Metamorphic Evolution of Amphibole-Bearing Aluminous

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

Metamorphic evolution of amphibole-bearingaluminous gneissesfrom the Eastern Namaqua Province, South Africa

Huw C. flurvtprtnnvsx Precambrian ResearchUnit, 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 isobaric cooling.Peak metamorphicassemblages equilibrated at 600-820 "C and 5-6.5 kbar and include 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- claseand quarlz, whereasbiotite is presentin the amphibolite facies.Cordierite and horn- blende have partially or completely altered to staurolite-, chlorite-, and gedrite-bearing assemblagesin rehydration reactions,whereas sillimanite-bearing rocks developedkyanite on cooling. Interpretation of these textures using a petrogeneticgrid in FMASH suggests a cooling path that is isobaric at pressuresabove that of the AlrSiO, triple point. The latest textures in the rocks reflect a regrowth of garnet over chlorite + quartz and gedrite + staurolite, indicative of a final period of reheating.This is consistentwith the metamorphic evolution of pelites along the strike of the amphibole-bearingrocks that show regrowth of garnet and staurolite from retrogradechlorite + muscovite substrates.Aluminous gneisses of the Copperton Formation preserveprograde and retrogradeexamples of the uncommon assemblagestaurolite + sillimanite + hornblende. The progradeassemblage occurs at the interface of a hornblende-bearingsemipelitic schist and a KrO-poor peraluminous gneiss; the retrograde assemblageoccurs as grain-boundary minerals at the margins of altered cordierite and plagioclasegrains.

InrnonucrroN pelitic compositionsand are describedin Cornell et al. (1992\. varieties are found in the Recent studiesof aluminous rocks containingamphi- Amphibole-bearing variety metamorphic grades. boles have revealed the wealth of pressure-temperature Copperton Formation at a of * horn- information availablefrom a considerationof their tex- They contain the unusual assemblagesillimanite (+ the more common tures and phaserelationships (Spear, 1977, 1978; Rob- blende + staurolite cordierite)and gedrite + garnet + cordierite * staurolite insonet al., 1982;Harley, 1985;Hudson and Harte, 1985; assemblages garnet * + gedrite. Spearand Rumble, 1986;Baker et al., I 987; Schumacher and + cordierite hornblende and Robinson, 1987;Visser and Senior, 1990).These in- GBor-ocrc sErrING clude the placement of peak temperaturesand pressures unit using calibrated petrogenetic grids in FeO-MgO-AlrO3- The Copperton Formation is a northwest-striking gneisses to the tec- SiO,-H,O (FMASH) or NarO-CaO-FeO-MgO-AlrOj- of banded and interlayered adjacent Belt, with an age be- SiOr-HrO (NCFMASH) and the constructionof P-Ipaths tonic boundary between the Kheis (Cornell 1986), and the from the successiveappearance of stable or metastable tween 3.0 and 1.7 Ga et al., gneisses Terrane, with an age between assemblageswithin a sample or group of samples.The of the Kakamas (Theart, Burger,1983). reactiontextures and mineral compositionsin aluminous 1.6and 1.0Ga 1985;Barton and is the single-zircon gneissesand schistsof the Copperton Formation in Cape The Copperton Formation dated by (Cornell 1990b)and is Province, South Africa, provide an opportunity to con- U-Pb method at 1.29 Ga et al., geochemical grounds as an struct a P-T path from such amphibole-bearingrocks. interpreted on tectonic and Namaqua The aluminous rocks at Copperton include those with island arc sequenceformed at the onset of the (Geringer 1986). The Copperton amphibole and thosewithout. The latter approximateto orogenic cycle et al., Formation was deformed and metamorphosedat 1.22- | .20 Ga and remetamorphosedat I . I 0- I .08 Ga (Cornell * Presentaddress: 6l Pendre Gardens. Brecon. Powvs. Wales et al., 1990a).The texturesdescribed in this paper date LD3 gEP, U.K. from these two periods of metamorphism. 0003-o04x/93109l 0-l 04l $02.00 l04l ro42 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

feldspar + orthopyroxene * cordierite that disrupted a pseudomorphedbiotite schistosity. Both granulite-facies metamorphism and accompanyingmelting occurredafter penetrative deformation. The three orthoamphibole-bearingspecimens (KDH5- 95, KDH5-105, and KDH5-420) are partially or fully struisbult retrogradedgranulites with heterogeneousdistribution of Member minerals.KDH5-95 and KDH5-105 have largesheaves of clove gray anthophyllite with gedrite or hornblende margins,coarse intergrowths of cordierite and qtrartz, and Copper coarse patches of plagioclase.Important retrograde tex- Mines tures in thesetwo rocks include (l) the replacementof cordierite grain boundaries by staurolite + chlorite + l-r gedrite (Fig. 2E), (2) the occurrence,at cordierite grain Karoo cover boundaries, of the assemblagesillimanite + gedrite as small euhedral grains (Fig. 2A), and (3) the coexistence of hornblende + staurolite + sillimanite as a grain boundary assemblagebetween plagioclase and cordierite. In l, the product grains are euhedral and postdate what remains of the structural fabric of the original granulite. Chlorite overgrows staurolite and is the final mineral to Kakamas form in the retrograde assemblage. lerrane Decompositionof biotite and cordieriteto the assem- blage muscovite + kyanite * staurolite * chlorite is seen in the amphibole-freegneiss KDH5-544 and offerscor- ollary evidence that the P-T path just after the peak of Fig. l. Local geology of the Copperton region, from Hum- metamorphism near Copperton passed across the silli- phreys et al. (1988b). Copperton is at the southeastcorner ofthe Kakamas Terrane (inset). manite-kyanitecurve. In KDH5-420, orthoamphibole has grown from As- semblageA along a band 2 mm wide, according to the reaction garnet + cordierite + orthopyroxene + potas- Loc,cl GEoLocy AND pETRocRApHIC sium feldspar+ HrO : biotite * anthophyllite+ quartz, DESCRIPTIONS representingthe earliest retrogressionof granulite facies The samples described come from the wall rocks of assemblagesat Kielder. Subsequentgrowth of secondary three zonesof pyritic mineralization near the prieska Zn- garnet grows across retrograde biotite and anthophyllite Cu mine. All the sampleswere collectedfrom within l5 (Fig. 3a). km of one another (Fig. l) and are found on the farms Kielder (three samples)and Smouspan (seven samples) Smouspanbiotite gneisses and near the VA Cu-Fe deposit (three samples). Peak Biotite * orthoamphibole-bearinggneisses with garnet metamorphic conditions lie in the granulite facies at occur in the VogelstruisbultMember of the Copperton Kielder and in the eastern part of Smouspanand in the Formation on the farm Smouspan. The Vogelstruisbult mid-upper amphibolite facies for the remainder. Mineral Member is a banded sequenceof amphibolite, calc-sili- assemblagesare in Table l. cate gneiss,biotite-hornblende gneiss, and pelite that forms the stratigraphic hanging wall to the Prieska mine Kielder deposit ore body. Five biotite-rich and two cordierite-rich gneiss- The Kielder Zn-Pb mineralization is hosted by mag- eswere sampled by bore holesSM6, SM7, SP2,V55, G2, nesian,mafic, and intermediategranulites (Gorton. 1982). and G5. Two paragenesescan be identified: Temperaturesare estimatedat 750-850.C by the garnet- garnet + + + plagioclase orthopyroxenethermometer of Harley (1984),and pres- orthoamphibole biotite suresof 5.5-6.7 kbar are estimatedfrom the garnet-or- + qtrartz (+ orthopyroxene) thopyroxene-plagioclase-quartzbarometer of Powell and (SM6-a95;SM7-69; SM7-82; SP2-B; V55-1090) (B) Holland (1988). The peak metamorphic assemblagein cordierite + orthoamphibole + plagioclase magnesiangranulites is + biotite + quartz (+ garnet + hornblende) orthopyroxene * cordierite * plagioclass + garnet (G2-D;c5-B). (C) -r qvarlz + hercynite spinel (KDH5-420). (A) AssemblageB was derived from a granulite-faciespre- Dehydration melting at the peak of metamorphism re- cursor, and in some cases,orthopyroxene is still extant sulted in crosscutting pods of plagioclase * potassium as heavily corroded and altered porphyroblasts.The pre- HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1043

TABLE1. Mineralassemblages

Sample Qtz Ky opx chl Opaque Kielder KDH5-95 x x x x Mag KDH5-105 xx x x xx x ps xxPy KDH5-420 xx / x x 7 x x Mag Smouspan v55-1090 13 25 6 24 31 Mag sM6-495 125 2 I 49 12 Mag x Mag sM7-69 xx x x 't2 sM7-82 47 3 I 30 Mag SP2-B xx x I x ps Mag G2-D 39 33 3 4 23 ? 15 2 Py G5.B 35 13 10 ps 10' 14? ? 15 2x Mag VA locality VA11-H xx ps x x xPy VA1lJ xx' ps xx x x xPy VAl2.A xx' ps 1x x x xxPY VA12.B xx x' ps xx x x Mag : Nofe: ps : pseudomorphs atler mineral observed; x : mineral present as primary phase; x: mineral present as secondary phase; x' mineral present as primary and secondary phases; ? : identity not confirmed. Numerals indicate modal percentages.Mineral abbr€viationsare after Kretz (1983).

dominant 52 fabric is definedby alignedbiotite and an- peraturesof 580-620 .C (calibrationsof Dasguptaet al., thophyllite prisms, which encloselarge poikiloblastic gar- 1991, and Williams and Grambling, 1990,with correc- net typical of this assemblage.The large primary garnet tions for Mn in garnet).A maximum pressureof 6.1 kbar grains contain quartz and magnetite but neither ortho- (at 580 'C) is estimated from a single barometric calcu- pyroxenenor orthoamphiboleand are assumedto have lation using the garnet-plagioclase-kyanite-quartzbarom- grown early in the parageneticsequence. A garnet sample eter of Powell and Holland (1988). from this assemblageyields a garnetgrowth ageof l20l Amphibole-bearing assemblagesoccur in two bore m.y., as determinedfrom Sm-Nd values obtained from holes,VAl l and VAl2, and include: the garnetand whole rock (Cornellet al., 1986).A second generation of garnet grows locally over biotite and or- cordierite * hornblende + sillimanite thoamphibole. * staurolite+ plagioclase+ quartz AssemblageC contains abundant retrograde staurolite (vAl2-A) (D) euhedra on gedrite-cordierite and cordierite-cordierite grain boundaries(G2-D). In G5-B, theseare intergrown, and with gedrite replacing cordierite * plagioclase(Fig. 2B, cordierite + garnet + orthoamphibole 2C). G5-B contains primary gedrite and hornblende,as * staurolite * plagioclase+ quartz well as smaller grains of secondary gedrite. The earlier (vAl2-B; VAll-J). (E) generationofgedrite is coarserand more equant than the later, retrograde, grains, many of which are fibrous or In addition, VAl2-A containsdomains where the assem- rhomb-shaped. Secondarygarnet has grown over retro- blage grade gedrite-stauroliteintergrowths (Fig. 2D). In G2-D, cordierite + phlogopite/biotite + quartz (F) sillimanite occurs with gedrite on cordierite grain bound- aries, but it is unclear whether sillimanite and gedrite was stableat the peak of metamorphism.Assemblage D were stable together. (Fig. 3b) contains primary staurolite and sillimanite in- tergrown with rare grains ofhornblende, surrounded par- VA mineralization tially by intergrowths of former cordierite and plagio- The VA mineralized zone (Fig. 1) is separatedfrom the clase. The occulTence of all five minerals together is biotite gneissesof Smouspan by the Klipgatspan shear restricted to the interface of domains containing Assem- zone, I km wide. It lies along the strike from the Annex blagesD and F. Hornblende is overgrown by sheavesof deposit, an uneconomiccopper iron sulfide deposit whose chlorite and subsequentlyby small grains of secondary pelitic wall rocks share a similar metamorphic history to staurolite (Fig. 2H), whereascordierite has been replaced those of the VA locality (Humphreys and Scheepers, by varying proportions of staurolite, chlorite, quartz, and 1993).The host sequenceof both sulfidezones is similar white mica (Fig. 2F). An outer rim of cloudy staurolite and contains amphibolites, quartz-rich gneisses,biotite seen in AssemblageD is separatedfrom the grain cores gneisses,and hornblende-biotitegneisses. Garnet-biotite by a narrow band of quartz inclusions a few tens of mi- thermometry from the Annex pelites indicates peak tem- crometers from the grain boundary (Figs. 2G, 3b). This 1044 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

F _ HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1045

Tlale 2. Sequenceof mineralparageneses

Smouspan VA mineralization Primary assemblage Crd+Opx+Pl crd+oam+ Pl+ crd+st+ Pl+ Bt [+ Grt, Hc, Qtz, Qtz (+ Grt, Hbl, + Otz (+ Hbl, Sil, Bt (rarel opx) Oam, Grt) Resultantassemblages on cooling/rehydration Oam + Chl Oam + Sl Ky+Chl+?Ms Sil + Oam st + chl Chl + Qtz sit+st+Hbl sit + chl st + chl Minerals grown upon reheating Grt (on Oam + Bt) Grt (on Oam + St) Grt (on Chl + B0 St (on Chl + Bt+ Hbt)

rim is resorbed against chlorite and thus predates the growth of secondarystaurolite, which grew subsequently acrosschlorite and hornblende (Table 2). In AssemblageE, primary garnet is intergrown with gedrite and staurolite; patches of fine-grained secondary micas, 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 before a secondgeneration of garnetformed as a rim upon the resorbedgarnet cores. Fig.3. Linedrawings oftextures in amphibole-bearingrocks: Cordierite in AssemblageF has broken down to form (a) Overgrowthsof second-generationgarnet upon preexisting acicular grains of kyanite, together with staurolite and coresin samplesVAI2-B (left)and G5-B (center,right). TU is chlorite. This is a key petrogeneticindicator and is cor- the texturalunconformity that representsthe hiatuscaused by prior (b) related with the retrograde growth of kyanite after silli- resorptionofthe core to overgrowthofthe rim. Stable assemblageof staurolite, sillimanite, and staurolite in VAl2-A. manite in the Annex pelites(Humphreys and Scheepers, l 993).

Mrxrnar, coMPosrrroNs potential was 15 kV, with a beam current of 20 nA for Minerals were analyzed on a Cameca Camebax elec- feldsparsand sheet silicatesor 40 nA for other minerals. tron probe microanalyzerhoused at the Department of Raw data were reduced using the method of Bence and Geochemistry,University of Cape Town. Accelerating Albee (1968).Mineral analysesare listed in Tables 3-8. The projection of mineral compositions from quartz, al- bite, and HrO is shown in Figure4. (- Garnet Fig. 2. Photomicrographsof textures from amphibole-bear- Kielder. Zoned garnet from the orthoamphibole-rich ing aluminous gneissesoccurring in the Copperton Formation. band in KDH5-420 has an outer rim that is distinct Scale bars : 0.5 mm. (A) Replacement of cordierite (Crd) by chemically and optically from the interior. A central core sillimanite (Sil), chlorite (Chl), and gedrite(Ged): KDH5-105. which (B) Replacementof cordierite (Crd) by gedrite (Ged) and srau- showsincreasing X*, toward 0.35 at the inner rim, rolite (St); fibrous sillimanite also occurs on grain boundaries: is succeededby a decreasein the outer rim to 0.23. The G2-D. (C) Alteration of plagioclase(Pl) and cordierite to gedrire opposite effectis seenin Xr,. X.uis asymmetric, reflecting and staurolite; gedrite has grown along plagioclasetwin planes: X", on one limb and Xr, on the other; Xr" increasesto G5-B. (D) Regrowth of euhedral garnet (Grt) upon a gedrire- both rims (Fig. 5A), indicating partial resorption of the staurolite substrate:G5-B. (E) Euhedral grains of orthoamphi- core before the rim grew. bole (Ged) resulting from the breakdown ofcordierite * ortho- Smouspan. Garnets in AssemblagesB and C have pyroxene:KDH5-105. (F) Alteration of cordieriteto kyanite (Ky), broadly similar profiles, characterized by flat cores, in- chlorite (Chl), and quartz (Qtz): staurolite (St) appearsto have creasingXr, toward the rims, and a decreaseat the outer grown later from chlorite: VA 12-A. (G) Intergrowh of silliman- parts the garnet (Fig. 5B). XF"varies in an inverse man- ite (Sil) and staurolite (St); staurolite has cloudy poikiloblastic of profiles vary nonsyste- margins and is replaced by chlorite and biotite in the center of ner to Xr' whereasXr" and X." the photograph:VAl2-A. (H) Hornblendeporphyroblast (Hbl) matically. In G5-B, X." is constant in the core before with margins corroded by chlorite (Chl) and biotite (Bt) and decreasingat the extreme edge and is broadly mirrored subsequentlyovergrown by staurolite (St): VAl2-A. by variations in Xr., which increasefrom near the mar- t046 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

TlaLE 3. Representativemineral analyses of garnet

KDH5-420 SM7-82 G5-B Core sio, 38.65 37.96 37.55 38.05 38.24 37.42 37.59 36.96 37.03 Alr03 22.48 22.14 21.75 21.98 22.01 2't.75 21.98 21.19 21.26 FeO 27.06 30.50 29.91 33.29 30.71 32.74 33.12 29.31 28.02 MnO 1.92 2.85 4.33 2.38 2.08 1.67 1.46 6.13 7.59 Mgo 8.68 6.08 4.38 5.03 6.14 5.12 4.64 3.22 4.19 CaO 2.81 1.87 3.28 1.30 2.47 2.45 2.65 4.61 2.52 't01.42 Total 101.60 101.40 101.20 102.03 101.65 r01 .15 101.44 100.61 Si 2.950 2.956 2.960 2.972 2.968 2.947 2.954 2.940 2.957 AI 2.022 2.O32 2.O20 2.O23 2.013 2.019 2.035 1.986 2.001 Fe 1.727 1.986 1.972 2.175 1.994 2.157 2.176 1.949 't.871 Mn o.124 0.188 0.289 0.158 0.137 0.111 0.097 0.413 0.513 Mg 0.987 0.706 0.515 0.585 0.710 0.601 0.543 0.382 0.499 Ca 0.230 0.156 o.277 0.109 0.206 o.207 o.223 0.393 0.215 Total 8.040 8.024 8.033 8.022 8.028 8.O42 8.028 8.06i! 8.056 Mg/(Mg + Fe) 0.364 o.262 o.207 0.212 0.286 0.218 0.200 0.164 0.211 Note-'atomic proportions calculatedon the basis of 12 O atoms.

gin to the extreme rim. The variation in zoning pattern X,, increasesfrom 0. I I (core)to 0. 17 (rim) with no hi- in thesegarnets supports the existenceof a secondaryrim, atus at the primary-secondary interface (Fig. 5C). which is indicated by inclusion patterns and optical ob- servations(cf. Figs. 3a, 5B). Amphibole VA mineralization.Garnet from VAl2-B has a spes- In this paper,all amphiboleswith cationsover l5 pfu sartinecomponent, which increasesfrom X*^: 0.14 at (23 O atoms) in the M4, octahedral,and tetrahedral sites the core to 0.23 at the interface between primary and have been correctedfor Fer*. Orthoamphiboleshave been secondarygarnet and drops to 0.17 at the outer rim. The subjected to minimum Fe3* corrections, whereas horn- high XM, at the interface is interpreted as the result of blende structural formulae are given as the mean of the resorption of primary garnet during retrogressionto bi- minimum and maximum Fer* correctionsrecommended otite and chlorite. The decreaseto the rims reflects the by Schumacher(199 l). Compositions of orthoamphi- normal fractionation of Mn in garnet and is interpreted bolesare summarizedin Figure 6 and indicate(l) a pos- as an indication ofprograde growth ofsecondarygarnet. itive correlation between I6lAl and Na,., (Fig. 6a, 6b) and

TABLE4, Mineralanalyses of orthoamphiboles

Kielder KDH5-420 KDHs-105 KDH5-105 Gores Cores sio, 50.14 50.00 54.92 54.86 54.57 42.50 43.88 50.43 47.16 Tio, 0.08 0.08 no no nd 0.10 nd nd nd Alros 8.25 8.30 2.31 2.47 1.93 20.29 18.82 8.90 13.56 FeO 19.37 19.69 17.59 17.74 17.92 19.18 17.92 18.25 17.52 MnO o.71 o.70 1.31 1.47 1.43 1.55 1.49 't.44 1.38 Mgo 18.97 18.68 21.82 21.46 21.64 14.O4 15.03 18.55 17.80 CaO 0.38 0.41 0.28 o.27 0.26 0.37 0.41 0.32 0.34 Na"O 0.90 0.95 0.11 0.16 0.09 2.17 1.86 0.78 1.12 Total 98.80 98.81 98.34 98.43 97.84 100.20 99.41 98.67 98.88 Si 7.135 7.130 7.751 7.753 7.753 6.043 6.243 7.174 6.683 {.lAl 0.865 0.870 0.249 0.247 o.247 1.957 1.757 0.826 1.317 Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 rotAl 0.519 0.524 0.094 0.164 0.076 1.443 1.398 0.667 0.948 Ti 0.008 0.008 0.010 Fe3+ 0.083 0.001 0.138 o.074 0.145 0.061 Mg 4.024 3.971 4.590 4.520 4.583 2.977 3.188 3.932 3.759 Fe. 0.366 0.496 0.178 0.242 0.196 0.570 o.414 0.401 0.232 Total 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Fe2* 1.856 1.851 1.760 1.781 1.788 1.710 '|.718 1.770 1.783 Mn 0.085 0.085 0.156 0.176 o.172 0.186 0.180 0.173 0.165 Ca 0.058 0.063 o.o42 o.o42 0.040 0.057 0.063 0.049 0.052 Na 0.001 0.001 0.031 0.001 o.o47 0.039 0.008 Total 2.000 2.000 1.989 2.000 2.000 2.000 2.000 2.000 2.000 Na 0.247 0.262 o.o42 0.026 0.553 o.473 0.208 0.308 Total o.247 0.262 0.000 0.o42 0.026 0.s53 0.473 0.208 0.308 Mg(Mg + Fe) 0.644 0.628 0.703 0.691 0.698 0.566 0.599 0.644 0.651 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES IO47

TAELE4.-Continued

Smouspan SP2-B sM7-82 G2-D Core Cores Core (profile) Rim sio, 47.33 47.03 46.66 44.53 44.13 46.39 44.40 Tio, o.42 0.15 0.28 0.18 0.16 0.10 0.14 Alr03 10.96 11.03 11.04 17.62 18.08 15.65 17.86 FeO 25.43 24.81 25.18 19.38 19.15 18.73 18.88 MnO 0.55 0.69 0.70 2.18 2.16 2.03 2.23 Mgo 14.10 14.64 14.00 14.26 14.16 14.98 14.28 CaO 0.38 0.27 0.41 0.44 0.45 0.37 0.4s Naro 1.18 135 1.36 1.67 1.75 1.34 1.67 Total 100.35 99.97 99.63 100.26 100.04 99.59 99.91 Si 6.846 6.810 6.807 6.327 6.282 6-589 6.317 rarAl 1.154 1.190 1.193 1.673 1.718 1.411 1.683 Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000 r6tAl o.714 0.696 0.705 1.277 1.316 1.209 1.312 Ti 0.04s 0.016 0.031 0.019 0.018 0.011 0.015 Fe3* 0.034 0.098 0.037 Mg 3.040 3.160 3.043 3.O21 3.004 3.171 3.028 Fe2+ 1j67 1.030 1.184 0.683 0.662 0.609 0.645 Total s.000 5.000 5.000 5.000 5.000 5.000 5.000 Fe2* 1.875 1.877 1.851 1.620 1.617 1.616 1.602 Mn 0.066 0.083 0.085 o.262 0.260 0.244 0.269 Ca 0.059 0.040 0.064 0.067 0.068 0.056 0.068 Na 0.051 0.055 0.084 0.061 Total 2.000 2.000 2.000 2.000 2.000 2.000 2 000 Na 0.331 0.379 0.385 0.408 o.428 0.285 0.400 Total 0.331 0.379 0.385 0.408 o.428 0.285 0.400 Mg/(Mg + Fe) 0.500 0.521 0.501 0.568 0.569 0.588 0.574

VA locality G5-B VA12.B Core (profile) Rim Cores Rim sio, 41.16 42.32 42.82 42.66 43.19 44.21 47.76 48.95 53.17 Tio, 0.16 0.13 0.09 0.10 0.10 0.11 0.14 0.13 0.08 Alro3 20.99 20.19 19.02 19.32 18.92 17.72 10.42 7.97 3.54 FeO 23.43 23.38 23.00 22.79 23.04 22.69 20.74 20.61 20.50 MnO 0.35 0.31 0.31 034 0.30 0.37 1.63 1.75 1.52 Mgo 12.11 12.54 12.77 12.76 12.71 13.33 16.87 17.84 20.03 CaO 0.41 0.43 0.43 0.35 0.40 o.44 0.54 0.45 0.34 Na.O 2.28 2.15 1.86 1.84 1.88 1.61 1.29 0.98 0.41 Total 100.89 101.45 100.30 100.16 100.54 100.48 99.39 98.68 99.59 Si 5.908 6.02s 6.149 6.126 6.182 6.312 6.849 7.O43 7.520 rrrAl 2.O92 1.975 1.851 1.874 1.818 1.688 1.151 0.9s7 0.480 Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 r6rAl 1.458 1.413 1.368 1.396 1.374 1.294 0.610 0.395 0.110 Ti 0.018 0.014 0.013 0.011 0.010 0.o12 0.015 0.015 0.009 Fg3+ 0.151 0.260 o.240 Mg 2.591 2.661 2.705 2.731 2.712 2.836 3.607 3.827 4.222 Fe2* 0.933 0.912 0.914 0.862 0.904 0.8s8 0.617 0.503 0.419 Total 5.000 5.000 5.000 5 000 5.000 5.000 5.000 5.000 5.000 Fe,* 1.880 1.872 1.822 1.876 1.854 1.851 1.720 1.717 1.766 Mn 0.043 0.037 0.038 0.041 0.036 0.044 0.197 o.214 0.182 Ca 0.062 0.065 0.070 0.054 0.061 0.068 0.083 0.069 0.052 Na 0.015 0.026 0 070 0.029 0.049 0.037 Total 2.000 2.000 2 000 2.000 2.000 2.000 2.000 2.000 2.000 Na 0.620 0.567 o.457 0.485 0.474 0.408 0.360 o.272 o.112 Total 0.620 0.567 o.457 0.485 0.474 0.408 0.360 0.272 0.1't2 Mg/(Mg + Fe) o.479 0.489 o.497 0.499 0.496 0.512 0.607 0.634 0.659 Note. atomic proportions on the basis of 23 O atoms; minimum Fe3* correctionemployed; nd : not determined.

(2) a negative correlation between t6rAl and Mg/(Mg + [SP2-B, SM7-82, KDH5-420, and KDH5-105 (cores Fe) calculatedwith a minimum Fe3+correction (Fig. 6b, only)1.The exceptionis VAl2-B, which strictly belongs 6c). Figure 6a illustratesthe separationof orthoamphi- to neitherofthese groups. The bestinterpretation ofthese bole compositionsinto two fields-those in rocks domi- data is that they reflectvariation in bulk composition. nated by secondarygrowth of staurolite and sillimanite Kielder. The coresof the large anthophyllite grains that and those that have clearly derived from orthopyroxene replacedorthopyroxene in KDH5-105 have minor 1048 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

TABLE6. Mineralanalyses of cordierite

KDH5-420 KDH5-105 +Qtz, Ab, H2O sio, 49.04 49.06 49.20 47.86 48.34 Alros 33.64 33.42 33.47 33.05 33.11 FeO 4.65 3.67 3.91 3.80 3.84 MnO 0.14 0.20 0.25 0.30 0.31 Mgo 10.46 10.37 10.36 9.65 9.80 Naro 0.15 0.57 0.60 0.85 0.89 Total 98.08 97.29 97.79 95.51 96.29 si 4.988 5.007 5.003 4.986 4.998 AI 4 026 4.020 4.011 4.058 4.035 Fe 0.395 0.314 0.333 0.332 0.332 Mn 0.012 0.018 0.021 0.027 0.027 Mg 1.583 1 577 1.571 1.499 1.511 Na 0.029 0.113 0.119 0.173 0.180 Total 11.033 11.049 11.058 11.075 11.083 Mg(Mg + Fe) 0.800 0.834 0.825 0.819 0.820 Note.'atomic proportions on the basis of 18 O atoms.

Fig.4. Projectionof mineralcompositions from albite, quartz, and HrO. Compositionsof orthoamphibolehave not beencor- phyllite is a ferroan pargasiteand more magnesianthan rectedfor Fe3*.Open symbolsare progrademinerals; closed its host orthoamphibole[Mg/Mg + Fe),o,: 0.71]. symbolsare retrograde minerals. Smouspan.Orthoamphiboles from AssemblageB are lessaluminous than gedritein AssemblageC but show a strongdepletion in Na and I6rAlnear their rims, accom- amountsof rlAl (0.094-0.164pfu) and Na (0.026-0.042 panied by an increasein Mgl(Fe + Mg). In Assemblage pfu), whereasthe rims are richer in both (16rA1: 0.667- C, gedrite is strongly zoned (Fig. 6b). Although there is 1.443pfu; Na : 0.216-0.600pfu). Rims are lessmag- some heterogeneity,rims of primary orthoamphibolein nesian[Mg/(Mg + Fe) : 0.57-0.64] than the cores[Mg/ G5-B are enriched in Mg and depletedin Na and Al. : (Mg + Fe) 0.69-0.711.Hornblende that rims antho- Hornblende is more magnesian[Mg/Mg + Fe) : 0.58] than coexistingorthoamphibole. VA locality. Analysesof orthoamphibolein VAl2-B TABLE5. Mineralanalyses of hornblende show an increasein Mg/(Mg + Fe) and a decreasein Na I6rAl KDH5- and from core to rim. Hornblendein VAl2-A (As- 105 VA12-A semblageD) is ferroan pargasitewith minor enrichment Core in Fe and Al at the rim. sio, 43.59 42.65 42.38 42.82 42.28 Cordierite Tio, 0.23 0.28 0.32 o.44 0.36 Al,o3 1718 18.16 17.98 16.87 1709 At Kielder, cordierite range '17.32 't7.47 analyses in Mg/(Mg + Fe) FeO 14.19 15.87 1581 from 0.70 to 0.84. The most Mg-rich analysesare from MnO 0.62 0.15 0.17 0.92 0.90 Mgo 11.01 9.23 9.22 10.44 10.28 KDH5-105, wherecordierite was replacedby gedrite,sil- CaO 10.92 10.83 10.77 10.14 10.28 limanite, and staurolite.In KDH5-420, cordieritein the Naro 1.59 1.70 1.70 1.86 1.81 unaltered AssemblageA is KrO 0.16 0.23 0.24 0.16 0.16 less magnesianthan that in Total 99.49 100.55 100.25 99.41 98.97 the orthoamphibole-richband. This resultsdirectly from si 6.181 6.063 6.045 6112 6.065 a continuous shift in Mgl(Mg * Fe).,o to higher values r4tAl 1.819 1.937 1.955 1.888 1.935 as it losesFe to reaction products with lower Mg/(Mg + Total 8.000 8.000 8.000 8.000 8.000 161Al 1.052 1.107 1.068 0.9s0 0.954 Fe). In the Smouspansamples, fresh cordieritein G2-D tl 0.024 0.030 0.034 0.028 0.039 lMg/(Mg + Fe) : 0.821has a Mg/(Mg * Fe)ratio similar Fe3* 0.616 0.634 0.684 0.857 0.814 to a heavily pinitized replacement Mg 2.326 1.955 1.960 2.222 2.198 in G5-B. The latter Fe2* 0.982 1.274 1.254 0.943 0.995 has AllSi ratios similar to cordierite,but a substantially Total 5.000 5.000 5.000 5.000 5.000 lower total (Humphreys, l99l). Fe2* 0.085 0.152 0146 0.082 0.088 Mn 0.075 0.018 0.020 0.112 0.109 Ca 1.660 1 650 1.645 1.550 1.580 Staurolite Na 0.180 0.180 0.189 0.256 0.223 Four analysesfrom KDH5-105 (Kielder) average0.51 Total 2.000 2.000 2.000 2.000 2.000 Na 0.256 0.289 0.281 0.258 0.280 wto/oZnO and span a range in Mg/(Mg + Fe) of 0.22- K 0.029 0.041 0.043 0.030 0.031 0-24.At Smouspan,staurolite from AssemblageC is Zn- Total 0.285 0.330 0.324 0.288 0.311 free,with Me/(Me + Fe) in rhe range0.22-0.25. Minor Mg/(Mg + Fe) 0.709 0.578 0.583 0.684 0.613 amounts of TiO, and MnO are present.In VAl2-A, pri- Note.-atomic proportions calculatedon the basis of 23 O atoms; Fe3+ mary and secondary staurolite have Mg/(Mg + Fe) : calculatedfrom the mean of minimum and maximum Fep+corrections. 0.23-0.24 in AssemblageD, whereassecondary staurolite HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1049

I G5B VAI2 B

Fe

I :-r ,|' \l-r" lo t'' :@ r{ t- I

\- /----v8< '*.--.._99-,o Mn I

(lmm)

Fig. 5. Compositionalprofiles 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 elementsdivided by the sum of Mg + Fe + Ca + Mn.

in AssemblageF has Me/(Mg * Fe) valuesof 0. l7-0. 18. Chlorite In addition, the former is Zn-free,whereas staurolite after cordierite in AssemblageF is mildly zincian. Staurolite All chlorite in the samplesdescribed resulted from ret- in VAl2-B is zincianwith Mg/(Mg + Fe) : 0.25. rogressionof orthopyroxene,orthoamphibole, cordierite, garnet, staurolite,or biotite. All are magnesianferroan Plagioclase clinochlore. Mg/(Mg + Fe) in KDH5-105 (Kielder) is Variations in Xo, are found in all three localities.At 0.78, midway betweenthat of cordierite and biotite. A Kielder, the rangein Xo" is small,between 0.29 and 0.31. singlechlorite sample from G2-D (Smouspan)has Mg/ At Smouspan,Assemblage B has a range of 0.24-0.28, (Mg + Fe) : 0.75 and is more Fe-rich than cordieritein whereasAssemblage C hasa rangeof 0.45-0.48; this dif- the same sample.At the VA locality, Mgl(Mg + Fe).* ferenceresults from bulk composition.The two VA sam- lies between0.65 and 0.66 for AssemblageD (chlorite ples have rangesin Xo^of 0.35 (core) to 0.41 (rim) in derived from biotite and cordierite) and between0.67 VAl2-A and 0.38 (core)to 0.36 (rim) in VAl2-B. The and 0.69 for AssemblageF (derived from cordierite alone). greatervariation in VAI2-A is possibly a result of Ca SinceMg/(Mg * Fe).,. > Mg/(Me + Fe).n,,the alter- enrichment due to the breakdownof hornblendein the ation of cordierite to chlorite + staurolite in all three rock during retrogression. localitieswas accompaniedby the breakdownof a suit-

TABLE7. Mineralanalvses of staurolite

KDH5-105 G2-D G5-B VA12-A VA12-B sio, 28.63 28.68 28 09 27.92 28.39 2787 27.84 Tio, o.47 o.24 0.58 0.44 0.47 0.57 0.62 Al,o3 54.69 54.71 52.86 52.41 53.93 5366 53.10 FeO 11.44 11.14 12.39 13.96 12.97 13.45 13.56 MnO o.37 051 0.60 nd 0.57 0.61 0.48 Mgo 1.85 2.02 2.13 2.46 1.54 2.37 2.58 ZnO 0.51 0.48 no nd 0.31 nd 0.30 Total 97.96 97.78 96.6s 97.25 98.18 98.s3 98.48 ai J./5b 3.767 3.756 3.739 3.747 3.676 3.683 Ti 0.o47 0.o24 0.058 0.044 0.046 0.057 0.062 AI 8.457 8.468 8.331 8.272 8.389 8.342 8.280 Fe 1.255 1.224 1.386 1.584 1 431 1.484 1.501 Mn 0.041 0.057 0.068 0.064 0.068 0.054 Mg 0.362 0.396 0.425 0.490 0.303 o.467 0.508 Zn 0.050 0.046 0.031 0.030 Total 13.968 13.982 14.024 14.117 14.011 | 4.094 14.118 Mg/(Mg + Fe) 0.224 0.244 0.23s 0.236 0 175 0.239 0.253 Note. atomic proportions on the basis of 22 O atoms; nd : not determined. 1050 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

Al-2077Na

Ca- 0.538Na

b G5B

= p Fig. 7. Quaternaryprojection of the progradeamphibolite faciesAssemblages C, D, and E from Anrr, quartz,and vapor (Spear,I 977).Note that garnet,being slightly calcic, is projected within the tetrahedron,so that the garnet-cordieritetie line in NuT -i l AssemblagesC and E passesbehind the staurolite-gedritetie t line, whichis in the front faceof the tetrahedron.The exploded , ..-\' f :r-+--1r -----..1. diagramon the right clarifiesthese relationships and showsthat the threeassemblages form interlocking(and hence compatible) Mg,/( Mg+Fe2+) tetrahedrain this projection.

. 0.4mm five contain the nondiagnostic AssemblageB (orthoam- c phibole + garnet + plagioclase+ biotite * quartz),and are not discussedfurther. The useful progradeparagenes- as follows: 07 es can be summarized Mg garnet + orthopyroxene + cordierite (A) Ivlg*Fe*g 6 garnet + hornblende* orthoamphibole+ a cordierite (C) o ooo staurolite+ hornblende+ sillimanite * cordierite (D) 0 0s tclAl t0 1.5 staurolite+ garnet * orthoamphibole* Fig. 6. Mineral chemistry of orthoamphiboles: (a) Plot of cordierite. (E) Na,o,vs. IurAl(per formula unit of 23 O atoms). All analyseshave All assemblagescoexist with plagioclase+ quartz,where- been corrected for Fe3* using a minimum Fer* correction. (b) present Zoning profile acrossa primary gedrite from G5-B. (c) Plot of as biotite is in AssemblagesC, D, and E. These M/(Mg + Fe) against t6tAl(per formula unit). three assemblagescan be representedin projection from plagioclase(An35), qtarrz, and vapor (Fig. 7). This sug- geststhat AssemblagesC, D, and E all representpeak able Fe-rich mineral, either amphibole or (more proba- amphibolite faciesassemblages formed at similar pres- bly) biotite, depending upon the original mineralogy. suresand temperaturesin rocks of slightly different bulk composrtron. pHASE RracrroN HISTORy AND RELATToNSHTps Retrogradeparageneses are subassemblagesofthe total Seven of the above samples contained cordierite and list of retrogrademinerals in a given rock. They are placed an amphibole at the metamorphic peak, whereas a further togetheron the evidenceof spatialproximity alonggrain HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1051

TABLE8. Mineralanalyses of plagioclase

VA12-A KDH5-420 KDH5-105 SP2-B sM7-82 G2-D VA12.B sio, EOnO 61.65 62.40 62.82 56.07 56.54 58.36 58.43 60.19 Alro3 25.99 23.73 23.40 23.O7 26.19 25.91 25.73 25.54 24.63 CaO 8.05 6.22 s.84 5.22 9.81 9.35 682 8.51 7.30 Naro 6.73 8.14 8.28 8.88 5.88 6.19 6.40 6.72 7.14 K.o 0.05 no 0.09 0.09 nd nd 1.10 nd nd Total 99.91 99.74 100.01 100.08 97.95 97.99 98.63 99.20 99.26 5l 2.640 2.748 2.769 2.781 2.570 2.591 2.644 2.638 2.697 AI 1.369 1.246 1.224 1.204 1.415 1.399 1.374 1.359 1.301 Ca 0.385 0.297 0.278 0.248 0.482 0.459 0.331 o.412 0.350 Na 0.583 0.704 0.712 0.762 o.522 0.550 0.562 0.588 0.620 K 0.003 0.005 0.00s 0.064 Total 4.980 4.995 4.988 5.000 4.989 4.999 4.975 4.957 4.968 Xo" 0.396 0.297 0.279 0.244 0.480 0.455 0.346 0.412 0.361 Note.'atomic proportions on the basis of eight O atoms; nd : not determined boundariesor becausethey constitutea pseudomorph(for ered with chlorite in VAl2-A. Pseudomorphingof cor- example after cordierite): dierite by kyanite-bearingassemblages in the samesam- ple can be describedby the analogousreactions: staurolite + chlorite + kyanite + quartz (A*) cordierite + HrO : kyanite + chlorite + quartz (3) staurolite* orthoamphibole* quartz + chlorite (B*) and hornblende+ sillimanite * staurolite+ ouartz cordierite + biotite + HrO : kyanite + orthoamphibole (C*) + muscovite + chlorite + quartz. (4) sillimanite * orthoamphibole* quartz (D*) The alteration of cordierite to gedrite in G5-B is de- orthoamphibole+ chlorite + quartz (E*) scribedby chlorite + biotite + quartz. (F*) cordierite + HrO The reactions that link prograde and retrograde assem- : gedrite * staurolite + quartz (5) blagesinvolve transitions from the stability fields of or- and the univariantreaction thopyroxene,cordierite, hornblende,and sillimanite to those of staurolite,chlorite, and kyanite. Although the cordierite + garnet + HrO detail ofeach reactiondiffers, these transitions in broad : gedrite + staurolite + quartz (6) terms involve a movement from lower pressuresand produce and gedritealong higher temperaturesto conditions of lower temperature which intergrowthsof staurolite grain Locally, twinned pla- alonga near-isobaricP-Z trajectory. the boundariesof cordierite. gioclaseis replacedby veinlets of gedriteand staurolite euhedraon the grain boundariesand is describedby Interpretation of textures in rocks without hornblende cordierite + albite + HrO : quartz. (7) The following reactions are interpreted from the tex- gedrite * staurolite+ with tural evidencein sampleseither a singleorthoam- In VAI2-B (AssemblageE), the opposite reaction is phibole The widespreadreplace- or no amphibole at all. inferred from the corona growth ofplagioclase from pri- by the reaction ment of cordieritecan be described mary staurolite: cordierite + HrO gedrite * staurolite : staurolite+ chlorite + quartz (l) : albite + garnet + chlorite + HrO. (8) but is problematicalbecause chlorite and stauroliteare the local coexistenceofgedrite and sillimanite both lessmagnesian than the reactantcordierite. The tex- At Kielder, grains grain boundariesof cordieritemay ture is better representedby the KFMASH univariant as small along reactron be due to the reaction cordierite + biotite + HrO cordierite + HrO : staurolite + chlorite * muscovite + quartz (2) : gedrite + sillimanite + qtrartz. (9) which is supported by biotite inclusions in cordierite This is usually regardedas a higher pressureassemblage pseudomorphsand random flakesof white mica interlay- (Harley, 1985; Visser and Senior, 1990), with kyanite r052 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

and chlorite + gedrite : garnet + qluartz + HrO (14) are negative, necessitatinga shift to higher temperature to produce garnet from the hydrous reactants.It is clear that garnet cannot be produced along the same P-7 vec- tor that produced staurolite, chlorite, and gedrite from cordierite(Fig. 8).

Interpretation of textures in rocks with hornblende Prograde hornblende-bearing assemblages(Assem- blagesC and D) formed at conditions similar to those that produced the hornblende-freeAssemblage E (Fig. 7). Apart from the grain-boundary replacement of horn- Fig. 8. Slopesof continuousreactions in FMASH predicted blende by staurolitein AssemblageD, calcic amphibole (1990) from the Hollandand Powell dataset. Products ofreac- produced at the metamorphic peak is not altered during tions interpretedto have grounds occurredon textural are in subsequentretrograde metamorphism. boxes.See text for discussion. Retrogradehornblende, however, occurs in KDH5- 105 as rims on cordieriteand orthoamphibole,together with staurolite, sillimanite, chlorite, and orthoamphibole of commonly being the stablealuminum silicate,although different composition to that of the substrate. General Hudson and Harte (1985)place the reactionaI 4-6 kbar. reactions describing the retrograde textures are In Smouspansamples G2-D and G5-B, chlorite sheaves have grown across other retrograde products. This final cordierite + plagioclase+ anthophyllite + HrO stageof rehydration can be describedin the FMASH con- : sillimanite * gedrite * hornblende tinuous reactions + staurolite+ quartz (15) cordierite + gedrite + HrO and : chlorite + quartz (10) cordierite + plagioclase+ anthophyllite + HrO and : staurolite + gedrite + chlorite staurolite + gedrite + HrO * hornblende+ quartz. (16) : chlorite + quarlz (11) Both reactionsoccur necessarilybelow the orthoamphi- and the univariant FMASH reaction bole solvus (i.e., below 600 "C at pressuresof 4-6 kbar; Spear,1980). cordierite + gedrite + HrO : staurolite * chlorite + quartz. (r2) Stability of hornblende with staurolite and sillimanite Reactionsthat are continuousin FMASH (Reactions Staurolite + hornblende + sillimanite occurs both as prograde l, 3, 5, 9, 10, I l) have been plotted to scalein Figure 8 a and retrograde assemblageat Copperton. It to illustrate the constraintsplaced upon the retrograde occursin rocksof aluminousbulk compositionand hence path. Predictably,the reactionsinvolving chloriterequire is not relatedchemographically to the occurrenceof ky- a proportionatelyhigher molar quantity of HrO and thus anite + staurolite + hornblende recorded both by Sel- (1984) (1987) have steeper slopes. Reactions interpreted to have oc- verstoneet al. and Helms et al. in rocks of prograde curred last in the retrograde sequence(Reactions l0 and demonstrablyamphibolitic composition.As a phase I 1)have the steepestslopes ofall. Although chlorite-pro- it is restrictedto the edgeof domains containing (sillimanite- ducing reactions place little constraint upon the retro- AssemblagesD and staurolite-dominated) F (cordierite-dominated). presence plagioclase grade path, the shallowerreactions (Reactions 5 and 9) and The of are consistent with an isobaric cooling path. in AssemblageD suggeststhat hornblendeis not simply phase part In all three localities, the regrowth ofgarnet from ged- an extra stabilizedby Ca and Na but is ofan premetamor- rite + staurolite(G5-B), gedrite + chlorite (VAl2-B), or unusual bulk composition resulting from phic peraluminous anthophyllite + biorite (KDH5-420, v55-1050) is inter- mixing of semipeliticand K-poor do- mainsin VAl2-A. preted as a later progrademetamorphic event. The slopes of the relevantFMASH reactions The replacement of hornblende by secondary fine- grained staurolite in AssemblageD is problematical,since staurolite + gedrite hornblende* stauroliteis alreadystable. Examination of : garnet + qxartz + HrO (13) the boundariesof hornblendesrains in VAl2-A reveals HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES I053 minor alterationof hornblendeto an assemblageinclud- discussedhere (Fig. 8), and on the Hudson and Harte ing chlorite, biotite, and quartz (Fig. 2H), whereasthe (1985)grid they would connectthe univariant reactions margins of staurolitein the same samplehave been al- sillimanite -+ orthoamphibole tered to chlorite (Fig. 2G). Thus secondarystaurolite grew : garnet quarlz from partly chloritic substrates near hornblende when * cordierite + + HrO (17) metamorphic conditions were lower than those required and to stabilize staurolite + hornblende + sillimanite, but abovethose responsible for the stability ofchlorite. Sec- staurolite + cordierite + HrO : ondary staurolite in VAI2-A thus belongsto the same sillimanite f orthoamphibole* quartz (18) metamorphic (reheating)event that produced garnet rims with a near-isobaricslope. Thermobarometric estimates in VAl2-B and euhedralgarnet upon gedrite* staurolite put peak conditions at pressureshigher than those de- in G5-B. fined by Reaction 17, whereasthe absenceof garnet in The retrogradeoccurrence in KDH5- 105is finegrained KDH5-105 in either progradeor retrogradeassemblages and may not representan equilibrium assemblage.The indicates that cooling was isobaric to near (above?) the assemblageformed in the samedomains as sillimanite + invariant point at which garnet is absent, where chlorite gedrite,but, as with the latter, chlorite is normally pres- and staurolite resulted from further breakdown of or- ent, and the division of mineralsinto thoseresulting from thoamphiboleand cordierite. retrogradehydration and thosegenerated by reheatingis not clear. Reheating path The reheatingpath indicated by the renewedgrowth of Pnnssunn-TEMPERATUREEyoLUTIoN garnet in all three localities must necessarilyleave the The P-T history of orthoamphibole-bearingrocks at chlorite stability field. The path indicated on Figure 9 is Copperton can be interpreted using the petrogeneticgrid constrainedby the observationthat secondarycordierite of Hudson and Harte (1985) for KrO-poor alumrnous * orthoamphiboleis not producedanywhere from stau- pelitic compositions(Fie. 9). The basis for this grid is rolite + chlorite (Reaction l2). fully discussedby Hudson and Harte; its usein this paper is more appropriatethan the grids of Spear(1978) and Kinetic factors Spearand Rumble (1986), sincethe bulk compositions The difference in grain size between large prograde are closerto thoseof pelites. grainsand commonly small retrogradeeuhedra supports the dominance of nucleation over grain growth during Amphibolite facies retrograde path cooling. This is indicative of the undercoolingof peak In amphibolite faciesparageneses (excluding Kielder), assemblagesinto lower Zstability fieldsprior to the rapid the peak conditions lie betweenthe [CHL], (CHL,ALS), influx of HrO. Somestable equilibria may thus havebeen and (OAM,CHL) invariant points. In Figure 9, this cor- crossedwithout leaving any trace of the reaction in the respondsto the temperaturerange 600-625'C at about rock record. It is therefore to be expected that the se- 4 kbar, the same range in temperature as calculated for quence of reactions predicted from the petrogeneticgrid the nearby Annex pelites (Humphreys and Scheepers, does not in every casematch the sequencedetermined 1993).The retrogressioncan largelybe attributed to iso- from the textures. baric cooling through the divariant field staurolite + or- thoamphibole+ cordierite(by meansof Reactions5 and TpcroNrc TMPLICATIoNS 6) into the chlorite stability field by Reactionsl2 and 3. The metamorphic sequencedescribed in Table 2 mir- The shallow slope of Reaction 5 is the most important rors the metamorphicevolution of pelitesfrom the near- constraint upon an isobaric path rather than one that by Annex deposit (Humphreys and Scheepers,1993). simply returns to the origin directly. The formation of However, retrogressionof high-gradecordierite-bearing kyanite by Reaction 3 suggeststhat the Hudson and Harte assemblagesto various combinations of garnet, stauro- (1985)grid is only partly correctlyplaced with respectto lite, biotite, chlorite, kyanite, and sillimanite is seenup the aluminum silicatetriple point. However,the produc- to 300 km along strike to the northwest (Humphreys, tion of retrogradekyanite in VAl2-A, which contains 1991)within the KakamasTerrane (Fig. I, inset).The primary sillimanite, is further evidence for isobaric typical postpeakpattern throughout the Kakamas Ter- cooling. rane is (l) a period ofisobaric cooling from peak pres- suresof 4-6 kbar followed by (2) rehydrationwithin the Isobaric cooling at Kielder kyanite or the lower temperaturepart of the sillimanite Coolingof the granulitefacies Assemblage A at Kielder stability fields, and (3) regrowth of garnet, staurolite, at pressures>5.5 kbar produced the retrogradeassem- and sillimanite from either hydrated (biotite + chlorite blage sillimanite + orthoamphibole + quartz (Assem- + kyanite or sillimanite) or unaltered(cordierite, ortho- blageD*) by Reaction9 in sampleKDH5-105. Isopleths pyroxene) substrates.On the bounding thrusts of the of Reaction9 are the shallowestof the divariant reactions Kakamas Terrane, there is clear field evidencelinking 1054 HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES

FASH _ MASH P FMASH kbar

6 Jcn € t (te - -s\\u-- ,/,

11 '-t/ peakmetamorphic condit ions (VA& W Smouspan) v" (OAM.CHL) Grt Als rd ;!i 1 -K- ---sr/ ' 4n}---- 550 600 650 Tt 700 Fig. 9. Pressure-lemperaturetrajectories superimposedupon an FMASH petrogeneticgrid after Hudson and Harte (1985). The orthoamphibole solvus is after Spear (1980). The lower pressurepaths are for west Smouspan(WS) and the VA mineralization, with the replacementof Als + Chl by St included in the path marked VA. The grid and rhe paths derived from it are misplaced with respect to the AlrSiO, diagram ofHoldaway (1971), since the occurrenceofkyanite cannot be accountedfor using the grid. The paths are thus representativeand may be I kbar lessthan therr tme pressure. late normal faulting on previous thrust surfacesto the retrogression.Many granulite faciespelites from through- developmentof hydrousretrograde assemblages in gran- out the Kakamas Terrane preservesmall euhedralgarnets ulites (Humphreys, 199l). Spatially, the effectsof re- within unrehydrated cordierite grains, indicating that the hydration diminish with distancefrom shearzones with final metamorphic event is regional in extent. The re- normal or strike-slip movement. At the Prieska mine, growth of these minerals is interpreted as the result of retrogradedevelopment of kyanite, phlogopite,musco- heating due to crustal overthickening by adamellite sills vite, staurolite, and chlorite from gneissescontaining and voluminous rift-related lavas above the presentlevel cordierite + perthite + sillimanite of the alterationzone of erosion,remnants of which are exposedwithin and on is zonedaround a normal faulting that cuts the ore body the easternmargin of the Kakamas Terrane. (Humphreys et al., 1988a).These lines of evidencesup- AcrNowr.rncMENTS port the interpretation that retrogressingfluids entered the rock pile along specificconduits during a period of This work was funded by the South African Foundation for Research extension,once the crust had cooled to near or below Development and the Precambrian ResearchUnit, University of Cape part thesis completed at the latter the sillimanite-kyaniletransition. Town, and represents of a doctoral institution. Excellent reviews by John Schumacherand SorenaSorensen The regrowth of garnet, staurolite, and sillimanite, not only pointed out some problems but went a long way toward helping however, is not confined to pelites that have undergone to solve them. HUMPHREYS: METAMORPHISM OF ALUMINOUS GNEISSES 1055

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