The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite

The tonalite–trondhjemite–granodiorite (TTG) series comprises silicic and sodic rocks that form a major component of preserved Archaean crust. TTG are widely considered to have formed during subduction by partial melting of hydrated oceanic crust. This hypothesis relies primarily on compositional similarities with Cenozoic subduction-related felsic rocks called adakites. However, simple geochemical parameters, such as silica content and Mg# [=Mg2+/(Mg2++FeTotal)×100, with FeTotal as Fe2+], show that TTG are distinct from adakite. Most adakite suites comprise, or include, high Mg# and low SiO2 samples which suggest that these slab melts interacted with the mantle, whereas virtually all samples of pre-3.0 Ga TTG, and more than half of the samples of post-3.0 Ga TTG, show no evidence for such interaction. Convincing evidence for a direct mantle component in TTG is primarily restricted to samples from the late Archaean Superior Province of Canada, where there is also independent evidence for subduction and arc-accretion. Consequently, most early Archaean, and many late Archaean, TTG suites are not analogues of Cenozoic adakite. The composition of these TTG suites does not provide evidence for modern-style subduction processes. Tectonic models that accommodate TTG production through melting of hydrous basaltic material at the base of thickened crust may be more appropriate to the Archaean.