Tonalite–trondhjemite–granodiorite (TTG) series are an aggregation of rocks that are formed by melting of hydrous mafic crust at high pressure. It is widely accepted that most Archaean granite–greenstones are dominated by TTG, although Late Archaean terranes, such as in the Yilgarn Craton, are dominated by potassium-rich granitoid rocks that are derived through remelting of older felsic TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton than is required by TTG-dominated crust.
The origin of TTG suites is debated; their chemistry is similar to modern-day subduction zone magmas, but there is disagreement as to whether modern plate tectonic processes operated during the Archaean. Some authors have suggested alternative styles of subduction, while others attribute the development of TTG to direct melting of the lithosphere by mantle plumes.