The fluid regime of granite formation in zones of ultramorphism

Thermobarometry of ultrametamorphic granitoids shows that in the zone of transition from the amphibolite to the granulite facies, the temperature of anatectic magma increases from 680° to 800°, corresponding to a decrease in H2O activity from 0.8–0.9 to 0.3–0.4. Gas chromatography also indicates a tendency toward a decrease in the H2O/CO2 ratio upon passing from the amphibolite to the granulite facies. On the basis of these data and fluid inclusion studies, a model is proposed to describe the phenomenon of ultrametamorphism in zoned metamorphic complexes based on a uniform concentration of hydrogen in the fluid column.