Carbonatite magmatism and fenitization of the epiclastic caldera-fill at Gross Brukkaros (Namibia)

Bulletin Volcanologique - Tập 57 - Trang 185-196 - 1995
Thomas Stachel1, Volker Lorenz1, Gerhard Brey2
1Institut für Geologie, Universität Würzburg, Würzburg, Germany
2Institut für Geochemie, Petrologie und Lagerstättenkunde, Universität Frankfurt, Frankfurt, Germany

Tóm tắt

The Gross Brukkaros inselberg is a dome structure with a crater-shaped central depression within Precambrian/Cambrian country rocks which was active as a depocenter during the Late Cretaceous. The formation of the structure was due to the intrusion and subsequent intermittent depletion of a shallow magma reservoir. Juvenile material has not been recognized hitherto. This is the first account of juvenile lapilli from within the epiclastic fill of the caldera structure. The lapilli are calciocarbonatites and magnesiocarbonatites in composition, but are characteristically low in elements such as P, Nb, Ba and Sr, otherwise typical of carbonatites. This signature, however, is also characteristic of carbonatites from surrounding volcanic centers and necks. The Brukkaros sediments suffered strong metasomatic-hydrothermal alteration, which 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 silicified on a large scale and locally enriched in P, Th and Cr. Si is derived from desilication of the wall rocks (basement?, Nama sediments) of the magma reservoir. Cr was probably mobilized during alteration of the abundant doleritic detritus within the Brukkaros depocenter.

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Bulletin Volcanologique

Tập 57

185-196

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