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779 Thc Canadian Mfurcralo gi st Vol. 36,pp. 779-791(1998)

THECRVSTAL CHEMISTRY OF AEGIRINE FROM MONT SAINT-HILAIRE. OUEBEC

PALILA C. PILONENI ANDANDREW M. McDONALD'z Departmentof EarthSciences, Laurentian University, Ram:ey Inke RoadSudbury, Ontario P3E 2C6

ANDREE. LALONDE'

Ottawa - Carleton Geoscience Centre, Depantnent of Earth Sciences, Untverstry of Onawa, 140 Ittuis Pastear Stree\ Ottawq Ontario KIN 6N5

ABSTRACT

Aegirine from tive different microenvironments within the peralkaline East Hill Suite @HS) at Mont Saint-Hilaire, Que- bec, were studied by SEM-EDS, Mdssbauer spectroscopy, wet-chemical and ICP-MS methods. compositions range from aegirine- (AqrDiry'Idrr) to end-member aegirine (Aq7Di1.5Hd,.r); the crystals exhibit strong 2ening, with a core enriched n Ca+ 7,r, and a rim enriched in Na + Ti. More than 857o ofthe Fe is present as octahedrally coordinated Fe3*,sug- gesting conditions of extreme oxidation prior to aegirine crystelliTation. Al1 samples of aegirine are enriched in the REE relative to chondrite and display a strong negative Eu anomaly. A concave pattern, with enrichments in both heavy and light REE, is found in all microenvironments exc€pt the one that led to fibrous aegirine. Such fibrous sprays show a sleep, negative slope and are strongly enriched in the light RZE. The aegirine is the product of fractionated batches of late-stage melt enriched in incom- patible elements (e.g. Zr,Ti aad REE and Na. The extremely low Fe2*/Fe* values restrict pyroxene fractionation trends to the Di-Ae tieJine, unlike those noted elsewhere. The Ae and trace-element contents of the were used to develop an evolutionary scheme for the EHS. Fractional crystallizalion of a parental mafic magma, possibly accompanied by liguid im- miscibility, resulted in the formation of and syenites. Further fractionation of the melt that gave sodalite syenite, effiched in volatiles and incompatible elements, led to crystallization of the aplites and dites, units which display identical aegirine, and to later igneous breccias. A secondary fluid-rich phase is considered responsible for very late- stage metasomatic overgrowths of fibrous aegirine in the pegmatite dikes.

Keywords: aegirine, peralkaline complex, clinopyroxene, chemical composition, M6ssbauer spectroscopy,rare-earth elements, fractionation, Mont Saint-Hilaire, Quebec.

SopnaenE

Nous avons 6urdi6 I'aegyrine provenant de cinq micrenviro*"-gng dans la suit€ hperalcaline de F4st Hill au mont Saint-Hilahe, Qu6bec, par microscopie 6lectronique d balayage avec d'6nergie, spectroscopie de Mdssbauer, analyse chimique par v6is firrmids et par plasma d couplage inductif avec specftometrie de masse. Les compositions de pyroxdne vont d'augite aegyrinique (AqrDirrlld") au p6le aegyrine (Aer7Di,rHd,.r); tous les cristaux font preuve de zonation marqu6e, i partir d'un noyau enrichi en Ca + Z vers une bordure enrichie en Na + Ti. PIus de85Vo du fer est d 1'6tat ferrique et.en coordinence octa6drique, ce qui t6moignerait d'une forte oxydation du magma avant sa cristallisation. Tous les 6chantillons d'aegy'ine sont enrichis en terres rares par rapport aux chondrites, et montrent une forte anomalie n6gative en Eu. Un spectre concave des terres rares, c'est-d-dire avec des teneurs relalivement 6lev6es en terres rares l6gbres et lourdes, caract6rise I'aegyrine dans tous les milieux sauf celui qui a produit I'aegyrine fibreuse. De tels amas fibroradids font preuve d'un spectre i forte pente n6gative, et donc avec un fort enrichissement en terres rares l6gbres. L aegyrine a cristallis6 l partir de venues tardives de magma 6vo- lu6, enrichi en 616mentsincompatibles (e.g.,7t,Ti etles t€rres rares) et Na. A cause des valeurs extrCmement faibles du rapport Fek/Feh, les trac6s d'6volution du clinopyroxbne sont paralldles au vecteur Di-Ae, ce qui diffbre des trac6s d'autres complexes alcalins. La composition du clinopyroxdne permet le d6veloppement d'un sch6ma 6volutif pour la suite de East Hill. Une cris- tallisation fractionnde d'un magma parental mafique, possiblement accompagn6ed'uns immi5gigililf, du magm4 a men6 i la t kesent address: Ottawa - Carleton Geoscience Centre, Deparment of Earth Sciences, Univenity of Ottawa, 140 Louis Pasteur Street, Ottawa, Ontario KlN 6N5. E-mail address: [email protected] 2 E-mail addresses:amcdonal @nickel.laurentian.ca. [email protected] 780 T}IE CANADIAN MINERALOGIST formation de sydnites tr n6ph6line et e sodalite. Un fractionnement plus pouss6 du magma qui a produit la sy€nite l sodalite, enrichi en composants volatils et en 6l6ments incompatibles, a men6 i la cristallisation d'aplites et de en filons et I des brbches ign6es tardives. L-aegyrine et I'assemblage de mindraux primaires sont identiques dans les aplites et peg@atites. Nous attribuons l une phase fluide secondaire la formation tardive m6tasomatique des amas fibroradi6s en surcroissance dans les filons de pegmatite. (Iraduit par la Rddaction)

Mots-cl4s: aegyrine, complexe hyperalcalin, clinopyroxbne, composition chimique, spectroscopie de Mdssbauer, terres rares, fractionnement, mont Saint-Hilake, Qu6bec.

INrnorucrroN OccuRnr,NceAND DEScRIP'IoN

Mont Saint-Hilaire @ISH), located 40 km east of The presentstudy is basedon samplesof aegirine Montreal,Quebec, is oneof tenearly Cretaceous alkaline collectedfrom five differentmicroenvironments exposed intrusions collectively referred to as the Monteregian in the Poudrettequarry: miarolitic cavitiesin nepheline Hills (Adams1903). The intrusionstrend east-west syenite,xenoliths of sodalitesyenite, aplites, pegmatite along the junction of the St. Lawrenceand Ottawa dikes,and igneous breccias. Table I is a summaryof the graben systems, from Mont Brome to the Oka assemblagesin thesemicroenvironments and a carbonatilecomplex, rising abruptlyfrom the surround- brief descriptionof the aegirinewithin each. ing St. LawrenceLowlands. Mont Saint-Hilaireis Dark grey,massive (containing composedof an older westernand a youngereastern essentialnepheline, plagioclase, kaersutite and annite) half, within which threemain suiteshave been defined contains angular to rounded xenoliths of sodalite (Currie 1983,Currie et aL.1.986):(l) the Sunrisesuite syeniteup to a meterin width (Horv6th& Gault 1990). consistingof amphibole-bearingalkaline gabbros, (2) The sodalitesyenite xenoliths are host to a wide array the Painde Sucresuite, composed of biotite gabbroand of rare andunusual mineralso many of which havenot nephelinedioriteso and (3) the EastHill Suite (EHS), been found in other microenvironments(Table 1). composedof nephelineand ssdalifs syenites, pegmatite Aegirine within thesexenoliths occurs as dark brown to dikesoigneous breccias and associated rocks. The entire green,prismatic, subhedral to euhedralcrystals that are intrusion is mantledby a hornfelsunit developedin the typically 0.5 cm long, embeddedin a matrix of sodalite. surroundingOrdovician host-rocks of the Lorraine and Optical studiesusing a Supperspindle stage and preces- Richmondgroups (Currie 1983,Cwie et aI. 1986). sion XRD revealttrat such grains are actually intergrown Radiometricdating of the intrusionhas yielded an age polycrystallineaggregates oriented parallel to Z. of 125Ma (Cunie et al. 1986,Gilbert & Foland1986, Sphericalmiarolitic cavities, ranging in diameterfrom Fahbaimet al.1963). onerrrm to l0 cm, occurwithin the ma,ssive,fine-grained The Poudrettequarry within the EHS is a unique nephelinesyenite. These cavities are lined by euhedral minerallocality from which 320 knownmineral species, analcime,aegirine and natrolite, and are hostto a wideanay including morethan 30 new to science,have been noted of REE and Ti silicates,oxides and carbonateminera.ls. (Horv6th & Gault 1990).The peralkalinenature of this Aegirineftom themiarolitic cavities acurs assmall (0.5 cm) intrusionaccounts for the significantenrichment in in- acicnlarcrystals with a squarecross-section and a cbmac/rrr- compatibleelements such as tle rare-earthelements istic blue-greencolo4 they commonly are coatedwith (REE,Zx andTi, which thusled to the crystallization elpiditeand an unknown Na-Tl silicatemineral. of many of the exotic mineral speciesfound there, Pegmatitedikes, which range from afew centimetersto Whereasa Ereatdeal of effort has been devotedto a nreteror morein width,cross-cut all orherrock types within 6ogrrmentingthe exotic minslal6gy of MSH (e.g.,Chao theEHS (Horvdrh& Gault 1990)and contain two genera- & Wight 1995),little emphasishas been placed on the tionsof aegirine.Coa$agrained, dak greento blackaegfuine rock-formingminerals or the evolutionthe EHS. Owing formsprismatic, subhedral to euhedralcrystals ranging in to the extremelycomplex nature of the intrusionand the lengthfrom oneto 10cnL with a squarecross-section. These const2ntremoval of materialfor road fill in the quarry crystalsa1s commonl] orientedperpendicular to the dike workings,it hasnot been possible to mapthe EHS in detail margin,a reflectionof inwardgrowth into thedike. larger to document the intrusive relationships among the crystalsare commonly asymmenical, exhibiting enhanced numerousmicroenvironments. We haveundertaken a growthand thickening in thedircction of magmaflow within crystalchemical study of aegirine,a major rock-forming thedike. In contast,a secondgureration of aegirineacr:rs as mineral within all microenvironmentsnoted in the light green,radiafing, acicular spra.ys of terminate4euhedral EHS, as a meansof exploring the paragenesisand crystals.These sprays are found on coarsergrains of evolutionaryhistory of this uniqueintrusion. aegirineand within small vugs. AEGIRINEFROM MONT SAINT-HTLAIRE,QUEBEC 781

TABIE 1. CHARACTERISTIC MINERAL ASSEMBLAGES OF TTIE MCROENVIRONMENTS WITIIINTHE EAST HIIISTIITE Microenvircnmeors

Miarolitic Cavitis analcime aciorlar, blue-grea (lvlc) ncrolite cry$als (0.5 m long) nepheline often coatedby elpidite elpidite Ti-silicatesmd oxides

Sodalite Syarile sodalite dark grear to browrL Xeioliths (SX) natrolite prisnatic crystals 0.5 crn in lorgth asrqhyllite biotite abrmdan REE

Aplit€s (AP) microcline prisrnatic, dark gre€n albile uysals 0.25{.5 cm in natfolite langth along with fibrous mass€s

P€mdite Dikes natrolite coarsegraind subhedral (PEG) cdspleiite to euhedral,prismatic microcline uystals rarging in lagth albite fromlto8crnwith thodocfirosite ahmdant fibrous" felt€d analcime overgrowth"s sermdite asCrophyllite zirqrt

Igneous Breccias qJarlz dark gre€nto bro\4n (IB) calcite crlcals wilh red-brown moly'Merite terminaions anphibole

A microenvironmentpreviously unde,scribed from the host to a uniquesuite ofminerals. Aegirine occursas EHS is herereferred to as aplite. This unit is very fine prismatic,subhedral to euhedral,dark greento brown grained, with a sugary texture. It is mineralogically crystalswith reddishbrown terminations. equivalentto the pegmatitedikes, consistingof albite, Ater-vrrcer Msrgops microcline,natrolite and aegirine, and presumably repre- sents the aplitic portion of zoned pegmatitedikes. Energy-dispersionspectrcfttetry (SEM-EDS) and induc- However,field evidencerequired to substantiatethis hy- tively coapledplnsma - massspectrometry (ICP-MS) pothesisis lackilg. Aegirine within the aplite occursas both fibrous,felted, acicular masses and as dark green, Major-elementdata were obtainedusing a JEOL prismaticcrystals ranglng from 0.25 to 0.5 cm in length. 6400SEM equippedwith a Link Analytical energy-dis- The igneousbreccias cont4in fragments of material persion (ED) detector.All analyseswere performed from all three suitesat MSH. They display complex underthe following operatingconditions: accelerating cross-cuttingrelationships with the previouslydescribed voltage20 kV beamcurrent 2.5 nA, and countingtimes microenvironmentsexposed in the quarry.According to of 100 seconds.Data reductionwas perforrnedusing Cunie (1983),the breccia locally contains fragments of Link Analytical soffrlvareand aZAF4 correction.Addi- youngersyenite and phonolile, but elsewhere,pegmatite tional analysesfor F were conductedon a Cameca dikes,along with gabbroicdikes, are found to cross-cut SX-50 electronmicroprobe, but concentrationsabove brecciaunits. Such fragmentsare generallyangular, detectionlimits werenot observed. rangefrom a few centimetersto a meterin diameter,and Six representativecrystals were selected from eachof may exhibitsigns of partialmelting and a reactionrim the 22 specimensthat were studied.These crystals were (Horv6th& Gault 1990).Cavities within the brecciasare thenmounted perpendicular to Zto evaluatethe extent of 782 THE CANADIAN MINERALOGIST chemical zoning. A traverse was done on one crystal (Fig. 1C).Table 2 representsaverage results of the SEM- ftom each specimen; in addition, two core and trvo rim EDS analysesand chemical formulae for eachsample. analyseswere perforrned on each of the remaining five grains. The structural formulae were calculated on tle basis of six atoms of , with the proportion of Fe2* and Fe* estimatedfrom charge-balanceconsiderations and later determined by Miissbauer spectroscopy and wet-chemical methods. Average concentrations of hace elements in aegirine concentrates were determined by ICP-MS, Crushed separalesof approximately 0.1 g were digested in acid, and solutions were analyzed for the REE. Analyses of three samples (PEGl7R IB15, MC18) could not be done owing to the paucity of material available. Deterrninarton of the rarto F e2+/Fe3+

Mtissbauer spectroscopy was employed to determine the proportion of Fe2*and Fe$ in selected specimens and to explore the possibility ofFek in tetrahedral coordina- tion. T\vo concentrates,PEG2 and SX10, representativeof the pegmatites and the xenoliths of sodalite syeniteore- spectively, were chosen for analysis.Absorbers for the two specimenswere preparedby crushing the separatesin acetoneto minimize oxidation of . Forboth absorbels, the ideal mass of material was calculated using the tech- nique developed by Rancourt er al. (1993) to ensure the maximum signal-to-noise ratio. The powdered aegirine was suspendedin penoleum jelly to ensure a random ori- entation of crystals within the circular aluminum mount (aperture:1.0 cm, thickness:0.6 cm). Spectrawere obtainedusing asTCosource in arhodium matrix at room lemperature (21"C). The hansducer was employed in constant-acceleration mode over a Doppler velocity range of t 4.0 mm/s, with the distancebetween ab- sorberand sourcefixed at 12.5mm" Spectrawere calibraled with an tFe-emiched iron foil. Data were collected in l0Z channsls and folded to obtain a flat background, Spectra were fitted with the Voigt-based quadrupole splifting distribution method of Rancourt & Ping (1991). In addition to the M

Back-scatteredelectron images reveal complex @SEI) Frc. l. Back-scattered electron images of aegirine from Mont compositional soning about Z n aII samples. In most Saint-Hilaire. A. Symmetrical, concentric zonation com- casesothis 2sning is concentricoshowing two or more monly observed in MSH aegirine, with a core (light grey) chemically distinct zones separatedby a sharp boundary being enriched in Ca+Ztandaim (dark gey) emiched in (Ftg. 1A). In addition, highly inegular, undulatory Na + Ti. Scale bar: I mm. 3. Inegular, undulatory zonation zoning also was observedoand possibly is due to the observed in many crystals, possibly due to late-stage inter- interaction of late-stage fluids with the grains (Frg. lB). action with metasomafic fluids. Scale bar: 100 p,m. C. Inclusions of catapleiite are common within cores of Catapleiite inclusion in aegirine core, indicating crystalliza- the pined or corroded grains that display this zoning tion of secondary alkali zirconosilicate. Scale bar: I mm. AEGIRINEFROM MONT SAINT-HILAIRE,QUEBEC 783

TABLE Z AVERAGE COMPOSINONSFC'R AEGIRINE FROM THE EAST HILL STJITBAT MONT SAINT.HII.AIRE SarplepEolc pEolFpECEc pEo2F pEcltc pEo3FpEo4 pEc5 pEc6 AP7 $g f'31434428431%313231 Coqonantoxdu(wt%o) sio2 5L61 52.7t 5L4t 5r.T2 51,39 51.19 sl.l7 51,E651.29 sl55 50.53 fio, 0.6t t.29 059 0.96 0.72 l.l7 o.EE 1.63 l.E9 0.74 l.v zfi, L4 1.00 1.07 0.43 Lm 0.73 t.2t 0.r3 0.gl 0.39 0.4 Alrq l.@ l.16 1.04 Lt6 1.07 l.3l t.t2 1.43 Ln l.ll o.% F%Ob 30.79 29.85 30.95 23.t5 30.41 30.15 3034 29.69 28.tt n57 2555 MnO 0.94 1.13 0.El 0.71 0.% 0.76 0.89 0.97 l.l7 0.75 0.84 M8o 0.29 0-20 0.33 0.I4 026 0.2r 0.19 0.n 0.31 0.4 2.9 CaO L4r 0.79 3.03 0.4 2.E9 0.n B7 0.88 1.60 L93 7.{l l.[ap lLn n32 11.871298 rr.7r rLTl l2ll rLgt lL28 rr.79 8.72 Tohl 102216101.45 rcLn 97.Et 100.6t 98.9 9.74 100.479955 10f,8 98.24 Nwber of catiottson tt c bdsisof sit oxmens si l.9t 1.9 r.97 t.g) 1.96 l.gE t.97 t.97 1.97 1.97 l.lb Afl o.ql o.ol 0.03 o.v2 0.04 o.m o.o3 0.03 0.03 0.03 0.a2 Sazt 100 LN Ln LOI 2.6 L@ 2.N 2.6 2.ffi LN l.9t r.IA 0.89 0.97 0.E5 0.9E 0.&t 0.95 0,93 0.Y 0.92 0.88 0.61 Ca 0.10 0.03 0.13 0.O2 0.14 0.03 0.05 0.05 0.07 0.12 037 Sun 0.9E l.0l 0.9t 1.00 0.98 0.9E 0,99 0.99 0.98 0.9 0.97 Fe$ 0.87 0.85 0.8? 0.84 0.87 o.E7 0.8E 0.86 0.83 0.88 0.72 Fel o.m o.o0 o.ot 0.01 0.01 0.00 0.01 0.00 0.m 0.00 0.00 Al vr o.v2 o.o4 o.o2 0.08 o.ot 0.04 o.m o.a o.o2 0.02 0.00 fi 0.m 0.04 0.02 0.03 0.v2 0.03 0.a 0.04 0.05 0.v2 0.04 Zt 0.02 0.V2 0.02 0.01 0.A 0.01 0.02 0.A 0.V) 0.01 0.00 Mn 0.03 0.04 0.03 0.V2 0.03 0.03 0.03 0.04 0.M 0.4 0.03 Mg 0.U2 0.01 0.V2 0.01 0.a 0.01 0.01 0.01 0.02 0.03 0.19 Sum 0.9E 0.99 0.9t 0.99 0.9t l.m 1.00 0.9 0.98 0.99

%Di 5.72 L?S 7.4 1.35 7.51 231 3.% 3.n 4.47 7.01 2457 %Hd 4.9r l.6E 6.90 t.2t 7.0t 1.68 3.20 2.63 3.v 5.78 16.39

SaEple sx9 sflo sxil tBt2 tBl4 tBr5 Apt6 pEolTcPEotTFMctB #'3/.n341826303039627

5154 Tiq 0.53 0.45 r.97 LX t.73 0.E5 Lll 0.E0 l.l7 2.19 bo, 0.52 0.42 0.4E 0.56 0.39 0.07 0.79 1.17 0.73 0.33 Alrq Ln l.l4 0.90 1.01 o.zrE 0.22 t.22 t.m l.3l 0.90 Fe2Ob 31.08 30.14 29.05 29.80 27.10 18.08 31.24 30.76 30.15 27.65 MrO 0.58 0.90 L.l1 0.91 0.63 059 1.05 0.n 0.76 1.26 Mso 0.29 0.4 0.47 0.45 2.45 7.93 0.30 0.24 0.21 1.25 CaO 1.97 4.48 1.70 2.52 4.93 14.4 1.20 2.32 0.77 4,47 MzO 12.15 1059 12.25 n.99 10.37 5.7E 12.36 ll.E6 12.7r lO.Cz Total 99,83 99rA 98.St 99.34 98.67 100.699E.91 100.7t 9E.99 93.13 Nwfier o! cubrs on tlo basisof six oxrgero si l.9E l.% 1.97 1.96 l.% 1.97 1.97 r.97 1.98 r.% Alv o.v2 o.o2 0.03 0.04 o.ol 0.01 0.03 0.03 o.u2 0.m Srzt LN 1,98 Z@ 2.n L97 l.9E LN 2.ffi 2.N \.99 Na 0.89 o.77 0.Cl 0.87 0.75 0.43 0.95 0.t6 0.91 0.79 Ca 0.09 O,2l 0.07 0.13 O.zs 055 0.05 0.ll 0.07 0.19 Sun 0.9E 0,98 1.00 1.00 0.99 1.00 1.00 0.97 0.98 0.98 Fe$ 0.E9 0.86 0.84 0.87 0.75 o5l 0.88 0,88 0.t5 0.79 Fea o.0l 0.00 0.00 0.01 0.00 0.m 0.00 0.00 0.01 0.00 AIU 0.03 0.03 0.0r 0.0r 0.00 o.o0 0.03 o.ol 0.03 0.00 T'i 0.02 0.01 0.05 0.03 0.05 0.o2 0.03 0.02 0.04 0.07 b 0.0r 0.01 0.01 0.01 0.0r 0.00 0.01 0.02 0.01 0.01 Mn 0.A 0.03 0.04 0.03 0.m 0.U2 0.02 0.03 0.03 0.04 Mg 0.02 0.03 0.03 o.m 0.17 0.4 0.01 0.01 0.03 0.07 Sun 0.99 0.9E 0.9 0.99 1.01 0.9 0.99 0.9 1.00 0.9E

%Di 5.25 ll.3E 5.06 7.31 19.23 39.89 3.45 6.23 5.28 1L97 %Hd 4.& lo.lE 3.68 6.6 rr.4 22.6 2.76 5.75 3.96 g.fi | - tr@bs of a@lys ildu&d i! '@ 784 TIIE CANADIAN MINERALOGIST

0.98

0.92

Zx

<- Rim-> <-Rim_-.-> <- Core

Frc. 2. Traverseacross PEG5 from rim to rim, indicating the presenceof a core enriched in Ca and Z,r, and at''m enrichedin Na and Ti. All traverseswere performedperpen- drctlJarto Z.

Tetrahedral(T) site !<-r tr<-r Aegirine at Mont Saint-Hilairedisplays almost d. co* completeoccupancy of the T siteby Si, with very lim- 0.9s ited substitutions. contentsrange from 1.96to 1.99atoms per formula urut (apfu), accompaniedby minimal Al (range:0.01--0.04 apfu), resultingin less _ 0.91 than2VoAl - Si substitution.The nearlycomplete oc- S o.sp cupancyof the T site by Si indicatesthat most of the z o"l remainingAl, alongwittr Fe9*,Zr andTl, mustreside at the Ml site.Previous investigators also have documented the presenceof Al-poor clinopyroxenewith negligiblesubsti- 0.83 tutionof Al for Si in theZsite in alkalineintrusions (e.9., 0.81 Larsen 1976, Ranlov & Dymek 1991, Mitchell & ooo o.o2 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Vladykin1996). Ca(aPfu)

M2 site Flc. 3. Concentration of Ca versas that of N4 depicting core Sodiumcontents inthe M2 site rangefrom 0.43 to (closed squares) and rim (open squares) compositions in 098 apfu.Aegirine from the pegmatitesand aplites is selected samples. the most enriched,whereas samples from the igneous brecciasand sodalite syenite xenoliths display emichmens in C4 resultingin up to 56VoNa* Ca substitution,and thuscorrespond to aegirine-augite.Traverses across indi- Na - Ca substitution,resulting in a positivecharge vidual grains(Fig.2) revealan antitheticrelationship "excess'oin theM2 site of up to +0.56.The saturation betweenNan and Ca2*,with coresrich in Ca andthe of.the M2 site with Na, alongwith the deficiencyin Al, rims enrichedin Na @g. 3), a phenomenonconsislent are presumablydue to the extremeperalkaliniry of the with increasingfractionation sf ths crystallizingmagma parentalmagma, and are compensated by a netpositive Qeer et al. 1978).The snong I : I correlation(R2 = 0.99) charge-deficiencywithin theMl sitedue to substitution benveenNa andCa noted in theaegirine points to a direct of Mg, Fe2*and Mn for Fe3*. AEGIRINE FROM MONT SAINI-HILAIRE, QI,'EBEC 785 Ml site The Mtjssbauerspectra are similar in the two specimens:both containtwo prominentpeaks posi- The calculatedFel* contentofthe aegfuineranges from tionedat approximately0.3 O.5ltD 0.89 apfil. with samplesfrom the pegmatites display- and 0.6 mm/sthat define the low- and high-energylines, respectively, ing ttrchighest contents. Ctrarge-balance calculations indicare of the dominantFe3* doublet (Fig. 5). In thatall Fe presentin the aegirineis Fe3-(d.01 apfuFe). addition,a less prominentand wide peakin velociry Thepyroxene compositions display low Mg contents,rela- the rangeof 1.6 to 2.6 mm/scorresponds to thehigh-energy tively constantlevels of NIn,and enrichments in bothTi (up line of the Fe2*doublet. The low-energyline of this doublet to 3.38wt%o TrOrin aegirinefrom themirolitic cavity)and showsup shoulder 7r (upto3.Z7wt%oZrOzn aegirine from the pegnatites). asa on the low-energyFe3* line in the region of 0 mm/s. IndividualSEM-EDS nzverses indicate that the core zone is The octahedrallycoordinated Fe3* was errichedin Zr, whereasthe rim is enrichedinTi, atend simi- modeled as having three Gaussiancontributions within a larto CaandNa @g. 4). Variationsin ironcontentfrom core prominentdoublet, with an averagecenter to rim andthe relationship of Feto othercations can change shift at (PEG2) (SX10). drarnatically,evelr within thesame microenvironmenl 0.39 mm/s and 0.41 mm/s These values are nearly identical to that obtained by 0.040 Annersten& Nyambok(1980) for pure,homogene------> a o 0.03s 6Eb ous aegirinefrom Arkansas(0.38 mm/s), but lower 0 030 than that obtainedby Singh & Bonardi (1972) tor aegirine-augitefrom JoanLake, Labrador(0.53 mm/s). 005 The threecontributions within the Fe3*doublet are con- - 0.020 sideredto representvariations in thelocal environment g 00r5 in which the cationsreside. N The was o.oroI Fe2* modeled as having rwo distinct I Gaussiancontributions, which can easily be resolvedas 000sI $ distinctpeaks on the high-energyside of the spectrum, oo* l- in the 1.5to 2.5mrn/s range of velocity(Frg. 5). Center 001 0.05 shiftsfor ths 5amplesare similar (PEG2: 1.15 mm/s; Ti (apfu) SXl0: 1.14mm/s), but lowerthan the values obtained Frc. 4. Concentr ation of.Ti v er sus that of Z,r, depicting core by Annersten& Nyambok(1980), 1.6 mrn/s, and Singh (closed circles) and rim (open circles) compositions in & Bonardi(1972), 1.30 mm/s. The Fez*doublets in selected samples. SX10 are more prominentthan those observedin The ratio Fe2+/Fe3+ PEG2, and indicate that 22.2Voof the total iron is presentas Fe2*, in contrastto Il,6Vo for PEG2.Average The stoichiometric formulae calculated from the Fe2*quadrupole splittings in thetwo spectraare nearly SEM-EDSanalyses all suggestvery low Fe2*contents. identical(PEG2: 2.29 mm/s,SXl0: 2.26 mm/s). This was confirmedby Mtissbauerspectroscopy and Similar valuesfor Fe2*/Fe,o,were obtainedby wet-chemicaldeterminations of FeO.It is importantto wet-chemistrydeterminations. Table 4 presentsa note that the proportionsof Fe2*and Fe3* obtained by summaryof resultsobtained by Miissbauerspectroscopy, thesetwo methodsapply to the bulk of eachsample Fe titrations,and SEM-EDS analyses. AII indicatethat anddo not takeinto considerationchemical zoning or greater than 85Voof the total iron is present as inclusionsthat arepresent. Results and fiuing param- octahedrallycoordiaated Fe3n within theMl site.Note etersfor eachspecimen can be found in Table3. that thereis a discrepancy4mong results obtained by the threemethods employed. The substantiallylower TABE3. MoSSBAUERspEcrRA FITTTNGPARAMETERS AND REsuLTs FORMSH AEGIRIM (PEC2AND SXIO) Fe2*contents suggested by the SEM-EDS dataappear Si@ Si@ Qudrupolesplict!8 CenftShtft #ofcoub Fe anomalous.Vacancies within the pyroxenestructure, Component (mE/9 (mn/g G+) coupledwith the initial calculationof total iron as wG2 Fe2O3,may haveresulted in iacorrectcharge-balance, Fe+ | 0.3t32 o.392t t23lL4.2 2 0.4937 45ya andthus low Fe2*/Fe3*values. 3 0;1t63 23672.2 F&. I 116 Ll5l9 1M59.9 2 2.6861 t3787.7 Reduc€d = f 1.333 TABLE 4, A COMPARISONOF RESULTSFROM MOSSBAUER SPECTROSCOPY,SEM/EDS AND Feo TITRATIoNS (WT. %) sxl0 Smple Spsi€ Mdssbruq SEM/EDS F€O Titntio$ Feg 0.3122 0.4054 103?99.1 77.E 0.4306 63.8 PEG2 Fe'- 27.36 30'75 287E 0.7 39917.1 Fe2' 3-59 o.32 2.17 2.176r' 1.1441 5t04?.1 22.2 2.7868 9436.2 sxl0 Fe3* 23.45 30.14 25.50 Reducedt2= 0.778 Fe2* 6.69 0.00 4.9 786 THE CANADIAN MINERALOGIST

n nnt 0.005 0.000 0.0@ -0.005 {.@5

I zI! z z 2, 0.48 0.65 Q

trl

zF z 0.60 0.44

2 0.55 0.40

0.50 0.36

aAt ,ZUL VELOCITY (mm/s) VELOCITY(mrnrs)

Frc.5. Mtissbauerspectra for aegirinefrom Mont Saint-Hilaire.A. PEG2:88.4VoF&-,ll.6c/oFd*.B. SX10:77.8VoFt*,22.2%o Fe3*.The solid line represenisthe best fit. Contributionscorresponding to octahedrallycoordinated Fe2" and Fe3+are super- imoosedon the data.

TABLE 5. RAITE-FAIITI{ EI IiMENT CONCENTRATIONS (PPM) IN AEGIRINE FROMTTIE EAST HILL SUTTEAT SAINT.HILAIRE Tm Yb

sx8 223.4398.3 43.e uo3 2n.l 1.9 13.8 L2 10.5 1.9 5.0 0.8 6.4 l.I SX9 298.4 nd 8L7 z}t.l 42.3 3.a 29.2 4.3 20.5 3.6 8.5 1.2 7.1 l.l sxlo 4r.7 103.7t2g 44.3 7.5 0.6 4.r 0.E 4.1 0.8 L9 0.8 8.9 2-l sxll 44.8 88.3 9.4 30.5 5.0 0.4 3.5 0.6 3.0 0.7 l.t 0.4 4.2 0.9 Aplioeaegirine AYI 26.1 4L2 4.7 16.7 L9 0.3 1.8 0.3 t.E 0.4 1.9 0.6 6.8 1.6 AP16 199.441E.0 ,16.9 151.5 n.l L2 17,3 Lg l3.l 22 5.0 0.7 5.2 1.0 Coane pegmatite aegirine PEGIC u.2 58.0 7.1 26.4 5.1 0.6 3.8 0.6 3.7 0.8 3.0 0.7 9.3 2.0 PEG2C 31.4 73.7 8.9 312 5.0 0.5 3.8 0.6 3.7 0.9 L9 0.7 8.2 1.7 PEG3C 20.6 56.2 8.0 n.9 5.7 0.6 3.9 0.6 3,2 0,7 L6 0.E 9.6 L2 PEG4 L7.5 4L3 5.6 2p.7 3.9 0.4 2.5 0.4 1.9 0.4 t.6 0.5 6.2 r.4 PEG5 28.1 55.2 6.1 18.7 LE 0.3 Lr 0.4 1.7 0.4 t.2 0.3 3.6 0.7 PEG6 n3 6D.Z 6.9 23.0 4.2 0.4 2.9 0.5 L2 0.4 1.3 0.3 3.5 0.7 PEGIT 24.5 U.6 8.5 31.4 6.0 0.6 4.1 0.6 3.2 0.6 2.5 0.7 E.2 1.9 Hbrous pegmatite aegiriae PEGIF814.0 nd 112.1354.4 6r.5 5.9 4L6 6.6 3?.4 6.t 17.9 3.0 19.3 2.7 PBG2F2U.3 4U.4 56.3 183.4 44.0 5.4 45.3 8.E 49.E 9.0 21.2 L9 15.2 1.9 PEG3F303.0 1170 75.6 239.3 47,7 5.3 3E.5 6.7 35.7 6.5 15.9 L4 l4.Z 1.9 Igneous breccia aegirine IBl4 67.7 lsLg 17.8 6L6 rr.3 1.4 9.1 1.5 8.8 1.8 s.7 r.2 rr,1 22 ffi

Rare- earth- element g eo chemis t ry europium anomaly that can be attributed to earlier fractionation of Ca-bearing minerals such as plagioclase ICP-MS data indicate that the aeghine is enriched and kaersutite, both of which occur in high modal per- in the REE (Table 5), and display a strong, negative centagesin the more primitive units of the EHS. AEGIRINE FROM MONT SAINT-HII.AIRE, QUEBEC 787

1000

a H E € loo

910

Flc. 6. Chondrite-normalized rare-earth-elementabundance,s in aegirine from Mont Saint-Hilairc. The profiles for fibrrous aegirine from pegmatite are shown as dashed lines.

Two distinct REE patterns were observed in the 1972),IIimaussaq, Greenland (Larsen 1976), and in aegirine (Fig. 6).The flust, observed in all specimens alkaline ring complexes described by Bonil & Giret except the fibrous aegirineois noticeably concave,with (1985), fractionation ftends extend from Di ftnther toward enrichments in both heavy REE and light REE relative Hd (up to 95Vo) pior to crystallization of Ae (Fig. 7). In to the middle REE, a pattern that seems inconsistent their study on clinopyroxenes from the Little Murun with REE patterns presentedfor other clinopyroxenes ultrapotassiccomplex (Aldan Shield, Russia), Mitchel (Rollinson 1994). Of note arc the REE pattems for the & Vladykin (1996) found that the Hd contents did not aegirine from aplite, which are identical to those of the exceed 15 mol.Vo, as at Mont Saint-Hilaire. In both aegirine from pegmatite, re-inforcing the hypothesis cases, oxidation of iron had largely occurred prior to that both rock t'?es are genetically related and possibly crystallization of the magma, thus inhibiting lhe crys- comagmatic. tallization of Fe2*-bearing clinopyroxenes such as The second REE pattem, observed only in the fi- hedenbergite. Pyroxene trends observed by Mitchell brous PEG aegirine, and to some extent in samples (1980) in the Fen alkaline complex (Norway) and by SX8 and APt6, exhibits greater enrichment in the tight Mitchell & Vladykin (1996) in the Little Murun Com- REE relatrve to both the middle and heavy REE, result- plex show more scalterthan al Mont Saint-Hilaire, a feature ing in a negatively sloped trend with a shongly negative that may be the result of competition for Ca-Mg-Na-Fe Eu anomaly. These samplesdisplay REE concentra- between the pyroxene and the other mafic phases.Within tions an order of magnitude greater than the previous the EHS, aegirineis commonly the dominant mafic phase, ones. resulting in a low degree of competition, thus produc- ing a fractionation trend with minimal scatter. The DlscussroN fractionation trend established for the Mont Saint- Hilaire suite of pyroxene compositions appears to be Morfmoto et al. (1989) defined aegirine as having more unique among peralkaline intrusions. 1d:anS0%oof theAe end-member(NaFe3.Si2O6). Using this The observed trend of continuous crystallization schemeofclassification, the clinopyroxene analyzedin this along the Di-Ae tie line indicates that the EHS is a prod- study covers the range from aegirine-augite in the igneous uct of fractionation of a volatile-rich, peralkaline magma breccias(Ag8D:rFIdB) tcrnear-end-member aegirine in the enriched in Na and Fe3*.The strong enrichments and pegmatites (Aq7Di1.5Hd,5).At Mont Saint-Hilaire, the distiact zonation of Zr and Ti within aegirine from MSH aegirine is unusual in that the Hd component never exceeds are directly related to this peralkalinity (Watson 1979, 23 mol.Vo,tnlike thaf ftom most peralkalineintrusions (Fig. Farges et al. 1994).In most natural systems,Zr and Ti 7). In sodic peralkalinein[usions such as Itapirapua Brazil tend to behaveindependently and partition into Zr or Ti (Gomeset al.1970), SouthQ0roq, Greenland (Stephenson phases,or, iffound in appreciable concentrations in a 788 TI{E CANADIAN MINERAI,OGIST Ae

t'+ / j" u / ,,i,t" .r! -.-..- i! iot:: 1ii',,i1,,,,,--,-,

Ftc. 7. Pyroxenecrystallization trend at Mon[ Saint-Hilaire(solid line) comparedto that obteinedin similal alkalineintrusions (dashed lines). 1. Ilfmaussaq, Greenland (Larsen 1976),2.Morutu, Japan (Yagi 1966),3.South Qdroq, Greenland (Stephenson 1972), 4. Auvergne,France (Varet 1969),5. Little Murun ultrapotassiccomplex, Russia (Mitchell & Vladykin 1996),6. Fen alkaline complex,Norway (Mirchell 1980).

single phase, will be strongly ordered (Batenko & Zr. As a result, zirconosilicates such as eudialyte or Voronkov 1978). Such is the caseat Mont Saint-Hilaire, catapleiite are more likely to crystallize, minerals in where the core of the aegirine is enriched inZr, andthe which the local environment around Zr is similar to that rim in Ti. This so-called "peralkaline effect' @argeset al. of Zr in the melt (Farges et al. 1994). Many minerals 1994) is the result of the chemical envtonment of the melt similar to theseare observedin late-stageenvironments from which the pyroxene crystallized. Infoduction of al- within the EHS at Mont Saint-Hilaire. kalis into a melt results in an increase of non-bridging Although systematic variations in the ratio Fe2+/Ile3+ oxygen atoms (1.e.,depolymerization of the melt), to are not observed in the aegirine, the sfrong enrichment which the Z p'referentially bonds. This depolymerization of Zr 1nthe core and its depletion in the rim of crystals inhibits the formation of r7t7,ror $tzt and provides ideal suggestthat oxidizing conditions within the magma sites for the forrnati on of t6tZ.r.Chain silicates crystallizing changed dramatically during crystallization of the from this melt provide six-fold coordinated sites (Ml) pJrroxene.Initial crystallization from a magma in which similar to those occupied by Z in the melt resulting in an reduced conditions predominated resulted in aegirine increased crystal-melt partition coefficient and increased enriched in the Di component and Zr. Continued en- incorporation of Z into the pyroxene structure. richment in the Ae end-memberresulted in Na and Fe+ This same mechanism may also be responsible for enrichment leading to decreased compatibility of Zr the incorporation of t6lTi.However, unlike Ti, the coor- within the pyroxene structureo the formation of dination number of Zr is affected by the oxidation state zirconosilicates, and Ae rims enriched in Ti. of the melt in which it resides (Gomes et al. 1970, Mont Saint-Hilaire is predominantly composed of Watson 1979, Jones & Peckett 1980, Farges et al. mafic to intermediate gabbros, diorites and monzonites 1994). Increaseddifferentiation of the magma, coupled (Sunrise and Pain de Sucre suites), lithologic units with decreasing Fe2*/Fe+ values, favor the formation of that cannot be ignored when discussing the highly charge-balancedt6lZr-O{Si,Na) bonds within the melt evolved syenites of the EHS. The EHS was emplaced and an excess bond-valence in the Ml site hostine the through both older alkaline and country rocks, leading AEGIRINE FROM MONT SAINT.HILAIRE, QUEBEC 789

Primarymagma - o enrichedin Na" F€+,Zr,Ti and Sodalitesyenite and xenoliths REE Aplites

Pegmatite E Dkes () Igneous Secondrymagma - Breccias Q enrichedin N4 Fd*, Fibrous,m etasomatic aegirine REEandwater rr)

Time

Frc. 8. Schemaricrepresentation of the parageneticsequence for the evolutionof the EastHill Suiteat Mont Saint-Hilaire.

to assimilation and entrainment of xenoliths, following aregenerally elevated. Elevated Ti contentsobserved in fractionation from a mafic parental magma (Currie aegirinefrom the miarolitic cavitiessuggest enrichment 1983).The Ae content of the aegirine can be used to de- in a residual,fluid-rich melt during cooling of the termine ftactionation trends within the EHS. Increasing nephelinesyenite and formation of late-stagepockets. fractionation results in higherAe contents, along with IncreasedTi contentsmay alsobe the resultof contami- enrichments in trace elements such as Zr,Ti and REE nation from earlier mafic phasesin the surrounding (Deer et al. 1978). These systematic variations can be Painde Sucresuite, which hasbeen shown in previous used to develop an evolutionary scheme for the EHS studies to contain Ti-rich amphiboles and micas (Fig.8). (Greenwood& Edgar1984, Lalonde et al. 1996). The nepheline syenites and xenoliths of sodalite Followingemplacement of thenepheline and sodalite syenite are considered to be the most primitive, and syenites,pegmatite dike unitsformed within the EHS. thus the earliest-formed microenvironments within the Thesemicroenvironments show strong enrichment in the EHS, as they display the lowest mol.VoAe and,concen- Ae component,Zr, Ti andREE, indicatingincreased trations of incompatible trace elements.Field evidence fractionationand alkalinity. The aplitic units areconsid- demonstrates the presence of xenoliths of sodalite ered to representthe outermostportions or chilled syenite in nepheline syenite and vice versa, with marginsof thesedikes. The aegirinein the aplite is gen- boundaries between host syenite and xenolith ranging erallyslightly depleted inh,Tt andREE relative to the from diffuse or smooth to irregular and angular. As aegirinein the pegmatites.The pegmatite- aplite dikes such, these fwo microenvironments are considered to appeartobe thepoduct of fractionalionof themagma from have formed contemporaneouslyfrom a single magma which the syenitescrystallized, as continuoushends are that may have undergone immiscibility at depth into seenin termsof thesystemAe-D-Hd Rare-earthpaftems rwo chemically distinct phases: (1) nepheline syenite, alsodisplay the same convex pattem as seen in thesye,nites, being less alkaline and more siliceous than (2) sodalite yet showstronger enrichmens. It appeanthat ffi magma syeniteoenriched in fluids and incompatible elements. wasfractionated from thefluid- andREE-rich barch from The sodalite syenite may represent an exsolved, volatile- which thesodalite syenite xenoliths formed. rich phaseof the magma-The local mine6l6gy supports The fibrous aegirinefound in the pegmatitedikes this hypothesis, as the sodalite syenite xenoliths are differs in both major- and trace-elementconcentrations dominated by incompatible- and volatile-enriched min- from the associatedcoarse aegirine. These fibrous sprays erals such as sodaliteonaEolite, eudialyte and villiaumite, arestrongly enriched in theAe componentand depleted along with a variety of REE silicates and carbonates. in both Z andTi. Although this may simply be a tunc- Simil2lly early crystnllization of amphibole in the tion of increasingperalkalinity and extremeremoval of nepheline syenitesindicates higher concentrations of SiO2 thesetwo elementsin earlier phases,the REE data sug- and suggestsa lower alkalinity, as alkali amphiboles such gestotherwise. Along with a strongenrichment in the as kaersutite and arfuedsonite become increasingly rrns1a- RE4 thefibrous aegirine displays a chondrite-normalized ble with increasing alkalinity in the magma (Stephenson patternwith a st€ep,negative slope, distinct from thecon- 1972).Alkalinity appearsto increasewithil ft1ssodalite cavetrend observed in theother specimens. The spraysof syenite xenoliths, as Ae contents and Zr concentrations fibrousoskeletal aegirine may originatefrom a distinct, 790 THE CANADIAN MINERALOGIST later-stage, metasomatic event. The homogeneity of geochemicallysimilar inffusions.Aegirine at Mont these samples (i.e., lack of chemical or optical zonarion) Saint-Hilaire is the productof fractionated,late-stage, is consistent with this hypothesis. This metasomatic fluid volatile-richmelts enriched in Na, Fe3*,and incompat- may or may not have been directly related to previously ible elementssuch as Zr,Tiandthe REE. crystallized microenvironments by normal processes of 5. An evolutionaryscheme was developedusing fractionation. The differences in major- and trace-element fractionationtrends and major- and trace-element dis- composition suggesttha,t a second magma may have been tributions. The presenceof nephelineand sodalite involved. syeniteis tentativelyattributed to liquid immiscibility In the past, there has been confusion surrounding of a parentalmafic magmafollowing crystallizationof the temporal relationship between the igneous breccias the earlierSunrise and Pain de Sucresuites. Pegmatite and the remainder of the microenvironments within the dikes and associatedaplites were later emplacedwithin East Hill suite. The breccias are concenhated along the the EHS. Late-stagemetasomatic activity resultedin boundaries benreen the older Pain de Sucre and younger the formationof fibrous,end-member aegirine within EHS, and contain clastsfrom both (Cunie 1983,Horv6th the pegmatites.The igneousbreccia unit, contaminated & Gault 1990). Complex cross-cutting relationships by mafic rocks of the Pain de Sucre suite, was em- noted by Cunie (1983) iadicate pegmatite units cut by placedcontemporaneously within thepegmatites along breccias, with the reverse also presen! yet other authors zonesof structuralweatr

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