MAXIMUM AND MINIMUM WATER CONTENTS OF GRANITIC MELTS - IMPLICATIONS FOR CHEMICAL AND PHYSICAL-PROPERTIES OF ASCENDING MAGMAS

New results obtained by the investigation of liquidus and solidus phas e relations in the system Qz-Ab-Or and by the determination of H2O sol ubility in haplogranitic melts are used to discuss the evolution of gr anitic (quartzo-feldspathic) magmas during their ascent in the crust. Particular attention is given to the role of H2O and its effect on phy sical properties of ascending magmas. The evolution of melt fraction, H2O content of the melt and viscosity of melt and magma during ascent are discussed for several examples (adiabatic ascent, ascent with cool ing, ascent with crystal fractionation). It is shown that decompressio n melting processes may be very important (significant increase of the melt proportion), especially at low pressure, in magmas ascending alo ng adiabats or with slow cooling rates. The comparison of the discusse d examples shows that melts formed at low T are significantly less vis cous than at high T. Although the degree of melting may remain very lo w in the case of dehydration melting at low temperature, the low visco sity of the generated melts may enhance the extraction of these melts from the source area to produce leucogranites.