CLOSED-SYSTEM FRACTIONATION IN A LARGE MAGMA CHAMBER - MINERAL COMPOSITIONS OF THE WEBSTERITE LAYER AND LOWER MAFIC SUCCESSION OF THE GREATDYKE, ZIMBABWE
Ah. Wilson et Jb. Chaumba, CLOSED-SYSTEM FRACTIONATION IN A LARGE MAGMA CHAMBER - MINERAL COMPOSITIONS OF THE WEBSTERITE LAYER AND LOWER MAFIC SUCCESSION OF THE GREATDYKE, ZIMBABWE, Mineralogical Magazine, 61(2), 1997, pp. 153-173
The Lower Mafic Succession of the Great Dyke is a 700 m thick sequence
of gabbroic rocks which shows remarkably regular mineral compositiona
l trends and trace element contents in whole rocks. Such chemical tren
ds are strongly indicative of undisturbed fractionation having taken p
lace 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 th
at 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 in
terface between the websterite layer (the topmost unit of the Ultramaf
ic Sequence) and the base of the Lower Mafic Succession indicate a cha
nge in crystallization conditions at this level. Modal percentages of
plagioclase and Al2O3 content of pyroxenes show the same trends indica
ting 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 ze
ro trapped liquid which contrasts with 10-15% trapped liquid in the un
derlying websterite There is a gradual rise in the amount of trapped l
iquid upwards in the Lower Mafic Succession. These results have implic
ations for the mechanisms by which porosity is reduced in mafic cumula
tes. An injection of a small amount (10%) of new magma at the interfac
e of the Ultramafic-Mafic Sequences of the Great Dyke was of a composi
tion slightly different to that which gave rise to the cyclic units of
the Ultramafic Sequence.