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Sm-Nd dating of eclogites from Northern Dabie Mountains and its constraints on the timing of granulite- facies retrogression

LIU Yi- can1,2, LI Shu- guang1, XU Shu- tong2, JAHN Bor- Ming3, ZHENG Yong- fei1, ZHANG Zong- qing4, JIANG Lai- li2, CHEN Guan- bao2, WU Wei- ping2 (1. Department of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026,  
Sm- Nd and oxygen isotope ratios of garnet and omphacite from eclogites in North Dabie Mountains were measured along with their petrological features. Unlike coesite- bearing eclogites in South Dabie Mountains which underwent amphibolite- facies retrogression, the eclogites in North Dabie Mountains suffered granulite- facies overprinting. Relationship between theδ 18O values of the eclogite minerals suggests oxygen isotope equilibrium fractionation, corresponding to the Sm- Nd isotope equilibrium between the minerals according to comparability between O and Sm- Nd diffusivities. Theδ 18O values of whole- rocks for the eclogites are+ 2.4‰ to+ 3.6‰ , indicating that the eclogites underwent the hydrothermal alteration of high- temperature surface water prior to the subduction of the Yangtze continental plate. The Sm- Nd isotope ages for the three samples of eclogites are identical with 210 Ma and they are interpreted to represent the cooling ages in the retrogressive stage of granulite- facies metamorphism. The initial ratios of Nd isotope for the eclogites are negative withε Nd(t)=- 10 (two samples) and- 2 (one sample), implying that they were derived from the subducted Yangtze continental crust like the eclogites in South Dabie. There is the isothermal or temperature rise history from 230 to 210 Ma for the eclogites in North Dabie, which is in contrast to that of the eclogites in South Dabie. This places important constraint on exhumation processes of the eclogites in northern and southern Dabie blocks.
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