Garnets, omphacite and the minerals of a clinopyroxene/amphibole/plagioclase symplectite in UHP eclogites from Yingshan, Dabieshan have been investigated by TEM and Micro-FTIR. TEM reveals that the predominant microstructures in eclogites and symplectite-forming minerals are chain multiplicity faults (CMFs), dislocation substructures, clusters of water molecules up to ∼50 nm in diameter and recrystallized grains ∼1.75 μm in diameter. This indicates dynamic recrystallization of omphacite, probably during an eclogite-facies metamorphic episode. The deformation structures in symplectite-forming minerals were produced by plastic deformation related to an amphibolite-facies retrograde metamorphic event. CMFs described in the present work demonstrate the existence of an infrequent ½<011> (010) slip system for P2/n omphacite from an UHP eclogite sample from Dabieshan. The frequent occurrence of CMFs in omphacite suggests that they indicate an important deformation mechanism in omphacite and shows that this slip system plays a significant role in the deformation and recovery of eclogite. The hydrous components of deformed minerals may cause plastic deformation of the rocks by dislocation movement and accelerate retrograde metamorphism. Micro-FTIR results show that all the garnets and omphacites contain structural water occurring as hydroxyl groups (OH) or water (H2O). The structural water contents in omphacite range from 110—710 ppm and in garnet from 0—180 ppm. Water released during decompression might supply an early-stage retrograde metamorphic fluid.

The hydrogen concentration and composition of garnets in the ultrahigh pressure eclogites at Shuanghe, eastern Dabieshan, were investigated using Fourier transform infrared spectroscopy and electron microprobe analysis. The OH absorption bands can be divided into four groups: (1) 3635–3655 cm–1; (2) 3600–3630 cm–1; (3) 3540–3580 cm–1; and (4) 3400–3450 cm–1 and the water content ranges from 45 to 2529 ppm. Based on the behaviour of the OH absorption band and the relationship between water content and the composition of garnets, the samples can be divided into two classes: samples with >400 ppm H2O and samples with ≤400 ppm H2O. The water content of the former shows an obvious positive correlation with Ca atoms and a negative correlation with the Si, Mg and Fe2+ atoms per 12 anions, whereas the water content of the latter shows no obvious linear correlation with cations. It is concluded that the major mechanism of hydroxyl incorporation in garnets with >400 ppm H2O is by the coupled substitution 4H +Z□ → □+ZSi in the tetrahedral site, and that several mechanisms are responsible for OH incorporation in garnets with ≤400 ppm H2O.