Metamorphic effect on zircon Lu-Hf and U-Pb isotope systems in ultrahigh-pressure eclogite-facies metagranite and metabasite

摘要

The Hf isotope composition of original igneous or detrital zircons in high-grade metamorphic rocks can be used to trace protolith origin, but metamorphic effect on the Hf isotope composition of newly grown domains remains to evaluate. We report a detailed in situ combined study of intragrain U-Pb and Lu-Hf isotopes in zircons front granitic gneiss and eclogite in the Dabie orogen of China that experienced ultrahigh-pressure eclogite-facies metamorphism. The results show correlations in Pb-206/U-238 age, initial HT isotope composition, and Th/U and Lu/Hf ratios between tire domains of different origins. The metamorphic domains are characterized by low Th/U and Lu/Hf ratios but high epsilon(Hf)(t) values relative it) (lie igneous core and mantle of pre-metamorphic ages. Positive correlations are observed between Th/U and Lu/Hf ratios. pointing to the similar effect of metamorphism on both U-Th-Pb and Lu-Hf isotope systems. Thus the metamorphic domains are distinguished from the igneous core and mantle by their low Lu/Hf ratios that are less than 0,001 for the granitic gneiss and less than 0.0001 for the eclogite. Despite differences in both protolith age and geochemical source between granitic gneiss and eclogite, rim epsilon(Hf)(t) values are variably 3.1 to 13.5 greater than core epsilon(Hf)(t) values when calculated at timing of protolith formation, This indicates that tire zircon overgrowth was associated with a metamorphic medium that has high Hf-176/Hf-177 but low Lu-176/Hf-177 ratios. While the metamorphic domains contain more radiogenic Hf isotopes than the original igneous core and mantle, their Lu/Hf ratios are significantly lower than those of core and mantle. Therefore, the metamorphic zircons acquired their initial I If isotope ratios from metamorphic fluids that have high Hf-176/Hf-177 ratios but low Lu/Hf ratios with sound variability depending on the Lu-Hf isotope compositions of pre-existing and co-precipitating phases. (C) 2005 Elsevier B.V. All rights reserved.