U–Pb, trace element and Lu–Hf properties of unique dissolution–reprecipitation zircon from UHP eclogite in SW Sulu terrane, eastern China

Abstract

A unique dissolution–reprecipitation zircon has been identified for the first time in a Qtz- and Phe-bearing eclogite from the drill hole CCSD-MH, southwestern Sulu UHP terrane by using laser Raman and cathodoluminescence (CL) imaging combined with trace-element analyses and U–Pb and Lu–Hf isotope data. Relic inherited domains of partially dissolved zircon show magmatic features and preserve mineral inclusions of Cpx+Pl+Qtz+Ap. Whereas newly formed zircons crystallized around or along cracks of the inherited (magmatic) domains and contain mineral inclusions of Coe+Grt+Omp+Phe+Ap typical for UHP metamorphism. In addition, primary fluid inclusions were identified in the coesite-bearing zircon domains, characterized by a single H2O-phase or CO2–H2O-phase, indicating the presence of fluids during UHP conditions. Strongly dissolved inherited (magmatic) zircons yielded SHRIMP 206Pb/238U ages of 782–588Ma with an upper intercept age of 780±14Ma. New recrystallized or metamorphic zircons yielded consistent Triassic UHP ages around 238–225Ma with a weighted mean of 229.2±2.3Ma. Zircon trace element and Hf isotope compositions and mineral inclusions classify the protolith of the Sulu eclogite as an intermediate to mafic Neoproterozoic igneous rock, which was subducted to mantle depth and experienced UHP metamorphism in Middle Triassic times. The juvenile εHf(780Ma) value of+7.4±2.9 in inherited (magmatic) zircon points to a significant input from the depleted mantle to the Dabie–Sulu UHP terrane during Middle Neoproterozoic. New UHP zircon domains have low Lu/Hf, Th/U and significantly higher 176Hf/177Hf ratios than the inherited (magmatic) zircons, consistent with formation during recrystallization and fractionation of the Lu–Hf system during UHP metamorphism.