The significance of metamorphic fluorite in the Adirondacks*

Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow fo,-fF> fields at constant P-T. The presence of fayalite-ferrohedenbergite-fluorite-quartz *magnetite and ferrosalite-fluorite-quartz-magnetite assemblages in orthogneisses from Au Sable Forks, Wanakena and Lake Pleasant, New York, buffered fluorine and oxygen fugacities during the granulite facies metamorphism in the Adirondack Highlands. These buffering assemblages restrict fF2 to 10-*9” bar and fO, to 10-‘b” bar at the estimated metamorphic temperature of 1000 K and pressure of 7 kbar. The assemblage biotite-magnetite-ilmenite-K-feldspar, found in the same Au Sable Forks outcrop as the fayalite-fluorite-ferrohedenbergite-quartz-magnetitie assemblage, restricts H,O fugacities to less than 103.’ bar. These fugacities limit H, and HF fugacities to less than 10’ bar for the Au Sable outcrop. The data indicate that relative to H,O, 02. H2, F, and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated Fz fugacities are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper fF2 limit for metamorphism in the Adirondacks and in other granulite facies terranes.