Canadian Mineralogist Vol. 30, pp. 145-152(1992)

A GAHNITE_GARNETRETROGRADE REACTION FROM THE PINNACLESDEPOSIT, BROKENHILL, NEW SOUTHWALES, AUSTRALIA

JOANNA PARR Deporlmentof Geologt, Universityof Newcastle,Newcastle, New South Woles2308, Australia

ABSTRAcT INTRODUcTIoN

A retrogradereaction involving the breakdown of the The spinel gahnite representsthe zincian end- zincian spinel gahnite [(Zn,Fe,Mg)Al2O4]to almandine member of the gahnite - hercynite solid-solution garnet occurs in samples from the Pinnacles mine, series Although gahnite is southwestof Broken Hill, New [(Zn,Fe,Mg)Al2Oa]. South Wales, Australia. it has been reported in Grains of prograde gahnite are rimmed by a retrograde relatively rare in nature, corona of gamet in a quafiz matrix. The gahnite grains association with metamorphosedmassive sulfide exhibit no compositional zoning, which suggeststhat deposits(Plimer 1977,Williams 1983,Sheridan & homogenization occurred during prograde metamor- Raymond1984, Spry & Scott1986, Spry 1987a,b). phism. The retrograde rim of garnet, however, exhibits The high Zn content of the spinel, together with some compositional zoning away from the gahnite - its hardness(8) and stability in metamorphicrocks, garnet interface and contains trace amounts of Zn. The make it a potential indicator in base-metal assemblagewas probably created by the formation of programs(Sheridan & Raymond 1984, garnet gahnite presence exploration from in the of solutionsenriched have been in Fe, Mn, Mg, Ca, Na and Si, with the loss of Zn. These Spry & Scott 1986). Several theories observations, together with sillimanite-bearing pelitic proposedfor the formation of gahnitein metamor- .assemblages,are consistentwith isobaric cooling during phic rocks. These include the desulfurization of retrogrademetamorphism. sphalerite(Sangster & Scott 1976),the breakdown of Zn-bearing silicates such as biotite (Dietvorst Keywords: zincian spinel, gahnite, electron-microprobe 1980) and staurolite (Stoddard 1979) during data, retrograde metamorphism, Broken Hill, prograde metamorphism, and the metamorphism Australia. of opaline silica and hydrothermal clays with adsorbedZn and zincian carbonate(Plimer 1988). SouuarnE Quartz - gahnite is the most common gahnite-bearinglithology in the Willyama Super- Une r€action r€trogradeimpliquant la d6stabilisation group, ; other rock typesinclude de la gahnite [(Zn,Fe,Mg)Al2Oa]pour donner un grenat psammitic to pelitic metasedimentsand quartz - riche en almandin fait l'objet de cette contribution e la feldspar- biotite - garnet- gahnitegneisses (Barnes min6ralogiede la mine Pinnacles,au sud-ouestde Broken et al. 1983), Gahnite is commonly observedas a Hill, au New South Wales, en Australie. Les cristaux de ganguemineral in the base-metalores of the Broken gahnite, prograde, d'origine ont une couronnede grenat Hill deposit,within the Willyama Supergroup(e.9., r€trograde dans une matrice de quartz. L'absence de Segnit 1961, Richards 1966, Plimer 1976, 19'77, zoflation en compositiondans la gahnite fait penserqu'il y a eu homogdndisationlors d'un 6pisodede m6tamor- Spry 1978, 1984, 1987a,Barnes et ol. 1983).Inthe phisme prograde. Les liser€s de grenat, par contre, main Broken Hill lode, there are at Ieast two montrent une zonation d partir de l'interface gahnite * distinct gahnite-bearing assemblages:quartz - grenat, et contiennentdes traces de . L'assemblage gahnite - pyrrhotite - - garnet - biotite (- r6sulterait de la formation du grenat au d6pens de la staurolite, muscovite, ilmenite, sphalerite) and gahniteen pr6sencede solutionsenrichies en Fe, Mn, Mg, garnet - biotite - chlorite - sphalerite- gahnite - Ca, Na et Si, et capables de mobiliser le Zn. Ces galena - pyrrhotite - q\aftz (+ ilmenite); mus- observations, et la pr6sence de sillimanite dans les covite, chlorite and staurolite are retrograde assemblagespdlitiques, concordent avec un moddle de (Spry 1978).This contribution documents refroidissementisobare pendant le m6tamorphismer6tro- grade. one pafiicularly unusual occurrence of gahnite from the Pinnacles Mine Sequence,New South Wales,in which retrogradereactions in sulfur-poor (Traduit par la R6daction) quartz - garnet - gahnite - biotite pelites Mo*-clds: spinelle zincifbre, gahnite, donn6es i la (sillimanite-deficient)have produced b garnet rim microsondedlectronique, m6tamorphisme rdtrograde, on grainsofgahnite. The associationof retrograde Broken HiIl. Australie. g:unet and prograde gahnite in sillimanite- and t45

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sphalerite-deficientrocks has not been described deposit, in the Cues Formation of the early previously and allows some speculation on Proterozoic Willyama Supergroup (Willis el a/. metamorphicconditions in the Broken Hill area. le83). The Willyama Supergroup has undergone two GsolocrceL SsrrrNc eventsof high-grademetamorphism and deforma- tion, followed by at leastone later retrogradeevent The Pb-Zn Pinnaclesmine lies 15 km southwest (Rutland & Etheridge1975, Glen et al. 1977,Laing of Broken Hill, westernNew South Wales(Fig. l), et al. 1978,Corbett & Phillips 1981,Stevens e/ a/. in the ProterozoicWillyama Supergroup.The mine 1988). Prograde regional metamorphism in the is the largest "Broken Hill"-type deposit in the Willyama Supergroup increasesfrom andalusite Broken Hill Block, after Broken Hill itself: grade in the northeast, through sillimanite - approximately 200,000 tonnes of have been muscovite and sillimanite - K-feldspar zones, to extracted,with averagegrades of l09o Pb, 420g/t two-pyroxenegrade in the southwest(Phillips 1978, Ag and 2.5W0Zn in the main lead lode, and 3.590 1980) (Fig. l). The Pinnacles deposit lies in the Pb, 100g/t Ae and 7.5-ll.59o Zn, in the smaller trvo-pyroxenezone and is characterizedby high- Zn lodes. Remainingore-reserves are estimatedat temperature,low- to intermediate-pressuremineral 200,000to 600,000tonnes (Departmentof Mineral assemblages.Retrograde metamorphism is most Resources1981). The depositconsists ofa seriesof strongly developedwithin, but not confined to, highly deformed strataboundsulfide lenseshosted zones of retrograde schists that transect the in pelitic schists. Stratigraphically, it lies ap- Willyama Supergroup (Stevens1986). There is a proximately 700 m below the main Broken Hill transition from lower-gradechloritoid-bearing as- semblagesin the central and northern regions, to staurolite-bearingassemblages in the central and southern regions, and zones of kyanite-bearing retrograde schist in the south (Corbett & Phillips Andaluslte + l98l). In the Pinnacles area, the is mugcovllo zons dominated by the kyanite - staurolite-bearing Thackaringa-PinnaclesRetrograde Shear Zone to Sllllmanlte + muocovllo zono the south ofthe deposit(Stevens 1986). The deposit itself also is cross-cutby severalretrograde shear Sllllmrnlto + K-loldspat zono zones.

2-pyroxone zone ANALYTICAL PROCEDURE

The chemical compositions of 35 gahnite and approximately250 garnet crystalswere determined using a JEOL JSM840 scanning electron micro- scopefiued with a TRACOR TN5500 energy-dis- persion spectroscopyEDS system.An accelerating potential of 15 kV and an emission current of 2 nA were used.The spectrawere analyzedusing the following standards:wollastonite (Si, Ca), corun- dum (AI), hematite (Fe), rutile (Ti), rhodonite (Mn), periclase(Mg), anorthoclase(Na, K), wil- lemite (Zn), V2Os (v) and chromite (Cr). Repre- sentativecompositions are given in Tables I and2.

PTTnocnepHYAND MINERAL CHEMISTRY

Within the PinnaclesMine Sequence,gahnite- bearingpelites are associatedwith the wallrocks to both the galena-richand sphalerite-richorebodies. The pelites form stratiform units with distinct compositional layering, interpreted as primary Frc. I . Outline of the Broken Hill Block showingprograde bedding (Ss.1)and defined by relative abundances metamorphic zones (after Phillips 1978), retrograde of garnet, gahnite, quartz and feldspar. Two metamorphic zones and location of the Pinnacles foliations (S2 and Sr) can be defined by the mine: modified after Stevens(1986). alignment of sillimanite and retrogrademuscovite

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TABT,EL NEPNBSENTATTVECTIEMICAL COMPOSITTONS OF OAITNIIE

s-qple Gha (h4 Ghr4 (h16 Chl? frr8 Ch31 Gh32 Gh3.ll GA1

aatu 6ntor edgs edgs .-_mtar-€dge

sior 0.@ 0.00 0.@ o.L? 0.18 0.00 0.18 0.85 0 0.n 0.00 0.00 o.12 0.00 Tio" 0.00 0.00 0.@ 0.16 0.00 0.0o 0.00 0.@ o.r1 0.00 0.(X! 0.ft1 0.@ 0.00 ALO" u.1o 66.02 60.30 64.96 66.(E 66.34 &2 66.6? &6.64 66.94 65.46 66.48 66.@ 6820 Cr2Os 0.00 0.00 0.68 L17 0.00 0.@ 0.16 0.(}0 0.00 0.00 0.(}0 0.00 0.@ 0.00 FeO' 10.89 12.66 9.8n rO,2 13.84 1Ls6 1358 $.?O 14.06 r.106 L3.67 13.60 14.01 L4Ll !,tno 0.00 02o 0.00 0.00 0.OO 0.00 0.@ 0.@ 0.00 0.00 0.@ o.fi) 0.00 0.00 MsO 0.m 031 O.U o.8t L72 029 0.91 0.66 0.U 0.48 0r9 0.U 0.16 027 7^O 83.08 3198 3:1.6? 82.4) .& 31.07 28.8:t W.62 28.8tt 2&68 n.6l 4.6|| 4.68 8.6 T0IAL 98.83 9€.36 [email protected] 99.91 99.& 98.66 [email protected] [email protected] [email protected] 100.76 98.92 Sg28 99.06 9910

Cation prcportione (based oa 4 oxygen atoEs) si 0.00 0.@ 0.00 0.01 0.01 0.@ 0.0r 0.01 0.01 0.01 0.@ 0.00 0.(p 0.00 Ti 0.00 0.@ o.fft 0.00 0.q, 0.(X} 0.m 0.(p 0.00 0.o0 0.@ 0.00 0.00 0.m Al r..98 LS? 1.98 L96 L.97 L90 1.98 LgE L9E L99 L90 L98 L98 L98 Cr 0.q, 0.@ 0.01 o.o:t 0.@ 0.00 0.00 0.@ 0.@ 0.(X) 0.00 0.@ 0.00 0.@ Fee o.28 0.t2 0 026 0.g6 0.80 0.84 0.34 0.36 0.35 0.35 0341 0.36 0.36 l[n 0.@ 0.01 0.00 0.00 0.00 0.00 0.@ 0.@ 0.00 0.q) 0.@ 0.@ 0.(b 0.(X, w 0.00 0.01 0.03 0,04 0.06 0.01 0.04 0.03 0.0:l o.t2 0.01 0.02 0.01 0.01 Za 0.75 0.?0 O.71 0.72 0.6:t 0.70 0.6:t 0.66 0.6t1 0.6:l 0.66 0.60 0.87 0.66

'Total Fe m FeO

TABLE 2. REPRESE!{TATM CIIEIiIICAL COMPOSITIONS OF GAB}IET

Smplo Gt139 ct142 ct203 at204 Gt901 c,t2g2

(orcna) wI% 1g"h";1s -_- qurtz) (con) teagsl (@) (adgo)

siol 36.76 fi.74 86.06 95.?6 35.88 36.86 A6.41 36.10 36.32 35.99 86 36.3? 35.91 85.86 Tio3 0.@ 0.00 0.00 0.00 0.@ 0.00 0.00 0.00 0.00 0.00 0.@ 0.00 0.m 0.00 Ato" 20.M 20.57 2t.tL 2a.97 2t.V2 20.81 2ttt 20.67 2L16 20.68 2L06 20.8{' 21.@ 20.68 Fgo fi.ll 36.66 36.10 3?.03 46.8? 36.83 36.16 56.6 36.60 38.72 36.841 S6.n 36.16 36.6? MnO 8.74 3.,18 4r0 3 3.36 3.&t 3.19 3.ftt 3.02 8.30 s.V2 3.@ 3.06 2.5? MsO L07 LL? 7.37 r.g2 L.8 L.U 1.68 1.66 1.66 r.61 1.66 L69 ,,?l 1.63 CaO r28 L 1.36 L39 1.36 L Lg l..u L4O 1.45 L.A r L4L L31 NaO 0.00 0.@ 0.r8 0.s1 02tr 0.oo o.oo o.(Xt 0.88 o.0o o.u 0.00 02I 0,7 zao 0.00 0.00 0.00 0.00 0.@ O.0O O.@ 0.@ O.0O 0.00 0.00 0.00 0.@ o.ql TCIIAL 99.00 99.s7' 10097 100.06 100.00 98.64 99.99 gS.n 100.63 N.12 1(X}28 99.38 99.45 9820

Catims pnportione (bqsed m 12 oxyBu ato@) si zsE 296 2.96 294 2.95 2.57 L97 2.57 LS6 2.X 2.96 498 2.55 2.98 Ti 0.00 0.(m 0.00 0.00 0.00 0.00 0.@ 0.(b 0.00 0.00 0.m 0.(n 0.(X, 0.(Xl AI L.97 L99 ao:l a0l| 2.03 a03 2-0l 2.d, a(x| 2.@ 2.V2 2.42 e0:l 2"42 tr's 2.68 LV L17 2.U Z6it 2.4 L& 2.60 L49 L62 ?-61 L& ?.8 L{l Mn oa 0 028 o23 0a o 0p ozL 02L 0.4 o2L 0.n 02L 02t MC 0.13 0.16 0.n 0.16 0.16 0.17 020 ofr 02a 020 030 0.19 o2L 0.19 Ca 0.11 0.11 0.72 o.12 0.12 0.11 0.11 0.4 0.t2 o.Ul 0.11 0.11 0.72 0.12 Na 0.@ 0.@ 0.o2 0.06 0.04 0.(b 0.00 0.00 0.06 0.00 0.Gt 0.@ 0.10 0.04 7a 0.00 0.04 0.00 0.00 0.@ 0.@ 0.00 0.00 0.@ 0.@ 0.(X) 0.q, 0.00 0.@

replacing prograde sillimanite in both 52 and S, crystalswithaninclusion-richcore(Fie.2b), similar (Fig. 2a). to that in grains of prograde garnet in the pelites. The pelites compri$e quartz, garnet, biotite, The inclusions commonly consist of quartz, but K-feldspar and gahnite, with varying amounts of also include subhedralbiotite, rutile and sphalerite. sillimanite, calcite, apatite, rutile, magnetite,tour- The rim is generally inclusion-free, although maline and zircon, and retrograde muscovite and commonly a crystal's margin has been partially staurolite. Sulfides present in these wallrocks corroded and removed,leaving only the inclusion- include sphalerite, galena, pyrrhotite, pyrite and rich core. The gahnite generallyhas relrogradedto arsenopydte.The gahnite occurs as dark green whitemica(cl, Corbett&Phillips 1981),areaction crystalsless than 0.5 mm in diameter. It seemsto that is particularly prevalent in sulfide-bearing be unaltered and commonly occurs as subhedral assemblages.A common associationis a gahnite

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Frc. 2. (a) Photomicrographshowing the pre-S2nature of garnet crystallization.Isotropic crystalsof garnet Gt) are aligned in 51 and partially resorbedalong subparallel52 foliation (Sj. Sillimanite is retrogradedto muscovite, which is aligned parallel to the 53 fabric (S), (b) Photomicrographof progradegahnite crystals(gh), eachwith a core rich in quartz inclusions.Crystals have embayedcontacts with matrix quartz (q) and are partially rimmed by biotite crystals(bi) alignedin 52. (c) Photomicrographof progradegahnite crystals (gh) with retrogradegarnet (gt) coronain subequigranularquartz (q). Note the sharpcontact between the mineral phases.(d) Photomicrograph of tourmaline crystal (t) within a retrogrademuscovite rim (mu) to a gahnite crystal (gh). Tourmaline also is a minor componentin muscovite-freegahnite-garnet-quartz assemblages.

core with a muscoviterim in a sphaleritehost. The few inclusions. The boundary between the two muscoviteis fine grained, mostly alignedin 51, but phasesis sharp and without intermediatephase or is also observedas a cross-cutting,axial planar, 52 symplectitic intergrowth; this is confirmed by fabric. back-scatteredimagery and electron-microprobe Less micaceous,sulfur-poor pelites make up a analyses (see below). Tourmaline (< lVo by minor, but important, component of the mine volume) is presentin somegarnet coronasand also sequenceand are the subject of this study. They is associatedwith a muscovite rim on gahnite are dominated by the assemblagequartz - garnet crystals elsewherein the sequence@ig. 2d). The - gahnite, with minor biotite and apatite. role of these small, subhedraltourmaline crystals Retrogradereactions observed in thesesulfur-poor is unclear,but they may indicateboron enrichment pelites include the formation of a garnet corona in the late-stagefluids that induced retrogression. around gahnite crystals (Fig. 2c), a garnet (in- Recently, Slack & Robinson (1990) reported the clusion-free)corona around garnet (inclusion-rich) breakdown of tourmaline during retrograde grains, and the minor breakdown of garnet to metamorphism in tourmaline - quartz - garnet - biotite. A garnet corona separatesmost examples biotite - sillimanite - ilmenite (minor gahnite, of gahnitein this rock from the quartz matrix. The plagioclase and apatite) assemblagesat Broken coronas are 30 to 150 pm thick, form a complete Hill; this may also be the casein pelites from the boundary betweenquartz and gahnite, and contain PinnaclesMine Sequence.

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The gahnite crystalshave a significant hercynite typical garnet coronasshow some small-scaleand component (up to 13.7q0FeO) but have low Mg consistentzoning involving Fe, Mn, Ca and Mg content and are virtually Mn-free (Table l). The (variations <2t/o; Fig, 3). Relativedepletions of Fe averagecomposition is (Zns.66Fes36JVIg6.stAl1.$O.,and Mn occur at the garnet - gahniteinterface and which is consistent with other compositions of at the outer margin of the garnet corona where, gahnite from pelitic metasediments (c/ Spry & conversely, there is enrichment in Mg and Ca. Scott 1986).Some gahnite also containshigh trace Variations are not large and suggesteither that amounts of Cr (up to l.l7r/o Cr2O3),indicating a partial metamorphic homogenization of these minor chromite component in the gahnite. There coronas occurred, or that changes in chemical is no systematicchemical zoning acrossthe gahnite composition during crystallization were only crystals(Table l: Gh 33, Gh l), indicating that any minor. The garnet coronas also contain trace primary grofih-zoning, if present,was removed by amounts of Zn (up to 0.66V0 ZnOi Table 2), homogenizationduring progrademetamorphism, a suggesting that Zn from the gahnite partially state that remained unchanged during the later substitutedfor Fe and Mn during retrogression. retrograde event. The lack of zoning is also observedin garnet in peliteselsewhere in the mine DIScUSSION successionand is reported for garnet from the Broken Hill deposit (Spry & Wonder 1989). P-T fields of stability for hercynite,the ferroan The garnet coronason gahnite grains consistof spinelend-member, have been calculated according Fe-rich almandine (Table 2) with an average to the prograde reactions: composition (Fe2.57Mn6.2aMg6.1aCa6.13Nq.6)Al2 Si3Ol2. Garnet elsewherein the same pelites, in Garnet+ Al silicate+ Spinel+ inclusion-rich cores and as an inclusion-free rim Cordierite+ Quartz (1) (retrograde?) on garnet cores, have a similar composition [average: (Fe2.51Mne.21Mgo.rzCao.rg(Loomis 1976)and: Nao.o,)AlrSirO,2l. Compositional profiles across Almandine+ Sillimanite+ Hercynite+ Quartz (2)

(Wall & 1979).The equilibria indicatethat hercynite- qvaftz is a high-temperature(>750'C), medium- to high-pressure(2-8 kbar) assemblage, 37 characteristicof granulite-faciesterranes (Bohlen el al. L986).The presenceof Zn in the crystalstructure 36 FoO is consideredto increasethe field of stability of the 35 spinel into the amphibolite-faciesdomain (Frost 1973,Dietvorst 1980,Montel et al. 1986). 34 The mineral assemblageobserved (quartz - 4 garnet- gahnitewith minor biotite) is simple,which MnO suggestseither l) completion of the metamorphic 3 reactionsabove with the removal of cordierite or 1 (or o 3 sillimanite both), or 2) a different reactionthat p does not involve an aluminosilicate.Cordierite is CaO x 2 not observedin other garnet-bearingpelites at the o I Pinnacles mine; Mg-bearing pyroxene, also a metamorphic product in the sillimanite - spinel - Mso r cordierite - garnet - orthopyroxene system (with 0 excessquartz) for pelitic rocks (Vielzeuf 1983),is rare in theserocks, suggestingthat neither of these 4 two phaseswere presentprior to the occurrenceof .,iIeo+CaO thesereactions. Sillimanite, on the other hand, is a common constituent in many pelites within the mine sequence, although it is nearly always retrogradedto muscovite,which is consistentwith -F+ hydration during retrograde metamorphism(Cor- jrm trom gahnlte/garnet contact bett & Phillips l98l). It is possible,therefore, that Ftc. 3. Compositional profiles of garnet coronas the observed texture of garnet corona around surrounding prograde gahnite, including that shown gahnite crystal in a qvartz matrix, with only minor in Fig. 2c. biotite and apatite, represents the incomplete

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prograde reaction of Wall & England (1979) amphibole rocks from North America; in the (reaction 2) after the removal of sillimanite. PinnaclesMine Sequence,the reverseappears to be However, evidence such as the sharp contacts true. Staurolite is observedin severalspecimens of betweencorona and core, the lack of muscovitein pelitic schists, but not in those exhibiting the this rock, the minor occurrence of retrograde gahnite - garnet reaction. The staurolite is biotite associatedonly with prograde garnet, and Zn-enriched (up to 4.2 wL.t/o ZnO) and forms the similar chemistriesof garnet coronas to both pseudomorphs of gahnite crystals; therefoie, gahnite and garnet crystals, indicates that this staurolite probably formed by the retrograde reaction is a retrograde one that induced the breakdown of prograde gahnite. Biotite is a formation of garnet during the breakdown of common retrograde product of the Fe-rich spinel gahnite. hercynite in hydrous conditions (Montel et ol. The formation of garnet as a product of the 1986).In the PinnaclesMine Sequence,retrograde breakdown of hercynite during retrograde biotite containsonly traceamounts of Zn (0.6 wt.9o metamorphismis predicted(e.g., Vielzeuf 1983)but ZnO detectedin one microprobe analysis),suggest- not widely documentedin the literature. Muscovite ing that Zn was not preferentially hosted in this is the more commonly observed product of mica. Sphalerite,however, is common in rocks of retrograde metamorphism in gahnite-bearing the mine sequence,although not in the immediate pelites;it is observedelsewhere in the mine sequence vicinity of the retrograde textures describedhere. and is widely reported for the Willyama Super- Consequently, the Zn seems to have been group (e.g., Barnes et al. 1983). In many of the transportedby hydrothermal fluids associatedwith gahnile-bearing,garnet-rich pelitic assemblagesof retrograde metamorphism and was deposited in the PinnaclesMine Sequence,garnet crystals are sulfur-rich reducing environments as sphalerite. subhedral, with an inclusion-rich core and an Kyanite- and staurolite-bearing late-stage shear inclusion-freecorona, similar to the gahnitecrystals zonesin the Pinnaclesarea (the Thackaringa-Pin- described above. In these rocks, both garnet nacles Shear Zone; Stevens 1986) indicate that coronas and garnet cores are partially resorbed high-pressureconditions were maintained during along the 52 foliation, which suggestsa pre-S2 the late-stagemetamorphic event that causedthe crystallization.Both coresand rims have the same formation of a garnet rim on gahnite grains as almandine chemistry (Table 2) as do the garnet describedin this paper. The reaction of Wall & coronas to the gahnite crystals describedabove, England (1979) (reaction 2), which has a steep which suggests that garnet grains and garnet positive gradient on a P-T diagram (Bohlen et al. coronasformed in closesu'ccession and, therefore, 1986),also is observedin the micaceousschists both representmetamorphic eventsduring 52. within the mine sequence.The transition from The lack of muscovite and sillimanite in gahnite to garnet in the absenceof sillimanite is association with the q:uartz - garnet - gahnite thermodynamicallyunconstrained, but it seemsto (minor biotite and apatite) assemblagedescribed represent conditions in which temperature here could be interpretedas a result of a gahnite - decreased more rapidly than pressure (cf. garnetreaction, representingdestabilization during petrogeneticgrids for sillimanite-bearingrocks of retrograde metamorphism of a quartz - gahnite Bohlenet al. 1986and Montel et al. 1986).All these assemblagein an anhydrous environment. A factors supp'ofi an isobaric retrogradeevent. Such retrograde reaction in an anhydrous environment an interpretation is consistentwith regional P-T would, however,be unusual, especiallyas evidence calculations that suggestan anticlockwise P-T-t for fluid migration is common elserpherein the path with final isobaric cooling (Corbett & Phillips sequence(e.9., in late-stageshear zones). Alterna- 1981,Phillips & Wall l98l). tively, the assemblagemay repre$enta reaction in the absenceof significant amounts of Al. The AcKNowLEDGEMENTS reactioninvolving the observedphases must include the addition of Fe with minor amountsof Mn, Mg, Ca and Na, and the removal of Zn. Therefore, the This work forms part of an ongoing research retrograde metamorphism took place in the project at the Pinnacles mine supported by the presenceof Fe- and Si-rich fluids that contained N.E.R.C. (UK). I thank MessrsE. and C. Williams minor concentrationsof Mn, Mg, Ca, and Na, but of the Pinnaclesmine for their help, D. Phelan at involved the removal of Zn away from the the University of Newcastle(NSW) for help with immediate assemblage.Possible "sinks" for the the electron-microprobeanalyses, and W. Crebert mobile Zn include staurolite,biotite and sphalerite. for drafting the diagrams. The manuscript has Stoddard (1979)observed the prograde formation benefittedfrom discussionwith I.R. Plimer and R. of gahnite from staurolite in granulite-facies Offler, and comments from P.G. Spry and J.M. metapelites and amphibolite-facies cordierite - Montel.

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