(KBR) Karroo Igneous Complex -- Birds River, South Africa

Title

Description

The Birds River Complex is a "Bell-Jar" intrusion first described by du Toit in 1905 and then by Ealey and Robey in 1976. One of the best exposed sections across the margin of the complex is on the farm Denwood, in the southwestern corner of the complex. The samples were collected from a mafic intrusion near Denwood farm. These samples from the Karroo Complex were collected from the previously studied section of the complex and neighboring areas.

Contributor

Dr. Brian Lock, Rhodes University

Western Minerals, Inc.

References

Start Here:
Cox, K.G., 1972, The Karroo Volcanic Cycle: Journal of the Geological Society, v. 128, p. 311-336.Â doi: 10.1144/gsjgs. 128.4.0311.

The petrogenesis of the Mesozoic igneous rocks of south-Eastern Africa is related to the tectonic event believed to be the initiation of the break up of Gondwanaland. The cycle began with the rise of potassium-rich picritic magma, the source material for rocks of the northern province, Rhodesia. On top of and peripheral to the main magma body, a zone of sodic magma was generated and formed some of the rocks of the north and most southern rocks.

Additional resources:
White, R.S., 1992, Magmatism During and After Continental Break-Up, Geological Society, London, Special Publications, v. 68, p. 1-16. doi:Â 10.1144/GSL.SP.1992.068.01.0.

Magmatism that goes along with continental break-up is caused primarily by decompression melting of the underlying mantle as it rises up beneath the rift. The amount of melt produced is determined by the asthenospheric mantle temperature and on the rate of rifting.Â

Eales, H.V., and van Robey, J.A., 1976, Differentiation of Tholeiitic Karroo Magma at Birds River, South Africa, Contributions to Mineralogy and Petrology 10. v. 56, p. 101-117. doi:Â 10.1007/BF00375423

Strongly fractionated residue from the partial crystallization of tholeiitic gabbroic magma was emplaced at depth within sediment of the upper part of the Karroo succession. The residue was most likely tapped from a deeper intrusion during cauldron subsidence, but was engulfed by a later intrusion of olivine gabbro on a large scale.

Walker, F., and Poldervaart, A., 1949, Karroo Dolerites of the Union of South Africa, Geological Society of America Bulletin, v. 60, p. 591-706. doi: 10.1130/0016-7606(1949)60[591:KDOTUO]2.0.CO;2

The Karroo dolerites are the result of the intrusive phase of the early Jurassic basalts that built up the Basutoland lava plateau. Most of the dolerites show little differentiation but olivine rich types and acidic veins have been recorded. The magma had an effect on the sedimentary rocks; many of the rocks described as granophyre were found to be transformed siltstone. The dolerites occur as sills and dikes but "bell-jar inclusions" occur (the Bird's River complex is a bell jar shaped inclusion).Â

Eales, H.V., and Booth P.W.K., 1974, The Birds River Gabbro Complex, Dordrecht District, Transactions of the Geological Society of South Africa, v. 77, p. 1-15.

On the gabbros of the Birds River Complex.

Collection Items

Tholeiitic Gabbro

KBR-3 through KBR-6 are a second gabbro mass, distinguished from the first by a repetition of the differentiation cycle. The gabbro is medium grained and holocrystalline with a hypidiomorphic-granular texture. There are gray and black crystals, both…

Tholeiitic Gabbro

Samples KBR-1, KBR-2, KBR-2A are from a vertical mass of gabbro showing progressive differentiation inward.This sample is the second sample for KBR-2. This sample is medium grained and holocrystalline with a hypidiomorphic-granular texture. There are…