Crustal growth during the Archaean

The dominant rock-types of the Archaean continental crust are those granitoids of the tonalite-trondhjemite-granodiorite (TTG) 

The critical reactions were summarized by Wyllie et al. (1997) and are shown in Fig. 4.17. The point at which melting begins is defined 

FIGURE 4.17 Pressure-temperature phase diagram showing the amphibolite solidus and the residual minerals in equilibrium with TTG melts at different pressures and temperatures (after Wyllie et al., 1997 Rollinson, 2006). The shaded field is the preferred melting region of Moyen and Stevens (2005) based upon their review of the experimental evidence. 

FIGURE 4.18 Plot of trace elements in average TTG (grey lines with symbols) (from Table 4.2) compared with trace elements in the average crust (heavy line with black symbols) (values of Rudnick Si Gao, 2003). (See Text Box 2.2.) 

These geochemical arguments imply that TTGs have been derived from the mantle in two stages. First the mantle melted to form a basalt and then that basalt became hydrated and remelted to form a TTG magma. However, the fact that TTGs have mantle-like initial Sr and Nd ratios means that the basaltic TTG precursor had a short crustal residence time (i.e. there was a short time interval between the creation and destruction of the basaltic crust), the normal case for oceanic crust. 

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