Understanding mineral reaction pathways is critical for determining the pressure- temperature-composition (P-T-X) histories of mountain belts. We use Monte Carlo simulations of crystal growth morphologies to assess chemical disequilibrium during deep crustal (25–35 km) garnet crystallization (Connecticut, USA). Initial garnet growth produced crystals with poorly developed crystal faces and/or branched, dendritic forms. This growth phase occurred after equilibrium conditions for devolatilization reactions were considerably overstepped during metamorphic heating as a result of energy barriers to garnet nucleation. Mineral compositions and assemblages may have been unable to track evolving P-T-X histories during both the period of overstepping when no reaction was occurring, and when reactions began far from chemical equilibrium. Growth during the overstepped stage must therefore preserve valuable information about the kinetic history of metamorphism, and could have released considerable volatiles, leading to rock weakening and potentially hydrofracturing. The position of regional garnet isograds in the study area may have been strongly dependent on the kinetics of garnet nucleation and growth, rather than the equilibrium P-T-X conditions for garnet-forming reactions.