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Stilpnomelane in Metasomatic Rocks Associated With

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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 rap- port 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 TI{E CANADIAN MINERALOGIST or-.^*t!orm -* l:P R_ II I I f' lzz o9''*" fi,.{ x"\'ee)rt \ li I Vt h fso \zz ll3 \ -j l\* trt it E \{o to\ "n? )f" - 5!t -3!t +.. 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 STILPNOMELANE IN METASOMATIC ROCKS, QUEBEC APPALACHIANS 425 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 (<lZ). 426 THE CANADIAN MINERALOGIST 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
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