465
S.Afr.l . Geol. ,1988,91 (4),465-476
The Keimoes Suite - a composite granitoid batholith along the eastern margin of the
Namaqua mobile belt, South Africa
G.J. Geringer and B.J.V. Botha
Department of Geology, University of the Orange Free State, Bloemfontein 9300, Republic of South Africa
M.J. Slab bert
Geological Survey, Upington 8800, Republic of South Africa
Accepted 28 August 1988
A syntectonic granitoid suite along the eastern margin of the Namaqua mobile belt, collectively known as the
Keimoes Suite, represents part of a granitoid batholith which in many respects resembles Phanerozoic
batholiths of destructive environments. The Keimoes Suite batholith consists of 13 individual granite plutons,
ranging from granodiorite to alkali-granite, and a number of small basic intrusions. Late tectonic movements
and tilting of crustal blocks along prominent shear zones caused deep-crustal katazonal granites to crop out
along the western edge of the batholith while mesozone and epizonal granites occur along the eastern margin.
The granites, which are calc-alkaline, are predominantly I-types although a few plutons reveal S- and A-type
characteristics. Chemically the Keimoes Suite granitoids show similarities with granites of destructive plate
margin environments. Chondrite-normalized REE curves demonstrate that the granite plutons share a common
magma source; that fractionation of the magma took place, and that the source region represents LREE
enriched material. It is concluded that the Keimoes Suite granitoids formed in a thick crust environment related
to crustal underriding of the Kaapvaal craton by the Namaqua plate following subduction of oceanic plate
material to give rise to a middle-Proterozoic calc-alkaline volcanic arc. The Keimoes Suite batholith is
comparable with batholiths of destructive margins involving crustal thickening such as the granites of southeast
Asia (Hutchison, 1983). A choked subduction model, similar to that of the southeast Asian plate, is proposed
for the eastern margin of the Namaqua mobile belt.
'n Sintektoniese granitoledsuite langs die oostelike rand van die Namakwalandse mobiele gordel, die
Keimoessuite, verteenwoordig 'n saamgestelde batoliet, wat met batoliete van destruktiewe
plaatgrensomgewings ooreenstem. Die Keimoessuite-batoliet bestaan uit 13 verskilIende granietplutone, wat in
samestelling van granodioriet tot alkali-graniet varieer. Verskeie kleiner basiese intrusies kom langs die
westelike rand van die batoliet voor. Laat-tektoniese bewegings en kanteling van korsblokke langs prominente
skuifskeure veroorsaak dat katasone-graniet langs die westelike rand van die batoliet voorkom, terwyl meso- en
episone graniet langs die oostelike rand aangetref word. Die kalkalkaliese graniete is oorwegend I-tipes terwyl
sekere graniete as S- en A-tipe geklassifiseer kan word. Chemies stem die graniete van die Keimoessuite ooreen
met graniete van destruktiewe plaatgrensomgewings met 'n neiging na 'n binneplaatomgewing.
Chondrietgenormaliseerde SAE-kurwes dui daarop dat die onderskeie granietplutone 'n gemeenskaplike
magmabron gedeel het; dat fraksionering en differensiasie van die magmas plaasgevind het en dat die bron
LSAE-verrykte materiaal verteenwoordig. Dit is in ooreenstemming met die lae inisiele Rb-Sr
isotoopverhoudings wat deur Barton & Burger (1983) vir die Straussburggraniet gerapporteer is en bevestig die
dominante rol wat korsmateriaal tydens die vorming van die granietmagmas gespeel het. Dit kan afgelei word
dat die Keimoessuite granitoi'ede in 'n verdikte kontinentale korsomgewing gevorm het. Die korsverdikking het
plaasgevind as gevolg van onderskuiwing van die Kaapvaalkraton deur die Namakwaplaat na 'n periode van
oseaanvloer-subduksie waartydens die middel-Proterozolese kalkalkaliese vulkaanboog, die Areachapgroep,
gevorm het. Die Keimoessuite-batoliet vergelyk goed met graniete van destruktiewe plaatgrensomgewings
waar korsverdikking 'n rol gespeel het soortgelyk aan die graniete van suidoos-Asie (Hutchison, 1983). 'n
Toegedrukte subduksiemodel, soortgelyk aan die van die suidoos-Asiatiese plaat word vir die oostelike rand
van die Namakwa-mobiele gordel voorgestel.
Introduction
A suite of syntectonic granitoids, collectively known as
the Keimoes Suite (Blignault & Geringer, 1980) and
associated gabbroic intrusions occur along the eastern
marginal zone of the Namaqua mobile belt, also known
as the Namaqua Front (Blignault et al., 1983). The Front
separates low-grade metamorphites of the Kheis
Province (1,7 -1,9 Ga) and Archaean basement granites
(2,6 - 2,9 Ga) of the Kaapvaal craton from high-grade
metamorphic gneisses of the Bushmanland terrane in the
west (Figure 1).
Granitoids of the Keimoes Suite have intruded
metasediments of the Korannaland Sequence (Malherbe
et al., 1980) and display an intimate relationship with the
calc-alkaline volcanic sequence of the Areachap Group
along the eastern margin (Geringer et al., 1986). The
suite is bounded in the west by the Waterval thrust
(Praekelt et al., 1986) and in the east by the Brakbosch
fault zone (Figure 1). A striking feature is the lack of
late- to post-tectonic granites outside the boundaries of
these two structures.
Along strike the batholith is exposed over a distance
of over 200 km from Putsonderwater in the southeast to
the Cnydas pluton in the northwest, beyond which it is
covered by the Nama Sequence (Figure 1). The intrusive
history of the batholith spans a considerable period
during the Namaqua tectogenesis, starting with the
intrusion of metagabbros, which predate the intrusion of
the granites (Von Backstrom, 1964). This was followed
by the intrusion of syntectonic granites, such as the
S.-Afr.Tydskr.Geol.1988,91(4)
466
....
CJ
Colston granite
~
~
Cnydas granite
G
~
Kleinbegin granite
~
Stukkende Dam adamellite
8i]
Strauss burg granite
II
Gouskop _granite
\\,;.~.::;.~\
Louisvale granite
.'\
Kanoneiland granite
~
Vaalputs adamellite
v
(.
0
[illmJ
V'll V V V
6
o
!
E2J
Gemsbokbult adamellite
Klip Koppies granite
Gifberg granite
-
Basic bodies
,
~
Koras Group
G
Areachap Group
Undifferentiated granite
20
40
km
!
Figure 1 Geological map showing the distribution of the various granitoid units within, and the major shear zones intersecting the
Keimoes Suite batholith along the eastern margin of the Namaqua mobile belt.
Straussburg granite (Van Zyl, 1981; Stowe, 1983),
followed by the late-tectonic Vaalputs-type granite
(Slabbert, 1985), and by late- to post-tectonic intrusions,
such as the Kleinbegin-type granites, the Zand Dam
pluton (Van Zyl, 1981), and the Ratel Draai pluton
(Stowe, 1983).
Radiometric ages, obtained by U/Pb and Pb/Pb
zircon, as well as by Rb/Sr and Pb/Pb whole rock dating,
show that most granites were emplaced within a limited
time span around 1100 Ma (Table 1). The Gemsbokbult
granite yields the oldest age of 1200 Ma whereas an age
of 807 Ma was reported from a nonporphyritic granite
within the Cnydas Complex. Barton & Burger (1983)
suggested that the granitoids were emplaced between
1250 and 1100 Ma, based on results obtained from the
Straussburg granite, covering the peak of the Namaqua
orogenesis.
The batholith is transected by several shear zones
from west to east (Straussheim, Boven Rugzeer,
Cnydas, Louisvale, and Koegrabe (Figure 1). The
granites intruded prior to the shearing and they have
been highly affected by these movements. A few
intrusions, including the post-tectonic granophyric dyke
in the Cnydas Subsuite (lankowitz, 1986), postdate the
shearing in the area.
Composition and general features
The Keimoes Suite is a composite batholith, consisting
of metagabbro intrusions collectively called the
Biesiepoort gabbro by Slabbert (1985), the Cnydas
Complex (Geringer, 1973) also called the Cnydas
Subsuite by lankowitz (1986), and 12 other plutons, each
with its own distinguishable textural, mineralogical, and
chemical characteristics (Figure 1). The Cnydas
Complex represents a 'super unit' according to the
classification used by Pitcher (1985) for the Coastal
batholith of Peru. The other plutons can be considered
467
S.Afr.J . Geol. ,1988,91 (4)
Table 1 Characteristics of the Keimoes Suite granitoids
Radiometric age (Ma)
Average
PbFoopb--zi rcon
Depth potassium Granite
level index(PI)
type unless otherwise stated)
e07
Pluton
Texture
Contact relationship
Bakrivier
Coarse-grained, porphyritic,folia- Sharp, lit-par-lit,
ted, xenoliths prominent
concordant
Katazone
216
I-type
Gitberg
Medium to fine porphyritic with Sharp contacts, finescattered phenocrysts, unfoliated, grained towards
numerous xenoliths
margins
Mesozone
328
I-type
Stukkende Medium to coarse, porphyritic,
dam
well foliated
GemsbokbultMedium to coarse, equigranular
with scattered phenocrysts
Klip Koppies Medium to coarse, scattered
phenocrysts to porphyritic
Louisvale
Medium to coarse, scattered
phenocrysts to porphyritic
Colston
Medium to coarse, scattered
Sharp, concordant
Katazone
Concordant,sheared
foliated near contact
Sharp, foliated near
contact
Sharp, foliated near
contact
Foliated near contact
Mesozone
224
I-type
Mesozone
221
I-type
I-type
Mesozone
Mesozone
Friersdale
granites
Medium to fine granophyric
223
1- and
193
I-type
Sharp, crosscutting, no Meso- to
192
I-type
Epizone
Meso- to
phenocrysts
near contact, met.
aureole present
Epizone
Medium-grained, scattered
phenocrysts to non-porphyritic,
Crosscutting, sharp
contacts
Mesozone
Li nstrom (1977)
I-type
Meso- to
Epizone
with rounded blue opaline quartz met. aureole
Kanoneiland Coarse to medium, scattered
Sharp, with foliation
Vaalputs
1200(U/Pb)
1156 ± 20
S-types
Medium to coarse, porphyritic in Sharp, crosscutting
places orbicules present in some
Geringer et al.
(1977)
phenocrysts to porphyritic,
numerous xenoliths
Cnydas
1080 ± 20
Reference
1155 (Grandiorite)
807 (Langklip granite)
1087
1085 ± (Rb/Sr)
180
157
Geringer et al.
(1977)
Jankowitz
(1986)
Barton &
Burger (1983)
I-type
1- and
S-types
numerous xenoliths·
Straussburg Coarse-grained, unequigranular, Sharp with met. aureole Meso- to
Kleinbegin
scattered phenocrysts
Medium to fine, non-porphyritic, Sharp, crosscutting
Epizone
Epizone
197
I-type
1080
1264 ± 604 (Rb/Sr)
174
Barton &
Burger (1983)
I-type
clots of mafic mineral aggregates
as 'units' within the Keimoes Suite batholith.
The composition of the various granites, their
characteristic features and radiometric ages are given in
Table 1. The variation in composition of the different
plutons of the Keimoes Suite batholith is also illustrated
in Figure 2.
A comparison of the compositional range of the
Keimoes Suite batholith with that of other batholiths,
i.e. those of the circum-Pacific margins (Figure 2), shows
that the Cordilleran batholiths of North- and South
America are characterized by a larger proportion of
quartz diorite and quartz monzonite (Cobbing &
Pitcher, 1983). The granitoids of the South Korean belt
(Kim & Lee, 1983) show a better compositional
resemblance with that of the Keimoes Suite
(granodiorite to alkali-granite).
The Keimoes Suite seems to be bimodal, consisting of
gabbroids and granitoids, whereas the intermediate
assemblages, i.e. diorite, quartz diorite, and tonalite are
seemingly absent.
There are, however, indications that small volumes of
ton ali tic melts existed during the early stages of the
emplacement of the batholith. Enclaves of tonalitic
composition occur in both the Colston and Straussburg
granites (Geringer et al., 1987). A comparison between
the normalized REE abundances of the granites and
their enclaves indicates that the enclaves can be
interpreted as early crystallized products of ton ali tic melt
fractions which formed at lower water-pressure
conditions than the accompanying host granites. The
enclaves, therefore, represent fragments of early formed
tonalite which were fragmented and assimilated by the
younger granite magmas (Geringer et al., 1987). If this
interpretation is correct, then tonalites indeed form part
of the magmatic sequence of the Keimoes Suite
batholith.
When the potassium index (PI) for the various units is
calculated the average values show an overall increase
from east to west. The porphyritic granites (Bakrivier,
Gifberg, Klip Koppies, and Gemsbokbult granites)
along the western margin of the batholith possess values
S.-Afr.Tydskr.Geo1.1988,91 (4)
468
Q
• Granodiorite (Cnydas-east)
o
Orbicular granite (Cnydas-west)
o
Frlersdale charnockltic adamellite
+
Gemsbokbult granite
• Strl\ussburg granite
A Bakrivler granite
X Vaalputs adamellite
• Kanonelland granite
o Klipkopples granite
U Klelnbegln granite
* Gouskop
t:,
granite
Gilberg granite
.. Colston granite
*
~
Enclaves in granites
Lima segment, Coastal Batholith
Metagabbro field
Gyongsang Basin, South Korea
50
Karamea Suite, New Zealand
Sierra Nevada Batholith
Alaska Range
Chilean graites
A
10
30
3b
4
50
70
90
p
Figure 2 A QAP-diagram showing the composition of the various granitoids of the Keimoes Suite batholith. The granitoids plot
predominantly inside the granite (3a), adamellite (3b), and granodiorite (4) fields.
10
8
6
4
::::::::::::::::
2
.
.:~ llllllllllllllllll'lllllllll ~ i:.
,;ji~!~~~~~~~~"
0.7
0.5
® Kleinbegin
.. I-type
• A-type
• S-type
0.3
CIJ Field of Keimoes suite granitoids
0.1~--------~--------~--------~----------~
________~________~________~________~~________L -__
FeO
MgO
CaO
Figure 3 Normalized major element concentrations of the various granitoids of the Keimoes Suite batholith compared with values
of 1-, S- and A-type granites (Standard quartz monzonite from east central Sierra Nevada; Carmichael et al., 1974, after Bateman,
1963).
higher than 210. The granites in the central region of the
batholith have values between 180 and 210, whereas the
values for the Kleinbegin-type granites along the eastern
boundary fall between 170 and 190. The Vaal puts
469
S.Afr.J. Geol., 1988,91 (4)
Vertical displacement or tilting of crustal blocks
between shear zones, as recognized by Stowe (1983),
exposed deep crustal levels in the west, whereas the
eastern part of the batholith is represented by high-level
intrusions.
granite in the central part of the batholith has values in
the order of 157, which are considerably lower than the
expected values for that part of the batholith (Table 1).
The higher PI values along the western margin probably
suggest greater crustal involvement, an aspect which will
be dealt with in the model, or may reflect potassium
metasomatism during the high-grade metamorphic event
towards the west.
The variation in PI values is synchronous with a
textural change in the granites, which may reflect their
tectono-stratigraphic position. In this sense the
porphyritic granites in the west (undifferentiated/
granites) resemble katazonal intrusions as suggested by
Geringer (1973) for the Bakrivier-type granites which
intruded high-grade metamorphites. The granites in the
central part of the batholith are predominantly mesozonal, whereas the granites along the eastern margin
(Klein begin-type ) are characteristic of epizonal
intrusions and are emplaced into gneisses of medium to
lower amphibolite grade.
I-type
S-type
Chemical characteristics
The major element concentrations of the various
granites of the Keimoes Suite are presented
diagrammatically on a logarithmically normalized oxide
concentration diagram (Figure 3). Oxide-element
concentrations have been normalized against a standard
of quartz monzonite from the east central Sierra Nevada
(Carmichael et al., 1974, after Bateman, 1963).
Compared to the standard quartz monzonite
composition, most granitoids of the Keimoes Suite are
enriched in Ti0 2, Fe203, FeO, MgO, and P 20 S , whereas
Si0 2, A1 20 3, and CaO, correspond with the standard
values while Na20 and K 20 are slightly depleted. The
I-type
1j1
1.1
I-type
S-type
I
I
I
1.1
S-type
I
Vaalputs
I
I Gifberg
Klipkoppies
I
Eksteenskuil
Stukkendedam
I
I
Strauss burg
I Kanoneiland
Friersdale
I
I
I
Bakrivier
granite
0.8
0.9
1.0
1.1
1.2
0.8
1.3
0.9
1.0
1.1
1.2
1.3
0.8
0.9
1.0
1.1
Molar [A120 3/(CaO+ Na 20 + K 20>J
I-type
1.1 S-type
I
.
'.'
0.7
0.8
0.9
1.0
1.1
1.2
•
1.3
~ 1.5.
1.4
Molar [AI20a/(CaO +Na 20 +K20>]
(Total batholith>
Figure 4 Histograms, based on molar AI203/(CaO+Na20+K20) values, showing the predominantly I-type character of the
individual plutons of the Keimoes Suite as well as for the batholith as a whole (Subdivision after Chappell & White, 1974).
S.-Afr.Tydskr.Geol.1988,91(4)
470
Klip Koppies and Kleinbegin granites have depleted
MgO values compared to the other granites of the
batholith. Average chemical compositions of A-, S- and
I-type granites from southeast Australia (White &
Chappell, 1983), normalized against the same standard,
are also plotted on the same diagram. A correlation
between granitoids of the Keimoes Suite and Australian
S- and I-type granites can be observed. A-type granites
show low concentrations of both MgO and CaO and
elevated Na20 values. In this respect the Klip Koppies
and Kleinbegin granites appear to resemble A-type
assemblages.
In order to test for S- and I-type characteristics in
granitoids of the Keimoes Suite, Molar AI203/(CaO +
Na20 + K20) values were plotted onto histograms (Fig.
4). Since I-type granites have Molar (AI203/(CaO +
Na20 + K20) values smaller than 1,1 and S-type
granites greater than 1,1 (Chappell & White, 1974) the
data indicate that the Keimoes Suite granitoids are
predominantly I-type although the Colston and Vaal puts
granites show tendencies towards S-type classification.
When (Na20 + K 20) and CaO are plotted against
Si0 2, the two curves intersect at 61 % Si0 2. According to
the classification of Peacock (1931) the Keimoes Suite
granitoids fall inside the calc-alkaline range. This is in
agreement with the findings of lankowitz (1986) for
granites of the Cnydas Complex.
The calc-alkaline character of the Keimoes Suite and
the resemblance with orogenic granites are also
illustrated by a plot of (An/An + Or) -x 100 against
normative quartz (Q)(Figure 5).
An AFM diagram (Figure 6), also reveals a distinct
calc-alkaline trend for the Keimoes Suite granitoids. On
the same diagram the composition of the gabbroic
intrusions, associated with the granitoids, are also
shown. A comparison with the calc-alkaline granites of
Papua, New Guinea (after Griffin, 1983), illustrates that
the Keimoes Suite granitoids chemically resemble calcalkaline granites of orogenic batholiths of the circum
Pacific terranes.
Normalized trace element values (Figure 7) show that
the Keimoes Suite granites correspond well with S-, 1-,
and A-type granites. Although the normalized major
element plot (Figure 3) reveals a closer resemblance with
S- and I-types than with A-types, the normalized trace
elements show a resemblance with A-type.
The normalized curves correspond with volcanic arc
related granites from Chile (Pearce et al., 1984).
According to Pearce et al. (1984) volcanic arc granites
have highly enriched Rb, Th with slightly lower Ba
values and a significant drop in Nb values, whereas Ce
and Sm are enriched relative to their adjacent elements.
It can be seen that the patterns of the Keimoes Suite
granites (Figure 7) fit this relationship almost perfectly.
The depletion in Y and Yb, characteristic of arc-related
granites, is also present in the Keimoes Suite rocks.
Pearce et al. (1984) did, however, mention that some
within-plate granites may obtain similar trace element
abundances (Figure 7). It is therefore not possible to
determine the tectonic environment from trace element
abundances unequivocally.
REE characteristics
There is a striking similarity in the chondrite-normalized
REE distribution patterns of the various granites of the
Keimoes Suite batholith (Figure 8). They are
characteristically LREE enriched with rather flat HREE
curves. The enriched LREE pattern is marked by high
La/Sm ratio (La/Sm = 3,1) whereas the HREE slope
(Gd/Lu = 1,8) is rather flat. All the granites are marked
by distinctly negative Eu anomalies and values for Eu/
Eu* are in the order of 0,5.
When compared with chondrite-normalized values of
the Coastal Batholith, it can be seen that the general
trends are similar although the concentration of LREE
in the Keimoes Suite batholith is considerably higher,
with La in the range 180 - 420, and Sm between 60 and
130. La in the Coastal Batholith is in the order of 100.
The Lu values in the Keimoes Suite batholith are
60
50r-~~------'\----------~~---------.----______~
• Granodiorite (Cnydaa-east)
.. Colston granite
40
• Strauaaburg granite
x Vaalputa adamallite
Q
• K anonelland granite
L!. Glfberg granite
30
o Kllpkopple. granite
* Gouakop granite
+Gemabokbult granite
o Frleradale
2°t------t~--~--~~--~----------~----~~~--------~r'
\
charnockltlc
adameUlte
\ Field of Cordilleran granites
-,J(after Bowden et al 1984)
10
90
~------~------L-----
ANOR= _A_"_ _ x100
A"+Or
1
____L -_ _ _ _ _ _ _ _- L________~____~
Fi~ure 5 ~ p.lot of Anor (An/(An + Or) x 100) versus Q (normative quartz) revealing the calc-alkaline character of the Keimoes
SUIte granItOIds.
471
S.Afr.J . Geol. ,1988,91 (4)
FeO·
=
FeO total
$ Strauss burg granodiorite/granite
l<
o
Vaalputs granite/Eksteenskuil granite
Friersdale charnockite
• K anon eiland granite
+ Gemsboksbult
granite
... Colston granite
V Stukkende dam granite
*' Gouskop
granite
o Klipkoppies granite
C:,.
Gifberg granite
II: Kleinbegin granite
17
U
A
.•:.::....::
Field occupied by the Keimoes
Suite granitoids
Field occupied by the
intrusions
gabbro
/"'/ Field occupied by the Cnydas complex
(Jankowitz. 1986)
(~/
J(
/
I
I
/
/
/
70
/
/
./
/
Calc-alkaline
/
////
,/
90
./
\. ...... .-//"
A
10
50'
30
70
90
M
Figure 6 AFM diagram showing the general trend of the Keimoes Suite granitoids and metagabbros.
70
50
30
10
~
7
0::
~ 5
..¥:
o
o
0::3
1.0T-------------------------~------~~~~~~~~--~
.5
Rb
Ba
Th
Nb
Ce
Zr
Sm
Y
Yb
Figure 7 Normalized trace element concentrations of the Keimoes Suite granitoids compared to S-, 1-, and A-type granites
(Standard Ocean Ridge Granite after Pearce et ai., 1984). The inset diagram shows the trace element distribution for volcanic arc
and within-plate granites.
S.-Afr. Tydskr.Geol.1988,91 (4)
472
likewise higher (between 26 and 40), whereas Lu values
for the Coastal Batholith plot around 10 - 15. The Eu
anomaly is fairly similar to that of the Coastal Batholith
with corresponding Eu/Eu* values (Eu/Eu* = 0,5)
(Pitcher, 1985). The REE concentrations of the Keimoes
Suite batholith also plot within the field for
monzogranites and syenogranites with moderate
negative Eu anomalies (Henderson, 1984)(Figure 8).
These relationships stand in sharp contrast to the REE
curves of typical anorogenic granites, i.e. the Madeira
pluton of northwest Brazil (Macambira et al., 1987)
which are characterized by enriched, downward concave
REE curves and extremely large negative Eu anomalies
(normalized against C 1-chondrite -- Evensen et al.,
1978). The highly enriched HREE (Lu in the range 80200) and LREE concentrations (La in the order of 600 1000 for the Madeira pluton) highlight the difference
between anorogenic granites and the Keimoes Suite
batholith.
Magmatic relationships and tectonic setting
Granites of specific chemical composition mark certain
tectonic domains (Hall, 1987). It is generally accepted
that calc-alkaline magmatism is characteristic of
destructive
plate
margin
environments.
The
predominant calc-alkaline character of the Keimoes
Suite granitoids thus suggests that they belong to such an
environment. It is, however, important to establish
whether a volcanic arc, active continental margin, or
continent-continent collision were involved since calcalkaline volcanism and magmatism mark all three these
tectonic regimes.
A comparison between the composition of the
Keimoes Suite granitoids and batholiths from destructive
margins (Figure 5), shows that the granitoids are high in
Si0 2 , and fall in the range between granodiorite and
alkali-granite. This composition is in agreement with
granitoids associated with thick continental crust, i.e. the
Coastal Range batholith in Peru, (Bowden et al., 1984),
the Lachlan Fold Belt of southeast Australia (White &
Chappell, 1983) and the Indonesian granites of southeast
Asia (Hutchison, 1983).
The calc-alkaline trend on the AFM diagram (Figure
6) is in agreement with that of the orogenic granites
(McCourt, 1981), whereas the plot of ANOR against
quartz (0) (Figure 5) also demonstrates this
relationship. The relationship of the Keimoes Suite
granites with destructive plate margin-type granites is
further corroborated by trace element ratios (Figure 7).
Pearce et al. (1984) described methods based on trace
element
concentrations
whereby
the
tectonic
500
• Kanoneiland granite
300
® Friersdale charnockite
Q)
• Louisvale granite
c
.&
.t:
e Straussburg granite
..II:
+ Gemsbokbult granite
:e"0
o
o.....
o
o
Colston granite
x Vaalputs granite
a:
100
80
60
40
20
Coastal Batholith Peru
5+---r---r------.------~--~--O-------~------r_--~~~-La
Ce
Nd
8m
Eu
Gd
Dy
Er
Vb
Lu
Figure 8 Normalized REE distribution curves for the various granitoids of the Keimoes Suite batholith compared to REE values
of the Coastal batholith, within-plate granites of Brazil, and normal monzogranites (Normalized against C 1-chondrite _ Evenson
et at., 1978).
473
S.Afr.J .Geol. ,1988,91 (4)
- 20L---------------------~U------------A
WPG 10RG(b)
E 40
Q.
Q.
.Q
Z
•••
• •
o
·l
~0 0
o
.u
u ·18
e
0. .
8.
U
U8
0
0
0
0
0
0
VAG/COLG/ORG
0
8 P3
•
18
8i0 2 (wt.%)
WPG and
90
.s
E
Q.
B
.: ...
70
50
ORG ab and c)
o
o
>
30
U
o
0
VAG and COLG(ORGd)
56
58
60
62
64
66
68
70
8i02 (wt.%)
74
72
76
78
80
Figure 9 Trace element ratio plots Nb vs Si0 2 (9A) and Y vs Si0 2 (9B) showing the relationship with volcanic arc and within-plate
granites. Collision granites (COLG), within-plate granites (WPG), oceanic-ridge granites (ORG), volcanic-arc granites (VAG).
Symbols for the various granites are given in Figure 10.
environment for certain granite types can be deduced. A
plot of Nb vs. Si0 2 (Figure 9A) indicates that the
Keimoes Suite granites plot in the field of volcanic arctype, collision-type or ocean ridge-type granites. When
Y is plotted against Si0 2 (Figure 9B) or Rb against Y +
Nb (Figure 10) the Keimoes Suite granites plot inside the
within-plate-type granite field.
Considering the resemblance between the trace
element concentrations of the Keimoes Suite granites
and arc-related granitoids of Chile as well as crustaldominant within-plate-type granites (Figure 7), it can be
deduced that crustal material played a significant role
during the origin and emplacement of the Keimoes
Suite.
Barton & Burger (1983) reported Rb-Sr initial ratios of
0,700 and 0,707 for the Straussburg granite which
correspond with the crust-dominant environment into
which the Keimoes Suite granites were emplaced. They
suggested that the granites were emplaced in a thickened
continental crust.
Normalized REE curves for the granites of the
Keimoes Suite batholith, show distinctly elevated LREE
values compared to that of the Peruvian Coastal
Batholith (Atherton & Sanderson, 1985). The granites
are, however, highly depleted in HREE compared to
anorogenic granites of the Madeira pluton of northwest
Brazil (Figure 8). It can also be seen that the Keimoes
Suite granites plot inside the field of orogenic monzoand syenogranites (Henderson, 1984).
u Eksteenskuil granite
800
• Straussburg granite
600
• Kanonelland granite
.. Smalvisch granite
00
400
o Orbicular granite
COLG
... Colston granite
o Charnocklte
• Granodiorite
200
WPG
E
Q.
.e
.Il
II:
90
VAG
70
50
30
ORG
10+-----.---.-.-~~~._----._~--~~~
10
30
50
70 90
Y
200
Nb (ppm)
400
600
Figure 10 A plot of Rb versus Y + Nb shows within-plate
affinities of the Keimoes Suite granitoids. Collision granites
(COLG), within-plate granites (WPG), oceanic-ridge granites
(ORG), and volcanic-arc granites (VAG).
The enriched patterns of the Keimoes Suite granites,
compared to that of the Coastal Batholith, also indicate
that crustal material could have played a dominant role
during their generation and/or emplacement. LREE
enrichment may also have originated from early
S.-Afr.Tydskr.Geol.1988,91(4)
474
plagioclase fractionation as indicated by the negative Eu
anomalies.
Initial Rb-Sr ratios of 0,700 and 0,707 for the
Straussburg granite (Barton & Burger, 1983) correspond
with the idea that the Keimoes Suite magmas were
generated and emplaced in thick crust environment.
Discussion and Conclusion
The late- to post-tectonic calc-alkaline granites of the
Keimoes Suite, which form part of a composite batholith
along the eastern margin of the Namaqua metamorphic
belt, are related to the Namaqua tectogenesis. The
batholith intruded during the closing stages of the
folding event between 1250 and 1100 Ma, but prior to
the transcurrent shear movements in the area. The
emplacement of the granites followed the extrusion of
calc-alkaline to high- K basalt (shoshonite) in the
Upington area and calc-alkaline to low-K basalt and
andesite in the Kleinbegin/Boksputs region, which also
form part of a calc-alkaline volcanic arc (Geringer et al.,
1986) known as the Areachap Group. These rocks are
developed along the eastern margin of the mobile belt,
and probably formed between 1600 and 1400 Ma
(Theart, 1985; Cilliers, 1987).
The chemical characteristics of the Keimoes Suite
granitoids resemble that of destructive margin-type
granites with a tendency towards within-plate granites.
The majority of the Keimoes Suite granites reveal I-type
properties while a few show S-type affinities. The
normalized trace elements show a close resemblance
with I-, S- and A-type granites of the Lachlan Fold Belt
of Australia (White & Chappell, 1983).
Normalized REE patterns indicate that the Keimoes
v v v vv v v v v v v v v vv v v v
vvv vvvvv vv,vvvvvvvvv
16001400
Suite granites are highly enriched in LREE compared to
that of the Coastal Batholith; that they show LREE
values similar to an orogenic granites but are highly
depleted in HREE compared to anorogenic granites of
Brazil; that they plot inside the field of orogenic granites
with moderate negative Eu anomalies (Henderson,
1984).
The REE patterns as well as the high Rb-Sr initial
ratios reported for the Straussburg granite by Barton &
Burger (1983) corroborate the idea that the Keimoes
Suite granites intruded in a crust-dominated
environment. In order to propose a model for the
Keimoes Suite granites, all the above-mentioned aspects
should be considered. Based on the isotopic evidence,
Barton & Burger (1983) proposed a continent collision
model for the eastern Namaqua domain while Stowe
(1983) favoured a Cordilleran tectonic model for this
region.
In order to accomodate and explain the intimate
relationship between the development of the Areachap
volcanic arc and the orogenic to within-plate granitoids
of the Keimoes Suite during a single orogenic event,
(Namaqua tectogenesis) a model which includes both
subduction of oceanic plate followed by underriding
(underplating) of continental plate, with subsequent
crustal thickening, is proposed for the eastern margin of
the Namaqua mobile belt. This model, schematically
illustrated in Figure 11, is based on the model proposed
by Hutchison (1983) to explain the origin and tectonic
setting of the granitoids of the Indonesian islands and the
Malay Peninsula of southeast Asia.
By analogy with this model the emplacement of the
Keimoes Suite batholith was preceded by subduction of
VV \lVVV
~ ~ ~ ~ ~ N~ ~ ~ri,~~ if~ ~ ~~J~(~~
"vvvV',
vv
v v v vv v v v v v v v v vv v v v v ~ ~ ~ ~ ~~
ower Crust
/
'0'///-.
Ma. ago
Mantle
.////./
Korannaland Sequence
I-type granites
Late tectonic
~s.yntectonic
12001000
Ma. ago
Figur~ ~ 1 Schemati~ 'choke~' subdu~tio~ model. with continental collision and subsequent crustal thickening by means of which
the OrIgIn of the Kelmoes SUIte granItes IS explaIned.
S.Afr.J. Geol., 1988,91 (4)
a small oceanic plate which resulted in the formation of a
calc-alkaline volcanic arc, the Areachap Group
(Geringer et al., 1986) along the eastern margin of the
belt between 1600 and 1400 Ma. (Theart, 1985; Cilliers,
1987).
Subsequent compression resulted in closure or
choking of the ocean and underriding of the Kaapvaal
Craton by the Namaqua Plate (Figure 11). This event
caused considerable crustal thickening and the
generation of plagioclase-rich magmas, which gave rise
to anorthosite complexes such as the Platsjambok
Complex (Geringer et al., 1985), and granitoid magmas
which intruded the gneisses of the Korannaland
Sequence on the down-moving plate, similar to the
model suggested by Hutchison (1983) for the granitoids
of southeast Asia. This event took place around 1100 1200 Ma ago.
This model offers a satisfactory explanation for the
chemical characteristics of the granites. It also provides
an explanation why the PI values of the granites increase
towards the west as the effect of the down-moving plate
becomes more dominant towards the west. It further
explains the intimate relationship between the granites
and the Areachap volcanic arc along the eastern margin
of the metamorphic belt.
Acknowledgements
The authors express their sincere gratitude to the
Central Research Fund of the University of the OFS and
the Geological Survey for financial support, Dr. W.A.
van der Westhuizen of the Department of
Geochemistry, University of the OFS for chemical
analyses, Mrs. l.H.C. du Bois, who prepared the
diagrams, and Mrs. M.l. van den Heever who typed the
manuscript.
References
Atherton, M.P. & Sanderson, L.M. (1985). The chemical
variation and evolution of the super-units of the segmented
Coastal Batholith. In: Pitcher et al., Eds. Magmatism at a
Plate Edge, The Peruvian Andes. John Wiley and Sons Inc.,
New York, 208-227.
Barton, E.S. & Burger, A.J. (1983). Reconnaissance isotopic
investigations in the Namaqua mobile belt and implications
for Proterozoic crustal evolution - Upington geotraverse.
In: Botha, B.J.V. Ed., Namaqualand Metamorphic
Complex. Spec. Publ. geol. Soc. S.Afr., 10, 173-192.
Blignault, H.J. & Geringer, G.J. (1980). The Keimoes Suite.
In: L.E. Kent (Compiler), Stratigraphy of South Africa,
Handbk. geol. Surv. S.Afr., 8, 307-310.
----, Van Aswegen, G., Van der Merwe, S.W. & Colliston,
W.P. (1983). The Namaqualand geotraverse and environs.
Part of the Proterozoic Namaqua mobile belt. In: Botha,
B.J.V. Ed., Namaqualand Metamorphic Complex. Spec.
Publ. geol. Soc. S.Afr., 10, 1-29.
Bowden, P., Batchelor, R.A., Chappell, B.W., Didier, J. &
Lameyre, J. (1984). Petrological, geochemical and source
criteria for the classification of granitic rocks: a discussion.
Phys. Earth Planet. Int., 35, 1-11.
475
Chappell, B.W. & White, A.S.R. (1974). Two contrasting
granite types. Pacific Geol., 8, 173-174.
Carmichael, I.S.E., Turner, F.J. & Verhoogen, J. (1974).
Igneous Petrology. McGraw-Hill, New York, 739 pp.
Cilliers, F.H. (1987). Isotope characteristics of the
sulphide-bearing sequence of the Areachap Group in the
Boksputs area, north-west Cape. M.Sc. thesis (unpubl.),
Univ. Orange Free State, Bloemfontein, 171 pp.
Cobbing, E.J. & Pitcher, W.S. (1983). Andean plutonism in
Peru and its relationship to volcanism and metallogenesis at
a segmented plate edge. In: Roddick, J.A. Ed.,
Circum-Pacific Plutonic Terranes, Mem. geol. Soc. Amer.,
159, 277-291.
Evensen, N.M., Hamilton, P.J. & O'Nions, R.K. (1978). Rare
earth abundances in chondritic meteorites. Geochim.
Cosmochim. Acta, 42, 1199-1212.
Geringer, G.J. (1973). Die geologie van die Argeiese gesteentes
en jongere formasies in die gebied wes van Upington met
spesiale verwysing na die verskillende granietvoorkomste.
D.Sc. thesis (unpubl.), Univ. Orange Free State,
Bloemfontein, 203 pp.
---- & Botha, B.J.V. (1977). Anatektiese graniete in die
mobiele gordel Namakwaland, wes van Upington. Bull.
geol. Surv. S. Afr., 61, 36pp.
----, ----, Strydom, D. & Potgieter, G.J.A. (1985). An
anorthosite-mangerite suite, indicative of crustal
thickening, along the eastern margin of the Namaqua
mobile belt, South Africa. Precambrian Res., 27, 321-335.
----, ----, Pretorius, J.J. & Ludick, D.J. (1986). Calc-alkaline
volcanism along the eastern margin of the N am aqua mobile
belt, South Africa - a possible middle Proterozoic volcanic
arc. Precambrian Res., 33, 139-170.
----, De Bruiyn, H., Schoch, A.E., Botha, B.J.V. & Van der
Westhuizen, W.A. (1987). The geochemistry and
petrogenetic relationships of two granites and their
inclusions in the Keimoes Suite of the Namaqua mobile
belt, South Africa. Precambrian Res., 36, 143-162.
Griffin, T.J. (1983). Granitoids of the Tertiary continent island arc collision zone, Papua New Guinea. In:
Circum-Pacific Plutonic Terranes. Mem. geol. Soc. Amer.,
159, 61-77.
Hall, A. (1987). Igneous Petrology. Longman Scientific and
Technical, John Wiley & Sons, Inc., New York, 572 pp.
Henderson, P. (1984). Rare Earth Element Geochemistry.
Elsevier, New York, 510 pp.
Hutchison, C.S. (1983). Multiple Mesozoic Sn-W-Sb
granitoids of southeast Asia. Mem. geol. Soc. Amer., 53,
35-59.
Jankowitz, J.A.C. (1986). A petrochemical investigation of the
Cnydas batholith, west of Upington. M.Sc. thesis (unpubl.),
Univ. Orange Free State, Bloemfontein, 205 pp.
Kim, O.J. & Lee, D.S. (1983). Summary of igneous activity in
South Korea. In: Circum-Pacific plutonic terranes. Mem.
geol. Soc. Amer., 159, 87-105.
Linstrom, W. (1977). Die geologie tussen Kenhardt en
Marydale met spesiale verwysing na die verband tussen die
Kheisgesteentes en die Namakwalandse mobiele gordel.
Ph.D. thesis (unpubl.), Univ. Orange Free State,
Bloemfontein, 251 pp.
476
Macambira, M.l.B., Teixeira, l.T., Walid el K.D. & Costi,
H.T. (1987). Geochemistry, mineralizations and age of
tin-bearing granites from Pitinga, Northwestern Brazil.
Ext. Abstr. Int. Symp. on Granites and Associated
Mineralization - Salvador, Brazil, 245-249.
Malherbe, S.l., Geringer, G.l., Parsons, C.F. & Blignault,
H.l. (1980). Korannaland Sequence. In: Kent, L.E.
(Compiler) Stratigraphy of South Africa. Handbk. geol.
Surv. S. Afr., 8, 261-267.
McCourt, W.l. (1981). The geochemistry and petrography of
the Coastal Batholith of Peru, Lima segment. Geol. Soc.
Lond., 138, 407--420.
Peacock, M.A. (1931). Classification of igneous rock series. J.
Geol., 39, 54-67.
Pearce, l.A., Harris, N.B.W. & Tindle, A.G. (1984). Trace
element discrimination diagrams for the tectonic
interpretation of granitic rocks. 1. Petrol., 25, 956--983.
Pitcher, W.S. (1985). A multiple and composite batholith. In:
Pitcher et al., Eds., Magmatism at a Plate Edge: the
Peruvian Andes. John Wiley & Son, New York, 93-107.
Praekelt, H.E., Botha, B.l.V. & Malherbe, S.l. (1986).
Discreet crustal fragments in the central part of the
Namaqua mobile belt in the Augrabies region. Ann. geol.
Surv. S.Afr., 20, 25--40.
S.-Afr. Tydskr.Geo\'1988,91 (4)
Slabbert, M.l. (1985). Die geologie in die omgewing van die
Melkboomkoepel, suid van Keimoes. M.Sc. thesis
(unpub\.), Univ. Orange Free State, Bloemfontein, 140 pp.
Stowe, C.W. (1983). The Upington geotraverse and its
implications for craton margin tectonics. In: Botha, B.l.V.,
Ed., Namaqualand Metamorphic Complex. Spec. Pub\.
geo\. Soc. S.Afr., 10, 147-171.
Theart, H.F.l. (1985). Copperton-Areachap Cu, Zn
mineralization. Ph.D. thesis (unpub\.), Univ. Stell en bosch ,
Stellenbosch. 329 pp.
Van Zyl, C.Z. (1981). Structural and metamorphic evolution
in the transitional zone between craton and mobile belt,
Upington geotraverse. Bull. Precamb. Res. Unit, Univ.
Cape Town, 31, 1-243.
Von Backstrom, l.W. (1964). The geology of an area around
Keimoes, Cape Province with special reference to
phacoliths of charnockitic adamellite-porphyry. Mem. geol.
Surv. S.Afr., 53, 206pp.
White, A.l. & Chappell, B.W. (1983). Granitoid types and
their distribution in the Lachlan Fold Belt, southeastern
Australia. In: Circum-Pacific Plutonic Terranes. Mem.
geo\. Soc. Amer., 159, 21-B5.