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Metamorphic Evolution of Amphibole-Bearing Aluminous

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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 retrogradeassemblage. 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
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