Alkaline Rocks and Carbonatites of the World

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Dicker Willem (Garubberg, Garub Berg, Great Tigerberg)


Occurrence number: 
Longitude: 16.02, Latitude: -26.47

This 3 km-diameter, approximately circular carbonatite intrusion is emplaced in gneisses of the Namaqualand Metamorphic Complex. There are two carbonatite centres both of which have steeply dipping concentric structures defined by flow banding, as well as cone sheets and dykes in the basement gneisses. Sovite forms broad, arcuate zones near the periphery of the complex but is disrupted by later alvikite, in which it also occurs as xenoliths. The sovites are texturally heterogeneous and commonly banded, which is defined by grain size variation as well as mineralogy, and consists of calcite, euhedral magnetite, aegirine-augite zoned to aegirine rims and occasionally replaced or rimmed by blue sodic amphibole, pyrochlore, biotite, apatite, which may be altered marginally to dahllite and, of lesser abundance, zircon, K-feldspar, nepheline and colourless mica. Irregular pods or segregations in the sovite up to 600 m in length consist of closely packed magnetite octahedra, aegirine, biotite, pyrochlore and apatite and Cooper (1988) considers that these rocks are sovite cumulates. The most abundant rock type of the complex is alvikite which is occasionally banded and forms moderately dipping cone sheets and ring-dykes and some cross-cutting dykes. Alvikites may contain calcite or dolomite phenocrysts and at one locality, illustrated by Cooper (1988, Fig. 10), in rhythmically banded alvikite dolomite rhombs and apatite are concentrated toward the base of the layers which grade upwards into spinifex-textured calcite, a texture which is widespread in these rocks. These textural features are described and discussed in some detail by Cooper and Reid (1991). Dolomite phenocrysts may be zoned to more iron-rich rims; other common phases include octahedral magnetite, tetraferriphlogopite and biotite, sodic pyroxene, quartz, barite and rare fluorite. Calcitic dykes with abundant goethite cut the alvikite and are referred to by Cooper (1988) as ferro-alvikites, and the last phase of carbonatite emplacement is represented by yellow-brown dykes and veins of carbonatite and carbonatitic microbreccia and tuffisite. In the central part of the complex are a number of pipes of carbonatite breccia up to several hundred metres across with marginal 30-40 cm diameter blocks decreasing inwards to 2-5 cm. The blocks consist of various types of carbonatite and clasts of calcite, apatite, magnetite, pyrochlore, biotite and zircon. Xenoliths of ijolite and feldspathic ijolite up to 100 m long occur in the sovite the local abundance of which suggests the former presence of a substantial body. The ijolites are texturally and mineralogically highly variable and comprise nepheline, aegirine-augite, biotite, melanite, titanite, apatite, calcite, occasional wollastonite and clear, interstitial K-feldspar which is sometimes abundant and may be a late addition. Trachytes and trachyte breccias are common in the basement gneisses close to the complex, but are also found up to 2.5 km away. They form cone sheets, dykes and small plugs. The breccias contain clasts of basement rocks, trachyte, carbonatite, feldspar porphyry and microsyenite which lie in a matrix that is generally carbonate-rich, and these grade into carbonatite breccias. The trachytes consist of sanidine and rarer zircon and magnetite phenocrysts in a groundmass of alkali feldspar microlites and aegirine; fluorite is a widespread late stage accessory. In a few places peripheral fenitization extends to 100 m, but generally it is at a low level. It is expressed mineralogically in basement granites by replacement of quartz by aegirine, replacement of plagioclase by K-feldspar, corrosion of biotite, some development of sodic amphibole and in places barite and fluorite. Four whole-rock analyses of carbonatite, including trace element data, are given by Cooper and Reid (1991) and a very detailed C and O isotope study of all rock types is that of Reid and Cooper (1992).

K-Ar determinations on biotite from three sovite samples gave an age of 48(2 Ma and Rb-Sr determinations on the same biotites together with a fourth sample gave an isochron age of 49.8(0.8 Ma (Reid et al., 1990).
COOPER, A.F. 1988. Geology of Dicker Willem, a subvolcanic carbonatite complex in south-west Namibia. Communications of the Geological Survey of South West Africa/Namibia, 4: 3-12.COOPER, A.F. and REID, D.L. 1991. Textural evidence for calcite carbonatite magmas, Dicker Willem, southwest Namibia. Geology, 19: 1193-6.JACKSON, M.P.A. 1976. High-grade metamorphism and migmatization of the Namaqua metamorphic complex around Aus in the southern Namib Desert, South West Africa. Bulletin, University of Cape Town, Department of Geology, Chamber of Mines Precambrian Research Unit, 18: 1-299.REID, D.L. and COOPER, A.F. 1992. Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia. Chemical Geology (Isotope Geoscience Section), 94: 293-305.REID, D.L., COOPER, A.F., REX, D.C. and HARMER, R.E. 1990. Timing of post-Karoo alkaline volcanism in southern Namibia. Geological Magazine, 127: 427-33.
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