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An oxygen isotope study of quartz veins within eclogites from the Dabie terrane

Science China Earth Sciences 2001年第7期44卷 621-634页

作者：李一良郑永飞傅斌 1. Department of Earth and Space Sciences University of Science and Technology of China 230026 Hefei China

The oxygen isotope composition of minerals from quartz veins and host eclogites in the Dabie terrane was measured in order to place geochemical constraints on the origin and transport of metamorphic fluid. The results are discussed together with structural and petrological relationships between quartz vein and wallrock. The quartz veins can be temporally classified into three groups: (1) synmetamorphic vein which would be formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels; (2) early retrogressive vein which was formed in the early stage of eclogite exhumation subsequent to the recrystallization, the vein-forming fluid is still relevant to the eclogites; (3) late retrogressive vein which was formed in the late stage of eclogite exhumation from deep crustal to upper crustal levels, oxygen isotope fractionation between vein quartz and host eclogite significantly deviates from equilibrium values and the vein-forming fluid was principally derived from granitic gneiss hosting the eclogites. For the synmetamorphic vein, it appears that local advective transport of fluid is the predominant mechanism in the processes of vein precipitation; the scale of oxygen isotope homogenization within the veins is much larger than that within the associated eclogites. The vein-forming fluid would be derived from the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. fluid flow prior to the eclogite-facies recrystallization and the early retrogression may occur mainly along pressure gradients.

关键词： oxygen isotopes quartz vein metamorphic fluid eclogite ultrahigh pressure metamorphism Dabie terrane

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SHRIMP Dating and Recrystallization of metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

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Acta Geologica Sinica(English Edition) 2004年第1期78卷 146-154页

作者： LI HongyanInstitute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 Geosciences Rennes, Universite de Rennes 1, 35042 Rennes Cedex, France Institute of Mineral Resources Chinese Academy of Geological Sciences Beijing 100037 Géosciences Rennes Université de Rennes 1 35042 Rennes Cedex France *Correspondence: LI Hongyan *Correspondence: E-mail: hyli@***

An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.

关键词： recrystallization of zkcon metamorphic fluid SHRIMP dating granitic gneiss Qinglongshan Sulu China

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