# Earth's earliest granitoids are crystal-rich magma reservoirs tapped by silicic eruptions

Oscar Laurent, Jana Björnsen, Jörn-Frederik Wotzlaw, Simone Bretscher, Manuel Pimenta Silva, Jean-François Moyen, Peter Ulmer, Olivier Bachmann

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## To cite this version:

Oscar Laurent, Jana Björnsen, Jörn-Frederik Wotzlaw, Simone Bretscher, Manuel Pimenta Silva, et al. Earth's earliest granitoids are crystal-rich magma reservoirs tapped by silicic eruptions. Nature Geoscience, 2020, 13 (2), pp.163-169. 10.1038/s41561-019-0520-6. hal-02934806

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# Earth's earliest granitoids are crystal-rich magma reservoirs tapped by silicic eruptions

Oscar Laurent<fnref n="1" />*, Jana Björnsen<fnref n="1" />, Jörn-Frederik Wotzlaw<fnref n="1" />, Simone Bretscher<fnref n="1" />, Manuel Pimenta Silva<fnref n="1" />, Jean-François Moyen², Peter Ulmer<fnref n="1" /> and Olivier Bachmann<fnref n="1" />

Granitoids of the tonalite-trondhjemite-granodiorite (TTG) series dominate Earth's earliest continental crust. The geochemical diversity of TTGs is ascribed to several possible geodynamic settings of magma formation, from low-pressure differentiation of oceanic plateaus to high-pressure melting of mafic crust at convergent plate margins. These interpretations implicitly assume that the bulk-rock compositions of TTGs did not change from magma generation in the source to complete crystallization. However, crystal-liquid segregation influences the geochemistry of felsic magmas, as shown by the textural and chemical complementarity between coeval plutons and silicic volcanic rocks in the Phanerozoic Eon. We demonstrate here that Paleoarchean (ca. 3,456 million years old) TTG plutons from South Africa do not represent liquids but fossil, crystal-rich magma reservoirs left behind by the eruption of silicic volcanic rocks, being possibly coeval at the million-year scale as constrained by high-precision uranium-lead geochronology. The chemical signature of the dominant trondhjemites, conventionally interpreted as melts generated by high-pressure melting of basalts, reflects the combined accumulation of plagioclase phenocrysts and loss of interstitial liquid that erupted as silicic volcanic rocks. Our results indicate that the entire compositional diversity of TTGs could derive from the upper crustal differentiation of a single, tonalitic magma formed by basalt melting and/or crystallization at <40 km depth. These results call for a unifying model of Hadean-Archean continent nucleation by intracrustal production of TTG magmas.

The backbones of Earth's first continents are granitoids of the tonalite-trondhjemite-granodiorite (TTG) series<fnref n="1" />. Deciphering the petrogenesis of TTGs is therefore a key issue to understand the growth and chemical evolution of early crust². Although there is consensus that TTG magmas result from melting and/or crystallization of hydrous, low-potassium (low-K) basalts<fnref n="1" />, their geodynamic setting of formation is still disputed. This is particularly true for TTGs emplaced more than 3 billion years ago and forming Earth's first continental nuclei, since the nature and even the existence of plate tectonics at that time remain elusive²,³. Actualistic hypotheses consider that these TTGs could have formed at convergent plate margins by melting of over-thickened arc roots⁴,⁵, subducted oceanic crust⁶ or fractionation of hydrous arc basalts;⁷,⁸ whereas non-uniformitarian scenarios call on the projected differentiation of thick basaltic plateaus above long-lived mantle upwellings³,⁹,¹⁰.

Much of the debate about the geodynamic context of TTG formation lies in the significance of their geochemical diversity. They indeed define distinct groups, delineated by the variations of elements controlled by minerals having a first-order pressure-sensitive behaviour in mafic systems:<fnref n="1" />,¹⁰ plagioclase (aluminum, sodium, strontium, europium), garnet (the heavy rare earth elements, yttrium) and rutile (niobium, tantalum). This diversity would therefore reflect magma genesis at very different pressures¹¹, from <1.0 GPa for 'low-pressure' (LP-) TTGs to 1.0–1.5 GPa for 'medium-pressure' (MP-) TTGs and ≥1.5 GPa for 'high-pressure' (HP-) TTGs. While the source region of LP- and MP-TTGs would have been located within the primitive Archean mafic crust (20–40 km thick; ref. ¹²), the HP-TTGs apparently require the burial of mafic rocks to mantle depths, along geothermal gradients comparable to 'hot' subduction⁶,¹¹. This conflicts with the results of thermo-mechanical

models showing that subduction does not occur at early Archean ambient mantle temperatures¹³, that is, >200 K hotter than today¹². For these reasons, some studies proposed melting of delaminated mafic crust to explain the HP-TTGs⁹, with the drawback that delamination is promoted in the case of refractory and anhydrous lower crust¹⁴ while TTGs require a fertile, hydrated mafic protolith.

The current discussion about TTG formation implicitly considers that their chemical compositions represent those of magmatic liquids so that any chemical variation they harbour translates into variable source conditions. However, the geochemistry of granitoids also reflects processes occurring during ascent and emplacement, such as crystal fractionation, accumulation¹⁵,¹⁶ and melt segregation to form coeval, silicic volcanic rocks¹⁷. Earlier studies may have overlooked such processes because TTGs do not commonly preserve magmatic structures owing to postemplacement deformation and/or metamorphism¹¹. Moreover, in modern systems, studying coupled plutonic and volcanic systems has proven essential to understand the geochemistry of silicic igneous rocks¹⁷. Although TTGs may coexist with silicic volcanic rocks and be related to them by crystal fractionation¹⁸, there is so far no detailed textural, geochemical and high-resolution geochronological comparison between them.

## The Paleoarchean Barberton TTGs and felsic volcanic rocks

Some of the Paleoarchean (ca. 3,456 million years (Myr) old) TTGs south of the Barberton Greenstone Belt (Kaapvaal Craton, South Africa) form well-preserved plutons¹⁹ (Stolzburg, Theespruit; Extended Data Fig. 1) temporally associated¹⁸,²⁰⁻²² with the felsic (sub-) volcanic and volcaniclastic rocks of the Buck Ridge Volcanic Complex (BRVC). Both were emplaced in the Onverwacht Group, the lowermost stratigraphic unit of the Barberton Greenstone

<footnote n="1">¹ETH Zürich, Department of Earth Sciences, Institute for Geochemistry and Petrology, Zürich, Switzerland. ²Université de Lyon, Université Jean Monnet, Laboratoire Magmas et Volcans (UMR6524-CNRS-IRD), Saint-Étienne, France. *e-mail: oscarlaurent86@gmail.com</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | The Paleoarchean Barberton TTG plutons and BRVC volcanic rocks are, respectively, crystal-rich and liquid-rich magmatic units. **a** The same region of a trondhjemite sample (JB17-C2a) imaged by optical microscopy with crossed polars (panel 1) and SEM-EDS spectral images, including phase map (panel 2) and plagioclase compositional map (panel 3). In panel 2, some plagioclase phenocryst cores altered to sericite are incorrectly assigned as K-feldspar. **b** Photomicrograph showing the phyric texture of a volcanic sample from the BRVC (JB17-F8) with altered groundmass (gm), plagioclase (Pl) and pristine quartz (Qz) containing melt inclusions (arrows). **c** Photomicrographs showing representative examples of the BRVC melt inclusions, unheated (top row) and after homogenization (bottom row; gl = glass; b = gas bubble). **d** Normative albite(Ab)-quartz(Q)-orthose(Or) triangle showing the normative composition of the BRVC melt inclusions compared with the H₂O-saturated temperature minima in the haplogranitic system⁵⁰ and high-silica rhyolitic glass from modern ignimbrites (compiled by ref. ⁵¹).</figcaption></figure>

Belt<sup>21,23-25</sup> made up of komatiites, basalts, minor felsic volcanoesedimentary rocks and their metamorphic counterparts (Extended Data Figs. 1 and 2). In the plutons, deformation is restricted to the margins and a southernmost 'gneissic' zone (Extended Data Fig. 1), while magmatic structures are well preserved in the pluton's intermediars<sup>19,26,27</sup> (Extended Data Fig. 2). These rocks thus offer the unique opportunity to investigate the role of crystal-liquid segregation in TTG suites and their relationships with silicic volcanism. For this purpose, we studied them through a combination of petrography, major- and trace-element geochemistry, high-precision uranium-lead dating and hafnium (Hf) isotopic analyses of accessory zircon.

The Paleoarchean Barberton plutons dominantly consist of trondhjemite, with minor diorite, tonalite and granite<sup>20,26</sup>, the latter forming dykes or small plutons (Extended Data Figs. 1 and 2). These rocks share a common mineral assemblage of plagioclase + quartz + biotite ± amphibole, K-feldspar and accessory minerals (apatite, epidote, titanite, zircon; occasional iron-titanium (Fe-Ti) oxides and garnet) in various proportions<sup>26</sup> (Extended Data Fig. 3). The undeformed trondhjemites show an ortho-cumulative texture<sup>21,26</sup> defined by inclusion-free, euhedral phenocrysts (1–5 mm) of normally zoned plagioclase (anorthite (An)<sub>20–30</sub> cores to An<sub>15–20</sub> rims) and minor amphibole (Fig. 1a). The interstitial domain consists of smaller (<1 mm) anhedral grains defining a granitic mineral assemblage, that is, quartz, unzoned calcium-poor plagioclase (An<sub>10–15</sub>), K-feldspar and variably chloritized biotite (Fig. 1a).

The BRVC volcanic rocks feature feldspar and quartz phenocrysts in devitrified groundmass<sup>25</sup> (Fig. 1b). These rocks underwent strong hydrothermal alteration<sup>21,24</sup> but melt inclusions hosted by pristine quartz (Fig. 1c) preserve the original melt compositions<sup>25</sup>. These are minimum granitic melts, compositionally overlapping with interstitial rhyolitic glass and melt inclusions from modern ignimbrite eruptions (Fig. 1d and Extended Data Fig. 4).

## Zircon evidence for a plutonic-volcanic connection

Zircon geochronology and Hf isotopic data support that the plutonic and volcanic rocks are coeval and genetically related. In all rock types, combined zircon uranium-lead (U-Pb) and lutetium- (Lu-) Hf isotopic data analysed by laser ablation (multi-collector) induc-tively coupled plasma mass spectrometry (LA-(MC-)ICP-MS) are consistent with a single population of zircon crystallized 3,456 Myr ago (Ma) that underwent variable extents ca. 3,200 Ma and recent lead loss (Extended Data Fig. 5). The age of ca. 3,200 Myr corre-sponds to a regional metamorphic event<sup>28</sup> that is recorded by apa-tite and titanite thermochronology in the Stolzburg pluton<sup>29,30</sup> and could therefore have induced incomplete lead loss in the zircons.

To circumvent this, we dated single zircon crystals and fragments from two trondhjemites, one diorite and one volcanic rock after chemical abrasion by isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS). The results show > 97% concordance and a spread of <sup>207</sup>Pb/<sup>206</sup>Pb dates between 3,457 and 3,448 Ma
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 2** | Zircon isotopic analyses showing that the Paleoarchean Barberton TTG plutons and silicic volcanic rocks are coeval and co-genetic.</figcaption></figure>

a. Wetherill concordia diagram showing U-Pb isotopic results of CA-ID-TIMS zircon analyses. The grey band along the concordia indicates uranium decay constant uncertainties. b. Plot of CA-ID-TIMS zircon $^{207}$Pb/$^{206}$Pb dates and weighted mean ages from samples of the Theespruit pluton and BRVC. c. Zircon Hf isotopic composition reported as $\epsilon$Hf calculated at 3,456 Ma; small symbols are individual analyses, and large symbols are average sample compositions. The red line indicates the median value of the entire dataset, and the red shaded area contains 90% of the data. The grey bars depict the range of $\epsilon$Hf$_{3,456 \text{ Ma}}$ for older country rocks: the >3,600 Myr old Ancient Gneiss Complex$^{52}$ (AGC), the ca. 3,530 Myr old Theespruit and Sandspruit formations$^{22,33}$ (T-SF) and the ca. 3,560 Myr old Moodies granite clasts$^{53}$ (MG). Error bars and ellipses indicate uncertainties at the 95% confidence level. MSWD, mean square weighted deviation.

(Fig. 2a). The youngest $^{207}$Pb/$^{206}$Pb dates could still be attributed to partial Pb loss ca. 3,200 Ma and were mostly obtained from the Stolzburg trondhjemite. Excluding this sample, 17 of 22 zircon fractions show statistically indistinguishable $^{207}$Pb/$^{206}$Pb dates between 3,456.74 ± 0.49 Ma and 3,455.80 ± 0.88 Ma (Fig. 2b).

This result implies that some of the Theespruit granitoids and BRVC volcanics crystallized within less than 1 Myr of each other, matching the typical lifetimes of the shallow silicic magma reservoirs that feed modern explosive eruptions$^{31}$. Most (eight out of ten samples) of the recent literature dating results$^{22,23,30,32,33}$ are within uncertainty identical to our new CA-ID-TIMS ages, indicating that the entire plutonic-volcanic system emplaced within

less than ~ 10 Myr, like modern crustal-scale plutonic systems$^{34}$. In terms of Hf isotopes, ~90% of the data from all samples span a narrow range of $\epsilon$Hf$_{3,456 \text{ Ma}}$ from +2 to -1, which supports their co-genetic character because this is on average more radiogenic than older rock types of the area at this time (from +1 to -8; Fig. 2c). Rare outliers show more negative values (Fig. 2c), suggesting minor contamination by such older rocks on emplacement.

## Formation of all TTGs by differentiation of a single magma

Geochemical data (Fig. 3) demonstrate that the trondhjemites, on one hand, and the BRVC silicic volcanic rocks, on the other hand, are, respectively, crystal- versus liquid-rich complementary magmatic units fractionated from the same evolved liquid (EL). The major- and trace-element compositions of the trondhjemites spread between those of the BRVC melt inclusions and a plagioclase-rich solid assemblage (Fig. 3a–d and Extended Data Fig. 6). Therefore, they are unlikely those of pure liquids and rather represent fossil crystal-rich magmas (mushes) characterized by variable respective proportions of cumulative plagioclase and residual granitic liquid, in line with textural evidence (Fig. 1). Least-square mass-balance fitting of the trondhjemite compositions by crystals and liquid represented by the BRVC melt inclusions (see Methods for details) confirms this and points to former crystal fractions of ~40–60 wt% (Fig. 3a,b). This crystal fraction overlaps with the typical crystallinities defining the rheological transition from magma to mush$^{35}$ (45–65%), thereby favouring solid compaction and residual liquid extraction in plutonic systems$^{36}$.

Relative to the trondhjemites, the BRVC melt inclusions are markedly depleted in elements compatible with plagioclase (strontium, europium), and to a lesser extent amphibole (titanium, nickel) and zircon (zirconium), and enriched in the other, incompatible, trace elements (cesium, rubidium, thorium, uranium, niobium, rare earth elements) (Fig. 3e). They represent therefore the residual liquid extracted from the underlying, complementary trondhjemite mush, the former of which made its way to the surface and erupted as the BRVC. In this model, dykes and small plutons of granite correspond to zones of collection and accumulation of the extracted liquid at depth, crystallized along with small amounts of cumulative plagioclase and biotite (Extended Data Figs. 2, 3 and 6). The sub-volcanic rocks of the BRVC show a plagioclase-cumulative texture$^{21}$ (Extended Data Fig. 2) and overlap in composition with the trondhjemites (Fig. 3a and Extended Data Fig. 6). They would either represent very shallow equivalents of the latter or result from the rejuvenation and remobilization of the plutonic mush, as proposed for modern phenocryst-rich ignimbrites$^{37}$. The presence of both crystal-rich and crystal-poor volcanic rocks, the latter being chemically similar to interstitial melt in coeval and co-genetic plutonic rocks, is one hallmark of recent ignimbrite eruptions purging shallow silicic magma chambers$^{17,38}$.

We used stochastic modelling to determine the most plausible composition of EL. Details about the modelling are provided in Methods and Supplementary Information. The major-element composition of EL was retrieved by mass balance, by 'remixing' the pure residual liquid end member (represented by the BRVC melt inclusions) and the trondhjemites assuming a possible range of relative mass fraction between volcanic rocks and trondhjemites. The trace element composition of EL was modelled by fractional crystallization calculations, using the phase proportions in trondhjemites to calculate partition coefficients. As expected, the composition of EL is on average more silicic (average $SiO_2$ = 73.3 wt%) and potassic (average $K_2O$ = 2.4 wt%) than that of the trondhjemites and TTG in general (Fig. 3a,b). The two compositional fields partly overlap, suggesting that some trondhjemite samples may approximate liquid compositions. Nevertheless, the compositional trend of the trondhjemites clearly departs from EL towards the plagioclase end member, that is, to higher $Na_2O$, Sr/Y ratio and lower $K_2O$, yttrium, niobium
a

<figure><img src="image_7.png" /></figure>

b

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c

<figure><img src="image_9.png" /></figure>

d

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e

<figure><img src="image_11.png" /></figure>

**Fig. 3** | Geochemical evidence that the compositional range of TTGs can be explained by crystal-liquid segregation associated with the differentiation of a single parental liquid. **a–d**, Diagrams of K₂O versus SiO₂ (**a**), Na₂O versus FeO_t (**b**), Sr/Y versus Y (**c**) and Nb versus Ta (**d**) showing the compositions of bulk samples and homogenized BRVC melt inclusions (Lit., literature data<sup>8,20,25–27</sup>; New, this study) together with amphibole (Am) and plagioclase (Pl) in the plutons. The compositions of Pl and EL were obtained by stochastic mass-balance modelling (Methods). The coloured fields contain 95% of the trial runs; darker shading means higher point density; stars indicate the average composition. The fields for TTG groups are after ref. <sup>11</sup>. The top-right inset in **b** shows the results of least-square fitted mass balance of phase proportions in bulk trondhjemite samples (Methods; ΣR<sup>2</sup> = sum of squared residuals). **e**, Box-and-whisker plot (horizontal line indicates the median; boxes indicate first to third quartiles) of the distribution of selected element contents in the BRVC melt inclusions (this study and ref. <sup>25</sup>) normalized to average trondhjemites.

and tantalum (Fig. 3a–d). This trend is the defining geochemical signature of HP-TTGs<sup>11</sup> (Fig. 3a–d and Extended Data Fig. 7), which may therefore be viewed as a plagioclase accumulation feature rather than a primary compositional signal of melt formed at high pressures.

Previous studies have either overlooked the diorites and tonalites from the Barberton TTG plutons or ascribed them to the assimilation of wallrock greenstone lithologies by trondhjemitic magma<sup>26,27</sup>. However, their igneous textures, co-magmatic relationships with the trondhjemites (Extended Data Figs. 2 and 3) and chemical compositions (Extended Data Fig. 6) rather indicate that they belong to the plutonic–volcanic system. Compared with the trondhjemites, the diorites and tonalites contain greater proportions of amphibole and calcium-richer plagioclase (Extended Data Fig. 3). We thus propose that they crystallized from a less-evolved, parental melt (PL) from which EL was produced through crystal fractionation (Fig. 3). The composition of PL, modelled using the same approach as EL (see Methods for details), is tonalitic and matches the major- and

trace-element signature of global LP- and MP-TTGs (Fig. 3a–d and Extended Data Fig. 7). The tonalites (>60 wt% SiO₂) could directly represent PL mingled with the resident trondhjemitic mush (Fig. 3a–d and Extended Data Fig. 2). The silica-poorer diorites (<60 wt% SiO₂) plausibly represent amphibole + plagioclase cumulate compositions formed out of PL (Fig. 3a,b and Extended Data Fig. 6).

These results entail that the compositional diversity of TTGs does not necessarily reflect different pressures of magma generation and a corresponding variety of geodynamic scenarios. The upper crustal differentiation of a single tonalitic magma through plagioclase- and amphibole-dominated fractionation combined with variable crystal–liquid unmixing equally explain it (Fig. 4). The parental magmas are compositionally similar to the LP-/MP-TTGs<sup>11</sup>, while the geochemical fingerprints of the HP-TTGs mirror crystallization from plagioclase-laden regions of magma reservoirs feeding genetic, silicic volcanic eruptions (Fig. 4). In this model, the distinctively variable La/Yb and Sr/Y ratios of TTGs<sup>11</sup> reflect a progressive increase with liquid evolution owing to amphibole fractionation
<figure><img src="image_12.png" /><figcaption>**Fig. 4** | Two possible geological interpretations of the geochemical diversity of Archean TTGs. **a**, Existing models of TTG magma genesis by melting of basalts at various depths, considering that the bulk-rock geochemistry of TTGs reflects those of liquids (modified after ref. ²). **b**, Alternative model proposed here, whereby the whole compositional spectrum of Archean TTGs is produced by crystal fractionation/ accumulation in a mid-/upper- crustal silicic mush and liquid extraction to form silicic volcanic rocks, all resulting from the differentiation of a single, tonalitic parent liquid formed in the lower crust.</figcaption></figure>

and/or plagioclase accumulation (Fig. 3c and Extended Data Fig. 7), as recently proposed on the basis of the study of Neoarchean TTG from the North China Craton³⁹. This possibly rules out the involvement of garnet at any stage of TTG petrogenesis and provides a straightforward explanation to the spatial and temporal association of LP-, MP- and HP-TTGs in grey gneiss units⁴⁰,⁴¹ without calling on the contemporaneous melting of mafic rocks at very different depths. More generally, our results highlight that uncovering the origin of silicic magmas emplaced in the upper crust requires first filtering out the geochemical signal resulting from crystal-liquid segregation.

## Implications for early Archean crust-forming processes

Our findings suggest that Archean silicic plutonic and volcanic rocks are connected by similar crystal-liquid segregation

processes as are Phanerozoic ones¹⁷,³⁶,³⁸. Yet the trondhjemites interpreted here as fossil plagioclase-rich mushes carrying a misleading HP-TTG signature are much rarer in the modern Earth than in the Archean Earth. We propose that this results from first-order differences in magma composition. First, the calculated parental liquids to TTG suites have lower K₂O/Na₂O than modern continental arc (medium- to high-K calc-alkaline) magmas, so that the former would form K-poorer cumulates (trondhjemites, tonalites) compared with the latter (granodiorites, monzodiorites³¹,³⁷). Second, experimental phase equilibria in silicic systems show that at given (upper crustal) pressure-temperature conditions, the plagioclase mass fraction and albite content both strongly increase with decreasing melt H₂O concentration⁴²⁻⁴⁴. At 800–900 °C, H₂O concentrations <4 wt% foster the formation of trondhjemitic cumulates with >75% of albitic (An₁₅₋₃₀) plagioclase⁴²⁻⁴⁴. Ultimately, the K₂O- and H₂O-poorer nature of Archean TTG magmas relative to modern granitoids would reflect the more primitive, less enriched nature of their mafic precursors compared with post-Archean calc-alkaline basalts².

Our new interpretation can be reconciled with an entirely intra-crustal production and differentiation of TTG magmas because the proposed tonalitic parent liquid to TTG suites could have been formed at pressures similar to that inferred for MP-/LP-TTGs, that is, 0.6–1.2 GPa (refs. ⁴,¹⁰,¹¹). This corresponds to a maximum 20 to 40 km (Fig. 4), well within the thickness range of the primitive Archean mafic crust¹²,¹⁴. Therefore, our model eliminates the geodynamic paradoxes encapsulated by the HP-TTGs, be it the controversial occurrence of ‘hot’ subduction or the petrologically challenging melting of delaminated mafic crust. Despite this, the data presented here constrain only the evolution of the parental tonalitic liquid in the upper crust, and the question remains about how this liquid was produced in the lower crust.

In the modern Earth, the production of tonalitic to granodioritic magmas relevant to the composition of PL occurs in two distinctive ways, by either (1) melting of mafic crustal rocks, water-fluxed⁴⁵ or via amphibole breakdown⁴⁶, or (2) fractional crystallization-controlled differentiation of mantle-derived, hydrous basalts⁷,⁴⁷. Interestingly, the composition of PL matches that of tonalitic liquids produced experimentally at ~900–1,000 °C in lower crustal conditions (0.7–0.9 GPa) by the fractional crystallization of moderately hydrous (2–3 wt% H₂O) tholeiites⁴⁷, compositionally comparable to common Archean basalts². Such liquids are highly mobile in the crust because they form <40% crystals between 1,000 and 800 °C (ref. ⁴⁷) and, as such, remain below the rheological lock-up threshold³⁵. They would therefore readily separate from their (ultra-) mafic cumulates, ascend, stall in the mid- to upper crust and further differentiate, mainly through plagioclase and amphibole fractionation, to produce granitic liquids similar in composition to EL⁴⁷. These experimental constraints suggest that fractional crystallization of basalt in the lower crust is a likely possibility to explain the origin of PL. Nevertheless, crystallization and melting are undistinguishable on the basis of the sole chemical signature of the liquids. Therefore, we cannot exclude that melting of compositionally similar source material within the same pressure-temperature conditions, as originally proposed for the formation of LP-/MP-TTGs¹⁰,¹¹, was a viable mechanism to generate PL.

Regardless, lower-mafic crust melting and basalt crystallization are end-member situations and not mutually exclusive. Both typically take place at modern convergent margins, in the crustal roots of the upper plate where the protracted injection and maturation of mantle-derived magmas build up ‘deep crustal hot zones’⁴⁸. The silicic magmas extracted from the latter further differentiate in the middle and upper crust, producing transcrustal magmatic systems whose surficial expressions can ramp up to large, silicic eruptions⁴⁹. The Paleoarchean plutonic-volcanic system described here in the southern Barberton Belt could represent
the upper part of one such transcrustal igneous column. However, we emphasize that this would not necessarily imply that TTG magmas were formed at convergent plate margins. Indeed, such transcrustal magmatic systems also occur in extensional settings in the modern Earth³⁴. This indicates that the processes involved in their construction remain valid in the case of the intrusion and differentiation of voluminous mantle-derived magma within a relatively thick crust, regardless of the geodynamic setting in which it was produced.

## Online content

Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41561-019-0520-6.

Received: 21 May 2019; Accepted: 28 November 2019;
Published online: 13 January 2020

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**Methods**

**Phase maps.** Compositional maps were obtained on carbon-coated thin sections using a Jeol JSM-6390LA scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS; Thermo Fisher Ultra Dry Detector SDD) at ETH Zürich (ETHZ, Switzerland). We used an acceleration voltage of 15 kV, a magnification of 80 times and a grid cell size of 128×96 pixels (resulting resolution of 12.2 µm pixel⁻¹). The spectral imaging of each grid cell consisted of 15 successive frames with a sweep time of 1 min each. Data processing was performed with the Igor Pro-based software iSpectra¹. The pixels assigned to plagioclase were further processed to determine their molar anorthite content using a calibration dataset in which the variation of the Al/Si ratio measured by EDS was fitted to a calibration spline to the measured mol% An content by quantitative analysis.

**Bulk-rock geochemical analyses.** Bulk-rock major-element compositions were determined with X-ray fluorescence (XRF) at ETHZ. About 1 g of rock powder was ignited at 1,000 °C for 2 h to determine the loss on ignition and subsequently mixed in proportions of 1:5 with lithium meta- and tetraborate flux (in proportions 1:2) to prepare glass pills by melting at 1,080 °C. The pills were analysed using a PANalytical Axios XRF spectrometer with a wavelength-dispersive XRF system. An acceleration voltage of 24–60 kV was used with a current of 40–100 mA. Typical uncertainties range from 2 to 10% relative for all elements. Bulk-rock trace-element compositions were analysed by LA-ICP-MS at ETHZ on shards from the broken XRF glass pills. We used a Coherent GeoLas excimer ArF (193 nm) laser ablation system coupled to a PerkinElmer NexION 2000 quadropole ICP-MS. We used a laser spot diameter of 100 µm, repetition rate of 10 Hz and energy density of 8–10 J cm⁻². The carrier gas consisted of high-purity helium (~11 min⁻¹) to which was admixed argon (~11 min⁻¹) downstream of the ablation cell, and signal homogenization was performed using an in-house squid tubing. The analyses consisted of 30 s gas blank acquisition followed by 30 s ablation. The National Institute of Standards and Technology (NIST) SRM610 glass⁵⁶ was used as the primary reference material (analysed with a spot diameter of 40 µm) for calibration using conventional standard-sample bracketing. The data were processed using the MatLab-based SILLS software⁵⁶. The CaO content of the samples obtained by XRF was used as the internal standard. Isobaric interferences from the flux were corrected using a lithium meta- and tetraborate blank prepared and analysed the same way as the samples. The accuracy and reproducibility of the analyses were checked by repeated measurements of the US Geological Survey (USGS) BCR-2 reference material prepared the same way as the samples. The data are reported in Supplementary Table 1. The results on BCR-2 show that analytical reproducibility ranges from 3 to 10% relative for most elements, and the results are accurate within uncertainty. On unknowns, uncertainties range from 1 to 25% relative depending on concentration (Supplementary Table 1).

**Mineral analyses.** The major-element compositions of amphibole and plagioclase were analysed on carbon-coated thin sections using a five-spectrometer JEOL JXA-8200 electron microprobe at ETHZ, equipped with five spectrometers, an argon X-ray detector (P-10 mixture) on spectrometers 1, 2 and 3 (thallium acid phthalate (TAP) and pentaerythritol (PET) crystals) and a xenon X-ray detector on spectrometers 3 and 5 (PET and LiF crystals). Acceleration voltage was set at 15 keV and the beam current at 20 nA. To prevent beam damage and diffusion of alkali elements, a defocused beam of 5 µm was used for all analyses. The counting time was 20 s on peak and 10 s on background. Calibration was performed using standard end-member mineral compositions. The data are reported in Supplementary Table 2. The trace-element compositions of amphibole, biotite, garnet, K-feldspar and plagioclase were analysed on thin sections by LA-ICP-MS at ETHZ, using an ASI RESOlution excimer ArF (193 nm) laser ablation system coupled to a Thermo Scientific Element XR sector-field ICP-MS. The samples and reference materials were analysed with a laser spot diameter of 29 µm, repetition rate of 5 Hz and energy density of ~3.5 J cm⁻². The carrier gas consisted of high-purity helium (~0.51 min⁻¹) and argon (~1.01 min⁻¹). Signal homogenization was performed using an in-house squid tubing. The analyses consisted of 30 s gas blank acquisition followed by 30 s ablation. The NIST SRM610 glass⁵⁶ was used as the primary reference material for calibration using conventional standard-sample bracketing. The data were processed using the MatLab-based SILLS software⁵⁶. For amphibole and plagioclase, the CaO content obtained by electron microprobe was used as the internal standard. For other minerals, we used stoichiometric contents of K₂O (biotite, K-feldspar) and SiO₂ (garnet). The accuracy and reproducibility of the analyses were checked by repeated measurements of the USGS GSD-1G reference material. The data are reported in Supplementary Table 3. The results on GSD-1G show that analytical reproducibility ranges from 2 to 10% relative for most elements, and the results are accurate within uncertainty. On unknowns, uncertainties range from 2 to >10% relative depending on concentration (Supplementary Table 3).

**Homogenization and analyses of melt inclusions.** Quartz phenocrysts from two samples of the BRVC (F2 and F5) containing crystallized melt inclusions were hand picked from the crushed samples under a binocular microscope. Homogenization experiments were performed in a cold-seal pressure vessel at ETHZ. The phenocrysts were set in a gold capsule and gradually heated and pressurized

(within 3–4 h) up to 800–850 °C and 1,500 bar. The experiment was kept at fixed temperature and pressure for 120 h owing to the slow melting kinetics of silicic melts⁷⁵⁻⁷⁸. Even so, only small melt inclusions (<25 µm) could be homogenized. The lower homogenization time (10 h) used by an earlier study⁷⁹ may therefore explain their scattered melt inclusion data, especially the low SiO₂ contents that may reflect incomplete resorption of the host quartz crystallized at the inclusion walls (Extended Data Fig. 4). After quenching, the quartz grains were extracted, mounted in epoxy and polished to expose the homogenized melt inclusions. The major element compositions of glass were measured only in the largest (>20 µm), fully homogenized inclusions to ensure that the analyses were representative of the bulk inclusion composition. The analyses were performed with the same electron microprobe system as for mineral analyses (see the preceding). Acceleration voltage was set at 15 keV. To prevent beam-induced alkali diffusion, we used a low-intensity (3 nA), defocused beam of 5 µm spot size; K and Na were analysed first in the element sequence and for a shorter time (5 s) compared with the other elements (20 s). Background counting times were set to 10 s. Between two and four spots were analysed on each melt inclusion to obtain an average composition. Trace elements contents of the homogenized melt inclusions were subsequently measured using the same LA-ICP-MS system as for trace element analyses in minerals (see the preceding). The same parameters have been used for the melt inclusions, with the exception being a laser spot diameter of 13 µm and energy density of ~2 J cm⁻². We also analysed the trace element contents of non-homogenized (crystallized), unexposed quartz-hosted melt inclusions on three thick sections of the BRVC samples (F2, F5 and F8), using a Coherent GeoLas excimer ArF (193 nm) laser ablation system coupled to a Thermo Scientific Element XR sector-field ICP-MS at ETHZ. We used a laser repetition rate of 10 Hz, energy density of 15–20 J cm⁻² and spot diameter ranging from 10 to 50 µm, set as to ablate the entire inclusion. For all analyses, the NIST SRM610 glass⁵⁵ was used as the primary reference material using conventional standard-sample bracketing. The data were processed using the MatLab-based SILLS software⁵⁶. The Al₂O₃ content of the homogenized melt inclusions was used as the internal standard. The accuracy and reproducibility of the analyses were checked by repeated measurements of the USGS GSE-1G reference material. The data are reported in Supplementary Table 4. The results on GSE-1G show that analytical reproducibility ranges from 2 to 8% relative for most elements, and the results are accurate within uncertainty. On unknowns, uncertainties are not directly quantifiable for crystallized inclusions but they range from 3 to 15% relative in homogenized inclusions depending on concentration (Supplementary Table 4).

**Zircon U-Pb and Lu-Hf isotopic analyses by LA-(MC-)ICP-MS.** Zircons were extracted from crushed samples out of the <500 µm fraction using conventional separation techniques, mounted in epoxy mounts and polished. Analysis of U-Pb isotopes was carried out by LA-ICP-MS at ETHZ using the same system as for mineral trace element analyses (see the preceding) and identical instrument parameters, with the exception of the laser spot diameter (19 µm) and energy density (~2 J cm⁻²). The zircon reference material GJ-1 was used as the primary reference material for the correction of laser-induced element fractionation, mass discrimination and drift using conventional standard-sample bracketing. The data were processed with the Igor Pro-based Iolite v2.5 software⁶⁹, using the VisualAge data reduction scheme⁶⁰,⁶¹. The accuracy and reproducibility of the analyses were checked by repeated measurements of secondary zircon reference materials AUSZ7-1 (ref. ⁶²), Plešovice (ref. ⁶³), 91500 (ref. ⁶⁴) and QGNG (ref. ⁶⁵). The obtained Concordia ages for all secondary standards are accurate within less than 1%. Age calculations and data plotting were performed using Isoplot v4.15 (ref. ⁶⁶). The data are reported in Supplementary Data Table 5. Zircon Lu-Hf isotopic analyses were carried out by LA-MC-ICP-MS at ETHZ using an ASI RESOlution 193 nm ArF excimer laser ablation system attached to a Nu Instruments Nu Plasma 2 multi-collector sector-field mass spectrometer. We used a laser spot size of 50 µm, a repetition rate of 5 Hz and an energy density of ~4 J cm⁻². The carrier gas consisted of high-purity helium (~0.351 min⁻¹) and argon (~1.01 min⁻¹). The data were processed offline with the Igor Pro-based Iolite v2.5 software using an in-house data reduction scheme. Instrumental mass bias for Yb and Hf isotopes were corrected to the natural abundance ratios ¹⁷⁰Yb/¹⁷¹Yb and ¹⁷⁰Hf/¹⁷ⁱHf of ref. ⁶⁷. The isobaric interferences of ¹⁷⁰Yb and ¹⁷⁶Lu on ¹⁷⁰Hf were corrected using the natural abundance ratios of ¹⁷⁰Yb/¹⁷³Yb = 0.79502 and ¹⁷⁶Lu/¹⁷⁵Lu = 0.02656. Accuracy and external reproducibility of the method were controlled by repeated analyses of reference zircon standards GJ-1 (ref. ⁶⁸), Plešovice (ref. ⁶³), 91500 (ref. ⁶⁴) and Temora (ref. ⁶⁹). The ¹⁷⁰Hf/¹⁷⁷Hf ratios obtained on all reference materials are within uncertainties identical to the recommended values. Calculations of initial ¹⁷⁰Hf/¹⁷⁷Hf ratios and εHf were performed using a ¹⁷⁶Lu decay constant of 1.865 × 10⁻¹¹ (ref. ⁷⁰) and ¹⁷⁰Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf values for the chondritic uniform reservoir recommended by ref. ⁷¹. The data are reported in Supplementary Data Table 6.

**CA-ID-TIMS zircon U-Pb geochronology.** Selected zircons were extracted from grain mounts after LA-(MC-)ICP-MS analyses for ID-TIMS. Extracted zircons were annealed for 48 h at 900 °C, washed in 4 M HNO₃ and loaded in 200 µl Savillek microcapsules with ~80 µl of concentrated HF and trace HNO₃ for chemical abrasion. Loaded microcapsules were assembled in Parr bombs and crystals
were chemically abraded at 180°C for 12 to 14h. The zircons of sample JB17-C6 (Stolzburg pluton) were generally more strongly affected by the abrasion process despite similar uranium content compared with the other samples. After chemical abrasion, residual crystal fragments were transferred to 3 ml Savillex beakers, fluxed in 6 M HCl and ultrasonically cleaned in 4 M HNO₃. Cleaned crystals were loaded back into their precleaned microcapsules with ~50 µl HF, spiked with 6–9 mg of EARTHTIME²⁰²Pb~205Pb~²³⁵U~²³⁵U tracer solution²⁷,²⁸ and dissolved at 210°C for 60 h in Parr bombs. After dissolution, samples were dried and redissolved in 6 M HCl at 180°C for several hours. Samples were dried again and redissolved in 3 M HCl for anion exchange chromatography. Uranium and lead were separated from the matrix employing an HCl-based single-column chemistry modified from ref. ⁷⁴. The U–Pb fractions were dried with a microdrop of 0.02 M H₃PO₄ and loaded on outgassed Re filaments with ~1 µl of silica gel emitter (procedure modified from ref. ⁷⁵). All U–Pb isotope analyses were performed on a Thermo TRITON plus thermal ionization mass spectrometer at ETHZ. The Pb was measured at typical run temperatures of 1,160–1,200 °C employing a dynamic peak-hopping routine on the axial secondary electron multiplier. The Pb isotope ratios were collected for 300 to 400 cycles (with 4 s on-peak integrations), and measured isotope ratios were corrected online for mass fractionation using the double spike⁷². Uranium isotopes were measured as uranium oxide (UO₂) using a static routine on the low-mass Faraday cups connected to amplifiers with 10¹³ ohm feedback resistors⁷⁶. The U isotope ratios were collected for 75–100 cycles (with 16 s on-peak integrations) at typical run temperatures of 1,360–1,420 °C. Isotope ratios were corrected for isobaric interferences from minor UO₂ isotopologues⁷⁶ and for mass fractionation using the double spike assuming a ²³⁸U/²³⁵U ratio of 137.818 ± 0.045 (ref. ⁷⁷) for samples and blanks. Uranium blanks were estimated by repeat analyses of total procedural blanks to 0.071 ± 0.035 pg. All common Pb in zircon analyses was assumed to be procedural blank and was corrected using the long-term average blank isotope composition. Data reduction, age calculation and uncertainty propagation were performed using Tripoli and ET_Redux software⁷⁸,⁷⁹. All U–Pb ratios and dates were calculated using the decay constants of ref. ⁸⁰ and the tracer calibration of ref. ⁷². The U–Pb and Pb–Pb ratios and dates were corrected for initial ²³⁰Th and ²³¹Pa disequilibria using a Th–U partition coefficient ratio of 0.2 ± 0.1 (ref. ⁸¹) and a Pa–U partition coefficient ratio of 3.0 ± 0.8 (ref. ⁸²). All uncertainties are reported at the 95% confidence level excluding contributions from the tracer calibration and decay constants. The accuracy and reproducibility of the analyses have been assessed by repeat analyses of the EARTHTIME ET2000 synthetic solution⁸³ and the natural reference zircon material OG-1 (ca. 3,466 Ma; ref. ⁸⁴). The data are reported in Supplementary Table 7.

**Geochemical modelling.** *Mass-balance determination of crystal versus liquid proportions in trondhjemites.* We tested whether a mixture of plagioclase, amphibole, accessory minerals (titanite, apatite and epidote) and liquid (average composition of melt inclusions analysed here) explains the major element composition of the trondhjemites. The phase proportions were calculated using the R software environment by least-square-fitted mass balance using all major elements and reported in the inset of Fig. 3b. Several runs were performed including biotite as a potential phase, but were discarded because they systematically led to negative proportions of either biotite or amphibole. This suggests that whereas amphibole is a cumulate phase relative to the composition of the residual granitic liquid, biotite crystallized out of it. Experimental data⁷⁷ show that in such systems, biotite saturates at low temperatures (~700 °C) compared with amphibole (~1,000 °C) and only in highly differentiated liquids (>75 wt% SiO₂). This is further indicated in our samples by the systematic presence of biotite in the granites (in contrast to amphibole, which is present only in the trondhjemites).

**Composition of the PL and EL.** The principle of the geochemical model of PL and EL compositions is sketched in Supplementary Fig. 1. The major element compositions were determined by mass balance; EL was modelled as a mixture between the average compositions of the melt inclusions (pure residual liquid end member) and the trondhjemites (= residual liquid + crystals); PL was modelled as a mixture between EL (pure residual liquid end member) and the diorites–tonalites (= residual liquid + crystals). The trace element compositions were modelled using the fractional crystallization equation $C_L = C_0 \times F^{D-1}$. The target composition is $C_0$ in this case; $C_L$ represents the composition of the residual liquid (average BRVC melt inclusions for EL; average EL for PL), F is the melt fraction and D is the global distribution coefficient between solid and liquid for a given element. The calculations were performed using the R software environment and consist of a stochastic model with 10,000 trial runs, each with a different set of randomly chosen values for the model parameters (element concentrations, mass fractions of end members and distribution coefficients). For major- and trace-element contents in the BRVC melt inclusions and trondhjemites, the values were selected according to a normal distribution defined by the average ± 2 s.d. concentrations. For element contents in the diorites–tonalites, SiO₂ was defined at each trial run according to a uniform distribution bound by the minimum and maximum concentrations in our samples (52–63 wt%); the concentrations for all other elements were calculated from the SiO₂ content on the basis of a linear relationship determined by element co-variation with SiO₂ in the samples. The mass fractions between the end members (used for mass balance on major oxides and the determination of F for trace-element fractionation) were selected following a uniform distribution;

we provide details on the choice of the bounding values in the Supplementary Information. For the trace element model, distribution coefficients were calculated from phase proportions obtained by mass balance and literature partition coefficients and randomly selected within ±50% uncertainty range (defined on log scale) according to a uniform distribution (Supplementary Table 8). The average compositions of PL and EL obtained using this model are reported in the Supplementary Information.

## Data availability

The authors declare that the data supporting the findings of this study were not deposited in a publicly available data repository because the disparity of the obtained results would not make it possible to access them as a single consistent dataset. All of the data are available within the article, extended data and Supplementary Information files. The literature dataset plotted in Fig. 3 is available from the corresponding author on request.

## Code availability

The R script files used for stochastic modelling are available from the corresponding author on request.

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## Acknowledgements

We thank J. Allaz, A. Fiedrich, M. Guillong and L. Zehnder for their help during experiments and analyses and M. Mühlberg for contribution to fieldwork and discussion.

## Author contributions

O.B., O.L. and J.-F.M. conceived the project. J.B., O.B., O.L. and J.-F.M. collected the samples. J.B. and O.L. performed petrographic, geochemical analyses and stochastic modelling. S.B., J.B., O.L. and P.U. carried out the melt inclusion study. J.-F.W. undertook CA-ID-TIMS zircon dating. M.P.S. and O.L. performed zircon U-Pb and Lu-Hf isotopic analyses by LA-(MC-)ICP-MS. All authors contributed to data evaluation, interpretation and paper writing.

## Competing interests

The authors declare no competing interests.

## Additional information

*Extended data* is available for this paper at https://doi.org/10.1038/s41561-019-0520-6.

*Supplementary information* is available for this paper at https://doi.org/10.1038/s41561-019-0520-6.

*Correspondence and requests for materials* should be addressed to O.L.

*Peer review information* Primary Handling Editor: Rebecca Neely.

*Reprints and permissions information* is available at www.nature.com/reprints.
<figure><img src="image_13.png" /></figure>

**Extended Data Fig. 1** | Geological map of the southern Barberton Greenstone Belt. Modified from published maps<sup>21,26,27</sup> based on our own field observations and interpretations, for example the presence of diorite enclaves in the Theespruit pluton and the fact that the Doornhoek pluton is largely granitic in character. The inset shows the location of outcropping Archean crust and the study area (red frame) on a map of South Africa.
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<figure><img src="image_21.png" /></figure>

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**Extended Data Fig. 2** | Representative field observations and rock textures of the Paleoarchean Barberton TTG plutons and Buck Ridge Volcanic Complex (BRVC). **a**, typical texture of undeformed, plagioclase (pl)-phyric trondhjemite (Theespruit). **b**, trondhjemite (Td) cross-cut by dykes of hololeucocratic, aplitic granite (G) and containing small xenolitihs of amphibolite (A) (Theespruit). **c**, Igneous-textured, mesocratic and undeformed tonalite (To) showing irregular and gradational contact with leucocratic trondhjemite (Td), both cross-cut by a granite (G) dyke (Stolzburg). **d**, Undeformed, coarse-grained granite (G) showing irregular layering defined by the accumulation of biotite (bt) (Doorhoek). **e**, undeformed, garnet (grt)-bearing, aplitic granite (Theespruit). **f**, Igneous-textured, undeformed diorite (Theespruit). **g**, Igneous-textured diorite (Dio) mingled with leucocratic trondhjemite/granite (Td/G) (Theespruit). **h**, trondhjemite (Td) containing an angular, sharp-edged xenolith of amphibolite (A) ('gneissic zone'). **i**, Angular blocks of amphibolite (A) brecciated by the intrusion of trondhjemite (Td) (Theespruit). **j**, Internally foliated, sharp-edged xenolith of amphibolite (A) in igneous-textured trondhjemite (Td) (Theespruit). **k**, Microphotograph showing the texture of a felsic volcanic rock from the BRVC, containing quartz (qz) and altered plagioclase (pl) phenocrysts in a devitrified groundmass (gm). **l**, Microphotograph showing the seriate texture of a felsic sub-volcanic rock from the BRVC, with phenocrysts of quartz (qz), altered plagioclase (pl) and biotite (bt).
<figure><img src="image_26.png" /></figure>

Extended Data Fig. 3 | Phase and plagioclase compositional maps for representative samples of the main igneous lithologies in the Paleoarchean Barberton TTG plutons. Phase maps (on the left-hand side) and plagioclase compositional maps color-coded for mol.% anorthite (An) (on the right-hand side) result from the analysis of SEM-EDS spectral images (see Methods). For more details about the texture of the trondhjemites, see Fig. 1b.
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**Extended Data Fig. 4** | Selected major-element diagrams showing the geochemistry of quartz-hosted homogenized melt inclusions from the Buck Ridge Volcanic Complex (BRVC). The compositions of the BRVC melt inclusions investigated in this study match those of high-silica rhyolitic glasses and melt inclusions from modern ignimbrites (as compiled by ref. 51 – see their work for the complete list of references). Some of the data from Agangi et al. (ref. 25) depart from those compositions along trends projected from pure quartz, likely indicating the incomplete homogenization of the host in their experiments (see Methods).
a

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b

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**Extended Data Fig. 5** | Summary of zircon LA-(MC-)ICP-MS U-Pb and Lu-Hf analyses. **a**, representative cathodoluminescence images of zircons, with the position of laser spots. **b**, U-Pb isotopic data plotted in Wetherill Concordia diagrams, with insets showing ¹⁷⁶Hf/¹⁷⁷Hf ratios calculated and plotted at the ²⁰⁷Pb/²⁰⁶Pb date of the corresponding spot (Pb loss trends defined by zircon radiogenic ingrowth according to the average ¹⁷⁶Lu/¹⁷⁷Hf ratio of zircon analyses from the sample). The data are consistent with a single ca. 3456 Ma generation of zircon that suffered variable extents of recent and ca. 3200 Ma Pb loss (age of the main orogenic event in the southern Barberton Belt²⁸⁻³⁰). We report upper intercept dates for information (dashed ellipses were excluded from the calculation), although some of them unlikely represent crystallization ages because of multiple Pb loss events and severe discordance. Error ellipses and bars all represent 2σ uncertainties.
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**Extended Data Fig. 6** | Selected Harker diagrams showing the composition of bulk rocks, melt inclusions, minerals and modelled EL and PL. The composition of the trondhjemites is consistent with a mixture of plagioclase-rich solids and residual granitic melt represented by the melt inclusions, both formed out of an evolved liquid (EL). The composition of the diorites and tonalites are consistent with amphibole + plagioclase cumulates, formed out of a parental liquid (PL) of tonalitic composition. In Harker plots for major oxides, we report the average composition of greenstone xenoliths<sup>26,27</sup> and a compositional trend of hybridization between the latter and the trondhjemites (arrows), showing that it does not satisfactorily reproduce the chemistry of the diorites and tonalites. In Harker plots for trace elements, the trends towards biotite and garnet were determined using the average composition of these phases for the element of interest. The fields for the modelled composition of EL and PL corresponds to 95% of the data obtained by stochastic calculations (point density increases with color darkness). The stars represent the average modelled compositions of EL and PL.
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**Extended Data Fig. 7** | Selected geochemical diagrams comparing the modelled compositions of EL and PL and those of the different TTG groups. The possible compositions of EL and PL were modelled by stochastic mass balance calculations (see Methods and Supplementary Information). The colored fields correspond to 95% of the data (point density increases with color darkness). The fields for low-, medium- and high-pressure TTGs¹¹ (LP-, MP-, HP-TTG) as well as the composition of the trondhjemites, diorites, tonalites and melt inclusions from the BRVC are showed for comparison. While the compositional field of HP-TTGs corresponds to the trondhjemites (that is plagioclase-laden regions of the silicic mush), the compositional range of liquids (EL and PL) are more consistent with MP- and LP-TTGs. The arrow shows the vector of plagioclase accumulation (based on plagioclase trace element compositions reported here) in all diagrams.