A granulite record of multistage metamorphism and REE behavior in the Dabie orogen: Constraints from zircon and rock-forming minerals

Abstract

A combined study of mineral inclusions, U–Pb ages and trace elements was carried for zircon and coexisting minerals from granulite in the North Dabie Terrane (NDT) of the Dabie–Sulu ultrahigh-pressure metamorphic (UHP) zone, east-central China. The results provide insights into the exhumation history of NDT and into rare earth element (REE) behavior during retrogression. Besides inherited cores and one magmatic rim, zircons separated from the granulite record three episodes of metamorphism under different P–T conditions: (1) 223.8±2.3Ma for domains that contain Grt+Cpx±Rt±F−Ap±Aln inclusions without plagioclase and show flat HREE patterns without negative Eu anomalies, representing peak eclogite-facies event; (2) 213.3±2.1Ma for domains that contain Pl±Cpx±Grt±Qtz±Ap inclusions and show rather flat HREE patterns with negative Eu anomalies, corresponding to granulite-facies retrogression; (3) 199.9±3.3Ma for domains that contain Amp±Pl±Qtz±Ap inclusions and show high REE contents with steep HREE patterns and remarkable negative Eu anomalies, representing amphibolite-facies overprinting. Therefore, the UHP eclogite in NDT experienced decompression heating during the initial exhumation, with local hydration in the late stage of the Triassic continental collision.Garnet in the granulite is composed of a corroded core with embayed outline and spongy texture and an overgrowth rim. There is equilibrium distribution of HREE between garnet rim and granulite-facies zircon domain, confirming the geological interpretation of 213.3±2.1Ma for the granulite-facies metamorphism. There is the prograde HREE depletion in porphyroblastic garnet from core to rim and the continuous decrease of HREE from the eclogitic to granulitic zircons, suggesting that the metamorphic transformation from eclogite-facies to granulite-facies took place in a closed system. On the other hand, the amphibolitic zircons show steep HREE patterns and significantly elevated REE contents, suggesting that the amphibolite-facies metamorphism took place in an open system with introduction of external fluids containing more REE to the granulite-facies rocks. Residual clinopyroxene partially replaced by amphibole has remarkably elevated REE contents relative to euhedral primary clinopyroxene. There is equilibrium distribution of REE between the residual clinopyroxene and the amphibole, marking fluid action during the amphibolite-facies overprinting.