Zircon dating reveals an Archaean crust beneath the Kalahari sand.

• Continents are subdivided into structural provinces based on the age of the last orogeny or major magmatic event which imprinted the structural fabric on the bedrock. These first-order subdivisions are important in understanding how the continents were assembled. The continuity and boundaries of provinces such as the Namaqua-Natal Province in Southern Africa have been established, however adjoining Kheis province which lies almost entirely under the Tertiary to Recent Kalahari sands, has thus far defied attempts to understand its age and geological history. A foreland thrust complex developed in 1.92 Ga redbeds at Olifantshoek (Fig. 1a) was postulated by Cornell et al. 1998. However the much younger 1.2 Ga to 1.1 Ga Sinclair Sequence volcanic and plutonic rocks exposed in the same area suggest that the Kheis Province could have experienced an Andean-type orogeny at ~1.2 Ga. Thus Moen (1999) suggested that much of the Kheis Province was coeval with and belonged to the ~1.2 Ga Namaqua-Natal Province. At Rietfontein on the eastern margin of the Kheis, tillites of the Permian Dwyka Group, were found to contain abundant cobble-sized clasts of crystalline bedrock. Bearing in mind the generally westward transport directions for the Dwyka Ice sheet (Fig. 1a), these exposures were investigated to see if they contained information about what lies under the Kalahari. These granotoid cobbles are badly weathered, but we succeeded in dating them using an ion probe and laser ablation ICPMS, by the U-Pb method. The data is largely discordant, reflecting ancient lead loss at the time of glaciation. Nevertheless we find compelling evidence that Archean and 2.0 Ga (Bushveld Complex age) granitic crust exists beneath the Kalahari. We also failed to find any evidence for the existence of younger granitic crust or high-grade metamorphic terranes. The published concepts of the Kheis Province as either a 1900 - 1750 Ma collisional orogen, or an extension of the 1200 Ma Namaqua Province find no support in our data. The possibility of Archean and Bushveld-aged rocks occuring as basement beneath the Kalahari has important implications for diamond, base and precious metal prospecting in the western Kalahari in South Africa, Botswana and Namibia. These possibilities should be tested by drilling with geophysical control, the last such campaign having been completed in Botswana in 1984.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

kalahari

Archean

Dwyka tillite

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