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

Dripta Dutta , Takeshi Imayama, Dyuti Prakash Sarkar, Jun-ichi Ando, Kaushik Das

Replacement reactions progress to varying degrees depending on the P-T conditions, exhumation rates, and fluid availability. The preservation of reactants and retrogressed products allows for the reconstruction of 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. Elliptical shapes, absence of chemical zonation, and scarce subgrains suggest that omphacite grains deformed via body diffusion creep. Because of the heterogeneous distribution of externally derived 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 <001>Omp//<001>Amp//<010>Plag, <010>Omp//<010>Amp, and <100>Omp//<100>Amp//<001>Plag. Both generations of amphibole grains, i.e., in S1 and S2, crystallised during exhumation. The S1 amphibole grains are poorer in Si (6.75–7.34 apfu) and crystallised earlier than those in S2 (Si = 7.29–7.79 apfu). Elevated stresses at the reaction interfaces deformed the plagioclase in S1 via dislocation creep. In contrast, due to fluid abundance, the plagioclase in S2 deformed via diffusion creep-accommodated grain boundary sliding. 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 grains in S1, eclogite matrix, and garnet amphibolites deformed via dislocation creep, whereas dislocation-accommodated grain boundary sliding deformed those in S2.