Geology and geochemistry of the carbonatitic Gross Brukkaros volcanic field and the ultrabasic Blue Hills Intrusive Complex, southern Namibia

Abstract

Abstract provided by author:

The carbonatitic Gross Brukkaros Volcanic Field is located at the southern margin of the Gibeon Kimberlite Province, southern Namibia. The enigmatic Gross Brukkaros occupies the centre of this Volcanic Field; it is a hill with a basal diameter of lO km and a height of 600 m above the surrounding plain of Precambrian to Cambrian Nama sediments (1000 m a. s. l.). The formation of the hill is attributed to the intrusion of a shallow seated magmatic body

The Gross Brukkaros Volcanic Field covers an area of about 25 km in north-south and 20 km in east-west direction. The Upper Cretaceous Volcanic Field consists of more than 100 dykes and 74 vents, mostly located on the dykes. The dykes have a radial orientation pattern with an intersection point at the southwestern boundary of the Balcony, a horst structure north of Gross Brukkaros, and Gross Brukkaros itself. The dykes have a width normally not exceeding 0. 5 m and a length up to 2 km. The most distant dyke occurs at a distance of 13 km away from the mountain. There exist specific areas, where the frequency of dykes is higher and where the carbonatite contains magnetite, which points to high production rates. These areas are called "trajectories"; between each other they have angles of quite exactly 120°

The erosion level of the carbonatite vents extends from the root zone to nearly horizontal maar beds. Since the time of eruption, strata of about 300 m were eroded (Cretaceous Kalahari, Permo-Carboniferous Karoo, and Nama sediments). The diameters of the vents may vary between several metres only and 200 metres. Frequently, the diatremes consist of several smaller, coalescing vents. These complex vents may be oriented parallel or - less frequently - radially in respect to Gross Brukkaros

The vents are normally filled with a volcanic breccia consisting of carbonatite as matrix and xenoliths of country rock. In part fenitised basement xenoliths may be a frequent component, indicating a localisation of the magma reservoir within the basement (about 2 km depth). In addition, dolerite blocks derived from nowadays eroded Jurassic dolerite sills, occur also. Spherical carbonatite lapilli with kernels normally consisting of basement fragments are a frequent component

The age relationship between the vents and the dykes indicates a penecontemporaneous time of activity, with the vents normally being the younger structures

The near absence of bubbles in the carbonatite matrix and in lapilli within the vents strongly points to an eruption mechanism that was not dominated by fluidisation as a result of exsolution of carbonatite volatile phases but by phreatomagmatism

It is shown that the Balcony is the older structure in respect to Gross Brukkaros. Brittle movement of the overlying crust was accompanied with the genesis of the Balcony. Related to the development of the Balcony the stress field for the carbonatite dykes was generated. Later slow intrusion of magma beneath Gross Brukkaros shifted the symmetry centre of the stress field from the Balcony/Gross Brukkaros boundary towards the present Gross Brukkaros crater centre. During the main phase of volcanic activity the stress field changed from compressive to extensional. This is responsible for the parallel orientation of the elongated vents in respect to Gross Brukkaros. The volcanic activity in the vents and dykes surrounding Gross Brukkaros also induced the subsequent depletion of magma beneath Gross Brukkaros resulting in the formation of a ring-anticline. This structure was contemporaneously filled with sedimentary material, the so called "Brukkaros sediments". Later intensive silification of these sediments and the underlying Nama strata was the cause for the younger evolution of Gross Brukkaros into an inselberg

Altogether there exist five types of beforsitic carbonatite which can be distinguished by petrographical and geochemical means. Petrographically, they are distinguishable by their colour and their content in mica, magnetite, and olivine or pyroxene pseudo-morphs. Geochemically, they may differ widely; some of the carbonatite types show affinities with alnoites, although no melilite was found

One carbonatite type is geochemically similar to the nearby Blue Hills magmatic rocks. This carbonatite is the only rock type that is isotopically clearly of mantle origin. The other rock types are more or the less influenced by crustal contamination. This observation supports the model of a crustal magma reservoir, whithin which fractionation and contamination towards the distinct carbonatite types took place

The nearby Blue Hills Intrusive Complex is located about 5 km south of Gross Brukkaros. It consists of 4 ultrabasic rock types, which show a mantle signature (80 to 100 km depth) and can be related to a fractionation model. Carbonatite, which is geochemically distinct from the Gross Brukkaros occurrences, was generated by liquid immiscibility at depths exceeding 6 kb in pressure. A pegmatite represents the last phase of magmatic activity. Both the carbonatite and the pegmatite show an unusual low REE content, which is related to perovskite fractionation

Radiometric age determinations (mica) yield an age of 75. 1 ± 0. 6 Ma for the Blue Hills, which is most probably also valid for the Gross Brukkaros magmatic activity