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The Rare-Element.Enrighed Monzogranite

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The Rare-Element.Enrighed Monzogranite 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. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/4/817/4006117/817_33_4_cm.pdf by guest on 25 September 2021 818 TrucANADIANMTNERALocTsT 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,
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