Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant constituent of Archean continental crust and as such are key in understanding the origin of Earth’s continents. Their geochemical variability is typically thought to reflect metabasite melting over a range of pressures (1 to ≥2 GPa). Recent studies challenge this interpretation and have proposed that a subset of “high-pressure” TTGs with very high Sr concentrations and Sr/Y instead formed by fractional crystallization of metasomatized mantle-derived sanukitoid magmas. Here we use titanium stable isotope variations to demonstrate that Neoarchean high-Sr TTGs from the Yilgarn craton (Western Australia) are equivalent to evolved sanukitoids, whereas other Neoarchean Yilgarn TTGs are best explained by metabasite partial melting in the garnet stability field, hence recording contemporaneous formation of TTGs by two distinct mechanisms. High-Sr TTGs are near ubiquitous in Archean cratons globally, became abundant after ca. 3 Ga, and most are spatially and temporally associated with sanukitoids. These observations point to widespread formation of TTGs from mantle-derived magmas in the late Archean and may reflect a change in geodynamic processes at this time.
Spencer et al. (Mon,) studied this question.