Download On the relationship between the Bushveld Complex and its felsic

Contrib Mineral Petrol (2015) 170:56
DOI 10.1007/s00410-015-1211-y
ORIGINAL PAPER
On the relationship between the Bushveld Complex and its felsic
roof rocks, part 2: the immediate roof
J. A. VanTongeren1,2 · E. A. Mathez2 Received: 31 July 2015 / Accepted: 12 November 2015
© Springer-Verlag Berlin Heidelberg 2015
Abstract Emplacement of large volumes of mafic
magma into the crust undoubtedly causes significant
thermal perturbation to the overlying crust. Despite
the clear importance of the country rock in modulating
the thermal evolution the Bushveld Complex, little is
known about the nature and extent of its roof zone. This
manuscript details the lateral variability of the rocks
that make up the immediate roof of the intrusion in the
Eastern Limb. In the Northern Segment of the eastern
Bushveld, the roof is dominated by thermally metamorphosed metapelites; in the Central Segment, the roof
is dominated by highly metamorphosed meta-volcanic
rocks and their partially molten equivalents; and in the
Southern Segment, the roof is likely composed of modestly thermally metamorphosed felsic volcanic rocks.
The variability of roof lithology is also reflected in the
variability of floor rocks to the intrusion. A new model
for the emplacement of the eastern Bushveld Complex
is proposed in which the mafic magmas intrude at a
deeper level in the north and become shallower to the
south.
Communicated by Timothy L. Grove.
Electronic supplementary material The online version of this
article (doi:10.1007/s00410-015-1211-y) contains supplementary
material, which is available to authorized users.
* J. A. VanTongeren
[email protected]
1
Department of Earth and Planetary Sciences, Rutgers
University, 610 Taylor Rd., Piscataway, NJ 08854, USA
2
Department of Earth and Planetary Sciences, American
Museum of Natural History, 79th and Central Park West,
New York, NY 10024, USA
Keywords Rooiberg Group · Bushveld Complex ·
Driekop Dome · Mphanama · Masekete · Roof zone ·
Residual Zone · Upper Zone · Droogehoek · Stoffberg ·
Hornfels · Leptite
Introduction
Layered mafic intrusions represent the primary observational record of igneous differentiation within a solidifying magma chamber. While the sequences of cumulate
rocks provide information on the magmatic responses to
solidification, it is the roofs of these intrusions that hold
the key to understanding the mechanisms of heat loss and
thermal evolution. Only six major layered mafic intrusions
have both their roofs and floors preserved and exposed: the
Skaergaard Intrusion of East Greenland, the Muskox, Sept
Iles and Kiglapait Intrusions of Canada, the much larger
Dufek Intrusion of Antarctica, and Bushveld Complex of
South Africa. The Bushveld Complex is part of the massive
Bushveld Igneous Province that also includes voluminous
ferroan granites and other felsic rocks, all of similar age of
≈2.06 Ga. The enormous extent of the Bushveld Complex
(250 × 350 km if a continuous sheet) belies the fact that it
is generally poorly exposed, with the exception that parts
of the eastern Bushveld are locally well exposed due to the
rugged topography. These regions thus offer the unusual
opportunity to understand how an enormous and long-lived
body of mafic liquid interacted with its roof.
One model of emplacement holds that the Bushveld
intruded along a regional unconformity between mainly
shales and quartzites of the underlying Pretoria Group
sediments and an overlying thick sequence of basaltic
to rhyolitic lavas known as the Rooiberg Group, with the
exception of one area in the southeast Bushveld where
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the Dullstroom Formation, the lowest unit of the Rooiberg Group, makes up the floor of the intrusion (Cheney
and Twist 1991). The Rooiberg Group consists of a lower
sequence of magnesian lavas and a petrologically distinct
upper sequence of ferroan lavas (Twist and Harmer 1987;
Mathez et al. 2013). In ascending stratigraphic order, these
lavas have been subdivided into the Dullstroom, Damwal,
Kwaggasnek, and Schrikkloof formations (SACS 1980).
Some of the ferroan lavas may have been generated by
fractional crystallization of the Bushveld mafic magmas
(VanTongeren et al. 2010, see below).
Understanding the roof of the Bushveld Complex is
complicated by the fact that the younger Lebowa Granite
Suite granites intruded at various levels within the roof
and lava sequences (Hill et al. 1996). The petrogenesis of
the granites and their relationship to the Bushveld Complex and Rooiberg lavas have been debated (e.g., Hill et al.
1996; Schweitzer et al. 1997), but due to the younger age of
the granites, their origin will not be considered here.
The Bushveld Complex is composed of the principle
eastern, western, and northern limbs along with a number of outliers. From east to west, the Bushveld extends
over 350 km, and the eastern limb alone crops out for
more than 150 km north–south. Due to the enormous size
of the Bushveld, the roof of the intrusion is neither everywhere laterally continuous nor exposed. In some places
the mafic rocks are capped by quartzite and/or metapelite,
and in others by a complicated lithology of leptite (see
below), hornfels, microgranite, granophyre, felsite, and
granite.
This paper describes the large-scale changes in contact
relationships as they are observed between the rocks of the
Upper Zone of the Bushveld Complex and its immediate
roof in four locations from north to south in the eastern
limb. The present report builds on the important works by
Groeneveld (1970), von Gruenewaldt (1968, 1972), Lombaard (1949), Molyneux (1970, 1974, 2008), and Walraven
(1987). The goal of our study is to provide a systematic
look at how the relationship between the final Bushveld
Complex magmas and the roof zone evolves when there are
different lithologies present in the overlying country rock.
Field relations
Throughout much of the eastern limb of the Bushveld, the
immediate roof of the intrusion consists of a distinctive,
100- to 300-m-thick rock layer composed of a complex
mixture of hornfels and microgranite. The term ‘leptite’
has been used by various authors (e.g., Von Gruenewaldt
1968, 1972; Molyneux 2008) to describe either the rock
or the hornfels or both. Leptite originated as a nineteenthcentury Swedish name for fine-grained, recrystallized,
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Contrib Mineral Petrol (2015) 170:56
quartz-feldspar, meta-volcanic or metasedimentary,
supracrustal rocks, such as those found in the Proterozoic Leptite Belt near Stockholm (e.g., Loberg 1980).
The Glossary of Geology (4th) states that the term is now
obsolete. Furthermore, where it exists in the roof of the
Bushveld, leptite has come to mean different things to different workers and does not actually describe the rock.
For these reasons, we dispense with the term entirely
and simply describe the rocks by listing the combination
of specific lithologies present. We define hornfels as a
fine- to very fine-grained, thermally metamorphosed rock
with classic granoblastic texture. It is important to note
that we use the term hornfels throughout the manuscript
to describe the rock texture, with no implication for its
protolith.
Here we divide the roof as it is exposed in the eastern Bushveld into three segments, each with a different
character. The roof of the most volumetrically significant
‘Central Segment’ (Fig. 1) is dominated by the distinctive
hornfels + microgranite rock noted above. This rock type
is well exposed in the Droogehoek and Masekete Sections,
described below. In the ‘Northern Segment,’ outcrops of the
Bushveld Upper Zone are limited and the map patterns and
seismic profiles imply significant structural complexity.
In this region the roof is dominated by metapelites and a
sedimentary hornfels that is significantly different than the
hornfels of the Central Segment. The roof lithologies characteristic of the Northern Segment are well exposed in the
Mphanama Section, also described below (Fig. 1). Finally,
least well exposed is the ‘Southern Segment,’ where Cawthorn (2013) asserted that the Bushveld cumulate rocks are
in immediately contact with the Rooiberg Group volcanics.
However, our observations suggest that the roof in this area
is similar to that in the Central Segment.
Central Segment
The Central Segment of the Bushveld roof extends from
Magnet Heights south to the Tauteshoogte area (Fig. 1,
24°50′S to 25°19′S, ≈24°50′E). The area from about
25°05′S to 25°19′S (near Tauteshoogte and Roossenekal)
was mapped by Von Gruenewaldt (1972) and the area north
of that to Magnet Heights (25°19′S–24°50′S) by Molyneux (1974). In this region, while there is variability at
the small scale, the same general lithologies are observed
for ~60 km along strike. Along this entire strike length,
the cumulate rocks of the Bushveld dip gently westward
beneath the granites exposed on the Nebo Plateau, and the
top of the Bushveld and the immediately overlying roof
rocks are locally well exposed in incised stream beds along
and below the steep eastern-facing escarpment of the Nebo
Plateau. Two such streams are Droogehoek and Masekete
(Fig. 1).
Contrib Mineral Petrol (2015) 170:56 Page 3 of 17 56
Fig. 1 General map of the Eastern Limb of the Bushveld Complex, adapted from Molyneux (2008) showing the geographic locations of all the
towns, sections, and segments mentioned in the text as well as geologic context
Droogehoek Section
The Droogehoek Section (24° 51.767′S, 29° 54.384′E) follows a streambed that provides nearly 100 percent exposure
of about a third of the section and perhaps 40 percent of the
rest (Fig. 2). It is accessible with permission of the local
community, nearest to the town of Ga-Maepa. The precise
contact between the cumulate diorites of the Upper Zone
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Contrib Mineral Petrol (2015) 170:56
Fig. 2 Field relationships in the Droogehoek Section of the Central
Segment. Large photograph shows the immediate roof zone above
the contact with the Upper Zone. Small photos taken of the rocks
observed immediately above the contact including the small metased-
imentary hornfels and the larger hornfels blocks with the microgranite. Arrow in bottom left of large photograph indicates stratigraphic
up
and the overlying microgranite–hornfels roof is usually not
exposed because it is covered by stream gravel, but it can
be placed within about 10 m in the stream cut. The immediate roof in this section is characterized by three distinct
lithologies:
4. A fine-grained granophyre dike or sill cutting the hornfels–microgranite. This is presumably the Stavoren
Granophyre, which was defined by Walraven (1987)
and is part of the Rashoop Granophyre Suite. To the
south in the Tauteshoogte area the Stavoren Granophyre exists as a locally discordant sheet up to 650 m
thick. The maps of von Gruenewaldt (1972) and Molyneux (2008) show that the granophyre is present as thin
(<10 m) sill- or dike-like bodies intruded into the hornfels–microgranite along the entire Central Segment
(e.g., von Gruenewaldt 1972).
1. Enormous hornfels blocks meters to tens of meters
across. Most if not all of the blocks are angular on the
meter scale, homogeneous, massive, and lithologically
identical to each other (see below).
2. Numerous small hornfels xenoliths in the centimeter
to tens of centimeter size range. Some xenoliths are
clearly fragments of once larger ones, and many of the
small xenoliths are thinly laminated, some with apparent cross-bedding, suggesting sedimentary protoliths.
3. A matrix lithology of microgranite/granodiorite exists
around the hornfels blocks (Fig. 2). Both the large and
small hornfels blocks are randomly distributed such
that the matrix microgranite is typically localized in
irregularly shaped masses on all scales from tens of
centimeters to tens of meters across.
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The contacts between the hornfels and microgranite
vary from sharp to gradational and may be straight, irregular, or sinuous. Some of the smaller xenoliths are partially
digested within the microgranite, so it is clear that the latter
contains, at least locally, a hornfels component. In places
where the hornfels and microgranite lithologies are intimately mixed, the microgranite characteristically displays
considerable heterogeneity in both mode (see below) and
grain size. In some cases, grain size in the microgranite
Page 5 of 17 Contrib Mineral Petrol (2015) 170:56 (a)
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(b)
Fig. 3 Field relationships in the Masekete Section of the Central
Segment. a Pristine outcrop exposure of hornfels–microgranite lithology in the Masekete River immediately above the contact with the
Upper Zone. Hornfels appears as a very fine-grained light gray blocks
with both rounded and sharp contacts to the microgranite. b Further up the Masekete River away from the Upper Zone contact, the
granite and granophyre lithologies are dominant. Large photograph
shows the outcrop exposure near the base of the Nebo Plateau scarp,
where both granite and granophyre are found in association with one
another. Small photographs highlight the difference in grain size and
contact relationships between the granite and granophyre. Arrow in
bottom left of large photograph indicates stratigraphic up
varies continuously from millimeter to centimeter size
away from the hornfels contacts.
The excellent exposure provided the opportunity to
determine the relative proportions of hornfels and microgranite and how those proportions change across strike by
pacing along two segmented traverses in reasonably flat
areas through the entire unit. Based on this exercise, the
rocks in the Droogehoek Section are estimated to comprise
38 % microgranite and 62 % hornfels, with no systematic
change in the proportion of lithologies away from the contact with the Bushveld cumulate rocks.
hornfels surrounded by a coarse-grained matrix of microgranite (Fig. 3), with the exception that the small metasedimentary hornfels xenoliths are notably absent.
Further upsection along the Masekete River, the hornfels–microgranite roof rocks are overlain by a granophyre
and fine-grained granite. Where granophyre–granite contacts are clearly exposed, the granite displays an intrusive
relationship with the granophyre but without chilled margins (Fig. 3), implying that the granophyre was hot when
the granite was emplaced. Therefore, this granite may simply be another phase of the magma that gave rise to the
granophyre.
Masekete Section
The Masekete Section (25° 11.979′S, 29° 46.398′E) is
approximately 20 km south of the Droogehoek Section
(Fig. 1). It is accessible along the Masekete River with
permission of the local game lodge on the road R555. The
immediate roof of the Bushveld Complex in the Masekete
Section is similar to that observed in the Droogehoek Section, with large, homogeneous blocks of fine-grained
Mineralogy and geochemistry of the Central Segment
lithologies
Hornfels blocks
The rocks that make up the hornfels blocks in the immediate roof zone of the Central Segment are made of equant,
rounded grains of quartz (diameters = 100–250 µm),
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Contrib Mineral Petrol (2015) 170:56
Fig. 4 Photomicrographs in plane and cross-polarized light of sample B10-044, representative of a typical hornfels–microgranite lithology. a Hornfels, b contact between hornfels and microgranite lithology, c microgranite. Field of view and scale bar of 500 um are the
same in all photomicrographs. *Geochemical analyses reported in
Supplementary Tables 1, 2 for B10-044 are from the microgranite
portion of the sample only
hedenbergite, and in some samples variable but minor
amounts of fayalite embedded in a matrix composed of
an equidimensional mosaic of plagioclase and orthoclase
grains (400–600 µm). The texture, as noted, is classic
granoblastic (Fig. 4). The rock also contains sparse plagioclase porphyroblasts up to 2 mm long. These display normal zoning near the margins, which encase abundant quartz
grains. No other porphyroblasts are present. Typical modes
are estimated in thin section to be 40–50 % feldspar, 50 %
quartz, 8 % hedenbergite + fayalite, and 2 % magnetite
plus exsolved ilmenite. In two of three samples analyzed,
the fayalite is nearly pure (>Fa98), so the assemblage and
mineral compositions fix the fO2 at essentially the fayalite–
magnetite–quartz (FMQ) buffer. The fayalite coexists with
hedenbergite of approximate composition En3Fs52Wo45.
The third analyzed sample contains Fa93–94, which coexists
with hedenbergite of En11Fs46Wo43. Hornblende and biotite
are encountered only rarely and exist almost exclusively in
rocks displaying mild sericitic alteration. Zircon and apatite
(30–60 µm) are the most abundant trace phases. A complete
dataset of mineral compositions is provided in Supplementary Table 1.
Plagioclase compositions in the hornfels blocks are
highly variable. Measured compositions from three analyzed hornfels samples in the Droogehoek Section range
from An1Ab98 to An39Ab59, though the range in any
given sample is typically ± 10 An units. Average plagioclase compositions for the hornfels samples range from
An19–24; K-feldspar compositions range from Or79–86. In
the Masekete Section, the average plagioclase and orthoclase of the hornfels samples are plagioclase An11–14 and
orthoclase Or80–87. The plagioclase compositions measured
in the hornfels blocks are similar to those of phenocrysts
from Damwal (An17–28; Buchanan et al. 2002) and the
Dullstroom high-magnesium felsite lavas (An11; Buchanan
et al. 1999) (no data have been published on plagioclase
phenocryst compositions from the Dullstroom low-magnesium felsites).
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Microgranite
The microgranite in the immediate roof of the Central Segment consists of equant quartz grains (0.5–1.0 mm), equant
to tabular, subhedral plagioclase grains (≤2 mm, usually
partially sericitized), and perthitic orthoclase that is interstitial to both. The mafic phase is hornblende (ferropargasite),
which is interstitial to plagioclase and quartz and may exist
as large, discrete grains up to 5 mm across poikilitically
enclosing those minerals. Some rocks also contain traces of
hedenbergite and fayalite. The textures suggest that these
minerals were originally more abundant but mostly altered
to amphibole. Biotite is present in minor amounts partially
Page 7 of 17 Contrib Mineral Petrol (2015) 170:56 replacing hornblende, and apatite and zircon are common
trace phases. The overall texture varies from poorly developed granitoid to locally allotriomorphic granular (Fig. 4).
Most of the microgranites are monzonitic, consisting of
20–30 % quartz and 8–10 % hornblende + biotite (bulk
SiO2 = 66–72 %), but some are distinctly more mafic,
containing ≈15 % quartz and 20 % mafic minerals (bulk
SiO2 = 62–63 %). The more mafic samples are the ones
that contain fayalite and hedenbergite. The microgranite
lacks layering, and the mode, texture, and variability of
each clearly distinguish it from the underlying Bushveld
layered cumulate rocks.
Average plagioclase compositions in the microgranites
are An5–25 in the Droogehoek Section and An14–17 in the
Maskete Section. The large plagioclase compositional differences are among samples, but within individual samples
the compositions are relatively homogenous. There is no
correlation between plagioclase An content and distance
away from the contact with the Bushveld Upper Zone.
In nearly all cases where pairs of hornfels and adjacent
microgranite were analyzed, the average plagioclase in
the microgranite was 3–5 An units higher than in the hornfels, implying a higher crystallization temperature or more
likely a higher water activity in the microgranite. Aside
from texture, the outstanding difference between the hornfels and microgranite assemblages is that the mafic mineral
in latter is almost exclusively hornblende, whereas hornfels
contains hedenbergite and fayalite but little or no hornblende. Therefore, the microgranite must have crystallized
under a substantially greater aH2O.
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(a)
(b)
(c)
Rock compositions
Bulk compositions of the Central Segment hornfels blocks
and microgranite (Supplementary Table 2) are compared
with those of the Rooiberg Group lavas and Stavoren
granophyres in the eastern Bushveld in Fig. 5. Due to their
small size and clear sedimentary nature, no bulk rock compositions were obtained for the small hornfels xenoliths
present in the Droogehoek Section.
Neither the hornfels blocks nor the microgranite has
exact compositional analogues among the Rooiberg Group
lavas, though they are remarkably similar in overall geochemical characteristics to some of the individual lava
formations. On average, the hornfels blocks have similar
SiO2 contents to the Kwaggasnek rhyolites (~72 wt% SiO2)
and are more silicic than either the Damwal or Dullstroom
LMF lavas (Fig. 5). The FeOtotal content of the hornfels
blocks is also similar to that of the average Kwaggasnek
lava (ca. 5 wt% FeO) and slightly lower than those of the
Damwal (ca. 6–7 wt%) or Dullstroom LMF lavas (ca.
5–7 wt%). The MgO content of the hornfels is typically
<0.10 %, whereas that of the Damwal and Dullstroom LMF
Fig. 5 Whole-rock major element geochemistry for the hornfels,
microgranite, and Rashoop Granophyre for comparison with the Rooiberg Group lavas. ASI is the Aluminum Saturation Index, MALI is
the Modified Alkali-Like Index, and Fe Index is the FeOtotal/FeOtotal + MgO, according to Frost and Frost (2008). The hornfels, microgranite, and granophyre all have compositions overlapping with the
upper Rooiberg Group lavas, the Damwal, Kwaggasnek, and Schrikkloof formations. Rooiberg lava and granophyre compositions are from
the compilation of Mathez et al. (2013) including data from Schweitzer (1998), Buchanan et al. (1999, 2002), Walraven (1987)
lavas is typically between 0.3 and 1.00 %, leading to higher
Fe/Mg ratios in the hornfels compared to the Dullstroom
and Damwal (high Fe index, Fig. 5a).
Despite the offset to higher SiO2 and Fe/Mg, both of
which may be altered during thermal or hydrous alteration,
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the hornfels blocks bear striking resemblance in most other
geochemical parameters to those of the Damwal and Dullstroom LMF lavas. The modified alkali lime (MALI) and
aluminum saturation indices (ASI) of the hornfels blocks
are strongly indicative of a volcanic protolith. Among the
Rooiberg Group lavas present within the region, the hornfels are most similar to the Damwal/Dullstroom LMF lavas
(Fig. 5b, c) and are distinct from those in the Kwaggasnek
and/or Schrikkloof formations.
Fluid-immobile trace element compositions in the
hornfels suggest that they are most closely related to the
Damwal lavas. The average Zr concentration of the hornfels samples is 386 ± 11 ppm (1σ). This is nearly identical
to the average of 376 ppm for the Damwal lava and distinct from the average Kwaggasnek (475 ppm) or Schikkloof (601 ppm) measured by Schweitzer et al. (1997). Rb,
Sr, Y, and REE contents also suggest similarity between
the average hornfels composition and the Damwal lavas
(Fig. 6).
There appears to be two distinct populations of microgranites in the Droogehoek Section. The majority of microgranite samples have high SiO2 (66–70 wt%) similar to
the hornfels and lavas, but two have distinctly lower SiO2
contents (62 wt%). The latter are also relatively FeO-rich
(10 wt% compared to 5–7 wt% in the high SiO2 samples)
and possess anomalously high Zr and Ba concentrations
and high Zr/Y.
In contrast to the hornfels, the Si-rich microgranites
have similar bulk rock SiO2 contents to the Dullstroom
LMF and Damwal lavas (Fig. 5). The bulk rock Fe/Mg
ratio (Fe index) of the microgranites, however, is higher
than the lavas, similar to that of the hornfels (Fig. 5a). Both
the hornfels and microgranite are distinctly metaluminous
and have MALI and ASI indices that overlap with the range
of values observed in the Dullstroom LMF and Damwal
lavas (Fig. 5b, c).
Trace element concentrations of the microgranites are
nearly identical to coexisting hornfels, except in the case of
Sr, Ba, and Zr, which are slightly higher on average in the
microgranites relative to the adjacent hornfels (Fig. 6).
The microgranites have a distinctly different composition from the Stavoren granophyre. While the microgranite
has a similarly high Fe index to the granophyres, it is much
lower in SiO2 and plots far from the granophyre field on the
ASI and MALI indices (Fig. 5).
Northern Segment: Mphanama Section
Field relations
In the northern portion of the eastern Bushveld, from
the towns of Jane Furse to Mohalatse (24°46′S, 29°54′E
to 24°33′S, 29°46′E), the roof rocks are dominated by
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Contrib Mineral Petrol (2015) 170:56
Fig. 6 Whole-rock trace element geochemistry for the hornfels,
microgranite, and granophyre for comparison with the Rooiberg
Group lavas. The granophyre has a trace element composition very
similar to that of the upper Rooiberg Group lavas and the Kwaggasnek and Schrikkloof formations. The hornfels and microgranite have
compositions distinct from the granophyre and upper Rooiberg Group
lavas. Aside from a single sample with high Zr/Y, the hornfels blocks
have very similar trace element ratios to those found in the Damwal
formation lavas. The microgranite, however, has a much higher trace
element compositional variability
metapelite and metasedimentary hornfels, which Molyneux
(2008) mapped as ‘leptite-after-quartzite’ (Fig. 1). Good
exposures of the Northern Section lithologies exist near the
town of Mphanama (24°35.532′S, 29°48.751′E; Figs. 1, 7a,
b).
A unit mapped as hornfels in this region consists of thermally metamorphosed mudstones and is thus radically different from the hornfels of the Central Segment. Erosional
surfaces on the mudstone lithologies preserve evidence
of rip-up clasts and clear sedimentary structures, though
rock interiors are fused (Fig. 7a). The most likely protolith
to this lithology is the Vermont Formation of the Pretoria
Group metasediments (P. Erikkson, personal comment).
A second unit mapped as ‘leptite-after-quartzite’ in the
Northern Section is dominated by thermally metamorphosed sandstones and contains large regions of metasediments with structures indicative of previously unconsolidated fine-grained material similar to the mudstone protolith
of the hornfels unit (Fig. 7b). The most likely protolith of
this lithology is the Lakenvalei formation of the Pretoria
Group metasediments (P. Eriksson, personal comment).
The ubiquitous microgranite that characterizes the Central Segment is nowhere present in the Northern Section.
Page 9 of 17 Contrib Mineral Petrol (2015) 170:56 (a)
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(b)
Fig. 7 Field relationships in the Mphanama Section of the Northern Segment. a Large photograph shows the regional relationships
between outcrops of quartzite, Upper Zone, and units mapped by
Molyneux (2008) as leptite-after-quartzite and hornfels. Smaller pho-
tographs show typical rocks of the metasedimentary hornfels. b Large
outcrop of unit mapped as ‘leptite-after-quartzite’ in the Mphanama
section. Rocks are thermally metamorphosed mudstones and sandstones and preserve their original sedimentary features
There is no evidence in either the metasedimentary hornfels unit or the ‘leptite-after-quartzite’ unit of any partial
melting during thermal metamorphism.
The patterns on Molyneux’s (2008) map reveal that
the Upper Zone and roof in this region do not display the
gentle westward dips that are typical of most of the rest of
the eastern Bushveld. Rather, there exists a NW–SE trending fold axis centered approximately 2 km to the east of
the Upper Zone–roof contact (Molyneux 2008). The fold
is also observed in the seismic reflection profile of Odgers
and du Pleiss (1993) as a broad anticlinal structure within
the Transvaal floor and Bushveld rocks, with the fold just
to the east of the Malopo Dome and Wonderkop Fault. The
fold results in shallow dips to the east near the Bushveld–
roof contact, which limits the stratigraphic exposure of the
Upper Zone cumulates (Molyneux 2008). Where the Upper
Zone is observed, only the uppermost few hundred meters
of stratigraphy from Magnetite Seam 17 (corresponding to
~375 m below the roof in the Magnet Heights type section of
Molyneux 1970) to the roof are typically exposed. The exposures in this region seem to be limited to areas capped by a
resistant layer of quartzite, such that the hills with outcrops
display Upper Zone in contact with either hornfels or quartzite, overlain by massive quartzite (Fig. 1; Molyneux 2008).
The Malopo Dome, also known in the literature as the
Driekop or Phepane Dome, has been interpreted to represent a metasedimentary diapir from the floor of the
intrusion (Johnson et al. 2004; Uken and Watkeys 1997;
Gerya et al. 2003) that penetrated partially molten Bushveld rocks. However, the rocks observed along the flanks
of the dome are similar to those observed in the roof exposures of the Mphanama section only ~4 km to the southwest, where floor diapirism is not inferred, suggesting that
the Malopo Dome is normal Bushveld Upper Zone with a
resistant quartzite cap. More work is required to understand
the significant structural complexity and outcrop patterns
of this region to understand the nature of the Malopo dome.
Mineralogy and geochemistry of the Northern Segment
lithologies
Hornfels
In the Northern Segment, the unit mapped as ‘hornfels’
on the map of Molyneux (2008) makes up the immediate
roof of the Upper Zone. From the field descriptions of the
Mphanama Section above, outcrops are clearly dominated
by thermally metamorphosed mudstones. Consistent with
this, in thin section the rocks are very fine-grained metapelites, with abundant orthoclase and only minor (<20 %)
quartz. Plagioclase is largely absent. Nearly pure magnetite
(Fe3O4) makes up an additional 10 % of the mode. Samples have abundant small planar lathes of fibrolite and large
elongated grains (0.5 mm) of sillimanite, which are also
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56 Page 10 of 17
present as inclusions in patches of biotite. Retrogressed
muscovite is present in some samples.
Leptite‑after‑quartzite
The unit stratigraphically above the hornfels in this region
is mapped as ‘leptite-after-quartzite,’ although quartzite
is absent where the unit crops out in the Mphanama Section. Thin sections are dominated by large euhedral grains
of quartz and orthoclase (Or98) with minor zircon, rutile,
and apatite. Plagioclase feldspar is notably absent, except
as exsolutions from the orthoclase (An0.2Ab99). Biotite and
muscovite were found in a very minor area of a single thin
section. In contrast to the hornfels unit, no sillimanite or
fibrolite was found in any of the samples investigated. Dark
nodules observed in hand sample were large (fist-sized
or slightly smaller) regions with pyroxene poikilitically
enclosing the quartz and orthoclase. The outcrop (Fig. 7b)
and phase assemblage suggest a distinctly sedimentary
protolith.
Upper Zone at contact
The diorite cumulates of the uppermost Upper Zone of the
Bushveld are dominated by plagioclase (An40), clinopyroxene (En3Fs44Wo52), and olivine (Fa99–95). Quartz is absent
and orthoclase is present only as antiperthite. Large euhedral grains of apatite are present mainly as inclusions in olivine and pyroxene. It is important to note that Fe–Ti oxides
are rare (<1 %) in the uppermost samples of the Upper Zone
in the Northern Section and where present are restricted to
ilmenite, not the magnetite-ulvospinel common elsewhere
in the Upper Zone. Grain sizes are similar to those throughout the Upper Zone and there is no evidence of a chilled
margin or enhanced rate of crystallization.
Southeastern Segment: Stoffberg Section
Field relations
South of the Nebo Plateau, near the town of Stoffberg (from
25°16.541′S, 30° 2.674′E to 25° 44.970′S, 29° 55.773′E),
the Lower and Critical Zones of the Bushveld Complex
are absent such that the cumulates of the Main Zone rest
directly on the Dullstroom volcanic rocks (Fig. 1).
In general exposure is very limited throughout the
area, but a few isolated outcrops exist in a small hill (S25°
26.363, E029° 49.158) on a farm accessible by a private dirt
road from road R33 with permission from the land owners. This section was previously described by Hall (1932)
and Groeneveld (1970), and more recently reinterpreted by
Cawthorn (2013). Several major features can be identified.
Rocks near the base of the hill are medium-grained diorites
13
Contrib Mineral Petrol (2015) 170:56
with no hornblende and likely correspond to the uppermost
cumulates of the Upper Zone. Approximately two-thirds of
the way up the hill, the rocks are amphibole-bearing diorites, termed hornblende quartz monzonites by Cawthorn
(2013). Further up in the section, the rocks become significantly more felsic, with quartz, orthoclase, and plagioclase
comprising >70 % of the total mineralogy. The uppermost
lithologies resemble the granophyres observed at the top of
the Masekete Section in the Central Segment. On a second
ridge of the hill slightly to the south, a minor amount of
thermally metamorphosed felsic rock forms a small cap at
the top and is laterally continuous with those further to the
NNW along the Bothasburg Plateau, according to Molyneux (2008). The rocks exposed along the Stoffberg section
were mapped as ‘leptite’ by Molyneux (2008); however,
they lack the characteristic hornfels–microgranite lithologies typical of the ‘leptite’ unit mapped in the Central
Segment. On the basis of trace elements, Cawthorn (2013)
proposed that this felsite corresponds to the Damwal Formation of the Rooiberg Group lavas.
A flat outcrop near the base of the hill reveals several
features of the lithologies that are not possible to observe
in the smaller outcrops elsewhere in the Stoffberg Section
(Fig. 8). The rocks possess a chaotic fabric of leucocratic
and melanocratic layers. There is a single small metasedimentary xenolith preserved (Fig. 8) within the mediumgrained fabric. There is no clear contact relationship
observed between the Bushveld cumulate rocks and the
roof rocks in this section.
Mineralogy and geochemistry of the Southern Segment
lithologies
Upper Zone near contact
The evolution of the uppermost Upper Zone in the Stoffberg Section of the Southern Segment is difficult to determine given the limited exposure. An Upper Zone cumulate
sample interpreted to be the stratigraphically lowest outcrop exposed in the section contains plagioclase (An40),
clinopyroxene (Mg#11), olivine (Fa95), minor (<1 %)
Fe–Ti oxides, and no orthoclase or quartz. Grain sizes of
the plagioclases are 1–3 mm and ~0.5 mm for the oxides
and mafic phases. Approximately 30 m stratigraphically
above the rocks also have plagioclase (An40), clinopyroxene (Mg# 20), and abundant magnetite. Olivine is notably
absent from the assemblage, and magnetite grains are typically rimmed by small quartz grains. Apatite and orthoclase
(Or74) are present as minor phases.
These rocks likely represent the uppermost Upper Zone
in this region. They possess cumulate textures in thin section as well as similar mineral compositions and grain sizes
to the cumulate rocks at the roof contact in the Central and
Page 11 of 17 Contrib Mineral Petrol (2015) 170:56 56
Fig. 8 Field relationships in the
Stoffberg Section of the Southern Segment. Large photograph
demonstrates the generally poor
outcrop exposure in this section
and shows the inferred regional
relationships between the rocks
of the Upper Zone, granophyre,
and felsite. Smaller photographs
show the complicated layering
found in diorite near the base
of the hill. It is very difficult to
determine exact petrogenetic
relationships between the units
in this region in the field. Arrow
in bottom left of large photograph indicates stratigraphic up
Northern Segments, and they resemble no other roof rocks
exposed elsewhere.
Monzonite
A highly altered sample obtained from a location described
as ‘monzonite’ by Cawthorn (2013) has significantly
smaller grain size from the diorite cumulates found below.
The rocks in this area contain, on average, 10–15 % quartz,
5–10 % orthoclase, 40 % plagioclase, 20 % fayalite and
hedenbergite, 10 % alteration of the pyroxene to hornblende, and 1–2 % Fe–Ti oxides (Cawthorn 2013). This
sample has plagioclase An23Ab75Or3 and two distinct
feldspar populations, one with An2Ab30Or67 and orthoclase with An0Ab3Or97, likely corresponding to subsolidus
reequilibration along the peristerite solvus. The monzonite
sample analyzed in this study also contains olivine (Fo7.5)
and clinopyroxene (Mg# 22), abundant magnetite and
ilmenite as well as apatite.
Felsite
The composition of the ‘monzonite’ sample analyzed in
our study is nearly identical to that of a nearby thermally
metamorphosed felsite collected approximately 400 m to
the south. The felsite has a slightly smaller grain size than
the ‘monzonite’ and is significantly less altered. The felsite
sample contains the same three feldspar populations with
An25Ab73Or2, An2Ab26Or72, and An1Ab3Or96 as the monzonite as well as abundant quartz and magnetite, and a trace
of clinopyroxene.
Granophyre
A single sample from the Stoffberg Section was identified
in outcrop as granophyre due to its slightly coarse grain
size and granophyric texture. In thin section, however, the
sample looks nearly identical to the ‘monzonite’ sample. It
contains abundant quartz, orthoclase, and magnetite, and
minor zircon and apatite. As in the monzonite and felsite
samples, the granophyre contains three feldspar populations, An16Ab82Or2, An4Ab95Or1, and An1Ab13Or86. The
compositions of the feldspars are more evolved than in
the monzonite and felsite samples and may have re-equilibrated to lower temperatures. Minor pyroxene has Mg# of
24, and no olivine was found.
The similarity in grain size, phase assemblage, and mineral compositions of the monzonite, felsites, and granophyre described here suggest that the ‘monzonite’ from this
region does not correspond to the uppermost Upper Zone,
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56 Page 12 of 17
as proposed by Cawthorn (2013), but is in fact thermally
metamorphosed and remelted roof material. If this is the
case, the cumulates at the top of the Upper Zone are similar
in composition and mineralogy to those present at the contact in the Central and Northern Segments. However, exposures are isolated and so severely limited in this region that
it remains unclear how the various lithologies are petrogenetically related.
Discussion
Due to the excellent exposure and large geographic range,
the Central Segment of the eastern Bushveld has received
the most attention in previous studies of the Bushveld Complex roof, and the lithologies present there have become
synonymous with ‘the roof.’ As described above, however,
the roof is laterally variable, with the Northern Segment
dominated by metapelite; the Central Segment dominated
by granophyre, granite, and felsite; and the Southern Segment dominated by felsite (Fig. 1, 9).
From the above descriptions two outstanding questions
remain:
1. How are the various lithological units petrogenetically
related to one another? In particular, what is the origin
of the microgranite/granodiorite present in the immediate roof of the intrusion, and how does it relate to the
other rocks in its immediate vicinity?
2. How were the mafic magmas of the RLS emplaced
within the shallow crust?
Contrib Mineral Petrol (2015) 170:56
Northern Segment Hornfels
The hornfels roof zone in the Northern Segment is dominated by thermally metamorphosed mudstones and sandstones. There are no blocks of felsic hornfels anywhere
present in the outcrops exposed in the Mphanama Section.
There does not appear to be any evidence for partial melting of the sedimentary lithologies during thermal metamorphism. In fact, original sedimentary structures, such as
cross-bedding and rip-up clasts, remain preserved, even in
areas stratigraphically close to the contact with the Bushveld Complex (Fig. 7). For these reasons, we conclude that
the immediate roof of the Bushveld Complex in the Northern Section is not petrogenetically related to the hornfels–
microgranite lithologies of the Central Segment, nor related
to the Rooiberg Group lavas or Stavoren granophyres in the
region.
Felsic Hornfels
Von Gruenewaldt (1972) mapped the immediate roof rocks
in the Central Segment in the Tauteshoogte and Paardekop
areas. There he reported that it consists of blocks of ‘leptite’ (i.e., hornfels) of variable size chaotically mixed with
and veined by ‘microgranite’ and locally by ‘micrographic
felsite,’ identical to the rocks observed in the Droogehoek
and Masekete Sections. Uncertain about the origins of
these rocks, he identified three possibilities, namely that
(a) the hornfels protolith is Rooiberg lava and the microgranite its molten equivalent, (b) the hornfels protolith is
Pretoria Group sandstone and the microgranite its molten
Fig. 9 Illustration of the change in lithology present in the immediate roof zone of the Bushveld Complex Upper Zone from N to S in the eastern limb. Thicknesses of units is not to scale, as it is not possible to pinpoint unit boundaries in many of the sections
13
Contrib Mineral Petrol (2015) 170:56 equivalent, or (c) the hornfels protolith includes both Rooiberg lava and sedimentary rocks, and the microgranite
mainly represents a residual melt from fractional crystallization of the Bushveld mafic magmas locally mixed with
melt derived by partial digestion of the hornfels. The first
of these was von Gruenewaldt’s (1968, 1972) preferred
choice.
On the basis of bulk rock major and trace element composition (Figs. 5, 6), mineral compositions, and field relations, we conclude that the hornfels blocks in the immediate
roof of the Central Segment are thermally metamorphosed
equivalents to the Dullstroom LMF and/or Damwal lavas.
Due to the similarity in composition between the Dullstroom LMF and the Damwal lavas, it is not possible to
distinguish between them. The small sedimentary hornfels
clasts that are present within the roof zone in the Central
Segment are consistent with the known occurrences of
minor intercalated sediments within the Rooiberg Group
(e.g., Twist 1985). Alternatively, the presence of the sedimentary hornfels in the Droogehoek Section, and absence
in the Masekete Section, could be due to the close proximity of the Droogehoek Section to the predominantly metasedimentary roof of the Northern Section.
Page 13 of 17 56
All samples of microgranite analyzed in this study have
trace element patterns similar to those of the hornfels
blocks. We conclude that the microgranite present in the
hornfels–microgranite lithology of the Central Segment
was formed by partial melting of the adjacent hornfels due
to thermal metamorphism with the underlying RLS. This
conclusion supports possibility (a) of von Gruenewaldt
(1972) that the microgranite is partially melted Dullstroom
LMF or Damwal lava. Walraven (1987) modeled the thermal effect of the emplacement of the Bushveld mafic
magma on the overlying roof rocks and concluded that the
amount of heat available from the crystallization of the
Bushveld was capable of partially melting up to 1000 m
of overlying material. While this is likely a maximum
estimate, given the necessary assumptions regarding the
emplacement depth, emplacement rate, and lithology, there
is no doubt that mafic magmas of the Bushveld Complex
were capable of partially melting significant volumes of
Damwal and/or Dullstroom lava to form the microgranite.
The lack of microgranite observed in contact with the
thermally metamorphosed felsite in the Southern Segment
is due to lower peak metamorphic temperatures in the
Southern Segment, where the RLS is considerably thinner
and was likely emplaced at a shallower stratigraphic level
as compared with the Central and Northern Segments (see
below). Lundgaard et al. (2006) documented a larger portion of trapped melt in the cumulate of the Main Zone near
the Southern Segment, which they attributed to faster cooling rates in this region. The inference of faster cooling rates
in this segment supports our hypothesis of lower peak metamorphic temperatures, as the heat from the emplacement
of the intrusion must has been more readily dissipated.
Based on trace element abundances, we can rule out possibility (c) of von Gruenewaldt (1972) that the microgranite represents the residual liquid of the RLS. VanTongeren
and Mathez (2012) calculated the rare earth element (REE)
concentration of the residual melt in equilibrium with apatites present at the top of the RLS. According to their estimate, the REE contents of the microgranite are too low to
represent the residual melt from the RLS (Fig. 10). Instead,
Fig. 10 Whole-rock rare earth element concentrations of the microgranite, granophyre, and Rooiberg Group lavas for comparison with
the estimated residual liquid composition from the Bushveld Complex mafic magmas. Residual liquid compositions for the Bushveld
Complex calculated from apatite–liquid trace element partitioning
by VanTongeren and Mathez (2012). Gray bar indicates the range of
compositions in equilibrium with the top of the Bushveld Complex.
The microgranite lithology found in the immediate roof zone is not a
match for the residual liquid from the Bushveld Complex. The granophyre and/or Kwaggasnek and Schrikkloof lavas are likely to represent the residual liquid erupted from the Bushveld (e.g., VanTongeren
et al. 2010; VanTongeren and Mathez 2012; Mathez et al. 2013)
Microgranite
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56 Page 14 of 17
the Rashoop granophyre has nearly identical REE concentration to that predicted for the RLS residual liquid and is
more likely to represent the missing magma (Fig. 10).
Granophyre
In the southern part of the Central Segment west of Roossenekal, the Stavoren Granophyre comprises a several
hundred-meter-thick sheet between underlying hornfels–
microgranite and overlying Rooiberg lavas, with which it
displays an intrusive relationship (von Gruenewaldt 1972;
Walraven 1987). Von Gruenewaldt (1972) interpreted the
granophyre to represent Rooiberg rhyolite completely
remelted by the RLS magmas, and Walraven (1987) interpreted it to represent the intrusive phase of those rhyolites.
Our results suggest that it is the microgranite that represents re-melted Rooiberg felsites (specifically Damwal or
Dullstroom formations) and that the compositional difference between the Stavoren Granophyre and the microgranite suggests that the Stavoren Granophyre does not represent partially remelted Rooiberg rhyolite. Furthermore,
although the major element compositions of the Stavoren
Granophyre and the Kwaggasnek and Schrikkloof members of the Rooiberg Group rhyolites are nearly identical,
Mathez et al. (2013) pointed out that the generally metaluminous character of the former distinguishes it from
the rhyolites, which tend to be weakly peraluminous. As
noted above, the granophyre also has REE concentrations
nearly identical to those expected of the residual liquids of
the RLS (e.g., VanTongeren and Mathez 2012). This along
with its highly ferroan and metaluminous major element
signature (e.g., Mathez et al. 2013) makes the Stavoren
Granophyre an ideal candidate to represent the residual liquid from the crystallization of the Upper and Upper Main
Zones of the RLS (e.g., VanTongeren et al. 2010). The general compositional similarity between the Stavoren Granophyre and Kwaggasnek and Schrikkloof rhyolites, however,
makes it difficult to definitively rule out the possibility that
these lavas may have been generated by fractional crystallization of the RLS.
For about 14 km along the east–west striking RLS contact south of the Kruis River (25°23′S, 29°37′E to 25°22′S,
29°44′E) (Fig. 1), the contact is marked by a <200-m-thick
sheet of granophyre. Here Lombaard (1949) documented
a progressive change over about 150 m stratigraphically in the rock mode from (45 % plagioclase + 18 %
hornblende + 14 % clinopyroxene + 8 % fayalite + 4 %
quartz + 1 % K-feldspar) to (59 % K-feldspar + 28 %
quartz + 13 % hornblende), illustrating that the granophyre
displays a gradational contact with the underlying olivine
diorite cumulate. Based on this, he interpreted the granophyre to represent the differentiated liquid from which the
cumulates formed. Walraven (1987) regarded the rock unit
13
Contrib Mineral Petrol (2015) 170:56
as distinct from the Stavoren Granophyre and named it
the Diepkloof Granophyre for the farm on which it is well
exposed. The Diepkloof Granophyre appears on both Lombaard’s (1949, Plate XIX) and Walraven’s (1987, p. 35)
maps. These are the only areas known to us where the field
evidence alone indicates that a granophyre may have been
produced by differentiation of RLS magmas.
Thin dikes and sills of granophyre are found associated
with the distinctive hornfels–microgranite unit within the
roof rocks throughout the eastern Bushveld (e.g., Molyneux 2008). In the Southern Segment along the edge of
the Bothasberg plateau, for example, the immediate roof of
the RLS is composed of layers of both rocks, which underlie the Rooiberg lavas (Groeneveld 1970). In the Central
Segment, the various granophyre bodies display considerable variability in the coarseness and development of their
characteristic micrographic texture (e.g., von Gruenewaldt
1968, 1972). The immediate roof rocks also host similarly
small bodies of relatively fine-grained granite, as observed
in the Masekete Section (Fig. 6). The relationship between
these small granite bodies and the Nebo Granite, which
forms the massive cliff face along the Nebo Plateau scarp,
is unknown. In fact, the intimate association and textural
variability of these small granophyre and granite bodies
render a clear distinction difficult, and it is conceivable that
some of these rocks are merely local variants of each other.
It is also not clear how the small granophyre bodies relate
to the large Stavoren Granophyre sill mapped by von Gruenewaldt (1972).
Emplacement model
Based on the above relationships, we propose a new model
for the emplacement of the RLS in the eastern Bushveld and its relationship to the roof and floor rocks. It is
clear from the map (e.g., Molyneux 2008) and the above
descriptions that the RLS did not simply intrude along a
single unconformity between the Rooiberg and the Pretoria Group, as previously proposed (Cheney and Twist
1991; Twist and French 1983). Over roughly 50 % of the
eastern Bushveld, the roof and floor rocks are composed
of mudstones and quartzites of the Pretoria Group. In the
regions to the north of Magnet Heights, there are no felsic meta-volcanic hornfels blocks, microgranite or felsic
granophyre anywhere present (Fig. 9). Immediately to the
south of Magnet Heights, the roof is dominated by felsic
meta-volcanic hornfels and its molten microgranite equivalent (plus minor metasedimentary xenoliths), with a greater
proportion of meta-volcanic hornfels further to the south
(Fig. 9). This is mirrored in the floor rocks that crop out
in this region, which grade from Pretoria Group sediments
in the Central region to Dullstroom volcanics in the south
Contrib Mineral Petrol (2015) 170:56 Page 15 of 17 56
(Fig. 1). Further to the south, meta-volcanic rocks and their
recrystallized equivalents dominate the roof of the intrusion, and metasedimentary rocks are entirely absent.
We propose that the mafic magmas of the RLS were
emplaced within the Pretoria Group sediments, now
quartzites, in the north whereas further to the south, where
both the floor and roof rocks are volcanic, the RLS was
emplaced within the lower portions of the Rooiberg Group
felsites (Dullstroom and Damwal formations) (Fig. 11).
The Central Segment just to the south of Magnet Heights
represents a transition between these two, in that the floor
rocks are dominated by sedimentary lithologies, whereas
the roof rocks are clearly meta-volcanic.
Our proposed emplacement scenario (Fig. 11) is supported by the presence of a large xenolith of meta-volcanic, metamorphosed Dullstroom lavas nearly entirely
transformed to microgranite within the cumulates of the
Main Zone and Upper Zone in the southeastern Bushveld
(Fig. 1). The microgranite/Dullstroom raft is present near
the Main Zone–Upper Zone boundary and appears to be
concordant with the igneous layering. It extends for nearly
40 km along strike from ~25° 20′S, 29°53′E to 25° 45′S,
29°50′E and possibly further to the south beyond the map
of Molyneux (2008). In our model, the microgranite/Dullstroom volcanic raft represents a portion of the pre-Upper
Zone roof that was intruded by the growing RLS magma
chamber, rather than a stoped block of material that fell
coherently from the roof, as was previously proposed by
Cawthorn (2013). A stoped block of felsic material, with
significantly lower density as compared with the mafic
magma, would not be likely to sink more than two kilometers vertically in a convecting magma chamber. The
microgranite raft is everywhere located at or above the
Main Zone–Upper Zone boundary, lending further support
to the hypothesis that the Upper Zone marks a major new
influx of magma (e.g., Cawthorn et al. 1991; VanTongeren
and Mathez 2013), which led to vertical and lateral magma
chamber growth (Kruger 2005). Our model is consistent
with the hypothesis of Clarke et al. (2009) suggesting that
the mafic magmas propagated as magmatic fingers in a
general NW–SE direction away from the Thabazimbi-Murchison Lineament.
The volcanic rocks that are present stratigraphically
above the RLS in the Southern Segment are not observed
in contact with the uppermost cumulates. Where the felsite
is present in the roof of the Stoffberg Section, it is not partially remelted and therefore likely did not experience the
same degree of thermal metamorphism, as the hornfels–
microgranite pairs seen in the Central Segment. One possibility is that the RLS in this region is significantly thinner,
and therefore the total thermal perturbation to the country
rocks was not as high as in the Central Segment where significant volumes of partial melt were created. This is consistent with the lack of Critical or Lower Zone in the Southern Segment. Another possibility is that Rooiberg volcanic
rocks in this region were erupted after significant crystallization of the RLS (e.g., VanTongeren et al. 2010) and were
not present during the majority of RLS emplacement, as
Fig. 11 New model for the emplacement of the Bushveld Complex
layered mafic magmas within the Transvaal Basin prior to emplacement of the later Nebo and Lebowa Granites. In the Northern Segment of the eastern limb, the roof and floor are defined by metasedimentary rocks of the Transvaal Supergroup. In the Central Segment,
the floor is metasedimentary, but the roof is meta-volcanic—likely
the Dullstroom and/or Damwal lavas of the Rooiberg Group. In the
Southern Segment, both the floor and roof are defined by meta-volcanic rocks of the Rooiberg Group, and there is a 40-km-long metavolcanic raft that is stratigraphically conformable along strike near
the geochemical boundary between the Upper Zone and the Main
Zone. We propose that the mafic magmas of the Bushveld Complex
intruded the crust shallowly and at different stratigraphic levels from
north to south
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56 Page 16 of 17
is commonly considered (e.g., Cawthorn 2013). We suggest that the field observations are most consistent with
extremely shallow emplacement of the RLS in the south.
Conclusions
We have described in detail the various lithologies present in the immediate roof zone of the Bushveld within
three segments from north to south in the eastern limb. The
immediate roof of the intrusion grades from dominantly
metasedimentary in the north to dominantly meta-volcanic
in the Central and Southern Segments. In the Central Segment, the roof of the intrusion is dominated by a complex
lithology consisting of hornfels blocks embedded in a
matrix of microgranite. On the basis of bulk rock and mineral compositions, we conclude that the hornfels blocks are
thermally metamorphosed Dullstroom LMF and/or Damwal lava. The microgranite does not bear any geochemical relationship to the ubiquitous Stavoren granophyre and
does not have the requisite REE concentration to be the
residual liquid from the Bushveld mafic magma. Instead,
the microgranite that forms the matrix of the hornfels–
microgranite lithology likely formed due to partial melting
of Dullstroom LMF or Damwal hornfels at extreme temperatures such as found in the pyroxene-hornfels facies.
The changing roof and floor lithologies present in the
eastern Bushveld implies that, instead of intruding along a
regional unconformity between sediments of the Pretoria
Group and the Rooiberg Group lavas (Cheney and Twist
1991), the Bushveld magmas were emplaced within units
of the Pretoria Group sediments in the north, most likely
Magaliesberg sandstone and quartzite in the floor and
Houtenbek, Lakenvalei, or Vermont formation in the roof.
In the Central Segment, the intrusion assumes the canonical relationship of sedimentary rocks of the Magaliesberg
quartzite in the floor and Dullstroom and/or Damwal lava
in the roof zone. Further to the south, however, Dullstroom
volcanics make up the immediate floor of the intrusion and
the top of the RLS may grade directly into granophyre with
a felsite roof. On the basis of these relationships, we infer
that the RLS was emplaced as an enormous hypabyssal sill.
The sill occupied varying structural levels, being deeper in
the north than in the Central and Southern Segments, where
it may have come very near to the surface (Fig. 11).
More detailed mapping of each of these segments will be
required in order to understand the nature of these emplacement and petrogenetic relationships as well as the thermal
evolution of the Bushveld magmas and its roof.
Acknowledgments This work was supported by NSF-EAR
0947247 awarded to E. A. Mathez.
13
Contrib Mineral Petrol (2015) 170:56
References
Buchanan PC, Koeberl C, Reimold WU (1999) Petrogenesis of the
Dullstroom formation, Bushveld Magmatic Province, South
Africa. Contrib Miner Petrol 137:133–146
Buchanan PC, Reimold WU, Koeberl C, Kruger FJ (2002) Geochemistry of intermediate to siliceous volcanic rocks of the Rooiberg
Group, Bushveld Magmatic Province, South Africa. Contrib
Miner Petrol 144:131–143
Cawthorn RG (2013) The Residual or Roof Zone of the Bushveld
Complex, South Africa. J Petrol 54:1875–1900
Cawthorn RG, Meyer PS, Kruger FJ (1991) Major addition of magma
at the Pyroxenite Marker in the Western Bushveld Complex,
South-Africa. J Petrol 32:739–763
Cheney ES, Twist D (1991) The conformable emplacement of the
Bushveld Mafic rocks along a regional unconformity in the
Transvaal Succession of South-Africa. Precambr Res 52:115–132
Clarke B, Uken R, Reinhardt J (2009) Structural and compositional
constraints on the emplacement of the Bushveld Complex, South
Africa. Lithos 111:21–36
Frost BR, Frost CD (2008) A geochemical classification for feldspathic igneous rocks. J Petrol 49:1955–1969
Gerya TV, Uken R, Reinhardt J, Watkeys MK, Maresch WV, Clarke
BM (2003) Cold fingers in a hot magma: numerical modeling
of country-rock diapirs in the Bushveld Complex, South Africa.
Geology 31:753–756
Groeneveld D (1970) The structural features and the petrography
of the Bushveld Complex in the vicinity of Stoffberg, Eastern
Transvaal. Geol Soc S Afr Spec Publ 1:36–45
Hall AL (1932) The Bushveld Igneous Complex of the central Transvaal. S Afr Geol Surv Mem 28:510
Hill M, Barker F, Hunter D, Knight R (1996) Geochemical characteristics and origin of the Lebowa Granite Suite, Bushveld Complex. Int Geol Rev 38:195–227
Johnson T, Brown M, Gibson R, Wing B (2004) Spinel–cordierite
symplectites replacing andalusite: evidence for melt-assisted
diapirism in the Bushveld Complex, South Africa. J Metamorph
Geol 22:529–545
Kruger FJ (2005) Filling the Bushveld Complex magma chamber:
lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti-Vmagnetite deposits. Miner Depos 40:451–472
Loberg BE (1980) A Proterozoic subduction zone in southern Sweden. Earth Planet Sci Lett 46:287–294
Lombaard AF (1949) Die geologie van die Bosveldkompleks langs
Bloedrivier (The geology of the Bushveld Complex along Blood
River). Geol Soc S Afr Trans 52:343–376
Lundgaard KL, Tegner C, Cawthorn RG, Kruger FJ, Wilson JR
(2006) Trapped intercumulus liquid in the Main Zone of the
eastern Bushveld Complex, South Africa. Contrib Mineral Pet
151(3):352–369
Mathez EA, VanTongeren JA, Schweitzer J (2013) On the relationships between the Bushveld Complex and its felsic roof rocks,
part 1: petrogenesis of Rooiberg and related felsites. Contrib
Miner Petrol 166:435–449
Molyneux TG (1970) The geology of the area in the vicinity of Magnet Heights, eastern Transvaal, with special reference to the magnetic iron ore. Geological Society of South Africa Special Publication 1:228–241
Molyneux TG (1974) A geological investigation of the Bushveld
Complex in Sekhukhuneland and part of the Steelpoort Valley.
Trans Geol Soc S Afr 77:329–338
Molyneux T (2008) Compilation on a scale of 1:50000 of the geology
of the eastern compartment of the Bushveld Complex. Johannes
Contrib Mineral Petrol (2015) 170:56 Willemse Bushveld Intelligence Center, Dept of Geology, University of Pretoria, Pretoria
Odgers A, du Plessis A (1993) Interpretation of a regional reflection seismic survey in the north-eastern Bushveld Complex. 3rd
SAGA Biennial conference and exhibition
SACS (1980) Lithostratigraphy of the Republic of South Africa,
South West Africa/Namibia and the Republics of Botswana,
Transkei, and Venda. S Afr Geol Surv Handb 8:690
Schweitzer JK (1998) The Dullstroom Formation and the Rooiberg
Group, volcanic rocks associated with the Bushveld Complex.
Ph.D. Thesis, University of Pretoria
Schweitzer JK, Hatton CJ, DeWaal SA (1997) Link between the granitic and volcanic rocks of the Bushveld Complex, South Africa.
J Afr Earth Sci 24:95–104
Twist D (1985) Geochemical evolution of the Rooiberg Silicic Lavas
in the Loskop Dam Area, Southeastern Bushveld. Econ Geol
80:1153–1165
Twist D, French BM (1983) Voluminous acid volcanism in the Bushveld Complex: a review of the Rooiberg Felsite. Bull Volcanol
46:225–242
Twist D, Harmer REJ (1987) Geochemistry of contrasting siliceous
magmatic suites in the Bushveld Complex: genetic-aspects and
Page 17 of 17 56
implications for tectonic discrimination diagrams. J Volcanol
Geoth Res 32:83–98
Uken R, Watkeys MK (1997) Diapirism initiated by the Bushveld
complex, South Africa. Geology 25:723–726
VanTongeren JA, Mathez EA (2012) Large-scale liquid immiscibility at the top of the Bushveld Complex, South Africa. Geology
40:491–494
VanTongeren JA, Mathez EA (2013) Incoming magma composition and style of recharge below the pyroxenite marker, Eastern
Bushveld Complex, South Africa. J Petrol 54:1585–1605
VanTongeren JA, Mathez EA, Kelemen PB (2010) A felsic end to
Bushveld differentiation. J Petrol 51:1891–1912
von Gruenewaldt G (1968) The Rooiberg Felsite north of Middleburg
and its relations to the Layered Series of the Bushveld Complex.
Trans Geol Soc S Afr 71:153–172
von Gruenewaldt G (1972) The origin of the roof-rocks of the Bushveld Complex between Tauteshoogte and Paardekop in the eastern Transvaal. Trans Geol Soc S Afr 76:207–227
Walraven F (1987) Textural, geochemical, and genetic aspects of the
granophyric rocks of the Bushveld Complex, Geological Survey of South Africa: Memoir, vol 72. Republic of South Africa,
Department of Mineral and Energy Affairs, Geological Survey
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