# Zircon geochronology and Hf-O isotopes of the Nulliak supracrustal assemblage (Saglek Block-Canada): Constraints on deposition age and setting, metamorphic age and environments of zircon crystallization

A. Vezinet, E. Thomassot, Y. Luo, D.G. Pearson, R.A. Stern, C. Sarkar

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## To cite this version:

A. Vezinet, E. Thomassot, Y. Luo, D.G. Pearson, R.A. Stern, et al., Zircon geochronology and Hf-O isotopes of the Nulliak supracrustal assemblage (Saglek Block-Canada): Constraints on deposition age and setting, metamorphic age and environments of zircon crystallization. Precambrian Research, 2022, 379, pp.106789. 10.1016/j.precamres.2022.106789. hal-04986705

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# Zircon geochronology and Hf–O isotopes of the Nulliak Supracrustal assemblage (Saglek Block–Canada): Constraints on deposition age and setting, metamorphic age and environments of zircon crystallization

A. Vezinet$^{a,b,*}$, E. Thomassot$^c$, Y. Luo$^a$, D.G. Pearson$^a$, R.A. Stern$^a$, C. Sarkar$^a$

$^a$ Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G

2E3, Canada

$^b$ Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, 38000 Grenoble, France

$^c$ Centre de Recherches Pétrographiques et Géochimiques (CRPG), UMR 7358, Université de Lorraine, CNRS, 54500 Vandoeuvre-lès-Nancy, France

* Corresponding author: adrien.vezinet@univ-grenoble-alpes.fr
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**Abstract**

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The Archean Saglek Block, North Atlantic craton, is one of the oldest crustal segments preserved on Earth. This area records two granulite facies metamorphic events: one in the late-Eoarchean at *ca.* 3.7 Ga and one in the Neoarchean, at *ca.* 2.75 Ga. Here, we report the results of combined U–Pb/Hf/O *in-situ* isotope analyses of detrital and metamorphic zircon domains from two meta-sedimentary samples, one psammitic, and one pelitic. Both samples are components of the oldest supracrustal assemblage of the Saglek Block: the Eoarchean Nulliak supracrustal assemblage. The isotopic data presented in this study: (i) confirm deposition age of the Nulliak supracrustal assemblage before *ca.* 3.65 Ga, (ii) indicate local provenance of the detrital zircon, (iii) suggest deposition of the sedimentary protoliths in a convergent setting, such as an arc, (iv) crystallization of the oldest zircon during the early Eoarchean in a protolith derived from a depleted mantle component, (v) involvement of material that experienced prolonged low temperature alteration during the late Eoarchean granulite facies metamorphism and (vi) selective metamorphic overprinting during the Neoarchean granulite facies metamorphism.

The new zircon U–Pb/Hf/O isotope data presented here in conjunction with previously published analyses suggest that a depleted mantle domain existed in the early Archean, as reflected in the positive εHf signature of zircon from the Saglek Block. Yet, such positive Hf isotope deviations are not the norm, but rather the exception in the preserved early Earth zircon record. In addition, mildly elevated zircon δ¹⁸O values, i.e. those ≥ 5.9 ‰, are as common in the early Archean as they are in the late-Archean zircon record, implying that crustal reworking was an active and efficient process all through the Archean. Hence, no significant variation in the nature of the reworking and broader geodynamic processes occurred during this period of time.
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# 1 Introduction

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Definite answers about the nature, composition and evolution of the Earth's crust and mantle during the Hadean-Archean eons (> 2.5 Ga) remain pending. Fragments of this period are seldom exposed at the modern Earth's surface and relics that escaped recycling have, in large-part, experienced high-grade metamorphism (Percival, 1994). Information about crust-mantle evolution and coupling during the Hadean-Archean can be extracted from the isotopic composition of minerals that have not been utterly reset during high-grade metamorphism. Zircon is such a mineral. Zircon U–Th–Pb isotope compositions, whenever combined with Hf and O-isotope compositions, constitutes a powerful tool to determine the nature and origin of the fluid within which crystallization occurred, whether the fluid is magmatic or metamorphic; a knowledge then used for constraining the evolution path of Archean lithospheric reservoirs (e.g. Amelin et al., 1999; Amelin et al., 2000; Bindeman and Valley, 2001; Valley et al., 2005; Hiess et al., 2009; Bell et al., 2011; Hiess et al., 2011; Zeh et al., 2014; Reimink et al., 2016; Bolhar et al., 2017; Vezinet et al., 2018; Vezinet et al., 2019; Petersson et al., 2020; Vezinet et al., 2020; Mole et al., 2021). Despite numerous Archean zircon isotopes studies, both the degree of differentiation of the outermost geochemical reservoirs of Earth and the tectonic processes operating during early Earth time remain unsettled (e.g. Gerya, 2014; Korenaga, 2021). This is partly due to the fact that post-crystallization disturbance of zircon isotopic compositions hamper identifying primary igneous signatures. Condition-controlled experiments have shown that disturbance of zircon isotopic signatures is enhanced by the effects of self-irradiation and the presence of aqueous fluid/melt (e.g. Geisler et al., 2007). Fluids, particularly water-rich fluids, are then thought to be effective in disrupting and erasing zircon U–Pb isotopic signatures.
The Saglek Block (North Atlantic craton, Canada, Figure 1 and Figure 2) hosts some of the oldest evidence of magmatic and metamorphic activity preserved at present-day Earth's surface with, notably, felsic magmatism as old as 3.87 Ga and granulite facies metamorphism at ≥ 3.6 Ga and ~ 2.75 Ga (Baadsgaard et al., 1979; Schiøtte et al., 1989b, a; Komiya et al., 2017; Vezinet et al., 2018; Vezinet et al., 2019; Wasilewski et al., 2021). The clastic components of the local supracrustal assemblages, which are intimately interlayered with the older felsic rocks, have however received only limited attention (Nutman et al., 1989; Schiøtte et al., 1989a; Nutman and Collerson, 1991; Schiøtte et al., 1992). In order to characterize the nature and source for the Nulliak supracrustal assemblage (NSA), and to investigate the impact of metamorphism and fluids on zircon isotopic compositions, we investigated the internal microstructures and U-Pb/Hf/O isotope signatures of two populations of early Archean detrital and metamorphic zircon from the NSA (Figure 1 and Figure 2). These results contribute to constraining better the ways and means of these early Archean supracrustal assemblages and, together with previously published analyses, contributes to understanding early Earth crust-mantle evolution and reworking processes.

<figure><img src="image_3.png" /><figcaption>**Figure 1:** A. Pre-drift reconstruction of the North Atlantic craton (yellow area on the map), dotted areas are pre-2.5 Ga terranes; green stars represent towns. CP: Churchill Province B. General geological map of the north-eastern Labrador and Québec (modified after Ryan and Martineau, 2012).</figcaption></figure>
## 2 Geological setting and sampling strategy

The Saglek Block, north-eastern Labrador – Canada, is the western termination of the North Atlantic craton (Figure 1). This terrane contains, along with its Greenlandic counterpart, among the oldest rocks preserved at Earth's surface, with felsic magmatism documented by zircon crystallisation ages of *ca.* 3.86-3.87 Ga (Schiøtte et al., 1989b; Vezinet et al., 2018). Numerous younger felsic igneous and metamorphic events, ranging from *ca.* 3.6 to *ca.* 2.5 Ga, have also been documented using U-Pb isotopic compositions of zircon and REE-rich minerals (Baadsgaard et al., 1979; Schiøtte et al., 1989b; Kusiak et al., 2018; Sałacińska et al., 2018; Vezinet et al., 2018; Vezinet et al., 2019; Dunkley et al., 2020; Fisher et al., 2020; Vezinet et al., 2022). Regional high-grade metamorphic events induced ductile, high-strain deformation that resulted in intercalation and mingling of sheared felsic igneous rocks and supracrustal lithologies forming grey gneiss complex (Figure 2 and Figure 4). Supracrustal lithologies found in the Saglek Block consist of chemical and clastic sedimentary rocks, basic to acid volcanic rocks and ultramafic pods (Bridgwater et al., 1975; Bridgwater and Schiøtte, 1991). Pioneering field investigations of the Saglek Block reported the occurrence of two distinct supracrustal sequences within the local grey gneiss complex: the Eoarchean Nulliak assemblage and the Mesoarchean Upernavik assemblage (Bridgwater et al., 1975; Bridgwater and Collerson, 1977; Baadsgaard et al., 1979). Distinguishing the two assemblages relies on field relationships such as the intrusion of mafic Saglek dykes, that in some instances host plagioclase megacrysts and crosscut "pegmatite veins and inclusions" (Baadsgaard et al., 1979). The Saglek dykes are however not exposed in every outcrop, and the elevated degree of ductile deformation across the area makes field relationships difficult to decipher in many outcrops (Bridgwater and Collerson, 1976; Schiøtte et al., 1989b; Whitehouse et al., 2019). Further, whole-rock geochemical and isotopic
measurements on the meta-basic constituents of these two supracrustal sequences have not confirmed earlier field observations. Instead, whole-rock analyses reveal strong similarities between the geochemical signatures of the (ultra)-mafic members of both sequences (Wasilewski et al., 2019). The age of the meta-mafic rock suites has been constrained with whole-rock Sm-Nd isotopes measurements. Three ultra-mafic suites have been recognized, yielding apparent Sm-Nd isochron ages ranging between 4.0±0.2 Ga and 3.4±0.1 Ga (Collerson et al., 1991; Morino et al., 2017). However, the meaning of these apparent ages has to be evaluated in the context of secondary, metamorphic, overprinting that are likely to have disrupted, to variable extents, the Sm and Nd isotope systematics (e.g. Bridgwater et al., 1989; Vezinet et al., 2021). Apart from petrographic descriptions, very little is known about the nature of the clastic sedimentary units of the supracrustal sequences exposed in the Saglek Block. Such units may shed light on the erosion and deposition histories of the region. Minimum detrital zircon age of ca. 3.78 Ga and > 3.24 Ga have been used to establish a deposition age for some Nulliak assemblage and the Upernavik assemblage units respectively (Schiøtte et al., 1989a; Nutman and Collerson, 1991; Schiøtte et al., 1992).

To provide further constraints on the Saglek Block sedimentary components we investigate two clastic meta-sedimentary rocks. Specimen LA13-19 is a quartz-rich rock sampled on the Nulliak Island (Figure 3, Figure 4A-B), where the Nulliak supracrustal assemblage was first described. This sample is a member of the clinopyroxene + biotite-bearing quartzofeldspathic gneiss sub-group defined on the latest available geological map (group 1g, Ryan and Martineau, 2012). Specimen LA9-16, from the Hebron Bay (Figure 4C and D), is less enriched in SiO₂ and Al₂O₃, richer in iron than sample LA13-19 and is part of the rusty-brown, biotite + garnet quartzofeldspathic gneiss (group 1q, Ryan and Martineau, 2012). This specimen has been
selected to constrain further the effect of Eoarchean metamorphism identified in a meta-igneous rock sampled in the same area and previously published in Vezinet et al. (2019). Providing better constraints on the nature and origin of zircon detrital populations from the Saglek Block is pivotal for the reconstruction of the geological history of this area and thus for the reconstruction of the geological history North Atlantic craton to which the Saglek Block belongs.

<figure><img src="image_4.png" /><figcaption>**Figure 2:** Geological map of the Saglek Block (Labrador, Canada). Stars are sampling sites; associated labels are sample names. Map modified after Ryan and Martineau (2012).</figcaption></figure>
### 3 Analytical work

Details of analytical protocols are reported in Supplementary Material and have been documented in detail in other published studies (Vezinet et al., 2018; Vezinet et al., 2019; Vezinet et al., 2020). Only the main features are presented here.

Zircon crystals were mounted in an epoxy mount and polished down to reveal mid-sections through grain at the Canadian Centre for Isotopic Microanalysis (CCIM), University of Alberta. Cathodoluminescence (CL) and backscattered electron (BSE) imaging was conducted on a Zeiss EVO MA15 instrument at the CCIM. Oxygen isotope analyses ($^{18}$O, $^{16}$O) of zircon were conducted using a Cameca IMS 1280 multicollector ion microprobe at the CCIM. For U-Pb dating, two distinct protocols were used depending on the size of the zircon domains of interest. Large domains, i.e. > ca. 40 µm, were analysed for their U–Pb and Hf isotopic composition using the laser ablation split stream protocol (LASS) established at the Arctic Resources Laboratory (CCIM, University of Alberta). Smaller domains, were analysed only for their U–Pb isotopic compositions using a Cameca IMS 1280 multicollector ion microprobe at the Centre de Recherches Pétrographiques et Géochimiques (CRPG, Nancy France). Combined U–Pb/Hf isotope analyses and U–Pb isotopes only analyses were always conducted on the top of the O–isotope spots to ensure spatial matching between isotope systematics. Ion probe isotopic analyses were processed with in-house spreadsheets. LASS isotopic analyses were processed using Iolite v.3 software. Results for the reference zircon materials used in this study agree with accepted values (cf. Supplementary Material).
# 4 Diagram construction, decay constants and uncertainty propagation workflow

Chronological and compositional information calculated from our isotopic analyses were generated with the IsoplotR software (Vermeesch, 2018). All regressions have been calculated using the Maximum Likelihood Estimation (MLE) algorithm available in IsoplotR. Decay constants for uranium U are the default values in IsoplotR (Vermeesch, 2018) which are: 0.000155125 Myr⁻¹ for <sup>238</sup>U and 0.00098485 Myr⁻¹ for <sup>235</sup>U. In cases where the MSWD value of the MLE regression is within the acceptable range at the 95 % confidence level (as defined in Wendt and Carl, 1991), the uncertainty reported is at 95 % confidence interval using the Student-t expansion and the appropriate number of degrees of freedom.

<figure><img src="image_5.png" /><figcaption>**Figure 3:** Geological map of Nulliak Island with sampling site given for LA13-19.</figcaption></figure>

Where the MSWD value of the MLE regression is not within the acceptable range at the 95 % confidence level, the uncertainty reported is z: "the approximate 95 % confidence interval
with overdispersion, calculated as $z = y \times \sqrt{MSWD}$. Ratios uncertainty (for U-Pb, Hf and O-isotope analyses) presented here includes the combined contribution of measurement and calibration uncertainties. For all the regressions presented here, calibration external (systematic) uncertainties have been propagated in final ages and initial ratios. Regarding Hf-isotopes, epsilon ($\epsilon$) deviations were calculated with the Model-1 algorithm of Ickert (2013). Two levels of uncertainty in the $\epsilon$ values are reported: (i) neglecting uncertainty in both the CHUR model and the decay constant and (ii) propagating uncertainty in both the CHUR model and the decay constant. These two levels are reported along with the $\epsilon$ value and separated by a vertical bar ("|" symbol). Uncertainty in the age is included in both calculations. If a single level of uncertainty is reported, it is the one including CHUR and decay constant uncertainty. SE: Standard Error.

$\epsilon^{176}\text{Hf} = \left[ \frac{\left( \frac{176\text{Hf}}{177\text{Hf}} \right)_{\text{sample}}}{\left( \frac{176\text{Hf}}{177\text{Hf}} \right)_{\text{CHUR}}} - 1 \right] \times 10^4$ with CHUR: Chondritic Uniform Reservoir; and

$\delta^{18}\text{O}_{\text{VSMOW}} = \left[ \frac{\left( \frac{18\text{O}}{16\text{O}} \right)_{\text{sample}}}{\left( \frac{18\text{O}}{16\text{O}} \right)_{\text{VSMOW}}} - 1 \right] \times 10^3$ with VSMOW: Vienna Standard Mean Ocean Water.
<figure><img src="image_6.png" /><figcaption>**Figure 4:** A. Field relationships of the sampling site of LA13-19 on Nulliak Island (Saglek Block) and B. Close view of the hand-specimen LA13-19. C. Representative outcrop of the Hebron Bay locality (sample LA9-16), showing field relations D. Close view of the hand-specimen LA9-16.</figcaption></figure>

# 5 Results

## 5.1 Petrography and whole-rock geochemistry

The rock specimen LA13-19 was sampled on the south-western shore of Nulliak Island (Figure 2 and Figure 3), at: Easting 523547, Northing 6463089 (UTM Zone 20V). The south-western shore of Nulliak Island (Figure 3) consists of centimetre- to metre-wide banded lithologies of (meta)-iron formation: Mag + Qz + Cpx ± sulphide (abbreviations after Whitney and Evans,
2010); garnet-bearing hornblendite; (ultra)-mafic pods: O1 + Opx ± Cpx; granitoids with mineral assemblages ranging from Hbl-bearing granite to charnockite; Late intrusive K-rich granite, and mafic dykes (Thomassot et al., 2015). Structural features—such as layering, transposition and folding—exposed in the Saglek Block have been interpreted as the result of multiple ductile deformation events (Collerson and Bridgwater, 1979). The mineral assemblage of the sample LA13–19 consists of: quartz (~70 %), microcline (~10 %), clinopyroxene (~10 %, hedenbergite), biotite (~10 %) and plagioclase (< ~1 %). The main accessory phases are zircon, titanite and apatite. This rock shows high silica content, 75 wt. %, with comparatively low alumina and sodium oxide contents, 6.96 wt. % and 0.96 wt. %, and high calcium and potassium oxide contents: 5.07 wt. % and 4.33 wt. %.

The rock specimen LA9–16 was sampled east of the Hebron Bay, on the eastern shore of Cape Nuvotannak. This outcrop is mainly made up of Uivak gneisses displaying a dense and regular pseudo-vertical NNW-SSE schistosity. These polyphase quartzo-feldspathic gneisses locally encompass boudinaged mafic (amphibolite) enclaves, ranging from 10-centimetres to few meters in size), as well as rusty lenses of garnet-biotite-rich paragneiss mapped as horizon “1s” by Ryan and Martineau (2012) probably having shales and siltstones as the protolith. Rock specimen LA9-16 was sampled at Easting 523966, Northing 6451637 (UTM Zone 20V), on the center of cape Nuvotannak, within a three-metre wide horizon that can be traced over one kilometre toward the North. This unit consists in repeated alternations of light green feldspathic quartzite and grey to rusty garnet-biotite gneiss. The mineral assemblage is quartz + plagioclase + biotite + garnet and can locally preserve evidence of partial melting as centimetre-large leucosomes. Whole-rock geochemistry of sample LA9–16 shows silica content of 54.62 wt. %, 18.50 wt. % Al₂O₃ and calcium and potassium oxide contents of 1.62 wt. % and 2.24 wt. %.
## 5.2 Zircon internal micro-structures

About 350 zircon grains were hand-picked under binocular microscope from the Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19. CL and BSE images of the zircon populations are reported as Supplementary Material. Zircon grains from this rock have rounded morphologies with widely varying aspect ratios (1 to 1:5). A large variation of CL internal microstructures is observed in this population: from dark homogeneous, broad-band zoning to oscillatory zoning (Figure 5). Although often limited, local recrystallization, convolute zoning and growth of new zircon truncates oscillatory zoning and show typical micro-structure of zircon grown under conditions of high-grade metamorphism can also be seen. CL and BSE images reveal mostly clean, i.e. unaltered and inclusion-free, internal microstructures (Figure 5 and Supplementary Material). Mineral inclusions are present although rare and if present, thin zircon growth rims often display radial cracks. Domains for isotopic analyses were selected away from cracks and mineral inclusions, in pristine looking domains, guided by BSE and CL images.

About 310 grains were hand-picked under binocular microscope for the biotite + garnet quartzofeldspathic gneiss LA9–16. Zircon grains from LA9–16 have generally rounded morphologies. While the elongation ratio is variable, a larger proportion of grains than in sample LA13-19 have a ratio closer to unity. CL imaging revealed dark grey, homogeneous at first glance internal microstructures (Figure 5) that can be attributed to partial or full transgressive recrystallization (Hoskin and Black, 2000; Corfu et al., 2003). Ghost microstructures are also observed in these dark grey domains (Figure 5). In this regard, zircon grains from sample LA9–16 are quite different than those from LA13–19. BSE/CL imaging shows that a large proportion of LA9–16 zircon grains have domains free of cracks and mineral inclusions (Figure 5 and Supplementary Material). These domains were selected for in-situ U–Pb/(Hf)/O isotope analyses.
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**5.3 Zircon U-Pb isotopes**

**5.3.1 Cpx-Bt-bearing quartzofeldspathic gneiss LA13-19**

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A total of 185 zircon U-Pb isotope analyses were obtained from the Cpx-Bt-bearing quartzofeldspathic gneiss LA13-19. Sixty-seven of these were analysed with the LASS protocol and the remaining 118 analyses with the SIMS protocol (Supplementary Material). These two datasets are presented independently below.

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Analyses obtained with the LASS protocols show a large range of U contents, from 68.8 ppm to 904 ppm (median=207 ppm). The Th contents are also very variable, ranging from 25.8 ppm to 720 ppm (median=110 ppm). Th/U values vary between 0.19 and 2.06 (median=0.57).

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<figure><img src="image_7.png" /><figcaption>**Figure 5:** Representative cathodoluminescence (CL) images of the two zircon populations investigated in this study. Spot size is 33 µm for LA13–19 (top) and 20 µm for LA9–16 (bottom). Note the significant differences in $\delta^{18}\text{O}_{\text{VSMOW}}$ values between core and rim domains. Uncertainty are reported at a coverage factor of 2.</figcaption></figure>

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Zircon common-Pb contents (calculated as the fraction of common Pb in the ²⁰⁶Pb signal, using Stacey and Kramers (1975) ²⁰⁶Pb/²⁰⁴Pb values, measured 204–Hg corrected if necessary–and 206 signals and reported as f²⁰⁶Pb) are overall negligible, with a maximum value of 0.50 %. Fifty-four analyses, out of the 67 obtained with the LASS protocol are within 95-105 % of concordance. ²⁰⁷Pb/²⁰⁶Pb dates (or apparent ages) range from 3386 ± 14 Ma to 3788 ± 10 Ma. It is worth noting that not all of these ²⁰⁷Pb/²⁰⁶Pb dates might relate to a geological event, ancient Pb-loss might have affected some of these zircon domains as suggested in other Saglek Block assemblages subjected to high grade metamorphism (Schiøtte et al., 1989b, a; Kusiak et al., 2018; Sałacińska et al., 2018; Vezinet et al., 2018; Vezinet et al., 2019; Whitehouse et al., 2019). The α-dose contents, i.e. the degree of lattice damaged cause by α-type radioactive decay, was calculated using equations reported in Murakami et al. (1991) and ²⁰⁷Pb/²⁰⁶Pb apparent age. Elevated α-dose contents favours both the loss of radiogenic lead produced by the decay of uranium and the ingress of common lead. For these zircon domains, the α-dose content ranges between 1.50×10¹⁵ α-decay events per mg to 1.81×10¹⁶ α-decay events per mg (median of 4.27×10¹⁵ α-decay events per mg). This range overlaps the first percolation threshold value of 2.2×10¹⁵ α-decay events per mg (Pidgeon, 2014)., i.e. where amorphous domains bridge and establish a connecting network.
<figure><img src="image_8.png" /></figure>

**Figure 6:** Th/U value versus apparent Pb-Pb age of zircon domains analysed in this study. Blue diamonds are analyses of LA13-19 zircon grains and dark orange diamonds are LA9-16 analyses. Clearly, the large majority of LA9-16 analyses show Th/U values below 0.1, the typical threshold used to discriminate igneous and metamorphic zircon (e.g. Rubatto, 2017). Conversely, LA13-19 analyses show largely Th/U above 0.1. Note that the ²⁰⁷Pb/²⁰⁶Pb apparent ages are an underestimation of the true zircon crystallisation age because of ancient Pb-loss (see details in text).

Analyses obtained with the SIMS protocol (n=118) show range in U and Th contents as large as those obtained with LASS measurements: U contents vary from 81 ppm to 1994 ppm (median=299 ppm) and Th contents from 3 ppm to 389 ppm (median=43 ppm). Th/U values range from < 0.01 to 0.42 (median=0.17). Common-Pb contents show a somewhat larger range than that obtained with the LASS protocol, with a maximum value of 2.34 %, the median value remains low however (0.09 %). One hundred and four (104) SIMS U-Pb isotope analyses of the full 118 SIMS U-Pb isotopes dataset, are within 95-105 % concordance. ²⁰⁷Pb/²⁰⁶Pb dates (or apparent ages) range from 3013 ± 3 Ma to 3872 ± 4 Ma and the median of 3689 Ma is close to the median value obtained with the LASS protocol. Thirty-four (34) SIMS U-Pb analyses relate to a δ¹⁸O<sub>VSMOW</sub> measurement (see section 5.5), but none of these analytical spots relate to a Hf-isotope measurement. The α-doses show the same range of values than the U-Pb isotopes obtained with the LASS protocol: from 1.55×10¹⁵ α-decay events per mg to 3.97×10¹⁶ α-decay
events per mg (median of 6.30×10¹⁵ α-decay events per mg). It is worth mentioning that zircon α-doses are positively correlated with ages; thus, ancient Pb-loss that would result in apparent rejuvenation of ²⁰⁷Pb/²⁰⁶Pb zircon apparent ages would also result in a decrease of calculated α-doses. Consequently, these values are only minimal estimates.

<figure><img src="image_9.png" /><figcaption>**Figure 7:** Zircon U–Pb Concordia diagram for sample LA13-19. A. U–Pb isotope analyses obtained on LASS (n=67). B. U–Pb isotope analyses obtained on SIMS (n=118). Colour code is relative to the O-isotope compositions. Colour code key is reported on the right-handside of diagrams. Empty ellipses are zircon U–Pb isotope analyses with no corresponding zircon O-isotope analysis. Ellipses are reported at the 95 % confidence level.</figcaption></figure>

### 5.3.2 Bt-Gt quartzofeldspathic gneiss LA9–16

A total of 121 U–Pb isotope analyses were obtained on zircon from the Bt-Gt quartzofeldspathic gneiss LA9–16. All U–Pb isotopes were obtained by the SIMS protocol. U contents range from 94 ppm to 2217 ppm, with a median value of 1153 ppm, higher than zircon analyses from the Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19. Th contents range from 2 ppm to 228 ppm, with a median value of 8 ppm, a value significantly lower than the median of the zircon population from sample LA13–19 (median Th contents of 110 and 40 for the LASS and SIMS protocols respectively). Consequently, Th/U values of zircon from sample LA9–16 are lower than those from sample LA13–19 and range from 0.001 to 0.21 with a median value of
0.01. Analyses with Th/U values > 0.1 are those displaying bright CL micro-structures. Common-Pb contents are overall insignificant, with a range from 0.01 % to 3.31 % and a median value of 0.07%. Seventy-five analyses out of the 121 have U-Pb dates within 95-105 % concordance, no analysis is above 105 %. <sup>207</sup>Pb/<sup>206</sup>Pb apparent ages range from 2605 ± 7 Ma to 3744 ± 10 Ma. Interestingly, while dark grey “ghost” zircon domains show ages ranging from the early Paleoarchean to the Neoarchean, bright CL cores analysed display overall older ages, mostly late Eoarchean to late-Paleoarchean, such as if these bright CL domains were less affected by Pb-loss. Lastly, these bright CL domains show Th/U values higher than 0.1 and higher than the ratio measured in dark grey domains (Supplementary data table). The Wetherill Concordia diagram in Figure 8A shows a spread of zircon U-Pb analyses from the Eoarchean at ca. 3.65 Ga to the Neoarchean at ca. 2.67 Ga. Both these dates correspond to known metamorphic events in the Saglek Block (Hurst et al., 1975; Wanless et al., 1979; Schiøtte et al., 1986; Schiøtte et al., 1989b; Kusiak et al., 2018; Vezinet et al., 2018; Sałacińska et al., 2019; Vezinet et al., 2019; Dunkley et al., 2020).

<figure><img src="image_10.png" /><figcaption>**Figure 8:** Wetherill concordia diagrams for zircon from sample LA9-16. Both diagrams show the same set of U-Pb isotope analyses, all obtained using the SIMS protocol. A. Filled ellipses are those used for the construction of the</figcaption></figure>

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discordia chord (56 out of 121 analyses). B. Filled ellipses are those with a corresponding O-isotope composition. Colour code is relative to the O-isotope compositions. Key of the colour code is reported on the right-handside of diagrams.

**5.4 Zircon Hf-isotopes**

Sixty-seven (67) Hf-isotope analyses were obtained on zircon grains from sample LA13-19. Results are presented in Figure 9. Measured ¹⁷⁶Hf/¹⁷⁷Hf values range from 0.280344 to 0.280651, corresponding to a range of current ε values of -75.5 to -86.3 at present time. ¹⁷⁶Lu/¹⁷⁷Hf values are typical of zircon, with a range from 0.000208 to 0.003310 and a median value of 0.000582. Corresponding (¹⁷⁶Hf/¹⁷⁷Hf)initial values, calculated from the zircon ²⁰⁷Pb/²⁰⁶Pb apparent ages, range from 0.280309 ± 44 to 0.280509 ± 45, equivalent to ε values of -7.5 ± 0.8 | 1.1 to +3.0 ± 2.6 | 2.6. Notably, lower ε values might be related to ancient Pb-loss. No Hf-isotope ratio measurements were conducted on zircon from sample LA9-16.
<figure><img src="image_11.png" /></figure>

**Figure 9:** Zircon U-Pb/Hf-isotopes results for LA13–19. Blue diamonds: U-Pb/Hf analyses of zircon grains from Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19, n=67. Green squares: Average U-Pb/Hf analyses from varied felsic rocks from the Saglek Block, n=18 (Vezinet et al., 2018; Fisher et al., 2020; Wasilewski et al., 2021). Time of the late Archean metamorphism is based on the upper discordia intercept obtained on zircon from LA9-16 (Figure 8) and from previous isotopic studies of the Saglek Block (Hurst et al., 1975; Baadsgaard et al., 1979; Wanless et al., 1979; Vezinet et al., 2019). Depleted mantle field boundaries are from Næraa et al. (2012) and Griffin et al. (2002). Uncertainty bars are reported at a coverage factor of 2. Green squares uncertainty bars are population weighted mean uncertainties. Blue diamond uncertainty bars are individual analysis uncertainties.

## 5.5 Zircon O-isotopes

### 5.5.1 Cpx-Bt-bearing quartzofeldspathic gneiss

One-hundred and twenty-seven (127) O-isotope analyses were obtained on sample LA13–19. For 97 of these, U-Pb were undertaken on the same sites. The typical individual 2SE uncertainty for O isotope data is 0.25 ‰ (see Supplementary Material for details on uncertainty calculation). $\delta^{18}\text{O}_{\text{VSMOW}}$ values range from 5.55 ± 0.31 ‰ to 9.27 ± 0.26 ‰ (median = 6.44 ‰). The lowest value of 5.55 ± 0.31 ‰ corresponds to a $^{207}\text{Pb}/^{206}\text{Pb}$ apparent age of 3703 ± 23 Ma.
The highest $\delta^{18}$O$_{VSMOW}$ value has a corresponding U–Pb isotope age of 7.83 ± 0.2 5‰ with a $^{207}$Pb/$^{206}$Pb apparent age of 3569 ± 12 Ma. One analysis from this dataset shows a $^{207}$Pb/$^{206}$Pb apparent age of 3788 ± 10 Ma and a corresponding $\delta^{18}$O$_{VSMOW}$ value 7.45 ± 0.26 ‰ (Figure 10).

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### 5.5.2 Bt-Gt quartzofeldspathic gneiss

Fifty-nine (59) O-isotope analyses were obtained on sample LA9-16. The typical individual 2SE uncertainty is similar that obtained for LA13-19 zircon: 0.22 ‰. $\delta^{18}$O$_{VSMOW}$ values range from 5.51 ± 0.20 ‰ to 10.84 ± 0.21 ‰ (median = 9.25 ‰). For twenty-five of the O-isotope analyses, U-Pb analyses were undertaken on the same sites. In this sub-set, the highest $\delta^{18}$O$_{VSMOW}$ value of 10.59 ± 0.21 ‰ has a $^{207}$Pb/$^{206}$Pb apparent age of 2764 ± 15 Ma and the lowest $\delta^{18}$O$_{VSMOW}$ value of 5.61 ± 0.20 ‰ has a $^{207}$Pb/$^{206}$Pb apparent age of 3365 ± 21 Ma. It is also worth noting that one analysis shows a $^{207}$Pb/$^{206}$Pb apparent age of 3711 ± 8 Ma and a corresponding $\delta^{18}$O$_{VSMOW}$ value 10.49 ± 0.18 ‰ (Figure 10). Elevated $\delta^{18}$O$_{VSMOW}$ signatures are found in both bright CL cores and dark grey (sometimes showing ghost features) domains (Figure 5 and Supplementary Material). Lastly, significant differences in $\delta^{18}$O$_{VSMOW}$ signatures are found between core and rims domains (Figure 5).

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<figure><img src="image_12.png" /><figcaption>**Figure 10:** Zircon U-Pb/O-isotope results for Saglek Block metasedimentary sample LA13-19 (dark and light blue diamonds) and sample LA9-16 (dark and light orange diamonds). Large diamonds represent analyses with concordance ranging between 95 and 105 %. Smaller diamonds are analyses with concordance either < 95 or > 105 %. Yellow diamonds, green diamonds and white dots are analyses and compiled dataset of zircon U-Pb/O-isotope analyses presented in Vezinet et al. (2018) and Vezinet et al. (2019). Uncertainty bars are reported at a coverage factor of 2.</figcaption></figure>

# 6 Discussion

## 6.1 Nature and origin of the meta-sedimentary rocks

In contrast to a number of Archean terranes showing low-strain domains preserving primary sedimentary features (Dimroth, 1982; Appel, 1998; Fedo et al., 2001), the Saglek Block lacks known domains that escaped from intense ductile deformation. This prohibits examination of primary sedimentary features in the metasedimentary rocks from this terrane. Determining the nature and origin of the sedimentary lithologies exposed in the Saglek Block consequently relies
chiefly on mineral assemblages and whole-rock geochemical compositions; features that can be
altered by metamorphism.

The Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19 is largely dominated by quartz
and feldspar with minor amphiboles and micas. This mineral assemblage may reflect a
feldspathic arenite protolith if metamorphism did not involve major fluid-induced
recrystallisation that may have caused geochemical perturbations. This rock is now a meta-
quartzite. The detrital nature of LA13–19 is clear from the large amount of zircon recovered
during mineral separation, a feature that would not be observed in a sediment originating from
chemical precipitation.

The Bt-Gt quartzofeldspathic gneiss LA9–16 has intermediate silica content of ca.
55 wt. % and higher Al₂O₃ and FeO contents than LA13–19 (18.5 wt. % and 13 wt. %
respectively). Biotite sheets form a well-defined foliation associated with garnet. These features,
however do not allow to discriminate between a pelitic or volcanic nature of the protolith. Yet,
the most recent extensive field survey (Ryan and Martineau, 2012) relates these rocks to
sedimentary protoliths, not volcanic. This interpretation is followed in this study. Further,
although sample LA9–16 has been classified as belonging to the “1q” sub-group defined in Ryan
and Martineau (2012): “rusty-brown, biotite + garnet quartzofeldspathic gneiss”, it could also be
a member of the “1s” sub-group: “grey to rusty-brown biotite + sillimanite-rich paragneiss”. A
smooth and continuous transition might exist from one lithology to the other in the field.

## 6.2 Timing of deposition of the studied Saglek Block metasedimentary rocks

6.2.1 Nature of the zircon domains analysed

It is clear from both the internal micro-structures (Figure 5 and Supplementary Material)
and the chemical/isotopic ratios (Figure 6) that the bulk chemistry and the constituent zircons of
LA13–19 and LA9–16 reacted in differing ways during surimposed tectonothermal events. While the CL microstructures of LA13–19 zircons generally show nicely preserved zoning patterns (either as broad bands or oscillatory zones, Figure 5) that are interpreted as indicative of minor secondary overprinting of these domains; LA9–16 zircon grains show blurry microstructures in CL, with remnants of older microstructures (some remnant oscillatory zoning patterns discernible, bright in CL), defined as ghost features. Such ghost features are typically interpreted as resulting from recrystallisation induced by a transgressive fluid during metamorphism (Hoskin and Black, 2000; Corfu et al., 2003). Further, Th/U values of analysed LA13–19 zircon sites are largely over 0.1, the threshold commonly used to distinguish between metamorphic zircon domains, those with Th/U < 0.1, and magmatic zircon domains, those with Th/U > 0.1 (Rubatto, 2017). In contrast, the median Th/U value for LA9–16 zircon is 0.01, a value that indicates Th uptake from the crystallizing medium before crystallization of these zircon domains. Thorium uptake can occur during (re)-crystallization of secondary phases such as apatite, allanite or monazite prior to or during metamorphic events.

The Wetherill Concordia diagram for LA9–16 zircon shows the analyses form a Discordia line intercepting the Concordia curve at *ca.* 3.7 Ga and *ca.* 2.7 Ga (Figure 8). This strong alignment is interpreted as the partial re-opening of the zircon crystal system, resulting in variable Pb-loss at *ca.* 2.7 Ga. Further details of the effect of metamorphism are provided below.

On the basis of this evidence, we interpret the zircon from sample LA9–16 as metamorphic, while those from LA13–19 as retaining significant amount of pre-metamorphic isotopic signature.

In addition, it should be highlighted that apart from rare CL bright core zircon domains, all LA9–16 zircon domains show the ghost microstructures, have Th/U values << 0.1 and align
about the Discordia chord in Figure 8. Thus, even the late Eoarchean domains showing *ca.* 3.7 Ga $^{207}$Pb/$^{206}$Pb apparent ages, are interpreted as formed during a metamorphic event. Our interpretation is that, based on micro-structural observations (Figure 5), these domains are formed through replacement/recrystallization during the late Eoarchean metamorphism. This late Eoarchean metamorphic event, already documented in earlier studies (Hurst et al., 1975; Schiøtte et al., 1989b, a; Vezinet et al., 2019), is confirmed by our data.

### 6.2.2 Deposition of the Nulliak assemblage

The depositional age of the sedimentary rocks LA9–16 and LA13–19 is difficult to estimate from their zircon populations. Zircon grains from LA9–16 are interpreted as metamorphic (see section 6.2.1) and can therefore only be used to mark a lower limit for deposition. Using the dataset produced on LA9–16 zircon grains, the lower limit for deposition is 3.65 Ga: the upper intercept of the Discordia chord formed by LA9–16 zircon U–Pb isotope analyses (Figure 8). Unfortunately, the LA13–19 zircon U–Pb isotope data do not provide a better estimate of the deposition age because these analyses likely suffered from ancient Pb-loss, which, although limited, slightly rejuvenated zircon Pb–Pb apparent ages (Figure 7). The limited Pb-loss event is confirmed by the horizontal trend on the (¹⁷⁶Hf/¹⁷⁷Hf)initial versus ²⁰⁷Pb/²⁰⁶Pb apparent age diagram presented in Figure 9 (see also section 6.3.1). As a consequence, the younger zircon apparent age from LA13–19 cannot be interpreted as the depositional age for this clastic sequence. With the dataset obtained in this study, we cannot provide tighter constraints on the depositional age than older than 3.65 Ga – a minimum estimate for the deposition age of the Nulliak supracrustal assemblage. Our estimate for depositional ages is considerably younger, yet no more precise than the estimate of Nutman and Collerson (1991) made from a quartzite from the same supracrustal assemblage, for which a depositional age of *ca.* 3800 Ma was proposed.
Lastly, the oldest zircon of our dataset shows a $^{207}$Pb/$^{206}$Pb apparent age of 3872 ± 4 Ma, a value
similar to the age of the oldest igneous activity recognized at the regional scale (Schiøtte et al.,
1989b; Komiya et al., 2017; Vezinet et al., 2018; Wasilewski et al., 2021).

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These lines of evidence might be interpreted as indicating that deposition of the Nulliak
supracrustal assemblage is not older than the most primitive igneous activity preserved in the
Saglek Block. This would support the earlier field observations collected by Collerson and
Bridgwater (1979) indicating that Nulliak supracrustal lithologies are found as xenoliths only
within Uivak II gneisses, while they are found interlayered with the older Uivak I gneisses,
emplaced at 3.87-3.86 Ga (Vezinet et al., 2018; Wasilewski et al., 2021). Alternatively, the
Nulliak assemblage might comprise rocks with different ages, some older than the Uivak I
gneisses and some younger. This latter view remains to be shown with further zircon isotope
studies.

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6.2.3 Tectonic setting of deposition

Determining a geodynamic context for the deposition of these clastic sediments is
cornerstone in the construction of an evolutionary model for the Saglek Block. One way to
determine the context of deposition in highly deformed terranes such as the Saglek Block is
using the CA – DA method (Cawood et al., 2012). Based on crystallisation ages (CA) of
individual zircon domains and the sediment depositional age (DA), this method indicates a
tectonic setting of deposition. It is worth mentioning here that the CA – DA method is grounded
on zircon age spectra from areas where tectonic settings are firmly constrained, i.e. Phanerozoic
areas. Thus, although being useful for zircon populations such as those studied here because of
the obliteration of tectonic constraints due to intense ductile deformation post-deposition, the
ancient character of these zircon populations should be kept in mind when interpreting CA – DA

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curves. These grains crystallized in a time period where even the operation of plate tectonics on Earth is debated.

Using a depositional age of 3.65 Ga, which is a minimal estimate of deposition, the LA13-19 zircon spectrum falls in the field of a convergent plate margin setting (Figure 11), i.e. trench, fore-arc, back-arc (Cawood et al., 2012). This observation is similar to the one made on Isua volcaniclastic rocks, also part of the North Atlantic craton (Figure 11).

<figure><img src="image_13.png" /><figcaption>**Figure 11:** CA-DA diagram linking zircon age spectra and tectonic settings. Zircon age spectrum for sample LA13-19 obtained using a deposition age of 3.65 Ga. Analyses with negative CA-DA values were screened out of the cumulative plot. U-Pb analyses were not screened for their concordance value for the construction of this diagram. Data for Isua cumulative curve is from Nutman et al. (2009). Fields are from Cawood et al. (2012).</figcaption></figure>

## 6.3 Nature of the crystallizing environment for zircon

### 6.3.1 Crustal reservoir

Ancient Pb-loss events in zircon make attempts to constraint the meaning of their associated Hf-isotope signatures very difficult, especially in detrital zircon populations of
Archean age (Kemp et al., 2010; Vervoort and Kemp, 2016). Even though we do not know the
exact age of all the detrital zircons, the horizontal alignment of LA13–19 zircon analyses on the
($^{176}$Hf/$^{177}$Hf)initial versus $^{207}$Pb/$^{206}$Pb apparent age diagram (Figure 9), extending out from ~ 3.8
Ga, is indicative of a Pb-loss process that likely affected these zircon domains. As such, the
analyses that we have the higher degree of confidence in, in terms of their Hf isotope
compositions, are the older ones because their hosts likely experienced less Pb-loss. The oldest
analysis of our dataset having a corresponding Hf-isotope composition shows a $^{207}$Pb/$^{206}$Pb
apparent age of 3788 ± 10 Ma and a corresponding $^{ε^{176}Hf}_{initial}$ of +3.0 ± 2.6 | 2.6 (see also Figure
5). This signature suggests crystallization from a supra-chondritic reservoir, i.e. in an
environment with a depleted mantle source. This result forms the most elevated zircon $^{ε^{176}Hf}$to
date in the Saglek Block. Previous Hf-isotope analyses of zircon from the Saglek Block returned
chondritic or sub-chondritic values (Vezinet et al., 2018; Vezinet et al., 2019) interpreted as
reflecting the lack of significant mantle depletion in the area. This view has however been
contested after the analysis of zircon isotopic compositions of other felsic igneous rocks from the
Saglek Block (Wasilewski et al., 2021). Our new results suggest that at least part of the mantle
domains contributing to the Saglek Block at *ca.* 3.8 Ga had experienced melt extraction during
the Hadean and the early-Eoarchean (Figure 9), in agreement with Wasilewski et al. (2021). This
also means that the igneous activity recognized at *ca.* 3.87 Ga in the Saglek Block might not
represent the oldest crustal activity in the North Atlantic craton, but only the oldest crustal
remnant preserved at present day. In this case, pre-3.87 Ga rocks would have been rapidly
recycled back into the mantle.
### 6.3.2 Reworking of material affected by low-temperature alteration

Zircon O-isotope compositions are a powerful tool to constrain the nature of the crystallizing fluid and particularly for identifying the presence of material that experienced low or high temperature alteration. Material that experienced low temperature alteration such as mafic crust that interacted hydrothermally with water-sea floor at temperatures below 150°C or 510 superficial sediments, are typically characterized by $\delta^{18}$O values above the mantle range of 5.3 ± 0.6 ‰, 2SE (Valley et al., 1998; Valley et al., 2005). Conversely, material that experienced high- 512 temperature hydrothermal interaction displays commonly $\delta^{18}$O values below the mantle range (Bindeman and Valley, 2001). Zircon precipitating from melts or media containing this reworked 514 material with elevated or low $\delta^{18}$O values will inherit these signatures. In addressing the source 516 of the O isotope variation in our zircon we should also consider the effect of secondary diffusion 518 on zircon O-isotope compositions. This effect has been recently described on natural zircon from 520 the Ukrainian shield (Claesson et al., 2016). In their model, the authors argue that fast diffusion 522 in the presence of a fluid phase, either high f(O$_2$) values or a fluid-medium, is a means to 524 significantly modify zircon O-isotope compositions without seriously affecting CL 526 microstructures or U-Pb/Hf-isotope compositions. There is little doubt that both meta- 528 sedimentary rocks investigated in this study went through, to some extent, metasomatic 530 alteration. This is clearly indicated by Pb-loss in both zircon populations. The main difference 532 however of our zircon O-isotope dataset with that of Claesson et al. (2016) is that we do observe 534 $\delta^{18}$O values that are significantly different between core and rims domains (Figure 5 and 536 Supplementary Material). This observation rules out an *en masse* resetting of the zircon O- 538 isotope compositions. A similar observation has been done on zircon grains from a meta-igneous 540 rock of the same area (Vezinet et al., 2019). Consequently, the rapid diffusion process presented
in Claesson et al. (2016) cannot be used to account for the elevated zircon O-isotope compositions obtained in this study. The term *reworking* is here used in the sense of any geological mechanism that brings material from the Earth surface down to mid- to lower crustal levels and reused. This definition follows that presented in Rollinson (2017).

Zircon $\delta^{18}$O deviations observed in the Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19 range from mantle-like values to mildly-elevated values up to +7.83 ± 0.25 ‰ (Figure 10). Zircon grains from the Bt-Gt quartzofeldspathic gneiss LA9–16 display higher $\delta^{18}$O deviations, up to +10.5 ‰ at ca. 3.7 Ga (Figure 10). These results are interpreted as indicative of involvement of material that underwent low-temperature alteration in the medium from which theses zircon precipitated. Interestingly, igneous zircon domains (LA13–19 and LA9–16 populations) and metamorphic domains (LA9–16 population) both show $\delta^{18}$O values > 7.5 ‰ implying that reworking processes are not restricted to a specific mechanism in the Saglek Block and that reworked material is widespread in this crustal pile by the mid-Eoarchean. The mildly evolved $\delta^{18}$O values measured in the igneous zircon domains from LA13–19 might be caused by ingestion in the melt of the pre-3.87 Ga rocks hypothesized in section 6.3.1 if these pre-3.87 Ga rocks experienced interactions with a low-temperature hydrothermal fluid before reworking (such as superficial alteration). The higher values measured in zircon from LA9–16 are likely the result of material with even higher $\delta^{18}$O values, such as sediments. Regardless of the exact nature of the material involved in the crystallizing medium of zircon, which can hardly be assessed with the present dataset, our new results indicate that reworking processes involving superficial material were efficient very early in the history of the Saglek Block, as soon as 3.7 Ga (Figure 10).
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**6.4** **Contrasting effects of metamorphism on Archean detrital zircon populations**

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The zircon populations investigated here both show variable degrees of modification due to metamorphic overprinting but in contrasting ways. While zircon grains from the Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19 show preserved CL micro-structures, elevated Th/U values, common > 3.8 Ga-old zircon domains and the most ancient Pb–Pb apparent ages in our dataset (up to 3.87 Ga); zircon grains from the Bt-Gt quartzofeldspathic gneiss LA9–16 show recrystallized microstructures in CL, no 3.8 Ga domains, minor > 3.7 Ga domains and a well-defined Discordia chord with intercepts at *ca.* 3.7 Ga and *ca.* 2.7 Ga (Figure 8). Hence, despite being interpreted as belonging to the same supracrustal sequence, these two rocks have strikingly distinct zircon microstructures and chemical/isotopic signatures.

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Experimental studies have shown that the re-opening of the zircon crystal lattice and alteration of its chemical/isotopic signatures during metamorphism hinge heavily on the dryness of the host medium during metamorphism. A hydrous local medium favours high diffusion rates of both cations and anions (Cherniak and Watson, 2003). In such an environment, zircon chemical and isotopic signatures can re-equilibrate through either a diffusion-reaction process or coupled dissolution-reprecipitation processes (Geisler et al., 2007). Zircon re-equilibration in a hydrous medium is the inferred process that would account for the observed CL microstructures and chemical/isotopic signatures of zircon from the Bt-Gt quartzofeldspathic gneiss LA9–16 (Figure 5, Figure 6 and Figure 8). The fluid is interpreted to originate from the breakdown of water-rich minerals such as micas following the prograde metamorphic path. In the Cpx-Bt-bearing quartzofeldspathic gneiss LA13–19 where the Bt modal proportion is significantly lower than in sample LA9–16, the amount of fluid produced during high grade metamorphism is limited, with a restricted effect on zircon grains present in the rock. Consequently, the initial parent lithology, arenite or pelite, appears to be a likely reason for the distinct evolution of these

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two zircon populations throughout regional metamorphism, despite belonging to the same supracrustal assemblage, having experienced the same pressure-temperature paths during metamorphism and being *ca.* 20 km apart. The Saglek Block offers then a natural comparison of the impact of relatively dry versus hydrous metamorphic environments on zircon populations.

All LA9–16 zircon domains showing (i) ghost CL microstructures, (ii) low Th/U values and (iii) adhering to the 3.7 Ga – 2.7 Ga Discordia chord, are interpreted as metamorphic. The Discordia is interpreted as being the result of ancient Pb-loss at *ca.* 2.7 Ga during the regional high-grade metamorphic event recognized in earlier studies (Baadsgaard et al., 1979; Schiøtte et al., 1989b; Kusiak et al., 2018; Sałacińska et al., 2018; Vezinet et al., 2018; Vezinet et al., 2019; Dunkley et al., 2020; Fisher et al., 2020; Vezinet et al., 2022). Domains with bright CL cores, Th/U > 0.1 are considered as remnants of igneous domains that—for some of them—experienced Pb-loss. Interestingly, both metamorphic and igneous domains in LA9–16 zircon population show elevated δ¹⁸O values, up to +10 ‰ (Figure 5 and Supplementary Material).

The metamorphic event occurring at *ca.* 3.7 Ga or before recorded in zircon from LA9–16 has re-equilibrated isotope systematics with a fluid showing lighter O isotope composition, a feature clearly indicated by the range of δ¹⁸O values obtained in metamorphic zircon domains (Figure 8B). Therefore, this Eoarchean metamorphic event involved dissolution and reprecipitation of older detrital zircon present in the sediment. The only remnants of these detrital grains are the scarce bright CL core domains (Figure 5 and Supplementary Material). The second metamorphic event, at *ca.* 2.7 Ga, is characterized by diffusion-reaction processes. Diffusion-reaction produced the “ghosted” CL microstructures and facilitated extensive Pb-loss but did not involve complete re-equilibration of isotopes systematics, creating the Discordia chord and preserving the elevated δ¹⁸O values.
## 6.5 Isotopic signatures of zircon from the Western Greenlandic and Scottish parts of the North Atlantic craton

The North Atlantic craton comprises three distinct areas (Figure 1): the Saglek Block in NE Canada, the Lewisian Gneiss Complex in NW Scotland and southern Greenland which includes the Itsaq Gneiss Complex at the western end. The latter is undoubtedly the best studied of the three, with numerous investigations of zircon isotope signatures (Bennett et al., 1993; Nutman et al., 1996; Vervoort et al., 1996; Nutman et al., 2002; Crowley, 2003; Nutman et al., 2004; Kamber et al., 2005; Hiess et al., 2009; Kemp et al., 2009; Nutman et al., 2009; Whitehouse et al., 2009; Hiess et al., 2011; Næraa et al., 2012; Nutman et al., 2014; Fisher and Vervoort, 2018; Kemp et al., 2019). The large set of zircon U-Pb isotope data available on the Itsaq Gneiss Complex reveals that the oldest crustal remnants in this terrane, emplaced at *ca.* 3.85 Ga (Nutman et al., 2004), are coeval with the oldest remnants exposed in the Saglek Block (Schiøtte et al., 1989b; Komiya et al., 2017; Vezinet et al., 2018; Wasilewski et al., 2021). Despite the similarity in age, differences between the Saglek Block and the Itsaq Gneiss complex are evident.

Zircon Hf-isotopes in the Itsaq Gneiss Complex display on the whole chondritic to sub-chondritic values (excluding analyses not obtained using spatially resolved techniques), with only few exceptions showing slightly positive values (Hiess et al., 2009; Kemp et al., 2019). Our results indicate the presence of differentiated mantle domains at *ca.* 3.8 Ga. The initial Hf isotope ratios measured in the cores of zircon from Saglek sample LA13-19 (Figure 5 and Figure 9), and those reported in Wasilewski et al. (2021), indicate either a source with a higher degree of fractionation from a chondritic Lu/Hf mantle domain or a fractionation with similar Lu/Hf values but over a longer period of time compared with the rocks exposed in SW Greenland. Regardless, the single value of $\epsilon^{176}\text{Hf} \sim +3.0$ would however have to be duplicated and confirmed by further
investigation to (i) improve the measurement uncertainty and (ii) better constrain the geodynamic evolution of the Saglek Block lithosphere during the early Archean. If confirmed, such an elevated zircon $\epsilon^{176}\text{Hf}$ would still remain a rare signature on a global scale as indicated by recent compilations of $\epsilon^{176}\text{Hf}$ signatures of zircon from igneous rocks showing that oldest crustal remnants on each Archean cratonic nucleus are either chondritic or sub-chondritic (Petersson et al., 2020; Vezinet et al., 2020).

The second difference between the Saglek Block and Itsaq supracrustal sequences is evident from zircon O-isotope signatures. Zircon O-isotope signatures in the Itsaq Gneiss Complex show no significant deviation above the mantle zircon field (Hiess et al., 2009; Hiess et al., 2011). This is interpreted by the authors as indicating the absence of material that undergone interaction with hydrothermal fluid at low temperature in the medium from which the local igneous felsic rocks were produced. In contrast, for the Saglek Block, metamorphic zircon domains from different rocks types (meta-igneous rocks and meta-sediments) show significant $\delta^{18}\text{O}$ deviations above the mantle zircon field. $\delta^{18}\text{O}$ values up to +9 ‰ for metamorphic zircon domains from meta-igneous rocks (Vezinet et al., 2019) and up to +10.5 ‰ for igneous and metamorphic zircon domains from meta-sedimentary rocks (this study, Figure 10). Such elevated $\delta^{18}\text{O}$ values are to date not recognized in zircon from SW Greenland. The lack of elevated $\delta^{18}\text{O}$ values in SW Greenland zircon might be related to the lower metamorphic grade of the rocks exposed, having limited metamorphic zircon overprint. However, *in-situ* garnet O-isotope analyses up to +9.7 ‰ (Gauthiez-Putallaz et al., 2020) also indicate the presence of a component weathered at superficial conditions in some SW Greenland rocks (Cates and Mojzsis, 2006). The Lewisian Gneiss Complex of NW Scotland has yielded only a limited number zircon of Eoarchean ages (Lancaster et al., 2015). These rare old zircon domains show chondritic Hf-
isotope signatures and δ¹⁸O suggesting restricted interaction with supracrustal material. Hence, across the North Atlantic craton Archean nucleus, the O isotope signatures of Eoarchean zircon domains from the Saglek block are presently unique.

Altogether, the evidence from our new zircon data from Saglek Block metasedimentary rocks shows that: (i) sedimentation in the Saglek Block occurred in an active convergent plate margin setting, such as trench, fore-arc, back-arc, if the CA-DA approach can be applied to Eoarchean sediments (Figure 11), (ii) mantle domains beneath the Saglek Block might be more depleted than those beneath other domains of the North Atlantic craton and (iii) crustal reworking was a common and effective process in the evolution of the Saglek Block, in contrast to other parts of the craton of similar age. These observations could be interpreted as indicating that the Saglek Block represented the active margin of the North Atlantic Eoarchean plate/proto-continent, while the Itsaq Gneiss Complex constituted the inner part of the proto-continent, despite that other geochemical evidence, such as the arc-like geochemical signatures of the SW Greenland supracrustal sequences, might mitigate this view (Hoffmann et al., 2010; Nutman et al., 2019).

## 6.6 A zircon perspective on pre-3.6 Ga crust-mantle evolution

### 6.6.1 A dominantly chondritic Lu-Hf mantle

The most recent zircon U-Pb/Hf isotope compiled datasets of early Earth material show at first glance that, on the whole, no mantle domain, with respect to its Lu-Hf isotope systematics, experienced prolonged melt extraction before 3.6 Ga. This is demonstrated by the observation that the overwhelming majority of pre-3.6 Ga zircon show either chondritic or sub-chondritic Hf-isotope signatures (Hiess and Bennett, 2016; Vezinet et al., 2018; Petersson et al., 2020). The positive ε¹⁷⁶Hf deviations observed in Eoarchean zircon from the Saglek Block (Figure 9) stand
out from this general global trend for Hadean and early-Archean zircon. Therefore, it is argued here that, except rare exceptions—such as the Saglek Block—Hf-isotopes can hardly be used to promote widespread, significant and protracted mantle differentiation by the end of the Eoarchean. The lack of significantly positive zircon $\epsilon^{176}\text{Hf}$ values in pre-3.6 Ga zircon does not have to mean that no large volumes of crust were produced at this time. A simple way to accommodate a broadly chondritic mantle and high crustal production rates is to consider the role played by fast and efficient recycling processes, as shown in Rollinson (2017). Production of large volumes of (mafic) crust during the early Archean would also promote the genesis of protocratonic lithospheric keels during the mid-Archean, a feature recently identified in the Slave craton by the presence of high Fo olivine within Mesoarchean diamonds (Timmerman et al., 2022).

### 6.6.2 Recycling and reworking processes during early Earth times

In a compiled dataset of igneous and metamorphic zircon from Archean meta-igneous rocks, Vezinet et al. (2019) showed that $\delta^{18}\text{O}$ values > 7.5‰ are restricted to rare zircon grains, including some from felsic meta-igneous rocks of the Saglek Block (see also Figure 10). Detrital zircon $\delta^{18}\text{O}$ values above 6.5‰ are reported from the Yilgarn craton-Jack Hills meta-sedimentary rocks (Cavosie et al., 2005; Trail et al., 2007; Bell et al., 2011; Bell and Harrison, 2013; Bell et al., 2016; Pidgeon et al., 2017), the North China craton (Wan et al., 2013; Zhang et al., 2014), the Zimbabwe craton (Bolhar et al., 2017), the Central Zone of the Limpopo Complex (Zeh et al., 2014) and the Saglek Block (this study, see Figure 10).

The global $\delta^{18}\text{O}$ signatures of Archean zircon from (meta)-sediments and (meta)-igneous rocks indicate that zircon with O-isotope signatures above the mantle range, i.e. $\ge 5.9$ ‰, are rather common, and even dominant, throughout the Archean, with about 46 % of analyses
showing a value above mantle range (Figure 12). A similar proportion is obtained if considering
only analyses older than 3.6 Ga. The pie charts presented in the top-right inset in Figure 12
suggest that the type of geological processes occurring within Earth's lithosphere did not change
significantly during the Hadean-Archean eon and this conclusion might be extended to the
associated tectonic style. On the whole, the currently available data show that a number of pre-
3.6 Ga zircon domains display δ¹⁸O values > 6.5 ‰, with even rare incidence of values > 8.0 ‰.

Yet, the large majority of O-isotopes are either within the mantle range or slightly below (Figure
12). Consequently, though it can be argued that both reworking and recycling of material that
interacted with low temperature hydrothermal fluids are likely to have started between the end of
the Hadean and the early Eoarchean in some places on Earth, these processes were not the
dominant crustal differentiation processes on Earth at this time.
<figure><img src="image_14.png" /><figcaption>**Figure 12:** Compilation of zircon U–Pb/O-isotope analyses of Archean age (n = 2946). Only 95-105 % concordant analyses are shown. Blue dots are zircon analyses showing δ¹⁸O values below mantle range (n = 289, i.e. 9.8 % of the dataset). Dark green dots are zircon analyses showing δ¹⁸O value within the mantle range (n = 1295, i.e. 44.0 % of the dataset). Yellow dots are zircon analyses showing δ¹⁸O value above the mantle range (n = 1070, i.e., 46.2 % of the dataset). Orange dots are zircon analyses showing δ¹⁸O values ≥ 7.5 ‰. (n = 292, i.e. 9.9 % of the dataset). Analyses with U–Pb age > 3.6 Ga show virtually the same distributions of δ¹⁸O values, except for the most elevated ones (≥ 7.5 ‰). Uncertainty bars are reported at a coverage factor of 2. Mantle range is from Valley et al. (2005).</figcaption></figure>

**6.7 Implications for local geodynamics**

Identification of elevated δ¹⁸O values in zircon from the early geological record lends weight to geodynamic models promoting some degree of both recycling and reworking on the pre-3.6 Ga Earth. Both mechanisms involve some form of vertical motion of material, whether it occurred within a subduction-like framework or another type of mechanism remains unsettled (Sobolev and Brown, 2019). The North Atlantic craton does not show clear evidence for paired metamorphism which, in a uniformitarian framework, is interpreted as a diagnostic feature of
present-day like plate tectonics (Brown, 2006; Holder et al., 2019), although this latter view is contentious (Capitanio et al., 2019). One can also consider that the high degree of tectonic disruption has, at least partly, erased metamorphic record, yet even in Isua, which is probably the best constrained domain of the North Atlantic craton in terms of metamorphic petrology, no firm consensus has been reached regarding the pressure-temperature path for all rocks exposed in the belt. Providing metamorphic constraints on rocks exposed in Saglek Block is essential in order to further the co-evolution of these two domains of the North Atlantic craton.

Based on the presently available set of data, geodynamic models of the Saglek Block must then include mechanism(s) that would account for the geochemical signatures measured in rocks and minerals from this area, namely: (i) significant fractionation (partial melting) of the early mantle, (ii) migration of the newly produced melt at the Earth's surface, (iii) interaction of this new crust with hydrothermal fluids at low temperatures (< 150°C) and (iv) burial of this altered crust down to partial melting conditions for generating more differentiated rocks. How widespread these processes were beyond the Saglek Block is unclear because even other units of the North Atlantic Craton display little evidence of recycling.

## Conclusion

Our study unequivocally indicates deposition of the meta-sedimentary units of the Nulliak supracrustal assemblage during the Eoarchean, before *ca.* 3.65-3.7 Ga. Yet, the patent lack of zircon domains showing > 3.87 Ga ages in Nulliak zircon spectra from Nulliak meta-sediments does not support the view that the Nulliak supracrustal lithologies precede emplacement of the oldest igneous rocks preserved in the Saglek Block at *ca.* 3.87 Ga. Furthermore, the metasediment zircon population gives no hint of un-discovered Hadean components within this portion of the North Atlantic Craton. Early mantle differentiation and recycling-reworking of
material altered at low-temperatures, inferred from zircon Hf- and O-isotope analyses make the Saglek Block a unique terrane, even with the context of its host craton, and provide an unprecedented glimpse of early Earth tectonics. This terrane may then represent an ideal playground for investigating the mechanisms of reworking and recycling during the early Earth. It is worth stressing that, in the light of previous geochemical studies of early Archean terranes, the mechanisms that produced the geochemical signatures inherent in the zircon sampled in the Saglek Block might have not reflected those dominant on Earth by the early- to mid-Eoarchean.

## Acknowledgements

The authors are grateful to W. Teixeira for efficient editorial work and two anonymous reviewers for constructive comments that helped improving the clarity of this manuscript. This study was funded by the Natural Sciences and Engineering Research Council of Canada Discovery Grants Program and the Canada First Research Excellence Fund: Metal Earth Project. This is Metal Earth contribution MERC-ME-2022-23. The field campaign was funded by the Agence Nationale de la Recherche (Grant ANR-11-JS56-0012) to E. Thomassot. This work has been part-supported by a grant from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Synergy Grant MEET, grant agreement no. 856555). The Centre de Recherches Pétrographiques et Géochimiques (C.R.P.G. Nancy, France) ion probe team is thanked for assistance in zircon SIMS U-Pb isotope analyses. AV and ET are indebted to Sebastien Roubinet and Anne-Lise Vacher-Morazzani for assistance in the field and unrivalled seamanship in the Labrador Sea.
764

766
Amelin, Y., Lee, D.-C., Halliday, A.N., Pidgeon, R.T., 1999. Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. Nature 399, 252-255, https://doi.org/10.1038/20426.

768
Amelin, Y., Lee, D.-C., Halliday, A., 2000. Early-middle Archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single zircon grains. Geochim. Cosmochim. Acta 64, 4205-4225, https://doi.org/10.1016/S0016-7037(00)00493-2.

772
Appel, P.W., 1998. Recognizable primary volcanic and sedimentary features in a low-strain domain of the highly deformed, oldest known (3.7-3.8 Gyr) Greenstone Belt, Isua, West Greenland. Terra Nova 10, 57-62, https://doi.org/10.1046/j.1365-3121.1998.00162.x.

774
Baadsgaard, H., Collerson, K.D., Bridgwater, D., 1979. The Archean gneiss complex of northern Labrador. 1. Preliminary U-Th-Pb geochronology. Can. J. Earth Sci. 16, 951-961, https://doi.org/10.1139/e79-081.

778
Bell, E.A., Harrison, T.M., McCulloch, M.T., Young, E.D., 2011. Early Archean crustal evolution of the Jack Hills Zircon source terrane inferred from Lu-Hf, <sup>207</sup>Pb/<sup>206</sup>Pb, and &delta;<sup>18</sup>O systematics of Jack Hills zircons. Geochim. Cosmochim. Acta 75, 4816-4829, https://doi.org/10.1016/j.gca.2011.06.007.

780
Bell, E.A., Harrison, T.M., 2013. Post-Hadean transitions in Jack Hills zircon provenance: A signal of the Late Heavy Bombardment? EPSL 364, 1-11, https://doi.org/10.1016/j.epsl.2013.01.001.

782
Bell, E.A., Boehnke, P., Harrison, T.M., 2016. Recovering the primary geochemistry of Jack Hills zircons through quantitative estimates of chemical alteration. Geochim. Cosmochim. Acta 191, 187-202, https://doi.org/10.1016/j.gca.2016.07.016.

786
Bennett, V.C., Nutman, A.P., McCulloch, M.T., 1993. Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth. EPSL 119, 299-317, https://doi.org/10.1016/0012-821X(93)90140-5.

788
Bindeman, I.N., Valley, J.W., 2001. Low-&delta;<sup>18</sup>O rhyolites from Yellowstone: Magmatic evolution based on analyses of zircons and individual phenocrysts. J. Petrol. 42, 1491-1517, https://doi.org/10.1093/petrology/42.8.1491.

792
Bolhar, R., Hofmann, A., Kemp, A.I., Whitehouse, M.J., Wind, S., Kamber, B.S., 2017. Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons. Geochim. Cosmochim. Acta 215, 432-446, https://doi.org/10.1016/j.gca.2017.07.008.

794
Bridgwater, D., Collerson, K.D., Hurst, R.W., Jesseau, C.W., 1975. Field characters of the early Precambrian rocks from Saglek, coast of Labrador, Report of activities. Geological Survey of Canada Ottawa, pp. 287-296.
Bridgwater, D., Collerson, K.D., 1976. The major petrological and geochemical characters of the 798 3,600 My Uivak gneisses from Labrador. Contrib. Mineral. Petrol. 54, 43-59.

Bridgwater, D., Collerson, K.D., 1977. On the origin of early Archaean gneisses: a reply. Contrib. 800 Mineral. Petrol. 62, 179-191, https://doi.org/10.1007/BF00372876.

Bridgwater, D., Rosing, M., Schiøtte, L., Austrheim, H., 1989. The effect of fluid-controlled 802 element mobility during metamorphism on whole rock isotope systems, some theoretical aspects and 803 possible examples, Fluid Movements—Element Transport and the Composition of the Deep Crust. 804 Springer, Dordrecht, The Netherlands, pp. 277-298, https://doi.org/10.1007/978-94-009-0991-5_24.

Bridgwater, D., Schiøtte, L., 1991. The Archaean gneiss complex of Northern Labrador-A review 806 of current results, ideas and problems. Bull. Geol. Soc. Den. 39, 153-166.

Brown, M., 2006. Duality of thermal regimes is the distinctive characteristic of plate tectonics since 808 the Neoarchean. Geology 34, 961-964, https://doi.org/10.1130/G22853A.1.

Capitanio, F., Nebel, O., Cawood, P., Weinberg, R., Chowdhury, P., 2019. Reconciling thermal 810 regimes and tectonics of the early Earth. Geology 47, 923-927, https://doi.org/10.1130/G46239.1.

Cates, N.L., Mojzsis, S.J., 2006. Chemical and isotopic evidence for widespread Eoarchean 812 metasedimentary enclaves in southern West Greenland. Geochim. Cosmochim. Acta 70, 4229-4257, https://doi.org/10.1016/j.gca.2006.05.014.

Cavosie, A., Valley, J., Wilde, S., 2005. Magmatic δ¹⁸O in 4400–3900 Ma detrital zircons: A 814 record of the alteration and recycling of crust in the Early Archean. EPSL 235, 663-681, 815 https://doi.org/10.1016/j.epsl.2005.04.028.

Cawood, P.A., Hawkesworth, C.J., Dhuime, B., 2012. Detrital zircon record and tectonic setting. 818 Geology 40, 875-878, https://doi.org/10.1130/G32945.1.

Cherniak, D.J., Watson, E.B., 2003. Diffusion in zircon. Rev. Mineral. Geochem. 53, 113-143, 820 https://doi.org/10.2113/0530113.

Claesson, S., Bibikova, E.V., Shumlyanskyy, L., Whitehouse, M.J., Billström, K., 2016. Can 822 oxygen isotopes in magmatic zircon be modified by metamorphism? A case study from the Eoarchean 823 Dniester-Bug Series, Ukrainian Shield. Precambr. Res. 273, 1-11, 824 https://doi.org/10.1016/j.precamres.2015.11.002.

Collerson, K.D., Bridgwater, D., 1979. Metamorphic development of early Archaean tonalitic and 826 trondhjemitic gneisses: Saglek area, Labrador, in: Barker, F. (Ed.), Trondhjemites, dacites and related 827 rocks Elsevier, Amsterdam, The Netherlands, pp. 205-274, https://doi.org/10.1016/B978-0-444-41765-828 7.50012-5.
830

Collerson, K.D., Campbell, L.M., Weaver, B.L., Palacz, Z.A., 1991. Evidence for extreme mantle
fractionation in early Archaean ultramafic rocks from northern Labrador. Nature 349, 209-214,
https://doi.org/10.1038/349209a0.

832

Corfu, F., Hanchar, J.M., Hoskin, P.W., Kinny, P., 2003. Atlas of zircon textures. Rev. Mineral. Geochem. 53, 469-500, https://doi.org/10.2113/0530469.

834

Crowley, J., 2003. U-Pb geochronology of 3810-3630 Ma granitoid rocks south of the Isua greenstone belt, southern West Greenland. Precambr. Res. 126, 235-257, https://doi.org/10.1016/S0301-9268(03)00097-4.

836

Dimroth, E., 1982. The oldest rocks on Earth: stratigraphy and sedimentology of the 3.8 billion
years old Isua supracrustal sequence. Sedimentary geology of the highly metamorphosed Precambrian
complexes, 16-27.

838

840

Dunkley, D.J., Kusiak, M.A., Wilde, S.A., Whitehouse, M.J., Sałacińska, A., Kielman, R., Konečný, P., 2020. Two Neoarchean tectonothermal events on the western edge of the North Atlantic Craton, as revealed by SIMS dating of the Saglek Block, Nain Province, Labrador. J. Geol. Soc. 177, 31-49, https://doi.org/10.1144/jgs2018-153

842

844

Fedo, C.M., Myers, J.S., Appel, P.W., 2001. Depositional setting and paleogeographic implications
of earth's oldest supracrustal rocks, the> 3.7 Ga Isua Greenstone belt, West Greenland. Sediment. Geol.
141, 61-77, https://doi.org/10.1016/S0037-0738(01)00068-9.

846

Fisher, C.M., Vervoort, J.D., 2018. Using the magmatic record to constrain the growth of
continental crust—The Eoarchean zircon Hf record of Greenland. EPSL 488, 79-91,
https://doi.org/10.1016/j.epsl.2018.01.031.

848

850

Fisher, C.M., Bauer, A.M., Luo, Y., Sarkar, C., Hanchar, J.M., Vervoort, J.D., Tapster, S.R., Horstwood, M., Pearson, D.G., 2020. Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite—Improvements, potential reference materials, and application to the Archean Saglek Block gneisses. Chem. Geol. 539, 119493, https://doi.org/10.1016/j.chemgeo.2020.119493.

852

854

Gauthiez-Putallaz, L., Nutman, A., Bennett, V., Rubatto, D., 2020. Origins of high δ¹⁸O in 3.7–3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks. Chem. Geol. 537, 119474, https://doi.org/10.1016/j.chemgeo.2020.119474.

856

858

Geisler, T., Schaltegger, U., Tomaschek, F., 2007. Re-equilibration of zircon in aqueous fluids and melts. Elements 3, 43-50, https://doi.org/10.2113/gselements.3.1.43.

860

Gerya, T.V., 2014. Precambrian geodynamics: Concepts and models. Gondwana Res. 25, 442-463,
https://doi.org/10.1016/j.gr.2012.11.008.
Griffin, W., Wang, X., Jackson, S., Pearson, N., O'Reilly, S.Y., Xu, X., Zhou, X., 2002. Zircon chemistry and magma mixing, SE China: in-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes. Lithos 61, 237-269, https://doi.org/10.1016/S0024-4937(02)00082-8.

Hiess, J., Bennett, V.C., Nutman, A.P., Williams, I.S., 2009. In situ U-Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust. Geochim. Cosmochim. Acta 73, 4489-4516, https://doi.org/10.1016/j.gca.2009.04.019.

Hiess, J., Bennett, V.C., Nutman, A.P., Williams, I.S., 2011. Archaean fluid-assisted crustal cannibalism recorded by low δ¹⁸O and negative εHf(T) isotopic signatures of West Greenland granite zircon. Contrib. Mineral. Petrol. 161, 1027-1050, https://doi.org/10.1007/s00410-010-0578-z.

Hiess, J., Bennett, V.C., 2016. Chondritic Lu/Hf in the early crust-mantle system as recorded by zircon populations from the oldest Eoarchean rocks of Yilgarn Craton, West Australia and Enderby Land, Antarctica. Chem. Geol. 427, 125-143, https://doi.org/10.1016/j.chemgeo.2016.02.011.

Hoffmann, J.E., Münker, C., Polat, A., König, S., Mezger, K., Rosing, M.T., 2010. Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochim. Cosmochim. Acta 74, 7236-7260, https://doi.org/10.1016/j.gca.2010.09.027.

Holder, R.M., Viete, D.R., Brown, M., Johnson, T.E., 2019. Metamorphism and the evolution of plate tectonics. Nature 572, 378-381, https://doi.org/10.1038/s41586-019-1462-2.

Hoskin, P.W.O., Black, L.P., 2000. Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon. J. Metamorph. Geol. 18, 423-439, https://doi.org/10.1046/j.1525-1314.2000.00266.x.

Hurst, R.W., Bridgwater, D., Collerson, K.D., Wetherill, G.W., 1975. 3600-My Rb-Sr ages from very early Archaean gneisses from Saglek Bay, Labrador. EPSL 27, 393-403, https://doi.org/10.1016/0012-821X(75)90058-8.

Ickert, R.B., 2013. Algorithms for estimating uncertainties in initial radiogenic isotope ratios and model ages. Chem. Geol. 340, 131-138, https://doi.org/10.1016/j.chemgeo.2013.01.001.

Kamber, B.S., Whitehouse, M.J., Bolhar, R., Moorbath, S., 2005. Volcanic resurfacing and the early terrestrial crust: zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland. EPSL 240, 276-290, https://doi.org/10.1016/j.epsl.2005.09.037.

Kemp, A.I., Whitehouse, M.J., Vervoort, J.D., 2019. Deciphering the zircon Hf isotope systematics of Eoarchean gneisses from Greenland: Implications for ancient crust-mantle differentiation and Pb isotope controversies. Geochim. Cosmochim. Acta 250, 76-97, https://doi.org/10.1016/j.gca.2019.01.041.

Kemp, A.I.S., Foster, G.L., Scherstén, A., Whitehouse, M.J., Darling, J., Storey, C.D., 2009. Concurrent Pb-Hf isotope analysis of zircon by laser ablation multi-collector ICP-MS, with implications
for the crustal evolution of Greenland and the Himalayas. Chem. Geol. 261, 244-260, https://doi.org/10.1016/j.chemgeo.2008.06.019.

Kemp, A.I.S., Wilde, S.A., Hawkesworth, C.J., Coath, C.D., Nemchin, A., Pidgeon, R.T., Vervoort, J.D., DuFrane, S.A., 2010. Hadean crustal evolution revisited: new constraints from Pb-Hf isotope systematics of the Jack Hills zircons. EPSL 296, 45-56, https://doi.org/10.1016/j.epsl.2010.04.043.

Komiya, T., Yamamoto, S., Aoki, S., Koshida, K., Shimojo, M., Sawaki, Y., Aoki, K., Sakata, S., Yokoyama, T.D., Maki, K., 2017. A prolonged granitoid formation in Saglek Block, Labrador: Zonal growth and crustal reworking of continental crust in the Eoarchean. Geosci. Front. 8, 355-385, https://doi.org/10.1016/j.gsf.2016.06.013.

Korenaga, J., 2021. Hadean geodynamics and the nature of early continental crust. Precambr. Res. 359, 106178, https://doi.org/10.1016/j.precamres.2021.106178.

Kusiak, M.A., Dunkley, D.J., Whitehouse, M.J., Wilde, S.A., Sałacińska, A., Konečný, P., Szopa, K., Gawęda, A., Chew, D., 2018. Peak to post-peak thermal history of the Saglek Block of Labrador: a multiphase and multi-instrumental approach to geochronology. Chem. Geol. 484, 210-223, https://doi.org/10.1016/j.chemgeo.2017.10.033.

Lancaster, P.J., Storey, C.D., Hawkesworth, C.J., 2015. The Eoarchaean foundation of the North Atlantic Craton. Geological Society, London, Special Publications 389, 261-279, https://doi.org/10.1144/SP389.11.

Mole, D., Thurston, P., Marsh, J., Stern, R., Ayer, J., Martin, L., Lu, Y., 2021. The formation of Neoarchean continental crust in the south-east Superior Craton by two distinct geodynamic processes. Precambr. Res. 356, 106104, https://doi.org/10.1016/j.precamres.2021.106104.

Morino, P., Caro, G., Reisberg, L., Schumacher, A., 2017. Chemical stratification in the post-magma ocean Earth inferred from coupled <sup>146,147</sup>Sm-<sup>142,143</sup>Nd systematics in ultramafic rocks of the Saglek block (3.25-3.9 Ga; northern Labrador, Canada). EPSL 463, 136-150, https://doi.org/10.1016/j.epsl.2017.01.044.

Murakami, T., Chakoumakos, B.C., Ewing, R.C., Lumpkin, G.R., Weber, W.J., 1991. Alpha-decay event damage in zircon. Am. Mineral. 76, 1510-1532.

Næraa, T., Scherstén, A., Rosing, M.T., Kemp, A.I.S., Hoffmann, J.E., Kokfelt, T.F., Whitehouse, M.J., 2012. Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago. Nature 485, 627-630, https://doi.org/10.1038/nature11140.

Nutman, A.P., Fryer, B.J., Bridgwater, D., 1989. The early Archaean Nulliak (supracrustal) assemblage, northern Labrador. Can. J. Earth Sci. 26, 2159-2168.
Nutman, A.P., Collerson, K.D., 1991. Very early Archean crustal-accretion complexes preserved in the North Atlantic craton. Geology 19, 791-794, https://doi.org/10.1130/0091-7613(1991)019<0791:VEACAC>2.3.CO;2.

Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., Kinny, P.D., 1996. The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma). Precambr. Res. 78, 1-39, https://doi.org/10.1016/0301-9268(95)00066-6.

Nutman, A.P., Friend, C.R.L., Bennett, V.C., 2002. Evidence for 3650-3600 Ma assembly of the northern end of the Itsaq Gneiss Complex, Greenland: implication for early Archaean tectonics. Tectonics 21, https://doi.org/10.1029/2000TC001203.

Nutman, A.P., Friend, C.R., Barker, S.L., McGregor, V.R., 2004. Inventory and assessment of Palaeoarchaean gneiss terrains and detrital zircons in southern West Greenland. Precambr. Res. 135, 281-314, https://doi.org/10.1016/j.precamres.2004.09.002.

Nutman, A.P., Friend, C.R.L., Paxton, S., 2009. Detrital zircon sedimentary provenance ages for the Eoarchaean Isua supracrustal belt southern West Greenland: juxtaposition of an imbricated ca. 3700Ma juvenile arc against an older complex with 3920-3760Ma components. Precambr. Res. 172, 212-233, https://doi.org/10.1016/j.precamres.2009.03.019.

Nutman, A.P., Bennett, V.C., Friend, C.R.L., Yi, K., Lee, S.R., 2014. Mesoarchaean collision of Kapisilik terrane 3070 Ma juvenile arc rocks and >3600 Ma Isukasia terrane continental crust (Greenland). Precambr. Res. in press., https://doi.org/10.1016/j.precamres.2014.12.013.

Nutman, A.P., Friend, C.R., Bennett, V.C., Van Kranendonk, M., Chivas, A.R., 2019. Reconstruction of a 3700 Ma transgressive marine environment from Isua (Greenland): Sedimentology, stratigraphy and geochemical signatures. Lithos 346, 105164, https://doi.org/10.1016/j.lithos.2019.105164.

Percival, J., 1994. Archean high-grade metamorphism, Dev. Precambrian Geol. Elsevier, pp. 357-410.

Petersson, A., Kemp, A.I., Gray, C.M., Whitehouse, M.J., 2020. Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia. Chem. Geol. 551, 119757, https://doi.org/10.1016/j.chemgeo.2020.119757.

Pidgeon, R., 2014. Zircon radiation damage ages. Chem. Geol. 367, 13-22, https://doi.org/10.1016/j.chemgeo.2013.12.010.

Pidgeon, R., Nemchin, A., Whitehouse, M.J., 2017. The effect of weathering on U-Th-Pb and oxygen isotope systems of ancient zircons from the Jack Hills, Western Australia. Geochim. Cosmochim. Acta 197, 142-166, https://doi.org/10.1016/j.gca.2016.10.005.
962 Reimink, J., Davies, J., Chacko, T., Stern, R., Heaman, L., Sarkar, C., Schaltegger, U., Creaser, R., Pearson, D., 2016. No evidence for Hadean continental crust within Earth's oldest evolved rock unit. Nat. Geosci. 9, 777-780, https://doi.org/10.1038/ngeo2786.

964 Rollinson, H., 2017. There were no large volumes of felsic continental crust in the early Earth. Geosphere 13, 235-246, https://doi.org/10.1130/GES01437.1.

966 Rubatto, D., 2017. Zircon: the metamorphic mineral. Rev. Mineral. Geochem. 83, 261-295, https://doi.org/10.2138/rmg.2017.83.9.

970 Ryan, B., Martineau, Y., 2012. Revised and coloured edition of 1992 map showing the Geology of the Saglek Fiord-Hebron Fiord area, Labrador (NTS 14L/2, 3, 6, 7), 1 ed. Government of Newfoundland and Labrador, Department of Natural Resources, Geological Survey.

972 Sałacińska, A., Kusiak, M.A., Whitehouse, M.J., Dunkley, D.J., Wilde, S.A., Kielman, R., 2018. Complexity of the early Archean Uivak Gneiss: insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland. Precambr. Res. 315, 103-119, https://doi.org/10.1016/j.precamres.2018.07.011.

976 Sałacińska, A., Kusiak, M.A., Whitehouse, M.J., Dunkley, D.J., Wilde, S.A., Kielman, R., Król, P., 2019. Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss. Int. J. Earth Sci. 108, 753-778, https://doi.org/10.1007/s00531-019-01677-y.

980 Schiøtte, L., Bridgwater, D., Collerson, K.D., Nutman, A.P., Ryan, B., 1986. Chemical and isotopic effects of late Archaean high-grade metamorphism and granite injection on early Archaean gneisses, Saglek-Hebron, northern Labrador. Geological Society, London, Special Publications 24, 261-273, https://doi.org/10.1144/GSL.SP.1986.024.01.23.

984 Schiøtte, L., Compston, W., Bridgwater, D., 1989a. U-Th-Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada. Can. J. Earth Sci. 26, 2636-2644, https://doi.org/10.1139/e89-224.

986 Schiøtte, L., Compston, W., Bridgwater, D., 1989b. Ion probe U-Th-Pb zircon dating of polymetamorphic orthogneisses from northern Labrador, Canada. Can. J. Earth Sci. 26, 1533-1556, https://doi.org/10.1139/e89-131.

990 Schiøtte, L., Nutman, A.P., Bridgwater, D., 1992. U-Pb ages of single zircons within" Upernavik" metasedimentary rocks and regional implications for the tectonic evolution of the Archaean Nain Province, Labrador. Can. J. Earth Sci. 29, 260-276, https://doi.org/10.1139/e92-024.

992 Sobolev, S.V., Brown, M., 2019. Surface erosion events controlled the evolution of plate tectonics on Earth. Nature 570, 52-57, https://doi.org/10.1038/s41586-019-1258-4.

994 Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. EPSL 26, 207-221, https://doi.org/10.1016/0012-821X(75)90088-6.
996 Thomassot, E., Kitayama, Y., Wind, S., Jovovic, I., Ternois, S., Barbey, P., Deloule, E., 2015. Petrography, U-Pb age constraints and metamorphic history of the Nulliak Supracrustal assemblage (Northeastern Labrador), Goldschmidt Conference, Prague.

998
Timmerman, S., Reimink, J.R., Vezinet, A., Nestola, F., Kublik, K., Banas, A., Stachel, T., Stern, R.A., Luo, Y., Sarkar, C., Ielpi, A., Currie, C.A., Mircea, C., Jackson, V., Pearson, D.G., 2022. Mesoarchean diamond formed in rapidly thickened lithosphere. EPSL, https://doi.org/10.1016/j.epsl.2022.117633.

1000
Trail, D., Mojzsis, S.J., Harrison, T.M., Schmitt, A.K., Watson, E.B., Young, E.D., 2007. Constraints on Hadean zircon protoliths from oxygen isotopes, Ti-thermometry, and rare earth elements. Geochem. Geophys. Geosyst. 8, https://doi.org/10.1029/2006GC001449.

1004
Valley, J., Lackey, J., Cavosie, A., Clechenko, C., Spicuzza, M., Basei, M., Bindeman, I., Ferreira, V., Sial, A., King, E., 2005. 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contrib. Mineral. Petrol. 150, 561-580, https://doi.org/10.1007/s00410-005-0025-8.

1006
Valley, J.W., Kinny, P.D., Schulze, D.J., Spicuzza, M.J., 1998. Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts. Contrib. Mineral. Petrol. 133, 1-11, https://doi.org/10.1007/s004100050432.

1010
Vermeesch, P., 2018. IsoplotR: A free and open toolbox for geochronology. Geosci. Front. 9, 1479-1493, https://doi.org/10.1016/j.gsf.2018.04.001.

1012
Vervoort, J.D., Patchett, P.J., Gehrels, G.E., Nutman, A., 1996. Constraints on early Earth differentiation from hafnium and neodymium isotopes. Nature 379, 624-627, https://doi.org/10.1038/379624a0.

1014
Vervoort, J.D., Kemp, A.I., 2016. Clarifying the zircon Hf isotope record of crust-mantle evolution. Chem. Geol. 425, 65-75, https://doi.org/10.1016/j.chemgeo.2016.01.023.

1016
Vezinet, A., Pearson, D.G., Thomassot, E., Stern, R.A., Sarkar, C., Luo, Y., Fisher, C.M., 2018. Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon derived from TTG of the Eoarchean Saglek Block, N. Labrador. EPSL 503, 95-107, https://doi.org/10.1016/j.epsl.2018.09.015.

1018
Vezinet, A., Thomassot, E., Pearson, D.G., Stern, R.A., Luo, Y., Sarkar, C., 2019. Extreme δ¹⁸O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga. Geology 47, 605-608, https://doi.org/10.1130/G46086.1.

1024
Vezinet, A., Pearson, D.G., Heaman, L.M., Sarkar, C., Stern, R.A., 2020. Early crustal evolution of the Superior craton–A U–Pb, Hf and O isotope study of zircon from the Assean lake complex and a comparison to early crust in other cratons. Lithos 368-369, 105600, https://doi.org/10.1016/j.lithos.2020.105600.
Vezinet, A., Pearson, D.G., Thomassot, E., 2021. Effects of contamination on whole-rock isochrons in ancient rocks: a numerical modelling approach. Lithos 386-387, 106040, https://doi.org/10.1016/j.lithos.2021.106040.

Vezinet, A., Thomassot, E., Luo, Y., Sarkar, C., Pearson, D.G., 2022. Diachronous redistribution of Hf and Nd isotopes at the crystal scale—Consequences for the isotopic evolution of a poly-metamorphic crustal terrane. Geosciences 12, https://doi.org/10.3390/geosciences12010036.

Wan, Y., Zhang, Y., Williams, I.S., Liu, D., Dong, C., Fan, R., Shi, Y., Ma, M., 2013. Extreme zircon O isotopic compositions from 3.8 to 2.5Ga magmatic rocks from the Anshan area, North China Craton. Chem. Geol. 352, 108-124, https://doi.org/10.1016/j.chemgeo.2013.06.009.

Wanless, R., Bridgwater, D., Collerson, K.D., 1979. Zircon age measurements for Uivak II gneisses from the Saglek area, Labrador. Can. J. Earth Sci. 16, 962-965, https://doi.org/10.1139/e79-082.

Wasilewski, B., O'Neil, J., Rizo, H., 2019. Geochemistry and petrogenesis of the early Archean mafic crust from the Saglek-Hebron Complex (Northern Labrador). Precambr. Res. 328, 321-343, https://doi.org/10.1016/j.precamres.2019.04.001.

Wasilewski, B., O'Neil, J., Rizo, H., Paquette, J.-L., Gannoun, A.-M., 2021. Over one billion years of Archean crust evolution revealed by zircon U-Pb and Hf isotopes from the Saglek-Hebron complex. Precambr. Res. 359, 106092, https://doi.org/10.1016/j.precamres.2021.106092.

Wendt, I., Carl, C., 1991. The statistical distribution of the mean squared weighted deviation. Chemical Geology: Isotope Geoscience Section 86, 275-285, https://doi.org/10.1016/0168-9622(91)90010-T.

Whitehouse, M.J., Myers, J.S., Fedo, C.M., 2009. The Akilia Controversy: field, structural and geochronological evidence questions interpretations of> 3.8 Ga life in SW Greenland. J. Geol. Soc. 166, 335-348, https://doi.org/10.1144/0016-76492008-070

Whitehouse, M.J., Dunkley, D.J., Kusiak, M.A., Wilde, S.A., 2019. On the true antiquity of Eoarchean chemofossils—assessing the claim for Earth’s oldest biogenic graphite in the Saglek Block of Labrador. Precambr. Res. 323, 70-81, https://doi.org/10.1016/j.precamres.2019.01.001.

Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. Am. Mineral. 95, 185, https://doi.org/10.2138/am.2010.3371.

Zeh, A., Stern, R.A., Gerdes, A., 2014. The oldest zircons of Africa—their U–Pb–Hf–O isotope and trace element systematics, and implications for Hadean to Archean crust–mantle evolution. Precambr. Res. 241, 203-230, https://doi.org/10.1016/j.precamres.2013.11.006.

Zhang, H.-F., Wang, J.-L., Zhou, D.-W., Yang, Y.-H., Zhang, G.-W., Santosh, M., Yu, H., Zhang, J., 2014. Hadean to Neoarchean episodic crustal growth: Detrital zircon records in Paleoproterozoic
1064

quartzites from the southern North China Craton. Precambr. Res. 254, 245-257,
https://doi.org/10.1016/j.precamres.2014.09.003.