FORMATION AND ASCENT OF GRANITIC MAGMAS

New results obtained by the investigation of liquidus and solidus phase relationships in the haplogranite system Qz-Ab-Or are used to discuss the evolution of magmas during their ascent in the crust. It is assumed that the magmas are formed at 720-degrees-C, 820-degrees-C, 920-degrees-C and at a depth corresponding to a pressure of 8 kbar. The starting composition of the magma is taken as 50% melt plus 50% quartz and feldspars. In case of a closed system (no heat exchange and no transfer of elements) the melt fraction of magmas, the water activity and the viscosity increase with decreasing pressure. The temperature slightly decreases. At 700-degrees-C the viscosity is approximately 2 orders of magnitude lower than at 900-degrees-C. This is related to the higher amount of water in the (H2O-undersaturated) melt at low T. It is also shown that dehydration melting is only realistic at high T (900-degrees-C). At lower temperatures water has to be added from outside to obtain an intrusive magma with approximately 50% melt.