The significance of metamorphic fluorite in the Adirondacks

Abstract

Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow [function of (italic small f)]O2-[function of (italic small f)]F2 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[function of (italic small f)]F2 to 10-29 +/- 1 bar and [function of (italic small f)]02 to 10-16 +/- 1 bar at the estimated metamorphic temperature of 1000K 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 H2O fugacities to less than 103[middle dot]3 bar. These fugacities limit H2 and HF fugacities to less than 101 bar for the Au Sable outcrop. The data indicate that relative to H2O, O2, H2, F2 and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated F2 fugacilies are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper [function of (italic small f)]F2 limit for metamorphism in the Adirondacks and in other granulite facies terranes.