Recrystallization and the origin of layering in the Bushveld Complex

LITHOS 0

ELSEVIER

Lithos 37 ( 1996) 15-37

Recrystallization and the origin of layering in the Bushveld Complex A.J. Zingg

’

Geology Depurtment, Rand Afrikaans University, 2ooO Johannesburg, South Africa Geoscience Department, Mining University, A-8700 Leoben, Austria Received 1 I

October 1993

Abstract Three rock types are distinguished in this study: (a) the layered rocks, (b) the Reef pegmatites which are parallel to the layering and form prominent marker horizons and (c) the iron rich ultramafic (IRUM) pegmatites which are transgressive to the layered rocks. Reaction textures are observed in the layered rocks adjacent to Reef pegmatites and are compared with similar reaction textures from alteration zones adjacent to and caused by the iron-rich ultramafic (IRUM) pegmatites. These include: (a) clinopyroxenecoronas around orthopyroxene, (b) hornblende lamellae along ( 100) of orthopyroxene, (c) orthopyroxene altered to olivine, serpentine, talc and/or chlorite. In the alteration zones adjacent to IRUM pegmatites the textures were generated by interaction between the pegmatite forming fluid and the wallrocks. A similar origin is proposed for reaction textures in the layered rocks. In the latter case they were formed by the fluid responsible for the formation of Reef pegmatites. In the layered rocks the reaction textures were modified to variable degrees during and after the emplacement of Reef pegmatites. These modifications include in leuconorites: (a) clinopyroxene-coronas overgrown by feldspar, (b) orthopyroxene overgrown by feldspar and hornblende lamellae transforming to biotite and in olivine-pyroxenites (c) olivine-serpentine textures overgrown by a second generation of orthopyroxene. The modifications suggest the subsolidus dissolution of orthopyroxene and precipitation of feldspar in leuconorites and the reverse process in pyroxenites. The textures suggest considerable transfer of matter in the solid rock due to a fluid gradient. Orthopyroxene recrystallized in areas with a high a nZO,plagioclase in regions of low aHZo. This process is believed to be responsible for the stratification of the rock into pyroxene and plagioclase-rich layers. Spherical mineral inclusions are relict textures in the three main minerals: chromite, orthopyroxene and plagioclase. A number

of similarities between the different inclusions suggest a similar origin. The high content of hydrous minerals in all three suggests their formation during the emplacement of Reef pegmatites. These hydrous minerals were then trapped in the recrystallizing plagioclase, orthopyroxene and chromite. Also spherical mineral inclusions witness the late crystallization of pyroxene in melanorite/pyroxenite and the crystallization of plagioclase in leuconorite/anorthosites after the emplacement of Reef

pegmatites.

1. Introduction The textures of layered intrusions are often explained in terms of the cumulus model (Wager et al., 1960; ’ Present Switzerland

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Jackson, 1961; Wager, 1963; Wager and Brown, 1968; Irvine, 1982). According to this model the rocks are the result of interaction between the melt and solid phases. Cumulus .crystals represent early precipitates from a melt dominated system, postcumulus crystals “crystallized from intercumulus liquid in the interstices

16

A.J. Zingg/Lithos

or pores of the cumulus framework” (Irvine, 1982, p. 13 1) . How the crystals accumulated and how differentiation and fractionation took place is the subject of debate (Wager and Brown, 1968; Campbell, 1979; McBirney and Noyes, 1979; Irvine, 1987). Layered intrusions are believed to cool over many thousands of years and should adapt, both chemically and texturally, to subsolidus conditions. Cooling rates of 1 degree over 3000 years have been postulated (Morse, 1980). Subsolidus reequilibration is thus expected to succeed and overprint the igneous stage. However, the subsolidus stage, preceding the hydrothermal stage, is not well documented. Hydrothermal activity has been described from the Bushveld (Schiffries and Skinner, 1987) and Skaergaard intrusions (Taylor and Forester, 1979; Bird et al., 1988). In the presence of a fluid phase, transfer and reaction rates are accelerated and reequilibration is enhanced. The importance of a fluid phase in the petrogenesis of layered rocks has become increasingly evident during the last decade (Boudreau and McCallum, 1985; Ballhaus and Stumpfl, 1986; Boudreau, 1988). The purpose of this study is to assess the influence and magnitude of subsolidus recrystallisation in the layered rocks of the Critical Zone. A number of reaction textures have been observed in the layered rocks, and the description of these textures will form the first part of this paper. These reaction textures will be shown to be due to interaction between solid rock and a supercritical fluid and not with a magma. These textures suggest considerable transfer of material in the subsolidus stage during and after the emplacement of Reef pegmatites. The transfer involves the dissolution of orthopyroxene (opx) from and precipitation of feldspar in rocks with a low Hz0 activity and the reverse process in rocks with a higher H,O activity. This mechanism could lead to the stratification into plagioclaserich and pyroxene-rich layers and could bring an answer to a long debated and still enigmatic topic the origin of the layering in the Bushveld Complex. The interpretation of the reaction textures in the layered rocks is based on similar textures discovered in alteration zones of iron rich ultramafic (IRUM) pegmatites (Cameron and Desborough, 1964; Viljoen and Scoon, 1985). The latter show numerous similarities to the Reef pegmatites (Jones, 1974; Ballhaus and Stumpfl, 1986) but also to the vertical pegmatite pipes from the Eastern Bushveld (Schiffries, 1982; Stumpfl

37 (1996) 15-37

and Rucklidge, 1982). Adjacent to IRUM pegmatites the reactions are due to rock-fluid interaction (Zingg, 1988). The same origin is proposed for the reaction textures in the layered rocks.

2. Geological setting and rock samples studied The Bushveld Complex is a mafic intrusion of Precambrian age (2050 Ma, Hamilton, 1977). The layered sequence, called the Rustenburg layered suite, has been subdivided into five different zones; Upper, Main, Critical, Lower and Marginal Zones (Wager and Brown, 1968; SACS, 1980). Irvine (1982) recommends the use of the terms Bushveld Complex and Basal Zone instead of Rustenburg Layered Suite and Marginal Zone, respectively. The present work is confined to the rock interval between the Upper Group Chromite seam No. 2 (UG 2) and the Merensky Reef (see SACS, 1980 for the location of the two horizons in the layered sequence). The UG 2 and Merensky Reef are the world’s richest platinum deposits and form prominent marker horizons in the upper part of the Critical Zone (UCZ) . Five borehole cores from the western part of the Complex were chosen for study. The core localities are shown in Fig. 1, the logging results in Fig. 2. The main part of the investigated sequence consists of the layered rocks (Fig. 3). The layering is due to rocks enriched in opx (i.e. pyroxenites, melanorites) alternating with rocks enriched in plagioclase (i.e. anorthosites, leuconorite) . Norite forms about 60 vol.% of the core material studied. Pyroxenites/harzburgites and anorthosites make up 22 and 18 vol.%, respectively. In general, it is observed that the melanocratic rocks are most abundant in the lower part of a macro rhythmic unit (Figs. 2 and 3) while the leucocratic rocks are preferably concentrated in the upper part. The succession from pyroxenite to anorthosite is not continuous but irregular including numerous reversals. Special types of anorthosites are the spotted and mottled ones underlying the Merensky Reef (Van Zyl, 1970). The spots and mottles are accumulations of orthopyroxene. The Reefpegmutites form prominent black marker horizons, with a high proportion of femic and hydrous minerals, The composition is pyroxenitic to harzburgitic. In the case of the Merensky and UG2 Reef they are bounded by thin (cm-range) chromitite seams. The

A.J. Zingg/Lithos 37 (19%) 15-37

17

Geological Map of the Bushveld Complex

cationofbore holes

lpoorl

l Fig,.

Johannesburg

I. Geological map of the BushveldComplex showing the locality of the studied core.

Merensky and UG2 Reef are laterally very persistent, also found in the eastern part of the intrusion. Locally they may be subject to considerable variation in thickness and at places may be structurally displaced. A special kind of unconformity are potholes (Ballhaus and Stumpf, 1985b) where the Reef experiences circular depressions (Fig. 3). The distinction between Reef and IRUMpegmatites is often difficult in the field (Jones, 1974), in particular if an IRUM one crosscuts a layered one. The IRUM ones postdate the Reef type and in general are transgressive to the lavering. They are preferably found at high angles or parallel to the layering. Reef and IRUM pegmatites are rich in femic minerals including olivine, opx, cpx, amphibole, spinel, biotite and sulfides. Olivine is surrounded by opx in both rocks, suggesting an increase in the activity of SiOZ during crystallization. Reef pegmatites may reach quartz saturation, while the IRUM ones are always quartz-undersaturated. Both are enriched in spine1 at the outer bounds. Reef pegmatites are accompanied by chromite; IRUM pegmatites contain magnetite. IRUM pegmatites in turn have numerous similarities with the pegmatite pipes of the Eastern Bushveld. Common to both is a high olivine content (Cameron and Desboraugh, 1964; Stumpfl and Rucklidge, 1982; Viljoen and Scoon, 1985)) which replaces plagioclase (Schiffries. 1982) and similar alteration reactions in the immediately adjacent wallrocks. It is

generally agreed (Fockema and Van Biljon, 1956; Cameron and Desborough, 1964; Schiffries, 1982; Stumpfl and Rucklidge, 1982; Viljoen and Scoon, 1985) that IRUM pegmatites and pipes were formed by replacement. The main argument for such a hypothesis is that chromitite layers from the layered sequence crosscut the pegmatite pipes without mechanical disturbance. Other evidence is found in reaction textures from the wallrocks that occur as relict textures inside IRUM pegmatites. The similarities between Reef and IRUM pegmatites on the one side and between IRUM pegmatites and vertical pipes on the other are evident from the field relations. IRUM pegmatites consist of a vertical part (Fig. 3a) crosscutting the lavered rocks, and subhorizontal parts preferably replacing leucocratic rock (Jones, 1974). While the pipes and the IRUM pegmatites show their transgressive character by alteration reactions in the adjacent wallrocks (Fig. 3b), no such reactions have been reported from the Reef pegmatites. This could be one reason why the origin of the latter is still obscure. A careful investigation of the textures of the layered rocks, however, reveals the same reaction types as found adjacent to IRUM pegmatites (and pipes). In the following, reaction textures from alteration zones of IRUM pegmatites are described first and then compared with observations from the layered rocks (Fig. 3b).

18

A.J. Zingg/Lirhos37(1996)

Turfbult

15-37

Marikana

Schildpadnest

Rustenburg

Brits

MR

UG2

I I I I

II

I I I I I I I i I I I I I I I I I I I I I I

-0

Scales mm 0

25 -

SOkm

20m Schildpadnest

: I I I I I I I I I

i

I I I I

i

I I I I I

Uarlkana

Fig. 2. Lithological profiles from five investigated bore hole cores from the Western Bushveld Complex showing the interval between the UG 2 and the Merensky Reef. Layered pegmatites are labelled by p. IRUM ones by p.

A..l. Zingg/Lithos 37 (1996) 15-37

19

(a) Reef Pegmatite (Merensky Reef)

melanocratic

rock

Layered Rocks

- pyroxenites

-__ Reef Pegmatite (Pseudo Reef) Reaction

Profile 1

___

IRUM Pegmatite __ with alteration zone Reef Pegmatite (UG2 Reef)

.

Reaction Profile 1 Cilnopyroxene coronas with sp~nel inclusions surrounding orthopyroxene

Hornblende lamellae parallel to (100) of orthopyroxene

J

Reaction Profile 2

Clinopyroxene coronas with spine1 inclusions surrounding orthopyroxene

Parallel btotite plates tracing the (100) direction of adjacent orthopyroxene

01 +

Srp + TIC

opx I

/

Hydration of orthopyroxene to olivine, serpentine and talc

Oliwne pseudomorph after orthopyroxene coexisting with serpentine and talc

Eiotite

Srp,Tlc

Fig. 3. (a) Schematic diagram showing the relationship between layered rocks, Reef pegmatites and IRUM pegmatites. The section from the bottom of one Reef pegmatite to a neighboring one is called macro rhythmic unit. Reaction profile 1 is through the alteration zone of an IRUM pegmatite, reaction profile 2 is through a macro rhythmic unit. (b) The three reactions observed in reaction profiles 1 and 2.

3. Analytical techniques

Mineral chemical analyses were performed on an ARL SEMQ electron microprobe. Accelerating voltage

was 15 kV, sample current on brass 20 nA and counting times were 20 s on the peak and 10 s on the background. The data obtained were corrected using the Bence and

20

Albee ( 1968) correction were used as standards.

A.J. Zingg/Lithos37(1996)

method.

Natural

minerals

15-37

orthopyroxene

.iT

A+,

4. Reaction textures in alteration zones of IRUM pegmatites IRUM pegmatites are recognized in the field as coarse grained rocks, enriched in femic and hydrous silicates and oxides, relative to the adjacent layered norites. They are preferentially oriented perpendicular or subparallel to the layering (Jones, 1974; Viljoen and Scoon, 1985) and are not horizontally persistent. Although the contact between pegmatite and host rock is sharp, adjacent norite has been affected by the fluid responsible for the formation of the pegmatites (Cameron and Desborough, 1964; Schiffries, 1982). The zone affected by the pegmatite has a width of tens of centimeters. Unaffected norite contains cumulus opx with a diameter of 2 to 4 mm and plagioclase (0 = 0.5 mm). The first sign of alteration is the appearance of clinopyroxene (cpx) subgrains surrounding orthopyroxene (opx) . All the opx crystals inside the alteration zone are accompanied by Cpx-coronas. The width of the coronas is typically 0.1-0.5 mm. The coronas contain numerous spine1 inclusions (Fig. 3b) with a size of about 0.02-0.05 mm. The spine1 compositions show a variable ratio of Cr3+ to Fe3+ ranging from 0.53 to 0.19 (Fig. 6). Cpx exsolution lamellae along ( 100) of opx are more abundant in the alteration zone. Closer to the pegmatite the cpx exsolution lamellae are superseded by hornblende lamellae of variable size and oriented parallel to ( 100) of opx. In the approximate center of the alteration zone, opx transforms to a spongelike intergrowth of olivine and talc which closer to the pegmatite, transforms to olivine and serpentine or chlorite (Fig. 3b). This texture may be surrounded by cpx megacrysts (up to cm diameter). The spongelike texture is interspersed with a lead-and nickel-rich pyrrhotite. The ratio between the different alteration products, olivine, talc and chlorite/serpentine, varies considerably. The contact between the alteration zone and the pegmatite is marked by the assemblage plagioclase, cpx and olivine with traces of hydrous minerals, magnetite and opx. Plagioclase may alter to a fine grained sericite. The composition of coexisting opx and cpx is displayed in Fig. 4. Two types of opx are distinguished, one with a high enstatite content

clinopyroxene

m g Lt ,A

0.2

0.15

0.1

; El

0.05

0

Distance from contact (in m)

Fig. 4. Chemical variation of opx and cpx in a cross section through alteration zone and IRUM pegmatite. Opx, affected by the pegmatite fluid, has a lower Mg/( Mg+Fe) ratio and is in equilibrium with cpx.

(Ens,) which has not been affected by the pegmatite forming fluid, and one with a lower enstatite content (En&, which shows equilibrium tie lines with cpx. Cpx-coronas and the formation of hydrous phyllosilicates have also been described from the Eastern Bushveld pipes (Cameron and Desborough, 1964; Schiffries, 1982; Stumpfl and Rucklidge, 1982). The contact between alteration zone and pegmatite is arbitrarily set where plagioclase increases in grain size. The contact is sharp and the increase in grain size abrupt from cumulus size to a uniform feldspar matrix over an entire thin section. Olivine and cpx do not change in shape or size. The large plagioclase crystals are interspersed with rounded magnetite crystals which may contribute up to 50 modal %. If magnetite is the dominant mineral at the contact, it forms a macroscopically visible, black layer. Some of the large magnetite crystals contain spherical mineral inclusions consisting of plagioclase, cpx, olivine and/or chlorite, talc, serpentine, amphibole and/or clinozoisite. Such inclusions have also been reported from the dunite pipes of the Eastern Bushveld (Lombaard, 1956; Cameron and Desborough, 1964). The main mineral compositional variation in the pegmatite is found in plagioclase which shows a decrease from Anss at the contact to An3o in the pegmatite’s center. This chemical variation parallels a modal decrease of plagioclase from about 70% (contact) to near zero (center). The modal abundance

A.J. Zingg / Lithos 37 (19%) 15-37

4-norite

14-J

harzburgite

Fig. 5. Abundances of: mineral inclusions in plagioclase in anorthites; cpx-coronasin norite; biotite plates adjacent to opx in norites and adjacent to olivine in harzburgites; hydrous minerals in chromitite; and spherical mineral inclusions in chromite of chromitites. Percentages refer to the number of samples with the particular feature relative to the total number examined.

of hydrous minerals increases from near zero (contact) to about 25% (center). The disappearance of plagioclase in favor of femic minerals, in particular olivine, is well documented from the Eastern Bushveld pegmatite pipes (e.g. Schiffries, 1982). Olivine in both IRUM pegmatites and pipes, shows an increase in Fa from contact to pegmatite core (Stumpfl and Rucklidge, 1982 and unpubl. data).

5. Reaction textures in the layered rocks The following textural description starts with the major rock type norite. Pyroxenite and anorthosite are generally located closer to Reef pegmatites (Fig. 2) and are described after the norite. Chromitite often forms the contact to thle Reef pegmatites and is discussed last. Nodes: In norites, plagioclase shows a transition in crystal shape from small and euhedral to large and anhedral. In the anorthosites/leuconorites, the feldspar consists of fresh euhedral to subhedral crystals poicilitically enclosing (overgrowing) opx, while in the pyroxenites/melanorites they form a uniform matrix enclosing euhedral opx. Opx is anhedral in leuconorites but is more euhedral the: higher its modal proportion. It shows a preferred orientation of the c-axis parallel to the layering (Schmidt, 1952). Opx in norite is often accompanied by a small corona of cpx or forms islands embedded in a cpx megacryst. This is a texture familiar

21

to most workers of layered intrusions. The abundance of this texture is given in Fig. 5 as percentage of thin sections where the observation was made in a specific rock type. The modal ratio of opx and cpx varies considerably. This is generally interpreted as a reaction relationship where cpx has grown at the expense of opx (e.g. Van Zyl, 1970). Enclosed in the Cpx-corona are small grains of a Cr-Fe spinel. The abundance of these oxide inclusions is variable and ranges from zero to a few modal percent. Microprobe analyses of the spine1 grains show considerable variation in the ratio Cr/Fe3 + while Al is roughly constant (Fig. 6). Opx in norite often is accompanied by parallel biotite plates that extend into adjoining plagioclase. The determination of the orientation of both opx and biotite by U-stage shows that (001) of biotite parallels ( 100) of opx. Fig. 7 shows some typical occurrences from the Rustenburg and Turfbult section. Opx in rare cases has been observed to contain hornblende lamellae along ( 100). This phenomenon seems more common in the Main Zone (G. Von Gruenewaldt, pers. commun.). Opx containing hornblende lamellae is euhedral while that associated with parallel biotite plates is anhedral and corroded. The fact that biotite lamellae are parallel to the (100) plane of adjoining opx and the pyroxene has only its c-axis parallel to the layering (a and b are in any inclination to the layering) explains why biotite is usually not parallel to the layering but is inclined between 0 and 90” to it. The relationship between bio-

Fe “’

alteration 0

Cr +++

0.2

0.4

layered

0.6

zone norites

0.8

Al++’

Fig. 6. Composition of oxide inclusions in Cpx-coronas of alteration zones of IRUM pegmatites and the layered rocks.

22

A.J. ZingglLithos

37 (1996) 15-37

a

L

J Fig. 7. Parallel biotite plates adjacent to orthopyroxene. (a and b) Small parallel biotite plates at the grain boundary of opx, sticking into adjacent plagioclase. The orientation of the plates is ( 100) of opx (ord. light, scale bar = 0.1 mm). (c) Parallel biotite plates enclosed in feldspar. The opx is corroded and overgrown by plagioclase (ord. light, scale bar= 0.1 mm). (d) Parallel biotite plates with interstitial plagioclase. The feldspar has adapted different crystal shapes between the plates (ord. light, and crossed polar& scale bar=O.l mm).

tite plates and host plagioclase is depicted in Fig. 7d. The shape of plagioclase adapts to the presence of the biotite plates suggesting the late crystallization of the

feldspar postdating the biotite plates. The composition of biotite lamellae and a comparison with other micas is given below.

A.J. Zingg/Lithos37(1996)

a

I

15-37

23

b

e

Fig. 8. Hydration of orthopyroxene. (a and b) Parallel biotite plates adjacent to olivine. In (b) biotite has increased in gram size. A newly formed rim of opx surrounds olivine (ord. light, scale bar = 0.1 mm). Orthopyroxene rim olivine (ord. scale bar 0.1 mm). Kidney-shaped inclusion talc in fresh, euhedral crystal (ord. scale bar 1 mm). Six to sided olivine similar shape size as in pyroxenite 8d) (ord. scale bar 1 mm).

24

A. J. Zingg / Liihos 37 (I 996) IS-37

d

Fig. 9. Spherical mineral inclusions in chromite. (a) Chromite grains enclosed in a matrix of plagioclase and containing spherical mineral inclusions (crossed polars, scale bar = 1 mm). (b) Oval shaped inclusion containing plagioclase (pl), a fine grained hydrous mineral (hm) and probably pyroxene (px) (ord. light, scale bar =O.Ol mm). (c) Spherical mineral inclusion where the old chromite gram boundaries (gb) and a reaction relationship among inclusion minerals is observed. The minerals in reaction relationship are opx, chromian phlogopite (phi) and talc (hm) ( ord. light, scale bar = 0.0 1 mm). (d) Perfectly spherical inclusion containing hydrous minerals only. These are chromian phlogopite (left side) and a fine grained hydrous mineral (ord. light, scale bar=O.Ol mm).

A.J. Zingg / Lithos 37 (19%) 15-37

Pyroxenites and H,zrzburgites: In hand specimen pyroxenites are characterized by euhedral opx grains poikilitically enclosed in a matrix of interstitial and colorless plagioclase. The feldspar in harzburgite in turn is no longer transparent but white and translucent. Thin sections from such samples show the alteration of plagioclase to sericite. While opx is always six to eight sided in pyroxenite, olivine in harzburgite may be either six to eight sided or rounded. Olivine-orthopyroxenites or harzburgites with six to eight sided olivine coexisting with serpentine and talc as well as altered feldspar are common near Reef Pegmatites at the UC2 and the Merensky Reef level. In places, olivine is accompanied by parallel biotite plates (Fig. 8a and b). The plates typically occur in parallel groups and are inclined to the layering at angles between 0 and 90” similar to those adjacent to opx in norites. Their orientation is not related to any specific crystallographic orientation of the olivine. In some cases olivine grains coexisting with serpentine and talc and accompanied by biotite plates are surrounded by a fresh rim of opx (Fig. 8~). The rim can increase to an extent such that the olivine/serpentine/talc assemblage is only preserved as small kidney-shaped inclusions in the opx (Fig. 8d). Fresh opx ranges in composition from 69 to 80 mole % En, while the corroded remnant has an En-content of 85 mole %. The composition of coexisting olivine ranges from 73 to 84 mole % Fo. Two serpentine analof 0.90 yses gave an average Mg/(Mg +FeO) (~0.01). Anorthosites: Plagioclase in anorthosites is subhedral and varies from less, than 0.1 mm to greater than 2 mm in dimension. Opx or cpx oikocrysts enclose euhedral plagioclase. In anorthosites adjacent to Reef pegmatites, plagioclase may contain small spherical inclusions. These inclusions contain hydrobiotite-vermiculite, plagioclase of a high and constant anorthite content (An,,+,, number of analyses = 6)) rutile, cpx, ilmenite and hydromuscovite. The minerals were identified by optical microscopy and electron microprobe analysis. Of 15 anorthosite specimens examined, three contained one or more spherical mineral inclusions (Fig. 5). It is noteworthy that only plagioclase grains larger than average contain these inclusions. The feldspar may be zoned and twinned and the inclusions are generally situated near the host’s center. The zonation of host

25

plagioclase may subdivide the grain into different areas as if the large plagioclase crystal were welded together from subgrains. The size of the inclusions does not exceed 0.1 mm. Chromitites: Chromite layers occur at different levels within macro rhythmic units (Fig. 2). The major ones occur at the levels of the UG2 and the Merensky Reef in the Western Complex. The thickness of these chromitite layers ranges from millimeters to tens of centimeters. The appearance of chromite is normally coupled with an abrupt increase in the mode of hydrous silicates; 87% of all layers were found to contain hydrous minerals (Fig. 5). Hydrous minerals are most prominent where chromitite layers form the contact between layered rocks and Reef pegmatites and include: biotite, brown amphibole, tremolite, clinozoisite, serpentine, talc and minor chlorite. Chromite grains commonly contain spherical mineral inclusions (Fig. 9). The mineral assemblage within the inclusions is similar to that outside the chromite grains. A number of related textures are observed in connection with the inclusions. Small chromite grains without inclusions coexist with large grains containing one or more inclusions. Silicates are partly embedded or enveloped by surrounding oxides or completely trapped at the grain boundaries or triple junctions of different chromite crystals. Inclusions show different shapes ranging from angular to spherical. Normally inclusions are angular if the grain boundaries of coexisting chromite are still visible (Fig. 9b and c). The larger the surrounding chromite mantle, the more spherical the inclusion. This is similar to what is observed at the contact to IRUMpegmatites with the difference that the spine1 is chromite instead of magnetite. Spherical inclusions usually contain a high percentage ( >90%) of hydrous minerals while angular ones are lower in hydrous mineral modes ( <90%). Minerals within the inclusions may be in reaction relationship. An opx grain forming a corroded remnant and separated from biotite by a reaction rim of a fine grained talc is shown in Fig. 9c.

6. Mineral compositions Pyroxene: The variation of Mgl (Mg + Fe) in coexisting opx and cpx from the Turfbult section is shown in Fig. 10. For most of the section between the UG 2

26

A.J. Zings /Lithos 37 (1996) 15-37

Depth (in m)

Sample No.

0 BIG

- -30

TB19 TE

- -40

q

@ 0.8

I 0.9

Mg/(Mg+Fe) Fig. IO. Chemical variation of pyroxene in the Turtbult section. Opx

and both show an abrupt drop in Mgl contactofthe Merensky below the MerenskyReef.

and cpx are in equilibrium

(Mg + Fe) by 0.1 at = IO m below the lower Reef. Depth is in meters

Dicmside

Hedenbaraite

The two sections display some common features: cumulus plagioclase has a smaller compositional range and a higher average An-content than postcumulus plagioclase, and the compositional variation from cumulus to postcumulus plagioclase is continuous. Differences between the two sections are: (1) there is a higher proportion of cumulus relative to postcumulus plagioclase at Rustenburg and therefore a higher average anorthite content of plagioclase in the Rustenburg section compared to Turfbult, (2) the Turfbult section has more pyroxenite and therefore more postcumulus plagioclase while the Rustenburg section is predominantly composed of leuconorite and therefore plagioclase of a high An-content prevails and (3) cumulus plagioclase from the Rustenburg rock section reaches An,,, compared to An,, in the Turfbult core, while postcumulus plagioclase at Rustenburg records a minimum An-content of 55%, compared to Anj3 at Turfbult. Mica: Textural differences among mica are reflected in compositional differences. Four types are distinguished: biotite-vermiculite enclosed in plagioclase; biotite enclosed in chromite; matrix biotite in pyroxeTable 1 Representative compositions of orthopyroxene from the Turfbult section

0.80

Fig.

I 1, Coexisting

0.60

0.40

0.20

Fenosilite

opx and cpx in the pyroxene quadrilateral. Opx

records a temperature of about 700°C

(t

150”), cpx a range between

500 and 800°C.

and the Merensky Reef a value of 0.81-0.84 (opx) and 0.86-0.90 (cpx) is recorded. At a distance of about 8 m below the Merensky Reef the ratios drop to 0.70 (opx) and 0.78 (cpx) respectively. The same data are shown in the pyroxene quadrilateral (Fig. 11) together with isotherms at 5 kbars (Lindsley, 1983). According to Lindsley’s geothermometer, opx records a minimum temperature of about 700°C ( + 150”) while cpx is characterized by a range from 500 to 800°C. Some representative microprobe analyses of opx and cpx are listed in Table 1 and Table 2. Plagiocluse: Plagioclase compositions from the Rustenburg and Turfbult sections are displayed in Fig. 12.

Sample no.

TB8

TBIO

TB14

TB15

54.71 54.95

TB19

TB20

TB28

SiO,

53.31

55.68

54.19

53.94

54.12

TiO;

0.26

0.16

0.16

0.16

0.29

0.15

0.20

AW,

0.75

1.01

1.12

0.99

0.80

1.45

1.69

FeO”

19.55

10.54

13.54

12.17

11.47

MnO

0.22

M8O

24.84

14.12 Il.04 0.32

0.32

0.40

0.33

0.26

29.51 30.49

0.24

30.89

29.99

29.87

29.35

cao

0.89

0.69

0.56

0.59

0.83

0.80

0.60

Na,O

0.01

0.02

0.06

0.00

0.02

0.01

0.05

CGO,

0.12

0.28

0.42

0.31

0.38

0.65

0.47

100.80 99.12

99.48

100.44

99.97

98.21

Wt.% total

99.95

Cations per 3 oxygen atoms Si 0.98 0.97

0.97

0.98

0.96

0.96

0.98

Ti

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Al

0.01

0.02

0.02

0.02

0.01

0.03

0.04

Fe2+

0.30

0.21

0.16

0.15

0.20

0.19

0.17

Mll

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Mg

0.68

0.78

0.81

0.81

0.79

0.80

0.79

Ca

0.01

0.01

0.01

0.01

0.01

0.02

0.01

NZI

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Cr

0.00

0.00

0.00

0.00

0.00

0.01

0.01

Total

I .98

1.99

1.97

1.97

1.97

2.01

2.00

Mg/(Mg+Fe)

0.69

0.79

0.84

0.84

0.80

0.81

0.82

“all iron as FeO.

A.J. Zingg/Lithos37(19%)

Table 2 Representative compositions of cliuopyroxene from the Turfbult section Sample no.

TB 8 TB 10

TB 14

TB 14

TB 15 TB 19 TB28

Si02

52.09

53.96

53.57

53.45

52.44

53.41

TiO?

0.25

0.4.5

0.16

0.45

0.32

0.45

0.71

A&O,

1.27

l.i9

1.00

1.47

1.03

1.79

1.61

Fee”

6.98

4.882

4.34

3.09

5.14

4.82

5.06

52.44

MnO

0.35

0.06

0.09

0.07

0.15

0.06

0.17

M@

14.43

I5.Es7

17.17

16.44

16.23

15.87

16.34

cao

22.65

22.44

22.2’7

22.18

23.43

22.73

22.37

Na,O

0.24

0.2,4

0.21

0.30

0.28

0.34

0.30

> 010%

0.25

O&5

0.13

0.87

0.50

0.85

0.66

98.30

98.99

99.24

99.69

99.83

99.01

100.91

wt.C/o total

Cations per 6 oxygen atoms Si

1.96

1.95

1.98

I.96

1.97

1.95

1.94

Ti

0.00

OS1

0.00

0.01

0.01

0.01

0.01

Al

0.05

0.08

0.04

0.06

0.05

0.08

0.06

Fe’ +

0.22

0.15

0.13

0.09

0.16

0.15

0.15

Mu

0.01

o.cIo

0.00

0.00

0.00

0.00

0.00

Mg

0.81

0.88

0.93

0.89

0.89

0.88

0.88

Cu

0.90

0.89

0.87

0.91

0.90

0.89

0.88

Nu

0.01

0.03

0.01

0.02

0.02

0.03

0.02 0.01

Cr

0.00

0.03

0.00

0.02

0.02

0.03

Total

3.96

4.Cl2

3.96

3.96

4.02

4.02

3.95

Mg/(Mg+Fe)

0.79

0.85

0.88

0.91

0.85

0.85

0.85

“all iron as FeO.

nites, and parallel biotite plates adjacent to opx and olivine in norite and pyroxenitelharzburgite (Fig. 13 and Table 3). As shown in Fig. 13 the hydrobiotite enclosed in plagioclase plots along the biotite-vermiculite transition. Mica as inclusions in chromite is enriched in Cr, Ti and Mg compared to the others. Mica as matrix biotite is intermediate between mica enclosed in plagioclase and in chromite. The parallel biotite plates associated with opx represent a separate group.

7. Discussion of reaction and relict textures Three reaction textures have been encountered in alteration zones of IRUM pegmatites (reaction profile 1 in Fig. 3b); Cpx-coronas around opx, hornblende lamellae along ( 100) of opx, and the transformation of opx to olivine and hydrous phyllosilicates. The same three reactions are observed in the layered rocks (reaction profile 2). Adjacent to IRUM pegmatites the reaction textures are due to interaction between fluid and rock (Zingg, 1988). It is proposed that the textures in

15-37

21

the layered rocks have a similar origin. A likely source of the fluid responsible for the reaction textures in the layered rocks is the Reef pegmatites. They are generally concordant to the layering and have a higher hydrous mineral content than the norites. Not only do they show similar alteration reactions in the wallrocks, though on a larger scale, they also show numerous similarities to the IRUM pegmatites. Among these are: (a) the formation of a spine1 layer at the contact to the wallrocks; (b) spherical mineral inclusions in spine1 contain a high percentage of hydrous minerals and show similar textural stages, suggesting a similar formation mechanism; (c) plagioclase increases in grain size at the contact, if the wallrocks are norite or anorthosite; (d) both pegmatites are enriched in femic and hydrous minerals, compared to the wallrocks; (e) in both, olivine is overgrown by opx witnessing an increase in the activity of silica during genesis. The fluid in both cases was chlorine-rich (layered peg.: Ballhaus and Stump& 1986; Boudreau et al., 1986; IRUM peg.: Schiffries, 1982; Stumpfl and Rucklidge, 1982) and led to the precipitation of graphite (layered peg.: Ballhaus and Stumpfl, 1985a, b; IRUM peg.: Tarkian and Stumpfl, 1975; Stumpfl and Rucklidge, 1982). In both pegmatite types, sulfides and platinum group minerals are present (layered peg.: Hiemstra, 1979; Naldrett et al., 1986, IRUM peg.: Kinloch, 1982; Stumpfl and Rucklidge, 1982; Ballhaus and Stumpfl, 1985a). Reaction textures in the layered rocks (reaction profile 2) were modified to variable degrees compared to the reaction textures in the alteration zone of IRUM pegmatites. These modifications are valuable indications for the rocks’ development uferthe emplacement of Reef pegmatites. Each of the textures will be discussed for each of the two environments and the succession of observed stages discussed. The technique to mode1 continuous reactions has been presented elsewhere (Zingg, 199 1) .

8. Reaction texture-s 8. I. Biotite plates In the alteration zone of IRUM pegmatites (reaction profile 1) hornblende lamellae are enclosed parallel to ( 100) of euhedral opx. They either replace cpx lamel-

A.J. Zingg /Lithos 37 (1996) 15-37

28

Turfbult Mineral proportions

Rock type

Rustenburq inclusions in opx

postcumulus and cumulus plagioclase 30

50

70 % An

40

Rock type Mineral proportions

70 % An

inclusions postcumulus and in opx cumulus plagioclase 50

70

90 60 80 %An

Scale (in m) 0

5

10

I

0 chromitite

pyroxenitel harzburgite

melano-

Fig. 12. Chemical variation of plagioclase from the left displays the variation in the type. The third column records the actual crystals. In the fourth column the chemical

leuconorite

anorthosite

cumulus

q postcumulus

plagloclase

in the Turtbult and Rustenburg section. For each locality four columns are shown. The first column modal proportions of plagioclase to pyroxene. The second column presents the corresponding rock compositional variation in rock forming plagioclase including cumulus (0) and postcumulus ( 0) variation of plagiochtse inclusions in opx is shown.

lae, also along (100) of opx or have formed independantly. They are most likely crystallographically controlled replacement features due to fluid infiltration ( McCallum, written commun.). Submicroscopic lamellae of amphibole (Desnoyers, 1975) enclosed in corona pyroxene are well known from granulite facies terrains (e.g. Griffin et al., 1985).

Also in the layered norites (reaction profile 2) may opx contain hornblende lamellae. After the hornblende lamellae were formed, the opx was overgrown by plagioclase. Hornblende did not survive the overgrowth process but transformed to biotite. The biotite plates are therefore not enclosed in opx but in the newly formed feldspar. If the opx contains hornblende, it is

A.J. Zingg/Lithos 37 (19%) 15-37

and McLelland, 1961)

29

1973). The KD is defined as (Kretz,

Table 3 Representative compositions of biotite/vermiculite

Fig. 13. Chemical variation of mica. Four different groups are distinguished: vermiculite in sflherical inclusions in plagioclase; chromian phlogopite in spherical inclusions in chromite; matrix biotite in norites and pyroxenite; parallel biotite plates adjacent to and parallel to ( 100) of opx. The four groups can be distinguished according to their concentration in Ti, K, Cr and in their Mg/(Mg+ Fe) ratio. The numbers accompanying the data points represent average per formula units of potassium.

euhedral; if it is accompanied by biotite it is anhedral. This texture is most common in leuconorites where opx is interstitial to plagioclase and was overgrown by the feldspar after the emplacement of Reef pegmatites. 8.2. Cpx-coronas The classical way that Cpx-coronas in layered rocks have been explained is by means of a peritectic reaction (Bowen, 1956; Jackson, 1961). Discussions of such a hypothesis are given in textbooks on phase equilibria in melt systems (e.g. Morse, 1980). The same texture can be explained as subsolidus reaction (Candia Fornoni et al., 1989), as observed in the alteration zones of IRUM pegmatites or granulite facies rocks. For the rocks under consideration one argument favoring the latter hypothesis is the composition of oxide inclusions. In Fig. 6 the oxides from alteration zones of IRUM pegmatites and layered rocks display a continuous transition from a chrome-rich to an iron-rich variety. The fact that the spine1 inclusions in the layered rocks are enriched in Cr compared to the oxides associated with IRUM pegmatites agrees with the fact that the fluid in the first case was Cr-rich and in the second was Fe-rich. This led to the forma.tion of chromitite layers in the Reef pegmatites and to magnetite layers in the IRUM pegmatites. Cpx-coronas are well known from granulite facies rocks (e.g. Griffin, 1’371; Whithney and McLelland, 1973). Coronas from the Adirondacks Mts. are characterized by K,,‘s ranging from 0.55 to 0.73 (Whithney

Si02 TiOl Al,& Cr& FeO” MgO KzO Na,O NiO Total

Enclosed in plagioclase

Enclosed in chromite

30.22 0.66 17.24 0.02 12.96 22.28 0.47 0.22 0.45 84.52

39.60 4.16 14.82 1.38 3.09 23.35 7.33 1.74 0.22 95.69

39.07 3.94 14.85 1.62 3.37 23.30 7.36 1.66 0.18 95.35

39.35 4.21 14.85 1.57 3.17 23.54 7.66 1.72 0.17 96.24

5.043 0.398 2.225 0.139 0.329 4.432 0.430 1.191 14.187

5.006 0.380 2.243 0.164 0.361 4.449 0.412 1.203 14.218

5.000 0.402 2.224 0.158 0.337 4.457 0.424 1.241 14.242

31.55 3.19 15.19 0.00 14.82 18.43 2.09 0.01 0.50 85.78

37.86 4.20 13.55 0.07 13.55 15.25 9.10 0.02 0.21 93.81

Cations per 22 oxygen atoms Si 4.428 4.641 5.188 Ti 0.073 0.353 0.433 Al 2.978 2.634 2.188 Cr 0.002 0.008 Fe2+ 1.588 1.823 1.553 Mg 4.865 4.040 3.114 Na 0.062 0.003 0.005 K 0.088 0.329 1.591 Total 14.084 13.823 14.080 Matrix biotite SiO, TiOi Al,Q Cr& FeO’ MgO K,O NazO NiO Total

34.34 1.35 16.00 0.86 7.18 22.76 7.57 0.27 0.19 90.52

Si Ti Al Cr Fe*+ Mg Na K Total

Cations per 22 oxygen atoms 4.735 5.010 4.973 0.144 0.140 0.140 2.601 2.495 2.522 0.094 0.084 0.089 0.828 0.704 0.646 4.677 4.425 4.433 0.072 0.087 0.114 1.332 1.302 1.441 14.479 14.251 14.358

“all iron as Fe0

38.15 1.46 16.12 0.81 6.41 22.61 7.77 0.34 0.21 93.88

Parallel plates 37.37 1.40 16.08 0.85 5.80 22.35 8.49 0.44 0.22 93.00

39.56 1.05 15.54 0.53 6.69 22.48 9.20 0.18 0.26 95.49

36.62 5.14 13.11 0.57 10.42 15.50 9.00 0.07 0.28 90.71

36.85 3.96 13.49 0.52 8.58 17.23 8.89 0.05 0.18 89.75

5.140 0.103 2.380 0.054 0.727 4.353 0.045 1.525 14.327

5.141 0.543 2.170 0.063 1.224 3.242 0.019 1.612 14.014

5.165 0.417 2.229 0.058 1.006 3.599 0.014 1.590 14.078

30

A.J. Zingg/Lirhos

37 (19%) IS-37

In the layered Bushveld rocks, precisely the same KD range is observed (0.55-0.73). Coronas from the Brits alteration zone have an intermediate KD of 0.60. Details on the formation mechanism can be obtained by a reaction model for an open system (Zingg, 1992). If cpx forms as a pseudomorph after opx, the plagioclase matrix need not be involved in the reaction. However, as soon as the bulk volume of pyroxene changes, it seems likely that plagioclase is involved. Eq. ( 1) describes the reaction of opx and plagioclase to cpx. Mg,,Fe, _ ,,Si03 + aNa,$a, oIlh<~py~0Xl?ll~

I

0.8

)(CPX

Mg 0.7

jy+r~

_ ,“A& _ ,,,Si2+ ,,,Os + iH+ = PlagiOChX

50.5

bCaMg,,Fe, _ ,,Si206 + cMg’+ + dFe*+ + eCazC +

fNaf

+gAl’+

+hSi4+

+-$I,0

(1)

Opx is assumed to be a solid solution of enstatite and ferrosilite, cpx of diopside and hedenbergite and plagioclase of anorthite and albite. The relationship between the stoichiometric coefficients b, c and d can be obtained by the massbalances of Mg and Fe Mg: u Fe: l-u

=bv+c

‘2

=b+c+d

1

=b(l-u)+d

(2)

The stoichiometric coefficients b, c and dare not independent variables. If two of the parameters are known, the third can be evaluated from Eq. (2). The compositional dependence between opx and cpx is obtained from the mass balance of Mg (or Fe) and by solving for 0 II

c (3)

v=b-b

The first term in Eq. (3) represents the slope, the second the intercept in a plot of u (PM:) versus u (x’,p,X). The composition of coexisting pyroxenes from the alteration zone of an IRUM pegmatite (Fig. 4) and from the layered rocks of Turfbult are plotted in Fig. 14. They yield the following equations for Brits : for Turfbult : for NiquelPndia

t

u = 1.OOOu+ 0.098 u = 0.939~ + 0.108 : u = 0.758~ + 0.307

0.6

0.7

0.6

Fig. 14. Compositionof coexistingopx and cpx in the alterationzone of a Brits IRUMpegmatite(Fig. 4). in the layered rocks of Turfbult and in Niquel&ndialayered rocks (CandiaFomoni et al., 1989). The compositional relationship of Bushveld pyroxenes is given by the equations suggest that the coronas in both rocks were formed in a similar process.

The equation for Niquelandiacoronas (Candia Fomoni et al., 1989) is shown for comparison. In the first two cases %=l(i.e.b=l) and g= -O.l(i.e.c-0.1). It follows that d = 0.1. If b * 1, the bulk volume of pyroxene doubles and plagioclase must be dissolved to keep the volume constant. If the rock volume is assumed to be constant (A VEaction = 0) the coefficient a can be determined. Such an assumption is proposed by the fact that the rock matrix is unaffected by the reaction and imposes a rigid framework in which the reaction takes place. No change in porosity between unaffected host rock and rock, affected by the reaction, is visible. Any change in porosity is small and will be neglected. Using molar volume terms from Helgeson et al. ( 1978), a is 0.35. Assuming a plagioclase composition of 75 mole % An, which represents an average composition of cumulus feldspar, the final equation is obtained, Mg,Fet - .SiQ + 0.35Nao.25Cao.7sAl,.,~Si2,2~O~ + Orthopyroxene

PlL+OClfLSt?

0.74Ca2+ + 0.21Si4+ + + O.lOMg*+ + 0.20H20 = lCaMg,Fe,

_,,Si,0,+0.61A13+

+0.10Fe2+

+

Clinopyroxene

O.O9Na+ + 0.40H+

(4)

A.J. Zingg/Lithos37(19%)15-37

The formation of Cpx-coronas requires the supply of Si4+, Ca2+ and Mg2+ and the removal of Fe’+, Na+ and Al’+. Equal volume proportions of opx and plagioclase transform to cpx. Coronas from the alteration zone of IRUM pegmatites and in the layered rocks can be described by identical equations, involving the same mole quantities ofionic species. This suggests that they formed in a similar way via a similar fluid supplying Si4+, Ca*+ and Mg*+ and removing A13+, Fe*+ and Na+. This is in agreement with the oxide inclusions which record a continuous trend from Cr3+ to Fe3+ rich varieties.

The fact that olivine is six to eight sided as well as rounded in layered harzburgite, and in IRUM pegmatites is rounded if contained in a matrix of feldspar, suggests that at least t.he six to eight sided type is a pseudomorph after opx. This conclusion is supported by parallel biotite plates adjacent to the olivine and the coupling of hydration of plagioclase with the appearance of olivine. Parallel biotite plates are typically observed adjacent to opx, where they mark the ( 100) direction. Adjacent to olivine they occur in parallel groups, but are not related to any specific c.rystallographic direction of olivine. Parallel biotite plates adjacent to olivine are not uncommon in harzburgites near the Reef pegmatites and they are the same size or larger than those near opx. Again using continuous reaction techniques it is possible to express a relationship between the three major minerals opx, olivine and serpentine. The details of this reaction mechanism are given elsewhere (Zingg, 1993). Seqnxtinc

Table 4 Reaction including opx, olivine and serpentine Reaction

(Mg,Fe,_,),Si,05(0H),+gH+ = aMgpe, _,SiO, +b(Mg,Fe, .J$iO, + cSi4+ +dMg’+ + eFe*+ +fH,O Stoichlometric a=

3(w-u)+d(l-w)-ew W-I)

b=3(u-v)

+d(v- 1) fve 2(w-v) 3u-2w-v+d(Zw-vc= w(2-v)

(1) (2)

srp srp

(3) (4) (5) (6) (7) (8)

Srp

,V)2Si04 +tSi4+

-7320

bOI bOI bOI bOI

+Si4+ +Si4+ +SP+ + SP’

=aOpx+bOI =aOpx+bOI =aOpx =Srp+aOpx =srp =srp =Srp +bOI = bO1

+cSi4+

+c +c +c +c

Si4+ Si4+ Si4+ SP’

system based on u, u and w. The stoichiometric coefficients a, b and c are expressed in terms of mole fractions as well as the coefficients d and e and are listed in Table 4. For each stoichiometric coefficient a, b and c there is a position in reaction space where it becomes zero. This is the position where the mineral disappears from the equation and changes from a reactant to a product phase or vice versa. These “zero contours” (Fisher, 1990) can be obtained by setting a stoichiometric coefficient equal to zero a=f(u,

u, w) =0

w)

+eFe2’

Olivinc

+4+f

1) +e(2w-v)

and solving for one of the mole fractions

=aMg,,F&1$i0~+ +dMg’+

+

aOpx+ aOpx+ a Opx Srp +aOpx

u=f(u, b(Mg,,Fe,

CoefJicients

Reaction configurations

8.3. Hydration of opx

(Mg,,Fe, _,,)3Si205(OH)4+fH+

31

(5)

Opx, olivine and serpentine are assumed to be solid solutions of their Fe and Mg end-members. This is expressed by the mole fractions u, v and w. Each of the mole fractions ranges from 0 (Fe end-member) to 1 (Mg end-member) and reaction space is the coordinate

Zero planes subdivide reaction space into segments, with each segment having its specific configuration of reactants and products. The different zero planes Opx = 0, 01 = 0 and Srp = 0 are displayed in the reaction cube of Fig. 15a and eight different reaction configuration are distinguished (Table 4). In order to find the correct reaction configuration, mineral compositions are plotted (Fig. 15b) and yield the following two reactions:

32

A.J. Zingg/Lithos

37 (1996) 15-37

:Srp

0

x

OPX

M!J

1

x&

0 0

x OPX

ML!

1”

-

Fig. 15. (a) Reaction cube for the equation Srp+gH+=aOpx+b01+cSi4++dMgZ++eFe2++fH,0 Eight reaction segments are distinguished. They are listed in Table 4. (b) Mineral compositions are plotted into the reaction cube and suggest the following two reaction configurations Opx, + Hz0 = Srp, + 01,. + Si“+ 01n.+H 20+Si4+=Srpu+Opxv (c) Real mineral analyses plot close to the plane of isovolumetric replacement.

Opx,, + H,O -+ Srp, + 01, + Si4+

(6)

01," + Hz0 + Si4 + -+ Srp,, + Opx,,

(7)

Eq. (6) was obtained using the composition of corroded Mg-rich opx while the fresh rim of opx surrounding olivine provides Eq. (7). First, a fluid phase poor in silica caused the transformation of opx to olivine and serpentine. An increase in the fluid’s silica content initiated Eq. (7)) producing more serpentine, and olivine transforms back to opx. The latter process is in agreement with the textures inside the Merensky Reef consisting of large opx crystals containing inclusions of

olivine. The volume of reaction is depicted in Fig. 1%. Mineral compositions in reaction space plot close to the plane of isovolumetric replacement. Based on the above observations, the following succession of events is postulated. In a first stage, a volatile phase infiltrated the rock and was responsible for the formation of hornblende lamellae in opx. Later, opx altered to olivine, serpentine and minor talc due to the removal of silica. The hornblende lamellae lost their original host ( =opx), transformed to biotite and in some cases survived at the outer edge of olivine. In a second stage, the fluid became enriched in silica and was responsible for the reverse transformation of olivine to opx. Hz0 is still supplied producing further serpentine. The sink and source of material was the pegmatite fluid.

A.J. Zingg/Lirhos 37 (19%) 15-37

9. Relict textures 9.1. Inclusions in chromite Spherical mineral inclusions in chromite, predominantly observed adjacent to Reef pegmatites, display a succession of stages. Small chromite grains without inclusions coexist with larger grains containing one or more inclusions. In an intermediate stage oxides are in contact with one another and trap silicates at grain boundaries. This is the same succession observed with magnetite at the contact of IRUM pegmatites. The observations are interpreted as follows. The fluid phase, responsible for the formation of pegmatites, caused the hydration of silicates. With IRUM pegmatites the fluid was iron-rich and caused the precipitation of magnetite, with Reef pegmatites it was chrome-rich and produced chromite. In both cases the oxide crystals start to grow, increase in grain size and trap silicates at grain boundaries. The trapped hydrous and anhydrous, silicates form the basic material for the inclusions. In a.n early stage, the inclusion has an angular shape and the old grain boundaries of the oxides are still visible. Such a case is shown in Fig. 9c. Further growth and sintering (Cawthorn et al., 1983) closes the original gra:in boundaries of spine1 and the inclusion becomes smadler. As soon as the fluid phase, in equilibrium with hydrous and anhydrous mineral phases, is trapped, the further growth or sintering of the host oxide causes a decrease in the inclusion’s size compressing the entrapped fluid phase. The compression or increase in pressure destabilizes the anhydrous phases in favour of hydrous ones -a reaction takes place. An example of a reaction texture between anhydrous and hydrous silicates is shown in Fig. 9c. 9.2. Mineral inclusions in plagioclase

and opx

Unlike those observed in chromite, spherical mineral inclusions in plagioclas#e and opx do not display a succession of stages in their formation. The elucidation of their genesis is thus rendered more difficult. The mineral assemblage enclosed in plagioclase is consistent with formation during a.period of increased fluid activity. This could be during the infiltration of the fluid responsible for the formation of Reef pegmatites. Like the parallel biotite platas, that suggest the recrystallization of plagioclase after the pegmatites were formed,

33

the spherical mineral inclusions also suggest recrystallization of plagioclase during and after the emplacement of Reef pegmatites. The large grains that contain the inclusions have zoning patterns that indicate they were welded together from subgrains, as is believed to have happened to the chromite. The similarities between inclusions in chromite and plagioclase are: a high hydrous mineral content, the rounded shape of the inclusion, the coexistence of two types of host crystals, (i.e. small inclusion-free grains and large grains containing inclusions) and the presence of rutile in both environments. It is therefore proposed that the inclusions in plagioclase formed by a mechanism similar to the one that formed the spherical chromite inclusions. The interpretation of rounded mineral inclusions in opx is based on three different textures which are supposed to succeed one another: ( 1) opx transforms to olivine and serpentine, (2) olivine and serpentine are surrounded by a small rim of opx with the opx rim widening in the course of time and (3) the final stage of this process yields rounded inclusions of olivine, serpentine and talc that are enclosed in a euhedral crystal of fresh opx (Fig. 9d). As with chromite and plagioclase, two grain sizes of opx can be distinguished; small ones without and large ones with inclusions.

10. Interpretation

and conclusions

The three reaction textures described from the layered rocks, Cpx-coronas and parallel biotite plates in the norites, and olivine-serpentine pseudomorphs in the pyroxenite-harzburgites, have their counterparts in the alteration zones of IRUM pegmatites. The reactions were caused in the first case by the fluid responsible for Reef pegmatites, in the second by the fluid responsible for IRUM pegmatites. The two pegmatites show numerous similarities like spine1 layers at the border with spherical mineral inclusions in the spinel, an increase in femic and hydrous minerals at the expense of feldspar inside the pegmatite and a fluid phase rich in chlorine and responsible for the precipitation of graphite and sulfide (Stumpfl and Rucklidge, 1982; Ballhaus and Stump& 1985a, b). The presence of graphite and sulfides both in the Merensky Reef and the pegmatite pipes is consistent with extremely reducing conditions during serpentinization (Frost, 1985). In the case of IRUM pegmatites the fluid was iron-rich

34

A.J. Zingg /Lithos 37 (1996) 15-37

and responsible for the formation of magnetite. In the case of Reef pegmatites it was chrome-rich and responsible for the formation of chromite. Based on similarities it is concluded that the reaction textures in the two environments -alteration zone of IRUh4 pegmatites and layered rocks - have a similar origin. In the layered rocks the reaction textures are modified to variable degrees and postdate the emplacement of the pegmatites. In leuconorites, opx is corroded and overgrown by plagioclase with parallel biotite plates witnessing the original position of the hornblende lamellae and host opx. Also the Cpx-coronas are overgrown by feldspar. In the pyroxenites/harzburgites serpentinization was first accompanied by the appearance of olivine and at a later stage by the formation of secondary opx. This observation has its counterpart in the IRUM pegmatites where opx is replaced by olivine near the contact and the reverse takes place inside the pegmatite. Mineral inclusions in all the major phases, chromite, opx and plagioclase contain a high content in hydrous minerals suggesting the recrystallization during and after the formation of Reef pegmatites. The temperature at which subsolidus reequilibration took place is witnessed by the two pyroxene geothermometers (Lindsley, 1983) and by mineral stabilities. While opx gives a temperature around 700°C with a large uncertainty of 150 to 200% cpx records a range between 500” and 800°C. Within this range fall the upper stability limits of the different hydrous phyllosilicates. Serpentine and talc have upper stability limits of about 550°C (Chernosky et al., 1988) and 750°C (Evans and Guggenheim, 1988)) respectively. These values are approximations as they depend on the type of reaction and pHzO. Mineral compositions in the layered rocks show some similarity with mineral compositions in IRUM pegmatites. In the latter an increase in the mode of mafic minerals (i.e. olivine and opx) is accompanied by a decrease in the An-content from An,, to Ansc,. This pattern corresponds well with what is observed in the layered rocks. With opx the similarities are less evident. In IRUM pegmatites, opx is affected first by the ironrich pegmatite fluid. The ratio Mg/ (Mg + Fe)Opx drops from 0.82 to 0.62 at the outer contact of the alteration zone and in a cross section through the pegmatite experiences minor variations only. The fact that the fluid responsible for the formation of Reef pegmatites was

not necessarily iron-enriched explains why opx may have the same Mg/(Mg+ Fe) ratio in the Reef pegmatites and layered rocks. This is not the case for the Merensky Reef at Turfbult, where the ratio Mg/ (Mg + Fe) in both opx and cpx drops by almost 0.1 at a distance of 8 m below the lower contact due to interaction with an iron-rich fluid (Fig. 10). Also Kruger and Marsh (1985) in their fig. 3 show major changes in the En-content in opx at different levels of Reef pegmatites. Depending on whether the fluid responsible for the formation of the Reef was Fe-or Mg-rich, the ratio Mg/ (Mg +Fe) decreases or increases towards the pegmatites. In brief, Reef pegmatites affected the composition of both major cumulus phases -opx and plagioclase. While Reef pegmatites are more magnesian than IRUM pegmatites and, therefore, may show a different effect on the opx, the decrease in the An-content in plagioclase in the Turfbult core, parallel with an increase in the modal ratio opx to feldspar corresponds well with the pattern observed in IRUM pegmatites. In a cross-section through Reef pegmatite and wallrock it seems reasonable to assume a change in the activity of H20. Opx as a member of the pyribole mineral group (Thompson, 1981) is probably more stable than plagioclase under a high c1u20. It crystallized in those parts with a high am0 and was replaced by plagioclase in areas with a lower aH20, further away from the Reef. Such a process could lead to the stratification into feldspar-rich and pyroxene-rich layers and could explain the layering. But why the assymetric pattern in the ratio opx:plagioclase in a cross section through a macro rhythmic unit? In a gravity field the fluid tends to move upwards. Depending on the rocks permeability, it is either trapped in the immediately overlying rocks of the Reef or invades further into the hanging rocks. The fact that a macro rhythmic unit in general starts with pyroxenite which gives way first to norite and later to anorthosite suggests the accumulation of the fluid in the immediately overlying rocks of Reef pegmatites. Small differences in the activity of HZ0 could explain the predominance of pyroxenite at the bottom and of anorthosites at the top of a macro rhythmic unit. The two types of pegmatite discussed in this study represent two stages during subsolidus cooling. The Reef pegmatites accumulated the fluid in the intergranular pores during an early stage where no fractures were present. The porous stage immediately succeeds

A.J. Zingg/Lithos

the igneous stage. The opening of fractures in the course of cooling provided the channels for fluid to escape and caused the formation of IRUM pegmatites. Thisfracrure stage is an early form of the hydrothermal stage. It is interesting to note that (a) the layering is best developed (Vermaak, 1976) at that level of the Bushveld stratigraphy, which hosts the laterally most persistent Reef pegmatites and (b) “Reef” pegmatites had a much stronger effect, as suggested by the larger penetration of alteration reactions into the wallrocks, compared to IRUM pegmatites. Gravity has been invoked as the factor responsible for the settling of crystals in a magma chamber; it could as well be held responsible for the structuring of the fluid in an intrusive body. If the layering is explained as a subsolidus phenomena due to a fluid gradient, the crystallization pattern in the rock becomes most simple. In anorthosites and leuconorites, opx crystallized first and was overgrown by plagioclase. In norites, opx and plagioclase crystallized simultaneously while in melanorites and pyroxenites the crystallization of opx succeeded that of plagioclase. The author was not able to establish a general crystallization order for the same rocks using cumulate criteria. Each thin section would require its own crystallization sequence which obviously does not make sense. The obs8ervations in the present study might offer a possible answer to a long debated topic -the origin of layerinlg in the Bushveld Complex.

Acknowledgements The present study presents the results of a Ph.D. project. It was made possible through the generous support of numerous individuals and institutions. First of all I wish to thank W.J. Van Biljon and E.F. Stumpfl for their everlasting encouragement and support. The Rand Afrikaans University is acknowledged for its hospitality, J.P.R. de Villiers and J. Markgraf for technical support, R.G. Cawthorn for inviting me to an excursion to the Eastern Bushveld Complex. Special thanks to B. Meurer (Duke University) for discussions on fluid versus cumulus hypotheses. Discussions and reviews at different stages with ‘W.J. Van Biljon, A. Boudreau, R.G. Cawthorn, W.L. Griffin, S.A. Hiemstra, T.N. Irvine, J.P. Lorand, A. McBirney, IS. McCallum, B.

37 (1996) 15-37

35

Meurer, S.A. Morse, H.R. Naslund, I. Parsons, E.F. Stumpfl and J.P.R. de Villiers substantially improved the clarity of the paper. The microprobe analyses were performed by MINTEK (Operators J. Russel and E.F. Viljoen) and at RAU (G. Bray). I also wish to thank Johannesburg Consolidated Investment, Western Platinum Mines and African Selection Trust for generously supplying core material and MINTEK and the CSIR of South Africa for financial support.

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