The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.
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We acknowledge financial support from The Institute of Geoscience Research (TIGeR) at Curtin University. R.H.S. publishes with the permission of the Executive Director, Geological Survey of Western Australia.
The authors declare no competing financial interests.
Reviewer Information Nature thanks J. Bédard, R. Rapp and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, La/Nb versus Ti/Gd ratios. b, La/Nb ratios versus MgO (wt%) concentration. c, La/Nb ratios versus wt% silica (SiO2, anhydrous) concentrations. d, Th/Nb ratios versus wt% SiO2 (anhydrous) concentrations. Five samples (green dots) that define a trend to higher La/Nb and Th/Nb ratios with increasing SiO2 levels and decreasing Ti/Gd ratios are thought to have undergone assimilation of felsic crust4 and were excluded from subsequent modelling. The remaining ten samples (pink dots), with low Th/Nb and La/Nb ratios, are considered to be free of major crustal influence22,23.
Full pressure–temperature (P–T) pseudosection for the average uncontaminated Coucal basalt (n = 10), using an Fe3+/ΣFe ratio of 0.1 (ref. 24) and a water content just sufficient to saturate the solidus at 1.0 GPa. The red dashed lines show melt proportions as mol% on a one-oxide basis to approximate vol%. The white dashed lines show linear geotherms in °C per gigapascal. The 900 °C GPa−1 geotherm discussed in detail in the text, is shown in yellow. This diagram forms the basis for the simplified version shown in Fig. 2. aug, augite; bi, biotite; g, garnet; hb, hornblende; H2O, aqueous fluid; ilm, ilmenite; ksp, K-feldspar; L, melt; mt, magnetite; muscovite (mu); opx, orthopyroxene; pl, plagioclase; q, quartz; ru, rutile; sph, sphene = titanite.
Extended Data Figure 3 Apparent geotherms recorded in metamorphic rocks dated to 2.4 Gyr ago or older.
The geotherms (T/P) are shown in units of °C per gigapascal. All but one rock of Archaean age (2.5 Gyr or older) record apparent geotherms of 500 °C GPa−1 or higher, and most are more than, or much more than, 700 °C GPa−1 (M.B. and T.E.J., manuscript in preparation). These findings are consistent with a global database of TTG compositions, 76% of which are LP- and MP-TTGs10 (see yellow and green fields in Fig. 2).
Extended Data Figure 4 Calculated melt compositions for melting along apparent geotherms of 700 °C GPa−1 and 1,100 °C GPa−1.
Calculated composition of melt produced from the average Coucal basalt along linear geotherms of 700 °C GPa−1 (top) and 1,100 °C GPa−1 (bottom) at various melt fractions (5–40%), normalized to the average composition of Palaeoarchaean East Pilbara TTG. The grey shaded region shows the 2σ envelope on the average Palaeoarchaean East Pilbara TTG data. Also shown are the average composition of TTGs and potassic granitoids worldwide10. As with the 900 °C GPa−1 data (Fig. 3a), the modelled melt compositions fit best at melt fractions of 20–30 mol% (approximate vol%).
Extended Data Figure 5 Calculated modal proportion for melting along apparent geotherms of 700 °C GPa−1 and 1,100 °C GPa−1.
Plots of modal proportions versus temperature (also known as modebox diagrams) showing the changing abundance of phases, modelled along linear geotherms of 700 °C GPa−1 (top) and 1,100 °C GPa−1 (bottom). Assemblages developed along the cooler geotherm (700 °C GPa−1) contain (at temperatures greater than 700 °C) abundant garnet, as well as a small quantity (<2 mol%) of suprasolidus sphene (titanite) at lower temperatures (less than 750 °C), of ilmenite at intermediate temperatures (700–860 °C), and of rutile at temperatures higher than 800 °C (that is, these assemblages are HP-TTGs). Little garnet and no rutile forms along the 1,100 °C GPa−1 geotherm, consistent with the formation of LP- and MP-TTGs; all of these assemblages contain a small amount (less than 2 mol%) of ilmenite, but no sphene.
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Johnson, T., Brown, M., Gardiner, N. et al. Earth’s first stable continents did not form by subduction. Nature 543, 239–242 (2017). https://doi.org/10.1038/nature21383
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