Creep behaviours of omphacite and amphibole-plagioclase symplectite: The role of heterogeneous hydration in the Tso Morari eclogite during retrogression

Replacement reactions progress to varying degrees depending on the P-T conditions, exhumation rates, and fluid availability. The collective preservation of the reactants and partly to completely retrogressed products allows reconstruction of the microstructural and mineralogical progression, which we investigated using electron backscattered diffraction and microprobe analyses on the omphacite, amphibole-plagioclase symplectite, and matrix amphibole of the Tso Morari eclogite. The elliptical shapes, absence of chemical zonation, and scarce subgrains suggest that the omphacite deformed via body diffusion creep. Because of the heterogeneous distribution of hydrous fluids in the eclogite, the omphacite is replaced by amphibole-plagioclase symplectite either partially along the peripheries (S1 symplectite) or completely (S2 symplectite). Strong omphacite CPOs, caused by growth anisotropy, are inherited by the symplectite constituents such that 001Omp//001Amp//010Plag, 010Omp//010Amp, and 100Omp//100Amp//001Plag. The amphiboles in S1 are poorer in Si (6.75–7.34 apfu) and crystallised earlier than those in S2 (Si = 7.29–7.79 apfu) during retrogression. Elevated stresses at the reaction interfaces deformed the plagioclase in S1 via dislocation creep. In contrast, the plagioclase in S2 deformed via grain boundary diffusion creep accommodated grain boundary sliding due to fluid abundance. The misorientations across the subgrain boundaries in the amphibole grains constituting S1 and S2 are similar to those in the amphibole of the eclogite matrix and the garnet amphibolites. The amphibole in S1, eclogite matrix, and garnet amphibolites deformed via dislocation creep, whereas dislocation creep accommodated grain boundary sliding deformed those in S2.