An experimentally based model for the origin of tonalitic and trondhjemitic melts

The results of melting experiments have been used to establish equations relating melt composition to temperature (T), pressure (P) and water content of the source (YH2O) for tonalitic and trondhjemitic melts produced by partial melting of average Archean tholeiite (AAT). A normative classification diagram for these rocks has been translated into a T-P-YH2O diagram showing the required conditions of partial melting. Tonalites form at high temperatures, low pressures and high water contents, while trondhjemites form at lower temperatures, higher pressures and lower water contents. As pressure increases, the K content increases, but the melts never achieve the composition of a granodiorite or granite. A method for estimating the conditions for the formation of natural tonalites and trondhjemites is proposed. The method has been successfully tested against other experimental studies and natural rocks where the temperature-pressure conditions have been constrained by traditional geochemical methods.