Dwyka cobbles reveal Archean basement beneath the Kalahari sands.

• We have found a new source of information about what lies beneath the Kalahari sands. The regions known as the Kheis and Rehoboth Provinces were thought to be underlain by either an ~1800 Ma orogenic belt, or a northern branch of the ~1200 Ma Namaqua-Natal Province, now largely covered by Cretaceous to Recent sand. Glacial diamictites of the Permian Dwyka Group exposed at Rietfontein west of the Kalahari carry cobbles plucked from the bedrock by the ice sheet which covered the Gondwana supercontinent about 300 Ma ago. Microbeam U-Pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200 Ma. Rather they testify to the presence of 2500 to 2900 Ma Archean crust beneath the Kalahari, with a lesser ~2050 Ma component, coeval with the Bushveld complex of the Kaapvaal Craton to the east. The mafic cobbles are much younger and are related to intrusions of the 1.1 Ga Umkondo Large Igneous Province along the Kalahari Line. Oxygen isotope analyses of zircon from the cobbles and western Kaapvaal Craton granites show a surprising difference, supporting the lithostratigraphic evidence that the granite cobbles do not originate from as far east as the Kaapvaal Craton. All the cobbles are most likely derived from either the Kalahari Line or the Rehoboth Province, whereas origins in the Kheis Province and Kaapvaal Craton are considered unlikely. The possible existence of Archean crust in the Rehoboth Province has important implications not only for the tectonic framework and assembly of Southern Africa, but also for exploration for diamonds and other ore deposits.

Ämnesord

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

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

Nyckelord

Kalahari

Archean

ion probe zircon dating

ion probe zircon oxygen isotope

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