The Adirondack Highlands were metamorphosed to granulite facies conditions during the Ottawan phase (1090 to 1020 Ma) of the Grenville Orogenic Cycle, whereas the Adirondack Lowlands were metamorphosed to mid to upper amphibolite facies conditions during the Shawinigan phase (1190 to 1140) of the Grenville Orogenic Cycle. Metamorphic temperatures ranged from 750 °C to 850 °C in the Highlands and 650 °C to 750 °C in the Lowlands. Metamorphic pressures were between 6.0 and 8.6 kilobars in the Highlands and 6.5 to 7.5 kilobars in the Lowlands. Following the peak of metamorphism, Adirondack rocks took a counter-clockwise path in pressure-temperature space. The activity of water is generally low in Adirondack metamorphic rocks, and many rocks did not contain a free fluid phase during metamorphism.



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