Multistage growth of garnet in ultrahigh-pressure eclogite during continental collision in the Dabie orogen: Constrained by trace elements and U-Pb ages
Major and trace elements for garnet and other minerals, and U-Pb ages and trace elements for zircon (as inclusions in garnet or as matrix) were determined on thin sections of UHP eclogite from the western part of the Dabie orogen. The results provide temporal constraints on growth of garnet during continental subduction-zone metamorphism. Major elements show homogeneous distribution across garnet grains, with weak or no zonation of Ca and almost flat distribution patterns of Mg and Fe. However, their REE compositions display two groups of patterns relative to chondrite. Group I garnet is characterized by steep patterns from LREE to HREE, whereas Group II garnet shows enrichment of MREE relative to LREE and HREE. These two groups of garnet also have different parageneses of mineral inclusions. Group I garnet always contains abundant inclusions of amphibole + epidote/allanite + rutile + plagioclase + quartz ± zircon, whereas Group II garnet occasionally contains inclusions of omphacite ± rutile ± apatite ± zircon. LA-ICPMS U-Pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet. Four types of zircon were identified based on their CL images, REE distributions and U-Pb ages. Types I and II zircons are metamorphosed after protolith magmatic zircon, with Type I being modified by solid-state recrystallization and Type II by dissolution recrystallization. Types III and IV zircons are metamorphic and grew in the stability field of garnet. They gave consistent 206Pb/238U ages of 239 ± 6 Ma and 222 ± 4 Ma, respectively, in association with Groups I and II garnets. This provides a temporal constraint on the two-stage growth of garnet, respectively, predating and postdating the peak UHP metamorphic event. Therefore, we conclude that there are two stages of zircon and garnet growth during continental collision, which occurred during the final subduction from HP to UHP phase and the early exhumation from UHP to HP eclogite facies, respectively.