The petrology and geochemistry of the lower pyroxenite succession of the Great Dyke in the Mutorashanga area

Mason-Apps, Alexander Dymoke. (1998) The petrology and geochemistry of the lower pyroxenite succession of the Great Dyke in the Mutorashanga area. Masters thesis, Rhodes University.

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Abstract

This thesis focuses on the petrology and geochemistry of the lower Pyroxenite Succession of the Great Dyke of Zimbabwe in an area to the south ofMutorashanga. Particular emphasis is placed upon the economically important chromitite C5, and on the pervasive serpentinization of olivinerich rocks. An overview of the Great Dyke, including the Satellite Dykes, the structure and stratigraphy of the Great Dyke, the economic resources of the Great Dyke, and the evolution of the Great Dyke magma, is given. A review of the geodynamic history of the Zimbabwe Archaean craton, which culminated in widespread cratonisation and emplacement of the Great Dyke is also provided. The silicate rocks of the lower Pyroxenite Succession are highly adcumulate dunites and orthopyroxenites, with well-developed granular textures and a restricted mineral assemblage of olivine and pyroxene, with very minor plagioclase and clinopyroxene. Within cyclic units, the silicate rocks commonly display a textural and modal progression from granular dunite through poikilitic harzburgite, granular harzburgite, and olivine orthopyroxenite, to granular orthopyroxenite. Chromitites commonly occur at the base of each cyclic unit, these are thin, massive, coarse-grained layers, and are shown to be modified, texturally and compositionally, by postcumulus annealing processes. The olivine-rich rocks are pervasively serpentinized to a depth of over 300 metres. The serpentites typically display well-developed pseudomorphic mesh textures, with a slight overprint of nonpseudomorphic interpenetrating textures and late-stage cross-cutting veins. X-Ray diffraction studies indicate that chrysotile is the dominant serpentine mineral, and also reveal the presence of a nickeliferous magnesium hydroxide, occurring as an intimate admixture with serpentine, and believed to be a nickel-bearing analogue of brucite. Mineral and whole rock compositions of chromitite and silicate rocks highlight the strongly magnesian nature of the Ultramafic Sequence. Studies ofthe footwall chromites below chromitite C5 are consistant with a model of replenishment of primitive magma into the Great Dyke magma chamber, at the base of each cyclic unit. The magma injection and subsequent mixing with the evolved resident magma gives rise to chromitite fonnation, and a causes a reversal of the fractionation trend, resulting in a return to more primitive compositions in the silicate rocks. The silicates display an overall fractionation trend that reflects the evolving composition of the parental magma.

Item Type:Thesis (Masters)
Uncontrolled Keywords:Dikes, Zimbabwe, Geology, Petrology, Geochemistry
Subjects:Q Science > QE Geology
Divisions:Faculty > Faculty of Science > Geology
ID Code:2822
Deposited By: Mrs Carol Perold
Deposited On:21 May 2012 10:06
Last Modified:21 May 2012 10:06
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