CLOSED-SYSTEM FRACTIONATION IN A LARGE MAGMA CHAMBER - MINERAL COMPOSITIONS OF THE WEBSTERITE LAYER AND LOWER MAFIC SUCCESSION OF THE GREATDYKE, ZIMBABWE

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.