RIFT-RELATED JURASSIC BASALTIC PHREATOMAGMATIC VOLCANISM IN THE CENTRAL TRANSANTARCTIC MOUNTAINS - PRECURSORY STAGE TO FLOOD-BASALT EFFUSION

The Middle Jurassic Kirkpatrick flood basalts and comagmatic Ferrar in trusions in the Transantarctic Mountains represent a major pulse of th oleiitic magmatism related to early stages in the breakup of Gondwana. A record of the volcano-tectonic events leading to formation of this continental flood-basalt province is provided by strata underlying and only slightly predating the Kirkpatrick lavas. In the central Transan tarctic Mountains, the lavas rest on widespread (greater than or equal to 7500 km(2)) tholeiitic pyroclastic deposits of the Prebble Formati on. The Prebble Formation is dominated by lahar deposits and is an unu sual example of a regionally developed basaltic lahar field. Related, partly fault-controlled pyroclastic intrusions cut underlying strata, and vents are represented by the preserved flanks of two small tephra cones associated with a volcanic neck. Lahar and air-fall deposits typ ically contain 50-60% accidental lithic fragments and sand grains deri ved from underlying Triassic - Lower Jurassic strata in the upper part of the Beacon Supergroup. Juvenile basaltic ash and fine lapilli cons ist of nonvesicular to scoriaceous tachylite, sideromelane, and palago nite, and have characteristics indicating derivation from hydrovolcani c eruptions. The abundance of accidental debris from underlying Beacon strata points to explosive phreatomagmatic interaction of basaltic ma gma with wet sediment and groundwater, which appears to have occurred in particular where rising magma intersected upper Beacon sand aquifer s. Composite clasts in the lahar deposits exhibit complex peperitic te xtures formed during fine-scale intermixing of basaltic magma with wet sand and record steps in subsurface fuel-coolant interactions leading to explosive eruption. The widespread, sustained phreatomagmatic acti vity is inferred to have occurred in a groundwater-rich topographic ba sin linked to an evolving Jurassic rift zone in the Transantarctic Mou ntains. Coeval basaltic phreatomagmatic deposits of the Mawson and Exp osure Hill Formations, which underlie exposures of the Kirkpatrick Bas alt up to 1500 km to the north along strike in Victoria Land, appear t o represent other parts of a regional, extension-related Middle Jurass ic phreatomagmatic province which developed immediately prior to rapid outpouring of the flood basalts. This is consistent with models which assign an important role to lithospheric stretching in the generation of flood-basalt provinces.