8L7
TTrcC anadian M ineralo gi st Vol. 33,pp. 817-833(1995)
THERARE-ELEMENT.ENRIGHED MONZOGRANITE - PEGMATITE - OUARTZVEIN SYSTEMSIN THE PREISSAC-LACORNEBAf,HOLITH. OUEBEC. II. GEOCHEMISTRYAND PETROGENESIS
T}IOMAS MI"ILIA, ANTI{ONY E. WILLIAMS-JONES, SCOTTA. WOOD1AND MICHEL BOILY2 Deparfinentof Eanhand Planetary Sciences, McGiIl University,3450 University Street, Montreql, Qucbec H3A 2A7
ABSTRACT
The ArcheanPreissac-Iacorne batholith in northwestemQu6bec includes four felsic plutons (Preissac,Moly Hill, Lamotte, Lacorne),which are zonedfrom biotite to muscovitemonzogranite. The l.amotte and Lacorneplutons are associatedspatially with rare-elementpepatites, whereaspegmatites are absentfrom the Moly Hill pluton and do not contain rare-element minerals in the Preissacpluton. The mre-elementpegmatites are zonally distributed from beryl-bearing in the plutons to spodumene-bearingin the county rocks. Molybenite-bearingquartz veins are associatedwith all four plutons, and in the case of the l,amotte andLacorne plutons, occur beyond fhe spodumenepegmatites. Dikes of molybdenite-bearingalbitite occurnort! of the Lacome.pluton.All the plutons are weakly to moderatelyperaluminous (A/CNK: 1.0-1.3) and exhibit a compositional continuumin major- andtrace-element contents from biotite to muscovitenonzogranite. This conpositional continuumextends to the rare-elementpegnatites, indicating that the monzogranitesand pegmatitesare comagmatic.The chemisty of the pegmatitessuggests that the!"utiderwent further evolution from beryl-bearingto spodumene-bearingvarieties. The monzo- granitesand pegmatitestrave 6l8O15progvalues (8.6 tl.3%oo). The zonation of the plutons, the geochenical trends,and the oxygen isotopic compositions incficate'that the vaxious types of moMogranite were mainly the products of fractional crystallization.Trace element(Rb, Ba, Sr) modeling of the l,amotte and Lacorneplutons suggeststhat the most fractionated monzogranitecould havebeen formed by 8V90Vocrystallization of the magmathat formed the biotite monzoglanite.A model is proposedfor the evolution of the Lamotte and Lacome plutons, in which side-wall crystallizationproduced their observed quasi-concenticzonation, and createdvolatjle-rich residualmelts. These nelts were subsequentlyinjected sequentially into the overlying parentalmonzogranite and later the country rocks, producing zonally dishibuted beryl and spodumenepegmatites, respectively.Fluids exsolvedfrom the most evolvedpegmatites back-reacted with earlier-crystallizedspodumene-bearing aplite to form albitite, or separatedfrom the melts, filling fractures as molybdenite-bearingquartz veins. The smaller Preissacand Moly Hill plutons, which host molybdenite-bearingquartz veins, did not evolve sufftciently to form rare-elementpegmatites. Vapor saturationoccured during late crystallization of the muscovite monzogranite,and culminated in the formation of molybdenite-bearingquartz veins, which filled fracnres in the overlying crust of previously solidified magma.
Keywords:rare-element monzogranite, granitic pegmatite,beryl pegmatite,spodumene pegmatite, molybdenite-bearing albitite, quartz veins,geochemistry, fractional crysrallization,Preissac-Lacome batholith, Archean,Qu6bec.
Sorwtans
Le batholite arch6ende heissac-l,acorne, dans le nord--ouestdu Qu6bec,comprend quatre plutons monzogranitiques @eissac, Moly Hill, Lamotte, Iacorne) pr6sentantune zonation p6rologique depuis un facibs i biotite jusqu'b un facibs i muscovite.Les plutons de l,amot0eet de Lacomerenferment de plus desmassifs pegoatitiques h 6l6mentsrares, tandis que le pluton de Moly Hill n'a pas de cortlge de pematites, et celles qui sont associ6esavec le pluton de Preissacne montrentpas d'emichissementen 6l6mentsrares. Les pegmatitese 6l6mentsrares d6finissent une zooatione paxtird'un facibsb b6ry1dans les plutons i un facibs b spodumbnedans les rochesencaissantes. Les veinesde quartz i molybddnitesont associ6esau( quatre plutons, et dans le cas des plutons de Lamotte et de l,acorne, se trouvent au deli des pegmatitesi spodumdne.Des filons d'albitite I molybd6nitesont disposdsle long du flanc nord du pluton de Lacome.Tous les plutons sont faiblementi mod6r6- ment hyTreralumineux(A/CNK: 1.0-1.3), et contiennentun spectrecontinu de compositions(6l6ments majeurs et traces),de monzogranitei biotite b monzogranitei muscovite.Ce specfiese poursuit jusqu'aux pep.atites a 6l6mentsrares, indication que cesdeux groupesde rochesseraient comapatiques. D'aprdsla compositiondes pegmatites, il y auraiteu 6volution progressive du facibs h b6ryl vers le facibs d spodumbne.les nonzograniteset les pegmatitespartagent des valeurs du rapport b'uOlsuowl (8.6 tl.3%o).la zonationdes plutons, les tendancesg6ochimiques, et la compositionisotopique de I'oxyg0nemontrent que les divers types de monzograniteont 6t6 produits par processusde cristallisation fractionn6e surtoul D'apres un moddle de
1 Presentaddress: DE)arbnent of Geologyand GeologicalEngineering, University of ldaho, Moscow, Idaho 83843,U.S.A. 2 Presentaddress: G6on, 10785,rue St-Urbaiq Montr6al, Qu6becH3L 2V4.
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fractionnementfond6 sur la distribution des6l6ments traces Rb, Ba et Sr daasles plutons de Iamotte et de l,acorne,le magma monzogranitiquele plus 6volu6aurait bien pu r6sulterpar cristallisationd 8G-907adu magna responsablepour le monzogranite I biotite. Une cristallisationle long des parois aurait produit une qistallisation quasi-concentrique,et un effichissementd'une phaseaqueuse dans les volumesde magmar6siduel. Ces venues de magmasont paxla suite 6t6 inject6esdans le monzogranite susjacent,et ensuite dans les roches encaissantes,pour produire un essaimde filons de pegmuite d b6ryl et i spodumbne, respectivement.Une phasefluide exsolv6edes pegmatites les plus 6volu6esa ensuiter6agi avecune aplite pr&oce i spodumbne pour produireles filons d'albitite, ou bien a rempli desfractures avec quarE + molyM6nite. Les plutonsplus petits de heissac et de Moly }Iill, qui renfermentdes veines de quartz+ molybd6nite,n'ont pas6volu6 suffisamment pour produiredes pegmatites enrichiesen 6l6mentsrares. Dans ces cas, la saturationen phaseaqueuse dans le magmamonzogranitique i muscovite6tait tardive, et mena6ventuellement i la formation de veinesde quartz dansles fracturesde la coquille sup6rieurede la chambre magmatiquesolidifi6e. Clraduit par la R6daction)
Mots'cl6s: monzogranite l dl6ments rares, pegmatite granitique, pegmatite n Mryl, pegmatite i spodumbne,albitib e molybd6nite,veines de quartz,g6ochimie, cristallisation fractionnde, batholite de heissac-Lacorne,arch6en, Qu6bec.
Ivmoouctton granite(cf. Mulja er al. L995b)nor analyzedthe asso- ciatedpegmatites. In this study,results of 85 chemical Recent investigations of rare-element-enriched analysesof samplesrepresenting the various subfypes granitesand granitic pegmatiteshave been concemed of monzograniteand pegmatite are used to decipherthe mainly with the very late stagesof the evolution of the geochemicalevolution of the plutons. Thesedata are magma, particuQrly the transition to subsolidus integrated with geological and mineralogical data processes(e.g., Cem! et al. 1986, London 1986, (Mulja at al. 1995b)to developa model that explains Trumbull 1993, Linnen & Williams-Jones1994). the processesresponsible for the developmentof zoned Howeverocomparatively few studieshave focusedon rare-element-endchedgranitic systernsin and around the early_history of -magmaticcrystallization (e.g., the heissac-Lacome batholith. Goad& Cernf 1981,Cem! & Meintzer1988, Breaks & Moore 1992,Shearcret al. 1992),which controlsthe GBotocrcar, Srrrnqc ANDMn{RALocy pattern of subsequentenrichment of the residual melt OF T}IE MONZOGRANUE in the rare elements @umham & Ohmoto 1980). Another featurethat is poorly documentedis the zonal The Preissac-Lacornebatholith crops out over an disribution of bodiesof granitic pegmatitewith respect areaof approximately600 km2 in the southernAbitibi to their predominn6 rare-element-bearingminerals, subprovince of the Superior province, and was e.g.,the occurrenceof beryl-bearinggranitic pegmatite emplaced in mafic to felsic volcnnic rocks of the within and close to the parental pluton, and of Kinojevis and Malartic groups, and biotite schist of spodumene-bearinggranitic pegmatitefurther away,in theKewagarna Group @g. 1;Dawson 1966). Infusive the county rocks. activity took place in two stages;the first stage at The Preissac-Lacomebatholith in Qu6bec(Frg. 1) 2671-2675Ma (Feng& Kerrich 1991)was marked by containsseveral excellent examplesof zonedmorzo- the emplacementof syntectonicgabbro to granodiorite, granitic intrusions and associatedbodies of rare- and the second stage at 2630-2655 Ma (Gari6py & element-enrichedgranitic pegmatite and Mo-bearing Alldgre 1985,Feng & Kerrich 1991,Feng et al. L993), quartz veins. The monzogranitesvary from biotite- to by the emplacementof post-tectonicmonzogranite and muscovite-bearing varieties, and show continuous pegmatite.This plutonismis believedto haveoccurred variationsin the compositionof plagioclase,biotite and late in the developmentof the Abitibi Greenstonebelt, muscovite, suggestingthat tle various subtypes of and to have involved collision of continents in a monzograniteare comagmatic(MuUa et al. I995b). convergent-platesetting @imroth et al. 1983). The This compositionalcontinuity extendsto the bodiesof early Preissac-Lacorneintrusions are interpreted to rare-elementpegmatite, thereby linking the evolution result from partial melting of a mantle wedgeabove a of the monzograniteto the formation of the Be-, Li-, subductionzone; the later intrusionsare consideredto M- and Ta-enrichedpegmatites. The distibution of be products of the melting of Pontiac Subprovince pegmatitebodies is zoned,from beryl-bearingvarieties sedimentaryrocks, which were paxt of a continental within the monzogranitic plutons to spodumene- crust thickenedby collision @eng& Kerrich 1992). bearing varieties in the county rocks @gs. lA-D). The monzograniteforms four plutons,two of which, hevious investigators(Dawson 1966, Boume & Danis Lamotte and Lacorne, host and are surroundedby 1987,Feng & Kerrich 1992)havepresentedsome geo- numerousrare-element-enriched pegmatite bodies. The chemicaldata on the Preissac-Lacomemonzograniles, other plutons, Preissacand Moly Hill, are associated but have not distinguishedbetween types of monzo- with fewer bodiesof pegmatite,most of which do not
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 THE PREISSAC_LACORNEBATHOLITTI, QTJEBEC 819 o o/"-, g:H.-F€F t \ €= b'.HE o - zG 4ot E 8"* lna=;;5 {".,d% slFflE€?ed 6oft gEs 0I Hlb f ?si q gE ,*d6. gFPotr39pEF:3E ob* 5uE.,g<3E / z4
- ao@ EEE$EE€ t bTgE;; F oQ o. >6 EEEFFE? E9s'5HF9 p >o)Y o ES{E.HEg E 1\ o ,t )ot E >E eguo:Boarou Ep o >E >CD g Ee E* E o o >g o o $eas€tgc rp o @ x 6 E *€E f, Ea*;E! o 6 nO EE t $8 o .a ,ilt .L '-a#t NH I o> ffiffigffiEffififfifiEffifig gEcE'csgg s I 'lrHxE.=Y:rF E P.ZH;E.E.EE o o rO EtsaaE$s gEEE6fgi Ei€{Ef,*E Z- !f;$gE Hi$ EiHE;3; g o :E d= ffi; qf9 t;tr€g;g & \ru '!"&w Eet$gtlePFE E FT Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 820 contain rare-element-bearingminerals. Molybdenite- muscovite-garnetmonzogranite subtytrles, columbite- bearing quartz veins and stockworksoccur in or near tantalite and molybdenitealso are presentas accessory all of the plutons. minerals. With the exceptions of xenotime and Detailedmapping by Mr;J;1aet aI. (1995b)has shown ilmenite, which are present in similar amounts that each of the monzogranite plutons comprises (< 0.1 vol.Vo)in the main subtypesof monzogranite biotite, two-mica and muscovite subtypes.The most (xenotime is not present in the muscovite-garnet westerly pluton, Preissac,is dominatedby two-mica monzogranitedike), the abundancesof all accessory and muscovite monzogranites;biotite monzogranite minerals decrease systematically from biotite to occurs only as a narow marginal facies adjacentto muscovitemonzogranite. muscovite monzogranitein the norfhern part of the The bodies of rare-element-enrichedpegmatite pluton. In addition to the principal subtypesof monzo- occursas vertical to subhorizontaldikes (up to 8 meters granite, there are also fine-grained dikes of wide and about 200 meters long), which generally muscovite-garnetmonzogranite (Frg. 1A). Contacts sftike east-west and randomly in the Lacorne and betweenthe principal subtypesof monzograniteare not Lamotte plutons, respectively. On the basis of exposed,except for that betweenbiotite and muscovite the predominant rare-element-hostingminslals, ths monzogranite,which is marked by na:row, anasto- pegmatite is subdivided into beryl-bearing, mosing veins (< 20 cm wide) containing quartz and spodumene-beryl-bearingand spodumene-bearing K-feldspar.The three subtypesof monzogranitein the types.In the text that follows, thesetypes of pegmatite Moly Hill pluton form discrete bodies, which are are referred to as beryl, spodumene-beryl and separatedby screens of mafic volcanic rocks and spodumenepegmatites. These bodies of pegmatiteare biotite schists. The contact relationships among the zonally distributed, from beryl pegmatite in the monzogranite subtypes are hidden, except at one monzogranitethrough spodumene-berylpegmatite at locality, nearthe edgeof the muscovitemonzogranite, and near the margins of the monzo$anite, to spo- where there is a gradual transition from fwo-mica to dumenepegmatite in the country rocks @igs. lA-D). muscoyitemonzogranite (Fig. lB). Both the Lamotte Beyond the spodumenepegmatiie to the north of the and Lacorne plutons are quasi-concentricallyzoned, Lacornepluton is a set of east-west-frendingdikes of with the outer unit of biotite monzogranitegiving way molybdenite-bearingalbitite, which occupy fractures inward to two-mica and muscovite monzogranites.A in the intercalatedschist and basalt.The albitite dikes separateintusion of muscovitemonzogranite forms an vary from 20 cm to 1 m wide and dip steeply75'to the L-shaped dike in the southern part of the Lacorne south. pluton. Contactsbetween subtypes of monzograniteare The beryl pegmatite and muscovite monzogranite not exposedoexcept in the northernpart of the Lacorne are mineralogically similar, except that the pegmatite pluton, where the biotite monzogranitegrades imper- containsberyl, more gamet(up to L0 vol.Vo),consider- ceptibly into two-mica monzogranile. ably more columbite-tantalite (up to I vol.Vo), and All subtypesof monzogranitevary from fine to traces of galnite. Spodumene-berylpegmatite in the coarse grained (crystals up to 0.5 cm across) and Lamotte pluton is distinguishedfrom beryl pegmatite consistessentially of quartz(25-35 vol.%o),plagioclase only by the presenceof spodumenein the centerof the (3O45Vo)and perthitic microcline (2545Eo), approxi- body of pegmatite. In contrast, spodumene-beryl mately 5Vobiotite + muscovite,up to 3Vogarnel and pegmatite associatedwith the Lacome pluton has minor epidote. Biotite monzograniteis distinguished spodumenethroughout variable amountsof lepidolite from the other types ofmonzogranite by the presence (up to 5 vol.Vo),and taces of schorl and pyrophanite. of biotite and a lack of primary muscovite;the two- The spodumenepegmatite differs mineralogicallyfrom mica monzogranite contains muscovite and biotite the spodumene-berylpegmatite in containing much roughly in the proportion 2:7, and muscovitemonzo- more spodumene(up to 30 vol.Vo),much less micro- granite containslittle or no biotite. Biotite and garnet cline (<10 vol.7o),less garneg and generally no beryl or show an antithetic relationship in the transition from lepidotte. Both beryl andspodumene-beryl pegmatites biotite to muscovite monzogranite. In the compo- are zoned from a garnet-richaplite border tlrough an sitional rangefrom biotite to muscovitemonzogranite, albite - perthite- qlJartz- muscovitezone to a core of the plagioclasecomposition varies from Anrr5 to An5, massive quartz. In contast, bodies of spodumene the Fe/@e+Mg)value of biotite increasesfrom 0.69 to pegmatiteare not zoned, or subtly zoned where large 0.85, and the Fe(Fe+Mg) value and NAI of muscovite crystals of perthite occur in the inner part of the increasefrom 0.65to 0.85and from 1.55to 1.72atoms pegmatite. The albitite is an almost monomineralic per formula unit, respectively. The garnet is a rock, composedmainly of euhedralto subhedralalbite, spessartine-almandinesolid solution (Spseo_oovariableamounts of molybdenite,and fracesof zircon, Mss_:z) and doesnot show systematiccompositional Ta-emichedilmenite and gamet. variationswith monzogranitesubg4les. The accessory The plagioclase composition in the pegmatite is minerals are xenotime, apatite, zircon, monazite, Anr-5, and the Fe/(Fe+Mg) and rvAl of muscovileare titanite, magnetiteand ilmenite. In the muscovile and similar to those of muscovite monzocranite.The Cs Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TIIE PREISSAC-LACORM BATIIOLITII, QIr'EBEC 82r conte[t of muscovite in the muscoviie monzogranite concenftationsof rare-earthelements (REE') in selected and rare-elementpegmatites correlates positively with samples were deterrnined by instrumental neutron- the contentsof Ta andRb, andnegatively with thoseof activation analysesat the Universit6 de Montr6al, and Sc and K/Rb. The compositionof columbite-tantalite by inductively coupled plasma - mass spectrometry ranges from ferrocolumbite in beryl pegmatite to (ICP-MS) at Memorial University of Newfoundland. manganotantalitein spodumenepegmatite (Mul1a et aI. Twelve whole-rock compositions from Bourne & 1995a). Danis (1987) and Feng (1992) were included in this study. Gpocruvrsrrv or fire Mol{zocRANrrE Major and trace elements Analytical mzthodx The Preissac-Lacomemonzogranites are siliceous Concentationsof the major andtrace elements were Q2.+76.5 tut.Vo SiO2, Tables 1A, B), and weakly determinedwith X-ray fluorescencespectromeuy at to moderately peraluminous lmolar ratio AltO3/ McGill University, the Universit6de Monfi6al, and the (CaO + NqO + K2O): 1.1-1.3; 1.2-2.8Vonormative Cenfte de RecherchesMin6rales du Qu6bec. The corunduml. Although there is considerablescatter in IA Ikluelmant oddq (vL 74 xnxnxnxn sio, 7L4 74.8 7 75,5 10 78.1 /J,6 742 2 75,4 7 T1O. 0.16 0.10 7 0.04 10 0.@. 0.16 0.(p 2 0.03 7 sK)! 74.4 A 75.14 76.05 2 745 75.4 8 76$ 2 762 8 40, 15,4 14.4 7 14.4 t0 142 14.6 145 2 145 7 n0, 0.08 6 o.of 4 o.ts 2 0,09 0.6 I 0.G 2 0.08 3 't.33 ',14.0 F€oq 1.75 0.68 7 0.40 10 044 1.60 2 0.41 7 A"q u2 a 1r',374 14,8 2 14.1 19.8 I 2 14.83 Mno 0,06 0.04 7 0.@ 10 08 n.a 0,$ 2 nat F€lq 0.74 6 0.BB4 0.6 2 1,97 0.82 I 0.4 2 0.0 3 Mgo 0.00 0.10 7 0.1010 0.6 0.31 0.14 2 MnO 0.6 6 0.014 0,142 0,ol o.fi I 0,w 2 0.06 C€0 0.5 O73 0.$ 10 0& 126 1.102 0.61 7 MgO 0.076 0.114 0.@ 2 0.16 0.10 I 0.082 0.04 Nqo 421 4,18 427 10 4:t0 3.73 4,8 2 427 C€O 0.92 6 0,784 0.gl 2 091 0,6-/ I 0.552 qo 5.m 420 4.10 t0 3.63 437 4.47 2 424 7 Na,O 3f2 0 4.0 4 4.@ 2 3.95 4.'t7 I 421 2 4.tu Pcq 0.00 0.N 0.04 t0 0.04 0.07 0.m 2 0,03 7 q0 4.36 6 4.434 3.39 2 428 8 4,3 2 4,10 P,q 0.03 6 0.044 o.w. 2 0.00 0.02 I 0.012 0,6 Twdenenb (ppn) Ba 6.76 419 10 6 4.62 788 m TtabGlmilb (p{n) Bb 450 385 496 13 476 348 465 4 Ba 5@6 1455 125 2 154 I 692 m3 Sr '10.5 S8 9.5 1S 781 105 154 Rb n46 3678 455 2 391 641 8 M2 7@ gr 2 183 Z! S3 69 58 13 16 80 &4 1106 46.18 105 118 598 s.5 r8.5 79 61 8 m2 40 Nbmz 37 t8 73 15 a 2s 4 Zt 86 5247 Nb 16 6 17.97 a 278 24,52 485 Y2412 28 13 24 8,7 t1 334 Y 17B 1928 42.5 172 n8 30.52 33 Ta 4.V. 8.3 6.6 2 9.6 3.8 5.6 Ta 6.134 64 12 6.6 4 4S 2 6:I Pt 2:t6 23 1,75 2 L6 45 23 1.43 'rh Hl 2gl4 214 215 3.35 1.8 3 2L4 10.4 13 7,W 2 12 14 12 1625 4.65 2, 21 5 155 2 8.3 g2, gos ft 166 u91 44 I34 118 18 u N.s 3403 78 247 150 6 1652 1793 Etrq5 s 2.6 L6 3.8 BE 3.6 5 5.754 4S 42, o.! o 652 83 u 4.'134 8s 7,3 321 4.A2 11.33 Faewh elanelE(ppn) Ga 215 24s u m 83 m52 m LA 52 212 4,69 2 0.64 50.32 9.4 7,46 Co 16:1 36 11:t 2 1.67 91.5 17,7 15.6 'l.w nsaeh elffierb (pW) Pr 1,51 1.@ 2 0 9:12 2 La 19.55 11.7I 5.1 243 1L8 9.36 5.54 '11.7 Nd 623 133 827 2 0t 31.7 7,1 7.4 Cs 37.5 5 21 A 16 41.7 ltA 192 Sm L47 23 L1 2 038 457 1.8 L* Pr 4.82 24 l.U 'tzl Eu 0,@ 02 0,12 0.02 0.07 0.31 0,14 Nd 14 5 0.126 11.6 8.gg 4:t3 Gd 0.05 1,07 e08 3.1 1.8 2.61 Sm 21 6 2.33 6 L4 313 L@. 1.63 'rb 0.49 0.35 0.35 0.62 0.34 026 0.4 Eu 0,38 5 023 6 0.1 038 024 0.10 0.14 Dy 3.14 2.1 42 1.S4 153 2.53 Gd 2.U, 328 Lg 1.93 Ho 0.6 0.s 0.36 0:7 0.31 0,zl 0.4{, ft 0385 0,40 0.s 06 0.55 0.5't 0.35 Er 1.51 1,87 0:14 0:t7 1.t8 Dy 0:7 L4 0:7 0.48 Tm 02.? of, 0.15 o;12 0,12 0.17 Ho 0.7 5 0.85 1.5 0.31 0.73 EI 0.7 L17 L1 1.S Yb 155 1.05 0.9r 0.64 0.76 1 Tm 0.65 5 0,13 1.3 0.11 0.s5 0.34 02 Lu 02 0.15 0.12 0.38 0.1 0.11 0.14 '196 Yb 0,885 t, 0.78 2U 24 t,e 'REE 42 783 14.6 433 49A Lu 0.S 5 08 0.3 0,12 us 0.35 0.16 >REE 76.3 46.9 gz1 n2 Eu/Eu' 0.6 0.08 0,08 0.01 0,17S 0.17 f 0,08 hyoa 3€39 3150 272 3852 3190 n98 @12 aS Bf,n nm I ]hru 4.16 2.4 U$ 7 3.75 3.4 1.45 'lmludlngtsompls, LGSed 6,from Fong (19S2),wih ftsmPton oftrs REE Eu/Eu' 093 0.13 0.03 0.18 0.07 0.07 0,6 MG:mmvltegiamd monagnnib f . (9m+ A4& Wh€reno d& torQd a$ Bpoded,b valuo'tb.18 approffid byft e 6dO 8.6 8,5 2 9.3 8.60 S 8.4 2 8.30 9'60 irnoB€rdonol a might Unslntsrpolaled vafues for 8m and x m&; n: numbolst malys Olankons amlld8) (14616@$e) ere delomh€dwllh t€ lcP-MsretlFda, partrl from Feng (1s2), wfh fi€ €epton ol tF REE conpldeREE analF€8 g (s REEare!€@ s@ d€temhodwlth lho !|AA retho&. MO 1: min mmv'lte monzogrnnr; M0 mus@vilsmm$adto dks Figm lD) Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 822 THE CANADIAN MINERALOGIST Preissacpluton Moly Hillpluton Lamottepluton 3" j!4l-----". 4 ll', ,ry*- 3#3 51-.51-D4 o5 5 .$.t 4 4 *=F,*,T.' z3 3 :.,.,T, J il '4 ,',, il 1.0 2 o gl:11 ll] *"h'F*. I 0.6 1 .- 0.2{---.---r---,---" 9.2 0.2 0 0.4a o.4 tr 0.4 0.4 9o,ls r I o.e 0.2 l:31.' ,'-{,i, "-. , ,l.i O.2t 0.2 0.0 0.0 ol o.2l 0.2 =^^l5l .- ' ' rr r -E- ? n,l-" o.Offir|lr|nv.v-ffiu.u 0.ol---t--f----+rl:A__-Fr_ > lr -n 21 o.ol--.--.tBar- 0.0 d2l r 5o4,1|E-*-tr I *-., '" r #4: or to ,S . 2 :1 11 pl8T_--ffi.:l------;li_ 15 1 { l:] ., r rs.f ','.ro Fil' , 01 ,€s*, 17 t*r, $iil :'',', T, ,,,J rl: ftjo,' tc $]i1' l2 9r.rl ,"rr, 0.2 .',',P,.,r., ,rrl ::':C+ . 73 74 75 76 77 73 74 75 76 Tl SlO2 SiO2 cs,il', q.' 0.0 72 74 76 72 74 Si02 Si02 Fig. 2. Plot of major elementoxides (wt.Vo)versw SiO2for all types of monzogranitein the Preissac,Moly Hill, l,amotte and Lacorne plutons. In this a:rd subsequentdiagrams, the symbols of the monzogranite,unless otherwise indicated are: r, biotite monzoganite;tr, two-micamonzogranite; o, rnuscovitemonzogranite; A, dikesof muscovite-gametmonzogranite (Preissacpluton) andmuscovite monzogranite (Lacome pluton). Thereis only one datapoint for biotite monzograniteof the heissac pluton owing to the small exposureof the rock (Fig. 1, and Mdjaet aI. 1985b). the data, generally, the SiO2 content increases from their contentsdecrease toward the two-mica monzo- biotite to muscovite monzogranite (to muscovite- granite (Fig. 4). On the orher hand, in the southern gamet monzogranite in the Preissac pluton), whereas biotite monzogranite,Ba and Sr contents decrease the K2O, CaO, MgO, FerO3 and TiO2 contents inward from the marginsof the intrusion. decrease (Fig. 2); the NarO, MnO and AlrO. The distribution of bodiesof spodumenepegmatite, contents do not vary. Two exceptions to these general- i.e., thetr restriction to the Lamotte and Lacorne izations are the trend of decreasing CaO content with plutons,is reflectedin the relative contentsof Li in the monzogranite evolution in the Lamofte pluton, and the monzogranitesof the four plutons. The content of low SiO, content of the highly evolved muscovite this elementin each of the monzogranitetypes in the monzogranite dike in the Lacorne pluton. Preissacand Moly Hill plutonsis low relative to that in lJte concentration ofBa, Sr and Th decreases from the correspondingmonzogranites in the Lamotte and biotite to muscovite monzogranite (Frg. 3). T\e Zr Lacorne plutons. Moreover, the content of Li in the content also decreases, but only in the Lamotte and muscovite monzograniteof each pluton is markedly Lacorne plutons. ln contrast, the Rb content increases lower than that of the other types of monzogranite.In from biotite to muscovite monzogranite, and the the Preissacand Moly Hill plutons,the conientsof Li content of Nb is highest in muscovite monzogranite. averagedover biotite and two-mica monzogranitesare The concentration of trace elements other than those 92 + L2 ppm and 126 + lL ppm, respectively.In mentioned above, and of Li, Be, U, Ga, F and REE. contrast,the correspondingvalues in the Lamotte and which are discussed below, do not exhibit systematic l,acorne plutons arc 265 I tr07 and 202 + 124 ppm. trends with monzogranite type. The amountof Li in the muscovitemonzogranite of the Besides varying with monzogranite type, the four plutons (ppm) are 30 + 2 @eissac), 18 (Moly contents of Ba and Sr also vary spatially within HiX), 78 x. 32 (Lamotte) and 173 t 70 (Lacorne, the Lacorne pluton. In the northern part ofthe pluton, including the muscovite monzogranite dike). The Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TIIE PREISSAC_I-A,CORNEBATI{OLITII, QTJEBEC 823 PreissacDluton MolyHill pluton Lamottepluton Lacomepluton 600 1000 800 800 Ba 300 400 n 0 120 300 sr 8o 2W 100 40 100 0 0 CUU Rb 500 500 400 300 300 0 40 50 60 Nb go 40 20 20 10 0 50 40 EN 100 Y30 Y20 0 10 0 200 140 I 8o 70 100 z, Zr 100 *;!1'h' 40 30 0 0 10 40 !10 20 Th 20 20 Th 20 0 0 0 10 7374757677rc74757677 Siq sioz Si02 Si02 Frc. 3. Plot of trace-elementcontents (pm) versrs SiO2for all types of monzogranitein the heissac, Moly Hill' Lamotte and l,acorneDlutotrs. distribution of Be is much more uniform among The averageF contentis 0.03 wt.7o,and doesnot vary the four plutons, and does not vary appreciablywith with rock type. monzogranitefype. However, it may be significant The sum of the rare-earthelements is 1qrv,lsss rhan that the Be content is lowest in the Moly Hill pluton 100 ppm, in all subtypesof monzogranite,except for (3 t 0.6 ppm versus4 t I.4, 4.5 t 1.8 and 5.5 + the Moly Hill biotite monzogranite, where it is 1.8 ppm in the heissac, Lamotteand Lacorneplutons, approrimately 200 ppm (Tables 1A, B). Chondrite- respectively);the Moly Hill pluton is the only one that normalizedprofiles display light rare-earthenrichment doesnot containberyl. and negative Eu anomalies,and there is a general Concentrationsof U and Ga were determinedonly tendencyfor the profiles to flatten and the Eu anomaly for the Lamotte and Lacome monzogranites,and were to become increasingly negative from biotite to found to increase systematically from biotite to muscovitemonzogranite (Ftg. 5). muscovite monzogranite (Table 1B). The average Th./U values in the Lamotte and Lacorne plutons Gsocrm\fisrRvoF THEPEGMATITES decreasefrom 5.4 and 7 in biotite monzogranite, aNp Ar:rrrrrs through 2.2 and 3.9 in fwo-mica monzogranite,to 0.55 and 0.4 in muscovitemonzogmnite, respectively. Major and trace ekments In the sameorder of types of monzogranite,the mean values of AVGa decrease:3639 -+ 3150 + 2n2 n Only a small number of the occurrences of the Lamotte pluton, and 3852 -+ 3196 -+ 2796 spodumenepegmatite in the Lamotte pluton, and in the Lacornepluton. of beryl and spodumenepegmatites in the Lacorne The fluorine content was establishedin only three pluton, could be analyzed(rare-element pegmatites do samplesof two-mica monzograniteand two samplesof not occurin the Preissacand Moly Hill plutons),owing muscovitemonzogranite, all from the Lacornepluton. to the diffrculty of obtaining representativebulk- Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 824 TIIE CANADIAN MINERALOGIST 1(m pegmatite.The contents of 84 Y and Th decrease, whereasthose of Rb and Ga increase.from muscovite I monzograniteto spodumenepegmatite in both plutons 800 (Frg. 6). The contenls of Zr and Iff decreasein the I 600 I Ea t f 400 II TABLE2. BULKCOMPOSMON OFPEGMAME AND A.BTTM Lamotteplulon Lasomepluton Rock Spd Brl Spd Ab1 Ab2 2N xnxn xnxn o/o) E oE|r Worelanant oxldes(vtL o. siq 74.4 2 7s.9 4 73.6 69 4 682 o. Tjo, 0.01 2 0.02 4 0.01 0.01 4 A.02 2 T I 140 40, 16 2 14.64 16 19.4 4 20,1 2 Fs,q 0.97 2 05.| 4 0.16 0.16 4 0.112 124 I MnO o.t6 2 0.3 4 0.13 0.07 4 022 I I Mgo A.24 2 0.og 4 0.01 0.01 4 0.01 2 Sr 100 r1 I CaO 0.n 2 0.9 4 0.06 0.53 4 0.132 Nqo 3.91 2 5.13 4 4,1 10.14 10.8 2 80 I I(?0 2.02 2 3.12 4 4 0.09 4 A32 P,oo 0,01 2 0.02 4 0.04 0.03 4 0.05 2 60 tr trtt Tracealanetts(ppm) 40 Ba 102 26.5 4 a.z & 4 672 Rb 110,.2 853 4 4875 13.6 4 2.25 2 Sr n2 31.34 53 72 4 5112 0 2 4 6 Zt m2 424 31 4 ffiz Nb ffi2 954 45.74 42 N s Y 14.5 2 154 u.0 2.55 4 2,25 2 Distance(km) Ta 4A 604 67 25 2 1D.2 th 2.4 9.5 4 2 45 Frc. 4. Bulk-rock contentsofBa and Sr along a north-south tt 1a B4 5.5 12 traverse(N-S) in the lacorne pluton (Fig. lD), showing Ga 462 434 57 2 602 chemicaltrends from biotite to muscovitemonzoganite in Li 6735 2 257 4 6.5 2 62 the northempart ofthe pluton, andfrom the marginsto the centerof the southernbiotite monzosanite. Aplite Rar*oarthelements (ppn) La 0.9 2 5.72 0.2 1.86 0.96 c€ 1.752 18 0.52 3.35 3.2. Pr 2,71 0.00 0.37 0.51 Nd 9.5 2 10.5 0.24 1.63 2 sm 0.65 2 8.68 0.34 1.r8 1,81 samplesof thesevery coarse-grainedrocks. In order to Eu 0,1 2 0.03 0.01 0.08 0.05 mini6lTs the problem of sample heterogeneity, Gd 5.63 0.33 1.V2 1.43 approximately4 kg of material were collected from Tb 0.15 2 0.6S 0,ffi 0.19 0.16 channelscut acrosssmall bodies of pegmatite( Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TIIE PREISSAC_LACORNEBAfiIOLITH, QIJEBEC 825 100 10 1 MOLYHIttPIUTON .1 .1 100 100 Biodte monzogranite Truo-micamonzogranite Muscbvite Muscovlte monzogranite dike I.ACORM PLIITON Ia CePr Nd SmEuGdTbDyHo ErTmYbLu I.a CePr Nd SmEuGdTbDyHoErTmYbLu Flc. 5. Chonclrite-normalize Lacomepluton butincreasein the Lamottepluton from 80 muscovitemonzogranite to spodumenepegmatite. The 60 contentof Sr decreasesinitially from the monzogranite Ba 40 Rbto to the beryl pegmatite, but then increases in the 20 spodumene pegmatite, in the Lacorne pluton. A 0 possible explanation for the high Sr content of the 60 spodumenepegugtite is the breakdownof radiogenic sr 4o Rb (c/ Clark & Cem! 1987).In the Lamotte pluton, 20 the Sr content is higher in the spodumenepegmatite 0 than in the muscovitemonzogranite. The contentof Nb 120 increases from the monzogranite to the beryl to pegmatite,and then dropsin the spodumenepegmatite. Nb This sharp decreaseis reflected by the presenceof 40 tantaliteas opposed to columbitein the latter pegmatite 0 (Mulja er al. 1995b).In the Lamotte pluton, the M Brl Spd 60 MG MG content is higher in the spodumenepegmatite than in dike 40 the muscovitemonzogranite. Y Spd:spodumsne 20 Brl:beryl pegmatite 0 MG:muscovlte 100 monzogranlts Lamottspluton Zt 60 E Frc. 6. Selected trace-element (ppm) concentrations in the muscovitemonzogranite and types of pegmatitein the Lacomepluton 0 I Lamotteand Lacomeplutons. The elevatedSr contentsof MG MG Brl 8F beryl and spodumenepegmatites in the Lacorne pluton dike could be due to fhe decayof 87Rb(see text). Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 826 T}IE CANADIAN MINERALOGIST 100 composition,but form two distinct populations with respectto their trace-elementconcentations (Iable 2). Group-l albitite has much higher Rb and Ta contents, and Group-2 albitite has a much higher contentof Sr. T\e ZREE content of the albitite (Group 1) is 10.7ppm, intermediatein value betweenthe beryl and spodumenepegmatites in the Lacome pluton. It is worth noting that the REEN profile of such albitite is almostidentical to that of aplite (Frg. 7). Oxycnq Isorops ANat.yses Oxygen isotope analyses(whole rock) ofrepresen- tative samplesof monzogranite,pegmatite, and albitite were performedat the Universit6 de Monft6al and the LaCePrNd SmEuGdTbDyHoErTmYbLu University of Saskarchewan.Details of the analytical techniqueare given in Hoy (1993).All 61EOvalues are reported relative to Standard Mean Ocean Water (sMow). The whole-rock 6lEO values of the Preissac, Lamotte andLacorne monzogranites range from 8.1 to 9.8%o,with most valueslying between8.1 and 8.8%o (Tablesl,2,Fig.8); the Moly Hill monzogranitewas not analyzed,As a group, the types of monzogranite are isotopically indistinguishablewithin each of the plutons, or from one pluton to another,i.e., they have not undergone significant interaction with meteoric water.A similarlangs of 6180values is alsodisplayed by the four samplesof beryl pegmatite,whereas the spodumenepegmatites show a somewhatwider range of 6180values (7.4 to l}.3Voo).Interestingly,the mean 6180 values of monzogranite,beryl and spodumene pegmatites are almost identical (Tables laCePrNd SmEuGdTbDyHo l, 2), and ErTmYbLu similar to the 6180values of the two samplesof albitite FIc. 7. REEN profiles of rare-element pegmatites in the analyzed. Lamotte and Lacome plutons. Also shown are the REE'* profiles for dikes of aplite and albitite. The stippled area indicatesthe rangeof i?EE* in the monzogranite. Alblfto oo Spodumsns T\e ZREE content of beryl pegmatite (Lacome Pegmdte AA pluton) is similar to that of the muscovitemonzogranite Beryl (54 versus 56 ppm). However, the ZREE content of Pogmdto oa a spodumenepegmatile in both plutons is sharply lower at 2.06 ppm (Lacorne) and 7.6 ppm (Lamotte). Both rrTnrl tr tl beryl and spodumenepegmatites display chondrite- 6.0 7.0 8.0 9.0 10.0 11.0 lennalizsd gull-wing profiles as a result of depletions of LREE andHREE(Frg. 7). An aplite dike, which cuts 618o2-(suow) the L-shaped muscovite monzogranite dike in the Lacorne pluton, has a chondrite-normalized REE Flc. 8. Resultsof oxygen isotope analysesof monzogranite, pegmatite(white patiern similar to that of the pegmatites(Frg. 7). and black symbolsare for Lamotte and Lacorne plutons, respectively), and albitite in the The albitite dikes, which occur only in association heissac-Lacorne batholith. The values are ouoted witl with the Lacornepluton, havea uniform major-element respecttoSMOW. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TTIE PREISSAC-LACORNE BATIIOLITH, QI'EBEC 827 DIscussIoN Geochemicalevolution Nature of sourcerocks The systematicchemical trends described above and the geological relationshipsdiscussed earlier, and in The chemicalcomposition of the heissac-Lacorne Mulja er al. (1995b), provide strong evidenceof the plutons, in terms of major and trace elements,and evolution of the Preissac-Lacorneplutons from notably the high level of K, Rb, Th, Ta and M, early biotite through two-mica and muscovite and the low level of CuZr, Hf, Y and Sr, is typical monzograniteto beryl and spodumenepegmatites. The of syncollisionalgranites Qlarris ar al. 1986),and has most plausible explanation for this evolution is beennoted by previous authors(e.9., Feng & Kerrich fractional crystallization of a parental monzoganitic L992). It has also been shown from Fb isotopic magma. data (Gari6py & Allbgre 1985) and Nd isotopic data Depletion of Ti, Fe, Ca, Ba, Sr, Zr and Th, and (Ducharrneet al. 1995) that the magmasforming the concomitantenrichment of Rb, M and U from biotite plutons representmixtures of juvenile, mantle-derived to muscovitemonzogranite (Figs,2,3,5) areconsistent and older, crust-derivedsources. As previously pro- with early fractionationof plagioclase,biotiten zircon, posedby Gari6py& Alldgre (1985),Feng & Kerrich monaziteand apatite,and late crystallizationof albite, (1992) and Boily (1995), we believe that the most muscovite,garnet, and columbite-tantalite.Similarly, plausiblesource for the magmasis the Pontiacsuite of the systematicincrease in the negative Eu anomaly clastic metasedimentaryrocks exposedto the south. with magmaevolution is consistentwith fractionation This sequenceis interpretedto have beenthrust under of early, more calcic plagioclase(cl Cullers & Graf the Abitibi Southem Volcanic Zone during an arc- 1984); the correspondingdecrease in total REE, continentcollision between2670 and 2630 Ma (Feng particularly b the IREE, can be explained by early & Kenich 1992).According to this model, the mixing fractionationof monaziteand apatite(Fig. 5). Finally, could indicateanatexis of the Pontiacmetasedimentary the linear relationship between Ba and Sr, and the sequenceand input of magma from the underlying marked decreasein Ba content from biotite to mantle,or the melting of sedimentaryrocks containing muscovitemonzogranite (Ftg. 9), are strong evidence both crustal and mantle components.Although it is for fractional crystallization of plagioclase and difficult to decide between the two alternatives.we K-feldspar. favor the latter becauseof evidencefor two populations Other processesthat have beenproposed to explain of zircon in the metasedimentaryrocks @eng er a/. compositionalevolution in monzograniticplutons 1993),one with an ageof up to 3050Ma, representing similar in extent of zonation to that observedin the an older basementcomponent, and the other with an Preissac-Lacorne plutons are restite unmixing age of 2700 Ma, possibly representing arc-related (Chappell et al. 1987), source-rock heterogeneity felsic igneousrocks. (Nabelek et al. 1992), and sequentialpartial melting (Holtz 1989).We can rule out restiteunmixing as the cause of the monzogranitezonation, becauseof the conspicuousabsence of xenolithsor xenocryststhat could representrestite, and the requirementthat the contents of all elements vary linearly with SiO2 contents (ChappeTlet al. 1987). Although the concentrationsof many of the major andtTace elements do show suchlinear variations(Tigs. 2, 3), a significant number shows trends that are clearly nonlinear, E EL e.g., Nb, Y, and Rb. Source-rockheterogeneity also CL 0 20 ,lO 60 80 100120 0 50 1(X) 150 200 can be ru1edout becauseof the remarkableconsistency ct to of 6180 values from biotite (8.6 t o.l%o) through two-mica (8.5 + O.lVoo)to muscovite(8.6 t 0.4Vao) monzognnite. Finally, sequentialpartial melting can be eliminated becausethis processtends to produce relatively constant concentrationsof compatible m0 elements(Hanson 1978). In the heissac-Laccrne plutons, several of the compatible elementsdisplay 0 wide variationsin concentrations,e.8., Ba and Sr' 0 50 1(x) 150 200 0 S0 100 iso 200 The relatively high contents of Ba and Sr in the Sr (PPm) biotite monzograniteand their extreme depletion in FIc. 9. Plots illustrating Ba versus Sr concentrationsin the tfie muscovite monzogranite simply cannot be four monzogranite plutons of the heissac-Lacome explainedby partial melting (c/. Mittlefehldt & Miller batholith. All valuesare in ppm. 1983). Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 828 TIIE CANADIAN MINERALOGIST TASLEs.MINEMUMELT (1o)usED Trace - elemznt rutdelin g INTRACE.B$TETT MODS.JNG, ANDVOIUME % OF PHASES FRACNOMTEDFFOM EOTITETO MUSCOVIIE MOMOGRINITE In order to test the hypothesisthat fractional crystal- Mhe|al Rb Vol.% lization was the principal processresponsible for the Plsdelaso 1.3 2.8 0,09 al compositional diversity of the Preissac-Lacorne K-tsld8paf 6.12 3.6 t.6 32 monzogranites, we applied Rayleigh fractionation Biotts 6.4 0.12 4.3 calculationsto simulatethe behaviorof Ba Sr and Rb, MrEcovile 12 0.4 1.5 I which are generally considered to be petogenetic indicators in granitic rocks (e.9., McCarthy & Hasty C: Lamotb 635 r3l 265 1976, Tindle & Pearce 1981). This modeling was CLaome 62. \U 3@ restrictedto the Lamotte and Lacorneplutons because Sourceotlg dsh:tlenoon('1978), Nash &Cmraft 0S85), Mahood & the data for the Preissacand Moly Hill plutons are Hfiedh(1sB] consideredinadequate. The following equation was (after d: odgtulcofistadonofolEnenl h tErgnf neff (ee text) usedto describefractional crvstallization Hanson 1978): CYCo= F@-1). 104 Lamottepluton 2O o/" 103 60% 1e 80o/o Ba 90 o/" 10 95o/o 1 1d 20 o/o 1& Sr 60 8Oo/" o/o 10 90 I 95 o/o to3 exto3to2 Rb Rb Frc. 10. Comparisonsof the Rayleigh fractionationmodel (solid line) and observeddata of all subtypesof monzograniteand pegmatitesin the Lamotte and l,acorne plutons. The agreementbetween the theoretically calculated and measuredssmpssitions strongly supportsthe conclusionthat ftactional crystelliTationwas the main process responsiblefor the compositionaldiversity of the monzogranite.Deviation of some measuredvalues for someof the pegmatitessuggest that the distibution coefficients used for the monzograniteare not applicableto volatile-rich batchesof psgmatite- forming liquid. Symbols: I biotiG monzogtanite, D two-mica monzogranite, . muscovitemonzogranite (main body), O muscovitemonzogranite (dike), A beryl pegmatite,+ spodumenepegrnatite. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TIIE PREISSAC_LACORNEBATI{OLITH, QT]EBFT 829 where Cr and Co arc weight concentrationsof a trace the absenceof this mineral in the beryl pegmatite,we elementin the derived melg and in the parentalmelt, interpretthe strongdepletion of t/REEinthe latterrock respectively;F is the fraction of melt, and D is the bulk to be a result of xenotimefractionation (in view of its disfibution-coefficient. The concentation of a ftace well-establishedtendency to concentate tJlreHREE). elementin the residual mineral phases,Cs, relative to The depletion n HREE may also be due in part to the parent, Coowas calculated from the relationship fractionationof gametin the muscovite-monzogranite- Cs = Cl * D. Barium. Sr. and Rb were used in the forming magmas.The depletionof IREE in the beryl modeling; amongthe traceelements whose concentra- pegmatitecan be interpretedto reflect fractionationof tion was established"thsse are the elementsfor which monazitq'monazite is considerablymore abundantin the mineraUmeltdistribution coefficients(Kr) are best the muscovite monzogranite than in the beryl known (Table 3). The values of Ko used in the pegmatite(Mulja er al. 1,995b).The extremedepletion calculationswere selectedfrom the lilerature because of both LKEE andHREE n the spodumenepegmatite the rocks from which they were estimated have can be explained by the continued fractionation of compositionssimilar to those of the heissac-Lacome moruEite and garnet in the beryllegmatite-forming monzogranites.Initial values of Ba, Sr and Rb rnagma, and the absenceof these minerals in the concentrationswere those of the least evolved biotite spodumenepegmatite. Other explanationsthat have monzogranite,i.e., that with the smallestnegative Eu been proposed for extreme depletion of REE in anomaly.The results of the modeling suggestthat the pegmatitesare F-REE complexing@ynn & Bumham various subtypesof monzogranitein the Lamotte and 1978) and subsolidus mobilization of REE by Lacorneplutons can be producedby moderateto high hydrothermal processes(e.9., Sverjensky 1984). degreesof fractional crystallization (6G-957o)from Although we do not have a test of the former hypo- sucha biotile monzograniteparent (Frg. 10). Suchhigh thesis,replacement of amphiboliteby biotite near the degrees of fractionation are consistent with tle spodumenepegmatite and replacementof hornblende relatively small volumes of the more evolved monzo- by holmquistite indi- granites relative to those of biotite moMogranite. catethere was si Although the calculations also produce good spodumenepegmatite to the country rocks. Moreover, correspondencebetween the model and observed altered amphibolitewithin a centimeterof the contact compositions of most of the occurrencesof beryl commonlycontains abundant apatite. It is thuspossible pegmatitein the Lacome pluton (over 90Vofuacnonal that some REE may have been hydrothermally crystallization, Fig. 10), spodumenepegmatites have transportedout of the pegmatiteand concentratedin substantially higher Rb, Ba and Sr contents than apatitein the county rocks. predicted.The reasonfor this is unclear.However, one possible explanation (at least for Rb) is that the Formation of albitite dikes magmasthat producedthe spodumenepegmatites had highercontents of volatile constituentssuch as F andB, The almost monomineralic nature of the albitite which are known to increasethe solubility of Group-l dtkes (97Voalbite and minor molybdenite) rules out elementsin the melt (London 1987, Manning & their formation by equilibrium magmatic crystal- Pichavant 1988). This explanationis consistentwith lization. A purely metasomaticreplacement of the host the presenceof tourrnaline and a higher content of basaltor biotite schistis unlikely becauseof the sharp fluorine in the muscovite (Mulja et aI. 1995b)n the contact between the atbitite and the country rocks. spodumenepegmatite. Moreover, the REE{ pattem of the albitite does not Thesearguments, the coherentrelationships among resemblethat of the biotite schist (cf Feng L992).On trace elements between the monzogranite and the other hand, the bodies of albitite could represent pegmatites(Figs. 6, 7), and the evidence from the felsic dikes that have been metasomaticallyaltered. frace-elementdata for muscovite(Fig. 8; Mtila et al. This interpretationis supportedby the similarity of the 1995b)illustrating a linear relationsbipof Sc, Rb and REE{ profile of the albitite to that of aplite (Fig. 7). We Ta versus Cs, demonstratethe genetic link of the thereforepropose that the dikes are the productsof a monzograniteto the rare-elementpegmatites. completereplacement of highly evolvedgranitic rocks by residual aqueous fluids, which evolved during Behavior of REE durtng pegmatiteevolation the late-stagecrystallization of the magm4 i.e., the precursor magmatic rock stewed in its own residual The concentrationsofEu, and ofthe light andheavy fluids. A questionthat needsto be addressed,howevel REd decreaseprogressively from beryl to spodumene is why quartzwas removedcompletely from the roch pegmatite in the Lacome pluton @g. 7). The large given that the residualfluids would have been quatu' negativeEu anomalyis most reasonablyinterpreted by saturated.The precursormay havebeen a spodumene- fractionation of plagioclase,as discussedpreviously. rich aplite, in which spodumeneand quartz reacted Given the relative abundanceof xenotime in the with the fluid phaseto form albite. This processhas muscovite monzogranite (Mulja er al. I995b) and been predicted thermodynamically by Wood & Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 830 THE CANADIAN MINERAI'GIST FIc. 11. Skerchesdepicting the proposedhistory of crystal- lization of the rare-element-enrichedgranitic magma of the l,amotte and Lacome plutons. Note that if drawn to scale,the roof would appearseveral times thicker to . i-:-::::t,.,.,.',,, ) ) ) ) represent its pre-intrusion thickness of about l0 km. A) Early side-wall crystallization lnoduces marginal biotite monzograniteand lessdense boundary-layer melts, which ascendto the roof of the magmachamber. These mels displacedownward the densercrystal-mushes that formed earlier. B) Continued fractional erystallization forms successivetwo-mica and muscovitemonzogranite layers and more differentiatedmelts. Late-stagecrystal- lization moves toward the center of the chamber.where the residual magma continuesto differentiate. A small portion of the more differentiated muscovite-bearing monzograniticmelt breachesthe chamberand intrudes the country rocks as a dike (MG) in the Lacorne pluton. C) Expulsion of pegmatite-formingvolatile-rich magma from the chamberowing fluid ove4nessureresults in the \ \\\--: emplacementof the beryl @rl) pegmatitein the overlying Muscovitd iotite monzogranite monzogranite.D) Later contractionof the pluton, partly monzogranite due to its advancedstage of crystallization, and to the Re.sidualmelt Tbo-mica monzogranite gooling of magma, reactivatesfractures in the county rocks and producesnew fractures,into which the more evolved melts are intruded. This gives rise to the spodumene-beryl (Spd-Brl) and spodumene (Spd) pegmatites.Aqueous fluids exsolvefrom the spodumene pegmatites;some back-react with earlier-formed aplite \ \ \ sq&wi5. and pegmatiteto form Mo-bearing albitite (Mo-Ab), and \\\\\\v \\\\\\v some precipitate quartz and molybdenite as veins \\\\\\v (Meqtz), in the country rocks. :-----J//'/ intuded country-rocks. In the Lacorne pluton, the geochemical(Frg. 4) and mineral-chemical(Fig. 6, Mda er al. 1995b) trends corroborate strongly the process of side-wall crystallization. The Lamotte \\\\\\_: pluton, which lacks observablesystematic chemical variations acrossthe monzogranites,could have been formed in a regimeof slow solidification, asis the case for homogeneousplutonic rocks (Sawkaet al. 1990). Furthermore,the comagmaticberyl and spodumene pegmatites, which represent the more and most evolved felsic liquids, respectively, cut all types of Williams-Jones(1993) to be an inevitableconsequenc€ monzogranite,indicating that the liquids must have of the cooling of a fluid initially in equilibrium with evolved toward the interior of the body of magma. spodumene-bearingpegmatite. On the basis of these geological and geochemical observations,we envisagean initial stageof side-wall THEMAGMACHettanm crystallization that produced a crystal mush (biotite monzogranite)and a less denseboundary-layer liquid The (quasi-) concentric zonation displayed by the (Fig. 11A) [seeMcBirney et aI. (1985)for a discussion Lamotte and Lacomeplutons, i.e., less evolvedbiotite of the dynamics of magma chambersl.According to monzogranitealong the margn of the innusions and this model,boundary-layer liquidrises buoyantlyto the the most evolved muscovite monzogranite in the top of the chamber,displacing earlier-formed,denser center, suggeststlat tle magna underwentside-wall crystal-mushlayers from the roof. Qely6sliea 6d high crystallization, in which the eartest solidification densityeventually drive theselayers into the lower part occurred at the contact between the liquid and the of the chamber,where they undergo further upward Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 TIIE PREISSAC_I.ACORNEBATHOLITII, QUEBEC 831 crystallization.This stageof continuedcrystallization spodumenepegmatite in the country rocks, molyb- along the walls producedthe more evolved two-mica denite in the heissac and Moly Hill plutons occurs monzogranite,which led to the ascentof an evenmore almost exclusively in quartz veins in the muscovite differentiated boundary-layer (Fig. 1lB). In the monzognnite. Pegmatite bodies are relatively Lacornepluton, a portion of the biotite monzogranitic uncommonin these two plutons; they rarely contain crystal-mush layer (southern biotite monzogranite) beryl, never contain spodumeneoand show no obvious must have separatedfrom the other layers prior to its spatial association with molybdenite-bearingquartz completeconsolidation, and must have begunits own veins. The small size of the plutons and the fact that pattem of side-wall crystallization.The separationand they did not undergodifferentiation to the sameextent independentcrystallization were necessaryto produce as the Lamotte and Lacome plutons are considered the observedgeochemical and mineral-+hemical trends responsible for these characteristics.Consequently, mentioned above. One cause for the separationof vapor saturationoccurred during crystallizationof the the southem biotite monzogranite was perhaps the muscovitemonzogranite under a relatively thin crustof emplacementof the magma along reactivated pre- solidified magma, which was readily fractured. The existing fracturesor along new ones.This tectonically exsolved fluid phaseescaped through these fractures inducedemplacement of magmaseems applicable also and formed molybdenite-bearingquartz veins in the to the emplacementof the L-shapedmuscovite monzo- muscovite monzogranite.This exsolution is reflected granite dike (labeled MG-dike, Fig. lD), which in the finer-gra:inednature of the monzogranitemound suggeststhat a batchof somewhatevolved felsic liquid the veins (Mulja er aI. 1995b),which developedas a breachedthe walls of the magma chamber through result of compositionalquenching of the melt. Support ftactures. for the above interpretationis provided by the fine- A consequenceof the inward crystallizationwas the grained muscovite-gametmonzogranite dike, which accumulationof more differentiatedand volatile-rich containssmall molybdenite-bearingvugs that probably batchesofresidual liqui{ which werebelow an already represetrt tle site of vapor-phase exsolution. lithified carapace.Fractures in the roofrocks, produced Significantly, this dike crystallizedlate in the history of tectonicallyor by overpressures,provided the conduits the pluton, andin fact cuts molybdenite-bearingquartz for the early, beryl-pegmatite-producing liquid veins. (Fig. 11C). At an advancedstage of crystallization, correspondingto the onsetof withdrawal of the above CoNclusIoNs liquid from the chamber, the pluton underwent contraction,producing conical fractures,analogous to Field, petrochemicaland oxygen isotope data for those associatedwith the main events of caldera four peraluminousmonzogranitic plutons that makeup formation, and reactivated pre-existing east-west- the Preissac-Lacornebatholith, taken in conjunction trendingfractures in the country rocks. Thesefractures with geologicaland mineralogicalindicators (Mulja er tappeddeep pockets ofmore evolvedfiquid, which was al. 1995b),have distinguishedsuites of comagmatic drawn upward and outward, solidifying as monzograniteand pegmatitesubtypes, and shownthat spodumene-berylpegmatite along the margins of the their compositionaldiversity was mainly the result of plutons, and eventually as spodumenepegmatite in fractional crystallization. Magma in the Lamotte and the county rocks @ig. llD). Crystallization of the Lacorne bodies underwent the most extensive spodumenepegmatite terminated with the exsolution differentiation, and evolved to the stageofproducing of aqueousfluids from the melt, which formed systems rare-element-bearingpegmatites. The mechanismof of quartz veins or, in somecases, caused the albitiza- fractionation in these plutons was side-wall crystal- tion of the associatedapttes. Molybdenum,which has lization, which developedquasi-concenfically zoned a fluid/melt distribution coefficient greater than I intrusivebodies. The rare-element-enrichedpegmatites (Candela& Holland 1984),partitioned into the residual were derived from batchesof residualmelt trappedin fluid and concentratedas molybdenitein the veins and the interior of the cooling plutons. Beryl and albitite. The proposed evolution of granite-related spodumenepegmatites were emplaced sequentially, mineralizationis consistentwith the spatialdisribution the former filling prlsssurg-inducedfractures in the of pegmatites,and the proximity of the most evolved overlying parentalmonzogranites, and the latter filling spoftrmenepegmatites to the dikes of albitite and the exiension fractures in the county rocks, reactivated veins of quartz. and producedby contractionof the adjacentsolidified plutons.The terminal stageof pegmatileevolution was OnrcN or Mor-yaosrnre-BEARTNGQUARTZ VEtr{s marked by exsolution of an aqueousflui4 which in IN TTIEPREISSAC AND MOLY IfuT huToNS some places back-reacted with earlier-formed spodumene-bearingaplite to form albitite, and in other In contrast to the Lamotte and Lacome plutons, placesseparated from the parentalbodies of pegmatite wheremolybdenite is concentratedin albitite dikes and to precipitate quartz, muscovite and molybdenite in quartz veins that are spatially associated with veins. 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