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

Abstract

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 176Lu/ 177Hf 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 176Hf/ 177Hf ratios, but almost unchanged 176Lu/ 177Hf 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 recrystallization 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.