Contrasting Lu-Hf and U-Th-Pb isotope systematics between metamorphic growth and recrystallization of zircon from eclogite-facies metagranites in the Dabie orogen, China

Different types of metamorphic zircon are recognized by a combined study of petrography, U-Th-Pb and Lu-Hf elements and isotopes in zircons from eclogite-facies metagranites in the Dabie orogen. The results provide petrological distinction between metamorphic growth and recrystallization with respect to protolith inheritance, fluid and melt effects. Zircon U-Pb dating for the metagranites yields two groups of ages at 778 +/- 13 Ma and 223 +/- 4 Ma, respectively, corresponding to protolith formation in the Neoproterozoic and metamorphic modification in the Triassic. Metamorphically grown zircons from the aqueous fluid are characterized by concordant Triassic U-Pb ages, relatively high U contents but low Th contents, low Th/U and Lu-176/Hf-177 ratios, and elevated Hf isotope ratios. Metamorphically grown zircons from the hydrous melt show concordant Triassic U-Pb ages, very high contents of both Th and U, elevated Hf-176/Hf-177 ratios, but almost unchanged Lu-176/Hf-177 ratios. Metamorphic recrystallization is commonly associated with discordant U-Pb ages between Neoproterozoic and Triassic, but availability of fluid/melt dictates the extent to which internal structure. morphology, U-Th-Pb and Lu-Hf element and isotope systems of protolith zircon were modified by metamorphic dehydration and partial melting. While the zircon U-Th-Pb isotope systems can be variably reset by solid-state recrystallization, its initial Hf isotope signature keeps unchanged at the same conditions. On the other hand. dissolution rectystallization causes almost complete resetting of the U-Th-Pb chronometric systems to concordant ages at the metamorphic time, but it does not significantly change the Lu-Hf isotope compositions. Replacement recrystallization resets the zircon U-Th-Pb and Lu-Hf isotope systems to variable degrees, depending on the activity of metamorphic fluid/melt. Consequently, the five types of metamorphic zircon are distinguished not only between growth from the aqueous fluid and the hydrous melt but also between the recrystallization via the solid-state, replacement and dissolution mechanisms. This provides insights into the behavior of zircon during subduction-zone metamorphism, particularly that concerning dehydration melting during exhumation. (C) 2009 Elsevier B.V. All rights reserved.