retrograde metamorphism

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retrograde metamorphism (diaphthoresis, retrogressive metamorphism) The recrystallization of pre-existing rocks in response to a lowering of metamorphic grade in the presence of a fluid phase. After reaching a metamorphic climax, lowering of metamorphic grade does not usually cause retrograde reactions to occur because all the water in the rock system has been expelled at the metamorphic climax, thus preserving high-grade mineral assemblages. If some water remains in the system, however, or is introduced as the grade decreases, the water can act as a catalyst to initiate retrograde reactions. The reactions produce hydrated mineral types (see HYDRATION), in contrast to the dehydration reactions of prograde metamorphism.