EXPOSED PYROCLASTIC VENT CONDUITS FOR MONOGENETIC ANDESITIC VOLCANOES IN A MESOPROTEROZOIC ARC SEQUENCE, SW NAMIBIA

Mesoproterozoic (1.2 Ga) volcanic arc rocks assigned to the Barby Formation are well exposed in SW Namibia but have received little previous attention from volcanologists. Our ongoing work indicates that significant parts of the volcanic succession consist of basaltic to andesitic pyroclastic rocks intercalated with lacustrine deposits and inferred to have been derived from monogenetic volcanoes undergoing various combinations of Hawaiian, Strombolian and phreatomagmatic eruptive activity.

Recent discovery of an andesitic vent-conduit system discontinuously exposed for ~1.5 km provides insight into processes occurring in the uppermost parts of the plumbing systems feeding individual small volcanoes within the arc succession. Outcrops of the vent conduits are ~50 to 150 m across, and their geometry suggests that eruptions began from linear fissures but in some places became focused to form central vents. The conduits are filled partly with a chaotic mixture of ellipsoidal bombs up to 1 m across, less common lithic blocks, variable amounts of lapillistone and lapilli tuff, and masses of coalesced spatter up to 20 m long. These features suggest that initial Hawaiian-style lava fountains produced spatter cones or ramparts at the surface, which collapsed inward as the conduits progressively widened during transitions to Strombolian eruptions that generated abundant fluidal bombs. Disaggregated sediment intermixed with juvenile pyroclasts in parts of the conduits suggests that phreatomagmatic processes in some cases played a role in explosive activity. Slab-like masses of lacustrine sediment ≥ 10 m long also locally occur in the conduits and show intense soft-sediment disruption, indicating that collapse of unstable sediment in conduit walls contributed to vent widening.

Chaotic pyroclastic material is exposed over vertical distances of as much as 80 m within the conduits, suggesting that either explosive disruption of magma occurred at least that deep beneath the paleosurface, or that magma drained back down in the conduit as pyroclastic eruptions continued. Degassed andesitic magma continued to rise at the end of explosive activity, intruding the pyroclastic material as irregular, meter-scale tongues or as larger masses of hypabyssal rock that partly fill the conduits.