CARBONATITE MAGMATISM AND FENITIZATION OF THE EPICLASTIC CALDERA-FILLAT GROSS-BRUKKAROS (NAMIBIA)

The Gross Brukkaros inselberg is a dome structure with a crater-shaped central depression within Precambrian/Cambrian country rocks which wa s active as a depocenter during the Late Cretaceous. The formation of the structure was due to the intrusion and subsequent intermittent dep letion of a shallow magma reservoir. Juvenile material has not been re cognized hitherto. This is the first account of juvenile lapilli from within the epiclastic fill of the caldera structure. The lapilli are c alciocarbonatites and magnesiocarbonatites in composition, but are cha racteristically low in elements such as P, Nb, Ba and Sr, otherwise ty pical of carbonatites. This signature, however, is also characteristic of carbonatites from surrounding volcanic centers and necks. The Bruk karos sediments suffered strong metasomatic-hydrothermal alteration, w hich introduced in a first stage fluids rich in Fe, Ti, Na, Nb, V, K ( Ca?, CO2?), and in a second stage the Brukkaros sediments were silicif ied on a large scale and locally enriched in P, Th and Cr. Si is deriv ed from desilication of the wail rocks (basement?, Nama sediments) of the magma reservoir. Cr was probably mobilized during alteration of th e abundant doleritic detritus within the Brukkaros depocenter.