Fluid processes during the exhumation of high-P metamorphic belts

Fluids can play a direct role in exhumation by influencing exhumation mechanisms and the driving processes for these mechanisms. In addition, the process of exhumation leads to the development of fluid-related features that in themselves may not drive exhumation. Fluids involved in exhumation are generally derived from dehydration reactions occurring during decompression, but at shallower crustal levels may also involve the introduction of exotic fluids. The composition of fluids attending exhumation are generally saline - CO2 mixtures, but N-2, CH4, H2O mixtures have also been recorded. Studies of fluid features related to exhumation have found that fluids may contribute to density changes and the initiation of partial melting during decompression, as well as the development of extensive vein systems. However, the preservation of geochemical signatures related to fluid processes occurring prior to high-P and ultrahigh-P metamorphism indicates that large-scale pervasive fluid flow systems, in general, do not operate at any stage during the exhumation history. Large-scale channelled fluid flow may have operated in detachment faults and shear zones related to exhumation, and this requires further study. The most significant role of fluids during exhumation appears to be their controlling influence on the preservation of high-P or ultrahigh-P rocks.