GEOCHEMISTRY OF JURASSIC DOLERITE INTRUSIONS IN THE OHIO RANGE AND SOUTHERN QUEEN MAUD MOUNTAINS, ANTARCTICA

Jurassic tholeiites of the Ferrar Large Igneous Province (FLIP) crop out along the Transantarctic Mtns. (TAM) from the Weddell Sea region to north Victoria Land, and into Australasia, a distance of ~4000 km. The FLIP was emplaced at 182.7 Ma over a short time interval (<0.4 myr) and is associated with Gondwana break-up. The most wide-spread intrusive phase of the FLIP consists of a high-level complex of dolerite sills that have intruded along the base of and within flat-lying Devonian to Triassic supracrustal strata of the Beacon Supergroup. Rocks of the FLIP have isotopic (Sr, Nd, Pb) and trace element characteristics indicative of crustal involvement in their genesis and evolution. The unique chemical characteristics of these rocks over such a broad area have been interpreted as evidence that magmas were generated from a center in the proto-Weddell Sea region and subsequently transported over long distances, either as dikes or sills.

Dolerites exposed in the 400 km segment of the TAM extending from the southern Queen Maud Mtns. through the Ohio Range have been poorly studied because of limited exposure, rugged terrain, and remoteness of the area. Reconnaissance geologic mapping of the region was conducted in the early 1960’s (Long, 1964; Treves, 1965). The petrography of the dolerite sills has been briefly described but no systematic examination of the chemistry has been previously conducted. Collections from the prior field work are currently housed in the U.S. Polar Rock Repository at Ohio State University. While the samples are small and not specifically collected for geochemical analysis (some suffer from secondary alteration and in situ differentiation), they contribute important information on the geographic variation in chemistry within the FLIP. We have measured the chemistry of ~40 samples using XRF and ICP-MS. The most primitive samples (SiO₂=52%, MgO=13%) come from the lower 70 m of a 250 m thick olivine-bearing intrusion in the Nilsen Plateau. The samples are all medium grained (~2 mm) and the chemistry may be slightly influenced by mafic mineral accumulation. The most evolved samples in the region (SiO₂=57%, MgO=3.3%) come from the sill capping the section in the Ohio Range. Overall the chemistry is consistent with models suggesting a single source for the FLIP and long distance lateral transport of magmas.