentary rocks in the south Rae craton to 1.98–1.93 Ga, constrained by and Christie-Blick, N., eds., Strike-Slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists (SEPM) Special Publication 37, detrital zircon analyses and timing of oldest metamorphic zircon growth. p. 193–209, https://​doi​.org​/10​.2110​/pec​.85​.37​.0193​. These metasedimentary units may have been sourced from localized uplift Aspler, L.B., Chiarenzelli, J.R., and McNicoll, V.J., 2002, Paleoproterozoic basement-cover and erosion of the Taltson magmatic zone (Fig. 11A) prior to peak meta- infolding and thick-skinned thrusting in Hearne domain, Nunavut, Canada: Intracratonic response to Trans-Hudson orogen: Precambrian Research, v. 116, p. 331–354, https://doi​ ​ morphism therein at 1.93 Ga. Metamorphic zircon growth occurred within .org​/10​.1016​/S0301​-9268​(02)00029​-3​. these metasedimentary rocks at 1.93–1.88 Ga and within high-pressure Baldwin, J.A., Bowring, S.A., and Williams, M.L., 2003, Petrological and geochronological mafic granulites at a deeper structural level at ca. 1.9 Ga (Fig. 11B). Sig- constraints on high pressure, high-temperature metamorphism in the Snowbird tectonic zone, Canada: Journal of Metamorphic Geology, v. 21, p. 81–98, https://doi​ ​.org​/10​.1046​ nificantly, we have documented a newly discovered 300-km-long, crustal- /j​.1525​-1314​.2003​.00413​.x​. scale shear zone, the WLsz, which partially accommodated exhumation Baldwin, J.A., Bowring, S.A., Williams, M.L., and Williams, I.S., 2004, Eclogites of the Snow- bird tectonic zone: Petrological and U-Pb geochronological evidence for Paleoprotero- of high-pressure mafic-granulites at 1.9–1.86 Ga during normal-oblique zoic high-pressure metamorphism in the western Canadian Shield: Contributions to sense displacement (Fig. 11C). Shearing initiated at granulite-facies con- Mineralogy and Petrology, v. 147, p. 528–548, https://​doi​.org​/10​.1007​/s00410​-004​-0572​-4​. ditions and was followed by amphibolite-facies shearing and mylonite Baldwin, J.A., Bowring, S.A., Williams, M.L., and Mahan, K.H., 2006, Geochronological con- straints on the evolution of high-pressure felsic granulites from an integrated electron development (Fig. 11D), constrained by variably deformed crosscutting microprobe and ID-TIMS geochemical study: Lithos, v. 88, p. 173–200, https://​doi​.org​ 1.86 Ga dikes (Fig. 8). Synchronous displacement along the Cora Lake /10.1016​/j​.lithos​.2005​.08​.009​. shear zone and WLsz partially exhumed large regions of middle to lower Baldwin, J.A., Powell, R., Williams, M.L., and Goncalves, P., 2007, Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Ca- crust in the southeastern Rae craton between 1.90 and 1.85 Ga by normal- nadian Shield: Journal of Metamorphic Geology, v. 25, p. 953–974, https://doi​ ​.org​/10​.1111​ oblique sense displacement, while the Legs Lake shear zone accommo- /j​.1525​-1314​.2007​.00737​.x​. Berman, R.G., Davis, W.J., and Pehrsson, S., 2007, The collisional Snowbird tectonic zone res- dated exhumation through thrusting and erosion. Lower-grade shearing urrected: Growth of Laurentia during the 1.9 Ga accretionary phase of the Trans-Hudson along more easterly trending segments of the WLsz may correspond with orogeny: Geology, v. 35, p. 911–914, https://​doi​.org​/10​.1130​/G23771A​.1​.

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 659

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

Berman, R.G., Pehrsson, S., Davis, W.J., Ryan, J.J., Qui, H., and Ashton, K.E., 2013, The Ar- northern Saskatchewan: Canadian Journal of Earth Sciences, v. 31, p. 1287–1300, https://​ rowsmith orogeny: Geochronological and thermobarometric constraints on its extent doi​.org​/10​.1139​/e94​-111​. and tectonic setting in the Rae craton, with implications for pre-Nuna supercontinent re- Heaman, L.M., Ashton, K.E., Hartlaub, R.P., and Harper, C.T., 1999, Preliminary U-Pb age con- construction: Precambrian Research, v. 232, p. 44–69, https://​doi​.org​/10​.1016​/j​.precamres​ straints on the timing of the Clearwater anorthosite complex metamorphism in the Train .2012​.10​.015​. Lake domain, and age of the Ourom Lake Group, in Summary of Investigations 1999, Bethune, K.M., Hunter, R.C., and Ashton, K.E., 2010, Age and provenance of the Paleoprotero- Volume 1: Saskatchewan Geological Survey, Saskatchewan Energy and Mines Miscel- zoic Thluicho Lake Group based on detrital zircon U-Pb SHRIMP geochronology: New laneous Report 99–4.1, p. 137–139. insights into the protracted tectonic evolution of the south-western Rae Province, Cana- Henderson, J.B., McGrath, P.H., Theriault, R.J., and Breemen, O.V., 1990, Intracratonic indenta- dian Shield: Precambrian Research, v. 182, p. 83–100, https://​doi​.org​/10​.1016​/j​.precamres​ tion of the Archean Slave Province into the early Proterozoic Thelon tectonic zone of the .2010​.07​.003​. Churchill Province, northwestern Canadian Shield: Canadian Journal of Earth Sciences, Bethune, K.M., Berman, R.G., Rayner, N., and Ashton, K.E., 2013, Structural, petrological and v. 27, p. 1699–1713, https://​doi​.org​/10​.1139​/e90​-177​. U-Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications Hoffman, P.F., 2014, The origin of Laurentia: Rae craton as the backstop for proto-Laurentian for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in amalgamation by slab suction: Geoscience Canada, v. 41, p. 313–320, https://doi​ ​.org​/10​ the SW Rae craton, Canadian Shield: Precambrian Research, v. 232, p. 89–118, https://doi​ ​ .12789​/geocanj​.2014​.41​.049​. .org​/10​.1016​/j​.precamres​.2013​.01​.001​. Hoskin, P.W.O., and Black, L.P., 2000, Metamorphic zircon formation by solid-state recrystal- Bostock, H.H., and van Breeman, O., 1994, Ages of detrital and metamorphic zircons and mona- lization of protolith igneous zircon: Journal of Metamorphic Geology, v. 18, p. 423–439, zites from a pre–Taltson magmatic zone basin at the western margin of the Rae Province: https://​doi​.org​/10​.1046​/j​.1525​-1314​.2000​.00266​.x​. Canadian Journal of Earth Sciences, v. 31, p. 1353–1364, https://​doi​.org​/10​.1139​/e94​-118​. Hoskin, P.W.O., and Schaltegger, U., 2003, The composition of zircon and igneous and meta- Bucher, K., and Grapes, R., 2011, Petrogenesis of Metamorphic Rocks: Berlin, Springer, 428 morphic petrogenesis: Reviews in Mineralogy and Geochemistry, v. 53, p. 27–62, https://​ p., https://​doi​.org​/10​.1007​/978​-3​-540​-74169​-5​. doi​.org​/10​.2113​/0530027​. Card, C.D., 2016, A Coming Together: The Juxtaposition of the Rae and Hearne Cratons along Kelsey, D.E., Clark, C., and Hand, M., 2008, Thermobarometric modelling of zircon and mona- the Virgin River Shear Zone (Snowbird Tectonic Zone), Saskatchewan, Canada, and the Im- zite growth in melt-bearing systems: Examples using model metapelitic and metapsam- plications for Proto-Laurentia [Ph.D. thesis]: Regina, Canada, University of Regina, 241 p. mitic granulites: Journal of Metamorphic Geology, v. 26, p. 199–212, https://doi​ .org​ /10.1111​ ​ Card, C.D., Bethune, K.M., Davis, W.J., Rayner, N., and Ashton, K.E., 2014, The case for a dis- /j.1525​ -1314​ .2007​ .00757​ .x​ .​ tinct Taltson orogeny: Evidence from northwest Saskatchewan, Canada: Precambrian Kiss, F., and Coyle, M., 2012, Aeromagnetic Survey of the South Rae Craton, Northwest Ter- Research, v. 255, p. 245–265, https://​doi​.org​/10​.1016​/j​.precamres​.2014​.09​.022​. ritories: Geological Survey of Canada Open-File 7122–7125, scale 1:100,000. Corrigan, D., 2012, Paleoproterozoic crustal evolution and tectonic processes: Insights from Knox, B., and Ashton, K.E., 2016, Thermotectonic events in the south Rae craton along its the LITHOPROBE program in the Trans-Hudson orogen, Canada, in Percival, J.A., Cook, easternmost margin in Saskatchewan: Geological Association of Canada–Mineralogical F.A., and Clowes, R.M., eds., Tectonic Styles in Canada: The LITHOPROBE Perspective: Association of Canada, Annual Meeting Program with Abstracts, v. 39, p. 46. Geological Association of Canada Special Paper 49, p. 237–284. Knox, B., and Biss, S., 2010, Bedrock geology of the Shagory Lake area, eastern Dodge Do- Davis, W.J., Rainbird, R.H., Aspler, L.B., and Chiarenzelli, J.R., 2005, Detrital Zircon Geochro- main, with emphasis on the history of mafic to ultramafic dykes and sheets (parts of NTS nology of the Paleoproterozoic Hurwitz and Kiyuk Groups, Western Churchill Province, 74P/10, /11, /14, and /15), in Summary of Investigations 2010, Volume 2: Saskatchewan Nunavut: Geological Survey of Canada Current Research 2005–F1, 13 p., https://doi​ ​.org​ Geological Survey, Saskatchewan Ministry of Energy and Resources Miscellaneous Re- /10​.4095​/216706​. port 2010–4.2, Paper A-1, 17 p. Davis, W.J., Hanmer, S., Tella, S., Sandeman, A., and Ryan, J.J., 2006, U-Pb geochronology Knox, B., and Lamming, J., 2015, Details from the 1:20000-scale mapping of the south-central of the MacQuoid supracrustal belt and Cross Bay plutonic complex: Key components Tantato domain, Rae Province, along the northern margin of the Athabasca Basin, in Sum- of the northwestern Hearne subdomain, western Churchill Province, Nunavut, Canada: mary of Investigations 2015, Volume 2: Saskatchewan Geological Survey, Saskatchewan Precambrian Research, v. 145, p. 53–80, https://​doi​.org​/10​.1016​/j​.precamres​.2005​.11​.016​. Ministry of the Economy Miscellaneous Report 2015–4.2, Paper A-1, 15 p. Davis, W.J., Pehrsson, S.J., and Percival, J.A., 2015, Results of a U-Pb Zircon Geochronology Knox, B., Card, C.D., and Ashton, K.E., 2011, Bedrock geology of the Grollier Lake area, south- Transect across the Southern Rae Craton, Northwest Territories, Canada: Geological Sur- eastern Dodge Domain (parts of NTS 74P/11, /12, /13, and /14), in Summary of Investiga- vey of Canada Open-File 7655, 101 p., https://​doi​.org​/10​.4095​/295610​. tions 2011, Volume 2: Saskatchewan Geological Survey, Saskatchewan Ministry of Energy Dickinson, W.R., and Gehrels, G.E., 2009, Use of U-Pb ages of detrital zircons to infer maximum and Resources Miscellaneous Report 2011–4.2, Paper A-3, 13 p. depositional ages of strata: A test against a Colorado Plateau Mesozoic database: Earth Kohn, M.J., Corrie, S.L., and Markley, C., 2015, The fall and rise of metamorphic zircon: The and Planetary Science Letters, v. 288, p. 115–125, https://doi​ .org​ /10​ .1016​ /j​ .epsl​ .2009​ .09​ .013​ .​ American Mineralogist, v. 100, p. 897–908, https://​doi​.org​/10​.2138​/am​-2015​-5064​. Dods, S.D., Teskey, D.J., and Hood, P.J., 1987, Magnetic Anomaly Map of Canada: Geologi- Krikorian, L., 2002, Geology of the Wholdaia Lake Segment of the Snowbird Tectonic Zone, North- cal Survey of Canada “A” Series Map 1255A, scale 1:5,000,000, 1 sheet, https://doi​ ​.org​ west Territories (Nunavut): A View of the Deep Crust During Assembly and Stabilization of the /10​.4095​/122836 Laurentian Craton [M.Sc. thesis]: Amherst, Massachusetts, University of Massachusetts, 90 p. Dumond, G., McLean, N., Williams, M.L., Jercinovic, M.J., and Bowring, S.A., 2008, High- Lafrance, B., and Sibbald, T.I.I., 1997, The Grease River shear zone: Proterozoic overprinting resolution dating of granite petrogenesis and deformation in a lower crustal shear zone: of the Archean Tantato domain, in Summary of Investigations 1997: Saskatchewan Geo- Athabasca granulite terrane, western Canadian Shield: Chemical Geology, v. 254, p. 175– logical Survey, Saskatchewan Energy and Mines Miscellaneous Report 97–4, p. 132–135. 196, https://​doi​.org​/10​.1016​/j​.chemgeo​.2008​.04​.014​. Mahan, K.H., and Williams, M.L., 2005, Reconstruction of a large deep-crustal terrane: Im- Dumond, G., Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2010, Subhorizontal fab- plications for the Snowbird tectonic zone and early growth of Laurentia: Geology, v. 33, ric in exhumed continental lower crust and implications for lower crustal flow: Atha- p. 385–388, https://​doi​.org​/10​.1130​/G21273​.1​. basca granulite terrane, western Canadian Shield: Tectonics, v. 29, TC2006, https://doi​ ​ Mahan, K.H., Williams, M.L., and Baldwin, J.A., 2003, Contractional uplift of deep crustal rocks .org​/10.1029​/2009TC002514​. along the Legs Lake shear zone, western Churchill Province, Canadian Shield: Canadian Dumond, G., Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2015, Monazite as a moni- Journal of Earth Sciences, v. 40, p. 1085–1110, https://​doi​.org​/10​.1139​/e03​-039​. tor of melting, garnet growth and feldspar recrystallization in continental lower crust: Mahan, K.H., Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2006a, Dating metamorphic Journal of Metamorphic Geology, v. 33, p. 735–762, https://​doi​.org​/10​.1111​/jmg​.12150​. reactions and fluid flow: Application to exhumation of high-P granulites in a crustal-scale Dumond, G., Williams, M.L., Baldwin, J.A., and Jercinovic, M.J., 2017, Backarc origin for Neo- shear zone, western Canadian Shield: Journal of Metamorphic Geology, v. 24, p. 193–217, archean ultrahigh-temperature metamorphism, eclogitization, and orogenic root growth: https://​doi​.org​/10​.1111​/j​.1525​-1314​.2006​.00633​.x​. Geology, v. 45, p. 943–946, https://​doi​.org​/10​.1130​/G39254​.1​. Mahan, K.H., Williams, M.L., Flowers, R.M., Jercinovic, M.J., Baldwin, J.A., and Bowring, S.A., Flowers, R.M., Bowring, S.A., Mahan, K.H., and Williams, M.L., 2006a, Timescales and sig- 2006b, Geochronological constraints on the Legs Lake shear zone with implications for nificance of high-pressure, high-temperature metamorphism and mafic dyke anatexis, regional exhumation of lower continental crust, western Churchill Province, Canadian Snowbird tectonic zone, Canada: Contributions to Mineralogy and Petrology, v. 151, Shield: Contributions to Mineralogy and Petrology, v. 152, p. 223–242, https://​doi​.org​ p. 558–581, https://​doi​.org​/10​.1007​/s00410​-006​-0066​-7​. /10.1007​/s00410​-006​-0106​-3​. Flowers, R.M., Mahan, K.H., Bowring, S.A., Williams, M.L., Pringle, M.S., and Hodges, K.V., Mahan, K.H., Goncalves, P., Flowers, R., Williams, M.L., and Hoffman-Setka, D., 2008, The 2006b, Multistage exhumation and juxtaposition of lower continental crust in the west- role of heterogeneous strain in the development and preservation of a polymetamorphic ern Canadian Shield: Linking high-resolution U-Pb and 40Ar/39Ar thermochronology with record in high-P granulites, western Canadian Shield: Journal of Metamorphic Geology, pressure-temperature-deformation paths: Tectonics, v. 25, TC4003, https://doi​ ​.org​/10​ v. 26, p. 669–694, https://​doi​.org​/10​.1111​/j​.1525​-1314​.2008​.00783​.x​. .1029​/2005TC001912​. Martel, E., and Pierce, K., 2006, An ArcView 3.x Digital Geological Atlas of the Snowbird Lake Flowers, R.M., Bowring, S.A., Mahan, K.H., Williams, M.L., and Williams, I.S., 2008, Stabiliza- Area, NTS 65D: Northwest Territories Geoscience Office NWT Open-File 2006–02, digital tion and reactivation of cratonic lithosphere from the lower crustal record in the western files and 2 maps, scale 1:125,000. Canadian Shield: Contributions to Mineralogy and Petrology, v. 156, p. 529–549, https://​ Martel, E., van Breemen, O., Berman, R.G., and Pehrsson, S., 2008, Geochronology and tec- doi​.org​/10​.1007​/s00410​-008​-0301​-5​. tonometamorphic history of the Snowbird Lake area, Northwest Territories, Canada: New Hadlari, T., and Rainbird, R.H., 2011, Retro-arc extension and continental rifting: A model for insights into the architecture and significance of the Snowbird tectonic zone: Precambrian the Paleoproterozoic Baker Lake Basin, Nunavut: Canadian Journal of Earth Sciences, Research, v. 161, p. 201–230, https://​doi​.org​/10​.1016​/j​.precamres​.2007​.07​.007​. v. 48, p. 1232–1258, https://​doi​.org​/10​.1139​/e11​-002​. McDonough, M.R., and McNicoll, V.J., 1997, U-Pb age constraints on the timing of deposition Hanmer, S., 1997, Geology of the Striding-Athabasca Mylonite Zone, Northern Saskatchewan of the Waugh Lake and Burntwood (Athabasca) Groups, southern Taltson magmatic zone, and Southeastern District of Mackenzie, Northwest Territories: Geological Survey of northeast Alberta, in Radiogenic Age and Isotopic Studies Report 10: Geological Survey Canada Bulletin 501, 106 p., https://​doi​.org​/10​.4095​/209170​. of Canada Current Research 1997-F, p. 101–111. Hanmer, S., Parrish, R., Williams, M.L., and Kopf, C., 1994, Striding-Athabasca mylonite zone: McDonough, M.R., McNicoll, V.J., Schetselaar, E.M., and Grover, T.W., 2000, Geochronologi- Complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, cal and kinematic constraints on crustal shortening and escape in two-sided oblique-slip

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 660

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018 THIESSEN ET AL. | Wholdaia Lake shear zone RESEARCH

collisional and magmatic orogen, Paleoproterozoic Taltson magmatic zone, northeastern Churchill Province: Exploring the continuity of the Rae-Hearne boundary and its role dur- Alberta, in Ross, G., ed., The Lithoprobe-Alberta Basement Transect: Canadian Journal ing the growth of Laurentia: Geological Society of America Abstracts with Programs, of Earth Science, v. 37, p. 1549–1573, https://​doi​.org​/10​.1139​/e00​-089​. v. 48, no. 7, paper 204-4, https://​doi​.org​/10​.1130​/abs​/2016AM​-286834​. McNicoll, V.J., Theriault, R.J., and McDonough, M.R., 2000, Taltson basement gneissic rocks: Regan, S.P., Williams, M.L., Chiarenzelli, J.R., Grohn, L., Mahan, K.H., and Gallagher, M., 2017, U-Pb and Nd isotopic constraints on the basement to the Paleoproterozoic Taltson mag- Isotopic evidence for Neoarchean continuity across the Snowbird tectonic zone, western matic zone, northeastern Alberta: Canadian Journal of Earth Sciences, v. 37, p. 1575–1596, Churchill Province, Canada: Precambrian Research, v. 300, p. 201–222, https://doi​ ​.org​/10​ https://​doi​.org​/10​.1139​/e00​-034​. .1016​/j​.precamres​.2017​.07​.022​. Mills, A.J., Berman, R.G., Davis, W.J., Tella, S., Carr, S., Roddick, C., and Hanmer, S., 2007, Regis, D., Acosta-Gongora, P., Davis, W.J., Knox, B., Pehrsson, S.J., Martel, E., and Hulbert, Thermobarometry and geochronology of the Uvauk complex, a polymetamorphic Neo- L., 2017a, Evidence for Neoarchean Ni-Cu–bearing mafic intrusions along a major litho- archean and Paleoproterozoic segment of the Snowbird tectonic zone, Nunavut, Canada: spheric structure: A case study from the south Rae craton (Canada): Precambrian Research, Canadian Journal of Earth Sciences, v. 44, p. 245–266, https://​doi​.org​/10​.1139​/e06​-080​. v. 302, p. 312–339, https://​doi​.org​/10​.1016​/j​.precamres​.2017​.09​.026​. Pattison, D.R.M., 2003, Petrogenetic significance of orthopyroxene-free garnet + clinopyroxene Regis, D., Martel, E., Davis, W.J., and Pehrsson, S.J., 2017b, U-Pb Zircon Geochronology of + plagioclase ± quartz-bearing metabasites with respect to the amphibolite and granulite Metaplutonic Rocks Across the Southern Rae Province, Northwest Territories: Geological facies: Metamorphic Geology, v. 21, p. 21–34. Survey of Canada Open-File 8254, 37 p., https://​doi​.org​/10​.4095​/302772​. Pehrsson, S.J., Berman, R.G., and Davis, W.J., 2013, Paleoproterozoic orogenesis during Nuna Roscoe, S.M., and Miller, A.R., 1986, Outliers of porphyritic alkaline volcanic rocks of the aggregation: A case study of reworking of the Rae craton, Woodburn Lake, Nunavut: Pre- Christopher Island Formation at Snowbird Lake, N.W.T., in Current Research, Part B: cambrian Research, v. 232, p. 167–188, https://​doi​.org​/10​.1016​/j​.precamres​.2013​.02​.010​. Geological Survey of Canada Paper 86–1B, p. 679–683. Pehrsson, S.J., Campbell, J.E., Martel, E., McCurdy, M.W., Agosta-Gongora, P., Thiessen, E., Rubatto, D., 2017, Zircon: The metamorphic mineral: Reviews in Mineralogy and Geochem- Jamieson, D., Lauzon, G., Buller, G., Falck, H., and Dyke, A.S., 2015, Report of 2015 Activi- istry, v. 83, p. 261–295. ties for the Geologic and Metallogenic Framework of the South Rae Craton, Southeast Sanborn-Barrie, M., Carr, S.D., and Theriault, R., 2001, Geochronological constraints on meta- Northwest Territories: GEM 2 South Rae Quaternary and Bedrock Project: Geological morphism, magmatism and exhumation of deep-crustal rocks of the Kramanituar Com- Survey of Canada Open-File 7958, 24 p., https://​doi​.org​/10​.4095​/297387​. plex, with implications for the Paleoproterozoic evolution of the Archean western Churchill Percival, J.A., Martel, E., Pehrsson, S.J., Acosta-Gongora, P., Regis, D., Thiessen, E., Jamison, Province, Canada: Contributions to Mineralogy and Petrology, v. 141, p. 592–612, https://​ D., Neil, B., and Knox, B., 2016, Report of 2016 Bedrock Activities for the Geologic and doi​.org​/10​.1007​/s004100100262​. Metallogenic Framework of the South Rae Craton, Southeast NWT: GEM 2 South Rae Sharpton, V.L., Grieve, R.A.F., Thomas, M.D., and Halpenny, J.F., 1987, Horizontal gravity gradi- Quaternary and Bedrock Project: Geological Survey of Canada Open-File 8142, 17 p., ent: An aid to the definition of crustal structure in North America: Geophysical Research https://​doi​.org​/10​.4095​/299469​. Letters, v. 14, p. 808–811, https://​doi​.org​/10​.1029​/GL014i008p00808​. Peterson, T.D., Van Breemen, O., Sandeman, H., and Cousens, B., 2002, Proterozoic (1.85–1.75 Shiels, C., Partin, C.A., and Eglington, B.M., 2016, Provenance approaches in polydeformed Ga) igneous suites of the western Churchill Province: Granitoid and ultrapotassic magma- metasedimentary successions: Determining nearest neighboring cratons during the tism in a reworked Archean hinterland: Precambrian Research, v. 119, p. 73–100, https://​ deposition of the Paleoproterozoic Murmac Bay Group: Lithosphere, v. 8, p. 519–532, doi​.org​/10​.1016​/S0301​-9268​(02)00118​-3​. https://​doi​.org​/10​.1130​/L537​.1​. Peterson, T.D., Jefferson, C.W., and Anand, A., 2015, Geological Setting and Geochemistry Stipp, M., Stunitz, H., Heilbronner, R., and Schmid, S., 2002, Dynamic recrystallization of of the ca. 2.6 Ga Snow Island Suite in the Central Rae Domain of the Western Churchill quartz: Correlation between natural and experimental conditions, in De Meer, S., Drury, Province, Nunavut: Geological Survey of Canada Open-File 7841, 29 p., https://doi​ .org​ ​ M.R., De Bresser, J.H.P., and Penock, G.M., eds., Deformation Mechanisms, Rheology, and /10​.4095​/296599​. Tectonics: Current Status and Future Perspectives: Geological Society, London, Special Petts, D.C., Davis, W.J., Moser, D.E., and Longstaffe, F.J., 2014, Age and evolution of the lower Publication 200, p. 171–190, https://​doi​.org​/10​.1144​/GSL​.SP​.2001​.200​.01​.11​. crust beneath the western Churchill Province: U-Pb zircon geochronology of kimberlite- Sun, S.-s., and McDonough, W.F., 1989, Chemical and isotopic systematics of ocean basalts: hosted granulite xenoliths, Nunavut, Canada: Precambrian Research, v. 241, p. 129–145, Implications for mantle composition and process, in Saunders, A.D., and Norry, M.J., https://​doi​.org​/10​.1016​/j​.precamres​.2013​.11​.010​. Magmatism in the Ocean Basins: Geological Society, London, Special Publication 42, Ply, D., 2016, Resolving Paragneiss Provenance at Grollier Lake in the Athabasca Granulite p. 313–345, https://​doi​.org​/10​.1144​/GSL​.SP​.1989​.042​.01​.19​. Terrane, Western Canadian Shield [M.Sc. thesis]: Fayetteville, Arkansas, University of Taylor, R.J.M., Kirkland, C.L., and Clark, C., 2016, Accessories after the facts: Constraining the Arkansas, 44 p. timing, duration and conditions of high-temperature metamorphic processes: Lithos, Rainbird, R.H., Stern, R.A., Rayner, N., and Jefferson, C.W., 2005, Age, provenance, and re- v. 264, p. 239–257, https://​doi​.org​/10​.1016​/j​.lithos​.2016​.09​.004​. gional correlation of the Athabasca Group, Saskatchewan and Alberta, constrained by Thiessen, E.J., Regis, D., and Gibson, H.D., 2017, U-Pb Zircon Geochronology of the Paleopro- igneous and detrital zircon geochronology, in Jefferson, C.W., and Delaney, G., eds., EX- terozoic Wholdaia Lake Shear Zone, South Rae Craton, Northwest Territories: Geological TECH IV: Geology and Uranium Exploration TECHnology of the Proterozoic Athabasca Survey of Canada Open-File 8193, 27 p., https://​doi​.org​/10​.4095​/300655​. Basin, Saskatchewan and Alberta: Geological Survey of Canada Bulletin 588, p. 1–17. Trouw, R.A.J., Passchier, C.W., and Wiersma, D.J., 2009, Atlas of Mylonites and Related Rainbird, R.H., Davis, W.J., Stern, R.A., Peterson, T.D., Smith, S.R., Parrish, R.R., and Hadlari, Micro­structures: Heidelberg, Germany, Springer, 322 p., https://​doi​.org​/10​.1007​/978​-3​ T., 2006, Ar-Ar and U-Pb geochronology of a late Paleoproterozoic Rift Basin: Support for -642​-03608​-8​. a genetic link with Hudsonian orogenesis, western Churchill Province, Nunavut, Canada: Whitney, D.L., and Evans, B.W., 2010, Abbreviations for names of rock-forming minerals: The The Journal of Geology, v. 114, p. 1–17, https://​doi​.org​/10​.1086​/498097​. American Mineralogist, v. 95, p. 185–187, https://​doi​.org​/10​.2138​/am​.2010​.3371​. Rainbird, R.H., Davis, W., Pehrsson, S., Wodicka, N., Rayner, N., and Skulski, T., 2010, Early Yakymchuk, C., and Brown, M., 2014, Behaviour of zircon and monazite during crustal melt- Paleoproterozoic supracrustal assemblages of the Rae domain, Nunavut, Canada: Intracra- ing: Journal of the Geological Society [London], v. 171, p. 465–479, https://doi​ .org​ /10.1144​ ​ tonic basin development during supercontinent break-up and assembly: Precambrian /jgs2013​-115​. Research, v. 181, p. 167–186, https://​doi​.org​/10​.1016​/j​.precamres​.2010​.06​.005​. Yakymchuk, C., Clark, C., and White, R.W., 2017, Phase relations, reaction sequences and petro­ Ramsay, J.G., 1967, Folding and Fracturing of Rocks: New York, McGraw-Hill, 568 p. chronology: Reviews in Mineralogy and Geochemistry, v. 83, p. 13–53. Regan, S.P., Williams, M.L., Leslie, S., Mahan, K.H., Jercinovic, M.J., and Holland, M.E., 2014, The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia: Canadian Journal of Earth Sciences, v. 51, p. 877–901, https://​doi​.org​/10​.1139​/cjes​-2014​-0015​. MANUSCRIPT RECEIVED 20 SEPTEMBER 2017 Regan, S.P., Williams, M.L., Grohn, L.J., Chiarenzelli, J.R., Mahan, K.H., Cousens, B., Aspler, REVISED MANUSCRIPT RECEIVED 8 JUNE 2018 L.B., and Jercinovic, M.J., 2016, Two transects across the Snowbird tectonic zone, western MANUSCRIPT ACCEPTED 5 JULY 2018

LITHOSPHERE | Volume 10 | Number 5 | www.gsapubs.org 661

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/5/641/4344156/641.pdf by guest on 08 December 2018