Sensitive high-resolution ion microprobe U-Pb zircon ages for late to posttectonic leucogranites fix the timing of extensional collapse of a portion of the Mesoproterozoic Grenville orogen of eastern North America. Plutons of Lyon Mountain Granite (LMG) were emplaced within the Carthage Colton shear zone synchronously with formation of extensional mylonite at 1045–1037 Ma. Leucogranite melts were generated in the hot granulite facies core of the Adirondack Highlands–Central Granulite terrane that served as the lower plate for down-to-the-northwest extension. The LMG suite is associated with high-temperature hydrothermal magnetite deposits in the Adirondack Highlands, and widespread Cl + CO2 hydrothermal alteration of upper-plate rocks is localized along the Carthage Colton shear zone where LMG granites are present. The relationships between melt generation, granite intrusion, high strain rates, extensional collapse, and high- temperature hydrothermal activity provide a framework for understanding midcrustal processes in modern and ancient orogenic belts.