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

• Published in: Nature geoscience 2020-02, Vol.13 (2), p.163-169
• Main Authors: Laurent, Oscar, Björnsen, Jana, Wotzlaw, Jörn-Frederik, Bretscher, Simone, Pimenta Silva, Manuel, Moyen, Jean-François, Ulmer, Peter, Bachmann, Olivier
• Format: Article
• Language: English
• Subjects: 140/146; 704/2151/209; 704/2151/213/4114; 704/2151/431; 704/2151/598; Basalt; Complementarity; Continental crust; Crystallization; Crystals; Differentiation; Earth; Earth and Environmental Science; Earth crust; Earth Sciences; Earth System Sciences; Eruptions; Fossils; Geochemistry; Geochronology; Geochronometry; Geology; Geophysics/Geodesy; Igneous rocks; Isotopes; Lava; Liquids; Low pressure; Magma; Magma chambers; Melting; Nucleation; Organic chemistry; Petrography; Phanerozoic; Plagioclase; Plate margins; Plateaus; Plutons; Pressure; Reservoirs; Rocks; Sciences of the Universe; Segregation; Uranium; Volcanic eruptions; Volcanic rocks; Volcanology
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-019-0520-6

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