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Published online 23 March 2009
Mineralogical Magazine; October 2008; v. 72; no. 5; p. 1057-1069; DOI: 10.1180/minmag.2008.072.5.1057
© 2009 Mineralogical Society of Great Britain and Ireland
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Defect microstructures in garnet, omphacite and symplectite from UHP eclogites, eastern Dabieshan, China: aTEM and FTIR study

Xiuling Wu1, Dawei Meng1,*, Xiaoyu Fan1,2, Xin Meng3, Jianping Zheng4 and Roger Mason4

1 Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, China
2 College of Materials Science and Chemical Engineering, Zhejiang University, Hangzhou 310027, China
3 Imperial College London, Sherfield Building, London SW7 2AZ, United Kingdom
4 Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

* E-mail: dwmeng{at}cug.edu.cn

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 1/2<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.

KEYWORDS: UHP eclogite, garnet and omphacite, symplectite, cluster of water molecules, CMFs, TEM and FTIR, Dabieshan






JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2009 by Mineralogical Society of Great Britain and Ireland