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Retrograde Metamorphism

Retrograde metamorphism (diaphthoresis, retrogressive metamorphism) is the mineralogical adjustment of relatively high-grade metamorphic rocks to temperatures lower than those of their initial metamorphism. A diaphthorite is a metamorphic rock in which minerals characteristic of a lower metamorphic grade developed at the expense of minerals formed at a higher metamorphic grade.

The term diaphthoresis (from the Greek "to degrade") was first applied by Becke (1909) to a phyllonite containing relict high-grade metamorphic minerals and relict gneissic layering in an otherwise low-grade mineral assemblage. The approximately equivalent term retrograde metamorphism is now commonly applied to metamorphic rocks that display any degree of replacement of a high-temperature mineral or mineral assemblage by one stable at a lower temperature. Where the process has not been carried to completion, the disequilibrium mineral assemblage will give evidence of the polymetamorphic