Photo gallery of the Dicker Willem carbonatite complex, a 49 Ma intrusion in the southern Namib desert
Photo gallery of the Kanabeam alkaline igneous complex, part of the 500 Ma Kuboos - Bremen igneous lineament
Check out a carbonatite poster on Dicker Willem presented at the Seventh International Kimberlite Conference in Cape Town 1998
Cretaceous dykes swarms and the Kogelfontein alkaline igneous complex, west coast of Namaqualand
The Richtersveld alkaline igneous complex straddles the border region along the lower Orange River and was emplaced between 860 and 780 Ma ago
Reference: Communications Geological Survey Namibia, 7, 3-13, (1991).
The oldest intrusive phase recognised in the complex produced a series of concentric rings of nepheline syenite, which was subsequently intruded by a plug of quartz syenite. Later phases involved saturated microsyenites and a variety of granitic rock types differing in texture and quartz content. The intrusive axis shifted progressively from SW to NE, following the regional trend of the Kuboos-Bremen line, and was accompanied by a change from undersaturated to oversaturated rock types. Late phonolitic breccia pipes pierce the plutonic rocks along major contacts and contain, amongst other lithologies, several alkali gabbroic types which may have been derived from deeper unexposed pans of the complex.
View south over the southern part of the Kanabeam Complex
Tour the Kanabeam Complex
Reference: Geology, 19, 1193-1196 (1991).
Oxygen isotope fractionation between silicate and oxide minerals in sovite cumulates indicate high temperatures (600-900 Degrees C), compatible with those determined experimentally for crystallization of liquidus calcite or dolomite. We propose that a Ca-rich carbonatite magma (proto-alvikite) separated immiscibly from a carbonate-rich ijolite and evolved by fractional crystallization of magnetite, acmitic pyroxene, and calcite to a dolomite-phyric alvikite.
Coarse spinefex-like texture developed in sovite at Dicker Willem. Stellate blades of calcite are intergrown with dark clinopyroxene (aegirine-augite) and biotite
Medium-grained spinifex-type texture in alvikite cone sheet. Pale coloured calcite intergrown with dark brown dolomite in divergent sheaths of plates
Banded sovite contains granular calcite, clinopyroxene and biotite. A late stage brown vein of Fe-rich alvikite can be seen at the top
Close up view of a cut rock slab of clinopyroxene rich sovite (interpreted as a cpx-calcite cumulate) from Dicker Willem
Reference: Chemical Geology (Isotope Geoscience), 94, 293-305 (1992).
The early sovites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsovite, bulk sovites and interstitial carbonate in the ijolites, with d13C (- 5 ‰ vs. PDB) and d18O (+ 7 to + 9 ‰ vs. SMOW). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sovites yields near magmatic temperatures of 600-900 degrees C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration.
Phenocrysts in dolomite-phyric alvikite are slightly enriched in d13C (average d13C = -3.6 ‰) and 18O (average d18O = +9.90 ‰) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing d13C with d18O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sovite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of 18O enrichment relative to the pheno- crysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.
A complex history of multiple vein injection is clear from this outcrop of the Dicker Willem carbonatite. It is this late stage vein activity that is thought to have caused the extensive interaction between the carbonatite and late stage fluids, resulting in significant shifts in d18O to isotopically heavy values
Reference: Mineralogical Magazine, 59, 401-408 (1994).
At Haast River, taeniolite occurs in sodic and ultrasodic fenites derived from quartzo-feldspathic schists and rarely in metabasites, adjacent to dykes of tinguaite, trachyte and a spectrum of carbonatites ranging from Ca- to Fe- rich types.
In Namibia, taeniolite is present in potassic fenites derived from quartz- feldspathic gneisses and granitoids at the margin of an early sovite phase of the complex and in a radial sovite dyke emanating from this centre.
The occurrence of taeniolite in these totally disparate carbonatite complexes together with examples of lithian mica from other carbonatite complexes worldwide, raises the question of the status of Li as a `carbonatitic element'. We argue that lithium is not a consequence of crustal assimilation or interaction, but reflects the geochemical character of the magmatic source. Li, an overlooked and little-analysed element, may be an integral part of metasomatic enrichment in the mantle, and of magmas derived by partial melting of such a source.
Keywords: lithium, taeniolite, carbonatite, fenite, fenitization, mantle metasomatism.
Dark green veins of fenite traverse altered granite in the contact aureole of the Dicker Willem carbonatite. It is this type of fenite that the Lithian mica taeniolite has been found
D. L. Reid
Department of Geochemistry University of Cape Town, Rondebosch, 7700, South Africa
A. F. Cooper
Department of Geology, University of Otago, Dunedin, New Zealand
Department of Earth Sciences, The University, Leeds, United Kingdom
R. E. Harmer
Division of Earth, Marine and Atmospheric Sciences, CSIR, Pretoria, Republic of South Africa
Reference: Geological Magazine, 127, 427-433 (1990).
New radiometric age data are reported for alkaline centres in southern Namibia,
and are discussed together with published age data in terms of models put forward
to account for post-Karoo (Mesozoic-Recent) alkaline magmatism within the African
Agreement between K-Ar and Rb-Sr ages indicate emplacement of the Dicker Willem arbonatite in southern Namibia at 49 ± 1 Ma. Alkaline rocks associated with the Gross Brukkaros volcano show a discordant radiometric age pattern, but the best estimate for the age of this complex is 77 ± 2 Ma, similar to that obtained for the neighbouring Gibeon carbonatite-kimberlite province.
The Dicker Willem carbonatite is therefore younger than the Luderitz alkaline province (133 ± 2 Ma), and the Gross Brukkaros volcano, but is older than the Klinghardt phonolite field (29-37 Ma).
The new age data argue against a distinct periodicity in alkaline igneous activity in southern Africa, thereby ruling out possible controls by episodic marginal upwarping of the subcontinent. Although the available age data do not appear to be consistent with the passage of one or even two hotspots under southern Namibia, it is argued that the surface expression of hotspots under continents may be so large and overlapping that within-plate magmatism attributed to these thermal anomalies need not necessarily be confined to narrow linear belts or show an age progression.
The role of hotspots in continental alkaline magmatism is most likely one of melt generation, while local crustal structure probably controls the distribution and timing of eruption. Major tectonic boundaries in the Precambrian basement underlying southern Namibia seem to have controlled the development of Tertiary alkaline centres in that region.
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Last Updated: 2006/03/24