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

Abstract

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.

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Fig. 1: The Paleoarchean Barberton TTG plutons and BRVC volcanic rocks are, respectively, crystal-rich and liquid-rich magmatic units.
Fig. 2: Zircon isotopic analyses showing that the Paleoarchean Barberton TTG plutons and silicic volcanic rocks are coeval and co-genetic.
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.
Fig. 4: Two possible geological interpretations of the geochemical diversity of Archean TTGs.

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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.

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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.

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Correspondence to Oscar Laurent.

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Extended data

Extended Data Fig. 1 Geological map of the southern Barberton Greenstone Belt.

Modified from published maps21,26,27 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.

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 xenoltihs 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).

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.

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).

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 Belt28,29,30). 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.

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 xenoliths26,27 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.

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.

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Supplementary Information

Supplementary information about modelling and Fig. 1.

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Supplementary Tables 1–8.

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Laurent, O., Björnsen, J., Wotzlaw, JF. et al. Earth’s earliest granitoids are crystal-rich magma reservoirs tapped by silicic eruptions. Nat. Geosci. 13, 163–169 (2020). https://doi.org/10.1038/s41561-019-0520-6

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