Investigation of the defect microstructures in both UHP metamorphic rocks and symplectite forming minerals from dabie mountains, China

Category Mineral processing
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wu, Xiuling۱; Meng , Dawei۱; Meng , Xin۲; Chen, Hong۱; Zhang, Zhengjie۱; Zheng, Jianping۱
Holding Date 23 September 2008

UHP eclogites and jadeite quartzite and symplectite forming minerals respectively from Yingshan and Shuanghe in Dabieshan, China, were investigated using transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR). TEM reveals that the predominant microstructures in the minerals are chain multiplicity faults (CMFs), dislocation substructures, the cluster of water molecules (< 50 nm) and recrystallized grains (~1.75 µm).
It indicates dynamic recrystallization for the omphacite during an eclogite-facies metamorphic episode probably. The deformation structures in symplectite aegirine-augite, albite, augite, taramite, hornblende and magnetite corona minerals were produced by plastic deformation related to an amphibolite-facies retrograde metamorphic event. The hydrous components (e.g. OH/H2O or cluster of water molecules on a nanometer-scale) in deformation omphacite, jadeite, quartze, aegirine-augite, albite and augite may induce the plastic deformation of the rocks by the movement of the dislocations, and can accelerate the retrograde metamorphism of rocks.
Structural water (OH/H2O) in nominally anhydrous minerals (NAMs) exists in structures of these minerals as defects. Micro-FTIR results show that all NAMs contain structural water occurring as hydroxyl (OH) or free water (H2O) or clusters of water molecules. The estimated whole-rock water content for eclogite and jadeite quartzite are 203 ppm and from 490 to 600 ppm, respectively. The water released during decompression might represent the early-stage retrograde fluid. Microstructure defects in the minerals as a potential indicator of extremely rapid and episodic exhumation of UHP metamorphic rock. We gratefully acknowledge financial support from NNSFC grants (Nos. 40572114, 40672136 and 40425002) and SRFDP (No. 20060491504).