COMPOSITION AND AGE OF EAST ANTARCTIC SHIELD IN WILKES LAND DETERMINED BY PROXY SAMPLES OF OLIGOCENE-HOLOCENE GLACIOMARINE SEDIMENT

The Precambrian East Antarctic shield (EAS) played an important role in the evolution of Rodinia and Gondwana. Beneath its polar ice cap, little is known about the composition and structure of the EAS interior, yet the ice itself provides proxy samples of underlying continental basement. Offshore glaciogenic deposits of Oligocene-Miocene and Holocene age along the Wilkes Land margin include glaciomarine diamicts containing basement rock clasts and fine-grained siliciclastic detritus. Rock clasts obtained by dredge comprise 81% metamorphic (mainly meta-arenite, phyllite, gneiss, granitic gneiss, granulite and charnockite), 14% igneous (calc-alkaline granitoid and tholeiitic diabase) and 5% sedimentary (arenite and graywacke). Granitoid clasts have trace elements indicating crustal sources in a volcanic-arc setting; two have U-Pb zircon crystallization ages of ~510 and ~515 Ma, likely derived from the Ross Orogen. Five other igneous clasts have ages between ~500-520 Ma, with two others at ~585 and 1725 Ma. Six metamorphic clasts of ~500 Ma age contain discrete zircon age populations of 670-780, 900-1300, 1740-2300 and >2700 Ma that reflect ages of primary source materials. A charnockite gneiss and granulite gneiss have recrystallization ages of ~1740 and 1720 Ma. Detrital zircon populations from seven glaciomarine sediments vary with depositional age, but show persistent terrigenous provenance ages of 540-590, 780, 1000-1260, 1460, 1610-1660, 1760, 2235-2450 and 3070-3485 Ma. These ages overlap significantly with pre-500 Ma inherited zircons in the Ross granitoids. Detrital zircon ages from Permian and Triassic terrestrial sediments indicate that the glaciogenic deposits do not contain significant second-cycle material from older cratonic basins. Together, these data suggest that EAS basement inland from the Wilkes Land margin is dominated by metamorphic rock units with distinctive Neoproterozoic, Paleoproterozoic and Archean ages, and that Ross-age granitoids either intruded or were derived by partial melting of this composite metamorphic basement. Integration of data from petrologically-distinctive individual clasts and large detrital zircon populations therefore provides a good first-order representation of the EAS terrains underlying the Wilkes Land ice sheet.