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The geometry and emplacement of the Pilanesberg Complex, South Africa

Published online by Cambridge University Press:  12 January 2015

R. GRANT CAWTHORN
R. GRANT CAWTHORN*
Affiliation:
School of Geosciences, University of the Witwatersrand, PO Box Wits, 2050, South Africa
*
*Author for Correspondence: Grant.Cawthorn@wits.ac.za
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Abstract

The circular 625 km2 alkaline Pilanesberg Complex, South Africa, contains coeval eruptive and several distinctive intrusive syenitic and foyaitic components, concentrically arranged at the surface. However, owing to poor outcrop the relationships between the different intrusive rocks, and their shape in the third dimension cannot be convincingly determined in the field. The original interpretation was a laccolith, whereas later models suggested a funnel shape, and appealed to ring-dyke and cone-sheet emplacement mechanisms. However, the radial widths of these coarse-grained bodies are over 1 km and so cannot have been emplaced as ring dykes or cone sheets, which are usually quite thin and fine grained. Creating the space for emplacement and removal of pre-existing country rocks for each postulated subsequent intrusive event presents a major challenge to this latter hypothesis. Extensive previously published and new field relationships are re-evaluated here to suggest that the body is a gently inward-dipping sheet and that subsequent injections of magma merely pumped up an existing and evolving magma chamber rather than intruded into solid rocks. A Bouguer gravity anomaly model is presented that supports the concept of a shallow, flat-bottomed body rather than one that continues to significant depth. There are many analogies with the Kangerlussuaq Intrusion, Greenland.

Keywords

Pilanesberg Complexlaccolithmultiple intrusionsBouguer gravity anomaly
Type
Original Articles
Information
Geological Magazine , Volume 152 , Issue 5 , September 2015 , pp. 802 - 812
Copyright
Copyright © Cambridge University Press 2015 

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References

Brotzu, P., Gomes, C. B., Melluso, L., Morbelli, L., Morra, V. & Ruberti, E. 1997. Petrogenesis of coexisting SiO2-undersaturated to SiO2-saturated felsic igneous rocks: the alkaline complex of Itatiaia, southeastern Brazil. Lithos 40, 133–56.Google Scholar
Corry, C. E. 1988. Laccoliths: Mechanisms of Emplacement and Growth. Geological Society of America, Special Papers no. 220.Google Scholar
Council for Geoscience , 1997. Geological Map of the Republic of South Africa and the Kingdoms of Lesotho and Swaziland: Gravity Edition; 1:1,000,000 Scale. Pretoria: Council for Geoscience.Google Scholar
Cruden, A. R. 1998. On the emplacement of tabular granites. Journal of the Geological Society, London 155, 853–62.Google Scholar
Hanson, R. E., Harmer, R. E., Blenkinsop, T. G., Bullen, D. S., Dalziel, I. W. D., Gose, W. A., Hall, R. P., Kampunzu, A. B., Key, R. M., Mukwakwami, J., Munyanyiwa, H., Pancake, J. A., Seidel, E. K. & Ward, S. E. 2006. Mesoproterozoic intraplate magmatism in the Kalahari craton: a review. Journal of African Earth Sciences 46, 141–67.Google Scholar
Harmer, R. E. 1999. The petrogenetic association of carbonatites and alkaline magmatism: constraints from the Spitskop Complex, South Africa. Journal of Petrology 40, 525–48.Google Scholar
Henderson, C. M. B., Pendlebury, K. & Foland, K. A. 1989. Mineralogy and petrology of the Red Hills alkaline igneous complex, New Hampshire, U.S.A. Journal of Petrology 30, 627–66.Google Scholar
Holmes, A. 1965. Principles of Physical Geology. London: Thomas Nelson and Son, 1288 pp.Google Scholar
Humphrey, W. A. 1912. The volcanic rocks of the Pilandsberg. Transactions of the Geological Society of South Africa 15, 100.Google Scholar
Kearey, P. 1993. The Encyclopedia of the Solid Earth Sciences. Oxford: Blackwell Science Ltd, 713 pp.Google Scholar
Kogarko, L. N., Kononova, V. A., Orlova, M. P. & Woolley, A. R. 1995. Alkaline Rocks and Carbonatites of the World. Part 2: Former USSR. London: Chapman and Hall, 226 pp.Google Scholar
Launeau, P. & Cruden, A. R. 1998. Magmatic fabric acquisition mechanisms in a syenite: results of a combined anisotropy of magnetic susceptibility and image analysis study. Journal of Geophysical Research 103, 5067–89.Google Scholar
Lurie, J. 1979. Mining magnet. Nuclear Active 20, 26–32.Google Scholar
Lurie, J. 1986. Mineralization of the Pilanesberg Alkaline Complex. In Mineral Deposits of Southern Africa, Vol. 2 (eds Anhaeusser, C. R. & Maske, S.), pp. 2215–28. Johannesburg: Geological Society of South Africa.Google Scholar
Magee, C., Stevenson, C. T., O’Driscoll, B. & Schofield, N. 2012. An alternative model for the classic Ardnamurchan cone sheet swarm, NW Scotland, involving lateral magma supply via regional dykes. Journal of Structural Geology 43, 73–91.Google Scholar
Maré, L. P. 2011. Council for Geoscience, Petrophysical Laboratory, Density Results. http://www/geoscience.org.za/index.php/geophysics/35-more-on-geophysics/118-petrophysical-laboratory Google Scholar
Mathias, M. 1987. Alkaline rocks of Southern Africa. In Alkaline Igneous Rocks (eds Fitton, J. G. & Upton, B. G. J.), pp. 189–202. Geological Society of London, Special Publication no. 30.Google Scholar
Mitchell, R. H. & Liferovich, R. P. 2006. Subsolidus deuteric/hydrothermal alteration of eudialyte in the lujavrite from the Pilanesberg alkali complex, South Africa. Lithos 91, 352–72.CrossRefGoogle Scholar
Molengraaff, G. A. F. 1906. Preliminary note on the geology of the Pilandsberg. Transactions of the Geological Society of South Africa 9, 208.Google Scholar
O’Driscoll, B., Troll, V. R., Reavy, R. J. & Turner, P. 2006. The Great Eucrite intrusion of Ardnamurchan, Scotland: re-evaluating the ring-dyke concept. Geology 34, 189–92.Google Scholar
Pantshi, B. & Theart, H. F. J. 2008. The Red Syenite of the Pilanesberg Complex: a potential raw material source for the South African ceramics and glass industry. South African Journal of Geology 111, 27–38.Google Scholar
Riishuus, M. S., Peate, D. W., Tegner, C., Wilson, J. R. & Brooks, C. K. 2008. Petrogenesis of cogenetic silica-oversaturated and undersaturated syenites by periodic recharge in a crustally contaminated magma chamber, the Kangerlussuaq Intrusion, east Greenland. Journal of Petrology 49, 493–522.CrossRefGoogle Scholar
Schreiner, G. D. & Van Niekerk, C. B. 1958. The age of a Pilanesberg dyke from the central Witwatersrand. Transactions of the Geological Society of South Africa 62, 198–9.Google Scholar
Shand, S. J. 1928. The geology of the Pilanesberg (Pilaan's Berg) in the western Transvaal: a study of the alkaline rocks and ring intrusions. Transactions of the Geological Society of South Africa 31, 97–156.Google Scholar
Snelling, N. J. 1963. Age Determination Unit. Annual Report of the Overseas Geological Survey, 30 pp.Google Scholar
Stevenson, C. T., Owens, W. H., Hutton, D. H. W., Hood, D. N. & Meighan, I. G. 2007. Laccolithic, as opposed to cauldron subsidence, emplacement of the Eastern Mourne pluton, N. Ireland: evidence for anisotropy of magnetic susceptibility. Journal of the Geological Society, London 164, 99–110.Google Scholar
Thompson, R. N., Gibson, S. A., Mitchell, J. G., Dickin, A. P., Leonardos, O. H., Brod, J. A. & Greenwood, J. C. 1998. Migrating Cretaceous–Eocene magmatism in the Serra do Mar Alkaline Province, SE Brazil: melts from the deflected Trinadade mantle plume? Journal of Petrology 39, 1493–526.Google Scholar
Van Niekerk, C. B. 1962. The age of the Gemspost dyke from the Venterrsdorp gold mine. Transactions of the Geological Society of South Africa 65, 105–11.Google Scholar
Verwoerd, W. J. 1986. Mineral deposits associated with carbonatites and alkaline rocks. In Mineral Deposits of Southern Africa, Vol. 2 (eds Anhaeusser, C. R. & Maske, S.), pp. 2173–91. Johannesburg: Geological Society of South Africa.Google Scholar
Verwoerd, W. J. 2006. The Pilanesberg Alkaline Province. In The Geology of South Africa (eds Johnson, M. R., Anhaeusser, C. R. & Thomas, R. J.), pp. 380–93. Johannesburg: Geological Society of South Africa.Google Scholar
Wager, L. R. & Brown, G. M. 1969. Layered Igneous Rocks. Edinburgh: Oliver & Boyd, 572 pp.Google Scholar
Wiebe, R. A. & Collins, W. J. 1998. Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma. Journal of Structural Geology 20, 1273–89.Google Scholar
Woolley, A. R. 2001. Alkaline Rocks and Carbonatites of the World. Part 3: Africa, pp. 269–70. Bath: The Geological Society of London.Google Scholar