the use of plagioclase composition as an indicator of...

Mineralogy and Petrology (1996) 56:91-103 Mineralogy

an(1 Petrology

© Springer-Verlag 1996 Printed in Austria

The use of plagioclase composition as an indicator of magmatic processes in the Upper Zone of the Bushveld Complex

D. M. W. Harney*, G. Von Gruenewaldt**, and R. K. W. Merkle

Institute for Geological Research on the Bushveld Complex, University of Pretoria, Pretoria, South Africa

With 6 Figures

Received November 30, 1993; accepted July 24, 1995

Summary

Analytical data on the composition of plagioclase from the lower part of the Upper Zone in the eastern Bushveld Complex is presented. Detailed electron microprobe investigations failed to establish any cyclic variation through that sequence but revealed similar variations in An content, potassium and iron concentrations below and above magnetite layers. These findings can be attributed to the heterogeneous nature of the plagioclase both within individual grains and within a given sample, which would mask any possible trends of cryptic variation. The Sr concentration and Sr/A120 3 ratio of plagioclase, determined by XRF on plagioclase separates, however change slightly at the level of the Main Magnetite Layer, which can possibly be related to the breakdown of density stratified liquid layers within the resident magma. Analyses of plagioclase separates are thus considered to be more suitable to indicate magmatic processes than plagioclase compositions determined by electron microprobe.

Zusammenfassung

Plagioklaszusammensetzung als Indikator ffir magmatische Prozesse in der Upper Zone des Bushveld Komplexes

Analytische Daten von Plagioklasen aus dem unteren Teil der Upper Zone im 6stlichen Bushveld Komplex werden prfisentiert. Detaillierte Untersuchungen mittels Elektronen-

Present addresses: * Anmercosa Exploration (Burkina Faso) Ltd., B.P. 1798, Ouagadougou 01, Burkina Faso, and ** Foundation for Research Development, P.O. Box 2600, Pretoria, 0001, RSA

92 D.M.W. Harney et al.

strahl-Mikrosonde ergaben keine Hinweise auf eine zyklische Variation in dieser Abfolge, zeigten aber eine/ihnliche Variation des An-Gehaltes, sowie der Kalium- und Eisengehalte im Liegenden und Hangenden von Magnetitlagen. Dies 1/il3t sich mit der heterogenen Natur der Plagioklase, sowohl in EinzelkiSrnern, als auch innerhalb einer Probe erkl/iren, die jeden m6glichen verborgenen Variationstrend verdecken wiirden. Der mittels XRF Analytik an separierten Plagioklasen bestimmte Gehalt an Sr und das Sr/AlzO 3 Verh~iltnis findern sich allerdings geringfiigig im Bereich des Main Magnetite Layer. Dies wird m6glicherweise mit dem Zusammenbruch yon dichtegeschichteten Schmelzlagen im Magma in Beziehung gebracht. Die Analyse von Plagioklaskonzen- traten scheint daher geeigneter zu sein magmatische Prozesse anzuzeigen als Mikroson- denuntersuchungen.

Introduction

Sudden changes in the fractionation trends of cumulus silicates have frequently been used to identify magmatic processes in layered intrusions, especially events such as magma mixing and magma replenishment. Well known examples from the Bushveld Complex are the Merensky Reef (Naldrett et al., 1986, 1987; Eales et al., 1988) and the Pyroxenite Marker (Von Gruenewaldt, 1973; Molyneux, 1974; Klemm et al., 1985; Cawthorn et al., 1991).

Controversy, however, still surrounds the formation of magnetite layers in the Upper Zone of the Bushveld Complex. While Sr-isotopic investigations by Kruger et al. (1987) indicate that the Upper Zone crystallized from an isotopically homogeneous magma, whole rock geochemistry as well as compositional data of pentlandite and olivine led Merkle and Von Gruenewaldt (1986) to conclude that magnetite layers possibly formed in response to magma mixing events. Changes in the platinum-group element concentration pattern at the level of the Main Magnetite Layer support this model (Harney et al., 1990).

Since plagioclase is the most prominent mineral phase in Upper Zone rocks, it was decided to conduct a detailed study on the composition of plagioclase, aiming to contribute towards a model for the formation of magnetite layers. The results of that investigation are presented in this paper.

Sampling and petrographic description

The stratigraphic interval under investigation comprises the lowermost 200 m of the Upper Zone in the eastern Bushveld Complex (Fig. 1). It extends from the bottom of the Upper Zone up to magnetite layer 7 (nomenclature after Molyneux, 1970) with specific emphasis on the Main Magnetite Layer and its immediate footwall and hanging wall contact.

Forty-one samples were taken from drill core through the portion between the Main Magnetite Layer and magnetite layer 5, drilled at Mapochs Mine, Roossenekal. Sample spacings range from a few meteres to a few centimetres in the immediate vicinity of magnetite layers. Further 17 samples, representing the remaining lower part of the Upper Zone, come from surface exposures in the vicinity of Roossenekal. For comparison only, two Main Zone samples from the Pyroxenite Marker and a typical gabbronorite stratigraphically 530 m above that layer were included in this investigation.

The use of plagioclase composition 93

300 -

250-

200"

•f'• <C 150-

,.NO Iil

=} lOO-

50"

O-

-5 (

- 100 -

gabbro

Layer 7

Layer 6

rite

gabbro

, Layer 5

, Layers 3 & 4

. . . . . . . . , . .

::i::i::i::iiiii Magnetite Layer 2 i:i:!:i:i:? ?i?i?i?i?i?i Magnetite Layer 1

Main Magnetite :: ~ : Layer

o o o =

o o o o

:i:i$i:i:! : : : : : : : : : : : :

e g ' C - p

, o o o

:~:~ Mottled anorthoalte , o o o ,

- 1 0 0 "

-15(

- 6 0 0 -

I.O o i z_

, : -85o-

-7oo-

- 750 -

-800 -

[U Z NO -850-

U)

~ Lower Magnetite Layers 2 & 3

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

i:i:i:!:i:i: Lower Magnetite . : . : . : . : . : . :

Layer 1 ::; o°: Mottled anorthoslte : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : - : : : : : : : : : : :

Fine-grained norlte

!iiii!! !iiiiii iii

Predominantly homogeneous gabbro

~ili~iiiiiiii iiiiiii!iiiil : : : : : : : : : : 2 > : + : + :

!i~ii[iiiii~i iiiii~iiiiiii iiiiiiiiiiiii : : : : : : : : : : : : :

iiiiiiii~ii~i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

::i:!:i:i:!:i . . . . . . . . y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . : + : + : + % % ' , m = l

* o ,

P e * e * ,

~::. . : ' P o r p h y r i t i c ' . n o r l t e

::11:: i:!:!:;i:!:i iiiiiiiiiiii :i:i:i:~:!:! . . . . . . . . . ,

iiiiiiiiiiii : : : : : : : : : : : : :

iiiiiiiiiiiii Homogeneous gabbro ...........,

: : : 5 : : : : : : : : : : : 5 : : : : : : : : : : : : : : : : : : : : :

5 : : : : : : : : : : . . . . . . . . . . . . : : : : : : : : : : : 5 : : : : : : : : : : : : :

: : : : : : : : : : : : : , . . . . . . . . . .

Pyroxenlte Marker

~ \ \ Fine-grained norlte \ \ \ \

Fig. 1. Generalized columnar section of a part of the Upper and Main Zones in the Roossenekal area, eastern Bushveld Complex (after Von Gruenewaldt, 1973). Height in metres relative to the base of the Main Magnetite Layer

Most prominent rock types in this stratigraphic succession are magnetite- bearing anorthosites, leuco-gabbronorites and gabbronorites, as well as layers of titaniferous magnetite. Plagioclase accounts for between 50 and almost 100 vol. % in all rocks under investigation, except in magnetite layers and a gabbronorite horizon 125 m above the Main Magnetite Layer. It forms large tabular cumulus grains which are usually 1.5 mm to 2 mm long, but frequently attain a length of up to 1 cm.


Pyroxene occurs as primary orthopyroxene, inverted pigeonite and clinopyroxene. The distribution of all three minerals is very irregular and their concentrations rarely exceed 5vo1.%. For most of the investigated stratigraphic interval, all three mineral phases are interstitial to cumulus plagioclase grains, except for minor gabbronorites towards the top of this sequence which contain cumulus Ca-poor pyroxenes.

The magnetite content of the rocks is highly variable, ranging in most cases from 5 to 30 vol.%, with almost 100 vol.% in magnetite layers. Within silicate-bearing rocks, magnetite occurs as discrete cumulus grains, while massive layers consist of closely packed polygonal magnetite grains. Traces of intercumulus material, mostly biotite as well as rare quartz and K-feldspar, occur throughout this portion of the Upper Zone. Variations in the modal amount of these interstitial minerals relative to magnetite layers were not observed.

Analytical procedure Fully quantitative electron-microprobe analyses were carried out with a JEOL 733 Superprobe. Plagioclase was analyzed for Na20, SiO 2, CaO, K20, A120 3 and FeO, using natural albite, quartz, wollastonite, sanidine and pure oxides, respectively, as standards. Accelerating potential was 15kV with a beam current of 2 x 10 - s~ , measured and monitored on a Faraday cup. The beam diameter used was 10 microns and counting times for each element were 20 and 10 seconds respectively for peak and background measurements. Full ZAF corrections were performed with the program FZA FM from JEOL. Using duplicate analyses of identical spots, repro- ducibilities were calculated to be 0.06 wt.% for Na20, 0.16 wt.% for SiO 2, 0.45 wt.% for CaO, 0.02wt.% for K20, 0.10wt.% for A120 3 and 0.03 wt.% for FeO.

In each sample, cores from 10 to 20 plagioclase grains were analyzed. In 36 thin sections, the rim of most of these grains was analyzed as well. To evaluate the validity of these simple core - rim analyses with respect to zoning, a total of 63 traverses through plagioclase grains from five samples were selected and 10 grains per thin section were analyzed along at least one traverse each. The distances between each measuring point increased from 20 microns at the rim to 50 and 100 microns in the centre of the grains. Furthermore, in three to four grains per section, both rims of the analyzed traverse were investigated in detail over a length of about 50 microns. In these cases, the distances between spot analyses as well as the beam diameter were reduced to 2 microns.

The results presented here are based on a total of 5095 microprobe analyses. Analyses with totals below 98.5 wt.% and above 101.5 wt.%, as well as those with total cation numbers below 4.98 and above 5.02, were rejected.

In addition to the microprobe analyses, trace element concentrations of 15 plagioclase separates were determined by XRF. The quality of all separates, pre- pared by magnetic separation, was tested microscopically, by electron microprobe and X-ray diffraction. The samples were analyzed for Sr, Rb, Ba, Ga, Cu, Ni, Pb, Zn, As, Sb, Mo, Nb, Y, Zr, U, Th, Cr, Se and Sc as pressed-powder briquettes of 5 g milled sample material with a wavelength-dispersion XRF equipment. A tube potential of 15 kV on a rhodium tube and a filament current of 15 mA were used. Counting times on peak and background positions were 40 seconds.


Due to lack of sample material, only 3 g were available to prepare powder briquettes of samples Z-2 and G649, and 4.5g from sample G426. To test the reliability of the results obtained from these samples, one sample (G568) was analyzed as a 3 g and 5 g powder briquette. A 17% lower Sr concentration was determined in the 3 g sample which is attributed to a lack of infinite thickness of the briquette. Consequently, the results of samples Z-2, G649 and G426 were extrapolated to a concentration equivalent to 5 g material.

Results

The detailed step analyses showed that virtually none of the plagioclase crystals is normally zoned. The grains are either not zoned at all or show reverse zoning which is detectable only at the outermost margins of the grains in a zone usually 10 to 20 microns wide (Fig. 2). The average difference between core and outermost rim was found to be about 16 mol.% An, but often exceeds 20 mol.%. Furthermore, zoning does not depend on the type of the adjacent mineral grain and is not necessarily developed along all boundaries of one plagioclase crystal.

6s

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55

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70

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10,u. 400 p. 10,u.

An %1 An%

1 75 ~ .I -- 75 6s g I ;es

10~ 300~ 10 p.

Fig. 2. Profiles of An content across four plagioclase grains in contact with different adjacent minerals. Please note the change in scale


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8 0 ~

70-

60-

50-

40-

30-

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c)

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6o A n %

d)

" ' 1 I ' q ' f ' J

50 55 60 A n %

Fig. 3. Frequency distributions of An content of plagioclase cores from 10 grains within a single sample (a) and three individual grains from the same sample (b to d)

The plagioclase cores are rather heterogeneous (Fig. 3). Within single grains, the core composition typically varies by 2 to 8 mol.% An. In individual thin sections, the An content was found to differ by up to 9 mol.%, in rare cases by as much as 18 mol.%.

The results of core analyses from all samples in the investigated stratigraphic succession are shown in Fig. 4 and 5. Selected Upper Zone analyses, demonstrating some extreme core compositions, are given in Table 1. As mentioned above, large variations of core composition is evident in all samples. Although fluctuating, the An content generally decreases with stratigraphic height from 58 to 64 mol.% An at the bot tom of the Upper Zone to between 53 and 59 mol.% An at the level of magnetite layer 7. However, given the large variation of An contents within individual horizons, no cryptic variation or compositional breaks can be recognized. A detailed investigation across the Main Magnetite Layer also failed to show any signs of systematic variation. Ten samples taken up to 2 m above and below the Main Magnetite Layer, as well as from a feldspathic parting within that layer, give similar core compositions ranging from approximately 53 to 63 mol.% An (Fig. 5).

The potassium and total iron content of plagioclase cores show similar results. Both concentrations fluctuate considerably within individual samples, up to 0.55 wt.% K20 and 0.72 wt.% FeO, and no compositional breaks can be recognized at any of the magnetite layers.

The Sr concentration of plagioclase, however, seems to increase at the level of the Main Magnetite Layer by about 20 ppm and is less variable above that layer than


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! i . . . . I . . . . I . . . . I . . . . I ' ' ' ' I . . . . I

50 5 5 6 0 6 5 70 7 5 80

An %

Fig. 4. Plagioclase core composition in the lower part of the Upper Zone, eastern Bushveld Complex, and in two Main Zone samples. Height in metres relative to the base of the Main Magnetite Layer


metres

3 0 -

2 5 -

2 0 -

1 5 -

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Main Magnetite Layer

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• 1 Data p o i n t

• 3 Data po in ts

k . . . . I . . . . I . . . . L . . . . I . . . . I . . . . I 5 0 5 5 6 0 6 5 7 0 7 5 8 0

An %

Fig. 5. Plagioclase core composition in the vicinity of the Main Magnetite Layer. Height in metres relative to the base of that layer

below (Fig. 6; samples with extrapolated Sr concentrations are identified by smaller symbols). A similar trend is shown by the Sr/A120 3 ratio in plagioclase (A120 3 content represents the mean value of all microprobe analyses from plagioclase cores within a single sample). Using the modified Tukey test (Neave, 1979), it can be shown with an error probability of less than 0.5% that plagioclase above the Main Magnetite Layer has a higher Sr/A120 3 ratio than below.


Table 1. Selected core compositions of plagioclase from the lower part of the Upper Zone

Sample Z3/1 G649 25 .91 56/57 72/73 76/77 90/91 G407 G608 Height 210.0 125.0 61.64 44.20 2.75 2.55 -1.45 -45.0 -125.0

SiO 2 53.81 53.68 5 4 . 4 8 55.23 54.64 50.86 53.69 51.86 52.33 A120 3 28.99 28.36 28.81 28.61 28.76 31.13 29.19 30.33 30.14 FeO 0.21 0.33 0.24 0.30 0.22 0.30 0.41 0.28 0.32 CaO 11.91 11.97 1 1 . 0 7 11.37 10.84 1 3 . 9 7 12 .15 13.07 13.07 Na20 4.66 4.45 5.04 4.89 5.07 3.48 4.54 3.94 3.95 K20 0.10 0.47 0.15 0.31 0.25 0.25 0.11 0.22 0.12

Total 99.68 99.26 99.79 100.71 99.78 99.99 100.09 99.70 99.93

Number of cations based on 8 oxygen atoms

Si 2.441 2 .451 2.463 2.477 2.470 2.319 2.430 2.364 2.377 A1 1.550 1 .526 1 .535 1.512 1.532 1.673 1.557 1.629 1.614 Fe 0.007 0.012 0.009 0.010 0.008 0.010 0.014 0.010 0.012 Ca 0.579 0.586 0.537 0.547 0.525 0.682 0.589 0.638 0.636 Na 0.410 0.394 0.442 0.425 0.444 0.307 0.398 0.348 0.348 K 0.006 0.027 0.009 0.018 0.015 0.014 0.006 0.013 0.007

Total 4.993 4.996 4.995 4.989 4.994 5.005 4.994 5.002 4.994

End-member proportions

An 58.22 58.17 54.36 55.22 53.36 67.97 59 .31 63.87 64.19 Ab 41.19 39.13 44.74 42.97 45.16 30.60 40.08 34.85 35.13 Or 0.59 2.70 0.89 1.80 1.48 1.43 0.62 1.29 0.68

Concentrations of oxides in wt.%; concentrations of end-members in mol.%; total Fe as FeO; height in metres relative to the base of the Main Magnetite Layer

Regarding any of the other trace elements, no such trend can be recognized. While the concentrations ofRb, Sc, Nb, Y, Sb, Se, Mo, U, Th, Cr and Sc are all below their respective lower limits of detection, Cu, Ni, Pb, Zn, Ga, Ba, As and Zr fail to display any systematic pattern within the investigated stratigraphic sequence (Table 2).

Discussion and conclusions

The results presented above clearly demonstrate the large composit ional variation of plagioclase cores from the lower part of the Upper Zone. In order to determine wether this is due to random variation or oscillatory zoning, smaller sampling distances in the centres and more traverses per grain would be required. However, the large size of the cumulus plagioclase grains suggests prolonged residence time of the growing crystals in the magma which could have caused more complex zonation patterns.


metres

1,50"

100-

50"

O-

-50"

-100-

M a i n Magn__~otite ~_~e__~r _ _

e

-295-~ •

_825.t Pyroxenite Marker . - . . . .

I 380 400 420 440

$r ppm

O

E 11 \ m

z " N

m

460 13 14 15 16 17 (Sr/AI203) x 104

Fig. 6. Sr concentration and Sr/A1203 ratio in plagioclase from the lower part of the Upper Zone and two Main Zone samples in the eastern Bushveld Complex. Height in metres relative to the base of the Main Magnetite Layer. The shaded areas in the right half of the diagram show the 2 sigma envelopes of the respective average values

The unsystematic narrow rims of reverse zoning are most likely related to postcumulus processes rather than being a primary magmatic feature. They can possibly be explained by reaction of the plagioclase rims with a late stage intercumulus melt or hydrothermal fluid, similar to the hydrothermal infiltration metasomatism observed by Schiffries (1982) in the upper Critical Zone of the Bushveld Complex.

However, due to the very narrow area of zoning and the heterogeneous cores, it is almost impossible to determine the extent of zoning within a grain with one single core and rim analyses each. In more than 1/3 of the plagioclase grains analyzed with the core-rim method, the zoning was interpreted incorrectly. Simple core-rim measurements do therefore not appear to be an appropriate method to determine plagioclase zoning.

The overall decreasing An content of plagioclase cores within the lower part of the Upper Zone or, as indicated by the two Main Zone samples, within the interval from


Table 2. Trace element concentrations in plagioclase separates from the lower part of the Upper Zone and two main zone samples

No Height Sr Zn Cu Ni Pb Ga Zr Ba As

G566 210 440 13 81 14 10 28 7 118 10 G568 195 440 14 30 8 10 26 25 148 4 G649 125 448 11 34 nd 9 22 4 142 nd G510 35 455 12 59 4 7 28 7 76 5 G509 10 451 10 99 l0 11 24 7 98 nd G400 5 442 10 58 l l 6 26 10 84 8 G406 - 10 431 11 26 6 7 24 5 58 nd U1A - 2 0 414 9 38 8 7 25 13 93 5 G612 - 2 5 431 14 22 nd 6 24 5 96 nd G407 - 4 5 433 10 25 5 7 27 6 147 nd Z2 - 8 0 387 9 83 nd 5 19 5 124 10 G609 - 110 458 12 16 nd nd 29 6 94 nd G608 - 125 434 11 35 nd 5 24 4 36 11 G600 -295 432 11 66 6 6 26 6 71 nd G426 -825 437 9 19 15 10 21 6 45 nd

Concentrations in ppm; nd not detected; height in metres relative to the base of the Main Magnetite Layer

the Pyroxenite Marker to magnetite layer 7 can be explained by fractionation processes in the magma chamber towards more sodic plagioclase in more differenti- ated rocks. No other trends of cryptic variation in the An content can be recognized. If such trends are present, they are completely masked by the internal heterogeneity of the plagioclase grains.

However, al though based on a much smaller data set, the Sr concentrat ion of plagioclase appears to change at the level of the Main Magneti te Layer. Strontium substitutes for Ca in the crystal structure of plagioclase (Bambauer, 1988) and being compatible into plagioclase (Drake and Weill, 1975), the Sr concentration in a melt generally decreases with progressive fractional crystallization. A sudden increase in the Sr concentrat ion above the Main Magnetite Layer at constant An content could therefore indicate a compositional change of the crystallizing magma.

It can be argued that a lower modal amount of clinopyroxene, representing the other major Ca-bearing mineral in the Upper Zone, could also lead to a higher Sr concentrat ion in plagioclase. However, no significant difference in the modal amount of clinopyroxene below and above the Main Magnetite Layer was observed and a Spearman correlation also failed to detect any statistical dependence between the modal amount of clinopyroxene and the Sr concentration in plagioclase (correlation coefficient: -0 .3077, error probability: 0.2646). Furthermore, clinopyroxene in the rocks under investigation formed during the intercumulus stage, i.e. after plagioclase had crystallized.

Possibly, the Upper Zone magma simply became enriched in Sr when plagioclase crystallization was replaced by crystallization of magnetite at the level of the Main Magnetite Layer.


However, a change in the Sr concentration of plagioclase, similar to that at the Main Magnetite Layer, was also documented from the J - M Reef in the Stillwater Complex (Naldrett et al., 1987) and considered as a strong indicator for an influx of a compositionally different magma. Furthermore, investigations by Eales et al. (1986, 1988) in the upper Critical Zone of the western Bushveld Complex demon- strated that different cyclic units, which are thought to represent discrete injections of primitive, mafic liquid, can be identified using Sr/A120 3 ratios.

By analogy, the different Sr concentrations and Sr/A120 3 ratios in plagioclase above and below the Main Magnetite Layer could possibly reflect a magma mixing event between the resident and a less fractionated melt at the level of the Main Magnetite Layer. This interpretation is supported by results from whole rock and mineral chemistry of Merkle and Von Gruenewaldt (1986) and Harney et al. (1990). As indicated by the small change in Sr concentration across the Main Magnetite Layer, magma mixing more likely resulted from the collapse of two or more density stratified liquid layers within the resident magma, rather than a fresh influx of melt. Hence, the Sr-isotope ratio will not change and thus explain the findings of Kruger et al. (1987).

In conclusion, the results of a detailed investigation on the composition of plagioclase in the Upper Zone of the Bushveld Complex clearly demonstrate the difficulties in interpreting microprobe data of that mineral. Given a sufficiently large number of plagioclase analyses, any trends of compositional variation are completely masked by the internal heterogeneity of the plagioclase grains. The strontium content of plagioclase, however, could possibly be a more sensitive indicator of magmatic processes. Geochemical analyses of plagioclase separates are therefore considered to be more meaningful than microprobe work on individual plagioclase grains to explain the formation of magnetite layers in the Bushveld Complex.

Acknowledgements

The authors thank the Anglo American Corporation of South Africa for making borehole cores available and the Foundation for Research Development for providing financial support for this investigation. Constructive criticism by K. L. WaIsh and an anonymous reviewer is greatly acknowledged.

References

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Marker in the western Bushveld Complex, South Africa. J Petrol 32:739-763 Drake M J, Weill DF (1975) The partition of Sr, Ba, Ca, Y, Eu 2 +, Eu 3 + and other REE

between plagioclase feldspar and magmatic silicate liquid: an experimental study. Geochim Cosmochim Acta 39:689-712

Eales HV, Marsh JS, Mitchell AA, de Klerk W J, Kruger F J, Field M (1986) Some geochemical constraints upon models for the crystallization of the upper Critical - Main Zone interval, northwestern Bushveld Complex. Min Mag 50:567-582

Eales HI~, Field M, de Klerk W J, Scoon RN (1988) Regional trends of chemical variation and thermal erosion in the upper Critical Zone, western Bushveld Complex, South Africa. Min Mag 52:63-79


Harney D MW, MerkIe RK W, Von Gruenewaldt G (1990) Platinum-group element behaviour in the lower part of the Upper Zone, eastern Bushveld Complex- implications for the formation of the Main Magnetite Layer. Econ Geol 85:1777-1789

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Authors' address: D. M. W. Harney, G. Von Gruenewaldt, and R. K. W. Merkle, Institute for Geological Research on the Bushveld Complex, University of Pretoria, Pretoria 0002, South Africa

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