RELICT COESITE EXSOLUTION IN OMPHACITE FROM WESTERN TIANSHAN ECLOGITES, CHINA

Relict coesite exsolution lamellae or rods in omphacite have been identified in eclogites from western Tianshan, China. They occur as strongly oriented rods or needles with up to 30um long and 2-3um wide within porphyroblastic omphacites and have a consistent optical orientation parallel to the c-axis of the host clinopyroxenes. Some quartz rods occur in bundles that are more than 10 um wide. BSE images show that the exsolution rods are more than 10 um length and 1-2 um width. Numerous compositional microprobe analyses on the exposed surfaces of the exsolution minerals show that they are pure SiO2. Beside quartz rods in omphacites, there are also some random rutile inclusions in omphacite. Raman spectra analyses show the presence of quartz exsolution at the diagnostic bands of 468 and 131 Cm-1 and their host omphacite with the bands of 1023, 680 and 224 Cm-1 in Western Tianshan eclogites, which are consistent with their mineral microprobe analyses. Among more than 20 Raman spectra analyses on the studied SiO2-rod-bearing omphacites, there are 4 spectra to show the presence of typical coesite vibrations at 521, 375, 270, 181, 151 118 Cm-1. Compared with the standard spectrum of coesite, the intensity of diagnostic band 521 Cm-1 of coesite exsolution in omphacite from Western Tianshan eclogites is not stronger. That maybe due to very small domain remains of coesite exsolution within quartz in omphacites during retrograde metamorphism. But the other subsidiary bands of coesite such as 375, 270, 181, 151 and 118 Cm-1 have been distinguished in the studied samples. That means the precursor of quartz exsolution in omphacite was coesite which changed into quartz during late retrograde metamorphism. Therefore, two stages of SiO2 exsolution rods in omphacite can be recognized, i. e., the first stage of coesite exsolution in omphacite and the second stage of the transition from coesite to quartz. According to the peak metamorphic reaction of magnesite + aragonite=dolomite reported in metapelites interlayered with eclogites in western Tianshan (zhang et al., 2003), the first stage of coesite exsolution in omphacite could occur at 5.0 Gpa and 600OC. The second stage from coesite to quartz may happen at 2.7~2.8 Gpa and near 600OC whicth is consistent with the calculation of Omp-Gt-Phen geothermometer and equilibrium grids of magnesite-bearing eclogite (Zhang et al., 2002a; b).