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Tram. geol. Soc. S. Afr.. 82 (1979), 343-348
THE VOLCANIC GEOLOGY OF THE LEBOMBO MONOCLINE IN SWAZILAND
by
R. W. CLEVERLY
ABSTRACf
The Lebombo monocline is a large flexure of late-Karoo age (late-Jurassic) extending along the
eastern South African border from Rhodesia to northern Natal. It consists mainly of a thick sequence of
Karoo basalts and rhyolites which overlie Archaean rocks and dip eastwards beneath the Cretaceous
cover at angles of up to 50°.
The lowest volcanic unit exposed in Swaziland is the Sabie River Basalt Formation. It consists of a
thick (c. 5 km) sequence of basalts and includes two groups of interbedded rhyolites: the Mkutshane
Beds and the Twin Ridge Beds.
A 5 km thickness of rhyolites overlies the Sabie River Basalt, the contact marked by a steep escarpment. The succession has been divided into the Jozini and Mbuluzi Rhyolite formations. The rhyolites
are thought to be pyroclastic or ignimbritic in origin. Detailed mapping of individual flows has enabled
the stratigraphy of the rhyolite sequence to be determined. Structurally the area is simple with broad
warps superimposed on the monoclinal flexure.
A number of granophyre plutons intrude the basalts close to the base of the rhyolites. A large dolerite
dyke-swarm intrudes the basalts; it is associated with a zone of strike-faulting and represents the locus of
tension developed along or parallel to the monoclinal axis.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
Page
CONTENTS
INTRODUCTION . . . . . . . . . . . . . .
GEOLOGICAL SETTING
..... .
STRATIGRAPHY . . . . . . . . . . . . . .
BASALTIC VOLCANICS: SABlE RIVER BASALTS
A. General Description
B. Mkutshane Beds
C. Twin Ridge Beds
RHYOLITIC VOLCANICS
A. Introduction
.....
B. Jozini Rhyolite Formation
C. Mbuluzi Rhyolite Formation
.....
D. Volcanology and Emplacement Mechanism
E. Structure and Stratigraphy
INTRUSIVE ROCKS . . . .
A. Granophyres . . . . . .
B. Dykes . . . . . . . . .
MONOCLINAL STRUCTURE
SUMMARY . . . . . .
ACKNOWLEDGMENTS
REFERENCES
I. INTRODUCTION
The Lebombo Monocline, a monoclinal flexure of lateKaroo age, is one of the largest tectonic features in southern Africa, and stretches 700 km from northern Natal to
the Limpopo River (Fig. 1). It consists of a thick sequence
of basic and acid Karoo volcanics which have been downwarped to the east along the eastern margin of the Karoo
volcanic province. The volcanic rocks of the Lebombo
have been dated at 190 ± 5 m.y. (Cleverly, 1977; Manton,
1968) and are thus early-Jurassic in age.
Surprisingly little work has been published on the Lebombo since it was first described 100 years ago. Du Toit
(1930) gave a general account of the geology of the Lebombo and a review of earlier work. More specific studies
have been done by Assuncao et al. (1962) and Wachendorf
(1971,1973) on the Mozambique side; Cox et at. (1965) on the
Nuanetsi district north of the Lebombo; Urie and Hunter
(1963) on the Swaziland section; and Stratten (1965, 1970)
on the southern end of the Lebombo. Cox (1970, 1972) discussed the regional tectonics and regional implications of
the Lebombo. Preliminary geochemical studies (Cox et al.,
1967) suggested the existence of geochemical provinces
within the Karoo volcanic group and the present study is
part of a larger investigation into the tectonic and pet-
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N
t
F::=3:::3 Post-Karoo
.
II VolcaniCS
I;~;~m~lmj Sediments
K
A
R
g
~Pre-Karoo
o
I
100 200 300 400 k m
I
I
I
I
Figure I
Outline geological map of part of southern Africa showing the
location of the Lebombo monocline (after Cox, 1972).
344
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
locality
map
Swazi
land
Geology of the
Lebombo in Swazi land
o
N
KAROO INTRUSIVE ROCKS
EZ:J
CJ
Granophyre
Dolerite
Extent of
dyke swarm
l>
IlJIII]
KAROO VOLCANIC ROCKS
I::::::::::::::~ Oribi Beds(1-4)
l
Ran~e.
c:::J Major Flow Units(a-s) N
MbUIUZI RhyolOt:.
Ie,············
0
~C::'~hti:~ess
of ornament used
to distinguIsh
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
5km.
~ Ity
~~~~~£~!::ite t,"".JMajor Flow Units(A-E)
3 km in Swaziland. unconformIty
Sabie River
Basalt
Formation
Max.
thickness 5 km.
~
Mkutshane Beds
Karoo
Sediments
m
Faults
~
Dip of strata
c:
m
~
Railway
,-/
Major roads
~~~:.-......~ .........
•
Towns
.
N
5
0
o
TWin Ridge Beds
5
b
10
15 km
z
AFRICA
c
Drawn and mapped by A W.Cleverly
Area north 01 Mbuluzi River mapped by
P.J.8erton
Geology 01 Karoa sedIments reproduced
from published maps with permission of the
Director, Swaziland Geological Survey
Figure 2
Geological map of the Lebombo monocline within Swaziland
0
GEOLOGY OF LEBOMBO MONOCLINE IN SWAZILAND
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
rological features of the Lebombo (cf. Bristow, 1976; Cleverly, 1977; Armstrong, 1978).
This paper is concerned with a description of the geology and structure of the Lebombo within Swaziland. Detailed I : 50000 geological maps of the area have been produced by Cleverly (1977); these are being prepared for
publication by the Geological Survey and Mines Department, Mbabane, Swaziland, and a summary version is
presented here (Fig. 2). Detailed discussion of volcanology
and geochemistry is beyond the scope of this paper and
will be published in the near future.
II. GEOLOGICAL SETIING
Most of Swaziland is underlain by Precambrian granitoid rocks of the Kaapvaal craton which have been described by Hunter (I 974a, b). The Karoo strata, tilted to
the east by monoclinal flexuring, form a north-south strip
along the eastern side of the country (Fig. I). The basal
Karoo sediments and the basalts are deeply eroded and
form the flat-lying lowveld; the overlying rhyolites are
more resistant to erosion and form the high ground of the
Lebombo Mountain Range. The upper basalts and the
rhyolites which form the Little Lebombo only crop out in
Mozambique. Intrusive rocks of the Karoo are represented by a doleritic dyke swarm which intrudes the
base of the basalts, and a number of granophyre plutons
which form rounded hills along the eastern edge of the
basalt outcrop. Farther east, on the Mozambique coastal
plain, the Karoo strata are overlapped by Cretaceous shallow-marine sediments.
The drainage of the Lebombo is unusual in that a total
of 14 rivers, three of them in Swaziland, transect the Lebombo at right-angles to the strike. Most of the rivers display incised meanders and represent antecedent drainage.
The consequent streams draining the Lebombo are largely
controlled by the regional joint sets and by the bedding in
the rhyolites.
III. STRATIGRAPHY
The nomenclature of the Lebombo volcanic rocks has
been formalised by Cleverly and Bristow (1979). They proposed a Lebombo Group to include all the volcanic rocks
of the Lebombo in contrast to the previous nomenclature
of Stratten (1965, 1970) who proposed a "Lebombo Stage"
that contained only the acid volcanics. Of the subdivisions
discussed by Cleverly and Bristow (op. cit.), the following
Formations are present in Swaziland:
1. Sabie River Basalt. This forms the base of the volcanic
succession and includes two groups of interbedded rhyolites: the Mkutshane Beds and the Twin Ridge Beds.
2. lozini Rhyolite. This overlies the Sabie River Basalt and
consists of a thick sequence of rhyolites which do not contain quartz phenocrysts.
3. Mbuluzi Rhyolite. This consists of a succession of
quartz-phyric rhyolites which overlies the lozini Rhyolite,
and includes a group of rhyolites, the Oribi Beds, characterised by particularly abundant quartz phenocrysts.
The basaltic and rhyolitic rocks are now described in
more detail.
IV. BASALTIC VOLCANICS: SABlE RIVER
BASALTS
A. General Description
The basalts of the Sabie River Basalt Formation overlie
the Karoo sediments but are deeply eroded and poorly exposed throughout the lowveld. They are fine-grained,
dark, relatively aphyric and commonly amygdaloidal.
Flows containing large feldspar phenocrysts are also
found. Olivine basalts, though common in areas of the Lebombo elsewhere, have not been observed within Swaziland in the present study.
Du Toit (1930) suggested that the abundant dolerite
dykes represent the feeders for the basalts; the dykes,
345
however, differ slightly in composItion from the basalts
and none has been observed directly feeding a basalt flow.
The structure of the basalts is difficult to determine because of the poor exposure, though dips measured on the
underlying sediments and the interbedded Twin Ridge
Beds suggest that the dip of the basalt flows varies from
about 15 0 at the western edge of their outcrop to about
35 0 at the eastern edge. In the south of Swaziland the contact between the Cave Sandstone (Clarens Formation) and
the Sabie River Basalt is repeatedly faulted to the surface
in a 5 km wide zone of strike-faulting with a total displacement of approximately 200 metres. This zone of faulting
trends obliquely away from the base of the basalts, but
within the basalt succession there are no marker horizons
to enable the amount of throw to be determined. The
Rooirand dyke-swarm, producing an extension of 40 per
cent (Armstrong, 1978), cuts the basalt outcrop and seems
to be associated with the strike-faulting. The strike-faulting and the dyking serve to increase the apparent thickness of the basalt by repetition and dilation respectively.
Partly because of this and partly because of overlap by the
overlying rhyolites the outcrop width varies from a maximum of 20 km at the latitude of Nsoko to a minimum of
6 km in the north of Swaziland. The best estimate, therefore, for the thickness of the Sabie River Basalt Formation
is in the range 4 000--5 000 metres.
B. Mkutshane Beds
The earliest acid volcanism in the Lebombo occurred
towards the middle of the Sabie River Basalt Formation.
A number of thin rhyolite flows, comprising the Mkutshane Beds, are found in Swaziland between the latitudes
of Siteki and Nsoko. The Mkutshane rhyolites are poorlyexposed and difficult to locate, but reasonable exposures
occur in and around the Mkutshane River, 20 km south of
Siteki, and on the road west of Nsoko. These rhyolites are
unremarkable in the field, being grey, fine-grained, relatively aphyric and slightly amygdaloidal, but are chemically distinct from the other rhyolites (Cleverly, 1977).
C. Twin Ridge Beds
The initial stages of the main Lebombo rhyolite sequence are represented by a group of three rhyolites interbedded in the upper part of the Sabie River Basalt Formation. These rhyolites are never more than 30 metres thick,
but are laterally very continuous and can be traced for distances of up to 100 km. Because of the overlap of the overlying rhyolite units, the Twin Ridge Beds occur in two
areas: to the north and to the south of Siteki. The rhyolites
from the two areas have similar geochemical characteristics and lithology (Cleverly, 1977).
V. RHYOLITIC VOLCANICS
A. Introduction
The Lebombo Group rhyolites overlie the Sabie River
Basalts conformably and, because of their superior resistance to erosion, form the high ground of the Lebombo
Mountains. A steep erosional escarpment is developed on
the west of the mountain range which follows, for most of
the length of Swaziland, the contact between basalt and
rhyolite.
The rhyolites have been divided into two main groups
(Cleverly and Bristow, 1979) and these are now discussed
in turn.
B. Jozini Rhyolite Formation
The lozini Rhyolite Formation is the basal unit of the
main Lebombo rhyolite sequence. It is distinguished from
the overlying Mbuluzi Rhyolite by the absence or rarity of
quartz phenocrysts. The lozini Rhyolite is typically massive and reddish, with phenocrysts of pink-stained plagioclase, small pyroxenes and occasional large magnetite
crystals. It forms the base of the succession and the
escarpment in Swaziland, except where overlapped by the
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
346
Mbuluzi Rhyolite, and most or all of the succession in
areas elsewhere. The rocks exposed in Swaziland are very
similar to those exposed at the type-section at 10zini Dam
(Cleverly and Bristow, 1979).
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
C. Mbuluzi Rhyolite Formation
The Mbuluzi Rhyolite Formation overlies the 10zini
Rhyolite Formation and is distinguished by the presence
of quartz phenocrysts. It forms the main part of the succession in Swaziland, overlapping the 10zini Rhyolite and
the Twin Ridge Beds to rest on basalt. The rocks are typically pale grey with quartz and white feldspar phenocrysts
in a fine-grained matrix. They are often completely massive, rarely showing flow banding or lamination. Railway
cuttings along the Mbuluzi Gorge afford good exposures
and represent the type-section. The Mbuluzi Rhyolite is
almost exclusively confined to Swaziland although a small
outcrop has been found in South Africa immediately south
of the Usutu River, and the top of the succession is exposed in Mozambique. A number of flows towards the
base of the Mbuluzi Rhyolite have a particular abundance
of quartz phenocrysts and these have been called the Oribi
Beds. They are resistant to erosion and form positive relief, especially in the vicinity of the farm Oribi, II km
north of Siteki.
D. Volcanology and Emplacement Mechanism
The volcanology and emplacement mechanism of the
rhyolites have generated considerable controversy. Urie
and Hunter (1963) in Swaziland, and Stratten (1965) and
Bristow (1976) in Zululand concluded that the rhyolites
were ignimbritic or pyroclastic in origin; whereas
Wachendorf (1971, 1973) decided that the upper portion
of the sequence, exposed in Mozambique, had originated
by lava flow. A detailed discussion of the problems involved is beyond the scope of the present paper and only a
brief account is included here.
One of the striking things about the Lebombo rhyolites
is their great thickness and extent. Some 30 flows have
been recognised, averaging in excess of 200 metres in
thickness and extending along strike for distances up to
60 km (Fig. 2). Because of the monoclinal structure, all the
strata have dips to the east and it is not possible to estimate
the lateral extent of the flows. However, similar flows in
the Nuanetsi syncline can be traced for distances up to
25 km along the syncline and 13 km across it (Monkman,
1961; Cox et al., 1965). In view of the sheet-like appearance of the flows, there is no reason to suppose that their
east-west extent is not of the same order of magnitude as
their length along strike.
At least 90 per cent of each rhyolite flow and the majority of rhyolite outcrops are completely massive or only
vaguely laminated. The upper portions of the flows generally show contorted flow-banding, varying from large-
........
w
scale open structures to small, intricate, ptygmatic-type
folds. The tops of the flows are frequently autobrecciated,
forming a rubbly surface. The flow bases are usually nearly
planar with massive or glassy laminated rhyolite resting
with sharp contact on underlying tuffaceous rocks. The
basal few millimetres of the flows generally show very fine,
streaky, eutaxitic-like banding which rapidly grades, with
devitrification, into the massive zone. Fiamme are found
at the bases of flows at only a few localities.
It can be seen that the Lebombo rhyolites possess both
the features of ignimbrites (i.e. sheet form, fiamme and
eutaxitic textures) and of lavas (i.e. contorted flow banding and autobrecciated flow tops). No relic shard textures
can be seen in thin section, but this may be the result of
devitrification or high degrees of welding.
It is considered by the present author, therefore, that
the rhyolites were emplaced by a pyroclast flow, in a manner similar to that of ignimbrites proper; but because the
flows were relatively high in temperature and low in volatiles, in the late stages of column collapse they formed a
fluidal mass, thus developing the features of a lava flow in
the upper parts of each unit (cf. Rutten and Van Everdingen, 1961; Schmincke and Swanson, 1967; Lock, 1972).
E. Structure and Stratigraphy
Detailed mapping of the area (Fig. 2) has enabled individual flows to be traced out and the stratigraphy of the
rhyolites to be determined. The structurally simple nature
of the monocline has produced parallel, north-trending
outcrops for the rhyolite flows and the order of eruption
can be clearly seen (Fig. 3).
The rhyolite outcrop has a pronounced sigmoidal warp
with a broad synform and antiform centred on Siteki and
the Mbuluzi River respectively. The warping is most pronounced near the base and is gradually flattened out by
successive flows so that the uppermost flows have a northerly strike. The two synforms, that centred around Siteki,
and that to the north of the Mbuluzi River, do not appear
to have any flows in common; the flows in the Siteki area,
except perhaps the very lowest flows which belong to the
10zini Rhyolite Formation, are structurally higher than
those in the north.
There are numerous unconformities and overlaps within
the rhyolite succession, some of the upper flows of the
Mbuluzi Rhyolite coming to rest on as many as six lower
flows. This overlap has resulted in an exaggeration of the
synformal structure, and may also explain the differences
between the width of the basalt outcrop in the north and
that in the south.
The rhyolites have a general easterly dip between 30 0
and 40 0 at the western edge of their outcrop, decreasing to
about 2 0 in the east. No major strike-faults occur within
the rhyolite succession and the total thickness of the rhyolites is probably about 5 km. This thickness, however, will
........
.........
~
zone of strike
faulting and
dyke intrusion
5km
5
Figure 3
Schematic east-west section across the Lebombo monocline approximately mid-way between Big P~nd and Sitekl. S\\a/iland. Legend as Figure 2.
GEOLOGY OF LEBOMBO MONOCLINE IN SWAZILAND
be a great overestimate if the rhyolite flows are not continuous to the east but consist of a sequence of overlapping lenticular flows.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
VI. INTRUSIVE ROCKS
A. Granophyres
Six separate granophyre bodies are found in Swaziland.
They form elongated north-trending ridges and have intruded the basalts close to the base of the rhyolites. They
are all medium- to coarse-grained and have a typical
granophyric texture.
No contacts of the granophyres are exposed and their
temporal relationship with the rhyolite is uncertain. Small
granophyre dykes cut the rhyolites and south of Siteki the
southern of the two granophyres appears to cut the base of
the rhyolites. In the Kruger National Park, however, large
bodies of granophyre intrude the rhyolites and it seems
probable that the granophyres in Swaziland also postdate
the rhyolites.
B. Dykes
A large dolerite dyke-swarm is present throughout most
of the Swaziland lowveld. The main swarm trends slightly
east of north, intruding the Karoo sediments in the south
of Swaziland and cutting obliquely across the basalt outcrop. It reaches its maximum development in the south of
Swaziland and in northern Zululand, forming the Rooirand ridge of hills, but becomes less prominent farther
north and seems to peter out around Siteki. Dykes are
present throughout the Lebombo, intruding the basalts,
but nowhere are they as abundant as in the Rooirand.
The dolerites are resistant to weathering relative to the
basalts and virtually all the outcrops in the lowveld are
formed by dykes or basalt baked by dykes. Dolerite dykes
in the rhyolites are extremely rare and tend to be very
weathered.
Individual dykes vary in width from I metre to 25
metres. They are fine- to medium-grained and generally
aphyric, though plagioclase phenocrysts are occasionally
present.
The percentage of intrusive material is often very high
in the central portion of the swarm. Road cuttings in the
Rooirand, to the south of Swaziland, contain over 50 per
cent of dolerite intrusive into basalt. Bristow (1976) and
Armstrong (1978) both quoted an average value of 40 per
cent dolerite for the area in northern Zululand. Because of
the poor exposure no estimate could be made for the areas
within Swaziland.
When exposed, the dykes are seen to dip steeply to the
west. The angle of dip seems to be lower near the monoclinal axis and increases away from it. The relationship between the dip of the dykes and the dip of the strata is complex, but it seems that the warping and dyke intrusion are
closely related. The rare dykes cutting the rhyolites are apparently all vertical.
The dyke-swarm trends obliquely across the basalt outcrop and postdates most of the basalt succession. It is unlikely that the dykes represent feeders for the basalts. The
lack of dolerite in the rhyolite succession suggests that
either the rhyolites are later than the dykes, or the structural environment for dyke intrusion was restricted to the
basalt outcrop and does not extend as far north as Siteki
nor as far east as the rhyolite outcrop.
Acid dykes are common around the granophyre intrusions, but are not found elsewhere in Swaziland. The dyke
rocks are similar to, though finer-grained than, the granophyres, and are indistinguishable on chemical grounds
(Cleverly, 1977).
VII. MONOCLINAL STRUCTURE
The monoclinal structure of the volcanic rocks has produced a relatively simple structure with dips of 10-15 0 at
the base of the basalts and the underlying sediments, increasing to values of 45 0 and higher towards the top of the
GEOL82/3-E1
347
basalts and the interbedded rhyolites, and increasing to
20-25 0 at the top of the rhyolites.
The axis of the monocline, or rather the axis of maximum dip, is difficult to locate in Swaziland because of the
poor exposure. Dips in the basalt succession, however, become steeper towards the top of the sequence and appear
to reach a maximum along, or just west of the contact with
rhyolite. This axis remains approximately along the base of
the rhyolites for the length of Swaziland except for a short
distance north of Siteki where the axis passes into the
rhyolites. The zone of maximum flexuring in the monocline seems to be developed along 10-15 km west of the
axis of maximum dip and corresponds to the locus of the
dyke-swarm and strike-faults. Both the dyke-swarm and
the strike-faulting reach their maximum intensity in the
Rooirand, in the south of Swaziland.
In the north of Swaziland, and the areas to the north, the
monocline appears to be largely flexural. Although strikefaults and dolerite dykes are still present, they are not
present in the same abundance as in the south of the
country. In the south of Swaziland and the north of Zululand the monocline is characterised by the presence of a
narrow zone of faulting and dyke intrusion corresponding
to a zone of extension presumably related to the axis of the
monocline. Dykes and large strike-faults do not appear to
be significant outside this zone. Farther south in Zululand
the structure of the monocline seems to be that of ~
faulted monocline (Bristow, 1976); north-trending faults
repeat the succession in a series of large fault blocks,
forming a "factory-roof" structure. At the extreme southern end of the monocline a horst-and-graben structure develops, faulting up basement on the eastern side of the volcanic rocks, and the monocline terminates in a series of
arcuate faults (Maud, 1961).
The Lebombo extends along the eastern edge of the
Kaapvaal craton but does not seem to correspond to any
Kaapvaal lineament. It is, however, parallel to the postulated extension of the Mozambique belt (Cox, 1970) and
may be constrained in its northerly trend by the tectonic
grain of the metamorphic belt.
VIII. SUMMARY
The Lebombo volcanic belt is formed by a thick sequence of Karoo volcanics exposed in a monoclinal structure. The beds dip to the east along the eastern margin of
the Karoo volcanic province and hence are exposed in
cross-section in the monocline.
The base of the volcanic succession in Swaziland is
formed by the Sabie River Basalt Formation. This formation contains a 4 km thickness of basalt and includes two
groups of interbedded rhyolites: the Mkutshane Beds and
the Twin Ridge Beds. Overlying the basalts is the main
sequence of rhyolites which forms the Lebombo Mountain
Range and is divided on petrographic grounds into a lower
10zini Rhyolite Formation and an upper Mbuluzi Rhyolite
Formation. A total thickness of about 5 km of rhyolite is
present. Field relationships suggest that the rhyolites are
probably of ignimbritic origin.
A number of granophyre bodies intrude the upper part
of the basalt sequence. A large dolerite dyke-swarm intrudes the lower part of the basalts; it is associated with a
zone of strike-faulting and represents the locus of maximum tension developed along the axis of the monocline.
The structure of the Lebombo changes progressively
from a graben in the south, through a series of fault blocks
in Zululand and repeating strike-faults in the south of
Swaziland and farther north in the Republic of South
Africa.
ACKNOWLEDGMENTS
The work for this study was carried out while a D. Phil.
student at the University of Oxford. The research was supported by the Natural Environmental Research Council. The
348
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
author is indebted to K. G. Cox for his supervision, and to
P. 1. Betton and J. W. Bristow for help and discussion
throughout the project. A. R. Duncan and 1. W. Bristow are
thanked for their comments on the manuscript, and E. P. Saggerson for his critical review.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
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(1972). The Karroo volcanic cycle. J. Geol. Soc., 128
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'
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9:
Department of Geology and Mineralogy,
Parks Road,
Oxford.
Present address:
Department of Geology,
University of Cape Town,
7700 Rondebosch.
Accepted for publication by the Society on 30.11.1979.