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STABLE ISOTOPE GEOCHEMISTRY OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS

ZHENG Yong- fei , FU Bin , GONG Bing (Department of Earth and Space Sciences,University of Science and Technology of China ,Hefei 230026,China)  
A comprehensive study of oxygen,hydrogen and carbon isotope geochemistry has been carried out for ultrahigh pressure metamorphic rocks in the Dabie orogen.A number of stable isotope anomalies have been discovered for the rocks.The results place constraints on the chemical geodynamics of the Yangtze plate subduction beneath the Sino- Korean plate:(1)eclogite and mafic granulite from North Dabie also have low δ 18O values as the ultrahigh pressure eclogite and gneiss in South Dabie,therefore their protoliths experienced hydrothermal alteration by surface water at high temperature before plate subduction,ultrahigh pressure metamorphic fluid is characteristic of 13C- depletion;(2)Oxygen isotope equilibrium has been preserved in part in lowδ 18O eclogites and yields reasonably concordant isotope temperatures,suggesting a short residence time when the eclogites were formed in the mantle and a rapid cooling in the early stage of their exhumation;(3)there are differential isotope exchange in the processes of retrograde metamorphism,the retrograde fluid in the early stage was internally buffered,whereas that in the later stage is of exotic origin;(4)marbles from eastern Dabie are significantly enriched in 13C and thus indicate a sedimentary environment of closed basin between the continental margins,where marbles from western Dabie haveδ 18O values as the normal marine limestone and suggest a sedimentary environment of open sea.
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