U-Pb Garnet, Sphene, Monazite, and Rutile Ages: Implications for the Duration of High-Grade Metamorphism and Cooling Histories, Adirondack Mts., New York

U-Pb ages were determined on metamorphic garnets, monazites, sphenes, and rutiles in metapelites and marbles from the Adirondacks to determine the duration of metamorphism, the timing of mineral growth, and the cooling history. Garnet ages for the Lowlands range from 1168-1127 Ma, those from the central and southern Highlands from 1154-1013 Ma. The ages indicate that metamorphism may have begun synchronously throughout the Adirondacks, but high-grade conditions existed over a period of about 150 Ma in the Highlands and only about 40 Ma in the Lowlands. Metamorphism in the Highlands may not have occurred as a single event but rather in several discrete thermal pulses. An age of$1153 \pm 3 Ma$was determined for garnets in the syn-regional metamorphic contact aureole of the Diana syenite, consistent with that of the syenite intrusion,$1155 \pm 4 Ma$(U-Pb zircon, Grant et al. 1986). Garnets just outside the contact aureole give an age of$1168 \pm 6 Ma$. In the Lowlands, monazite yielded an age of$1161 \pm 1 Ma$, rutiles yielded ages of$1005 \pm 2 Ma$and$953 \pm 4 Ma$, and sphene ages range from 1156 to 1103 Ma. In the Highlands, monazite yielded an age of$1033 \pm 1 Ma$, rutiles yielded ages of$911 \pm 2 Ma$and$885 \pm 2$and sphenes from 1033 Ma to 991 Ma. The rutile and monazite ages indicate that both terranes cooled at time-integrated rates of ca. 1.5°C/Ma for at least 150 Ma following the last phase of high-grade metamorphism. The Lowlands cooled to ca. 400°C by ca. 1000 Ma and the Highlands by ca. 900 Ma. The slow, nearly isobaric cooling (as deduced from garnet zoning and fluid inclusions indicates that during the final stages of metamorphism, the terranes were close to isostatic equilibrium, and vertical tectonic movement was limited. The mineral ages indicate that metamorphic pressures and temperatures recorded by thermo-barometry correspond to conditions attained polychronically over 150 Ma or more. Mineral ages combined with temperature estimates for peak metamorphism indicate that the closure temperature for the U-Pb system is >800°C in garnet, 640-730°C in monazite, and 500-670°C in sphene.