Feldspar and oxide thermometry of granulites in the Adirondack Highlands

AbstractThermometry of regionally metamorphosed granulites of the Adirondack Highlands has been undertaken using feldspar and iron-titanium-oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An18–30) and microcline perthite (Or57–87) pairs from charnockites and granitic gneisses give KD[Na/(Na+Ca+K]plag/[Na/(Na+Ca+K)]or = 2–3 yielding temperatures of 650 ° to 750 ° C in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pressures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magnetite (ulvöspinelss 16–45) and ilmenite (hematitess 4.7–6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ° to 800 ° C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10−20 and 10−16 and always lie within 10+1 of the f $$f_{{\text{O}}_{\text{2}} } $$ buffered by fayalite-magnetite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulvöspinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically reasonable results. The inferred 750 ° and 700 ° C isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790 ± 50 ° C) occur between Saranac Lake and Tupper Lake, New York.

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