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Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

Published online by Cambridge University Press:  02 January 2018

R. G. Cawthorn ,
K. L. Lundgaard ,
C. Tegner  and
J. R. Wilson
R. G. Cawthorn*
Affiliation:
School of Geosciences, University of the Witwatersrand, PO Wits, 2050 South Africa
K. L. Lundgaard
Affiliation:
Department of Geoscience, University of Aarhus, DK 8000 Aarhus C, Denmark
C. Tegner
Affiliation:
Department of Geoscience, University of Aarhus, DK 8000 Aarhus C, Denmark
J. R. Wilson
Affiliation:
Department of Geoscience, University of Aarhus, DK 8000 Aarhus C, Denmark
*
*E-mail: grant.cawthorn@wits.ac.za
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Abstract

Many layered intrusions are considered to have been repeatedly inflated by magma additions, but rates of magma mixing relative to rates of layer accumulation are difficult to model. The nature of magma recharge through the interval including the Pyroxenite Marker (PM), Main Zone, Bushveld Complex, South Africa, is examined with regard to such processes. The plagioclase compositions (An value) in five previously published and three new profiles (presented here and focusing on the core compositions) that are at least 600 m in vertical extent and spread along a strike length of 110 km are evaluated. The compilation of the eight profiles shows the following trends. Upward reversals in compositions show considerable lateral as well as vertical variations. Lateral variations show a range in: (1) the minimum An value reached in each profile prior to the onset of magma recharge (An51 to An59); (2) the depth below the PM at which the minimum value is observed (50 to 575 m); (3) the An value close to the PM (An54 to An75); (4) the maximum value recorded above the PM (An63 to An76); (5) the height above the PM at which this maximum value is reached (0 to 300 m) – in all cases, the highest values of An occur at the northern end of the studied sections; and (6) the vertical extents over which the reversals occur range from 150 to over 600 m indicating very protracted magma additions and/or slow mixing. The PM terminates toward the south, and close to this termination the immediate footwall rocks to the PM change from north to south from gabbronorite to magnetite gabbronorite. A cross-section through these profiles defines two basins, with an intervening structural upwarp. The magma pulses that were added to produce very gradual and protracted reversals in mineral compositions through the PM interval ponded initially at the base of the northern basin, and did not homogenize the entire magma column. These added magmas did not overflow and have an effect on mineral compositions in the southern basin until after considerable replenishment and crystallization (including the PM) had taken place in the northern basin. We emphasize the prolonged period(s) of magma input and slow rate of vertical homogenization of the magma column during the formation of this sequence of as much as 400 m of the Main Zone.

Keywords

Bushveld ComplexPyroxenite MarkerMagma AdditionMineral CompositionsLateral Variations
Type
Research Article
Information
Mineralogical Magazine , Volume 80 , Issue 2 , April 2016 , pp. 213 - 225
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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References

Ashwal, L.D., Webb, S.J. and Knoper, M.W. (2005) Magmatic stratigraphy in the Bushveld northern lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue Core. South African Journal of Geology, 108,199–232 CrossRefGoogle Scholar
Barnes, S.J. (1986) The effect of trapped liquid crystal¬lization on cumulus mineral compositions in layered intrusions. Contributions to Mineralogy and Petrology, 93, 524–531. CrossRefGoogle Scholar
Campbell, I.H. (1996) Fluid dynamic processes in basaltic magma chambers. Pp. 45-76 in: Layered Igneous Rock. (R.G. Cawthorn, editor). Elsevier Science, Amsterdam.Google Scholar
Cawthorn, R.G. (Editor) (1996) Layered Igneous Rocks. Elsevier Science, Amsterdam, 580 pp.Google Scholar
Cawthorn, R.G. (2013) The residual or roof zone of the Bushveld Complex, South Africa. Journal of Petrology, 54, 1875–1900. CrossRefGoogle Scholar
Cawthorn, R.G. and Ashwal, L.D. (2009) Origin of anorthosite and magnetitite layers in the Bushveld Complex; constrained by major element compositions of plagioclase. Journal of Petrology, 50,1607—1637 CrossRefGoogle Scholar
Cawthorn, R.G., Cooper, G.R.J.. and Webb, S.J. (1998) Connectivity between the western and eastern Bushveld Complex. South African Journal of Geology, 101,291–298 Google Scholar
Cawthorn, R.G., Meyer, P.S. and Kruger, F.J. (1991) Major addition of magma at the Pyroxenite Marker in the western Bushveld Complex, South Africa. Journal of Petrology, 32, 739–763. CrossRefGoogle Scholar
Clarke, B.M., Uken, R. and Watkeys, M.K. (2000) Intrusion mechanisms of the southwest Rustenburg Layered Suite as deduced from the Spruitfontein inlier. South African Journal of Geology, 103,120–127 CrossRefGoogle Scholar
Coombs, D.S. (1963) Trends and affinities of basaltic magmas and pyroxenes as illustrated on the diopside-olivine-silica diagram. Mineralogical Society of America, Special Paper, 1, 227–250. Google Scholar
Davey, S.R. (1992) Lateral variations within the upper Critical Zone of the Bushveld Complex on the farm Rooikoppies 297JQ Marikana, South Africa. South African Journal of Geology, 95, 141—149 Google Scholar
Groeneveld, D. (1970) The structural features and the petrography of the Bushveld Complex in the vicinity of Stoffberg, eastern Transvaal. Pp. 36-45 in: Symposium on the Bushveld Igneous Complex and Other Layered Intrusion. (D.L. Visser and G. von Gruenewaldt, editors). Geological Society of South Africa, Johannesburg, South Africa.Google Scholar
Klemm, D.D., Ketterer, S., Reichhardt, F., Steindl, J. and Weber-Diefenbach, K. (1985) Implication of vertical and lateral compositional variations across the Pyroxenite Marker and its associated rocks in the upper part of the Main Zone in the eastern Bushveld Complex. Economic Geology, 80,1007–1005CrossRefGoogle Scholar
Lombaard, B.V. (1934) On the differentiation and relationships of the rocks of the Bushveld Complex. Transactions of the Geological Society of South Africa, 37, 5–52. Google Scholar
Lundgaard, K.L. (2003) Magma chamber processes studied in two layered intrusions. Unpublished PhD thesis, University of Aarhus, Denmark.Google Scholar
Lundgaard, K.L., Tegner, C., Cawthorn, R.G., Kruger, F.J. and Wilson, J.R. (2006) Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa. Contributions to Mineralogy and Petrology, 151,352–269CrossRefGoogle Scholar
Mitchell, A.A. (1990) The stratigraphy, petrography and mineralogy of the Main Zone of the northwestern Bushveld Complex. South African Journal of Geology, 93, 818–831. Google Scholar
Molyneux, T.G. (1974) A geological investigation of the Bushveld Complex in Sekhukuneland and part of the Steelpoort valley. Transactions of the Geological Society South Africa, 77, 329–338.Google Scholar
Morse, S.A. (1984) Cation diffusion in plagioclase feldspar. Science, 225,504–505 CrossRefGoogle ScholarPubMed
Nex, P.A., Kinnard, J.A., Ingle, L.J., van der Vyver, B.A. and Cawthorn, R.G. (1998) A new stratigraphy for the Main Zone of the Bushveld Complex, in the Rustenburg area. South African Journal of Geology, 101, 215–223. Google Scholar
Quadling, K. and Cawthorn, R.G. (1994) The layered gabbronorite sequence, Main Zone, eastern Bushveld Complex. South African Journal of Geology, 97, 442–454. Google Scholar
Roelofse, F. and Ashwal, L.D. (2012) The Lower Main Zone in the northern limb of the Bushveld Complex -a >1.3 km thick sequence of intruded and variably contaminated crystal mushes. Journal of Petrology, 53, 1449–1476. CrossRefGoogle Scholar
Schweitzer, J. and Hatton, C. (1997) Plumes, Plates and Mineralization ‘97. Bushveld Excursion, University of Pretoria, South Africa, 39 pp.Google Scholar
Scoon, R.N. and Mitchell, A.A. (1994) Platinum-group element mineralization in the Critical Zone of the Bushveld Complex. Economic Geology, 90, 1094–1121. CrossRefGoogle Scholar
Sharpe, M.R. (1985) Strontium isotope evidence for preserved density stratification in the Main Zone of the Bushveld Complex. Nature, 316, 119–126. CrossRefGoogle Scholar
Tanner, D., Mavrogenes, J.A., Arculus, R.J. and Jenner, F.E. (2014) Trace element stratigraphy of the Bellevue core, northern Bushveld: multiple magma injections observed by diffusive processes. Journal of Petrology, 55, 859–882. CrossRefGoogle Scholar
VanTongeren, I A. and Mathez, E.A. (2013) Incoming magma composition and style of recharge below the Pyroxenite Marker, eastern Bushveld Complex, South Africa. Journal of Petrology, 54, 1585–1605. CrossRefGoogle Scholar
VonGruenewaldt, G. (1973) The main and upper zones of the Bushveld Complex in the Roossenekal area, eastern Transvaal. Transactions of the Geological Society South Africa, 76, 207–227. Google Scholar
Walraven, F. (1993) Significance of interlayered granitic rocks for emplacement models for the Rustenburg Layered Suite, Bushveld Complex. Symposium on Layering in Igneous Rocks. University of the Witwatersrand, Johannesburg, South Africa.Google Scholar
Wilson, J.R. and Sørensen, H.S. (1996) The Fongen-Hyllingen layered intrusive complex, Norway. Pp. 303—330 in: Layered Igneous Rock. (R.G. Cawthorn, editor). Elsevier Science, Amsterdam.Google Scholar
Wilson, J.R., Robins, B., Nielsen, F.M., Duchesne, J.C. and vander Auwera, J. (1996) The Bjerkreim-Sokndal layered intrusion, southwest Norway. Pp. 231-256 in: Layered Igneous Rock. (R.G. Cawthorn, editor). Elsevier Science, Amsterdam.Google Scholar
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