|dc.contributor.author||Lehloenya, Pelele Barnard||
|dc.description.abstract||The in situ compositional (major element, trace element and Sr-isotopic) and petrographic
results of plagioclase as obtained from the gabbroic cumulates across the boundary
between the Main and Upper Zones of the western Bushveld Complex, as studied from
the 1119.13 m, BK-2 drill core are reported. The data are compared with similar dataset
on this petrogenetically important stratigraphic sequence and a model that better explains
the petrogenesis of the Pyroxenite Marker interval is proposed.
There is a significant variation within and between coexisting plagioclase crystals across
the studied stratigraphic interval, which is not a new phenomenon in the Bushveld
Complex and other layered intrusions. In situ major element compositions recorded a
continuous upward trend of increasing plagioclase anorthite (i.e. a reversed differentiation
trend) content from ~ 342 m below the Pyroxenite Marker. The REE abundances of
plagioclase show LREE enrichment and slight depletion of HREEs relative to chondrites.
The initial 87Sr/86Sr ratios of plagioclase averaged 0.7086 in the lower Main Zone and
0.7078 in the Upper Zone, showing an isotopic ratio decrease up the stratigraphy of BK-
2. The Sr-isotopic composition of plagioclase in the Upper Zone was relatively constant
with stratigraphic position, and this is coupled with a normal differentiation trend as
exemplified by the anorthite (An%) content of plagioclase.
The disappearance of inverted pigeonite in the vicinity below the Pyroxenite Marker,
coupled with a reversal in mineral compositions and an inflection in initial 87Sr/86Sr ratios,
mark a zone of prolonged magma mixing, culminating in the Pyroxenite Marker.
The proposed model for petrogenesis of the Pyroxenite Marker is suggestive that the
interval may have formed due to; stabilization of pyroxene at the expense of plagioclase
because of rapid homogenization of two stratified magma layers that might have caused
a minor shift in phase equilibria, perhaps as a result of a transient fluctuation in pressure
within the chamber.
The isotopically heterogeneous integration stage of the BK-2 (i.e. the lower parts of the
Main Zone below the Pyroxenite Marker) was caused by several influxes of magma of
distinct composition, and the isotopically homogeneous differentiation stage (i.e. that part
of the Main Zone above the Pyroxenite Marker and the overlying Upper Zone) was
manifested by magma evolution that was dominated by fractional crystallization
processes without strong evidence to suggest further large-scale magma influxes.
Isotopic variations at the mineral scale are of great use in the monitoring of magma
evolution, processes and timescales, together with core-rim variations that are good
tracers of magma mixing.||en_ZA
|dc.description.sponsorship||National Research Foundation (NRF)||en_ZA
|dc.publisher||University of the Free State||en_ZA
|dc.subject||Dissertation (M.Sc. (Geology))--University of the Free State, 2017||en_ZA
|dc.title||Chemical and isotopic variations in plagioclase across the transition between the main upper zones, Western Bushveld complex||en_ZA
|dc.rights.holder||University of the Free State||en_ZA