Resolving Paragneiss Provenance at Grollier Lake in the Athabasca Granulite Terrane, Western Canadian Shield Dustin Ply University of Arkansas, Fayetteville

University of Arkansas, Fayetteville ScholarWorks@UARK

Theses and Dissertations

5-2016 Resolving Paragneiss Provenance at Grollier Lake in the Athabasca Granulite Terrane, Western Canadian Shield Dustin Ply University of Arkansas, Fayetteville

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Resolving Paragneiss Provenance at Grollier Lake in the Athabasca Granulite Terrane, Western Canadian Shield.

A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology

Dustin Ply University of Arkansas Bachelor of Science in Geology, 2014

May 2016 University of Arkansas

This thesis is approved for recommendation to the Graduate Council.

______Dr. Gregory Dumond Thesis Director

______Dr. Adriana Potra Dr. Doy L. Zachry Committee Member Committee Member

Abstract

U-Pb crystallization ages of metamorphic and detrital zircons from all three paragneiss samples fall into the range of ca. 1.85-2.59 Ga, excluding two much older grains. Evidence suggests that the paragneiss of Grollier Lake record deformation exclusively from the Taltson and Trans-Hudson orogenies. It is apparent from geochronological data that the Taltson orogeny played an exceedingly larger role in the deformation of these rocks than the Trans-Hudson.

Deposition of the paragneiss protoliths most likely culminated between ca. 2037-1994 Ma with metamorphism ceasing by 1852.1 ± 11.1 Ma. The oldest overgrowth considered to be concordant is 1994 ± 12 Ma and interpreted to represent the first signature of burial facilitated by the Taltson orogeny. U-Pb crystallization ages ranging from ca. 1872-1900 Ma can be attributed to metamorphisms from both the late Taltson or early Trans-Hudson orogenies given that the transition between these events is hard to delineate. Zircons dated > ca. 2.04 Ga are detrital in origin with U-Pb crystallization ages for these grains possibly being discordant as supported by the concordia diagrams. However, these ages can still be explained by provenance from sources such as the ca. 2.17-2.13 Ga Rutledge River basin to the west of Grollier Lake, and the ca. 2.3

Ga Arrowsmith subduction-related plutons north of Lake Athabasca. Older zircons (2955.6 ±

10.7 Ma and 3078 ± 13.9 Ma) in the migmatitic paragneiss are inferred to be inherited from

Neoarchean and early Paleoproterozoic crust below. It is possible that rocks of the lower structural levels will record deformation from the Arrowsmith orogeny.

Acknowledgments

Thanks are extended to the staff of the University of Arkansas Graduate School for the many services that they provide to the students.

The hospitality and assistance provided by the staff at the Arizona LaserChron Center is greatly appreciated.

Thanks are also extended to Dr. Dumond for being a tremendous mentor in addition to funding the research with his NSF grant #1255277, and to Dr. Potra and Dr. Zachry for their cooperation on this thesis. Additionally, a special thanks to Dr. Zachry, who during his time with the Department of Geosciences at the University of Arkansas has been an outstanding professor, not just to me, but to many students before me.

Table of Contents

Page

Introduction ...... 1

Geologic Setting...... 1

Petrography ...... 6

Methodology ...... 7

U-Pb Geochronology Results ...... 9

Discussion ...... 10

Conclusions ...... 19

References ...... 21

Appendix ...... 25

Introduction

Provenance studies of highly deformed terranes, although difficult, can provide valuable information towards reconstructing the tectonostratigraphic framework. Provenance studies of paragneiss are usually more complex than those of its sedimentary counterpart due to metamorphic processes such as heterogeneous recrystallization (Flowers et al., 2008) of zircon that can alter geochronological results leading to discordance. Even more obscure than the provenance is the tectonic processes that facilitated the deformational events recorded by these rocks i.e. the events that led to the formation of the Snowbird Tectonic Zone (STZ). The geological complexity of areas such as Grollier Lake is why the origin of the STZ and its components, like the Grease River shear zone (GRsz), remain debated (e.g. Hoffman, 1987;

Hanmer, 1994; Mahan and Williams, 2005; Berman et al., 2007). The study area is within the

Athabasca granulite terrane at Grollier Lake which, until now, has been lacking of any geochronological data. This area was chosen mainly for two reasons: 1. It represents the wall rocks on the northwest side of the GRsz, and 2. Restoration of post 1.85 Ga strike slip offset along the GRsz (Mahan and Williams, 2005) places Grollier Lake adjacent to the East Athabasca mylonite triangle, an area that has been studied vigorously and has abundant data (Hanmer,

1994; Baldwin et al., 2003; Mahan et al., 2006a; Flowers et al., 2008). It is imperative to document lithological and geochronological data on both sides of the GRsz in order to fully understand its implications. Presented are the first U-Pb geochronological results from Grollier

Lake, which are used to resolve provenance, place constraints on maximum date of deposition, and determine date of peak metamorphism.

Geologic Setting

Located in the western Canadian Shield, the Western Churchill Province is one of the largest cratonic constituents of Laurentia. It is comprised of the Rae and Hearne domains which

are separated by the ~2800 km long, northeast-trending Snowbird Tectonic Zone, a geophysically-defined cratonic discontinuity (Hoffman, 1988) whose origin has major implications for the cratonic assemblage of the western Churchill Province (Figure 1a). Grollier

Lake is located in the southernmost portion of the Rae, which coincides with northern

Saskatchewan. It is part of the Dodge domain within a region named the Athabasca granulite terrane (Figure 1c). This terrane represents >20,000 km2 of exhumed continental lower crust uplifted from depths of 30-50 Km in the hanging wall of the 1.85 Ga thrust-sense Legs Lake shear zone (LLsz) (Mahan and Williams, 2005). Offsetting the LLsz is the GRsz, which represents the boundary between the Rae and Hearne at this location. The GRsz also denotes the southeastern termination of the study area. The significance of this shear zone will not be the focus of this study, however, results from this project will help to reconstruct the history of the northwest wall rocks of this tectonic phenomenon. The East Athabasca mylonite triangle subsequently occupies the southeast wall rocks of the GRsz (Figure 2).

Gently-dipping, northwest-striking migmatic S1 fabrics and recumbent F1 folds are re- folded into upright, gently southwest-plunging F2 folds that define a 20 km-scale Type 2 fold interference or “mushroom” pattern at Grollier Lake (Figure 4). S2 foliation on S1 defines an L2 intersection lineation displayed in the stereonets in Figure 5. Possible orogenies experienced by these rocks are presented in Figure 6. Although not sampled in this study, there is a presumed

Archean-aged basement suite of deformed granite, granodiorite, and migmatitic paragneiss. The structurally higher and inferred to be significantly younger paragneiss cover package consists of garnet + sillimanite ± cordierite- bearing felsic granulite, quartzite, and migmatitic paragneiss.

Field images of these rocks are presented in Figure 3. The felsic granulite is structurally the highest of all three samples and presumed to be a constituent of the youngest sequence here. It is also possible that the felsic granulite is analogous to the psammopelitic gneiss mapped in this

Figure 1. a) Simplified geologic map of the western Canadian Shield compiled and modified after Hoffman (1988), Tella et al. (2000), Ross (2002), and Hajnal et al. (2005). Note location of inset b) at center. Abbreviations are: GSLsz = Great Slave Lake shear zone; CLsz = Charles Lake shear zone; BBsz = Black Bay shear zone; GRsz = Grease River shear zone; LLsz = Legs Lake shear zone; CBsz = Cable Bay shear zone; VRsz = Virgin River shear zone; NFsz = Needle Falls shear zone; TNB = Thompson Nickel Belt; Tsz = Tyrell shear zone; Cd = Chesterfield domain; Cfz = Chesterfield fault zone (Berman et al., 2007); TWBd = Tehery Wager Bay gneiss domain (van Breeman et al., 2007). Locations of Lithoprobe seismic reflection profiles from http://www.lithoprobe.ca. b) Inset depicting present day disposition of Athabasca granulite terrane. OMBsz = Oldman-Bulyea shear zone (Card, 2001). c) Circa 1.85 Ga reconstruction of Athabasca granulite terrane after Mahan and Williams (2005). region by Knox et al. (2011). The quartzite is a discontinuous layer that has been mapped at three localities at Grollier Lake. Knox et al. (2011) states it is possible that the quartzite was once a continuous unit, but now occurs as isolated packages due to an originally thin thickness that was subjected to multiple deformation events. The quartzite sampled in this study was taken directly above the migmatitic paragneiss. Before geochronological analysis, the migmatitic paragneiss was thought to be an orthogneiss due to the abundance of large, euhedral feldspar phenocrysts.

However, after petrographic and geochronological analysis it was apparent that this rock had a sedimentary protolith that underwent melting during burial thus giving it characteristics of an

orthogneiss.

Similar tectonostratigraphic frameworks, folding patterns, and fabric overprinting to that at Grollier Lake have been observed via workers with study areas in the Rae. Northeast of

Grollier Lake at Snowbird Lake, Martel et al. (2008) documents a similar Type 2 Fold interference pattern and migmatic S1 fabric. Southwest of Grollier Lake at the Beaverlodge

Domain, Bethune et al. (2013) also documents a similar migmatic S1 fabric. The occurrence of this fabric at all three of these locations suggests it is possible the same event is responsible for creating it. A constraint on the timing of melting could provide support for this claim.

Figure 2. ASTER satellite image of Grollier Lake with restoration of 110 km strike-slip offset (Mahan and Williams, 2005) demonstrating juxtaposition with the East Athabasca mylonite triangle. Contents of this figure provided by personal communication with Dr. Dumond.

Figure 3. Field images of the felsic granulite, quartzite, and migmatitic paragneiss. The pen is marking the intersection lineation. The surface of the felsic granulite is defined by coarse-grained Kfs+Pl+Qtz leucosome. Pictures used with permission from the photographer: Dr. Gregory Dumond.

Figure 4. 1-50k interpretive geologic map of Grollier Lake displaying field measurements of S1 and S2 foliations and intersecting lineation. Some S4 foliation data (S2 in this study) taken from Knox et al. (2011). Remaining field data provided by personal communication with Dr. Dumond.

Figure 5. Stereonets of S1 and S2 foliation along with the L2 intersection lineation.

Figure 6. Table demonstrating timing of the three orogenies possibly recorded by the rocks at Grollier Lake. Dates of Arrowsmith orogeny are from Hartlaub et al. (2007), Taltson and Trans- Hudson dates are from Hoffman (1988, 1989).

Petrography

Figure 7. Thin section photomicrographs all of the a) felsic granulite, b) migmatitic paragneiss, and c) quartzite. All images are illuminated under cross-polarized light.

05G-062: garnet + sillimanite ± cordierite-bearing felsic granulite.

Thin section analysis shows that biotite is rare in this rock, but when present is aligned with foliation. Quartz grains are highly fractured with serrated to lobate grain boundaries and crystal lattices that display undulose extinction. Garnet porphyroblasts are abundant, contain many inclusions, and are highly fractured with mica filling these fractures. K-feldspar is abundant and at undulose extinction with serrated to lobate grain boundaries.

14V-030A: migmatitic paragneiss

The migmatitic paragneiss is abundant with fine-grained, euhedral quartz and feldspar of unimodal distribution. Microcline is the most common form of K-feldspar. Biotite is also common and shows no signs of alignment with foliation. Many quartz grains display straight extinction in addition to undulosic. Euhedral grains with straight extinction are phenocrystic in nature and are characteristic of crystallization from a magmatic source, most likely during partial melting of this rock. Due to the abundance of grains with undistorted crystal lattices, it is assumed that exhumation of these rocks must have occurred relatively soon after melting.

14V-030B: quartzite

The quartzite is abundant with large domains of quartz that have been partially recrystallized. Biotite is also common, and similar to the migmatitic paragneiss, is not aligned with foliation, and appears to be mostly recrystallized. As expected, feldspar is sparsely distributed throughout. The abundance of grains displaying signs of repair in the quartzite and migmatitic paragneiss, in addition to checkerboard extinction of quartz in the migmatitic paragneiss is evidence that these rocks experienced significantly higher temperatures than the felsic granulite.

Methodology

Samples 14V-030A and 14V-030B required cutting into smaller slabs at the University of

Arkansas before they could be sent along with sample 05G-062 for preparation at the Arizona

LaserChron Center. Samples 14V-030A and 14V-030B were also cut into billets using a template that would allow for production of thin sections for petrographic analysis. A thin section for 05G-062 already existed. At the Arizona LaserChron Center, zircon grains were extracted from all three samples by traditional methods of crushing and grinding. Next, separation was carried out with a Wilfley table, heavy liquids, and a Frantz magnetic separator.

Samples were then processed where all zircons remained in the final heavy mineral fraction. Since detrital analyses were being conducted, a large split of grains was incorporated into a 1” epoxy mount together with loose grains of Sri Lanka, FC-1, and R33 zircons that are used as primary standards. The mounts were sanded down to a depth of approximately 20 microns, polished, imaged, and cleaned prior to isotopic analysis. Grains were imaged so that the spot placement of pits could be positioned in the optimal locations. The images were made using a Hitachi 3400N SEM and a Gatan CL2 detector system. BSE images were produced for all three samples, and CL images were only made for samples 14V-030A and 14V-030B. Due to the high concentration of rutile in 05G-062, a CL image could not be produced for this sample.

U-Pb geochronology of zircons was conducted by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at the Arizona LaserChron Center (Gehrels et al., 2006,

2008; Gehrels and Pecha, 2014). The analyses involved ablation of zircons with a Photon

Machines Analyte G2 excimer laser equipped with HelEx ablation cell that used a spot diameter of 20 microns. Spots were picked using the targeting software Chromium2.1. The laser was set at an energy density of ~5 J/cm2, a repetition rate of 8 Hz, and an ablation time of 10 seconds, making ablation pits ~12 microns deep. The ablated material is carried by helium into the plasma source of an Element2 HR ICPMS, which rapidly sorts through U, Th, and Pb isotopes. Once analyses of each sample was complete, data reduction was performed with an in-house Python

decoding routine and an Excel spreadsheet that produced the zircon datasets which, combined with Isoplot (Ludwig, 2008), were used to create concordia and probability density diagrams.

U-Pb Geochronology Results

05G-062: garnet + sillimanite ± cordierite-bearing felsic granulite.

The zircons in 05G-062 were both smaller and less abundant than those of the other samples. It was difficult to find grains that could accommodate the 20-micron spot diameter used by the laser. There were 77 of the original 110 grains deemed sufficient to represent this sample after further review of spot selection. Rutile was much more abundant than zircon in the epoxy mount. The abundance of rutile made CL images impossible to produce by the imaging methods used. Subsequently, there was no way of determining zonation patterns in the zircons from this sample. All uncertainties in U-Pb crystallization dates for each of the three samples are reported at the 1-sigma level. U-Pb dates for this sample ranged from 1888.1 ± 10 Ma – 2575 ± 9.7 Ma.

206Pb/207Pb concordia and probability density diagrams for this sample are presented in Figure 8.

The majority of zircons plot between ca. 1880 – 2100 Ga in the probability density diagram. The most prominent peaks in the probability density diagram occur at 1970 Ma and 2011 Ma. Even though the abundance of rutile complicated the U-Pb geochronology analysis of this rock, rutile could still have important applications in future studies of this rock and those like it. Rutile is a major host for Nb, Ta and other high field strength elements, which have been used as monitors of geochemical processes in the crust and mantle such as magma evolution and subduction-zone metamorphism, while elements such as Cr, Nb, Sb, and W are used to trace the provenance of rutile (Meinhold, 2010).

14V-030A: migmatitic paragneiss

There were 126 cores and 10 rims that were sampled from 14V-030A. It was possible to differentiate cores from rims given the brighter response of rims to CL imaging than cores. Rims

were selected to constrain the date of metamorphism. 206Pb/207Pb concordia and probability density diagrams are illustrated in Figure 8. The majority of zircon grains in this sample displayed signs of metamorphic overgrowth. U-Pb crystallization dates for the cores ranged from

1852.1 ± 11.1 Ma – 2507.2 ± 13.9 Ma. Two much older zircons had dates of 2955.6 ± 10.7 Ma and 3078 ± 13.9 Ma. Approximately 70% of cores fall in the age range of ca. 1852 - 2100 Ma with the highest peak in the probability density diagram being at 2000 Ma. U-Pb crystallization dates for rims ranged from 1896 ± 14.7 Ma - 2205.4 ± 8 Ma. Rims plot in three distinct groups with peaks at 1902 Ma, 2090 Ma, and 2202 Ma, the most prominent peak being at 1902 Ma.

14V-030B: quartzite

There were 82 cores and 21 rims that were sampled from the quartzite. Where possible, cores and rims of individual grains were selected. 206Pb/207Pb concordia and probability density diagrams were also constructed for this sample and presented in Figure 8. As expected, metamorphic overgrowths were present on the majority of zircons in this sample. Cores yielded a range of U-Pb crystallization dates from 1899.9 ± 12.3 Ma – 2587.5 ± 10.1 Ma. The highest peak in the probability density diagram of cores occurs at ca. 1921 Ma. Rims plot in three distinct groups and yielded dates from 1872 ± 13.5 Ma – 2184 ± 24.3 Ma. The highest peak for rim ages in the probability density diagram occurred at 1893 Ma. Two smaller peaks existed at 1994 and

2160 Ma. It was possible to date three zircons from this sample for both core and rim crystallization ages. These grains, spot selections, and ages are shown in Figure 9. Cores were dated at 2450.7 ± 11.7 Ma, 2165 ± 10.8 Ma, and 1923.6 ± 10.5 Ma, with rim ages of 1894.7 ±

13.5 Ma, 1896 ± 12.4 Ma, and 1912.4 ± 12.0 Ma, respectively.

Discussion

U-Pb geochronology of the zircons from these samples record a ~740 Ma complex geologic history, generally owing to the high-T metamorphism and multiple deformation events

experienced by these rocks. All three samples have a majority of zircons that fall somewhere into the range of 1.85-2.59 Ga. Given this range of dates, orogenies possibly recorded by these rocks include the ca. 2.4-2.3 Ga Arrowsmith orogeny (Hartlaub et al., 2007), 2.02-1.9 Ga Taltson orogeny, and 1.9-1.8 Ga Trans-Hudson orogeny (Hoffman 1988, 1989). The zircon population from ~ 1900-1994 Ma is attributed to metamorphism from the Taltson orogeny, while those ca.

1872-1900 Ga could be from metamorphism during late Taltson or early Trans-Hudson orogeny.

Given that the dominating population of zircons occur between ca. 1900-2000 Ma for all three samples, it is apparent that the Taltson orogeny left the largest signature on these rocks. The transition from Taltson to Trans-Hudson is somewhat difficult to delineate from the geochronology data. Perhaps the most definitive evidence of Trans-Hudson related metamorphism is the 1852.1 ± 11.1 Ma core from the migmatitic paragneiss. Being the latest orogeny possibly experienced by these rocks, it would be expected that a significant number of zircons younger than 1900 Ma would exist, but this is not the case. It is therefore determined that the Trans-Hudson orogeny played a minor role in deformation of the paragneiss at Grollier Lake.

Some zircon rims in the migmatitic paragneiss and quartzite existed outside of the ca. 1850-2000

Ma metamorphic group. These ages had peaks in the probability density chart at 2160 Ma in the quartzite, and 2090 Ma and 2202 Ma in the migmatitic paragneiss. The dates of these overgrowths are problematic because they correspond to times of inferred cratonic stability for

Grollier Lake given that this is after Taltson and before Arrowsmith orogeny. Even though only a few of these rims exist, it is practical to consider their implications. Two of the most probable explanations are 1. U-Pb crystallization ages of these rims are discordant, or 2. These grains experienced metamorphism (from an unidentifiable orogeny) prior to transportation to Grollier

Lake and managed to preserve their older metamorphic ages. Given complications such as heterogeneous recrystallization that can occur to zircon during metamorphism (Flowers et al.,

a) Felsic Granulite

Felsic Granulite

Taltson + Trans-Hudson?

Relative Probability

Detrital Number

Age (Ma)

b) Migmatitic Paragneiss

Migmatitic Paragneiss

Taltson + Trans-Hudson

Relative Probability

Detrital

Number

Age (Ma)

c) QuartziteQuartzite

QuartziteQuartzite

Taltson + Trans-Hudson?

Relative Probability

Number Detrital

Age (Ma)

Figure 8. Concordia and probability density charts of all zircons used for interpretive analysis of the a) felsic granulite, b) migmatitic paragneiss, and c) quartzite. Concordia plots are determined with Isoplot (Ludwig, 2008) with error ellipses that are 2-sigma. Analyses conducted by LA- ICPMS, as described by Gehrels et al. (2008) and Gehrels and Pecha (2014). Analyses with >10% uncertainty (1-sigma) in 206Pb/207Pb age are not included. Common Pb correction is from measured 204Pb with common Pb composition interpreted from Stacey and Kramers (1975). Common Pb composition assigned uncertainties of 1.5 for 206Pb/204Pb, 0.3 for 207Pb/204Pb, and 2.0 for 208Pb/204Pb. U/Pb and 206Pb/207Pb fractionation is calibrated relative to fragments of large Sri Lanka zircons and individual crystals of FC-1, and R33.

Figure 9. CL images and spot selections for zircons from the quartzite. These images represent the only three zircons that were selected for both core and rim analysis. Spot 135 and 145 are on cores inferred to be detrital in origin. Within error, all three rims can be attributed to late Taltson or early Trans-Hudson orogeny.

2008), and the fact that there are no known orogenies that could have been responsible for these metamorphic ages, the first explanation is the favorable one.

Metamorphic zircon growth related to the Arrowsmith orogeny cannot be proven due to the fact that there were no overgrowths dated that correspond to this event. However, this

possibility is not ruled out due to the existence of zircon cores dated between 2.4-2.3 Ga and that it is possible overgrowths that once recorded this older orogeny were recrystallized. Structural evidence cannot deny this possibility either. The recumbent F1 folds at Grollier Lake could have formed from both the Arrowsmith and Taltson orogenies considering that collisional forces were from the west (McDonough et al., 2000; McNicoll et al., 2000; Hartlaub et al., 2007) of Grollier

Lake during both of these events. F2 folds would still be attributed to the Trans-Hudson orogeny.

If future studies from Grollier Lake can prove that in-situ grown metamorphic zircons exist from the Arrowsmith orogeny, then this could imply a much longer lower crustal residence time for these rocks i.e. from ca. 2.4-1.85 Ga instead of ca. 2.04-1.85 Ga. This would further imply that all zircons younger than ca. 2.4-2.3 Ga could not be detrital. However, the analytical methods used in this study simply cannot prove that these zircons exist, therefore, further interpretations of provenance and constraints on timing of deposition of the protoliths are made under the assumption that the paragneiss at Grollier Lake record exclusively Taltson and Trans-Hudson orogenies.

Analyzing dates from both the cores and rims of zircons is one method to make constraints on deposition along with burial to peak metamorphism. This has been used by other workers such as those studying paragneiss provenance of the Swakane Gneiss in the crystalline core of the North Cascades (Matzel et al., 2004; Gatewood & Stowell, 2012), and on metasedimentary rocks from Snowbird Lake, which could also be analogous to the paragneiss at

Grollier Lake (Martel et al., 2008).

A fundamental issue encountered when constraining dates of deposition and metamorphism of the paragneiss at Grollier Lake was differentiating detrital from in-situ grown metamorphic zircon. One technique that can possibly distinguish these two types is to constrain the dates of metamorphism related to orogeny so that older dates correspond to deposition.

Zircon dates older than the Taltson orogeny correlate to a period of deposition of the protoliths.

The youngest zircon that is not contributable to metamorphism will provide a constraint for the maximum age of deposition. This zircon came from the migmatitic paragneiss and has a core dated at 2037.3 ± 7.8 Ma. Within error, it is the youngest zircon older than the onset of the 2.02

Ga Taltson orogeny, and is thus considered the youngest detrital zircon. Using the oldest metamorphic overgrowth age of 1994 Ma implies deposition of the migmatitic paragneiss to have culminated between ca. 2037 – 1994 Ma. This range can be extrapolated to the above quartzite and felsic granulite given that no recognizable unconformities exist between these units. An additional technique of differentiating metamorphic from detrital zircons was used which involved dating both the core and rims from individual grains. The youngest zircon with a core that is older than the onset of Taltson orogeny and a rim that coincides to Taltson or Trans-

Hudson metamorphism would provide a constraint for maximum date of deposition followed by burial. The zircon that best fits these requirements came from the quartzite and yielded a core of

2165.6 ± 10.8 Ma and rim of 1896 ± 12.4 Ma, which places deposition of the quartzite by ca.

2165 Ma. Even though the rim of this zircon is dated at 1896 Ma, the latest recorded date of metamorphism is considered to be 1852.1 ± 11.1 Ma, being the overall youngest zircon sampled that must have a metamorphic origin. It is likely that this method gives a looser constraint due to the lack of grains that were sampled in this way and is therefore not considered the most accurate technique of the two used. A max depositional age of 2165 Ma is also problematic. Many zircons younger than this exist that must have a detrital origin due to the fact that is an inferred period of cratonic stability for the Grollier Lake region. To gain a tighter constraint using this method, techniques that can resolve a smaller spot size than 20 microns, such as an ion microprobe, are recommended due to rarity of zircons with overgrowths that can accommodate such a size.

Geochronology supports that zircons > ca. 2.04 Ga are detrital in origin. Evidence exists

that the western termination of the Rae Craton represented a rift passive margin before 2.0 Ga

(Hoffman, 1988) which led rise to the Rutledge River basin. This basin had an influx of pre-

Taltson 2.17-2.13 Ga detrital zircons (Bostock and van Breeman, 1994). The Rutledge River basin is could have sourced the paragneiss protoliths during uplift and closure resultant from the arrival of the Slave province. It is likely that ca. 2.3 Ga detritus from this region was sourced from the Arrowsmith orogeny as suggested by van Breeman and Aspler (1994). These sources include several Arrowsmith subduction-related plutons such as the 2326 ± 15 Ma Macintosh Bay

Monzogranite, the 2321 ± 3 Ma Gunnar Monzogranite, the 2320 ± 44 Ma Geebee Lake Tonalite, the 2287 ± 14 Ma Yahya Lake Monzogranite, and the 2297 ± 10 Ma Hayter Bay Monzogranite, all near Uranium City, Saskatchewan (Hartlaub et al., 2007). Zircons that fall into the ca. 2.4-

2.59 Ga range are common in the quartzite and rare in the migmatitic paragneiss and felsic granulite, which may indicate transport from different sources during deposition of these units.

Undifferentiated Neoarchean-Paleoproterozoic basement rock of the Dodge and nearby domains seem like the most probable sources for this older detritus. It is furthermore possible that these zircons may represent eroded remnants of the Arrowsmith orogenic belt.

Two significantly older zircons from the migmatitic paragneiss were dated at 2955.6 ±

10.7 Ma and 3078 ± 13.9 Ma. It was determined that these grains must have been inherited from

Archean-aged country rock possibly correlating to the 2999 ± 7 Ma Cornwall Bay Granite and/or the 3060 ± 40 Ma Lodge Bay Granite (Hartlaub et al., 2005). This interpretation supports the existence of a lower level Archean-aged crustal root at Grollier Lake, which can be added to the aforementioned list of similarities that exist between Grollier Lake, Snowbird Lake, and the

Beaverlodge domain. It is considered likely that rocks of this age at Grollier Lake will record metamorphism from the Arrowsmith orogeny.

One caveat that could alter the interpretation of these geochronological results is the

possibility that some of the U-Pb crystallization dates from these samples are discordant as supported by the problematic metamorphic rims dated from the quartzite and migmatitic paragneiss. Flowers et al. (2008) provides evidence for discordance that can be attributed via heterogeneous recrystallization of zircons that occurred during metamorphism. Additionally,

Kroner et al. (2014) cautions against the use of destructive techniques such as LA-ICPMS on rocks of high-T terranes due to the likelihood of sampling invisible heterogeneous domains that will yield discordant ages difficult to interpret. The concordia diagrams further support the possibility that these zircons have discordant U-Pb crystallization ages. For ages > ~ 2.0 Ga, the majority of error ellipses appear to fall below concordia in the migmatitic paragneiss and quartzite. Error ellipses from ~ 1.85- 2.0 Ga appear to plot more precisely on concordia for all three samples. These observations support the idea that zircons grown during metamorphism from the Taltson and early Trans-Hudson orogenies are more concordant than the > ~ 2.0 Ga detrital zircons due to the subjection of these grains to recrystallization. Zircons from ~ 1.85- 2.0

Ga possibly appear more concordant because it is less likely that they underwent recrystallization considering the evidence presented by Mahan and William, (2005) for post 1.85 Ga exhumation of these rocks along the LLsz.

Conclusions

Geochronology coupled with structural evidence indicates that the paragneiss of Grollier

Lake record deformation exclusively from the Taltson and Trans-Hudson orogenies. Lower crustal residence of these rocks is inferred between ca. 2.04-1.85 Ga. Geochronological evidence suggests that the Taltson orogeny had a much larger impact on the deformation of these rocks than the Trans-Hudson. Although unlikely, the possibility that these rocks experienced metamorphism from the Arrowsmith orogeny cannot be ruled out, and could imply that a much longer lower crustal residence time from ca. 2.4-1.85 Ga. Assuming that Arrowsmith orogeny is

not recorded by these rocks, deposition of the paragneiss protoliths culminated ca. 2037-1994

Ma with cessation of metamorphism occurring by 1852.1 ± 11.1 Ma. Peak metamorphic conditions were likely reached during the Taltson orogeny. The oldest overgrowth considered to be concordant is 1994 ± 12 Ma and interpreted to represent the first signature of burial facilitated by the Taltson orogeny. The younger zircons ranging from ca. 1872-1900 Ma can be attributed to metamorphism from late Taltson or early Trans-Hudson orogeny being that the transition between these two are difficult to define in the data. There is evidence that suggests zircons dated > ca. 2.0 Ga are discordant which would make provenance interpretation of these grains difficult. However, most of these ages are still attributable to derivation from local sources.

These being the ca. 2.17-2.13 Ga Rutledge River basin (Bostock and van Breeman, 1994), and the ca. 2.3 Ga Arrowsmith subduction-related plutons north of Lake Athabasca identified by

Hartlaub et al. (2007). Older detrital zircons most likely represent undifferentiated Neoarchean and Paleoproterozoic crust from the Dodge and proximal domains and possibly eroded remnants from the Arrowsmith orogenic belt. Much older zircons (2955.6 ± 10.7 Ma and 3078 ± 13.9 Ma) in the migmatitic paragneiss are evident of an Archean-aged basement at Grollier Lake. Given that they have yet to be directly dated, the question of whether rocks of this age do in fact exist at

Grollier Lake remains. However, given the similarities that exist in regional framework between

Grollier Lake and nearby localities in addition to the presence of these two grains, it is inferred that rocks of the lower structural levels (further to the east) will in fact record these older dates. It is furthermore possible these rocks will display signatures of Arrowsmith-related metamorphism.

The significance of studying Grollier Lake stems primarily from its application towards reconstructing the geologic history of the wall rocks on the northwest side of the GRsz. This study and future ones like it from Grollier Lake will be useful in aiding to determine whether the relationship between the Rae and Hearne represents a suture created during early Trans-Hudson

orogeny (Hoffman, 1987; Berman et al., 2007), or a long-lived intracratonic discontinuity initiated in the Archean (Hanmer et al., 1994; Mahan and Williams, 2005). However, being a region that is still underrepresented by data, additional geochronological constraints are still needed to determine exactly what type of regime the rocks of Grollier Lake occupied throughout the Paleoproterozoic and Archean.

References

Baldwin, J.A., Bowring, S.A., Williams, M.L., 2003. Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada. Journal of Metamorphic Geology, v. 21 (1), p. 81-98.

Berman, R.G., Davis, W.J., Pehrsson, S., 2007. Collisional Snowbird tectonic zone resurrected: Growth of Laurentia during the 1.9 Ga accretionary phase of the Hudsonian orogeny. Geology, v. 35 (10), p. 911-914.

Bethune, K.M., Berman, R.G., Rayner, N., Ashton, K.E., 2013. Structural, petrological, and U- Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogeny in the SW Rae craton, Canadian Shield. Precambrian Research, p. 1-30.

Bostock, H.H., van Breemen, O., 1994. Ages of detrital and metamorphic zircons and monazites from a pre-Taltson magmatic zone basin at the western margin of the Rae Province. Canadian Journal of Earth Sciences v. 31, p. 1352-1364.

Card, C.D., 2001. Geology and tectonic setting of the Oldman-Bulyea shear zone, northern Saskatchewan, Canada. Unpublished M.Sc. thesis, University of Regina, Regina, 188 p.

Flowers, R.M., Bowring, S.A., Mahan, K.H., Williams, M.L., Williams, I.S., 2008. Stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian shield. Contributions to Mineralogy and Petrology, v. 156, p. 529-549.

Gatewood, M.P., Stowell, H.H., 2012. Linking zircon U-Pb and garnet Sm-Nd ages to date Loading and metamorphism in the lower crust of a Cretaceous magmatic arc, Swakane Gneiss, WA, USA. Lithos, v. 146-147, p. 128-142.

Gehrels, G.E., Valencia, V., Pullen, A., 2006. Detrital zircon geochronology by Laser-Ablation Multicollector ICPMS at the Arizona LaserChron Center, in Loszewski, T., and Huff, W., eds. Geochronology: Emerging Opportunities, Paleontology Society Short Course: Paleontology Society Papers, v. 11, 10 p.

Gehrels, G.E., Valencia, V., Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–

mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, Q03017, doi:10.1029/2007GC001805.

Gehrels, G. and Pecha, M., 2014, Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America: Geosphere, v. 10 (1), p. 49-65.

Hajnal, Z., Lewry, J., White, D., Ashton, K., Clowes, R., Stauffer, M., Gyorfi, I., Takacs, E., 2005. The Sask Craton and Hearne Province margin: seismic reflection studies in the western Trans-Hudson Orogen. Canadian Journal of Earth Sciences, v. 42 (4), p. 403- 419.

Hanmer, S., Parrish, R., Williams, M., Kopf, C., 1994. Striding-Athabasca mylonite zone: complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan. Canadian Journal of Earth Sciences, v. 31, p. 1287-1300.

Hartlaub, R.P., Chacko, T., Heaman, L.M., Creaser, R.A., Ashton, K.E., Simonetti, A., 2005. Ancient (Meso- to Paleoarchean) crust in the Rae Province, Canada: Evidence from Sm- Nd and U-Pb constraints. Precambrian Research, v. 141, p. 137-153.

Hartlaub, R.P., Heaman, L.M., Chacko, T., Ashton, K.E., 2007. Circa 2.3-Ga Magmatism of the Arrowsmith Orogeny, Uranium City Region, Western Churchill Craton, Canada. The Journal of Geology, v. 115, p. 181-195.

Hoffman, P.F., 1987. Continental transform tectonics: Great Slave Lake shear zone (ca 1. 9 Ga), northwest Canada. Geology, v. 15 (9), p. 785-788.

Hoffman, P.F., 1988. United Plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia. Annual Review of Earth & Planetary Sciences, v. 16, p. 543- 603.

Hoffman, P.F., 1989. Precambrian geology and tectonic history of North America. In: Bally, A.W., Palmer, A.R. (Eds.), The Geology of North America – An Overview. Geol. Soc. Am., Boulder, Colorado, p. 447-512.

Knox, B., Card, C.D., Ashton, K.E. 2011. Bedrock Geology of the Grollier Lake Area, Southeastern Dodge Domain (parts of NTS 74P/11, /12, /13, and /14). Summary of Investigations 2011, v. 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2011-4.2, Paper A-3, p. 1-13.

Kroner, A., Wan, Y., Liu, X., Liu, D. 2014. Dating of zircon from high-grade rocks: Which is the most reliable method? Geoscience Frontiers, v. 5, p. 515-523.

Ludwig, K., 2008. Isoplot 3.6: Berkeley Geochronology Center Special Publication 4, 77 p.

Mahan, K.H., & Williams, M.L., 2005. Reconstruction of a large deep-crustal terrane: Implications for the Snowbird tectonic zone and early growth of Laurentia. Geology, v.

33, p. 385-388.

Mahan, K.H., Goncalves, P., Williams, M.L., Jercinovic, M.J., 2006a. Dating metamorphic reactions and fluid flow: application to exhumation of high-P granulites in a crustal-scale shear zone, western Canadian Shield. Journal of Metamorphic Geology, v. 24 (3), p. 193–217.

Martel, E., Breeman, O.v., Berman, R.G., Pehrsson, S. 2008. Geochronology and tectonometamorphic history of the Snowbird Lake area, Northwest Territories, Canada: New insights into the architecture and significance of the Snowbird tectonic zone. Precambrian Research, v. 161, p. 201-230.

Matzel, J.E., Bowing, S.A. 2004. Protolith age of the Swakane Gneiss, North Cascades, Washington: Evidence of rapid underthrusting of sediments beneath an arc. Tectonics, v. 23, p. 1-18.

McDonough, M.R., McNicoll, V.J., Schetselaar, E.M., Grover, T.W., 2000. Geochronological and kinematic constraints on crustal shortening and escape in a two-sided oblique-slip collisional and magmatic orogeny, Paleoproterozoic Taltson magmatic zone, northeastern Alberta. Canadian Journal of Earth Sciences, v. 37, p. 1549-1573.

McNicoll, V.J., Réginald, T.J., McDonough, M.R., 2000. Taltson basement gneissic rocks: U-Pb and Nd isotopic constraints on the basement to the Paleoproterozoic Taltson magmatic zone, northeastern Alberta. Canadian Journal of Earth Sciences, v. 37, p. 1575-1596.

Meinhold, G., 2010. Rutile and its applications in earth sciences. Earth-Science Reviews, v. 102, p. 1-28.

Ross, G.M., 2002. Evolution of Precambrian continental lithosphere in Western Canada: results from Lithoprobe studies in Alberta and beyond. Canadian Journal of Earth Sciences, v. 39 (3), p. 413-437.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two stage model: Earth and Planetary Science Letters, v. 26, p. 207-221.

Tella, S., Hanmer, S., Ryan, J.J., Sandeman, H.A., Davis, W.J., Berman, R., Wilkinson, L., Mills, A., 2000. 1:100000 scale bedrock geology compilation map of the MacQuoid Lake-Gibson Lake-Cross Bay-Akunak Bay region, western Churchill province, Nunavut, Canada. Geological Association of Canada-Mineralogical Association of Canada Program with Abstracts, 25: Conference CD 676. van Breeman, O., and Aspler, L. B. 1994. Detrital zircon ages from Nonacho Basin, western Rae Province, Northwest Territories. Geological Survey of Canada, Current Research 94-F, p. 49-59. van Breeman, O., Pehrsson, S., Peterson, T.D., 2007. Reconnaissance U-Pb SHRIMP geochronology and Sm-Nd isotope analyses from the Tehery-Wager Bay gneiss domain,

western Churchill Province, Nunavut. Geological Survey of Canada, Current Research 2007-F2.