The nature of olivine-rich cumulate rocks of the lower critical and lower zones of the northwestern Bushveld Complex

Haikney, Susan Ann (1993) The nature of olivine-rich cumulate rocks of the lower critical and lower zones of the northwestern Bushveld Complex. Masters thesis, Rhodes University.

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Abstract

Boreholes NG1 and NG2 were drilled on the farm Nooitgedacht 406 KQ to intersect the lower Critical and lower Zones of the western Bushveld Complex. The aim of this study is to describe the textural features and chemical characteristics of the olivine-bearing rocks in the intersections, as determined by petrographic studies, XRF analysis and microprobe analysis. The olivine-bearing rocks are dunites, harzburgites and olivine pyroxenites. They comprise olivine and orthopyroxene, with minor chromite, clinopyroxene and plagioclase, and their textures vary between adcumulate, mesocumulate and poikilitic. The sequence intersected can be broadly correlated with that in the eastern Bushveld Complex. Of the whole-rock inter-element ratios, the MMF (MgO)/[MgO+FeO])ratio is the clearest indicator of cyclicity. The olivine-rich rocks are more primitive than the associated rocks, and seem to become more primitive with height in most intervals. The plagioclase in the olivine-bearing rocks is unusually sodic in corrposition, having a maximum Na₂0 content of 8.12%. A comparison of olivine and plagioclase compositions with those in other intrusions has revealed that the only other major intrusion with sodic plagioclase is the Kiglapait intrusion of Canada. In the Kiglapait intrusion the sodic plagioclase occurs in conjunction with fayalitic olivine as opposed to the forsteritic variety of this study. Chemical variations in the rocks sampled indicate that periodic replenishment of the magma from which the rocks crystallised must have occurred. In some of the olivine-bearing intervals where little fractionation is evident, replenishment seems to have been continuous. In other intervals fractionation appears to have continued uninterrupted for significant periods, prior to rejuvenation by fresh influxes of magma.

Item Type:Thesis (Masters)
Uncontrolled Keywords:Bushveld complex, South Africa, Textual features, Chemical characteristics, Olivine-bearing rocks, Dunites, Harzburgites, Olivine pyroxenites, Sodic plagioclase, Fractionation
Subjects:Q Science > QE Geology
Divisions:Faculty > Faculty of Science > Geology
Supervisors:Eales, Prof. and Scoon, Roger
ID Code:4087
Deposited By: Philip Clarke
Deposited On:19 Nov 2012 06:39
Last Modified:19 Nov 2012 06:39
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