Ruhr-Universität Bochum, Institut für Mineralogie, Research group “High-Pressure metamorphism”, Postfach 102148, D-4630, Bochum, Germany

1. (1) phengite±quartz+K,Mg-rich siliceous fluid=feldspar+phologopite+H₂O

2. (2) phengite+talc+K,Mg-rich siliceous fluid=phlogopite+quartz/coesite+H₂O

at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C.

It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas.