geochemical and isotopic evolution of the anorthositic plutons

925

The Canadian MineralogistVol.48,pp.925-946(2010)DOI:10.3749/canmin.48.4.925

GEOCHEMICAL AND ISOTOPIC EVOLUTION OF THE ANORTHOSITIC PLUTONS OF THE LARAMIE ANORTHOSITE COMPLEX: EXPLANATIONS FOR VARIATIONS

IN SILICA ACTIVITY AND OXYGEN FUGACITY OF MASSIF ANORTHOSITES

CarolD.FROST§andB.RonaldFROST

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

DonaldH.LINDSLEY

Department of Geosciences and Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100, USA

KevinR.CHAMBERLAINandSusanM.SWAPP

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

JamesS.SCOATES

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

Abstract

TheLaramie anorthosite complex includes three anorthositic plutons: thePoeMountain,Chugwater, andSnowCreekintrusions.ThePoeMountainandChugwaterbodiesexhibitmappablemagmaticstratigraphythatrecordsfractionationprocessesinamagmachamberatornearthepresentlevelofexposurefollowedbydomingandexpulsionofresidualliquid.TheSnowCreekplutonismorepoorlyexposed,althoughlayeringislocallyobserved,andthereisgeochemicalevidenceforremovalofinterstitialliquidintheeasternportionofthebody.Thethreeplutonswereemplacedinclosesuccession.TheU–PbzirconandbaddeleyitegeochronologyofanorthositefromtheChugwaterandSnowCreekplutons,andamonzodioritedike,indicatesthattheChugwateristheoldestofthebodies,withaweightedaverage207Pb/206Pbageoffivesamplesof1435.5±0.3Ma.TheSnowCreekanorthositeintrudesthepreviouslydated1434MaPoeMountainanorthositeandis,inturn,cutbythe1432.8±2.4Mamonzodioritedike.TheLACandassociatedgranitesoftheShermanbatholithsuitewereemplacedoveraperiodnogreaterthan12millionyears,andpossiblyinaslittleasthreemillionyears.Despitetheirsimilarages,eachoftheLACanorthositeplutonsdisplaysadistinctiveassemblageofminerals.ThePoeMountainanorthositeischaracterizedbyolivine,augite,lowCa-pyroxene,ilmeniteandmagnetite,andplagioclaseintherangeAn43–53.TheChugwateranorthositegenerallycontainsnoolivinebutiscomposedofaugite,lowCa-pyroxene,ilmeniteandmagnetite,andiridescentplagioclaseintherangeAn50–56.TheSnowCreekanorthositeischaracterizedbyalackofolivineandmagnetiteandthepresenceofiridescentplagioclaseofcompositionAn47toAn56; it iscommonlyquartz-bearing.Theseassemblages recorda rangeof silicaactivitiesandoxygen fugacities.Higheractivitiesofsilicaandfugacitiesofoxygenarecorrelatedwithgreateramountsofcrustalassimilation,asindicatedbyNdandSrisotopiccompositionsandthepresenceofinheritedcomponentsinzircon.WedemonstratethatcrustalassimilationcanproducearangeofmineralassemblagesandSr–NdisotopiccompositionsinLACanorthositescrystallizedfromcommonmantle-derivedparentalmagmas,andthatassimilationofcrustislikelyanimportantcontrolonthecompositionalvariationsdocumentedinotherProterozoicmassifanorthosites.

Keywords: anorthosite, silica activity, oxygen fugacity, geochronology,Nd andSr isotopes,Laramie anorthosite complex,Wyoming.

Sommaire

LecomplexeanorthositiquedeLaramiecontienttroisplutonsmajeurs,PoeMountain,Chugwater,etSnowCreek.LesmassifsdePoeMountainetdeChugwaterfontpreuved’unestratificationignéequitémoignedeprocessusdefractionnementdansunechambremagmatiqueprèsduniveauactueld’affleurement,cecisuivid’unsoulèvementetd’uneexpulsionduliquiderésiduel.

§ E-mail address:[email protected]

926 thecanadianmineralogist

LeplutondeSnowCreekaffleuremoinsbien,quoiqu’unlitagesoitobservélocalement,etilexistedessignesgéochimiquesfavorisantl’évacuationd’unliquideinterstitieldanslapartieorientaledupluton.Lestroisplutonsontétémisenplacedansuncourtlapsdetemps.L’étudegéochronologiqueU–Pbeffectuéesurlezirconetlabaddeleyiteprélevésdel’anorthositedesplutonsdeChugwateretdeSnowCreek,ainsiqued’unfilondemonzodiorite,montreleplutondeChugwaterseraitleplusanciendecesvenues,avecunemoyenned’âges207Pb/206Pbpondéréede1435.5±0.3Ma(cinqéchantillons).L’anorthositedeSnowCreekrecoupelemassifdePoeMountain,déjàdaté(1434Ma)etilest,àsontour,recoupéparlefilondemonzodiorite(1432.8±2.4Ma).LecomplexeanorthositiquedeLaramieetlesgranitesassociésdubatholitedeShermanontétémisenplaceaucoursd’unintervalledepasplusque12millionsd’années,etpeut-êtremêmeaussipeuquetroismillionsd’années.Malgréleursâgessemblables,chacundesplutonsanorthositiquesprésenteunassemblagedistinctifdeminéraux.L’anorthositedePoeMountain contientolivine, augite, pyroxène à faible teneur enCa, ilménite etmagnétite, et unplagioclasedans l’intervalleAn43–53.L’anorthositedeChugwaternecontientpasd’olivine,engénéral,maispossèdel’assemblageaugite,pyroxèneàfaibleteneurenCa,ilméniteetmagnétite,unplagioclaseiridescentdecompositionAn50–56.Dansl’anorthositedeSnowCreek,l’olivineetlamagnétitesontabsentes,etleplagioclase,decompositionAn47toAn56,estiridescent,etlequartzestprésent,engénéral.Cesassemblagestémoignentd’unintervalled’activitésdesiliceetdefugacitésd’oxygène.Lesvaleursplusélevéesdecesparamètresmontrentuncorrélationavecuneassimilationaccruedelacroûte,commel’indiquentlescompositionsisotopiquesduNdetduSr,ainsiquelaprésencedecomposanteshéritéesdanslezircon.Nousdémontronsquel’assimilationdelacroûtepeutproduireunéventaild’assemblagesdeminérauxetdecompositionsisotopiquesSr–NddanslecomplexeanorthositiquedeLaramieàpartirdumêmemagmaparentdérivédumanteau,etqu’elleexerceraituncontrôleimportantsurlesvariationsencompositiondocumentéesdansd’autresmassifsprotérozoïquesd’anorthosites.

(TraduitparlaRédaction)

Mots-clés:anorthosite,activitédesilice,fugacitéd’oxygène,géochronologie,isotopesdeNdetdeSr,complexeanorthositiquedeLaramie,Wyoming.

logicalandgeochemicalresultstodocumentthattheseplutonswereintrudedoverashortperiodoftime,andthattheyrecordasimilararangeofsilicaactivityandoxygen fugacity as themuch larger and longer-livedNainPlutonicSuite,Labrador.Because the anortho-siteplutonsof theLACwere intrudedclose in spaceand timeandunder the same tectonic conditions, thevariablesthatcontrolthedifferencesinoxidationstateandsilicaactivitycanbereasonablywellconstrained.Moreover, because the LAC intrudesArchean andProterozoicwallrocksthatareconsiderablyolderthantheanorthositicplutons,NdandPbisotopiccomposi-tions of theLAC are sensitive indicators of crustalcontamination.Thus, theLACaffordsanopportunitytoevaluatetheprocessesbywhichmassifanorthositesobtaintheirvaryingcompositions.

AnorthositicPlutonsoftheLaramieAnorthositeComplex

The LaramieAnorthosite Complex (LAC) isexposed in the LaramieMountains of southeasternWyoming,aLaramideupliftthathasbeenthrustoverPhanerozoic rocks along its easternmargin (Fig. 1).Thewestern contact of the complex is overlain byPhanerozoicsedimentaryrocks.Onlythenorthernandsouthern contacts of the complex arewell exposed.TheLACintrudedtheca.1.75GasuturebetweentheArcheanWyomingprovinceandProterozoiccrust.Tothe north lies theArcheanWyomingprovince that isdominated byLateArcheangranitic gneisses.To thesouth lie Proterozoic rocks thatwere formed at 1.76Gaorearlier.TheArchean–Proterozoicboundarydips

Introduction

Although allmassif anorthosites are by definitiondominated by plagioclase, they evince awide rangeofcompositionalandmineralogicalvariability.Massifanorthositesmaybesubdividedonthebasisofplagio-clasecomposition intoandesineand labradoriteanor-thosites(Anderson&Morin1969).Massifanorthositesalsodiffer in theirsilicaactivityandoxygenfugacity(Hébertet al. 2005,Morse 2006).Andesine anortho-sites,suchasLabrieville(Owens&Dymek2001),arecommonly quartz-saturated, contain exsolved ferrianilmenite (commonly known as “hemo-ilmenite”, sonamedforthepresenceofexsolvedlamellaeofhema-titeinanilmenitehost),andtypicallylackmagnetite.Labradoriteanorthositesusuallylackquartz,commonlycontain olivine, and are characterized by coexistingilmenite andmagnetite. However, evenwithin thelabradorite anorthosites such as theNain PlutonicSuite,thereisaconsiderablerangeinsilicaactivityandoxygenfugacity,fromreducedandolivine-saturatedtorelativelyoxidizedandquartz-saturated(Xue&Morse1993).The causes for these variations in intensiveconditionsremainuncertain.Owens&Dymek(2001)contended that they reflectdifferences in theprimarymagmasofanorthosites,whereasXue&Morse(1993)argued that the range of compositions is related tocrustalassimilation.

Inthiscontribution,wedescribethreeanorthositicplutonsfromthe1.44GaLaramieAnorthositeComplex(LAC) that are exposed in a relatively small area(~800km2)intheLaramieMountainsofsoutheasternWyoming,USA.Wepresentgeochronological,petro-

evolutionofanorthositicplutons,laramiecomplex,wyoming 927

Fig.1. SimplifiedgeologicalmapoftheLaramieanorthositecomplex,southeasternWyoming,showingthelocationsofthePoeMountain,SnowCreek,andChugwateranorthositeplutons,andsamplelocationsforgeochemical,isotopicandmineralchemistryresultspresentedinthisstudy.TheinsetmapshowsthelocationoftheLAConthesoutheasternmarginoftheArcheanWyomingprovince.TheCheyennebelt,whichmarksthetraceoftheArchean–Proterozoicboundary,isconstrainedto lienorthof theProterozoicgneisses intrudedby theChugwateranorthositeand southof the southernmostexposuresofArcheanrocksintrudedbythewesternSnowCreekanorthosite.Thesuturedipstothesoutheastatapproximately55°(Allmendingeret al.1982);therefore,thePoeMountainanorthositeascendedexclusivelythroughArchean-agecrust,andthesouthernChugwateranorthositetraversedthegreatestthicknessofProterozoic-agecrust.


approximately 34° to the south (Allmendingeret al.1982,Chamberlain1998).

TheLACconsistsofthreeanorthositeplutons:PoeMountain(Scoates1994,2000,Scoates&Frost1996),Chugwater (Lindsley et al. 2010), and SnowCreekanorthosites,twomaficintrusions,theStrongCreekandGreaserintrusions(Mitchell1993,Mitchellet al.1995,1996),andthreemonzoniticplutons,Sybille(Fuhrmanet al. 1988, Scoates et al. 1996),Maloin (Kolker&Lindsley 1989,Kolker et al. 1990, 1991), andRedMountain (Andersonet al. 2003)plutons, alongwithnumeroussmallplutonsanddikesofhigh-Algabbro,leucotroctolite,ferrodiorite,monzodiorite,andgranite.TheLAC is spatially and temporally associatedwiththe 1438–1431Ma ferroan, alkalic–calcic Shermanbatholith,whichisexposedalongthenortheasternandsouthernmarginsoftheLAC(Geistet al.1989,Frostet al.1999,2001,2002,Edwards&Frost2000).

The anorthositic plutons

The anorthositic plutons are the oldest intrusiverocksintheLAC.Theyformthecoreofthecomplexand are rimmed by younger intrusions (Fig. 1).ThePoeMountain anorthosite,which is thenorthernmostanorthositic pluton, is a distinctly layered intrusionthatgradesfromanorthositeinthelowerstratigraphiclevels to leucogabbro in the higher levels (Scoates1994,2000).LayeringinthePoeMountainanorthositeformsadomalpatternwithdipsbecomingprogressivelyshallower toward lower stratigraphic levels.Most oftheeasternportionofthedomehasbeendisruptedbyLaramidefaults,sothatonlythewesternandnorthernmarginsarepreserved.ThePoeMountainanorthositeis characterizedbyolivine, augite, low-Capyroxene,ilmeniteandmagnetite,andplagioclase(An43–53).ThePoeMountain anorthosite contains numerous blocksofleucogabbrothatdeformtheplagioclaselaminationbeneath them; these are interpreted as representingblocksthatdroppedthroughanopenmagmachamberontotheflooroftheintrusion.ThesourceoftheblocksmaybepreservedaslargexenolithsofleucogabbrothatoccurwithinthenorthernportionoftheSybilleintru-sion.Locallywithinthesexenoliths,theleucogabbroischilledagainsttheArcheancountryrock.Threesamplesof PoeMountain anorthosite giveU–Pb zircon orbaddeleyiteagesthatareidenticalwithinerror:1434.4±0.6,1434.5±0.6,and1434.1±0.7Ma(Scoates&Chamberlain1995).

The southern anorthositic pluton of the LAC istheChugwateranorthosite(Lindsleyet al.2010).TheChugwateranorthositeconsistsofbroadlayersofanor-thositeandgabbroicanorthosite.UnlikethePoeMoun-tainanorthositethatformedbyin situfractionationinamid-crustal-levelmagmachamber(Scoateset al.2010),theChugwateranorthositewasemplacedasaseriesofcrystal-richmagmasfollowedbydoming,probablyinresponsetogravitationalinstabilityofaplagioclase-rich

crystal-richmush(Lindsleyet al.2010).LikethedomeofthePoeMountainanorthosite,theeasternlimbofthedomehas beengreatly affectedby later deformation.Someofthisdeformationoccurredatahightempera-tureandmayhavebeenrelatedtotheemplacementoftheMaloinRanchpluton,whereassomeofitisclearlyassociatedwithbrittledeformationduringtheLaramideorogeny.TheChugwater anorthosite contains augite,low-Capyroxene,ilmeniteandmagnetite,andplagio-claseintherangeAn50–56(Lindsleyet al.2010).Anor-thositeintheChugwaterplutongenerallycontainsnoolivineandislocallyquartz-bearing.Olivineispresentwheretheanorthositehasbeenintrudedbyandmixedwith leucotroctolite. Scoates&Chamberlain (1995)reportedoneU–Pbbaddeleyitedate,1435.5±0.5Ma,from the northern part of theChugwater anorthosite,whichoverlapswithinerrorthedatesobtainedforthePoeMountainanorthosite.

The relative agesof thePoeMountain andChug-water anorthosites cannot be determined fromfieldrelations because the SnowCreek anorthosite liesbetweenthem(Fig.1).TheSnowCreekanorthositehadreceivedrelativelylittleattentionpriortothisstudy,inpartbecauseoutcropislimitedandcontactsarepoorlyexposed.TheSnowCreekanorthositeisintrudedbythetroctoliticStrongCreekandferrodioriticGreasermaficintrusions(Mitchell1993),whichdividetheanorthositeintowesternandeasternportions.TheplutonisnamedforexposuresintheSnowCreekareaonthecentral–western portion of theLAC. In this area, the SnowCreekanorthositeisweaklylayered,withthelayeringhavinganearlyflatorientation.IntheeasternportionoftheSnowCreekanorthosite,thelayeringissteeperandbetterdeveloped,butthisareaismorehighlydeformedthanthewesternSnowCreekregion.TheSnowCreekanorthositeischaracterizedbyalackofmagnetiteandthepresenceofiridescentplagioclase.Itiscommonlyquartz-bearing, particularly thewesternportion.Boththe southern and northern intrusive contacts of thewesternbodyaresharpandaredelineatedbychangesinfabricorientation.Incontrast,thelimitsoftheeasternbody are very difficult to delineate.We establishedtheselimitsbynotingthelocationsofsamplesthatlackmagnetiteinthinsection.

ThereisasizableareaofanorthositeinthenorthernportionofthecomplexthatisboundedonallsidesbyLaramide faults. Because the rocks are extensivelyaltered,we are unable to determine towhich plutontheserocksbelong.Consequently,wehavemappeditas“anorthositeundivided”.

GeochronologyoftheChugwaterandSnowCreekAnorthosites

OnlyonedatehasbeenpublishedfromtheChug-waterpluton, andnone from theSnowCreekpluton.Accordingly,we present in this studyU–Pb isotopiccompositionsoffourbaddeleyiteand18zirconfractions


(Table1) fromsixsamples: four fromtheChugwateranorthosite,onesamplefromtheSnowCreekanortho-site,andonesamplefromalatemonzodioritedikethatcross-cuts theSnowCreekanorthosite.Betweennineand20grainsofbaddeleyiteandbetweenoneand17grainsofzirconweredissolvedforeachanalysis.

Zircon inmost samples is anhedral and colorless.Scoates&Chamberlain (1995) interpreted anhedralzircon in theLaramie anorthosite complex as a late-

crystallizingphase interstitial tocumulusplagioclase.Two samples contain predominantly euhedral zircon:GM148 from theCoyoteFlat area of theChugwateranorthosite, andBM5 from themonzodiorite dike.Baddeleyite was analyzed from two samples. Theanalyzedgrainsarebrownand translucent,andmanygrainsdisplaywell-developedpolysynthetictwinning.

Concentrations ofU and Pb are extremely low,rangingfrom79to588ppmUand19to158ppmPb.


TheseconcentrationsaretypicalofProterozoicmassifanorthosites(e.g.,Higgins&vanBreemen1992,Owenset al.1994,Scoates&Chamberlain1995,1997,2003,Frostet al.2000).Valuesof206Pb/204Pb(correctedforfractionation, spike composition, andblank contribu-tion)arehigh,rangingfrom2,950to320,160(Table1),indicating lowamounts of commonPb in zircon andbaddeleyite.

Chugwater anorthosite (KM13)

SampleKM13 is amedium-grained, tabular anor-thosite.Thiswell-foliatedrockisfromKingMountain,theAn3unitoftheChugwateranorthosite(Lindsleyet al. 2010).Stratigraphic reconstructionsplace it 8,860mabovethelowestexposedanorthositesinthislayeredanorthosite intrusion (Lindsleyet al.2010).The rockis composed of 93%plagioclase,which is deformedonlylocally,2–3%ilmenite,andminorquartz,calcite,whitemica and epidote. TheU–Pb data from twoanhedral single crystals of zircon andone nine-grainbaddeleyite fraction are overlapping and concordant,with207Pb/206Pbagesfrom1435.8to1436.1Ma(Fig.2A).Theweightedaverage207Pb/206Pbageof1436.0±0.7MaisconsideredthebestestimatefortheageofcrystallizationofKM13.

Chugwater anorthosite (GM144)

SampleGM144isastronglydeformedanorthositelocated in the southeastern portion of theChugwateranorthosite.Itiscomposedof96%plagioclase,whichismostlyneoblastic.Relictlargegrainsofplagioclaseare kinked or bent.The sample containsminor late-stagecalcite,whitemicaandchlorite.Fourfractionsofanhedralzirconwereanalyzed,eachfractioncontainingbetweenfourandeightgrains.TheU–Pbdataforthreefractionsarenearlyconcordant(1.4to1.8%discordant),whereasdatafromafourthfractionaremorediscordant(7.4%discordant;Fig.2B).Anupperinterceptageof1435.7±1.1Mawasobtainedfromalinearregressionofthefourdatasets,whichisinterpretedasabestesti-matefortheageofcrystallizationofGM144.

The Chugwater(?) anorthosite (GM148)

SampleGM148comesfromasmallbodyofanor-thosite that liessoutheastof theChugwater intrusion,and is less deformed thanGM144. It is composedof 94% plagioclase,which is kinked and bent, butneoblastsareminor.Ferromagnesianmineralsincludesubequalamountsofaugite,orthopyroxeneandinvertedpigeonite. Ilmenite ismore abundant thanmagnetite.Calcite,whitemicaandchloriteareminor.Fourfrac-tions,eachcomposedofsixtoninegrainsofeuhedralzircon,wereanalyzed.Thedatafromallfractionsarenearlyconcordant(lessthan1%discordant),andyield

207Pb/206Pbagesof1434.9to1435.5Ma(Fig.2C).Theweightedaverage207Pb/206Pbage,1435.3±0.6Ma,isconsideredthebestestimatefortheageofcrystalliza-tionofGM148.

The Chugwater gabbronoritic anorthosite (KM6)

SampleKM6was taken froma leucogabbro layerin theAn2 unit of theChugwater intrusion located4250mabove thebaseof thepluton (Lindsleyet al.2010).Therockiscomposedof80%plagioclase,5%orthopyroxene,5%augite,5%olivine,2%biotite,2%titaniferousmagnetitewith green spinel (hercynite)and1%ilmenite.Itisinterpretedtobetheproductofmixingofleucotroctoliticmagmawithpartiallysolidi-fiedChugwateranorthosite(Lindsleyet al.2010).Threefractionsofbaddeleyitewereanalyzedfromthesample,eachfractioncontainingbetween12and20grains.TheU–Pbdatafromthesefractionsincludetwoconcordantanalyses and one nearly concordant analysis (0.92%discordant;Fig.2D).Alinearregressionthroughthesethreedatasetsyieldsanupper interceptageof1435.2± 0.9Ma, interpreted as the age of crystallization ofthissample.

The Chugwater anorthosite (BM136) from Scoates & Chamberlain (1995)

In addition to the ages reported above, an ageacquiredon afifth sample ofChugwater anorthosite,BM136, is available (Scoates&Chamberlain 1995).Thismegacrystic anorthosite from the northern partof theChugwater anorthosite (An1 unit) is from the1404mstratigraphiclevel,makingitstratigraphicallythe lowest of the dated samples. It yielded abundantbaddeleyite, but little zircon.TheU–Pb data fromfour baddeleyite fractions gave aweighted average207Pb/206Pbageof1435.4±0.5Ma.

The Snow Creek anorthosite (BM243)

TheSnowCreekanorthositeintrudestheChugwateranorthositetothesouthandthePoeMountainanortho-sitetothenorth.SampleBM243islocatedwestoftheStrongCreekandGreaserintrusions.Itiscomposedof90%plagioclase,which is strongly bent and suturedandhasathinrimofmesoperthite.Itcontainsaugite,inverted pigeonite, ilmenite and quartz.Data fromfourfractionsofanhedralzirconarenearlyconcordant(0.89 to 1.17%discordant), and yielded 207Pb/206Pbagesof1436.7to1439.2Ma(Fig.2E).Thisrangeof207Pb/206Pbagesexceedsanalyticaluncertainty,andaweighted average has nogeological significance.Weinterpret the data as evidence for a small componentof inherited zircon, and conclude that the youngest207Pb/206Pb age of 1436.7 ± 1.2Ma is amaximumageofcrystallization.This interpretationisconsistent


Fig. 2. U–Pb concordia diagrams for analyzed zircon and baddeleyite fractions from theChugwater and SnowCreekanorthositesandthemonzodioritedike.Thewidthoftheconcordiabandrepresentseffectsofuncertaintiesindecayconstantsoftheuraniumisotopes.


withfield evidence that the SnowCreek anorthositeis younger than the Chugwater and PoeMountainanorthosites.

Monzodiorite dike (BM5) cutting the Strong Creek Complex, Chugwater anorthosite and Snow Creek anorthosite

Thissampleisfromamonzodioritedike10kmlong,1–3mwidethatintrudestheStrongCreekcomplex,theChugwateranorthositeandtheSnowCreekanorthosite.Onthebasisoffieldrelations,itisoneoftheyoungestunits in the central portion of theLaramie complex.Therockisfinegrained,andcomposedofantiperthiticplagioclase,alkalifeldspar,augite,olivine,Fe–Tioxide,andapatite.Fourfractionsofzirconyielded207Pb/206Pbagesof1432.8to1493.0Ma(Fig.2F).Datafromthefractionswith the twoyoungest 207Pb/206Pb ages arelessthan1%discordant.Theolder207Pb/206Pbagesoftheotherfractionssuggestthepresenceofaninheritedcomponent,consistentwithNdandSrisotopicevidenceof crustal contamination (Mitchell et al. 1996).Weinterprettheyoungest207Pb/206Pbageof1432.8±2.4Maasamaximumagefortheintrusionofthemonzo-dioritedike.

Summary of geochronology

ThefivedatedsamplesofChugwateranorthositearegeographically and stratigraphicallywell distributed.They includeboth leucogabbro and anorthosite, bothhighlydeformed andundeformed rocks, and sampleswherezirconismoreabundantthanbaddeleyiteaswellastheconverse.Despitethisdiversity,allagesfromtheChugwaterintrusionoverlapwithinerror.Theweightedaverageageforthesefivesamplesis1435.5±0.3Ma,withanMSWDof0.78.TheageofKM–6,interpretedas the result ofmixing of troctoliticmagmawith ananorthositic crystalmush, is indistinguishable fromtheageofotherChugwatersamples,implyingthatthetroctoliticmagmamusthaveintrudedandmixedveryshortlyaftertheanorthositebodywasemplaced.

TheindividualdeterminationsofagemadefortheChugwaterintrusionimplyolderagesthanthoseofthePoeMountainanorthosite,buttheyoverlapwithinerror.However,theweightedaverageageoftheChugwaterintrusion(1435.5±0.3Ma)isolderoutsideoferrorthantheweightedaverageageofthePoeMountainanortho-sitesamples(1434.4±0.4Ma;MSWD=0.4).TheageoftheblockofleucogabbroicanorthositefromwithinthePoeMountainanorthositeisindistinguishablefromthe age of theChugwater anorthosite.This xenolithcontainsiridescentplagioclasemegacrysts(Scoates&Chamberlain1995),asdotheChugwateranorthosites.However,Nd andSr isotopic compositions of xeno-lithsinthePoeMountainanorthositecontrastwiththeisotopic compositions of theChugwater anorthosite.ThissuggestseitherthattheChugwateranorthositewas

more contaminatedwithArchean crust farther to thenorthwheretheblockislocated,orthattheblockwasderivedfromacoeval,butdifferent,intrusionthantheChugwateranorthosite.

Because zircon from theSnowCreek anorthositecontains inherited components, only amaximumagecouldbeobtainedbyU–Pbdating.Therefore,althoughthe SnowCreek intrusion is constrained to be theyoungest intrusionofanorthositeonthebasisoffieldrelations,theabsoluteagecouldnotbepreciselydeter-mined.However, it iscutbythedike(BM–5),whichhasamaximumageof1432.8±2.4Ma.TheU–Pbdatapermittheinterpretationthatthedikeisasyoungastheyoungest pluton in theLaramie anorthosite complex,theRedMountain pluton (1431.3± 1.4Ma,Vertset al. 1996).This interpretation is compatiblewithfieldobservationsofmonzodioritedikescuttingallunitsoftheLaramieanorthositecomplex,butfewercuttheRedMountain pluton than cut the other bodies.This datealsoconstrainstheageoftheSnowCreekanorthositetobeolderthanthemonzodioritedike.

TheU–PbagedeterminationsforLACmonzoniticplutons(Fig.3)indicatethattheyintrudedtheanortho-sitesshortlyafterthesewereemplaced.GraniteofthenorthernShermanbatholithslightlypredatestheanor-thosites(1437.7±2.4Ma;Frostet al.2002),whereasthe Lincoln and Sherman granites of the southernShermanbatholithareyounger(1430.6±2.6and1433.0±1.5Ma;Frostet al.1999).TheentireLAC–Shermanbatholithsuitewasemplacedoveraperiodnogreaterthan12millionyears,andpossiblyinaslittleasthreemillionyears(Fig.3).

Mineralogy,PetrologyandGeochemistryoftheSnowCreekAnorthosite

Themineralogy,petrology,andgeochemistryofthePoeMountainandChugwateranorthositeshavebeendescribed in other studies (seeLindsleyet al. 2010,Scoateset al.2010,andreferencestherein).Below,wedescribe themineralogy, petrology, andgeochemistryoftheSnowCreekanorthosite.

Snow Creek anorthosite: mineralogy

TheSnowCreekanorthosite isdistinguishedfromthePoeMountainanorthositebytheabsenceofolivineandthepresenceofiridescentplagioclaseandlocallyofquartz,especiallyinthewesternportionofthepluton,andfromtheChugwateranorthositebytheabsenceofmagnetite.

Plagioclase. Plagioclase from the SnowCreekanorthosite ranges in composition fromAn47 toAn56andshowsarangeofsolid-staterecrystallization.Theearliestplagioclaseistabularandisblackinhandspec-imenbecauseitcontainsabundantneedlesofilmenite.This plagioclase generally shows various degrees ofdeformation,includingbending,withthedevelopment


ofdeformationtwins.Inareasofintensedeformation,theplagioclasehasbeenrecrystallizedtogranoblasticgrains that commonly exhibit ca. 120° triple graininterfaces.Theseneoblasticgrainsofplagioclaselacktheilmeniteneedlesandappeargreytowhiteinhandsample.

Pyroxene.Pyroxeneoccursinallsamplesandformstypical“post-cumulus”texturesinwhichgrainsenclosethemargins of the tabular plagioclase. In stronglyrecrystallizedrocks,pyroxeneformsequantgrainsthatarethesamesizeastheassociatedplagioclase.Augiteisfoundinallrocks,althoughinonesample(SR357),itisahighlyexsolvedsubcalcicaugitethatwouldhavebeenstableattemperaturesofca.1100°C(Lindsley&Frost1992).Highlyexsolvedinvertedpigeoniteisthemost abundant low-Ca pyroxene, although themost

magnesianrockscontainorthopyroxene(Opx)instead.TheXFethelow-Capyroxenerangesfrom0.37to0.63(Table2).

Ilmenite.Mostsamplescontainilmenite,althoughinsomesamples, theamount is less that0.1%. Ilmenite

Fig. 3. Summary of geochronology for the Laramieanorthosite complex and the Sherman batholith.Datafrom this study, Frost et al. (1999, 2001), Scoates&Chamberlain(1995,2003),Vertset al.(1996).


containslessthan5mole%oftheFe2O3endmember(Table 2), similar to ilmenite fromother labradoriteanorthosites.

Biotite.Most samples contain small amounts ofred-brownbiotite. In some samples, it surrounds theilmenite, in others it appears alone or in associationwithpyroxene.

Potassium feldspar. Orthoclase is present inmostrocks.Itoccursassmallgrainsassociatedwithrecrys-tallized plagioclase in the highly deformed zones oras irregular blebs in tabular plagioclase.The texturessuggest that the orthoclase formed by exsolution oftheOrcomponentofplagioclaseduringlate-magmaticrecrystallization.

Quartz. Quartz is present inmany rocks. Likeorthoclase, it occurs as small grains associatedwithrecrystallized plagioclase.This texture suggests thatthequartzmayhaveformedfromexcesssilica incor-porated in plagioclase (Schwanke’smolecule) duringrecrystallizationofplagioclase.Quartzmayalsooccurinzonesofintensealteration,atexturesuggestingthatsomequartzwas produced as a by-product of low-Thydration reactions. In no rocks didwefind texturesindicatingthatquartzcrystallizeddirectlyfromthemelt,soourinterpretationis that theserockswerenearbutnotatquartz-saturation.

Alteration. Few samples are pristine;most showsomedegree of low-temperature alteration.Themostcommonlyobservedalterationmineralsarecalciteandmuscoviteafterplagioclase,chloriteandactinoliteafterpyroxene,andtitaniteandrutileafterilmenite.

Geochemistry

In this paper,we report results of 18whole-rockanalyses of samples from the SnowCreek anortho-site(Table3),eightofwhichwerereported inearlierpapers (Mitchellet al. 1995,Scoates&Frost 1996).In those papers, however, itwas not recognized thatsomeoftherocksanalyzedbelongtotheSnowCreekanorthosite;we include themhere for completeness.The SnowCreek anorthosite has awider spread innormativeanorthitecontentthaneitherthePoeMoun-tain or Chugwater anorthosites (Fig. 4). Formostsamples, the normativeAnof the other SnowCreekanorthositesamplesaresimilartothatoftheChugwateranorthosite.Thisisnotunexpected,astheybothhaveiridescentplagioclase.However,foursamplesfromtheeasternpartoftheSnowCreekanorthositehavemoresodic compositions.The easternmost portions of theSnowCreekanorthositealsorangetomoremagnesiancompositionsthanarefoundinthewesternportion,asindicatedbythelocaloccurrenceoforthopyroxene.Ifthis area is part of the samedome that is exposed inthewest,itwouldbestratigraphicallyhigher.ThePoeMountain andChugwater anorthosites becomemoredifferentiated (andmore ferroan) upward.ApossibleexplanationforthelackofthistrendintheSnowCreek

is that the eastern portion of the pluton intrudes thePoeMountainanorthosite.Itislikelythatatransitionzone exists between theSnowCreek anorthosite andPoeMountainanorthositewhere theSnowCreekhaspartiallyassimilatedportionsofthePoeMountainanor-thosite.Thiswouldexplainwhysomeportionsof theSnowCreekhavecompositionsthatdonotfitasimpledifferentiationtrendandalsowhyitisverydifficulttolocate the contact between theSnowCreek andPoeMountainanorthositicplutonsinthefield.

Similar to other anorthositic rocks in the LAC(Scoates1994,Lindsleyet al.2010),chondrite-normal-ized rare-earth-element (REE) patterns of the SnowCreekanorthositehaveapositiveEuanomalyandarestronglydepletedinheavyREE(Table3,Fig.5).ThetroctoliteandleuconoritehavehigherREEabundances,with smaller positiveEu anomalies.The anorthositicrocks from the eastern body have lower abundancesof REEwith a larger Eu anomaly than those fromthewestern body.The abundanceof theREE is alsopositivelycorrelatedwithP2O5content,consistentwiththeinterpretationthatmuchoftheREEintheserocksresides in the interstitialminerals, including apatite.Wesuggest that the intensedeformationaffecting theeastern rocks efficiently extracted intercumulusmelt,causing the depletion in theREE.This is consistentwith the observations ofLafranceet al. (1998),whoproposedthatthedeformationobservedintheanortho-siticrocksoftheLACtookplacebyfastgrain-boundarymigration,whichwasprobablyenhancedbythepres-enceofamelt.ThefactthattheEucontentofallthesamplesfromtheSnowCreekanorthositeisaboutthesame suggests that throughout the process, the bulkpartition-coefficientforEuwasnearly1.0.

IsotopeGeochemistry

TheNdandSrisotopicdatafromtheLaramieanor-thosite complex have been reported bySubbarayuduet al. (1975),Goldberg (1984),Geist et al. (1990),andMitchellet al.(1995,1996).Kolkeret al.(1991),Scoates et al. (1996), andAnderson et al. (2003)focusedon the isotopic characteristics of themonzo-niticMaloin, Sybille, andRedMountain intrusions,andScoates&Frost(1996)describedthemforthePoeMountainanorthosite.Inthisstudy,wereportNdandSr isotopiccompositionsof theChugwaterandSnowCreek anorthosites (Table 4). In addition,wepresentresults for one anorthosite sample from the eastern,undividedportionof the anorthositic area.Finally, toaugment the isotopic dataset formonzodiorites fromtheLaramieanorthositecomplex,wepresentdataforthreesamplesfromtheSybilleintrusion.

The Chugwater anorthosite

Samples from theChugwater anorthosite exhibita narrow range of initial Sr isotope ratios (0.70393


to 0.70445), but a large range in initialNd (+2.4 to–1.5) (Fig.6).The samples includeanorthositesensu stricto and anorthositic rockswith 80–90% plagio-

clase,includinggabbroic,gabbronoritic,andtroctoliticanorthosites.Themoremafic anorthositic rocks havebeen interpreted asmixtures of troctoliticmagmaswithpartiallycrystallizedanorthosite in apluton thatconsistsofthreemainlayers,eachofwhichareanor-thositicat thebaseandgabbroicat the top.Fraction-ationissuggestedbyupwardincreasesinincompatibleelements (Lindsley et al. 2010).According to thisinterpretation, at least three differentmagmaswereinvolvedintheproductionoftheChugwateranorthositebody:ananorthosite-producingmagma,andatleasttwoleucotroctoliticmagmas.

TheChugwater anorthosite includes sampleswiththemost stronglypositive initialNdof any samples,regardless of rock type, anywhere in the Laramieanorthositecomplex.BecausecrustalassimilationwillresultinlessradiogenicinitialNd(lowervalues),theparentmagma forChugwater anorthositemust havehad initialNd of +2.4 or higher.The samplewithinitialNdof+2.4(BM137)istheanorthositefromtheloweststratigraphiclevelintheChugwateranorthosite.Theanorthositesample fromthehigheststratigraphiclevel (KM13) has themost strongly negative initialNdof–1.3,andsamplesfromintermediatelevelshaveintermediateinitialNdvalues.ThisrangeofinitialNdsuggeststhatcrustalassimilationoccurredatthelevelof emplacement, not duringmagma ascent, and that

Fig. 4. Mg# versus normativeAn for anorthosites of theSnowCreekanorthositecomparedtofieldsfordatafromChugwater (Lindsley et al. 2010) and PoeMountainanorthosites (Scoates 1994).ALZ: anorthositic layeredzone,LLZ:leucogabbroiclayeredzone.

Fig. 5. Chondrite-normalized rare-earth-element plots for thewestern and eastern SnowCreek anorthosite.The lowerabundancesofrare-earthelementsandmorepronouncedEuanomaliesinthewesternSnowCreeksamplesareinterpretedtoreflectmoreefficientexpulsionoftrappedliquidfromtheeasternSnowCreekanorthositelateinitscrystallizationhistory.Chondrite-normalizedvaluesfromWakitaet al.(1971).


magmasintheupperportionsofthebodynearesttheroofrockswerethemoststronglycontaminated.

The gabbroic and troctolitic anorthosites vary ininitialNdfrom+0.9 to0.1.Again,assuming that theparentmagma becomes less radiogenicwith crustalassimilation, then the parent troctoliticmagma hadinitialNdof+0.9orhigher.ThisvalueisslightlylowerthantheleastcontaminatedoftheLAChigh-Algabbros(+2.0;Mitchellet al.1995).Thehigh-Algabbroswereinterpretedasmantle-derivedmagmasparentaltoLACanorthosite, and these least-contaminatedsamplesarefrom the southernLAC.The high-Al gabbros havea lower initial 87Sr/86Sr value than the gabbroic andtroctoliticanorthositesoftheChugwateranorthosite.

The sample of undivided anorthosite located eastof severalLaramide faults (Fig. 1) consists ofwell-laminated,medium-grained (crystals 2–3 cm across,2–3mmthick)tabularanorthosite.WeanalyzedtheNdandSr isotopiccompositionsof this sample (95DR2)becauseitcloselyresemblesKingMountainAnortho-site,themostdistinctiveunitoftheChugwateranortho-site.ItsrelativelyhighinitialNdof+0.7anditsinitialSrisotopicratioof0.70435placeitwithinthefieldofChugwateranorthosites.Inaddition,atroctoliteandagabbrosamplefromthisarea,95IG2and3,alsohavesimilarSrandNdisotopiccharacteristicstoChugwateranorthosite (initial 87Sr/86SrandNdequal to0.70443and1.43,and0.70421and0.11,respectively;Frostet al. 2001). If this portion of theLaramie anorthositecomplex is part of theChugwater anorthosite, thenLaramide faulting has translated the eastern portionsoftheanorthositearea15kmormoretothenorth,orthissampleispartofwhatwasoriginallyamuchmoreextensiveChugwaterintrusion.

SampleGM146isaninclusionofwhite,fine-grainedanorthositeinChugwateranorthosite.Itandanumberof other inclusions found in the central Chugwateranorthosite and along the contactwith theMaloinRanchpluton are characterizedbyplagioclasewith ahigheranorthitecontentandaregeochemicallydistinctinmanyotherways(Lindsleyet al.2010).Thehigherinitial 87Sr/86Sr value of this sample distinguishes itfromtheChugwateranorthositesamplesaswell.Thisinclusion is interpreted as a block of an older bodyof anorthosite that is present at depth beneath thesouthernLAC,inclusionsofwhichwerebroughtuptothepresentlevelofexposureasChugwateranorthositemagmasascendedthroughthisbody.Theolderanortho-sitecouldbeasoldas1.76Ga,whentheHorseCreekanorthositewasemplacedapproximately5kmsouthofthesouthernlimitoftheChugwateranorthosite(Scoates&Chamberlain1997,Frostet al.2000).

The Snow Creek anorthosite

TherearenineNdandSrisotopicanalysesofrocksfromtheSnowCreekanorthosite(Table4),includingsome previously published analyses of samples that

are now recognized as part of theSnowCreek anor-thosite.Like theChugwater anorthosite, the rangeofinitial 87Sr/86Sr ratios of theSnowCreek anorthositeis relatively narrow,whereas the variation in initialNdislarge.However,theChugwaterandSnowCreekanorthositeshavedistinctSrandNdisotopiccomposi-tions.TheSnowCreekanorthositehasmoreradiogenic87Sr/86Sr values, 0.7051–0.7056, andmore negativeNd,from–1.3to–4.7.Theseisotopiccompositionsareconsistentwithalargeramountofcrustalcontaminationin theSnowCreekanorthosite than in theChugwateranorthosite.Alternatively (andmore likely), agreaterproportionofArcheancontaminantmayhaveaffectedtheSnowCreek anorthosite,whereas theChugwateranorthositemayhavepassedthroughprimarilyProtero-zoiccrust,whichwouldhaveresultedinlessextremeNdandSrisotopiccompositions.

Ferrodioritic rocks

Included onTable 4 are three additional analysesof ferrodioritic rocks collected from the vicinity oftheSybille–Poeboundary toaugment theanalysesofMitchellet al. (1996).These samples are among theferrodioriteswiththehighestinitial87Sr/86Srratios,andtheyhaverelatively lowinitialNd.This isconsistentwiththegeographiccorrelationobservedbyMitchellet al.(1996),wherebytheferrodioriteslocatedclosesttoArcheanoutcropsalong thenorthernboundaryof theLAChavethehighestinitial87Sr/86Srvalues.Mitchellet al. interpreted this as reflecting a greater amountofArchean assimilant inmagmas intruded farthernorth.Theisotopiccompositionsofthethreesamplespresented inTable 4 imply a larger degreeof crustal

Fig. 6. Initial Sr andNd isotopic compositions of LACanorthosites, calculated for 1435Ma. Squares:maficrocks associatedwith the PoeMountain anorthosite.OpendiamondissampleGM146,ablockofmorecalcicanorthositefoundintheChugwaterpluton.Datafromthisstudy,Scoates&Frost(1996),andMitchellet al.(1995).


assimilationthantheSybillemonzosyenitetheyintrude.Therefore, these particular ferrodiorites cannot beparentalmeltstoSybille,butmayrepresentadifferentgenerationofferrodioriteformedbyasimilarprocess.

Discussion

Evidence for crustal assimilation in anorthosite plutons of the LAC

ThePoeMountain,Chugwater, andSnowCreekanorthositeplutonswereemplacedinclosesuccession.

TheChugwater anorthosite ismarginally the oldest,emplaced at 1435.5Ma, closely followedby thePoeMountainanorthosite.TheSnowCreekisconstrainedtobetheyoungestintrusionofanorthositeonthebasisoffieldrelations,andmustbeolder thanthe≤1432.8Ma cross-cuttingmonzodiorite dike. Geochemicalevidencefor interactionofSnowCreekmagmaswithresidualliquidremaininginthePoeMountainanortho-siteplutondescribedaboveisfurtherevidencethattheplutonswereemplacedclose together in timeaswellasinspace.Domingandhigh-temperaturedeformationaffectedallthreeplutons,evidentlywhiletherewasstill


residualmeltpresent.Thecontinuityofthelayeringthatiswell-developedinthePoeMountainandChugwateranorthosites(Fig.1;Scoates1994,Lindsleyet al.2010)suggeststhatthedomingaffectedbothoftheseplutonstogether.Whether this deformation tookplace duringor after emplacement of theSnowCreek anorthositeislesscertainbecauseoftherelativelypoorexposuresof this youngest anorthosite pluton. In any case, thethree plutons intruded each otherwithin a relativelyshortperiodofapproximately1to5millionyears.Thetectonicenvironmentandconditionsofmagmagenera-tioninthislocationwereunlikelytochangeinthisshorttimeinterval.Itthereforeseemslikelythattheseplutonssharedacommonparentalmagma.

Despite their nearly simultaneous emplacement,eachoftheLACanorthositeplutonsdisplaysadistinc-tivemineralassemblage.ThePoeMountainanorthositeis characterizedbyolivine, augite, lowCa-pyroxene,ilmenite andmagnetite, and plagioclase in the rangeAn43–53.TheChugwateranorthositegenerallycontainsnoolivine,butiscomposedofaugite,lowCa-pyroxene,ilmenite andmagnetite, and plagioclase in the rangeAn50–56; it locally contains quartz.The SnowCreekanorthosite is characterized by a lack of olivine andmagnetite and the presence of iridescent plagioclasewithAn47 toAn56. It is commonly contains quartz,particularlyinthewesternportion.

Isotopicevidencesuggeststhatdifferencesinminer-alogybetween the plutonsmaybe related to varyingamountsofcrustalassimilation.Twolinesofevidencesuggest that the quartz-bearing SnowCreek plutonassimilated themost strongly felsic crust. First, theSnowCreekanorthositeistheonlyplutonthatexhibitsaninheritedcomponentinitszircon.Fourfractionsofzirconyielded207Pb/206Pbagesfrom1436.7to1439.2(Fig.2e),allofwhichexceedthemaximumageoftheplutonof1434.4±0.4MarepresentedbytheageofthePoeMountainanorthositeitintrudes.Second,theinitialNdarethemoststronglynegativeofthethreeplutons(Fig.6),againsuggestingassimilationofPrecambriancrust.TheSnowCreekanorthositeliesalongthetraceofthesouth-dippingsuturebetweenArcheanandProtero-zoiccrust;hence,thecrustalassimilantisconstrainedtobealmostentirelyArcheaninage.

ThePoeMountainanorthosite,whichliesnorthofthe trace of the south-dippingArchean–Proterozoicsuture, has less strongly negative initialNd than theSnowCreekanorthosite(Fig.6).AsonlyArcheancrustwas available as an assimilant to the PoeMountainanorthosite, itmusthaveexperienced lesscontamina-tion than the SnowCreek anorthosite. Simple bulk-assimilationmodelssuggestnomorethan3%Archeancomponent in PoeMountain anorthosites comparedtoupto10%inSnowCreek(Scoates&Frost1996).Thepresenceof olivine inPoeMountain anorthositeisconsistentwiththislesserextentofcontamination.

TheChugwater anorthosite intrudes Proterozoiccrust at the current level of exposure, andArchean

crustispresentatdepth.ItsmoreradiogenicinitialNdcomparedtotheotherplutons(Fig.6)probablyreflectstheassimilationofProterozoiccrustwithlessstronglynegativeNdthantheArcheanrocks.ThisinterpretationisconsistentwiththerelativelyhighactivityofsilicaintheChugwateranorthosite (0.7 to1.0;Lindsleyet al.2010);contaminationofamantle-derivedmagmawouldraisethesilicaactivityabovethatforolivinesaturation(0.67–0.70;Lindsleyet al.2010).

The relationship of oxygen fugacity and silica activity in anorthosite plutons

The effects of crustal assimilation in the anortho-sitic plutons of theLACare reflected in themineralassemblages of these rocks.Thismaybe understoodfrom silica-dependent equilibria relating the relationsamongFe–Tioxides,orthopyroxene,olivine,andquartz(Table 5).The first five of the reactions inTable 5areQUIlF-related reactions tabulated byLindsley&Frost (1992).The secondfive are very similar to thefirst five; in fact, somehave the same stoichiometryas a QUIlF-related equilibrium.We have writtenthemasseparatereactionstoemphasizethat inthese,unlike theQUIlF-relatedequilibria, silica is amobilecomponent.We showhow these reactions are relatedin Figure 7, as calculatedwith theQUILF programofAndersen et al. (1993). The stability of Opx +Mgt+Ilmonthisdiagramisboundedat lowoxygenfugacitiesby the stabilityofolivine (OPUIlO)andathigh f(O2)bythestabilityofquartz(QUIlOp).Inour


calculations,weassumeda temperatureof1000°C,apressureof3000bars,andsaturationoforthopyroxenewith a calcic phase.Wechose a compositional rangeatmoderatevaluesofmFeMgwhereolivine+quartzdonot coexist, because this is the range represented bymost anorthosites.On this figure,we do not specifywhere pigeonite is stable relative to orthopyroxene+augite for two reasons. First, the difference betweenthe pigeonite-saturated and orthopyroxene-saturatedcurvesisminimal(seeLindsley&Frost1992).Second,pigeonitestabilityisclearlyafunctionoftemperature,andthereforeitisreasonabletoexpectthatsomeplutonswill contain pigeonite,whereas other plutonswithsimilarbulk-compositionsmayhaveorthopyroxene.Itisimportanttorecognizethatthisfigure(andotherslikeitinthetext)isbestusedasatopology,ratherthanaphasediagram.ThisisbecauseitisunlikelythatrockscrystallizinganorthopyroxenewithXMgrangingfrom0.2 to 0.7would crystallize at the same temperature.Whereas1000°Cmaybeareasonabletemperatureforthe iron-richrocks,a temperaturecloser to1100°Cis

likely for themagnesian end.Most of the equilibriaonthefigure,especiallyOpUIlOandQUIlOpslidetomoreoxidizingconditionswithdecreasingtemperature;thus any parameter, such as the relative fugacity ofoxygeninarockcontainingafixedorthopyroxeneorilmenitecompositionwillbevalidonlyforthepressureand temperature atwhich the figurewas calculated.Despitethisshortcoming,thetopologicalrelationswedescribe below that are inferred from the figure arevalid,regardlessofthetemperatureofequilibrationfortherocksinvolved.

Itiswellknownthatolivineisasinkforsilicaand,aslongasolivineispresentinamagma,silicaactivitywillbekeptfixedby reaction (1) (Table5). It is lessobviousthatinarockcontainingFe–Tioxideminerals,oxygenfugacityisdirectlyrelatedtosilicaactivity.Thisrelationshipisclearfromthestoichiometryofreactions(2)and(3),butitisnotsimpletoquantifythisrelation-ship, however, becauseoxygen fugacity is a functionofmFeMg–1 in the silicates andmFeTi–1 in theoxideminerals, bothofwhich are sensitive to variations insilicaactivity.

As longasolivine ispresent in themagma, silicawillbeconsumedbyreaction(1)toproduceorthopy-roxene.However,onceolivinehasbeendepleted,silicawillstillbeconsumedbyreactions(2),(3),(4)and(5).Thestoichiometryof reactions (4)and(5),whichareclosedwithrespecttooxygen,indicatesthateachmoleofmagnetite intherockhasthepotential toconsumeonemole of silica.To quantify how this affects thecompositionofthesilicates,letusconsiderarockwiththe assemblageOpx–mgt–ilm and trace amounts ofolivine (i.e., sittingon theOpUIlObuffer,Fig.7).At1100°Cand3kilobars,arockwithXFe

Opx=0.50willcoexistwithilmenitewithXHem=0.131andmagnetitewithXUsp=0.549 (according to theQUILFprogramofAndersenet al.1993).Anincreaseinsilicaactivitywilldrive therocktowardtheQUIlOpsurface.If themagnetiteintherockislessthan2%ofthevolumeoftheorthopyroxeneintherock,theXFeofthesilicatewillnotbeaffectedbythereaction(ifMgt/Opx=0.02,theorthopyroxenewilldecreasefromXMg=0.50toXMg=0.496),andthereactionpathwillfollowtheblackarrowinFigure7.For larger proportionsofmagnetite overorthopyroxene, thefinalorthopyroxenewillbe corre-spondinglymoreFe-enriched,butevenifthemagnetite:orthopyroxenevolumeratiois0.15,theresultingXFeofthe orthopyroxenewhenmagnetite is consumedwillonlychangefrom0.50to0.475(grayarrowonFig.7).Thisoxidationeventcoulddepletemagnetitefromtherock and cause increases in theFe2O3 component ofilmenite.ForrockswitharelativelyMg-richorthopy-roxene(forFig.7,thiswouldbearoundXFe

Opx<0.45),the ilmenite coexistingwithquartzwill containmorethan20%oftheFe2O3componentandshouldexsolvehematiteoncooling.

Althoughthedetailswillbedifferentdependingonthe temperature and startingXFe

Opx used, this simple

Fig. 7. Dlog f(O2) (relative toFMQ)versus fictiveXFeOpx

diagram showing relations amongmagnetite–ilmenite,pyroxene, olivine, and quartz calculated at 1000°C and3 kilobars.QUIlOp: quartz – ulvöspinel – ilmenite –orthopyroxene.TheheavylinelabeledQUIlOpindicatesthe locus of saturation in quartz, dashed lines give thelocationoftheQUIlOpcurveatlowersilicaactivitiesof0.9,0.8,and0.7.AlsoshownaretwocontoursforilmenitecompositionshowinghowilmenitebecomesricherintheFe2O3 component as oxygen fugacity increases.Darkarrow shows path followed during the assimilation ofsilicabyarockwitholivine+orthopyroxene+magnetite+ ilmenite inwhichpyroxene is farmoreabundant thanmagnetite.Gray line shows trajectory followed if themodal abundance of magnetite or pyroxene is 0.20.Calculations from theQUILF program ofAnderson et al.(1993).


model leads us to predict four important conclusionsforasystemwhereamaficrockreactswithasilica-richassimilant:

1)Oxygenfugacitywillincrease.TheextentofthisincreasewillbedependentonXFeoftheoriginalsilicateminerals.Relativelymoremagnesianrockswillbecomemoreintenselyoxidized.Thisisbecauseatalltempera-turestheOpUIlOandQUIlOpcurvesgetprogressivelycloser in f(O2)with increasingXFe

Opx.Rocksreactingat lower temperatureswillbemore stronglyoxidizedbythisprocessbecause,asnotedabove,OpUIlOandQUIlOpslidetohigherfugacitiesofoxygenwithlowertemperatures.

2)Magnetitewill be consumed, and ilmenitewillbecomemoreabundant.

3)IlmenitewillbecomeenrichedinFe2O3.Forrockswith originalXFe of orthopyroxene less than around0.40–0.45, the ilmenitemay become rich enough inthehematitecomponenttoexsolvehematiteoncooling(i.e.,XHem = 0.20).The relatively lowXHem in theilmenite fromSnowCreek is probably an indicationthat the rockswerenotveryclose to silica saturationorthattheassimilationtookplaceattemperatureswellabove1000°C.

4)OrthopyroxenewillbecomeenrichedinFe2Si2O6,but onlymarginally so unless themagma initiallycontained very large proportions of magnetite toorthopyroxene.

The oxygen fugacity and silica activity in LAC anorthosite plutons

WecanusetheserelationstounderstandthedifferentoxygenfugacitiesrecordedbytheanorthositicplutonsintheLAC(Fig.8a).ThePoeMountainanorthositeisolivine-saturatedthroughoutandliesupontheOpUIlOsurface.BecauseitliesontheOpUIlOsurface,theiron-enrichmenttrendassociatedwithdifferentiationcausedthePoeMountain anorthosite to lie at progressivelylower relative oxygen fugacity (Fig. 8A) and higherrelative activity of silica (Fig. 8B).TheChugwateranorthosite is dominated by the assemblageOpx (orPig)–Aug–Mgt–Ilm,andthusliesatoxygenfugacitiesbetween those of theOpUIlO andQUIlOp surfaces.Olivine only is present in the relativelymagnesiantroctoliticintrusions(whichalsocontainminorortho-pyroxene),andquartz ispresent in themost iron-richrocks.We show the trajectory to lie as a band thatrunsfromslightlybelowOpUIlOforthetroctolitestoQUIlOpatthemostiron-richconditions.

The Snow Creek anorthosite is dominated bythe assemblageOpx (or Pig)–Aug–Ilm.Someof thesamples are quartz-bearing, but the quartz does nothave amagmatic texture, indicating that the originalmagmawas near but not quite at quartz saturation.Theabsenceofmagnetiteindicatesthattherockswererelatively oxidized, but the lowXHem in the ilmenitesuggests that theywerenotnear theQUIlOpsurface.

We show thefield for theSnowCreekanorthosite indashedlinesinFigure8becausetheonlylowerlimitsforoxygenfugacitythatwecanbecertainofwouldbeolivinesaturation.

Because silica activity and oxygen fugacity arecovariants,wecandisplaythesameinformationonaplotofsilicaactivityversusXFe

Opx(Fig.8B).BecausethePoeMountainanorthositeisolivine-saturated,iron-enrichmentduringdifferentiationcausedsilicaactivityto increase.An increase in silica activity is also seenin theChugwateranorthosite,butbecause itdoesnotcontainolivine,thistrendwasnotinternallycontrolledand probably reflects crustal contamination in themostFe-richportionsoftheanorthosite,whicharetheoutermostportionsoftheintrusion.Wedonotseeanyclear trend in silica activitywith respect to pyroxenecomposition in theSnowCreek anorthosite; as notedabove,theonlyobvioustrendisthefactthatthewesternbodyisquartz-saturated,whereastheeasternoneisnot.

Oxygen fugacity and silica activity in Canadian anorthosites

The anorthosites of the Nain Plutonic Suite,Labrador,showsimilartrendstothoseoftheLAC.ThetroctoliticKiglapait intrusion contains orthopyroxeneonly as a rim on olivine.The Fe–Ti oxidemineralsappear at a point atwhich olivine is Fo60,which isequivalenttoafictiveXFe

Opx=0.332).OnFigure8C,weshowittolieclosetobutslightlybelowtheOpUIlOsurface.Many anorthositic rocks fromHarp Lake,Labrador(Emslie1980), like thePoeMountainanor-thosites, are olivine-saturated and lie on theOpUIlOsurface.Magmas that crystallized both theKiglapaitintrusionandHarpLakeanorthositesunderwentrelativereduction(Fig.8C)andincreasesinsilicaactivity(Fig.8D)duringdifferentiation.TheNainanorthositeinthePuttuaalukLakearealacksolivine(Ranson1981)andhence lies between theOpUIlO andQUIlOp curves.Ranson (1981) reported a tight cluster of pyroxenecompositionsforanorthositeswithoneoutlier.Forthisreason,we show twofields for the PuttuaalukLakeareainFigures8Cand8D.Theheavystipplingisthefieldwheremostofthepyroxenecompositionsfall;thelightstipplingextendsthefieldtoencompasstheoutlier.Ranson (1981) reported that some samples from thePuttuaalukLakeareaarequartz-saturatedandthattheFe–Tioxideassemblage is ilmenite±magnetite. It ispossiblethat,liketheChugwateranorthosite,theanor-thositesofthePuttuaalukLakeregionhaveassemblagesthat recordadistinct trajectory inoxygen fugacityorsilica activity, but that information cannotbederivedfromtheinformationprovidedinRanson(1981).

Finally, the Labrieville anorthosite is a classicexampleofanandesineanorthosite.Ithastheassem-blage orthopyroxene – augite – ilmenite (Owens&Dymek2001).Mostofthesamplesarequartz-saturated,andsmallamountsofmagnetitearepresentintheleuco-


gabbros.ItliesonorneartheQUIlOpsurface(Fig.8C),but unlike the quartz-saturated rocksofSnowCreek,theLabrieville anorthosite contains exsolved ferrianilmenite (“hemo-ilmenite”).Wemaintain that this isbecausemostof theLabrievilleanorthosite isconsid-erablymoremagnesian than theSnowCreek,whichmeansthatincreasesinsilicaactivitywouldhavedriventheLabrievilletomuchhigherfugacityofoxygenthanthemore iron-rich rocks of SnowCreek [Note thatcontrarytotheconventionofOwens&Dymek(2001),theLabrievilleanorthositeisnotanalkalicanorthosite.AsintroducedbyPeacock(1931),thetermalkalichasa

distinctpetrologicdefinition.Asdefinedbythemodifiedalkali–limeindexofFrostet al.(2001),theLabrievilleanorthosite,ifanything,isalkali–calcic.]

SummaryandConclusions

In this study,wehave shown that the three anor-thositic plutons of theLaramieAnorthositeComplexcontainassemblagesthatrecordarangeofsilicaactivi-tiesandoxygenfugacities,andthatthisrangeislikelyaproductofvariableamountsofcrustalassimilationofmantle-derivedmagmas.Becausetheparentalmagmas

Fig.8. A)Dlogf(O2)(relativetoFMQ)versusfictiveXFeOpxdiagramandB)silicaactivityversusfictiveXFe

OpxshowingtheconditionsofcrystallizationofanorthositicplutonsintheLaramieAnorthositeComplex.DatafromFrost&Lindsley(1992),Lindsleyet al.(2010),andthisstudy.C)Dlogf(O2)versusfictiveXFe

OpxdiagramandD)silicaactivityversusfictiveXFeOpx

showingthecrystallizationconditionsoftheNainplutonicsuiteandtheLabrievilleanorthosite.DatafromEmslie(1980),Ranson(1981),andOwens&Dymek(2005).


to theLACplutonsformedinaverynarrowwindowofspaceandtime,wesuggest that thesevariationsinsilicaactivityandoxygenfugacityarenotprimarilyafunctionofdifferences in thecompositionofparentalmeltsderivedfromthemantle-sourceregion.Instead,wehave shownhow the addition of silica by crustalcontamination produces both the variations in silicaactivityandoxygen fugacity in theLAC.Theoppor-tunities for crustal assimilation prior, during, andfollowingdifferentiationofanorthositicintrusionsareshown schematically onFigure 9.Because gabbroicandtroctoliticmagmasfromtheLACdidnotpondanddifferentiateatdepth,theyexperiencedlessinteractionwith crust than the anorthositic plutons. In contrast,theSnowCreek anorthosite, themost oxidizing andsilica-richoftheanorthositeplutons,alsohasNdandSrisotopiccompositionsindicatingincorporationofthegreatestamountofacrustalcomponent.Byextension,thehypothesisthatsilicaactivityandoxygenfugacity,and the resultingmineral assemblages in cumulateanorthosites, are controlled by assimilation of conti-nental crust alsomustbe considered forothermassifanorthosites. Isotopic identification of assimilation,especially based on theSr andNd isotopic systems,can be difficultwhere anorthosite intrudes juvenilecrust thatwas extracted from themantle a few tensto hundreds ofmillions of years prior to anorthositemagmatism(e.g.,intheGrenvilleprovince);however,themineral assemblages of Proterozoic anorthositesareespeciallysensitivetochangesinextentofcrustalassimilation irrespective of the ageof the underlyingandenclosingcrust.

Themajor-elementchemistryofanorthositesrepre-sents a poor record of crustal assimilation, becauseaddition of a graniticmelt to a crystallizing plutondominatedbyfeldsparwillnotgreatlyaffecttheweightpercent of themajor oxides. For this reason,Morse(2006) could successfully invert themajor-elementcompositionsofplagioclasecrystalsfromNainanortho-sitesandidentifyanolivine-normativeparentmagmatothetroctoliticrocksandasilica-saturatedparentmagmaforthenoriticsuite.ExperimentaldataallowedMorse(2006)torelatebothsuitestoacotecticparentmagma.Isotopic results from theNainPlutonicSuite indicatethatthetroctoliticrocksunderwentcomparativelylittlecrustal contamination upon ascent, but the anortho-sitic rocks assimilated a greater volume ofwallrockduringtheirresidencetimeinthelowercrust(Emslieet al.1994).Incontrast,thetrace-elementbudgetofarockdominatedbyplagioclasemaybemorestronglyaffectedbyinputfromafelsicassimilant.Wecautionthattheseeffectsmustbeexplicitlyquantifiedandtakeninto account to obtain accuratemodels based uponinversionofthetrace-elementcontentsofanorthosites(e.g.,Bédard2001).Becauseoftheircumulatenatureandmultistagepetrogenesis,describingtheoriginandevolutionofmassifanorthositesandthenatureof theparentmagma(s) is challenging.Nevertheless, thisstudy has shown how crustal assimilation stronglyaffectsbothsilicaactivityandoxygenfugacityof theparentmagmastoanorthositicrocksandhowarangeofmineralassemblagesandSr–Ndisotopiccompositionscanbe produced frommagmas that share a commonmantle-derivedlineage.

Fig. 9. Schematic diagram showing the opportunities for assimilation of crust prior,during,andfollowingdifferentiationofanorthositeintrusions.Becauseolivinegabbrosandtroctolitesdonotpondanddifferentiateatdepthinthecrust,theymayexperiencelessinteractionwithcrustduringascent.


Acknowledgements

ThisresearchwassupportedbygrantsEAR–9017465andEAR–9218360 toB.R.FrostandC.D.Frost,andEAR–9218329and itspredecessors toD.H.Lindsley.Wearepleased todedicate this paper to thememoryofRonEmslie,whose comprehensive studyof anor-thosites and related rocks, includingfield, petrologic,geochemicaland isotopicaspects,was the inspirationforustodolikewise.

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Received December 18, 2008, revised manuscript accepted July 4, 2010.

geochemical and isotopic evolution of the anorthositic plutons

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