Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks

The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism. Such structures are most common in immature sandstones and siltstones. Mineral reactions, involving modification of detrital grains and diagenetic minerals, and forming a logical extension to diagenetic processes, are an integral part of the deformation mechanism, and the cleavage stripes represent accumulations not just of inert particles, but mostly of newly crystallized micas, the products of these reactions. The mechanism of deformation by pressure solution is now seen to involve mmetamorphic reactions, change in volume of solid phases during reaction, removal of some silica from the rock, rearrangement of reaction products to produce fabrics, solution of some detrital grains in cleavage stripes.Formal reactions have been written to describe the alteration of detrital felspar and epidote to white mica, the modification of greywacke matrix to white mica, and the transformation of diagenetic chlorite to white mica, all of which are observed to occur during formation of the pressure solution cleavages. These reactions emphasize the importance of metamorphic processes during pressure solution deformation, suggest that pressure solution may involve removal of silica released as a product of the reactions, indicate that the pH of the aqueous phase may be buffered to a level that silica solubility is increased, involve a volume reduction that contributes to the overall shortening during deformation, and also involve dehydration, the large scale circulation of released water possibly being important to the removal and redistribution of silica during pressure solution.