Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen)

Abstract

Omphacite microstructures are important to decipher the metamorphic pressure-temperature (P-T) conditions of eclogite during subduction and exhumation processes. Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy (TEM) in an ultrahigh-pressure (UHP) eclogite sample from Shuanghe, Dabie orogen, China. The omphacite can be divided into two phases:(1) Omp-1, exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains (APDs). (2) Omp-2, shows as banded subgrains with C2/c space group containing large amount of dislocations. These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls. Along these dislocation walls, we found some barroisite and albite microcrystals. The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite. While the albite microcrystals contain small amount of dislocations and does not show topological relation with host. These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly, this two omphacite space groups should be evolved from the same precursor C2/c omphacite, which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage. Secondly, the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2-2.6 GPa and 650-700℃ at the early amphibole eclogite stage. Thirdly, large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries, which is the necessary for the space group of C2/c surviving under the lower P-T conditions (1.7-1.9 GPa and 630-690℃) during the middle amphibole eclogite stage. Lastly, the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of ~1.5 GPa and 650-680℃ during the late amphibole eclogite stage.