NEW EVIDENCE OF PROTEROZOIC METAMORPHIC EVENTS IN EAST ANTARCTICA FROM IN-SITU U-PB AGE DATING OF MONAZITE IN METAMORPHIC GLACIAL CLASTS, CENTRAL TRANSANTARCTIC MOUNTAINS, ANTARCTICA

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 assembly and break-up of major supercontinents. The EAS has affinities with Africa, India, and Australia, but an ice cap up to 4 km thick prevents most direct geologic access. In addition to glacial cover, the Precambrian history of the EAS is obscured by the Ross Orogeny, which caused local high-grade metamorphism and thermal re-equilibration of crystalline basement at ~500 Ma. Petrologic, geochemical, and isotopic analysis of igneous and metamorphic clasts in glacial moraines along the central Transantarctic Mountains improve our understanding of the geologic history of the ice-covered shield. Seven metamorphic clasts from three different sample sites are semi-pelitic leucogneisses with a moderate- to high-P assemblage of Grt + Bt + Pl + Qz ± Mc ± Als ± Ms, plus accessory Mnz and Zrn. In-situ SHRIMP U-Pb monazite analysis gave pre-Ross Proterozoic ages in all seven samples. Two samples from Lonewolf Nunataks, at the head of Byrd Glacier, gave Proterozoic ages; one has an age of ~1190 Ma with older inherited components up to ~1900 Ma, and the other has a metamorphic age of ~1690 Ma with an older component at ~1870 Ma. Five other samples, from moraines near the Miller Range, yielded Neoproterozoic ages. Three have metamorphic ages of ~545-555 Ma whereas the others have ages between ~570 and ~595 Ma, the latter having an older component at ~660 Ma. These ages coupled with P-T analysis provide a greater understanding of the crustal and tectonic history of the EAS. Together, they record previously unknown Paleoproterozic tectonometamorphic events in central East Antarctica, and the younger Mesoproterozoic metamorphism corresponds to recently identified magmatic events. Samples from Lonewolf Nunataks may document to a Paleoproterozoic event within the EAS related to assembly of the Nuna supercontinent (~1870-1900 Ma). An age component of one sample relates to Rodinia break-up (~650 Ma) recorded elsewhere in Antarctica and Australia. Ages from the younger samples (~545-595 Ma) precede metamorphic dates typically reported from the Ross Orogen (~470-540 Ma); these samples may therefore provide evidence that Ross metamorphism was initiated significantly earlier than previously thought.