Ultrahigh-temperature metamorphism in interior Antarctica reveals mature orogeny during assembly of Gondwana

Ultrahigh-temperature (UHT) metamorphism, recording extreme crustal thermal regimes, provides pivotal insights into the evolution of ancient orogens. Terranes with UHT metamorphism distributed within the continental fragments of the former Gondwana megacontinent hold critical clues to the processes and dynamics associated with Gondwana’s assembly and evolution. The Kuunga Orogen documents the collision between northern and southern Gondwana, marking the final assembly of the Gondwana megacontinent. However, the evolution of this orogen, especially its eastern section, which is largely covered by the Antarctic ice cap, remains poorly constrained. Based on integrated petrological, phase equilibrium modeling, and petrochronological analyses, we report UHT metamorphism in spinel-bearing pelitic granulites from the glacial moraines in the Grove Mountains, East Antarctica. Phase equilibrium modeling and mineral thermometry reveal peak temperature conditions of ≥900 °C and ~ 0.7 GPa for pelitic granulites characterized by the assemblage garnet + spinel + K-feldspar + plagioclase + ilmenite ± sillimanite ± rutile ± biotite + melt, with a subsequent near-isobaric cooling path to ~800 °C. Petrochronological analyses of multiple dateable phases were conducted to constrain the timing of the UHT metamorphism. Zircon and monazite U–Pb dating constrain peak UHT metamorphism to predate ~535 Ma, with cooling to the solidus by ~520 Ma. Biotite/K-feldspar Rb-Sr isochron ages of ~456–440 Ma are interpreted to represent the post-peak cooling stage at ~300–400 °C. Combined with existing regional data, our new results define a two-stage cooling history: an initial rapid cooling at ~18 °C/Myr from ~535 to 500 Ma, followed by a significantly slower stage at <3 °C/Myr from ~500 to 440 Ma. The UHT metamorphism recorded in the moraine samples indicates that extreme thermal perturbations may have facilitated crustal evolution and maturation of the subglacial orogen in East Antarctica. These findings shed new light on the geodynamic evolution of the eastern Kuunga Orogen and the assembly history of Gondwana.