Two fundamentally different scales of element mobility have been identified in the upper‐amphibolite‐facies Hunts Brook fault zone of south‐central Connecticut, USA. Field relations, mineral chemistry, and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic Rope Ferry orthogneiss. Two‐sample mass balance calculations between the Rope Ferry Gneiss and fault rocks strongly suggest that the Rope Ferry Gneiss was segregated into biotite‐rich schists and quartz–plagioclase‐rich gneisses through the centimetre‐scale transfer of Si, Al, Ca, Na, Ba, and Zr from schists to gneisses. Three‐ and four‐sample mass balance calculations provide convincing evidence that Al, Ca, Na, and Ba were lost and Si and K gained by the entire fault zone. Both scales of metasomatism have been identified at two localities separated by 20 km, implying that the metasomatic processes were pervasive throughout the fault zone.

From the mass balance calculations, an apparent disparity in K mobility arises. K appears to be gained by the entire fault zone, but is immobile during the centimetre‐scale segregation. This is because mass balance calculations can only detect differential element mobility. Differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients. Therefore, the identification of differentially mobile elements provides insight into the mechanisms of metasomatism. The immobility of K during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt. However, the element mobility is reconcilable with deformation‐driven metamorphic differentiation. The gains of Si and K and losses of Ca, Na, Al, and Ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt, but may reflect the infiltration of a metasomatizing aqueous fluid.