Production of granodiorite melt by interaction between hydrous mafic magma and tonalitic crust. Experimental constraints and implications for the generation of Archaean TTG complexes

Two-layer experiments where a hydrous mafic glass is attached to a powdered tonalite have been performed at conditions of 850 to 1000 °C and 6 to 10 kbar. The aim of these experiments is to simulate the effects of fluids released from a crystallizing hydrous magma emplaced within a tonalitic continental crust. This process has been inferred from field observations and geochemical relationships in Archaean cratons where the first K-rich granite batholiths were developed. The results of these experiments indicate that conditions of 950 °C and 6 kbar, about 50 vol.% of granodiorite melt is generated in the tonalitic crust. The process implies the transfer of H2O, originally dissolved into the mafic melt, together with potassium to the adjacent tonalite. It is proposed that the Archaean granodiorites (of tonalite–trondhjemite–granodiorite (TTG) complexes) result of recycling of older tonalitic crust, and that their generation required the intrusion of mantle-derived mafic magmas. These mafic magmas are sanukitoid in composition and their water content is higher than the water coordinated by hydrous phases such as biotite and amphibole. A simple model to determine the fertility of granitic crust to produce new granite batholiths based on the water content of the hydrous mafic magmas emplaced into the continental crust is presented. This model indicates that for conditions prevailing in the lower and middle crust, the volume of granitic melt may be twice the volume of basic magma.