Download In the published descriptions of igneous complexes in various

LATE-KARROO INTRUSION BRECCIAS FROM THE NUANETSI DISTRICT
OF SOUTHERN RHODESIA, WITH SPECIAL REFERENCE TO THE
GRANITIC COMPLEX OF DEMBE-DIVULA
By J. R. Vail,
[PLATES
M.Sc., Ph.D.
I-II]
ABSTRACT
Igneous breccias, consisting of angular or rounded xenoliths of basalt, gabbro, and
dolerite contained in a medium-grained granitic matrix, are frequently intimately
associated with granitic ring complexes in the south-eastern part of Southern Rhodesia.
In the Dembe-Divula Complex the breccias are marginal to the granitic ring-dykes and
roof-sheets which are thought to have been passively emplaced into late-Karroo
(Jurassic) basalts and rhyolites. During intrusion the country rocks suffered net-veining,
brecciation, contact metamorphism, and some metasomatism.
The nomenclature of igneous breccias is reviewed briefly.
CONTENTS
(I) Distribution and field relations
139
140
140
143
143
(2) Petrology ...
145
INTRODUCTION
NOMENCLATURE
REGIONAL GEOLOGY AND PREVIOUS WORK
THE INTRUSION BRECCIAS IN THE DEMBE-DIVULA COMPLEX
148
150
150
151
COMPARISONS WITH OTHER AREAS
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES CITED
INTRODUCTION
In the published descriptions of igneous complexes in various geological
settings from different parts of the world reference has been made to breccias
occurring in or around intrusive masses. Depending on the geological environment
these breccias are made up of a variety of components, and it is clear that the
different types owe their formation to several causes.
Some breccias are associated with a volcanic environment, in which case
they appear to be the result of violent explosive activity, and are usually to be
seen in the fillings of pipes and vents. Other breccias owe their formation to
tectonic movement. Sometimes breccia formation accompanies igneous intrusion,
with concomitant fracturing of the country rocks and the incorporation of fragments of the walls in the matrix of the invading magma. Such mixed rocks have
recently been studied in Southern Rhodesia, and are referred to in this paper as .
intrusion breccias . .A study of the literature shows a vagueness in the usage of
terms, and the opportunity is now taken to discuss the definition of some of the
terms used before describing a typical intrusion breccia from Southern Rhodesia.
140
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
NOMENCLATURE
The term intrusion-breccia was first used by Harker (1908, p. 69) in his
description of the contact of plutonic rocks on the Isle of Rhum. He states that
"the newer rock has sometimes penetrated the older in a network of ramifying
veins; and has enclosed detached blocks of it, of all sizes, also intimately veined.
The junction is thus not a line but a zone, which may have a width of a hundred
yards or more. Such a zone of mechanical admixture may be termed for convenience an intrusion-breccia". This usage was followed for marginal features of
the Ardnamurchan complex by Richey, et al (1930), who also distinguished
net-veining. (op. cit., p. 256) .
.Another term that has been used to describe similar rocks, but which has
the connotation of a phreatic origin, is explosion breccia. This was first used by
Tyrrell (1928a, p. 168) to indicate "the material formed by the shattering of a
sedimentary cover (in this case), and by the auto-brecciation of the invading
igneous rock that caused the explosive action".
Richey (1932b, p. 811) extended the usage "to include all breccia-form
rocks due to explosions, whether these occurred below ground, along a vent-wall
or within the interior of a vent".
This term is almost synonymous with volcanic breccia as applied originally
by Tyrrell (1928b, p. 290) "to material (mostly angular) resulting from the
general shattering of country rock by volcanic activity ... " and is in general
use today as applied to "more or less indurated pyroclastic rock, chiefly of accessory and accidental angular ejecta 32 mm. or more in diameter, lying in a fine
tuff matrix". (Wentworth and Williams (1932, p. 46)).
There is thus a change in usage from Harker's intrusion breccia to explosion
breccia, or volcanic breccia. Wegmann (1936, p. 40) attempted to supply a
general term to cover these rocks in his definition of agmatite to mean "a fragmental rock with a more or less granitic cement". Sederholm (in Wegmann, 1936)
applied agmatite to migmatites (products of granitization) which contained rock
fragments. Although Wegmann's agmatite might be applied to the breccias of
the Dembe-Divula Complex, it does not cover the basic rocks of Rhum and Skye,
which Harker included in his definition of "intrusion breccia"; nor would it
include synitic and other breccias.
In this paper intrusion breccia is used in the sense of Harker to describe
fragmental rocks usually found at the contacts of the igneous masses. The rocks
may grade from rare xenoliths of country rock in the intruding host, through
a mixture of host and fragments (many of which are unoriented, rounded or
angular, of local or cognate origin), to that part of the surrounding rock which
contains only net-veins and dykelets of the invading rock.
REGIONAL GEOLOGY AND PREVIOUS WORK
The presence of a cluster of intrusive complexes in the south-eastern part of
Southern Rhodesia has recently been referred to in a short paper (Cox, Vail
and others, 1961). Most of these igneous masses are in the vicinity of the Mateke
Hills, which lie between the Nuanetsi and Bubye Rivers at about latitude 21 0
40' S. and longitude 31 0 10' E. Examples of intrusion breccia have been found in
..
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142
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
the course of the detailed mapping of the various igneous complexes in the Nuanetsi District, and particularly during the examination of the Dembe-Divula
Complex (Vail, 1960).
The intrusive complexes are localized along an east-north-east trending line
running from Messina to the Sabi River (Fig. 1). They cut through Basement
complex (Messina Formation type) and the overlying Karroo (upper Palaeozoiclower Mesozoic) rocks. These latter consist of a thin sedimentary formation of
sandstones and shales, followed by a thick sequence of basaltic (Stormberg) lavas,
which give way to rhyolitic ignimbrites (Monkman, 1961) near the top.
The "younger" intrusive cycle consists of quartz gabbro and olivine gabbro
masses, in part at least, in the form of sheets or large sills. These were followed
by the emplacement of a large, slightly discordant, granophyre sheet, up to
several hundreds of feet thick, known as the Main Granophyre. Further intrusion
of granites and granophyres represented the final stage in the emplacement of
the complexes. A detailed account of the geology of the Nuanetsi area is being
prepared (Cox, Vail and others.)
The complexes generally display a characteristic ring structure, and DembeDivula is no exception. Divula, the eastern half, is made up essentially of a
number of acid intrusive phases of fine-grained granites and granophyres. These
are considered to be ring-dykes, with their horizontal roof-sheets largely preserved. Dembe, the western part, is less clearly understood structurally. It
o
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FlO. 2
Outline of the Dembe-DivulB. Complex.
Stippled--granitic rocks of the ring intrusions.
Marginal hachuring-margins of the Dembe depression.
Curved lines in Dembe--arcuate acid dykes.
LATE-KARROO INTRUSION BRECCIAS FROM THE NUANETSI DISTRICT
143
appears to be a ring complex, only part of which is preserved as an arcuate ringdyke, most of the centre being made up of gabbroic country rocks. The two arcs
of the Dembe and Divula rings are in juxtaposition in the low ground of the
Dembe depression (Fig. 2).
Until recently little has been published on the geology of the south-eastern
corner of Southern Rhodesia, apart from two accounts of reconnaissance work
in the area by members of the Southern Rhodesian Geological Survey.
Lightfoot (1939) carried out an investigation of the country to the east of
the Nuanetsi River and mentioned an occurrence of intrusion breccia at the contact of a large mass of granophyre with the Karroo basalts in the N uanetsi River.
Later work at this locality (Cox, 1960) has shown the intrusive rocks of this
contact to be part of the Masukwe Igneous Complex.
In 1939 Tyndale-Biscoe (1950) visited the western part of the area and
recorded (p. 409) "intrusion breccias, consisting of angular blocks of basalt in
granophyre ... on the eastern margin of the curious depression or amphitheatre
in the granophyre ... "-the depression that is now known as Dembe (Vail, 1960).
Mention is made in Memoir 40 of the Southern Rhodesian Geological Survey
(Swift, et al, 1953) of intrusion breccias from the Sabi River, and an illustration
is given of a granite containing fragments of basalt from what has since been
recognised as the Mutandawhe quartz syenite ring complex (Wood, 1961).
Descriptions of the breccias in the various ring complexes in the vicinity of the
Mateke Hills are to be found in unpublished theses (Stillman, 1959; Cox, 1960;
Vail, 1960; Wood, 1961).
THE INTRUSION BRECCIAS IN THE DEMBE-DIVULA COMPLEX
1. Distribution and Field Relations
The intrusion breccias in the Dembe-Divula Complex are best developed
around parts of the margins of Dembe, at some places inside the Dembe amphitheatre, and along the northern margin of Divula where the granitic rocks are
intruded into the gabbros. (Fig. 3).
Structurally, the breccias invariably occur as narrow sub-vertical or subhorizontal bodies 20 or 30 ft. wide.
The sub-vertical intrusion breccias occur along the steeply dipping margins
of the granitic rocks. They are best seen along the south side of Object Hill;
around the base of parts of Jenama; and along much of the Divula granite contact
on the south side of the microgabbro strip north-east of Masingo Mountain.
(Fig. 4). At these localities the contact breccia can be followed along the contact
for many hundreds of feet. The rock consists of xenoliths of basic country rocks
enclosed in granite. The xenoliths are of microgabbro in the north, but on the
south side of Object Hill, basalts, as well as gabbros are the included rock types.
Since basalt is not exposed at the surface at this locality the xenoliths are presumed to have been transported from depth by the invading acid magma, in the
same way that angular blocks of Basement complex gneisses have been brought
up through many hundreds of feet of basalts by the intruding magma of dolerite
dykes, some 20 miles to the north of the Dembe-Divula Complex. (Cox, Vail
and others).
144
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
PART
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Geological sketch map of part of the Dembe-Divula Complex.
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FIG. 4
Field sketch of section across the northern contact of the Divula ring-dyke.
1. Microgabbro.
4. Divula granite.
2. Granophyre.
5. Granophyre dykes.
3. Intrusion breccia.
.
'.
LATE-KARROO INTRUSION BRECCIAS FROM THE NUANETSI DISTRICT
145
The sub-horizontal intrusion breccias are found particularly in central
Dembe; north of Object Hill; and around Masingo Mountain. (Fig. 3). They are
in the form of sheets a few tens of feet thick, with xenoliths of basalt and gabbro,
and less widespread dolerite, and rarely adamellite. In many localities there is
a gradational change from the basic rocks, net-veined by granite, through
numerous xenoliths of basic rock within a granite matrix, to granite containing
only a few basic xenoliths. (Fig. 5). Many of the flat sheets of breccia in central
Dembe dip to the east, generally at about 10° to 15°. This structure is well
illustrated by the breccia sheets in the adamellites of eastern Dembe, and by the
breccia around the western end of Object Hill (Plate IA.).
o
I
FEE T
I
5
I
FIG. 5
Net-veining and brecciation of basic rocks (black) by granite (white). West end of
Object Hill. (From photographs)
Similar intrusion breccias have been found in the other ring complexes of
the Mateke area. In the Northern Ring Complex, for example, the gabbros are
net-veined by microgranites, rounded, disoriented xenoliths only being developed
in a few localities. (Stillman, 1959). The Causeway microgranite dykes of the
Masukwe Complex also show brecciation, and many grades of fracturing and
assimilation of microgabbros and basalts can be seen. (Cox, 1960).
2. Petrology
Two principal petrological types of breccia have been found although they
are not always easy to distinguish: (i) basalt, gabbro or adamellite xenoliths in
146
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
a granitic matrix; and (ti) basic xenoliths in a granophyric or adamellite matrix.
Acid xenoliths, other than the contaminated adamellites of probable hybrid
origin, have not been found in the breccias.
The intrusion breccias are striking rocks in the field, (Plate IB), dark xenoliths contrasting strongly with the light colours of the invading acid rocks. The
xenoliths range in size from small fragments to large blocks several feet across,
the most common size being about six inches in diameter. The shape of the
fragments is variable, and gradations occur from angular blocks on the one hand
to completely rounded xenoliths on the other. Usually a variety of shapes is to
be found in anyone outcrop, and while some of the xenoliths are rounded or
angular, others are broken up by veining or assimilation to irregular patches of
dark material in the granite. As a result the latter is highly contaminated in some
places. There are two petrological aspects of the intrusion breccias and these
can be related to the two major components of the rocks: (i) the contamination
of the acid matrix by basic xenoliths, and (ti) the metamorphism and breakdown
of the basic fragments by the acid magma.
Variations in the acid rocks of the intrusion breccias.
Where the granitic rocks of the breccias have been contaminated it may be
difficult to identify the acid rock type. However, in the main, the Divula granite
forms the matrix of most of the breccias in the Dembe-Divula Complex, and an
examination of this rock will serve to illustrate the changes in the rocks of the
matrix.
The Divula granites seldom chill to a very fine grain size against the margins,
and this is especially seen in the intrusion breccias, where the size of the minerals
of the granite on the whole is maintained to within a few millimetres of the contact
of the xenoliths, and even in the small veins which penetrate the basic rocks
(Plate II).
Usually the Divula granite is made up of crystals about 2 mm. in length
arranged in an equigranular, granitic texture. Potash feldspar is the most abundant mineral, accounting for between 50 and 60 per cent of the rock. It is usually
in the form of microperthite and is frequently highly sericitized. Quartz occurs
as slightly rounded grains, the edges of some of which are corroded; in a few
places a micrographic texture is developed. Quartz accounts for about 30 to
36 per cent of these granites. Plagioclase, of composition AnSI to An46! is present
in amounts ranging from 3 to 12 per cent. As a rule it is highly altered, but
twinning and some slight zoning can usually be distinguished . .Accessory minerals
make up less than 5 per cent of the rock and include pleochroic, brown and green
hornblende, which alters to pleochroic, green, yellow, and brown biotite; this in
turn alters to penninite (chlorite). Small crystals of zircon, apatite, epidote, and
fluorite are present, as well as about one per cent of ore minerals, which are
usually hematite and magnetite . .An average of nine modal analyses of the Divula
granite from southern Divula near Object Hill is given in Table I, analysis 1.
With increasing contamination the hybridization of the Divula granite can
be traced from unaffected granite, through slight changes in composition, to grey
hybrid rocks. The granite becomes darker, while retaining its texture, until with
further contamination small clusters of dark minerals, xenocrystR, and released
amygdales are scattered through the rock.
(i)
147
LATE-K.ARROO INTRUSION .BRECCIAS FROM THE NUANETSI DISTRICT
Under the microscope the progressive change in composition of the granite
can be seen. With slight contamination there is a decrease in the amount of
potash feldspar and a corresponding increase in the amount of plagioclase
(analysis 2, Table I). Rare grains of anorthoclase are also present; they probably
represent high-temperature feldspar retained in the more rapidly cooled contact
zone of the granite mass.
The dark inclusions are usually made up of small clots of highly altered
plagioclase, clinopyroxene, chlorite, biotite, hornblende, and magnetite, contaminated by irregular masses of quartz. Sphene, zircon, and epidote are also
present. These basic aggregates break up into separate xenocrysts, which further
add to the basification of the granite. Occasional well-preserved, rounded amygdales, separated from their basalt host, occur within the acid matrix.
Analyses 3 and 4, Table I, illustrate advanced stages in the contamination
of the granite. Ferromagnesian minerals comprise up to 20 per cent of the rock,
which as a result is quite dark in colour. In the extreme case (analysis 4) clinopyroxene is present and shows marginal alteration to chlorite and hornblende.
Magnetite, although not abundant, is scattered throughout the rock. Sphene is
present, but apatite appears to be absent. Potash feldspar and quartz occur as in
the normal granite, but the amount of quartz and orthoclase is down by about
one third. Up to 20 per cent plagioclase is present in this rock, and it is highly
altered, zoned, and rimmed by potash feldspar. The plagioclase varies considerably in composition; one zoned specimen measured from A~6 to Ana" other
crystals are up to An"s in composition.
TABLE
I
1
Quartz ...
33·7
Potash feldspar
54·8
Plagioclase
7·4
Biotite and chlorite
1·9
Hornblende ...
0·8
Clinopyroxene
Ore minerals
1 ·0
Accessories
0·4
Colour index
3·7
1. Average of 9 granites from south-west Divula..
2. Divula granite from intrusion breccia. Object Hill.
3. Contaminated granite. Object Hill.
4. Highly contaminated granite. Object Hill.
2
33·9
45·6
14·2
2·1
2·4
3
24·6
44·5
17·9
0·3
1·2
3·7
1·8
11·2
0·6
5'7
7·2
4
22·2
36·7
19·4
4·4
7·8
8·2
0·6
0·7
21·0
Variations in the basic xenoliths of the intrusion breccias
There are various types and sizes of xenoliths in the intrusion breccias. Most
of the fragments are of basalt, but gabbro and dolerite xenoliths are also present.
As is to be expected, the basic rocks of the breccias show two principal effects
of being caught up in an acid magma: (a) The mechanical breakdown of the rock,
and (b) metamorphism and metasomatism. In the first case the fragments of
basic rock become digested by the granite, resulting in the contaminated acid
rocks discussed above. In the other case, the larger blocks show the effects of the
intrusion of the granite around their margins, while the smaller fragments are
highly metasomatized.
All the xenoliths of basalt examined show signs of contact metamorphism.
These metamorphic effects are represented by a crystalloblastic texture, which is
clearly seen when the rock is examined in thin-section. The pleochroic, olive(ti)
148
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
green pyroxene is partially altered to brown biotite, and forms a poikilitic texture
with magnetite grains, which are also scattered throughout the turbid groundmass. In the groundmass of the basalt xenoliths there is an unusually high proportion of quartz, and this increases with the breakdown of the fragments until
the rock grades into a contaminated granite. Orthoclase is also present in the
groundmass of the basalt fragments. It is difficult to distinguish from turbid,
anhedral, and poorly twinned plagioclase.
In the metamorphosed basalt xenoliths the amygdales contain minerals
somewhat di1Ierent from the normal amygdale mineral assemblage. Crystalline
quartz, feldspar, sericite, chlorite, biotite, and magnetite contrast with the
amygdale minerals of the unmetamorphosed basalts, which are usually chalcedony, chlorite, magnetite, calcite, fibrous zeolites, zoisite and epidote.
Basic xenoliths having a doleritic texture, containing vesicles and a few
phenocrysts of plagioclase were noted in some of the breccias. These rocks are
very similar to feeder dykes cutting the basalt country rocks some distance from
the Dembe-Divula Complex. The groundmass of these diabasic xenoliths is made
up of an ophitic intergrowth of pale-green, pleochroic pyroxene, and small feldspar laths of calcic andesine. Chlorite, brown biotite, and magnetite are also
present. The grain size of the crystals is generally larger than that of the basalts.
The plagioclase phenocrysts are similar in size and shape to those of some of the
basalt flows. The feldspars are zoned slightly, and are of labradorite composition
(approximately An7s)' The vesicles of these rocks are about 3 mm. in diameter,
and consist of an outer rim, about 0·5 mm. in width, of small flakes of pleochroic,
reddish-brown biotite, and a little magnetite and calcite.
COMPARISONS WITH OTHER AREAS
A comparison of the Nuanetsi igneous rocks with those from other igneous
complexes in which intrusion breccias have been described helps to throw light
on the origin and mode of emplacement of the Dembe-Divula breccias.
The carefully studied explosion breccias from Ireland, such as the Kilkenny
"appinitic intrusion breccia" (Pitcher and Read, 1952), the well-known complex
of Slieve Gullion (Richey, 1932bj Reynolds, 1951 and 1954), and the Dunmore
"intrusion breccia" (French and Pitcher, 1959) make an interesting comparison.
In all these localities detached fragments, many rounded by abrasion, occur in
the breccias. As a rule the fragments show signs of intense metasomatism, and
all the breccias are considered to be the result of hot gas streams rising under
pressure up fissures ahead of magmatic intrusion. The enclosed fragments may
be of local origin where they match the fissure wall-rocks, but many h~ve either
risen up to a higher stratigraphic level, or sunk to a lower level in the fissure.
From the detailed descriptions it is clear that this type of breccia di1Iers from the
igneous breccias of the N uanetsi area.
Some of the net-veining of the basic rocks in Southern Rhodesia, such as in
the gabbros of the Northern Ring Complex (Stillman, 1959), and possibly also
some in the Dembe gabbros, may be explained by an hypothesis proposed by
Emmons (1940) and applied by Wells (1954) to the granitic net-veining of the
gabbros of the Ardnamurchan complex in Scotland. (Richey, et al, 1930). The
LATE-KARROO INTRUSION BRECCIAS FROM THE NUANETSI DISTRICT
149
mechanism for the process involves the migration of acid residual and interstitial
liquids of a crystallizing mass by the reduction of pressure. In this case the
material filling the veins is limited in amount and is derived locally from the basic
magma. The concentration of volatiles into a gaseous phase could result in the
explosive brecciation of the gabbroic wall-rocks. The restricted development of
the breccias at Ardnamurchan, and in some of the other igneous masses in
Scotland, is thought by Richey (1932a) to be related in part to gas pressure and
relative depth in the complex. Where breccias are absent, emplacement is thought
to have been more deep-seated, explosive brecciation being active only near the
surface. As far as can be judged, this is not the case in the Dembe-Divula Complex,
since the acid rocks are not considered to be in situ differentiates of the basic
magma.
On the question of metasomatic, as opposed to magmatic origin, such as was
raised regarding the rocks and breccias of the Coast Range Batholith of Canada,
near Vancouver (Phemister, 1945, and Read, in discussion), there seems no
reason to doubt that the bulk of the Dembe-Divula granitic rocks are magmatic.
Indications of this are seen from several features consistent with igneous intrusion, such as chilled margins and cross-cutting contacts. That some metasomatism of the country rocks, particularly when fragmented, has occurred is not
surprising. The presence of potash feldspar crystals in the basalt inclusions,
and the presence of "ghost" xenoliths in the contaminated granite is indicative
of metasomatic activity. Processes of reciprocal reaction (Read, 1924) have
taken place between the basic fragments and the acid matrix. The minerals of
the contaminated granitic rocks indicate increasing amounts of magnesia, ferrous
iron, and lime, with a corresponding increase in the basic xenoliths of silica,
alumina, and potash. This is in keeping with the findings of Nockolds (1933
and 1934). It is noted, however, that this phenomenon is almost entirely confined
to the smalJer, frequently well-rounded xenoliths, which are usually not of local
origin, and metasomatism is not considered to have been the principal process
involved in the development of the Dembe-Divula breccias .
.An interesting comparison of the Southern Rhodesian intrusion breccias can
be made with those associated with very similar complexes of much the same
age and character in Damaraland, South West .Africa. (Martin, Mathias and
Simpson, 1960). Of these the most fully studied is the Messum Igneous Complex
(Rorn and Martin, 1955). Breccias at Messum are well developed. Evenly
distributed, unoriented, basalt xenoliths of varying size and shape contained in a
hybrid matrix are considered to have formed after initial shattering of the wallrocks during movements connected with cauldron subsidence, and then to have
been metasomatized along the fracture planes by granitizing emanations which
transformed the brecciated rocks into acid and intermediate types (Mathias, 1957).
Both metasomatic and magmatic granite is present. In some parts hybrid rocks
are formed through the incorporation of basic material in a granitic matrix.
The origin of the Messum breccias falls between those formed by fluidization
(such as the explosion breccias of Ireland) and those formed primarily by igneous
activity.
The breccias of the Okonjeje igneous complex (Simpson, 1955) are not as
conspicuous as those at Messum. They consist of fragments of basic material in
J 50
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
an acid rock, considered by Simpson to be the product of contamination of the
alkali rocks of the complex. Several of the other Damaraland plutons have breccias
associated with them, but they have not yet been described in detail. The granodiorite stock of Erongo is "marginally crowded with xenoliths of basement rock
types" (Martin, Mathias and Simpson, 1960). "Xenoliths of Kaoko lavas and
sediments in all stages of assimilation are conspicuous" at Brandberg (op. cit.).
The outer granite of the Kalkfeld complex also contains fragments of Damara
basement rocks (op. cit.). Like the intrusion breccias of the Nuanetsi petrographic
province "passivity of the magma is characteristic of these (Damaraland) plutons" (Kom and Martin, 1955), but metasomatism is apparently an important
aspect of these South West African rocks.
CONCLUSIONS
The Dembe-Divula intrusion breccias are contemporaneous with, and intimately related to, the intrusion of the granites. Emplacement took place as a
result of relatively passive (non-explosive) upwelling of magma along arcuate
fractures, accompanied by a certain amount of brecciation of the walls and
penetration of the magma along planes of weakness, joints, flow boundaries and
geological contacts. Attrition in the magma caused the rounding of many of the
xenoliths. Volatiles originally present in the granite magma are now represented
by normal accessory minerals such as hornblende, biotite, and fluorite, and high
gas pressures and violent explosion are not indicated. The Southern Rhodesian
breccias are considered to have had a mode of origin that was largely magmatic
and entirely passive. This is compared with the South West African breccias
which are in part metasomatic and represent a stage removed from the Irish
breccias which are largely the result of explosive gas activity.
The Dembe-Divula breccias are locally of importance in aiding the structural
interpretation of the Complex, in that they give an indication of the rock types
lying beneath the surface. This is especially so in Dembe where gabbro is the
only rock exposed. The xenoliths, on the other hand, are usually of basalt. This
is taken as further evidence that the Dembe gabbros, like those of the neigh
bouring Northern Ring and Masukwe complexes, are a floored intrusion.
The study of Karroo igneous activity in Southern Rhodesia described here
has shown that the views of the earlier workers that the breccias were related to
a "granophyre batholith"-now recognized as the Main Granophyre sheet (Cox,
Vail and others, 1961)-were incomplete. It is now established that the breccias
are intimately related to the ring complexes, and although limited in areal extent,
they form an interesting and conspicuous part of these fascinating structures.
ACKNOWLEDGEMENTS
This paper forms part of an investigation being carried out under the
direction of Professor W. Q. Kennedy at the University of Leeds Research
Institute of African Geology. The work was financed by an Oppenheimer Scholarship provided through the University from funds donated by the Anglo-American
Corporation. The author acknowledges with thanks the co-operation of colleagues
during the field work, and the help received from P. W. Bro<)k, who assisted with
the photographs. The author is indebted to members of the Institute for constructive criticism of the manuscript.
LATE-KARROO INTRUSION BRECCIAS FROM THE NUANETSI DISTRICT
151
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RESEAECH INSTITUTR OF AFRICAN GEOLOGY.
LEEDS UNIVERSITY. LERDS, ENGLAND.
Accepted for pUblication by the Society
on 4th August, 1962.
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