Generation of Archaean TTG Gneisses Through Amphibole‐Dominated Fractionation

Abstract

AbstractThe early continental crust is characterized by the tonalite‐trondhjemite‐granodiorite gneisses (TTG) of Archaean terrains. There is a consensus that TTGs were formed by partial melting of hydrated mafic rocks. High‐pressure melts tend to be more trondhjemitic with higher Sr/Y ratios due to the presence of garnet and absence of plagioclase, whereas low‐pressure melts are more tonalitic with relatively lower Sr/Y ratios. Here we present a geochemical study of diorites, TTGs, and hornblendites from Eastern Hebei, North China Craton. Geochemical analyses show that dioritic parental magmas may evolve to tonalites and trondhjemites through a maximum of 27% fractional crystallization of amphibole and apatite. We also identified the cumulate apatite‐bearing hornblendite, a predicted solid fractionate and chemical complement to the TTGs. Mixing of the trondhjemite and the hornblendite can generate similar chemical compositions to the diorites in this region. The mixing trends are consistent with those predicted for fractional crystallization. Based on these analyses, we conclude that these trondhjemites and tonalites can be formed by fractional crystallization of amphiboles and accessory minerals from the calc‐alkaline diorites. Partial melting is not a unique solution to the problem of how to form the Archean TTG gneisses and how to make an Archean continent. Thick crust or lithosphere (in the garnet stability field) is also not a unique requirement. Amphibole fractionation may also cause an abrupt increase of Eu/Eu* of Archean granitoids at 3.2–3.0 Ga, which may be linked to a transition period in tectonic style at ~3 Ga in Earth's evolution.