INTRUSIVE PYROCLASTIC ROCKS FORMED DURING EXPLOSIVE ANDESITIC ERUPTIONS IN A MESOPROTEROZOIC VOLCANIC ARC SETTING, BARBY FORMATION, SW NAMIBIA

Mesoproterozoic volcanic arc rocks in the Barby Formation are extensively exposed in mountainous desert terrain in SW Namibia. Here we report results of detailed mapping of a representative succession of pyroclastic fall deposits intercalated with lacustrine strata consisting mostly of planar-bedded mudstone, siltstone and sandstone. Individual pyroclastic fall deposits are up to ~20 m thick and contain various amounts of fluidal to angular lapilli intermixed with ellipsoidal bombs up to 60 cm across, indicating deposition close to source vents that were primarily undergoing Strombolian-style eruptions.

Also present are enigmatic pyroclastic rocks (basaltic andesite in composition) that intrude lacustrine sedimentary packages at a number of locations spread out laterally for ~600 m over a vertical stratigraphic distance of ~300 m. The intrusive pyroclastic rocks consist of moderately to highly vesicular lapilli and dispersed, less vesicular bombs and are lithologically similar to some of the extrusive pyroclastic units. In one area the pyroclastic material forms a 1-m-wide dike intruded into lacustrine strata. More commonly the pyroclastic intrusions form structureless masses ≥ 12 m across that transgress bedding in the lacustrine sequences and in places enclose rafts of sediment that have been rotated relative to intact strata nearby. The sediment rafts show varying degrees of disruption and local peperitic mixing with intrusive pyroclastic material. How these pyroclastic intrusions were emplaced is unclear. Possibly they represent pyroclastic jets injected laterally into weak layers of unconsolidated, fine-grained sediment from vent conduits that fed explosive eruptions at higher levels (not exposed); we plan to test this hypothesis during future field work.

In most cases, zones of peperite up to 3 m thick separate the pyroclastic intrusions from host lacustrine sequences and consist of abundant fluidal bodies of vesicular basaltic andesite up to 50 cm across mingled with fine-grained sediment preserving planar lamination to various degrees. We infer that these peperites formed after explosive activity had ceased, when small batches of magma preferentially intruded along zones of weakness between the intrusive pyroclastic material and adjacent sediment.