Hydrous fluids play an important role in the development of an decollement below the convergent margins. Pore-fluids compacted below the decollement flow along it and are expelled a-long the frontal thrust zone or are injected into the trench. Large amounts of aqueous fluids de-pent upon the magnitude of sediment accretion. Significant mass losses of elements Si, Ca and Sr and gains in elements Ti, Fe and Mg are a common feature in the ductile shear zone. Volume loss in the deep ductile deformational zone is the main source for the silica in the upper ductile-brittle and brittle deformational zones. Fluid immiscibility and phase separation are important mechanisms for gold deposition. Fault zones and. especially, the deep fault zones are characterized by a high fluid / rock ratio, and herein the volume loss of deformed rocks and depletion of soluble elements can be as high as 65%. Fluid-rock interaction within the low-grade metamorphic terrain of an orogen generally occurred under the rock-buffered conditions, with the water / rock ratio far less than 1, and the mass transport mechanism is infiltration-diffusion composite control. Mineral-fluid reaction during metamorphism is close to equilibrium. Regional patterns of high fluid / rock ratios concentrically centered on deep-seated synmetamorphic plutonic intrusions further argue for a consideration of metamorphic hydrothermal cells at midlevels of the crust. Fluid flow through rocks therefore causes metamorphic mineral reactions to occur and acts as a fundamental driving force behind metamorphism. Mass transport mechanisms during deformation and metamorphism include; (1) diffusion; (2) pervasive infiltration; and (3) channellized infiltration or flow.