Most of the mineralogical, chemical and textural features that can be observed in metamorphic rocks are produced during devolatilization reactions (Fyfe et al. 1978; Thompson 1983). In many cases only the last such event is recorded. Between devolatilization pulses, fluid-rock interaction will be minimal and microstructural evidence of even the last event is often modified by recrystallization (Ridley & Thompson 1986). The mechanisms by which the fluids escape varies depending upon whether a connected porosity, or fracture network, is available and the magnitudes of temperature and fluid-pressure gradients (e.g. Norton & Knight 1977; Etheridge et al. 1984; Yardley 1986). These factors will change during the metamorphic process. Fluids released deeper in the metamorphic pile do not necessarily flow pervasively through the overlying rocks but rather appear to be focused into high permeability channelways through a variety of processes (Fyfe et al. 1978; Wood & Walther 1986). The patchiness of retrogressive alteration points also to the focused flow of fluid after a metamorphic peak and perhaps under different rheological conditions. Metamorphism in the absence of fluid results in incomplete mineral reactions and other disequilibrium features in metamorphic rocks (Thompson 1983; Ridley & Dixon 1984).
Many papers have appeared in recent years dealing with aspects of fluid motion during metamorphism. They have variously demonstrated the transient nature of fluid flow, indicated some controls on different modes of fluid behaviour, and suggested that mechanical responses will be different depending upon whether a rock is undergoing devolatilization or not. Attention is focused here on