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Geochemical effects of magma addition: compositional reversals and decoupling of trends in the Main Zone of the western Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

P. A. M. Nex ,
R. G. Cawthorn  and
J. A. Kinnaird
P. A. M. Nex*
Affiliation:
School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
R. G. Cawthorn
Affiliation:
School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
J. A. Kinnaird
Affiliation:
School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
*
*E-mail: 065nex@cosmos.wits.ac.za
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Abstract

The Main Zone of the Bushveld Complex, which is ˜3–3.5 km thick, comprises a sequence of gabbronorites with minor anorthosites and pyroxenites. The Pyroxenite Marker (PM), a thin orthopyroxenite layer occurring towards the top of the Main Zone in the eastern Bushveld, marks the change from an inverted pigeonite-bearing microgabbronorite below, to a primary orthopyroxene-bearing porphyritic gabbronorite above. In the western Bushveld the PM has only been observed in core material although its surface position can be inferred from the mineralogical and textural changes. Whole-rock geochemistry of surface and core samples from the Brits and Marikana areas, together with mineral compositional data, have been integrated with published analyses to elucidate the magmatic processes that occurred during the addition of new magma into the chamber, just below the level of the PM. Modal and major-element data show that most lithologies lie close to the plagioclase-two pyroxene cotectic. However, there are three units below the PM in which distinct modal layering is developed. Changes in geochemical trends for trace-element abundances in both whole-rock and mineral-separate data occur at ˜150 m below the PM, below a layered package, the Hexrivier Unit. However, there is a displacement of at least 40 m between the beginning of the reversals in An content in plagioclase and in Mg# in pyroxene which occur at 124 m and 80 m below the PM respectively. In addition, a gradual change occurs in the mineral parameter Mg# in opx minus An in plagioclase, with the plagioclase becoming more primitive relative to the pyroxene. This decrease and the decoupling of geochemical trends has not been noted in the Main Zone of the Bushveld Complex before. The decoupling between plagioclase and pyroxene compositional reversals is not easily explained by modal effects, an influx of phenocrysts in the new magma, or by infiltration metasomatism. Instead we propose that new magma pulses first entered the chamber some 150 m below the PM and that this magma had a composition that crystallized more primitive plagioclase but similar pyroxene to the magma residing in the chamber. The chamber was intruded by progressively more magma that had a more primitive composition particularly in terms of pyroxene composition. This model can explain the decoupling between plagioclase and pyroxene compositional trends. Continued mixing between resident magma and the new influxes occurred over an interval of ˜150–200 m. Above the PM fractional crystallization processes dominated and continued into the Upper Zone.

Keywords

Bushveld ComplexSouth AfricaMain Zonemagma additiongeochemical trendspyroxeneplagioclase
Type
Research Article
Information
Mineralogical Magazine , Volume 66 , Issue 6 , December 2002 , pp. 833 - 856
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

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References

Ashwal, L.D. (1993) Anorthosites. Springer-Verlag, Berlin.CrossRefGoogle Scholar
Atkins, F.B. (1969) Pyroxenes of the Bushveld Intrusion, South Africa. Journal of Petrology, 10, 222–249.CrossRefGoogle Scholar
Boudreau, A.E. and Meurer, W.P. (1999) Chromatographic separation of the platinum-group elements, gold, base metals and sulphur during degassing of a compacting and solidifying igneous crystal pile. Contributions to Mineralogy and Petrology, 134, 174–185.CrossRefGoogle Scholar
Cawthorn, R.G. (1999) Permeability of the footwall cumulates to the Merensky Reef, Bushveld Complex. South African Journal of Geology, 102, 293–310.Google Scholar
Cawthorn, R.G. and Davies, G. (1983) Experimental data at 3 Kbars pressure on parental magma to the Bushveld Complex. Contributions to Mineralogy and Petrology, 83, 128–135.CrossRefGoogle Scholar
Cawthorn, R.G., Meyer, P.S. and Kruger, F.F. (1991) Major addition of magma at the pyroxenite marker in the western Bushveld Complex, South Africa. Journal of Petrology, 32, 739–763.CrossRefGoogle Scholar
Eales, H.V. and Cawthorn., R.G. (1996) The Bushveld Comple. Pp. 181–229 in: Layered Intrusions (Cawthorn, R.G., editor). Elsevier, Amsterdam.CrossRefGoogle Scholar
Eales, H.V., de Klerk, W.J. and Teigler, B. (1990) Evidence for magma mixing processes within the Critical and Lower Zones of the northwestern Bushveld Complex, South Africa. Chemical Geology, 88, 261–278.CrossRefGoogle Scholar
Grove, T.L. and Juster, T.C. (1989) Experimental investigations of low-Ca pyroxene stability and olivine-pyroxene-liquid equilibria at 1-atm in natural basaltic and andesitic liquids. Contributions to Mineralogy and Petrology, 103, 287–305.CrossRefGoogle Scholar
Irvine, T.N. (1978) Infiltration metasomatism, adcumulus growth and secondary differentiation in the Muskox intrusion. Annual report of the director, Geophysical Laboratory, Carnegie Institution of Washington Yearbook, 77, 1755, 743–751.Google Scholar
Irvine, T.N. (1980) Magmatic Infiltration metasomatism, double-diffusive fractional crystallization, and adcumulus growth in the Muskox intrusion and other layered intrusions. Pp. 325–383 in: Physics of Magmatic Processes (Hargreaves, R.B., editor). Princeton University Press, Princeton, New Jersey.CrossRefGoogle Scholar
Klemm, D.D., Ketterer, S., Reichhardt, F., Steindl, J. and Weber-Diefenbach, K. (1985) Implication of vertical and lateral compositional variations across the pyroxene marker and its associated rocks in the upper part of the Main Zone in the eastern Bushveld Complex. Economic Geology, 80, 1007–1015.CrossRefGoogle Scholar
Kretz, R. (1963) Distribution of magnesium and iron between orthopyroxene and calcic pyroxene in natural mineral assemblages. Journal of Geology, 71, 773–785.CrossRefGoogle Scholar
Kruger, F.J. (1990) The stratigraphy of the Bushveld Complex: a reappraisal and the relocation of the Main Zone boundaries. South African Journal of Geology, 93, 376–381.Google Scholar
Kruger, F.J. (1994) The Sr-isotopic stratigraphy of the western Bushveld Complex. South African Journal of Geology, 97, 393–398.Google Scholar
Kruger, F.J. and Marsh, J.S. (1982) Significance of 87Sr/86Sr ratios in the Merensky cyclic unit of the Bushveld Complex. Nature, 298, 53–55.CrossRefGoogle Scholar
Kruger, F.J., Cawthorn, R.G. and Walsh, K.L. (1987) Strontium isotope evidence against magma addition in the Upper Zone of the Bushveld Complex. Earth and Planetary Science Letters, 84, 51–58.CrossRefGoogle Scholar
Markgraaff, J. (1976) Pyroxenes of the western Bushveld Complex, South Africa. Transactions of the Geological Society of South Africa, 79, 217–224.Google Scholar
McBirney, A.R. (1989) The Skaergaard Layered Layered Series I. Structure and average compositions. Journal of Petrology, 30, 363–397.CrossRefGoogle Scholar
Mitchell, A.A. (1986) The petrology, mineralogy and geochemistry of the Main Zone of the Bushveld Complex at Rustenburg Platinum Mines, Union Section. PhD thesis, Rhodes University, Grahamstown, South Africa.Google 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
Mitchell, A.A. (1996) Compositional cyclicity in a pyroxenitic layer from the Main Zone of the western Bushveld Complex: evidence for repeated magma influx. Mineralogical Magazine, 60, 149–161.CrossRefGoogle Scholar
Morse, S.A. and Nolan, K. (1984) Origin of strongly reversed rims on plagioclase in cumulates. Earth and Planetary Science Letters, 68, 485–498.CrossRefGoogle Scholar
Naslund, H.R. (1989) Petrology of the Basistoppen Sill, East Greenland: A calculated magma differentiation trend. Journal of Petrology, 30, 299–319.CrossRefGoogle Scholar
Nex, P.A., Kinnaird, 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
Parsons, I. (1987) Origins of Igneous Layering. Reidel Publishing Company, Dordrecht, The Netherlands.CrossRefGoogle Scholar
Quadling, K.E. (1996) The Layered Marker Sequence, Main Zone, Eastern Bushveld Complex – A Model for modal layering. MSc thesis, University of Witwatersrand, South Africa.Google Scholar
Quadling, K.E. and Cawthorn, R.G. (1994) The layered Gabbronorite Sequence, Main Zone, eastern Bushveld Complex. South African Journal of Geology, 97, 442–454.Google Scholar
Raedeke, L.D. (1982) Petrogenesis of the Stillwater Complex. PhD thesis, University of Washington, Seattle, USA.Google Scholar
SACS (South African Committee for Stratigraphy) (1980) The Stratigraphy of South Africa, Part 1 Lithostratigraphy of the republic of South Africa, South West Africa/Namibia and the Republics of Bophuthatswana, Transkei and Venda. Pp. 223–231 in: Bushveld Complex (Kent, L.E., compiler). Handbook of the Geological Survey of South Africa, 8, Pretoria, South Africa.Google Scholar
Snyder, D., Carmichael, I.S.E. and Wiebe, R.A. (1993) Experimental study of liquid evolution in an Fe-rich, layered mafic intrusion: constraints of Fe-Ti oxide precipitation on the T-f O2 and T-P paths of tholeiitic magmas. Contributions to Mineralogy and Petrology, 113, 73–86.CrossRefGoogle Scholar
von Gruenewaldt, H.G. (1970) On the phase-change orthopyroxene-pigeonite and the resulting textures in the Main and Upper Zones of the Bushveld complex in the eastern Transvaal. Special Publication of the Geological Society of South Africa, 1, 67–73.Google Scholar
von Gruenewaldt, H.G. and Weber-Diefenbach, K. (1977) Coexisting Ca-poor pyroxenes in the main zone of the Bushveld Complex. Contributions to Mineralogy and Petrology, 65, 11–18.CrossRefGoogle Scholar
Wager, L.R. and Brown, G.M. (1968) Layered Igneous Rocks. Oliver and Boyd, Edinburgh and London.Google Scholar