On the last explosions of carbonatite pipe G3b, Gross Brukkaros, Namibia

Abstract

 Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick. The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume. Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space. The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above. Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin.