Incipient continent formation by shallow melting of an altered mafic protocrust

Domain: earth_science
Published: 2025-05-16
Editorial outcome: minor_revision

Abstract
Understanding how Earth’s continental nuclei first formed in the Archean eon (4.0–2.5 Ga) underpins our notions of early Earth geodynamics. Yet, the nature of Earth’s early protocrust and the primary mechanism for its transformation are poorly understood, as very ancient rocks preserving petrological evidence for these processes are incredibly rare. Here we report the discovery of a formerly melt-bearing amphibolite from the Sylvania Inlier of the Pilbara Craton in Western Australia. Radiometric dating of zircon and titanite in these rocks constrain the time of partial melting to 3565 Ma, providing evidence for a metamorphic event that predates most exposed rocks in the Pilbara Craton by ~30 million years. Low δ18O compositions and modelled melt compositions comparable to evolved Hadean rocks in the Acasta gneisses indicate Earth’s oldest continental crust may have sourced rocks of a similar composition. Thermodynamic modelling suggests partial melting at temperatures of 680–720 °C and pressures of 0.8–1.0 GPa, implying a maximum burial depth of ~30 km. These results support models of continental nuclei formation via shallow partial melting of hydrothermally altered mafic protocrust in high heat flow environments.

Introduction
Although there is evidence for the existence of some felsic (silica-rich, continental) crust on Earth as early as 4.4 billion years (Gyr) ago1, data from rare exposures and very ancient detritus indicate Earth’s continents were built from pre-existing mafic nuclei (protocrust) and subordinate precursory continental crust (PCC) formed between the late Hadean and early Paleoarchean (4.03–3.5 Gyr ago)2–7. However, the physical processes involved and tectonic setting in which these ancient continental nuclei formed are debated8.

One prominent occurrence of ancient rocks resembling PCC are the 4.02 Gyr old Idiwhaa tonalite gneisses within the Acasta Gneiss Complex (AGC) of the Slave Craton in northwest Canada2,3. These rocks are geochemically distinct from younger Archean tonalite–trondjemite–granodiorite (TTG) series rocks in having high total iron (FeOT) and correspondingly low Mg# (atomic Mg/(Mg + Fe2+)), low Sr/Y and Gd/Y ratios, and containing primary (magmatic) zircon with sub-mantle (isotopically light) oxygen isotope compositions (δ18O < 4.7‰)2,3. These geochemical features resemble those of modern felsic crust produced by shallow (uppercrustal; ≤15 km) magmatic fractionation and assimilation in Iceland (so-called Icelandites), suggesting Earth’s first continental nuclei may have formed in a similar manner2,3. However, an alternative model for Icelandite-like rocks involves shallow-level (uppermost few kilometres) partial melting of hydrothermally altered mafic crust9. Unlike with modern Icelandites, this could have occurred in response to intense meteorite bombardment on a planet with surface water during the Hadean to early Archean10,11.

Felsic rocks containing zircon with isotopically light δ18O compositions have since been documented amongst the oldest preserved parts of other Archean cratons, including in the Western Australian Pilbara and Yilgarn cratons11, indicating that the processes that produced the Idiwhaa tonalite gneisses may have been typical. Key to the origin of these ancient (c. 4.02 Gyr old) gneisses are their mafic host rocks, which include Fe-rich garnet-bearing amphibolites that are older than 4.02 Ga based on cross-cutting relationships, and represent rare remnants of the Hadean protocrust12. However, the protracted polymetamorphic history of the AGC has seemingly destroyed the primary mineralogical/petrological features of these rocks, obscuring any genetic link to the ancient felsic rocks3,12.

Here, we report formerly melt-bearing (migmatitic) Paleoarchean amphibolites from the Sylvania Inlier of the Pilbara Craton. Integrating detailed petrological observations with geochemistry, geochronology, phase equilibrium and geochemical modelling, we show that these rocks are suitable analogues for Earth’s reworked early (Hadean to early Archean) mafic protocrust. By constraining the pressure (P), and temperature (T) conditions these rocks formed under, we probe the thermal regime in which Earth’s first enduring continental nuclei originated.

Results
Geological setting, petrology and mineral chemistry
The Pilbara Craton of Western Australia preserves well-exposed weakly deformed and metamorphosed Archean rocks that represent a key geological repository of the early Earth13. In the dominantly Paleoarchean (3.6–3.2 Ga) East Pilbara Terrane, metamorphic grades are low (< or <<500 °C) and higher grade (amphibolite to granulite facies) migmatitic metabasic rocks that might be the residual protoliths of the TTG-dominated crust are not exposed. However, in the Sylvania Inlier, south of the East Pilbara Terrane and at the margin of the Pilbara Craton (Fig. S1), Proterozoic orogenesis has exhumed such high-grade rocks14, potentially providing a window into Earth’s primitive protocrust.

The northern part of the Sylvania Inlier contains a package of ‘hybrid intermediate rock’ on the western margin of a greenstone belt (Fig. S1a, b)15 from which two samples of metatonalite (samples 216545 & 216594) have been investigated previously11. The rocks comprise 35–40% plagioclase, 35–40% quartz and 2–3% garnet with minor biotite, ilmenite, and titanite. Secondary epidote replaces plagioclase (5–6%) and chlorite locally replaces garnet (Fig. S2). Zircon from the metatonalite dated by SHRIMP U–Th–Pb geochronology yield ages of 3565 ± 3 Ma and 3566 ± 5 Ma11. Notably, these grains have low δ18O oxygen isotopic compositions (3.76–3.95‰)11, a feature shared by some Hadean zircon from the Idiwhaa tonalite gneiss of the AGC2,3. The focus of this study concerns two additional samples from the same geological unit within the Sylvania Inlier that are considerably more mafic than those previously reported (Fig. 1a–d). Macroscopically, the rocks are dominated by a fine-to-medium grained melanocratic matrix (melanosome) with variable quantities of quartzofeldspathic leucosome (Figs. 1c, d, S1 & S2), interpreted to reflect partial melting at the metamorphic peak.

The melanosome is mostly composed of amphibole + quartz + plagioclase + ilmenite + titanite with secondary epidote (Fig. 1a–d). Electron-probe microanalyser (EPMA) analyses classify the calcic amphibole as hornblende with AlT=0.47–1.84 (Si = 7.53–6.16), Na + K(A)=0.09–0.53, and Mg/(Mg + Fe)=0.29–0.57, ranging from ferro-tschermakite to magnesio-hornblende/actinolite in a plot of Mg# versus Si content (Fig. S3). The leucosome is dominated by quartz and plagioclase in roughly equal proportions, and contains abundant accessory zircon. Plagioclase is extensively replaced by sericite and epidote, but where fresh, preserves lamellar twinning and is albite to oligoclase in composition (X (atomic Ca/Ca + Na + K)=0.03–0.31; Fig. S3). Preserved igneous textures in the leucosome include coarse-grained idiomorphic plagioclase crystals surrounded by finer-grained intergrowths of quartz and plagioclase (Fig. 1c), the latter resembling graphic textures common in pegmatites formed by crystallisation of volatile-saturated eutectic (minimum) granitic melts16. We interpret these quartzofeldspathic domains as the crystallized products of a hydrous partial melt formed during the prograde-to-peak metamorphic history.

Titanite is almost exclusively found as clusters of randomly oriented granules forming coronae around ilmenite crystals within the melanosome (Fig. 1d). Detailed backscattered electron (BSE) imaging of this titanite reveals relic grains of ilmenite, with titanite forming interconnected fracture networks in the cores of ilmenite where preserved (Figs. 1d, 2b). These observations indicate the titanite grew via the breakdown of ilmenite. In places, entrained fragments of matrix containing hornblende and titanite–ilmenite within larger leucosome segregations suggest ilmenite breakdown (and titanite growth) predated melt crystallisation and may have predated or been coeval with partial melting.

Geochemistry & Geochronology
Compared to the metatonalite samples, the amphibolites have low to moderate SiO2 (58–63 wt% versus 74–76 wt%), higher FeOT (8–12 wt% versus 3–4 wt%), MgO (2–3 wt% versus 0.2 wt%), CaO (6–8 wt% versus 2–3 wt%) and TiO2 (1–2 wt% versus 0.5 wt%), and similar Al2O3 (~13 wt%) contents. The most mafic samples (e.g. 255836) have flat to slightly positively sloping rare earth element (REE) patterns (Fig. 1f), with chondrite-normalised La/Yb ratios between 0.39 and 0.86. The concentrations of heavy REE (HREE) and high field-strength elements (HFSE; including Zr, Hf, Ti) in the amphibolites are similar to those of high-TiO2 basalts and the Coucal basalts (CF-2 in Fig. 1f) from the lower Warrawoona Group of the East Pilbara Terrane17,18. However, lower light REE (LREE) contents in the studied rocks result in strong positive Zr and Hf anomalies in primitive-mantle normalised distribution diagrams (Fig. S3a). This observation suggests that depletion in LREE might reflect fluid alteration that preferentially removed LREE over more immobile elements. In more leucocratic (formerly more melt-rich) samples (e.g. 255834), the depletion in LREE is more pronounced, resulting in stronger positive Eu anomalies (Eu/Eu*=1.49–2.38, where Eu/Eu* is chondrite normalised Eu = (Sm×Gd)) relative to more mafic samples (255836, Eu/Eu*=1.21–1.31) (Fig. 1f).

In situ zircon and titanite U–Th–Pb geochronology was undertaken to determine the age of the studied amphibolites. Detailed analytical and standardisation procedures are described in the Methods section, and a table of results are provided as supplementary materials. In BSE and cathodoluminescence (CL) images, most zircon grains are strongly altered and exhibit mottled textures, although some crystals preserve relatively unaltered areas with oscillatory zoning (Figs. 2a and S4). In sample 255834, a cluster of concordant analyses from these unaltered domains within zircon yield a date of 3567 ± 4 Ma, identical to that of zircon grains in the metatonalites (3565 ± 3 Ma and 3566 ± 5 Ma)11. After filtering for U–Pb concordance and secondary water content (measured via 16O1H, see Methods and Supplementary Information for details), in situ 18O/16O isotope measurements on these domains (presented as δ18O normalized to Vienna Standard Mean Ocean Water) give δ18O values between 3.40 and 4.26 ‰ with a median value 3.86 ± 0.44 ‰ (2 s.d., n=14), which is identical to zircon from the metatonlite11. A cluster of concordant U–Pb isotope analysis of the granular titanite overgrowths on ilmenite yield an age of 3565 ± 10 Ma (Fig. 2b–d). An in-situ Lu–Hf isotope analysis of garnet from a sample of the metatontalite yields a date of 3550 ± 60 Ma (216594, Fig. 2e, f).

Although it is possible that the zircon in the amphibolites is igneous in origin and crystallised during the emplacement of the mafic precursor, we infer an anatectic origin given the clear evidence for partial melting and similarities with zircon in the metatonalite noted above. Collectively, these data provide strong evidence that metamorphism and partial melting of the amphibolite was coeval with formation of the tonalites, implying a genetic link. The scatter in U–Pb isotope systematics correlates with the presence of mottle-textures and/or recrystallisation fronts within zircon (Fig. 2a), and minor disturbance in the U–Pb systematics of titanite is likely related to hydrothermal activity associated with deposition of the Fortescue and Hamersley basins around 2800–2700 Ma (Fig. 2c)19. Nevertheless, the unaltered zircon, titanite and garnet ages coincide with a distinct age mode of detrital and inherited zircon crystals from the East Pilbara Terrane (Fig. 2g)20,21, indicating that partial melting of the Sylvania amphibolite occurred during a major (tectono)thermal event that may have driven the earliest phase of felsic crust production within the Pilbara Craton. As such, these rocks offer a rare glimpse into the mafic source regions for the first felsic crust that formed in the Pilbara Craton.

Phase equilibrium modelling & thermobarometry
To determine the P–T conditions at which partial melting occurred, and hence estimate the depth of felsic crust production, phase equilibria were modelled using the software THERMOCALC22. Calculations were performed in the NaO–CaO–KO–FeO–MgO–AlO–SiO–HO–TiO–O (NCKFMASHTO) chemical system using the measured (XRF) bulk compositions of samples 255836 and 255834 assuming an Fe3+/ΣFe ratio of 0.2, and measured loss on ignition as a proxy for H2O contents of 1.15 wt% and 0.82 wt% respectively.

A simplified P–T phase diagram calculated in the range 0.2–1.2 GPa and 600–800 °C (Fig. 3a) shows the inferred peak stability fields for both rocks, along with phase boundaries denoting the appearance and disappearance of major phases. In the most mafic sample (255836), the inferred peak metamorphic assemblage of hornblende + plagioclase + quartz + titanite + melt implies temperatures of 650–720 °C and pressures of 0.7–0.8 GPa (Figs. 3a & S8a). The upper pressure limit is defined by the garnet-in phase boundary, whereas the lower pressure limit is constrained by the biotite-out phase boundary to lower temperatures and ilmenite-in phase boundary to high temperatures. A minimum temperature of 650 °C is demarcated by the appearance of melt (solidus), and an upper temperature limit by the appearance of ilmenite.

At the inferred peak metamorphic conditions, the models predict 2–5 mol% melt, consistent with the observed proportion of leucosome in hand specimen (Fig. S1c). The phase diagram for sample 255834 is broadly similar, although the inferred peak stability field spans a larger pressure range as the stability of garnet is restricted to higher pressures (Fig. 3a, Fig. S8b). The model for sample 255834 has no clinopyroxene-free assemblages in the P–T window of interest, with minor clinopyroxene (<5 mol%) predicted to be present. This could reflect limitations in the current activity–composition models for metabasic rocks, which have been shown to underestimate the amount of hornblende and over estimate that of clinopyroxene, particularly in rocks with high modal abundances of the former23. Nevertheless, within the inferred peak stability field for sample 255834, the predicted titanite Zr concentrations (10–110 ppm, see Methods) are in excellent agreement with measured values (19–151 ppm, n=128), providing an independent average temperature constraint of 693 ± 35 °C using the temperature–concentration relations of ref. 24 (Fig. 3a).

The good agreement between the phase equilibrium models for the two different samples and Zr-in-titanite thermometry suggests that the inferred peak P–T conditions are robust. These data translate into an apparent geotherm (or thermobaric ratio, T/P) between 700 and 1000 °C.GPa−1. From the topology of both diagrams the formation of the observed ilmenite–titanite reaction textures can be explained by near isobaric cooling from temperatures greater than 740–780 °C. If so, the absence of any garnet in the metamorphic assemblage indicates these rocks may have been buried along a geotherm warmer than 1000 °C.GPa−1, so as not to intersect the garnet stability field during their prograde P–T evolution. Using finite-difference models for Zr diffusion in titanite25 we show that, for grains with radii of 5–30 µm, the predicted apparent Zr-in-titanite temperatures match the naturally observed distribution over a large range of cooling rates (1–100 °C.Myr−1, Fig. 3b).

The replacement of ilmenite by titanite has also been documented in mafic granulites hydrated during their retrograde evolution26,27. In these cases, titanite growth is generally associated with the conversion of anhydrous minerals like clinopyroxene and orthopyroxene into (OH–-bearing) amphibole. Although there is no evidence for any pre-existing pyroxene in these rocks, the titanite grains do exhibit lower F contents relatively to Al and Fe than predicted by (Al + Fe3+) + F = Ti + O substitution reactions27 (Fig. S3d). The titanite chemistry implies additional OH– within the titanite lattice to maintain the charge balance, consistent with a fluid phase being present when titanite grew27. On a temperature–M (mol% H2O) diagram (Fig. S9a) this pathway is depicted by increasing MH2O at constant temperature. Importantly, in this region of the temperature–MH2O diagram, the titanite-in, ilmenite-out and melt-in (solidus) lines all have steep negative slopes and are sub-perpendicular to the MH2O axis. Hence, a fluid-fluxed melting model can also explain many of the petrological features of these rocks, including the titanite coronae on ilmenite, and lack of anhydrous peritectic minerals. However, given that at low MH2O garnet is also predicted to be part of the subsolidus mineral assemblage, and the low zircon δ18O compositions imply a hydrothermally altered protolith, a model of closed-system (constant MH2O) burial, heating and cooling along a high geotherm is perhaps most likely.

Constraints on the source of early felsic crust
The new geochronological data, petrological observations and thermodynamic modelling indicate that the studied amphibolites are suitable source rocks for the metatonalites that represent some of the earliest felsic crust in the Pilbara Craton (Fig. 3). Accordingly, 15–35% partial melting of source rocks with a mineralogy similar to the Sylvania amphibolites can produce melts with major element and HREE chemistry matching naturally observed Eoarchean-to-Paleoarchean Fe-rich tonalites from the Pilbara craton over the observed range of apparent geotherms (700–1200 °C.GPa−1, Fig. 3c, d).

However, there are several key differences. The depletion in LREE relative to HREE is not typically observed in other (known) old mafic components of the Pilbara Craton21, which generally show flat to positive sloping chondrite normalised REE patterns12,28. Thus, the depletion of LREE relative to HREE and other HFSE like Zr, Hf and Ti may reflect the hydrothermal history of the rocks. Detailed work on element gains and losses from the 3481 Ma Dresser hydrothermal system in the East Pilbara Terrane shows that LREE are notably mobile in high-temperature amphibole–chlorite-dominated alteration assemblages, which in some places show depletion in LREE relative to HREE and other immobile elements29. Importantly, the mobility of LREE is not uniform across the system, even among rocks with similar alteration assemblages. Assuming the hydrothermal system that affected the basalts in the Dresser Formation is analogous to that which affected the Sylvania amphibolites, a spectrum of compositions, including LREE-enriched and LREE-depleted components might be expected. The apparent dominance of LREE-enriched compositions among ancient crustal melts shown in Fig. 3d could reflect the fact that LREE-depleted rocks like the Sylvania amphibolites are minor components of these ancient hydrothermal systems. Such complexity will not affect metamorphic phase relations, which will be similar for rocks with similar major element chemistry, irrespective of their trace element concentrations.

Despite the fact that, in both cases, the modelled melts are depleted in LREE relative to the naturally observed rocks, highlighting important contributions from enriched material, the Sylvania amphibolites are good petrological analogues for the source of the metatonalites. The major element composition of the metatonalites from the Sylvania Inlier also lies close to the melting paths depicted in Fig. 3c. However, none of the modelled trace element distributions for the melt match their observed trace element patterns (Fig. 1f), indicating that the metatonalites are not simply partial melts of the amphibolite. Rather, the concave up (‘U-shaped’) REE profile of these rocks and strong positive Eu anomaly suggests they formed via plagioclase accumulation, with the slight positive sloping HREE attributed to the presence of garnet, indicating a more complicated petrogenetic history.

Interestingly, the Sylvania amphibolites also share many geochemical features of the Hadean rocks of the AGC (the Idiwhaa tonalitic gneiss and garnet amphibolites)2,3,12, including intermediate SiO2, low Al2O3, elevated FeOT/MgO, and low FeOT/TiO2, the latter on account of high TiO2 contents. Although the absolute FeOT content is lower than the most Fe-rich components of the AGC (garnet amphibolites), the more Fe-rich samples of the Sylvania amphibolites (255836) have similar FeOT/MgO and FeOT/TiO2 ratios to the AGC garnet amphibolites (Fig. 1e). Moreover, the presence of low δ18O magmatic zircon in rocks from the AGC and the Sylvania Inlier indicate that, in both cases, the mafic source rocks experienced high-temperature isotopic exchange with Archean seawater2,3. The major and trace element modelling also confirms the Sylvania amphibolites as suitable sources to the Idiwhaa tonalite gneiss, although this would require heating to temperatures beyond amphibole stability (>825 °C) along very high geothermal gradients exceeding 1200 °C.GPa−110. Therefore, Earth’s first felsic crust may have had similar source rocks.

Implications for early Achean crust-forming processes
The ‘hybrid-intermediate rock unit’15 from the Sylvania Inlier represents the metamorphosed remnants of some of the earliest mafic crust from the Pilbara Craton. Given that similar chemical characteristics are shared by the putative Hadean mafic crust in the AGC, we propose a similar petrogenetic history2,3. The elevated FeOT and TiO2 content (relative to MgO) are indicative of anhydrous (H2O < 0.2 wt%) reduced mantle-derived magma whose liquid lines of descent are controlled by low-pressure fractionation crystallisation with delayed Fe–Ti oxide saturation, similar to those inferred to have produced the mafic rocks in the AGC2,30. That the absolute FeOT and TiO2 content of the garnet amphibolites in the ACG are higher than the majority of mafic rocks in the Pilbara Craton could reflect differences in composition, hydration state and oxygen fugacity of the parental melts. However, it could also indicate that the Sylvania amphibolites formed from melts more chemically evolved than those that produced the Acasta garnet amphibolites, particularly since Fe–Ti oxide saturation will rapidly deplete the residual melt in Fe and Ti as differentiation proceeds. We infer that magmatic differentiation was partly responsible for these differences as the Sylvania amphibolites have comparable FeOT/MgO and FeOT/TiO2 ratios to the Acasta garnet amphibolites at lower MgO and higher SiO2 contents. Nevertheless, the isotopically light δ18O composition (3.86‰) of zircon grains from the amphibolite indicate the protolith also experienced high-temperature (>300–400 °C) hydrothermal alteration before partial melting9. Therefore, the original magma must have erupted onto or intruded the shallow crust where it was hydrothermally altered by Archean seawater at high temperature, then finally transported to depths where it began to melt.

Despite the good correspondence between the major element composition of the Sylvania metatonalites and the modelled melt compositions (Fig. 3c), partial melting alone cannot reproduce their trace element chemistry (Fig. 3d). During low pressure differentiation of tholeiitic magmas FeOT enrichment is largely driven by the removal of plagioclase and Mg-rich clinopyroxene, where plagioclase tends to dominate fractionation assemblages (60–75% cumulatively)30, such that the metatonalites could be metamorphosed cumulates formed during differentiation of the parental magmas. However, the Sylvania metatonalites generally lack ferro-magnesian phases, excluding minor garnet and secondary chlorite (Fig. S2), and consequently have lower MgO and CaO, and higher Na2O and K2O contents than mafic cumulates31. Since the age and oxygen isotopic composition of zircon from the metatonalites are identical to the anatectic zircon in the amphibolite, and there is clear textural evidence that plagioclase was an early crystallising phase from small volumes of melt trapped within the amphibolite (e.g. Fig. 1d, e), we interpret the metatonalites to be plagioclase-rich cumulates formed by fractionation of tonalitic crustal melts generated deeper in the crust32. Although no garnet is observed in the exposed amphibolite, the phase equilibrium models predict that garnet will be present at pressures greater than 0.8 GPa, only marginally deeper than the crustal level presently exposed. Therefore, this model can also explain the presence of garnet with an identical age to the zircon and metamorphic titanite in the metatonalites (Fig. 2), which is inferred to be of either magmatic or peritectic origin.

It has been argued that the absence of stable garnet under crustal pressure conditions (<1.4 GPa) in partial melting experiments using basaltic glasses retrieved from modern ocean plateau environments invalidates an intracrustal origin for early Archean felsic crust33. The results presented here contradict this claim. This discrepancy reflects the strong control of the proportion of FeO to MgO on garnet stability34. We illustrate this point using a pressure–composition (P–XFeO) diagram based on sample 255836B constructed at 700 °C (Fig. S9b), where XFeO = FeOT/(FeOT + MgO) in mol%. The pressure at which garnet appears in the metamorphic assemblage is highly dependent on XFeO. At Fe-contents comparable to the bulk composition of modern MgO-rich ocean island basalt used in experimental studies (XFeO = 0.35–0.36) garnet is predicted to be unstable below 1.4–1.5 GPa33. However, at XFeO > 0.62 garnet stability is significantly less sensitive to Fe content and is stabilised between 0.72 and 0.85 GPa (Fig. S9b). Since XFeO for the more mafic Sylvania amphibolites is ~0.68–0.72, the absence of garnet as a residual phase in these melt-bearing metabasic rocks provides a robust maximum pressure (and thereby depth) constraint on melt generation.

Even so, most felsic rocks in Archean cratons were derived from sources with lower XFeO ratios than the mafic rocks described here (e.g. Fig. 1e), and the presence of trace element signatures related to residual garnet in more Mg-rich crustal melts clearly has pressure/depth significance (e.g. Fig. S9). As high XFeO mafic rocks are rare among Archean mafic rocks in general, it is intriguing that such rocks appear to have been involved in the earliest crust formation events in both the Slave and Pilbara cratons2,4. Given their antiquity, we interpret these rare Fe-rich mafic rock to be fragments of the primordial crust that surfaced the early Earth and was largely reworked to become more magnesian during the growth and maturation of the continents (Fig. 4). Considering the two disparate continental nuclei that preserve these Fe-rich rocks formed nearly 500 million years apart suggests that whatever processes produced these continental nuclei were also highly asynchronous.

Our thermodynamic modelling indicates that partial melting of Sylvania amphibolites occurred at depths of around 30 km, close to the estimated thickness of primary mafic (mantle-derived) crust predicted for higher Archean potential temperatures (25–35 km)35, and the average crustal thickness from modern oceanic plateaux like Iceland (29 km)36 and the Ontong-Java plateau (33 km)37. Our results support the notion of continental nuclei formation via intracrustal partial melting and differentiation in high heat flow environments (Fig. 4)2,32.

Methods
Whole-Rock Geochemistry
New data for samples 255834 and 255836 were obtained for this study, following the analytical procedures outlined by Smithies et al.38,39. The analyses were conducted by Bureau Veritas, Perth, Western Australia. Sample preparation involved crushing with a plate jaw crusher and milling in a low-chromium steel mill to produce pulp with a nominal particle size of 90% passing <75 µm. Major and minor elements (Si, Ti, Al, Cr, Fe, Mn, Mg, Ca, Sr, Ba, Na, K and P) contents were measured using X-ray fluorescence (XRF) spectrometry on fused glass discs, while loss on ignition was determined by thermogravimetric analysis. Trace elements—including Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, La, Lu, Nb, Nd, Ni, Pb, Pr, Rb, Sc, Sm, Sn, Sr, Ta, Tb, Th, Tl, Tm, U, V, W, Y, Yb, Zn and Zr—were measured by laser ablation ICP–MS on a fragment of each glass disk earlier used for XRF analysis. Data quality was monitored by blind insertion of sample duplicates, internal reference materials, and the certified reference material OREAS 24b. BV Minerals also included duplicate samples, certified reference materials (including OREAS 24b), and blanks. Total uncertainties for major elements are ≤1.5%, those for minor elements are <2.5% (at concentrations >0.1 wt.%) and those for most trace elements are ≤10% (Lu ± 20%). Newly acquired (samples 255834 & 255836) and compiled (samples 216545 & 216594) geochemical data are provided in Supplementary Data 1. Chondrite- and primitive mantle–normalized trace element plots use the reference compositions of Sun & McDonough40.

SIMS Analytical Methods
Zircon separated from the leucosome-rich sample 255834 was analysed via secondary ion mass spectrometry (SIMS) on the SHRIMP II ion microprobe at the John de Laeter Centre at Curtin University. Procedures followed those described by Compston et al.41, with modifications summarized by Williams42. A 25 µm diameter primary beam of O2 ions at 10 keV, purified by means of a Wien filter, was employed to sputter secondary ions from the surface of each target zircon grain. The net primary ion current, as measured leaving the sample, was typically 1.7–1.9 nA. Secondary ions are accelerated to 10 keV, energy-filtered by passage through a cylindrical 85° electrostatic analyser with a turning radius of 1.27 m, and mass-filtered using a 72.5° magnet sector with a turning radius of 1 m. Secondary ions are counted by switching the magnetic field to direct the secondary ion beam into an electron multiplier used in pulse-counting mode. During the analytical session, the secondary ion analyser is set to a mass resolution of ≥5000 (1% peak-height definition), which is sufficient to resolve lead isotopes from most potential molecular interferences. During the session analyses of the 91500 zircon reference indicated an internal spot-to-spot (reproducibility) uncertainty of <0.63% (1σ), and a 238U/206Pb* (*=radiogenic) calibration uncertainty of <0.5% (1σ). Analyses of OGC zircon treated as an unknown indicated a yield concordia date of 3464 ± 5 Ma (n=7, MSWD=1.5) identical to the reported age, indicating that a 207Pb/206Pb fractionation correction is unnecessary. As a secondary check, GSWA sample 142535 yielded a date of 3246 ± 7 Ma (n=5, MSWD=0.86), within uncertainty of the reported SIMS age (3236 ± 3 Ma). Our date being slightly older reflects minor Pb loss that was not accounted for in the previously reported data43. Common-Pb correction used the measured 204Pb and contemporaneous common-Pb isotopic compositions determined according to the model of Stacey and Kramers44. All SIMS U-Th-Pb data are provided in Supplementary Data 2. Cathodoluminescence images showing individual spot locations are provided in Fig. S4a, b. Tera-Wasserburg concordia diagrams for secondary reference materials are shown in Fig. S6a, b.

Following U-Th-Pb isotope analysis, the mount was repolished using a 0.5 µm colloidal silica solution to remove the upper 1–2 µm of the sample surface and the U-Th-Pb analysis pits. Before oxygen isotope analysis, the mount was cleaned in ethanol and distilled water before coating with ~60 nm Au. The mounts were loaded in the vacuum chamber three days prior to analysis to degas. Measurement of oxygen isotopes (18O, 16O, 16O1H) was carried out in the same location as the U-Th-Pb isotope analyses in July 2023 using a CAMECA IMS 1300-HR3 in the John de Laeter Centre at Curtin University. The 133Cs+ primary beam conditions included a +10 kV acceleration potential, a beam diameter of ~15 µm, and a beam current of ~2 nA.

An individual oxygen isotope analysis lasted approximately ~4 minutes and involved pre-sputtering (rastering) for 60 s over a 10×10 µm area to approach sputter equilibrium, automated secondary ion tuning, and 120 s of simultaneous integration of masses 16O, 16O1H, and 18O. A normal incidence electron gun was utilized for charge compensation. The negative secondary ions were extracted from the sample mount (–10 kV potential) into the transfer section of the secondary ion column. Secondary ion transfer conditions included a 5 mm wide field aperture, a 121 µm entrance slit, a 202 µm exit slit, and a 50 eV energy window. The mass-separated oxygen isotopes were detected simultaneously in Faraday cups at the L’2 (16O), FC2 (16O1H) and H1 (18O) positions in the multi-detector array. Mass resolution (M/ΔM at 10% peak height) at L’2 and H1 was nominally 2500, and count rates for 16O were ~1.5–2.0 × 109 c/s.

Instrumental mass fractionation was monitored by duplicate analyses of 91500 zircon reference before and after every six unknowns. No significant drift in the IMF was identified during a session. The data for unknowns were corrected in blocks of six unknowns bracketed by two 91500 analyses at the start and end of each block. Individual spot uncertainties at 95% confidence are typically ±0.26‰ (range: 0.16–0.42‰), and include errors relating to within-spot counting statistics, internal reproducibility of the 91500 reference material, and instrumental mass fractionation. The analytical results for the Plesovice and Mud Tank zircon reference materials, which were treated as unknowns throughout the session, give an error-weighted mean for δ18O=+7.94 ± 0.08‰ (2se, MSWD=1.19, n=13), and +4.81 ± 0.07‰ (MSWD=0.69, n=13) respectively, with no outliers rejected, and within the uncertainty of the reported values (Fig. S7a). All oxygen isotope data are provided in Supplementary Data 3, and individual spot locations on each crystal are denoted on S4a & b.

δ18O(VSMOW) was calculated as follows:
δ18O VSMOW = (18O/16O)sample / (18O/16O)VSMOW - 1
where 18O/16O (VSMOW)=0.0020052, and δ18O(VSMOW) is reported in ‰.

In addition, it has been shown that the presence of water in zircon, typically as hydroxyl ions (OH-), indicates potential secondary overprint45,46. Elevated 16O1H/16O values greater than the crystalline reference material implies water within the zircon structure possibly affecting the original δ18O value. Therefore, we also calculated raw 16O1H/16O ratios as a qualitative measure of zircon OH- content47. Measured 16O1H/16O ratios in zircon reference materials were relatively consistent throughout the run, but show a steady decrease with time, likely related to diminishing outgassing of the epoxy mount with time. This background 16O1H/16O is accurately modelled using a power law function shown in Fig. S6b (R2=0.88). The fractional deviation of unknown samples away from the power law model at the same position in the analysis sequence is denoted in Fig. S7b by XOH:
XOH = (16O1H/16O)unknown / (16O1H/16O)model - 1

XOH provides a relative measure of the addition OH- present within the zircon crystal. There is a weak correlation (R2=0.14–0.16) between the IMF corrected δ18O values and XOH (Fig. S7c). Unknown analyses with XOH <0.3 show relatively consistent δ18O compositions, within uncertainty of one another, whereas samples with XOH >0.3 trend to lower δ18O values with increasing XOH. We therefore apply a cutoff XOH value of 0.3, which roughly corresponds to a 3σ envelope of the model for the standard analyses, to separate zircon preserving primary δ18O compositions from those that have been affected by secondary processes (Fig. S7b and c).

LA-ICP-MS Analysis
Titanite geochronology. U-Pb isotope analysis of titanite was performed in situ on polished thin sections of sample 2555834. Given the small grain size and low U contents, U-Pb isotopic data were acquired on a Nu Plasma II multicollector LA-ICP-MS using ion counter detectors to allow for maximum spatial resolution and sensitivity. The analyses were carried out at the GeoHistory Facility in the John de Laeter Centre, Curtin University. The sample was ablated using a RESOlution 193 nm ArF excimer laser system with a beam diameter of 16 µm. Analyses were performed at a 10 Hz laser repetition rate with a laser energy of 2.6 J.cm-2 as measured on the sample surface. Throughout the ablation, the sample cell was flushed with ultra high-purity He (350 ml/min) and N2 (3.8 ml/min).

U–Pb data were processed in Iolite48 using the built-in “Geochronology4” data reduction scheme with MKED1 (1517 ± 0.32 Ma)49 as a calibration standard as its radiogenic Pb/common Pb ratio is fixed. The titanite BLR (1047.1 ± 0.4 Ma)50 was employed as a secondary reference material and treated as an unknown to verify the procedure. For BLR, a model-1 regression through the U-Pb isotopic data from common Pb (approximated Stacey-Kramer common Pb model44) yielded a lower intercept date of 1048 ± 3 Ma (MSWD=1.6, n=16), identical to the recommended value. Titanite U–Pb geochronology data are presented in Supplementary Data S4. All uncertainties are reported at a 2σ level. Representative BSE and electron back-scatter diffraction image (EBSD) detailing the textural complexity of the titanite are provided in Fig. S5a, b respectively with several U-Pb analyses locations highlighted. A histogram showing the grain-size distribution (measured as the maximum Feret diameter) of individual titanite crystals is provided in Fig. S5c.

Garnet geochronology. Garnet Lu-Hf geochronology followed methods outlined by Simpson et al.51 and Ribeiro et al.52. In situ Lu–Hf geochronology was performed on polished thin sections at the John de Laeter Centre GeoHistory Facility, Curtin University, Australia. The instrumentation consisted of a RESOlution 193 nm ArF excimer laser equipped with a Laurin Technic S155 sample cell, coupled to an Agilent 8900 triple quadrupole ICP-MS/MS operating with a NH₃/He gas mixture in the reaction cell (20% NH₃ in premixed He). This setup follows the methodology outlined by Simpson et al.51 or Lu and Hf isotope measurements. The laser, gas flow, and mass spectrometer setup are summarised in Supplementary Data 1. A Laurin Technic ‘squid’ aerosol mixing device was used to smooth aerosol pulses between the laser and ICP-MS. Garnet samples were ablated using a fluence of ~3.5 J.cm-2, repetition rate of 10 Hz and a circular laser beam of 90 µm. Garnet reference materials (GWA–1 and GWA–2)52 and NIST SRM 610 were ablated using a circular laser beam of 130 µm and 33 µm, respectively. A low flow rate of N2 (4 ml.min-1) was added to the carrier gas before the ICP torch to enhance sensitivity53, passing through inline Hg traps. To optimise sensitivity, nitrogen (4 ml/min) was added to the carrier gas stream prior to the ICP torch, Instrument tuning was first carried out in single quadrupole mode (no collision gas) using NIST SRM 610 to optimize plasma conditions, suppress oxide interferences, and enhance ¹⁷⁵Lu sensitivity, with a 50 µm beam. Upon switching to MS/MS mode, the ICP-MS was tuned to maximise sensitivity for Hf reaction products using NH3/He. To check for potential systematic errors in Lu/Hf from pulse/analogue (P/A) conversion in the 8900-detector system, we performed repeated measurements of the NIST SRM 610 standard using different spot sizes and detector modes for 175Lu (33 and 64 µm), resulting in a P/A factor of ~0.18. Regression of Lu–Hf isochron data was calculated using the IsoplotR software54,55, with 176Lu decay constant calculated using the value of Söderlund et al.56. Inverse isochron ages and analytical uncertainties are stated at the 2 σ level of confidence (95 % confidence). Garnet GWA–1 (1267.0 ± 3.0)52 was employed as reference material to account for instrumental and matrix effects, and GWA–2 (934.7 ± 1.4 Ma)52 was employed as a secondary age check. GWA–1 yielded a matrix uncorrected inverse isochron age of 1281 ± 24 Ma (N=34, MSWD=1.2), indicating a positive offset of ~1.1 % compared to the isotope dilution Lu–Hf reference age. After applying this correction factor to 176Lu/176Hf ratios (garnet reference materials and unknowns), GWA–2 yielded a matrix corrected inverse isochron age of 944 ± 24 Ma (N=31, MSWD=1.1) in agreement with the reference age. All the garnet Lu-Hf isotope data are provided in Supplementary Data 5. Inverse isochron diagrams showing the results for secondary garnet reference materials are provided in Fig. S6d, e.

Titanite Zr concentrations. Zr concentration (and several other major and trace elements) were measured in titanite in a separate analytical session. These measurements were made using the same laser and instrument settings used for U-Pb isotope analysis and element beam intensities were measured on a triple quadrupole Agilent 8900 ICP-MS. The trace element glass standard GSD1 was used as the internal calibration standard with 49Ti as the standard element. Glasses BHVO, NIST SRM 610, NIST SRM 612, and NIST SRM 614 were used to check the accuracy of the data reduction procedure. The median measured Zr concentration and standard deviations (2σ) for these secondary reference materials are provide in Table S1 below. A full table of the LA-ICP-MS trace element data are provided in Supplementary Data 6.

Thermodynamic and Trace Elment Modelling
Pseudosections were calculated using thermocalc v3.5022 and the internally consistent dataset ds63 (created on 05.01.2015) of Holland and Powell57. Pressure-temperature diagrams were constructed in the 10-component Na2O—CaO—K2O—FeO—MgO—Al2O3—SiO2—H2O—TiO2—O2 (NCKFMASHTO) model system. The activity-composition (a-X) models used in this study are: clinoamphibole (actinolite-glaucophane-hornblende-cummingtonite), clinopyroxene (augite) and tonalitic melt of Green et al.58; epidote of Holland and Powell57; ternary plagioclase model (C1) of Holland et al.59,60; chlorite, biotite, muscovite-paragonite, garnet, orthopyroxene of White et al.61; spinel-magnetite of White et al.62; ilmenite of White et al.63; albite, quartz, rutile, titanite and H2O are pure end-member phases. Bulk compositions were converted from weight % oxides to molar % oxides by ignoring minor MnO and P2O5 and assuming that 20% of total Fe is present as Fe3+. The H2O-content was chosen in order to stabilize the fluid-present solidus over the explored pressure range. Uncertainties for the assemblage field boundaries are approximately ± 1 kbar and ± 50 °C at a 2σ level64. Pressure-temperature (P-T) pseudosections for each modelled composition are provided in Fig. S8. Temperature-composition (T-X) and pressure-composition (P-X) pseudosections for sample 255836 are provided in Fig. S9.

Utilizing the same thermodynamic dataset and a–X models as outlined above, Gibbs free energy minimization software PerpleX was used to calculate the composition and abundance of melt and solid (residual) phases at a given pressure and temperature for sample 255836. The calculated residual stable phase assemblage (X, mass fraction), corresponding melt proportion (F), and mineral-melt partition coefficients (Kd) appropriate for partial melting of metabasic rocks65 were used to calculate bulk partition coefficients [Ds = Ʃ(Kd × X)], and thereby constrain the trace element compositions of melts produced along different geothermal gradients. For these calculations, we used the modal batch melting equation66:
Cm = Co / (F + (1 - F) × Ds)
where Co and Cm are the concentration of elements in the source rock and melt, respectively.

The Zr contents of titanites can be used as an additional independent geothermometer24; however, the Zr-in-titanite (ZiT) temperature calibration is dependent on several other variables, including the activity of titania (aTiO2), activity of silica (aSiO2) and pressure24. To account for these variables, we calculate titanite Zr ppm P–T contours for the thermodynamic model of sample 255834 using the approach of Yakymchuk et al.67,68. Quartz is predicted to be present across the entire modelled P–T range therefore aSiO2 is taken to be 1.0. aTiO2 at each P–T point is related to the chemical potential of titania via the expression:
μP,T,TiO2 = μ0TiO2 + RT * ln(aTiO2)
where μP,T,TiO2 is the chemical potential of rutile at a given P–T point, μ0TiO2 is standard state chemical potential of reference (i.e. rutile), R is the universal gas constant and T is temperature in Kelvin. As μP,T,TiO2 and μ0TiO2 may be calculated using thermocalc, one can solve for aTiO2 at a given P-T point within the model domain via the expression:
aTiO2 = e^((μP,T,TiO2 - μ0TiO2) / RT)

The Zr concentration at each P–T point is then calculated by substituting the relevant values for aSiO2, aTiO2, P and T into the equation from Hayden et al.24. Titanite Zr concentration contours for sample 255834 calculated via this approach are shown as dashed white lines in Fig. S8b.

EPMA Analysis
Mineral compositions were determined using a JEOL JXA-8530F Plus Hyper-Probe electron probe microanalyzer (EMPA) equipped with an energy-dispersive (ED) and five wavelength-dispersive (WD) spectrometers housed at the Institute of Earth Sciences, NAWI Graz Geocenter, University of Graz, Austria. Measurement conditions were 15 kV accelerating voltage, 10 nA beam current, and a beam diameter of 1–5 µm depending on the analysed mineral. Calibration was carried out using a range of synthetic and natural mineral reference materials. Mineral compositions measured using EPMA are provided in Supplementary Data 7. The calculation of structural formulae from measured weight percent oxide was carried out using the MATLAB programme MinPlot69.

Diffusion Modelling
Apparent Zirconium in titanite temperatures are a function of cooling rate, initial temperature and grain radius, and were calculated using ZirTiDiS (Zirconium in Titanite Diffusion Software)25 executed in MATLAB. ZirTiDiS solves the diffusion equation for a spherical geometry using an implicit finite difference scheme and experimentally-determined diffusion parameters for Zr in titanite70. The initial Zr concentration is calculated for specified starting temperature, pressure and activities of TiO2 and SiO2 using the equation of ref. 24 and allocated to the entire grain. Cooling rates are kept constant for individual runs. During cooling, the boundary condition for equilibrium Zr concentration at the grain rim is calculated at each timestep using the Hayden equation. The resulting concentration gradients drive Zr diffusion from grain core to rim, resulting in variable effective Zr concentrations for each cooling history. The final Zr concentration remaining at the grain core is then converted to apparent ZiT temperature with the Hayden equation. The diagrams in Fig. 3b are calculated for cooling from variable initial temperatures (coloured lines) to 600 °C at a constant pressure of 8.5 kbar and fixed activities (aTiO2=0.8, aSiO2=1). It is pointed out that since these values are kept constant during each run, the choice of pressure and activities does not affect the recorded apparent Zr-in-titanite temperature. See Schorn and Moulas25 for details.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reports geochronology, geochemistry and petrography, and P-T-X modeling for two samples of migmatitic amphibolite in the Sylvania Inlier south of the Pilbara craton in western Australia. The methods used are modern and meet the expected standards in the field. There is enough detail provided in the methods and other supplementary information for the work to be evaluated and reproduced. It is really nice work.\nThe paper makes a convincing case that the 3.56 Ga migmatitic amphibolite experienced partial melting in the same event that produced metatonalite rocks located 5 km to the east that were the subject of a previous study. The authors make a case for the amphibolite was hydrothermally altered then partially melted to form early felsic crust. Their modeling gives conditions of 650-720°C and 7-8 kb that produced 2-5% melt. The processes they describe for the Sylvania Inlier are convincing and support the hypothesis that partial melting of altered mafic rocks produced Paleoarchean tonalitic felsic crust.\nI am less persuaded by their argument that the Sylvania migmatitic amphibolites are a good analogue for early mafic crust in Pilbara, and particularly for Acasta. The comparator basalts that they chose from Pilbara seem very different in terms of trace elements—see Fig 1f and S3a—and Pilbara basalts have a much larger range in composition in terms of major elements (Fig 1e). Rather than explaining a craton from two rocks in an adjacent area, to me it would make more sense to look at the variety of mafic rocks in Pilbara and the various petrologic processes that formed them to determine how representative this particular migmatitic amphibolite is of early mafic crust.\nThe comparison to Acasta seems even more tenuous.\n• The major element chemistry of Sylvania is not as Fe-rich as the Acasta amphibolites, and the Sylvania metatonalites are distinct from the Acasta felsic rocks.\n• The ages are different by nearly 500 Ma at a time when the Earth was evolving quickly, cooling rapidly and impacts tailing off. The authors did not make the case that early felsic crust formed asynchronously and that therefore processes in Pilbara should apply to Acasta 500 Ma earlier.\n• The mismatches in chemistry described on line 201 are concerning.\n• The significance of the iron-enrichment and its origin is not well explained. I’m curious how plagioclase drives melts to high FeOT (line 235). It isn’t clear that the ferroan lunar anorthosites are relevant even beyond the mismatches described on line 238.\nThere are quite a few typos and small mistakes in the manuscript (l 213 analogous for analogues; l 221 Achean for Archean) and misnumbered figures (1f, 2cde) and missing information (Fig S3a “Ref X”). Figure 4 discusses the origin of high pressure TTG signatures (not taken up in the text) but in any case high-P TTG may be misinterpreted (see Laurent et al. 2024 Elements).\nThere is such good science here, but my impression is that the authors would be better off with an expanded manuscript focused on the Sylvania rocks instead of stretching to try to make a splashy case for 4 Ga early crustal processes based on two 3.5 Ga rocks so they can be published in Nature.", "reviewer_2": "This paper presents a detailed study of new amphibolite-facies samples that are convincingly demonstrated to be of early Paleoarchaean age.\nThe observations and data are well documented, including via the supplementary files. The age information, and interpretation of oxygen isotope data, are convincing, and the implications of having such samples to investigate in detail is very exciting: analogues of Hadean protocrust. The interpretation appears to be sound and sufficiently substantiated, although there are a one or two aspects that could be explored a bit further (likely constrained by the short-format). Overall, I think this is a quite rigorous and exciting study that would be of strong interest to the journal readership, and thus appropriate for publication. My comments are really very minor indeed, and intended constructively.\nI am particularly pleased to see the rigor of documentation of the analyses (Sup files), and mineral textures. The figures included in the main text are all of value and well chosen to support the descriptions and discussion. There is a lot packed into this manuscript, but I found the discussion and interpretation to be well supported by the data presented.\nMy main comment regarding interpretation of the data is with respect to the trace element systematics, and particularly REE of the amphibolites. Lines 112 – 115 the Zr-Hf anomalies are noted to be more pronounced and attributed to loss of LREE elements via hydrothermal alteration. This, and the difference in LREE between the modelled partial melts and the observed rocks, is a bit dissatisfying (although that may be my personal biases showing).\nWould enrichment in Zr-Hf be a potential alternative (or why not?). The mobility of the LREE relative to HREE (and HFSE) would have implications for the nature of the fluid involved (ligands available for transporting, e.g. F vs Cl). This loss of LREE (presumably) relates to hydrothermal alteration near the seafloor (i.e. early), which was also responsible for the low oxygen isotopes. Does this environment of alteration fit with LREE mobility relative to HREE ? Conversely, could this have any implications for composition of Archaean seawater?\nNoting also that the more melted sample (255834) has higher LREE depletion, does this fit with presumably more hydrothermal alteration (e.g. resulting in a more hydrated protolith that could melt more readily). Presumably, the discrepancy in matching the LREE of the gneisses from the melting modelling (fig 3d) means they would require a less hydrothermally altered source. Is this reasonable, and what would this look like?\nTo be fair, I think I’m fairly convinced by your evidence, and it’s likely an issue with length constraints, but I feel there’s more here than is currently discussed.\nLines 252-260. The effect of higher Fe compositions on garnet stability is generally well argued. However, the caption for Fig S9b is lacking in some key information – primarily how XFe relates to the Fe-rich amphibolites in the study - while the XFe for modern MgO-rich basalts is given in the article text (line 256) the equivalent for the rocks here is not.\nAlso, how sensitive is this result (pressure of garnet stability field) on the assumed Fe2+:Fe3+ ratio?\nMinor typos, suggestions.\nFig 1 – mislablled f. As e. in caption. Also, need to explain Avg. Apex, ave. CF-2 etc. (explained in text only, and if you go into the supplementary data file).\nFig 2. – mixed up c and d in the caption.\nFig 3d. While an aesthetically appealing colorway here, it is a bit more difficult than it should be to distinguish the models (reds) fro the red naturally observed rocks. Please use more distinct colors.\nFig S3a – ref X (?)\nFigs S8 and S9 – admittedly people who can read such figures likely know this, but it would still be useful, and good practice, to include a key to mineral symbols here (esp. as titanite is abbreviated to sph). Particularly as I would expect this paper to be of interest to a wider audience than those conversant with pseudosections."}, "author_response": "Reviewer Comments and Responses (in red)\nReviewer 1\nThis paper reports geochronology, geochemistry and petrography, and P-T-X modeling for two samples\nof migmatitic amphibolite in the Sylvania Inlier south of the Pilbara craton in western Australia. The\nmethods used are modern and meet the expected standards in the field. There is enough detail provided in\nthe methods and other supplementary information for the work to be evaluated and reproduced. It is really\nnice work.\nThe paper makes a convincing case that the 3.56 Ga migmatitic amphibolite experienced partial melting in\nthe same event that produced metatonalite rocks located 5 km to the east that were the subject of a previous\nstudy. The authors make a case for the amphibolite was hydrothermally altered then partially melted to\nform early felsic crust. Their modeling gives conditions of 650-720°C and 7-8 kb that produced 2-5% melt.\nThe processes they describe for the Sylvania Inlier are convincing and support the hypothesis that partial\nmelting of altered mafic rocks produced Paleoarchean tonalitic felsic crust.\nThank you.\nI am less persuaded by their argument that the Sylvania migmatitic amphibolites are a good analogue for\nearly mafic crust in Pilbara, and particularly for Acasta. The comparator basalts that they chose from Pilbara\nseem very different in terms of trace elements—see Fig 1f and S3a—and Pilbara basalts have a much larger\nrange in composition in terms of major elements (Fig 1e). Rather than explaining a craton from two rocks\nin an adjacent area, to me it would make more sense to look at the variety of mafic rocks in Pilbara and the\nvarious petrologic processes that formed them to determine how representative this particular migmatitic\namphibolite is of early mafic crust.\nThis is a valid concern, but we believe there are reasonable explanations for many of the differences\nbetween the Acasta and Pilbara rocks. For example, younger hydrothermal systems in Archean mafic crust,\nsuch as the one preserved in the Dresser Caldera, produce rocks with a range of trace element chemistries,\nincluding LREE depleted material like the Sylvania amphibolites (see Caruso et al., 2021). We should be\nclear that we are not trying to “explain a craton”, but rather intend to point out that rocks with similar\nchemistry are preserved amongst the oldest preserved material in two disparate cratons formed at vastly\ndifferent times. In our opinion this is a rather intruiging observation.\nRegarding their suitability as analogues, yes, there are differences in terms of trace elements. However, as\nwe outline in detail in detail below, the two groups of rocks are actually quite similar in terms of bulk\nFeOT:MgO ratios, which influence metamorphic phase equilibria more than absolute concentrations (see\nThompson, 1957). So while we accept the Sylvania rocks might not be the perfect analogues, we believe\nthey represent the closest equivalent available for study. Please see more detailed comments that follow.\nThe comparison to Acasta seems even more tenuous.\n• The major element chemistry of Sylvania is not as Fe-rich as the Acasta amphibolites, and the Sylvania\nmetatonalites are distinct from the Acasta felsic rocks.\nThe absolute FeOT and TiO contents may be lower, but so is the MgO content, and the FeOT/MgO and\n2\nmetamorphic phase relations are more sensitive to the relative proportions of FeOT:MgO rather than\nabsolute FeOT abundances (Thompson, 1957). This explains why the modelled melts for the Sylvania rocks\nstill reproduce the major element compositions of the Idiwhaa felsic gneisses reasonably well, despite their\nabsolute FeOT and TiO content being lower than the garnet amphibolites in Acasta.\n2\n• The ages are different by nearly 500 Ma at a time when the Earth was evolving quickly, cooling rapidly\nand impacts tailing off. The authors did not make the case that early felsic crust formed asynchronously\nand that therefore processes in Pilbara should apply to Acasta 500 Ma earlier.\nasynchronous, history of craton formation and maturation.\n• The mismatches in chemistry described on line 201 are concerning.\nIndeed, the discrepancy between LREE chemistry is disatifying. However, hydrothermally altered basalts\nwith trace elements compositions resembling the Sylvania are found in younger hydrothermal systems (e.g.\nthe 3481 Ma Dresser Caldera in the East Pilbara Terrane, see discussion of Caruso et al. (2021)). LREE\ndepleted rocks are a minor component of these systems and indeed LREE enriched rocks are more\nabundant. So while we accept the Sylvania rocks might not be representative of average trace element\ncompositions of Archean mafic rocks, metamorphic phase relations are controlled by major element\nchemistry and constraints on any subsequent metamorphic conditions these rocks were buried to will be\nequally valid for any LREE enriched counterparts. To address these concerns, we have exanded the\ndiscussion, with specific reference to the Dresser hydrothermal system.\n• The significance of the iron-enrichment and its origin is not well explained. I’m curious how plagioclase\ndrives melts to high FeOT (line 235). It isn’t clear that the ferroan lunar anorthosites are relevant even\nbeyond the mismatches described on line 238.\nThe significance is that it indicates the parental magmas must have been relatively anhydrous and reduced\nsuch that Fe-Ti oxide saturation occurred late in the differentiation history. This is required to produce\nmagmas with relatively high FeO and TiO2 and low MgO content. These aspects and the role of plagioclase\nin driving FeO enrichment in tholeiitic magmas is well described by Grove and Kinzler (1986). They note\nthat FeO enrichment is primarily achieved by removal of plagiolcase and Mg-rich clinopyroxene, with\nplagioclase making up between 60-75% of cumulative fractionating assemblages in their experiments. To\naddress this comment we have removed the comparison to ferroan lunar anorthosite, and expanded the\ndiscussion on the possible mechanisms of FeO enrichment in the parental magmas for the Sylvania rocks.\nThere are quite a few typos and small mistakes in the manuscript (l 213 analogous for analogues; l 221\nAchean for Archean) and misnumbered figures (1f, 2cde) and missing information (Fig S3a “Ref X”). Figure\n4 discusses the origin of high pressure TTG signatures (not taken up in the text) but in any case high-P\nTTG may be misinterpreted (see Laurent et al. 2024 Elements).\nWe have corrected any remaining errors in the manuscripts and thank the reviewer for pointing these out.\nThere is such good science here, but my impression is that the authors would be better off with an expanded\nmanuscript focused on the Sylvania rocks instead of stretching to try to make a splashy case for 4 Ga early\ncrustal processes based on two 3.5 Ga rocks so they can be published in Nature.\nWe are grateful for the compliments on the science. While we may only have studied two samples here, we\nbelieve the thermodynamic constrains on melting conditions offer valuatble insight into conditions of early\nArchean crust formation. Samples/outcrops of Hadean crust are also extremely scarce. Despite there being\nseveral differences to Hadean rocks, we believe there are enough similarities to warrant a comparison.\nGiven the acute scarcity of preserved Hadean crust, we believe our best option to be able to better\nunderstand Hadean crustal processes is through studying younger rocks that have similar chemistry.\nTherefore, in our opinion it would be remiss to not point out these similarities. However, we do\nacknowledge there are limitations to this approach and hope that elaborating on some of these in the revised\nmanuscript will make for a more nuanced discussion. Given the rarity of rocks of this age and composition\nwe think these findings will be of interest to a broad readership of a journal like Nature Communications,\nand note that reviewer #2 shares these sentiments.\nReviewer 2\nThis paper presents a detailed study of new amphibolite-facies samples that are convincingly demonstrated\nto be of early Paleoarchaean age. The observations and data are well documented, including via the\nsupplementary files. The age information, and interpretation of oxygen isotope data, are convincing, and\nthe implications of having such samples to investigate in detail is very exciting: analogues of Hadean\n2\nprotocrust. The interpretation appears to be sound and sufficiently substantiated, although there are a one\nor two aspects that could be explored a bit further (likely constrained by the short-format). Overall, I think\nthis is a quite rigorous and exciting study that would be of strong interest to the journal readership, and\nthus appropriate for publication. My comments are really very minor indeed, and intended constructively.\nThank you for the compliments.\nI am particularly pleased to see the rigor of documentation of the analyses (Sup files), and mineral textures.\nThe figures included in the main text are all of value and well chosen to support the descriptions and\ndiscussion. There is a lot packed into this manuscript, but I found the discussion and interpretation to be\nwell supported by the data presented.\nThank you. We are please the information provided in the supplementary files are helpful.\nMy main comment regarding interpretation of the data is with respect to the trace element systematics,\nand particularly REE of the amphibolites. Lines 112 – 115 the Zr-Hf anomalies are noted to be more\npronounced and attributed to loss of LREE elements via hydrothermal alteration. This, and the difference\nin LREE between the modelled partial melts and the observed rocks, is a bit dissatisfying (although that\nmay be my personal biases showing).\nThis is an interesting point, and indeed, the discrepancy between the modelled melts and many natural rock\nsamples is a bit dissatisfying. However, we think there is reasonable explanation for this. Please see the\nresponses to the comments below:\nWould enrichment in Zr-Hf be a potential alternative (or why not?). The mobility of the LREE relative to\nHREE (and HFSE) would have implications for the nature of the fluid involved (ligands available for\ntransporting, e.g. F vs Cl). This loss of LREE (presumably) relates to hydrothermal alteration near the\nseafloor (i.e. early), which was also responsible for the low oxygen isotopes. Does this environment of\nalteration fit with LREE mobility relative to HREE ? Conversely, could this have any implications for\ncomposition of Archaean seawater?\nIn theory yes, but we do not have clear processes for how this might occur. Assimilation of zircon rich\nsedimentary material? At the time these rocks where emplaced there was probably very little felsic crust\naround (let alone relatively mature zircon-rich sedimentary rocks) so in our opinion such scenarious are\nunlikely. We would need additional isotopic data to begin to explore these possibilities.\nInstead, we believe the alteration environments preserved in Archean mafic crust do support mobility of\nLREE relative to HREE. For example, Caruso et al. (2021) conducted a detailed study of major and trace\nelement mobility in the North Start Basalts from the Dresser Caldera in the East Pilbara Terrane. They\nnote that propylitic, intermediate phyllic, and phyllic styles of alteration (their nomenclature) – which reflect\nhigher-temperature chlorite-dominated alteration assemblages - were associated with depletions in LREE\nover HREE and Zr-Ti-Hf on the order of 10-20% in some places. Unfortunately, this study did not go into\ndetail on the petrological processes involved in LREE depletion. One explanation for the mobility of LREE\nover HREE and Zr-Hf-Ti could be alteration of primary magmatic apatite during seafloor hydrothermal\nalteration, as depletion of LREE relative to HREE is observed in fluid-modified apatites in some shear\nzones for example (Ribeiro et al., 2020).\nTo address these comments we have expanded the discussion, noting that younger hydrothermal systems\nin Archean mafic crust, like that found in the 3481 Ma Dresser Caldera, produce rocks with similar\nchemistry to the Sylvania amphibolites. We also note that LREE depleted rocks are indeed a minor\ncomponent of these systems, which might explain why the majority of Archean felsic crust appear to be\nderived from LREE enriched sources.\nNoting also that the more melted sample (255834) has higher LREE depletion, does this fit with presumably\nmore hydrothermal alteration (e.g. resulting in a more hydrated protolith that could melt more readily).\nPresumably, the discrepancy in matching the LREE of the gneisses from the melting modelling (fig 3d)\n3\nmeans they would require a less hydrothermally altered source. Is this reasonable, and what would this look\nlike? To be fair, I think I’m fairly convinced by your evidence, and it’s likely an issue with length constraints,\nbut I feel there’s more here than is currently discussed.\nWe believe the more mafic and melt-poor samples are probably more representative of altered mafic\nprotoliths. The lower LREE in the more melt rich samples could indeed reflect difference in hydration state\nand melt fertility. However, there are probably multiple processes involved here. The lower LREE in the\nmore melt rich samples might also be explained by small amounts of melt loss, with preferential retention\nof plagioclase.\nLines 252-260. The effect of higher Fe compositions on garnet stability is generally well argued. However,\nthe caption for Fig S9b is lacking in some key information – primarily how XFe relates to the Fe-rich\namphibolites in the study - while the XFe for modern MgO-rich basalts is given in the article text (line 256)\nthe equivalent for the rocks here is not. Also, how sensitive is this result (pressure of garnet stability field)\non the assumed Fe2+:Fe3+ ratio?\nThanks for pointing this out. We have modified the caption and provide this additional information.\nThe point about Fe3+/Fe2+ is also relevant. To test this, we have produced a quick model in PerpleX using\nthe bulk composition for sample 255836B (see figure below). This result shows that changing Fe3+/Fe2+\ndoes influence the pressure of the garnet stability field, which varies from ~7.5 kbar at 0% Fe3+ and 10 kbar\nat 50% Fe3+. This result is well within the range of results from our P-T modelling (7-10 kbar) and its\nassociated uncertainty (within the uncertainty of the modelling (± 1 kbar).\nIt is important to note that changing Fe3+/Fe2+ will not only affect garnet but also change the P-T\nconditions at which ilmenite and titanite are stable. Since the chosen Fe3+content (20%) reproduces the\npreserved assemblage at similar temperatures to the independent Zr-in-titanite thermometry, we believe it\nis a reasonable approximation of the conditions these rocks experienced. In general, in the Archean it is\nunlikely that conditions were highly oxidising. Given that variations in the pressure of the garnet stability\nfield are relatively limited for Fe3+ < 40-50% (see figure below), we infer that any variations in garnet\nstability resulting from differences in Fe3+/Fe2+ were probably minor compared to those resulting from\nvariation in bulk rock X .\nFeO\nDiagram of Pressure versus X(Fe3+)\n(Fe3+/[Fe2+ + Fe3+]) for sample\n255836B calculated at 700°C using\nPerpleX. All calculations in our\nstudy were conducted with X(Fe3+)\n= 0.2.\nFig 1 – mislablled f. As e. in caption. Also, need to explain Avg. Apex, ave. CF-2 etc. (explained in text\nonly, and if you go into the supplementary data file).\nWe have corrected the mislabled figure and provided explanations for the rock compositions shown.\nFig 2. – mixed up c and d in the caption.\nWe have corrected the caption in Figure 2.\nFig 3d. While an aesthetically appealing colorway here, it is a bit more difficult than it should be to\ndistinguish the models (reds) fro the red naturally observed rocks. Please use more distinct colors.\nWe have changed the color and line weighting for the naturally observed rocks so they are better\ndistinguished from the modelled melts.\nFig S3a – ref X (?)\nWe have added the corrected references here.\nFigs S8 and S9 – admittedly people who can read such figures likely know this, but it would still be useful,\nand good practice, to include a key to mineral symbols here (esp. as titanite is abbreviated to sph).\nParticularly as I would expect this paper to be of interest to a wider audience than those conversant with\npseudosections.\nThanks for point this out. We have added the description of the mineral abbreviations to the relevant figure\ncaptions.\nCaruso, S., Van Kranendonk, M.J., Baumgartner, R.J., Fiorentini, M.L., Forster, M.A., 2021.\nThe role of magmatic fluids in the~ 3.48 Ga Dresser Caldera, Pilbara Craton: New\ninsights from the geochemical investigation of hydrothermal alteration. Precambrian\nResearch, 362: 106299.\nGrove, T.L., Kinzler, R.J., 1986. Petrogenesis of andesites. Annual Review of Earth and\nPlanetary Sciences, 14(1): 417-454.\nRibeiro, B.V. et al., 2020. Strain localization and fluid-assisted deformation in apatite and its\ninfluence on trace elements and U–Pb systematics. Earth and Planetary Science\nLetters, 545: 116421.\nThompson, J.B., 1957. The graphical analysis of mineral assemblages in pelitic schists.\nAmerican Mineralogist: Journal of Earth and Planetary Materials, 42(11-12): 842-\n858.\n6"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for your response to reviewer comments. Although many of my concerns remain I appreciate that the Archean material available is sparse and imperfect analogs may be all we have at present.", "reviewer_2": "I reviewed the earlier version of this manuscript. As noted then, this paper presents a detailed study of new amphibolite- facies samples that are convincingly demonstrated to be of early Paleoarchaean age. The observations and data are well documented, including via the supplementary files. The age information, and interpretation of oxygen isotope data, are convincing, and the implications of having such samples to investigate in detail is very exciting: analogues of Hadean 2 protocrust. The interpretation appears to be sound and sufficiently substantiated. Overall, I think this is a quite rigorous and exciting study that would be of strong interest to the journal readership, and thus appropriate for publication. My comments in my review were relatively minor.\nI would like to thank the authors for their thoughtful and considered responses. The expanded discussion regarding the LREE depletion and the 3481 Ma Dresser rocks in the manuscript makes this section a more convincing and satisfactory read - thank you.\nI am happy with the edits made and have no additional comments of substance.", "reviewer_3": "Review « Incipient continent formation by shallow melting of an Fe-rich mafic protocrust» by Hartnady et al.\nThis paper presents new data on 2 amphibolites and a metatonalite from the Sylvania Inlier. The authors convincingly show robust Paleoarchean ages for both lithologies. In particular, they date the metamorphism of both rock types using titanite in the amphibolite and garnet in the metatonalite. They argue that one of the amphibolites represent a potential mafic protocrust affected by high temperature hydrothermalism which is revealed by low oxygen isotopic composition. Then, they propose via modelling that this amphibolite can be a good candidate for the source of the metatonalite. In a second part, they apply this interpretation to Acasta gneisses even though strong dissimilarities for trace elements exist. They suggest that a similar mafic protocrust could also be the source for the genesis of the Idiwhaa Gneiss (Acasta). This is the less convincing part as this is not fully supported by their data. They, however, elaborate on these discrepancies and clearly expose to the reader potential scenarios to explain them. The paper is well-written and the rationale behind the analyses and the modelling is well-demonstrated as well as the limitation of using such technique to generate some of the trace elements signatures of the most felsic rocks present at a similar time.\nI particularly appreciated the extensive supplementary data that allow to assess the quality of the acquired data. Overall, I think that this is excellent paper for the community. The data are of very good quality and the interpretation convincing. The figures are of good quality and the impact of these results for the community makes it a good manuscript for publication in nature communication.\nI have only few comments:\n- The title infers a Fe-rich mafic protocrust, but in reality, the Fe content of the studied amphibolites is not higher than for mafic rocks already published in the literature (eg Condie et al., 2016). The paper proves that using the studied amphibolite as the potential source of the genesis of Metatonalite and other Fe-rich gnreisses (AGC) is a viable process and works with the Fe-richer endmember of the amphibolite studied in this contribution. However, this type of composition (about FeOT = 10%) is common for Archean mafics. The current title is misleading and inaccurate. Fe-rich should be remove.\n- L58 Remove “that preserve geochemical features remarkably similar to those of Hadean amphibolites from the AGC” This sentence is exaggerated as the compositional fields of these amphibolites and the amphibolites from Sylvania overlap (e.g; Fig. 1b). Indeed, major and trace elements differ in the presented figures. The only similar geochemical feature is the low oxygen zircon values, which is not a strong argument by itself to invoke an overall similarity.\n- L126 add age value with uncertainties for the metatonalite.\n- L120-L132 Comment on the age of the dated zircon from the amphibolite: In this section, the authors use the wording “primary crystalline age” which might be interpreted as igneous. Everywhere else in the text, authors imply that the zircon ages in the amphibolites are dating the HT metamorphism. What are the arguments for attributing this age (3567 Ma) to metamorphism and not magmatism? This is unclear in the paper. I see in supplementary that pictures of zircon revealed secondary processes (metamorphic textures) but the picture Fig.2a could be interpreted as oscillatory zoning.\nIt could be that zircon ages are magmatic and that the titanite grains date a HT metamorphic event that occurred rapidly after the magmatic event? This would be hidden within the wider uncertainties associated to this event. This has no major implications on the conclusions of the paper but is rather important for the interpretation of ages and needs to be clarified in the current version.\n- L140 “that could have drove…”\n- L199-L 239 Many metamafics would share similar major element features. This does not mean that they have the same history. The section L197 in its current state is a bit of a stretch. Indeed, there are more dissimilarities than similarities in the geochemistry of these amphibolite (AGC vs Sylvania) and AGC gneisses. This is the only part of the paper where the reading is not pleasant. I would advise the authors to reorganize this section by starting with the section L224-229, focus on the East-Pilbara Interpretation and then at the end extrapolate on a potential application for “mafic protocrust” for Acasta. The current version swings the reader’s mind from interpretation applied to East-Pilbara to Acasta several times within few lines which is not satisfying.\n- L244 “we propose that a similar…”\n- L251 “could also indicate that…”\n- L286 If below then why having a”>” just after?\n- Fig. 1 f , the legend is not self-explanatory. Please describe in caption the Avg Apex, high TiO2, AvgCF-2. Not sure either to which line corresponds the intermediate TiO2 annotated on the figure.\n- Fig. 2 a Please add information on ages reported on (a). Are they concordant ages, 206/207 ratios? Additional explanation on white circles corresponding to ox isotopes should also appear.\n- Fig. 3(a), for consistency, please replace sph by titanite in the peak assemblage fields\n- S6 a-c there are no indications of the names of the secondary reference standard. Please, add this information in caption and or figure."}, "author_response": "Reviewer Comments and Responses (in red)\nReviewer 1 & Reviewer 2\nThank you, we very much appreciate the constructive and helpful reviews.\nReviewer 3\nWe also thank the third reviwer for the constructive review that has helped us to clarify a few remaining\npoints. Detailed response to their specific comments are provided below:\n- The title infers a Fe-rich mafic protocrust, but in reality, the Fe content of the studied amphibolites is\nnot higher than for mafic rocks already published in the literature (eg Condie et al., 2016). The paper\nproves that using the studied amphibolite as the potential source of the genesis of Metatonalite and other\nFe-rich gnreisses (AGC) is a viable process and works with the Fe-richer endmember of the amphibolite\nstudied in this contribution. However, this type of composition (about FeOT = 10%) is common for\nArchean mafics. The current title is misleading and inaccurate. Fe-rich should be remove.\nOk, we have modifed the title accordingly.\n- L58 Remove “that preserve geochemical features remarkably similar to those of Hadean amphibolites\nfrom the AGC” This sentence is exaggerated as the compositional fields of these amphibolites and the\namphibolites from Sylvania overlap (e.g; Fig. 1b). Indeed, major and trace elements differ in the presented\nfigures. The only similar geochemical feature is the low oxygen zircon values, which is not a strong\nargument by itself to invoke an overall similarity.\nOk, we have modified this sentence.\n- L126 add age value with uncertainties for the metatonalite.\nWe have added the age values and uncertainties for the metatonalites\n- L120-L132 Comment on the age of the dated zircon from the amphibolite: In this section, the authors\nuse the wording “primary crystalline age” which might be interpreted as igneous. Everywhere else in the\ntext, authors imply that the zircon ages in the amphibolites are dating the HT metamorphism. What are\nthe arguments for attributing this age (3567 Ma) to metamorphism and not magmatism? This is unclear in\nthe paper. I see in supplementary that pictures of zircon revealed secondary processes (metamorphic\ntextures) but the picture Fig.2a could be interpreted as oscillatory zoning. It could be that zircon ages are\nmagmatic and that the titanite grains date a HT metamorphic event that occurred rapidly after the\nmagmatic event? This would be hidden within the wider uncertainties associated to this event. This has\nno major implications on the conclusions of the paper but is rather important for the interpretation of\nages and needs to be clarified in the current version.\nThis is a fiar point, and is something we have considered However, we think that an anatectic origin is\nmost likely given the age and oxygen composition of zircon in the migmatitic amphibolites and\nmetatonalites are identical. Although it is possible that zircon in the metabsite might be magmatic, the\nzircon in the tonalite must refelct the solidification of the melts / cumulate formed by metamorphism of\namphibolite-like sources. Given clear evidence for partial melting in the amphibolite, we don’t think it is\nnecessary to ascribe a different origin to the amphibolite-hosted zircon. With further detailed\ninvestigations of these rocks we may be able to distinguish these two possibilities.\nTo clarify this point, we have changed the nomenclature we use and instead refer to the “primary\ncrystalline zircon” simply as “unaltered crystalline zircon”, since we only mean refer to the textural\ncharacteristics of the zircon without any genertic implication. We have also modified the text to clearly\nstate that we interpret these zircon as anatectic in origin, although we do agree that an igneous origin is\nalso plausible.\n1\n- L140 “that could have drove…”\nWe have modfiied this sentence accordingly\n- L199-L 239 Many metamafics would share similar major element features. This does not mean that they\nhave the same history. The section L197 in its current state is a bit of a stretch. Indeed, there are more\ndissimilarities than similarities in the geochemistry of these amphibolite (AGC vs Sylvania) and AGC\ngneisses. This is the only part of the paper where the reading is not pleasant. I would advise the authors\nto reorganize this section by starting with the section L224-229, focus on the East-Pilbara Interpretation\nand then at the end extrapolate on a potential application for “mafic protocrust” for Acasta. The current\nversion swings the reader’s mind from interpretation applied to East-Pilbara to Acasta several times\nwithin few lines which is not satisfying.\nThanks you for highlighting this. We have rearrange this section to better distinguish dicussion relted to\nthe East-Pilbara from that related to Acasta, and hope this makes for easier reading.\n- L244 “we propose that a similar…”\nWe have modified the sentence accordingly.\n- L251 “could also indicate that…”\nWe have modified the sentence accordingly.\n- L286 If below then why having a”>” just after?\nThanks for spotting this, we have removed the “>”\n- Fig. 2 a Please add information on ages reported on (a). Are they concordant ages, 206/207 ratios?\nAdditional explanation on white circles corresponding to ox isotopes should also appear.\nThanks for pointing this out. We have added the following text to the figure caption: “Red ellipses\ndenote SHIRMP U-Pb analysis spots with corresponding common-Pb corrected 207Pb/206Pb dates, and white\nellipses denote SIMS oxygen isotope analyses spots and δ18O (VSMOW) composition.”\n- Fig. 1 f , the legend is not self-explanatory. Please describe in caption the Avg Apex, high TiO2, AvgCF-\n2. Not sure either to which line corresponds the intermediate TiO2 annotated on the figure.\nOk, we moved the labels for the compositions around so it is clear which lines refer to which\ncomposition. We have also added the following text to the caption to clarify the what each composition\ncorresponds to: “The average Apex and CF-2 compositions denote a typical tholeiitic and enriched basalt\nrespectively (following Smithies et al.1), which are similar to the high TiO and intermediate TiO groups of Mt\n2 2\nAda basalt from Murphy et al.2”\n- Fig. 3(a), for consistency, please replace sph by titanite in the peak assemblage fields\nok, we have replaced sph with ttn here and in the supplementary information.\nS6 a-c there are no indications of the names of the secondary reference standard. Please, add this\ninformation in caption and or figure.\nThanks for pointing this out. We have added the secondary reference mateiral names to the top pof the\nrelevant panels in this figure.\nReferences\ncontinental crust through infracrustal melting of enriched basalt. Earth and Planetary\nScience Letters 281, 298-306 (2009).\nfrom the East Pilbara Terrane, Western Australia. Chemical Geology 578, 120301\n(2021).\n2"}]

Decision letter