ERUPTIVE MECHANISMS, LITHOFACIES PATTERNS AND SUBVOLCANIC MAGMA PLUMBING SYSTEMS IN A 1.2 GA EXTENSIONAL CONTINENTAL MARGIN ARC ON THE EDGE OF THE KALAHARI CRATON, SW NAMIBIA

Mesoproterozoic continental margin arc rocks in SW Namibia are part of a major volcano-plutonic terrane within the 1.4-1.0 Ga Namaqua-Natal orogenic belt, which extends along the southern margin of the Kalahari craton in southern Africa. The most extensive volcanic unit in the terrane is the 1.2 Ga Barby Formation, which is as much as 8.5 km thick and is typically only weakly deformed and metamorphosed. In spite of this, little modern work has been carried out on the physical volcanology of these rocks. Here we report results of new fieldwork within representative parts of the unit exposed in mountainous desert country, providing cross-sectional views of a complex array of lavas, abundant proximal pyroclastic deposits, vent feeder conduits and hypabyssal magma plumbing networks. Ongoing geochemical work indicates that these rocks are calc-alkaline basaltic andesite and andesite to shoshonite in composition. Intercalated lacustrine deposits ≤ 200 m thick are common. The facies patterns suggest volcanism took place in a setting with relatively low topography and numerous lakes, consistent with previous interpretations that the Barby Formation accumulated in extensional basins during oblique subduction along the convergent margin.

The pyroclastic deposits formed partly from Strombolian- and Hawaiian-style eruptions producing ribbon, ellipsoidal and fusiform bombs ≤ 60 cm across, as well as massive beds of agglutinated to densely welded spatter. Spatter deposits grade up into lapilli tuff with up to 50% intermixed sediment, suggesting that collapse of vent walls led to explosive phreatomagmatic interactions between rapidly heated pore fluids in the sediment and rising magma. Exposed vent conduits indicate that eruptions began from linear fissures but in some cases became focused to form small central vents. Magma was transported at shallow levels in the subsurface by a series of dikes and sills, including sill packages that underwent nonexplosive quench fragmentation against wet sediment when they encountered thick lacustrine intervals. Unusual intrusive pyroclastic rocks that transgress bedding are also present and are inferred to have formed when pyroclastic jets were blasted laterally into weak sediments from active vent conduits feeding explosive eruptions at the surface.