epigenetic tourmaline in sedimentary red.beds: an ... - GeoScienceWorld

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sandstone of the Rose Hill Formation from westem Virginia. Rounded quartz with minor detrital tourmaline and clay-mineral ooids are cemented by berthierine ...
599 The Canalian Mineralogist Vol. 32, pp. 599-605 (1994)

RED.BEDS: IN SEDIMENTARY EPIGENETIC TOURMALINE VIRGINIA AN EXAMPLEFROMTHESILURIANROSEHILLFORMATION, DARRELLJ. TIENRY.GANG LU ANPCHAD MCCABE Departnvnt of Geology and Geophysics, Inuisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

ABSTRACT Acicular epigenetic tourmaline is developed hemimorphically on the c pole of detrital tourmaline in a Silurian red-bed sandstone of the Rose Hill Formation from westem Virginia. Rounded quartz with minor detrital tourmaline and clay-mineral ooids are cemented by berthierine and hematite that formed epigenetically during a late Paleozoic influx of fluid associated with the Alleghenian orogeny. Detrital grains of tourmaline are very Fe- and Ti-rich but Al-poor. Compositional signatures of the detrital tourmaline suggest a provenance from a highly oxidized granitic rock or a quartz-tourmaline vein. Relative to the detrital substrates, the composition of the epigenetic overgrowths is distinctly different, consisting of Al-rich schorl-dravite with a Fe(Mg+Fe) value of 0.66 and a significant X-site vacancy. Al-enrichment and the X-site vacancy reflect operation of a consequence of growth in an Al-rich but Na- and Ca-poor environment. A high proporthe exchange vector: IAINa-rMg-r, tion of X-site vacancies is commoniy observed in epigenetic tourmaline from sandstones, but the epigenetic tourmaline in this red-bed sequence is the most Fe-rich reported. Epigenetic tourmaline is a sensitive recorder of the chemical environment during early stages of metamorphism of the red-bed sequence. Keyvords: tourmaline, epigenetic, red-bed, sandstone, hemimorphic, detrital, vacancy, hematite, berthierine, exchange vectors, Rose Hill Formation, Virginia.

SomaarnE hdmimorphiquede tourmalineaciculaircauthigbnes'est developp6esur le pole c de grainsd6tritiquesde Un surcroissance tourmalinedansune sdquencede g€s rougesde la Formationde RoseHill, dansl'ouest de la Virginie. Des grains arrondis de quartz, avec une faible concentrationde tourmaline d6tritique et de nodulesde min6raux du groupe des argiles, ont 6t6 cimentdspar un assemblagede berthierineet d'h6matite,dont la formation 6pig6n6tiqueserait due A une incursionde fluide versla fin du Pal6ozoique,lors de I'orogenbsealldghenienne.l,a tourmalined6tritiqueesttrbs riche en Fe et Ti, et appauvrieen Al. Une telle compositionindiqueraitune provenanced'un socle granitiquefortementoxyd6 ou bien d'une association hydrothermalede quartz + tourmaline en fissures.Par rapport au substrat,la tourmaline en surcroissance6pig6n6tiqueest chimiquementdistincte; elle est faite de schorl-dravitealumineuseayant un rapport Fe/(Fe+ Mg) de 0.66 et une proportion en A1 et les lacunesr6sultentdu vecteurd'6changenAlNa-1Mg-1, importantede lacunesdansle site X. L'enrichissement consdquence d'une croissancedansun milieu riche en Al maispauvreen Na et Ca. Une teneur6lev6ede lacunesdansle site X de gros rougesmontre le plus fort est couftmtedansla tourmaline6pig6n6tiquedes grbs; pil contre, celle de cette s6.quence enrichissementen fer qui soit. La tourmalinedpig6n6tiques'avbreOtreun indicateursensibledu milieu chimiqueau coursdes stadesprdcocesdu m6tamorphismedesgtds rouges. Clraduit par la Rddaction) Mots-cl6s:tourmaline,6pigdn6tique,grls rouge,h6mimorphique,d6tritique,lacune,hdmatite,berthierine,vecteursd'6change, Formationde RoseHill. Virsinie.

could not havesurvivedtransportin a clastic sedimentary environment(Spencer1925,Stow 1932, Alty Tourmalinehaslong beenknown to be a diagenetic 1933, Deverin L934, Krynine 1946,Mader 19'78, or epigeneticmineral in sedimentaryrocks. For over Rickefts 1978,Gautier 1979).An additionalfeatureof 100 years,tourmalinehas beenrepodedas an over- diageneticor epigenetictourmalineis its tendencyto growth on cores of derital tourmaline (Wichmann nucleatehemimorphicallyon pre-existingdetrital tour1880,Van Hise 1885,Lory 1887).A diageneticor malirreonly at the positive end of the c axis (antiepigeneticorigin was inferred for theseoccurences logouspole) of the detrital grain(e.g.,Stow 1932, on the basis that tourmaline overgrowthstypically Alty 1933, Krynine 1946, Chatterjeeet al. L975, which Dietrich 1985).Generally,overgrowthshave been developas fragile, slender,syntacticneedlesn INTRODUCTToN

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describedas having pale colors and, on the basisof optical properties,havebeenpostulatedto be elbaite or intermediateschorl-dravite(e.9.,Alty 1933, Awasthi,196l,GovindaRajulu & Nagaraja1969, Mader 1978).However,owing to complexcompeting substitutions in logrnaling,an approachbasedon optical propertiescan lead to equivocaldeterminationsof tourmalinecomposition(Henry & Dutrow 1992). Consequently,a microanalyticalapproachis wananted for thesefine overgrowths. The only microanalyticaldata availableon epigenetic overgrowthson tourmalinewere publishedby O'Connor(1990).He examinedcompositionsof detrital cores and epigeneticrims of tourmaline from Ordovician arenitesof the TuscaroraFormation, centralVirginia. Dark greento opaque,roundeddetrital cores of tourmaline feature overgrowtls of clear, cockscomb-likeepigenetictourmaline.The analytical data of O'Connor (1990)revealedgenerallyFe- and F-rich detrital cores,with a relatively wide compositional variation(14-20VoFeO,0.2-6.2VoMgO and 0.2-1.257oF). Epigeneticovergrowtls are generally richer in Mg and containmoreX-site vacancies(xn), and are distinctly poorer in Fe, F, Na, Ca and Ti (0.16 c BA el@ratod F%q Mtnlro wt%oK2O) is consistentwith this interpretation(see @rdlaated Elts Gice et al. 1993). In accordancewith the preferred Adauttolsl F%oj

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p€! fomla udt. 3 B ato6 @uDhg rc wmde b the @tahednllv dqulated ssudrg (gL gawthoeE et al. 1s93) ed ohatge bale@. g d@!tt1l@. lnwl@ tE tDsslble lf Fs-eddado!

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EPIGENETICTOTJRMALINEIN SEDIMENTARYRED-BEDS

The overgrowthsof epigenetictourmalinein one samplehave a relatively uniform composition, strikingly different from the cores of Al-poor detrital tourmaline.The overgrowth is substantiallyricher in A1 (Al = 6.31 aptu) andxn (457oalkah-defrcient),but poorerin Ti (Tur 3 in Table 2). It is also relatively Fe-rich [Fet/(Fet+Mg) = 0.66], with a minimum of 970 of the Fe as Fe3+,consistentwith growth in a hematite-bearingassemblage. This epigenetictourrnaline compositionis significantly richer.in Fe than the epigeneticovergrowthsreportedby O'Connor(1990), owing to the Fe-rich bulk compositionof the sample. However, both setsof data indicate that epigenetic tourmalinein eachof the investigatedsandstones is rich in Al and x[I, primarily reflecting the exchange vector: tlAlNa-rMg*t. This exchangevector will be most strongly favored in those rocks that are both peraluminousand Na- and Ca-deficient,such as the aluminousberthierine-richmatrix of this red-bed sequence(c/. Werding & Schreyer1984,Foit et al. r989). DlscussroNANDCoNCLUSIoNS Epigenetictounnaline is responsiveto reactionsin the local chemicalenvironmentsand can be an effective chemicalmonitor even at relatively low-temperature conditions.The compositionof the epigenetic tourmaline in the Rose Hill Formation generally mirrors the Fe- and Al-rich, and Na- and Ca-poor nature of the red-bedmatrix. Furthermore,Mg-Fe partitioning betweentourmalineand associatedproximal berthierine[Koru-ner= (Mg/Fe)ro/(Mg/Fe)s.r] is 2.77, wlttch is similar to Mg-Fe partitioning between tourmalineand berthierinein hydrothermalmassive sulfide depositsequilibratedat 350-400'C (Slack & Coad 1989,Slack el al. 1992).The compositionsof layer silicatessuchasberthierinetend to adjustreadily to the epigeneticconditionsassociatedwith tourmaline development(c/. Guidotti 1984). The systematic chemicalcharacteristicssuggestthat chemicalequilibrium was approached,at leastlocally, in the epigenetic settingof the RoseHill Formation. Monopolargrowth of epigenetictourmalineat the c pole proceedswith little or no chemicalinteraction witl the substrateof detrital tourmaline. This hemimorphic growth contrastswith overgrowthsthat form under higher-temperaturemetamorphicconditions of the chlorite zone,in which tourmalinedevelopsasymmetrically at both poles of a detrital grain of tourmaline, resulting in striking compositionaldifferences betweenthe poles (Henry & Dutrow 1992).With an increasein metamorphicgrade,textural asymmetry and compositionalpolarity disappearroughly at staurolite-zoneconditions(Henry 1992).The monopolar growth of tourmalinein the red-bedsamplefrom the Rose Hill Formationreaffirms the low-temperature characterof the reactioninferredfor this seouence.

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The origin of the boron necessaryto form the epigenetictourmalineis uncertain;it could have been derived from four possible sources:(a) dissolution of detrital tourmaline, (b) dissolution of detrital K-feldsparwith minor contentsof B, (c) releaseof adsorbedboron from clay mineralsundergoingthe smectite-to-illite transformationor the lM to 2M1 polytypic transition in illite (Reynolds 1965), or (d) introduction of boron from an external source, suchasthe probablelate Paleozoicbrinesthat affected theserocks(Hearnet al. 1987).Becauseof the refractory natureof tourmaline,the latter three possibilities are consideredthe most likely (c/ Henry & Dutrow 1992). The sourceof the Fe and Al necessaryfor the formation of the diageneticmineralsis still enigmatic. The aluminum could have been derived locally throughthe breakdownof detrital K-feldsparor aluminous mafic silicatesor from clays. However, Fe is more problematicin that the amountof Fe necessary to form the Fe-rich oxidesand silicatesof the matrix is greaterthan that which can be derived from local detrital grains and must have, in part, beenderived ofberthierineas extemally(Lu 1994).The occurrence an apparentlyepigeneticmineral in the red-bedis relatively unusualin that berthierinecommonlydisappears during burial metamorphismat temperaturesof s f S l a c ke r a l . 1 9 9 2 t . < 1 0 0 ' C ( s e ed i s c u s s i o n o However"berthierinehas beenidentified as a stable phasein other,higher-temperature settingscomparable to that of the red-bed(e.9.,Slack et al. 1992,Xl & Veblen1993). Epigenetictourmalineis an ideal mineralto provide information on the diageneticand very low grade metamorphic history of a sedimentaryrock. Tourmaline has a wide range of possiblechemical compositionsthat vary dramaticallyin responseto local chemicalreactionsand, once formed, tends to retain that chemicalimprint. However,therearefew microanalyticaldata availableon epigenetictourmaline.Futurestudiesof the diagenesisandepigenesis of sedimentaryrocks shouldinclude systematic investigationsof the compositionsof any epigenetic tourmaline. as this mineral can aid in the evaluation of the geochemicalevolution of the sedimentary rocks. ACKNOWI-EDGEIvIENTTS

This work was supportedby National Science FoundationGrantsEAR 8805220to Henry and Dutrow and EAR 900534to McCabe.We thank Wanda LeBlanc for the X-ray-diffraction analysis. The manuscript benefitted greatly from insightful and rapid reviewsby J.T. O'Connor,J.F. Slack and B.L. Dutrow. We are grateful to J.T. O'Connor for providing a preliminary version of his manuscripton epigenetictourmaline.

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ReceivedAugust 18, 1993,revised manuscript accepted NovemberlT, 1993.

APPENDX: Et-scrRot {-MrcRopRoBE PRocEDUREs

Minerals were quantitatively analyzed by wavelength-dispersion spectrometry(WDS) using the automatedJEOL 733 electronmicroprobeat Louisiana StateUniversity. WDS analyseswere done at an acceleratingpotential of 15 kV and a sarnplecrurent of l0-15 nA. The electronbeam was focusedto a l-pm spot for the microprobetraverseof tfie tourmaline, and was defocusedto a 5-pm spot for the analysesof associatedminerals.Well-characterized syntheticand naturalsilicatesand oxideswere usedas standards,and the datawere correctedon-line with the proceduresof Bence & Albee (1968) and with the improvedcl-factorsof Armstrong(1988).On the basis of replicate analysesof several secondary

standards,analyticalprecisionfor selectedoxides1s estimatedto be t 0.177osio2,+0.l2vo Al2o3,fl.Mvo FeO, X0.09Vo MgO, t0.0270CaO and t0.037oNa2O. Tourmalineformulae were nonnalizedon the basisof 15 cationsexcludingNa, Ca, andK, i,e., assumingno vacanciesin the tetrahedralor octahedralsites (cl, Hawthorneet al. L993).A minimum value of Fe3+was estimatedassumingcharge-balanceconstraintsand maximal H in the OH site. Any dehydroxylation reactionsinvolving Fe would produceadditionalFe3+. The amount of BrO, necessaryto producethree B cations in the structural formula was calculatedfrom stoichiometricconstraints.