|dc.description.abstract||The petrology, mineralogy and geochemistry of the Karroo-age
basaltic and rhyodacitic volcanics present in the Lebombo Belt
near Komatipoort, Eastern Transvaal, Republic of South Africa
The volcanics comprise a succession of extrusives within
the Komatipoort area, consisting of:
(3) Rhyodacitic lavas and tuffs
(1) Olivine Basalts.
These volcanics dip eastwards at angles between 10° and 40°,
with dips increasing as the Eastern boundary of the area, the
Mozambique border, is approached. Further Karroo-age volcanics,
constituting the upper part of this succession, lie across the
border in neighbouring Mozambique.
Various intrusives, similar in composition to the extrusives,
are also present in the area, commonly as north-south trending
dykes forming part of a large dyke-swarm. These, as else\Olhere
in the Lebombo, are considered to be feeders to the extrusives.
Three relatively major intrusions occur in the area, the largest
being the Komatipoort Intrusion, here interpreted as a 700m thick,
sub-concordant, composite sheet-like body, ~onsisting of five
major lithological units. From the base upwards these are:- .
unit 1 - olivine gabbro, unit 2 - igneously laminated gabbro,
unit 3- granophyric gabbro, unit 4 - granophyre, unit 5 - feldspathic
gabbro. The olivine gabbro, the granophyre and the feldspathic
gabbro appear to form separate intrusive bodies, but the
igneously laminated gabbro and the granophyric gabbro, could represent
the products of in situ differentiation. If so, the
granophyric gabbro has suffered subsequent disturbance as shown
by evidence including a homogeneous composition, reaction and
corrosion textures and the presence of deformed pyroxene grains,
which are described in detail.
Another major intrusive of probable Karroo age is the
Crocodile River Intrusion, which occurs to the north-east of the
main mapped, and represents the southern- most end of a northsouth
trending line of mafic intrusives, which parallel the
Lebombo for approximately 200 km. The intrusion here has a dykelike
form, and shows evidence of fractionation by crystal settling.
A smaller, obviously composite intrusion occurs near the
base of the basaltic sequence in the Komatipoort area, (the Basal
Representative samples of a variety of extrusive and intrusive
rock types have been analysed. These analyses include major
and trace element determinations of a series of samples of the
major units of the Komatipoort Intrusion. In 'addition, analyses of
a number of minerals from several different rock types occurring
in the Komatipoort Intrusion, are presented. The analytical data
available for the Komatipoort volcanics confirms the previously
known southward variation in the geochemistry of the basalts
and the presence of bath a high and a low-Ng basalt series in the
The majority of the basic rocks in the Komatipoort area
belong to the low-Mg series. Compositional variation in this
series may be explained largely by low-pressure fractionation of
olivine and pyroxene only, despite the presence of abundant plagioclase
phenocrysts. Some of the variation in the high-Mg series , basalts
can be explained by the fractionation of olivine, and what appear
to be relatively highly fractionated rocks formed in this manner
occur in the area.
The Lebombo volcanics ln general display a bimodal silica
distribution and rocks with an intermediate silica content are rare.
In the Komatipoort area intermediate rocks do occur in the, form of
two classes of granophyre, (high and low silica varieties), present
in the granophyre unit of the Komatipoort Intrusion. Liquid immiscibility
is a possible mechanism for the formation of the two types
of granophyre, and a widespread development of this process in
intermediate magmatic liquids could provide an explanation for the
,scarcity of rocks of this composition in the Lebombo belt.
Relatively few analyses of the rhyolitic volcanics are presented
in this study, but it appears possible that those available could
be representative of two processes, firstly, partial melting of the
lower crust or upper mantle, and secondly, fractionation by
crystallisation of the commonly observed phenocryst phases,(feldspar,
pyroxene, quartz, magnetite), or addition of these phases to the