Metamorphism in the Adirondacks. I. Petrology, Pressure and Temperature

Grenville Supergroup sediments and suites of pre- and syn-tectonic igneous rocks have been metamorphosed to the upper amphibolite and granulite facies in the Adirondacks of northern New York during the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomite and garnet-clinopyroxene thermometry indicate that metamorphic temperatures (T) increase from about 650 °C in the area west and northwest of Gouverneur to 700–750 °C near Colton and along the Lowlands-Highlands boundary to 750–800 °C in areas within and around the Marcy anorthosite massif. The presence of grossular-rich garnet + quartz without wollastonite + plagioclase in calc-silicate rocks and the apparent absence of metamorphic ferropigeonite in charnockites restrict maximum metamorphic T to less than 800–850 °C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite, garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite, fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz, kyanite-sillimanite, anorthite-garnet-sillimanite-quartz and the stability of akermanite, are 6·5–7·0 kb near Gouverneur and increase to 7·5–8·0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemical systems are interpreted as peak or near-peak conditions for Adirondack metamorphism. The compositions of thin retrograde rims on garnets indicate a post-peak-metamorphic P-T path for the Adirondacks with appreciable cooling (200–300°) before decompression. Peak and retrograde P-T conditions inferred for the Adirondacks are similar to numerous other granulite terranes suggesting that similar tectonothermal events are necessary for the formation of many granulite belts.