The chemical composition of continental crust significantly evolved though time, in particular at the Archaean-Proterozoic transition (3000–2500 Ma), which witnesses major geodynamic changes at that time. The nature and origin of these changes are poorly constrained so far. To better constrain them, I studied a range of granitoid emplaced at that time at the northern margin of the Kaapvaal Craton, in South Africa. In the light of my work, the typology and origin of this magmatism has been reappraised; in particular, the late-archaean granitoids can be split in three different groups : (1) Sanukitoids are represented in South Africa by the Bulai pluton. They are hybrid magmas derived from interaction between mantle peridotite and a component rich in incompatible elements (generally a melt derived from either metabasalts or metasediments). They can be separated in two groups, depending on the hybridation process: low-Ti sanukitoids derive from one-step interaction of silicate melt with peridotite, while high-Ti sanukitoids result from melting of a metasomatic, amphibole- and phlogopite-bearing assemblage equilibrated during the interactions. Finally, the differentiation mechanisms of sanukitoid suites at crustal levels are mainly controlled by fractional crystallization or, less likely, partial melting. (2) « Marginal » sanukitoids, as represented in the Kaapvaal craton by Mashashane, Matlala, Matok and Moletsi plutons, are produced by interactions between sanukitoids and crust-derived melts. Because the source of the latter can be very different from a craton to another, this group of granitoids is extremely diverse. Their magmatic evolution is mostly controlled by fractional crystallization, such as sanukitoids. (3) Some granites, such as those from the Turfloop batholith in South Africa, directly derive from melting of older crustallithologies (TTGs, metasediments, mafic rocks). The evolution of late-archaean magmatism in the Kaapvaal craton follows a very typical sequence: genesis of TTG took place between ~3300 and ~2800 Ma, and give way to the emplacement of all granitoid types presented above, which occurs in a short time span between 2780 and 2590 Ma. This succession of events is identical within every craton worldwide at the end of the Archaean. It witnesses the advent of crust recycling processes, as late-archaean magmas derive from both intracrustal differentiation and interactions between peridotite and continental material introduced within the mantle. This sharply contrasts with the genesis of TTG through melting of juvenile metabasalts only. This duality of petrogenetic processes is also typical of magmatic events in late- to post-orogenic settings, at the end of present-day subduction-collision cycles. As a result, the evolution of the crust composition between 3000 and 2500 Ma likely reflects the initiation of modernstyle plate tectonics. This would be the consequence of global cooling of Earth, which has induced a threshold effect in parameters such as (1) the thickness of oceanic crust and (2) the rheology and volume of continental crust. Indeed, these parameters exert a primary control on the thermo-mechanical stability of subduction and collision, and both became possible at the end of the Archaean only.
