[S-MP26] Supercontinents and Crustal Evolution

[SMP26-08] Oxygen isotope equilibration of zircon in metacarbonate rocks: An example from Sør Rondane Mountains, East Antarctica

*Madhusoodhan Satish-Kumar¹, Kenji Horie², Ian S. Williams³, Mami Takehara², Tomokazu Hokada², Naho Otsuji-Makino¹ (1.Department of Geology, Faculty of Science, Niigata University, 2.National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan, 3.Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia)

Keywords:Oxygen isotopes, zircon, metacarbonate rocks, East Antarctica

Zircon geochronology and geochemistry is extensively studied for understanding the age and provenance of metasedimentary rocks. However, during metamorphism zircon overgrowth is commonly reported, which is characterized by trace element characteristics and oxygen isotopic composition. What is the behavior of zircon in high-grade metacarbonate rocks? In this presentation we present an example from the Sør Rondane Mountains (SRMs), East Antarctica. The SRMs, located in the Neoproterozoic to Early Cambrian East African-Antarctic collisional orogen, are composed of medium- to high-grade metasedimentary, metaigneous and intrusive rocks of diverse composition. Multidisciplinary geological studies have revealed that this region can be separated into two distinct terranes, a metasedimentary and metaigneous dominated Northeastern (NE) and a meta-tonalitic and meta-sedimentary dominated Southwestern terrane (SW), that collided at around 650-660 Ma along the Main Tectonic Boundary (Osanai et al., 2013; Adachi et al., 2023). Pure metacarbonate rocks in the SRMs are suggested to have deposited in late-Tonian (880-850 Ma) in the SW terrane, whereas those in the NE terrane recorded slightly older early Cryogenian ages (820-790 Ma) (Otsuji et al., 2013). Pure metacarbonate rocks in the SRMs are characterized by typical marine O and C isotopic composition, low concentrations for mobile trace elements and flat REE patterns Otsuji et al., 2013; Satish-Kumar et al., 2021). Petrographic observations revealed that zircons are abundant in impure metacarbonate rocks, superficially the grains appear detrital, but internal textures are suggesting metamorphic growth. SHRIMP analyses of zircons in three impure metacarbonate rocks gave well defined tight concordia U-Pb zircon ages of 545 +/- 1 Ma, 546+/- 2 Ma and 549 +/- 2 Ma, younger than the peak metamorphism in the SRMs. Oxygen isotope analyses of zircon in these rocks yielded interesting results, extremely high δ18O(V-SMOW) for zircon of about 23.4‰, and 21.5‰ in two samples, which are found to be in equilibrium with associated carbonate minerals. These high values are comparable with the previous reports on high oxygen isotope ratios for zircons in metacarbonate rocks from Sri Lanka and Myanmar (Cavosie et al., 2011) as well as those from marble hosted UHP eclogites from the Sulu orogen (Chen et al., 2016).
The zircons in the impure metacarbonate rocks seems to have re-equilibrated at c.550 Ma. Consistent with the recently reported carbon isotope thermometry results between calcite and graphite (Satish-Kumar 2023), both titanium-in zircon thermometry zircon-calcite oxygen isotope thermometry suggested temperature conditions of around 800-850 °C, implying high temperature equilibration of zircon. We also discuss the possible role of fluids that might have been instrumental for the recrystallization process of zircon in metacarbonate rocks.

References
Adachi et al. (2023), Journal of Mineralogical and Petrological Sciences, 118, S014.
Cavosie et al. (2011), Contributions to Mineralogy and Petrology, 162, 961–974.
Chen et al. (2016), Chemical Geology, 436, 35-53.
Osanai et al. (2013), Precambrian Research, 234 8-29.
Otsuji et al. (2013), Precambrian Research 234, 257-278.
Satish-Kumar et al. (2021), Gondwana Research, 96, 163-204.
Satish-Kumar (2025), Geological Journal 60, doi.org/10.1002/gj.5095.