509
S.Afr.J . Geol. ,1988,91 (4) ,509-521
Two occurrences of ash-flow tuff from the lower Beaufort Group in the HeilbronFrankfort area, northern Orange Free State
N. Keyser and P.K. Zawada
Regional Geology Division, Geological Survey of South Africa, Private Bag X112, Pretoria 0001, Republic of South Africa
Accepted 28 September 1988
Two occurrences of ash-flow tuff in the northern part of the Karoo Basin near Frankfort and Heilbron
respectively, are reported for the first time. The two outcrops are approximately 70 km apart and are situated in
the lowermost strata of the Beaufort Group, 27-47 m above the Ecca-Beaufort contact. A tuffaceous sediment
overlying the ash- flow tuff contains a subordinate amount of glass shards. Petrographic examination of the tuffs
have shown they comprise microlitic acicular plagioclase laths, euhedral biotite flakes, pseudomorphous
chlorite, interstitial quartz and albite, and devitrified glass. Amygdales and lapilli occur throughout the tuffs.
An amygdaloidal lava bomb has been classified as K-rich andesite. It is suggested that the source of the volcanic
material was situated northward of the outcrops.
Twee voorkomste van asvloeituf in die noordelike deel van die Karookom, naby Frankfort en Heilbron
respektiewelik, word vir die eerste keer gerapporteer. Die twee dagsome is ongeveer 70 km uitmekaar en kom
voor in die onderste lae van die Groep Beaufort, 27-47 m bokant die Ecca-Beaufort-kontak. 'n Tufagtige
sediment wat die asvloeituf oorle bevat 'n ondergeskikte hoeveelheid glasskerwe. 'n Petrografiese ondersoek
van die asvloeituf het gewys dat dit uit mikrolitiese naalderige plagioklaaslatte, eievormige biotietplaatjies,
tussenruimtelike kwarts en albiet, en gedevitrifiseerde glas bestaan. Amandels en lapilli kom dwarsdeur die
tuwwe voor. 'n Amandelhoudende lawabom is geklassifiseer as K-ryke andesiet. Dit word voorgestel dat die
bron van die vulkaniese materiaal noordwaarts van die dagsome ge\ee was.
Introduction
During mapping of the Frankfort 1:250 000 geological
map by the first author, outcrops of ash-flow tuff
interbedded with sediments of the lower Beaufort Group
strata were noted. This, to the author's knowledge, is the
first reported occurrence of ash-flow tuff from the
Beaufort Group and testifies to the presence of a local
volcanic source in the northern part of the Karoo Basin.
Evidence of volcanic activity in the Karoo Basin was
first reported by Bourdon et al. (1966) from the Prince
Albert Formation in the southwestern Cape Province.
Later published accounts by Martini (1974), Elliot &
Watts (1974), Lock & Johnson (1974), Lock & Wilson
(1975), Ho Tun (1979), and Viljoen (1987) have
confirmed the widespread occurrence of tuffaceous beds
ranging in stratigraphic position from the uppermost part
of the Dwyka Formation to near the base of the Beaufort
Group. Pebbles of volcanic origin have also been
recorded from the Katberg Formation of the Beaufort
Group in the East London area (Mountain, 1939).
Tuffaceous beds have to date been recorded only from
the southern part of the Karoo Basin. However,
McLachlan & Jonker (in prep.) described a 'deformed
bed' in the northwestern part of the Karoo Basin which
shows features resembling vesicles and chilled margins,
and is regarded as a possible tuff layer.
General characteristics
Two outcrops of ash-flow tuff have been found. The first
occurs north of Heilbron on the farm Oranje 342 and the
second is on Blydschap 1390, which lies about 20 km east
of Frankfort. The two localities are approximately 70 km
apart and are shown on a generalized geological map of
the area (Figure 1).
Oranje ash-flow tuff
This outcrop lies close to the trigonometric beacon
situated about 1 km southeast of the farmhouse. It
barely protrudes through the soil cover and is exposed
for a distance of 50 m, with a width of 0,5 m. The tuff is
mostly weathered and green in colour, but in fresh
samples the colour ranges from light grey to dark grey.
Three holes have been drilled in the vicinity of the
tuff. Borehole BOil, which was collared topographically
higher than boreholes BO/2 and BO/3, did not intersect
any tuff. This is due to the intrusion of dolerite below the
ash-flow tuff, displacing it upwards. The other two
boreholes were collared on the outcrop and intersected
an average thickness of 1 m of tuff (Figure 2).
The ash-flow tuff consists of a single bed and is
underlain by a medium-grained sandstone which shows
some hydrothermal alteration close to the contact. The
lower contact is uneven and diffuse because of mixing
with the underlying sandy material. Grey streaks of
sandy material, and xenolithic material of lapilli size,
occur throughout the layer.
Blydschap ash-flow tuff
The tuff occurs about 100 m north of the farmhouse in a
small terrace on the western bank of the tributary that
flows into Langspruit. Four boreholes (BBIl-/4) have
been drilled in the vicinity of the tuff, three of which
have intersected it (Figure 2). The ash-flow tuff occurs as
a thin (85 cm) layer and is exposed for 30 m along strike.
Like the ash-flow tuff at Oranje this occurrence is also
made up of one bed only and is extensively weathered
and altered. Dolerite intruded at the lower contact of the
ash-flow tuff.
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S.Afr.l . Geol. ,1988,91 (4)
ORANJE
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Dolerite
Tuffaceous sediments
Ash-flow tuff
Medium-grained sandstone
Coarse-grained sandstone
Figure 2 Generalized stratigraphic profiles
of shallow
boreholes drilled at Oranje and Blydschap.
The ash-flow tuff is overlain, with a sharp contact, by
1,7 m of tuff and tuffaceous sediments which consist of
interlaminated to thinly bedded silty and sandy material.
Stratigraphic position of the ash-flow tuffs
Both outcrops of ash-flow tuff occur in the lowermost
strata of the Beaufort Group, approximately 29-47 m
above the Ecca-Beaufort contact. This contact is taken
at the base of the first prominent sandstone overlying the
Volksrust Shale Formation of the Ecca Group (Figure
3).
Figure 3 illustrates that the ash-flow tuff on Blydschap
occurs 47,22 m above the Ecca-Beaufort contact
(dolerite has been excluded in this calculation). It can
furthermore be seen that the major part of the EccaBeaufort transition zone comprises two upwardcoarsening units. These units comprise shale,
interbedded shale and siltstone, and a capping of
sandstone. This sequence closely resembles small-scale
upward-coarsening,
regressive
deltaic
sediments
described by Elliot (1974), Van Dijk et al (1978),
Coleman (1982), and Farquharson (1982). The 103,17 m
thick interval of dark shale represents either a basin
shale, or a distal region of the prodelta environment
(Elliot, 1978) where deposition occurred through fall-out
of suspended clay-grade material in quiet, possibly
anoxic water conditions. The upward change from shale
to interbedded shale and siltstone represents the
proximal or distal delta front (Coleman & Gagliano,
1964). The 6,80 m thick sandstone overlying the
interbedded sequence records deposition in a delta-front
regime. The presence. of siltstone partings within this
sandstone testify to periods of bedform emergence
resulting either from 'healing' or stranding of the
channel during periods of low-river stage (Elliot, 1978).
Although the overlying upward-coarsening unit is
thinner it shows essentially similar features to the
underlying unit. High depositional rates are, however,
indicated by the presence of slump features (Allen,
1985). The ash-flow tuff itself occupies a position within
a sequence of sandstone and siltstone beds arranged in
upward-fining motifs. Although no other details are
available for this part of the stratigraphy it would seem
probable that, as these upward-fining units overlie a
regressive deltaic sequence, the ash-flow tuff was
deposited on a fluvial environment possibly on the upper
delta plain.
Figure 4 is a measured profile of the core drilled on
Oranje by the Geological Survey. Drilling commenced
at a position along strike from the outcrop of ash-flow
tuff, but none was intersected. The probable position of
the tuff is therefore just above the top of the profile.
Although it was not possible to core to the base of the
Ecca-Beaufort contact, field mapping in the area
suggests that the ash-flow tuff occupies a position
approximately 29 m above the Ecca-Beaufort contact.
From Figure 4 it is clear that the probable position of the
ash-flow tuff occurs above a sequence comprising
upward-fining units. The middle sandstone (3,89 m
thick) is similar to Allen's (1985) example of an upwardfining motif characterized by a change from large-scale
cross-bedding to cross-lamination and succeeded by
siltstone and mudstone. According to Allen (1965) such
sequences are produced by low- or high-sinuosity fluvial
channels.
Ash-flow tuff
The ash-flow tuff exhibits combined features of
pyroclastic rocks and lava flows and can best be
described as a welded ash-flow tuff (tuffolava). The
presence of numerous large amygdales immediately
gives the impression of a lava flow. In fresh samples,
though, the pyroclastic character comes to the fore when
the individual lapilli clasts, which are darker in colour,
are observed together with some banding in places.
Lapilli
The lapilli range from 2 mm to about 30 mm in diameter
and comprise juvenile lava and pumice clasts as well as
accidental (xenolithic) clasts of sedimentary origin. The
clasts are not graded (sorted) and also not imbricated.
The contacts of some of the clasts are diffused probably
due to intense welding.
A petrographic study of an amygdaloidal lava clast
revealed a fine-grained hypocrystalline texture. The lava
is made up of microlitic feldspar and biotite flakes set in
a groundmass of devitrified glass and secondary chlorite
and calcite. A few xenolithic quartz grains occur in
places. Because the clast is juvenile (originates directly
S.-Afr.Tydskr.GeoI.1988,91 (4)
512
Coarse sand
Med ;um .and
I
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1,26
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_ _ _ __
Ash-flow tu ff
±
1 m thick
Dolerite
Dolerite
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Medium-grained sandstone
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35,95
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103,17
Homogenous black shale
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LEGEND
Slumping
Cross-bedding
~
~ Cross-lamination
~ Clay-pellet conglomerate
I
6
Upward-fining sequence
V
Coarsening-upward sequence
Figure 3 Stratigraphic profile of a borehole drilled at Blydschap showing the position of the ash-flow tuff.
from volcano), the contamination by quartz grains must
have taken place in the conduit, prior to ejection. Small
amygdales in the clast are filled up with quartz, calcite,
and analcite. A similar amygdaloidal lava clast (bomb)
has been analysed for major and trace elements.
Ash
The ash-sized « 2 mm) volcanic and xenolithic material
of the ash-flow tuff is set in a thoroughly welded
aphanitic, hypocrystalline groundmass which consists of
microlitic, acicular plagioclase, euhedral biotite flakes,
513
S.Afr.l.Geol., 1988,91 (4)
Coarse sand
m
Probable position of ash-flow tuff
Core loss
0,10 ~h;::;:::j
Sandy overburden
4,89
Dolerite
SharD planar con tact
Gradational contact
~
Feldspathic, better sorted
than sandstone below
3,89
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1,33
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LEGEND
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----
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Dark-grey, carbonaceous, pyritic shale
Micaceous siltstone
Figure 4 Stratigraphic profile of borehole drilled at Oranje showing the probable position of the ash-flow tuff.
pseudomorphous chlorite, interstitial quartz, devitrified
glass and secondary chlorite, calcite, and analcite.
The plagioclase phenocrysts are, on average, 0,4 mm
long but do not exceed 1,5 mm in length (Figure 5). A
fair amount of broken feldspar phenocrysts, which are
indicative of tuffs (Fisher & Schmincke, 1984), occur
throughout the layer. Albite twinning of plagioclase is
common in the larger phenocrysts. Extinction angles
measured on twin lamellae indicate that the plagioclase
is andesine.
The acicular plagioclase laths are in places arranged in
radial clusters resembling spherulites. This texture
originates from the devitrification of volcanic glass.
Biotite occurs as very thin, pseudo-hexagonal flakes
not more than 0,1 m across (Figure 6). It ranges from
light to dark reddish brown and black in colour. The
flakes cut across mineral boundaries or are poikilitically
enclosed in feldspar, and are of primary origin.
Lath-shaped chlorite pseudomorphs occur as a result
of alter·ation of either pyroxene or amphibole (Figure 7).
S.-Afr.Tydskr.Geo1.1988,91(4)
514
In conclusion, it can be said that the petrographic data
support an andesitic to dacitic (intermediate)
composition for the ash-flow tuff.
Figure 5 Photomicrograph showing twinned plagioclase lath
about 1,5 mm long. Crossed nicols.
Xenolithic material
The xenolithic grains of quartz, feldspar, and occasional
heavy minerals are present throughout the ash-flow tuff.
Quartz constitutes up to 95% of the total xenolithic
material and shows resorption and reaction rims. This
indicates that the quartz grains were in chemical
disequilibrium with the volcanic material during
deposition of the tuff.
Feldspar makes up 5% of the xenolithic material and
comprises orthoclase and microcline. It is clouded due to
saussuritization, with only a few cores still clear. This is
in contrast with the feldspar phenocrysts of volcanic
origin which are still fresh.
Amygdales
Amygdales occur throughout the ash-flow tuff (Figure
8). They vary from microscopic to about 4 cm in
diameter and are usually spherical or ellipsoidal in
shape. The larger amygdales are found towards the top
of the layer. Locally they are elongated in a southerly
direction, indicating the direction of flow.
Figure 6 Photomicrograph of pseudo-hexagonal biotite in an
aggregate of feldspar (white) and chlorite (grey). Plane
polarized light.
Figure 8 Quartz and calcite amygdales in ash-flow tuff. Scale
in cm.
Figure 7 Photomicrograph of pseudomorphous chlorite (dark
grey) in an aggregate of feldspar (white), biotite and opague
oxides (black), and devitrified glass (light grey). Plane
polarized light.
The original mineral has been completely replaced so
that it can no longer be identified. The chlorite laths are
of the same size and abundancy as the plagioclase laths.
The groundmass comprises quartz, albite, devitrified
glass, and secondary chlorite and calcite. The amount of
chlorite and calcite varies according to the degree of
alteration that has taken place in the groundmass.
Figure 9 Photomicrograph of a glass shard draped over a
quartz grain, in a tuffaceous sediment.
515
S.Afr.l.Geol., 1988,91( 4)
The amygdales consist of quartz, calcite, chlorite, and
analcite. Quartz usually forms the outer zone of the
amygdales while the core is made up of calcite, chlorite,
and analcite. In general the larger amygdales consist of
calcite while the smaller ones are mainly chlorite and
analcite. This probably indicates that calcite was the last
to precipitate since it would take longer for a big
amygdale to fill up than for a small one.
Small amygdales occur in the sediments underlying the
ash-flow tuff. It is envisaged that the amygdales were
formed when the hot ash reacted with moisture present
in the underlying sediment to form gas bubbles which
could not escape through the overlying ash layer. The
vesicles are filled up with quartz and chlorite.
Sedimentary tuff
A sedimentary tuff which grades into a tuffaceous
sandstone and then into sandstone, overlies the
ignimbrite at Blydschap. Numerous glass shards (Figure
9), pumice lapilli, and a few amygdales occur near the
base of the tuff but becomes less as the tuff grades
upwards into a sandstone. The largest shards occur in
coarse-grained sandy material, whereas the finer-grained
sand contains smaller shards. This is indicative of some
reworking and sorting of the volcanic material. The
source of the volcanic material could not have been too
far away, otherwise the glass shards would not have been
preserved.
Conspicuous flat clasts, black in colour and ranging
from 1 mm to 25 mm in length, occur in sandy layers in
the tuffaceous sediments. In thin section a streaky
texture comprising alternating chloritic and quartzose
layers is visible. These flat inclusions probably are
fiamme.
Geochemistry
It is clear from the petrographic study that
contamination of the magma took place at various stages
of emplacement and extrusion. Together with
devitrification and hydrothermal alteration this causes
significant changes in the original geochemical
composition and severely limits the scope for
geochemical investigation.
As the effects of
contamination and alteration can be reduced by careful
sampling and manipulation, six samples (four of
ignimbrite, one of an amygdaloidallava bomb, and one
of dolerite) have been analysed for major and trace
elements (Table 1). All chemical analyses were done by
the Geological Survey of South Africa. Major elements
(except Na) were analysed by X-ray fluorescence
spectrometry on glass fusion discs, whereas Na20 and
the trace elements were determined on pressed powder
briquettes.
Two other analyses of intermediate rocks which are
associated with the main volcanic episode of the Karoo,
I.e. the Belmore Andesite and Pronksberg Dacite
(Duncan et al., 1984), are also included in Table 1 for
general comparison with the Oranje-Blydschap samples.
The Belmore and Pronksberg analyses are the average
compositions of eight and twelve samples respectively.
The freshest available material of the ash-flow tuff,
containing a certain amount of xenocrysts (quartz,
feldspar), but devoid of any lapilli, xenoliths, and
amygdales was analysed and the average composition of
the four samples was used for calculation of the CIPW
norm and plot on classification diagrams.
The amygdaloidal lava bomb which occurred in the
ash-flow tuff, was sawn out of the rock, crushed to < 3
mm and hand picked to rid it of amygdaloidal material
(mostly calcite).
Table 1 Major elements, trace elements and CIPW
norms of ignimbrite (NK01), lava bomb (XE01),
Belmore Andesite (BA01), Pronksberg Dacite (PD01)
and dolerite (NK09)
Sample
NKOI
Major elements (wt%)
66,71
Ti0 2
0,49
AI 2 0 3
15,89
Si0 2
XEOI
BAOI
PDOI
NK09
60,83
0,71
63,54
65,68
0,78
17,20
2,29
50,88
0,96
16,79
0,50
8,32
0,19
7,29
11,78
2,70
19,80
0,52
3,84
0,13
2,39
5,40
2,49
Fe203
FeO
MnO
MgO
CaO
1,84
2,96
0,20
1,09
2,70
Na20
K 20
1,55
6,47
P2 0 s
0,11
3,84
0,05
Total
CO 2
99,17
1,37
1,95
0,67
99,24
2,68
1,59
0,12
H 2 0+
H 2 0-
0,79
15,91
2,30
4,34
0,18
3,25
4,97
2,27
2,26
0,17
3,68
0,17
2,37
2,78
3,39
1,45
0,21
0,50
0,09
98,77
0,08
0,71
0,42
Trace elements (ppm)
Th
3
Rb
Zn
209
200
31
302
15
60
23
91
Pb
26
Sr
y
Zr
Nb
Ni
Cu
13
141
248
20
146
17
53
o
112
6
11 ,5
104
295
31
14
146
15
17
195
203
323
31
203
11
32
37
84
22
13
28
27
97
27
19
90
23
85
58
71
7
23,88
13,34
19,18
24,69
0,67
29,17
8,56
28,66
13,79
4,99
0,00
0,00
2,95
22,79
32,19
0,00
21,07
9,81
0,00
9,36
8,90
CIPW norms
Q
24,26
Or
Ab
An
C
38,25
13,10
12,65
1,68
0,00
Di
14,11
22,74
21,06
26,44
1,83
0,00
0,00
11,52
0,00
13,21
01
6,20
0,00
mt
il
2,67
0,93
0,77
1,35
3,34
1,50
3,32
1,49
0,72
1,82
Ap
0,25
0,12
0,37
0,47
0,19
Hy
0,00
S.-Afr. Tydskr.GeoI.1988,91 (4)
516
A sample of the dolerite underlying the ash-flow tuff
at Blydschap was also analysed for comparison with the
ash-flow tuff and lava analysis. The sample was taken 40
cm below the tuff-dolerite contact to avoid any possible
contaminated material near the chilled contact.
Since hydrothermal alteration is assumed to have
increased the Fe203/FeO ratio of the rocks, Fe203 in
excess of (1,5 wt % + Ti0 2)) was converted to FeO and
the analysis recalculated to 100% excluding H 20 and
CO 2 (Irvine & Baragar, 1971). This recalculation was
also done for the Belmore and Pronksberg analyses. The
original totals as well as the wt% loss of the volatiles
(C0 2, H 20+, and H 20-) are also included in Table 1
because it gives some indication of the hydrated and
carbonated nature of the rocks.
It can be concluded from Table 1 that the ash flow tuff
is depleted in Na20 and enriched in K20 and Si0 2 in
comparison with the lava bomb. This enrichment can
partly be attributed to contamination of the ash flow tuff
by sedimentary material while the sodium depletion is
due to the devitrification of volcanic glass which is
responsible for the leaching of sodium from the system
(Noble, 1967; Lipman et al., 1969). The high degree of
alteration of the ash flow tuff as compared to the lava
bomb probably is due to a greater permeability which
makes it more prone to hydrothermal alteration.
Classification of the Oranje-Blydschap rocks therefore
will have to be based on the chemical analysis of the lava
bomb.
It is further evident from Table 1 that the geochemical
composition as well as the CIPW norm of the dolerite
(olivine and diopside normative) differs markedly from
that of the Oranje-Blydschap lava and other rocks
(quartz and hypersthene normative). It is thus not
related to the Oranje-Blydschap lava and is excluded
from any further classification diagrams.
CIPW norms (Table 1) calculated from the analyses
show that all of the samples are corundum normative.
This is in sharp contrast with the petrographic data which
shows no corundum. The appearance of corundum in the
norms can be attributed to leaching of sodium and
calcium from the system (Noble, 1967) which leaves an
excess of Al 20 3 over (Na20 + K20 + CaO) to form
corundum.
The classification procedure of Irvine & Baragar
(1971) has been used to classify and determine the
geochemical nature of the Oranje-Blydschap rocks. To
10
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,---"
// 0- ""
/"
{!
"-
calc-alkaline
A
M
Figure 11 AFM diagram classifying the rocks as calc-alkaline.
~
Calc-alkaline field
-- --
Tholeiitic field
10
100
80
NORMATIVE
Figure 12 Plot of Al 2 0
3
60
80
Figure 10 Alkalies versus silica plot showing the division
between the alkaline and subalkaline fields.
26
0
0
40
PLAGIOCLASE
20
o
COMPOSITION
versus normative plagioclase composition (NPC) shows all the rocks to be calc-alkaline.
517
S.Afr.l.Geol., 1988,91( 4)
establish whether the lava is alkaline or sub-alkaline the
samples are plotted on a Alk vs Si0 2 diagram (Figure
10). All the rocks fall within the sub-alkaline field. The
AFM diagram (Figure 11) as well as the Al 20 3 vs NPC
(normative plagioclase composition) diagram (Figure
12) classify the Oranje-Blydschap lava as calc-alkaline.
To define names for the different rocks under
discussion, Irvine & Baragar (1971) make use of the NCI
(normative colour index) vs NPC plot (Figure 13) which
defines fields for basalt, andesite, tholeiitic andesite,
dacite, and rhyolite. The Oranje-Blydschap lava plots
close to the andesite-basalt boundary in the andesite
field. It is expected that the NPC value of 56% An
calculated for the lava is too high. The petrographic data
indicates the presence of andesine in the lava and thus
limits the plagioclase composition at a maximum of 50%
An. The high An content can be ascribed to the leaching
of sodium which leaves an excess of CaO over Na20 and
results in a more calcic plagioclase. A lower An content,
which would plot away from the basalt field deeper into
the andesite field, would be more realistic for the
Oranje-Blydschap lava.
The An-Ab-Or diagram (Figure 14) of Irvine &
Baragar (1971) finally classifies the rocks as being
potassium-rich, 'average' or soda-rich. All the samples
plot in the K-rich field of the diagram in the area
between K-rich andesite and K-rich basalt. Again, a
Oranje-Blydschap lava with less potassium and more
sodium would shift the composition of the rock towards
the 'average' field.
On the RI-R2 classification diagram of De la Roche et
at. (1980), which uses the cations of most of the major
elements, the Oranje-Blydschap lava is classified as an
andesite (Figure 15). The sample which contains 60,83
wt% Si0 2, plots close to the 60 wt% Si0 2 contour. This
probably indicates that all the major oxides are in
equilibrium with each other and that the sample has not
been contaminated or altered to any great extent. The
Belmore Andesite and Pronksberg dacite plot in the
dacite and rhyodacite fields respectively.
Since there is doubt as to the degree of secondary
alteration and its effect on the major elements and thus
the classification of the Oranje-Blydschap lava, further
use of incompatible trace elements will be made to
confirm the classification based on the major elements.
According to Winchester & Floyd (1977) Ti, Zr, Nb,
and Yare immobile during post-consolidation alteration
and are reliable geochemical discriminants of different
magma series. Figures 16 and 17 classify the OranjeBlydschap lava as andesite and trachyandesite
respectively. On Figure 18 the lava plots on the
trachyandesite-alkali basalt boundary. The classification
of the Oranje-Blydschap lava, with the aid of trace
elements, as a trachyandesite, is thus in agreement with
the major element classifications of Irvine & Baragar
(1971) as K-rich andesite and De la Roche et at. (1980) as
andesite.
80~---------------------------------'
~
I.o.l
60
0
~
~
;:J
0
.....l
0
40
basalt
tholeiitic
U
I.o.l
:>
~
<t:
::E 20
~
0
Z
o
80
40
60
NORMATIVE
o
20
PLAGIOCLASE COMPOSITION
Figure 13 Plot of normative colour index (NCI) versus
normative plagioclase composition (NPC) classifies the
Oranje-Blydschap lava as andesite.
An
I
I
I
K-rich
,
I I
I ~ \ ba1t
I
~~
,£\
\
(b::
\
{t
K-rich
andesite
.
Sodic \ l' \
'\ RhyodaCite
.
\ ~
dacne
'1!.
\
Sodic \
rhyolite \
Ab
!I>
0
'\
"" "
Potassic rhyolite
Or
Figure 14 The An-Ab-Or diagram classifies the OranjeBlydschap lava as K-rich andesite.
Source of volcanic material
Because of the identical petrography and stratigraphic
position of the ash-flow tuffs at Oranje and Blydschap, it
is concluded that they had a common source.
Palaeocurrent data (Keyser, 1983) indicate a southerly
transport direction for the Frankfort Member and the
sediments overlying it, suggesting that the volcanic
material occurring in the tuffaceous sediments were also
transported from the north. Aeromagnetic data have not
revealed any circular anomalies in the immediate vicinity
of the outcrops, but such features could be masked by
the intrusion of dolerite in the area. However, a circular
S.-Afr.Tydskr.Geol.1988,91 (4)
518
F
+
QJ
U.
L/)
N
+
r--
t
N
d
::::.:::
+
0
Z
..0
0
0
I
V$
C'I'I
~
II
ci:
o
o
o
N
o
o
o
«+
Ol
~
N
+
0
U
lD
II
N
a:::
0
§
0
C'I'I
N
8
Figure 15 The De la Roche R1-R2 classification diagram classifies the Oranje-Blydschap lava as andesite.
~
519
S.Afr.J. Geol., 1988,91 (4)
RHYOLITE
72
COMENDITE
RHYODACITE
PANTELLERITE
DACITE
TRACHYTE
ANDESITE
SUB-ALKALINE
BASALT
TRACHYBASANITE
NEPHELINITE
40
~----------------~------------------~----------------T----1,00
0,10
0,01
Zr/Ti02
Figure 16 SiO r Zr/Ti0 2 diagram showing the fields for common volcanic rocks. The Oranje-Blydschap lava plots in the andesite
field.
RHYOLITE
72
RHYODACITE
DACITE
C'J
o
o
o
COMENDITE
PANTELLERITE
ANDESITE
56
C/)
SUB-ALKALINE BASALT
40 ~----------------~-----------------r----------------~
0,10
1,00
10,00
Nb/Y
Figure 17 SiOrNb/Y diagram showing the fields for common volcanic rocks. The Oranje-Blydschap lava plots
trachyandesite field.
feature (possible plug) has been identified on the farm
Beerlaagte 494 about 40 km north-northeast of Oranje
and 60 km north-northwest of Blydschap. This anomaly
is shown on the latest 2628 East-Rand 1:250 000
geological map.
In
the
The nearest known volcanic pipe in the area occurs at
the Voorspoed Mine situated some 32 km northnortheast of Kroonstad. According to Wagner (1914),
the pipe is middle Jurassic in age, and is therefore much
younger than the late Permian Oranje and Blydschap
ash...flow tuffs.
S.-Afr. Tydskr.Geol.1988,91 (4)
520
-------------------IT------,
1,00 __
COMENDITE
P ANTELLERITE
\
PHONOLITE
'" '" '"
RHYOLITE
-- --- ---
0,10
\
~
TRACHYTE
RHYODACITE
ANDESITE
0,01
ANDESITE
-
ALKALI-BASAL T
NEPHELINITE
BASALT
------_../
SUB-ALKALINE BASALT
0,01
0,10
Nb/Y
1,0
10,0
Figure 18 Zr/Ti0z-Nb/Y diagram showing the fields for common volcanic rocks. The Oranje-Blydschap lava plots on the
trachyandesite-alkali basalt boundary.
Conclusions
The Oranje and Blydschap ash-flow tuffs indicate that
igneous activity was not restricted to the southern and
south-eastern Cape Fold Belt, but also occurred in the
northern
Karoo
Basin.
Further
petrographic
investigation of Karoo sediments of the north-eastern
part of the basin might confirm the existence of
tuffaceous material and also throw more light on the
source of the volcanic material.
Acknowledgements
The Chief Director of the Geological Survey is thanked
for permission to publish these results. Thanks are also
due to Mr H.F.G. Moen for critical reading of the
manuscript. Anglo American Mining Corporation kindly
provided borehole information.
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