Processes leading to eclogitization (densification) of subducted and tectonically buried crust

The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. However, a complex of partially eclogitized granulites on the island of Holsnøy in the western part of the Norwegian Caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust. The rocks on Holsnøy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex. Metamorphism then occurred in a spatially heterogeneous manner, with several distinct physical processes involving feedbacks between deformation, fluid infiltration, and chemical reactions operating simultaneously in different parts of the rock mass. Field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks. A reservoir‐flux systems model is used to represent the various processes contributing to eclogitization of the granulites, which operated at vastly different rates over disparate intervals of time. The model suggests that the metamorphic process may have been brief (≪1 Myr), and ultimately self‐limiting, arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite.