ZIRCON GEOCHEMISTRY RECORDS THE ACTION OF METAMORPHIC FLUID IN ULTRAHIGH-PRESSURE JADEITE QUARTZITE FROM THE DABIE OROGEN

Jadeite quartzite is a specific type of metamorphic lithology at convergent plate margins, but its genesis is still enigmatic with the framework of subduction-zone metamorphism. In order to decipher the action of metamorphic fluids in its genesis, we have carried out a combined study of zircon for UHP jadeite quartzite from the Dabie orogeny, east-central China. The results provide insights into fluid metasomatism of the UHP metamorphic rock during continental subduction to mantle depths and thus constraints on the protolith nature of the jadeite quartzite.

The zircons extracted mostly show core-rim structures. The overgrown rims contain rare mineral inclusions, and exhibit concordant U-Pb ages of 219 to 244 Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the residued cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U-Pb ages varying from 541 to 2116 Ma and steep REE patterns with remarkable negative Eu anomalies. We interpret the rims as the new growth of metamorphic origin and the cores as the protolith detrital of magmatic origin. The protolith magmatic zircons underwent solid-state recrystallization and metasomatic recrystallization. The solid-state recrystallized zircons exhibit slightly discordant U-Pb ages close to the protolith age, steep MREE-HREE patterns, nearly unchanged Hf isotope ratios. In contrast, the metasomatic recrystallized zircons exhibit partial resetting in REE composition and U-Pb and Lu-Hf isotopic systems. The all zircon domains show relatively homogeneous δ¹⁸O values of 4.0±0.2‰. The protolith zircon cores underwent the metasomatic recrystallization by the action of an internal fluid at UHP conditions. In this regard, the fluid metasomatism would firstly take place along fractures of the detrital zircons during prograde subduction of the continental crust and then experience the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. The composition of mineral inclusions within the detrital zircon cores suggests that the metasomatic fluids were rich in Si, Ti, Na and Al. Thus, the internal fluid would be derived from the breakdown of hydrous minerals such as phengite and amphibole during the continental subduction-zone metamorphism.