ISOTOPIC STUDIES OF NEAR CONTEMPORANEOUS ZIRCONS FROM PERMIAN AND TRIASSIC STRATA IN CENTRAL ANTARCTICA AND THE CONTEMPORANEOUS MAGMATIC ARC IN WEST ANTARCTICA: PRELIMINARY RESULTS

The record of the Permo-Triassic Gondwana Orogen in Antarctica occurs in isolated granitoids and gneisses in West Antarctica (Marie Byrd Land; MBL) and the Antarctic Peninsula, and in related stratigraphic sequences in the Transantarctic Mountains (TAM) and Ellsworth Mountains. Detrital zircon geochronology of Permian and Triassic sandstones from central TAM (CTM) has demonstrated that near contemporaneous igneous zircons are present in Upper Permian and Triassic strata of the Victoria Group. These zircons, which paleocurrent data show were derived from West Antarctica, indicate that at least intermittent magmatic activity occurred during this time interval. Ellsworth Mountains Permian strata, which include several tuffs, have yielded elongate zircons of probable volcanic or sub-volcanic origin of late Permian age. In acceptable Gondwana reconstructions, the CTM zircons were most likely derived from MBL, whereas the Ellsworth Mountains zircons were probably derived from the Antarctic Peninsula.

Zircons from two MBL granitoid samples have been re-analyzed by SHRIMP II, and together with the contemporaneous detrital zircons from CTM and the Ellsworth Mountains have been analyzed for Lu-Hf and O isotopes. Victoria Group samples show a range of Hf- and O-isotope characteristics with a trend from relatively evolved Hf but markedly depleted O values toward more enriched Hf compositions and crustal O isotopic values. These data indicate increasing crustal interaction and residence time from upper Permian through late Triassic time. These results and those for the Ellsworth Mountains tuffs are compared with data for magmatic arc rocks from the Gondwana margin.