The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

Textural and mineral chemistry data for the ultramafic sequence of the Hartley Complex are presented with the object of evaluating emplacement mechanisms, crystallization history and sub-solidus re-equilibration processes for the Great ‘Dyke’. Mineral chemistry indicates in situ crystallizaration for the ultramafic sequence, whereas textural evidence suggests that limited crystal settling of chromite took place. It is concluded that crystallization of cumulus phases occurred at or near the floor of the magma chamber. The mineral chemistry indicates that the volume of magma from which each unit crystallized was significantly smaller than that represented by the stratigraphic succession of the Hartley Complex. The magma chamber may effectively have been part of an open system during the crystallization of the ultramafic sequence. The results are consistent with the concept of a stratified magma chamber and the process of double-diffusion convection. Modelling of the liquid line of descent and crystallization sequences indicate that none of the previously proposed initial liquid compositions are likely to have constituted the parental magma of the Great ‘Dyke’. Rather than komatütic or exceptionally high magnesium liquids, as previously suggested, a parental magma with about 15 per cent MgO, similar to the composition of the chill phase of a dyke parallel to and in close proximity to the East Dyke is in closest agreement with the observed and modelled results. Chromite compositions are strongly related to textural and mineralogical environments. Seam chromitites are higher in Cr, Mg and Fe3+ than chromites enclosed in silicates. Chromite enclosed in cumulus olivine is higher in Fe2+ than that in coexisting pyroxenes but there is little difference in the proportions of the trivalent captions. Seam chromitites are considered to have precipitated in response to increases in foi associated with periodic influxes of magma into the magma chamber. The higher ferric iron content of the seam chromitites compared with the chromite enclosed in the silicates is consistent with such a mechanism. Compositional zoning in olivine and pyroxene adjacent to enclosed chromite grains is interpreted as reflecting subsolidus re-equilibration with cooling. Zoning profiles exhibit strong crystallographic control. Computer modelling using finite difference approximations has allowed controlling factors to be assessed by optimization of the modelled parameters to give closest agreement to the measured results. Interdiffusion coefficients and distribution coefficients for Fe2+ and Mg for olivine and pyroxene with chromite are modelled and compared with published data. Indicated blocking temperatures for olivine are of the order of 600 °C to 700 °C and 750 °C to 850 °C for orthopyroxene. Thus chromites enclosed in orthopyroxene are more Mg-rich than those enclosed in olivine. Coarse-grained seam chromitites have been little modified subsequent to crystallization but the compositions of the associated silicates have been influenced by the modal abundance of the chromite. Geothermometers based on chromite-silicate equilibria are probably not applicable to layered intrusions, but information on thermal histories may be provided by evaluation of the diffusion profiles.