TEM and HRTEM study of microstructures in omphacite from UHP eclogites at Shima, Dabie Mountains, China

Transmission electron microscope (TEM) and high resolution transmission ele ctron microscope (HRTEM) analyses have been performed on omphacite from ult ra-high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocat ion substructures, including free dislocations, loops, tiltwalls, dislocati on tangles and subboundaries. They were produced by high-temperature ductil e deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470 +/- 6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APD s in ordered omphacites from Shima, coincident with ages given by single-zi rcon U-Pb dating (471 +/- 2 Ma). HRTEM reveals C2/c and P2/n space groups i n different parts of one single omphacite crystal, and no exsolution is obs erved in the studied samples, which is attributed to rapid cooling. It is s uggested that the UHP eclogites underwent a long period of annealing at hig h temperatures, followed by relatively rapid cooling. These data provide va luable information for the formation and exhumation mechanism of UHP eclogi tes in the Dabie high-pressure (HP) and UHP metamorphic belt.