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

Metamorphic baddeleyite and zircon in metacarbonates can provide important insights into the timing and processes of a metamorphic event involving fluid-rock interaction. Here, we carry out whole-rock C-O isotope analysis and integrated in-situ analyses of U-Pb-Lu-Hf-O isotopes and trace elements for metamorphic zircon and baddeleyite from impure marbles from the Tongbai orogen, central China. U-Pb dating by secondary ion mass spectroscopy indicates that the baddeleyite and zircon grains were formed at 1848.0 ± 3.3 Ma (MSDW=1.2; n=12) and 1840 ± 13 (MSWD=1.7; n=13), respectively. The low Ti content, positive Ce anomaly (Ce/Ce*=1.24-11.2), and inclusions of an assemblage of minerals (hematite, calcite, dolomite, lizardite) and H₂O indicate that the zircon grains were crystallized at relatively low temperature, oxidizing, and water-rich fluid conditions likely during retrograde metamorphism. Both baddeleyite and zircon grains have similar weighted average ɛ_Hf(t) values, i.e., -8.9 ± 0.3 (MSWD=3.2; n=11) and -9.3 ± 0.5 (MSWD=6.0; n=15), respectively, suggesting that they were derived from a uniform hafnium source. However, the baddeleyite and zircon show contrasting δ¹⁸O_VSMOW of 7.1 ± 0.6‰ (MSWD=0.91; n=22) and 20.4 ± 0.1‰ (MSWD=3.5; n=15), respectively, indicating that they recorded distinct hydrothermal effects. Because the whole-rock δ¹⁸O values (16.2‰ and 16.3‰) are markedly lower than the expected values equilibrated with the metamorphic zircons, and the high δ¹⁸O values of these zircons preclude any percolation of external meteoric fluids during their formation, we consider a "detrital zircon dissolution" mechanism to provide the zirconium for the formation of zircon and baddeleyite (i.e., [1] ZrSiO₄+CaMg[CO₃]₂→CaMgSiO₄+ZrO₂+CO₂; [2] ZrO₂+SiO₂→ZrSiO₄). In this model, the O isotope composition of zircon is believed to be more faithful to record the signature of its host rock, due to its low oxygen diffusion rate and strong resistance to hydrothermal alteration. The relatively low δ¹⁸O values of baddeleyite is attributed to the fast rate of the transformation reaction [1] which did not allow isotope equilibrium between the baddeleyite and host rock. The disequilibrated whole-rock O isotope compositions of the metacarbonates are most likely resulted from metamorphic devolatilization.