Tectonic Setting and Evolution of Anatectic Melt Composition During Prograde Metamorphism up to UHT Metamorphism: Constraints From P–T–t–Melting Path From Rundvågshetta, Lützow–Holm Complex, East Antarctica

Abstract

The pressure–temperature–time (P–T–t) evolution of a metapelitic ultrahigh-temperature (UHT) granulite from Rundvågshetta (Lützow–Holm Complex, East Antarctica) and chemical evolution of partial melt during the prograde metamorphism up to UHT metamorphism are studied in detail. The presence of different phosphorus (P) concentration zones in garnet is used to distinguish four phases of garnet growth. The P–T conditions for the P-poor garnet core, P-rich mantle and two P-poor rim growths are estimated, respectively, at ~840°C–920°C/7.7–12.5 kbar, ~920°C–1015°C/12.5–14.3 kbar, ~950°C–1000°C/7 kbar and ~800°C/5 kbar using Al₂SiO₅ inclusions, Zr-in-rutile thermometry, P–T grid and pseudosection analyses. The glassy inclusions in the P-poor core of garnet plot approximately on the Qz-Or cotectic line for 10 kbar in the CIPW normative Qz-Ab-Or diagram, representing prograde to UHT melt formed through the dehydration melting of biotite + sillimanite. Furthermore, the earlier prograde P–T–t–melting information was constrained from inclusions in zircon. The inner mantle of zircon dated at 564 ± 10 Ma includes prograde inclusions of muscovite + quartz + nanogranitoids (NIs) that predate the garnet growth. The NIs in zircon remelted by piston-cylinder experiments plot approximately on the Qz-Or cotectic line for 5 kbar, representing the early melt formed through dehydration melting of muscovite at ~700°C/5 kbar. The P-rich garnet mantle and the CL-bright inner rim of zircon dated at 532 ± 5 Ma were in equilibrium at 900°C–1100°C, based on the REE distribution between them. This suggests that the peak UHT metamorphism occurred at 532 ± 5 Ma, with the prograde metamorphic period lasting ~30 Myr and overall anatectic period exceeding ~40 Myr. Negligible Pb diffusion between zircon zones possibly indicates that peak UHT was short lived, lasting less than 10 Myr. The systematic compositional change of the above-mentioned two stages of melt inclusions is consistent with compositional evolutions in published melting experiments and thus reflects the near-equilibrium compositional evolution of partial melts as the P–T conditions change over ~30 Myr of prograde metamorphism. Therefore, the UHT metamorphism in Rundvågshetta was probably caused by radiogenic self-heating in the thickened crust during the continental collision.