Volcanology of the carbonatitic Gross Brukkaros volcanic field (Namibia)

Gross Brukkaros is a volcano-shaped inselberg rising 600 m above the surrounding Nama plain in southern Namibia. It formed in several stages starting with the intrusion of a laccolithic body which caused uplift of the overlying Cambrian, Permocarboniferous and Upper Cretaceous Kalahari sediments as well as a Jurassic dolerite sill. Uplift of these sediments and consequent formation of almost radial fissures facilitated emplacement of more than 100 carbonatite dikes and 74 closely-associated carbonatite diatremes. Eruption of these subvolcanic feeder structures caused a mass deficiency in the laccolith and subsidence of its roof, i.e. a down-sag caldera formed, collecting thick reworked sediments and pyroclastic debris. In this near-surface environment, approximately 150-650 m below the original surface, the magma rising within the dikes rarely exsolved volatile phases, even in localities where diatremes formed. The carbonatite diatremes present evidence of phreatomagmatic explosive activity because of the occurence of typically non-vesicular carbonatite ash grains and lapilli. The paleo-hydrogeology was conductive to such explosive activity. A near surface groundwater table is indicated by unconsolidated Kalahari sediments and the free groundwater table of the lake inside the down-sagging caldera of Gross Brukkaros.