FIELD-BASED VOLCANOLOGICAL STUDIES OF AN EXTENSIONAL 1.2 GA CONTINENTAL MAGMATIC ARC ON THE EDGE OF THE KALAHARI CRATON, SW NAMIBIA

We report results of the first modern volcanological studies of the 1.4-1.0 Ga Konkiep volcano-plutonic terrane in SW Namibia, which has an area of ~10,000 km² and forms a major part of the collisional Namaqua-Natal orogen (NNO) along the margin of the Kalahari craton in southern Africa. We focus on the most extensive extrusive unit in the terrane, the 1.2 Ga calc-alkaline to shoshonitic Barby Formation, which is several km thick and formed during arc volcanism preceding assembly of the Rodinian supercontinent. In contrast to other parts of the NNO, the volcanic rocks are surprisingly well preserved, permitting detailed 3D studies of volcanic lithofacies and hypabyssal magma plumbing systems exposed in mountainous desert terrain.

Evidence for major stratovolcanoes is lacking. Instead, reconnaissance mapping coupled with detailed work in well-exposed areas shows the most common lithofacies to consist of proximal pyroclastic deposits (containing bombs to 2 m across) intercalated with sequences of fine-grained lacustrine volcaniclastic sediments ≤ 200 m thick. Pyroclastic deposits show vertical transitions between Hawaiian, Strombolian, and phreatomagmatic eruptions, inferred to record variations in magma ascent rate as well as collapse of wet lacustrine sediments into active vents. Preserved vent conduits show that eruptions in some cases began from linear fissures but became focused to form central vents fed by diatremes exposed at deeper levels and filled with a chaotic mixture of juvenile pyroclasts, down-slumped portions of ejecta rims, etc. The facies patterns suggest that volcanism took place in a subdued topographic setting where numerous lakes repeatedly formed during ongoing subsidence, consistent with accumulation of the Barby Formation in one or more extensional arc basins during oblique subduction or slab rollback.

Magma was transported at shallow levels in the subsurface by a complex array of dikes, sills and inclined sheets that commonly have peperitic margins and developed highly sinuous or irregular shapes where they encountered wet lacustrine sediment. Networks of unusual intrusive pyroclastic rocks that transgress bedding are inferred to have formed from pyroclastic jets blasted laterally into weak sediments from diatremes feeding explosive eruptions at the surface.