# Igneous apatite geochemistry indicates early cratonization of continents

Adrien Vezinet, Javiera Flores-Rojas, Alexander Sobolev, Julien Léger, Aleksandr Chugunov, Valentina Batanova, Marlina Elburg, Axel Hofmann, Mélanie Balvay, Nouméa Paradis

► To cite this version:

Adrien Vezinet, Javiera Flores-Rojas, Alexander Sobolev, Julien Léger, Aleksandr Chugunov, et al. Igneous apatite geochemistry indicates early cratonization of continents. Precambrian Research, 2025, 430, pp.107927. 10.1016/j.precamres.2025.107927. hal-05337278

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# Igneous apatite geochemistry indicates early cratonization of continents

Adrien Vezinet<sup>a,* ID</sup>, Javiera Flores-Rojas<sup>a</sup>, Alexander V. Sobolev<sup>a</sup>, Julien Léger<sup>a</sup>, Aleksandr V. Chugunov<sup>a</sup>, Valentina G. Batanova<sup>a</sup>, Marlina A. Elburg<sup>b</sup>, Axel Hofmann<sup>b</sup>, Mélanie Balvay<sup>a</sup>, Nouméa Paradis<sup>a</sup>

## ABSTRACT

Processes and mechanisms accounting for the stabilization of Archean (4.0–2.5 Ga) continental crust remain a matter of debate. Over the last decades, major efforts have been made to determine the chemical and isotopic composition of rocks belonging to the Tonalite-Trondhjemite-Granodiorite (TTG) suite, i.e. those forming the bulk of Archean continental crust, as well as late-Archean sanukitoids, the typical marker of cratonization. The extensive use of zircon elemental and isotopic signatures has indisputably been an unrivalled source of information; yet it has also biased interpretations through the prism of a single mineral, hence not reflecting the whole geological history of these magmas. To extend our understanding of early continent stabilization, a pivotal aspect of Earth’s evolution, a fresh perspective is necessary. Here, we present in-situ analyses of igneous apatite, Ca₅(PO₄)₃(OH,Cl,F), from Archean granitoids exposed in the eastern Kaapvaal craton for major/ trace elements and U–Pb/Sr isotopes. The trace element signatures of these apatite crystals, with a clear enrichment in LREE and an elevated LREE/HREE, resemble that of apatite from sanukitoids and Phanerozoic I-type granites, a signature which can be blurred at the whole-rock scale. We interpret this signature as indicating that the studied granitoids are formed via interaction between (i) a TTG melt, formed via partial melting of a subducting oceanic crust and (ii) a mantle component, causing chemical depletion of the mantle domain involved and thus production of long-lived and stable lithospheric keels pivotal in the long-term preservation of Archean lithosphere at the Earth’s surface. Therefore, the identification of this signature—in igneous apatite from Paleo to Meso-Archean TTGs of the eastern Kaapvaal—indicates an early onset of cratonization in this region.

## 1. Introduction

The presence of substantial amounts of long-lived, differentiated, felsic, crust makes our planet unique amongst the known celestial bodies. Chemically differentiated crust has been inferred to have existed as early as the Hadean (>4.0 Ga) mainly based on the Jack Hills zircon population, which includes some crystals interpreted as originating from felsic magmas (Cavosie et al., 2005), and the Acasta gneiss (Bowring and Housh, 1995; Iizuka et al., 2007; Reimink et al., 2016). The identification of felsic igneous rocks of Eo-Archean age (3.6–4.0 Ga) in preserved Archean terranes all over the world (Hiess et al., 2009; Hiess et al., 2011; Ge et al., 2018; Vezinet et al., 2018b; Petersson et al., 2020) and elevated O-isotope signature in early Archean zircon reinforces the view that crust was present at the Earth’s surface during in the Hadean (Bell et al., 2016; Pidgeon et al., 2017; Vezinet et al., 2019; Vezinet et al., 2022; Mixon et al., 2023). Continental crust of Eo-Archean age represents, however, only a negligible volume (<<1 %) of preserved Archean crust and Archean continental exposures mostly consist of ≤3.2 Ga material. Despite clear evidence of substantial generation of continental crust during the Meso-Archean, the petrogenesis of this crust and its genetic

relationship with the sub-continental mantle (SCM) as well as with mafic–ultramafic igneous rocks in supracrustal sequences associated with rocks of the Tonalite-Trondhjemite-Granodiorite (TTG) suite, remain to be fully grasped (Pearson et al., 2002; Arndt, 2023).

The processes that govern the co-evolution of Archean crust and the SCM are generally captured into the umbrella term ‘cratonization’ (Pollack, 1986; Rapp et al., 2010). Cratonization processes include both the formation of low density felsic continental crust as well as the accumulation of chemically depleted SCM making the resulting lithosphere buoyant, rheologically strong (McKenzie and Priestley, 2008; Arndt et al., 2009) and thus leading eventually to the long-term preservation of these blocks at Earth’s surface for billions of years (Kusky, 1993). Since the SCM is the volumetrically dominant component in Archean lithosphere (continental crust + SCM), investigating the chemical evolution of the SCM is decisive to understanding the timing of cratonization of Archean terranes.

One key observation is that most terrains of Archean age were intruded by a specific type of igneous rocks by the end of the Archean (ca. 2.8–2.5 Ga): sanukitoids (Stern et al., 1989). These rocks have a distinct geochemical signature (Martin et al., 2005; Heilimo et al., 2010)

<footnote><sup>a</sup> Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, 38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Department of Geology, University of Johannesburg, P.O. Box 524, Auckland 1 Park 2006, Johannesburg, South Africa</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: adrien.vezinet@univ-grenoble-alpes.fr (A. Vezinet).</footnote>
made of a dual character: enrichment in both crustal (e.g. Ba, Sr, REE, HFSE) and mantle (e.g. V, Ni, Cr) elements. Even if the precise petrogenesis of this rock suite is still a matter of debate (e.g. Laurent et al., 2014), there is a consensus that they are formed through interaction between a (metasomatized) mantle component and a crustal component (granitoids ± sedimentary rocks in variable proportions). Such petrogenesis exclude any typical Hf-isotope signature in zircon retrieved from sanukitoids. Since sanukitoids s.l. form the only suite of Archean felsic rocks for which a direct contribution of the SCM is unequivocal in their petrogenesis, production and emplacement of sanukitoids is considered to record the onset of cratonization, i.e. formation of a chemically depleted and rheologically strong SCM (Kusky and Polat, 1999). However, sanukitoids form only minor igneous bodies when compared with the volume of exposed TTG crust in Archean terranes (Martin et al., 2005), and some places, such as the eastern Kaapvaal craton, do not show sanukitoids at exposure level, despite showing a thick lithospheric keel (James and Fouch, 2002). The presence of diamond inclusions also indicating thick lithospheric keels (>150 km) under the Kaapvaal craton from 3.3-3.2 Ga (e.g. Richardson et al., 1984) do not, however, imply that these keels were already physically coupled with the preserved, presently exposed, continental crust (Moser et al., 2001). To provide evidence of such coupling, one need to demonstrate a direct contribution from the SCM into the source of rocks that form the Archean continental crust (Pollack, 1986).

In order to further understand early cratonization, a process central in the establishment of long-lived continents, we focus our research on TTGs. The petrogenesis of the TTG suite is considered to occur in two steps: partial melting of a (chemically undepleted) mantle domain to form a mafic rock that is hydrated at surface, then transported into the mantle where it contributes to the formation of TTG (Martin et al., 2005; Moyen and Martin, 2012). If TTG production occurs in a geometry somewhat similar to present-day subduction zones then a contribution

of rocks from the ‘mantle-wedge’ could be expected. Here, we investigated two TTGs exposed in the Archean Kaapvaal craton (South Africa, Fig. 1) which have been extensively studied in terms of major and trace elements at the whole-rock (WR) scale (Moyen et al., 2024), long- and short-lived radiogenic isotope systems at the WR scale (Connolly et al., 2011; Puchtel et al., 2013; Blichtert-Toft et al., 2015) as well as long-lived isotope systems at the crystal scale (Kröner et al., 1989; Armstrong et al., 1990; Kröner and Tegtmeyer, 1994; Amelin et al., 2000; Zeh et al., 2009; Koglin et al., 2010; Zeh et al., 2011; Guitreau et al., 2012; Zeh and Gerdes, 2012; Block et al., 2013; Kröner et al., 2013; Zeh et al., 2013a; Zeh et al., 2013b; Kröner et al., 2014; Laurent and Zeh, 2015; Hoffmann et al., 2016; Vezinet et al., 2018a; Hofmann et al., 2019). Unlike previous studies that have largely focused on zircon, we present new in situ geochemical data for a largely underexplored mineral, apatite, Ca₅(PO₄)₃(OH,Cl,F), for which we obtained major, minor and trace elements contents as well as U-Pb and Sr-isotope compositions. We use these results to discriminate the nature of the magma from which these TTGs have solidified from and especially whether a direct mantle contribution was involved. Indeed, the Sr-isotope and LREE contents in apatite have been shown to be ideally suited to constrain the geochemical signature of the geological environment, whether igneous or metamorphic, from which apatite forms (e.g. Bruand et al., 2017; Laurent et al., 2017; Henrichs et al., 2018; Janots et al., 2018; O'Sullivan et al., 2018; Bruand et al., 2020; Caton et al., 2022). Apatite geochemistry therefore plays a crucial role in enhancing our understanding of the petrogenesis of ancient felsic rocks and the role these rocks play in the cratonization of Archean crust.

## 2. Geological Settings: Paleo- to Mesoarchean Barberton TTGs

The Kaapvaal craton, largely exposed in South Africa and Eswatini (Fig. 1), contains some of the best-preserved Archean exposures and

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** A. Simplified geological map of the Archean Kaapvaal craton. Unfilled areas are post-Archean cover. Figures modified after Eglington and Armstrong (2004) and Vezinet et al. (2018a). B. Geological map of the Barberton Greenstone Belt and surrounding granitoids. Samples' locations are reported by the large stars along with pluton name. Lithology delimitations are from the Geological Survey of South Africa (https://www.geoscience.org.za/). Abbreviations: Da: Dalmein granitoid, Hee: Heerenveen granitoid, Mba: Mbabane late Archean intrusion, Mpa: Mpageni late Archean intrusion, Nel: Nelshoogte granitoid, Sin: Sinceni late Archean intrusion, Ste: Steynsdorp gneiss, Stz: Stolzburg granitoid, Tsp: Theespruit granitoid, Usu: Usutu granitoid.</figcaption></figure>
hence is among the most studied cratons. The eastern part of the craton is made of an assemblage of (i) sedimentary and volcanic rocks with ages between about 3.5 to 3.2 Ga, all variably metamorphosed/altered and (ii) plutonic rocks: the older ones (≥ca. 3.2 Ga) belong to the Tonalite-Trondhjemite-Granodiorite (TTG) suite, typical of Archean exposures, while the younger ones (≤3.2 Ga) are enriched in potassium (K) compared to TTGs, with syenite documented in the area as well as sanukitoids (Moyen et al., 2024).

For details on the nature, generation and emplacement of igneous rocks in the eastern Kaapvaal craton, the reader can refer to Robb et al. (2006), Zeh et al. (2009), Hofmann et al. (2019), Kröner et al. (2019), Robb et al. (2021) and Moyen et al. (2024); only a summary, based on these studies, is given in hereafter. During the early Paleoarchean, between ca 3.66 and ca. 3.45 Ga, the oldest of igneous rocks preserved in the eastern Kaapvaal craton are emplaced. These igneous rocks (e.g. Stolzburg, Theespruit, Ngwane and Tsawela gneisses), located in the southern part of the Barberton greenstone belt area (Fig. 1A), show a

geochemical affiliation to the TTG suite and an overall chondritic or slightly sub-chondritic zircon Hf isotope signature. There is no consensus on the geodynamics accounting for the generation of these oldest igneous rocks. That said, there is a clear difference between the zircon Hf isotope signatures in igneous rocks emplaced coevally in the southern part (e$^{176}$Hfinitial < 0) and the northern part (e$^{176}$Hfinitial > 0) of the Barberton greenstone belt. This has been used to interpret the Barberton Lineament (also referred to as the Inyoka fault system) as a suture zone between two blocks: the Eswatini block to the South and the Witwatersrand to the North (Fig. 1A), likely formed during the late Paleoarchean. This crustal accretion event is coeval with emplacement, at the northern edge of the Barberton greenstone belt, of granitoids with zircon e$^{176}$Hfinitial > 0 signature (e.g. Kaap Valley, see below) and ca. 0.1 Ga later, of granitoids with zircon e$^{176}$Hfinitial < 0 signature (e.g. Salisbury Kop and Nelspruit, see below too). The latter also show an enrichment in potassium (K) compared to the older igneous rocks exposed. This enrichment is interpreted as a first stage of stabilization of the Kaapvaal

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Main petrographic observations of the two granitoids from the Kaapvaal craton investigated here. **A. Top:** Photomicrograph of Kaap Valley sample, revealing a coarse grained granitoid with biotite and amphibole as main mafic phase forming a weak foliation. Plagioclase is Na-rich (oligoclase) as expected in a felsic, K-poor, granitoid of Archean age. **A. Bottom:** Photomicrograph (BSE) showing apatite and oxide crystals hosted within a large amphibole crystal indicating that these phases are either coeval or that apatite/oxide are older than amphibole. **B. Top:** Photomicrograph of Salisbury Kop sample, revealing a coarse grained granitoid with biotite (and minor epidote) as main mafic phase. No clear preferred orientation can be observed at the thick section scale. The modal ratio quartz/feldspar is higher in Salisbury Kop than Kaap Valley sample. **B. Bottom:** Photomicrograph (BSE) showing a large apatite crystal (> 500 µm long) within a matrix made of quartz, K-feldspar and biotite. The latter also hosts smaller apatite crystals with their elongation roughly parallel to the basal cleavage of biotite. Apatite crystals appear at textural equilibrium with their surroundings. A titanite crystal is also observed at the bottom of the picture (also observed in Kaap Valley sample, not shown). Mineral abbreviations after Whitney and Evans (2010).</figcaption></figure>
craton at ca. 3.1 Ga. During the late Archean (<3.2 Ga), the eastern Kaapvaal craton experienced the intrusion of granitoids all showing a sub-chondritic zircon Hf isotope signature ($ε^{176}\text{Hf}_{\text{initial}} < 0$), which combined with other evidence has been interpreted as indicating reworking of older crust, possibly the Paleoarchean one exposed in the eastern Kaapvaal craton today. Altogether, previous studies largely focused on whole-rock and zircon geochemical signatures to build petrogenetic interpretations.

Here, we investigated two felsic plutons sampled in the Northern Barberton area (Figs. 1 and 2): Kaap Valley and Salisbury Kop. Their whole-rock geochemistry, given by previous work (Condie and Hunter, 1976; Kleinhanns et al., 2003; André et al., 2019; Moyen et al., 2024), see also Table 1 and Fig. 3 for a summary, indicates a metaluminous character with Aluminum Saturation Index (ASI) < 1 and down to 0.89 for Kaap Valley, a geochemical feature supported by the presence of amphiboles in our Kaap Valley sample (Fig. 3A). Kaap Valley geochemistry is interesting in the way it does not fit in any of the recent classification of Archean granitoids (e.g. Laurent et al., 2014), neither TTGs, nor Bt- or two-micas granites, nor sanukitoids. Its high maficity and high compatible elements (V + Cr + Co + Ni) contents, more than 150 ppm, might be indicative of a mixed signature between TTG and sanukitoid. On the other hand, the geochemistry of Salisbury Kop does not show high maficity nor high compatible element contents and is much closer to typical Archean granitoids of the TTG suite.

Zircon U-Pb/Hf isotope composition from these plutons give the following information (Zeh et al., 2009): an age of 3.23 Ga for Kaap Valley with supra-chondritic Hf ($ε^{176}\text{Hf}_{\text{initial}} \approx +2.5$) close to estimated depleted mantle composition at emplacement age and an age of 3.10 Ga for Salisbury Kop, with sub-chondritic zircon Hf-isotope signature ($ε^{176}\text{Hf}_{\text{initial}} \approx -1$). It should be stressed that the ($ε^{176}\text{Hf}/^{177}\text{Hf}_{\text{initial}}$) of both zircon populations is the same: 0.28076 ± 0.00002 for Kaap Valley and 0.28076 ± 0.00003 for Salisbury Kop (Zeh et al., 2009), and that the difference in $ε^{176}\text{Hf}_{\text{initial}}$ reflects the difference in the timing of zircon crystallization only. Zircon U-Pb dates are interpreted as crystallization ages of the host rocks whereas Hf-isotope signatures reflect that of the melt from which zircon precipitated. The two samples studied here are

representative of two major geological events in the Eastern Kaapvaal craton during the late Paleozoic to early Mesozoic (Robb et al., 2021).

### 3. Methods

Apatite from the two plutons investigated here have been studied both in thick section (130 µm-thick) and as mineral separates mounted in regular one-inch epoxy mounts. Cathodoluminescence (CL), and back-scattered electron (BSE) images as well as major and minor element compositions were obtained through electron microprobe analyses (EPMA, JEOL JXA-iHP200F). U-Pb isotope and trace element (TE) analyses were conducted via laser ablation (LA)-SC-ICP-MS. The laser ablation system is a Resolution-SE (Applied Spectra) and the ICP-MS used is a 8900 (Agilent) instrument, used in single-quad mode. Sr-isotope compositions were obtained via LA-MC-ICP-MS with the same laser ablation system and a NeptuneXT (Thermo-Fisher) instrument. Details about all analytical protocols used in this study and results obtained on validation materials are reported in the Electronic Supplementary Material (ESM). No common-Pb correction was applied on the data.

Apatite internal micro-structures can be complex (Fig. 2 and ESM), consequently location of analytical spots was chosen in consideration of both CL/BSE images and no analysis was made where the laser spot would overlap different growth zones. Following analysis, time-resolved signals were examined and shorter integration intervals were selected where different apatite domains were analysed in the vertical direction. Excluding such data in favour of longer integration times would have led to a significant loss in the number of data points presented, such is the complex growth zonation of these apatite crystals (we will see below that these complex internal microstructures do, however, not relate to differences in crystallization age). Hence, we present isotope data with the caveat that their fully propagated uncertainties can, in some instances, be larger than optimal data, due to the complexity of these apatite and their ancient age. All laser ablation analyses were reduced on the latest version of Iolite-4 (Paton et al., 2011).

The absence of a correlation between measured REE$^{2+}$ and $^{87}\text{Sr}/^{86}\text{Sr}$

<caption>**Table 1**</caption>

Main petrological and geochemical features of the investigated felsic igneous rock from the eastern Kaapvaal craton. ASI: Aluminum Saturation Index, calculated as molar Al/[($Ca-3.33 \times P + Na + K$)], see Frost and Frost (2008), maficity (wt. %) = $FeO_t + MgO + TiO_2 + MnO$ (all in wt.%).

<table><thead><tr><th>Pluton</th><th>Mineral assemblage (Fig. 2)</th><th>Whole-rock geochemistry (Condie and Hunter, 1976; Kleinhanns et al., 2003; André et al., 2019; Moyen et al., 2024)</th><th>Zircon microstructure (Zeh et al., 2009)</th><th>Zircon U-Pb (Ma) (Zeh et al., 2009)</th><th>Zircon Hf isotope (ε<sup>176</sup>Hf<sub>initial</sub>) (Zeh et al., 2009; Guitreau et al., 2012)</th><th>Apatite microstructure (Fig. 2 and ESM)</th><th>Apatite trace elements (Fig. 4 & Fig. 5), see Laurent et al. (2017); Bruand et al. (2020).</th><th>Apatite Sr-isotope (Fig. 6)</th><th>Apatite U-Pb date in Ma (Fig. 7) & interpretation of the U-Pb date</th></tr></thead><tbody><tr><td>Kaap Valley</td><td>Pl + Qtz + Kf + Bt + Hbl + Rt + Fe-Ti-ox + Zrn + Ttn + Ap</td><td>ASI = 0.89<br>Mg# = 0.51<br>Maficity ≈ 7 wt%<br>K<sub>2</sub>O/Na<sub>2</sub>O = 0.2<br>to 0.5<br>Sr/Y ≈ 48<br>(La/Yb)<sub>N</sub> = 12.5<br>V + Cr + Co + Ni > 150 µg/g</td><td></td><td>3231 ± 9 Ma</td><td>(<sup>176</sup>Hf/<sup>177</sup>Hf)<sub>i</sub> = 0.28076 ± 2<br>corresponding to:<br>ε<sup>176</sup>Hf<sub>i</sub> > +2.1 ± 0.2</td><td></td><td></td><td>Heterogeneous, from <sup>87</sup>Sr/<sup>86</sup>Sr = 0.70091 ± 53 to 0.70382 ± 44</td><td>3243 ± 30 Ma<br>Initial Pb: 1.089 ± 0.031<br>Interpretation: Magma crystallisation age</td></tr><tr><td>Salisbury Kop</td><td>Pl + Kf + Qtz + Bt + Ep + Fe-Ti-ox + Zrn + Ttn + Ap</td><td>ASI = 0.92 to 0.96<br>Mg# = 0.11 to 0.44<br>Maficity ≈ 1.5 to 3 wt%<br>K<sub>2</sub>O/Na<sub>2</sub>O = 0.70 to 1.59<br>Sr/Y = 13 to 29<br>(La/Yb)<sub>N</sub> = 21<br>V + Cr + Co + Ni < 50 µg/g</td><td>Oscillatory zoning patterns interpreted as igneous</td><td>3100 ± 14 Ma</td><td>(<sup>176</sup>Hf/<sup>177</sup>Hf)<sub>i</sub> = 0.28076 ± 3<br>corresponding to:<br>ε<sup>176</sup>Hf<sub>i</sub> = -0.8 ± 1.1</td><td>Core and rim domains, no clear cross-cutting relationships</td><td>Analogous to apatite from Phanerozoic I-type granites or from late-Archean sanukitoids. Distinct from “typical” TTG apatite.</td><td>Heterogeneous, from <sup>87</sup>Sr/<sup>86</sup>Sr = 0.70074 ± 101 to 0.70240 ± 34</td><td>3109 ± 42 Ma<br>Initial Pb: 1.120 ± 0.022<br>Interpretation: Magma crystallisation age</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Main whole-rock (WR) geochemical features of Kaap Valley and Salisbury Kop granitoids showing that neither of the two granitoids investigated here display unequivocal affiliation to any of the three geochemical fields reported (sanukitoids, TTGs and biotite-granites). WR geochemical data after Condie and Hunter (1976); Kleinhanns et al. (2003); Moyen et al. (2024). Fields after Laurent et al. (2014). Geochemical diagrams were produced using GCDKit (Janousek et al., 2006).</figcaption></figure>

on unknowns implies that REE²⁺ correction is either fully successful or
that the fraction of REE²⁺ on masses 87 and 86 total signals is
insignificant.

4. Results

4.1. Petrography, apatite internal micro-structures, major and minor element contents

The main petrographic and geochemical results are summarized,
along with previously published results on rocks from the same pluton, in Table 1. Both rocks are coarse-grained and little preferred orientation of mineral grains was observed (Fig. 2). In both samples, feldspar is partially altered as in the grey cloudy areas in Fig. 2. Apatite is found as inclusion in amphibole crystals, biotite sheets as well as in contact with other phases. Our microscope investigation shows that in the Kaap Valley sample studied, zircon is found in similar textural relationships as apatite. Small elongated crystal, interpreted as apatite, are also observed within zircon grains. In the Salisbury Kop sample investigated, zircon is less abundant and found only in inclusion with micas, associated with large apatite and titanite crystals.

Apatite crystals from both samples investigated via CL and BSE exhibit inner and outer domains yet no unambiguous cross-cutting relationships between inner and outer apatite domains were observed. A total of 409 EPMA analyses were conducted. Out of those, 389 returned CaO and P₂O₅ values matching known apatite compositions (the others were either not apatite or a mixed composition between apatite and a neighboring phase). The P₂O₅ contents of apatite from both samples cluster around 42 wt%, similar to values reported in apatite from sanukitoids in Bruand et al. (2020) and distinct from P₂O₅ contents measured in apatite crystals from Archean TTGs which have a lower value of ca. 33 wt%, while CaO contents in apatite from both TTG and sanukitoids is ca. 55 wt%. Volatile contents in apatite domains studied here are strongly leaning towards the fluorapatite endmember with average F contents 3.30 wt% for Kaap Valley and 3.93 wt% Salisbury Kop. The CL contents are below detection limit (with DL≈70 µg/g) and SO₃ contents are 0.12 wt% and 0.24 wt%.

## 4.2. Trace element contents

A total of 23 trace elements (plus one internal standard for reference) were analysed. This includes Rb, Sr, Y, Zr, Nb, Ba, U, Pb, Th and all REEs. The full dataset is reported in the ESM. A chondrite-normalised REE diagram (Fig. 4) show a significant enrichment in LREE (La to Nd) compared to HREE (Ho to Lu) in the apatite domains analysed. This is confirmed with (La/Lu)N of 15.4 and 24.4 for Kaap Valley and Salisbury Kop apatite, respectively. Elevated (La/Lu)N are typical of apatite from sanukitoids, high Ba-Sr granites and plutonic members of the calc-alkaline series (BADR) whereas apatite from TTGs shows a much

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Chondrite-normalized REE patterns of apatite investigated in this study compared with previously published fields of apatite from given rock sources: TTG and sanukitoids (fields from Bruand et al., 2020). REE signature of apatite from Salisbury Kop perfectly matches that of apatite from sanukitoids while REE patterns of apatite from Kaap Valley are slightly more enriched in MREE yet still preserving elevated (La/Lu). Chondrite values are from Nakamura (1974). Geochemical diagram produced using GCDkit (Janoušek et al., 2006).</figcaption></figure>

lower (La/Lu)N, typically < 1.5 (Bruand et al., 2020).

A similar conclusion can be reached regarding (La/Sm)N, the compiled dataset of Bruand et al. (2020) shows that apatite from TTG has (La/Sm)N ≤ 0.5 while apatite from sanukitoids shows (La/Sm)N ≥ 1, like those studied here (see ESM).

The LREE-rich nature of our apatite populations is clearly observed in Fig. 5A–B where LREE enrichment is more pronounced than Y-enrichment. Such a pattern is typically observed in apatite extracted from “mantle-derived granitoids” (Laurent et al., 2017) and “sanukitoids” (Bruand et al., 2020). A similar observation can be made when looking at apatite Th/U v. La/Sm (Fig. 5C). With a both Th/U and La/Sm > 1, apatite investigated here is closer to chemical compositions of apatite from “mantle-derived granitoids”. Elevated Th/U in apatite indicates no crystallization of Th-rich phases such as monazite before apatite crystallisation. This observation is in line with our petrographic observations in which no monazite has been observed (Table 1 and Fig. 2). Lastly, the ternary diagram in Fig. 5D, which in addition to LREE and Y also includes Sr indicates that, according to compiled dataset of Bruand et al. (2020), chemical composition of the apatite investigated here is closer to the composition of apatite extracted from sanukitoids and I-type granitoids than from those from TTG and S-type granitoids.

## 4.3. Sr-isotope signature

Analyses of the two apatite populations were conducted along with several reference apatites: MRC and BRZ (Apen et al., 2022) and MAD (Thomson et al., 2012; Yang et al., 2014). Analyses of the reference apatites yielded values within uncertainty of the accepted Sr-isotopic ratios (see ESM for details). All apatite analyses have ⁸⁷Rb/⁸⁶Sr below 0.0008 (except analysis 2239_i1c_apatite5). An ⁸⁷Rb/⁸⁶Sr ≤ 0.0008 would affect the ⁸⁷Sr/⁸⁶Sr on the 5th digit after 3.2 GPa of ⁸⁷Rb radioactive decay. This is an order of magnitude more precise than the precision obtained during our LA-MC-ICP-MS experiments. As a consequence, ⁸⁷Sr/⁸⁶Sr data presented here are not age corrected (age correction of analysis 2239_i1c_apatite5 does not change the ⁸⁷Sr/⁸⁶Sr outside analytical uncertainty).

A total of 42 Sr-isotope analyses were obtained on apatite crystals from the Kaap Valley pluton sample. ⁸⁷Sr/⁸⁶Sr range from 0.70091 ± 0.00053 to 0.70382 ± 0.00044 (Fig. 6). The weighted mean of all 42 analyses returns Chi-squared statistics below 5 %, i.e. p(χ²) < 0.05, indicating that, at the analytical precision obtained in this study, the Sr-isotope signature of apatite from Kaap Valley is statistically heterogeneous. The lowest ⁸⁷Sr/⁸⁶Sr of our dataset (unradiogenic end-member) is within uncertainty of the bulk silicate Earth (BSE) value at the inferred crystallization age of this population of apatite (see below).

Analysed apatite domains from Salisbury Kop show a similar pattern, despite a more limited range in ⁸⁷Sr/⁸⁶Sr: 0.70074 ± 0.00100 to 0.70240 ± 0.00033 (Fig. 6). Here also, p(χ²) of the weighted mean is <0.05 indicating a statistical heterogeneity. As for the Kaap Valley sample, the Sr-unradiogenic end-member is within uncertainty of the bulk silicate Earth (BSE) value at the inferred crystallization age of this population of apatite.

## 4.4. U–Pb isotope geochronology

Apatite U–Pb isotope analyses were conducted with the same reference material as for Sr-isotope analyses. MRC apatite was used as calibration material while BRZ and MAD as validation materials. All provided intercepts similar to published ages (see ESM).

A total of 73 U–Pb isotope analyses were conducted on apatite from Kaap Valley (Fig. 7A). Since U–Pb analyses were not corrected for common-Pb, they are presented in a ²⁰⁷Pb/²⁰⁶Pb versus ²³⁸U/²⁰⁶Pb diagram (Tera and Wasserburg, 1972). If all analyses are considered, a discordia chord featuring a lower intercept at 3241 ± 93 Ma and a (²⁰⁷Pb/²⁰⁶Pb)initial of 1.081 ± 0.098 is obtained. MSWD and Chi-squared statistics indicate heterogeneity of this sample population at the
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Minor and trace element contents measured in apatite from Kaap Valley and Salisbury Kop. A. Y v. Ce binary diagram showing apatite contents obtained via laser ablation. Our new data are compared with previously published fields of Laurent et al. (2017) suggesting that apatite investigated here have chemical affinities similar to apatite from sanukitoids, the latter being interpreted as produced with the contribution of a mantle component (SCM). B. La v. Y binary diagram with our new data and previously published fields of Bruand et al. (2020). Based on published fields for apatite it is suggested that apatite from the two late-Paleo to Mesoarchean granitoids from the Kaapvaal craton investigated here have chemical affinities similar to apatite from sanukitoids. Note that diagrams A and B are virtually similar and thus as expected are pointing in the same direction. C. Th/U v. La/Sm binary diagram showing our new apatite data and fields of Laurent et al. (2017) again showing the similarity of our apatite with those extracted from mantle-derived granitoids. D. Ternary diagram and fields after Bruand et al. (2020) allowing the identification of igneous chemical composition based on apatite minor and trace element contents. Based on this diagram, our new apatite analyses indicate the Source rock is either sanukitoid in origin or an I-type granitoid. Geochemical diagrams produced on GCDKit (Janoušek et al., 2006).</figcaption></figure>

precision obtained. A qualitative data filtering approach can be applied to the full dataset. Ellipses that smear towards the right-hand-side of the regression line—interpreted has having experienced Pb-loss and/or a higher ²⁰⁷Pb/²⁰⁶Pb—can be eliminated. Similarly, those plotting clearly on the left-hand side of the main regression are excluded since they could be old cores (of unknown age) showing common-Pb. From there, a sub-set of 44 analyses (filled ellipses in Fig. 7) yields acceptable statistics, a lower intercept of 3243 ± 30 Ma and an upper intercept of (²⁰⁷Pb/²⁰⁶Pb)initial of 1.089 ± 0.031. Both approaches return upper and lower intercepts similar within uncertainty. In addition, this lower

intercept is within uncertainty of the U–Pb date obtained on zircon from the same pluton (Kamo and Davis, 1994; Zeh et al., 2009; Guitreau et al., 2012).

For Salisbury Kop, 59 apatite U–Pb analyses were conducted. If the full data set is considered, the lower intercept is 3114 ± 127 Ma while the intercept with the y-axis is 1.102 ± 0.059. This chord does not return statistics indicating homogeneity (p(χ²) < 0.05). A subset of 42 analyses provides a better constrain with acceptable statistics (p(χ²) = 0.06). This subset has been selected the same way as for apatite from Kaap Valley. The lower intercept of this discordia chord is 3109 ± 42 Ma while the
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Results of the apatite Sr-isotope analyses conducted in this study. A. $^{87}$Sr/$^{86}$Sr of apatite from Kaap Valley granitoid. Clearly, this population is not statistically homogeneous. Mean uncertainty: 0.00043. B. $^{87}$Sr/$^{86}$Sr of apatite from Salisbury Kop granitoid. Although less marked, this population is also statistically heterogeneous ($p(\chi^2) = 0.000022$) indicating here also a mixing between two end-members (see text for interpretation). Mean uncertainty: 0.00046. All uncertainties (figures and bars) are reported at 95 % confidence interval. Sr-isotopic composition of the BSE obtained with the initial $^{87}$Sr/$^{86}$Sr of Papanastassiou and Wasserburg (1968) and the Rb/Sr of McDonough et al. (1992). Diagrams constructed on IsoplotR (Vermeesch, 2018).</figcaption></figure>

upper intercept is 1.120 ± 0.022. As before, lower and upper intercepts of both approaches are similar within uncertainty indicating that although improving statistics, our data filtering does not alter the material conclusions that will be drawn from it. Lastly, the apatite U–Pb date retrieved here is also similar to the U–Pb date obtained on zircon from the same pluton (sample KK20 in Zeh et al., 2009).

# 5. Discussion

## 5.1. Significance of apatite U–Pb dates and apatite intra-crystalline diffusion

The trace element analyses obtained on apatite from the Kaap Valley and Salisbury Kop plutons show elevated U, Th, and LREE contents unlike the lower trace element contents typically observed in metamorphic apatite populations (Henrichs et al., 2018; O'Sullivan et al., 2018). Further, since apatite U–Pb analyses return dates that are within uncertainty of zircon U–Pb crystallization dates (Fig. 7 and Table 1) and that the latter are interpreted as reflecting emplacement ages (Kamo and Davis, 1994; Zeh et al., 2009; Guitreau et al., 2012), we consider that the apatite analyzed here record the same event as the zircon and hence are magmatic in origin. From there, we can assess whether the trace element contents in apatite reflect that of the igneous mineral. Compiled experiments in Cherniak (2010) show that REE diffusion in apatite is slower than Pb diffusion. Since Pb diffusion is limited in apatite and apatite U–Pb dates are indistinguishable from zircon U–Pb dates (even if considering the whole set of apatite U–Pb of analyses presented here), all interpreted as igneous crystallization ages (Fig. 7), we consider the trace element contents obtained here as representative of the magma from which apatite crystallized.

## 5.2. Significance of the observed range of $^{87}$Sr/$^{86}$Sr in apatite

The range of $^{87}$Sr/$^{86}$Sr in the apatite populations studied here (Fig. 6) is not dissimilar to the results presented in Caton et al. (2022). These authors attributed their observed range in apatite $^{87}$Sr/$^{86}$Sr to several potential causes. First, fluid-rock interaction which would increase Rb/Sr of apatite and hence—after time integration—the $^{87}$Sr/$^{86}$Sr. This option can be ruled out since only analyses with Rb/Sr << 0.005 are

presented here. In most cases, $^{85}$Rb signal is analogous to background signal (ESM). Second option presented in Caton et al. (2022): isotopic re-equilibration. The observations that (i) the apatite populations studied here return U–Pb dates that are within uncertainty of the igneous crystallization age and (ii) Sr diffuses at slower rates than Pb in apatite (Cherniak, 2010) indicate limited Sr isotopic re-equilibration in Kaap Valley and Salisbury Kop apatite. That said, since Sr-isotopes in apatite are measured at much greater precision than U–Pb isotopes (MC–SF–ICP–MS v. SC–Q–ICP–MS respectively, see Methods and ESM), minor diffusion of Pb would go unseen at the precision obtained (individual precision of ca. 2 %) while causing detectable deviation in $^{87}$Sr/$^{86}$Sr signature which have a typical individual uncertainty of ca. 0.065 % in this study. As a consequence, even if we regard the most radiogenic $^{87}$Sr/$^{86}$Sr as a consequence of assimilation of a continental crust end-member, elevation of $^{87}$Sr/$^{86}$Sr by secondary processes cannot be excluded with a sufficient degree of confidence. Consequently, only the lowest values of each dataset can robustly be used to extract geological information of the magma from which apatite crystallized. It flows from this interpretation that no geological correlation can be made between (i) apatite major/trace element contents/ratios and (ii) apatite $^{87}$Sr/$^{86}$Sr which are higher than the minimum value obtained here (see below and Fig. 6) since only the lowest apatite $^{87}$Sr/$^{86}$Sr are interpreted as reflecting an igneous signature.

In both apatite populations studied here, the Sr-unradiogenic end-member is within uncertainty of the BSE value at the crystallization age of the sample (Fig. 6), and only slightly (≈ 10 ε-units) more radiogenic than the 30 Myr-older Weltevreden ultramafic volcanic flows exposed at the southern end of Kaap Valley which show $^{87}$Sr/$^{86}$Srinitial of ca. 0.7000 (Barton et al., 1983; Vezinet et al., 2025). Therefore, we interpret the Sr-unradiogenic end-member identified in apatite (Fig. 6) as a component with BSE or more depleted composition. The unradiogenic Sr-isotope composition of apatite from Kaap Valley is in line with the radiogenic Hf-isotope composition of zircon (see Zeh et al. (2009) and Table 1) both indicate that component of the Kaap Valley granitoid was derived from the mantle shortly before this igneous body solidified. The opposite observation can be made for Salisbury Kop, however: apatite domains analysed show an unradiogenic Sr-isotope signature (i.e. juvenile, mantle-like) while zircon Hf-isotope compositions are sub-chondritic (see Zeh et al. (2009) and Table 1). The implication of this last
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Apatite U–Pb isotope analyses for Kaap Valley (panel A) and Salisbury Kop (panel B) in a Tera–Wasserburg diagram (Tera and Wasserburg, 1972). Note that for both apatite populations regression (discordia chord) of the sub-set (filled ellipses) returns upper and lower intercepts which are within uncertainty of the values obtained with the full dataset. Both lower intercepts are also within uncertainty of the U–Pb dates obtained on zircon in previous studies (Kamo and Davis, 1994; Zeh et al., 2009; Guitreau et al., 2012) and thus interpreted as igneous crystallization age. Ellipses and uncertainties in both lower and upper intercepts are reported at 95% confidence interval. The second uncertainty in the lower intercept (date) includes a 0.45% long-term in house variance of secondary validation material (Léger et al., 2024). Diagrams constructed on IsoplotR (Vermeesch, 2018).</figcaption></figure>

observation will be discussed below.

**5.3. Assessment of previous apatite trace element classification and limitations**

This work uses the earlier classification of Bruand et al. (2020) to identify the nature of the magma from which apatite crystallized. Hence, it is worth looking critically at this classification before moving ahead. The data presented in Bruand et al. (2020) reflect a range of WR compositions, all moderately to strongly felsic (SiO₂ > 65 wt%) including TTGs, sanukitoids, high Ba–Sr granites and adakites. They were able, based on apatite trace element contents, to connect apatite compositions and WR compositions. Bruand et al. (2020) proposed that the alumina saturation index (ASI) has a pivotal role in the solubility of apatite (positive correlation) and all TTG investigated in their study have ASI > 1. This contrasts with both the Kaap Valley and Salisbury Kop granitoids which have an ASI of ca. 0.9 (Table 1). Therefore, although remaining valid, the TTG field in Bruand et al. (2020) should be labelled “peraluminous TTG”. Our results indicate that apatite extracted from the two Archean granitoids we studied have ASI < 1 and trace element contents similar to those observed in apatite from sanukitoids, high Ba-Sr granite and Basalt-Andesite-Diorite-Rhyolite (BADR) suite (Figs. 4 and 5) despite being referred to as belonging to the TTG suite (Table 1 and references cited). The key observation here is that these 3 types of felsic rocks (sanukitoids, high Ba-Sr and BADR) all have, in their source, a direct contribution from the mantle. One way to interpret this observation is that apatite trace element chemistry, does not only reflect the host-rock chemical composition as previously suggested, but also records the contribution of the SCM in the source rock. One can say that the difference might be slight, yet, since some key elements in the definition of sanukitoids and high Ba-Sr granite such as Sr and LREE (e.g. Laurent et al., 2014), are strongly partitioned into apatite, this mineral phase is ideally suited to record—and preserve—variations in these elements. Consequently, we interpret the trace element pattern observed in apatite as indicating the, so far undocumented, direct contribution of the SCM to the source of both the Kaap Valley and Salisbury Kop granitoids. Apatite is thus a more precise tracer of such contribution

than whole-rock compositions.

Our apatite trace element results (Figs. 5 and 6), and those presented in earlier studies (e.g. Laurent et al., 2017; Bruand et al., 2020), show a range of trace element distribution in apatite populations from individual hand-specimens, that might be interpreted as mixing between mafic–ultramafic and felsic sources. Despite a growing amount of apatite trace element data in the literature, there is currently not enough data for robustly attributing a geological source to a geochemical end-member. Considering this limitation, such correlations are not conducted in this study.

**5.4. Petrogenesis of Paleo- to Mesoarchean felsic rocks of the Eastern Kaapvaal craton**

The new geochemical information presented in this study, combined with previously published geochemical information, both at the WR and mineral scale, obtained on these mid-Archean felsic igneous rocks enables us to further constrain their petrogenesis. Information extracted from liquid lines of descent (LLD) cannot be used in this work since the two hand-specimens investigated are phaneritic rocks and thus do not show the textural requirements for applying LLD (Rollinson and Pease, 2021). That said, the observation that apatite is found as inclusions within amphibole as well as biotite and shows an acicular habit in the thin section investigated (Fig. 2) suggests that apatite is a primary, igneous, mineral (Piccoli and Candela, 2002). Further, the trace element pattern in apatite is similar between Kaap Valley and Salisbury Kop (Figs. 4 and 5), two igneous bodies showing distinct mineral assemblages and hence a distinct WR geochemistry. This suggests that although differences in trace element contents can be affected by local crystallization of other minerals (e.g. amphiboles, REE-rich accessory phases), the global geochemical signal of low-Y and high-Sr, high-LREE (Fig. 5) reflects the composition of the source of the magma(s) from which apatite crystallized rather than local, small scale, petrological processes during magma evolution. Importantly, we interpret the LREE enrichment and high LREE/HREE observed in Kaap Valley and Salisbury Kop apatite (Figs. 4 and 5) as the result of the lack of monazite crystallization in the magma before apatite formed due to an ASI < 1 (Chu
et al., 2009; Bruand et al., 2020). Lastly, in the petrogenetic model suggested below, no recycled sediments are involved. If sediments were indeed involved, they would likely pull the zircon Hf-isotope signature towards negative values, which is not observed, zircon Hf-isotope signature of Kaap Valley is > +2 and that of Salisbury Kop is at -0.8

± 1.1, see Table 1. Therefore, if sediments were present, they were either volumetrically insignificant and/or showing juvenile Hf (and Sr) signature.

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Schematic cross-sections of the Eastern Kaapvaal craton by the late-Paleoarchean/early Mesoarchean (ca. 3.2–3.1 Ga), redrawn and modified from Schoene and Bowring (2010). This schematic has been constructed using interpretation of data from this study and previous works (Smithies, 2000; Martin et al., 2005; Zeh et al., 2009; Schoene and Bowring, 2010; Connolly et al., 2011; Zeh et al., 2013b; Vezinet et al., 2025). Sketches not to scale.</figcaption></figure>
### 5.4.1. Kaap Valley petrogenesis

The WR chemistry of the Kaap Valley *tonalite*, more mafic and enriched in LREE than other igneous bodies of the same age or older in the area, has long been interpreted as indicating a source produced via mixing between a trondhjemitic magma and a mafic component (Condie and Hunter, 1976; Robb et al., 1986). The former study suggests that “Kaap Valley has partially digested parts of the Onverwacht [now Weltevreden] mafic and ultramafic volcanics” while the latter suggests that the mafic component could be “the residue to partial melting of a typical greenstone assemblage”, see also Fig. 8. More recently Smithies (2000) has suggested that the unusual geochemistry of Kaap Valley tonalite results from interaction TTG melt and mantle and interpreted Kaap Valley tonalite as an Archean analogue to Cenozoic adakites. Either way, it seems there is a consensus on the hybrid nature (crust + mantle) of Kaap Valley tonalite. Our interpretation based on both trace element and Sr-isotope analyses of Kaap Valley apatite align with these earlier claims, indicating that igneous apatite robustly records the direct contribution of a mantle component in the magmas from which crystallization occurs. An interesting aspect though is that in situ U-Pb/Hf isotope data of zircon from Kaap Valley, interpreted to be igneous, return an age of 3.23 Ga with a supra-chondritic, and a range of initial Hf-isotope: $ε^{176}\text{Hf}_{\text{initial}} = +2.1 ± 0.2$ to $+2.9 ± 0.2$, see Zeh et al. (2009); Guitreau et al. (2012) and Table 1. This signature is interpreted by Zeh et al. (2009) as indicative of new felsic crust production at ca. 3.23 Ga, formed through partial melting of subducting slab itself made of mafic rocks originating from partial melting of a chemically depleted mantle domain. The unradiogenic Sr component, close to the Sr-isotope signature of the depleted mantle (DM) at ca. 3.2 Ga, is interpreted to be the same as the source of the supra-chondritic signature of zircon reported by Zeh et al. (2009) and Guitreau et al. (2012).

Altogether, the key geochemical features of Kaap Valley magma are: SiO₂-rich (ca. 65 wt% or >), elevated maficity (FeO + MgO + TiO₂ + MnO > 5 wt%), aluminum saturation index (ASI) ≈ 0.9, Mg# > 0.5, high V and Cr contents (ca. 50 µg/g each), high Ba and Sr contents (up to ca. 500 µg/g each), no Eu anomaly, apatite trace element contents analogous to those from I-type or sanukitoid host rocks, suprachondritic Hf-isotope ($ε^{176}\text{Hf}_{\text{zircon}}$ ≈ +2.5) and unradiogenic apatite Sr-isotope (see also Table 1). With respect to the geochemical evidence accumulated here and previously, we concur with Moyen et al. (2024) that Kaap Valley *tonalite* “does not (or barely) fulfill the defining criteria for TTGs”; here we go further and propose that the 3.24 Ga Kaap Valley granitoid is, in fact, intermediate between TTG and *sanukitoid s.l.* end-members (following the geochemical classification of Laurent et al., 2014) and hence reflects a first unequivocal stage of cratonization in the eastern Kaapvaal craton as soon as ca. 3.24 Ga.

Consequently, for Kaap Valley, the geochemistry of apatite aligns well this the whole-rock geochemistry as expected, both indicating the direct contribution of a mantle component, mixing with the melt produced via partial melting of the subducting slab before 3.24 Ga (Fig. 8).

### 5.4.2. Salisbury Kop petrogenesis

More interesting is the geochemical signature retrieved in apatite from Salisbury Kop granitoid. Unlike Kaap Valley, geochemical features of this pluton have not been interpreted to include a mantle component in its source, see Table 1 and Condie and Hunter (1976), the latter interpreting Salisbury Kop granitoid as anorogenic. In situ zircon U-Pb/Hf isotope data from Salisbury Kop granitoid show an upper intercept discordia date of ca. 3.10 Ga, interpreted as crystallization age and a corresponding zircon $ε^{176}\text{Hf}_{\text{initial}} = -0.8 ± 1.1$, i.e. chondritic to slightly sub-chondritic, see Zeh et al. (2009) and Table 1. These authors interpret this zircon Hf-isotope signature as indicating intra-crustal reworking.

This interpretation is supported by the presence of inherited zircon in the Nelspruit batholith, yielding 3.21 Ga to 3.24 Ga dates and sub-chondritic Hf-isotope signature, hence distinct from the Hf-isotope signature obtained in Kaap Valley zircon. The Sr-isotope signature of apatite from Salisbury Kop (Fig. 6) gives overall a similar information as

apatite from Kaap Valley, i.e. an unradiogenic Sr-isotope signature. This observation is in line with the trace element pattern obtained here on apatite from Salisbury. All analyses show trace element signatures similar to Kaap Valley apatite and hence similar to sanukitoid-like and I-type granitoids. Following our interpretation of the trace element pattern of apatite from Kaap Valley, we interpret that apatite from Salisbury Kop granitoid records a direct contribution of the SCM in the parental magma of Salisbury Kop. The chemical signature reflects mixing between a TTG melt and a mantle component, such as Kaap Valley (Fig. 8).

The contribution of SCM component in the source of Salisbury Kop magma is, likely, volumetrically less important than that inferred for Kaap Valley since it hardly affects the WR geochemical signature, yet it remains unambiguous when looking at apatite trace element patterns (Figs. 4 and 5). We argue, based on trace element contents in apatite, that the generation of Salisbury Kop magma, and likely the cogenetic Nelspruit batholith, includes a SCM component. Consequently, the generation of Salisbury Kop magma was involved in the development of a chemically depleted sub-continental mantle (cratonization) under the (proto)-eastern Kaapvaal craton. This event favored the protracted preservation of the eastern Kaapvaal craton at Earth surface. Since the Nelspruit batholith is one of the biggest igneous bodies in the eastern Kaapvaal craton, even a small SCM contribution has likely played a central role in the overall chemical depletion-and hence preservation-of the SCM under the proto-eastern Kaapvaal craton.

### 5.5. Implications for the cratonization of Archean cratons worldwide

Archean cratons, which are largely made of granitoids and their high-grade equivalents, form the nucleus adjacent to which Proterozoic and then Phanerozoic belts grew. Hence, they represent the primordial building blocks of present-day continents onto which the sub-aerial life as we know it today developed. Identifying the geological processes that favored the ongoing persistence of these building blocks at the Earth's surface and pin these processes onto the geological time scale therefore lies at the center of our understanding of early Earth lithospheric evolution. Since the formation-petrogenesis-of Archean continental crust, is intimately coupled with the lithosphere underneath (Pearson et al., 2002; Rapp et al., 2010), therefore deciphering the geological processes at play for Archean continent formation necessitates to look at the lithospheric scale.

In the Kaapvaal craton, mantle melting is evidenced by the presence of mafic-ultramafic igneous rocks in greenstone belts sporadically exposed in the Kaapvaal craton (Brandl et al., 2006). Among those rocks are komatiites, first identified in the eastern Kaapvaal (Viljoen and Viljoen, 1969), with MgO contents in inferred parental magmas up to 32 wt% (Robin-Popieul et al., 2012) testifying to emplacement of high-temperature magmas, up to 1530 °C (Sobolev et al., 2016), in this area during the Paleo-Archean. The presence of such lavas is largely interpreted as indicating a mantle plume activity in the eastern Kaapvaal craton at ca. 3.48 and ca. 3.26 Ga (Condie and Hunter, 1976; Armstrong et al., 1990; Connolly et al., 2011; Blichtert-Toft et al., 2015; Sobolev et al., 2016). These high-temperature ultra-mafic lavas are juxtaposed with felsic igneous rocks, chiefly belonging to the TTG suite, see Fig. 1 and Robb et al. (2006). Komatiite formation does indicate mantle melting, produced through deep mantle-plume activity (Herzberg, 1992; Shimizu et al., 2001; Arndt et al., 2008; Kamber, 2014; Sobolev et al., 2016; Sossi et al., 2016; Sobolev et al., 2019; McKenzie, 2020). Although this process does produce a melting residue matching the geochemical features of xenoliths sampling cratonic lithosphere (Arndt et al., 2009), there is little felsic, continental like, igneous activity associated with mantle plume activity. Hence, mantle plume are not (the only) drivers of chemical depletion of SCM required to form stable, long-lived, keels that have survived geological ages. Another mechanism must account for chemical depletion of the SCM. We argue that the most favourable configuration to juxtapose a crustal component (such as TTG
melt) and (metasomatized) mantle is in a subduction-like type of geometry (Fig. 8). First, the chemical changes in the ‘mantle-wedge’ as a result of burial—and partial melting—of young oceanic lithosphere giving rise to TTG rocks during the early Archean. Second, with time, the proportion of metasomatic mantle increases and hence its incorporation into newly formed TTG melts is now affecting the geochemistry of the melts, visible in apatite geochemistry as shown here. This event marks the onset of cratonization, in the late Paleo-Archean in the case of the eastern Kaapvaal craton.

## 6. Conclusion

We showed that the combination of in-situ U-Pb, trace element contents and Sr-isotope geochemistry of apatite can provide pivotal information on the petrogenesis of (Archean) igneous rocks, some of which is not robustly recorded at the WR scale or in zircon. This is used here to identify direct SCM contribution in the petrogenesis of TTGs from the Archean Kaapvaal craton. Our results indicate that chemical depletion of the SCM occurred as early as the Paleozoarchean in the Kaapvaal craton, i.e. >0.5 Ga before generation and emplacement of sanukitoids and >0.1 Ga before emplacement of potassic granites, both usually seen as indicator of cratonization. Vertical tectonics, such as continued resurfacing, sagduction and stagnant lid can hardly account for the geochemical observations obtained on ca. 3.2 Ga apatite from the eastern Kaapvaal craton since juxtaposition of TTG melts and mantle rarely occurs in these geometries. Our observations better fit a dynamic framework of horizontal tectonics, possibly induced by a mantle plume, for which activity is documented only few tens of Ma earlier in the area. Lastly, apatite trace element geochemistry is a valuable tool to unravel Archean continental crust petrogenesis and particularly in the recognition of direct contribution of mantle components that have generated large volumes of melts, driving depletion—and hence—stabilization of the SCM as well as emplacement of large volumes of low density, unsinkable, rocks at mid- to upper crustal levels.

### CRediT authorship contribution statement

**Adrien Vezinet:** Writing – original draft, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Javiera Flores-Rojas:** Writing – review & editing, Validation, Methodology, Investigation, Data curation. **Alexander V. Sobolev:** Writing – review & editing, Validation, Supervision, Investigation, Conceptualization. **Julien Léger:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Aleksandr V. Chugunov:** Writing – review & editing, Validation, Investigation, Data curation. **Valentina G. Batanova:** Writing – review & editing, Validation, Methodology, Investigation, Data curation. **Marlina A. Elburg:** Writing – review & editing, Validation, Investigation, Data curation. **Axel Hofmann:** Writing – review & editing, Validation, Investigation, Data curation. **Mélanie Balvay:** Writing – review & editing, Data curation. **Nouméa Paradis:** Writing – review & editing, Data curation.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The first two authors contributed equally to this work. Chris Hawkesworth and an anonymous reviewer are thanked for constructive criticism. Victoria Pease is thanked for efficient editorial handling. Mineral separation and mounting were conducted in the Geo-Thermochronology (GTC) facility of ISTerre-Grenoble. Colette Guilbaud (LGL-

TPE, Saint Etienne, France) is thanked for providing high quality thick sections. Emilie Bruand, John W. Valley and Steve S. Sobolev are thanked for fruitful discussions. A.H. thanks the Mpumalanga Tourism and Parks Agency and Sappi Forests for access and support. This project is supported by a grant of the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Synergy Grant MEET, grant agreement no.856555) to A.V. S., Auvergne-Rhône-Alpes region (2020 AURA P3 - CPER 2015/2020) and CNRS-INSU.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.precamres.2025.107927.

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