INTERMIXED HAWAIIAN, STROMBOLIAN AND PHREATOMAGMATIC ERUPTION STYLES IN A MESOPROTEROZOIC VOLCANIC ARC SEQUENCE (BARBY FORMATION) IN SOUTHWEST NAMIBIA

The Barby Formation makes up part of the Konkiep Terrane, which is a major Mesoproterozoic arc complex within the Namaqua-Natal Orogenic Belt along the Kalahari craton margin in southwest Namibia. Previous mapping indicates that the Barby Formation contains a laterally and vertically complex series of basaltic to andesitic lavas and rhyolitic ignimbrites and lavas, interbeded volcaniclastic deposits and associated hypabyssal intrusions. New work indicates that thick basaltic to andesitic pyroclastic successions are also present within the unit and record a wide variation in eruption styles.

Our detailed mapping indicates the presence of Hawaiian, Strombolian and phreatomagmatic pyroclastic deposits forming successions up to 45 m thick emplaced close to source vents and intercalated with fine-grained lacustrine strata in an area ~20 km². The most abundant deposits consist of basaltic to andesitic spatter accumulations formed from vigorous lava fountains during Hawaiian-style eruptions. These sequences show random vertical transitions on the scale of a few meters from moderately agglutinated to densely welded spatter, which reflect variations in magma flux rates. Individual spatter pieces are up to 35 cm long. The densely welded spatter forms lava-like units but we see no evidence of clastogenic lava flows. Sequences of basaltic lapillistone with dispersed ribbon and fusiform bombs up to 50 cm long record Strombolian eruptions during episodes of lower magma flux rate without involvement of external water. The spatter accumulations typically grade upward into phreatomagmatic deposits containing minor amounts of spatter and cauliflower bombs mixed with poorly vesicular lapilli tuff, in which particle shapes are controlled by fracture surfaces rather than broken bubble walls; up to 30% lacustrine sediment is intermixed on a fine scale with juvenile lapilli and ash. A decrease in magma flux lowered the mass ratio of magma to external water, which caused repeated shifts from Hawaiian style to phreatomagmatic eruptive behavior. We infer that changes in eruptive style are controlled by variations in magma ascent rates at shallow depths as documented in numerous other volcanic provinces.