Stilpnomelane in Metasomatic Rocks Associated With

Canadian Mineralogist Yol. 22, pp. 423-435(1984)

STILPNOMELANEIN METASOMATICROCKS ASSOCIATED WITH STEATITEAND IN REGIONALSCHISTS, OUEBEC APPALACHIANS

TOMAS FEININGER'I' Ddpartementde Gdoloeie,Universitd Lavol, Sainte-Foy,Qudbec GIK 7P4

ABSTRACT originelle.A St-Pierre-de-Broughton,un dpisode de d6for- mationposterieur au mdtamorphisme' peut-Otre d'6ge aca- Stilpnomelaneis widespreadat St-Pierre-de-Broughton dien,a d6tach6ca et li lesroches st6atitis6es ultrabasiques and Robertsonvillein the QuebecAppalachians, 65 km de leur enveloppemdtasomatique. south of QuebecCity. The mineral occurs in regionally metamorphosed,intermediate-pressure, biotite-grade rocks Mots-cl4s:stilpnom6lane, salbandes chloriteuses' Appala- of appropriatebulk chemicalcomposition, and in metaso- chesqu6b6coises. matic rocks adjacentto blackwall that enclosessteatitized parts of the ultrabasic Pennington sheet. The ratio GENERALSrarsl4gNt Fe3+/(Fe3* + Fd+) in the stilpnomelaneranges from 0 probably primary Fe2* to 0.83. All Fe3+ represents oxi- Stilpnomelaneis a dark brown to black layer- dizedduring weathering.Stilpnomelane coexists with iron- low-graderegionally rich biotite and chlorite in which the Fe*/(Fe* + Mg) ra- silicatemineral found in some tio exceeds0.57 and 0.53,respectively. Other minerals com- metamorphosedterranes. Although particularly well monly associatedare albite, phengite,quartz and titanite. developedin iron formations, stilpnomelanemay The Xo"./Xs, partition coefficient of mineral pairs and also occur in pelitic, quartzofeldspathicand mafic the distributidn of tie lines lines on the AFM projection rocks. The megascopicappearance and optical prop- suggestthat regional metamorphismand metasomatism ertiesof stilpnomelanemimic closelythose of other, werecoeval. The blackwall formed during steatitizationis more common minerals, and in cursory studies,it considereda residuewhose volume is about half that of may easilybe overlooked.In hand specimen,for ex- precursorrock, Postmetamorphicdeformation, possi- its ample, stilpnomelaneresembles biotite, whereasin bly of Acadian has in placessheared the steatitized age, for biotite, oxidized ultrabasicrocks from their blackwall shellat St-Pierre-de- thin section, it is easilymistaken Broughton. chlorite or green phengite. From the QuebecAppalachians, stilpnomelane has Keywords: stilpnomelane, blackwall, Quebec, Ap- beenreported heretofore only as an accessoryminer- palachians. al in weakly metamorphosedrocks located 30 km southwestof QuebecCity (Zen 1974)and near the SoMMAIRE town of Richmond (Trzcienski1976). The intent of the presentreport is to indicate that stilpnomelane La stilpnom6laneest rEpandued St-Piene-de-Broughton occurswidely in low-graderocks of Early Paleozoic et e Robertsonville,i 65 km au sud de Qudbec,dans les agein the Appalachians,and that the mineral qudb6coises. roches Quebec Appalaches On la rencontredans des is a major constituentof schistsin an area from St- de composition approprieem6tamorphis6es d l'6chellerdgio- Robertsonville. Although nale,i pressionmoyenne, jusqu'au stadede la biotite, ainsi Pierre-de-Broughtonto que dansl'enveloppe m6tasomatique des salbandeschlo- someexposures occur in modern quarries, others are riteuses("blackwall") qui renfermentles parties stdatiti- found in wooded outcrops where, for more than a s6esde l'6caille ultrabasiquede Pennington. Le rapport century, coarse-grainedstilpnomelane presumably * * * Fe3 /(Fe3 + Fd ) de la srilpnomdlanevarie de 0 i 0.83; has been misidentified as biotite. le Fe3+ serait surtout dt d I'oxydation du Fd* primaire par alt6ration atmosphdrique.La stilpnom6lanecoexiste Geot-octcnt- Ssrrtl.{c avecbiotite et chlorite richesen fer danslesquelles le rapport respectivement. Fe*/(Fe* + Mg) d6passe0.57 et 0.53, and metamorphicrocks of the Sont pr6sents: phengite,quartz et titanite. Le Sedimentary Que- aussi albite, into two parts coefficient de partage Xgg./X\4rentre min6raux et I'atti- bec Appalachians are divided broadly tude desassemblages coexistants dans les projections AFM separatedfrom one another by a regional fault or indiquent que le m€tamorphismerdgional et le mdtasoma- unconformity. Rocks of Cambrian and Ordovician tisme 6taient contemporains.Le "blackwall" serait un agein the north constitutethe "Taconian orogen" rdsidu occupantenviron la moitid du volume de la roche (St-Julien & Hubert 1975); rocks of Silurian and Devonian ageof the ConnecticutValley - Gasp6syn- *Present address:Earth PhysicsBranch, I Observatory clinorium occur to the south. Stilpnomelaneis ap- Crescent,Ottawa, Ontario KIA 0Y3. parently restrictedto low-grademetamorphic rocks Contribution from the Earth PhysicsBranch No. 1126. of the Taconian orogen. 423

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Roberlsonville

Frc. l. Generalizedmap of the St-Pierre-de-Broughton- Robertsonville area, showing part of the Penningtonsheet (grey),Bennett Schist (blank), and samplesites Qarge numbers; those in circlesare the compositesamples of schist listed in Table 5). Geologysimplified from unpublishedmap by Pierre St-Julien.

In the St-Pierre-de-Broughton- Robertsonville is found in a strictly regionalenvironment in schists area, located 65 km south of Quebec City (Fig. l), of appropriate bulk chemicalcomposition and meta- stilpnomelane occurs in two contrasting settings. It morphic grade. Stilpnomelane also occurs in a

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metasomatic enyironment in rocks that have under- St-Pierre-de-Broughton(oc. 17, Fig. l) contains gone major chemical exchangeswith abutting ultra- minute flakes of stilpnomelaneless than 0.1 mm mafic bodiesdudng their alteration to talc-carbonate long. One hundred km southwestof St-Pierre-de- rock and steatite. Broughton, a massiveto faintly laminafsd, dssp brown to black metavolcanicrock in the Lower Or- Sru,pNotrlsLANE-BgAFJNcRocrs dovician St-Daniel Formation is exposedalong auto- route 55, 10 km northwest of Sherbrooke. The Regional environment rock containsl09o stilpnomelaneas nearlycolorless (Z pale green)flakes 0.6 mm long, but only 0.015 Stilpnomelanein regionally metamorphosedrocks mm thick. The flakes are arranged in radial ag- has been identified by the author in the Bennett gregatesset in a muddy-appearing matrix composed Schistin the St-Pierre-de-Broughton- Robertson- of finely granular (< 0.02 mm) albite and epidote, ville area (Fig. I and Group E in Table l) and in the with accessorychlorite, sericite, rutile and titanite. St-Daniel Formation near Sherbrooke. Dark brown stilpnomelane in flakes 5 mm in di- Met asom at ic enviro nment ameter is concentrated in laminae in a fine-grained, light yellow-greenepidote schist, 4 km north of St- Unusual stilpnomelane-richmetasomatic rocks are Pierre-de-BroughtonQoc. 16, Fig. 1). Layers of associatedspatially with the ultramafic Pennington coarse-grained chlorite schist in the same outcrop sheetat St-Pierre-de-Broughton,and betweenthat contain flakes of stilpnomelane as much as I cm in village and Robertsonville(Fig. l, Groups A to D diameter. Brown, medium-grained,somewhat gneis- in Table l). Active and abandonedtalc quarriesaf- sic stilpnomelaneschist constitutessmall outcrops ford exceptionalexposures. on a hillside overlooking the Pabner River valley, The Pennington sheet is a relatively thin, gently 6 km southeastof St-Pierre-de-Broughton(loc. 15, dipping, areally extensive ultramafic body that Fig. l). The rock is complexly deformed and con- marks the sole of a regional thrust-fault. The sheet tains sericitized albite. Fine- to medium-grained bi- was emplaced tectonically as serpentinite and sub- otite schist in a roadcut 4 km southeast of sequentlywas altered (steatitized) in many placesto

TAELE1. GENERALIZEDMDAL COMPOSITIONSOF]7 SCHISTSFROII IHE ST-PIERRE-DE-BROUGHTON- ROBERTSONVILLE AREA E

F@ g gg . " ppo-!oEooc-F=F*"5$:eEg.:,9.9 !TUPOFF€UdF96o 6o4OFFPr!ppdA; SarnpleB e ; E 5 A €,? F tr ; e A 3 rielonwberandcomnts .I I IA r A a I l59A Hanging-wall schist. Biotlte pseudonorphic after stllpnomelane. 2 La rt A a I 174q Hanging-wallschlst. 3 AT !A a a a a l74AD Smll t€ctonlc lnclusion. Late rccrysta]lizatlon of sorE albite, 4a A a a a a 174AHl Blackwall shell of large tectonic lncluslon in talc-cartonate mck. 5 ra r A a a a l74AJl lnner shell of sametectonic lnclusion. 6 ar ar a a a a l74A( Core of sanE tectonlc lncluslon. 7 La ar AAAA IT4ALIldem. I ra I AA A 174AVFoot{all schist.

9 Ataa t A AA 177A Hanging-wallschlst. ]0 araaal A A 177F Footcal'lschist. 'll r r ^ a a r A AA 177J Hanging-r,allschlst.

C't2r I At I A 23lA Verymafic mck. '13 '178E rrr I A A ]4 ar^ I A A l78I fts r r a a I I a a a a 774-64 EJ ro r r a a r a a a zoTc Itf r r r a a a t a a t 228 Stllpnonelaneisnotlntexturalequlllbriumwithotherphases. A, Talc-cartonatercck quarry, 2 kmnortiwest of 5t-Plerre-de-Brcughton;B, Steatite quarry norti of rcute ll2, 5 knrnortieast of Robertsonvllle;c' Abandonedsteatite quarry, 0.7 krnsouthwest of Bi D, Abandonedsteatlte quarty at the Broughton soapstoneCo. mlll' 7 kmsoutheast of St-Plerre-d€-Broughton;E, Sanplesnot associatedrrith ultrabaslc rocks. see Flg. I for Iocalitles. I tilajormlneral (tl0%); e Mlnormineral (l-l0Z); AAccessoryrnlneral (

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talc-carbonaterock or to steatitecomposed of talc severalmetres thick and disposedoutward from the alone.Contact relationships ofthe Penningtonsheet ultramafic body. The thicknessand compositionof with the surroundingcountry-rocks mimic thoseof eachzone are determinedby the original composi- small ultramafic bodies to the south in the Ap- tion of the host rock, the composition and availa- palachiansof Vermont and Massachusetts;these bility of hydrothermal fluids, and the metamorphic weredescribed in detailby Chidester(1962), Jahns T and P that prevailedduring the talc-forming reac- (1967)and Sanford(1982). tions (Sanford 1982).Two well-definedzones of Alteration of serpentiniteto talc-bearingassem- metasomaticrocks enveloptalc-rich parts of the Pen- blagesinvolves major chemicalexchanges with sili- ningtonsheet. The innermostzone is blackwall,com- ceoushost-rocks, and leadsto a regular succession posednearly entirely of chlorite. As in many New of metasomaticzones from a few centimetresto England examples(Jahns 1967, p. 147; Sanford

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1982,p.605-606), blackwall is succeededby a zone zoned and contain a core of more felsic rock with enriched in Na, comparable to the albite- stilpnomelaneGamples 6 and7, Table l) that differs porphyroblast rock of Chidester (1962, p. e-65). from stilpnomelane-rich rock on the hanging wall The accompanyingenrichment in iron, which in New in that it contains grey quartz, is relatively darker England is manifested by an abundance of biotite colored, and is layeredand gneissic(Fig. 28) rather (Chidester 1962, p. 123) or by a relatively higher than schistose.Stilpnomelane occrus as undeformed Fetr'/(Ire'r+ Mg) ratio in biotite and amphibole (San- plates more than 5 mm across,some of which are ford 1982,p. 560,565, and Figs. 18,20,22), in Que- arrangedin radial aggregates(Fig. 2C). Other miner- bec is expressed in the crystallization of als present are titanite, zircon, rutile, apatite and stilpnomelane. pyrite.

TALC-CARBoNATE-RoCKQUARRY Ar OrHsn LocALITIES St-Prcnnr-or-BnoucnroN Stilpnomelaneis a major phasein fine- to medium- A singular, fine-grained albite-stilpnomelane gained, dark brown to black, salt-and-pepperalbite- schistmakes up a discontinuouslayer asmuch as 30 rich schist just outside the blackwall of both the cm thick, admixed with albite-porphyroblastrock hanging wall and footwall of the active steatitequar- immediately overlying blackwall on the hanging wall ry (ocalities 9 to ll, Fie. l) 5 km northeast of of the Penningtonsheet. The schistis composedof Robertsonville. On the hanging wall of an aban- untwinned, saccharoidal,white albite in erains <0.1 donedtalc pit oocality l2,Fig. 1), 0.7 km southwest mm in diameter, and subhedral to euhedral black of the active quarry, chlorite and stilpnomelanemake stilpnomelanein bent plates from 5 to l0 mm across up 35 and 3090, respectively,of a dark, fine- to and arranged in radial aggregates(Fig. 2A). Other medium-grained mafic schist similar to blackwall. minerals are titanite (about 490 modally) and minor Stilpnomelane occurs sporadically in fine-grained, chlorite, apatite and pyrite. The associatedalbite- dark brown schistadjacent to blackwall and talc-rich porphyroblast rock is fine- to medium-grained, dark ultramafic rocks at the large abandoned quarry at green-greymuscovite schist pepperedwith euhedral the Broughton SoapstoneCo. mill (localitiesl3 and porphyroblasts of dusted white albite I to 3 mm in 14, Fig. l). The schistcontains bent flakes of mus- diameter. Ilmenite, iron-rich chlorite, titanite and covite and is more quartzose than stilpnomelane apatite are accessories. schist elsewherein the metasomatic environment. Talc-carbonaterock in the quarry containsnumer- ous tectonicinclusions of host rock. Most of the in- MrNsRArocY clusionshave the form of thin slabstens of metres long and have been altered wholly to blackwall. Microprobe analyseswere carried out on 11 grains However, two bulky inclusions are concentrically of stilpnomelane,7 of biotite, 16 of chlorite, 8 of

scHtsTsFRor'r .BROUGHTON. ROBERTSONVILLE AREA Sarple no. 17 sro, 46.5 45.0 45.5 44.3 46.6 M,7 45.1 45.5 45.6 46.5 T10; 0.1 $ 0.0 tr 0.0 0.0 0.0 tr tr Alz0s 6,2 6.0 6.0 6.5 6.6 7.1 5,6 5.6 Fe203 16.7 0.1 0,0 0.4 2.0 23.0 6.6 21.6 l4,5 Feo 15.1 27.5 27.4 27.8 27.0 12.7 8.8 8.4 l2.l l'lno 0.5 0.6 0.4 0,3 0.4 0.9 0.7 0.6 1.6 Mgo 5.7 6.7 7.2 o.u 5.4 b.b Cao 0.1 0.2 0,2 0.2 0.1 0.0 0.3 tr 0.1 0.1 Na20 0.2 0.3 0.6 0.0 0.0 0.0 0.0 KzO 0.6 1.2 1.5 1.7 2.2 0.4 ;.i 0.7 t .of

Nwber of lons on the basls of ll (0)

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muscovite,2 of actinolite and 3 of calcite. Stilpnome- (sample l1) is frorn a natural outcrop enlarged by lane compositions are grven in Table 2, whereasre- quarrying. All rock is weathered in tle shallow pit maining compositions are available, at nominal to the southwest, where sample 12, containing fer- charge,from the Depository of UnpublishedData, ristilpnomelane, was taken. Sample 13, also contain- CISTI, National ResearchCouncil of Canada, Ot- ing ferristilpnomelane, is from a near-surface sile tawa, Ontario KIA 0S2. exposedby quarrying in the last century. All stilpnomelanein natural outcrops (samples15 to 17) is Stilpnomelane ferristilpnomelane. Furthermore, pale green ferrostilpnomelane commonly is encasedin thin shells The author has used the nomenclature of of light brown stilpnomelaneeasily interpreted as the Hashimoto (1969) to distinguish ferristilpnomelane product of incipient oxidation, a r.elationship also from ferrostilpnomelane, with a division between reportedby Hutton (1938),Brown (1971)and Craw the two at a value of the atomic ratio (re81). Fd+/(Fd+ +F*+) of 0.50. The term stilpnomelane Some authors have proposed that progressive is reserved for the entire series. Fe3+ contents were leaching of K accompaniesthe oxidation of ferro- calculated by solving the following pair of simulta- to ferristilpnomelane. Frey et al. (1973) found that ueous equations: green (ferro-)stilpnomelane contains as much as ten times the amount of K in brown (ferri)stilpnomelane. Fe,N_ Fe2++ Fe3+ (l) Brown (1971,p. 282-283)went further and, basing his argument on the composition of stilpnomelane (100 - ferrostilpnomelane) = coexisting with the chemically equivalent pair (Fd+ +Al)/(Fe3+ +Al+Fd+ +Mg+Mn) A) chlorite-minnesotaite(on an Al-Fe-Mg plot) in an iron formation, suggestedthat the minimum content The value (10 - fenostilpnomelane)was determined of potassium in natural occurrencesof stilpnome- eraphically @eer e/ al. 1962,p. 106)based on the lane is 1.8890KrO. n, of each Crain of stilpnomelane measured in The Quebecstilpnomelane sheds some light on sodium light. theseproblems, but one must first emphasizethe er- In the Quebec rocks, stilpnomelaneconstitutes ratic behavior of potassiumand the uncertainty as- euhedral to subhedral plates that in most samples sociated with its analysis in stilpnomelane (Graham have a porphyroblastic habit and a radial arrange- L976, Craw l98l). Commentson the presentana- ment (Fig. 2C). Color and pleocbroismrange widely, lyses (Table 2) are in order. The microprobe at the and dependon the Fe3+,/(Fe3*+ Fd+) ratio of the Universit6Laval is a model 121000ARL fitted with mineral. Ferrostilpnomelanewith little or no Fe3* two crystal spectrometers.Spots to be analyzedwere (samples6 and 7 , Table 2) is weakly pleochroic: X selectedbeforehand and located prechely on thin sec- colorless, Y : Zpale gey-green. With only slightly tion drawings by the author. Analyses by repeated higher Fe3+ contents (samples9, l0), pleochroism ten-secondintegrations, two elementsat a time, were becomespronounced: X light yellow-tan, Y = Z carried out on each spot. Potassiumwas analyzed medium olive-brown. Grains of ferristilpuomelane on the third or fourth run. By completion of the five are stronglypleochroic: Xmedium bright yellow, Y required runs (for nine elements),target spots on fer- : Z deep orange-brown to opaque. rostilpnomelane (exceptsample 3) were darkened to The role of Fe3+ in stilpnomelane has been a deepred-brown color, and many had strongly en- deliberatedextensively @obretsov el a/. 193). Based hancedbirefringence. Target spots on ferristilpnome- on evidence from crystal chemistry (Egeleton & lane were unchanged. Alterations of potassium Bailey 1965),phase equilibria(Zen 1960,Craw 1981) content may have taken place at target spots in the and field relationships(Robinson 1969),agreement courseof analysis,particularly in ferrostilpnomelane. is general(but not unanimous: seeEggleton 1972) For example,whereas analysis of ferrostilpnomelane that stilpnomelane forms as ferrostilpnomelane that in sample7 by electronmicroprobe gave 1.5V0KrO, subsequentlymay undergo oxidation, commonly by analysis of a concentrate ()9590 stilpnomelane) weathering. This view is supported by observations from the samesample gave 0.8690KzO bV atomic in the QuebecAppalachians. Samples6 and 7, con- absorption, and 0.92V0K2O by X-ray fluorescence. taining pure ferrostilpnomelane,were taken from the Stilpnomelanein sample17 behaved in a particular- core of the largesttectonic inclusion, riven by blast- ly unusualfashion: under the electronbeam during ing only months prior to sampling in the quarry at analysis,potassium counts increased progressively. St-Pierre-de-Broughton. Ferrostilpnomelane from Nowhere on the mineral were counts stable, and a the hanging wall of the active steatite quarry north- value of l0/oK2O, the approximatevalue at the on- east of Robertsonville (sample 9) comesfrom schist set of analysis, was assigned(Table 2). The mechan- uncovered by quarrying in the past few years, ism proposed by Graham (197Q and modified by whereas ferrisflpnomelane from fl1e langing wall Craw (1981)best explainsthis behavior: OH- ions

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TABLE3. AI{HYDROI'SFONilULAE OT STTLPIIOIiELAI{E

IDEAL StILPMlrlEUlllE qUEBECSnLPI{0|EL ilES ) Averaqe ferrcstl lpn*i unu(1 | nuno" (K, Na,Ca)0.00 (K, Na,C.)0..15 (K, Na, ca)0.23 (K, - 0.84 Na,Ca)'.* - O.34 (Al, F€, l.lg,tn)2.04 (Al' Fe' lt'lq,ttn)r.r', (Al, Fe, Hs, tln)2.93 (Al, Fe, I'ls, _ l..tt W)Z.qA- Z.gu slg.ro Alo.go sl:.s4 Alo.oo sl:.04 Alo.g6 slr.:l A1o.az - 3.7s - o.zg* - :,oo - o.u (K, Na, Ca)n* (r, (r. (K, o" U* Na,ca)2.6 m, ca)q.r Na,ca)0.7 _ 5,6 (Tlo.r A12.3F":s.s ilgg.: ho.g) (Alz.t (A1l.g F"32.0iglz.7 ffn0.7) F":z.oil9rg.5 ilno.s) (Alo.o- q.s F za.z- rs.r M9r0.8- r5.3 Mn0,3 - (st63Al9)* (st$ A18.3) (s153A15.3) (sr63A15.9 _ .11.6)

(l) saEples6, 7, 9 and l0; 'Chalvel (1972)bqsd on ll (g); s Egglaton(1972) * blsodon Sl " 63i Eggletont Chqppell(1978) bas€d on st , 6t.

volatilized by the beam are replacedby nearby, loose- flakes, is the only white mica in the samplesstudied. ly bound K+ ions that migrate to fill the structural In the metasomatic environment it is widespread at vacanciescreated. The problem of charge balance the three southernmostquarries (samples9 to l4). remains unresolved. Basedon 22 oxygenatoms, Si rangesfrom 6.12 to The correlation between K and Fe3+/(Fe3+ 6.73. The value of Xpmgoniteis between 0.04 and +Fd+) is erratic in the Quebec occurrencesof 0.10, and even lower, 0.03, in the only albite-free stilpnomelane. On average,ferrostilpnomelane con- sample(no. l2). Although generallycommon in the tains more K than ferristilpnomelane (respectively Bennett Schist, muscoviteis presentin only one sam- 1.4 and 0.990 K2O by weight). Nevertheless,the ple (no. 15) from the regional environment. It is most oxidized ferrisiilpnomelane (in sample 12) con- weakly phengitic, with Si = 6.09 and ds6genilg= tains more K (0.9% K2O) than ferrostilpnomelane 0.06. Relatively iron-rich muscovite [Fe*/(Fe* + in sample 3 (0.690). The data do not support the Mg) > 0.53.1 is found with stilpnomelane. In minimum KrO content of unaltered stilpnomelane stilpnomelane-freesample 8, the ratio is 0.32. proposedby Brown (1971). The ideal formula of stilpnomelaneis in dispute. Actinolite Stilpnomelane from QuebecClable 3) shows general agreement with formulae proposed by Chauvel Actinolite occurs in two samples.In sample l, ac- (1973)and Eggleton (1972). tinolite is bright blue-ereenin thin section [Fe*,/(Fe* + Mg) = 0.46i Alzo: : 2.6Vol, whereasin sam- Biotite ple 17 it forms nearly colorlessanhedra [Fe*/(Fe* + Mg) : 0.30; Al2O3= 0.9901. Biotite is anhedral, with Z-direction colors that range from olive to dark red-brown with increasing Calcite Fe*/(Fe* + Mg) ratio. Iron-rich biotire [Fe*/(Fe*+Mg) >0.57] coexistswith stilpnomelane, Anhedml carbonate,in part presentas porphyrob- although relatively magnesian biotite [Fe*/(Fe* t lasts, is a major constituent in the hanging wall at Mg) : 0.501coexists with stilpnomelanein sample the St-Pierre-de-Broughtonquarry. It is also a com- t7. mon accessorymineral in the regional sshists. Car- bonatesin samplesl, 2 and 17 consist of nearly pure Chlorite calcite(>97n/o CaCO3), with minor MgCO3(0.2 to 0.6V0),FeCO, (0.5 to l.2t/o) and MnCO, (0.8 to Progradeanhedral chlorite occursin 15 ofthe 17 1.590). samples.It is secondaryafter biotite in sample l, and is absentin sample9. Compositionsvary chiefly in PstnoLocv the ratio Fex'/(Fe'*+Mg). Strongly pleocbroic, bright grey-green,iron-rich chlorite [Fe*/(Fe* + Mg) >0.53] The discussionthat follows aims to establish:(l) coexistswith stilpnomelane. More magnesianchlo- that the grade of regional metamorphism is uniform rite is pale green, and the chlorite in the blackwall throughout the St-Pierrede-Broughton - Robertson- [sample 4; Fe*/(Fe*+Mg) 0.19] is nearly ville area, Q) that conditions of T and P adjacent colorless. to the Pennington sheetduring steatitization resem- bled those in surrounding host-rocks during region- Muscovite al metamorphism, and (3) that metamorphism of the Bennett Schist and stealitization in the Pennington Phengiticmuscovite, mostly asrobust, subhedral sheetwere coeval.

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Unfortunately, the petrology of stilpnomelane- porphyroblasts occur where the reverseheld, i.e., bearing rocks is not without pitfalls. Firstly, the where deformation outlastedmetamorphic recrystal- chemical composition of stilpnomelane is known lization. Stilpnomelane plates are aligued with imprecisely, and K2O may be in error in many anal- schistosity where deformation and metamorphic yses. Secondly,the role of Fe3* in the mineral is recrystallization were coeval. uncertain. Finally, the porphyroblastic habit charac- Two sampleswere judged on textural grounds teristic of stilpnomelaneis open to multiple interpre- alone to representdisequilibrium assemblages.Sam- tations. ple l, without stilpnomelane,contains cross-cutting, Prograde stilpnomelane typically occurs as bold- lathlike porphyroblasts of anhedral biotite pseudo- ly discordant porphyroblasts that locally are filled morphic after stilpnomelane.In sample 17' stilp- with tiny inclusions (Fte. 2C). Such a texture nomelaneoccurs as minute platesno more than 0.005 dominatesthe Quebecrocks in both the metasomatic mm thick that are not commensuratewith the grain and regional environments, although stilpnomelane size of the schist, where biotite forms flakes from may also be bent or lie weakly oriented with respect 0.1 to 0.35 mm thick. to schistosity.In only one sample(no. 13) is stilpnomelane rigorously parallel with schistosity. Metamorphic grade The porphyroblastic textures are open to two in- terpretations. Either stilpnomelane postdates the Metamorphic grade appears constant in passing regional metamorphismand recordsa later, static, from the more or lessisochemical metamorphism of thermal overprint, or it formed late in the meta- the regional environment to the metasomatic en- morphic episodeafter virtually all deformation had vironment characterizedby substantial chemical ex- ceased.A history of repeatedstilpnomelane-forming changes.The widespreadocqurence of biotite' albite events that differentially affected rocks in the res- and actinolite, the presenceof chloritoid in highly tricted St-Piene-de-Broughton- Robertsonvillearea, aluminouspelites at the Harvey Hill Cu mins, aad particularly in the absenceof intrusive igneousrocks, the absenceof staurolite or garnet, evenin iron-rich is improbable. Rather, the author interprets the tex- pelites, all point to regional metamorphism having tura, varied as they are, to be consonantwith miner- taken place in the biotite zone of an intermediate- alogical equilibrium achieved during a single pressure facies series. Stilpnomelane itself is of lit- metamorphic episode.Stilpnomelane constitutes un- tle use as an isograd minslal (Zen & Thompson deformed porphyroblasts in rocks where metamor- 1974). 11may appearin the lowest-temperaturepart phic recrystallization outlasted deformation. Bent ofthe greenschistfacies (Dobretsov et al.1973), or

(Xpr/x^rn)biot't" ( X p"/X vn)ttilPnomelane (X p"/ X Mr) "tllPnomelane

1xr"/xrn1"n'o'"u 1xr"/xrnlo'ot't" 1xr"/xun)"n'ot"" pairs. Frc.3. Fe,/Mg Fartitioning in biotite-chlorite, stilpnomelane-biotite and stilpnomelane-chlorite All Fe calculated as Fd+. Samplenumbers are from Table l.

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even at lower grades(Seki 1961),and persistsinto X*"./XMspartition coefficients, Ko, have been plot- the amphibolite facies(Brown 1971,Korikovskiy et ted for the mineral pairs biotite-chlorite, al. 1975). stilpnomelane-biotite and stilpnomelane-chlorite (Frg. 3). SamplesI and 17, judged already on tex- Phase equilibria tural grounds to be disequilibrium assemblages, depart significantly. Three ferromagnesian silicates, chlorite, biotite In sample 1, biotite is relatively too iron-rich with and stilpnomelane in order of increasing Fe"'/(Ire'r respect to coexisting chlorite. The biotite is in part + Mg) ratio, arewidely dispersedin the study area. pseudomorphicafter an earlier stilpnomelane,which

c HLO !TE

STILPNOMELANE Feo/ Mgo

tt t UtOt Frc.4. AFM (fhompson) projection of chlorite, stilpnomelaneand biotite compositions,All Fe calculatedas Fd+. Sample numbers are from Table l. Filled circles, sampleswith quartz + muscovite; open circles, sampleslacking quartz or muscovite or the t'wo minerals and therefore not compatible topologically with the projection. Biotite in sample l0 is too fine grained to permit analysis.

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may have influenced its XFe'/X*sratio during the of partition coefficientsand the congruenceof miner- replacementreaction (Brown l97l): al tie-lines from widely spacedsamples provide further evidencethat temperature was broadly uniform muscovite + stilpnomelane during the peak of progrademetamorphism tlrough- - biotite (+ chlorite + H2O) (3) out the area. Moreover, the overlap of samplesfrom the regional and metasomaticenvironments is in Ideally, to be in equilibrium with the chlorite keeping with the view that prograde regional (Xp*/Xue = 0.98) of sample l, a biotite should metamorphism and steatitizatioo in 1hspsnnington have an XF"./XM, ratio of 1.06, whereasthe ratio sheetwere coeval. of a coexistingstilpnomelane should be 1.62 (Fig. 3). The ratio of the biotite i1 sample I is 1.30, an Stilpnomelane and blackwall intermediate value. Presumably, the newly formed biotite, during the replacementreaction, equilibrat- The association of stilpnomelane-rich rocks with ed continuously with matrix biotite to achieve the blackwall is conspicuousin all the talc quarries. Es- medial value. Why the chlorite did not react to be- peciatly sienificant examplesare afforded by tectonic come more iron-rich and reach equilibrium with the inclusions with cores of albite-stilpnomelane rock new biotite is not clear. Some of the chlorite is sec- that are enclosedin talc-carbonaterock in the quarry ondary and replacesbiotite. Perhaps the late chlo- at St-Pierre-de-Broughton. The unique spatial set- rite formed during sanling and equilibrated witl pro- ting of the inclusions permits semiquantitativemass- grade chlorite under conditions unlike those of the transfer calculations of the chemical and volumet- progade metamorphism. ric changesthat may have accompanied formation Fine-grained,accessory stilpnomelane in sample of the blackwall. 17 is not in equilibrium with coexisting magnesian The largest tectonic inclusion has the form of an biotite or chlorite (Fig. 3). The stilpnomelane- ellipsoid of revolution. It is made up of a core of forming reaction, which postdates the prograde albite-stilpnomelanerock (no. 7, Table 4) whose regional metamorphism, may have been @obretsov major and minor axesare 6 and 2 m, respectively. et al. 1973): The core is envelopedby a shell 0.4 m thick, com-

biotite + quartz + magnetite + H2O - stilpnomelane + chlorite + muscovite (4) TABLE4. CHEI'IICALAI{D I,IODAL COMPOSI. TIOIIS AIiD BULK DENSITIESOF THREE ROCKSTHAI @MPOsEA TEC1ONTCINCLU- The presenceof two generationsof chlorite, one SIOII OF COUiTIRYROCK IN TALC.CARBOMIE primary and the other a $econdaryreplacement of ROCKAT ST.PIERRE-DE.BROUGHTOI{ Sarpleno. 4 5 7 biotite, supports the proposedreaction, altlorrgh the si02 30.13 60,02 63.m absenceof muscoviteis unexplained.As in 5ample Tio2 t.60 1.40 1.40 A1203 18.68 18.36 14.85 l, retrograde reactions are ambiguous. Fe203 I .75 l. 39 1.33 Feo 8.75 3.73 3.80 All compositions determined for chlorite, stilp- l,ho 0.08 0.05 0.06 plotted Mso U,87 1.45 l.4s uomelane and biotite have been on an AFM Cao 0.06 0.58 l.l8 (Thompson) projection (Fig. 4). The following ob- Na20 0.13 9.84 8.76 K20 0.00 0.05 0.46 servationsare pertinenfi (1) All iron in stilpnomelane Pzos 0.02 0.32 0.21 c0, 0.02 0.13 0.07 was calculated as Fd*. The restricted scatter ttro" rz.a l.8o 1.Bz of points that represent stilpnomelane with H20- 0.22 0.13 0. t4 Fe3*/(Fd* + Fd+) ratios from 0 to 0.83 (Table2) 99.51 99.25 94.73 Quartz 5.4 lends support to the view that primary iron in stilp- Albite 42.9 66.5 I'lus@ylte 0.1 nomelane is largely or wholly ferrous (Zen 196O, Or'lorlte 97.8 Stl lpnomlile Robinson 1969).Q) Stilpnomelanethat coexistswith Zlr@n 0.3 0.2 iron-rich chlorite, iron-rich biotite, or with both Tl tani te 1.0 Rutile 1.9 dt mins1als,defines a n:urow fan of tie lines. Small Apatite tr 1.8 0paque 1.7 departuresare interpretedas artifacts that result from Calclte tr unreliable analysa for potassium,preferential leach- ,0 100.1 ing of potassiumfrom ferristilpnomelaneby weather- D (g/qt) 2.79 2.71 2.67 4, Blacktrallshell thatsurrcmds tie ing, or a combination of the two. The widely tectonic lncluslon; 5, Inner shell of divergent.tie-lines of samples I and 17 confirm the albite-chlorlte schlst; 7, Core of stllDnorelaneschlst. llet-chemlcal nonequilibrium nature of these samplesestablished analyses by Centre de Recheriheslilln- 6ra1es, Qulbec. lilodal Malyses b6ed earlier. The tie line joining coexisting chlorite and on 1435,1445, ild 1289 polnts, re- spectiwly, on standardthln sectlons biotite in stilpnomelane-free sample 2 falls beyond by the autior. Prcportlons of quartz the most iron-poor side of the tfuee-phasefield chlo- ud alblte detemlned by relatlre peak lntenslties on rhole-rcck dif- rite - stilpnomelane - biotite. (3) The concordance fractogrm.

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posed of albite-chlorite rock (no. 5, Table 4). A TABLE5. Na^OAND TiO^ IN COMPOSITESAI4PLES OF BENNFTTSCHIST 1.S-m-thickshell of blackwall (no. 4, Table 4) con- Sample* Na20 Ti0^ stitutes the exterior of the inclusion. The core and '1.04 shells of the inclusion have chemical compositions I wt.i * *., 2.57 2.08 unlike those of common rocks, and all are metaso- la? '| 0.70 matic replacementsof precursor 4 .48 I .03 a rock. Isolation of 6 'I0.42 0.06 the inclusion in the Pennington sheet, 30 m below .09 U. JC 1[s hangingwall and 75 m abovethe footwall, 7 0.93 0.83 makes 6 I .69 1.02 it likely that metasomaticexchanges were limited ex- clusivelyto the testonic inclusion and enclosingultra- Average 1.38 0.89

mafic rocks. Gradients of chemical potential were r See Figure I for smple sltes, whlch are all from the restricted to inwardly and outwardly directed radial St-Plerre-de-Broughton - Robertsonvil le area. paths, more or less normal to the surface of the inclusion. Diffusion away from the ultramafic rock was restricted to TABLE6. DEllSlTl! VoLUllE,'IIIEBUIK IEIGHT, Na20AND Tl02 lN A IECT0NICBLoC( IN a confined space,and components TAIC-CARBOMTEROCK AT QUARRY,ST.PISRRE.OE-BROUATON, AI{D POSSIBLE could not disperseimperceptibly into vast volumes PRECURSORROC(S OF THE INCLUSION of host rock as presumablytook place at the hang- Rock type Densliy Vol@ Bulk relght l{a20 Tl02 ing wall and footwall. Core 2.67 37.7 100.7 8.8 1.4 ^'. Innef shel l 2.71 30.t 81.6 8.0 l. l The assumption is made here that the chemical Blackrall shell 2.79 223.9 624.7 0.8 10.0 compositionof the precursorrock of the inclusion .7 CoE - lnner shell was similar to that of the pelitic to semipelitic host pEcuBor schlst 2.7 67.8 183.1 2.5 1.6 B. Blackrell 2.7 223.9 604.5 Bennett Schist expo$edbeyond the metasomatic in- precursor schlst fluence of the Pennington sheet.Accordingly, eight compositesamples of BennettSchist were taken from 3l*";:1"::ni:itt 2.7 67.8 rs3.r 2.s r.6 fresh roadcuts and stream-bank exposureswithin a '' Slii*ll *n," 2.7 447.8 rzoe.o 16.7 r0.g few km of St-Pierre-de-Broughton(Fig. l). Two elements, sodium and titanium (expressedas Na2O 515.6 1392.1 19.2 12,4 A. The lncluslo4 as exposed.Core of alb{te-stllpnomlane rek: a 6 E. b 2 and TiO) were selectedfor analysisbecause of their m, D 2.67 g.@-3; Table 4, no. 7. Inner shell of albite-chlorlte rccka 0.4 n tfilck, D 2.71 g.@3i Table4, no. 5. Blackrall shell: 1.5 d thlck.0 respective mobility and immobility during the 2.79 g.ffri Table 4, no. 4. Vol@ ln 03. l{a,Oand TtO, tn bnnes- B. Vol@-for-vol@ replace@nt of pEu6or-schist by-blackmll. precur- metasomatic reactions (Table 5). sor schlst: a 9.8 !, b 5,8 n, D 2.7 q.m3, lta,0 1.382, TlO, 0.89r. C. Vol@ of bllckrall is 50t of tjle vol@ of-DrecuEdr scfrlst. Blck of Three casesare considered (Table 6). Case A is precuGor schlst: a ll.5 !, b 7.5 a, D 2.7 9.m.3, Na201.38t, Tl02 O.O9g. the present state of the inclusion: its core of albite- stilpnomelane rock (volume 37.7 ff), inner shell of albite-chlorite rock (30.1 m3), and shell of black- Pierre-de-Broughton. At first glance it may appear wall Q23,9 m3). Based on the chemical composi- improbable that a volume loss perhaps as gxeat as tions and densitiesof the three rocks, the inclusion 5090 sould be unaccompanied by direct structural contains 17.6 tonnes of Na2O and 12.5 tonnes of evidence. However, the linear shrinkage of the in- TiO2. In B, it is assumedthat metasomatism oc- clusion during metasomatismand volume loss would curred at constant volume; that is, the body of havebeen much less.For example,the long axis of precursorrock had the volume of the presentlyex- the inclusion in the quarry is about 9.8 m; with a posedinclusion Q91.7 m3), a density of 2.7 g/crf , shell of precursor rock of twice the volume in place and the tenors of Na2O and TiO, in averagehost- of the present blackwall shell, the long axis would rock (Table 5). The contents of Na2O and TiO, be 11.5. This is a diminution of lessthan 1570,an (respectively10.8 and 7.0 tonnes)of such an inclu- amount perhaps easily passed unnoticed. Sanford sion fall far short of the amounts of the oxides ac- (1982) has already remarked on this relationship in tually presentin the metasomatizedrocks (Iable O. a study of blackwalls in New England. An equal volume of precursorrock containsless than two thirds of the required NarO, and just over one Why stilpnomelane? half the TiO2. Case C assumesthat the blackwall shell rqrresents a residue whose volume is only one Stilpnomelaneis not common in metamorphic ter- half of the precursor rock. Here the calculated con- ranes. Its surprising abundance in the area of this tents of Na2O and TiO2 are remarkably close (with- report is due to a fortuitous combination of favora- in 10 and lVo, respectively)to the contentsofthese ble metamorphic conditions and appropriate com- oxides observed in the inclusion. position of the rocks. Around the world, The figures usedin the calculationsabove are semi- stilpnomelane is restricted chiefly to low-grade and quantitative at best. Their purpose is solely to show high- or medium-pressure metamorphic rocks the likelihood that significant volume-lossaccompa- Miyashiro 1973). In the St-Pierre-de-Broughton - nied the metasomatic formation of blackwall at St- Robertsonville €uea, the Bennett Schist is in the

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greenschistfacies. Contiguous rocks to the south- dreas,took the photographsof the hand specimens westwith blue amphibole(Trzcienski 1976) and om- (Fig. 2). I acknowledgehelpful commentson the phacite(Laird & Albee l98l) showthe high-pressure manuscriptby ProfessorWalter E. Trzcienski, Jr., affinities of the metamorphic belt. of the Ecole Polytechnique,Montr6al, Dr. Richard Among pelites and psammites,only rocks with F. Sanford, of the U.S. GeologicalSurvey, and an high Fe*/(Fe* + Mg) ratios and low contentsof Al anonymousreviewer. This study has beenfinanced and K can contain stilpnomelane.As illustrations, by grant 47348 of the Natural Sciencesand En- even though iron-rich, aluminous pelites contain gineeringResearch Council of Canada,and FCAC chloritoid in placeof stilpnomelane,and K-rich rocks grant EQ-1005 of the Ministdre de I'Education du contain biotite. Thus biotite in samplesl0 and ll Qu6bec. is restrictedto small flakes adjacent to aggregates of muscovite,the only K-rich domainsin the respec- REFERENCES tive rocks.Mineralogical evidence points to the Ben- nett Schist being poor in K relative to Na. BnowN, E.H. (1971): Phase relations of biotite and Accordingly, in the alkali metasomaticrock adja- stilpnomelane in the greenschist facies. Corfr. cent to blackwall,enrichment was chiefly in Na, and Mineral. Petrologlt 31, 27 5-299. stilpnomelaneis the ferromagnesianmineral ob- (1973): Contr. servedrather than biotite, which is characteristicin CseuvEr-, J.-J. Les stilpnomdlanes. Mineral. Petrology 3E, 37-44. someVermont occurrences(Chidester 1962, p. 123, 125).In fact, molar Na2O,/(Na2O+ K2O) ratios of CsroEsrBn,A.H. (1962): Petrology and geochemistry albite-rich metasomatic rocks at St-Pierre-de- of selectedtalc-bearing ultramafic rocks and adja- Broughton range from 0.97 to 0.99 (Table 4), cent country rocks in north-central Vermont. U.,S. whereasan albite-porphyroblastrock with biotite Geol. Sum. Proj. Pop.345. from Vermont has the ratio 0.83 (Chidester1962, Table 3, no. 24). Cnew, D. (1981):Oxidation and microprobe-induced potassium mobility in iron-bearing phyllosilicates from the Otago schists, New Zealand. Lithos 14, ACADIAN(?) DEFORMATION 49-57.

The history of deformationand metamorphismin DpBn,W.A., Howle, R.A. & Zussrr,rar,I. (1962):Rock' the St-Pierre-de-Broughton- Robertsonvillearea is Forming Minerals. 3. Sheet Silicates. Longmans, complexand incompletelyunderstood. Nevertheless, London. the fact that multiple deformationand the main epi- sodeof metamorphismtook placeduring the Taconic Dosnrrsov, N.L., Knrcsrov, V.V. & S6soI-Bv,V.S. orogenyis well established(St-Julien & Hubert 1975). (1973): The Facies of Metamorphism at Moderute Postmetamorphicshearing has separatedthe Pen- Pressures(trans. by D.A. Brown). Australian Na- nington sheetfrom its blackwall shell along the foot- tional University, Canberra. wall in quarry The the at St-Pierre-de-Broughton. Eccnrol, R.A. (1972): The crystal structure of stilp- former presenceof blackwall is recordedby trains nomelane. II. The full cell. Mineral. Mag. 3E, of coarse,mechanically broken grains of pyrite on 693-7It. the surfaceof the host schistwhere overlain by steatite or talc-carbonaterock. Pyrite is prominent only & Bensv,S.W. (1965):The crystalstructure of in blackwall, where commonly it forms euhedral stilpnomelane. I. The subcell. Clays Clay Minerals cubesseveral mm acrossthat may reach390 modally. 13, 49-63. The shearingmay be of Acadian age.St-Julien e/ (1978): al. (1983)have speculatedthat Acadian deformation -& Crnpppn, B.W. The crystal structure III. Chemistry and physical prop- in the Notre Dame (site of stilpnomelane. anticlinorium of the Pen- erties. Mineral. Mag. 42' 361-368. nington sheet), heretofore unrecognized,may be widespread.Evidence in the quarry at St-Pierre-de- Fnev, M., HuNzIren, J.C., Roccwtu-Bn,P. & ScutN- Broughton supports this view. orpn, P. (1973): Progressive niedriggradige Metamorphose glaukonitfiihrender Horizonte in den AcrNowt-scH\4sNrs helvetischenAlpen der Ostschweiz. Contr. Miner- al. Petrologlt 39, 185-218. For the analysesof samples,I thank J.-P. Trem- GnaHau, C.M. (1976): A note on some Dalradian blay (microprobe), R. Fillion (X-ray fluorescence) stilpnomelanes and their oxidation. Mineral. Mag, and M. Gigudre (atomic absorption). Professor 40,467-472. PierreSt-Julien graciously allowed access to his un- published geologicalmapping of the St-Pierre-de- Hessruoro, M. (1969): A note on stilpnomelane Broughton - Robertsonvillearea. My father, An- mineralogy. Contr. Mineral. Petrology 23, 8688.

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HurroN, C.O. (1938):The stilpnomelanegroup of and Geophysicsof Mountain Chains (R. Hatcher minerals. Mineral. Mag 25, 172-206. H. Williams & l. Zietz, eds.).Geol. Soc.Amer. Mem. lSt, 103-111. JesNs,R.H. (1967):Serpentinites of the Roxburydis- trict, Vermont. 1z Ultramafic and Related Rocks SeNrono,R.F. (1982):Growth of ultramaficreaction @.J. Wyllie, ed.). JohnWiley & Sons,New York. zonesin greenschistto amphibolitefacies metamorphism.Amer. J. Sci.?$2,543-616, Kop.xovsnr,S.P., BonoNrxruN, V.A. & Lapurnle,I.P. (1975):High-temperature boundary of the stability Snrr, Y. (1961):Pnmpellyite in low-grademetrmor- field of stilpnomelanein the kyanite metamorphic phism. "/. Petrology2, 407423. complex, Patom Mountuns. Acad. Sci. USSR Dokl., Earth Sci.Sect. 222,205-207. TnzcrcNsrr,W.8., Jn.(1970: Crossiticamphibole and its possibletectonic significancein the Richmond Lerno, J. & Alsrs, A.L. (1981): High-pressure area,southeastern Quebec. Can. J. Earth Sci.13, metamorphismin mafic schist from northern Ver- 7lt-7t4. morft. Amer. J, Sci, 281,97-126. ZBN,Bar (1960):Metamorphism of Lower Paleozoic Mryasnrno,A. (1973): Metamorphism and Meta- rocksin the vicinity of the Taconicf,ange in west- morphic.Bells.Halsted, New York. centralVermont, Amer. Mineral. 45, 129-175. RosrxsoN, P. (1969): Weathering of ferro- (1974): Prehnite- and p'tmpellyite-bearing stilpnomelaneto ferri-stilpnomelanein the Otago mineralassemblages, west side of the Appalachian Schist, East Otago, New Zealand. Geol. Soc, metamorphicbelt, Pennsylvaniato Newfoundland. Amer. Abstr. Programs l' 288'289. J. Petrology 15, 197-242.

Sr-JuunN,P. & Husnnr, C. (1975):Evolution of the & TnoupsoN,A.B. (1974):Low graderegional Taconian orogen in the QuebecAppalachians. metamorphism:mineral equilibriumrelations. ,422. Arner. J. Sci. 2ll5A,337-362. Rev. Earth Planet. Sci. 2, 179-212.

-, Slrvrrsrv,A. & FerNrNcrn,T. (1983):A deep structural profile across the Appalachians of ReceivedMarch 16, 1983,revised manuscript accepted southernQuebec. .fu Contribution to the Tectonics September24, 1983,

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