Tonalite-Trondhjemite-Granodiorite

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 (e.g. McCall, 2003). Some authors have suggested alternate styles of subduction, while others attribute the development of TTG to direct melting of the lithosphere by mantle plumes.

References

McCall, G. J. H. (2003). A critique of the analogy between Archaean and Phanerozoic tectonics based on regional mapping of the Mesozoic-Cenozoic plate convergent zone in the Makran, Iran. Precambrian Research 127: 5-17.Rapp, Robert P. (1999) "First Origins of Archean Continental Crust: Assessing Experimentally the Roles of Mafic Versus Ultramafic Sources." Dept. of Geosciences, State University of New York. [http://www.the-conference.com/JConfAbs/4/140.html]