Constraints on the form of the P-T loop in the Southern Marginal Zone of the Limpopo Belt, South Africa

Precambrian Research, 55 (1992) 279-296

279

Elsevier Science Publishers B.V., Amsterdam

Constraints on the form of the P-Tloop in the Southern Marginal Zone of the Limpopo Belt, South Africa Gary Stevens and Dirk van Reenen Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park, Johannesburg 2000, South Africa (Received March 6, 1991; accepted after revision July 12, 1991 )

ABSTRACT Stevens, G. and Van Reenen, D., 1992. Constraints on the form of the P-T loop in the Southern Marginal Zone of the Limpopo Belt, South Africa. In: D.D. Van Reenen, C. Roering, L.D. Ashwal and M.J. de Wit (Editors), The Archaean Limpopo Granulite Belt: Tectonics and Deep Crustal Processes. Precambrian Res., 55: 279-296. A generalised, clockwise P - T path, consisting of a prograde portion, a high-grade isothermal decompression portion and an essentially isobaric cooling portion, has been suggested for the metamorphic evolution of the Southern Marginal Zone. Previous studies have concluded that the high-grade decompression path started in the kyanite stability field; peak-metamorphic mineral compositions have been reset during the equilibrium decompression event; and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field. The present petrographic study from the Bandelierkop Quarry, a locality in the granulite zone of the Southern Marginal Zone, identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700°C to 850 ° C. During this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite. The maximum temperature melting event is closely followed by two divariant reactions, ( G a + Qz + Sil= Cord and Ga + Q z = Cord + Hy), which occur in response to a period of general isothermal decompression. A subsequent cooling dominated rehydration period, is indicated by a crystallisation-hydration reaction (Cord + Ksp + Melt = Bi + Ky + Qz) which occurred at a minimum temperature of approximately 640°C and pressure greater than 6.5 kbar. The present data confirms the clockwise form of the loop and also supports the previously suggested fiat slope of the retrograde cooling portion. Additional mineral chemistry data, relating to the second decompression reaction, indicate that the zonation profiles of cordierite and garnet, as well as the growth of small quantities of garnet after the reaction, are in strong support of the suggested P-Tpath. The present mineral compositions in the Southern Marginal Zone reflect a near total re-equilibration during the high-grade, cooling dominated portion of the P - T path and as a result, reflect P-T conditions considerably lower than those of the peak metamorphism.

Introduction

The Limpopo Belt consists of a narrow band of E-W trending high-grade metamorphic rocks located along the South African-Zimbabwe/Botswana borders. It is regarded as the product of an -,~2700 Ma collision between the Kaapvaal and Zimbabwe Cratons (Van Reenen et al., 1990). The Limpopo Belt can be subdivided into a Central Zone and two flanking marginal zones, termed the Northern and Southern Marginal Zones. The Central Zone consists largely of epicontinental rocks,

whereas the Northern and Southern Marginal Zones consist essentially of the deformed, attenuated and ultra-metamorphosed equivalents of the granite-greenstone succession of the bounding cratons (Du Toit et al., 1983; Van Reenen et al., 1990). In the Southern Marginal Zone, the grey-gneiss of the craton is represented by the migmatised Baviaanskloof Gneiss. The greenstone lithologies of the craton have been transformed into the pelitic, mafic and ultramafic units of the Bandelierkop Formation (Du Toit et al., 1983) (Fig. 1 ). Previous metamorphic studies, which have

0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

280

G. STEVENS AND D.D. VAN REENEN

THE SOUTHERN MARGINAL ZONE AND ADJOINING KAAPVAAL CRATON

~

Greenschist Facies I

~

Amphibolite Facies

L~

Granulite Facies Charnockite

~_J

Baviaanskloof Gneiss

[ ] Greenslone Bells lit = Rheno'sterkoppies Belt P : Pietersburg Bell $ = Sutherland Belt IC = Khavagari Hills ~0 = Murchison Belt G = Granulite Outcrops B - Bandelierkop Formation

~

2650 Ma Granite

[ I [ U ~ 2450 Ma Palmiettontein Granite Schiel Alkaline Complex 2050 Ma

~ ~

Zone of Retrogression Shear Zones

Hout River Shear Zone

Cover []

Retrograde Orthoamphibole Isograd

Locality of the Bandelierkop Quarry

Late Faults

Fig. 1. Map illustrating the distribution of the various metamorphic facies in the Southern Marginal Zone of the Limpopo Belt. The regional orthoamphibole isograd and major shear zones are also shown (after Van Reenen et al., 1990). The localities of the Bandelierkop Quarry and sample point DV84 are indicated on the map.

focused on the metapelite unit of the Bandelierkop Formation, have identified three distinct metamorphic phases as part of a complex metamorphic history (e.g. Van Reenen et al., 1990) (Fig. 2). A prograde granulite facies event (M1) was followed by a decompressional event under granulite facies conditions (M2), ending in a high-grade, retrograde hydration event (M3) (Van Reenen, 1978, 1983, 1986). The purpose of this paper is to review the existing metamorphic model, and to present new data from two localities in the Southern Marginal Zone. These data support the general form of the published P - T loop (Fig. 2) but petrographic information from the Bandelierkop Quarry locality (Fig. 1) allows

part of the previously unknown, high-grade prograde path to be resolved. Data from locality DV84 (Fig. 1), reveals new information pertaining to the processes operating during and after the M2 period of decompression. Review of the previous work

The M1 metamorphic event

The metapelite of the Southern Marginal Zone is characterised by variable XMg (XMg=MgO/MgO+FeO) values. The mineral assemblage of the relatively Fe-rich metapelite garnet-granulite (XMg< 0.60) records textural equilibration at granulite facies con-

CONSTRAINTS

ON THE

P-T

11

LOOP

1N THE

281

SMZ

~

,

~T=

"

XCO 2 = 0 , 8

160'C

If

10 9

7

-12 Ctl 2

~ ,~

•

. ~,~,,~'~-.'. " "

6

, 400

I

,

I

,

I

,

I

500

,

600

I

=.~... " "'

,

I 700

-....... " "

,

. ..... "" ' "

I

,

• "

I

13

,

I

800

T - (°C) Fig. 2. Published P - T diagram for the Southern Marginal Zone, illustrating the conditions of the M1, M2 and M3 metamorphic events (after Van Reenen, 1986). The constraints of fluid-inclusion data, [ - 12; - 3 ; + 13, refer to the Th of CO2-inclusions for three calculated isochors (Van Reenen and Hollister, 1988)] and the displacement of the reaction, En + Qz + H20 = Anth (Van Reenen, 1986 ), on the form of the retrograde path, are also illustrated.

ditions, with no evidence for decompression reactions, and is therefore, assumed to represent the peak prograde metamorphic assemblage (Van Reenen, 1983). This assemblage also provides no micro-textural evidence for deformation, supporting the suggestion that maximum metamorphic conditions were attained after the main phase of deformation, (D~) had subsided (Van Reenen, 1983). The peak metamorphic conditions attained during M~ are assumed to be at least 780°C (the temperature of M 2 ) , and > 9 kbar (based on an interpretation of intergrowths of biotite and kyanite as being pre-M2) (Van Reenen, 1983 ).

The Me metamorphic event The M I peak prograde metamorphic event was followed by a retrograde granulite facies decompression event ( M 2 ) , which resulted in the formation of cordierite and second generation hypersthene symplectic coronas replacing garnet in more Mg-rich rocks (XMg> 0.6 ). The continuous reaction responsible for the

development of this texture was G a + Q z = C o r d + H y (Van Reenen, 1978, 1983). During the reaction, the composition of all the Fe-Mg minerals shifted to higher Fe/Mg ratios, as a result of equilibration at lower pressures (Fig. 3) (Van Reenen, 1983). Evidence that chemical equilibrium was attained during the M2 event is indicated by the lack of significant mineral zonation in the garnet, cordierite and hypersthene of the M: symplectic coronas, as well as by constant KD values for different mineral pairs (Van Reenen, 1983). This results in very similar compositions for both the coarse M~ and fine-grained, symplectic M 2 generations of hypersthene. The mineral compositions that developed during the M t event have therefore, subsequently been reset during M 2 (Van Reenen, 1983). In addition to pressure, the M2 divariant reaction was controlled by the whole-rock XMg value (Fig. 4 ). The continuous reaction did not occur in metapelites with XMg<0.6 (the garnet-granulite). It was arrested while in the process of occurring in metapelites with an XMg

282

G. S T E V E N S A N D D . D . V A N R E E N E N

A

F

Regional geobarometry and geothermometry data, based on the garnet-cordierite geobarometer and on the garnet-biotite geothermometer respectively, indicate that the M2 decompression event culminated at minimum pressures of approximately 7.2 kbar and that decompression occurred at approximately isothermal conditions of 780°C (Van Reenen, 1983). The excellent preservation of the textural instabilities, represented by the M2 symplectites, in the southern part of the granulite zone, indicates rapid cooling to temperatures below those at which recrystallisation would occur, after the M2 decompression. This is supported by the preservation of the M~ assemblage in the most Fe-rich metapelite, which escaped the decompressional reaction and by fluid inclusion data (Fig. 2 ) (Van Reenen and Hollister, 1988), which precludes a steep P - T trajectory during the interval between M2 and

M

Fig. 3. AFM diagram (projection through K-feldspar), illustrating the phase relations between garnet (Ga), cordierite (Cord) and biotite (Bi) in three samples of the garnet-cordierite gneiss, collected at different localities in the Southern Marginal Zone. XMg=XMgwhole-rock values. The systematic shift of the three phase field in the direction of F is due to the minerals adjusting their compositions during the reaction G a + Q z = C o r d + H y , as garnet is consumed (after Van Reenen, 1983).

M3.

value of between 0.6 and 0.7 (the garnet-cordierite granulite) and proceeded to completion in metapelites with XMg>0.7 (the cordierite-granulite) (Van Reenen, 1983).

The Ms metamorphic event

A second retrograde event (M3) is the result of two hydration reactions, which consume

15 XMg = 0.7

XMg = o.s . . . . . XMg 0.3 ......... =

10

Ga Qz

| D.,

5

;a Qz J

~

2ord H y 1

...........

[ I

I

-

-

......

.:.-. . . .

._:... . . .

Cord H y I

800

I

I

I

I

1000

I

I

I

1200

T- ('C) Fig. 4. Influence of whole-rock XMg value on the position and width of the P-Tinterval, within which garnet (Ga), quartz (Qz), cordierite (Cord) and hypersthene (Hy) form a stable association (after Van Reenen, 1983).

283

CONSTRAINTS ON THE P - T LOOP IN THE SMZ

cordierite and hypersthene (Van Reenen, 1986 ). One of these reactions involves the hydration of cordierite according to the reaction Cord + HzO = Ged + Ky and is therefore, restricted to Mg-rich metapelites which recorded the M: cordierite producing reaction. A second hydration reaction, consuming hypersthene according to the reaction H y + Q z + H20 =Anth, occurs in all varieties of the metapelite gneiss. The similar FeO/MgO ratio of hypersthene and anthophyllite imply an approximately univariant reaction, which defines a retrograde orthoamphibole isograd, dividing the Southern Marginal Zone into a granulite facies domain to the north and a zone of retrogression in the south (Fig. 1 ) (Van Reenen, 1986). The cordierite hydration reaction, in contrast, is a divariant reaction and, as such, is not restricted to the area south of the isograd. The degree to which cordierite has been consumed by this reaction and the grainsize of the reaction products, however, increases dramatically at the isograd, along which cordierite bearing lithologies are characterised by the presence of both hydration reactions (Van Reenen, 1986). The application of biotite-garnet thermometry to metapelites along the orthoamphibole isograd indicates a temperature of close to 600°C for the M 3 hydration event (Van Reenen, 1986). A maximum temperature of about 620 °C is also supported by the position of the solvus in the anthophyllite-gedrite series (Van Reenen, 1986). The presence of kyanite as a hydration product of cordierite at the isograd, suggests that pressure must have been at least 6 kbar at the time of M 3 (Van Reenen, 1986). The reaction Hy + Qz + H20 = Anth has therefore, been displaced to about 600 °C at 6 kbar due to the dilution of the fluid phase present at the time of the reaction, to a value of XH2O<0.2 (Fig. 2) (Van Reenen, 1986; Van Reenen and Hollister, 1988). This is supported by the results of a fluid inclusion study on metapelites sampled across the isograd (Van Reenen and Hollister,

1988), and by thermodynamic modelling (Baker et al., 1992).

Summary Four important conclusions which followed from regional metamorphic studies by Van Reenen (1978, 1983, 1986)and Van Reenen et al. ( 1990 ) are: (i) direct evidence for "peak" metamorphic conditions during the prograde stage has been almost completely obliterated by the resetting of mineral compositions during the subsequent metamorphic events; (ii) chemical equilibrium was approximated during the M2 reaction; (iii) isothermal decompression ( M 2) probably commenced within the stability field of kyanite (Fig. 2); and (iv) cooling and subsequent rehydration (M3) ended in the kyanite stability field at a temperature of approximately 600 ° C, indicating that the retrograde part of the P-T loop must be very flat (Fig. 2 ). Data from the present study

Petrographic data from the Bandelierkop Quarry exposure The migmatitic metapelites of the Bandelierkop Quarry locality (Fig. 1 ) are well suited for a detailed petrographic study of metamorphism in the Southern Marginal Zone, since all the chemical varieties of metapelite are present in this fresh exposure. In addition, various generations of anatectic leucosome are developed within the metapelite. Data from the Bandelierkop Quarry should therefore, be useful in further refining and evaluating the previously published P-T loop, which was based on regional metamorphic studies. Petrographic evidence from the Bandelierkop Quarry generally substantiates the previous subdivision of the Southern Marginal Zone metamorphic history into a peak M I metamorphic event, a decompresson dominated M2 metamorphic event and a cooling

284

G. STEVENSAND D.D. VAN REENEN

dominated period, prior to the high-grade M 3 hydration event. Petrographic studies at this locality have, however, also supplied new information about the high-grade portion of the prograde path and about the conditions of M I.

Pre-Mz textures Many of the leucosomes and the selvages to the leucosomes at the Bandelierkop Quarry contain textural evidence of metamorphic reactions not preserved in the associated metapelites of similar composition to the leucosome selvages (Stevens, 1991 ). These textures are, therefore, genetically related to the origin of the leucosomes, many of which can be shown to have formed prior to the end of the D~ fabric forming phase of deformation (Stevens and Van Reenen, 1992; Stevens, 1991 ), i.e., preM¿. The textures represent relict prograde melting reactions, which are summarized in Table 1, along with the estimated P - T conditions at which the various reactions have occurred. Detailed descriptions of these textures, as well as the relationship of the melting reactions to the mode of formation of the Southern Marginal Zone granulites, are discussed by Stevens and Van Reenen ( 1992 ). M1 peak metamorphic textures MI textures are only preserved in the relatively Fe-rich garnet-granulite ( Y M g < 0 . 6 )

which has not undergone the subsequent M 2 and M 3 reactions due to compositional controis. At the Bandelierkop Quarry the garnetgranulite is represented by a medium-grained, equigranular rock consisting of quartz, plagioclase, biotite, hypersthene and garnet. The micro-texture of this rock (Fig. 5 ) records no evidence of the D~ deformation event, confirming the previous hypothesis that the peak prograde (M1) assemblage crystallised after the end of the main fabric-forming event (DI).

Me decompression textures Petrographic observations at the Bandelierkop Quarry support the presence of two different decompression reactions. The M2a reaction. The large poikilitic, embayed garnet crystals in the sillimanite-bearing leucosomes and the selvages to these leucosomes, are surrounded by fine-grained granoblastic aggregates of cordierite (Fig. 6). Large sillimanite needles, in close proximity to the garnet, are surrounded by similar aggregates of cordierite. Garnet, sillimanite and quartz are not observed to be in mutual contact in these samples. This texture is the result of the continuous reaction, G a + Qz+ Sil =Cord ( M 2 a ) . At the Bandelierkop Quarry, this reaction is confined to sillimanite-bearing leucosome

TABLE 1 Melting reactions responsible for the formation of the various generations of Bandelierkop Quarry leucosomes Leucosomes

Melt-forming reactions

P-T conditions of the reactions

L2 and L3

Mus + Qz + Plag = Sil + Ksp + Melt

Approximate Tofoccurrence o f 7 0 0 ° C (Storre, 1977; Pet& 1976). The production of sillimanite confines P to a maxim um of 7 kbar (Holdaway, 1971 ).

L2, L3 and L4

Bi + Sil + Qz + Plag = Ga + Ksp + Melt

This reaction will occur between 760 and 800°C (Le Breton and Thompson, 1988 ). The crystallization of a second generation of sillimanite in the L3 leucosome after this reaction (Stevens, 1991 ) confines pressure to a maximum of 8.5 kbar (Holdaway, 1971 ).

L5

Bi + Qz + Plag = Hy + Ksp + Cord + Melt

A temperature of approximately 850 °C is indicated for this reaction (Hoschek, 1976 ). The production of cordierite by the reaction possibly indicates that the reaction occurred during the documented Me decompression period (Stevens, 1991 ).

285

CONSTRAINTS ON THE P - T LOOP IN THE SMZ

Fig. 5. Texture of the garnet-gneiss records no deformation in the parts of the outcrop unaffected by late shearing. M~, therefore, postdates the D t fabric forming phase of deformation. Ga = garnet, /4), = hypersthene, Bi = biotite, Qz = quartz and Plag= plagioclase.

Fig. 6. The large garnet (Ga) of the L2 leucosome and its selvage are often rimmed by granular cordierite (Cord) aggregates, which also developed around sillimanite.

generations and the selvages of these leucosomes. The reaction never occurs in the matrix of the metapelites, which did not contain sillimanite at the time of the M2 decompression.

The M2b reaction. Garnet in the garnet-cordierite granulite (XMg= 0.6-0.7 ) is always surrounded by a symplectic intergrowth of cordierite and hypersthene. The original form of the euhedral garnet is defined by a coarse hypersthene rim (Fig. 7 ). Garnet, surrounded by

Fig. 7. The garnet-cordierite gneiss is characterised by the presence of cordierite (Cord) and hypersthene (Hy) symplectic rims around embayed garnet (Ga) grains. The form of the original garnet grain is defined by the pseudomorphic outer rim ofhypersthene.

the symplectic textures, is never in contact with quartz, except where the quartz has been produced by a post-M2 reaction, consuming cordierite. The origin of these textures can be explained by the continuous reaction; Ga + Qz = C o r d + H y ( M 2 b ) . The cordierite-granulite (XMg> 0.7) is characterised by identical cordierite and hypersthene symplectic intergrowths. Garnet has, however, been completely consumed by the reaction in this Mgrich lithology. All the cordierite in the garnetcordierite granulite and cordierite-granulite at the Bandelierkop Quarry is confined to the symplectic intergrowths. These rock types, defined according to their mineralogy, have therefore, developed out of the M~ texture garnet-granulite, by the M2b reaction. The M2b cordierite-hypersthene symplectite textures are also developed in the leucosome and selvage of the last leucosome generation to be produced at the Bandelierkop Quarry. This indicates that all the melting reactions occurred prior to, or during M2.

Textures resultingfrom the hydration of cordierite Two reactions, termed M3a and M3b , resuited in the partial replacement of cordierite

286

in all the cordierite-bearing rock types at the Bandelierkop Quarry.

The M3a reaction. This reaction, Cord+ Ksp + Melt = Bi + Ky + Qz, resulted in a relatively coarse-grained intergrowth of biotite and kyanite, which partially replaced cordierite (Fig. 8). In some cases this reaction has proceeded to completion, resulting in biotite with abundant kyanite inclusions, associated with hypersthene and garnet. The M3a reaction is best developed in the leucosomes and leucosome selvages. The M3b reaction. Cordierite at the Bandelierkop Quarry that is not rimmed by biotite and kyanite, after the M3a reaction, was consumed by a subsequent reaction, producing a very fine-grained fibrolitic intergrowth of clear, high relief phases, which invade cordierite from grain boundaries (Fig. 8 ). This reaction is assumed to be identical to the cordierite hydration reaction, south of the orthoamphibole isograd, i.e., C o r d + H 2 0 = G e d + K y ( M 3 b ) ,

Fig. 8. Fine-grained, light brown biotite (Bi) with kyanite (Ky) inclusions, replacing cordierite (Cord). This reaction occurs in most of the cordierite-bearing lithologies in the Bandelierkop Quarry but is best developed in cordierite-bearing leucosomes and leucosome selvages. Cordierite in all cordierite-bearing lithologies is further rimmed by a narrow (30/~m), fibrolitic intergrowth (Fi) ofkyanite and gedrite. Ga=garnet. This texture clearly developes after the biotite texture.

G. STEVENSAND D.D. VANREENEN

which has produced a similar, but much more coarse-grained texture (Van Reenen, 1986). The two generations of kyanite resulting from the M3a and M3b retrograde reactions are the only generations of this mineral to have been identified at the Bandelierkop Quarry.

Significance of the petrographic observations Petrographic data from the different rock types exposed at the Bandelierkop Quarry allows a general metamorphic model to be created for this outcrop. The widespread geographical occurrence of the metamorphic textures observed at the Bandelierkop Quarry throughout the Southern Marginal Zone (Van Reenen, 1983, 1986), indicate that the model should be valid for the entire area. The proposed model is strongly dependant on the observed chronology of metamorphic reactions, and on the relict thermometers and barometers, represented by reactions with respectively steep or shallow dP/dT slopes. It is generally not constrained by geothermometry and geobarometry and, as a result, possibly avoids the uncertainty, which some authors feel, surrounds the application of these methods to granulite facies assemblages (e.g. Frost and Chacko, 1989; Spear, 1992 ). Prograde conditions and the MI event. The sequence of melting reactions identified at the Bandelierkop Quarry and listed in Table 1, indicates a period of prograde metamorphism from approximately 700 ° C, the temperature of the reaction Mus + Qz + Plag = Sil + Ksp + Melt (Storre, 1977; Pet6, 1976), to approximately 850°C, the temperature of the reaction Bi + Qz + Plag = Hy + Ksp + Cord/Ga + Melt (Hoschek, 1976 ). The production of sillimanite by the muscovite reaction confines pressure at this first reference point on the prograde path to a maximum of 7 kbar (Holdaway, 1971 ). The subsequent preservation of this phase confines the entire prograde path to the sillimanite stability field, indicating a maximum possible pressure of approximately 9.5 kbar

287

C O N S T R A I N T S O N T H E P - T L O O P IN T H E SMZ

(Holdaway, 1971), for the Bandelierkop Quarry exposure. The incipient nature of Q z + P l a g + B i melting at the Bandelierkop Quarry (Stevens and Van Reenen, 1992), indicates that the temperature of first occurrence of this divariant reaction has not been overstepped to a large degree, thereby confining the temperature of M ~to close to 850 oC. The production of cordierite and not garnet, as an accessory phase, by this reaction at the Bandelierkop Quarry, possibly indicates that pressure was declining prior to the M~ peak temperature being attained. No evidence for prograde kyanite has been found at the Bandelierkop Quarry. It is suggested that the previously described association of prograde kyanite-biotite intergrowths with garnet, on which the hypothesis of decompression starting in the kyanite stability field was based, resulted from the misidentification of the M3a reaction texture. '"

M2a

F

/ M2b

~ A

I l

M

M2a"

Ga + Sil + Qz = Cord

M2b:

Ga + Qz = Cord + Hy

From the phase relations in the system MgOFeO-A1203-SiO2, it is clear that the M2a reaction must occur at higher pressure than the M2b reaction, for a rock of given XMg value (Hen-

}

A

i ft.

The M2 period of decompression. Garnet of the M~ peak metamorphic assemblage at the Bandelierkop Quarry, is partially or totally replaced by cordierite and hypersthene, in respectively the intermediate and the MgO-rich metapelite varieties. A similar reaction, consuming garnet and sillimanite, and producing only cordierite has occurred in the sillimanitebearing leucosomes and the selvages to these leucosomes. The two generalised decompression related reactions which have been identified on the basis of petrographic observations at the Bandelierkop Quarry are:

:¢~¢;¢SS;3~$3;S

'

;~G; .......... . . . . . . .

tx TI:l P4 P2

I

- M2a (Ga + Qz + Sil : Cord}

DCcolnpression at constant v, hole rock coW,position (XMs=°'65) [

A 12.

P4_

o

o.s

XMg.-

f~J/l -M2b (Ga * Qz : Cord + "y)

Bulk compositions x and y have identical XMg = 0.65 but different A values.

Fig. 9. AFM, P-T and P--XMgdiagrams depicting the M2a and M2b divariant reactions (after Hensen, 1971; Hensen and Green, 1973 ). Note that M2a will occur at higher pressure than M2b for any given XMs whole-rock value. The preservation of garnet in the garnet-cordierite gneiss and the L2 leucosome ()(MS~ 0.65 ), implies that the P - T path (a) indicated on the diagram would be incorrect for the Bandelierkop Quarry and that the terrane probably followed a path similar to (b). Phase changes, occurring with decreasing pressure during the M2a reaction (sillimanite-bearing leucosomes and selvages ) and the M2b reaction (sillimanite-free metapelites and leucosomes), are illustrated on the AFM diagrams. The wholerock compositions chosen are representative of the L 2 leucosome composition (x) and garnet-cordierite gneiss composition (y).

288

G. STEVENS AND D.D. VAN REENEN

HI

1I

10

Whole rock XM8 dependant M2b reaction curves. - Cordlerlte-gneiss .... Garnet-cordierite gneiss ........... Garnet-gneiss +Pla 8 -Q z + Pla S - P i s s + FI - -

Sil+Ksp+Melt G a + Ksp + Melt Opx + Melt

120 - Anth ( X H 2 0 = 1.0) t 2 0 - Anth ( X t t 2 0 = 0.2) !hydration fthe aelting

sp + Melt = Bi + Ky + Qz

600 ~--

Reaction M3ainvolvcs ~ a granitic melt and produces kyanitc,

700 T - (°C) --

800

Maximum pressure at the time of reaction I which produces sillimanJte.

Fig. 10. Simplified P-T diagram for the Bandelierkop Quarry exposure in the Southern Marginal Zone, illustrating the conditions of the M~, M2 and M3 metamorphic events (after Stevens, 1991 ). The constraints of fluid-inclusion data ( - 12; - 3; + 13) refer to the Th of CO2-inclusions for three calculated isochors (Van Reenen and Hollister, 1988) and the displacement of the reaction, En + Qz + H20 = Anth, due to the dilution of H20 in the fluid phase to a value ofXmo = 0.2, on the form of the retrograde path, are also illustrated. The reaction En + Qz + H20 = Anth is confined to south of the retrograde, orthoamphibole isograd (Van Reenen, 1986). The positions of the reaction curve En + Qz + H20 = Anth, for Xu2o = 1.0 and XH2o=0.2, were calculated with the use of the GeO-Calc program of Berman et al. (1987), using the thermodynamic data base of Berman et al. ( 1985 ) and software of Perkins et al. ( 1986 ). sen, 1971; H e n s e n a n d G r e e n , 1973). Both rea c t i o n c u r v e s are sub-parallel to the t e m p e r a ture axis in the P - T d i a g r a m o f Fig. 9 a n d are t h e r e f o r e , m a i n l y c o n t r o l l e d by a decrease in pressure. T h e fact that the Mzb reaction has not r u n to c o m p l e t i o n in the g a r n e t - c o r d i e r i t e granulite a n d has not o c c u r r e d in the garnetgranulite, m u s t be the result o f significant cooling p r i o r to f u r t h e r d e c o m p r e s s i o n ( p a t h B in Fig. 9). T h i s h y p o t h e s i s is s u p p o r t e d by the excellent p r e s e r v a t i o n o f the M2b symplectic coronas, which w o u l d h a v e recrystallised, if subjected to p r o l o n g e d high t e m p e r a t u r e s after formation.

The M3 hydration event T h e l o c a t i o n o f the B a n d e l i e r k o p Q u a r r y in the granulite zone, several k i l o m e t e r s n o r t h o f the r e t r o g r a d e o r t h o a m p h i b o l e isograd (Fig. 1), results in h y p e r s t h e n e r e m a i n i n g stable t h r o u g h o u t the r e t r o g r a d e m e t a m o r p h i c history. C o r d i e r i t e has been, h o w e v e r , a f f e c t e d by two r e t r o g r a d e reactions:

M3a: C o r d + Ksp + Melt = Bi + K y + Qz; M3b: C o r d + H 2 0 = G e d + Ky. T h e M3b r e a c t i o n is r e p r e s e n t e d by the inc i p i e n t - h y d r a t i o n o f c o r d i e r i t e to p r o d u c e a fibrolitic i n t e r g r o w t h o f gedrite a n d kyanite.

289

CONSTRAINTS ON THE P - T LOOP IN THE SMZ

Fig. 1 I. Small, euhedral garnets (Ga.2) developed within the cordierite (Cord) produced by the M2b reaction. The form of this generation of garnet contrasts with that of the large corroded relicts (Ga. 1 ), remaining after the M2b reaction (Fig. 7 ). Hy = hypersthene.

Thermodynamic considerations indicate that this is a relatively high-pressure assemblage (Spear and Rumble, 1986). This is supported by the M3a crystallisation-hydration reaction, Cord + Ksp + Melt = Bi + Ky + Qz, which textural relationships indicate occurred prior to the M3b reaction (Fig. 8 ). The degree of development of this reaction can be positively correlated with the proximity of the sample site to an anatectic leucosome and the reaction can be shown to have removed significant modal quantities of K-feldspar from small volume leucosomes (Stevens and Van Reenen, 1992). This type of reaction will occur as aH20 in the in-situ crystallising melt rises to the point where the reversal of a melting reaction, involving a hydrous phase, becomes inevitable (Ashworth, 1985). As a result, the temperature of the H20-saturated granite solidus is a minimum temperature of occurrence for this reaction which therefore, must have occurred at a temperature of greater than 630-650°C for all pressures between 6 and 8 kbar (Johannes, 1984). The production of kyanite by the reaction confines pressure to a minimum of approximately 6.5 kbar at the above temperatures (Holdaway, 1971). This further illustrates that shortly after the occurrence of

the M2 reactions, the P-T path must have reverted from a decompression dominated path to a cooling dominated path. The resultant P - T loop, based on petrographic observations from the Bandelierkop Quarry exposure (Fig. 10), is in good general agreement with the previously published P-T loop (Fig. 2), and differs only in that the new data illustrates that the high-grade prograde path was confined to the sillimanite stability field. The additional new data, relating to the Mza and M3a reactions, is in support of the published decompression and subsequent cooling dominated portions of the loop.

Mineral zoning profiles and retrograde equilibration Previous studies (e.g. Van Reenen, 1983) have maintained that the M2b symplectite phases are largely unzoned, and that chemical equilibrium had been attained during the M2b reaction. This resulted in the conclusion that the mineral chemistries from the Southern Marginal Zone metapelite no longer reflect the MI peak metamorphic conditions, including the most Fe-rich metapelite which did not record the M2 reactions. The M2 symplectic coronas in the garnet-cordierite gneiss and the cordierite-gneiss have however, formed in response to limitations in inter-crystalline diffusion during the M2 reactions. It is therefore, extremely unlikely that intra-crystalline diffusion would have been rapid enough to prohibit the formation of any zonation during this reaction. This suggests that evidence of zoning has subsequently been obliterated by post-M2 diffusion processes, which would have been relatively rapid at the high temperatures of M 2 (Spear, 1992). The pervasive overprint of the M3a reaction, which consumed cordierite at the Bandelierkop Quarry, however, prohibits the detailed investigation of this hypothesis at this highly migmatised locality. Consequently an additional sample of the garnet-cordierite granulite (DV84), from a related area (Fig. 1 ),

290

G. STEVENS AND D.D. VAN REENEN A1.

Sp.

Gr.

Py.

60 -

2.3 -

4.4~

49 -

58

2.1

4.2

48 47 -

Fig. 12a I

56 _

1.9 -

4.0L

54

1.7

3.8

46

52 --

1.5 --

3.6L

45 --

I

1

Sp.

50

1.3

3.4

44

48 --

1,1 --

3.2L

43 --

46

0.9

3.0

42

44 _

0.7--

' 2.8L

41 I--

42

0.5

2.6

40

40 _

0.3-

2.4L

39 !

py. Or._

I

I

I

60

120

180

240

Distance from rim (urn) I5.6

Fig. 12b I

Fig. 12c I

I

I

T

15.0 14.4

garnet

XFe

XFe 13.8 132 12.6

cordierite

I

I

I

I

20

40

60

80

100

Distance from rim (urn)

Distance from rim (urn) Fig. 12. (a, b ) Z o n a t i o n profiles for garnet and cordierite in the M2b corona textures, occurring in sample D V 8 4 . ( c ) Hypothetical a n d s i m p l i f i e d z o n a t i o n profiles which would probably have resulted in garnet and cordierite during the M2b reaction. Hypersthene would have also developed a similar z o n a t i o n p r o f i l e . T h i s is however, not i n c l u d e d o n the d i a g r a m . T h e z o n a t i o n p r o f i l e s i n the garnet and cordierite of sample DV84 are markedly different from the proposed, hypothetical M2b z o n a t i o n profiles, illustrating that the composition of garnet, cordierite and hypersthene has been reset d u r i n g the cooling dominated p o r t i o n o f t h e P - T path. AI= a l m a n d i n e , Sp. = s p e s s a r t i n e , Gr= g r o s s u l a r a n d Py. = pyrope. Xre = 100 × Fe + 2 / F e + 2.~_ M g + 2. The mineral compositions on which the profiles are based, as well as those of hypersthene a r e l i s t e d i n T a b l e s 2, 3 a n d 4.

was chosen for detailed study. The bulk rock composition of sample DV84 is almost identical to that of the intermediate XMg variety metapelite from the Bandelierkop Quarry.

Sample D 1784 The symplectic products of the M2b reaction in this sample are relatively coarse-grained. Cordierite and the M2b generation of hypersthene in the symplectite coronas characteristically have an average grain-size between 0.5 mm and 0.75 mm. Cordierite often has inclusions of small, perfectly euhedral garnet crys-

tals with a size distribution from 0.05 to 0.1 mm (Fig. 11 ). The form of these crystals is in sharp contrast to the embayed and corroded border of the large relict garnet. This indicates that these crystals have been produced after the M2b reaction. The large relict garnet shows no zonation with respect to almandine, spesartine or pyrope across a traverse from the grain boundary to 190/lm towards the crystal core (Fig. 12a) and Table 2. The minor grossular component, however, decreases towards the rim. The composition of the small euhedral garnet inclusions in cordierite is slightly more

291

CONSTRAINTS ON THE P - T LOOP IN THE SMZ

TABLE2 Garnet composition variation with increasing distance from the crystal rim 84.b10 84.bl SiO2 TiO2 A1203 Cr203

FeO MnO MgO CaO Total Si 4+ Ti 4+

38.73 39.02 38.56 38.73 0.02 0.02 0.04 22.88 22.64 22.59 22.58 0.06 0.11 0.21 0.13 27.00 28.33 28.14 27.24 0.82 0.66 0.85 0.86 10.39 10.77 9.93 10.31 1.36 1.36 1.24 1.25 101.24 102.91 101.54 101.14

Cr 3+ Fe 2+ Mn 2+ Mg:+ Ca 2+ 02

4.08 0.01 2.32 0.07 2.06 0.16 24.00

6.62 0.00 4.00 0.01 2.42 0.06 2.11 0.16 24.00

Aim% Spes% Py% Gros% /tin-rim

50.37 1.56 44.54 3.54 3.00

50.91 1.21 44.47 3.41 4.00

AI 3+

84.b12 84.b9

6.63 -

84.b2

84.b8

84.b3

84.b13 84.b14 84.b7

39.16 39.03 39.02 38.98 0.03 0.01 0.06 22.84 22.80 22.92 22.40 0.21 0.16 0.15 0.30 28.03 28.19 27.80 28.90 0.87 0.84 0.85 0.88 10.69 10.69 10.94 10.28 1.27 1.14 1.16 1.06 103.10 102.85 102.85 102.86

38.74 0.06 22.49 0.22 20.60 0.82 9.65 1.24 93.82

6.63 0.00 4.05 0.02 2.43 0.07 1.97 0.15 24.00

6.65 0.00 4.04 0.01 2.35 0.08 2.05 0.15 24.00

6.62 0.00 4.02 0.02 2.38 0.08 2.09 0.15 24.00

6.62 4.02 0.01 2.40 0.07 2.09 0.14 24.00

6.60 0.00 4.04 0.01 2.37 0.07 2.14 0.14 24.00

6.64 0.01 3.97 0.03 2.48 0.08 2.02 0.13 24.00

6.85 0.01 4.14 0.02 1.83 0.07 1.97 0.15 24.00

52.55 1.61 42.61 3.23 5.00

50.87 1.63 44.24 3.25 12.00

50.74 1.60 44.46 3.20 20.00

51.07 1.55 44.50 2.88 40.00

50.16 1.56 45.36 2.92 50.00

52.65 45.44 1.63 1.84 43.03 48.91 2.69 3.81 50.00 60.00

38.98 22.85 0.17 28.12 0.88 10.69 1.11 102.80 6.61 4.03 0.02 2.40 0.08 2.10 0.13 24.00

84.b4

84.b6

84.b5

84.b15

38.97 38.58 38.85 38.70 0.01 0.01 22.86 22.80 22.87 22.79 0.16 0.14 0.10 0.15 27.98 27.53 27.72 27.90 0.84 0.88 0.81 0.90 10.94 10.58 10.63 10.92 1.03 1.05 1.06 1.05 102.79 101.57 102.04 102.41 6.60 0.00 4.03 0.01 2.38 0.07 2.14 0.12 24.00

51.01 50.50 1.62 1.54 44.56 45.37 2.81 2.59 90.00 100.00

6.61 0.00 4.06 0.01 2.37 0.08 2.09 0.13 24.00

6.62 4.06 0.01 2.38 0.07 2.09 0.13 24.00

6.61 4.05 0.01 2.38 0.06 2.13 0.12 24.00

50.80 50.91 1.65 1.51 44.85 44.86 2.70 2.71 130.00 150.00

50.54 1.56 45.37 2.53 185.00

All analyses are from o n e Mzb corona texture in sample DV84. Analyses b12, b13 and b14 are from the small euhedral garnet inclusions in cordierite. All the other analyses are from the large relict garnet. The cordierite and hypersthene analyses in Tables 3 and 4 are from the same corona texture. (#m-rim) indicates the distance of the analysis point from the crystal edge in microns.

FeO-rich than the rims of the large embayed garnet crystals. Cordierite is characterised by a systematic increase in XMg towards the rim (Fig. 12b, Table 3 ). The M 2 b symplectite generation of hypersthene is unzoned (Table 4). The M I matrix generation of hypersthene has a slightly higher Xve value than the M2 symplectite hypersthene, with no zonation of the XFe. The Al-content of the M~ hypersthene rims is identical to that of the M2bhypersthene, i.e., 0.32. The matrix hypersthene core, however, has a slightly higher Al-content (0.37).

Interpretation of the zonation profiles As the M 2 b continuous decompression reaction proceeded, the participating Fe-Mg phases must have continually adjusted their compositions towards the FeO-rich end-members

(Fig. 9) (Hensen, 1971 ). Intra-crystalline diffusion constraints during decompression, probably resulted in the rims of the cordierite, hypersthene and garnet becoming more FeOrich than the cores. The hypothetical and simplified zonation profile which would have resuited from the M 2 b reaction in garnet, cordierite and hypersthene is illustrated in Fig. 12c. These zonation patterns are markedly different from those for the minerals in sample DV84. The garnet in this sample is characterised by a distinct lack of zonation (Fig. 12a). This is interpreted as supporting the previously advanced hypothesis, that the mineral zonation profiles resulting from the M2b reaction have been destroyed by post-M2 diffusion. Cordierite is characterised by Mg-enriched rims relative to the cores (Fig. 12b). This zonation would not have formed during the M z b

292

G, STEVENSAND D.D. VAN REENEN

TABLE 3 Cordierite composition variation with increasing distance from the crystal rim 84-b4

84.b5

84.b6

84.bl

84.b2

84.b3

SiO2 A1203 FeO MgO Na20 Total

48.15 33.68 3.28 11.54 0.05 96.70

48.09 33.65 3.30 11.21 0.03 96.28

48.79 33.66 3.06 11.14 0.02 96.67

48.99 32.83 3.35 11.10 0.03 96.30

48.90 33.52 3.43 10.95 0.04 96.84

48.96 33.62 3.60 10.93 0.05 97.16

Si 4+ AI 3+ Fe z+ Mg 2+ Na '+ 02

4.93 4.06 0.28 1.76 0.01 18.00

4.94 4.08 0.28 1.72 0.01 18.00

4.98 4.05 0.26 1.70 0.00 18.00

5.03 3.97 0.29 1.70 0.01 18.00

4.99 4.03 0.29 1.67 0.01 18.00

4.99 4.04 0.31 1.66 0.01 18.00

XFe ~tm-rim

13.75 5.00

14.17 15.00

13.35 30.00

14.48 60.00

14.95 100.00

15.60 160.00

All analyses are from cordierite in one Mzb corona texture in sample DV84. (/1m-rim) indicates the distance of the analysis point from the crystal edge in microns. Xve= 100 × Fe 2+/Fe2+ + Mg 2+ . TABLE 4 Hypersthene composition variation with increasing distance from the crystal rim 84.b I

84.a I

84.a3

84.b2

84.b3

84.b4

84.a4

SiO, TiO2 AI_,O3 Cr203 FeO MnO MgO CaO Total

49.46 0.12 6.85 0.35 20.58 0.16 22.15 0.05 99.72

48,96 0,14 7,62 0.42 22.16 0.13 20.68 0.05 100.16

47.55 0.05 7.82 0.34 21.36 0.32 20.98 0.02 98.44

49.18 0.06 7.14 0.36 19.82 0.15 21.76 0.06 98.53

49.37 0.21 7.36 0.36 20.66 0.14 22.17 0.03 100.30

49.28 0.11 7.56 0.36 20.16 0.16 22.18 0.03 99.84

46.85 0.13 8.16 0.39 21.31 0.18 20.59 0.05 97.66

47.97 0.30 8.30 0.39 21.90 0.20 20.37 0.06 99.49

49.00 0.09 7.73 0.33 20.04 0.11 22.20 0.05 99.55

Si 4+ Ti 4+ AI3+ Cr 3+ Fe 2+ Mn 2+ Mg2+ Ca 2+ 0:-

1.83 0.00 0.30 0.01 0.64 0.01 1.22 0.00 6.00

1.82 0.00 0.33 0.01 0.69 0.00 1.14 0.00 6.00

1.80 0.00 0.35 0.01 0.67 0.01 1.18 0.00 6.00

1.84 0.00 0.31 0.01 0.62 0.00 1.21 0.00 6.00

1.82 0.01 0.32 0.01 0.64 0.00 1.22 0.00 6.00

1.82 0.00 0.33 0.01 0.62 0.01 1.22 0.00 6.00

1.78 0.00 0.37 0.01 0.68 0.01 1.17 0.00 6.00

1.79 0.01 0.37 0.01 0.68 0.01 1.14 0.00 6.00

1.81 0.00 0.34 0.01 0.62 0.00 1.22 0.00 6.00

34.23 65.67 0.11 5.00

37.50 62.39 0.11 15.00

36.34 63.62 0.04 15.00

33.77 66. l0 0.13 20.00

34.31 65.63 0.06 50.00

33.75 66.19 0.06 70.00

36.69 63.20 0.11 80.00

37.57 62.30 0.13 120.00

33.58 66.31 0.11 180.00

Aw, XMg JQa /~m-rim

84.a2

84.b5

All analyses are from hypersthene in sample DV84. Analyses bl, b2, b3, b4 and b5 are from one M2b corona texture. Analyses a 1. a2, a3 and a4 are from an M~ matrix hypersthene. (/tm-rim) indicates the distance of the analysis point from the crystal edge in microns. Xve= 100 × Fe 2+/Fe 2+ + Mg 2+ + Ca 2+. XMg and )(ca were determined in a similar manner.

('ONSTRMNTSON THE P-T LOOPIN THE SMZ

293

10

"'"

• ~

XFe ~

. ~

"

~

/

-

-

0.45

M3a p 6 ,

/

k b

/

/

a

r

4

do/

46"/ /

/ /

/

/ /

5il And

0L 400

500

600

700

800

900

1000

T ('C) Cordierite Hypersthene Garnet Fig. 13. P - T diagram illustrating the position of lines of constant XF~ for hydrous cordierite, garnet and hypersthene participating in the reaction Ga + Qz = Cord + Hy. The cooling dominated portion of the P - T loop, which follows the M26

reaction is indicated on the diagram. Cordierite, undergoing diffusion during this period, would have developed more Mg-rich rims, as is evident from the compositions for cordierite in sample DV84. Garnet forming during this time would be more Fe-rich than that in equilibrium at the start of the cooling path. This is proposed as the mode of origin for the small, euhedral garnet inclusions in cordierite of sample DV84. See text for further discussion. (Fig. 13 courtesy of F.S. Spear. ) reaction, which would have produced zonation in the opposite direction. This characteristic must, therefore, also be the result of postM2 diffusion processes. The P - T loop proposed on the basis of the sequence of reactions identified at the Bandelierkop Quarry, indicates that decompression halted and that the path converted to a cooling-dominated path, at relatively high metamorphic grades. If this cooling dominated portion of the bath is superimposed on a P - T diagram, illustrating changes in F e / M g ratio in garnet, cordierite and hypersthene, during the M2b reaction, it is

observed that the lines of constant composition of hydrous cordierite have a strong positive slope, with higher values of XMg in cordierite corresponding to lower temperatures, as well as higher pressures (Fig. 13). Approximately isobaric cooling, following the M2b reaction, could therefore, have resulted in cordierite developing the observed zonation profiles by diffusion occurring after M2b. The lines of constant garnet composition have a weak negative slope, with higher values of Ave in garnet corresponding to both lower pressures and lower temperatures (Fig. 13). It is

294 therefore possible that during the cooling dominated period, a small amount of slightly Fe-rich garnet could have been produced, after the destruction of the initial Mzb zonation profile. This is proposed as the mechanism by which the small euhedral and Fe-rich garnet inclusions in cordierite (Fig. 11 ), which clearly formed after the decompression reaction, originated. The lines of constant composition for hypersthene are very close to parallel to a path of isobaric cooling (Fig. 13 ). This is consistent with the fact that hypersthene, in the Mab textures and in the coarse-grained matrix, does not show zonation similar to that of cordierite. The observed higher A1203 content of the M~ generation of hypersthene in both samples possibly reflects a relict of the higher pressures at which this generation would initially have equilibrated. This would, however, imply that these coarse-grained hypersthene crystals had not completely reequilibrated during the highgrade cooling path, when there is good evidence that garnet, which is regarded as a mineral in which diffusion rates are slow (Tracy, 1982 ), has undergone almost total reequilibration. Conclusions

The relict reaction textures observed in the rocks of the Bandelierkop Quarry exposure support the identification of three phases in the metamorphic history, during which the P - T path followed different trajectories. A highgrade prograde proportion has been identified on the basis of relict melting related textures, with heating from approximately 700°C to 850°C. During this time, pressure was confined to the sillimanite stability field. The maximum metamorphic temperature (M~), was probably attained during a period of approximately isothermal decompression (M2), which is indicated by the relict textures of two, decompression driven, garnet breakdown reactions: G a ÷ S i l + Q z = C o r d (M2a) and G a +

G. STEVENS AND D.D. VAN REENEN

Q z = C o r d + Hy (M2b). These reactions are controlled by whole-rock XMg value, in addition to pressure and the absence of the M2b reaction in the most Fe-rich metapelites indicates that the decompression-dominated path converted to a cooling-dominated path at relatively high pressures. The presence of this cooling dominated portion of the P - T loop is confirmed by the identification of a crystallisation-hydration reaction: Cord + Ksp + Melt=Bi+Ky+Qz (M3a), which has occurred at a minimum temperature of approximately 630°C at a minimum pressure of 6.5 kbar. During the period of near-isobaric cooling which followed the M2 decompression, temperatures were initially sufficiently high to allow the destruction of the mineral zonation profiles, which probably resulted from the M2 reactions. As a result, geothermometry and geobarometry data from the Southern Marginal Zone metapelites probably no longer reflects the peak M~ conditions, or the true conditions which existed at the end of M2. Diffusion, during this cooling-dominated period, can possibly account for the lack of zonation in the large garnet crystals, the zonation of cordierite towards more MgO-rich compositions at the rim and the growth of small, euhedral garnet crystals after the M2 reactions, which consumed garnet. The published P - T loop has been interpreted as a typical clockwise P - T loop, resulting from a continent-continent type collision (Van Reenen et al., 1990). This accounts for the fact that the surface sedimentary sequence, represented by the Bandelierkop Formation metapelites, records pressures equivalent to burial of approximately 30 km. The peak metamorphic assemblage overprints the main metamorphic fabric and a period of approximately isothermal decompression has occurred, probably as a result of rapid tectonic erosion as the terrane rebounded isostatically (Van Reenen et al., 1990). The form of the P T loop proposed in the present study is in gen-

CONSTRAINTS ON THE P - T LOOP IN THE SMZ

eral agreement with the published loop. As a result, the data of this study also support the previous tectonic interpretation of the metamorphic history of the Southern Marginal Zone (Van Reenen et al., 1990). It is, however, interesting to note that the approximately isobaric cooling portion of the loop probably resuits from the re-establishment of a steady state geotherm after the elevation ofgeotherms during the uplift, associated with the M2 decompression (Ellis, 1987). Uplift must therefore, have halted in the mid-crustal region ( 6.5 to 6 kbar), for a sufficiently long period for the thermal reequilibration of the crust. The documented thrusting of the Southern Marginal Zone granulites in a southerly direction, over the Kaapvaal Craton (Van Reenen et al., 1990), might therefore, not be part of a continuous tectonic event related to uplift and decompression.

Acknowledgements The authors would like to express appreciation to the Foundation for Research Development and the Rand Afrikaans University for financial and other assistance and to Deon De Bruin of the Geological Survey of South Africa for assistance in obtaining the microprobe data. Reviews of the manuscript by F.S. Spear and G.T.R. Droop contributed a great deal to its improvement.

References Ashworth, J.R., 1985. Introduction to migmatites. In: J.R. Ashworth, (ed.), Migmatites. Blackie, Glasgow, pp. 135. Baker. J., Van Reenen, D.D., Van Schalkwyk, J.F. and Newton, R.C., 1992. Constraints on the composition of fluids involved in retrograde anthophyllite formation in the Limpopo belt, South Africa. Precambrian Res., 55: 000-000, this volume. Berman, R.G., Brown, T.H. and Greenwood, H.J., 1985. An internally consistent thermodynamic data base for minerals in the system Na20-K/O-CaO-MgO-FeOFe203-A1203-SiO2-H20-CO2. Tech. Rep. 377, Atomic Energy of Canada Ltd., 62 pp.

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