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 plagioclase, 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 compositions 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 feature 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 of "myrmekites" that are grained anorthosite that contains orthopyroxene found in the St-Urbain massif, and speculateon their megacrysts,which are dessibed in Dymek & Gromet origin through corrosiveinteraction betweencumulate (1984).The remainder of the samplesare anortho- plagioclasecrystals and lak-stage, high-temperature sites sezsa slrlclo (those from "Mont du lac-des- aqueousfluid. Thesereults are combined with addi- Cyges" are examplesof anorthositedyke) that con- tional data and observationson the anorthosite,in the sist of more than 9590plagioclase togetler with small interest of developing a preliminary model for the amounts of Fe-Ti oxide (as exsolvedlamellar inter- emplacementand crystalization history of the St- growths with approximate compositions llmrrHemp Urbain massif. and Ilm3sHemrs), clinopyroxene (CaarMgorFelsto CALCIC VTYRMEKITE IN THE ST.URBAIN ANORTHOSITE 293

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Frc. l. Simplified regional geology of the St-Urbain area, Charlevoix County, Quebec (adapted from Rondot 1979). Numbered localities correspondto the following samples:l(Mont du Lac-des-Cygnes:MLC P-368, Mr .C 20-4, MLC l9-2, MLC 20-3, cHV 80-76); 2(STU 24-19); 3(STU 24-20X 4(STU 224);5(SIU 22-25);6(STU 24-7);1$'rU 22-9,STU 22-11); 8(STU l4-rA); 9(STU 2a-18); 10(STU22-23); II(STU 25-7); rZ(STU 24-21).

Caa5Mgaef'err),orthopyroxene (Enrs-rJ, biotite (X1* nent in finer-grained materials and in anorthosite - 0.75-0.85;- 24 wt.tlo TiO2; - l-3 wt.qo F), dykes. very rare fluorapatite and, of course "m)nmekite", Factors that control the distribution of plagioclase which may constitute up to I volume Vo of a given megacrysts are unclear. Crystal sorting during sample. primary igneouscrystallization is suggestedby align- In the field, the anorthositesare seento consist ment of megacrystsin somedykes. The involvement fypically of 1-20 cm (ocally up to 50 cm) dark blue- of igneous processes is supported further by grey plagioclase megacrysts set in a finer-grained widespread examples of progressive coarsening of (typically mm-sized) matrix of pink to pale grey grain sizeinto pegmatitic zoneswhere giant crystals plagioclase.The proportions of megacrystsmay vary of plagioclase, orthopyroxene and Fe-Ti oxide are from 0 to 100q0in a given outcrop area, such that found commonly with biotite and, rarely, quartz. On the anorthosites range from somewhat homogene- the other hand, it is possible that the anorthositic ous, medium-gFainedrocks to very coarse-grained matrix resulted from granulation or recrystallization "pegmatitic" types, although grain size seemsbest of megacrysts.In severalcases, individual megacrysts characterizedas seriateporphyritic. Of particular sig- appear fractured and seemingly invaded by the nificance to the present study is the petrographic matrix. This problem is consideredfurther in the next observationtlat "myrmekite" appearsmore promi- section. 294 THE CANADIAN MINERALOGIST

FELDSPARSoF THEANIEsINE ANORTHoSITE multiple sets of intersecting arrays that produce a "Widmansttitten" texture. Contrary to the generali- TexturaI charact erist ics zation drawn by Carstens(1967), however, the blebs in the present caseare not localized at composition In thin section,plagioclase exhibits subrounded, planes of albite twins, but seemto occur at almost polygonal cross-sections,and tabular forms are typi- any angle to them; they may even lie parallel to the cally lacking, although some megacrystsare some- compdsition planes of pericline twins. what elongate l- [010]. Grain boundaries show The plagioclassalse coalains orientedplatelets and straight segments,but are highly inegular and even rodlets of exsolvedFe-Ti oxide (which comprisetiny suturedin detail, commonly with interlocking pro- lamellar hematite-ibnenite intergrowths), and acic- trusions between adjacent crystals. We seeno com- ular to stubby clinopyroxene(<2O p,mlong) crys- pelling reasonswhy such textures could not have tals or crystal aggregatesthat may also be of exso- resulted from the mutual interference of plagioclase lution origin. In a few cases,the clinopyroxene crystals growing from a melt. aggregatesoccur with granular Fe-Ti oxide and, All "matrix" grains display well-developedalbite rarely, apatite. and pericline twins, which are curved or bent in some At plagioclase grain-boundaries, granular Fe-Ti cases. Other features indicative of internal strain oxide and subequantto i'vermicular" clinopyrox- include unduloseextinction, mosaicismand incipient ene(both < 100pm) may be found. Theseintersti- polgonization. Plagioclasemegacrysts are also well- tial materialsmay havecrystallized from a trapped twinned, but exhibit very little evidenceof internal melt component. strain, although bending of albite twins and minor zonesof polygonization have beennoted. The lat- Chemical compositions ter appearto be localizednear pyroxeneand oxide inclusions, and are terminated abruptly at megacryst The compositions of plagioclase and alkali feld- margins. spar from the andesineanorthosite (Tables l, 2), Theseobservations permit us to addressthe rela- togetherwith the compositionsof feldsparsand other tionship betweenmegacrysts and matrix plagioclase minerals (Tables 4-O, were determined by electron- in a preliminary way. For example,if the finer grain- microprobe analysis using methods outlined in size of the matrix is a result of deformation and Dymek & Gromet (1984). recrystallization of megacrysts,then one would Plagioclase compositions are generally within a expectthis "new" feldsparto be largely strain-free few mole percent of Ano6. Individual gains are and the "relist" megacryststo be highly strained, remarkably homogeneous, with typical composi- whereasthe opposite appearsto be the case.Thus tional variation within any sample being less than we cannot accountfor gxainsizevariation in the ande- 3 mole VoAn. However, there are important (and sine anorthosite entirely by in situ degradation of not well-understoodt) exceptionsto this generaliza- megacrysts.Moreover, we cannot provide textural tion. For example, every boundary with interstitial evidence for dynamic recrystallization to a finer pyroxene is characterizedby a thin (- 10-50 pm grain-size, such as that illustrated by Kehlenbeck wide) rim of reverselyzoned plagioclase.Moreover, (1972) for the Lac Rouvry massif. everypyroxilrc inclusion in plagioclaseis sunounded Nevertheless,it is evident that the feldspar grains by a l0-50-pm wide "halo" of reversely zoned experiencedsome type of deformation, in which the plagioclase. In both situations, compositions as cal- applied stress was accommodated principally by cic as An, are encountered,K contents of the modification of "matrix" plagioclase, with only plagioclasedecrease to less 0.05 wt.9o K2O, and minor effects on the megacrysts.Whether the resul- both exsolved oxides and alkali feldspar blebs are tant strain was inducedduring movementof a largely absent. consolidated crystal-mush or was related to exter- The processes leading to the development of nal forces is unclear, although we prefer the former reversezoning may havesome bearing on tle present interpretation. problem. Some preliminary interpretations are Plagioclase in the samples studied occurs as presentedin this report, but a detailed consideration antiperthite, and examples of exsolution textures of reverse 2sning and its implications will be resembling nearly every variety figured by Carstens provided in a subsequentpaper (cl, Dymek 1981, (1967)from the Egersund(Norway) anorthosite can Dymek & Gromet 1984). be found at St-Urbain. The blebs of exsolvedalkali A representative plagioclase composition from feldspar range in shape from rectangular to ellipti- each of the 17 samplesinvestigated for this project cal to highly elongate, have apparent widths rang- is listed in Table l. These data were selectedto ing from I to 15 pm, and are oriented parallel to approximatethe mean composition in that specimen, several crystallographic directions ofthe host. The and collectively outline only a small range of over- blebs may comprise single setsof parallel arrays, or all composition (An, to An ). The valueslisted for CALCIC MYRMEKITE IN THE ST-URBAIN ANORTHOSITE 295 17.0 \K Alsi308 I a t6.5 I ol a a a a a ) o o l' a 'a l't-l fa ta a o l.' .tt a a \} I a t aa be 0 i t5.5 I = ? ./" io.,i'/ ol o t5.0 fol o rl (\I ,, ol o 0l o o, ol o a/ \l oc 5| poinls ol c) QI NI \it'/ -1.YI 14,0r- 0,0 05 1.0 l,5 2,0 Wt.%Bo0 Frc.2. Abundances of BaO and K2O in exsolvedalkali feldspar blebs in antiperthite from andesineanorthosite.

TABLE1, REPRESENTAIIIECOI4POSITIONS OF PLAG1OCLASEHOSI iN AI{IIPEIITHITE

$t. z L. 2. 3. 4. a L2. 13. 14. IA si02 57.34 57.90 58.14 57.96 53.86 58.33 58.28 58.71 58.09 58.20 59.27 58.45 59.2! 59.24 57'9? 58.07 51-52 l1o2 0.04 0.03 0.00 0.00 0.03 0.03 0.03 0.01 0.00 0.03 0.02 0.02 0.05 0.01 0.05 0.03 0.00 A1203 26.94 26.60 26.62 26.58 25.69 26.58 26.47 25.5O 26.r7 25.79 25.88 25.7O 25.73 25.82'26.53 26.53 26.44 Feo 0.14 0.08 0.10 0.13 0.15 0.10 0.20 0.13 0.11 0.18 0.14 0.08 0.I3 0.15 0.12 0'10 0.09 MSo 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.90 0.00 0.00 0.00 0.00 0.0rJ 0.00 0.00 0.00 Cao 8.73 8.56 8.34 8.45 8.50 8.38 8.69 7.91 8.25 8.30 7.64 8.03 8.06 7.83 8.49 8'72 8.56 sro - 0.13 - o.I2 0.12 0.07 0.13 0.10 0.14 0.13 0.15 0.07 0.13 - 0.11 0.06 0.11 Bao 0.00 0.00 u.04 0.04 0.06 0.u0 0.05 0.u5 0.07 0.05 0.03 0.00 0.02 0.00 0.00 0.06 0.00 KzO 0.41 0.39 0.34 0.39 9.39 0.36 0.32 0.43 0,21 0.27 0.30 0.19 0.J1 O,44 0.J3 0.39 0.37 Na20 6.28 6.46 6.77 6.81 6.40 6.69 6.49 6.84 6.86 6,76 7.I4 6.97 6.91 6.85 6.58 6.62 6.43 loral 99.90 100.16t00.38 1U0.491rJ0.20 100.55 r00.66 99.75 99.88 99.71 100.57 99.51 100.54100.35 100.14 lrjo.58 99.61

FORMULAPROPORTIONS BASED ON 8 OXYGENATOI'IS si 2.514 2.592 2.596 2.590 2.631 2.599 2.598 2.636 2.607 2.617 2.637 2.628 2.636 2.639 2.593 2.592 2,590 Ti 0.001 0.001 0.000 0.000 0.001 0.001 0.001 0.000 0.000 0.001 0,001 0.00I 0.002 0.000 0,002 0,001 0.000 Al 1.425 1.403 1.401 1.400 i..353 1.396 1.391 1.350 1.381 1.367 1.357 1.362 1.350 1.356 1.400 1.396 1.404 Fe 0.005 0.003 0.004 0.005 0.006 0.0M 0.007 0.005 0.004 0.007 0.005 0.003 0.005 0.006 0.005 0.004 0.007 Mq 0.001 0.001 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.4?0 0.411 0.399 0.405 0.407 0.400 0.415 0.383 0.397 0.400 0.364 0.387 0.384 0.374 0.407 0.417 0.413 Sr - 0.003 0.003 0.003 0.002 0.003 0.003 0.004 0.003 0.004 0.002 0.003 0.003 0.002 0.003 Ba 0.000 0.000 0.001 0.001 0.001 0.000 0.001 0.001 0.001 0.001 0.001 0.000 0.00rJ 0.000 0.000 0.001 0.000 K 0.023 0.022 0.021 0.o22 0.022 0.021 0.018 0.025 0.012 0.015 0.017 0.011 0.018 0.025 0.019 0.022 0.021 NA 0.547 0.561 0.586 0.590 0.555 0.578 0.561 0.596 0.597 0.589 0.61.6 0.608 0.596 0.592 0.571 0.573 0.561

4.997 4.997 5.007 5.016 4.980 5.001 4.995 4.999 5.005 5.0u1 5.001 5.000 4.995 4.991 5.000 5.007 4.999

ENo-I'IEMBERPR0P0RTI0NS

An 42.4 41.5 39.7 40.0 4t.4 40.2 41.9 38.3 39.7 40.0 36.8 38.6 38.7 37.7 4r.0 41.3 41.7 lu 58.3 57.8 56.4 57.7 56.3 59.2 59.1 58.4 61.5 60.3 59.6 59.7 57.1 56.5 56.2 0r 2.3 2.2 z.L 2.2 2.3 2.! 1.8 2.5 r.2 1.5 1.7 1.1 1.8 2.5 1.9 2.2 2,L

trl sru 24-19. t2l Sru 24-20. t3l STU22-22. [4] I'ILCp-368*. [5] MLC20-4*. 16l Sru 22-25. [7] MLc19-2*. [8.] STU24-7. [9] l4LC20-3*. [10] CHV90-76*. [1r] sru 22-9. [r2l STU14-14. [13] STU22-ll. [14] SrU24-18. [l5l STU22-23 [16] SIU25-7. [Ul Sru 24-21. - . not ana]yzed *sanplesfrm anorthoslt'lcdykes, Mont du Lacdes Cygnes 296 THE CANADIAN MINERALOGIST

TAELE2. AVERAGECOMPOSTTIONS OF POTASSTUM FETDSPAR ELEBS IN ANIIPERTHITE

Yt. z L. 2. 3. 4. 5. 7. 10. ll. 14. 16 L7.

sr02 63.34 63.94 64.75 65.17 64.9t 64.17 64.90 63.58 63.79 64.23 64.86 64.57 64.L3 64.35 63.85 62.47 63.59 Ii02 0.00 0.01 0,0i 0.00 0.05 0.02 0.05 0.00 0.u0 0.05 0.03 0.03 0.05 0.00 0.01 0.05 0.00 A1203 1a.58 18.69 18.77 18.06 17.65 18.71 17.87 18.77 18.65 18.72 18.49 18.39 18.77 18.76 18.88 18.42 18.77 Feo 0.05 0.16 0.06 0.05 0.07 0.01 0.05 0.03 0.00 0.05 0.06 0.05 0.08 0.08 0.05 0.03 0.04 r'190 0.00 0.04 0.00 0.00 0.00 0.00 0.02 0.(D 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cao 0.00 0.06 0.0i 0.10 O.0Z 0.04 0.00 0.0s 0.02 0.09 0.00 0.02 0.03 0.14 0.03 0.06 0.03 sro - 0.14 0.24 0.13 0.10 0.t7 0.25 0.t0 0.03 0.26 0.12 0.09 0.16 - 0.21 0.18 0.08 Bao 1.04 0.97 1.07 0.40 0.63 0.59 0.82 0.75 1.08 0.78 0.66 0.'t7 0.12 1.01 r.20 r.32 1.45 K20 16.03 16.20 16.33 15.24 16.18 16.10 15.77 16.30 15.99 16.L8 15.72 16.43 16.44 15.89 16.16 16.45 15.96 t{a20 0.33 0,25 0.24 0.zg 0.31 0.33 0.29 0.22 0.28 0.34 0.32 0.24 0.19 0.20 0.24 0.18 0.27

Total 99.37 100.46 101.54 99.43 99.93 100.14 100.02 99.00 99.84 100.70 100.26 100.49t0o.s7 100.43 t00.64 99.16 1rr0.19

ON8 OXYGEN si 2.972 2.970 2.976 3.020 3.017 2.978 3.013 2.968 2.97ri 2.972 2.995 2.990 2.976 2.978 2.965 2,955 2.967 t1 0.000 0.000 0.000 0.000 0.001 0.000 0.002 0.000 0.000 0.001 0.001 0.001 0.002 0.000 0.000 0.002 0.000 A1 1.028 1.024 l.0lB 0.987 0.967 1.023 0.9i8 1.033 1.026 1.0?t 1.006 1.004 1.025 1.024 1.033 1.028 1.032 Fe 0.002 0.006 0.002 0.002 0.003 0.000 0.002 0.001 0.000 0.001 0.002 0.002 0.003 0.003 0.002 0.001 0.002 llg 0.000 0.003 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Ca 0.000 0.002 0.003 0.005 0.001 0.002 0.000 0.002 0.001 0.005 0.000 0.001 0.001 u.007 0.001 0.002 0.001 - 0.006 0.006 0.004 0.003 0.005 0.007 0.003 0.001 0.007 0.003 0.002 0.005 0.006 0.005 0.002 Ba 0.020 0.018 0.019 0.008 0.011 0.015 0.014 0.014 0.020 0.014 0.012 0.012 0.013 0.018 o.022 0.024 0.027 0.960 0.960 0.958 0.901 0.960 0.953 0.934 0.971 0.952 0.955 0.926 0.971 0.971 0.938 0.95/ 0.994 0.950 Na 0.030 0.022 0.023 0.0?5 0.028 0.030 0.026 0.020 0.025 0.031 0.035 0.022 0.0u 0.0180.024 0.017 0.024

t 5.012 5.009 5.005 4.952 4.991 5.002 4.97' 5.012 5.00r 5.0u7 4.980 5.005 5.0t3 4.986 5.010 5.031 5.005

END-MEMBERPROPORTIONS

An 0.0 0.8 0.9 1.0 0.4 0.7 0.7 0:q 0.2 0.2 0.3 0.3 0.6 0.7 D.7 0.7 0.3 Ab 3.0 2.2 2.3 2.7 2.4 3.0 2.6 2.O 2.5 3.1 3.6 2,2 r.7 1.8 2.4 1.6 2.4 0r 9s.0 95.2 94.9 95.5 95,7 95.3 95.1 96.I 95.2 94.4 94.9 96.3 96.4 95.6 94.8 95.4 94.6 Cn 2.0 1.8 1.9 0.S 1.1 1.1 1.5 1.4 2.0 1.4 1.2 t.2 1.3 1.8 2.2 2.3 2.7

[1] sru a4-19. [2] STU24-20. L37SrU 22-22. [4] i'lLCp-368r. [5] MLC20-4*. [6] STU22-25, l7l i,lLCt9-2r. l8l SrU24-7. !91_HLc20-3*. [10] cHvso-20*. [ul sru 22-9. lrzl sru 14-1A.tr3] sru z2-1r. l14l sru z4-1s. lrsj sru 22-43 Lr6.l sTU 25-7. ILTJ sTU 24-21. - o not analyzed .samples from anorthoslilc dykes, Mont du Lac des Cygnes

KzO (- 0.2-0.4wt.Vo), FeO (- 0.1-0.2wt.9o) and SrO ( - 0.1 wt.9o) aretlpical of most compositions. SI. URBTII{ Tracesof Mg, Ba and Ti may also be present,but ANORIHOSIIE: most values are below limits of detection (<0.05 Andesine wt.9o each). Antipatthile Compositions of blebs of exsolvedalkali feldspar, representingthe averageof 3-5 analysesin eachcase, are listed in Table 2. Overall, the blebs show little variation in composition (ca. Ore5),being characterized by very high K, very low Na, and virtually no Ca. Similar K-rich compositionsfor blebs of exsolved alkali feldsparwere reported byAnderson (1966')and Kay (1977). Small amounts of Fe (- 0.1 wt.9o FeO) and Sr (- 0.1-0.3 wt.% SrO) are present,but only traces of Mg and Ti occur. However, a noteworthy feature HOSI ofthe alkali feldspar is a large relative variation in 'ai'" " l{s r; ri ii m oo ?o 0o e0 Co Ba content (- 0.4-1.6 wt.9o BaO), such that Frc.3. A plot of atom 9o K-Na-Ca for antiperthite from K2O/BaO ratios range from l0 to 30 (Fig. 2). andesine anorthosite. Although this variation in Ba content may simply CALCIC MYRMEKITE IN TI{E ST-I'RBAIN ANORTHOSITE 297 reflect selectivepartitioning into potassium feldspar is the low K content of the host, and the low Na and during exsolution, it is possiblethat changesin K,/Ba Ca contentsof the blebs.Application of two-feldspar ratio may ultimately provide important information geothermometry (e.g., Whitney & Stormer 1977)Io on the courseofdifferentiation ofthe andesineanor- thesecompositions yields temperaturesof equilibra- thosite (cl, Anderson 1966). Data on whole-rock tion ranging from 310 to 380oC,averaging 345"C. samplesare currently being gatheredto evaluatethis It is highly unlikely, however, that these values possibility. represent the temperature at which exsolution was Compositions of the plagioclasehost and blebs of initiated, but probably reflect closure of K-Na exsolved alkali feldspar are shown in Figure 3 in exchange between host and bleb following a terms of atom 9o Ca, Na and K. Of particular note prolonged and complex thermal history.

Frc. 4. A. Type-A myrmekite occurring as a subrounded aggregate between two plagioclase crystals in sample MLC l9-2; note elongate protrusions into top plagioclasegrain (transmitted cross-polarizedlighu bar scale500 pm). B. Type-A myrmekite extending from grain boundary into plagioclase in sample STU 24-7 (transmitted cross-polarized lighq bar scale 5@ pm). 298 THE CANADIAN MINERALOCIST

MynNasKnE norm from the initial analytical data, determination of correction factors for each element weighted Methods of analysis and data presentotion against the abundance of that element in the nor- mative minerals, recalculation of the bulk analysis, Three varieties of "myrmekite" have been iden- and reiteration. Sincethe myrmekitesars "simple" tified (termed types A, B and C, respectively),which quartz-feldspar intergrowths that lack a heavy are characterized in detail below. These have been absorber(e.9., iron), the additional correctionsare analyzed for their bulk compositions and for the small, and of the sameorder of magnitudeas the compositions of their constituent plagioclase. Myr- counting error associatedwith the analysesperse(- mekite bulk-compositions were determined by l-290 relative). microprobe analysis using an expanded electron- The myrmekite bulk-compositionshave been recal- beam, where the spot diameter (30-50 pm) was culated to end-member proportions of An adjusted to accommodatethe sizeof the intergrowth (CaAl2Si2O8),Ab (NaAlSi3Or),Or (KAlSi.Or) and in each case. At least three analyseswere obtained Qtz (as Si4OJ, where Qtz = 100 - An - Ab - Or. on each sample. The trace amounts of Mg, Fe, Ti, Ba and Sr were The electron microanalysis of such multiphase neglected in this calculation. aggregatesis problematic, however,because standard Volume vlo quartz (as SiO) in the myrmekites matrix-correction procedures assumea homogene- was calculated from their bulk compositions using ous target as well as a general coincidence between the following expression: the path lengths of the incident electron-beam and all generatedX-rays. In the presentcase, these stipu- lations are not fulfilled, and an additional correc- Vol. VoQtz: tion'for an inhomogeneoustarget has been applied (4 x mole 7o SiaOJ(V-qJ using the approach outlined by Albee et al, (1977), , This method involves the calculation of a cation (4 x mole 9o Sip)(Vo) + (mole 9o Fsp)(Vr'n)

Ftc.5. Back-scatteredelectron (BSE) and X-ray imzgeeof a homogeneoustype-A nyrmekite from andesineanorthosite. CALCIC MYRMEKITE IN THE ST-URBAIN ANORTHOSITE 299 where = Vpro X*\L, + XAbVAb+ Xo.Vo' and the which it seemsto replace (Fig. B). Relict "islands" factor of 4 reflectsconversion of SiaOsto SiOr. We of the host plagioclasehave been observed,which have used the molar volume data of Robie Zr a/. is consistent with a replacement origiu. 0979) (Vatz:22.59 cm3; Voo: 100.79 cm3; Var : [email protected] cm3;Vor: 108.62cm3). Boundaries between the intergrowths and surrounding plagioclase are extremely sharp, being gently curved with some straight segments,although TYPE-A MYRMEKITE they may undulate on a pm scale. Observations in reflected light and use of back-scattered electron (BSE) imaging confirm the sharpnessof the bound- Occltrrence,form and distribution of elements ary and reveal the eutectic-like texture of the inter- Type-A intergrowths comprise subrounded 1o grofihs. A portion of the most commonly observed subangular to plumose aggregates$ituated typically variety of type-A myrmekite is illustrated in Figure at plagioclasegrain-boundaries (Fig. 4A), although 5. The BSE imageshows the intergrowth to consist a few appearto be enclosedentirely by plagioclase. of a homogeneousbright-reflscting "matrix" (which Theseare the most corrunonones observed, and con- is plagioclase) containing 20-30s/o uniformly dis- stitute up to - I volume 9o of somesamples. Most tributed dark-reflecting l-S-pm wide curvilinear are 100to 500 pm in maximum dimension, although "blebs" (which consist of quartz). This figure also a few are as large as about 3 mm. The borders of showsthe contrast in reflectivity betweenplagioclase the intergrowths are generally convex toward in the host anorthosite and that in the intergrowth plagioclase,and in many casesthe myrmekite pene- (which indicates a strong compositional difference), trates far into one of the adjacent plagioclasegrains, whereas the X-Ray images confirm that the myr- llr"i

ff,$sm, ry14i Fto.6, BSE and X-ray imagesof a heterogeneoustype-A myrmekite from andesine anorthosite. 300 THE CANADIAN MINERALOCIST mekite "matrix" is enriched in Ca and depleted in A secondand less-commonvariety of type-Amyr- Na compared to the surrounding plagioclase, and mekite is illustrated in Figure 6. It is characterized that the intergrowth as a whole is devoid of K-rich by a variation in the width of quartz blebs (produc- material. The tendency of many quartz blebs to be ing a "cauliflower" texture) and the occurrenceof oriented normal to the external interface can also be minor K-rich areasat the innermost boundary with observedin Figure 5. the host.

TABLE3. BULKCOMPOSIIiONS OF TYPE-AI4YRMEKITE

l"/t,. % 1. 2. 3. 4. 6. 7. 8. 9. 10.

si02 60.38 5B.15 57.25 58.39 59.93 60.54 59.56 59.24 58.61 59.88 Tioz 0.06 0.00 0.02 0.02 0.00 0.03 0.04 0.05 0.02 0.05 A1203 25.22 26.90 27.29 26.34 25.03 25.59 25.6L 24.74 27.08 25.25 Fe0 0.L6 0.05 0.06 0.22 0.06 0.07 0,L2 0.17 0.13 0.18 l4g0 0.03 0.00 0.01 0.00 0.02 0.00 0.01 0.00 0.00 0.00 Ca0 ll.79 L3.16 13.iB 13.31 L3.zz r.3.06 L3.zs 13.10 13.44 13.3s Sr0 0.13 0.10 - 0.06 0.07 0.07 0.10 - 0.06 0.07 Ba0 0.02 0.00 0.02 0.03 0.02 0.01 0,02 0.04 0.01 0.01 Kzo 0.19 0.10 0.03 0.23 0.00 0.03 0.05 0.06 0.01 0.00 Naro 2.05 1..89 1.59 1.41 1.51 1.42 1.34 L.zL 1.15 1.07

Total r.00.03r.00.35 100.05 100.01 99.86 100.82100.15 98.60 t00.50 99.86

FORI4ULAPRUPORTIONS BASED ON 8 OXYGENATOMS si 2.677 2.585 2.556 2.605 2.665 2.663 2.644 2.667 2.593 2.66L tt 0.002 0.000 0.001 0.001 0.000 0.001 0.001 0.002 0.001 0.002 AI 1.3t7 1.409L.437 1.385 L.3L2 L.327 1.341 1.313 1.41.3L.322 Fe 0.006 0.002 0.002 0.008 0.004 0.002 0.005 0.006 0.005 0.006 Mg 0.002 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.0q0 Ca 0.561 0.627 0.660 0.636 0.630 0.615 0.632 0.632 0.638 0.636 Sr 0.003 0.003 - 0.002 0.002 0.002 0.003 0.002 0.002 Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.00L 0.000 0.000 K 0.011 0.006 0.002 0.00B 0.000 0.002 0.003 0.003 0.000 0.000 Na 0.176 0.1630.r.37 0.L22 0.130 0.L2L 0.115 0.1050.099 0.093 x 4.753 4.795 4.195 4.767 4.745 4.733 4.744 4.729 4.75t 4.722 si+Al 3.994 3.994 3.993 3.990 3.977 3.990 3.985 3.980 4.006 3.983 si/0* 0.335 0.323 0.320 0.326 0.333 0.333 0.331 0.333 0.324 0.333 xc.** 0.751 0.788 0.826 0.830 0.830 0.833 0.843 0.854 0.864 0.872 Vof. Z Qtzt 23.3 18.5 i.8.5 ZL,6 ZZ.2 Z4.Z 23.1 24.0 24.3 ZS.L

END-MEMBERPROPORTIONS

An s6.1 63.0 66.0 63.6 63.0 61.5 63.2 63.2 63.8 63.6 Ab L7.6 16.3 13.7 L2.2 13.0 L2.L 11.5 t0.5 9.9 9.3 0r" 1.1 0.6 0.2 0.8 0.0 0.2 0.3 0.3 0.1 0.0 Qtztt 25.2 20.L 20.L 23.4 24.0 26.2 25.0 26.0 26.2 27.L [1] STU24-Le (4). [2] STU24-20 (3). [3] sTu 22-22(5',. [4] MLCP-368 (5). t5l NLC2o-4 (3). 16l STU22-2s (3). [7] MLC1e-2 (10). [S] STU24-7 (s), igj MLC20-3 (7',t. trol cHi,80-76 (r0). ( ) = # of analyses averages - = not analyzed * Si cations/oxygenassociated with all cations * Xau = Ca/(Ca+Na*K)atomic tas Sl0, tt as o SiOO, CALCIC MYRMEKITE IN THE ST.URBAIN ANORTHOSITE 301

tr si408 o si408

TYPEA .. :,,lb tr . t'-T ':1 lypEc Aaa d

(No,K)Al Sir0s 20 80 Co AlrSir0,

KAtSi308 KAlSisos

TYPEC ^A I a^ a a A- |YPEA

NoAlSir0, CoAlrSir0t

FIo.7. Proportions of the feldsparsand "quartz" (as trSiaOf in St-Urbain myrmekites, calculated from bulk compositions.

In transmitted plane-polarized light, the inter- In general, the intergrowths are inclusion-free, growths are seento consist of two colorless phases although some contain oxide granules oriented with differing indicesof refraction, which indicesare parallel to (and possiblyrelated to) those found in in turn different from thoseof the host plagioclase. nearby plagioclase.In only one casewas a K-feldspar Some intergrowths are brown and turbid, however, bleb found within an intergrowth; this exceptionmay s\4ring to the presenceof Ca,Al-silicate alteration representthe effect of a gently sloping grain bound- phases. In transmitted cross-polarizedlight, the ary with the adjaqent antiperthitic plagioclase. plagioclaseofthe intergrowthsis seento consistof up to severalgrains that extinguish in different posi- Bulk compositions tions. It hasnot beenpossible to determinewhether. the quartz blebs representoptically continuous sin- Bulk compositions of type-A myrmekite from l0 gle grains or numerous smaller ones. samplesare listed in Table 3, wherethey are arranged r0

80 85

Mole% Anorthite Fto. 8. Histogram of plagioclase compositions in type-A myrmekite. 302 THE CANADIAN MINERALOGIST

in order of increasing Xc"[: Cal(Ca+Na+K)]. from 20 to 27 mole 9o flSiaOs. This correspondsto lAnalyses were also obtained on myrmekites in 6 a range of calculated volume tlo quartz (as SiO) other samples, for which additional mineral- from 18.5 to 25.W0, which is entirely compatible chemicaldata are presentedin Tables 1,2, 4 andT, with visual estimates. but these results are not reported becausethey are Other noteworthy features of these analyses consideredunreliable sying to low analytical totals include variable X* (corresponding to a range of (85-95wt.9o) causedby combinationsof alteration average plagioclase compositions from Aq5 to and poor polishing. Nevertheless,the relative oxide Anrr), relatively constant formula-proportions for abundancesin there.otheranalyses are in good agree- (Si + Al) at 4.0 (a value expectedfor a quartz-feldspar ment with the compositions that are listed in Table intergrowth), and relatively constant Si/O at 0.333 3.1These compositions are characterizedby high soa- (an unanticipated result, which is discussedin detail tents of Ca, Al and Si, relatively low Na, and virtu- later). Further inspection of the data reveals a very ally no K. Values for Mg, Fe and Ti are also very slight excessin Al p.e., Al > (2Ca + Na + K)1, low, which precludesthe presenceof mafic silicates which most likely reflects analytical error rather than or oxidesfrom the intergrowths. Quartz contentscal- anything significant about tle bulk compositions. culated from theseaverage bulk-compositions range In order to show that averaginghas not biasedour

TABLE4. coMposITIoNs0F pLAGiocLAsEIN TypE-A I'IvRMEKITEi

WN.Z L, 6. 8. 10. 11. L?, l? L7.

si02 46.33 46.74 46.2L 47.40 45,25 46.65 46.95 45.95 46.70 47.81 52.30 46.46 47.23 51.55 51.0r Ti02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.04 0,02 0.00 0.05 At?03 34.29 34.23 34.10 33.54 34.37 34.58 33.96 34.24 33.91 JJ.U6 JU.OI J4.JJ 33.77 3L.76 30,76 Feo 0. 12 0.L2 0.00 0.L2 0.13 0.08 0.14 0.08 0.11 0.1r 0.2L 0.13 0.20 0.09 0.L2 i{90 0.00 0.00 0.02 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.00 Cao L7.04 17.25 L7.75 17.73 17.50 L7,32 L7,42 t 7.08 16.9i. t6,34 L2,24 16.99 L7,L5 L4.07 13.4t Sr0 0. 17 0.27 U. II U. IU 0.lr. 0.13 0.09 0.13 0. t1 0. l0 0.2i. 0. 14 0. i5 0.L0 Bao 0.00 0.00 0.00 0.02 0.00 0.04 0.00 0.00 0.00 0.01 0.00 0.04 0.00 0.02 0.00 KzO 0.05 0.05 0.00 0.04 0.04 0.03 0.06 0.07 0.06 0.05 0.16 0.04 0.07 0.28 0.15 NaZo t.58 L.67 1.60 0.94 1.39 1.65 1.64 1.56 1.80 2.01 3.98 1.70 1.48 3.20 3.65

Totai 99.57 100.33100.28 99.93 98.86 100.46100.29 gg.Lz s9.62 99.51 99.65 99.96 100.07 101.12 99.25

FORr,luLAPR0L0RTI0NS BASEp 0N 8 0XYGENAToMS

st 2.L39 2.L44 2.L20 2.176 2,LLl 2.L36 2,L53 2.L3L 2.L54 2.201 2.376 2.L38 2.L68 2.320 2.337 Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.001 0.001 0.000 0.002 Al 1.865 1.851 1.876 1.815 1.889 1.866 1.836 1.872 1.8rU 1.795 1.639 1.862 1.827 1.684 1.661 Fe 0.005 0.005 0.000 0.005 0.005 0.003 0.005 0.003 0.004 0.004 0.008 0.005 0.008 0.003 0.005 Mg 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.004 0.000 0.000 0.000 0.000 Ca 0.843 0.848. 0.873 0.872 0.874 0.850 0.856 0.849 0.836 0.806 0.596 0.838 0.844 0.678 0.658 Sr 0.005 0.007 - 0.004 0.005 0.003 0.004 0.002 0.003 0.003 0.003 0.006 0.004 0.004 0.003 Ba 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 K 0.003 0.003 0.000 0.002 0.002 0.002 0.003 0.004 0.003 0.003 0.009 0.002 0.004 0.016 0.009 Na 0.L42 0.148 0.142 0.084 0.125 0.147 0.146 0.141 0.161 0.179 0.350 0.152 0.132 0.280 0.325 r 5.000 5.006 5.012 4.958 5.010 5.006 5.004 5.003 5.006 4.992 4.984 5,006 4.986 4.986 4.998

END-MEMBERPROPORTiONS

An 85.4 85.0 86.0 91.1 87.4 85.1 85.2 85.4 83.6 81.6 62.5 84.6 86,2 69,7 66,4 An 14.3 L4.7 14.0 8.7 L2.4 L4.6 14.5 L4.2 l6.t 18.1 36.5 L5,2 13.4 28.6 32,1 0r 0.3 0.3 0.0 0.2 0.2 0,2 0.3 0.4 0.3 0.3 0.9 0.2 0.4 1.6 0.9

[i] sru 24-1e. [2] STU24-20. 13) sru 22-22. l4l MLCp-368*. isl MLC2o-4*. [6] sn' 22-25.l7l MLCle-z*. L8l STUz4-7. [9] MLc20-3*. lr0l cHv80-76t. !r1l STU22-9. [12] STU14-1A. i13l STU22-11. [15j STU22-23. [17.] STU24-21; * sanplesof anorthosit€dykes from Mont du Lac des Cygnes

f analysis #rs teyed !o data in Tables 1 and 2 - - not analyzed CALCIC MYRMEKITE IN TIIE ST-URBAIN ANOKTHOSITE 303

Frc.9. A. Type-B myrmekite appearing as a dark filn decorating plagroclasegrain-boundaries in sample MLC 20-3 (transmitted plane-polarizedlight; bar scale I mm), B. Close-upview of type-B myrmekite shown in Figure 9A (transmitted plane-polarized light; bar scale 100 pm). C. Type{ myrmekite forming blocky "inclusions" in plagioclase megacryst in sample MLC l9-2 (ransmitted cross-polarized lighu bar scale I mm). D. Close-up view of type-C myrmekite showtr in Figure 9C (transmitted cross-polarized light; bar scale 1@ pm. presentation, we have plotted in Figure 7A the rela- range, from Anro to Ano. This is not an artifact of tive proportions of Qtz-An-(Ab + Or) for the 59 sanpling, but a realistic representationof the actual individual bulk-compositions;the samedata are illus- distribution of plagioclasecompositions. In fact, trated on Figure 78, plotted in terms of relative someof the more sodic compositionsmay not be part proportions An-Ab-Or. In both cases,the cluster- of a "new" myrmekite feldspar, but may be modi- ing of the data points is evident, indicating that the fied "relics" of the original host plagioclase.Addi- averagesprovide a useful representationof the entire tional information sa this point is presentedin a sub- data-set. sequent section.

Compositions of constituent plagioclase TYPE.BMyfuTSKITE A representativecomposition of plagioclasefrom each of the ten examples of myrmekite described Type-B intergrowths comprise lG to l0X).pmwide above, and for five others on which useful bulk- semicontinuous "films" that decorate plagioclase analysescould not be obtained, are listed in Table grain-boundaries.These locally widen and appearto 4. These miaeral analysescomplement and extend grade into 8pe-A intergrowths, and their textural the bulk analyses, and confirm tlnt plagioclase in (aud chemical) characteristics are largely indistin- the intergrowths is universally more calcic than that guishable from those outlined above. Type-B inter- in the host anorthosite (c/ Table l). growths seem more susceptible to alteration than A histogram ofall analyzedplagioclase in type-A thoseof type A, and in severalsamples, plagioclase myrmekite is illustrated in Figure 8. Although the grain-boundaries are accentuated by this brown total range of measured compositions is An5s to material. Photomicrsgraphs of typical type-B inter- dn , the majority fall within a more restricted growths are illustrated in Figures 94 and 9B, and 3M THE CANADIAN MINERALOGIST

FIc. 10. BSE and X-ray images of a type-B myrmekite from andesineanorthosite.

combinedBSE and X-ray imagesare shownin Figure As shown in Figure 9C, tle intergrowths are found 10. Becauseof the obvious similarity betweentype- typically at albite twin lamellae, but both the albite A and type-B intergrowths, detailed chemical data and pericline twins pass unimpeded through them. are not presented for the latter. Each intergrofih shows a characteristic "bow-tie" structure @ig. 9D), which in many casescontains a kernel of an Fe-Ti oxide granule. Many are altered Tvpe-C MYRMEKITE to a brown turbid material that has a composition similar to the zeolite mineral phillipsite (fable 5, #4). Occarrence and krture BSE and X-ray imaging (Fig. 1l) confirms that these intergrowths are enriched in Ca and depleted Type-C intergrowths comprise aggregatesup to in Na compared to the enclosing plagioclase 2@ pm acrossthat occgr as rectangular "inclusions" megacryst. Moreover, they contain K-rich regions, in plagieslas. megacrysts,but as yet have beeniden- and seemto be surrounded in part by grains of K- tified in only one sample(MLC 19-2), whereapprox- feldspar whose size, shape and chemical composi- imately 2 dozenwere found. Howwer, thesemay not tion (Table 5, #3) are not unlike tlte exsolvedblebs be inslusionsat all, but may actually be cross-sirtions-- found throughout the megacrysts. It is unclear of "tubes" penetrating into the megacrysts from whether these K-feldspax "rinds" indicate that the thelr margins. A systematicstudy of megacrystswill myrmekite acted as a nucleation site for their later be required to assessthe appearanceof theseobjects growth, or whether the myrmekites formed later and in three dimensions. "pushed aside" the K-feldspar as they grew. CALCIC MYRMEKIIE IN THE ST.URBAIN ANORTI{OSITE 305

Composition grofihs with a texture like that of myrmekite. However, a bulk analyris (Table 6, #11)indicates a The averagecomposition of a type-C intergrowth mineralogy consisting predominantly of quartz + from sampleMLC l9-2 is listed in Table 5 (#l). Like albite + chlorite. Detailed examination also reveals the type-A myrmekites discussedabove, this variety the presenceof rare more calcic plagioclase with a is also characterizedby high Ca, Al and Si, and rela- composition near Anro (Table 6, #10). The origin of tively low Na. However,it is distinguishedfrom all this type of intergrowtl is unknown, although the other examplesby a high K-content. In terms of rela- presenceagain of quartz * calcic plagioclase per- tive proportions of quartz and feldspar (Fig. 7A), mits some type of relationship to myrmekite. the individual sompositionsof theseintergrowths are The third intergrowth (Fig. l3), which is also shifted toward the (Ab + Or) corner compared to found at plagioclase grain-boundaries of the host data for type A. However, this shift appears tb be anorthosite, consists of quartz and feldspar with a causedprincipally by a higher Or-content, since the remarkablediversity of compositions.These are sum- Anl(An+Ab) of the individual bulk-compositions marized in Figure 14, and representative composi- are within the range observedfor type-A myrmekites tions are listed in Table 6 (#l-5; the locationsof the (Fig. 9B). In addition, plagioclasewithin the type-C analyzedspots are shown in Fig. 13). myrmekite has a composition Anr, (Table 5, #3), The most prominent features found here are l0- also similar to values for type A (c/ Table 4, Fig. to 25-pm wide "veinlets" of albite (compareNa and 8). Consequently,it is unqlear whether the type-C Ca X-ray maps) intergrown with quartz, which inter$owth represents a type-A myrmekite that separate grains of matrix plagioclase and enclose somehowincorporated K-feldspar (from exsolved subangular plagioclase that is reversely zoned in a blebs?) during its formation, or represents a fundamentally different entity. Further considerationof type-C myrmekite is best deferred until additional TASLE5. COMPOSITIONSFROM A.I9-2 TYPE-C examples are identified. I,fYR}IEKITEIN SAIiIPLEMLC

ut, z t. 2. MISCELLANEoUSINTERGRoWTHS 5lo2 57.67 47.9t 64.15 48.15 The useof BSE imaging has facilitated greatly the Tl02 0.05 0.00 0.o2 0.02 recognition and characterizationof a variety of A1203 25.74 33.69 18.38 25.03 other Fe0 o.L2 0.14 types of intergrowths. 0.I2 0.04 Three examplesare discussed ilg0 0.03 0.00 0.o0 o.o0 below that sharesome textural and chemical charac- Ca0 1L,25 L6.79 U.13 tL96 teristics with the myrmekites. In addition, data are 5r0 0.16 0.20 0.20 0.02 presentedon some rare occurrencesof what appear Ba0 0.02 0.00 0.79 0.00 to be secondaryalteration-phases that introduce fur- Kzo 3.03 0.06 15.79 0.14 ther complications into a full understanding and NaA0 I.65 1.98 0.24 l.ll interpretation of the history of the anorthosite mas- Total 99.71 100.69 99.73 A6.47 sif. The main purpose for presentinginformation of .thistype is to call attention to the extraordinary level FORI'IULAPROPURTIONST of unanticipatedsmall-scale complexity that has been sl 2.610 2.185 2.9A9 9.960 Ti 0.002 0.000 0.001 0.004 observed. It is difficult to evaluate the abundance AI 1.374 1.807 1.009 6. 113 of theseother 0.004 0.005 0.001 0.008 intergrowths,but our impressionsat ilg 0.002 0.000 0.000 0.000 this time suggestthat they are quantitatively insig- 0.546 0.820 u.006 2.655 Sr 0.004 0.005 0.005 0.002 nificant. Ba 0.000 0.000 0.014 0.000 The frst variety comprises very fine-grained, 0.144 0.003 0.938 0.038 Na 0.1i5 0.175 0.022 0.444 porous-looking quartz-plagioclase aggregatesalong plagioclasegrain-boundaries of the host anorthosite t 4.861 5.00r 4,986 19,224 @ig. 12A). Plagioclasein theseintergrowths is com- END-I'IEI'IBERSPROPORTIONS plex, and consistsof a rounded core (with a sompo- An 55.0 A2.2 1.1 Ab L4.4 17.5 2.2 sition like that of the host anorthosite, Le., Ann6) 0r 17.5 0.3 95.2 surroundedby a more calcic rim (fable 6, #9) which, Qtzt Cn in a few cases,penetrates the core along what appear 1.4 to be cleavageplanes (Fig. 128). Thesetextures indi- Il] gulk c@posltlon (avg. of l1 analyses)., [2] Plagioclase. [3] Alkall feldspar. cate a replacement origin for the intergrowths, and [4] srorn alteration materlal uhose formula, (Na,()o.5ca2.5Al6st the association of quartz + calcic plagioclase sug- 10016.6H20,Is close to phlllipslle (avg. gests some type of link to the myrmekites. of 3 analyses). * f1-3 based on 8 oxygen at@s; f4 based The secondvariety @ig. l2C,D) forms very fine- on 32 orygen atoms tas uSla0g grained pale-green quartz-rich vermicular inter- 306 THE CANADIAN MINERALOGIST

TABLE6. COMPOSITIONSOF FTLDSPARS,PREHNITE, AND BULK COMPOSITIONS FROMMISCELLAITEOUS INTERGROI.ITHS

Sample 24-21 24-21 24-21 24-21 24-21 24-L8 24-18 22-25 24-19 22-rr 22-rr \t. z L. 2, 3. 4. 5. 6. 7. 8. 9. 10. 11.

st02 53.24 67.46 56.32 57.98 63.68 64.55 42.8J 45.03 55.21 55.69 83.89 fi02 0.03 0.03 0.06 0.02 0.06 0.17 0.22 0.09 0.00 0.02 0.00 A1203 29.14 20.05 27.27 24.08 19.27 21,04 22.98 23.10 28.U A.72 8.53 Feo 0.18 0.02 0.09 o.22 0.21 0.00 0.45 1.30 0.06 0.18 1.11 MSo 0.00 0.00 0.00 o.8 0,29 o.1l 0.00 0.12 0.00 0.03 1.48 Cao 11.87 0.58 9.69 3.34 0.19 r.09 25.92 23.19 9.81 10.51 0.32 Sr0 o.Lz 0.16 0.03 0.19 0.10 0.17 0.04 0.00 0.25 0.23 0.06 Bao 0.00 0.00 0.22 0.15 0.24 0.00 0.00 0.01 0.00 0.00 0.02 KzO 0.25 0.06 0.27 r0.10 13.52 0.54 0.01 0.25 0.15 0.23 0.2s Na20 4.51 11.17 5.84 1.73 1.07 10.41 0.13 1.17 5.62 5.29 3.15

Total 99.4I 99.54 99.78 98.09 98.64 98.09 92.58 94.26 99.26 100.89 98.81

FORI,IULAPROPORTIONS*

sl 2.425 2.965 2.540 2.712 2.959 2.896 3.041 3.126 2.503 2.488 8.078 Ti 0.001 0.001 0.002 0.001 0.002 0.000 0.011 0.005 0.000 0.002 0.000 At 1.565 1.039 1.450 1.327 1.056 1.113 1.923 1.890 1.505 1,512 0.967 Fe 0.007 0.001 0.004 0.008 0.008 0.006 0.027 0.075 0.0r.)2 0.007 0.089 t,tg 0.000 0.000 0.000 0.0?0 0.020 0.007 0.000 0.000 0.013 0.oo2 0.2r2 ca 0.580 0.027 0.468 0.168 0.010 0.053 1.972 I.725 0.477 0.503 0.033 Sr 0.003 0.004 0.006 0.005 0.003 0.004 0.002 0.000 0.007 0.006 0.003 Ba 0.000 0.000 0.000 0.003 0.004 0.000 0.000 0.000 0.000 0.000 0.000 ( 0.015 0.003 0.016 0.603 0.802 0.031 0.001 0.022 0.008 0.013 0,030 Na 0.404 0.952 0.511 0.157 0.097 0.905 0.0r8 0.158 0.494 0.459 0.587

r 5.000 4.992 4.996 5.OO3 4.960 5.015 6.995 7.014 4.995 4.990 36.504

END-ME}IBERPROPORIIONS

An 58.2 3.1 47.4 18.4 1.4 5.7 - 49,0 51.9 - Ab 40.3 96.6 51.0 16.8 10.6 91.I 50.1 46.8 0r 1.4 0.3 1.6 64.4 8i.6 3.r - 0.4 1.3

lll Resorbed(?) plagloclase enclosed ln albite; conpositlons in this assoclatlon range from Anro-"n. [2J Replacementalblte; cmposltlons range fron An2-5' [3J-Reversely*t-o"zoned pligloclase near albite veinlet; composltions.-'range frm Anrn-rr. [4] Bulk cmpostion of globular u/o-feldspar intergrowth lcmposl- tlon ""-"' coniidereo apirroximate).-[6] sponsysecondary alblte. [7] Bladed secondaryprehnlte. [8] secondaryprehnlte. [9] Replacementrlm-on An40 plagloclase. [10] Rellct plagloclase frm quartz-rlch vemlcular lntergrowth. If1] Bulk composition of quartz-rlch lntergrowth.

' #1-6, 9 & 10 nomalized to 8 oxygen atdns; f7 & I nornalized to 11 oxygen a&ms; SII norralized to tO catlons for convenlence(smmation represents calculated total positive charge).

radial fashion from Anre (core) to Aneo(rim). The reversezoning in the plagioclasedeveloped at this plagioclaseof the surrounding host anorthosite is time is unclear. It is possible that the albite was reverselyzoned in the vicinity of the intergrowth as superimposedon featuresestablished ai grain bound- well; the wavy arrows in Figure 13 indicate the direc- aries during an earlier episodeof myrmekite formation of increasingAn-content, which changesfrom tion; plagioclasein type-A myrmekite from this sam- An40 to An* adjacent to the albite. Also observed ple is more sodicthan in most others(Table 4, #17), in this intergrorvth are globular two-feldspar inter- perhaps indicating late modification. growths whose locations are indicated by the K-rich Albite also occursin a few other samples,where areas(see Fig. 13). The approximatecompositions it has a "spongy" texture, and may be intergrown of this object and of its constituentK-feldspar are with bladedprehnite. Compositionsof thesephases listed in Table 6 (#4, 5); a low Ba-content distin- from sample STU Z-18 are listed in Table 6 (#6, guishesthis K-felr',spar from the antiperthit€ blebs 7). This samplehas a mottled, bleachedappe,uance, (c/ Table 2, Fie. 2). suggestingsome type of secondaryalteration. It appearsthat this intergrowth as a whole signi- Prehnitehas also been identified in other samples fies sometype of late "albitization" event;whether (without albite), where it occurs as coarsefracture- CALCIC MYRMEKITE IN THE ST-URBAIN ANORTHOSITE 307

Flc. I l. BSE and X-ray imagesof a type-C myrmekite; note abundantK-rich areas,in contrastto myrmekitesof types A and B. filling material or as tiny (5-25 pm) grains that + quartz and lack the associatedalkali feldspar, they replace plagioclase.A composition of the latter var- appear to be fundamentally different from most iety from sampleSTU 22-25 appearsin Table 6 (#8). occurrencesdiscussed by previous investigators. Thesedifferences must be taken into account when D$cussloNr Ovnnvrsw applylng earlier ideas on myrmekite genesisto St- Urbain. An extensiveliterature existson myrmekite, which The anorthosite samplesreported on in this paper hasbeen reviewed recently by Phillips (1974,1980), were collectedfrom widely separatedlocalitic within Shul-diner(1972), and Smith (1974).Some aurhors the St-Urbainmassif (Fig. l). Nevertheless,the data usethe term myrmekite in a descriptive, nongenetic show very similar compositions for the host antiper- sense applied to vermicular intergrowths of thite (Fig. 3) and only a limited range in myrmekite plagioclase feldspar + quartz, and note that such cornpositions(Table 3). It seemsreasonable to sug- intergrowths are found only rarely in mafic igneous gestthat a similar processhas operatedto produce rocks, but are common accessoryconstituents of these myrmekites, and that the widespread occur- many granitic plutonic rocks and quartzofeldspathic rence of such intergrowths at St-Urbain has an gneisses.Phillips (1974),however, restricts the def! important bearing on the details of crystallization nition of myrmekite to intergowths of sodic of this anorthositicpluton. However,it is important plagioclase + quartz that separatepotassium feld- at this juncture to inquire as to whetherthis occur- spar from plagioclase (which is typically more cal- renceof calcic myrmekite is unique to St-Urbain, or cic than that found in the myrmekite). Becausethe whether the results obtained here have more general St-Urbain myrmekites consist of colcic plagioclase applicability. 308 TI{E CANADIAN MINERALOGIST

MvRMEKTTES(?) rN Orunn ANonrnosruc Rocrs Damiao massif, People's Republic of China, and illustrated andesine antiperthite in which grain The literature on anorthosites contains several boundaries are decoratedwith fine-grained bulbous descriptions of features that broadly resemble the aggregatesthat appear similar to those shown here myrmekite found at St-Urbain. For example, Pav- (cf. Fie.4,9A); although he did not draw attention lov & Karskii (1949) described and illustrated myr- to theseobjects, we suspectthat they are myrmekite. mekite in labradorite anorthosite from an unspeci- Carstens(1967, Figs. 9-12) illustrated curious ver- fied locality in the Soviet Union that consists of micular intergrowths in anorthosite from Egersund, intergrorvn Ans6 * quartz. Morin (1956) noted the Norway, which bear an uncanny resemblanceto the presenceof "myrmekitic structures" interstitial to St-Urbain myrmekites. He indicated that the Eger- plagioclase in andesine anorthosite from the sund intergrowths consist of plagioclase + alkali Labrieville area, Quebec,but unfortunately provided feldspar, representing an antiperthite formed by a no details of this ocsurrence.Emslie (1980,Fig.27C) "discontinuous preclpitation" mechanism.However, illustrated a vermicular intergrowth in anorthosite neither optical nor chemical data were presented; from the Harp Lake Complex, Labrador, which con- theseoccurrences should be exanined in detail as our sists of calcic plagioclase + K-rich alkali feldspar; own cursory study on samplesofEgersund anortho- perhaps further study will reveal quartz in addition. site indicate that tlese intergrowths are myrmekite. Guanghsag (1982)dessribed anorthosite from the In addition, we have observed myrmekite in anor-

Fto. 12. A. BSE image of quartz-rich grain-boundary intergrowth. B. BSE image of area near arrow in Figure l2A, showing "mantled" feldspar in which An s sur- rounds Ana6. C,D. BSE and Si X-ray imagesof quartz-chlorite-albiteintergrowth. CALCIC MYRMEKITE IN THE ST.URBAIN ANORTHOSITE 309 thosite from the Lac Allard massif, Quebecand from and will emergeas a common feature of massif anor- the River Yalley massif, Ontario. thosites and other plagioclase-rich igneous cumu- Myrmekite also occurs in layered mafic intrusive lates. complexes.Salpas et al. (1983)noted ils development in anorthositesfrom the Middle Banded Zone of the ORrcrN oF CALcrc MynNasxrrE Stillwater Complex. Wager & Brown (1968)and Yon Gruenewaldt (1979) described and illustrated myr- The origin of myrmekite has been the subject of mekite from the Bushveld Complex and observed sustainedand arduous controversy for more than a that it is associatedwith reverselyzonedplagioclase. century. Many attempts to resolve the debate have Preliminary results of our own study of myrmekite been hamperedby a lack of quantitative data. Most from the Bushveld Complex indicate that there are of the controversy has centered on three classesof textual and chemicalsimilarities betweenthese inter- explanations: (l) cotectic crystallization of quartz growths and the calcic myrmekite from St-Urbain. and plagioclase(e.g., Sugi 1930,Spencer 1938); (2) In summary, although the literature doesnot con- exsolution of the "Schwantke" component (e.g., tain a largebody of corroborativeobservations, there Schwantke1909, Spencer 1945, Phillips 1964,lW3, are a sufficient.number of intriguing intergrowths Hubbard 1966);and (3) metasomaticreplacement of reportedin other anorthositicrocks to warrant fur- the host feldspar(e.9., Becke 1908, Sederholnr 1916, ther investigationof this problem. It is possiblethat Barker 1970).In addition to evaluatingthese possi- calcic myrmekite has been overlooked frequently, bilities, we also introduce and discuss two other

FIc. 13. BSE and X-ray imagesof intergranular albite-rich area in sampleSTU 24'-21; numbered points correspond to compositions listed in Table 6. 310 THE CANADIAN MINERALOGIST

itebtebs) Feldspor \ntirertlr Compositions \(f from globulor sTU24-21 inlergrotlh

o-G lobulor lvo- feldspor inter g ror t h (bulk conp.)

Plog suroundingolbite zone Secondoryolbite

103050(a! Ftc. 14, Summary of feldspar compositionsin sampleSTU 24-21.

hypotheses:(l) isochemicalbreakdown of a pyrox- growths is not unlike that produced by eutectoid eneprecursor, and (2) exsolutionof minor-element decompositionin many metal-alloysystems; it also components. resemblescertain examplesof exsolution-inversion texturesobserved in pigeonitic clinopyroxene.Hence, Cot ect ic cryst a I lization it seems worthwhile to evaluate whether the St- Urbain myrmekites could representthe products of In order to test the hypothesisthat myrmekite can isochemicalbreakdown of some precursor phase. form by simple cotectic crystallization of plagioclase As noted earlier, the bulk compositions(Table 3) and quartz, the bulk compositionsof myrmekites yield Si/O = 0.333, which correspondsto the from St-Urbain have been plotted on a liquidus dia- (SiOJ, stoichiometry of pyroxene. Accordingly, gram for the systemalbite-anorthite-quartz-water at recalculation of the myrmekite, for example,in sam- P(Hro) = 5 kbar (Fig. 15, Yoder 1968b). The ple STU 22-25, as a "pyroxene" (on the basisof 6 quartz + plagioclasecotectic does not passthrough oxygen atoms) yields the formula Nfu.osr Ca0.o6z the bulk compositionsof myrmekite. This observa- n0.45rAl0.ee6si2.eq0O6,which corresponds to 90.2 tion argues against the formation of these myr- mole tlo of the Eskola-component mekites by simple cotectic crystallization. It is (no.rCao.rAlSi2odl,9.1 mole 9o Jadeite and 0.7 unlikely that the small amounts of K and other minor mole 9oWollastonite. The other bulk-compositions constituent$ of the myrmekite could shift the cotec- yield similar reults, although they may contain a few tic by a sufficient amount so that it would pass mole 9o of a Tschermakcomponent (CaAISiAtO6) through the bulk compositionsof the myrmekite. It in addition. is also unlikely that variations in total pressure or Evaluation of thesedata by an alternative method water pressurecould produce the required shift of is shown in Figure 16, where bulk compositionshave the cotectic. been plotted in terms of relative mole 9o SiOr, NaAlSi2O6 and CaAlSiAlO6, using the following Breakdown of a precursor phase conversions: nsi4os: 4 SiOzi CaAl2Si2O6: CaAlSiAlO6+ SiO; NaAISLOs= NaAlSi2O6+ SiOr. The curious vermicular texture of the type-A inter- The data points cluster near the line connecting CALCIC MYRMEKITE IN THE ST-URBAIN ANORTHOSITE 311

Eskola pyroxenewith Jadeite,which representsthe si02 locusof compositionswith Si/O : 0.333,and passes through the plagioclasefield at Anrr. These intriguing results are difficult to reconcile with experimental constraints on stability relations for pyroxene. It is well known that the stability field of jadeitic clinopyroxeneis restrictedto high pressure (e.9., Birch & Lecompte 1961). Recent 5 kbar experimental work on the system CaO-MgO- Al2O3-SiO2at 25-30 kbar has yielded clinopyroxene AGIOCLASE with 2G-26mole Voof the Eskola-component(Wood & Henderson 1978, Gasparik & Lindsley 1980), Myrnekile whereasSmyth (1980)has reported up to 17 mole /r'S Conposilions' 9o of the Eskola-component in kimberlitic ompha- cite. The pressuresindicated from these studies are inconsistent with the geological setting of the St- NoAlSi.0. CoAl"Si"0" Urbain massif. FIc. 15.Bulk compositionsof type-Amyrmekites plotted It has beensuggested that exsolutionlamellae of on a liquidusdiagram for the systemAn-Ab-Or-H2O. plagioclasefound in orthopyroxenemegacrysts at St- Urbain formed as a consequenceof metostablerncor- poration (and subsequentdecomposition) of a few mole 9o of the Eskola-component (Dymek & Gro- unlikely that the St-Urbain myrmekites formed by met 1984).However, in the presentcase, it seems the isochemicalbreakdown of pyroxene becausethe unrealistic to propose the prior existenceof a pyrox- data seemto require a rather preposterouscompo- ene with composition JdlqEskes.Consequently, it is sition for the precursor Phase.

QUARTZ si02

SCHVAIITKE ST,URBAIN HYRITEKITE CoAlrSir0,r(oo,rCoo,uAlSir0r) (nole %) CoAlrSio0,r (nou Coo.uAlSir0r) ESKOLA

NoAlSio0s CoAlrSir0t ALEITE '/ 10,66zco.rr3Alr,3risiz,eeloBANONTHITE

f,YRXETITE o Butrcoupostttotts ncl0

NoAlSir0, CoAlSiAl0, ,]ADEITE TSCHERUAK

FIc. 16. Bulk composilions of type-A myrmekites plotted in terms of relative mole9o SiOrNaAlSi2O6-CaAlSiAlO6 (see text for discussion). 3t2 THE CANADIAN MINERALOGIST

Exsolution of minor-element components (4) produces rutile + calcium aluminate. Linear combinationsof (l) through (4) can be devisedthat The presenceof abundant blebs of K-feldspar and yield a variety of additional products of exsolution, oriented needlesof oxide in the plagioclaseindicates such as Fe-Ti oxide + aluminosilicate (5), ilmenite that exsolution has been an impofiant process in + rutile + corundum (6), and rutile + spinel (7). modifying its original magmatic composition. As a consequenceof all of thesereactions, the resul- Moreover, it is possiblethat someof the tiny inclu- tant feldspar is nonstoichiometric (unlessthe other sions of clinopyroxene in the plagioclaseare also of phasesprecipitate in addition to pyroxeneand oxide). exsolutionorigin. Therefore,it is important to con- In the presentcase, we have not beenabldto iden- sider whether the exsolution of K-feldspar, Fe-Ti tify exsolution products other than Fe-Ti oxide and oxide and, possibly, clinopyroxene may be coupled K-feldspar (and possiblypyroxene). Moreover, the in some way to the production of myrmekite. data on the St-Urbain plagioclasesyield stoichiomet- The substitution of potassium into plagioclase ric proportions of cations, within the limits of ana- represents simple KNa_, exchange that maintains lytical error. Therefore, thesesimplified exsolution- feldspar stoichiometry. Subsequentexsolution of K- reactions do not seemcapable of forming the calcic feldspar also preservesstoichiometry, but may be myrmekites. [Other investigators have identified sil- inhibited somewhat kinetically as it involves unmix- limanite (Sturt 190) and spinel (Whitney 1972)inclu- ing of (Alsrl) domains from (Al1.aSi2.),which sionspossibly of exsolution origin in plagioclase,and would necessitatesome reordering on tetrahedral it may turn out that information on minor-element sites.Neverthelcs, we seeno reasonwhy the develop- substitutions in feldspar can be gained from obser- ment of myrmekite should be related to the forma- vations on exsolution products.l tion of anliperthite. An additional possibility is that these putative A full grasp of the implications of oxide or pyrox- exsolution-reactionsare coupledin a complexway. ene exsolution, however,is hamperedby a lack of For example, a reaction of the type: understanding of tlte precisesubstitutional relationships for Mg, Fe and Ti in plagioclase as well as 4 CaFeTiSi2O6+2 MgAl2Si2Os= uncertainty in the original valence-state of iron. 4 FeTiO3+ 2 CaMgSirO6+ 2 CaN2Si2O8+ 4 SiO2 Despitethese problems, it is possibleto formulate (A) a variety of stoichiometric end-member feldspar componentsinvolving simple or coupled Fe, Mg, Ti exchangeon.anorthite. Theseare listed in Table ?, provides a convenient mechanism for producing together with suggestedbreakdown-products. Reac- ilmenite, clinopyroxene, plagioclase and quartz by tion (l) produces clinopyroxene + quartz, (2) decomposition-exsolutionof minor-element compo- produces orthopyroxene, + aluminosilicate, (3) nents. Production of ilmenite (t hematite), producesFe-Ti oxide + wollastonite + quartz, and plagioclaseand quartz (without clinopyroxene) can be accommodated by the following reactions:

TABIE7. SOI'1EPOSSIBLE MIIIOR-ELEMENT SUBSIIIUTIONAL RELATIoNSHIPS (CaAl2Si208) FoRNoRTHITE 4 CaFeTiSi2O6+ 4 FeAl2SiTiOs = EXCIIANOE END.I.1EI48ER DECOMPOSIIIONPRODUfiS 8 FeTiO3+ 4 CaAl2Si2Os+ 4 SiO2 (la) (B) rezrslAl_, caFesl3og CaFoSl206a Si02 ( lb) MgslAl_z cal,iss,t3o8 Cal.lSSlZ06+ SlUz

(2a) Fez+ca-, FeAlZSlZ0g Fesio3i Al2Sl05 4 CaFd*2Si2O8+ 2 CaAl2Ti2O8+ 4 Fe2+AlzSizOe: (2b) llgca_l l.lgAlZSl208 l,lgsl03r Al2Sl05 4 FeTiO, + 4 Fe,O3+ 6 CaAI2Si2Os+ 4 SiO2 (c) Fez*TtAl_, caFeTisrzo8 FeTlo3rCasio3aS.toZ (3b)l,rl Fer-2At_z caFezslz-8 Fe203iCaslo3rSi02

(4) n2S1_2 CaAtzTt2og 2 Tl02 + CaAlZo4 Given "appropriate" circumstances,the plagioclase + quartz producedin (A), (B) and (C) could com- (5) Feztltca_rst_, FeAl2stTtoS FeTlo3+ AlZSi0S bine to yield myrmekite. t-QbttL/2G)1 The abovereactions have beenbalanced deliber- (6) such that the products Fe2rlirca-,st_, FeAl2lizoS Fello34Tl02+A1203 "ately contain 4 SiO2in each [-(2b)+(4)J (or2Tl02+FeAl204) case. The combination of 4 SiO2+ I CaAlrSirO, (4 Qtz + I An) yields CaAl2Si6O,u (or 2 ll l MSTl2Ca_lSl-Z {gAlZTt2oS 2 Ti02 + l'19A1204 Ino.rCaa.rAlSi3Os],which correspondsto the feld- l.(zal+L/z(4)) (or l.tgTto3r Tl02 + A1203) spar "molecule" of Schwantke(1909). As discussed by Phillips (1974), breakdown of the Schwanrke CALCIC MYRMEKITE IN THE ST-URBAIN ANORTHOSITE 313 component represents the most commonly cited A curve having this form appears on Figure 17 mechanismproposed for the origin of myrmekite by (abeled An), which does not passthrough the bulk previousinvestigators. This model is consideredin compositions of the St-Urbain myrmekites. more detail in the following section. Moreover, these bulk compositions do not lie any- wherenear a line (not drawn) between"Albite" and Exsolution of the Schwontke component "Schwantke" in Figure 16, nor do they lie on the line between "Schwantke" and An40 (observed Most discussionsof the Schwantkecomponent composition of the host plagioclasein the St-Urbain make the assumption that all the Ca present in the samples).These features provide strong evidencethat original feldspar phase occurred in the Schwantke the St-Urbain myrmekites did not form by exsolu- "molecule" (e.9., Phillips et al. 1972),This assumption or decomposition of a feldspar in the Ab- tion restricts the original feldspar to the Ab-Or- Schwantke-(Or) plane or on the An*-Schwantke Schwantke plane in composition space. An even join. more restrictive assumptionhas been applied in some The Schwantke"molecule" can be thought of as cases,where compositional variation is limited to the a feldspar component related to albite by Na-2! Ca Ab-Schwantke binary join. Under these assump- exchange or 10 anorthite by Ca-1Al-2[J Si2 tions, complete breakdown of the Schwantke exchange. The first case was discussed above, "molecule" would lead to the formation of myr- whereasthe latter essentially representssolid solu- mekite whose quartz content is proportional to its tion of "excesssilica" in plagioclase. plagioclasecomposition 6phillips 1964).A number The suggestionthat excesssilica occurs in feldspar of investigators (Hubbard 1969,Phillips & Ramson has beenobjected to strenuouslyby many workers 1968,Ramson & Phillips 1969,Shelley 1969,Ash- (e.9,, Orville 1972), but at least two recent worth 1972, 1973)have noted such a correlation, experimental studieshave demonstrated substantial which they interpret as supporting decomposition of solubility of SioOrin CaAl2Si2O6(up to 17 wt.t/o; the Schwantke"molecule". Bruno & Facchinelli1974. Longhi & Hays 1979).It

QUARTZPRODUCTION DURING PLAGIOCLASE REPLACEIiIENT

^ MYRilEKtTE " BULKCOUPOSIT'ONS

N s 5 o ;e o, E = o

Mole % Anorthite Frc. 17. Bulk composition of St-Urbain type-A myrmekites plotted in terms of volume q0 quartz versasmole9o anorthite. The labeled curves illustrate the relationship between the amount of quartz produced and change in feldspar composition during progressivereplacement of sodic by calcic plagioclase, assumingconservation of Al. Note that the curve for An4s, which representsthe initial composition of St-Urbain plagioclase, passesthrough the cluster of data points corresponding to the bulk composition of the myrmekite. The curve for Ane corresponds numeri- cally to breakdoyn of the Schwantkecomponent and bears no relationship to the present data-set (seetext for discussion). 314 THE CANADIAN MINERALOCIST

is conceivabletherefore that solid solution of SiaO, We have remarked eadier that all clinopyroxene in plagioclaseforms a continuous seriesof com- inclusionsin plagioclaseare surroundedby a "halo" pounds,with the Schwantke"molecule" represent- showingreverse zoning. Theserelationships suggest ing the limit of such solid solutions. Plagioclasein a scenario whereby exsolution of Fe-Ti oxides and lunar mare basalts appears to contain abundant clinopyroxene produced silica and anorthite, some excesssilica (Werll et ol, 1970,Crawford 1973),but of which combinedin proportions approximating the we are unaware of any unequivocal examples of Eskola-component (that precipitated along grain naturally occurring terrestrial plagioclasewith excess boundariesas myrmekite), while the lsrnaining anor- silica. [Previously, Carman & Tuttle (1963, 1967)had thite produced local reversedzoning in the vicinity reported excesssilica in dkali feldspar and specu- of the pyroxene. However, this model requires a lated on its bearing to the myrmekite problem, but modal abundanceof ("exsolved") clinopyroxenefar this interesting study has not been followed up with in excessof the observedamount (somesamples con- additional quantitative analytical data.l tain essentiallynone). It also requires-that the origi- In the previous section, we discussedhow the cou- nal feldspar contain at least I wt.9o eachofFeO and pled exsolution-reactionsyield anorthite in excessof TiO, (based on trial calculations) in order to make that neededto make the Schwantkecomponent; the the requisite amount of exsolved ilmenite; such correspondingsilica./anorthite proportions from (A), valuesare higher by a factor of 2-3 than those found (B) and (C) are indicated by the arrows on Figure in bulk compositionsof St-Urbain feldspar @ymek, 16. We note that the bulk compositionsof the myr- unpubl. results).Although it is possiblethat some mekites are contained within the triangle An40 - type of exsolution process has contributed to the Anorthite - Schwantke,and lie closeto the line from developmentof reversezoning in the plagioclase,we An40 to Eskola. Moreover, the silica/anorthite seeno way of linking the formation of myrmekite proportions from reaction (A), which involved the to exsolution or to any theoretical formulation of exsolutionof clinopyroxene,are preciselythose of the Schwantkecomponent. the Eskola-component. Replacement

PLAG-FLUIDNo-Ca PARTITIONING The nature of the boundary between myrmekite and host plagioclaseis stronglysuggestive of a reaction front at which An* was convertedto An* concomitant with the precipitation of quartz. In our qt u, opinion, sometype of replacementprocess involv- € ing corrosive interaction between (t "cumulate" C' plagioclase crystals and either a late-stageaqueous Ctt fluid or water-rich intercumulus melt provides the s explanation for a- best the origin of the St-Urbain myr- s mekite. Sucha replacementprocess can be modeled = in terms of the simplified end-member reaction: {- ()E 2 NaAlSi3Os+ Ca2* (aq) : ()F CaAl2Si2O8+ 2Na+ (aq) + 4SiOz (D)

in which the products (calcic plagioclaseand quartz) 0,0 0.2 0,4 0,6 0.8 t,0 combine to form myrmekite. Equilibrium sonditions for cation-exchange Co/(Qs*u^t'""'fruid equilibria betweenplagioclase and aqueouschloride solutions have beendetermined by Orville (L972; see Ftc. 18. Schematic representationof an ..infiltration additional discussionby Wyart & Sabatier 1974,and isotherm" for a plagioclase- aqueouschloride solution, Helgeson 1974), who also establishedthat reaction basedon results of Orville (1972).As discussedby Hof- (D) proceedsrapidly in the quartz-producing direc- mann (1972), ..isotherm', the fact that the is concave tion but slowly in the quartz-coasuming direction. toward fluid composition indicates that interaction The plagioclaseinvolved in reaction will change betweensodic plagioclaseand aqueous fluid will yield @) calcic plagioclase(and quartz), and tlat the replacement composition by coupled CaAlNa-rSi-t exchange. will tend toward the dwelopment of a self-sharpening We chooseto considerthe casewhere Al is conserved reaction front. Thesepredictions are consistentwith our in the replacement reaction. This is consistent with observations on the St-Urbain myrmekite. experimentaldata (Orville 1972),and with the obser- CALCIC MYRMEKITE IN THE ST.URBAIN ANORTHOSITE 315 vation that Al is a relatively immobile element in recent (and slightly different) determinationsof sub- most hydrothermal systems. solidus phase-relations(Grove ef'al. 1983, Smith The model reaction can be applied to the St-Urbain 1983).We contend that our observationsare best myrmekite by "adjusting'l the compositionsof the explainedby the chromatographic theory of infiltra- reactant and product plagioclase to the values tion metasomatism(Hofmann 1972).An infiltration observedin our samples(c/. Pavlov & Karskii 1949). processinvolving reaction @) has beenproposed to Accordingly, the conversion of andesine (Anj to explain 1[e sligin of myrmekite and reversezoning bytownite (Ans) + quartz can be expressedas: of plagioclasein the BushveldComplex (Schiffries 1982). A distinctive characteristic of infiltration 1.2857(Nq.uCao.J(Alr.aSrz.e)Os + 0.2857 C** = metasomatismis the occurrenceof "plateaus" in the (Nan.2Ca6.6)(Al,.sSir.r)Or+ 0.5714 Na* + 1.1428 composition of a mineral solid-solution(Hofmann si02 1972).The plateaus are separatedby reaction fronts (E) that may be either sharp or diffuse, depending on t}te shapeof the ion-exchangeisotheim (the locus of equilibrium compositions for coexistingmineral and The relative proportions of the products are fixed fluid at a given temperature). Experimental data by the stoichiometry of reaction (E). The molar ratio (Orville 1972)for cation-exchangeequilibria between of silica to calcic plagioclasecorresponds to 20.5 vol. plagioclase and aqueous chloride solutions demon- i/o quartz (if all the silica is precipitated as quartz). strate that the ion-exchangeisotherm for this system This value is in close agreementwith the observed is strongly concave toward the fluid composition amount of quartz in the St-Urbain myrmekites (Fig. l8), and thereforethe reactionfront shouldbe Cfable 3). We considerthis agreementto be substan- self-sharpening(Hofmann 1972).This is consistent tial support in favor of a replacementorigin for the with the occurrence of extremely sharp contacts myrmekite. between the plagioclase of the myrmekites and The replacementprocess can be viewed as a con- plagioclase of their hosts at St-Urbain. tinuous reastion that raisesthe anorthite content of There is no requirement that the infiltrating fluids the plagioclaseand producesquartz. The proportion be concentrated chloride brines. Orville (1972) of quartz increases as the reactant plagioclase is showedthat the curvature of the infiltration isotherm progressively replaced by an increasingly calcic besomesgreater as the chloride concentration of the product. Figure 17 showsthe proportionality rela- fluid decreases.The end-membercase for "pure" tionship for three different starting compositions of waterwas studied byAdams (1968),who determined plagioclase. The curves labeled Ano and An66 are that intermediate plagioclaseundergoes incongruent for referenceonly; none ofthe data points plots near dissolution to anorthite and silica. Thus the infiltrat- these curves. The curve for An*, which cor- ing fluids may have been very dilute: small quanti- responds to the composition of the reactant ties of magmatically derived water would probably plagioclaseat St-Urbain, passesthrough the cluster suffice. of data points representingthe bulk compositions Another possibility is that the infiltrating fluid was of the St-Urbain myrmekites. This confirms that the a silicate liquid rather than a hydrothermal solution. theoretical replacement-reaction@) can account for In principle, the effects of magmatic infiltration- the relative proportions of quartz and plagioclase, metasomatismare analogousto.those of hydrother- as well as the plagioclase compositions that are mal infiltration-metasomatism (Irvine 1980). A observedin the St-Urbain myrmekites. It should be difficulty with this possibility, ho'ffever,lies in gener- evident from Figure 17 that an infinite family of ating a silicate liquid that would be in equilibrium curyes, corresponding to a reactant plagioclase of with both calcic plagioclase and quartz. any composition, could be calculatedfor the replace- However, a replacementorigin involving igneous ment reaction. crystallization may be related to the 'effects of HrO Two important observations that remain to be on the melting behavior of plagioclasefeldspars. explained are: (l) the extremely sharp contact Fieure 19 illustrates a schematicisothermal section betweenthe myrmekitesand plagioclaseof their host; in the system An-Ab-H2O, which will serve as the and (2) the relatively uniform composltion of framework for the ensuing discussion. Initially, the plagioclase on either side of the contact (with the anorthosites consisted of abundant cumulus exception of rare relict "islands" of host plagioclase plagioclase (Plr) and a small amount of intercumu- within the myrmekite). The contrast in plagioclase lus melt (Lr), and contained only an infinitesimal compositionsmay indicate that the replacementreac- quantity of water (bulk composition given by X). As tion occurred at conditions where a two-feldspar the water content increases,the bulk composition solws was intersected. However, the observed rvould move along a line radial to H2O. The silicate plagioclasecompositions ardnot compatiblewith rwo melt migates isothermally from Lt toward La, dur- 3t6 THE CANADIAN MINERALOGIST ing which time it would be in equilibrium with SUMMARY,CoNcLUsIoNs AND PRoPoSALSFoR progressivelymore calcic plagioclase(i.e., Pl, to FUTURESrunres Pl). Note that a discretevapor phasewould appear only after sufficient HrO has been added, such that The results presentedin the previous sestioru lead the bulk composition intersects the to the following scenariofor the developmentof cal- Plag + Melt + Vapor field (alternative paths and cic myrmekite in andesineanorthosite from the St- geometriescould be constructed such that the melt Urbain massif: l) accumulationof plagioclasecrys- field could be intersected fust). tals (An40) together with minor pyroxene and The increase in water content could be brought ilmenite - hematite solid solution from a water- about by the progressivecrystallization of anhydrous bearing anorthositic magma of unspecified bulk- minerals, or by the introduction of external water. composition; 2) compaction and upward movement In either case, the net result of the above is that of this cumulate pile as a crystal mush, resulting in increasesin H2O result in corrosion of previously the protoclastic developmentof a seriateporphyritic formed plagioclase.This would be a silica-producing texture in anorthosites and banded structure in reaction if aluminum is conservedin the plagioclase. border leuconorites; 3) adcumulus growth of Although there are strong reasonsfor advocating Al plagioclase concomitant with crystallization of conservation for the caseof hydrothermal replace- trapped melt to produce additional ilnenite - hema- ment, the evidenceis less compelling for the caseof tite solid solution, orthopyroxene, clinopyroxene, magmatic replacement. If silica is not precipitated, elc., leading ultimately to the evolution of an aque- then this scenariomay provide a partial explanation ous fluid lesalized at grain boundaries; 4) replace- for the abundant reversezoning observed in some ment of An, plagioclaseby this fluid, resulting in plagioclase. The juxtaposition of An* and Anro the widespreadformation of calcic myrmekite, aud may signal the instantaneousappearance of an aque- 5) migration of Na-enriched fluid which, upon cool- ous fluid phase. ing, causedlocalized "albitization" (e.9., sample Heo

Ab L, Ptt PtzPb Pt4 An

Fig. 19. Schematic representation of the ternary system An-Ab-H2O; progressive increase in H2O-content could cause feldspar resorption or ieverse 2ening, whereasinstantaneous appearance of a separatefluid phasewould causea ,!ump', in the composition of equilibrium plagioclase from p\ to pl4 (seetext for additional discussion). CALCIC MYRMEKITE IN TI{E ST-URBAIN ANORTHOSITE 3t7 sTU24-2t). AxpsnsoN,A.T., Jn., (1966): Mineralogy of the A further consequenceof the presenceof small Labrievilleanorthosite, Quebec..4 mer. Mineral. 51, amounts of an interstitial aqueous fluid is that it t67r-l7ll. could have assistedin the "hydraulic" fracturing of (1982): mafic and Fe-Ti older labradorite anorthositewall-rock, while it acted Asnwer.,L.D. Mineralogyof differentiates of the Marcy anorthosite as a grain-boundary during the emplace- oxide-rich "lubicarlt" massif, Adirondacks, New York. Amer. Mineral. ment of anorthositic dykes and veins. 67, l+27, At present, we are unable to specify either the quantity and composition of fluid involved or the Asnwonur,J.R. (1972):Myrmekites of exsolutionand temperatures at which the myrmekites formed; replacementorigins. Geol. Mag. lW' 45-62. stable-isotopestudies are planned that may assistin these areas. Direct evidenceof an aqueous fluid at (1973):Myrmekites of exsolutionand replace- - some stagein the evolution of the St-Urbain massif mentorigins a discussion.Geol. Mag.110' 77-80. is preservedin the form oftwo-phase (liquid-vapor) (1970):Compositions of granophyre, fluid inclusions found in interstitial quartz Banxen,D.S. from myrmekite and graphic granite. Geol. Soc.Amer. several samples of andesine anorthosite. Regretta- Bull. tL,3339-3350. bly, the grain size of quartz in the myrmekites is generally too fine to recognize fluid inclusions even Bscrr, F. (1908): Uber Myrmekit. Tschermaks if they are present. Nevertheless,the searchfor fluid Mineral, Petrog. Mitt. 27' 371-390. inslusionsand their charasterizationassume high pri- ority in view of our postulates for the origin of the Brncs, F. & LBcol'.rprr,P. (1961): Temperature- myrmekite. Data on homogenization temperatures, pressureplane for albite compositro* Amer. J. Sci, salinity, etc., wilI enableus to assessthe role of fluids 258, 207-217. in a more quantitative fashion. Bnuxo. E. & FaccnrNelu, A. (1974): Experimental studies on anorthite crystallization along the join CaAl2Si2O8-SiO2,Soc. Frang. Mindral. Crist. Bull. ACIC{OWLEDGEIvIENTS 97,422432. (19@): re- This researchwas supported in part by NSF erant Buppnqorou,A.F. The origin of anorthosite EAR82-06741to RFD. We thank L.P. Gromer for evaluated.Records Geol. Sum. India t6' 421432. numerousdiscussions, S.A. Morse for suggestingthe use of the ternary system An-Ab-H2O as a means CanuaN,J.H. & Turns, O.F. (1963):Experimental of evaluating the effects of water on magmatic studybearing on the origin of myrmekite.Geol. Soc. plagioclase,and R.B. Hargravesfor providing access Amer., Spec.Pap. 76,29 (abstt.). to several suites of anorthosite samplesin order to (1967): verification searchfor otler examplesof myrmekite. -& Experimental of In addition, solid solution of excesssilica in andesinefrom rhyo- we providing thank Mss Inna Feldman for a trans- hte*. Geol, Soc,Amer,, Spec.Pap. 115,33 (abstr.) lation of the paper by Pavlov & Karskii, Mrs. Freda Sofian for typing the manuscript, and two anony- CansrsNs,H. (1967):Exsolution in ternary feldspars. mous reviewersfor their commentson an earlier ver- I. On the formation of antiperthites. Contr. Mineral. sion of the paper. Finally, we are especiallygrateful Petrology14, n-35. to R.F. Martin for his thougbtful and comprehen- sive editing, and for encouragingpublication of our Crawporp,M.L. (1973):Crystallization of plagioclase data which, we hope, will aid other investigators of in marebasalts . hoc. Lunar Sci.Conf. 4th,705:717. anorthosites in general and the myrmekite problem in particular. DvN,rsr,R.F. (1980): Petrogeneticrelationships betweenandesine anorthosite dikes and labradorite anothositewall rock on Mont du Lac des Cygnes, St-Urbainmassif, Quebec. Geol. Soc.Amer., Abstr. REFERENcES Program, t2r 4l9.

Aoaus, J.B. (1968):Differential solution of plagioclase (1981): Reversezoning in plagioclasefrom in supercriticalvater. Arner. Iulinerol.53, 1603-1613. labradoriteanorthosite, St. Urbain anorthositemassif, Quebec:retrograde Plag-Pyx relcttot? Geol. Ar,rre, A.L., Qurcr, J.E. & CHoDos,A.A. (1977): Soc.Amer., Abstr. Prograrn13,444. Source and magnitude of errors in "broad-beam analysis" (DBA) with the electronprobe. .Iz Lunar (1984):Sapphirine of possibleigneous origin ScienceVIII. The Lunar ScienceInstitute, Houston, from the St-Urbain anorthosite massif, Quebec. 7-9 (ext. abstr.). Trans.Amer. Geophys.union 16,295 (abstr.). 318 THE CANADIAN MINERALOGIST

& Gnorvcr,L.P. (1984):Nature and origin of IGmnrBBcr, M.M. (1972):Deformation textures in the orthopyroxenemegacrysts from the St-Urbainanor- Lac Rouway anorthosite mass.Con. J. Earth Sci, thosite massif, Quebec.Can. Minerol. 22, 297-326. 9, 1087-1098. ENasLrs,R.F. (1980):Geology and petrology of the LoNou, J. & Hevs, J.F. (1979):Phase eguilibria and Harp Lake complex,central Labrador: an example solid solution along the join CaAl2Si2O8-SiO2. of Elsonian magmatism. Geol. Sum. Can., BuU, Amer. J. Sci.tl9' 876-890. 293. ManncNoue,J. & ScHnrrvBn,K. (1970):Tectonic set- Gaspenrr, T. & LtNpsI-nv,D.H. (1980): Phase ting and evolution of the Morin anorthosite,Gren- equilibria at high pressureof pyroxenescontaining ville Province,Quebec. Bull. Geol. Soc.Finland 42, monovalentand trivalent ions. Iz Pyroxenes(C.T. t65-209. Prewitt, ed,), Mineral. Soc,Amer., Rev.Minerql. 7,309-339. Mewpsrrv, J.B. (1927):St-Urbain area, Charlevoix District, Quebec.Geol. Sum. Can., Mem. 152. Gnolvrct,L.P. & Dvtr4sr,R.F. (1980):Evidence for at leastt\po geochemicallydistinct anorthositetypes in MoruN,M. (1950: Geologyof the Labrieville Map the St-Urbainmassif, Quebec. Geol. Soc.Anter., Area, SaguenayCounty, Quebec. Ph.D. thesis, Abstr. Program 12,438. Laval Univ., Qu€bec,QC.

& - (1981):Petrological and geochemical Monss,S.A. (1982):A partisanreview of massifanor- characterizationof the St-Urbain anorthositema$- thosites.Amer. Mineral. 67' 1087-1100. sif, Quebec:$ummary of initial resuhs.-ln Work- shop on Magmatic Processesof Early Planetary Onvrlr-p,P.M. (1972): Plagioclasecation exchange Crusts.The Lunar and PlanetaryInstitute, Hottston, equilibria with aqueouschloride solution: resultsat 30-32(ext. abstr.). ?@oC and 20@ bars in the presenceof quartz. Amer, J. Sci.?i12,234-272, Gnow,T.L., Frnnv,J.M. & Sppen,F.S. (1983): Phase relations and decomposition relations in calcic (1949): plagioclase.Amer. Minerql. 6t, 4l-59. Pewov, N.V. & Kansrrr,B.E. On the myrmekites in certain basic rocks. Izv. Akad, Nauk (in GuaNorouo,Xs (1982):Petrological characteristics of SSS& Ser.Geol.5,128-133 Russ.). the Damiao anorthositecomplex in Hebei Province, (1964): China. Geochemistry1, 369-385. Prullrps, E.R. Myrmekite and albite in some granitesof the New Englandbatholith, New South Wales."I. Geol. Soc.Aust. 11,49-60. Hancnews,R.B. (1962):Petrology of the Allard Lake anorthositesuite, Quebec.Geol. Soc.Amer., Bud- (1972): A comment on "Plagioclase cation dington Vol,, 163-189. exchangeequilibria with aqueouschloride solution: results at 700'C and 2000 bars in the presenceof HrLcnsor,H.C. (1974):Chemical interactions of feld- quartz". Amer. J. Sci.2i12,969-971. sparsand aqueoussolutions. Iz The Feldspars(W.S. Mackenzie& J. Zussman,eds.). Manchester Univ. (1973):Myrmekites of exsolutionand replace- Press,Manchester, England. mentorigins - a discussion.Geol. Mag.110,7+71 .

Horuamq, A. (1972): Chromatographic theory of (1974):Myrmekite - onehundred years later. infiltration metasomatismand its applicationto feld- Lithos7, 181-194. spars,Amer. J. Sci.272,69-90. - (1980):on polygeneticmyrmekite. Geol, Mag HunncRD,F.H. (1966):Myrmekite in charnockitefrom t17,29-36, southwestNigeria. Ame r. Mineral. 57, 762-7 7 3. & RaNsov, D.M. (1968):The proportionality (1969):The proportionalityof quartzin myr- of quartz in myrmekite, Amer. Mineral. 53, mekite: a contribution to the discussion.Amer. l4l l-1413. MineraL54. 988-989. -, & VEnNoN,R.H. (1972):MYrmekite Invwe, T.N. (1980):Magmatic infiltration metaso- and muscovitedeveloped by retrograde metamor- atism, double-diffusive fractional crystallization, phism at Broken Hill, New South Wales.Minerql. and adcumulusgrowth in the Muskox intrusion and Mag. 3t,570-578. other layered intrusions. ,Iz Physicsof Magmatic [email protected]. Hargraves,ed.). PrincetonUniv. Pownr.r,J.A., Gnourr, L.P. & Dvlaex,R.F. (1982): Press,Princeton, New Jersey. Quartzmonzodiorites and oxide-apatitenorites mar- ginal to the St-Urbain anorthositemassif: products Kav, S.M. (1977):The origin of antiperthitesin anor- of liquid immiscibility? Trans. Amer. Geophys, thosites.Amer. Mineral. 62, 905-912. Union 63,456(abstr.). CALCIC MYRMEKITE IN THE ST.URBAIN ANORTHOSITE 319

RaNsou,D.M. & Prur.r.ps,E.R. (1969):The propor- Sr'rvrn,J.R. (1980):Cation vacanciesand the crystal qignality of quartz in myrmekite - a reply. Amer. chemistry'of breakdown reactions in kimbeilitic Mineral.s4,98+987. qmphaciles.Amer, Mineral.65, lr85-ll9l.

Ro!ry,R.A,, HsMNoway,B.S. & Frsnsn,J.R. (1979): Srunr,B.A. (1970):Exsolution during metamorphism Thermodynamicproperties of mineralsand related with particular referenceto feldsparsolid solutions. substances298.15iK and I bar pressureand at higher Mineral. Mag. 37, 815-832. temperatures.U.S, Geol. Sun. Bull. t452. Sucr,K. (1930):On the graniticrocks of Tsukubadis- RoNpor,R. (1979):Reconnaissances g€ologiques dans trict and their associatedinjection-rocks. Jap. J. Charlevoix-Saguenay. Ministdre Richesses Geol. Geog,13,29-112. Naturelles du Qudbec,DPY-662. VoN GnuBNswAmr,G. (1979):A reviewof somerecent Roy, D.W., Roloor, J. & Dyrr,rsr,R.F. (1972):A conceptsof the BushveldComplex, with particular crypto-explosionstructure at Charlevoixand the St- referenceto sulfide mineraliz41ig11. Can. Mineral. Urbain anofihosite.24th Int. Geol. Congress(Mon- 17,233-256. treal). Guidebook, Excursion Bffi . Wacen,L.R. & Bnomr,G.M. (1968):Layered Igneow Sar.pas,P.A., HasrrN,L.A. & McCanurr,r,I.S. (1983): Roc&s. Oliver & Boyd, Edinburgh, Scotland. Stillwateranorthosites: a lunar arnlog?proc. Lunar Planet. Sci. Conf, I4th, J, Geophys. Ras. 88, Werlr, D.F., McCar,luu, I.S., Borrnca, Y., Dnarn, Bn-839. M.J. & McKay, G.A. (1970):Petrology of a fine- grained rock from the Seaof Tranquility. Science Scrrrrnrcs, C.M. (1982): The petrogenesisof a 167,635-638. platiniferous dunite pipe in the BushveldComplex: infiltration metasomatismby a chloridesoluiion. Wnmrpv, J.A. & S'ro*!rsn,J.C., Jn. (1977):The dis- Econ. Geol. 77, 1439-t453. tribution of NaAlSi3Os between coexisting microclineand plagioclaseand its effect on geother- -691, Scswarrxn, A. (1909):Die Beimischungvon Ca im mometriccalculations. Amer. Mineral. 62, 687 f4ifeldspat und die Myrmekitbilduig, Centralbt. Mineral.,311-316. WnrrNev,P.R. (1972):Spinel inclusions in plagioclase of metagabbrosfrom the AdirondackHighlands. SrosRHor.r,t,J.J. (1916):On synantecticminerals and Amer. Mineral. 57, 1429-1436. related phenomena.Comm. G6ot. Finlande Bull. 153. Woop,B.J. & HeNDrnsoN,C.M.B. (1978):Composi- tions and unit-cell parametersof synthetic non- SHsr,Lsy,D. (1969):The proportionality of quartz in stoichiometrictschermakitic clin opyroxenes, A mer. myrmekite: a discussion.Amer. Minelal. 54, Mineral. 63,66-12. 982-984. Wvenr, J. & SaserrEn,G. (1974):Can Schwantke's Strur-orNen,Y J. (1972)tThe problemof myrmekites. compoundbe formedby reactionof alkalic feldspars Int. Geol.Rev. \4,354-358. with hydrothermal calcic solution? A commenton the recentpaper by P.M. Orville: Plagioclasecation SpBNcsn,E. (1938):The potash-sodafeldspars. II. exchangeequilibria with aqueouschloride solutions to petrogenesis.Mineial. Mag. at 700oCand 2000bars in the presenceof quartz. !9nry_app_licationsg7-118. 25, Amer. J. Sci,tl4, 633-637. .- (1945):Myrmekite in graphicgranite and in vein Yoorn, H.S., Jn.,(1968a): Experimental studies bear- perthite.Mineral. Mag. t|,79-98. ing onthe origin of anorthosite./z Origin of Anor- tlosite and RelatedRocks (Y.W. Isachsen,ed.)..ly'ew Surrs, J.V. (1974): Feldspar Minerals. 2. Chemicol York StateMuseum Sci. Serv.,Mem. 18, 13-22, and-Textural Properties. Springer Verlag, New York. - (1968b):Albite-anorthite-quartz-water at 5 Kb. CarnegieInst. Wush. Year Book 66, 477478. - (1983):Phase equilibria of plagioclase..& Feld- sp-arMine-ralogy (2nd edition, p.H. Ribbe, ed.). ReceivedJanaary 9, 1985,revised monuscript sccepted Mineral. $oc.Amer., Rev.Mineral. 2,223-239. Augast 25, 1986.