Closed System Fractionation in a Large Magma Chamber: Mineral Compositions of the Websterite Layer and Lower Mafic Succession of the Great Dyke, Zimbabwe

A. H. Wilson and J. B. Chaumba*
Department of Geology and Applied Geology, University of Natal, P.Bag X10, Dalbridge 4014, South Africa
*Present address: Department of Geological Sciences, University of Cape Town, P. Bag Rondebosch, South Africa

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 Succession • Great Dyke • Zimbabwe • closed system fractionation • websterite

Mineralogical Magazine; April 1997 v. 61; no. 405; p. 153-173; DOI: 10.1180/minmag.1997.061.405.01
© 1997, The Mineralogical Society
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