Hostname: page-component-76fb5796d-r6qrq Total loading time: 0 Render date: 2024-04-25T21:00:06.069Z Has data issue: false hasContentIssue false
  • English
  • Français

Closed system fractionation in a large magma chamber: mineral compositions of the websterite layer and lower mafic succession of the Great Dyke, Zimbabwe

Published online by Cambridge University Press:  05 July 2018

A. H. Wilson  and
J. B. Chaumba
A. H. Wilson
Affiliation:
Department of Geology and Applied Geology, University of Natal, P.Bag XI0, Dalbridge 4014, South Africa
J. B. Chaumba
Affiliation:
Department of Geology and Applied Geology, University of Natal, P.Bag XI0, Dalbridge 4014, South Africa
Get access Rights & Permissions [Opens in a new window]

Abstract

The Lower Mafic Succession of the Great Dyke is a 700 m thick sequence of gabbroic rocks which shows remarkably regular mineral compositional trends and trace element contents in whole rocks. Such chemical trends are strongly indicative of undisturbed fractionation having taken place within the magma chamber and contrast with the major development of cyclic units which characterize the underlying Ultramafic Sequence of the Great Dyke. The style of fractionation is quite different to that in the equivalent Main Zone of the Bushveld Complex with the latter possibly reflecting a ‘leaky’ input system, whereas in the Great Dyke the magma chamber was sealed. Major compositional reversals at the interface between the websterite layer (the topmost unit of the Ultramafic Sequence) and the base of the Lower Mafic Succession indicate a change in crystallization conditions at this level. Modal percentages of plagioclase and Al2O3 content of pyroxenes show the same trends indicating a strong control by temperature and magma composition.

Modelling of the fractionation processes and the influence of trapped liquid was carried out for Mg#, Cr2O3, and NiO in pyroxenes and for Zr in whole rock. The lowermost gabbroic rocks are adcumulates with effectively zero trapped liquid which contrasts with 10–15% trapped liquid in the underlying websterite There is a gradual rise in the amount of trapped liquid upwards in the Lower Mafic Succession. These results have implications for the mechanisms by which porosity is reduced in mafic cumulates. An injection of a small amount (10%) of new magma at the interface of the Ultramafic-Mafic Sequences of the Great Dyke was of a composition slightly different to that which gave rise to the cyclic units of the Ultramafic Sequence.

Keywords

Lower Mafic SuccessionGreat Dyke, Zimbabweclosed system fractionationwebsterite
Type
Petrology
Information
Mineralogical Magazine , Volume 61 , Issue 405 , April 1997 , pp. 153 - 173
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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.)

Footnotes

*

Present address: Department of Geological Sciences, University of Cape Town, P. Bag Rondebosch, South Africa

References

Ashwal, L.D. (1995) Trace element geochemistry of Bushveld plagioclase. Centenial Geocongress Extended Abstracts. Johannesburg: Geological Society of South Africa. 492—3.Google Scholar
Atkins, F.B. (1969) Pyroxenes of the Bushveld intrusion, South Africa. J. Petrol., 10, 222-49.CrossRefGoogle Scholar
Barnes, S.J. (1986a) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contrib. Mineral. Petrol., 93, 524-31.CrossRefGoogle Scholar
Barnes, S.J. (1986b) The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochim. Cosmochim. Acta, 50, 1889-909.CrossRefGoogle Scholar
Beattie, P. (1993) Olivine-melt and orthopyroxene-melt equilibria. Contrib. Mineral. Petrol., 115, 103—11.CrossRefGoogle Scholar
Beattie, P., Ford, C. and Russell, D. (1991) Partition coefficients for olivine-melt and orthopyroxene-melt systems. Contrib. Mineral. Petrol., 109, 212-24.CrossRefGoogle Scholar
Boudreau, A. E. (1987) Pattern formation during crystallization and the formation of fine-scale layering. In: Parsons, I., (ed.) Origin of Igneous Layering. NATO ASI Series. Dordrecht: D. Reidel, 287–312.Google Scholar
Boyd, F.R. and England, J.L. (1964) The system enstatite-pyrope. Carnegie Inst. Washington, Ann. Rept. Dir. Geophys. Lab., 1963-64. 157-61.Google Scholar
Brown, G.M. (1957) Pyroxenes from the early and middle stages of fractionation of the Skaergaard intrusion, East Greenland. Mineral. Mag., 31, 511-43.Google Scholar
Cameron, M. and Papike, J.J. (1981) Structural and chemical variation in pyroxenes. Amer. Mineral., 66, 1-50.Google Scholar
Campbell, I.H. (1987) Distribution of orthocumulate textures in the Jimberlana intrusion. J. Geol., 95, 35-54.CrossRefGoogle Scholar
Campbell, I.H. and Borley, G.D. (1974) The geochemistry of pyroxenes from the lower layered series of the Jimberlana intrusion, Western Australia. Contrib. Mineral. Petrol., 47, 281-97.CrossRefGoogle Scholar
Eales, H.V., Teigler, B. and Maier, W.D. (1993) Cryptic variations of minor elements AI, Cr, Ti and Mn in Lower and Critical Zone orthopyroxenes of the western Bushveld Complex. Mineral. Mag., 57, 257-64.CrossRefGoogle Scholar
Fujii, T. (1976) Solubility of Al2O3 in enstatite coexisting with forsterite and spinel. Carnegie Inst. Washington, Ann. Rept. Dir. Geophys. Lab., 1975-76, 566–71.Google Scholar
Green, T.H. (1994) Experimental studies of trace-element partitioning applicable to igneous petrogenesis - Sedena 16 years later. Chem. Geol., 117, 1-36.CrossRefGoogle Scholar
Hamilton, J. (1977) Sr isotope and trace element studies of the Great Dyke and Bushveld mafic phase and their relation to early Proterozoic magma genesis in southern Africa. J. Petrol., 18, 24-52.CrossRefGoogle Scholar
Hanghoj, K., Rosing, M.T. and Brooks, C.K. (1995) Evolution of the Skaergaard magma: evidence from crystallized melt inclusions. Contrib. Mineral. Petrol., 120, 265-9.CrossRefGoogle Scholar
Hart, S.R. and Dunn, T. (1993) Experimental cpx/melt partitioning of 24 trace elements. Contrib. Mineral. Petrol., 113, 1-8.CrossRefGoogle Scholar
Hatton, C.J. and von Gruenewaldt, G. (1985) Chromite from the Swartkop Chrome Mine - an estimate of the effects of subsolidus reequilibration. Econ. Geol., 80, 911-24.CrossRefGoogle Scholar
Henderson, P. (1970) The distribution of phosphorus in the early and middle stages of fractionation of some basic layered intrusions. Geochim. Cosmochim. Acta, 32, 897-911.CrossRefGoogle Scholar
Hunter, R.H. and McKenzie, D.P. (1989) Compaction and textural equilibration in layered intrusions. 28th Int. Geol. Cong., Abstracts, 2, 284.Google Scholar
Hunter, R.H. and Sparks, R.S.J. (1987) The differentiation of the Skaergaard Intrusion. Contrib. Mineral. Petrol., 95, 451-61.CrossRefGoogle Scholar
Hunter, R.H. and Sparks, R.S.J. (1990) The differentiation of the Skaergaard Intrusion: a discussion. Contrib. Mineral. Petrol., 104, 248-54.CrossRefGoogle Scholar
Irvine, T.N. (1980) Magmatic infiltration metasomatism, double diffusive fractional crystallization and adcumulus growth in the Muskox intrusion and other layered intrusions. In: Hargraves, R.B., (ed.) Physics of Magmatic Processes. Princeton: Princeton Univ., 325-83.CrossRefGoogle Scholar
Irvine, T.N., Keith, D.W. and Todd, S.G. (1983) The J-M platinum-palladium reef of the Stillwater Complex, Montana: II. Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex. Econ. Geol., 78, 1287-334.CrossRefGoogle Scholar
Kruger, F.J. and Marsh, J.S. (1985) Significance of Sr87/Sr86 ratios in the Merensky cyclic unit of the Bushveld Complex. Nature, 298, 53-55.CrossRefGoogle Scholar
Lindsley, D.H. and Dixon., S.A. (1976) Diopside-enstatite equilibria at 850° to 1400° 5 to 35 kb. Amer. J. Sci., 276, 1285-301.CrossRefGoogle Scholar
McBirney, A.R. and Naslund, H.L. (1990) The differentiation of the Skaergaard Intrusion: a discussion. Contrib. Mineral. Petrol., 104, 235—40.CrossRefGoogle Scholar
Meurer, W.P and Boudreau, A.E. (1996) Compaction of density-stratified cumulates: effect on trapped-liquid distribution. J. Geol., 104, 115-20.CrossRefGoogle Scholar
McKenzie, D.P. (1984) The generation and compaction of partially molten rock. J. Geol., 25, 713-65.Google Scholar
Mitchell, A.A. (1990) The stratigraphy and mineralogy of the Main Zone of the northwestern Bushveld Complex. S. Afr. J. Geol., 93, 818-31.Google Scholar
Mysen, B.O. and Boettcher, A.L. (1975). Melting of hydrous mantle: II. Geochemistry of crystals and liquids by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen and carbon dioxide. J. Petrol., 16, 549-93.CrossRefGoogle Scholar
Nathan, H.D. and Van Kirk, C.K. (1978) A modal of magmatic crystallization. J. PetroL, 19, 66-94.CrossRefGoogle Scholar
Podmore, F. and Wilson, A.H. (1987) A reappraisal of the structure, geology and emplacement of the Great Dyke, Zimbabwe. In: Halls, H.C. & Fahrig, W.F., (eds). Mafic Dyke Swarms. Geol. Assoc. Canada Spec. Paper 34, 317-30.Google Scholar
Prendergast, M.D. (1987) The chromite ore field of the Great Dyke, Zimbabwe. In: Stowe, C.W., (ed.) Evolution of Chromium Ore Fields. New York: Van Nostrand Reinhold, 89–108.Google Scholar
Prendergast, M.D. (1988) The geology and economic potential of the PGE rich Main Sulphide Zone of the Great Dyke, Zimbabwe. In: Pritchard, H.M., Potts, P.J., Bowles, J.F.W. and Cribb, S.J., (eds.) Geo-Platinum 87. London: Elsevier, 281–302.CrossRefGoogle Scholar
Prendergast, M.D. and Keays, R.R. (1989) Controls of platinum-group element mineralization of the PGE-rich Main Sulphide Zone in the Wedza Subchamber of the Great Dyke, Zimbabwe: implications for the genesis of, and exploration for, stratiform PGE mineralization in layered intrusions. In: Prendergast, M.D. and Jones, M., (eds.) 5th Magmatic Sulphides Field Conference, Harare, Zimbabwe. London: Institution of Mining and Metallurgy, 43–69.Google Scholar
Prendergast, M.D. and Wilson, A.H. (1989) The Great Dyke of Zimbabwe. II Mineralization and mineral deposits. In: Prendergast, M.D. and Jones, M., (eds.) 5th Magmatic Sulphides Field Conference, Harare, Zimbabwe. London: Institution of Mining and Metallurgy, 21–42.Google Scholar
Roeder, P.L. and Emslie, R.F. (1970) Olivine-liquid melt equilibrium. Contrib. Mineral. Petrol., 29, 275-89.CrossRefGoogle Scholar
Rollinson, H. (1993) Using geochemical data: Evaluation, Presentation, Interpretation. New York: Longman Scientific and Technical. 352 pp.Google Scholar
Saxena, S.K. (1968) Distribution of elements between coexisting minerals and the nature of solution in garnet. Amer. Mineral., 53, 994-1014.Google Scholar
Sharpe, M.R. (1985) Strontium isotopic evidence for preserved density stratification from the Main Zone of the Bushveld Complex, South Africa. Nature, 316, 119-26.CrossRefGoogle Scholar
Shirley, D.N. (1986) Compaction of igneous cumulates. J. Geol., 94, 795-809.CrossRefGoogle Scholar
Sparks, R.S.J., Huppert, H.E, Kerr, R.C., McKenzie, D.P. and Tait, S.R. (1985) Postcumulus processes in layered intrusions. Geol. Mag., 122, 555-68.CrossRefGoogle Scholar
Wager, L.R. (1960) The major element variation of the layered series of the Skaergaard intrusion and a re-estimation of the average compositions of the hidden layered series and of the successive residual magmas. J. Petrol., 1, 364-98.CrossRefGoogle Scholar
Wager, L.R. (1963) The mechanism of adcumulus growth in the layered series of the Skaergaard intrusion. Mineral. Soc. Amer. Spec. Paper, 1, 1—19.Google Scholar
Wilson, A.H. (1982) The geology of the Great Dyke, Zimbabwe: The ultramafic rocks. J. Petrol., 23, 240-92.CrossRefGoogle Scholar
Wilson, A.H. (1992) The geology of the Great Dyke, Zimbabwe: Crystallization, layering, and cumulate formation in the P1 pyroxenite of Cyclic Unit 1 of the Darwendale Subchamber. J. Petrol., 33, 611-663.CrossRefGoogle Scholar
Wilson, A.H. and Prendergast, M.D. (1989) The Great Dyke of Zimbabwe. 1: Tectonic setting, stratigraphy, petrology, structure, emplacement and crystallization. In: Prendergast, M.D. and Jones, M., (eds.) 5th Magmatic Sulphides Field Conference, Harare, Zimbabwe. London: Institution of Mining and Metallurgy, 1–20.Google Scholar
Wilson, A.H. and Tredoux, M. (1990) Lateral and vertical distribution of the platinum-group elements and petrogenetic controls on the sulfide mineraliza-tion in the P1 pyroxenite layer of the Darwendale Subchamber of the Great Dyke, Zimbabwe. Econ. Geol., 85, 556-84.CrossRefGoogle Scholar
Wilson, A.H. and Wilson, J.F. (1981) The Great ‘Dyke'. In: Hunter, D.R., (ed.) Precambrian of the Southern Hemisphere. Amsterdam: Elsevier, 572—8.Google Scholar
Wilson, A.H., Naldrett, A.J. and Tredoux, M. (1989) Distribution and controls of platinum-group element and base metal mineralization in the Darwendale Subchamber of the Great Dyke, Zimbabwe. Geology, 17, 649-52.2.3.CO;2>CrossRefGoogle Scholar