Apparent retrograde metamorphism: Another example of the influence of structural deformation on metamorphic differentiation

A model relating apparent retrograde metamorphism to folding is proposed. According to the model, pressure gradients brought on by folding in the course of regional metamorphism cause the migration of water to low pressure fold crests. The greater concentration of water renders the mineral assemblage unstable relative to a more hydrous, lower grade assemblage. The result is that several mineral assemblages, each of which is characteristic of what classically has been called a different metamorphic facies, can be formed during the same metamorphic event, under the same pressure-temperature conditions.As an example, the paragenetic relationships between three mineral assemblages in a metamorphosed basalt flow are explained in terms of chemical changes accompanying structural deformation. During regional metamorphism the basalt was recrystallized to hornblende-, oligoclase-, and biotite-bearing assemblages. Folding late in the course of the regional metamorphism brought about the destruction of pre-existing assemblages in the fold crests and formation there of a more hydrous chlorite and albite assemblage.