Download Geochemistry of intermediate to siliceous volcanic rocks of the

Contrib Mineral Petrol (2002) 144: 131–143
DOI 10.1007/s00410-002-0386-1
P.C. Buchanan Æ W.U. Reimold Æ C. Koeberl
F.J. Kruger
Geochemistry of intermediate to siliceous volcanic rocks
of the Rooiberg Group, Bushveld Magmatic Province, South Africa
Received: 3 December 2001 / Accepted: 4 June 2002 / Published online: 6 September 2002
Ó Springer-Verlag 2002
Abstract The volcanic Rooiberg Group represents the
earliest phase of Bushveld-related magmatism and
comprises, in some areas, the floor and roof rocks of the
mafic–ultramafic intrusive units of the Bushveld Complex. The lower to middle Dullstroom Formation is
composed of two interbedded series of low Ti and high
Ti volcanic strata, which are predominantly basaltic
andesites. Volcanic units above these strata range from
andesites to dacites in the upper Dullstroom Formation
and to predominantly rhyolites in the overlying Damwal
and Kwaggasnek Formations. Compositional data suggest that these intermediate to siliceous volcanic rocks
are petrogenetically related to the low Ti volcanic suite
and suggest that the low Ti magmas resided in a shallow
magma chamber where they experienced fractional
crystallization and assimilation of crustal material. In
contrast, the high Ti volcanic suite is petrogenetically
unrelated. These data confirm previous suggestions that
Bushveld-related magmas experienced significant
amounts of assimilation of continental crust.
P.C. Buchanan (&) Æ W.U. Reimold
Impact Cratering Research Group, School of Geosciences,
University of the Witwatersrand, Private Bag 3,
WITS 2050 Johannesburg, South Africa
E-mail: [email protected]
C. Koeberl
Institute of Geochemistry, University of Vienna,
Althanstrasse 14, 1090 Vienna, Austria
F.J. Kruger
Economic Geology Research Institute-Hugh Allsopp Laboratory,
School of Geosciences, University of the Witwatersrand,
Johannesburg, South Africa
Present address: P.C. Buchanan
Antarctic Meteorite Research Center,
National Institute of Polar Research,
1-9-10 Kaga Itabashi-ku, Tokyo 173-8515, Japan
Editorial responsibility: T.L. Grove
Introduction
The Rooiberg Group, which includes, in ascending
order, the Dullstroom, Damwal, Kwaggasnek, and
Schrikkloof Formations (Schweitzer et al. 1995), is
located in the eastern part of South Africa (Fig. 1) and
represents a voluminous (6-km thick) series of predominantly volcanic rocks with minor interbedded
sediments (Fig. 2). The original areal extent of this
sequence was large, as the present outcrop exposures
extend over parts of an area of 65,000 km2 (Tankard
et al. 1982; Schweitzer et al. 1995). In some areas,
these volcanic units make up the floor and roof rocks
of the Rustenburg Layered Suite (RLS), the mafic–
ultramafic phase of the intrusive Bushveld Complex.
The Lebowa Granite Suite, the granitic phase of the
Bushveld Complex, intrudes the Rooiberg Group in
other areas.
Most of the isotopic age dates acquired for samples of
the Rooiberg Group and the Bushveld Complex range
from 2061±2 to 2052±48 Ma (e.g.; Walraven et al.
1987, 1990; Walraven 1997; Harmer, personal communication 2000; Buick et al. 2001). The close spatial relationships of these intrusive and extrusive rocks and the
similarity of their crystallization ages suggest that they
were deposited during the same magmatic event. Irvine
(1982) suggested that all of these rocks should be included in the Bushveld Magmatic Province. The intrusive relationship of the Bushveld Complex with the
Rooiberg Group indicates that these volcanic rocks
represent the initiation and early development of Bushveld magmatic activity.
Three hypotheses have been proposed for the origin
of the Rooiberg Group. First, several authors (e.g.,
Rhodes 1975; Elston 1992) attributed these strata and
the Bushveld Complex to the simultaneous impact of
several comets or asteroids. However, Buchanan and
Reimold (1998) discounted this hypothesis. Second,
Hatton (1988) suggested that subduction-related processes associated with a nearby plate margin formed
132
Fig. 1. Generalized geologic
map of the southeastern portion
of the Bushveld Complex after
Council of Geosciences (1978).
The inserts at the bottom indicate the regional position of the
study area. Sampling areas for
rocks analyzed in this study are
noted here and in Buchanan
et al. (1999)
these units. Third, Harmer and von Gruenewaldt (1991),
Hatton (1995), and Hatton and Schweitzer (1995) attributed the Rooiberg Group and the Bushveld Complex
to partial melting of subcontinental lithosphere and
lower crust by a mantle plume.
Recently, Buchanan et al. (1999) studied the high Ti
and low Ti andesites and basaltic andesites of the
Dullstroom Formation and suggested that they represent magmas that were derived by partial melting of
compositionally distinct source areas and that they resided in at least two magma chambers in which magma
mixing, fractional crystallization, and assimilation of
crustal material occurred. The goal of this study is to
elucidate the petrogenesis of the overlying intermediate
to siliceous volcanic rocks. Bulk geochemical compositions should help to trace the development of these
early stages of Bushveld-related magmatism and to
determine whether these magmas were derived by
crustal melting or by differentiation and contamination
of the same melts represented by the more mafic
volcanic rocks of the lower to middle Dullstroom
Formation.
Samples and analytical techniques
Most of the samples analyzed in this study were collected in 1996–
1997, as described in Buchanan et al. (1999). Supplementary
samples representative of strata from the upper Dullstroom
Formation were collected in 2000 in the northeastern part of the
Bushveld Magmatic Province. Sampling areas are noted in Fig. 1
and in Buchanan et al. (1999). Preliminary compositional data for
these units suggested that Rooiberg Group strata in the western
and central parts of the province, close to outcrops of the Lebowa
Granite Suite, were pervasively altered. To avoid this alteration as
much as possible, most of the samples considered in this study
were obtained from the southeastern part of the province. This
proved possible for the Dullstroom, Damwal, and Kwaggasnek
Formations, but outcrops of the Schrikkloof Formation were
more difficult to find in this area and, hence, this unit was excluded from the present study. Samples were processed and analyzed using the same techniques described in Buchanan et al.
(1999).
133
Fig. 2. Generalized stratigraphic column of the Rooiberg Group as
proposed by Schweitzer et al. (1995). Adapted from Cheney and
Twist (1991), Council of Geosciences (1978), Eriksson et al. (1994),
and Schweitzer et al. (1995). Strata designated as white are volcanic
and pyroclastic rocks, those designated with horizontal lines are
sedimentary rocks, and those designated as solid black are
granophyric rocks. Scale, in meters, shows approximate thickness
of units
Results
Petrography
Buchanan et al. (1999) described the petrography and
mineral chemistry of the low Ti and high Ti rock suites
of the lower to middle Dullstroom Formation. Metavolcanic units of the upper Dullstroom Formation also
display both primary volcanic textures and metamorphic
textures. These rocks contain rare altered phenocrysts,
porphyroblasts, and zoned amygdules in a fine-grained
to very fine-grained groundmass, which is predominantly composed of subequal proportions of equant
quartz and lath-shaped feldspar with varying proportions of amphibole, biotite, and opaque minerals
(Fig. 3a). Groundmass feldspar is twinned and is predominantly plagioclase (An3 to An59) with rare grains of
alkali feldspar (Or99 to Or86). Phenocrysts apparently
were originally feldspar and commonly are sericitized.
Hornblende porphyroblasts in units of the upper
Dullstroom Formation are variably developed and
generally comprise anhedral to subhedral, lath-shaped,
and somewhat poikilitic crystals. In some cases, opaque
mineral grains are concentrated in the outer portions of
Fig. 3. Photomicrographs of the various units considered in this
study: a representative photomicrograph of a dacite from the upper
Dullstroom Formation, b photomicrograph of augen-shaped
bodies of amphibole within an individual biotite grain, c representative photomicrograph of a rhyolitic volcanic rock from the
Damwal Formation. All photomicrographs are the same scale and
the scale bar is 1 mm. All photomicrographs taken with
transmitted light
these porphyroblasts. Amphibole has an average abundance of TiO2 of 0.57 wt% and Mg# [100 Mg/
(Mg+Fe), atomic] ranges from 40 to 56. In general,
these amphiboles are more Fe-rich than those in the low
Ti and high Ti units of the Dullstroom Formation
(Buchanan et al. 1999). Minor proportions of prehnite,
pumpellyite, magnetite, chlorite, and possible apatite
also occur in these samples. Biotite is also present in a
few samples and, in some cases, augen-shaped inclusions
of green hornblende occur within individual grains of
biotite (Fig. 3b).
Zoned amygdules in samples from the upper Dullstroom Formation have quartz-lined outer edges and
134
cores that contain pleochroic, green/tan hornblende. In
rare cases, these amygdules also contain feldspar. The
matrices of a few of these volcanic rocks display concentrations of quartz in linear or curving concentrations
that apparently represent fractures filled with secondary
quartz. Where possible, these fractures were avoided
during sample preparation.
The metavolcanic rocks of the Damwal Formation
also display both primary volcanic textures and metamorphic textures. These strata occur above the Rustenburg Layered Suite (RLS) and range from rocks with
spherulitic textures to rocks that are somewhat granophyric in texture and grade into the Rashoop Granophyre. This study concentrates on the spherulitic units
(e.g., Fig. 3c) and those that have pyroclastic textures,
including units that appear to be lapilli tuffs. The
spherulitic units have a fine-grained groundmass predominantly composed of quartz and feldspar. These
feldspars commonly have albite-rich compositions (Ab72
to Ab83) with rare grains of K-feldspar (Or90 to Or95).
Rare euhedral to subhedral feldspar phenocrysts occur
and are commonly altered. In some samples, these
phenocrysts occur in clusters and these rocks have a
glomeroporphyritic texture.
Significant metamorphism of these units of the
Damwal Formation is indicated by the development of
hornblende porphyroblasts. These porphyroblasts range
in Mg# from 13 to 35 and abundances of TiO2 range
from 0.86 to 1.18 wt%. Rare amygdules are filled with
quartz. Some of these rocks contain quartz-filled fractures and some are layered with concentrations of quartz
in undulating zones that have diffuse boundaries and
may reflect primary pyroclastic textures.
Volcanic rocks of the Kwaggasnek Formation are
fine- to very fine-grained and are predominantly composed of quartz and feldspar with varying proportions
of opaque minerals, which include ilmenite and magnetite. These opaque minerals occur as discrete grains and
as intergrowths that may be the result of exsolution.
Textures are, to a greater or lesser degree, spherulitic,
with centers of spherules that are fine-grained, equigranular aggregates. Samples that are not spherulitic are
very fine-grained and are equigranular in texture. In
some of these samples, concentrations of quartz have
linear or elongated shapes and the units have a vague
layering, which may also reflect primary pyroclastic
textures. Feldspars are twinned and are either predominantly albite (e.g., Ab97) or K-feldspar (e.g., Or98).
Rare, sericitized, euhedral to rounded feldspar phenocrysts also occur in these units and may be partially
resorbed. In some cases, these phenocrysts occur in
clusters. Rounded quartz grains are also present, but are
rare.
Chemical compositions
Geochemical compositions of these volcanic rocks are
contained in Tables 1, 2, and 3 and suggest that the
upper Dullstroom Formation and the Damwal and
Kwaggasnek Formations in the southeastern part of the
Bushveld Magmatic Province suffered minimal hydrothermal alteration. Loss on ignition (LOI) values are
generally close to 1 wt% and Na/K values are low and
relatively constant. Specimens appear relatively fresh
with only rare, minor iron oxide staining. Abundances
of the element Ba, which is commonly considered to be
mobile during hydrothermal alteration, increase with
increasing SiO2 content, which suggests differentiation
rather than significant degrees of post-crystallization
remobilization. Data for Rooiberg Group strata in other
parts of the province suggest that hydrothermal alteration was more significant at locations near present-day
outcrops of the Lebowa Granite Suite and, hence, confirm the suggestion by Schweitzer and Hatton (1995)
that significant alteration was associated with granite
intrusion at 2,053.4±3.9 to 2,057.5±4.2 Ma (Harmer
personal communication 2000).
As suggested by previous authors (e.g., Schweitzer
et al. 1995), there is a general increase in abundance of
SiO2 moving upward in the Rooiberg Group succession
(Fig. 4). Harmer and von Gruenewaldt (1991), Hatton
and Schweitzer (1995), Schweitzer et al. (1995), and
Buchanan et al. (1999) determined that most of the
volcanic rocks of the lower to middle Dullstroom Formation, which range from basalts to andesites, but are
predominantly basaltic andesites, fall into two geochemical groups: high Ti units (TiO2>1.0 wt%;
SiO2<60 wt%) and low Ti units (TiO2<1.0 wt%;
SiO2<60 wt%; Fig. 5a). Rare dacite volcanic units are
interbedded with these basaltic andesites in the middle
Dullstroom Formation (Schweitzer et al. 1995; Buchanan et al. 1999). These dacites become more common
moving upward into the upper Dullstroom Formation
and basaltic andesites diminish in number and volume.
Volcanic strata in the overlying Damwal Formation
include both rhyolites and dacites and units of the
Kwaggasnek Formation are predominantly rhyolites
(Fig. 4).
Twist (1985) originally divided the intermediate to
siliceous units of the upper Dullstroom Formation and
the Damwal Formation into two groups: high Mg felsites (MgO >1.7 wt%) and low Mg felsites (MgO
<1.0 wt%). High Mg felsites predominate in the upper
Dullstroom Formation and low Mg felsites predominate
in the Damwal Formation. Detailed sampling in the
study area and laboratory analyses of these samples
suggest that these two groups of volcanic rocks may
have originally represented a spectrum of related compositions. In many variation diagrams (Fig. 5a–f), these
rocks form clusters along trends that also include samples of the low Ti suite and samples from the Kwaggasnek Formation. This division into two groups based
on abundances of MgO is complicated by the fact that
the intrusive Rustenburg Layered Suite may have assimilated significant portions of wallrock with intermediate abundances of MgO from the central portion of
the Rooiberg Group succession (e.g., Fig. 2). Hence,
135
Table 1. Compositions determined by XRF and INAA for dacites of the upper Dullstroom Formation. V, Ni, Cu, Y, Nb, and most
major elements determined by XRF. Na, Fe, Cr, Zn, Sr, Zr, and Ba determined by a combination of both XRF and INAA. The remainder
of the trace elements determined by INAA. n.d. Not determined
Sample
wt%
SiO2
TiO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K2 O
P2 O5
LOI
Total
ppm
Sc
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
Th
U
a
hs1
hs2
hs5
hs7
hs8
66.0
0.65
13.4
6.87
0.14
2.24
4.66
2.77
2.50
0.14
1.12
100.5
64.8
0.66
13.5
7.35
0.15
2.51
4.69
3.34
2.16
0.15
1.08
100.4
65.5
0.62
13.3
6.95
0.13
2.35
4.83
3.30
2.28
0.13
1.08
100.5
68.1
0.60
12.7
5.96
0.11
1.29
3.57
3.28
2.62
0.13
0.81
99.2
67.2
0.59
12.9
5.86
0.14
1.33
3.74
3.21
3.01
0.12
0.94
99.0
16.3
109
132
19.0
23
11
100
96
284
31
219
12
1.87
630.
43.5
80.6
36.2
5.66
1.27
4.26
0.81
0.47
2.63
0.42
5.15
0.75
12.8
2.51
18.8
121
126
25.0
34
8
83
74
272
32
213
12
1.79
722.
37.4
73.7
34.7
5.59
1.35
4.75
0.75
0.43
2.61
0.42
5.07
0.63
11.1
2.45
17.6
109
102
21.0
34
14
98
85
286
31
213
12
1.83
694.
42.7
78.4
37.6
6.04
1.37
5.59
0.83
0.46
2.91
0.41
5.17
0.66
12.4
2.51
13.1
88
64
16.0
18
16
74
102
272
31
232
11
1.35
798.
39.7
75.3
35.3
5.47
1.24
4.66
0.76
0.35
2.42
0.38
5.19
0.63
11.8
2.56
12.7
83
55
13.0
15
18
98
118
296
38
217
12
1.79
831.
42.2
76.7
37.9
6.45
1.32
5.31
0.85
0.45
1.85
0.43
5.29
0.67
12.1
2.34
hs10
69.2
0.52
12.3
5.07
0.04
1.26
3.78
2.12
3.29
0.10
1.19
98.9
n.d.
87
226
21.0
17
9
57
135
239
28
202
11
n.d.
880.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
hs16
hs17
hs18
hs20
hs21
Avg.
65.9
0.67
13.5
7.31
0.07
1.87
4.47
3.16
2.48
0.14
0.83
100.4
65.3
0.66
13.7
7.05
0.07
2.28
4.43
2.88
2.58
0.14
1.10
100.2
65.7
0.66
13.4
7.03
0.08
2.17
4.73
3.35
2.19
0.14
0.99
100.4
65.2
0.64
13.5
7.06
0.17
2.19
4.46
3.33
2.68
0.14
1.13
100.5
65.9
0.66
13.3
7.09
0.19
1.94
4.66
3.11
2.46
0.14
0.97
100.4
66.3
0.63
13.2
6.69
0.12
1.95
4.37
3.08
2.57
0.13
1.02
100.1
16.9
113
155
20.0
27
9
81
92
288
30
222
11
1.74
752.
41.9
76.2
35.4
5.64
1.34
4.99
0.76
0.41
2.63
0.39
4.97
0.55
11.9
2.03
16.3
116
143
21.0
29
15
92
95
277
32
221
12
2.01
713.
38.5
75.1
30.7
5.48
1.29
4.65
0.76
0.44
2.41
0.38
4.82
0.57
11.5
1.76
15.7
113
156
21.0
26
4
88
77
271
32
216
11
1.47
620.
37.9
72.7
32.4
5.54
1.48
5.06
0.77
0.43
2.64
0.41
5.06
0.55
11.2
2.14
n.d.
118
151
24.0
29
22
85
99
261
32
221
12
n.d.
711.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
16.8
117
142
20.0
21
11
97
86
273
31
220
12
1.62
657.
41.7
76.8
36.5
5.93
1.29
5.05
0.71
0.41
2.64
0.42
5.17
0.59
12.3
2.23
16.0
107
132
20.1
25
13
87
96
274
32
218
12
1.72
728.
40.6
76.2
35.2
5.76
1.33
4.92
0.78
0.43
2.64
0.41
5.10
0.62
11.9
2.28
All iron reported as Fe2O3
because it is not clear that this division into high Mg
felsites and low Mg felsites is a function of original
petrogenetic processes, in this study, these volcanic units
are classified by the formations in which they occur,
rather than by abundances of MgO.
Compositional data for the volcanic units of the
Dullstroom, Damwal, and Kwaggasnek Formations
show consistent variations from the basaltic andesites of
the Dullstroom Formation to the rhyolitic rocks of the
Kwaggasnek Formation (Fig. 5a–f). Samples of volcanic
units from the Mg-rich extreme of this spectrum commonly are relatively enriched in Al2O3, CaO, and Sc. In
contrast, samples of volcanic units from the Mg-poor
extreme of the spectrum are relatively enriched in K2O
and the rare earth elements (e.g., Sm, Fig. 5f). Increasing
abundances of some elements correlate with those of the
incompatible element Zr and other elements display a
reverse correlation. For example, abundances of Sr, Sc,
and Co decrease with increasing abundances of Zr
(Fig. 6a–c), whereas abundances of Ba, Ta, and Hf increase as abundances of Zr increase (Fig. 6d–f).
Chondrite-normalized abundances of rare earth elements for representative samples from each suite are
plotted in Fig. 7a–c. Abundances of rare earth elements
for low Ti and high Ti volcanic units of the Dullstroom
Formation were previously reported in Buchanan et al.
(1999). Abundances increase with increasing stratigraphic height and patterns are generally parallel for
most of these samples. The magnitude of the negative
chondrite-normalized Eu anomaly increases with stratigraphic height.
Figure 8 contains spider-diagrams in which the element abundances for samples of the volcanic suites of
the upper Dullstroom Formation, Damwal Formation,
136
Table 2. Compositions determined by XRF and INAA for rhyolites of the Damwal Formation. V, Ni, Cu, Y, Nb, and most major
elements determined by XRF. Na, Fe, Cr, Zn, Sr, Zr, and Ba determined by a combination of both XRF and INAA. The remainder of the
trace elements determined by INAA. b.d. Below detection limits
Sample
kk20
wt%
SiO2
68.4
0.57
TiO2
Al2O3
12.0
a
8.06
Fe2O3
MnO
0.13
MgO
0.76
CaO
2.68
3.60
Na2O
K2 O
3.38
0.10
P2 O5
LOI
0.43
Total
100.1
ppm
Sc
13.4
V
5
Cr
7.40
Co
5.81
Ni
5
Cu
43
Zn
149
Rb
143
Sr
155
Y
54
Zr
333
Nb
18
Cs
2.34
Ba
1,041
La
65.9
Ce
131
Nd
65.3
Sm
10.9
Eu
1.61
Gd
10.3
Tb
1.66
Tm
0.89
Yb
6.14
Lu
0.81
Hf
9.45
Ta
1.12
Th
21.6
U
4.84
a
kk21
nk10
nk11
nk15
pb3
pb8
pb9
pb10
l17
ld1
67.3
0.64
11.9
8.82
0.14
0.95
2.71
3.11
4.01
0.16
0.61
100.4
69.0
0.53
11.9
7.26
0.15
0.29
1.99
3.18
4.11
0.15
0.36
98.9
69.5
0.54
11.9
7.04
0.15
0.38
2.41
3.59
3.56
0.15
0.51
99.7
69.2
0.51
12.2
7.14
0.11
0.27
2.07
3.63
3.83
0.15
0.65
99.8
69.2
0.65
11.8
6.57
0.11
0.33
2.33
3.23
4.71
0.18
0.81
99.9
69.9
0.53
11.8
6.86
0.11
0.14
1.92
2.64
4.68
0.12
0.99
99.7
69.6
0.55
11.3
7.64
0.13
0.15
2.10
3.01
4.97
0.13
0.79
100.4
71.8
0.41
11.7
6.22
0.08
b.d.
1.41
2.66
5.27
0.06
0.87
100.5
67.7
0.57
11.9
8.09
0.12
0.34
1.76
3.19
4.42
0.14
1.06
99.3
69.9
0.64
11.9
7.27
0.18
0.24
0.77
3.77
4.56
0.14
0.94
100.3
15.7
4
5.00
8.49
5
45
155
151
140
51
317
17
1.03
981
61.8
128
57.2
10.3
1.59
7.96
1.38
0.77
5.59
0.77
8.52
0.97
19.2
3.79
11.9
<15
4.45
10.1
<9
26
133
154
148
46
294
18
3.84
961
69.2
122
56.5
10.5
1.79
10.2
1.47
0.83
5.01
0.75
8.59
1.26
21.6
5.47
11.6
<15
6.12
9.22
<9
25
129
124
166
44
331
16
4.91
818
61.8
116
55.0
9.23
1.84
8.60
1.31
0.70
4.48
0.71
8.08
1.06
19.6
4.25
12.5
<15
5.96
9.18
<9
21
176
153
143
47
309
17
3.56
922
69.1
127
62.1
10.6
1.87
10.6
1.49
0.76
5.32
0.79
8.94
1.23
21.8
5.47
12.8
<15
2.71
8.14
<9
55
138
184
168
47
329
17
4.45
1,168
50.9
105
51.3
9.21
1.67
9.75
1.50
0.73
4.95
0.74
8.85
1.48
20.3
4.28
10.5
<15
2.84
6.75
<9
15
142
167
163
58
339
18
2.10
1,036
78.0
144
62.0
11.0
1.96
10.3
1.65
0.77
5.11
0.76
8.43
1.17
19.3
4.05
12.1
<15
4.14
7.48
<9
35
179
185
123
49
329
17
2.24
1,030
75.9
131
63.6
11.5
2.12
10.7
1.59
0.81
5.46
0.82
9.42
1.49
22.2
5.40
7.96
<15
2.48
2.40
<9
13
115
201
111
62
405
20
3.75
1,263
73.8
140
64.8
12.2
2.11
11.4
1.87
0.99
6.93
0.98
10.3
1.66
22.3
5.28
12.8
b.d.
2.81
9.95
b.d.
37
132
160
149
52
329
18
2.27
970
67.8
126
55.9
11.3
1.96
9.09
1.68
0.94
5.66
0.81
8.97
1.43
20.9
5.90
11.0
b.d.
10.8
10.6
b.d.
8
341
176
100
50
370
22
0.73
1,014
69.6
115
54.8
11.0
1.69
8.96
1.61
0.79
5.26
0.77
9.23
1.38
20.8
4.59
Avg.
69.2
0.56
11.9
7.36
0.13
0.35
2.01
3.24
4.32
0.13
0.73
99.9
12.0
4.97
8.01
29
163
163
142
51
335
18
2.84
1,019
67.6
126
59.0
10.7
1.84
9.81
1.56
0.82
5.45
0.79
8.98
1.30
20.9
4.85
All iron reported as Fe2O3
and Kwaggasnek Formation are normalized to the
composition of pyrolite (silicate earth) of McDonough
and Sun (1995). Similar diagrams for low Ti and high
Ti volcanic units were previously displayed in Buchanan et al. (1999). Patterns for volcanic rocks from
the Dullstroom, Damwal, and Kwaggasnek Formations are consistently enriched in most incompatible
trace elements, which commonly increase in abundance
with increasing stratigraphic height and increasing
abundance of SiO2. However, negative pyrolite-normalized anomalies for Nb, Ta, Sr, P, and Ti occur.
With increasing stratigraphic height, the Sr, P, and Ti
anomalies become more pronounced. In contrast,
abundances of Nb and Ta increase with increasing
stratigraphic height and increasing SiO2 abundance,
and, hence, the magnitude of the Nb–Ta anomaly
remains relatively constant.
Discussion
Magmatic processes
The sequence of volcanic rocks in the Rooiberg Group is
distinctive because of the presence of a wide spectrum of
compositions (Fig. 4). These rocks range from two types
of relatively mafic rocks (low Ti and high Ti suites) in
the lower to middle Dullstroom Formation through
strata with intermediate compositions (andesites and
dacites) in the upper Dullstroom Formation to rhyolites
in the overlying formations. Abundances of various incompatible trace elements parallel this broad range of
compositions (Figs. 5 and 6).
The abundances of some incompatible trace elements
constrain the petrogenetic relationships of these volcanic
137
Table 3. Compositions determined by XRF and INAA for high-Si
units of the Kwaggasnek Formation. V, Ni, Cu, Y, Nb, and most
major elements determined by XRF. Na, Fe, Cr, Zn, Sr, Zr, and Ba
determined by a combination of both XRF and INAA. The remainder of the trace elements determined by INAA. Composition
VG1 is an average of analyses of granitoids and gneisses from the
Vredefort Dome and the Johannesburg Dome that were sampled
and analyzed by C. Lana, University of the Witwatersrand,
Sample
wt%
SiO2
TiO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K2 O
P2 O5
LOI
Total
ppm
Sc
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
Th
U
a
Johannesburg (personal communication 2001) using the same analytical techniques. For some major and trace elements, the composition of VG1 is the average of 38 analyses acquired by XRF.
For other elements, VG1 is the average of 16 analyses acquired by
INAA. The averages of low Ti and high Ti units are from Buchanan et al. (1999). b.d. Below detection limits; n.d. not determined
m2
m3
m5
m6
m7
m8
m9
m10
76.9
0.28
12.3
1.05
0.01
b.d.
b.d.
2.75
4.98
0.02
1.11
99.4
76.6
0.28
11.8
1.64
0.03
b.d.
b.d.
2.91
4.99
0.03
0.83
99.1
76.2
0.26
11.5
2.89
0.05
b.d.
0.06
2.60
4.44
0.02
1.11
99.1
75.5
0.27
11.5
3.98
0.04
b.d.
0.26
2.32
5.06
0.03
1.09
100.1
74.1
0.26
11.6
4.31
0.04
b.d.
0.29
2.62
5.08
0.02
0.98
99.3
74.7
0.26
11.2
5.00
0.09
b.d.
0.05
2.17
5.06
0.02
1.05
99.6
75.8
0.25
11.0
4.54
0.03
b.d.
0.02
2.53
5.06
0.03
0.96
100.2
74.8
0.28
11.2
4.08
0.03
b.d.
0.13
3.53
5.09
0.02
0.83
100.0
75.6
0.27
11.5
3.44
0.04
1.39
1.35
4.76
0.85
3.35
0.71
3.60
0.73
8
112
184
49
78
435
22
6.99
1,241
185
199
117
23.9
3.38
18.4
2.76
1.22
8.18
1.24
12.9
2.17
26.9
6.20
b.d.
11
58
185
54
77
469
23
3.68
1,202
97.9
179
74.6
15.0
2.23
14.2
2.52
1.26
8.11
1.24
13.4
2.15
27.7
5.40
9.33
107
179
47
76
476
23
5.72
1,147
111
163
81.8
15.3
2.41
14.4
2.30
1.19
7.90
1.18
12.6
1.87
25.2
5.15
1.54
b.d.
1.46
b.d.
1.30
b.d.
2.33
1.89
2.40
1.76
4.41
1.71
4
58
164
35
77
506
25
2.88
1,206
49.2
105
48.3
10.1
1.84
13.2
2.05
1.28
8.51
1.30
13.0
1.89
25.1
6.57
b.d.
b.d.
67
186
47
70
481
24
2.70
1,111
130
157
87.8
16.0
2.48
13.7
2.15
1.09
7.22
1.07
12.6
1.76
24.2
4.89
b.d.
22
194
154
53
86
450
23
10.2
1,191
146
142
106
15.5
2.85
14.9
2.60
1.27
8.33
1.20
12.1
1.71
23.7
5.21
b.d.
1.26
b.d.
1.22
b.d.
4.06
1.66
1.31
b.d.
1.31
b.d.
2.24
0.65
5.25
0.73
b.d.
b.d.
8
109
193
47
71
491
23
5.41
1,114
103
188
78.1
15.0
2.36
13.7
2.12
1.07
7.31
1.05
12.0
1.66
23.8
4.64
3
129
187
46
70
473
21
8.47
1,080
89.0
161
70.1
13.5
2.07
13.1
2.10
1.09
7.33
1.11
12.3
1.84
24.9
3.87
b.d.
b.d.
128
180
42
82
499
24
5.43
1,031
90.8
170
72.3
13.5
2.10
13.8
2.07
1.23
8.18
1.23
12.5
1.74
24.9
4.43
Avg.
Kwag.
0.10
2.68
4.97
0.02
1.00
99.6
b.d.
b.d.
Avg.
low Ti
Avg.
high Ti
VG1
55.0
0.59
15.2
9.60
0.16
5.81
8.95
2.54
1.11
0.09
1.03
100.1
56.5
1.58
12.9
11.0
0.16
4.51
7.46
2.95
1.67
0.17
0.81
99.7
71.0
0.43
15.2
2.39
0.05
0.43
2.11
5.26
2.72
0.13
0.64
100.4
34.2
190
123
35.7
98
50
98
34.9
250
16
98
8
1.21
334
17.1
34.3
17.2
3.31
0.93
3.42
0.54
0.26
1.75
0.24
2.44
0.29
3.86
0.68
22.5
208
141
35.7
87
118
111
70.0
394
31
194
15
1.66
339
30.5
59.0
33.8
7.10
1.94
6.78
1.00
0.44
2.69
0.37
4.64
0.74
5.82
1.28
2.70
23.2
23.9
4.77
9.64
2.72
63
109
406
12.4
452
11.6
1.40
723
68.0
105
41.6
5.31
1.33
4.65
0.57
0.22
1.16
0.15
5.85
0.61
48.0
n.d.
All iron reported as Fe2O3
rocks. For example, the abundances of some rare earth
elements are greater for the high Ti suite than for the low
Ti suite or for the overlying strata of the upper Dullstroom Formation (Fig. 5f; see also Buchanan et al. 1999).
Hence, dacite liquids represented by strata of the upper
Dullstroom Formation probably were not derived from
high Ti magmas because such derivation requires differentiation of liquids with lower abundances of incompatible trace elements from liquids with higher
abundances. These data suggest that all of the intermediate to siliceous volcanic rocks of the upper Dullstroom
Formation and the Damwal and Kwaggasnek Forma-
tions were derived from low Ti magmas, which had
lower abundances of incompatible trace elements than
the high Ti magmas. This is also well illustrated by a plot
of Hf (ppm) vs. Ti/Zr (both in ppm; Fig. 9). On this plot,
compositions of low Ti volcanic rocks and overlying
intermediate to siliceous rocks form a well-defined trend,
whereas compositions of high Ti volcanic rocks form a
separate trend.
Considering all of these data together, it is likely that
fractional crystallization affected the magmas represented by the Rooiberg Group. The magnitude of the
negative chondrite-normalized Eu anomaly increases
138
Fig. 4. Classification system based on SiO2 (wt%) vs. Na2O +
K2O (wt%; after Le Bas et al. 1986) for low Ti and high Ti volcanic
rocks of the Dullstroom Formation through rhyolitic volcanic
rocks of the Kwaggasnek Formation
with increasing stratigraphic height (Fig. 7a–c), suggesting crystallization of feldspar. Crystallization of
feldspar is also consistent with the decrease in abundances of Al2O3 (Fig. 5b) and CaO (Fig. 5c) with decreasing abundances of MgO and with increasing
stratigraphic height. Feldspar crystallization is also
consistent with the negative pyrolite-normalized Sr
anomalies, which increase in magnitude with increasing
stratigraphic height (Fig. 8a–c). Abundances of Sr decrease with increasing abundances of the commonly incompatible element Zr (Fig. 6a), suggesting that Sr was
compatible with the suite of crystallizing minerals and
further supporting the crystallization of feldspar.
Crystallization of other minerals is also suggested by
these geochemical data. Abundances of Sc decrease with
increasing abundances of Zr (Fig. 6b) and decreasing
MgO content (Fig. 5e), suggesting that Sc was compatible with the suite of crystallizing minerals, which
probably included pyroxene. Abundances of Co (and
Ni) also decrease with increasing abundances of Zr
(Tables 1, 2, and 3, Fig. 6c) and suggest the crystallization of olivine or pyroxene. Negative pyrolite-normalized P anomalies for these strata also increase in
magnitude with increasing stratigraphic height and
suggest crystallization of apatite (Fig. 8a–c). Increasing
magnitudes of negative pyrolite-normalized Ti anomalies with increasing stratigraphic height may indicate
fractionation of ilmenite, magnetite, pyroxene, or
amphibole (Fig. 8a–c).
In light of the geochemical data presented in Tables 1,
2, and 3, it is also possible that assimilation of upper
continental crust played a role in the petrogenesis of
these magmas. The low Ti and high Ti volcanic suites of
the Dullstroom Formation, which are mostly basaltic
andesites, have a distinct signature of continental crust
(e.g., Buchanan et al. 1999). This crustal signature is
even more pronounced for the intermediate to siliceous
volcanic rocks considered in the present study (Fig. 8a–
c). Assimilation seems the most likely explanation for
this crustal signature in volcanic rocks that range from
Fig. 5. Variation diagrams for units of the Dullstroom, Damwal,
and Kwaggasnek Formations: a MgO (wt%) vs. TiO2 (wt%),
b MgO (wt%) vs. Al2O3 (wt%), c MgO (wt%) vs. CaO (wt%),
d MgO (wt%) vs. K2O (wt%), e MgO (wt%) vs. Sc (ppm), and
f MgO (wt%) vs. Sm (ppm). Data for low Ti and high Ti volcanic
units are from Buchanan et al. (1999)
139
Fig. 7. Chondrite-normalized REE diagrams for Rooiberg Group
volcanic units: a upper Dullstroom Formation, b Damwal
Formation, and c Kwaggasnek Formation. Abundances are
normalized to those of C1 chondrites from Anders and Grevesse
(1989). Compare with a similar diagram for high Ti and low Ti
units of the Dullstroom Formation in Fig. 6 of Buchanan et al.
(1999)
relatively mafic to very siliceous. However, it is important to recognize that, in some cases, it may be difficult
to distinguish between the effects of assimilation of
crustal material by a mantle-derived melt and the effects
associated with partial melting of crustal material, particularly if the crustal melt is mixed with a mantlederived melt in a shallow magma chamber.
Modeling
Fig. 6. Variation diagrams for units of the Dullstroom, Damwal,
and Kwaggasnek Formations: a Zr (ppm) vs. Sr (ppm), b Zr (ppm)
vs. Sc (ppm), c Zr (ppm) vs. Co (ppm), d Zr (ppm) vs. Ba (ppm),
e Zr (ppm) vs. Ta (ppm), and f Zr (ppm) vs. Hf (ppm). Data for low
Ti and high Ti volcanic units are from Buchanan et al. (1999)
In an attempt to determine whether the compositions of
the intermediate to siliceous volcanic rocks of the
Rooiberg Group are consistent with fractional crystallization and assimilation of continental crust by an
originally mantle-derived melt, several models (models 1
to 3) were calculated. The broad range of compositions
from low Ti basaltic andesites in the Dullstroom Formation to rhyolites in the Kwaggasnek Formation allowed more detailed calculations than those of
Buchanan et al. (1999). Major element abundances were
calculated in a series of steps assuming equilibrium
crystallization of the relevant minerals. Each step represented 10% by weight of fractional crystallization.
Trace element abundances were calculated using the
140
Fig. 8. Spider-diagrams for Rooiberg Group volcanic units:
a upper Dullstroom Formation, b Damwal Formation, and
c Kwaggasnek Formation. Abundances are normalized to the
composition of pyrolite (silicate earth) of McDonough and Sun
(1995)
Fig. 9. Variation diagram of Hf (ppm) vs. Ti/Zr (both in ppm) for
Rooiberg Group volcanic units. Data for low Ti and high Ti
volcanic units from Buchanan et al. (1999)
methods and equations discussed in DePaolo (1981).
Mineral/melt partition coefficients for trace elements
used in these calculations were based on the discussion
and references contained in Wilson (1989). Although
these models probably aren’t unique, they represent
reasonable approximations of assimilation and fractional crystallization processes that might have affected
these magmas.
Instead of the approximate composition of the upper
continental crust of Taylor and McLennan (1985), the
assimilant was assumed to be the average composition
(VG1, Table 3) of a significant number of granitoids and
gneisses from the Vredefort Dome and the Johannesburg Dome, immediately south of the Bushveld Complex (personal communication 2001, C. Lana, University
of the Witwatersrand). Compositions of these granitoids
and gneisses were determined in the same laboratories
and using the same analytical techniques as the other
geochemical data in this study. For major elements and
some trace elements, the composition of VG1 was calculated by averaging compositions determined by XRF
for 38 samples. For other trace elements, the composition of VG1 was calculated by averaging compositions
determined by INAA for 16 samples. There is evidence
that these granitoids and gneisses are representative of
the basement in the study area and in surrounding parts
of the Kaapvaal Craton (e.g., Anhaeusser 1999, and
papers discussed and quoted in Anhaeusser 1983). It is
important to note, however, that there are obvious
limitations associated with any attempt to estimate the
average composition of a heterogeneous continental
crust. Nevertheless, the composition of VG1 is generally
similar to the more limited compositional data for siliceous Archean basement rocks from the Vredefort
Dome reported by Hart et al. (1990).
The suites of crystallizing minerals used to calculate
each of these models is based on several factors.
Feldspar phenocrysts are rare, but ubiquitous, among
strata of the Dullstroom and Damwal Formations and
support the geochemical evidence for the crystallization
of feldspar. Other phenocrysts are present in the low Ti
and high Ti volcanic rocks of the Dullstroom Formation and are commonly altered, but apparently were
originally mafic silicates (e.g., Buchanan et al. 1999).
These phenocrysts support the geochemical evidence
for the crystallization of pyroxene and/or olivine. In
rare cases, it is possible to determine that these phenocrysts were originally pyroxene, whereas, in other
cases, it is not possible to confidently identify them and
they may have been olivine. Significant proportions of
olivine in the suite of crystallizing minerals in the early
stages of differentiation seems to best reproduce the
levels of enrichment of SiO2 of the Rooiberg Group
volcanic units. This enrichment could probably also be
reproduced with crystallization of larger proportions of
ilmenite and magnetite. However, crystallization of
large amounts ilmenite should result in a more significant decrease in TiO2 content with increased differentiation from the low Ti suite to the overlying
intermediate to siliceous volcanic rocks (Fig. 5a). There
is, however, indirect geochemical evidence for the
crystallization of small proportions of apatite and
possibly ilmenite and magnetite from these magmas
(see above).
Where possible, compositions of the crystallizing
phases used in these calculations were assumed to be the
compositions of phenocrysts. In other cases (e.g., mafic
silicates), approximate compositions of crystallizing
phases were estimated. Proportions of crystallizing
phases were varied within reasonable limits and the
models described below are generally the calculated
141
compositions that reproduce most closely the average
compositions of each of the rock groups.
Model 1 was calculated to approximate the average
composition of the andesites and dacites of the upper
Dullstroom Formation. For this calculation, the average
composition of the low Ti suite of volcanic rocks was
subjected to 30% assimilation of VG1 and 50% crystallization by weight of a mixture of 60% plagioclase,
20% olivine, and 20% of a mixture of subequal proportions of augite and pigeonite. Composition of the
plagioclase ranged from Ab23 (the approximate composition of plagioclase phenocrysts in the low Ti strata)
to Ab63, that of the olivine ranged from Fo78 to Fo50,
and the Mg# of the pyroxene ranged from 54 to 80. The
calculated model 1 composition is compared with the
average composition of the dacites in Fig. 10 and in
Table 4.
Model 2 was calculated in an attempt to reproduce
the average composition of the dacites and rhyolites of
the Damwal Formation. The average composition of the
volcanic rocks of the upper part of the Dullstroom
Formation was subjected to 30% crystallization by
weight of a mixture of 65% plagioclase and 35% of a
mixture of subequal proportions of pigeonite and augite.
Composition of the plagioclase ranged from Ab63 to
Ab70 and the Mg# of the pyroxene ranged from 46 to 62.
The calculated composition of model 2 is compared with
the average composition of the volcanic rocks of the
Damwal Formation in Fig. 10 and in Table 4.
Model 3 was calculated to approximate the average
composition of the rhyolites of the Kwaggasnek Formation. The average composition of the volcanic rocks
of the Damwal Formation was subjected to 25% crystallization by weight of a mixture of 40% albite, 10% Kfeldspar, 40% augite (Wo45En10), and 10% of a mixture
of ilmenite and magnetite. Model 3 composition is
compared with the average composition of rhyolites
from the Kwaggasnek Formation in Fig. 10 and Table 4.
It is worth noting that the abundance of phosphorus in
model 3 is a bit higher than the average abundance of
that element in the Kwaggasnek rhyolites. This probably
is the result of crystallization of significant, but small,
proportions of apatite.
Petrogenesis
Fig. 10. Spider-diagram comparing compositions of models 1–3
from this study with average compositions of the dacites of the
upper Dullstroom Formation, the rhyolites and dacites of the
Damwal Formation, and the rhyolites of the Kwaggasnek
Formation. Also included is the average composition of the low
Ti volcanic units of the Dullstroom Formation (Table 3). All data
are normalized to the composition of pyrolite (silicate earth) of
McDonough and Sun (1995). See text for details of calculations
Table 4. Comparison of major
and minor element abundances
for models 1–3 with average
compositions of dacites of the
upper Dullstroom Formation,
rhyolites of the Damwal Formation, and siliceous units of
the Kwaggasnek Formation.
b.d. Below detection limits
Sample
wt%
SiO2
TiO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K2 O
P2 O5
LOI
Total
a
Model 1
64.2
0.90
13.7
7.13
0.22
1.22
5.72
2.82
2.40
0.16
1.52
100.0
Buchanan et al. (1999) suggested that the high Ti and
low Ti volcanic rocks of the Dullstroom Formation
represent liquids that were derived by partial melting of
compositionally distinct source areas and probably resided in different magma chambers. The data presented
in the present study (e.g., Fig. 9) indicate that the
overlying intermediate to siliceous volcanic units of the
Bushveld Magmatic Province are not petrogenetically
related to high Ti volcanic rocks, but are probably
closely related to the low Ti suite.
Avg. upper
Dullstroom
66.3
0.63
13.2
6.69
0.12
1.95
4.37
3.08
2.57
0.13
1.02
100.1
All iron reported as Fe2O3
Model 2
70.5
0.90
11.8
6.08
0.17
0.70
2.48
2.11
3.67
0.19
1.46
100.1
Avg. Damwal
Model 3
Avg. Kwaggasnek
69.2
0.56
11.9
7.36
0.13
0.35
2.01
3.24
4.32
0.13
0.73
74.6
0.25
12.7
3.16
0.17
0.04
b.d.
2.75
5.20
0.17
0.97
75.6
0.27
11.5
3.44
0.04
b.d.
0.10
2.68
4.97
0.02
1.00
99.9
100.0
99.6
142
Schweitzer et al. (1997) used the similarity between
the bulk compositions of the dacites of the upper
Dullstroom Formation and the continental crust (Taylor
and McLennan 1985) to suggest that some of the volcanic strata of the Rooiberg Group represent crustal
melts generated by a mantle plume. However, average
abundances of some incompatible trace elements (e.g.,
the rare earth elements) in the low Ti strata (Table 3) are
extremely similar to those of the estimated composition
of the lower continental crust (Taylor and McLennan
1985). Hence, it is unlikely that these rocks represent
partial melts of lower crustal material. In contrast, although the average composition of the high Ti volcanic
suite (Table 3) is generally enriched in incompatible
trace elements compared with the estimated composition
of the lower continental crust (Taylor and McLennan
1985), the average abundances of SiO2 and many other
major element oxides are similar and are difficult to
reconcile with partial melting of lower crustal material.
Further, the compositions of the overlying intermediate
to siliceous volcanic rocks and the compositions of low
Ti strata form very well defined trends on many major
and trace element variation diagrams (Figs. 5, 6, and 9).
The continuity of these trends is unlikely if these volcanic rocks represent partial melts of different source
areas (i.e., lower crust and mantle, respectively). Hence,
based on the data presented in this study it is unlikely
that any of these rocks represents a crustal melt sensu
stricto.
Another possible interpretation is related to the suggestion by Maier et al. (2000) that some magmas of the
lower part of the Rustenburg Layered Suite represent
mixtures of mantle-derived melts with partial melts of
crustal material. Could volcanic rocks of the Rooiberg
Group represent mixtures of mantle-derived melts and
increasing proportions of crustal melts with increasing
stratigraphic height? Although this hypothesis explains
some of the spectrum of compositional data for these
strata, the decrease in abundances of Al2O3 with decreasing MgO content (Fig. 5b) requires that the crustal
melts would have had to contain significantly less than
11% Al2O3 (see also Table 3). Based on a variety of
experimental studies (e.g., Huang and Wyllie 1981), this
seems unlikely. Considering the discussion presented
above, a more reasonable interpretation is that these
rocks were affected by increasing amounts of feldspar
crystallization.
Hence, based on the model calculations presented in
the preceding section and the above discussion, the most
reasonable interpretation is that the volcanic rocks of
the Rooiberg Group represent mantle melts that underwent large amounts of differentiation and assimilation of crustal material in two or more shallow magma
chambers. This seems to be the only way to explain the
very well-defined trends formed by geochemical data
from the low Ti suite of volcanic rocks and the overlying
intermediate to siliceous volcanic rocks. This interpretation may have significant implications for the petrogenesis of the related intrusive rocks of the Rustenburg
Layered Suite. Magmas that played a role in the petrogenesis of these igneous rocks also have a distinct crustal
signature (e.g., Maier et al. 2000). The data presented in
the present study indirectly confirm the suggestion by
Maier et al. (2000) that some of these magmas assimilated significant amounts of continental crust. These
data also confirm the suggestion by Irvine (1982) that
the intrusive and extrusive phases of the Bushveld
Magmatic Province are related and experienced similar
petrogenetic processes.
What do these features imply about the tectonic
framework in which these volcanic rocks were deposited? On the one hand, some evidence supports a
mantle plume origin (Hatton 1995; Hatton and
Schweitzer 1995). For example, these magmas apparently were very hot and very voluminous. The abundances of trace elements in all of these volcanic rocks
bear a striking signature of continental crust. However,
the data presented in this study suggest that none of
these volcanic rocks represent crustal melts sensu
stricto as suggested by Schweitzer et al. (1997). Particularly distinctive of these strata are the sequential
changes with vertical stratigraphic height from more
mafic compositions to siliceous compositions. Although
these features are consistent with a mantle plume origin
(Hatton 1995), they are also consistent with a variety of
other continental environments, including subduction
zones, which are noted for shallow, long-lived magma
chambers (e.g., Wilson 1989). This question may be
elucidated by isotopic analyses presently underway in
this laboratory.
Conclusions
1. Dacites and rhyolites of the upper Dullstroom Formation and the Damwal and Kwaggasnek Formations are petrogenetically related to the low Ti suite of
volcanic rocks of the lower to middle Dullstroom
Formation. Compositions of these volcanic rocks
comprise well-defined trends on a variety of variation
diagrams and they are best explained as the products
of increasing assimilation and fractional crystallization by a mantle-derived melt in a shallow, long-lived
magma chamber.
2. High Ti magmas are not petrogenetically related to
the low Ti magmas.
3. The characteristics of the volcanic rocks of the
Bushveld Magmatic Province are consistent with
formation of these magmas in a variety of continental
settings, including plume-related environments, but
also including subduction-related environments.
4. The data presented in this study indirectly confirm
the suggestion by Maier et al. (2000) that significant
amounts of assimilation of crustal material may
have also affected magmas associated with the
Rustenburg Layered Suite of the intrusive Bushveld
Complex.
143
Acknowledgments The University Research Council (URC) and
the Impact Cratering Research Group of the University of the
Witwatersrand, Johannesburg, South Africa, provided support for
P.C.B. The National Research Foundation (NRF) of the Republic
of South Africa also supports W.U.R.’s research. Support was
provided by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung, project Y58-GEO to C.K. This publication
represents University of the Witwatersrand Impact Cratering Research Group Contribution #34. Sharon Farrell and Matt Kitching
provided excellent technical support and Lynn Whitfield and Di du
Toit assisted with expert drafting. Excellent reviews were provided
by J.S. Marsh and W.D. Maier.
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