ANDESITIC PYROCLASTIC INTRUSIONS INJECTED LATERALLY INTO WEAK LACUSTRINE SEDIMENTS WITHIN A MESOPROTEROZOIC VOLCANIC ARC SUCCESSION, BARBY FORMATION, SW NAMIBIA

The Konkiep terrane in SW Namibia forms a major tectonic element within the 1.4-1.0 Ga Namaqua-Natal orogenic belt, which represents one of the main convergent margins active during Rodinia supercontinent assembly. The 1.2 Ga Barby Formation records establishment of a major continental margin arc within the terrane following earlier accretionary events. Our recent fieldwork shows that significant parts of the formation consist of proximal pyroclastic fall deposits ranging in composition from basaltic andesite to shoshonite, which were erupted from small volcanoes (fissures and scoria or spatter cones) in a poorly drained region where abundant lakes existed.

Here we report results of detailed mapping in a well-exposed part of the Barby Formation that provides an oblique cross-sectional view of a thick succession of andesitic pyroclastic fall units intercalated with fine-grained, planar bedded lacustrine sequences. Massively bedded units ≤ 80 m thick contain abundant bombs in a matrix of moderately to highly vesicular lapillistone and lapilli tuff and are inferred to have accumulated near small vents that were undergoing dominantly Strombolian-style eruptions. Hypabyssal dikes, sills and irregular discordant intrusions of similar composition are common. We have also found pyroclastic rocks in 12 separate locations over a lateral distance of ~600 m and a vertical stratigraphic height of ~300 m that contain similar types of bombs and lapilli to those in the pyroclastic fall deposits but show clear intrusive relations with adjacent rocks; the full extent of these pyroclastic intrusions remains to be determined. The examples so far found are restricted to lacustrine intervals and form small pyroclastic dikes or larger masses ≥ 12 m wide that cut discordantly across bedding in the host strata and enclose meter-scale rafts of sediment showing partly disrupted bedding. We infer that jets of intrusive pyroclastic material were blasted laterally into weak, unlithified lake sediments from one or more vent conduits feeding explosive eruptions at the surface. Liquefaction and fluidization of the sediments as their pore water was converted to steam likely facilitated lateral motion of the pyroclastic jets. Pyroclastic intrusions of this type may be more common than generally realized.