Hostname: page-component-78c5997874-m6dg7 Total loading time: 0 Render date: 2024-11-17T14:47:15.820Z Has data issue: false hasContentIssue false
  • English
  • Français

Compositional cyclicity in a pyroxenitic layer from the Main Zone of the Western Bushveld Complex: evidence for repeated magma influx

Published online by Cambridge University Press:  05 July 2018

Andrew A. Mitchell
Andrew A. Mitchell*
Affiliation:
Department of Geology, University of Durban-Westville, Private Bag X54001, Durban 4000, South Africa
Get access Rights & Permissions [Opens in a new window]

Abstract

Lacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, 1100 m above the Main Zone — Critical Zone contact, a 10 m thick pyroxenitic layer was intersected in an exploration borehole from the southern sector of the western Bushveld.

Within the pyroxenitic layer, mineral chemistry defines a series of five cycles of upward Mg and Cr enrichment in pyroxenes, and Ca enrichment in plagioclase. The mineral chemistry, supported by textural evidence, suggests the influx of successive surges of magma. Sustained streaming of magma gave rise to adcumulate textures in the central portion of each cycle, with orthocumulate textures at bases and tops of cycles representing waxing and waning stages of magma surges.

Keywords

Bushveld ComplexMain Zonepyroxenitemagma influxcompositional cyclicity
Type
The 1995 Hallimond Lecture
Information
Mineralogical Magazine , Volume 60 , Issue 398 , February 1996 , pp. 149 - 161
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Barnes, S.J. (1986) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contrib. Mineral. Petrol., 93, 524–31.CrossRefGoogle Scholar
Bédard, J.H., Sparks, R.S.J., Renner, R, Cheadle, M.J. and Hallworth, M.A. (1988) Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex. J. Geol. Soc. London, 145, 207–24.CrossRefGoogle Scholar
Campbell, I.H. (1987) Distribution of orthocumulate textures in the Jimberlana Intrusion. J. GeoL, 95, 35–54.CrossRefGoogle 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. J. Petrol., 32, 739–63.CrossRefGoogle Scholar
Dunlevey, J.N. and Mitchell, A.A. (1994) MODEPLUS: a program for computer-assisted modal measurements. Computers and Geosciences, 20, 369–93.CrossRefGoogle Scholar
Eales, H.V., Field, M., de Klerk, W.J. and Scoon, R.N. (1988) Regional trends of chemical variation and thermal erosion in the Upper Critical Zone, western Bushveld Complex. Mineral. Mag., 52, 63–79.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. Chem. Geol, 88, 261–78.CrossRefGoogle Scholar
Eales, H.V., Maier, W.D. and Teigler, B. (1991) Corroded plagioclase feldspar inclusions in orthopyroxene and olivine of the Lower and Critical Zones, western Bushveld Complex. Mineral. Mag., 55, 479–86.CrossRefGoogle Scholar
Bales, H.V., Botha, W.J., Hattingh, P.J., de Klerk, W.J., Maier, W.D. and Odgers, A.T.R. (1993) The mafic rocks of the Bushveld Complex: a review of emplacement and crystallization history, and mineralization, in the light of recent data. J. Afr. Earth Sci., 16, 121–42.Google Scholar
Eales, H.V., de Klerk, W.J., Teigler, B. and Maier, W.D. (1994) Nature and origin of orthopyroxenites in the western Bushveld Complex, in the light of compositional data. S. Afr, J. GeoL, 97, 399–407.Google Scholar
Hill, R.E.T., Barnes, S.J., Gole, M.J. and Dowling, S.E. (1990) Physical volcanology of komatiites. Geol. Soc. Australia (Divn, W.A..), Excursion Guide Book 1, 100 pp.Google Scholar
Hill, R.E.T., Barnes, S.J., Gole, M.J. and Dowling, S.E. (1995) The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia. Lithos, 34, 159–88.CrossRefGoogle Scholar
Huppert, H.E. and Sparks, R.S.J. (1985) Komatiites I: eruption and flow. J. Petrol, 26, 694 —725.CrossRefGoogle Scholar
Kruger, F.J. (1994) The Sr-isotopic stratigraphy of the western Bushveld Complex. S. Afr. J. GeoL, 91, 393–8.Google Scholar
Lee, C.A. and Butcher, A.R. (1990) Cyclicity in the Sr isotope stratigraphy through the Merensky and Bastard Reef units, Atok section, eastern Bushveld Complex. Econ. GeoL, 85, 877–83.CrossRefGoogle Scholar
Lesher, C.M. (1989) Komatiite-associated nickel sulfide deposits. In: Ore deposition associated with magmas: Reviews in Economic Geology, 4 (Whitney, J.A. and Naldrett, A.J., eds.), Soc. Econ. Geol., 45–101.Google Scholar
Maier, W.D. and Mitchell, A.A. (1995) Grain-size variations of cumulus plagioclase in the Main Zone of the Bushveld Complex. Eur. J. Mineral, 7, 195–204.CrossRefGoogle Scholar
McBirney, A.R. and Hunter, R.H. (1995) The cumulate paradigm reconsidered. J. Geol., 103, 114–22.CrossRefGoogle Scholar
Meyer, C. (1969) Some petrological aspects of mafic rocks from four borehole sections between the Merensky Reef and the Main Zone gabbro in the western and eastern Bushveld Complex.Unpubl. M.Sc. thesis, Potchefstroom University, 91 pp.Google Scholar
Mitchell, A.A. (1986) The petrology, mineralogy and geochemistry of the Main Zone of the Bushveld Complex at Rustenburg Platinum Mines, Union Section. Unpubl. Ph.D. Thesis, Rhodes Univ. 122 pp.Google Scholar
Mitchell, A.A. (1990a) The stratigraphy, petrography and mineralogy of the Main Zone of the northwestern Bushveld Complex. S. Afr. J. Geol, 93, 818-31.Google Scholar
Mitchell, A.A. (1990a) The Main Zone of the southwestern Bushveld in the vicinity of Brits. Geol Soc. S. Afr., Geocongress 90, Abstracts, 419—22.Google Scholar
Mitchell, A.A. and Scoon, R.N. (1991) Discussion on 'the stratigraphy of the Bushveld Complex': a reappraisal and the relocation of the Main Zone boundaries. S. Afr. J. Geol., 94, 183–7.Google Scholar
Morse, S.A. (1986) Convection in aid of adcumulus growth. J. Petrol, 27, 1183–214.CrossRefGoogle Scholar
Palacz, Z.A. and Tait, S.R. (1985) Isotopic and geochemical investigation of unit 10 from the Eastern Layered Series of the Rhum Intrusion, northwest Scotland. Geol Mag., 122, 485–90.CrossRefGoogle Scholar
Perring, C.S., Barnes, S.J. and Hill, R.E.T. (1995) The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia. Lithos, 34, 189–207.CrossRefGoogle Scholar
Quadling, K. and Cawthorn, R.G. (1994) The layered gabbronorite sequence, Main Zone, eastern Bushveld Complex. S. Afr. J. Geol., 97, 442–54.Google Scholar
Robinson, P., Ross, M., Nord, G.L. Jr., Smyth, J.R. and Jaffe, H.W. (1977) Exsolution lamellae in augite and pigeonite: fossil indicators of lattice parameters at high temperature and pressure. Amer. Mineral., 62, 857–73.Google Scholar
Scoon, R.N. and Mitchell, A.A. (1994) Discordant iron- rich ultramafic pegmatites in the Bushveld Complex and their relationship to iron-rich intercumulus and residual liquids. J. Petrol, 35, 881–917.CrossRefGoogle Scholar
Scoon, R.N. and Teigler, B. (1994) Platinum-group element mineralization in the Critical Zone of the western Bushveld Complex: I. Sulfide-poor chromi- tites below the UG-2. Econ. Geol., 89, 1094–121.CrossRefGoogle Scholar
Sharpe, M.R. (1985) Strontium isotope evidence for preserved density stratification in the main zone of the Bushveld Complex, South Africa. Nature, 316, 119–26.CrossRefGoogle Scholar
Tait, S.R. (1985) Fluid dynamic and geochemical evolution of cyclic unit 10, Rhum, Eastern Layered Series. Geol. Mag., 122, 469–84.CrossRefGoogle Scholar
Tail, S.R. and Kerr, R.C. (1987) Experimental modelling of interstitial melt convection in cumulus piles. In: Origins of igneous layering(Parsons, I., ed.) D. Reidel, Dordrecht, 569—87.Google Scholar
Tait, S.R., Huppert, H.E. and Sparks, R.S.J. (1984) The role of compositional convection in the formation of adcumulate rocks. Lithos, 17, 139–46.CrossRefGoogle Scholar
Wager, L.R. and Brown, G.M. (1968) Layered igneous rocks. Edinburgh, Oliver & Boyd: 588 pp.Google Scholar
Wilson, A.H. (1982) The geology of the Great 'Dyke', Zimbabwe: the ultramafic rocks. J. Petrol., 23, 240–92.CrossRefGoogle Scholar