Wright dykes : a geochemical study of the dyke-forming rock types of the Wright Valley, Southern Victoria Land, Antarctica

Hood Hills, Simone Belinda.

Jurassic fragmentation of Gondwanaland initiated the widespread voluminous outpouring of tholeiitic magma that is recorded on four continents. In Antarctica the Ferrar Supergroup, which outcrops along the length of the Transantarctic Mountains, formed within a wet failed rift during this break-up. It includes the extensive Ferrar Dolerite sills and dykes, and the extrusive Kirkpatrick flood basalt, and explosive volcanic deposits of the Mawson Formation. The Allan Hills, in southern Victoria Land, provides excellent exposure of these volcaniclastic deposits and was the site of study for this project. A small area was mapped in detail to determine the mode of origin of a part of the Mawson Formation and its relationship to the recently described 'phreatocauldron' in the nearby Coombs Hills. The Mawson Formation in the Allan Hills has previously been interpreted as part of a regional lahar field that formed as a precursor to Kirkpatrick flood-basalt eruption. Structural relationships and componentry of the Mawson Formation in the area, supplemented with results of coal vitrinite analysis, are difficult to reconcile with a laharic origin. During field work three different lithofacies were defined; a) thick, structureless, lithic-rich ponded pyroclastic density current deposits, b) several block-rich explosion or lag breccia horizons, and; c) a diatreme-like breccia-filled conduit (Ninnis Neck), one of the sources of other Mawson Formation deposits. These deposits are here interpreted to have formed within a phreatomagmatic vent complex, created by the coalescence of multiple vents similar to and including Ninnis Neck. Phreatomagmatic activity was fuelled by Fuel-Coolant-Interaction (FCI) as Ferrar Supergroup magmas intercepted water-saturated sedimentary materials of the Beacon Supergroup. Abundant peperite fragments formed by incomplete FCI, involving mingling of Beacon Supergroup clastic debris with magma preceding explosive fragmentation. Peperitic margins on basaltic dykes provide further evidence for involvement of wet sedimentary debris. Ninnis Neck was formed by upwards-directed tephra jets late in the overall complex's history. Recycling of tephra within individual vents, such as Ninnis Neck, was important in slowing the 'drying out' process and lengthening explosive activity. Growth of the Allan Hills vent complex occurred by lateral expansion facilitated by repeated failure of Beacon Supergroup strata along the margins due to volcanic and/or tectonic activity. The vent complex is interpreted to be relatively shallow, indicated by the limited sampling depth of accidental lithic clasts. The style of volcanism expressed in the Allan Hills of a shallow, possibly broad phreatomagmatic vent complex is comparable to that of the nearby Coombs Hills, and appears to have been widespread through much of the area covered by Kirkpatrick la vas now exposed in the Transantarctic Mountains.

viii, 107, [22] leaves : col. ill., col. maps ; 30 cm.

2004Hood_Hills

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