Metamorphic growth and recrystallization of zircons in extremely 18O-depleted rocks during eclogite-facies metamorphism: Evidence from U–Pb ages, trace elements, and O–Hf isotopes

Abstract

A combined in situ SIMS and LA-(MC)-ICPMS study of U–Pb ages, trace elements, O and Lu–Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U–Pb ages of 769 ± 9 Ma have positive δ 18O values of 0.1–10.1‰, high Th/U and 176Lu/ 177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ 18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U–Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ 18O values of −10.0‰ to −2.2‰, low Th/U and 176Lu/ 177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ 18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ 18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ 18O magmatic core and the negative δ 18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ 18O fluids. The zircons recrystallized in solid-state maintained positive δ 18O values, and REE and Lu–Hf isotopes of protolith zircon, but their U–Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ 18O values similar to the metamorphic growths, almost completely reset U–Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ 18O values, and partially reset REE, and U–Pb and Lu–Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.