THE GONDWANA SEQUENCE IN ANTARCTICA: SOME RESULTS OF DETRITAL ZIRCON STUDIES

The Permian to Triassic Gondwana sequence crops out in Antarctica in the Transantarctic Mountains and Ellsworth Mountains. The most complete and thickest sequence is in the central Transantarctic Mountains and only Permian strata are present farther south and in the Ellsworth Mountains. In CTM there is a significant change in sandstone petrology to a magmatic provenance in mid Permian time. Detrital zircons from sandstones from CTM, the Ohio Range and the Ellsworth Mountains have been analyzed for provenance studies, first for detrital age spectra and then, by targeted geochronological analyses of the youngest grains of magmatic origin, for stratigraphic age constraints, and for Hf- and O- isotopes for source rock characteristics. Two CTM Permian Buckley Formation samples yield late Permian ages, indicating deposition no older than ~267 Ma (Guadalupian); two Triassic Fremouw Formation samples have ages of ~245 Ma and ~242 Ma (ca. Olenekian-Anisian boundary); and one Falla Formation sample gives an age no older than ~207 Ma (Norian). A sandstone from the Mt. Glossopteris Formation of the Ohio Range gave an age no older than ~270 Ma. Two samples from the Polarstar Formation, Ellsworth Mountains, contained elongate volcanic zircons and gave essentially identical ages of about 260 Ma. These results demonstrate the onset of magmatism, as recorded in the Gondwana strata, and that magmatism continued through to late Triassic time, although interpretation is limited by the lack of Triassic Gondwana strata in the Ohio Range and the Ellsworth Mountains.

Oxygen and Lu-Hf isotopic data for the youngest zircon grains in each CTM sample demonstrate a range of igneous sources, some having a strong Mantle imprint and others exhibiting significant crustal evolution. There is no systematic temporal change in the isotopic characteristics of the igneous sources being eroded, although the oldest and youngest samples are distinct.

The results suggest magmatism was more continuous and possibly more widespread than inferred from scattered plutonic rocks of that age range along the West Antarctic Panthalassic margin. Further, the isotopic results provide new information on the West Antarctic magmatic sources, which currently lack in situ Hf- and O-isotopic data.