American Mineralogist, Volume 78, pages 104I-1055, 1993

Metamorphic evolution of amphibole-bearing aluminous gneisses from theEastern Namaqua Province, South Africa

Huw C. flurvtprtnnvsxPrecambrian Research Unit, Department of Geology, University of Cape Town, 7700 Rondebosch, South Africa

AssrRAcr

Amphibote-bearing aluminous rocks from the 1.3 Ga Copperton Formation of the Na-maqua Province, South Africa, preserve reaction textures consistent with near isobariccooling. Peak metamorphic assemblages equilibrated at 600-820 "C and 5-6.5 kbar andinclude orthopyroxene + garnet + cordierite in the granulite facies and garnet + gedrite+ cordierite + staurolite, gedrite * hornblende + cordierite + garnet, and cordierite +staurolite + sillimanite + hornblende in the amphibolite facies. All rocks contain oligo-clase and quarlz, whereas biotite is present in the amphibolite facies. Cordierite and horn-blende have partially or completely altered to staurolite-, chlorite-, and gedrite-bearingassemblages in rehydration reactions, whereas sillimanite-bearing rocks developed kyaniteon cooling. Interpretation of these textures using a petrogenetic grid in FMASH suggestsa cooling path that is isobaric at pressures above that of the AlrSiO, triple point. The latesttextures in the rocks reflect a regrowth of garnet over chlorite + quartz and gedrite +staurolite, indicative of a final period of reheating. This is consistent with the metamorphicevolution of pelites along the strike of the amphibole-bearing rocks that show regrowth ofgarnet and staurolite from retrograde chlorite + muscovite substrates. Aluminous gneissesof the Copperton Formation preserve prograde and retrograde examples of the uncommonassemblage staurolite + sillimanite + hornblende. The prograde assemblage occurs at theinterface of a hornblende-bearing semipelitic schist and a KrO-poor peraluminous gneiss;the retrograde assemblage occurs as grain-boundary minerals at the margins of alteredcordierite and plagioclase grains.

InrnonucrroN

Recent studies of aluminous rocks containing amphi-boles have revealed the wealth of pressure-temperatureinformation available from a consideration of their tex-tures and phase relationships (Spear, 1977, 1978; Rob-inson et al., 1982; Harley, 1985; Hudson and Harte, 1985;Spear and Rumble, 1986; Baker et al., I 987; Schumacherand Robinson, 1987; Visser and Senior, 1990). These in-clude the placement of peak temperatures and pressuresusing calibrated petrogenetic grids in FeO-MgO-AlrO3-SiO,-H,O (FMASH) or NarO-CaO-FeO-MgO-AlrOj-SiOr-HrO (NCFMASH) and the construction of P-Ipathsfrom the successive appearance of stable or metastableassemblages within a sample or group of samples. Thereaction textures and mineral compositions in aluminousgneisses and schists of the Copperton Formation in CapeProvince, South Africa, provide an opportunity to con-struct a P-T path from such amphibole-bearing rocks.

The aluminous rocks at Copperton include those withamphibole and those without. The latter approximate to

* Present address: 6l Pendre Gardens. Brecon. Powvs. WalesLD3 gEP, U.K.

pelitic compositions and are described in Cornell et al.(1992\. Amphibole-bearing varieties are found in theCopperton Formation at a variety of metamorphic grades.They contain the unusual assemblage sillimanite * horn-blende + staurolite (+ cordierite) and the more commonassemblages gedrite + garnet + cordierite * stauroliteand garnet + cordierite * hornblende + gedrite.

GBor-ocrc sErrING

The Copperton Formation is a northwest-striking unitof banded and interlayered gneisses adjacent to the tec-tonic boundary between the Kheis Belt, with an age be-tween 3.0 and 1.7 Ga (Cornell et al., 1986), and thegneisses of the Kakamas Terrane, with an age between1.6 and 1.0 Ga (Theart, 1985; Barton and Burger, 1983).The Copperton Formation is dated by the single-zirconU-Pb method at 1.29 Ga (Cornell et al., 1990b) and isinterpreted on tectonic and geochemical grounds as anisland arc sequence formed at the onset of the Namaquaorogenic cycle (Geringer et al., 1986). The CoppertonFormation was deformed and metamorphosed at 1.22-| .20 Ga and remetamorphosed at I . I 0- I .08 Ga (Cornellet al., 1990a). The textures described in this paper datefrom these two periods of metamorphism.

0003-o04x/93109 l 0-l 04 l $02.00 l 04 l

ro42 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

Karoo cover

Kakamaslerrane

Fig. l. Local geology of the Copperton region, from Hum-phreys et al. (1988b). Copperton is at the southeast corner oftheKakamas Terrane (inset).

Loc,cl GEoLocy AND pETRocRApHICDESCRIPTIONS

The samples described come from the wall rocks ofthree zones of pyritic mineralization near the prieska Zn-Cu mine. All the samples were collected from within l5km of one another (Fig. l) and are found on the farmsKielder (three samples) and Smouspan (seven samples)and near the VA Cu-Fe deposit (three samples). Peakmetamorphic conditions lie in the granulite facies atKielder and in the eastern part of Smouspan and in themid-upper amphibolite facies for the remainder. Mineralassemblages are in Table l.

Kielder deposit

The Kielder Zn-Pb mineralization is hosted by mag-nesian, mafic, and intermediate granulites (Gorton. 1982).Temperatures are estimated at 750-850.C by the garnet-orthopyroxene thermometer of Harley (1984), and pres-sures of 5.5-6.7 kbar are estimated from the garnet-or-thopyroxene-plagioclase-quartz barometer of Powell andHolland (1988). The peak metamorphic assemblage inmagnesian granulites is

orthopyroxene * cordierite * plagioclass + garnet-r qvarlz + hercynite spinel (KDH5-420). (A)

Dehydration melting at the peak of metamorphism re-sulted in crosscutting pods of plagioclase * potassium

feldspar + orthopyroxene * cordierite that disrupted apseudomorphed biotite schistosity. Both granulite-faciesmetamorphism and accompanying melting occurred afterpenetrative deformation.

The three orthoamphibole-bearing specimens (KDH5-95, KDH5-105, and KDH5-420) are partially or fullyretrograded granulites with heterogeneous distribution ofminerals. KDH5-95 and KDH5-105 have large sheavesof clove gray anthophyllite with gedrite or hornblendemargins, coarse intergrowths of cordierite and qtrartz, andcoarse patches of plagioclase. Important retrograde tex-tures in these two rocks include (l) the replacement ofcordierite grain boundaries by staurolite + chlorite +gedrite (Fig. 2E), (2) the occurrence, at cordierite grainboundaries, of the assemblage sillimanite + gedrite assmall euhedral grains (Fig. 2A), and (3) the coexistenceof hornblende + staurolite + sillimanite as a grainboundary assemblage between plagioclase and cordierite.In l, the product grains are euhedral and postdate whatremains of the structural fabric of the original granulite.Chlorite overgrows staurolite and is the final mineral toform in the retrograde assemblage.

Decomposition of biotite and cordierite to the assem-blage muscovite + kyanite * staurolite * chlorite is seenin the amphibole-free gneiss KDH5-544 and offers cor-ollary evidence that the P-T path just after the peak ofmetamorphism near Copperton passed across the silli-manite-kyanite curve.

In KDH5-420, orthoamphibole has grown from As-semblage A along a band 2 mm wide, according to thereaction garnet + cordierite + orthopyroxene + potas-sium feldspar + HrO : biotite * anthophyllite + quartz,representing the earliest retrogression of granulite faciesassemblages at Kielder. Subsequent growth of secondarygarnet grows across retrograde biotite and anthophyllite(Fig. 3a).

Smouspan biotite gneisses

Biotite * orthoamphibole-bearing gneisses with garnetoccur in the Vogelstruisbult Member of the CoppertonFormation on the farm Smouspan. The VogelstruisbultMember is a banded sequence of amphibolite, calc-sili-cate gneiss, biotite-hornblende gneiss, and pelite thatforms the stratigraphic hanging wall to the Prieska mineore body. Five biotite-rich and two cordierite-rich gneiss-es were sampled by bore holes SM6, SM7, SP2, V55, G2,and G5. Two parageneses can be identified:

garnet + orthoamphibole + biotite + plagioclase+ qtrartz (+ orthopyroxene)

(SM6-a95; SM7-69; SM7-82; SP2-B; V55-1090) (B)

cordierite + orthoamphibole + plagioclase+ biotite + quartz (+ garnet + hornblende)

(G2-D;c5-B). (C)

Assemblage B was derived from a granulite-facies pre-cursor, and in some cases, orthopyroxene is still extantas heavily corroded and altered porphyroblasts. The pre-

struisbultMember

CopperMinesl-r

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

TABLE 1. Mineral assemblages

1043

Sample Qtz Ky opx chl Opaque

KDH5-95KDH5-105KDH5-420

v55-1090sM6-495sM7-69sM7-82SP2-BG2-DG5.B

VA11-HVA1lJVAl2.AVA12.B

3ps

pspspsps

/

62x3x

1 0

x'

xx xx x

13 251 2 5x x

47x x

39 3335 13

x xx x 'x x 'x x

xxx

xxx

psx

1 2

't2ps

2 31 4 ?

24IxII

410 '

Kielder

x x

Smouspan

VA locality

xxx

3149x

30x

1515

xxxx

7

??

Magx x P y

Mag

x P yx P y

x x P Yx Mag

22 x

MagMagMagMagMagPyMag

xxx

Nofe: ps : pseudomorphs atler mineral observed; x : mineral present as primary phase; x: mineral present as secondary phase; x' : mineralpresent as primary and secondary phases; ? : identity not confirmed. Numerals indicate modal percentages. Mineral abbr€viations are after Kretz(1983).

x x1 x

x x

dominant 52 fabric is defined by aligned biotite and an-thophyllite prisms, which enclose large poikiloblastic gar-net typical of this assemblage. The large primary garnetgrains contain quartz and magnetite but neither ortho-pyroxene nor orthoamphibole and are assumed to havegrown early in the paragenetic sequence. A garnet samplefrom this assemblage yields a garnet growth age of l20lm.y., as determined from Sm-Nd values obtained fromthe garnet and whole rock (Cornell et al., 1986). A secondgeneration of garnet grows locally over biotite and or-thoamphibole.

Assemblage C contains abundant retrograde stauroliteeuhedra on gedrite-cordierite and cordierite-cordieritegrain boundaries (G2-D). In G5-B, these are intergrown,with gedrite replacing cordierite * plagioclase (Fig. 2B,2C). G5-B contains primary gedrite and hornblende, aswell as smaller grains of secondary gedrite. The earliergeneration ofgedrite is coarser and more equant than thelater, retrograde, grains, many of which are fibrous orrhomb-shaped. Secondary garnet has grown over retro-grade gedrite-staurolite intergrowths (Fig. 2D). In G2-D,sillimanite occurs with gedrite on cordierite grain bound-aries, but it is unclear whether sillimanite and gedritewere stable together.

VA mineralization

The VA mineralized zone (Fig. 1) is separated from thebiotite gneisses of Smouspan by the Klipgatspan shearzone, I km wide. It lies along the strike from the Annexdeposit, an uneconomic copper iron sulfide deposit whosepelitic wall rocks share a similar metamorphic history tothose of the VA locality (Humphreys and Scheepers,1993). The host sequence of both sulfide zones is similarand contains amphibolites, quartz-rich gneisses, biotitegneisses, and hornblende-biotite gneisses. Garnet-biotitethermometry from the Annex pelites indicates peak tem-

peratures of 580-620 .C (calibrations of Dasgupta et al.,1991, and Williams and Grambling, 1990, with correc-tions for Mn in garnet). A maximum pressure of 6.1 kbar(at 580 'C) is estimated from a single barometric calcu-lation using the garnet-plagioclase-kyanite-quartz barom-eter of Powell and Holland (1988).

Amphibole-bearing assemblages occur in two boreholes, VAl l and VAl2, and include:

cordierite * hornblende + sillimanite* staurolite + plagioclase + quartz

(vAl2-A)

and

cordierite + garnet + orthoamphibole* staurolite * plagioclase + quartz

(vAl2-B; VAl l -J) . (E)

In addition, VAl2-A contains domains where the assem-blage

cordierite + phlogopite/biotite + quartz (F)

was stable at the peak of metamorphism. Assemblage D(Fig. 3b) contains primary staurolite and sillimanite in-tergrown with rare grains ofhornblende, surrounded par-tially by intergrowths of former cordierite and plagio-clase. The occulTence of all five minerals together isrestricted to the interface of domains containing Assem-blages D and F. Hornblende is overgrown by sheaves ofchlorite and subsequently by small grains of secondarystaurolite (Fig. 2H), whereas cordierite has been replacedby varying proportions of staurolite, chlorite, quartz, andwhite mica (Fig. 2F). An outer rim of cloudy stauroliteseen in Assemblage D is separated from the grain coresby a narrow band of quartz inclusions a few tens of mi-crometers from the grain boundary (Figs. 2G, 3b). This

(D)

1044 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

F _

Tlale 2. Sequence of mineral parageneses

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1045

Fig. 3. Line drawings oftextures in amphibole-bearing rocks:(a) Overgrowths of second-generation garnet upon preexistingcores in samples VAI2-B (left) and G5-B (center, right). TU isthe textural unconformity that represents the hiatus caused byresorption ofthe core prior to overgrowth ofthe rim. (b) Stableassemblage of staurolite, sillimanite, and staurolite in VAl2-A.

potential was 15 kV, with a beam current of 20 nA forfeldspars and sheet silicates or 40 nA for other minerals.Raw data were reduced using the method of Bence andAlbee (1968). Mineral analyses are listed in Tables 3-8.The projection of mineral compositions from quartz, al-bite, and HrO is shown in Figure 4.

Garnet

Kielder. Zoned garnet from the orthoamphibole-richband in KDH5-420 has an outer rim that is distinctchemically and optically from the interior. A central coreshows increasing X*, toward 0.35 at the inner rim, whichis succeeded by a decrease in the outer rim to 0.23. Theopposite effect is seen in Xr,. X.uis asymmetric, reflectingX", on one limb and Xr, on the other; Xr" increases toboth rims (Fig. 5A), indicating partial resorption of thecore before the rim grew.

Smouspan. Garnets in Assemblages B and C havebroadly similar profiles, characterized by flat cores, in-creasing Xr, toward the rims, and a decrease at the outerparts of the garnet (Fig. 5B). XF" varies in an inverse man-ner to Xr' whereas Xr" and X." profiles vary nonsyste-matically. In G5-B, X." is constant in the core beforedecreasing at the extreme edge and is broadly mirroredby variations in Xr., which increase from near the mar-

Smouspan VA mineralization

Primary assemblagec r d + o a m + P l + c r d + s t + P l + B tC r d + O p x + P l

[+ Grt, Hc, Qtz,Bt (rarel

Oam + ChlSil + Oams i t + s t + H b lst + chl

Grt (on Oam + Bt)

Qtz (+ Grt, Hbl,opx)

+ Otz (+ Hbl, Sil,Oam, Grt)

Resultant assemblages on cooling/rehydrationOam + Slst + chlsit + chl

K y + C h l + ? M sChl + Qtz

Minerals grown upon reheatingGrt (on Oam + St) Grt (on Chl +

St (on Chl +Hbt)

B0B t +

rim is resorbed against chlorite and thus predates thegrowth of secondary staurolite, which grew subsequentlyacross chlorite and hornblende (Table 2).

In Assemblage E, primary garnet is intergrown withgedrite and staurolite; patches of fine-grained secondarymicas, staurolite, and quartz indicate the former exis-tence of cordierite in the peak assemblage. The first gen-eration of garnet was locally replaced by chlorite beforea second generation of garnet formed as a rim upon theresorbed garnet cores.

Cordierite in Assemblage F has broken down to formacicular grains of kyanite, together with staurolite andchlorite. This is a key petrogenetic indicator and is cor-related with the retrograde growth of kyanite after silli-manite in the Annex pelites (Humphreys and Scheepers,l 993) .

Mrxrnar, coMPosrrroNs

Minerals were analyzed on a Cameca Camebax elec-tron probe microanalyzer housed at the Department ofGeochemistry, University of Cape Town. Accelerating

(-

Fig. 2. Photomicrographs of textures from amphibole-bear-ing aluminous gneisses occurring in the Copperton Formation.Scale bars : 0.5 mm. (A) Replacement of cordierite (Crd) bysillimanite (Sil), chlorite (Chl), and gedrite (Ged): KDH5-105.(B) Replacement of cordierite (Crd) by gedrite (Ged) and srau-rolite (St); fibrous sillimanite also occurs on grain boundaries:G2-D. (C) Alteration of plagioclase (Pl) and cordierite to gedrireand staurolite; gedrite has grown along plagioclase twin planes:G5-B. (D) Regrowth of euhedral garnet (Grt) upon a gedrire-staurolite substrate: G5-B. (E) Euhedral grains of orthoamphi-bole (Ged) resulting from the breakdown ofcordierite * ortho-pyroxene: KDH5-105. (F) Alteration of cordierite to kyanite (Ky),chlorite (Chl), and quartz (Qtz): staurolite (St) appears to havegrown later from chlorite: VA 1 2-A. (G) Intergrowh of silliman-ite (Sil) and staurolite (St); staurolite has cloudy poikiloblasticmargins and is replaced by chlorite and biotite in the center ofthe photograph: VAl2-A. (H) Hornblende porphyroblast (Hbl)with margins corroded by chlorite (Chl) and biotite (Bt) andsubsequently overgrown by staurolite (St): VAl2-A.

t046

TlaLE 3. Representative mineral analyses of garnet

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

KDH5-420 SM7-82 G5-B

Core

sio,Alr03FeOMnOMgoCaO

TotalSiAIFeMnMgCa

TotalMg/(Mg + Fe)

38.6522.4827.061.928.682.81

101.602.9502.0221.727o.1240.9870.2308.0400.364

37.9622.1430.502.856.081.87

101.402.9562.O321.9860.1880.7060.1568.024o.262

37.5521.7529.914.334.383.28

101.202.9602.O201.9720.2890.515o.2778.033o.207

38.0521.9833.292.385.031.30

102.032.9722.O232.1750.1580.5850.1098.0220.212

38.2422.0130.712.086.142.47

101 .652.9682.0131.9940.1370.7100.2068.0280.286

37.422't .7532.741.675.122.45

r 01 .152.9472.0192.1570 . 1 1 10.601o.2078.O420.218

37.5921 .9833.121 .464.642.65

101 .442.9542.0352.1760.0970.543o.2238.0280.200

36.96 37.0321.19 21.2629.31 28.026.13 7.593.22 4.194.61 2.52't01.42 100.61

2.940 2.9571.986 2.0011.949 't.8710.413 0.5130.382 0.4990.393 0.2158.06i! 8.0560.164 0.211

Note-'atomic proportions calculated on the basis of 12 O atoms.

gin to the extreme rim. The variation in zoning patternin these garnets supports the existence of a secondary rim,which is indicated by inclusion patterns and optical ob-servations (cf. Figs. 3a, 5B).

VA mineralization. Garnet from VAl2-B has a spes-sartine component, which increases from X*^: 0.14 atthe core to 0.23 at the interface between primary andsecondary garnet and drops to 0.17 at the outer rim. Thehigh XM, at the interface is interpreted as the result ofresorption of primary garnet during retrogression to bi-otite and chlorite. The decrease to the rims reflects thenormal fractionation of Mn in garnet and is interpretedas an indication ofprograde growth ofsecondary garnet.

TABLE 4, Mineral analyses of orthoamphiboles

KDH5-420Kielder

KDHs-105 KDH5-105

X,, increases from 0. I I (core) to 0. 17 (rim) with no hi-atus at the primary-secondary interface (Fig. 5C).

Amphibole

In this paper, all amphiboles with cations over l5 pfu(23 O atoms) in the M4, octahedral, and tetrahedral siteshave been corrected for Fer*. Orthoamphiboles have beensubjected to minimum Fe3* corrections, whereas horn-blende structural formulae are given as the mean of theminimum and maximum Fer* corrections recommendedby Schumacher (199 l). Compositions of orthoamphi-boles are summarized in Figure 6 and indicate (l) a pos-itive correlation between I6lAl and Na,., (Fig. 6a, 6b) and

Gores Cores

sio,Tio,AlrosFeOMnOMgoCaONa"O

TotalSi{.lAl

TotalrotAlTiFe3+MgFe.

TotalFe2*MnCaNa

TotalNa

TotalMg(Mg + Fe)

50.1 40.088.25

19.37o.71

18.970.380.90

98.807.1350.8658.0000.5190.0080.0834.0240.3665.0001.8560.0850.0580.0012.0000.247o.2470.644

50.000.088.30

19.69o.70

18.680.410.95

98.817.1300.8708.0000.5240.0080.0013.9710.4965.0001.8510.0850.0630.0012.0000.2620.2620.628

54.92no2.31

17.591.31

21.820.280 . 1 1

98.347.7510.2498.0000.094

0.1384.5900.1 785.0001.7600.156o.o420.0311.989

0.0000.703

54.86no

2.4717.741 .47

21.46o.270.16

98.437.7530.2478.0000.164

o.0744.5200.2425.0001.7810.176o.o420.0012.000o.o420.o420.691

54.57nd1.93

17.921.43

21.640.260.09

97.847.753o.2478.0000.076

0.1454.5830.1965.0001.788o.1720.040

42.500.10

20.2919.181.55

14.O40.372.17

100.206.0431.9578.0001.4430.010

2.9770.5705.0001.7100.1860.057o.o472.0000.5530.s530.566

43.88nd

18.8217.921.49

15.030.411.86

99.416.2431.7578.0001.398

3.188o.4145.000'| .7180.1800.0630.0392.000o.4730.4730.599

2.0000.0260.0260.698

50.43 47.16nd nd8.90 13.56

18.25 17.52't.44 1.3818.55 17.800.32 0.340.78 1 .12

98.67 98.887.174 6.6830.826 1.3178.000 8.0000.667 0.948

0.0613.932 3.7590.401 0.2325.000 5.0001.770 1.7830.173 0.1650.049 0.0520.0082.000 2.0000.208 0.3080.208 0.3080.644 0.651

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES IO47

TAELE 4.-Continued

SmouspanSP2-B sM7-82 G2-D

Core Cores Core (profile) Rim

sio,Tio,Alr03FeOMnOMgoCaONaro

TotalSirarAl

Totalr6tAlTiFe3*MgFe2+

TotalFe2 *MnCaNa

TotalNa

Total

Mg/(Mg + Fe)

46.660.28

1 1.0425.180.70

14.000.411.36

99.636.8071 . 1 9 38.0000.7050.0310.0373.0431.1845.0001.8510.0850.064

2.0000.3850.3850.501

VA localityG5-B

46.390.10

15.6518.732.03

14.980.371.34

99.596-5891.4118.0001.2090.011

3.1710.6095.0001 .6160.2440.0560.0842.0000.2850.2850.588

VA12.B

47.33o.42

10.9625.430.55

14.1 00.381 . 1 8

100.356.8461 .1548.000o.7140.04s0.0343.0401 j 6 7s.0001.8750.0660.059

2.0000.3310.3310.500

47.030 . 1 5

11 .0324.810.69

14.640.271 3 5

99.976.8101 .1908.0000.6960.0160.0983.1601.0305.0001.8770.0830.040

2.0000.3790.3790.521

44.530.18

17.6219.382.18

14.260.441.67

100.266.3271.6738.0001.2770.019

3.O210.6835.0001.620o.2620.0670.0512.0000.4080.4080.568

44.130.16

18.0819.152.16

14.160.451.75

100.046.2821.7188.0001 .3160.018

3.0040.6625.0001 .6170.2600.0680.0552.000o.428o.4280.569

44.400.14

17.8618.882.23

14.280.4s1.67

99.916.3171.6838.0001.3120.015

3.0280.6455.0001.6020.2690.0680.0612 0000.4000.4000.574

Core (profile) Rim Cores Rim

sio,Tio,Alro3FeOMnOMgoCaONa.O

TotalSirrrAl

Totalr6rAlTiFg3+

MgFe2*

TotalFe,*MnCaNa

TotalNa

TotalMg/(Mg + Fe)

41.160.16

20.9923.430.35

12.110.412.28

100.895.9082.O928.0001.4580.018

2.5910.9335.0001.8800.0430.0620.0152.0000.6200.620o.479

42.320.13

20.1 923.380.31

12.540.432.15

101 .456.02s1.9758.0001 . 4 1 30.014

2.6610.9125.0001.8720.0370.0650.0262.0000.5670.5670.489

42.820.09

19.0223.000.31

12.770.431.86

100.306.1491.8518.0001.3680.013

2.7050.9145.0001.8220.0380.0700 0702 000o.457o.457o.497

42.660.10

19.3222.790 3 4

12.760.351.84

100.166.1261.8748.0001.3960.011

2.7310.8625 0001.8760.0410.0540.0292.0000.4850.4850.499

43.190.10

18.9223.040.30

12.710.401.88

100.546.1 821 .8188.0001.3740.010

2.7120.9045.0001.8540.0360.0610.0492.0000.4740.4740.496

44.210 . 1 1

17.7222.690.37

13.33o.441.61

100.486.3121.6888.0001.2940.o12

2.8360.8s85.0001.8510.0440.0680.0372.0000.4080.4080.512

47.760.14

10.4220.741.63

16.870.541.29

99.396.8491 . 1 5 18.0000.6100.0150.1513.6070.6175.0001.7200.1 970.083

2.0000.3600.3600.607

48.95 53.170.13 0.087.97 3.54

20.61 20.501.75 1 .52

17.84 20.030.45 0.340.98 0.41

98.68 99.597.O430.9s78.0000.3950.0150.2603.8270.5035.0001 . 7 1 7o.2140.069

2.000o.2720.272

7.5200.4808.0000 . 1 1 00.009o.2404.2220.4195.0001.7660.1820.052

2.000o.1120.1't2

0.634 0.659

Note. atomic proportions on the basis of 23 O atoms; minimum Fe3* correction employed; nd : not determined.

(2) a negative correlation between t6rAl and Mg/(Mg +Fe) calculated with a minimum Fe3+ correction (Fig. 6b,6c). Figure 6a illustrates the separation of orthoamphi-bole compositions into two fields-those in rocks domi-nated by secondary growth of staurolite and sillimaniteand those that have clearly derived from orthopyroxene

[SP2-B, SM7-82, KDH5-420, and KDH5-105 (coresonly)1. The exception is VAl2-B, which strictly belongsto neither ofthese groups. The best interpretation ofthesedata is that they reflect variation in bulk composition.

Kielder. The cores of the large anthophyllite grains thatreplaced or thopyroxene in KDH5-105 have minor

+ Q t z , A b , H 2 O

1048

Fig.4. Projection of mineral compositions from albite, quartz,and HrO. Compositions of orthoamphibole have not been cor-rected for Fe3*. Open symbols are prograde minerals; closedsymbols are retrograde minerals.

amounts of rlAl (0.094-0.164 pfu) and Na (0.026-0.042pfu), whereas the rims are richer in both (16rA1 : 0.667-1.443 pfu; Na : 0.216-0.600 pfu). Rims are less mag-nesian [Mg/(Mg + Fe) : 0.57-0.64] than the cores [Mg/(Mg + Fe) : 0.69-0.711. Hornblende that rims antho-

TABLE 5. Mineral analyses of hornblende

KDH5-105 VA12-A

Core

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

TABLE 6. Mineral analyses of cordierite

KDH5-420 KDH5-105

sio,AlrosFeOMnOMgoNaro

Total

siAIFeMnMgNa

TotalMg(Mg + Fe)

47.86 48.3433.05 33.113.80 3.840.30 0.319.65 9.800.85 0.89

95.51 96.294.986 4.9984.058 4.0350.332 0.3320.027 0.0271 .499 1.5110.173 0.180

11.075 11 .0830.819 0.820

49.04 49.06 49.2033.64 33.42 33.474.65 3.67 3.910.14 0.20 0.25

10.46 10.37 1 0.360.15 0.57 0.60

98.08 97 .29 97.794.988 5.007 5.0034 026 4.020 4.0110.395 0.314 0.3330.012 0.018 0.0211 .583 1 577 1.5710.029 0 .113 0 .119

11.033 11 .049 11 .0580.800 0.834 0.825

Note.-atomic proportions calculated on the basis of 23 O atoms; Fe3+calculated from the mean of minimum and maximum Fep+ corrections.

Note.' atomic proportions on the basis of 18 O atoms.

phyllite is a ferroan pargasite and more magnesian thanits host orthoamphibole [Mg/Mg + Fe),o, : 0.71].

Smouspan. Orthoamphiboles from Assemblage B areless aluminous than gedrite in Assemblage C but show astrong depletion in Na and I6rAl near their rims, accom-panied by an increase in Mgl(Fe + Mg). In AssemblageC, gedrite is strongly zoned (Fig. 6b). Although there issome heterogeneity, rims of primary orthoamphibole inG5-B are enriched in Mg and depleted in Na and Al.Hornblende is more magnesian [Mg/Mg + Fe) : 0.58]than coexisting orthoamphibole.

VA locality. Analyses of orthoamphibole in VAl2-Bshow an increase in Mg/(Mg + Fe) and a decrease in Naand I6rAl from core to rim. Hornblende in VAl2-A (As-semblage D) is ferroan pargasite with minor enrichmentin Fe and Al at the rim.

Cordierite

At Kielder, cordierite analyses range in Mg/(Mg + Fe)from 0.70 to 0.84. The most Mg-rich analyses are fromKDH5-105, where cordierite was replaced by gedrite, sil-limanite, and staurolite. In KDH5-420, cordierite in theunaltered Assemblage A is less magnesian than that inthe orthoamphibole-rich band. This results directly froma continuous shift in Mgl(Mg * Fe).,o to higher valuesas it loses Fe to reaction products with lower Mg/(Mg +Fe). In the Smouspan samples, fresh cordierite in G2-DlMg/(Mg + Fe) : 0.821 has a Mg/(Mg * Fe) ratio similarto a heavily pinitized replacement in G5-B. The latterhas AllSi ratios similar to cordierite, but a substantiallylower total (Humphreys, l99l).

Staurolite

Four analyses from KDH5-105 (Kielder) average 0.51wto/o ZnO and span a range in Mg/(Mg + Fe) of 0.22-0-24. At Smouspan, staurolite from Assemblage C is Zn-free, with Me/(Me + Fe) in rhe range 0.22-0.25. Minoramounts of TiO, and MnO are present. In VAl2-A, pri-mary and secondary staurolite have Mg/(Mg + Fe) :0.23-0.24 in Assemblage D, whereas secondary staurolite

sio,Tio,Al,o3FeOMnOMgoCaONaroKrO

Totalsir4tAl

Total161Al

t l

Fe3*

MgFe2*

TotalFe2*MnCaNa

TotalNaK

TotalMg/(Mg + Fe)

42.65 42.380.28 0.32

18.16 17 .98'17.32 't7.470.15 0 .179.23 9.22

10.83 10.771.70 1 .700.23 0.24

100.55 100.256.063 6.0451 .937 1.9558.000 8.0001.107 1 .0680.030 0.0340.634 0.6841.955 1.9601.274 1.2545.000 5.0000.152 01460.018 0.0201 650 1.6450.180 0 .1892.000 2.0000.289 0.2810.041 0.0430.330 0.3240.578 0.583

42.82 42.28o.44 0.36

16.87 17 0915.87 15 810.92 0.90

10.44 10.2810.14 10 .281 .86 1.810.16 0 .16

99.41 98.976112 6 .0651.888 1.9358.000 8.0000.9s0 0.9540.028 0.0390.857 0.8142.222 2.1980.943 0.9955.000 5.0000.082 0.0880.112 0.1091 .550 1.5800.256 0.2232.000 2.0000.258 0.2800.030 0.0310.288 0.3110.684 0.613

43.590.23

1 7 1 81 4 . 1 90.62

1 1 . 0 110.921.590.16

99.496.1811 .8198.0001.0520.0240.6162.3260.9825.0000.0850.0751.6600.1802.0000.2560.0290.2850.709

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1049

G 5 B I

Fe

\- /----v8<

'*.--.._99-,o

M n

Fig. 5. Compositional profiles in garnet: (A) KDH5-420 (Kielder); (B) SM7-69 and G5-B (Smouspan); (C) VA12-B (VA min-eralization). Values are ratios of the individual elements divided by the sum of Mg + Fe + Ca + Mn.

in Assemblage F has Me/(Mg * Fe) values of 0. l7-0. 18.In addition, the former is Zn-free, whereas staurolite aftercordierite in Assemblage F is mildly zincian. Staurolitein VAl2-B is zincian with Mg/(Mg + Fe) : 0.25.

Plagioclase

Variations in Xo, are found in all three localities. AtKielder, the range in Xo" is small, between 0.29 and 0.31.At Smouspan, Assemblage B has a range of 0.24-0.28,whereas Assemblage C has a range of 0.45-0.48; this dif-ference results from bulk composition. The two VA sam-ples have ranges in Xo^ of 0.35 (core) to 0.41 (rim) inVAl2-A and 0.38 (core) to 0.36 (rim) in VAl2-B. Thegreater variation in VAI2-A is possibly a result of Caenrichment due to the breakdown of hornblende in therock during retrogression.

TABLE 7. Mineral analvses of staurolite

Chlorite

All chlorite in the samples described resulted from ret-rogression of orthopyroxene, orthoamphibole, cordierite,garnet, staurolite, or biotite. All are magnesian ferroanclinochlore. Mg/(Mg + Fe) in KDH5-105 (Kielder) is0.78, midway between that of cordierite and biotite. Asingle chlorite sample from G2-D (Smouspan) has Mg/(Mg + Fe) : 0.75 and is more Fe-rich than cordierite inthe same sample. At the VA locality, Mgl(Mg + Fe).*lies between 0.65 and 0.66 for Assemblage D (chloritederived from biotite and cordierite) and between 0.67and 0.69 for Assemblage F (derived from cordierite alone).

Since Mg/(Mg * Fe).,. > Mg/(Me + Fe).n,, the alter-ation of cordierite to chlorite + staurolite in all threelocalities was accompanied by the breakdown of a suit-

VAI2 B

I: - r , | '\l-r"

t ' '

I

l o: @

r {t -I

( lmm)

KDH5-105 G2-D G5-B VA12-A VA12-B

sio,Tio,Al,o3FeOMnOMgoZnO

Totala i

TiAIFeMnMgZn

TotalMg/(Mg + Fe)

28.63o.47

54.6911.44o.371 .850.51

97.96J . / 5 b

0.o478.4571.2550.0410.3620.050

13.9680.224

28.68o.24

54.711 1 . 1 40 5 12.020.48

97.783.7670.o248.4681.2240.0570.3960.046

13.9820.244

28 090.58

52.8612.390.602.13no

96.6s3.7560.0588.3311.3860.0680.425

14.0240.23s

27.920.44

52.4113.96

nd2.46nd

97.253.7390.0448.2721.584

0.490

14.1170.236

28.390.47

53.9312.970.571 .540.31

98.183.7470.0468.3891 4310.0640.3030.031

14 .01 10 175

27 870.57

53 6613.450.612.37nd

98.s33.6760.0578.3421.4840.068o.467

| 4.0940.239

27.840.62

53.1013.560.482.580.30

98.483.6830.0628.2801.5010.0540.5080.030

14.1 1 80.253

Note. atomic proportions on the basis of 22 O atoms; nd : not determined.

1050

. 0.4mm

c

a

oo o o

0 0s tc lA l t0 1 .5

Fig. 6. Mineral chemistry of orthoamphiboles: (a) Plot ofNa,o, vs. IurAl (per formula unit of 23 O atoms). All analyses havebeen corrected for Fe3* using a minimum Fer* correction. (b)Zoning profile across a primary gedrite from G5-B. (c) Plot ofM/(Mg + Fe) against t6tAl (per formula unit).

able Fe-rich mineral, either amphibole or (more proba-bly) biotite, depending upon the original mineralogy.

RracrroN HISTORy AND pHASE RELATToNSHTps

Seven of the above samples contained cordierite andan amphibole at the metamorphic peak, whereas a further

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

Ca-0.538Na

=p

0 7

Mg

Ivlg*Fe*g 6

Fig. 7. Quaternary projection of the prograde amphibolitefacies Assemblages C, D, and E from Anrr, quartz, and vapor(Spear, I 977). Note that garnet, being slightly calcic, is projectedwithin the tetrahedron, so that the garnet-cordierite tie line inAssemblages C and E passes behind the staurolite-gedrite tieline, which is in the front face of the tetrahedron. The explodeddiagram on the right clarifies these relationships and shows thatthe three assemblages form interlocking (and hence compatible)tetrahedra in this projection.

five contain the nondiagnostic Assemblage B (orthoam-phibole + garnet + plagioclase + biotite * quartz), andare not discussed further. The useful prograde paragenes-es can be summarized as follows:

garnet + orthopyroxene + cordierite (A)

garnet + hornblende * orthoamphibole +cordierite (C)

staurolite + hornblende + sillimanite *cordierite (D)

staurolite + garnet * orthoamphibole *cordierite. (E)

All assemblages coexist with plagioclase + quartz, where-as biotite is present in Assemblages C, D, and E. Thesethree assemblages can be represented in projection fromplagioclase (An35), qtarrz, and vapor (Fig. 7). This sug-gests that Assemblages C, D, and E all represent peakamphibolite facies assemblages formed at similar pres-sures and temperatures in rocks of slightly different bulkcomposrtron.

Retrograde parageneses are subassemblages ofthe totallist of retrograde minerals in a given rock. They are placedtogether on the evidence of spatial proximity along grain

Al-2 077Na

bG 5 B

NuT -i l

, . . - \ ' t

f : r -+--1r - - - - - . .1.

Mg,/( Mg+Fe2+)

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

TABLE 8. Mineral analyses of plagioclase

1051

VA12-A

KDH5-420 KDH5-105 SP2-B sM7-82 G2-D VA12.B

sio,Alro3CaONaroK.o

Total

EO nO25.998.056.730.05

99.912.6401.3690.3850.5830.0034.9800.396

61.6523.736.228 .14no

99.742.7481.2460.2970.704

4.995

0.297

62.4023.40s.848.280.09

100.012.7691.2240.2780.7120.0054.9880.279

62.8223.O75.228.880.09

100.082.7811.2040.2480.7620.00s5.0000.244

56.0726.199.815.88nd

97.952.5701 .4150.482o.522

4.9890.480

56.5425.919.356 .19nd

97.992.5911.3990.4590.550

4.9990.455

58.3625.73

6826.401 . 1 0

98.632.6441 .3740.3310.5620.0644.9750.346

58.4325.548.516.72nd

99.202.6381.359o.4120.588

4.9570.412

60.1924.637.307.14nd

99.262.6971.3010.3500.620

4.9680.361

5l

AICaNaK

Total

Xo"

Note.'atomic proportions on the basis of eight O atoms; nd : not determined

boundaries or because they constitute a pseudomorph (forexample after cordierite):

ered with chlorite in VAl2-A. Pseudomorphing of cor-dierite by kyanite-bearing assemblages in the same sam-ple can be described by the analogous reactions:

cordierite + HrO : kyanite + chlorite + quartz (3)

and

cordierite + biotite + HrO : kyanite+ muscovite + chlorite + quartz. (4)

The alteration of cordierite to gedrite in G5-B is de-scribed by

cordierite + HrO: gedrite * staurolite + quartz (5)

and the univariant reaction

cordierite + garnet + HrO: gedrite + staurolite + quartz (6)

which produce intergrowths of staurolite and gedrite alongthe grain boundaries of cordierite. Locally, twinned pla-gioclase is replaced by veinlets of gedrite and stauroliteeuhedra on the grain boundaries and is described by

cordierite + albite + HrO: gedrite * staurolite + quartz. (7)

In VAI2-B (Assemblage E), the opposite reaction isinferred from the corona growth ofplagioclase from pri-mary staurolite:

gedrite * staurolite: albite + garnet + chlorite + HrO. (8)

At Kielder, the local coexistence ofgedrite and sillimaniteas small grains along grain boundaries of cordierite maybe due to the reaction

cordierite + HrO: gedrite + sillimanite + qtrartz. (9)

This is usually regarded as a higher pressure assemblage(Harley, 1985; Visser and Senior, 1990), with kyanite

staurolite + chlorite + kyanite + quartz

staurolite * orthoamphibole * quartz +chlorite

(A*)

(B*)

hornblende + sillimanite * staurolite + ouartz+ orthoamphibole (C*)

sillimanite * orthoamphibole * quartz (D*)

orthoamphibole + chlorite + quartz (E*)

chlorite + biotite + quartz. (F*)

The reactions that link prograde and retrograde assem-blages involve transitions from the stability fields of or-thopyroxene, cordierite, hornblende, and sillimanite tothose of staurolite, chlorite, and kyanite. Although thedetail ofeach reaction differs, these transitions in broadterms involve a movement from lower pressures andhigher temperatures to conditions of lower temperaturealong a near-isobaric P-Z trajectory.

Interpretation of textures in rockswithout hornblende

The following reactions are interpreted from the tex-tural evidence in samples either with a single orthoam-phibole or no amphibole at all. The widespread replace-ment of cordierite can be described by the reaction

cordierite + HrO: staurolite + chlorite + quartz (l)

but is problematical because chlorite and staurolite areboth less magnesian than the reactant cordierite. The tex-ture is better represented by the KFMASH univariantreactron

cordierite + biotite + HrO: staurolite + chlorite * muscovite + quartz (2)

which is supported by biotite inclusions in cordieritepseudomorphs and random flakes of white mica interlay-

r052

Fig. 8. Slopes of continuous reactions in FMASH predictedfrom the Holland and Powell (1990) data set. Products ofreac-tions interpreted to have occurred on textural grounds are inboxes. See text for discussion.

commonly being the stable aluminum silicate, althoughHudson and Harte (1985) place the reaction aI 4-6 kbar.

In Smouspan samples G2-D and G5-B, chlorite sheaveshave grown across other retrograde products. This finalstage of rehydration can be described in the FMASH con-tinuous reactions

cordierite + gedrite + HrO: chlorite + quartz

staurolite + gedrite + HrO: chlorite + quarlz

(10)

and

( 1 1 )

and the univariant FMASH reaction

cordierite + gedrite + HrO: staurolite * chlorite + quartz. (r2)

Reactions that are continuous in FMASH (Reactionsl, 3, 5, 9, 10, I l) have been plotted to scale in Figure 8to illustrate the constraints placed upon the retrogradepath. Predictably, the reactions involving chlorite requirea proportionately higher molar quantity of HrO and thushave steeper slopes. Reactions interpreted to have oc-curred last in the retrograde sequence (Reactions l0 andI 1) have the steepest slopes ofall. Although chlorite-pro-ducing reactions place little constraint upon the retro-grade path, the shallower reactions (Reactions 5 and 9)are consistent with an isobaric cooling path.

In all three localities, the regrowth ofgarnet from ged-rite + staurolite (G5-B), gedrite + chlorite (VAl2-B), oranthophyllite + biorite (KDH5-420, v55-1050) is inter-preted as a later prograde metamorphic event. The slopesof the relevant FMASH reactions

staurolite + gedrite: garnet + qxartz + HrO

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

and

chlorite + gedrite: garnet + qluartz + HrO (14)

are negative, necessitating a shift to higher temperatureto produce garnet from the hydrous reactants. It is clearthat garnet cannot be produced along the same P-7 vec-tor that produced staurolite, chlorite, and gedrite fromcordierite (Fig. 8).

Interpretation of textures in rocks with hornblende

Prograde hornblende-bearing assemblages (Assem-blages C and D) formed at conditions similar to thosethat produced the hornblende-free Assemblage E (Fig. 7).Apart from the grain-boundary replacement of horn-blende by staurolite in Assemblage D, calcic amphiboleproduced at the metamorphic peak is not altered duringsubsequent retrograde metamorphism.

Retrograde hornblende, however, occurs in KDH5- 105as rims on cordierite and orthoamphibole, together withstaurolite, sillimanite, chlorite, and orthoamphibole ofdifferent composition to that of the substrate. Generalreactions describing the retrograde textures are

cordierite + plagioclase + anthophyllite + HrO: sillimanite * gedrite * hornblende

+ staurolite + quartz (15)

and

cordierite + plagioclase + anthophyllite + HrO: staurolite + gedrite + chlorite

* hornblende + quartz. (16)

Both reactions occur necessarily below the orthoamphi-bole solvus (i.e., below 600 "C at pressures of 4-6 kbar;Spear, 1980).

Stability of hornblende with staurolite and sillimanite

Staurolite + hornblende + sillimanite occurs both asa prograde and retrograde assemblage at Copperton. Itoccurs in rocks of aluminous bulk composition and henceis not related chemographically to the occurrence of ky-anite + staurolite + hornblende recorded both by Sel-verstone et al. (1984) and Helms et al. (1987) in rocks ofdemonstrably amphibolitic composition. As a progradephase it is restricted to the edge of domains containingAssemblages D (sillimanite- and staurolite-dominated)and F (cordierite-dominated). The presence of plagioclasein Assemblage D suggests that hornblende is not simplyan extra phase stabilized by Ca and Na but is part ofanunusual bulk composition resulting from premetamor-phic mixing of semipelitic and K-poor peraluminous do-mains in VAl2-A.

The replacement of hornblende by secondary fine-grained staurolite in Assemblage D is problematical, sincehornblende * staurolite is already stable. Examination ofthe boundaries of hornblende srains in VAl2-A reveals( 1 3 )

minor alteration of hornblende to an assemblage includ-ing chlorite, biotite, and quartz (Fig. 2H), whereas themargins of staurolite in the same sample have been al-tered to chlorite (Fig. 2G). Thus secondary staurolite grewfrom partly chloritic substrates near hornblende whenmetamorphic conditions were lower than those requiredto stabilize staurolite + hornblende + sillimanite, butabove those responsible for the stability ofchlorite. Sec-ondary staurolite in VAI2-A thus belongs to the samemetamorphic (reheating) event that produced garnet rimsin VAl2-B and euhedral garnet upon gedrite * staurolitein G5-B.

The retrograde occurrence in KDH5- 105 is fine grainedand may not represent an equilibrium assemblage. Theassemblage formed in the same domains as sillimanite +gedrite, but, as with the latter, chlorite is normally pres-ent, and the division of minerals into those resulting fromretrograde hydration and those generated by reheating isnot clear.

Pnnssunn-TEMPERATURE EyoLUTIoN

The P-T history of orthoamphibole-bearing rocks atCopperton can be interpreted using the petrogenetic gridof Hudson and Harte (1985) for KrO-poor alumrnouspelitic compositions (Fie. 9). The basis for this grid isfully discussed by Hudson and Harte; its use in this paperis more appropriate than the grids of Spear (1978) andSpear and Rumble (1986), since the bulk compositionsare closer to those of pelites.

Amphibolite facies retrograde path

In amphibolite facies parageneses (excluding Kielder),the peak conditions lie between the [CHL], (CHL,ALS),and (OAM,CHL) invariant points. In Figure 9, this cor-responds to the temperature range 600-625'C at about4 kbar, the same range in temperature as calculated forthe nearby Annex pelites (Humphreys and Scheepers,1993). The retrogression can largely be attributed to iso-baric cooling through the divariant field staurolite + or-thoamphibole + cordierite (by means of Reactions 5 and6) into the chlorite stability field by Reactions l2 and 3.The shallow slope of Reaction 5 is the most importantconstraint upon an isobaric path rather than one thatsimply returns to the origin directly. The formation ofkyanite by Reaction 3 suggests that the Hudson and Harte(1985) grid is only partly correctly placed with respect tothe aluminum silicate triple point. However, the produc-tion of retrograde kyanite in VAl2-A, which containsprimary sillimanite, is further evidence for isobariccooling.

Isobaric cooling at Kielder

Cooling of the granulite facies Assemblage A at Kielderat pressures >5.5 kbar produced the retrograde assem-blage sillimanite + orthoamphibole + quartz (Assem-blage D*) by Reaction 9 in sample KDH5-105. Isoplethsof Reaction 9 are the shallowest of the divariant reactions

I 053

discussed here (Fig. 8), and on the Hudson and Harte(1985) grid they would connect the univariant reactions

sillimanite -+ orthoamphibole: garnet * cordierite + quarlz + HrO (17)

and

staurolite + cordierite + HrO: sillimanite f orthoamphibole * quartz (18)

with a near-isobaric slope. Thermobarometric estimatesput peak conditions at pressures higher than those de-fined by Reaction 17, whereas the absence of garnet inKDH5-105 in either prograde or retrograde assemblagesindicates that cooling was isobaric to near (above?) theinvariant point at which garnet is absent, where chloriteand staurolite resulted from further breakdown of or-thoamphibole and cordierite.

Reheating path

The reheating path indicated by the renewed growth ofgarnet in all three localities must necessarily leave thechlorite stability field. The path indicated on Figure 9 isconstrained by the observation that secondary cordierite* orthoamphibole is not produced anywhere from stau-rolite + chlorite (Reaction l2).

Kinetic factors

The difference in grain size between large progradegrains and commonly small retrograde euhedra supportsthe dominance of nucleation over grain growth duringcooling. This is indicative of the undercooling of peakassemblages into lower Zstability fields prior to the rapidinflux of HrO. Some stable equilibria may thus have beencrossed without leaving any trace of the reaction in therock record. It is therefore to be expected that the se-quence of reactions predicted from the petrogenetic griddoes not in every case match the sequence determinedfrom the textures.

TpcroNrc TMPLICATIoNS

The metamorphic sequence described in Table 2 mir-rors the metamorphic evolution of pelites from the near-by Annex deposit (Humphreys and Scheepers, 1993).However, retrogression of high-grade cordierite-bearingassemblages to various combinations of garnet, stauro-lite, biotite, chlorite, kyanite, and sillimanite is seen upto 300 km along strike to the northwest (Humphreys,1991) within the Kakamas Terrane (Fig. I, inset). Thetypical postpeak pattern throughout the Kakamas Ter-rane is (l) a period ofisobaric cooling from peak pres-sures of 4-6 kbar followed by (2) rehydration within thekyanite or the lower temperature part of the sillimanitestability fields, and (3) regrowth of garnet, staurolite,and sillimanite from either hydrated (biotite + chlorite+ kyanite or sillimanite) or unaltered (cordierite, ortho-pyroxene) substrates. On the bounding thrusts of theKakamas Terrane, there is clear field evidence linking

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

Pkbar

6

1054 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

FASH_ MASH

FMASH

,/,

Jcnt € (teu--- -s\\

peak metamorphic condit ions(VA & W Smouspan)

Grt Alsrd

11'-t/

v" (OAM.CHL)

;!i1 K-

- ---sr/' 4n}----

Tt 700Fig. 9. Pressure-lemperature trajectories superimposed upon an FMASH petrogenetic grid after Hudson and Harte (1985). The

orthoamphibole solvus is after Spear (1980). The lower pressure paths are for west Smouspan (WS) and the VA mineralization,with the replacement of Als + Chl by St included in the path marked VA. The grid and rhe paths derived from it are misplacedwith respect to the AlrSiO, diagram ofHoldaway (1971), since the occurrence ofkyanite cannot be accounted for using the grid.The paths are thus representative and may be I kbar less than therr tme pressure.

650600550

late normal faulting on previous thrust surfaces to thedevelopment of hydrous retrograde assemblages in gran-ulites (Humphreys, 199 l). Spatially, the effects of re-hydration diminish with distance from shear zones withnormal or strike-slip movement. At the Prieska mine,retrograde development of kyanite, phlogopite, musco-vite, staurolite, and chlorite from gneisses containingcordierite + perthite + sillimanite of the alteration zoneis zoned around a normal faulting that cuts the ore body(Humphreys et al., 1988a). These lines of evidence sup-port the interpretation that retrogressing fluids enteredthe rock pile along specific conduits during a period ofextension, once the crust had cooled to near or belowthe sil l imanite-kyanile transition.

The regrowth of garnet, staurolite, and sillimanite,however, is not confined to pelites that have undergone

retrogression. Many granulite facies pelites from through-out the Kakamas Terrane preserve small euhedral garnetswithin unrehydrated cordierite grains, indicating that thefinal metamorphic event is regional in extent. The re-growth of these minerals is interpreted as the result ofheating due to crustal overthickening by adamellite sillsand voluminous rift-related lavas above the present levelof erosion, remnants of which are exposed within and onthe eastern margin of the Kakamas Terrane.

AcrNowr.rncMENTS

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MeNuscrrsr REcETVED JeNunnv 13, 1992M.c.Nuscnrpr AccEprED ApnIl 30, 1993

HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES