Multiple timings of garnet-forming high-grade metamorphism in the Neoproterozoic continental collision zone revealed by petrochronology in the Sør Rondane Mountains, East Antarctica

Abstract

The Sør Rondane Mountains (SRM), East Antarctica is located in a key area of Gondwana formation where the East African-Antarctic Orogen and the Kuunga Orogen cross. Timing of peak metamorphism has been considered at ca. 650–600 Ma in the SRM, on which previous tectonic models were built. In this study, timing of garnet-forming metamorphisms is determined using in situ U-Pb dating of zircon and petrochronological approach utilizing distribution coefficient of rare earth elements (DREE) between zircon and garnet and REE patterns of zircon. As a result, we obtained several different periods of garnet-forming metamorphism at ≥600 Ma and ≤580 Ma both from the NE- and SW-terranes. This is the first report of multiple timings of garnet formation detected within single samples from the SRM. The latest garnet formation in each sample is probably caused by regional metamorphism, because the garnets accompany pressure shadows and are wrapped by the penetrative foliation defined mainly by the arrangement of biotite. Therefore, multiple timings of garnet formation during a single metamorphism or polymetamorphism are both likely. In the former case, the SRM may be affected by a single long-lived regional metamorphism. This implies that the regional metamorphism belongs only to the East African-Antarctic Orogeny or that the East African Orogeny continuously transformed into the Kuunga Orogeny in the deep crust. In the latter case, the former regional metamorphism corresponds to the East African(-Antarctic) Orogeny, whereas the latest metamorphism may correspond to the final stage of East African-Antarctic Orogeny or the Kuunga Orogeny. Any temperature drops during these two garnet-forming timings support the polymetamorphism, which needs to be cleared in the future works.