JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 9
PAGES 1607^1637
2009
doi:10.1093/petrology/egp042
Origin of Anorthosite and Magnetitite Layers in
the Bushveld Complex, Constrained by Major
Element Compositions of Plagioclase
R. GRANT CAWTHORN* AND LEWIS D. ASHWAL
SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, PO WITS, 2050, SOUTH AFRICA
RECEIVED NOVEMBER 5, 2008; ACCEPTED JUNE 4, 2009
The Bushveld Complex, a layered mafic intrusion in South Africa,
shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 28 km of the
intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An
content of plagioclase has previously been determined for 420 samples
and decreases upward from An78 to An36, with six minor, slow
reversals. Variations in modal proportions of plagioclase have been
calculated based on 2200 density determinations on whole-rocks.
Forty-five anorthosite layers have been identified, ranging from 1 to
23 m thick. None of these layers is associated with the abovementioned reversals in An content in plagioclase and nearly all have
leucocratic rocks below and above, with more than the likely cotectic
proportions of plagioclase. These observations argue against an
origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization,
possibly associated with pressure changes, followed by crystal settling
and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have
been identified. There is a significant reversal in An content in the
overlying plagioclase compared with the underlying sample across
only one such layer. Again, this observation challenges hypotheses
that such layers result from magma addition, but is consistent with
a pressure-change hypothesis for triggering magnetite crystallization.
The upper contacts of magnetitite layers that grade into anorthosite
over many centimetres possibly also reflect settling and sorting.
Rocks forming the uppermost 100 m of the intrusion contain the
most sodic plagioclase compositions, demonstrating that there is no
downward crystallizing roof facies. Furthermore, this uppermost
*Corresponding author. Telephone: þ27 11 717 6557. Fax: 6579.
E-mail: [email protected]
100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged
suspension of plagioclase.
anorthosite; Bushveld Complex; magnetitite; modal
layering; plagioclase compositions
KEY WORDS:
I N T RO D U C T I O N
The processes operating in large, differentiated mafic magmatic systems ought to be reflected in the changing compositions of the major minerals. In the classic example of the
Skaergaard Intrusion, it was shown by Wager & Brown
(1968) that the An content of plagioclase decreases with
increasing differentiation from An77 to An30. All subsequent studies have reaffirmed their stated basic principle,
namely that the intrusion was formed from a single injection of magma. However, those workers and Maaloe
(1976) noted that the cores of plagioclase grains in this
intrusion are far from homogeneous, probably resulting
from the contributing effects of supersaturation, nucleation
and variable crystal growth rates (and hence diffusion
rates through the magma), resorption, recycling within
convecting magma and settling through possibly stratified
magma columns. In other intrusions additional processes
and complications are introduced when periodic magma
addition is considered, including the extent of stratification
versus mixing (e.g. Campbell, 1996). One of the ultimate
aims of studies of layered intrusions is to determine the
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JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 9
SEPTEMBER 2009
Fig. 1. Simplified geological map of the Bushveld Complex, showing location of the Bellevue borehole core, BV-1, in the northern lobe studied
here, and Bierkraal core, BK, in the west (Tegner et al., 2006). Transvaal and Rooiberg refer to the Transvaal Supergroup, a major sedimentary
succession, and the Rooiberg Group, a thick volcanic succession, both of which predate the mafic rocks of the Bushveld Complex.
cause of mineral layering. Plagioclase compositions are
potentially the most valuable of all mineral parameters in
such studies because of the extremely slow diffusive
exchange between CaAl and NaSi (Morse, 1984) such that
primary compositions are likely to be preserved in the
cores of grains. Processes such as re-equilibration with
trapped liquid, compaction and annealing can add complications to the interpretation of original cumulus mineral
compositions. However, the study by Lundgaard et al.
(2006) on rocks from the eastern Bushveld Complex
showed that the presence of variable proportions of
trapped liquid did not affect the core compositions of plagioclase grains. Hence, we believe that the plagioclase
compositions reported here generally represent the compositions of the primary accumulating grains. In contrast,
mafic minerals rapidly exchange Mg^Fe, obliterating any
primary zonation. Trace elements and isotope ratios can
also provide valuable constraints, although their diffusion
rates permit some partial homogenization that may variably overprint the original variations (e.g. Davidson et al.,
2001, 2007).
In this study we focus on the major-element concentrations in plagioclase analysed from a 28 km deep borehole
core drilled through the upper part of the Bushveld
Complex, South Africa (Ashwal et al., 2005). A generalized
map and various stratigraphic sections through the
Bushveld Complex are shown in Figs 1 and 2. Within this
succession there are numerous anorthosite, magnetitite
and (four) feldspathic pyroxenitic layers, and we test various hypotheses about their geneses, using plagioclase mineral compositions and modal proportions. The
compositions of plagioclase in this section are shown in
Fig. 3, together with the locations of the anorthosite and
magnetitite layers.
1608
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
Bierkraal
KEY
pig +
opx
Bellevue
Ashwal et al.
(2005)
von Gruenewald
(1973)
Magnet Heights
Union Section
pig
Molyneux (1974)
Mitchell (1990)
opx
0
Roossenekal
Walraven &
Wolmarans (1979)
Cawthorn et al. (1991)
Tegner et al. (2006)
1400
ol +
opx
250
500
apatite in
apatite in
apatite in
apatite in
Depth, meters
750
olivine in
olivine in
1000
1250
UZ
MZ
olivine in
olivine in
magnetite in
magnetite in
PM
PM
Marikana area
1500
magnetite in
0
Nex et al. (1998)
magnetite in
magnetite in
PM
1750
Pyx Horizon
2000
PM
PM
2250
2750
, meters
2500
-1400
EOH
Northern Lobe
Western Lobe
Eastern Lobe
Fig. 2. Various vertical sections through the Main and Upper Zones in different lobes showing probable correlations and differences. (The different shadings denoted opx, pig and ol refer to whether the Ca-poor pyroxene was originally orthopyroxene or pigeonite, the latter now
inverted to orthorhombic symmetry, and where olivine is present. Plagioclase and a Ca-rich pyroxene are present throughout. PM is
Pyroxenite Marker, dominated by orthopyroxene. Pyroxenite horizon is dominated by Ca-rich pyroxene.) The scale on the Bellevue section
from 0 to 2900 m (scale on left) refers to absolute depth in the borehole core, as used by Ashwal et al. (2005). The scale from 1400 to 1400 m
(scale on right) refers to location relative to the Main Zone^Upper Zone boundary, which is used in the subsequent figures.
SOM E GEN ER A L PR I NC I PLES
Here we briefly summarize how various plausible magmatic processes, both chemical and physical, can be identified by studying trends of changing plagioclase
composition and modal proportions of plagioclase in vertical successions within layered intrusions. In a Main Zone
rock, dominated by plagioclase and pyroxene, the density
of the rock can be used as a proxy for the modal proportion
of the two minerals. The consequences of such processes in
terms of An content in plagioclase and modal proportions
(deduced from density) are shown schematically in the
vertical sections in Fig. 4. It should be noted that in these
conceptual diagrams we are referring to the cumulus
phase compositions only. In a large, homogeneous differentiating magma chamber the An content of plagioclase
ought to decrease systematically upward, as shown in
Fig. 4a (i) (excluding any roof facies, as in the Skaergaard
Intrusion, but absent in the Bushveld Complex). Sorting
between mafic and felsic minerals during settling should
have no effect on such a trend, although the rate of
change of An mol % will be influenced by the proportions
of phases crystallizing and accumulating (Morse, 2006).
In our dataset there are four data points that do not fit
such a trend, as shown in Fig. 4a (ii). We choose to ignore
them as being rogue values, but they could represent a
very small-scale version of one of the following processes.
Magma addition versus mineral sorting
If the new magma is more primitive, a reversal in An content may be observed [Fig. 4a (iii) and (iv)]. The reversal
could be abrupt [Fig. 4a (iii)], which could represent
instantaneous mixing and homogenization of new and resident magmas, or it could be that the new magma was
emplaced under the resident magma, with no mixing at
all, because of its higher density. If there was protracted
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VOLUME 50
NUMBER 9
1500
100
1400
0
1300
–100
1200
–200
900
800
apatite in
olivine in
–500
–600
–700
–800
500
–900
400
–1000
300
–1100
200
–1200
100
–1300
0
Upper Zone
olivine out
P
–1400
Main Zone
(a)
–1500
50
–100
40
P
–400
700
600
Upper Zone
Main Zone
Anorthosite layers
1000
SEPTEMBER 2009
–300
Magnetitite layers
1100
Anorthosite layers
Depth (m) Relative to MZ/UZ Boundary
JOURNAL OF PETROLOGY
50
60
(b)
60
70
80
mole % An
mole % An
Fig. 3. Plot of average An content in plagioclase as a function of position relative to the Main Zone^Upper Zone boundary where magnetite
becomes a major phase. (Data from Ashwal et al., 2005.) The locations of the anorthosite and magnetitite layers are shown on the left and
right of the two columns, respectively. Plagioclase and two pyroxenes are present throughout the sequence. The appearance and disappearance
of other minerals is indicated. P refers to thin feldspathic pyroxenite layers. (Note the slight overlap of the data from ^100 to 100 m in the two
columns.)
mixing, or very slow addition of magma, a gradual reversal might result [Fig. 4a (iv)]. Addition of magma that
was more differentiated could lead to forward jumps in
the An content, which could either be abrupt [Fig. 4a (v)]
or gradual [Fig. 4a (vi)]. If the magma chamber became
stratified then reversals of the form in Fig. 4a (iii) and
(iv) could be generated by downward convection or circulation of less differentiated magma from the upper part
of the magma chamber. Distinguishing between externally and internally derived magmas emplaced into the
zone of crystallization would not be possible using only
An values.
Changes in whole-rock density in layered intrusions largely reflect changes in modal proportions, primarily the
proportion of plagioclase. Thus, the whole-rock density
can be used as a proxy for the proportion of plagioclase to
pyroxene. In a simplified binary phase diagram of plagioclase and pyroxene, different liquid compositions can crystallize a cumulus assemblage of plagioclase only, pyroxene
only, or plagioclase and pyroxene in their cotectic proportions, which are about 60:40 (by weight) for the An and
mg-number [Mg/(Mg þ Fe)] values of the minerals in
this study. Modal proportions different from this ratio are
not easily explained unless sorting by some physical
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CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
(a) (i)
(ii)
(iii)
(iv)
(v)
(b) (i)
(ii)
(iii)
(iv)
(v)
(vi)
/
Fig. 4. (a) Schematic representation of possible trends in An content in plagioclase as a function of height in an intrusion, resulting from various magmatic processes discussed in the text. (b) Schematic representation of possible trends in bulk-rock density (assuming only plagioclase
and pyroxene as cumulus phases) as a function of height, resulting from various magmatic processes discussed in the text. A, P and G refer to
anorthosite, pyroxenite and gabbro.
mechanism occurs that redistributes the proportions of
phases crystallizing as they accumulate. We note that in
many publications on layered intrusions the distinction
between the phase assemblage and proportions that are
crystallizing from a magma and the phase assemblage
and proportions that are actually accumulating in layers
is not clearly drawn, and we suggest here that this distinction is crucial. A pattern in Fig. 4b (i) that shows an
abrupt change from anorthosite (density528 g/cm3) to
gabbronorite with the cotectic proportion (density about
295 g/cm3) could be explained by crystallization and accumulation from a liquid initially saturated only in plagioclase that reaches the cotectic. Patterns of regularly
decreasing density [Fig. 4b (ii)], culminating in anorthosite, could be explained by settling and sorting from a suspension of plagioclase and pyroxene grains crystallized
from a magma lying at the cotectic for these minerals.
However, in this model, for an anorthosite to accumulate
requires that no further cumulus pyroxene continues to
crystallize because it would rapidly sink as a result of its
high density. The graded layers in the Skaergaard
Intrusion were attributed by Wager & Brown (1968) to
such a process. Pulsed crystallization was envisaged so
that the low-density plagioclase could sink to form
extremely leucocratic layers without being joined by continued crystallization of dense minerals.
A pattern that shows an abrupt change from relatively
homogeneous gabbronorite to anorthosite cannot easily be
explained from a single magma, even incorporating settling processes [Fig. 4b (iii)]. Gradual increases in the proportion of pyroxene upward [Fig. 4b (iv)] are similarly
difficult to explain from a single magma.
Maaloe (1978) and Morse (1979a, 1979b) suggested that
supersaturation and delayed nucleation processes were
important. In such a model, pyroxene might continue as
the sole crystallizing phase even though the magma had
reached the pyroxene plus plagioclase cotectic composition. Subsequently, rapid nucleation of plagioclase could
produce that mineral in great excess of the cotectic proportion. The magma would then be driven to become supersaturated in pyroxene. Thus, the magma would oscillate
about the cotectic rather than remaining at the cotectic.
Such a process ought to produce a density pattern as
shown in Fig. 4b (v), and so should be easily identifiable.
The ability to identify these various processes will be
partially dependent upon the frequency of vertically
spaced data points. If samples are widely spaced, two adjacent data points may suggest a sharp break, whereas
1611
JOURNAL OF PETROLOGY
VOLUME 50
detailed sampling of the intervening interval may show a
gradual trend over a vertical interval comparable with or
slightly less than the actual sample spacing. Equally, wide
sample spacing may produce an apparently smooth trend,
whereas more detailed sampling might reveal distinct
breaks.
We present our data below in the form of graphs analogous to Fig. 4, and discuss their implications.
R E L AT I O N S H I P T O P R E V I O U S
BUS H V E L D ST U D I E S
The enormity of the areal (65 000 km2 preserved) and vertical extents (up to 8 km) of the Bushveld Complex makes
comprehensive studies almost impossible. Existing information has been reviewed by Eales & Cawthorn (1996)
and Cawthorn et al. (2007). The intrusion crops out for
over 100 km in each of three major lobes, the eastern, western and northern lobes (Fig. 1). The eastern and western
lobes have many distinctive layers and packages of rocks
in common, appear to be mirror images and are probably
connected at depth (Cawthorn et al., 1998; Cawthorn &
Webb, 2001). The northern lobe is different from the eastern
and western in its lower succession, but becomes similar
to the other two upward (Fig. 2). The vertical subdivisions
consist of the Lower Zone dominated by cumulus orthopyroxene and olivine, Critical Zone (orthopyroxene, plagioclase and chromite), Main Zone (Ca-rich and Ca-poor
pyroxene and plagioclase) and Upper Zone (plagioclase,
magnetite, two pyroxenes, olivine and apatite). With
respect to this study we emphasize that there is no early
accumulation of layers at the top of the intrusion as
occurred for the Skaergaard Intrusion (Wager & Brown,
1968). Mineral compositions show that fractionation was
unidirectional to the extreme top of the Bushveld
Complex (Eales & Cawthorn, 1996; Ashwal et al., 2005;
Tegner et al., 2006).
We focus on samples from a borehole (BV-1 in Fig. 1)
drilled through the upper 28 km of the intrusion in the
northern lobe. The hole was collared in Bushveld granite,
six thin sheets of which and two younger dolerite bodies
intruded the uppermost portion of the layered mafic intrusion (Ashwal et al., 2005). Thin-section inspection of the
layered rocks more than 1m away from these dykes
showed them to be perfectly fresh.
The Upper Zone is readily characterized in the field and
core because it is based on the presence of cumulus magnetite in rocks containing varying proportions of cumulus
plagioclase, clinopyroxene and orthopyroxene, and joined
progressively upward by olivine and apatite, and then a
variety of minor, non-cumulus minerals: ilmenite, biotite,
hornblende, quartz and alkali feldspar. In the BV-1 core it
is 1200 m thick, and correlates closely with the succession
in the eastern and western lobes (Fig. 2). Below this Upper
NUMBER 9
SEPTEMBER 2009
Zone lies the Main Zone, which consists of plagioclase
and various pyroxenes as cumulus minerals. In the northern lobe, studied here, there is also a troctolite succession
in the Main Zone (where the drill core was terminated)
that is not found in the eastern or western lobes
(Ashwal et al., 2005). Subdivision of the Main Zone
(Fig. 2) has been based on whether the low-Ca pyroxene is
inverted pigeonite or primary orthopyroxene (Nex et al.,
1998). In the eastern and western lobes there is a
Pyroxenite Marker horizon (dominated by orthopyroxene)
across which there is a protracted reversal in plagioclase
and pyroxene compositions and a distinct change in
the initial Sr isotopic ratio (von Gruenewaldt, 1970,
1973; Sharpe, 1985; Kruger et al., 1987; Cawthorn et al.,
1991). In the northern lobe there is a pyroxene-enriched
layer (Fig. 2); however, it contains abdundant clinopyroxene and there is no mineral compositional change across
it (Ashwal et al., 2005), and hence it is not considered the
analogue of the Pyroxenite Marker in the eastern and
western lobes. Apart from the anorthosite, magnetitite
and four feldspathic pyroxenite layers described here,
modal layering is poorly developed, variation in mineral
proportions is subtle, and the boundaries between rock
types are difficult to locate precisely. A measure of this
paucity of modal layering can be demonstrated by the similarity between adjacent density measurements as discussed in the Appendix.
In the present study, a borehole core that traversed the
entire Upper Zone (1400 m) and penetrated 1400 m of
the Main Zone was sampled. All depths here are quoted
relative to the Main Zone^Upper Zone boundary, and are
not corrected for a dip of 20^258 to the west (van der
Merwe, 1976). Hence, these values slightly exaggerate the
true thicknesses.
Based on the cumulus mineralogy, a correlation between
this core and the better-known eastern and western lobes
can be inferred (Fig. 2). The nature of the modal layering
and mineral compositions in the Main and Upper Zones
of the better-exposed eastern lobe have been presented by
von Gruenewaldt (1973) and Molyneux (1974). In their studies, plagioclase compositions were determined by optical
methods and X-ray diffraction, which are considered
valid, but do not permit detailed examination of zoning.
Von Gruenewaldt (1973) and Molyneux (1974) analysed
101 and 26 samples through 3900 m of Main Zone, and 48
and 21 samples through 2200 m of Upper Zone, respectively. Their data were based on field samples, so exact vertical heights were not as precise as from the borehole core.
Poor outcrop in the western lobe does not permit such
detailed field sampling. Borehole core (BK in Fig. 1) was
sampled by Tegner et al. (2006), who reported electron
microprobe analyses for nine samples from 500 m of the
uppermost Main Zone and 46 samples through the entire
1800 m of the Upper Zone. Nex et al. (1998) reported on
1612
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
the underlying succession based on field samples,
and obtained major- and trace-element analyses of plagioclase on mineral separates by X-ray fluorescence spectrometry. In all these studies such sampling represents a
vertical spacing of 30^100 m. Ashwal et al. (2005)
reported aspects of the layering and evolution of
2800 m of mafic rocks from the Upper Zone and part of
the Main Zone from borehole core in the northern
lobe. They obtained electron microprobe data for 420 samples over that interval, at an average spacing of 6 m,
although some sections were investigated in extreme
detail and others much more widely spaced, depending
upon the presence of modal layering. Here we focus
on the cumulus plagioclase compositions reported in
that study with special reference to the anorthosite and
magnetitite layers.
W H Y P L AG I O C L A S E A N D N O T
MAFIC MI N ER A LS?
In the study of Ashwal et al. (2005) electron microprobe
analyses of plagioclase, pyroxenes, olivine, magnetite,
ilmenite, amphibole and biotite were summarized. Here
we focus specifically on the plagioclase compositions,
given in Supplementary Dataset 1 (available for downloading at http://petrology.oxfordjournals.org/), for a number
of reasons. Most of the rocks contain at least 50% plagioclase, and plagioclase contents average 60% for all samples analysed. In 15 magnetite-rich and 13 pyroxene-rich
samples analysed there was less than 20% plagioclase.
Thus, an essentially continuous record of cumulus plagioclase compositions has been obtained. In contrast, in 144
samples there was less than 10% total pyroxene, and less
than 20% in a further 89 samples. Olivine was present
only in the lowest 200 m of the section, and intermittently
in the upper 800 m. Thus, the mafic minerals do not give
a continuous record of their cumulus compositions.
Of more importance is the fact that primary cumulus
compositions of the minerals should be compared. In the
case of the mafic minerals, any primary zonation that
may have existed has been destroyed by diffusion between
Mg and Fe. Also, when a mafic mineral is present in low
abundance its bulk composition may have been variably
influenced by the relative proportion of interstitial liquid
with which it equilibrated (Hess, 1960; Barnes, 1986).
Finally, in the Upper Zone, the mafic minerals coexist
with variable proportions of magnetite. Subsolidus
exchange between oxide and mafic phase in these rocks
has been reported, with Mg from oxide redistributing into
the mafic phase with decreasing temperature (Reynolds,
1985b). For all these various reasons, the Mg:Fe relationships in the mafic phases will not be considered in this
interpretation.
D E F I N I NG C U M U LU S
P L AG IOC L A S E C OM P O S I T ION S
Interpreting vertical trends in plagioclase composition
depends initially upon knowing the initial cumulus composition. The original definition of a cumulus crystal by
Wager et al. (1960) emphasized the presence of a cumulus
core and an intercumulus rim, which might be normally
or reversely zoned (e.g. Morse & Nolan, 1984). During electron microprobe analysis it is possible to select what
appear to be the centres of relatively euhedral grains.
However, given that a thin section is a two-dimensional
cut through three-dimensional grains estimating proximity to an edge of a grain in the dimension perpendicular
to the thin section is not always possible. In the case of
the Bushveld Complex, the proportion of trapped liquid
in Main and Upper Zone rocks is typically about 10%
(Cawthorn & McCarthy, 1985). This component would
contribute about 6% more plagioclase and 4% of mainly
pyroxene (and oxide in the Upper Zone). Thus, in an average rock with 60% plagioclase, 54% would be cumulus
and 6% intercumulus. Statistically, nine analyses out of 10
might be of the cumulus component, and one the intercumulus component. As shown below, the most An-depleted
compositions in a single sample typically encountered in
this study are within 10% An of the most calcic. Thus, in
taking an average of 10 analyses, one value of an An-poor
rim might cause a decrease in the average of 1% An relative to the cumulus composition. This uncertainty could
therefore exist in all the averaged data used below.
An alternative approach might be to take the most Anrich composition obtained from any sample, rather than
the average, and assume that it represents the nearest
approach to the true cumulus composition. However, such
an analysis might represent one area of reverse zoning, or
possibly an antecryst (Davidson et al., 2001) that might
have been recycled, exotic (from a partially remelted previous layer), a xenocryst, or simply a ‘rogue’ analysis.
Within the so-called cumulus core, there can be very significant zoning. In the case of the Skaergaard Intrusion,
Maaloe (1978) showed that some grains had a small central
area with low An content, overgrown by either a gradually
or sharply reverse-zoned region, succeeded by a plateau
zone. Even within the plateau zone random variations in
An occurred. The central nucleus could be as much as
10% lower in An content than the plateau values.
Similarly, single grains with a broad reversed centre followed by a normally zoned pattern were shown to be
common within the Stillwater Intrusion (Czamanske &
Scheidle, 1985). For the euhedral portion of these grains
variations of up to 8% An were reported.
We have optically examined the central portions of
grains in many samples from the Bushveld Complex,
using the principle of the relationship of extinction angles
to composition. All the petrographic features described in
1613
JOURNAL OF PETROLOGY
VOLUME 50
detail by Maaloe (1978) can be identified in the Bushveld
rocks. Reverse-zoned cores are fairly common, but less
common than grains that are dominated by normal
zoning. Irregular-shaped and patchy cores with slightly
variable extinction angles exist, but are relatively rare.
The average number of analyses per sample was 10, with
a minimum and maximum of five and 29 (Ashwal et al.,
2005). Analyses with poor totals or dubious stoichiometry,
or that were obviously very different from all other analyses in the sample were excluded. In terms of the extent of
zoning, as defined by the highest value less the lowest An
value recorded, over 90% of the samples have ranges
between 2 and 7% variation in An content. The average
value for each sample has a standard deviation that
ranges from 1% to over 3%, with most standard deviations less than 2%. In almost all samples plagioclase
was a major phase, and identifying and analyzing cores of
grains was relatively straightforward. However, in two
short vertical intervals of feldspathic pyroxenite plagioclase was less common and there is textural evidence that
there was a considerable proportion of intercumulus
growth. In such cases, it is more difficult to assess whether
cores or rims were being analysed. As a result, the averages
obtained for these samples may not represent the true
cumulus composition.
A detailed study of plagioclase composition from a short
interval in the Upper Zone in the eastern Bushveld was
presented by Harney et al. (1996). They made between 40
and 50 electron microprobe analyses on each of 10 grains
from a single sample. The total dataset of 443 analyses
gave an anorthite content and standard deviation of
566 20 if obviously different compositions (from
reversed and normal zoned rims) are excluded. Hence, we
will consider all averages quoted here to have an uncertainty of 2% An. These uncertainties limit some of the
tests and interpretations that are presented in the following
discussion. Bearing these limitations in mind, the average
An determined for all analyses from each sample is
quoted here, and referred to as the cumulus composition,
and used for the modeling of processes. In Fig. 3 we show
this average An content of all samples analysed as a function of height relative to the Main Zone^Upper Zone
boundary.
RO C K D E N S I T I E S
Rock densities can provide valuable information on modes,
especially where plagioclase and a mafic phase are the
dominant minerals, as shown by Morse (1979a) for troctolites in the Kiglapait intrusion and by Cawthorn & Spies
(2003) for pyroxenite to anorthosite packages in the
Critical Zone of the Bushveld Complex. Densities were
determined by the water-immersion method on over 2200
core samples (Ashwal et al., 2005) averaging 10 cm in
length and 46 cm diameter from the layered rocks,
NUMBER 9
SEPTEMBER 2009
equivalent to one sample every 1^2 m, more closely
spaced where modal layering is apparent, and further
apart where the rock is homogeneous. Densities are given
in Supplementary Dataset 2 (available at http://petrology
.oxfordjournals.org/). Replicate weighing gave an uncertainty of 0005 g/cm3. The numbers of measurements falling into different density ranges are shown in Fig. 5,
where the data have been divided into samples from the
Main Zone and those from the Upper Zone. In the Main
Zone, densities range from 27 g/cm3 for anorthosites
through typically 3 g/cm3 for gabbronorites and troctolites,
to 33 g/cm3 for feldspathic pyroxenites. In the Upper
Zone densities show a much greater range (27^46 g/cm3)
because of the variable abundance of magnetite.
The weighted averages for these two zones are 291 and
312 g/cm3, respectively. These whole-rock densities can be
viewed in relation to densities of single minerals in Fig. 6.
This figure shows the densities of pure end-member minerals in the plagioclase, pyroxene and olivine series (Deer
et al., 1966). The actual mineral compositions at the base
of the borehole core, the Main Zone^Upper Zone boundary, the level of the appearance of olivine, and the top of
the intrusion are shown, based on the data from Ashwal
et al. (2005). These density data are used for a number of
different purposes in the following discussion.
D E F I N I N G A N O RT H O S I T E
L AY E R S
Streckeisen (1976) recommended that anorthosite be
defined by having more than 90% plagioclase by mode.
If pyroxene were the only other mineral, 90% by mode
would equal 88% by mass. Visual estimates are subjective,
and we prefer to use the density determinations, as they
provide consistent and objective criteria. Also, there are
2200 density determinations on discrete samples, as
opposed to 430 modal determinations from thin sections.
Figure 6 shows how the bulk-rock density will change
throughout the studied interval as a function of various
proportions of the silicate phases. (Obviously, it cannot
be applied rigorously to the Upper Zone where magnetite
is variably abundant.) Rocks composed of 88% by mass
of plagioclase and 12% pyroxenes in equal proportion will
have a bulk density of about 28 g/cm3 throughout the
entire interval.
A histogram of the density data for the Main Zone
(Fig. 5) shows a maximum frequency at 297 g/cm3, a subsidiary maximum at 277 g/cm3, and an intervening minimum at 283 g/cm3. We therefore propose to define
anorthosite here as those samples having a density less
than 280 g/cm3, with the following proviso. Samples with
densities of 280^283 g/cm3 present a problem as to how
they should be named and related to anorthosite layers.
The problem and our proposed solution are shown in
1614
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
/
Fig. 5. Histogram of densities of core, typically about 10 cm in length, from the Upper Zone and Main Zone. Data are in 002 g/cm3 bins. It
should be noted that 52 samples with densities 436 g/cm3 (dominated by magnetite) in the Upper Zone are excluded. Main Zone troctolites
are distinguished by diagonal shading.
Fig. 7, where two short sections containing multiple
anorthosite layers are shown. In Fig. 7a, two anorthosite
layers are present. The boundary to the lower layer
(denoted layer ^17 in Fig. 7a and Table 1) is uniquely
defined from the distinct break in density value for the
underlying sample, showing a decrease from over 30 to
less than 28 g/cm3 for adjacent samples. Within the 11m
thick layer there are three values, denoted (i), that lie
between 280 and 283 g/cm3. Each has overlying and
underlying samples with values of less than 280 g/cm3. We
consider these three samples as part of a single anorthosite
layer. Between layers ^17 and ^16 is a sample, denoted
(ii), with a density slightly less than 283 g/cm3. It is overlain and underlain by samples with densities greater than
283 g/cm3. That sample is not considered to constitute an
anorthosite layer. Above layer ^16 there are three samples,
denoted (iii), that have densities of 280^283 g/cm3. The
question is whether those samples should be considered
anorthosite or not. Our preference is that if such samples
fall totally above or totally below an anorthosite layer
they should be excluded from the defined layer. In Fig. 7b
there are three layers in close proximity. At ^1026 m there
is a change in density from 29 to 283 g/cm3 [two samples,
denoted (i)], to 278 g/cm3. Again, we exclude these two
samples, denoted (i), from the anorthosite layer. Within
the layer ^25, at ^1022 m and ^1014 m, there are two isolated samples with densities of 281g/cm3, which have samples with densities of less than 280 g/cm3 both above and
below. Here, we define these samples as part of the
anorthosite layer. Between layer ^25 and layer ^24 there
is a single sample with a density of 289 g/cm3. This value
is considered sufficiently high that it definitely splits the
package into two distinct, although closely spaced,
anorthosite layers.
Based on these criteria, 45 anorthosite layers were identified, and their positions, thicknesses and other compositional data are given in Table 1. Their locations in the core
are given relative to the Main Zone^Upper Zone boundary in Fig. 3. Anorthosite layer numbers are positive and
increase upwards in the Upper Zone, and are negative,
increasing downward, in the Main Zone. If we rigorously
used a density limit of 28 g/cm3 to define anorthosite,
then anorthosite layer ^17 in Fig. 7a would become split
into three anorthosite layers. Applying this principle
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/
Fig. 6. Plot of mineral densities vs mineral composition for plagioclase (An %), clino- and orthopyroxene, and olivine (mg-number). Mineral
densities are from Deer et al. (1966.) It should be noted that because these values decrease upward in the Bushveld Complex, the y-axis
corresponds qualitatively to stratigraphic height. Typical mineral compositions at various horizons in the intrusion are indicated and joined by
tie-lines (from Ashwal et al., 2005). The bulk density of a rock containing 90% plagioclase and 10% silicate minerals is shown for these specific
horizons. No account for magnetite in the Upper Zone is included.
throughout the core would add a total of a further 16
anorthosite layers, with all these added layers having very
gradational or diffuse boundaries. The identification of
these 45 anorthosite layers based on density measurements
agreed closely, but not always exactly, with the logging
reported by Knoper & von Gruenewaldt (1996).
Thicknesses range up to 23 m, and so are very different
from the extremely thick anorthosite layers reported in
the Stillwater Complex (Hess, 1960; McCallum, 1996).
N AT U R E O F C O N TAC T S T O
A N O RT H O S I T E L AY E R S
The lower and upper contacts to anorthosites are variable,
and the following discussion is based partly on observations of the core reported by Knoper & von Gruenewaldt
(1996), and also on the density determinations. Subtle variations in mafic mineral content between 10 and 20% are
extremely difficult to estimate in core samples, and so in
that regard a density measurement is considered to be
more objective and reproducible. Conversely, density measurements were made on pieces of core about 10^15 cm in
length every 1^15 m, and so there would be typically at
least 1m of core for which no density measurement is available, and so sharp vs gradual variations in mode would be
difficult to assign from the density measurements alone.
Extreme cases of the nature of the lower contacts are
shown by Figs 7a and 8. The base of layer ^17 (Fig. 7a)
shows a change in density from over 30 to 275 g/cm3
between adjacent samples. In contrast, below the base of
layer ^15 in Fig. 8, the density decreases regularly from
430 to 28 g/cm3 over a distance of 10 m, based on 10 density determinations. All other lower contacts have variable
density contrasts between these two extreme examples.
Distinguishing sharp and gradual boundaries is therefore
rather arbitrary. Thus, we feel that no generalizations can
be made about the nature of anorthosite contacts to adjacent rocks.
1616
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
(a)
/
(b)
/
Fig. 7. (a) Plot of density vs depth for the interval including anorthosite layers ^17 and ^16 (see Table 1), and (b) for the interval including
anorthosite for layers ^25 to ^23. The dashed line at 28 g/cm3 indicates the preferred upper density limit for anorthosite density. The dotted
line at 283 g/cm3 denotes the upper limit for anorthosites, as discussed in the text. The encircled and labeled points are discussed in the text.
P L AG IOC L A S E C OM P O S I T ION S
W I T H I N A N O RT H O S I T E L AY E R S
The number of separate samples analysed within each
anorthosite layer is shown in Table 1. Most anorthosite
layers are thin (less than 3 m) and only one analysis of plagioclase was obtained for the entire layer. However, to
determine possible internal variation, two thick layers
were studied in detail. In the 23 m thick layer ^13, seven
samples were analysed. The results are shown in Fig. 9.
The total variation in average An content is from 575 to
605 (Table 1). If each datum point has an uncertainty of
2 mol % An then there is little variation within the
anorthosite layer, and also within the two samples immediately above and below the layer. In the second example, 11
samples were analysed in the 11m thick layer ^7. Also, 22
samples were analysed from the underlying 6 m. The
lowest 4 m of this succession (from ^395 to ^391m) shows
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Table 1: Anorthosite layers, their numbering system, thickness, densities and An contents in plagioclase of the layers and
adjacent rocks, relative to height above Main Zone^Upper Zone boundary
No.
Height
Thickness
Density below
Density above
No. of samples
(m)
(m)
(g/cm3)
(g/cm3)
in layer
Plagioclase compositions (An %)
Below
anorthosite
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
1222
715
695
681
593
526
504
491
471
461
444
421
401
213
192
152
112
98
55
1
30
69
182
200
369
389
402
409
413
428
440
546
558
605
737
753
872
880
901
918
925
1001
1007
1026
1134
51
51
3
51
51
3
51
7
15
51
5
6
10
6
2
14
19
2
12
51
51
2
8
52
10
11
3
3
51
8
11
23
9
13
2
11
2
51
15
14
5
1
1
13
6
2862/2892
3082/2934
2815/3072
3375/3262
2873/2862
2969/2859
3079/2906
2828/2911
2857/2852
2828/2908
2819/2894
2831/2817
2886/2937
2836/3024
2890/2904
2824/2876
2884/2909
2986/2923
2848/2884
2810/2843
2914/3008
2839/2910
2940/2844
2830/2929
2887/2882
2852/2848
2871/2809
2861/2990
2880/2893
2943
3047/2932
2835
2812/2823
2823/2815
2846/2828
2999/3025
2851/2870
2879
2811
2881/2816
2983
2876/2916
2885
2829/2834
2907/3009
3537/2900
2966/3037
2863/2880
3001/3155
2976/2963
2846/2883
2825/3012
2958/2906
2861/2824
2840/2886
2847/2848
2936/2845
2910/2840
2880/2926
2968/3828
2876/3569
2896/2906
2881/2815
2916/2947
2819/2918
2965/3038
2926/2899
2903/2883
2990/2959
2817/2919
2872/2855
2986/3048
2906/2890
2990/2861
2834/2880
2943
2834
2811/2869
2842/2817
2854/2844
2847/2828
2825/2849
2827/2860
2879
2811
2814/2816
2905/2925
2869/2884
2885
2903/3076
3
(523)
547/540
554/(523)
564/561
557/559
(571)/(593)
555/571
(597)/575
(597)
(567)/(560)
(571)/565
(550)/(564)
(571)/572
(591)
616
(591)/606
(556)/619
569/557
531/530
(537)/(544)
(564)/(545)
(534)/(564)
2
12
4
2
7
5
580/578
593
587/593
(596)/595
594/594
593/612
614/605
(633)/616
(654)/686
(711)/(693)
711/701
(712)
(756)/(727)
754/741
2
(725)/(702)
(753)/(739)
Within
anorthosite
457
509
510
540
558
579
593y
567–586
566
557
607y
556
562
590
573z
610
573
550
553–575
560–595
?549–602
Above
anorthosite
470/483
(527)/497
(523)
(541)/547
552/567
572/(544)
561/564
579/561
586/(570)
(575)/(586)
(597)
570/581
560/(559)
550/550
(560)/578
591
616
591/(606)
561/573
(548)/(540)
(537)/526
(591)/535
(545)
(545)/(562)
589/574
586–587
596
575–605
593
605–629
647
593
585
601/586
613
611/(600)
649/(615)
694
693
698
729
730
(686)/(663)
(686)/(694)
(698)/(701)
(712)
(755)/(743)
734
750–757
(741)/(754)
(750)/(734)
Column ‘No.’ refers to the layer number, which is given relative to the Main Zone–Upper Zone boundary; positive layer
numbers refer sequentially to layers above this boundary, and negative numbers to layers below it. Columns ‘Density
below’ and ‘Density above’ contain two density values; the first is the value for the sample closest to the anorthosite layer
and the second is for the next sample further away. If the further sample is more than 10 m from the anorthosite its value
is given in parentheses. If the nearer sample is greater than 5 m distant its value is also in parentheses. If no, or only one,
value is given it means that there is no or only one determination between this anorthosite and the adjacent anorthosite.
Where multiple analyses have been obtained from within an anorthosite layer the number of such determinations is given
in the column ‘No. of samples in layer’, and the range of An contents is given in the column ‘Within anorthosite’. For
layer 8 a value of 549 is prefixed by ?, because this value is very different from the other three values, for which the
range is 589–602.
A forward jump in An content occurs at the base of the anorthosite layer.
y
A reversal occurs at the base and a forward jump at the top of the layer.
z
A forward jump occurs at the base and a reversal at the top.
1618
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
/
Fig. 8. Plot of density vs depth for samples straddling the base of layer ^15. (See legend to Fig. 7.)
Fig. 9. Plot of multiple average plagioclase An contents vs height for samples just below and through the anorthosite layer ^13. It should be
noted that the base of the anorthosite layer has been drawn at an angle. Two closely spaced samples, one below and one above the contact,
were analysed, and the angled base is purely to clearly emphasize this relationship. Within the anorthosite layer there is a random vertical variation in An content.
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Fig. 10. Plot of multiple average plagioclase An contents vs height for samples just below and into the anorthosite layer ^7. Here, a regular
decrease in An vertically in the layer is recorded, in contrast to that seen in Fig. 9.
random scatter within the 2 mol % An uncertainty. In
contrast, for the 2 m below layer ^7 and the entire layer,
although all lie within analytical uncertainty of each
other, they show a progressive upward decrease in An content from 59 to 56 (Fig. 10). In the other five layers, for
which between two and five samples were analysed, the
range of values is typically about 2% An. (One sample in
layer ^8 has a value of 549, but excluding that value, the
other three samples have a range of 2% An.).
M A G N E T I T I T E L AY E R S
Defining the boundaries of magnetitite layers is slightly
easier than defining the anorthosite layers. Usually bases
are sharp, although upper contacts are very commonly
gradational. Pure Bushveld magnetite has a density of
48 g/cm3 (Molyneux, 1972), and we use a minimum density of 38 g/cm3 (60% magnetite) as a lower density limit
for defining magnetitite layers. The locations and thicknesses of 20 magnetitite layers, and compositions of plagioclase in the rocks above and below, are given in Table 2.
There are three layers included here that were not
included in the scheme of Knoper & von Gruenewaldt
(1996), for which the density exceeded 38 g/cm3 (60%
magnetite). Knoper & von Gruenewaldt referred to these
sequences as ‘plagioclase 50% magnetite’, and ‘magnetite
gabbronorite’. There are also two extremely thin magnetite
concentrations that we identified based on densities of
378 and 366 g/cm3 at heights above the Upper Zone
boundary of 1270 and 1324 m, which are within 100 m of
the roof, but which are not included in the table. In seven
instances, there are two or more layers within 2 m of each
other, and these were combined as one composite layer
in the tabulation with the letters A to D identifying the
single layers. Their spatial grouping into four clusters,
as seen in the eastern Bushveld (Molyneux, 1974), could
be considered applicable here, with two layers between
13 and 46 m (group A of Molyneux), four layers between
171 and 241m (Group B), eight layers between 526 and
744 m (Group C), and six layers between 927 and 976 m
(Group D). There are further similarities between this section and the eastern lobe. The topmost layer in this core
matches that in the eastern lobe in terms of being by far
the thickest (6 m) and in its very low vanadium content
(Molyneux, 1974; Ashwal et al., 2005). A slight difference
exists in the western lobe, where there are some very thin
magnetite layers overlying the extremely thick analogue
(Tegner et al., 2006). The second thickest at 2 m, close to
the base of the Upper Zone, also matches that in the eastern and western lobes in terms of thickness and vanadium
content.
1620
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
Table 2: Magnetitite layers, numbering system, their densities, thicknesses and An contents in plagioclase in adjacent rocks,
measured relative to height in Upper Zone
Layer no.
Height in
UZ (m)
Density
1
Thickness
3
(max.) (g/cm )
Gp2
Plag3
Plagioclase compositions
Contact4
(cm)
Below
Above
Lower
Upper
20
972
44
1300
D
500/497
498/501
N
19
963
43
60
D
532/536
536/510
N
Gr
40
D
532/536
R
Gr
Gr
13
D
V Gr
V Gr
18b
18a
9565
433
17b
506/507
27
D
3
D
(506)
19
D
(481)/(502)
7
C
(530)
45
70
C
509/527
14
C
6725
45
18
C
565/561
644
465
50
C
557/566
10
C
17a
9305
16
927
15
744
425
14
724
13a
12
13b
11b
540/526
Sh
R
Sh
Sh
R
Gr
Sh
526/540
R
Sh
Sh
513/512
N
Sh
Sh
530
N
V Gd
V Gr
554/523
N
Sh
Sh
Sh
Sh
Sh
545/561
N
V Gr
(556)/(557)
F
Sh
Sh
Sh
Sh
11a
6145
406
12
C
(571)/583
10
594
41
10
C
571/565
552/561
N
No data
No data
9
583
39
55
C
561/552
571/583
N
Sh
Sh
8
572
45
29
C
552/554
564/(561)
N
Sh
Sh
7
526
5
C
564/572
559/558
N
Sh
Sh
6c
22
B
561/558
N
Sh
Gr
6b
2
B
Sh
Sh
6a
2415
10
B
(559)/575
5
195
383
7
B
555/541
50
B
1775
445
65
B
3d
30
B
3c
130
3b
4b
4a
3a
1675
445
550/550
N
(571)/(556)
N
528/570
Sh
Sh
Sh
V Gr
Sh
Sh
Sh
Sh
Sh
B
Sh
Gr
50
B
Gr
Gr
170
B
Gr
Sh
V Gr
V Gr
574/528
N
560/578
2
46
39
10
A
555/556
619/(610)
R
1
13
425
25
A
546/536
551/543
N
Gr
Numbering of layers is from the base of the Upper Zone upward. Very closely spaced multiple layers are denoted with a
number and a letter (a–d).
1
Density of pure magnetite is 48 g/cm3 (Molyneux, 1972).
2
(Gp) Group lettering according to Molyneux (1974) for clustering of magnetitite layers in the eastern Bushveld.
3
Plag refers to whether there is no change (2 mol % An) in the An content across a layer (N), a reversal (R), or a
forward jump (F).
4
The nature of the upper and lower contacts to the magnetitite layers may be sharp, Sh; or (very) gradational, (V) Gr (see
text for amplification).
5
Combined thickness of multiple closely spaced layers.
F E L D S PAT H I C P Y ROX E N I T E
L AY E R S
There are only four layers, all in the Main Zone, enriched
in mafic silicate minerals. Feldspathic pyroxenite samples
are here defined by a density greater than 315 g/cm3,
representing 65% mafic phase. The scarcity of samples in
the Main Zone with higher densities is apparent in Fig. 5.
There is a 2 m thick layer at ^1234 m, with a sharp base
and gradational top (based on closely spaced density measurements). Its average density is 328 g/cm3, which corresponds to about 85% pyroxene. At ^395 m is a thin layer
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Table 3: Stratigraphic intervals through which the plagioclase compositions show a reversal in An content upward, or in
which plagioclase compositions show considerable scatter in An contents
Base of reversal (m)
Top of reversal (m)
Vertical interval (m)
Min. An value.
Max. An value
Increase in An value
No. of samples involved
Reversal in An content upward
880
930
50
47
54
7
5
547
605
58
54
58
4
11
295
430
135
55
58
3
14
40
52
12
55
62
7
2
0
40
53
57
4
9
960
115
72
76
4
6
40
1075
Base of anomaly (m)
Top of anomaly (m)
Vertical interval (m)
Min. An value
Max. An value
Range in An value
No. of samples involved
Scatter in An contents
406
390
16
56
60
4
33
1245
1225
20
70
78
8
24
Top and base of anomalous intervals are given in metres relative to Main Zone–Upper Zone boundary.
that reaches a density of 318 g/cm3 or 68% pyroxene over
40 cm, having gradational contacts. At ^85 m there is a
2 m thick layer with densities reaching 335 g/cm3, and at
^75 m and ^74 m are single samples with densities of 336
and 321g/cm3 (490 and 70% pyroxene, respectively). No
mafic silicate-rich rocks occur in the Upper Zone. Many
samples occur with density 4315 g/cm3 in the Upper
Zone (Fig. 5), but these result from enrichment in magnetite, not mafic silicate minerals.
V E RT I C I A L D I F F E R E N T I AT I O N
We now apply the principles depicted in Fig. 4 to the data
collected by Ashwal et al. (2005), with particular attention
to the genesis of anorthosite and magnetitite layers.
We note that the weighted bulk density for rocks
from the Main Zone (average of 1063 determinations) is
291g/cm3. Using a density of 273 g/cm3 for plagioclase
and 338 g/cm3 for the average of clinopyroxene and orthopyroxene, with minor olivine (actual densities given in
Fig. 6) indicates an average plagioclase content of about 70%
(by weight). The same calculation cannot be done for the
Upper Zone because of the variable proportions of magnetite.
The entire length of core discussed here shows a change
in plagioclase composition from An77 to An36. However,
the evolution is not always uniform or regular (Fig. 3).
Figure 10 shows a very short vertical section of 6 m, of typical gabbronoritic rocks, from which 22 samples were analysed. They show a total variation of 4% An, with an
average of 582% An and a standard deviation of 10%
An. We therefore suggest that sample-to-sample variation
that is less than 2% An (e.g. An58 to An60) lies within the
typical inherent variation of the magmatic process of crystallization. Changes that exceed that value, and especially
if sustained over several samples, are considered to indicate
some perturbation in the crystallization process. A few
single anomalous values exist, where for a particular
sample there is an abrupt stratigraphically upward
increase or decrease in An content. However, where the
next overlying sample shows a value that reverts to that of
the underlying sample it is not considered a sustained
break [as shown in Fig. 4a (ii)]. Four such samples exist in
Fig. 3, at ^2155, 2200, 2565 and 6649 m relative to the
Main Zone^Upper Zone boundary, and are ignored in
the following discussion. None are in, or immediately adjacent to, anorthosite or magnetitite layers.
Six sustained reversals in the An content of plagioclase
can be identified in Fig. 3 and Table 3. Four reversals display a change of less than 4 mol % An, and two a change
of 7%. The vertical intervals through which the reversals
occur range from 12 to 135 m, and involve up to 14 samples
(Table 3). No forward jumps are identified. However,
because differentiation and forward jumps define trends
in the same direction, but with possibly different rates
of change vertically [see Fig. 4a (vi)] such breaks
could be more difficult to identify. Two intervals are also
identified in Table 4, where there is considerable random scatter of the plagioclase compositions (4 and
8 mol % An) over short vertical intervals. They do not
define sustained breaks, but suggest some aberration to
the normal crystallization process (discussed further,
below).
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BUSHVELD PLAGIOCLASE COMPOSITIONS
/
Fig. 11. Plot of sample density vs average An content in plagioclase for a melanocratic to leucocratic section from ^1245 to ^1225 m relative to
the Main Zone^Upper Zone boundary. A density of 33 g/cm3 indicates a pyroxenite with less than 10% plagioclase. These pyroxenitic samples
contain plagioclase with an average An content 10% lower than in anorthosite samples. The continuous line denotes a best-fit line; the dashed
lines are 2% An on either side, indicating the likely uncertainty in these data.
Effect of intercumulus component
All the sections discussed thus far have abundant cumulus
plagioclase. When plagioclase becomes a minor phase the
contribution from intercumulus crystallization needs to be
considered. The two profiles in Fig. 3 and Table 3 that
show a scatter of data coincide with the intervals of feldspathic pyroxenite layers at ^1225 to ^1245 m and ^390 to
^406 m. The An values and the bulk densities of the samples through the depth range ^1245 to ^1225 m are shown
in Fig. 11. Plagioclase ranges in composition from An70 to
An78. The samples with densities of less than 3 g/cm3 are
relatively rich in plagioclase, and the grains contain
demonstrably large cumulus cores. Their compositions are
An75^78. Within the pyroxene-rich part of the unit, with
increasing density (decreasing plagioclase proportion) the
plagioclase becomes progressively depleted in An, reaching An70 for samples with densities of 33 g/cm3, equivalent
to less than 10% plagioclase. In these rocks the plagioclase
is dominantly intercumulus. We repeat that the values plotted here are the averages of multiple analyses that show
considerable range in composition. Samples with progressively less cumulus plagioclase and a greater relative proportion of intercumulus component will have averaged
An contents between the true cumulus and intercumulus
compositions. There is a scatter of 2% An on either side
of a perfect line in Fig. 11. Thus, in this case, the rapid
change in An value over a small vertical interval can be
related to the changing relative proportions of cumulus to
intercumulus compositions, and is not related to differentiation or magma addition.
In the upper pyroxene-rich layer the pattern is less
clearly defined (Fig. 12). In the vertical interval ^406 m to
^390 m some samples have densities 528 g/cm3, and so
consist largely of cumulus plagioclase and have compositions of An59^60. Samples with progressively higher densities have less cumulus plagioclase and slightly lower An
values in the range An57^59. However, the densest rock
only has a density of 32 g/cm3, compared with 33 g/cm3
in the feldspathic pyroxenite layer shown in Fig. 11, and so
samples with dominantly intercumulus plagioclase are not
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Fig. 12. A similar plot to Fig. 11 for the section from ^406 to ^390 m. Samples only reach a maximum density of 32 g/cm3 and so are melagabbronorite rather than pyroxenite. A continuous line and two dashed lines of identical slope to those shown in Fig. 11 are included for comparison.
The correlation is not as convincing as in Fig. 11.
present in this section. As a result, the total range of An
values is not as great as seen in Fig. 11. Given the same scatter as in the previous diagram of 2% An, the trend in
Fig. 12 is less well-defined, but is considered to result from
the same process. There are insufficient determinations of
plagioclase compositions through the other two thin pyroxene-enriched intervals to make similar interpretations,
but no hiatus is seen through these intervals in Fig. 3.
It is apparent that the scatter in plagioclase composition
included in the two sections listed in Table 3 at depths of
1225 and 390 m can be attributed to a dominance of
intercumulus plagioclase in the averaged An values for
pyroxene-rich samples with less than 40% plagioclase (or
density 431g/cm3). This interpretation emphasizes the
importance of comparing only cumulus compositions in
this kind of study.
Reversals
Five of the reversals toward more An-rich compositions
identified in Table 3 are gradual. They occur over vertical
intervals of 40^135 m, and involve 6^14 samples. One
reversal is sharp in that the maximum change in An content is recorded between adjacent samples, which are 12 m
apart. The magnitude of these reversals ranges from 4 to
8% An. If there are any other more subtle reversals of less
than 3% An, they are lost in the scatter of the data. The
reversal near the top of the Main Zone (^40 to 0 m)
appears to reach its culmination with the appearance of
cumulus magnetite (defined as being at 0 m). Whether
these reversals result from addition of new magma or are
the result of convective overturn within the chamber
cannot be resolved using only the present database. The
constancy of initial Sr isotope ratios has been used as an
argument against magma addition in the Upper Zones of
the eastern and western lobes (Tegner et al., 2006), but no
such data exist for the northern lobe. However, what is
important here is that the intervals of these reversals
cannot be specifically identified as coincident with distinct
mineralogical or modal changes, such as the presence of
magnetitite or anorthosite layers (compare depths in
Tables 1 and 2 with Table 3).
The observation that these reversals are protracted,
involving many samples and many tens of metres of section
suggests that if they are related to magma addition and/or
mixing they represent extremely slow processes. At the
level of the upper part of the Main Zone, Cawthorn &
Walraven (1998) calculated the rate of accumulation to be
1m every 40 years. Thus, the longest reversal of 135 m
took about 5000 years to be achieved.
O R I G I N O F A N O RT H O S I T E
L AY E R S
The following hypotheses have been proposed to explain
the origin of anorthosite layers within layered complexes:
1624
(1) injection of new magma with plagioclase as the only
liquidus phase at the crystal mush^magma interface
(Czamanske & Scheidle, 1985; Irvine et al., 1983);
(2) end-product of differentiation after injection of pyroxene-saturated magma (Eales et al., 1990);
(3) intrusion of a plagioclase mush into a pre-existing
package; that is, below the mush^magma interface
(Czamanske & Bohlen, 1990; Be¤dard et al., 2007);
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
(4) supersaturation and nucleation effects (Maaloe, 1978;
Morse, 1979a, 1979b);
(5) rhythmic pulses of crystallization followed by settling
and sorting under the influence of gravity (Wager &
Brown, 1968);
(6) changes in pressure within the magma chamber
(Naslund & McBirney, 1996).
We now discuss the applicability of these processes to the
succession described here.
Injection of plagioclase-saturated magma
Periodic replenishment of magma chambers is almost certainly the norm for large intrusions. Reversals in all mineral compositions (e.g. at the Pyroxenite Marker in the
Bushveld Complex, von Gruenewaldt, 1970), changes in
the order of crystallization (e.g. at the J-M reef in the
Stillwater Complex, Irvine et al., 1983), and changes in isotopic ratios (at the Merensky Reef and Pyroxenite Marker
in the Bushveld Complex, Kruger, 1994; and in the Rum
intrusion, Davidson et al., 2001) have all been used as evidence for such processes. In these examples, the addition
of new magma is associated with specific modal variations.
Applying this magma-addition model to the present data,
it might be suggested that the resident magma was crystallizing plagioclase and pyroxene(s) (in the Main Zone),
possibly with magnetite (in the Upper Zone).
Accumulation was interrupted by the addition of a plagioclase-saturated magma at the instantaneous base of the
chamber that produced an anorthosite layer (Irvine et al.,
1983; Czamanske & Scheidle, 1985). If this process were
the case, there is no reason for the An content of the
newly forming plagioclase to bear any relationship to the
plagioclase in the immediate footwall (as discussed by
Eales et al., 1990). In Table 1 the An contents of plagioclase
in the anorthosite and the immediate underlying footwall
and overlying hanging-wall samples are given, and they
are also shown in Figs 13 and 14. If a difference of greater
than 2% An is considered significant, Fig. 13a suggests
that anorthosite layer 6 could be considered to show a
reversal in An content within the anorthosite relative to
its footwall, whereas layers 2, 3, 4 and 17 could show a forward jump. The other 24 layers show no change. For the
other 16 layers we have no data for the immediately underlying footwall rock or the anorthosite layer itself.
However, only one discontinuity exceeds 3% An. It is also
apparent in Table 1 and Fig. 14 that the An contents of the
anorthosite layers show a fairly regular decrease upward
in the succession from An75 to An51. A hypothesis that
requires addition of magma 43 times out of 45 events that
produced a plagioclase composition matching within 3%
An (and within 2% for most of the layers) the resident plagioclase as it steadily evolved upward is not considered
plausible.
End-product of differentiation after
injection of pyroxene-saturated magma
The cyclicity observed in the Critical Zone of the Bushveld
Complex has been attributed by some workers to addition
of new magma (Eales et al., 1990). The principle behind
this model is that the resident magma differentiated to
yield products from pyroxenite through norite to anorthosite. The reappearance of pyroxene above the anorthosite
in a subsequent cycle requires addition of magma in this
model. The anorthosite layers represent the last cumulates
formed from one magmatic event, prior to further magma
addition. In the dataset presented here, the rocks above
the anorthosite layers are not specifically enriched in
mafic minerals. These observations are shown in Fig. 15
and Table 1. Most samples immediately above the anorthosite layers have densities of less than 295 g/cm3, showing
that they contain much more plagioclase than the cotectic
proportion between pyroxene and plagioclase. Thus, there
is no suggestion of mafic enrichment above anorthosite
layers. Also, the compositions of the plagioclase overlying
the anorthosite layers do not differ significantly from
those of the anorthosite layers themselves (Figs 13b and
14). Above only two anorthosite layers (2 and 9) is there a
possible reversal, and above one other layer (layer 6)
there is a forward jump. Again, such correspondence is
unlikely if there had been magma addition. Hence, we
conclude that there is little evidence for addition of mafic
magma above each anorthosite layer.
Intrusion of a plagioclase suspension
into the succession
The above two models invoke magma addition at the level
of the crystal mush^liquid interface. Another possibility
exists; namely, that the material added was intruded into
(as opposed to onto) the crystal pile. This concept has
been proposed for the thick anorthosite layers seen in the
Stillwater intrusion (Czamanske & Bohlen, 1990). A similar argument to that presented immediately above can be
applied to test this hypothesis. If the anorthosite is genetically unrelated to both its footwall and hanging-wall
rocks there should be no reason for the An contents of the
plagioclase in the anorthosite to match those of its adjacent
hosts. The close agreement in An content between that of
the anorthosite layers and their hosts (Table 1 and Figs 13
and 14) suggests that this process is not applicable here.
Be¤dard et al. (2007) presented a different model to
explain the presence of anorthosites in thick sills in the
Antarctic. They suggested that plagioclase grains were relatively locally derived from the surrounding crystal mush
by elutriation, and so no difference in An content would
be expected between the remobilized plagioclase in
anorthosites and the adjacent plagioclase grains. In such a
model there should be layers, relatively enriched in pyroxene grains, from which the plagioclase grains had been
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(a)
(b)
Fig. 13. (a) Plots of average An content in plagioclase from the sample in the anorthosite layer vs composition of the sample in the immediate
footwall, and (b) for each anorthosite layer and its immediate hanging-wall sample. In general, samples are between 2 and 10 m apart. The continuous line denotes identical compositions in both samples. The dashed lines denote 2% An uncertainty limits. Numbers refer to the layer
number in Table 1. Five samples fall outside those boundaries, one implying a small upward reversal, the other four a forward jump in composition from footwall into the anorthosite layer (a). Two reversals and one forward jump are indicated from the anorthosite layer into the hanging
wall rocks (b).
extracted. Inspection of Table 1 shows no layers of anorthosite that have associated packages significantly enriched in
pyroxene (i.e. densities significantly greater than 3 g/cm3).
In the present instance we also note that the anorthosite
layers are associated with underlying and overlying leucocratic rocks (Table 1). If the typical cotectic proportion of
plagioclase (density 27 g/cm3) to pyroxene (density
335 g/cm3) in the Main Zone is 60:40, the cotectic bulk
density would be 295 g/cm3. Similarly, using cotectic proportions of 60:35:5 for plagioclase (265 g/cm3) to pyroxene
plus olivine (35) to magnetite (48), gives a bulk density of
307 g/cm3 for the Upper Zone. Inspection of the densities
in Table 1 indicates that 40 of the 45 anorthosite layers
have underlying and overlying rocks that have lower density and so are more leucocratic than these averages. (Not
one layer has both underlying and overlying rocks with
higher density.) It is suggested that in almost all cases the
anorthosite layers are associated with distinctly leucocratic
packages of rocks, suggesting a genetic association between
the anorthosites and adjacent layers, rather than injection
of a crystal mush, which even if locally extracted and
remobilized would have left pyroxene-enriched residual
layers.
Supersaturation effects
The model proposed by Maaloe (1978) requires supercooling, such that supersaturation of magma occurs, resulting
in rapid bursts of nucleation of alternating minerals.
Significant changes in the An content of the plagioclase
would not be expected with this process. It might be
expected that supersaturation would result in texturally
identifiable features. Finer grain size might be associated
with such processes. Although we have not determined
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CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
crystallization, without significant cooling, drives the
liquid beyond the cotectic and into the plagioclase stability
field. Thereafter, an overabundance of plagioclase nucleates and grows. With respect to the present dataset, we
note that the density data are not consistent with this
model. The expected density profile consequent upon this
oscillatory nucleation process is shown in Fig. 4b (v), and
we note that no resemblance to that pattern, specifically
high-density, pyroxene-enriched rocks on either side of an
anorthosite, is noted anywhere (see Fig. 15 and Table 1).
Crystal settling
Fig. 14. Average composition of plagioclase in footwall (shown by
horizontal bar), anorthosite layer (filled circle or bar ended with circles if more than one analysis from within the layer) and hangingwall samples (cross or bar ended with crosses), plotted as a qualitative
function of height in the intrusion. The number of the anorthosite
layer is given on the left. In this plot, the values of the nearest two
samples in both footwall and hanging wall are given, provided they
are within 5 m of the anorthosite layer. It should be noted that the vertical axis is not a uniform index of vertical distance, and so different
slopes in different parts of the diagram do not indicate different rates
of differentiation, merely different spacing between anorthosite layers
(see Table 1).
grain size quantitatively, variations are not apparent
macroscopically. In such a large intrusion as the Bushveld
Complex, the cooling of the magma was extremely slow.
The calculations of Cawthorn & Walraven (1998) indicate
that at the top of the Main Zone cooling would have been
at the rate of 18C per 300 years, and in the middle of the
Upper Zone had slowed to 18C per 500 years. The thermal
inertia of this entire magma chamber and its country
rocks makes it impossible for faster cooling rates to be
achieved. Thus, we doubt that significant supercooling
could occur during such slow cooling. An alternative process to supercooling is compositional supersaturation
(Morse, 1979a, 1979b), which is only subtly different from
supercooling. In such a process excess pyroxene
The analogy of field observations between modal layering
in the Skaergaard Intrusion and clastic sedimentary rocks
led Wager & Brown (1968) to propose that differential settling between mafic minerals and plagioclase led to
graded modal layering. In the extreme case, this process
could lead to plagioclase being the sole accumulating
phase, producing a pure anorthosite, at the top of a modally graded unit. If this mechanism were operative, it
might be expected that there would be a gradual upward
increase in plagioclase content, and hence gradual
decrease in density, culminating in the anorthosite layer.
Measurements made for this study of the sample densities
were taken typically at 1m intervals or where there was
an obvious change in mode. Excluding the anorthosites,
in only five cases was there very obvious small-scale
modal layering and enrichment in pyroxene. For the thickest of these packages measurements were taken of every
piece of core, typically about 10 cm in length, over a vertical section of 6 m. This section was chosen for detailed
measurements because the succession progressed (rather
erratically) from a feldspathic pyroxenite through gabbronorite to anorthosite. The density data are shown in
Fig. 16, and demonstrate an overall, if slightly irregular,
decrease in density upward from ^395 to ^389 m, with possible smaller cycles within that interval. Also, the preponderance of rocks with greater than 70% plagioclase
immediately below almost all the anorthosite layers
(Fig. 15) shows that such graded contacts are the norm.
Both these features are consistent with the process of crystal settling and sorting. Several other profiles, where measurements of density in every single sample of core, over
considerable vertical intervals, have demonstrated that
such regular variations in density and hence modal proportions that differ markedly from the cotectic proportions, are commonly observed lower down in the Bushveld
Complex (Cawthorn & Spies, 2003).
There is one major difference between the observations
made here and those reported by Wager & Brown (1968).
In the Skaergaard Intrusion plagioclase-rich layers are
overlain by mafic-rich layers (mafic mineral abundance in
excess of the cotectic proportion). Wager & Brown (1968)
attributed this pattern to the pulsed crystallization of a
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Fig. 15. Histograms of densities of two samples immediately adjacent to an anorthosite layer (distance typically less than 4 m). A separate diagram is shown for samples above and below anorthosite layers, and from Main (MZ) and Upper Zones (UZ). The preponderance of densities
of less than 29 g/cm3 corresponds to greater than 65% plagioclase, showing that rocks immediately adjacent to anorthosite layers are themselves distinctly leucocratic.
new crop of both plagioclase and mafic grains in their
cotectic proportions, which then underwent differential
settling. That exact mechanism does not appear applicable
in the present dataset, because almost every anorthosite
layer is overlain by a leucocratic rock (Fig. 15). In the
Main Zone none of the anorthosite layers is overlain by
rocks with more than the cotectic proportion of pyroxene,
indicated by a density 4295 g/cm3 (Table 1). However,
near the top of the Upper Zone four anorthosite layers
(numbered 12, 16, 18 and 19 in Table 1) are overlain by
rocks of higher density, but the high density may result
from the presence of magnetite, not a deficiency in plagioclase. Thus, in our model, we would suggest that the crop
of plagioclase grains from the first crystallization event
had not all totally settled before a second period of crystallization of plagioclase and pyroxene was initiated. Sinking
pyroxene grains from the second batch then merged with
the plagioclase grains from the first batch to produce the
gradational upper boundaries to anorthosite layers.
The original concept of settling and sorting was challenged by a re-examination of the density of plagioclase
relative to likely magma compositions (Campbell et al.,
1978). This issue was rigorously evaluated by Scoates
(2000) with reference to anorthosite complexes, and the
difficulty of plagioclase sinking was emphasized. The densities of his calculated liquid compositions were typically
003^02 g/cm3 greater than that of the plagioclase. The
modeled magma compositions and densities (on an anhydrous basis) through the Upper Zone of the Bushveld in
the western lobe were presented by Tegner et al. (2006),
and the magma densities were found to be less than
003 g/cm3 greater than that of plagioclase. Addition of
1% H2O would lower the magma density by about
004 g/cm3. The differences in density between the inferred
Bushveld and anorthosite liquids (Scoates, 2000) result
from two differences in the calculated liquid compositions.
The uppermost Main Zone and lower Upper Zone of the
Bushveld Complex contain abundant orthopyroxene
rather than olivine [as in the anorthosite discussed by
Scoates (2000)] and indicate a higher SiO2 content for the
Bushveld magma. Also, in the Bushveld Complex oxide
minerals begin to crystallize when plagioclase has a composition An55 (Table 2) and clinopyroxene has an mgnumber of 70 (Ashwal et al., 2005). The oxide appearance
in the anorthosite studied by Scoates (2000) is delayed to
more evolved compositions (his fig. 10). Hence, the evolving Bushveld magma does not go through the same
degree of iron enrichment prior to oxide precipitation.
These two effects (higher SiO2 and lower total iron)
lower the predicted densities of the evolving Bushveld
magma. However, both inferred evolving magma compositional trends have their uncertainties, and so a definitive
answer to whether the plagioclase would sink or float
based on density calculations on inferred liquid compositions is not forthcoming.
We resort to direct observations to discuss this aspect
further. In Fig. 17 we present the densities of the rocks
forming the uppermost 100 m of the layered suite. (Note
that the locations and thicknesses of two granite and one
dolerite sheets have been excluded in the construction of
this graph.) The An content of the plagioclase decreases
continuously upward (Fig. 3), demonstrating that there is
no roof facies formed by downward crystallization. These
preserved uppermost rocks have a density of 305 g/cm3
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BUSHVELD PLAGIOCLASE COMPOSITIONS
/
Fig. 16. Plot of density vs depth for a short section of relatively melanocratic rocks in the Main Zone. Densities of 43 g/cm3 indicate an abundance of pyroxene in excess of its cotectic proportions with plagioclase. Small-scale oscillations in modal proportions are indicated, within an
overall sequence that becomes more leucocratic.
and grade regularly downwards through 80 m to densities
of nearly 38 g/cm3. The bulk density of the Upper Zone is
312 g/cm3, and so we see no evidence in this section for
the preferential concentration of plagioclase in the uppermost rocks that might support a flotation process.
On a much smaller scale, we re-examined the range of
An contents in plagioclase in single samples through this
uppermost interval. If grains of plagioclase had floated or
even been held in suspension during protracted periods of
the fractionation process then some An-rich cores might
have been preserved in the last rocks to solidify. Thus,
occasional analyses with atypically high An content might
be expected in these uppermost rocks. We note that in the
complete analytical database there is no evidence for any
grains that are anomalously An-rich in this uppermost
part of the studied section. We conclude that the density
and mineral compositional evidence do not support the
concept of plagioclase flotation.
We emphasize a major difference between the top of the
Skaergaard Intrusion and the top of the Bushveld
Complex. A roof facies exists for the Skaergaard
Intrusion, in which the mineral compositions indicate
solidification from the top downward. In the Bushveld
Complex there is no comparable roof facies.
Differentiation in terms of An in plagioclase and Mg:Fe
ratios in the mafic minerals continues monotonically to
the top (Ashwal et al., 2005). Hence, we do not consider
the inward fractionation or solidification front model of
Marsh (2006) applicable here.
A subtle, but important, distinction between prolonged
flotation and suspension of plagioclase grains has been
emphasized by Davidson et al. (2001, 2007). They suggested
that grains of plagioclase may have remained suspended
in the magma and transported from where they grew (in
their case, near the edge of a magma chamber) to be
deposited in a very distant and different setting. They
used the term antecrysts to describe such minerals that
were foreign to their immediate setting but had formed
within the same magmatic system. Given the small density
contrast between plagioclase and magma, temporary suspension of plagioclase in a convecting magma is plausible.
Evidence for delayed accumulation of plagioclase on
scales of tens of metres vertically in the Critical Zone of
the Bushveld Complex has been presented by Cawthorn
(2002). Specifically, he showed that some pyroxenite layers
had formed, not from a magma crystallizing only pyroxene, but from a magma crystallizing both pyroxene and
plagioclase, but that the plagioclase grains had been held
in suspension. The processes of crystal settling and sorting
and temporary suspension of less dense grains lead to a
very important caveat, that there can be a significant difference in the mineralogy that is instantaneously crystallizing from a magma and the modal mineralogy that may
be accumulating at the floor of the magma chamber.
Lateral distribution
A single vertical section may not be representative of the
entire intrusion if there has been lateral redistribution of
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Fig. 17. Density of samples from the uppermost 100 m of the layered suite. From 80 m below the top contact they define a gross trend of upward
decrease in density. Only two samples have densities of less than 30 g/cm3. These data give no support to the concept that plagioclase, because
of its low density, may have been concentrated near the top of the intrusion.
minerals. Such processes have been documented in sills by
Fro«elich & Gottfried (1988) and Be¤dard et al. (2007) on a
scale of several kilometres, where single vertical sections
change from mafic-rich to mafic-poor, distally away from
a presumed feeder to the sills. A similar lateral variation
has been reported over a horizontal distance of 170 km in
the Critical Zone in the western Bushveld Complex
(Maier & Eales, 1997). In the vertical borehole core section
in the present study, we cannot comment on whether the
intersected layering has lateral continuity. Exposure in
this northern lobe of the Bushveld Complex is extremely
poor. However, in the well-exposed eastern lobe lateral
continuity of all distinctive layers is revealed for many
tens of kilometres (von Gruenewaldt, 1973; Molyneux,
1974). Where thicknesses of layers can be measured, constancy is maintained over these distances, although it is
usually magnetitite layers, rather than anorthosite layers,
to which this statement applies (Eales & Cawthorn, 1996).
By inference, therefore, we suggest that the anorthosite sections identified in the core have lateral continuity.
However, even if there were lateral distribution of the different accumulating phases, that simply reinforces the concept of settling and sorting, merely with a horizontal
component introduced.
Pressure changes
It has been suggested that chromitite layers could result
from an increase in pressure in the magma
chamber (Osborn, 1980; Lipin, 1993). The relative thermal
stabilities of all silicate minerals, as well as chromite, are
influenced by pressure. The liquidus temperatures of plagioclase and pyroxene increase with increasing pressure, but
at different rates, as shown in Fig. 18. Thus, a liquid may
have a composition at the eutectic for plagioclase and pyroxene at a particular pressure. If there were a reduction
in pressure, possibly as a result of magma eruption
(Naslund & McBirney, 1996; Cawthorn & Walraven,
1998), that same liquid would become superheated. On
slight cooling it would crystallize plagioclase only until
the cotectic was again reached. In this way, it is possible
to temporarily terminate pyroxene crystallization, and
permit the accumulation of an anorthosite layer
(Cawthorn, 2003). Proving that a pressure reduction
occurred within a magma chamber remains an elusive
challenge.
We note that the relationship between the adiabatic gradient and the liquidus as a function of depth or pressure
was first identified by Hess (1960) as a process of
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CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
Fig. 18. Effect of pressure on the relative stabilities of plagioclase and pyroxene crystallizing from a basic magma [constructed from experimental studies at shallow crustal pressures reviewed by Cawthorn (2003)]. It should be noted that the dT/dP gradient is less for plagioclase than
for pyroxene, 004 vs 0168C/MPa, and so changing the pressure changes the relative stability of the two phases. The adiabatic gradient is
0018C/MPa, and so pressure release on a magma lying at the cotectic for these two phases (filled dot) causes superheating. An increase in pressure will promote rapid crystallization.
inducing crystallization. He suggested that cooling
magma at the upper contact of an intrusion would
become more dense and begin to sink. As it did so, it
would begin to crystallize (as shown by Fig. 18). The
crystallizing minerals being typically mafic minerals
would increase the combined density of the sinking blob
(Morse, 1986), so accelerating the process. However, in our
case, we are investigating the origin of plagioclasedominant layers, which cannot be induced by pressure
increase, and so sinking magma packets cannot be the
mechanism here.
M A G N E T I T I T E L AY E R S
Hypotheses for the origin of magnetitite layers include:
liquid immiscibility (Reynolds, 1985a, 1985b); magma addition and/or mixing (Harney et al., 1990); settling and sorting of minerals (Wager & Brown, 1968); change in
oxygen content (Klemm et al., 1985) and pressure changes
(Cawthorn & McCarthy, 1980).
Liquid immiscibility
The separation of an evolving magma into iron-rich and
silica-rich immiscible liquids was proposed by Bateman
(1951) for the origin of magnetitite layers. As applied to
the Bushveld Complex, this model has been elaborated
upon by Reynolds (1985a, 1985b), von Gruenewaldt (1993)
and Scoon & Mitchell (1994). Recent experimental data
have substantially amplified our understanding of the complexity of such immiscibility (Veksler et al., 2007). We do
not question the concept of immiscibility, but rather how
it applies to the formation of monomineralic layers. In
terms of field observations, the basal contacts of magnetitite layers, typically underlain by anorthosite, are sharp
and planar (see Cawthorn et al., 2005, fig. 20). An ironrich liquid would have a low viscosity and a high density,
and would percolate through a mush of plagioclase
grains, but this is not observed. Also, the graded tops to
magnetitite layers, often over tens of centimetres thick,
show a regular increase in plagioclase proportion from 0
to 80% (Cawthorn et al., 2005, fig. 20). Such a gradual
change would not be expected at the top of an extremely
dense layer of iron-rich liquid.
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JOURNAL OF PETROLOGY
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The most iron-rich immiscible liquids produced in
experimental studies, as reviewed by Veksler et al. (2007),
contain no more than 28% total FeO (and up to 5%
TiO2). The magnetitite layers contain over 95% of these
two elements. Thus, monomineralic magnetitite layers do
not approximate to the composition of immiscible liquids.
An enrichment of Fe and Ti by a factor of three relative to
the immiscible liquid composition and the near-complete
removal of all other elements are still required. Iron-rich
immiscible liquids strongly partition phosphorus relative
to the silicate magma, possibly by a factor of greater than
10 (Watson, 1976). We note that the lowest magnetitite
layers do not contain apatite at all. It is present in magnetitite layers only above the level of the appearance of cumulus apatite in the silicate-rich rocks (Cawthorn & Walsh,
1988; Ashwal et al., 2005). Hence, we believe that there are
several lines of evidence that suggest that immiscibility is
not the mechanism that produced the oxide-rich layers.
Magma mixing
Addition of magma may trigger the formation of magnetite, possibly by magma mixing or if the new magma was
magnetite-saturated. Kruger & Smart (1987) and Harney
et al. (1990, 1996) suggested that the resident magma
column might be internally stratified and that the magma
mixing event resulted from the collapse of two or more
density stratified liquid layers. If there was addition of
magma, the composition of the plagioclase above a magnetitite layer need not show any relationship to that
below. If the mixing was with a less differentiated magma
from within the chamber a reversal in An content would
be expected, although the magnitude cannot be predicted.
Tegner et al. (2006) suggested that this process occurred
intermittently in the western lobe of the Bushveld, but
that magnetitite layers were not obviously related to such
reversals in An content. In Table 2 and Fig. 19 it is shown
that for three magnetitite layers (16, 17 and 18) there is an
increase of between 2 and 3% An above the magnetitite
layer, and for one layer (2) an increase of 6% An. Across
one magnetitite layer (11) there is a forward jump of
slightly more than 2% An. For the other 15 layers there is
no significant difference. The preponderance of matches
between the compositions above and below layers suggests
that magma addition is not likely to be the common mechanism for layer formation. This problem is addressed
below.
Settling and sorting of minerals
The general principle of settling and sorting proposed by
Wager & Brown (1968) may be applied here. Many of the
magnetitite layers display a sharp base and gradational
top, which could be considered indicative of settling and
sorting (Cawthorn et al., 2005, fig. 20). However, there is
an apparent inconsistency in its application here. If it is
assumed that the magma is saturated in magnetite,
NUMBER 9
SEPTEMBER 2009
pyroxene and plagioclase then that ought to be the order
in which the phases accumulated from the base upward.
In fact, the gradational tops are dominated by the increasing abundance of plagioclase, and no or little cumulus pyroxene is present. Thus, whereas the graded modal
relationship between magnetite and plagioclase suggests
sorting, the absence of pyroxene, which has intermediate
density between magnetite and plagioclase, suggests that
pyroxene is not a crystallizing phase.
Change in oxygen content
It has been suggested that an increase in oxygen fugacity
may initiate magnetite crystallization (Ulmer, 1969;
Klemm et al., 1985). This mechanism requires no change
in magma composition in terms of elements that would
affect the plagioclase composition. Hence, there should be
no change in plagioclase composition above a magnetitite
layer compared with that below, once plagioclase resumed
crystallization, as is generally observed in Table 2.
Problems with this model are how this extra oxygen could
have been added to the magma, and why it produced such
an abrupt and uniform basal contact to magnetitite layers
over such enormous distances. The V and Ti contents of
the only layer that has been studied are extremely constant
over distances of hundreds of kilometres (Cawthorn &
Molyneux,1986). Because the Vand Ti partition coefficients
into magnetite are extremely sensitive to the fO2 of the
magma (Toplis & Corgne, 2002) these observations
demand, not merely addition of oxygen, but exactly the
same change in fO2 over these huge distances.
Change in pressure
By analogy with the suggestion that chromitite layers
might originate from an increase in pressure, such an
increase in pressure has also been proposed as a mechanism for producing magnetitite layers (Cawthorn &
McCarthy, 1980). As for the model that envisaged an
increase in oxygen fugacity, such a pressure change would
not result in any differences between the An content of plagioclase below and above the magnetitite layers.
A consequence of this pressure change and the formation
of copious amounts of magnetite in making a layer would
be the lowering of the iron content in the remaining melt.
A magma that was originally crystallizing plagioclase
and pyroxene (and possibly magnetite), at a cotectic, may
no longer be saturated in pyroxene as a result of excessive
magnetite formation. Thus, the absence of pyroxene
within the modally graded tops to magnetitite layers
becomes explicable by this process.
We note that models of magma addition and changes in
intensive parameters do not preclude the subsequent settling and sorting of minerals. Hence, both of these processes can occur together; they do not have to be mutually
exclusive.
1632
CAWTHORN & ASHWAL
BUSHVELD PLAGIOCLASE COMPOSITIONS
Fig. 19. Average composition of plagioclase in footwall and hanging-wall samples, below and above magnetitite layers. The central line defines
the trend with zero difference in An content between the underlying and overlying samples. The other lines are 2% An on either side, indicating
the likely uncertainty in these data. Numbers refer to the magnetitite layer number in Table 2.
CONC LUSIONS
The An content of plagioclase can be used as an index of
differentiation. In large layered intrusions steady upward
decrease in An is both predicted and generally observed.
In this study of the uppermost 2800 m of the Bushveld
Complex, such a trend is generally observed, with the
exception of six small reversals in An content. These reversals are not sharp breaks, but occur over many samples
(6^14) and considerable vertical heights (up to 135 m).
Whether such reversals result from addition of magma
from an external source or convective overturn within a
stratified magma chamber cannot be resolved with the current data, but the reversals suggest that such processes,
and especially the rate of mixing, must be extremely
protracted.
Forty-five layers of anorthosite (with greater than 90%
plagioclase) are identified. None exceeds 23 m in thickness.
There are three possible reversals in An content either
within or above these layers, whereas for 42 of the layers
the An contents are the same in the immediate underlying
and overlying layers and the anorthosites themselves.
Furthermore, the immediately underlying and overlying
rocks to most anorthosite layers have a greater proportion
of plagioclase (based on density measurements) than a
predicted cotectic proportion of plagioclase and pyroxene
(plus magnetite). These two observations challenge many
of the previously proposed hypotheses for the origin
of anorthosite layers. We suggest that settling and sorting
of plagioclase and mafic minerals under the influence of
gravity may best explain these features.
Twenty magnetitite layers are observed. Across 16 of
these layers there is no significant change in the An content
of the plagioclase; only one shows a significant reversal.
We suggest that these observations indicate an internal
mechanism for the origin of these layers rather than addition of new magma. A change in pressure represents one
mechanism that can induce magnetite crystallization without causing any change in the composition of the
plagioclase.
AC K N O W L E D G E M E N T S
We thank Tony Morse, James Scoates and Jean Be¤dard
for their constructive reviews, and Marjorie Wilson
for her comments and careful editing of our
manuscript. R.G.C. acknowledges the financial support
of Lonplats, Implats and Angloplats mining
companies. Di du Toit and Lyn Whitfield drew many of
the diagrams.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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VOLUME 50
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APPENDIX
Intensity of modal layering
The creation of a mathematical parameter that defines the
intensity of modal layering in an intrusion would be a difficult concept. Both the vertical interval over which such a
feature occurs and the change in modal proportion are
important in this regard. We attempt a graphical representation of the latter here.
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NUMBER 9
SEPTEMBER 2009
(a)
(b)
Fig. A1. Histogram showing number of determinations of density differences between adjacent samples (typically slightly more than 1 m apart)
in the Upper Zone (a) and Main Zone (b). Absolute differences are calculated (not actual differences), so all values are positive regardless of
whether density increases or decreases upward. It should be noted that the last column in each diagram includes all determinations with differences in density greater than 023 and 022 g/cm3 (a and b, respectively). The effects of changing modal plagioclase proportions on the differences in density between adjacent samples are shown.
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BUSHVELD PLAGIOCLASE COMPOSITIONS
The scale of layering in a vertical sense has been investigated by Webb et al. (2008), usng the same density database
that we use here. In the continuous borehole core studied
here, small-scale layering of the order of 1cm to tens of
centimetres is only extremely rarely observed, specifically
of thin magnetitite layers. There are 2003 determinations
of density of the layered rocks (1014 in the Upper Zone
and 989 in the Main Zone), equivalent to one determination every slightly more than 1m. A wavelet and statistical
analysis of these density data (Webb et al., 2008) revealed
possibly three scales of periodicity or layering in the density data, with wavelengths of about 25, 75 and 150 m.
Obviously these cannot be appreciated in field outcrop or
even in core sections.
Because the density measurements were taken approximately every metre they represent a scale of sampling
between those two scales (tens of centimetres and tens of
metres), which can be appreciated in the field or in borehole core. We determined the absolute difference (and
hence, all differences are positive values) in density
between each adjacent measurement. Such differences are
shown in the histogram, Fig. A1.
We approximate such differences in density to the variation in the proportion of plagioclase to all other (mafic)
minerals, which are inferred not to vary in their relative
proportions. This latter assumption may be reasonable in
the gabbronorites of the Main Zone because the densities
of the mafic silicate phases are similar. However, it is an
oversimplification for the Upper Zone given its magnetite
content. In the Main Zone, plagioclase and the pyroxenes
have a density of about 27 and 33 g/cm3, respectively.
Thus a 1% change in plagioclase proportion equates to
0006 g/cm3 change in density. Given a magnetite content
of about 10% in the Upper Zone, and silicate mineral densities of 35 g/cm3, the combined mafic mineral content of
the Upper Zone would have a density of 36 g/cm3.
For these rocks, a 1% change in plagioclase proportion
equates to a change in density of 0009 g/cm3. Differences
of 5^10% plagioclase are probably very to fairly subtle,
and hence difficult to clearly recognize in hand specimen.
In Fig. A1, we note that for the Main Zone, adjacent
measurements that differ by less than 003 g/cm3 (equivalent to 55% plagioclase modal variation) make up nearly
60% of all measurements. Measurements with a difference
of less than 006 g/cm3 (510% plagioclase variation) make
up 85% of all determinations. For the Upper Zone determinations, measurements with a variation of less than
0045 g/cm3 or less than 009 g/cm3 (equivalent to a
change of 5 and 10% plagioclase, respectively) make up
55% and 75% of all determinations. Hence, on a 1m
scale, modal layering is not easily recognizable in over
75% of all observations. Layering that displays a greater
than 20% modal variation in plagioclase over 1m occurs
in less than 5% of Main Zone and less than 15% of
Upper Zone sections. Thus, we conclude that modal layering that exceeds 10% variation in plagioclase proportion
is a rare phenomenon in the Main and Upper Zones of
the Bushveld Complex. The anorthosite, magnetitite and
pyroxenitic layers discussed here are therefore the exceptions, rather than merely extremes in a continuously variable modally layered succession.
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