Genesis of baddeleyite and high δ¹⁸O zircon in impure marble from the Tongbai orogen, Central China: insights from petrochronology and Hf–O isotope compositions

Coexistence of baddeleyite and zircon is rarely documented in metacarbonate rocks and poorly understood. In this study, metamorphic baddeleyite and zircon were recognized in marbles from the Tongbai orogen, central China. We carried out high spatial resolution analysis of trace element geochemistry and U‒Pb‒Lu‒Hf‒O isotope systematics for the zircon and baddeleyite. SIMS (Secondary Ion Mass Spectroscopy) U‒Pb analysis yields weighted average ²⁰⁷Pb/²⁰⁶Pb ages of 1848.0 ± 3.3 Ma (MSDW = 1.2, n = 12) for baddeleyite and 1840 ± 13 Ma (MSWD = 1.7, n = 13) for zircon. The baddeleyite grains have relatively low REE (Rare Earth Element) contents (ΣREE = 1.30–47.3 ppm) with flat chondrite-normalized patterns and moderate contents of HFSE (High Field Strength Element), which are ascribed to the nature of its host metacarbonate rock and mineral assemblage. The zircon grains have low Ti contents (1.63–3.56 ppm), positive Ce anomalies (Ce/Ce* = 1.91–6.96), and mineral inclusions of hematite, calcite, dolomite, lizardite and H₂O. These characteristics indicate zircon formation in a relatively low-temperature setting under oxidizing and water-rich conditions. The baddeleyite and zircon have similar ranges of ε_Hf(t) values of – 7.7 to – 10.5 and – 7.8 to – 11.6, respectively, which suggest their derivation from a relatively uniform Hf source. The high δ¹⁸O values of these zircon grains (20.0–21.0‰) preclude any percolation of external meteoric fluids during their crystallization. Therefore, a “detrital-zircon-dissolution” reaction is preferred to provide the Zr for the metamorphic zircon and baddeleyite in this study. The O isotope disequilibrium between whole-rock (δ¹⁸O = 16.2–16.3‰) and metamorphic zircon is most likely caused by metamorphic devolatilization or interaction with an H₂O-dominant fluid during later metamorphism. On the other hand, the baddeleyite grains yield δ¹⁸O values ranging from 4.5 to 8.7‰, which is in disequilibrium with metamorphic zircon at the temperature of “detrital-zircon-dissolution” reaction (ca. 660–710 °C; Zircon + Dolomite → Baddeleyite + Forsterite + Calcite + CO₂). The genesis of metamorphic zircon can be interpreted by a baddeleyite-to-zircon reaction of ZrO₂ + SiO₂ → ZrSiO₄ at a retrograde stage, consistent with the low Ti-in-Zircon temperatures. The results highlight the importance of integrating trace element and multiple isotopic compositions in studying the genesis of metamorphic baddeleyite and zircon. Additionally, the U–Pb geochronology data also indicate that the protoliths of the marble were deposited in the early Paleoproterozoic and experienced a metamorphism at 1.85 Ga. The ca. 1.85 Ga high-grade metamorphic event might have been induced by the coeval magmatism in an extensional setting.