Experimental constraints on TTG petrogenesis: Implications for Archean geodynamics

Archean TTGs (tonalite-trondhjemite-granodiorite) are sodic granitoids that represent the bulk of the Archean continental crust. They are formed by fluid-absent partial melting of amphibolites. A compilation of the published data on experimental melting of amphibolites allows a mineralogical model for amphibolite melting to be derived for three different starting lithologies. A major and trace element model for melt compositions is produced using the mineralogical model. This model suggests that TTGs formed at P > 15 kbar and T between 900 degrees C and 1100 degrees C, corresponding to low (15 degrees C/km) geothermal gradients that are likely to be attained only in subduction zones. Furthermore, it appears that Nb/Ta, La/Yb, Eu/Eu*, Sr, and HREE contents are intercorrelated in TTGs and are indicators of the pressure of melting. TTGs were generated over a large range of depths, from at least 10 to 25 kbar, and this is reflected in TTG compositions.