| Summary: | The Sani Pass in the Natal Drakensberg is situated in the north-eastern sector of the Lesotho Highlands
which forms a major Karoo-age basaltic massif in the Karoo Igneous Province. The volcanic section
exposed in the pass is approximately 800m thick, and comprises a succession of regularly stratified,
massive and amygdaloidallavas which were extruded mainly by fissure-type eruptions. Dolerite dykes,
which now occupy thefissures,form a network ofpredominantly NE-SW and NW-SE trending topographic
features.
During post-eruption cooling hydrothermal solutions percolated through the volcanic succession and
produced an amygdale zonation which was controlled predominantly by ambientpressure and temperature
conditions. An original maximum thickness of 1 820m of the volcanic succession has therefore been
estimated and an average fossil geothermal gradient of 111° C/km is conceived to have persisted during
amygdale formation.
New electron microprobe data are presented for the silicate phases in the Sani Pass basalts and
dolerites. These data do not effectively separate the Sani Pass volcanic succession into different
geochemical units. Microprobe analysesfor olivine, albeit limited, are in the forsterite range and indicate
that a proportion of olivine in the high-MgO basalts is due to cumulus enrichment. The pyroxenes are
predominantly augite and minor pigeonite, with some ofthe augites displaying a tholeiitic trend similar
to that recognised at Skaergaard. Plagioclase is mainly in the labradorite to bytownite range, the
phenocrysts being slightly enriched in anorthite compared to the groundmass.
The use ofwhole-rock geochemistry for 67 basalts and 8 dolerites has permitted the recognition of
five geochemically distinct magma types, namely, the Giant's Cup, Agate Vale, Sakeng, Mkhomazana and
the Phinong. The Phinong basalts comprise the upper two-thirds ofthe volcanic succession and although
are generally homogeneous, there is a slight tendency for the more evolved rocks to be found higher up
in the stratigraphic sequence. The remaining magma types precede the Phinong succession and are
generally enriched in silica and have higher Zr/Nb and lower PfZr ratios than the Phinong basalts. Within
the pre-Phinong succession the Giant's Cup basalts are generally depleted in the compatible elements,
while the overlying Agate Vale basalts are enriched in incompatible elements. Except for a marginally
lower Na20 and Sr content, the chemistry ofthe Sakeng basalts is variable, generally overlapping with the
other magma types. The Mkhomazana basalts are slightly enriched in MgO, Ni, Cr and Sc compared to
all other pre-Phinong basalts. The dolerites in the area adjacent to the Sani Pass are geochemically
similar to the Phinong basalts.
The Phinong magma type is considered to be equivalent to the Lesotho magma type, based on their
geochemical and stratigraphical similarities. In terms ofdiscriminant diagrams the Giant's Cup, Sakeng
and Mkhomazana basalts generally show some compositional overlap with the Phinong, or plot in
incoherentfields, but the Agate Vale basalts are distinctly different and might indicate a new magma type
within the Karoo Central area. Broad compositional overlap between the Phinong basalts and those
preserved at Kirwan and Heimefrontfjella, Antarctica, indicates juxtaposition of Antarctica along the
southern African east coast in a reconstructed Gondwanaland.
Petrogenesis of the Sani Pass basalts has been examined in terms of alteration, open and closed
system fractional crystallization, partial melting procesess and a heterogeneous source. Although limited
alteration and conduit contamination have occurred, the most feasible mechanism responsible for the
geochemical variation lies in the existance ofinhomogeneities in the upper mantle at the time ofgeneration
of the Sani Pass magmas. === Thesis (M.Sc.)-University of Durban-Westville, 1992.
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