The Buchan area of Dalradian rocks represents a distinct tectonothermal domain within the Scottish Highlands, and the Portsoy-Duchray Hill lineament on the western margin of this block is one of the best examples of a long-lived domain boundary. Rb-Sr mica cooling ages, from metamorphic rocks from across the trace of this lineament, document a prolonged Ordovician cooling history. Delayed, but then relatively rapid, cooling to the west of the Portsoy-Duchray Hill lineament is recognized whereas to the east the age data point to a high heat flow regime lasting more than 50 Ma. The isotopic ages suggest that c. 460 Ma was a time of active tectonism, resulting in final cooling of the low-grade rocks and differential uplift and rapid cooling of deeper structural levels to the west. The radiometric ages do not support previous tectonic models for the region that have invoked thrust thickening along the line of the Portsoy-Duchray Hill lineament and a new model for the metamorphism is proposed. The overprinting of andalusite-bearing assemblages by kyanite to the west of the Portsoy-Duchray Hill lineament is linked to pressure increases associated with the intrusion of the Newer Gabbros and represents a magmatic loading related to the same tectonothermal event that generated the low-pressure metamorphism, rather than a separate tectonic episode.