Fluid regime of granite formation in zones of ultrametamorphism

Petrological data on the Ladoga, Muzkol, Belomorian and Aldan complexes provide a great body of evidence for P-T conditions of ultrametamorphism, the bulk fluid composition and the nature of mineral-forming media. It is shown that the onset of ultrametamorphism, when anatexis occurred, is likely to be characterized by the temperature 680-770°C (Ladoga: P=4.5-5 kbar; Aldan: P=5 kbar), 705-750°C (Muzkol: P=8-9 kbar), 700-715°C (Belomorian: P=7.5-8 kbar), 800-850°C (Aldan: 6 kbar). These temperatures correspond to a decrease in H₂O activity from 0.8-0.9 to 0.3-0.4 in the transition from amphibolite to granulite facies. A model composition of the fluid participating in ultrametamorphism is calculated. The proposed model "equal hydrogene concentration' seems to provide a consistent explanation of the H₂O mole fraction decreasing under high temperature within single fluid flow. -from Authors