SPACE-TIME-GEOCHEMICAL CONSTRAINTS ON THE EMPLACEMENT OF THE FERRAR LARGE IGNEOUS PROVINCE IN SOUTH VICTORIA LAND, ANTARCTICA

Jurassic tholeiites of the Ferrar Large Igneous Province (FLIP) crop out in a 3000 km linear belt along the Transantarctic Mtns (TAM) extending into SE Australia. The FLIP along with the Karoo Province in southern Africa is temporally associated with the initiation of Gondwana break-up in the Weddell Sea region and has been identified as a trigger of global changes in biodiversity and seawater chemistry associated with the Pliensbachian-Toarcian boundary. The FLIP consists primarily of dolerite sills and subordinate connecting and cross-cutting dikes that have intruded near the base of and within flat-lying supracrustal strata at depths of <5km. Locally pyroclastic rocks and lavas cap the intrusive complex. Ferrar rocks in South Victoria Land (SVL) provide a well exposed vertical cross section of the Ferrar magmatic architecture within an area of less than a few 100’s of km². The SVL sequence consists of at least four vertically stacked, laterally extensive sills overlain by ~550 m of lavas. Major and trace element compositions of chilled margins of the sills are remarkably consistent over distances of 10’s of km and form several compositional clusters that are related mainly by low-pressure gabbro fractionation. Marsh (2004) proposed that the most evolved rocks in SVL were erupted first as lavas, and sills were emplaced at progressively greater depth as increasingly more magnesian magmas and crystal mushes were injected into supracrustal and finally basement rocks. This model is supported locally by cross-cutting relations showing more mafic intrusions cutting less mafic compositions. Previous geochronologic data has yielded a multigrain zircon/baddeleyite U-Pb date of 183.8±1.6 Ma, and a cluster of ⁴⁰Ar/³⁹Ar dates ca. 180 Ma with several younger dates down to ca. 173 Ma. The range of dates has been attributed to either protracted magmatic activity and/or cooling. Stratigraphically and chemically characterized Ferrar rocks from SVL have yielded new U-Pb single crystal zircon dates, which better constrain the timing of magma emplacement. The chemistry of the intrusive rocks provides important context for identifying related magma batches, and overcomes the lack of stratigraphic control. The results suggest that the entire suite of Ferrar rocks in SVL was emplaced over an exceptionally short time frame.