Mineral equilibrium modelling and calculated chemical potential relations of reaction textures in the ultrahigh‐temperature In Ouzzal terrane (In Hihaou area, Western Hoggar, Algeria)

Abstract Mg–Al‐rich orthopyroxene–sillimanite‐bearing granulites from In Hihaou, In Ouzzal terrane, NW Hoggar were previously determined to have experienced ultrahigh‐temperature metamorphism at 1,050°C and 10 kbar. Multilayer corona and symplectite reaction textures that pervasively replace peak metamorphic porphyroblasts were interpreted to have formed during near‐isothermal decompression. Pseudosection calculations using new and realistic activity–composition models give estimates of 900 ± 20°C and 8 ± 0.5 kbar for the reaction texture assemblage. Calculated chemical potential relations show that reaction textures formed due to very minor decompression from peak conditions, when the removal of intracrystalline melt led to the development of local equilibrium domains around garnet and sillimanite porphyroblasts. The growth of initial monomineralic coronas further isolated the porphyroblasts from the matrix, leading to the later development of the symplectites. These results indicate that the derivation and interpretation of UHT conditions using the occurrence of diagnostic assemblages on classic petrogenetic grids can overestimate temperature and/or pressure conditions. Similarly, the interpretation of reaction textures to derive retrograde conditions and P–T paths without considering their spatial development is unlikely to be reliable, as textures do not typically form in this way or routinely record this information.