National Science Foundation, United States

New geochronologic data from two sample sites in offshore Antarctica are used to further constrain the submarine and subice limits of Gondwanaland's orogenic belts. DSDP 270 was drilled in the central Ross Sea (77.4415 degrees S 178.5032 degrees W) in 1973, and bottomed in calcsilicate gneiss. New LA-ICP-MS U-Pb dating of titanite from the gneiss gives a 440 Ma age. We interpret this to be consistent with typical (410-450 Ma) Lachlan-Tuhua-Swanson Orogen metamorphic ages; it is too young to be a typical (>460 Ma) Ross Orogen metamorphic age. The Iselin Bank (73.7000 degrees S 176.4667 degrees W) is 400 km north of DSDP 270, on the Antarctic continental shelf edge facing Zealandia. It was dredged on a cruise by the R/V S.P. Lee in 1984. A variety of igneous and metamorphic rocks, including some obvious dropstones, were obtained. One rock, a meta-rhyolite, had a freshly broken surface and was the only sample thought by the shipboard scientists to be possibly in situ. New LA-ICP-MS U-Pb dating of zircon from the metarhyolite gives an age of 544 + or - 21 Ma which we interpret as an eruption age. New Ar-Ar dating of K-feldspar phenocrysts from the same sample gives 270-300 Ma low temperature step ages, rising monotonically to 540 Ma in the high temperature steps. The protolith age is substantially older than any rocks in the Lachlan-Tuhua-Robertson Bay-Swanson Orogen. We interpret these results to indicate a correlation of the Iselin Bank rhyolite with known Neoproterozoic-Cambrian igneous rocks in the Delamerian-Ross Orogen. If the Iselin Bank material is not ice-rafted debris, then it represents a further intriguing occurrence of older Ross-Delamerian basement found within the younger Pacificward greater Lachlan Orogen. Similar occurrences have been reported from the South Tasman Rise, Fiordland New Zealand, and West Antarctica. The size, mechanism and timing of dispersal of these pieces of Ross Orogen into the greater Lachlan Orogen is speculative. One likely explanation is rifting of the Ross-Delamerian Orogen during Ordovician deposition of Lachlan sediments. Alternative possibilities involve post-Lachlan subparallel strike slip faulting, and/or low-angle extensional exhumation.