LATE- TO POST-TECTONIC SETTING OF SOME MAJOR … · sutures montre aussi une corrélation avec le...

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The Canadian MineralogistVol.48,pp.000(2010)DOI:10.3749/canmin.48.4.000

LATE- TO POST-TECTONIC SETTING OF SOME MAJOR PROTEROZOIC ANORTHOSITE – MANGERITE – CHARNOCKITE – GRANITE (AMCG) SUITES

Jamesm.mcLeLLaND§aNDBruceW.seLLecK

Department of Geology, Colgate University, Hamilton, N.Y. 13346, U.S.A.

michaeLa.hamiLTON

Jack Satterly Laboratory, Department of Geology, University of Toronto, Toronto, Ontario M5S 2B3, Canada

mariONe.BicKFOrD

Department of Earth Sciences, Syracuse University, Syracuse, N.Y. 13244–1070, U.S.A.

aBsTracT

Recentzircon-basedgeochronologicalinvestigationsintheAdirondackMountains,inthestateofNewYork,demonstratethat1155±6Mamassifanorthositewasemplacedduringthepost-contractionalphase(1160–1140Ma)oftheca.1200–1140MaShawiniganorogeny.Emplacementofmanyother anorthosite–mangerite– charnockite–granite (AMCG) suites alsocorrelateswith thewaningstageoforogenyand includes theca.1155MaMorinandLac-St-Jeancomplexes, theca.1650MaMealyMountaincomplex,andtheca.1450MacomplexesofeasternLabrador.Thecorrelationalsoappliestotheca.930MaRogalandanorthositecomplexofNorwayandtheca.1060–1020Malate-topost-OttawananorthositesofcentralQuebecandtheAppalachiansofVirginiaandsoutheasternPennsylvania.Thesecorrelationssuggestmodelsinvolvingdelaminationofoverthickenedorogenic lithospherebyfounderingorconvectiveremoval, followedbyathenosphericascent,andpondingofgabbroicmeltat thecrust–mantle interface.Orogen rebound followingdelamination results in stable,dynamicallybalancedsettingsinwhichgabbroicmagmaevolvesslowlyathighpressuretoproducehigh-aluminumpyroxeneandcoarse,intermediateplagioclasecharacteristicofmassifanorthosite.Relatedmeltingofthelowercrustproducesmangeriticandcharnockiticmagma.Ultimately,bothanorthositicandgraniticmagmasascend,and lower-pressure fractionationyields theclassicAMCGsuites.Transtensionalreactivationoflithospheric-scaleshearzonesandoldsuturesalsocorrelateswithimportantAMCGmagmatismandprovidesconduitsforgabbroicmagmathatpondsatthecrust–mantleinterfaceorinthedeepcrusttoproduceAMCGsuites.Flat-slabsubduction,back-arcextension,slabbreakoff,andhotspotsrepresentalternativesettingsthatcanaccountforgabbroicunderplatingandfractionationintoAMCGsuites,ifconsistentwithgeochronologicalconstraints.

Keywords:anorthosite,post-orogenic,geochronology,delamination,shearzone,AMCG(anorthosite–mangerite–charnockite–granite)suites.

sOmmaire

NostravauxgéochronologiquesrécentsportantsurlesmontsAdirondack,étatdeNewYork,démontrentquelesmassifsd’anorthosite ont étémis en place à 1155± 6Ma, lors de la phase post-contractionnelle (1160–1140Ma) de l’orogenèseShawinigan,dans l’intervalleca.1200–1140Ma.Lamiseenplacedeplusieursautres suitesde typeAMCG(anorthosite–mangérite – charnockite – granite) semble associée aux stadesfinauxd’uneorogenèse, par exempleca. 1155Mapour lescomplexesdeMorinetdeLac-St-Jean,ca.1650MapourlecomplexedeMealyMountain,ca.1450Mapourlescomplexesdel’estduLabrador.Lacorrélations’appliqueaussiaucomplexeanorthositiquedeRogaland,enNorvège,àca.930Ma,etlesanorthositesducentreduQuébecetdesAppalachesenVirginieetdePennsylvanie,liéesaustadestardifsdel’orogenèseOttawaàca.1060–1020Ma.Cescorrélationsfontpenseràunévénementdedélaminationparaffaissementd’unelithosphèreorogéniquesurépaissieoubienparérosionconvective,cequiauraitprovoquéunemontéedemanteauasthénosphérique,etaccumulationdemagmagabbroïqueàl’interfacecroûte–manteau.Lesoulèvementdel’orogènesuivantladélaminationaproduituncontextestable, dynamiquement équilibré, dans lequel lemagmagabbroïque a évolué lentement à pression élevéepour produire unclinopyroxènealumineuxetunplagioclaseintermédiaireàgrainsgrossiers,caractéristiquesdel’anorthositeenmassif.Enmêmetemps,unefusiondelacroûteinférieureaproduitlesmagmasmangéritiquesetcharnockitiques.Éventuellement,lesmagmas

§ E-mail address:[email protected]

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anorthositiquesetgranitiquesmigrentverslasurface,etleurfractionnementàplusfaiblepressionproduitl’associationclassiquedessuitesAMCG.Uneréactivationtranstensionnellelelongdezonesdecisaillementàl’échellelithosphériqueetd’anciennessuturesmontre aussi une corrélation avec lemagmatismeAMCG, et fournit des conduits pour lemagmagabbroïque, quis’accumuleàl’interfacecroûte–manteauoubiendanslacroûteprofondepourproduirelesassociationsAMCG.Unesubductionsubhorizontale,uneextensionderrièrel’arc,l’affaissementd’uneplaque,etl’existencedepanachesmantelliquesreprésenteraientd’autres contextes possibles pour expliquer la venuedemagmasgabbroïques et leur fractionnement pour donner les suitesAMCG,comptetenudescontraintesgéochronologiques.

(TraduitparlaRédaction)

Mots-clés:anorthosite,post-orogénique,géochronologie,délamination,zonesdecisaillement,suitesAMCG(anorthosite–man-gérite–charnockite–granite).

(1996, 2004), Corrigan&Hanmer (1997), Scoates(1994,2008),andMorissetet al.(2009),amongothers,havesummarizedtheseissues.

In the present paper, we provide evidence andarguments that stress 1) the importance of gabbroicparentalmagmas that pond and differentiate underrelatively quiescent conditions at the crust–mantleinterface,yieldinganorthositicderivativesthatascendintothemiddletouppercrusttogetherwithgranitoidsderived by crustal anatexis, and 2) results of precisezircongeochronologythatplacestheseeventsinlate-to post-tectonic settings. Such settings are prone todelaminationofoverthickenedlithosphereandtopost-collisionalextensionpromotingtheinfluxandpondingofgabbroicmagmaformedbydecompressionmelting.

iNTrODucTiON

Proterozoicmassif anorthosites have attractedinterestformanydecadesowing,inpart,totheirlargesize, almostmonomineralic compositions, occurrenceas plutonic rocks only, and their somewhat restrictedoccurrence (Fig. 1) to Proterozoic belts (Herz 1969,Rämöet al.2003).Theyarecharacterizedbycompositebatholithsconsistingofmultiple,coalescedplutonsofvariable composition, but dominated by anorthositeand leuconorite.Althoughmanyof their petrologicalproblemshavebeenresolved,theissueoftheirtectonicsetting is still unsettled, and their spatial–temporalrestriction remains enigmatic.Ashwal (1993),Emslie(1978, 1985),Emslieet al. (1994),McLellandet al.

Fig.1. DistributionofAMCGsuitesandferroan-potassic(A-typeandrapakivi)granitesacrossNorthAmericaandwesternEurope.Symbols:AD:AdirondackMountains,G:Grenvilleorogen,HC–L:HorseCreekandLaramieAMCGcomplexes,M:Mazatzalorogen,N:NainAMCGcomplex,R:RogalandAMCGcomplexintheSveconorwegianorogen,SF:SaintFrancoisMountains(blackcross),SG:SanGabrielAMCGcomplex,Y:Yavapaiorogen.Dashedlinesapproximateboundariesoforogenicbelts.ModifiedafterHaapala&Rämö(1999).

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TecTONicseTTiNgOFsOmemaJOrprOTerOzOicamcgsuiTes 1027

Suchmagmas can also be tapped by the interactionof steep shear-zoneswith ancient crustal boundaries.Examples of anorthosites in all of these settings arepresentedalongwithrelevantzircongeochronology.

peTrOLOgicaLcONsTraiNTsONaNOrThOsiTegeNesis

There has been general agreement (seeAshwal1993forasummary)thatmassifanorthositeisderivedfrom a high-alumina gabbroicmagmaponded at thecrust–mantle boundary, where it underwent early(Phase 1) fractionation of olivine and pyroxene thatsankback into themantle,aswellas fractionationofintermediate plagioclase that floated in the increas-inglydense residualmagmas (Kushiro 1980,Scoates2000). Ultimately, plagioclase-rich crystalmushesascended into themiddle andupper crust,where thestabilityfieldofplagioclaseexpandedandliquidfrac-tions underwent lower-pressure differentiation (Phase2) that yieldedmore anorthosite and leuconorite aswell as small volumes of late ferrodiorite andFe–Tioxide ores.During early ponding, advected heat, aswell as heat of crystallization of the high-aluminagabbros, caused partialmelting in the lower conti-nental crust, resulting in quartz-poor, potassic, iron-rich, orthopyroxene-bearing granitoids that, togetherwith the anorthositicmagmas, form coeval (but notcomagmatic) anorthosite –mangerite – charnockite– granite (AMCG) suites.Experimental data (Wyllie1977) indicate that high-pressure, anhydrousmeltingof continental crust produces silica-poor, potassium-richminimummeltsthatwillfractionateduringascentinto orthopyroxene-bearingA-type granitoids similartomangerites,charnockites,andgranites. Inaddition,fractionationofmagnesianmaficphaseswilldrivethecompositionstowardironenrichmentcharacteristicoftheMCGrocks.ThevolumeofMCGgranitoidsgreatlyexceeds thatof anorthosite inmost complexes and isconsistentwith a coeval, but not comagmatic origin.InacompetingpetrogeneticmodelproposedbyDuch-esne,VanderAuwera and coworkers (e.g.,Duchesneet al.1999,VanderAuweraet al.1998),mantle–crustinteractionduringpartialmeltingofslabsofanhydrousgabbronoriticlowercrustthrustintotheuppermantleproducesjotuniticmagmasthatfractionatedplagioclase(i.e.,anorthosite)andevolvedtowardsilicaenrichment(i.e., theMCGgranitoids). Scoates (1994, 2008) hasshown that these jotuniticmagmas cannot produceolivine-normativemagmas characteristic of the earlyphases ofmany anorthosites, nor do they follow theiron-enrichment, silica-depletion differentiation trendcharacteristic of anorthositicmagmas (Scoates 1994,2008,Morissetet al.2009).

It is generally agreed that a link exists betweenlayeredmaficintrusionsandmassifanorthosites(Morse1968,Ashwal 1993), and it is not uncommon tofindthe two spatially associated (e.g.,Kiglapait andNain

complexes,Morse1968).Thelineageistobeexpected,since both are products of gabbroicmagmatism, butidentifyingandexplainingthecausesofthedifferencesbetweenthetwoareothermatters.Weproposethattoalargeextent,thedifferencesreflectvariationsinbothdepths of fractionation andmagma-residence time atthose depths,with anorthosites evolving at greaterdepths and for longer times thanmafic layered intru-sions. Inwhat follows,we shall endeavor to demon-stratehowtheappropriateconditionsforthegenerationofmassifanorthositearise.

Emslie(1975)notedthatpondingofahigh-aluminagabbroicmagmaatthemantle–crustinterfaceisconsis-tentwiththepresenceofmegacrysts(5–50cmandupto1m)ofaluminouspyroxene(mostcommonlyortho-pyroxene)withinmanymassifanorthosites.Thesemayoccurassubroundedsinglecrystalsorgroupsofcrystals(TypeI)orinsubophiticintergrowthwithequallycoarseplagioclase (Type II), the latter type forming rafts ofmegacrystic anorthositewithinfiner-grained anortho-site and leuconorite.Both types ofmegacrysts haveelevatedconcentrationsofAl,butType2isespeciallyhigh, rangingfrom~4 to9wt.%Al2O3, indicativeofpressuresofcrystallizationof5–12kbaror~15–40kmdepth(Green1969,Fram&Longhi1992,Longhiet al.1999).Thehigh-pressurecrystallizationdocumentedbythe pyroxenemegacrysts is consistentwith composi-tionsofassociatedmegacrystsofplagioclase(An48–55)inthecoarserafts.AsshownexperimentallybyFram&Longhi (1992), theAn content of plagioclase inequilibriumwithhigh-aluminagabbrodecreaseswithrisingpressuresothatthecompositionofplagioclaseontheliquiduschangesfromAn74at1kbartoAn54at11.5kbar,whichoverlapsmegacrysticplagioclase(~An48–60)inthemassifsandisconsistentwithcrystallizationfromabasicmeltatpressuresof10–12kbar.Thesepressuresagreewith those associatedwith giantmegacrysts ofhigh-Alpyroxeneaswellasthefieldevidencethatbothcrystallizedunder thesamedeepconditionsandweretransportedas raftsorcrystalmushes to theirpresentpositions.

Alsoimpliedforthedeep(Phase1)environmentarehigh-temperature,anhydrousconditions,andsufficienttectonicstabilitytopermitthegrowthoflargecrystalsand retention ofmelts for long periods of time.Theabsenceofsignificantzoningwithinmostplagioclaseimplies replenishments of newhigh-aluminumbasicmelt to keep magma composition approximatelyconstantaswellas toincreasethequantityofplagio-clasecumulatebeyondthatproducedbyasinglemelt.AsdescribedbyEmslieet al.(1994)andBickfordet al.(2010),AFCinteractionofthegabbroswithpyroxenegranulite restites in the lower crust imparts amild,but very significant, crustal trace-element signatureto them,and to theirderivativeanorthosites,andalsoextends the lifetimeofplagioclaseonthe liquidus.Ina relatedmanner,Duchesneet al. (1999) argued thatelevatedvaluesof187Os/188OsintheSuwalkianortho-

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site inPoland require a predominantly crustal sourcefor theanorthosite.However,Hannah&Stein(2002)have shown that theOs isotope characteristics canbe explained by assimilation of crustalmaterials bymantle-derivedanorthositicmagmas.

FieLDreLaTiONshipsaNDTecTONicseTTiNgsOFaNOrThOsiTegeNesis

ThefieldrelationsofmassifanorthositeshavebeenclearlysummarizedbyEmslie(1978),whopointedoutthatwhereas these bodies commonly occur in high-grade terranes, they rarely, if ever, showevidenceofbeing synchronouswith regional contraction. Someareoverprintedby latermetamorphism(e.g.,Marcy),andothersappeartobepristine(e.g.,Nain,Fig.2).Onthe basis of suchobservations, anorthosites and theirassociatedMCGgranitoids came to be regarded as“anorogenic”plutonicrocksthat,togetherwithferroan,potassicgranitoids(i.e.,A-typeandrapakivigranites),formed a Proterozoicmagmatic belt across NorthAmerica(Fig.1).Althoughtheterm“anorogenic”wasneverclearlydefined,ithasgenerallybeenunderstoodto implyaregionofmild toabortiverifting inwhichpondedbasicmagmascouldevolveasdescribedabove(Emslie1978,1985).Within thisenvironment, itwaspresumed that riftingwas sufficiently slow to ensurethatthepondedmagmaswerenotprematurelyremovedfrom their site of fractionation, and that the absenceofmajor contraction precluded large-scalemixingwithcrustalmaterialsthatwouldproduceintermediatemagmas. In short, an “anorogenic” environmentwasoneofthick,tectonicallystablecrustthatcouldprovidethe hot, high-pressure, tectonically stable, and anhy-drousconditionsrequiredbypetrologicalmodelskeyedtodeep,protractedfractionationofthebasicmagmas.Insomecases,fieldevidenceisconsistentwithaback-arcsetting (e.g.,Mistastin,Michikamau, andHarpLakemassifs:Rivers&Corrigan 2000,Rivers 2008b). Inotherinstances,reactivationalongmajorfaultzonesorterraneboundaries (e.g., theHorseCreekanorthosite:Scoates&Chamberlain1995,1997,Frostet al.2000;theNain complex:Myers et al. 2008) correspondswithfieldrelations.Someinvestigators(e.g.,Hoffman1989) suggested that subcontinental hot spots andplumes could produce the conditions necessary for“anorogenic”magmatism,butdemonstrating that thishasproventobeelusive.Allofthesemechanismsarepotentiallyconsistentwiththepetrogeneticconstraintsoutlinedabove;however,acriticalboundary-conditionforactualanorthositegenesisisprovidedbygeochro-nology,whichwediscussbelow.

u–pBzircONgeOchrONOLOgyOFamcgsuiTes

By the1980s,U–Pbzircondatingbecamewidelyaccessible andwas applied to high-grade terranes,

includingmany in theGrenville Province (Fig. 2).Within theAdirondacks,McLelland et al. (1988b)produced ages bymultigrain thermal-ionizationmassspectrometry (TIMS) on an array ofmetamorphosedigneousrocks,includingsevengranitoidsamplesfromtheMarcyAMCGsuitethatrangedfrom1155to1125Ma(Table1).SimilareffortsbyEmslie&Hunt(1990)inthecentralGrenvilleProvinceyieldedfivemultigrainTIMSages (1160–1123Ma) formassifMCG suites,[excludingtheLabrieville(1018Ma),RivièrePentecôte(1365–1350Ma), andMealyMountains (1646–1635Ma), allofwhich suggestedmore thanoneperiodofAMCGmagmatism]. Itwas assumedon the basis offieldrelationshipsthatallmembersofthesesuiteswerecoeval,butintheabsenceofdirectdatingofanorthositewith zircon, this represented littlemore than awell-informedassumption.

Difficulty in dating anorthosite arose from thepaucityofigneouszirconintheserocks.Silver(1969),in his landmark application of zircon geochronologytotheAdirondacks,notedthattheanorthositescontainnumeroussmall,multifacetedgrainsofzirconhavinga“soccerball”morphologythatyieldedmultigrainTIMSagesofca.1050Ma,buthestressedthatthesezircongrainswere clearlymetamorphic in origin.Machado&Martignole (1988) andMartignole et al. (1993)obtainedthefirstU–Pbzirconageformagmaticzirconfromananorthosite(i.e.,1354±3MafortheRivièrePentecôte intrusion inQuebec).Subsequentdatingofcharnockiteassociatedwiththatmassifyieldedanageofca.1350(Emslie&Hunt1990),helpingtosupportthecoevalnatureofAMCGsuites.InWyoming,Frostet al.(1993)obtainedasingle-grainzirconageof1439±7MaforamonzoniticrockfromtheLaramieanorthositecomplex.Directdatingof theAdirondackanorthositedidnotgetunderwayuntiltheJ.C.RoddickSHRIMPfacilitybecameavailableat theGeologicalSurveyofCanadaandmadeitpossibletoobtainageswithhighspatial resolutiononasmallnumberofsinglegrains.Atthatpoint,McLellandet al.(2004)andHamiltonet al. (2004) initiated a series ofSHRIMPU–Pb zirconinvestigations of theAdirondackAMCGsuite.Theseresults(Table1)demonstratedthatallmajormembersoftheAdirondackAMCGsuitewereemplacedwithinerrorof eachother at1155±8Maandarebasicallycoeval (Fig. 3).As canbe seen inTable 1,SHRIMPages aremore consistent and reliable for these rocksthanresultsofmultigrainTIMSanalyses,andwehaveusedthemwhereverpossible.

Uranium–Pbzirconagesforanorthositesnowexistelsewhere in theGrenville Province (e.g., Higgins&vanBreemen1992,Hébert&vanBreemen2004,Morisset et al. 2009).These ages not only clarifiedissues regarding petrology; they also provided anessentialframeworkenablingAMCGsuitestobeplacedwithinthecontextofregionaltectonicsettings,aswassubsequently done byMartignole (1996),McLelland

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et al.(1996)andCorrigan&Hanmer(1997).Itisthepurposeofthispapertopresentthisevidencetogetherwiththeconclusionthatratherthanbeing“anorogenic”,mostAMCGmagmatismtookplacewithinthecontextof regional orogenic settings.However, this does notruleoutextensional,transtensional,orhotspotsettings

wherethesesettingscanbeshowntobelate-topost-tectonic.Thekeytosuchdemonstrationappearstoliewiththorough,precisegeochronologyandfieldrelation-ships.A detailed discussion beginswithAdirondackmassif anorthosite,withwhich the authors aremostfamiliar.

Fig.2. GeneralizedmapshowingtheGrenvilleProvince,whosethreemajortectonicdivisions(Rivers1997)areindicated.Symbols:ABT:AllochthonousBoundaryThrust,CCZ:Carthage–ColtonShearZone,CMBBZ:CentralMetasedimentaryBeltboundaryzone,F:Frontenacterrane,H:Highlands,L:Lowlands,LDZ:LabelleDeformationZone,MH:MountHollycomplex,MSZ:MaberlyShearZone,MT:Morin terrane,PIN:Pinware terrane,SLR:St.LawrenceRiver,TO:Torngatorogen,X:oceanicperidotite.Themajoranorthositemassifsoftheregion(withagesandnumberedreferencestoage)are:MM:Marcy(ca.1150Ma,1,6),Mo:Morin(ca.1153Ma),LSJ:Lac-St-Jean(ca.1155Ma,3,4,7,8),HSP:HavreSt-Pierre(ca.1126Ma,2),dashedwhitelineisAbbe-Huardlineament,LA:LacAllard(ca.1060Ma,10),MR:MagpieRiver(ca.1060Ma,4)A:Atikonak(ca.1130Ma,2),HL:HarpLake(ca.1450Ma,9),Labrieville(1060Ma,12),C:charnockitesofParcdesLaurentides(ca.1050Ma,11),M:Mistastin(ca.1420Ma,90,Mi:Michikamau(ca.1460Ma,9),N:Nain–Kiglapait(ca.1383–1269Ma,1,9),P:Pentecôte(ca.1350Ma,5),S:St.Urbain(ca.1060Ma,10),SG-MG:Shabogamo–Michaelgabbros,ca.1470–1460Ma,9).Agereferences:1)Hamiltonet al. (2004),2)Emslie&Hunt(1990),3)Higgins&vanBreemen(1992,1996),4)vanBreemen&Higgins(1993),5)Machado&Martignole(1988),6)McLellandet al.(2004),7)Hébert&vanBreemen(2004),8)Hervetet al.(1994),9)Gower&Krogh(2002),10)Morissetet al.(2009),11)Corrigan&vanBreemen(1996),12)Owenset al.(1994).

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geOLOgicseTTiNgOFTheaDirONDacKhighLaNDsaNDLOWLaNDsTerraNes

Overview

TheAdirondackMountains are a southwesternextension of theGrenville Province towhich theyconnectviatheFrontenacArchandThousandIslandsregionoftheSt.LawrenceRiver(Fig.2).TheregionistopographicallydividedintotheAdirondackHighlandsandLowlands,separatedbytheCarthage–ColtonShearZone(CCZ,Fig.4).Theformerisunderlainlargelybyorthogneissmetamorphosedtothegranulitefacies,andthelatterbyupper-amphibolite-grademetasedimentaryrocks,notablymarble.Bothsectorshaveexperiencedmultiple deformations resulting in refoldedmajor

isoclines.TheCarthage–ColtonZonedips~45°NW,andthelatestdisplacementalongit(ca.1050Ma)wasdowntothenorthwest,juxtaposingtheLowlandsandHighlands (Buddington 1939, 1969, Johnson et al.2004,Streepeyet al.2001,Sellecket al.2005,Bairdet al.2008).

Themajor events ofAdirondack tectonic evolu-tion are summarized in Figure 5.The oldest rocksrecognizedintheregionarecalc-alkalinetonalitesandgranodioritesdatedatca.1350–1250MaandexposedinthesouthernandeasternAdirondackHighlands(McLel-land&Chiarenzelli1990).Rocksofsimilarcomposi-tionandagehavebeenrecognizedintheMountHollycomplex(Fig.4)ofVermont(Ratcliffeet al.1996),theProterozoicofwesternConnecticut(Walshet al.2004),and theNew JerseyHighlands (Volkert&Aleinikoff

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2008).Inaddition,calc-alkalineplutonicrocksranginginagefrom1450to1300MaoccurwithinthewesternCentralMetasedimentaryBeltoftheGrenvilleProvince(D, Fig. 2),which includes all of theAllochthonousMonocyclicBelt,exceptfortheAdirondackHighlandsandMorinterranes(Fig.2).Dickin(2000)andRivers&Corrigan(2000),amongothers,interpretedthesecalc-alkalineplutonsaspartofasoutheast-facingAndean-stylearcdevelopedalongtheLaurentianmarginduringtheintervalca.1450–1300Ma.TectonicslicesofthisarcarepreservedatthewesternmarginoftheCentralMetasedimentaryBeltBoundaryZone (D,Fig. 2) aswellasinthecompositeElzevirianarc(Fig.5A),andarereferredtoastheDysartsuite(Easton1992,Dickin&McNutt2007).McEachern&vanBreemen(1993)andHanmeret al.(2000)proposedthatthe1400–1300Ma tonalitic rocks of the southernAdirondackHigh-lands and theMountHolly complex of theGreenMountains, inVermont (Fig.5A,AD–GM), representremnants of this arc rifted from Laurentia duringformationof theCentralMetasedimentaryBelt back-arcbasin,andreferredtothemastheDysart–MountHolly suite (Rivers&Corrigan2000).From1280 to1220Ma,theseremnantsservedasnucleiforoutboardarcmagmatism(Fig5A).Atca.1220Ma,thecompositeElzevirianarcunderwentaccretiontoLaurentiaalongthelong-livedCentralMetasedimentaryBeltBoundaryZone,shuttingoffElzevirianmagmatismasthesubduc-tion zone stepped eastward (Fig. 5B).This sequenceof events, aswell as the configuration of terranes inFigure5,differsfromthoseofCarret al.(2000)withrespecttotheplacementoftheAdirondackLowlands,whichishereconsideredtorepresentthetrailingedgeof theElzevirian-ageCentralMetasedimentaryBelt(Fig. 5A).This assignment is based on the presenceof ca. 1210–1200Ma calc-alkaline plutonic rocksof theAntwerp–Rossie suite (Figs. 2, 5B)within theLowlands,togetherwithoxygenisotopeevidencethatthese plutonswere emplaced above awest-dippingsubductionzone(Pecket al.2004),asshowninFigure5B.NotethattheserocksareabsentfromtheHighlands,consistentwithitsseparationfromtheLowlandsatthattime.Recently,Chiarenzelliet al.(2010)havereportedthediscoveryof anophiolitic peridotitewithoceaniccrust affinities in the northeasternLowlands (X,Fig.2).TheserockshaveNdmodelagesofca.1400–1350Maandcontainmetamorphiczircondatedatca.1214Ma.Thisimportantresultiswhollyconsistentwiththetectonicmodelpresentedabove.

TheFrontenacterranenorthwestoftheSt.LawrenceRiverisstructurallyseparatedfromtheLowlandsbythelateOttawan,steeplydippingBlackCreekShearZone(Fig.4),whichtheca.1160KingstondikesofCanadado not cross.Although similarities exist between theFrontenac andLowlands terranes, there thusmaybea significant discontinuity between them.Among thesimilaritiesbetweenthetworegionsisthefactthatbothexhibithigh-gradeShawiniganmetamorphism(~650°<

T<800°C,~6.5<P<8MPa),butnothermalOttawaneffects exceeding the closure temperature of titanite(~650°C); local 40Ar/39Ar hornblende ages indicatethatOttawan temperatures did not exceed ~500°C(Mezgeret al. 1991, Streepeyet al. 2001,Heumannet al. 2006).These results have been interpreted toindicate thatduringpeakShawinigancontraction(ca.1190–1160Ma), theLowlandswere thrust over theHighlands along a suture zone coincidentwith thepresentCarthage–Colton zone (Fig. 4E).TheDianaquartz syenite complex intruded the suture zone at1164±11Ma,anditsoldestkinematicindicatorsareconsistentwith thrustdisplacement to theeast (Baird2008).At1050Ma,reactivationofnormalfaultsalongtheCarthage–Coltonzoneresultedintopsidedown-to-the-westdispacement(Fig.4F),asrecordedbyinematic

Fig.3. PlotofU–PbzirconagesforAdirondackanorthositesandassociatedcharnockite–mangerite–graniteAMCGmembers.NumbersinparenthesesarekeyedtonumbersinTable1.ModifiedafterMcLellandet al.(2004).

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indicatorsintheca.1050MaLyonMountaingranite,thatlocallyintrudedthezone(Sellecket al.2005).TheFrontenacandLowlandsterranesareinterpretedaspartsoftheOrogenicLidofRivers(2008a).

The Shawinigan orogeny

Atca.1200Ma,theAdirondackHighlands–GreenMountainterranewasaccretedtotheLaurentianmargin(Fig. 5C), and the early, contractional phase of theShawiniganorogenyensued.Deformationincludedtheformationofearly(F1)nappes,lithosphericthickening,high-grademetamorphism,andtheemplacementofthesynkinematicHermon(ca.1190Ma)andHydeSchool(ca.1170Ma)granitoids(Fig.5C).Simultaneously,theLowlands,attheleadingedgeoftheupperplate,were

thrusteastwardovertheHighlandsalonganearly,west-dipping precursorCarthage–ColtonZone (Fig. 5C).During the thrusting, the precursorCarthage–ColtonZonewasintrudedbythe1164±11MaDianaquartzsyenite, which contains numerous early kinematicindicatorsshowingtop-side-up-to-the-east(Baird2008,Bairdet al. 2008).Followingaccretion,delaminationof overthickened lithosphere took place and influxesof asthenosphere led to underplating of the crust bygabbroicmagmaandtheevolutionoflateShawiniganAMCGsuites(Figs.5C,D).

Corrigan (1995) appears to have been thefirst torecognize ca. 1190–1140MaShawinigan tectonismduring investigations near the small town of thatname northeast ofQuebec.At about the same time,Friedman&Martignole(1995),Martignole&Friedman

Fig.4. Generalizedgeological–geochronologicalmapof theAdirondacks.Unitsdesignatedbypatternsand initialsconsistofigneousrocksdatedbyU–Pbzircongeochronology,withagesindicated.UnitspresentonlyintheHighlands(HL)are:RMTG:RoyalMountaintonaliteandgranodiorite(southernHLonly),HWK:Hawkeyegranite,LMG:LyonMountaingraniteandANT:anorthosite.UnitspresentintheLowlands(LL)onlyare:HSRG:HydeSchoolandRockportgranites(HydeSchoolalsocontains tonalite),RANT:RossiedioriteandAntwerpgranodiorite.Granitoidmembersof theAMCGsuite (MCG)arepresentinboththeHighlandsandLowlands.Unpatternedareasconsistofmetasediments,glacialcover,orundividedunits.A:Antwerp,BCSZ:BlackCreekshearzone,CCZ:Carthage–Coltonshearzone,LL:Lowlands,HL:Highlands,MM:Marcyanorthositemassif,OD:OregonDomeanorthositemassif,SM:SnowyMountainanorthosite,IL:IndianLakeLM:LyonMountain,IL:IndianLake,R:Rossie,C:Canton,SLR:St.LawrenceRiver.ModifiedafterMcLellandet al.(2004).

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(1998)andMartignole(1996)reportedwidespreadca.1190–1140MametamorphismandtectonismwithintheAllochthonousMonocyclicBelt(Fig.1).Subsequently,Rivers (1997) andRivers&Corrigan (2000) definedtheShawiniganorogenyasencompassingtheinterval

1190–1140Ma.However,bothMartignole&Friedman(1998) andMartignole (1996, and references therein)stressed that contractionalorogenyhad largely termi-natedby1160Ma,andcontinuingmetamorphismwasduetothermalinfluxesfromca.1160–1140MaAMCG

Fig. 5. Plate tectonic summary for theAdirondack region.AH–GM:AdirondackHighlands –GreenMountains belt,AL:AdirondackLowlands,BSZ:Bancroftshearzone,CCZ:Carthage–Coltonshearzone,CGB:CentralGneissBelt,CMBBZ:CentralMetasedimentaryBeltboundaryzone,E:Elzevir terrane,F:Frontenacterrane,HSG:HydeSchoolgneiss,MSZ:MaberlyShearZone.ModifiedafterHeumannet al.(2006).

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plutons (e.g., theMorin suite at 1155± 3Ma:Doig1991).Wodickaet al. (2004)utilizedU–PbSHRIMPanalysisanalysestodatethepost-tectonicmonzonitic–gabbroicChevreuil suite at ca. 1170–1160Ma, thuspost-datingregionalca.1200–1180Magranulite-faciesmetamorphismin theAllochthonousMonocyclicBeltnorthwestof theMorinanorthosite (Fig.1).Corrigan&Hanmer(1997)stressedthat,“…evidencefordefor-mation related to contractional tectonics appeared toberestrictedtotheinterval1.19–1.16Ga”.Inarelatedway,theundeformedca.1160MaKingstondikeswarmcross-cutsdeformed1176–1162plutonsand1180–1160Magranulite-faciesassemblageswithintheFrontenacterrane(Marcantonioet al.1990).WithintheAdiron-dackLowlands, titaniteandmonaziteagesof1156±and1171±Ma, respectively, exhibitno resetting,andnopost-1172Maregionaltectonismormetamorphismisobserved(Mezgeret al.1991,Heumannet al.2006).Local ca. 1155MaAMCGplutons exhibitminimaldeformation outside of shear zones, and tectonismappearstohavepeakedatca.1180–1160Ma.Thelargeca.1170MaMaberlyshearzoneatthewesternmarginof the Frontenac domain (Fig. 1) terminated thrustmotionat1156±2Ma(Corfu&Easton1992,Rivers&Corrigan2000).Insummary,theShawiniganregionalcontractionoccurredthroughoutthisentireareafromca.1200to1160Ma,whereasmetamorphismfrom1160to1140MawaslargelylimitedtoheatfromAMCGsuitesortolocalshearzones.AssumingthattheShawiniganorogeny in theGrenville Province continues to beassignedtotheinterval1200–1140Ma,itshouldalwaysbeemphasizedthatregionalcontractionceasedbyca.1160Ma.Within thesouthernAppalachians,Shawin-igancontractionalorogenesisdoesextendfromca.1200to1140Ma,andanorthositesaresmall,scarce,orabsent(Tolloet al.2006).

FollowingtheShawiniganorogeny, theredoesnotappeartohavebeenanysignificantregionaldeforma-tionintheAdirondacksuntilca.1090Mawhen,afterabriefinterval(ca.1103–1090Ma)ofgraniticmagma-tism(theHawkeyesuite,McLellandet al.1996), theentireGrenvilleProvincebegan to experience effectsof theOttawanorogeny, considered tohavebeen theresult of collision ofLaurentia andAmazonia (Figs.5F,G),with the suture zone lying somewhere to thesoutheastof thepresent-dayAppalachians.Accordingtogeothermometry,geobarometry,andseismic inves-tigations,duringtheOttawanorogeny,theAdirondackHighlands regionattainedadouble thicknessofcrustandunderwentgranulite-faciesmetamorphismandF2nappeemplacement(Bohlenet al.1985,Valleyet al.1990,Kitchen&Valley1995,McLellandet al.1996).ThemajorcontractionalphaseoftheOttawanorogenyappearstohaveterminatedbyca.1050–1040Ma,andminimallydeformedplutonsofLyonMountaingranite(Figs.4,5G)wereintrudedatthistime(McLellandet al.2001).Locally,youngereventsoflessermagnitudecontinuedtoaffecttheregionuntilca.950Maandare

recordedbymetamorphiczircon,titanite,andmonazite.TounderstandhowAdirondackanorthositesfitintothistectonicframework,weneedtofocusupontheresultsofrecentgeochronologyforboththeAMCGsuiteandtheShawiniganorogeny.

Details of the Shawinigan Orogeny in the Adirondacks

Evidence ofShawinigan plutonism andmetamor-phismiswelldevelopedintheAdirondackLowlands,perhaps better so than elsewhere in theGrenvilleProvince, and thus deserves some additional discus-sion.Figure 4 shows the presenceof largequantitiesof igneous plutons in the Lowlands, and SHRIMPU–Pbzirconages,aswellassingle-grainTIMSages,demonstrate that all of these fall into the interval1214–1155Ma.The oldest rocks in the LowlandsconsistoftheRossiediorite–quartzdioritesuiteandtheAntwerpgranitoid suite, consistingprincipally ofbiotitegraniteandgranodiorite.Bothunitsarestronglycalc-alkaline.TheRossiedioritehasbeendated(LA–ICP–MS) byBaird et al. (2008) at 1214 ± 38Ma,andtheAntwerpgranitoidsuite(SHRIMP),at1210±10MabyWasteneyset al. (1999).The calc-alkalinebiotite–hornblendeHermonmegacrystic granite hasbeendated(SHRIMP)at1182±8Ma(Heumannet al.2006).Nextinageistheca.1172±2Ma(single-grainTIMS)Ma calc-alkalineHydeSchool gneiss,whichconsistsofsubequalamountsofpinkleucograniteandgraytonalite(McLellandet al.1992,Wasteneyset al.1999).Thegranitic facies of theHydeSchool gneissisthesamecompositionandageasthepinkRockportleucogranitethatoccursalongtheSt.LawrenceRiver,and the two are interpreted as belonging to a single,widespreadShawinigan event.Both theHermon andHydeSchoolunitshaveemplacement ages that coin-cidewithmetamorphicagesdiscussedinthefollowingparagraphandareinterpretedassyntectonicintrusionsconsistentwith their fabrics (McLellandet al. 1992).Notethatnoneoftheca.1210–1170Macalc-alkalinerocks of theLowlands are present in theHighlands,indicatingthatthetwoterraneswereseparatepriortothisinterval,andduringmuchofit,butwereaccretedtooneanotherbyca.1155MawhenAMCGstitchingplutonswereemplacedacrossbothregions.

InarecentSHRIMPU–Pbzirconstudyofanatexisinmigmatiticmetapelitesof theLowlands,Heumannet al.(2006)showedthatpartialmeltingtookplaceat1180–1160Ma,andzircongrainsexhibitnoyoungerovergrowths.Thisresultisconsistentwiththetitanitedataanddemonstrates that theonlyhigh-grademeta-morphism experienced by the Lowlandswas at ca.1210–1160Ma(i.e.,theShawiniganorogeny).Shawin-iganmetamorphicagesoccurintheHighlandsaswell(Heumannet al.2006,Bickfordet al.2008),andxeno-lithsofhigh-gradeShawinigangarnet–sillimanitegneissarepreservedwithinan~1106Ma(J.Aleinikoff,pers.

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commun. 2006,SHRIMP)metagabbroof the easternHighlands(McLellandet al.1988a).ItissignificantthatShawinigan-age regionalBarrovianmetamorphism intheHighlandswasnotdocumenteduntilHeumannet al.(2006)andBickfordet al.(2008)obtainedcrystalliza-tionagesof1180–1170Maforanatecticassemblages.These findings imply the following: 1) Shawiniganca. 1200–1180Maplutons are commononlywithinthe Lowlands, and 2) there is extensive Ottawangranulite-faciesmetamorphism in theHighlands hasoverprintedearlierevents.Similarconsiderationsmayhave obscured recognition of Shawiniganmetamor-phisminpartsoftheeasternGrenvilleProvince,anditwouldbe instructive toobtainzirconagesand in situmonazite ages for leucosome in paragneisses olderthanca.1200Ma.Importantly,Wodickaet al.(2003)havereportedU–Pbzirconagesof1170–1180MaforgranitoidplutonsinthevicinityofHavreSt-Jeanmassif,andfurtherdatingislikelytorevealmoreoftheseages.

Whenthe1156±7Maaverageage(Table1)oftheAMCGsuiteiscomparedwiththeageofShawiniganplutonismandanatexisintheAdirondacks(ca.1210–1160Ma), it is clear that the ages closely approachoneanother at 1160Ma. In addition, the threeoldestmembers of the granitoid suite have average agesexceeding1170Ma,andthenextfouroldestmembersarewithinerrorofmid-1170MaagesoftheShawiniganorogeny.Eventhefouroldestanorthositesamplesarewithin error ofca. 1170Ma.Given this age overlap,and the proximity of all of these rocks, it cannot berealisticallysupposedthattheAdirondackAMCGsuiteisinanyfashion“anorogenic”.Instead,theformationandevolutionofthesuitemustbeincorporatedintoaworkingmodelwithinwhichitcanformandevolveinasettingthatislate-toearlypost-tectonic.Asweshalllaterargue,thistypeofenvironmentappliestoseveralmajorAMCGcomplexes.

amODeLFOrThegeNesisOFamcgsuiTes

Themodel is based upon the quantitative inves-tigations ofHousemanet al. (1981),Dewey (1988),Turneret al.(1992)andPlatt&England(1994),amongothers,who have applied geophysical reasoning andcalculations to orogeny,mountain building, and thelate collapse of orogens.These authors concludedthat contractional orogeny results in overthickenedlithospheric roots, and that the latter are likely to bedenser than the asthenosphere.A largeportionof theoverthickened lithosphere is then removed either bymechanicaldelaminationorbyconvectiveremovalofthe thermalboundary-layer (Fig.6).Thedelaminatedlithosphereisreplacedbyasthenospherethatundergoescompressionmeltingtoproducegabbroicmagmasthatpondatthecrust–mantleinterface.Duringremovalofthe lithospheric root, the orogen rises in elevation ashotter,lessdensematerialreplacesthedensekeelandproduces buoyancy forces.The increased elevation

results in increased potential energy of the plateau,whichrespondsbyexertingoutwardlydirected,exten-sionalhorizontal(FB)thatcausetheunderlyingcrusttopushsidewaysuntilitbalanceswhatevercontractionalforces (FC) continue to exist andprovide support fortopographicelevations.At thispoint, theorogenmaybesaidtohaveattainedastateofmechanicalequilib-rium that supportsmountainous topography and thatsimultaneously experiences collapse along low-anglenormal faults until isostatic equilibrium is attained.It is estimated that the equilibrium statemay last for10–20Ma(Platt&England1994).Duringthisinterval,AFCprocessesmayresultincrust–mantleinteractionsrecorded in geochemical signatures of trace elements(Emslieet al.1994,Bickfordet al.2010).

The creation of amechanically equilibrated statein the delaminatedorogen is conducive to the devel-opment of anAMCG suite, because: 1) it remainstectonically stable for protracted periods of time, 2)it is associatedwith the ponding of large quantitiesof gabbro at the crust–mantle interfacewhere hot,dry conditions exist, 3) it promotes the formation ofcoarse,commonlyunzonedplagioclaseofintermediatecomposition thatfloats indenserhigh-pressuremelts,and4)itfacilitatesthereturnofolivineandpyroxenecumulates back to themantle. Finally, 5) themodelprovides the heat necessary to partiallymelt largevolumesofdeepcontinentalcrust.Althoughlatentheatprovidesmuchoftherequiredthermalenergy,Platt&England(1994)showedthattheconvectiveremovalofthelithosphericboundary-layerproceedssorapidlythathotasthenospherearrivesatthebaseofthecrustwithouthaving lostmuchheatbyconduction, thuscreatingathermal environmentmuch like that associatedwithplumesorhotspots.Inshort,thelate-topost-tectonicenvironmentthatexistsinoverthickened,delaminatedorogenspossessesallofthepropertiesoncethoughttocharacterize“anorogenic”environments,andmoreover,it is consistentwith geochronological data andfieldevidence.

During the late- to post-tectonic phase of theShawiniganorogeny,regionalextensiontookplaceascontractionalforceswaned.MajormanifestationsofthisextensionaretheFlintonandTwelveMilebasins(Fig.5D),whichhostbasinalsedimentscontainingca.1155Madetritalzirconandwhichhavebeenconstrainedtoca. 1150–1100Ma in age (Sager-Kinsman&Parrish1993,Wodickaet al.1996).

amcgmassiFsBeyONDTheaDirONDacKs

The Central Grenville Province

Figure 2 shows themajor anorthositemassifs ofthe northeasternGrenvilleProvince, aswell as thosesituatednorth of theGrenvilleFront inLabrador.Ofspecial importance to this discussion are theMorin(1153±2Ma,Doig1991)andtheLac-St-Jeanmassifs

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(Higgins&vanBreemen1992, 1996,Higginset al.2002,Hébert&vanBreemen2004),whichwediscussbelow.TheAtikonak(1133±10Ma,Emslie&Hunt1990) and the northern part of theHavre St. Pierre(1126±7Ma,Emslie&Hunt 1990)massifs remainproblematic,becausetheseagesareinferredfromdatingadjacent granitoids.However,Wodickaet al. (2003)haverecentlydatedanorthositeinthenorthwestlobeoftheHavreSt.Pierremassifandobtainedanagerangeof1139–1150Ma,withinerroroftheAdirondack,Morin,andLac-St-JeanAMCGsuites.

Asstatedpreviously,Friedman&Martignole(1995),Martignole(1996),Corrigan&Hanmer(1997),Marti-gnole&Friedman (1998)andWodickaet al. (2004),amongothers, stressed that intrusionof the1153±3MaMorinAMCGsuitepostdatesregionalShawinigancontractionalorogeny(Fig.7).Accordingly,itappearsthattheMorinanorthositeandrelatedgranitoidsformedinatectonicsettingcloselyresemblingthatproposedfortheAdirondacks(i.e.,earlypost-tectonicdelaminationwithinfluxesoffreshasthenospheretothebaseofthecrust,wheretheAMCGmagmasevolved).

TheentireMorin terranewasaffectedbyShawin-igancontractionalmetamorphismfromca.1190to1160Ma, and the type localityof theShawiniganorogenyliesatthesoutheasternedgeoftheMorinterrane~250km southwest of the enormousLac-St-Jeanmassif.AlthoughtheLac-St-Jeanmassifwasdisplacedfromitsoriginalsiteofintrusion,itseemsunlikelythatShawin-igandeformationandmetamorphismdidnotaffecttheregionintowhichthebatholithwasemplaced.Notwith-standing, therearecurrentlynoShawiniganages thathavebeenreportedforthearea.However,asdescribedearlier,ca.1190–1160MaShawiniganmetamorphismwasnotrecognizedintheAdirondackHighlandsuntilSHRIMP analysis of zircon grains in the leucosomeofmigmatiticmetapelites revealed its presence.AgeochronologicalreconnaissanceofsimilarlithologiesinthecentralGrenvilleProvincemightwellyieldsomeShawiniganagesandshouldbeundertaken.Moreover,asdescribedbelow,intrusionoftheLac-St-Jeanbatho-lithisdemonstrablyassociatedwithlarge-scale,ductilestrike–slipshearzonesthatmanifestcompressiveforcesatca.1157Ma.

Fig. 6. Schematic representation of an overthickened collisional orogen undergoinglithospheric delamination and consequent orogen rebound and collapse along low-angle normal faults during late phases of orogenesis.Delamination represented byfounderingof lithospherickeel; a convectiveerosionof the thermalboundary-layerisjustaslikely,however.ASreferstoasthenospherethatascendstothecrust–mantleinterfaceandundergoesdecompression-inducedmelting toyieldagabbroicmagmathatfractionatesolivine(ol)andpyroxene(px),whichsinkbackintothemantle,andintermediateplagioclase(pgf),whichfloats.Theplagioclaseaccumulatesintoacrystalmush(blacksquares)atthebaseofthecrust.Ambientheatandheatofcrystallizationresult inmelting of the lower crust to yieldmangeritic, charnockitic, and graniticmagmas(MCG)oftheAMCGsuite.OrogeniccontractionalforcesarerepresentedbyFC,andthehorizontalcomponentsofbuoyancyforces,byFB.

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Datingof the largeLac-St-Jeananorthositemassifwas undertaken byHervet et al. (1994),Higgins&vanBreemen (1992, 1996), Higgins et al. (2002),and byHébert& vanBreemen (2004).The resultsof these investigations (Table 2) are commonly citedas documenting a~20m.y. interval of crystallizationfrom1160–1140Ma(e.g.,Rivers1997,Hébert&vanBreemen 2004).However, examination of the datainTable2 reveals that support for this interval isnotcompelling, and amuchmore restricted interval ofcrystallizationismorelikely.Specifically,therearesixagesthatdirectlydatethecrystallizationofanorthositeorpetrogeneticallyrelatedrocks(samples1,2a,3,8,10,and13).Fiveofthesesixfallbetween1157and1148Mawitherrors(2s)of2–4Ma.Thesixth(sample13)isdatedat1140±10Ma.Theaverageofallsixsamplesis1154±4Ma,whichweconsidertobetheemplacementage of the anorthosite. It is an age that is coincidentwith the 1160–1140Ma post-contractional phase ofthe Shawinigan event andwith emplacement of theMarcyandMorinmassifs(Fig.7).Thisageisfurthersupportedbytheobservationthat1)the1146±3MaLacLaBrèquegraniticplutoniscompletelyenclosedbytheanorthosite,and2)theDuBrasgranophyricplutoncross-cutstheanorthosite.Inaddition,lateanorthositicpegmatiteswithin theLac-St-Jeanmassif commonlycontainacoreofgranophyre,twoofwhichyieldagesof1155±2Maand1152±2Ma.Thustheanorthositemustbeofthisageorolder.Onegranophyriccorehas

been dated at 1142± 2Ma, andwas interpreted byHiggins&vanBreemen(1992)asa latedifferentiateof the anorthositicmagma, although no persuasiveevidenceispresentedforthis.Moreover,residualgrano-phyricliquidsarenotlikelytoformfromanorthositicparentmagmas,whichfollowastrongiron-enrichmentFennertrend,leadingtoferrodioriticresiduasuchasinsamples1,8,and10.WesuggestthatthegranophyresarecrustalA-typepartialmeltsproducedbyheatfromunderplatedgabbroandthemassifsthemselves.Finally,we note thatmetamorphic zircon in the granophyresyieldsagesof1142±2Ma,clearlydemonstratingthatthisagedoesnotreflectemplacementoftheanorthosite.Infact,Higgins&vanBreemen(1992)arguedpersua-sively that field and textural evidence in theNNE-trendingmegadykes (samples 1, 2) demonstrate thattheywereemplacedwhiletheanorthositewasstill,inpart,notfullycrystallized.Bythetimethattheyoungerofthetwomegadykes(Sample2)hadbeenemplaced,theanorthositewasundergoingsolid-statestrain.Theyused this evidence to argue for rapid emplacement(i.e.,aonetotwomillionyearinterval,not~20m.y.)of the anorthositicmagma,withwhichweagree.Wealso agree that the steepNNE strike-slip faults (withreleasing bends) that cut the anorthositemightwellhaveservedasimportantconduitsfortheLac-St-Jeanmagmastoreachthemiddlecrustfromitsstagingareaat the crust–mantle interface.Such faultingwouldbeconsistentwiththelatetransitionoftheregionalstress-

Fig.7. ChartshowingthecorrelationbetweenemplacementofmajorAMCGmassifsofnortheasternNorthAmericaandthewaningphaseoforogeny.Blackrepresentscontractionalorogeny,lightgrayrepresentsmagmatism,anddarkgrayrepresentspre-contractionalarcmagmatism.

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fieldfromcompressiontoregionalextension.Theagesof associatedgranitoids (samples 1, 9a,b, 11, 15) areconsistentwiththeforegoingarguments.

South of theAbbe–Huard lineament in theHavreSt.Pierremassif (Fig.1), theLacAllardandMagpieRiver lobes of theHavreSt. Pierre anorthosite havebeendirectly dated atca. 1060Ma (vanBreemen&Higgins1993,Morissetet al.2009),whichcoincideswith the terminal stages of theOttawan orogeny, asemphasizedbyMorissetet al.(2009).InadditiontotheHavreSt.Pierreexample,Owenset al.(1994),Dymek&Owens (1998)andOwens&Dymek (2001,2004)havemapped,analyzed,anddatedanumberofsmallbodies ofca. 1080–1010Ma anorthosite and related

ca. 1050MaA-type, post-orogenic granitoid plutonsin theParcdesLaurentides area (Fig.2) that lie in aNNE-trending belt (CRUMLbelt, Fig. 1) extendingfromQuebecCity to the northeasternmargin of theLac-St-Jeanmassif.Theca. 1080–1010Ma anortho-sitesoftheCRUMLBeltandthecentralAppalachiansdiffer from the largemassifs of theAdirondacksandGrenville Province in several importantways,e.g., they contain Fe3+-rich ilmenitewith exsolutionlamellae of hematite rather than ilmenite–magnetite,their antiperthitic plagioclase compositions are richerin sodium and potassium (An23–48Or4–25) than in theca.1150Mamassifs(An48–62Or2–3),andtheycontainorthopyroxene,butneitherclinopyroxenenorolivine.

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Theyareconsideredtohaveevolvedunderhigherf(O2)conditions andwithin somewhatmore hydrous envi-ronments,andassimilatedmorecontinentalcrustthanthelargemassifsoftheGrenvilleProvince.Theyhavebeenreferredtoasalkalicanorthosites(e.g.,Owenset al.1994,Owens&Dymek2001,2004).Incontrasttothe alkalic anorthosites, the pre-1100Ma anorthositemassifs are referred to as pyroxene–labradorite orpyroxene – olivine – labradorite anorthosites on thebasisoftheirdominantmineralogy.

Alloftheagescitedinthepreviousparagraphfallinto the late- to post-tectonic phase of theOttawanorogeny (Fig. 7) and are approximately synchronouswith late extension and collapse of the orogen atca.1060–1020Ma (Rivers 2008a)making them goodexamplesofthelate-topost-tectonicsettingforAMCGmagmatism (Morissetet al. 2009).More specifically,peak and late phases of theOttawanorogenywithintheAdirondacks were followed by delamination,pop-up,andcollapse,accompanied(andenhanced)bytheascentoflargevolumesofca.1050MaLyonMt.leucogranite(Figs.4,5G),consideredtobecorrelativewith ca. 1060–1050Ma Parc des Laurentides (Fig.2) granitoids.TheCRUMLBelt projects farther totheeastoftheAdirondacks,whereitmayliebeneathPhanerozoic cover.This belt re-emergeswithin theProterozoic inliers of the Pennsylvania andVirginiaAppalachians (i.e., theMontpelier anorthosite, 1045±10Ma,andtheRoselandanorthosite,1045±44Maanorthosites:Aleinikoff et al. 1996, Pettingill et al.1984).Thesesmallbodiesareassociatedwithca.1050Magranites and leucogranites similar to those in theCRUMLBeltMCGgranitoids andAdirondackLyonMountainGranite.Basedupontheseobservations,were-emphasizethatOttawanorogenesiswasfollowedbylate-topost-orogenicAMCGmagmatismwiththesamebasictimingandmechanismsasproposedforthelate-to post-ShawiniganAMCG complexes.Differencesbetweenthealkali-richanorthositesandthelabradoritemassifscanbeaccountedforbydifferencesinfluidsandmagmaticinteractionwithcrustalmaterials.

The AMCG Complexes complexes of eastern Labrador

Importantmassifsofolivine–pyroxenelabradorite-type anorthosite, leuconorite, and leucotroctolite,together withMCG granitoids, occur in Labradorimmediately north of the easternGrenville Province(i.e.,Michikamau,ca.1460Ma,HarpLake,ca.1448–1426Ma,Mistastin,ca.1420Ma,mainlyMCG,andNain, 1350–1290Ma complexes).TheseundeformedandpristineEarly toMiddleElsonian(1.46–1.29Ga)massifshavebeenregardedastypeexamplesof“anoro-genic”AMCGmagmatism and exhibit characteristiclargecrystalsofintermediateplagioclaseandaluminouspyroxene(Emslie1978,1985,Emslieet al.1994).TheNain complex is localizedwithin a steep Paleopro-

terozoicshearzone (theTorngatorogen)between theChurchill andNainprovinces, and theother three liejusttothesouthwest.Recentgeochronological,petro-logical,andtectonicreviewsbyGower(1996),Gower&Krogh(2002)andMyerset al.(2008)haveopeneda new chapter in our understanding of theseAMCGcomplexesandaresummarizedinnthissection.

Utilizingbothdetailedfieldworkandzircondating,Gower&Krogh (2002) have demonstrated that theeasternGrenvilleProvince experienced strongdefor-mation,metamorphism, andplutonismduring theca.1.52–1.46MaPinwarianorogeny,whoseAndean-typeplatemarginborderedsoutheasternLaurentiadirectlysouthof thefuturesitesof theearlyElsonianAMCGsuitesinLabrador.TerminalPinwarianevents(ca.1.46Ga)coincideintimewiththeonsetofAMCGmagma-tism(ca.1460Ma)intheHarpLake,Michikamau,andMistastinmassifs(Fig.7).Inaddition,theShabogamoandMichael gabbros, located south of themassifsandmostlywithintheGrenvilleProvince(Fig.2),aredatedat1459±23Maand1472±27Ga,respectively,andmanifest extension overlapping and followingPinwarian contraction. Gower & Krogh (2002)accounted for thePinwarian orogenybyproposing anorth-dipping subduction zone thatwas active from1520to1460Ma.Subsequently,orogenyandmagma-tism shut off along the continentalmargin, and theextension-related 1469–1426Ma Shabogamo andMichaelgabbros(Fig.2)wereemplacedinproximitytothepresent-dayGrenvilleFront.Ataboutthesametime(1460–1420Ma),theHarpLake,Michikamau,andMistastinAMCGcomplexeswereemplacedinpulsesthatgrewyoungerfromsouthtonorth.

Inorder to account for theseobservations,Gower&Krogh (2002), followingGutscher et al. (2000),advocatedflat-slabsubductioninvolvinganoverriddenspreadingridgethatprovidedthermalbuoyancyforthelowerplate.ThisproposalexplainstheshuttingoffofPinwarianmagmatismat thesubductionzoneaswellasthesouth-to-northyoungingofintrusiveages.Gower&Krogh(2002)suggestedthatthespreadingridgenotonlyprovidedbuoyancy,butmightalsohavefedathe-nosphericgabbrothroughtheoverlyinglithospheretothebaseofthecontinentalcrust,whereitpondedandfractionated.Gower&Krogh (2002) also noted thatthe“entireregionunderpinnedbyflat-subductedcrustisoneoftectonicinstabilityandpocketsofmagmatismmight be anticipated in anyweak spot”.We expandthis suggestion to include delamination or breakoffof the cooling lithosphere(s), resulting in the ascentof deeper, hotter asthenosphere and decompressionmelting,producinggabbro thatpondedat thebaseofthe crust.Anothervariation is that thenarrowwedgeof asthenosphere above the leading tip of the now-dehydratedoceaniccrustofthelowerplatewasreplen-ishedbyconvectivecirculationprovidingfresh,hotterasthenosphere that underwent decompressionmeltingtoyieldtheunderplatedgabbros.Althoughdifficultto

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demonstrateatpresent,theflat-slabmodelisattractiveandconsistentwithwhatisobservedatthesurfacefortheHarpLake,Mistastin,andMichikamauplutonsaswellastheMichael–Shabogamodikeswarms(Fig.1).Moreover, it isconsistentwithalate-topost-tectonicsetting(Fig.7).Rivers&Corrigan(2000)andRivers(2008b)haveproposedthattheAMCGmagmatismofLabrador is the consequence of back-arc extension,whichmay be compatiblewith the flat-slabmodeldescribedhere.

It is difficult to reconcile theflat-slabmodelwiththeNaincomplex,whichis74millionyearsyoungerthan the neighboringAMCGplutons.However, therecent investigations ofMyerset al. (2008) has shedconsiderable lighton theoriginof theNaincomplex,whichweaddressinthenexttolastsectionofthepaper.

Anorthosites and AMCG suites of southern Norway

The1100–930Ma(Bingenet al.1993)Sveconorwe-gianOrogenbearsresemblancestotheGrenvilleProv-incewithwhichitwaspresumablycontiguousduringmuch ofmid-Proterozoic time; it alsowas affectedby deformation analogous to the ca.1100–1030MaOttawan(i.e.,Grenvillian)orogeny.Duringthepasttwodecades,agreatdealofhigh-qualitygeochronologicaldatahaveemergedontheSveconorwegianregionandhas served to clarify important details of its evolu-tion.Wenotethatthewesternportionoftheorogenisallochthonousandhasbeentransportedandoverprintedbymetamorphism in the JotunNappeComplex ofsouthwesternNorway.Theeasternpartof theorogenislocatedonthesouthwesterntipofNorwayandreap-pears in eastern Sweden.Anorthosites andAMCGsuitesexistinbothsectors.IntheJotunNappeportionoftheorogen,regionalconvergenceandmetamorphismappeartohavebegunatca.1110MaandwerefollowedbysubductionbeneathBalticafromca.1050to1040Ma (Bingen& vanBreemen 1998). Subsequently,thrustingmigrated eastward.Relaxation occurred atabout970Ma,andamassifanorthositewasemplacedatthattime(Lundmark&Corfu2008,Bingenet al.2005).Farther southeast, regionalmetamorphism extendeduntil930MawhentheverylargeRogalandanorthositecomplex, including theBjerkreim–Sokndal layeredleuconoriticlopolith,wasrapidlyemplaced(Schäreret al.1996,Bingenet al.2005).Alloftheseinvestigatorsremarkedon thebarelypost-tectonicemplacementoftheseAMCGmassifsandtheirrelationshiptoanexten-sional,collapsephaseoftheSveconorwegianOrogeny.

Miscellaneous examples of AMCG complexes in lithospheric-scale shear zones

Myers et al. (2008) have recently published theresultsofathoroughinvestigationoftheNainigneousplutoniccomplex(Figs.2,7)thatmustbepartofany

discussionofthetectonicsettingofAMCGsuites.Herewesummarize the resultsof theirwork.The~300 3 75–100kmNainbatholithwasintruded(1363–1289Ma)intothe~100km-wideTorngatorogen(Hamilton2008)thatmarksthesuturebetweentheChurchillandNainprovinces(Fig.2).TheNaincomplexconsistsoflargecoalescingandcross-cuttingplutonsofanortho-site,monzonitic, ferrodioritic, troctolitic, andgraniticrocksthatintrudeintosharp,well-definedfracturesandgivelittlesupportfordiapiricmodelsofintrusion.Ringcomplexes,cauldronsubsidence,dikesandsheetschar-acterizetheintrusivemodes.Emplacementoccurredinseveral distinct pulses ofAMCGmagmatism spreadover75millionyears.TheoriginoftheNaincomplexiscloselyrelatedtothe~1000 3 50kmlithospheric-scaleGardar–VoisceyBayFaultZonethat trendseast–westfrom theGardar Province,Greenland (Upton et al.2003)and intersects theNW–SETorngatorogenatahigh angle.The fault zone is steep, exhibits sinistraloffsetof~20km,andcontainsAMCGandotherintru-sionsthathavebeendatedandconstrainthetimingofdisplacement.Theagesofmagmatismanddisplacementcorrelatecloselywiththefive1363–1289MaintrusivepulsesthatbuilttheNaincomplexfrom1363to1289Ma.The interpretation is that transtensionaldisplace-menton the fault zone reactivated theTorngat suturecausingfracturing,pressuredrops,andpotentialopen-ings that led to decompressionmelting in themantleandprovidedconduitsforgabbroicmagma.

Emplacement ofAMCG suites into ancient,lithospheric-scale shear belts and suture zones hasbeen demonstrated in several areas other than theNainComplex, e.g., the Laramie andHorseCreekcomplexes(Fig.1)inWyoming(Scoates1994,Scoates&Chamberlain1995,1997),aswellasthePhanerozoic(410–420Ma)AMCGcomplexesinAïr,Niger(Moreauet al. 1994).TheWyoming examples occurwithintheCheyenneBelt thatmarks the suturebetween theArcheanWyomingProvinceandtheProterozoicColo-radoProvince.InthecaseoftheHorseCreekanortho-site,Scoates&Chamberlain(1995,1997)havedateditsemplacementatca.1.76Ga,whichcoincideswiththe late stages of theMedicineBoworogeny,whichinvolved a local transtensional zone along the suturethatservedasaconduitforAMCGmagmas(Frostet al.2000).Asimilaroriginmayapplytotheca.1.43GaLaramieanorthositecomplex(Frostet al.1990,2000,Scoates&Chamberlain1995,1997),but the tectonicsettingatthattimeremainsuncertain.

Although not Proterozoic, the ca. 410–420MasubvolcanicAMCG ring-dike complexes of theAïrregion inNiger (Brown et al. 1989,Demaiffe et al.1991,Moreauet al. 1994)deservemention.ThebeltwasoncethoughttoformaN–Shot-spottrack,~1200km long.However,more recent dating indicates thatconsistent younging to the south does not exist, andmost workers have abandoned the hotspotmodel.

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Instead,itnowappearslikelythatthebeltformedatca.410–420Mainresponsetoreactivationalongthe~1000km long,N–S trending, dextral strike–slipRaghanelithospheric-scaleshearzonethatgaverisetoacomplexarrayofN20–50°EReidelshearsthatservedasmagmaconduits(Moreauet al.1994).

summaryaNDDiscussiON

DetailedgeochronologyintheGrenvilleProvince,Labrador,andsouthernNorway indicates that signifi-cantexamplesofAMCGmagmatismarecloselycorre-latedwiththelate-topost-tectonicstagesoforogenyorwith strike–slip displacement along lithospheric-scalefault zones and reactivated sutures, or both (Fig. 7).Theredoesnotappeartobesupportforatruly“anoro-genic”emplacement.ThemostsignificantfactorinthegenesisofProterozoicmassifanorthositeisdeliveryofasthenosphere to the base of thick, stable continentalcrustwheredecompressionmeltingyields agabbroicmagma.Thesemelts undergo slow, high-pressure,anhydrous fractionation and are replenishedby freshinfluxes of gabbroicmagma.Relatedmelting of thelower crust produces felsicmembers of theAMCGsuite,andbatchesofthesegranitoidseventuallyascend,alongwith early, high-pressure anorthositic plagio-clase cumulates containedwithinmagmasof broadlyleucogabbroic to leuconoritic composition.Followingemplacementinthemiddletouppercrust,lower-pres-surefractionationtakesplaceintheproto-anorthositicmagmas.DetailedU–Pb zircon geochronology hasestablishedthatthispolybaricevolutionoccursinlate-topost-tectonicsettings.Althoughprecisemechanismsaredifficulttoprove,severalcandidatesseemplausibleandareconsistentwithevidence.Theseincludedelami-nation of overthickened lithosphere in contractionalorogens, flat-slab subduction, perhaps accompaniedby delamination, extension in back-arc basins, andslabbreakoff.Whatallofthesehaveincommonistheabilitytodeliverasthenospheretothebaseofthecrustandtodosowithinthecontextoflate-topost-tectonicconditions.Aspecialsubclassof thesesettings is thatof the transtensional reactivationof lithospheric-scaleshearzones,especiallythoseinvolvingthereactivationofancientsutures.Intheexamplesinvestigatedtodate,therearenogoodexamplesofAMCGsuitesgeneratedbyhotspotsorplumes.

In retrospect,we propose that the localization ofAMCGmagmatisminNorthAmericaalongarelativelynarrowbeltisduetothefactthatthebeltrepresentsaprotracted (ca. 1800–1000Ma) orogenic continentalmargin arc system ~4000 km long thatwas activealongthesoutheasternmarginofLaurentia(Anderson1983,Rivers&Corrigan2000,Karlstromet al.2001).Within this context, late- topost-tectonicmagmatismcouldproduceAMCGcomplexesviathemechanismspresentedinthispaper.However,byitself,thisproposal

does not explainwhy equivalentAMCGcomplexesofNeoproterozoic agehavenot beendiscovered.Onthe other hand, the PhanerozoicAMCG complexesofAïr document that these rockswere not restrictedto theProterozoic.TheAïr examplesare subvolcanicintrusions,whereasAMCG suites developedwithincompressionalbeltsareemplacedatmid-crustaldepthsandrequirelongerintervalsoferosiontoarriveatthesurface.

acKNOWLeDgemeNTs

OurAdirondack researchwas supportedNationalScienceFoundation,grantEAR–0125312toBickfordandMcLelland. The Colgate University ResearchCouncilandtheH.O.WhitnallChairprovidedfundstosupportSelleck.WethankTobyRivers,JamesScoates,andananonymousreviewer,andRobertF.Martinforveryhelpfulsuggestionsandinsights,andediting.

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Received January 9, 2009, revised manuscript accepted March 24, 2010.

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