Jurassic Volcanic Rocks in the Transantarctic Mountains: A Test of a Model for Continental Flood Basalt Magmatism in Antarctica

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Abstract This award supports a project to study the Jurassic volcanic rocks which overlie the Permian to Triassic Gondwana sequence in Antarctica. These rocks constitute the Antarctic part of the magmatic record of the break-up of the supercontinent Gondwana. These Jurassic rocks, which occur along the length of the Transantarctic Mountains, include both silicic and tholeiitic compositions. The most complete stratigraphic record of the time interval from the Late Triassic termination of deposition in the Beacon foreland basin to the Middle Jurassic eruption of the flood basalts of the Ferrar Group is located in the central Transantarctic Mountains. In that region, the upper Falla Formation is dominated by volcanogenic rocks including plinian airfall tuffs and phreatomagmatic deposits. The overlying Prebble Formation is dominated by fine to coarse grained phreatomagmatic deposits, including accretionary lapilli tuffs and breccias. Compositionally the rocks are bimodal with silicic activity initiating volcanism and continuing into the flood basalt eruptive episode, whereas the basaltic activity begins with explosive eruptions in the Prebble and changes to quiet effusion of lava. The basaltic pyroclastic rocks are comagmatic with the lavas, having the same geochemical characteristics. The silicic rocks are probably anatectic. Coarse quartzose sandstones with large angular clasts and the presence of monoclinal structures active late in the magmatic activity, indicate contemporaneous basement block faulting. Stratigraphic, structural, petrologic and paleovolcanologic data indicate that these igneous rocks accumulated in a tectono-magmatic rift setting. In south Victoria Land, the correlative strata include flood lavas, basaltic pyroclastic deposits and some recently reported silicic pyroclastic rocks. At one locality a major disconformity separates the basaltic pyroclastic rocks from the underlying Beacon. The relations of the silicic tuffs to the basalt ic pyroclastic rocks and to the disconformity are unclear. The silicic tuffs are reported to include an ignimbrite. The objectives of this project are to gather data to test the tectono- magmatic model proposed for the Jurassic magmatism in Antarctica. Specifically, the objectives are to: 1) establish a detailed stratigraphic framework for the Jurassic pyroclastic rocks in order to assess syndepositional extensional normal faulting, 2) investigate the paleovolcanology of the rocks, in particular the rocks formed by phreatomagmatic processes, in order to understand the physical environment, 3) further characterize the geochemistry of the rocks in order to better define time dependent changes in magma compositions, relations between silicic and basaltic rocks, and source region characteristics of the silicic magmas, and 4) establish the age of onset of the silicic volcanism. These data will be used to validate or modify the proposed model and to assess the model's applicability to continental flood basalt magmatism in general.