COMPLEX SHALLOW-LEVEL ANDESITIC MAGMA PLUMBING SYSTEMS AND MAGMA/SEDIMENT INTERACTION IN A MESOPROTEROZOIC VOLCANIC ARC SETTING IN NAMIBIA

SW Namibia contains widespread outcrops of 1.2 Ga volcanic arc rocks, including the andesitic to shoshonitic, 8-km-thick Barby Formation, which is the most extensive unit within the arc package. Rugged desert topography in a representative study area allows excellent cross-sectional views of parts of the unit, which contains pyroclastic fall deposits intercalated with abundant lacustrine strata, as well as crosscutting diatremes and a wide array of hypabyssal intrusions inferred to represent feeders for the growing volcanic pile.

Hypabyssal intrusions in the form of dikes and sills are common, but more complicated geometries occur where magma came in contact with unlithified sediments or pyroclastic deposits. Larger plug-like intrusions have marginal zones of monomict breccia, which we infer developed from contained steam explosions caused by heated groundwater. Diatremes with vertical walls also occur and represent feeders to small maar volcanoes. They are filled with a chaotic mixture of bombs, blocks, spatter, disaggregated sand and mud, and large tabular bodies of lacustrine sediment and pyroclastic deposits formed from collapse of diatreme walls and ejecta rims.

Peperite formed by nonexplosive mixing of quenched magma and sediment is abundant along intrusive margins, as irregular masses within diatremes, or in isolated domains ≥40 m across within bedded strata, and contains fluidal bodies or clasts of andesite surrounded by partly disrupted host sediment. Pyroclastic rocks showing clear intrusive relations with lacustrine strata contain fluidal bombs in a matrix of vesicular lapillistone or lapilli tuff, and are similar to extrusive pyroclastic fall deposits in the area. These unusual pyroclastic rocks transgress bedding in sedimentary sequences as dikes or larger discordant masses over 30 m thick, and in some cases enclose tabular sediment masses. In places they form an interconnected 3-D network cutting across parts of the succession and can be traced at least one kilometer laterally over a vertical stratigraphic distance of several hundred meters. An intrusive origin is only obvious when the contacts of these pyroclastic masses with the host sedimentary strata are exposed. They may have formed by laterally directed subsurface blasts from diatremes undergoing explosive behavior.