Canadian Minelalogist Vol. 28, pp.25l-266 (1990) AGPAITIC AND MIASKITIC NEPHELINESYENITES OF THE MCGERRIGLEPLUTONIC COMPLEX. GASPE, OUEBEG: AN UNUSUAL PETROLOGICALASSOCIATION* GRAEME M. WALLACE Depaftmentof GeologicalSciences, McGill University,3450 UniversityStreet, Montreal, QuebecH3A 2A7 JOSEPH B. WHALEN GeologicalSumey of Canado,601 Booth Strcet, Ottawa,Ontario KIA 0E8 ROBERT F. MARTIN Department of Geological Sciencesand Mineral Exploration ResearchInstitute, McGill University, 3450 UniversityStreet, Montreal, QuebecH3A 2A7 ABSTRAcT dansun r€servoircrustal pourrait avoir fourni le flux de chaleurrequis pour une fusion partielle du protolithedes The dominantly granitic Early Devonian McGerrigle plu- rochesgranitiques. Cette hypothbse explique Ia miseen place tonic complex, in the Gasp6region of Quebec,contains demagmas contemporains sous-satur6 et sursaturden silice minor volumesof 1) miaskitic and 2) sodalite- aenigma- dansle massifde McGerrigle. Ce sch6ma requiert une cham- tite - astrophyllite - arfvedsonite-bearingagpaitic nephe- bre magmatiquetrCs stable, donc un milieu en extension line syenites. Significant mineralogical variations in these le long d'un rift ou d'unefaille transformante. rocks principally reflect the aepaitic index andflO) of the magma,These geochemically highly evolvedundersaturated Mots-clds:sy6nite n6ph6linique, miaskitique, agpaitique, rocks probably representthe products of fractionation of granite,contamination crustale, complexe de McGer- mildly alkaline mafic magma(s). Isotopic data imply that rigle,Gaspdsie, Qu€bec. batchesof juvenile syenitic magma were variably contami- nated by minor volumes of crustal material. Fractionation of mafic magmasin a crustal reservoir could have supplied INTRODUCTION heat to partially melt the protolith of the granitic rocks. This proposal accounts for the contemporaneoussupply The nephelinesyenites of the generallygranitic of silica-undersaturatedand silica-oversaturatedrock-types McGerrigle plutonic complex, Gasp6 Peninsula, in the McGerrigleplutonic complex.Our proposalis con- (Fig. l; Whalen 1987)constitute the only with a very stable magma chamber, as would be Quebec sistent known exampleof silica-undersaturatedfelsic mag- expectedin a tensional environment, possibly associated with rifting or strike-slip faulting. matism of Devonian age in the Canadian Ap- palachians(Currie 1976,Woolley 1987).The associ- Keywords: nepheline syenite, miaskitic, agpaitic, granite, ation in the McGerrigle complex of small volumes crustal cont.rmination, McGerrigle complex, Gasp6, of both agpaitic and miaskitic nephelinesyenite with Quebec. largevolumes of granite and lesservolumes of gab- bro (Whalen 1987)is petrologicallyhighly unusual. SOMMAIRE In this paper, we present new mineralogical and geochemicaldata bearing on the nature and petro- plutonique Mccerrigle, en Gasp6- Le complexe de situ6 genesisof theserocks. Where usedwithout a quali- sie (Qudbec),d'6ge d6vonienpr€coce et i cardctbresurtout granitique, contient de petites quantit€s de sy6nitesn€ph6- fier, "nepheline syenite" will refer to both agpaitic liniques miaskitique et agpaitique (association de sodalite and miaskitic varietiesof nephelinesyenite. + aenigmatite + astrophyllite + arfvedsonite). Lesvaria- tions min6ralogiquesimportantes dans cesroches reflbtent surtout I'indice d'agpaicitd et la fugacit6 d'oxygbne du GsNsRAr GeoLocY AND PETRocRAPHY magma. Cesroches sous-satur6s fortement 6volu€esr6sul- teraient du fractionnement de magma(s) mafique(s) l6gb- The McGerrigle plutonic complex, which intrudes rement alcalin(s). Les donn€esisotopiques t6moipent aussi the Cambro-Ordovician Shickshock and Quebec d'une contamination locale du magma sydnitique juvdnil groups (de Rdmer 1977),hasbeen subdivided into par la cro0te. Le fractionnement de magma(s) mafique(s) two suites, a predominantly northern hybrid suite *GeologicalSurvey of CanadaContribution Number 34789. (Fig. 1), with which the undersaturatedrock$ are Mineral Exploration ResearchInstitute contribution num- associated,andagranitic suitein the southernpart ber A9031. (Whalen1985, 1987). The hybrid suiteexhibits a het- 251 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/28/2/251/3446190/251.pdf by guest on 02 October 2021 2s2 THE CANADIAN MINERALOGIST FIG. l. Location of the McGerrigle Plutonic Complex, GaspdPeninsula, and a generalizedgeological map of the area giving locations of the nephelinesyenite samples studied in detail (closedcircles). Sample 16 (WXMGI6) consists of miaskitic nephelinesyenite, whereas the others consistof agpaitic nephelinesyenite, exposed as dykes or sills. The triangles correspondto the main peaks in the McGerrigle Mountains. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/28/2/251/3446190/251.pdf by guest on 02 October 2021 NEPHELINE SYENITES. MCGERRIGLE COMPLEX, GASPE 253 TABLE 1. !,{INERTIO€Y AND IdODAI, CO{POSIf,XON OT SAUPLES OF erogeneousappearance on both the outcrop and NEPHSLINE SYENITE FROM XllE UCGERRIGI.E PLUTON regional scale;xenoliths of gabbro, diorite, nephe- line syenite, metavolcanicrocks and hornfels are 7234567 Rock Tylp MIAS AGPA AGPA AGPA AGPA AGPA CINC found in quartz-bearingmonzodiorite, monzonite s@ple !,tc16 MG3? UG39 UG67 I{G94 l.tc98 MG98 and syenite(Whalen & Gari6py 1986).Felsic inclu- K-feldspar I 78.361.9XXXXx sions are rare. Remarkablefeatures suggestiveof AIbit6 trlrxXxXx complexinteraction and hybridization of the mafic NepheJ.in6 9.219.3XXXX sodaltte - 2.8 X ? X X and felsic magmasled Whalen (1985)and Whalen FlEoxone 3.0 11.9 X X X X X Agt Aa Ae Agt As & Gari6py (1980 to proposea magma-mixingmodel AnphLbole - 3.5 - X - X X Arf for tlte origin of the hybrid suite. On the other hand, Btott!€ 4.60.2XXXXX granitic Bt Lep UgBt I€P I€P the suite doesexhibit textural variations, but uagnqtlte 0.8 - x x x - x in generalit correspondsmineralogically and com- l1tsenLte x- A€nLgmtitE - 0.6 positionally to a homogeneoussubsolvus biotite- l'9 x rr zLxcoa 2.2 - X - X - X amphibole gratite sensustricto. Mafic inclusions are AlEtlto trtrx-XXX Aatlophyl1lte _trx rare. Fluollte -trtxx Pectollte -trx-? Radiometric data indicate that the complex is 9l6hle!1go x- Devonianin age(de R6mer L974,LaRocque 1986, I}i6och1oro x- Eudlal!rc6 -tr Whalen& Roddick 1987).A K-Ar age of 377+9 RoaenbuachLte -96rzenlte ?- Ma, determinedon biotite from a sampleof miaskitic nephelinesyenite (WXMGIO, falls within the range litLnoralogy atrd nodal contrDsLtLon of selocted 6tup1oa ol of rock nepholLng gyenlta (whalen f, Gart6py 1986). Abbrevlatlons agesexhibited by the silica-oversaturated of rock tf'tE6! ltlAs nLaskttlc nepholiae syentte' AGPA types in the pluton (Whalen & Roddick 1987).U- [email protected] nephelLte syonit€, cINc J,eucoclatLc cognat€ tiiluslon tn-wxMG98 (;orlm 6). stdbolsr Ao aegLrlno Pb agesfrom the samesample of nephelinesyenite, (UG3?, lfc39 also cootaln tltml.an aeqLrlne)' Agt aegLr- Lne-auglte, Arf arfvedaon.tio. Bt biotlto, l4gBt ug-rlch a sampleof gabbro and one of granite all overlap biotlte, I€p lapLdonelanei X proaont, tr lraca tuounts, - abgont, r altsred to aecondaty ninerals, *r lDsstbly at about 390Ma, which suggeststhat all components aocondary. Tho full 6dple nu&ber contalns tho prafia tlx of the pluton are contemporaneous(J.B. Whalen and J. Mortenson, unpubl. data). Equilibrium assemblagesof mineralsin a 1.5- to 3-km contactaureole ofthe complexindicate a con- indicated by mapping. fining pressureof lessthan 2 kbars (de Rdmer 1977, Gabbro, which occursas inclusionsup to 1500m Van Bosse& Williams-Jones1988). The aureolehas in length or as thin dykes(Whalen 1987)in the hybrid a much broader extent than 3 km, as revealedby suite, is the only other silica-undersaturatedrock type regional contact-relatedgradients in illite crystallinity in the complex.The gabbroscontain up to 6 wt.9o and reflectanceof organic matter in the enclosing normative nepheline and exhibit a chemistry and Cambro-Ordovician flysch sequence(Islam et al. mineralogy (e.g., presenceof titaniferous diopside 1982).The extent of the aureolesuggests that the plu- and kaersutite)consistent with an alkaline charac- ton flares out at a shallow depth. ter (Whalen & Gari6py 1986,Wallace 1986). The agpaitic nephelinesyenite is exposedin dykes Inclusions are present in the nephelinesyenite. and sills up to 3 m thick that cut syenite and other Someare mafic to intermediate,angular to irregu- rocks of the hybrid suitein the northern half of the larly shaped,metasomatized xenoliths up to 10 cm complex (Fig. l) The dyke rocks are fine grained, across. The more strongly agpaitic samples(e.9., grey to dark green,and porphyritic. A chilled mar- WXMG37, WXMG39) contain rare polymineralic gin commonly is observed,with little or no evidence clots @igs.2a,b) that may representpseudomorphs of metasomatismof the host rock at the contact. The after, or remnantsof, xenocryststhat werepossibly miaskitic nephelinesyenite, texturally similar to the' derived from such xenoliths. A\o presentare ovoid, agpaitic variants, appearsas grey pods or ovoid bod- equigranular to porphyritic cognateinclusions up to ies about 100by 20 m that cut syeniteof the hybrid 10 cm acrossthat havea mineralogysimilar to that suite northeast of Mont Jacques-Cartier(Fig. l). of their host rock. The larger cognateinclusions may Leucocraticaplitic veinscommonly cross-cutor are exhibit disaggregationfeatures. associatedwith the miaskitic nephelinesyenite, and the host rocks may exhibit local evidenceof alkali Mnnnelocv oF THE NEpHELINESvsNtrss metasomatism(c/.