PEPERITES, STEAM-EXPLOSION BRECCIAS, INTRUSIVE PYROCLASTIC ROCKS AND PHREATOMAGMATIC DIATREMES FORMED DURING INTRUSION OF ANDESITIC MAGMA INTO WET LACUSTRINE STRATA IN A 1.2 GA INTRA-ARC BASIN (BARBY FORMATION, SW NAMIBIA)

Well-preserved 1.2 Ga volcanic arc rocks of the 8-km-thick Barby Formation form extensive exposures in mountainous desert terrain in SW Namibia. Our recent work indicates that significant parts of the unit consist of lacustrine sedimentary sequences up to 200 m thick intercalated with proximal pyroclastic fall deposits produced from small andesitic volcanoes undergoing Hawaiian, Strombolian and/or phreatomagmatic explosive activity. Here we report detailed mapping of a particularly well-exposed area in which lacustrine and pyroclastic strata form a gently tilted succession > 1 km thick. The study area provides a window into processes occurring in the shallow subsurface during eruptions of small monogenetic volcanoes in a subsiding intra-arc basin.

Diatremes up to 80 m wide penetrate parts of the succession and are inferred to be feeders to volcanic vents now eroded away. They contain chaotic mixtures of disaggregated sand and mud, bombs, scoria, spatter, lithic clasts and coherent tabular sediment masses recording collapse of parts of the diatreme walls. Hypabyssal andesitic intrusions are abundant and include dikes, sills and more complex intrusive networks of meter-scale fluidal tongues or thin, inclined, curving sheets, particularly where magma penetrated lacustrine strata. Widespread peperite records fine-scale, nonexplosive interactions between magma and lacustrine beds in domains up to 40 m across in which fluidal andesite clasts are complexly intermixed with disrupted sand and mud. Massive monomict breccia in thick zones along the margins of large plug-like intrusions likely formed from contained subsurface steam explosions generated from heated groundwater. Perhaps the most unusual hypabyssal rocks have pyroclastic textures, with fluidal bombs set in matrix of scoriaceous lapillistone or lapilli tuff, but show clear intrusive relations against the host strata. The pyroclastic intrusions form meter-scale dikes or larger masses ≥30 m thick that partly transgress bedding in lacustrine sequences and surround disrupted rafts of the host sediment. We infer that these intrusions formed when pyroclastic jets were blasted laterally into weak sediments during collapse of parts of the walls of feeder conduits that were erupting explosively at higher levels.