On the interpretation of retrograde reaction textures in granulite facies rocks

Retrograde reaction textures, such as partial pseudomorphs, coronae and symplectic coronae are a relatively common feature of granulite facies rocks and represent evidence of change of mineral modes or assemblages. Thus, such reaction textures represent one of the more potentially useful indicators of retrograde P-T evolutions. However, the interpretation of such textures requires the simultaneous consideration of the formation of the texture itself in terms of the spatial relationships in a rock, and of the changes in P-T conditions that drove the development of the reaction texture. Calculated phase diagrams provide a robust framework in which to consider reaction textures, especially via the combined use of pseudosections to understand the P-T changes and mu-mu diagrams to understand the spatial arrangement of the minerals. Consideration of the chemical potential gradients that may exist across a given texture means that elucidating P-T conditions and P-T paths must be done with care. The evolution of a given reaction texture may be complex with different layers or individual minerals in a texture forming at different times. Thus, the approach of writing a reaction to represent the inferred local reactant and product phases and simply ascribing such reaction textures to crossing equivalent univariant reactions from petrogenetic grids or traversing through fields in pseudosections without due consideration of chemical potential gradients may result in an incorrect interpretation. Furthermore, the potential for a non-continuous metamorphic evolution of a rock by polymetamorphism or some other hiatus in the textural/mineral-assemblage evolution creates further uncertainty in the interpretation of a texture in terms of its reaction history and P-T path. A common interpretation of coronae and symplectic coronae, on the basis of the preservation of the apparent reactants, is that they represent reactions that failed to go to completion for various kinetic reasons. While this may be true in some cases, many other textures may actually represent reaction having gone to completion with, for example, silicate melt being the phase completely consumed. Furthermore, even where a reaction has failed to go to completion, the possible reasons for this may be different for different rocks