PERMIAN-TRIASSIC BEACON STRATA, ANTARCTICA: PROVENANCE AND AGE FROM DETRITAL ZIRCON GEOCHRONOLOGY

Permian-Triassic strata in the central Transantarctic Mountains form the most complete Gondwana sequence in Antarctica. In mid Permian time the depositional setting changed from an (?)intra-cratonic basin to a range-parallel foreland basin. Paleocurrent data suggest sediment input from both flanks: the East Antarctic craton and sources in West Antarctica, with the latter sources changing to a volcanic terrain by the late Permian.

Detrital zircon studies on sandstones from the Shackleton Glacier region, with paleocurrent data indicating a West Antarctic source, demonstrate onset of contemporaneous magmatism in middle to upper Permian time and continuing through the Triassic. These results expand on evidence for a magmatic arc, inferred from scattered plutonic rocks of that age range along the West Antarctic Panthalassic margin, that constitutes the major part of the so-called Amundsen Province of Marie Byrd Land. Follow-up U-Pb data obtained in geochronological mode (SHRIMP II) for the youngest igneous zircon populations support ages assigned from fossil microfloral, macrofloral and vertebrate assemblages.

Overall, the West Antarctic provenance of strata on that flank of the Beacon basin is dominated by a contemporaneous magmatic arc and sources with Late Neoproterozoic and Cambrian (Pan-African and Ross Orogen respectively) zircon age populations. The present Ross Orogen outcrop, essentially coextensive with Beacon strata, might have formed a significant belt outboard of its presently known distribution. Alternatively, the Ross Orogen-age source and the significant Pan-African component suggest a sedimentary sequence of Early Paleozoic age in West Antarctica (possibly Swanson Formation or equivalents) that was reworked into the Beacon basins. Minor Carboniferous and Devonian igneous source rocks reflect older magmatic arcs in West Antarctica, also recognized in the Ross Province of Marie Byrd Land. A minor but persistent component of late Proterozoic ("Grenville") age grains suggests unexposed Grenville age rocks in West Antarctica or original distal sources in East Antarctica and/or Africa reworked through Lower Paleozoic strata in West Antarctica.