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Calcic Myrmekite:Possible Evidence for the Involvementof Water Duringthe Evolutionof Andesineanorthosite from St-Urbain,Ouebec

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Calcic Myrmekite:Possible Evidence for the Involvementof Water Duringthe Evolutionof Andesineanorthosite from St-Urbain,Ouebec CanodianMineralogist Vol.25, pp. 291-319 (1987) CALCIC MYRMEKITE:POSSIBLE EVIDENCE FOR THE INVOLVEMENTOF WATER DURINGTHE EVOLUTIONOF ANDESINEANORTHOSITE FROM ST-URBAIN,OUEBEC ROBERTF. DYMEK Department of Earth and Planetory Sciencesand McDonnell Center for the SpaceSciences, Washington University, St. Louis, Missouri 63130, U.S.A. CRAIG M. SCHIFFRIES Deportmentof GeologicolScienca, Harvard University,Combridge, Mossachusetts 0213E, U,S.A, ABSTRACT de myrmdkite le long de la bordure des grains de plagio- clase, en agr€gat bulbeux (l mm) ou en liserC (entre l0 et Vermicular intergrowths of quartz + plagioclase feld- 1@ pm de largeur). Le plagioclase de l'intercroissance a spar are found in andesineanorthosite from the St-Urbain uoe composition entre An62 et An92, et prbs de Ansg dans massif, Quebec,where they compriseup to about I volume la plupart des cas. Par contre, le plagioclase de l'anortho- 9o of individual samples.Such "myrmekite" occurs along site hote est une antiperthite de composition An4613. plagioclasegrain-boundaries of the host anorthosite as bul- L'intercroissance contient environ 23t/o de quartz p,u bous aggregates (up to - I mm across) to thin films volume. L'interface entre le domaine myrm€kitique et le ( - l0-l@ pm wide). Plagioclasein the myrmekitesranges plagioclase h6te est franche, le changement en composi- from An62 to An92, with most compositions near Ans6. In tion du plagioclase 6tant abrupt. Ces caractdristiquestex- contrast, plagioclase of the host anorthosite is an antiper- lurales e1chimiques font penser que l'interface repr6sente thite with composition An40*3. Quartz contents of the un front de r€action arr€t6e, of le plagioclase sodique se myrmekites average abottt ?3a/oby volume. Boundaries fait remplacer par un plagioclase plus calcique, r6action between myrmekite and host plagioclaseare sharp, across accompagn&par la pr€cipitation du quartz. Un calcul mon- which the change in plagioclase composition is virtually tre que la conversion de AnaA d An6g, avec conservation instantaneous, These textural and chemical features sug- de I'aluminium, produil presqueexactement la quantitd de gestthat the boundariesrepresent an arrestedreaction-front, quartz observde. Cette myrmdkite calcique rdsulterait de where sodic plagioclasewas replaced by calcic plagioclase I'interaction corrosive d'un fluide aqueux d'origine mag- accompaniedby precipitation of quartz. Calculation shows matique, d tempdrature €levde,avec les cristaux de plagio- that the conversion ofAna to Ans6 (conservingAl) yields clase des cumulats. Cette interprdtation concorde avec les almost exactly the amount of quartz observedto occur in r€sultats oe6riment4ux sur la rdaction entre plagioclaseet the myrmekites. We interpret these calcic myrmekites to fluide aqueux,tandis que l'interprdtation d'un front de r6ac- be the products of corrosive interaction betweencumulare tion concorde avec les pr&ictions de la th€orie de finfil- plagioclase crystals and a magnatically derived, high- tration d'un fluide. Une origine de l'intercroissancepar cris- temperature aqueous fluid. This interpretation is consis- tallisation cotectique, par exsolution d'un feldspath ou par tent with experimental results on plagioclase-fluid ddcomposition d'un pr6curseurne peut p.rs 6tre envisagde. equilibria, whereasthe sharpnessof the myrmekite bound- Il est probable qu'une intercroissancesemblable se trouve ary is consistentwirh predictions from fluid-infiltration the- dansles autresmassifs d'anorthosite ainsi que dans desplu- ory. Altemative origins for thesemyrmekites through cotec- tons mafiques stratiformes. Rdpanduemais pas facilement tic crystallization, exsolution from feldspar, or rep6r6e ou tout simplement ignor6e, elle serait un indice decomFositionof a precursor phaseaxe trot consideredvia- de la pr6senced'un fluide aqueux dans desroches jusqu'ici ble. Ostensibly similaf iqfslg'6wths occur in other massif jug€es anhydres. anorthosites and also in layered mafic plutons. Thus cal- (Traduit par la R6daction) cic myrmekite may be a common (but easily overlooked or ignored) feature of anorthositic rocks in general,provid- Mots-clds: anorthosite, antiperthite, exsolution, phase ing evidencefor the presenceof aqueous fluids in systems fluide, infiltration, myrm6kite, plagioclase, renplace- heretofore considered anhydrous. ment, St-Urbain, Qudbec. Keywords: anorthosite, antiperthite, exsolution, fluids, infiltration, myrmekite, plagioclase, replacement, St- Urbain, Quebec. INTRoDUgfioN propo$ed Souulnn In 1960,A. F. Buddington that massif anorthosites crystallized from a water-bearing mag- Une intercroissancevermiculaire de quartz et de plagio- ma of overall "gabbroic anorthosite" composition. claseoccupe environ l9o du volume de l'anortho$ite e and6- His principal reason for invoking a hydrous situa- sine du massif de St-Urbain, au Qu6bec.On trouve ce genre tion was the experimentally demonstrated shift of 291 292 THE CANADIAN MINERALOGIST the plagioclase-clinopyroxene cotectic boundary in GEoLocIcAL Ss'rm{c the systemDi-An-Ab toward more feldspathic com- positions with increased HrO-content. Hargraves The St-Urbainmassif is locatedabout 120km north- (1962) developed similar arguments and suggested eastof QuebecCity, aud is situatedwithin the central that outward escapeof watsl aessmpaniedsolidifi- high-gradegrarulite terraneof the Grenvillestructural cation of anorthosite, thereby promoting anatexisof province. It is a relatively small, oval pluton (about country rocks, which could account for the 15 x 30 km; Fig. l) composedalmost exclusivelyof widespreadassoiiation of anorthositewith the so- anorthositic rocks, predominanfly andesineanortho- called "charnockite" suite. Yoder (1968a)reviewed site of severaltextural types, with minor leuconorite various lines of experimentalevidence bearing on the and a few small Fe-Ti oxide deposits(Mawdsley 1927, petrogenesisof massif-type anorthosites, and also Roy et al. 1972,Rondot 199). Available petrological noted that water favors the production of highly feld- and geochemicaldata indicate a comagmaticrelation- sPathicmagYnas. ship for theselitlologies (Gromet & Dymek l98l). In Alttrough there is still no consensuson the sub- addition, as much as l59o of tle massif is underlain ject of parent-magmacomposition, many investiga- by relativelyolder labradoriteanorthosite that is petrc. tors (e.9., Ashwal 1982)have subsequentlyembraced logically, gmchemicallyand isotopicallydistinct from, Buddington's idea of gabbroic anorthosite, but lir- and apparently not direcdy reliatedto, the andesine tle if any definitive evidencehas been marshaled in anorthosite (Dymek 1980, Gromet & Dymek 1980). support of a significant role for HrO in massif Whereverlabradorite anorthositeis found in tle mas- anorthosites. In fact, evidence to the contrary is sif, it is cut by dykes of the andesineanorthosite (see seemingly abundant, as an anhydrous plagioclase- Fig. 2A in Dynek & Gromet 1984). pyroxene-olivine-oxide mineralogy is symptonatic ln general,the massiflacks peiretrative deforrnation, of massif anorthositesin general.plsyailing opin- corona structurs and other evidencefor significant ion, as summarizedin Morse (1982),is that the par- posfiragmatic modification. The border zona (outer ent msgna for anorthosite was extremely poor in I km) locally consist of layered or banded rocls water. enrichedin orthopyroxene,Fe-Ti oxide, or both. This However, the andesineanorthosite at St-Urbain, "foliation" app€rs to repres€ntan igrmus f€ture that Quebec,which is the subject of the presentstudy, was accentuatedduring emplaceinentof tle massif to contains a variety of features that may be explained its presentlevel in the crust (Roy et ol. 1972;cf. Mar- as plausibleconsequences of a water-bearingmagma. tignole & Schrijver 1970).The country rock includes These include: highly oxidized primary magmatic pyroxene-bearinggranitic intrusive rocks of overall ilmenite - hematite solid solution, high Fe3+ in quartz monzodiorile composition,banded "charnock- pyroxene, lack of significant F&+ /Mg variation in itic" gneisses,and a complenlydeformed sequenceof silicates, widespread presenceof biotite, abundant paragneissesand granites. At one locality near the reversezoning in plagioclase(even in rocks where northwest corner of tle massf, a thin (100m or las) plagioclaseis but a minor constituent, such as in Fe- unit of apatite-, ilmenite-, magnetite-rich "norite" Ti oxide ores), and the presenceof myrmekitic inter- @owell et al. ,98,) separatesanorthosite from coun- growths of quartz + calcic plagioclase.The suppres- try rock. Other oxide-apatite rocks that lack magne- sion of F*+ /Mg fractionation in silicatesmay be tite are associatedwith the ilm€nite orebodies,where explainedby the magmaticreaction: 2FeO + H2O: they may containaccessory rutile and sapphirineas well FgO, + I{2 (which should also promote precipiadon as abundant biotite @ymek 1984). of Fe2Or-rich ihnenite), whereas reverse 2siring in plagioclasemay reflect tle welldocumented shift of the plagioclase"melting loop" to more anorthitic com- SAMPLE DESCRIPTIoN positionsunder hydrousconditions. Each ofthese fea- ture merits detailed corsideration, and possibly has Data presentedin this paper are restricted to sam- alternative explanations. ples of andesineanorthosite, the locationsofwhich In this paper we focus primarily on only one of the are shown on Figure 1. One of these(STU 24.-7)is above: we report on t}le texfirres, bulk compositions the plagioclase-rich host to an extremely coarse- and plagioclasecompositions
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