New evidence of Proterozoic high P-T metamorphism in East Antarctica from thermobarometry and in-situ U-Pb age dating of monazite in metamorphic glacial clasts, central Transantarctic Mountains, Antarctica

Abstract

The East Antarctic shield (EAS) is a key component in the study of early crustal evolution due to its ancient geologic history and involvement in the amalgamation and break-up of major supercontinents. The EAS has documented affinities with the cratons of Africa, India, and Australia based on limited coastal outcrop, but an ice cap up to 4 km thick prevents direct access to the interior bedrock of the EAS. Additionally, thermomechanical effects of the Ross Orogeny (~500 Ma) obscure the Precambrian history in rarely-exposed crystalline basement. Metamorphic rock clasts from glacial moraines near the central Transantarctic Mountains were studied for petrologic, geochemical, and isotopic characteristics in order to further understand the geologic history of the EAS. These clasts were presumably eroded from the interior of the EAS and may provide unique natural samples of the ice-covered basement. Metamorphic rock clasts selected in this study are semi-pelitic gneisses with high-pressure mineral assemblages, as well as accessory minerals including monazite and zircon. New in-situ SHRIMP U-Pb analysis of monazite yielded Paleoproterozoic to Neoproterozoic ages in six clasts. Among these, two clasts from Lonewolf Nunataks, at the head of Byrd Glacier, gave previously unrecognized ages of ~1900-1700 Ma, with one having a Mesoproterozoic overprint at ~1200 Ma. Lonewolf Nunataks clasts preserve evidence of high-pressure granulite-facies metamorphic conditions associated with Proterozoic crustal convergence and thickening during orogenic activity. Clasts sampled from moraines near the Miller Range yielded Neoproterozoic U-Pb ages; one clast yielded ages of ~660 and ~590 Ma, whereas other clasts gave ages of ~570- 545 Ma. Neoproterozoic samples conducive to thermobarometric analysis record high P-T conditions comparable to previously documented Ross Orogen activity in reactivated Precambrian crystalline rocks of the Nimrod Group. Together, the clast ages coupled with P-T analysis record previously unknown Paleoproterozoic tectonometamorphic events in central East Antarctica, overprinted by younger Mesoproterozoic metamorphism. Clasts from Lonewolf Nunataks may reflect a Paleoproterozoic event within the EAS related to development of the Nuna supercontinent (~1870-1900 Ma), overprinted by a Mesoproterozoic orogenic event, possibly related to the final amalgamation of Rodinia (~1200 Ma). One clast with a metamorphic age of ~660 Ma may record rift-margin activity associated with supercontinent breakup. Neoproterozoic ages of ~590-545 Ma from some of the clasts are demonstrably older than Ross Orogen ages known from Nimrod Group metamorphic basement; these older ages may therefore provide evidence that Ross orogenic activity was initiated earlier than previously thought, and that its metamorphic overprint extends farther inboard of the Transantarctic Mountains.