887 The Canadian Mineralogist Vol. 48, pp. 887-923 (2010) DOI : 10.3749/canmin.48.4.887 PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING Donald H. LINDSLEY § Department of Geosciences and Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100, USA B. Ronald FROST and Carol D. FROST Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA James S. SCOATES Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Abstract The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa, on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north (in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush. Keywords: anorthosite, gabbroic anorthosite, leucotroctolite, plagioclase, layering, magmatic stratigraphy, magma mixing, filter pressing, Chugwater pluton, Wyoming. Sommaire L’anorthosite de Chugwater est un des nombreux massifs intrusifs qui constituent le complexe anorthositique de Laramie, au Wyoming, mis en place il y a 1.43 milliard d’années. La partie du complexe de Chugwater se trouvant au sud-ouest du pluton contient une stratigraphie magmatique cartographiable sur une épaisseur d’environ au moins 8,000 mètres, et probablement même jusqu’à 10,000 m ou plus. Nous définissons trois unités majeures, chacune comprenant une portion inférieure à dominance anorthositique (>90% plagioclase par volume) et une unité supérieure surtout à anorthosite gabbroïque (80–90% plagioclase). Chaque portion anorthositique contient des couches d’anorthosite gabbroïque et vice versa, dont l’échelle s’étend jusqu’au centimètre. Le plagioclase se rapproche de An 50–55 , quoique des valeurs plus élevées et plus faibles sont aussi présentes. Cette “série principale” du massif de Chugwater est en général dépourvue d’olivine modale et normative. La fugacité de l’oxygène a varié entre le tampon FMQ et FMQ + 0.5. Nous interprétons cette “série principale” comme résultat de trois venues majeures § E-mail address: [email protected]
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887
The Canadian MineralogistVol.48,pp.887-923(2010)DOI:10.3749/canmin.48.4.887
PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING
DonaldH.LINDSLEY§
Department of Geosciences and Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100, USA
B.RonaldFROSTandCarolD.FROST
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
TheChugwaterAnorthosite is oneof several 1.43Ga intrusions thatmakeup theLaramieAnorthositeComplex in theLaramieMountains,Wyoming,USA.The southwestern portion of theChugwaterAnorthosite has amappablemagmaticstratigraphy totaling at least 8000meters, andprobably 10,000mormore. It consists of threemajor units, eachofwhichcomprisesalower,dominantlyanorthositeportion(>90vol.%plagioclase)andanupper,dominantlygabbroicanorthositesection(80–90vol.%plagioclase).Eachanorthositeportioncontainslayersofgabbroicanorthositeandvice versa,onavarietyofscalesrangingdowntocentimeters.PlagioclaseisdominantlyAn50–55,althoughhigherandlowervaluesarealsofound.This“mainseries”oftheChugwaterAnorthositemainlylacksmodalandnormativeolivine.OxygenfugacityrangedfromFMQtoFMQ+0.5.Weinterpretthatthe“mainseries”wasproducedbyatleastthreemajorinjectionsofmildlyhyperfeldspathicmagmacontainingapproximately40%entrainedplagioclasemegacrysts(tabularcrystalsatleast5cmacross).TheTicontentsofthemegacrysts(0.3–0.4wt%TiO2)suggestthatthesehadcrystallizedatpressuresnear10kbar,consistentwithinitialformationinamagmachamberatornearthebaseofthecrust.Emplacementpressureispoorlyconstrained,butappearstohavebeennear3.5–4kbar.However,beforethemainserieshadcompletelysolidified,itwasrepeatedlyintrudedbyatleasttwo(andprobablymore) leucotroctoliticmagmas.Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse againstgabbroicanorthosite,suggestingthatsufficientresidualmeltremainedinthelattertopermitlocalmixingwithleucotroctolite.Weestimatethat80–85%oftheChugwaterAnorthositeismain-series,10–15%ismixedrock,and5%isleucotroctolite.Priortofinalsolidification,theentireChugwaterAnorthositewasdomed,probablyasaresultofgravitationalinstabilityoftherelativelybuoyantplagioclase-richmaterial.TheemplacementoftheChugwaterAnorthositeasaseriesofcrystal-richmagmas,followedbycontinuedfractionation inamagmachamberatamid-crustal levelandsubsequentdoming,arecharacteristics thatmaketheChugwaterAnorthositeintermediatebetweenthePoeMountainAnorthositetothenorth(in situfractionationinamagmachamber)andtheclassicdiapiricemplacementofacrystal-richmush.
thatbehavedas“drop-stones”ontheunderlyingsemi-consolidatedcumulates,ofhaving formed inanopenmagmachamber.Assuch,itappearstobeintermediatebetweenthelayered-intrusionnatureofthePoeMoun-tainAnorthositeandthediapiricoriginofmanyothermassif anorthosites (e.g.,Barnichonet al. 1999).Assuggested byWiebe (1992, p. 225), there appears tobeacontinuumofemplacementmodesforProterozoicanorthosites.
GeologicalSettingandPreviousWork
The ChugwaterAnorthosite is one of severalplagioclase-rich intrusions in the southern LaramieMountainsthat,togetherwithanumberofgraniticandmonzoniticbodies,makeuptheyoungestPrecambrianrocks of southeasternWyoming. These intrusionsstraddle theCheyenneBelt, cuttingArchean rocks tothenorthandnorthwest(Fig.1)andProterozoicrocksto the south. Elsewhere, their contacts aremainlycoveredbyPhanerozoicsediments.Dartonet al.(1910)were thefirst to report anorthositic rocks in the area,althoughthepresenceof thecloselyassociatedFe–Tioxideoredepositshadbeenknownsince theHaydenSurveyof1871.Fowler(1930a,1930b)madethefirstdetailedstudyoftheanorthosites.Newhouse&Hagner(1957)mapped the area at a scaleofonemile to theinch(1:63,360)andidentifiedanorthernandasmallersouthern anorthositicmass.They suggested that thenorthern anorthosite forms a doubly-plunging anti-cline,withan“anticlinalaxis”orientedapproximatelynorth–south.Theyreportedthatmostofthemajoroxidedepositsoccuronornearthisaxis.ThenameLaramieAnorthositeComplex (LAC)wasfirst applied to theentire area byHodge et al. (1973).WhereasFowler(1930a, 1930b) recognized themagmatic nature ofthese rocks, some laterworkers argued that they aremetasomatic (Newhouse&Hagner 1957).Klugman
Introduction
Proterozoicmassifanorthositeshavelongintriguedpetrologists, both for the enigmas posed by theirplagioclase-richnatureandforthedistinctivesuitesofrocksthatalmostinvariablyaccompanythem.Clearlytheyformedbyigneousprocesses,but,aspointedoutbyBowen (1917), they cannot have formed from amagmaof theirownbulkcomposition.Somemecha-nismmustconcentratehugevolumesofplagioclasetomakeanorthositespossible.During thepast40years,aremarkableconsensushasdevelopedonatwo-stagemodel for their origin (Morse 1968, Emslie 1985,Longhi&Ashwal1985,Emslieet al.1994):magma,probably basaltic, ponds anddifferentiates at or nearthebaseofthecrust,crystallizingolivineandpyroxene.Theseferromagnesianphasessink,leavingtheresidualmagmaenrichedinplagioclasecomponentsandinironrelativetomagnesium.Plagioclaseeventuallycrystal-lizesandfloatstothetopofthemagmachamber;thebuoyant plagioclasewith some trapped liquid thenascends diapirically into the crust. Finally,much ofthetrappedliquidisexpelledthroughavagueprocessknownas“filter-pressing”,andtheresultisanorthosite,anigneousrockcontainingmorethan90%plagioclase.
Although theMorse–Emslie–Longhi–Ashwalmodel described above successfully explainsmanymassifanorthosites,thereareexceptions.Scoates(1994,2000, Scoateset al. 2010) has shownunequivocallythat the PoeMountainAnorthosite of the LaramieAnorthositecomplexinsoutheasternWyomingcrystal-lizedmainlyin situwithinamid-crustal-levelmagmachamber.WedescribeheretheChugwaterAnorthosite,whichoutcropssouthofthePoeMountainAnorthosite,andwhichalsoshowsmagmaticlayeringandmagmaticstratigraphy. However, the ChugwaterAnorthositeshows abundant evidence of solid-state deformationand lacks clear evidence, such as scours or blocks
Fig. 1. Simplifiedgeologicalmapof theLaramieAnorthositeComplex, southeasternWyoming, showing the position of theChugwaterAnorthosite.Abbreviations:CR:Archean (north)andProterozoic (south)country rock,RMP:RedMountainPluton,Phan:Phanerozoic sediments, SnCA:SnowCreekAnorthosite, SCC:StrongCreekcomplex,GI:GreaserIntrusion,Ltr:leucotroctolite(olivineanorthositeofNewhouse&Hagner1957),CFA:CoyoteFlatsAnorthosite,MRP:MaloinRanchPluton.MajorLaramide faults are indicated.Dotted lines showmajor roads.Not labeled:ButtesGranite(immediatelywestofGI)andtwosmallbodiesofmonzosyenite(inSnCAandwestofChugwaterAnorthosite),withsimilaritiestotheSybillePluton.
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(1969)restatedthecaseformagmaticorigin,andwiththe exceptionofDeVore (1975),whoargued that theanorthositesarerefractoryresiduesafterpartialmeltingofmetasediments, subsequentworkers have acceptedtheir igneousnature.Subbarayudu(1975)andSubba-rayuduet al.(1975)conductedthefirstRb–Srisotopicstudies of theLAC,whereas Fountain et al. (1981)andGoldberg(1984)presentedgeochemicalresultsformonzoniticandanorthositicrocksinthenorthernpartoftheplutonicsuite.
Wehavebeenstudying theLACsince1981,withmostofourearliereffortsconcentratingonthegraniticandmonzoniticrocksofthearea(seereferencesinFrostet al.1993).Scoates(1994)conductedadetailedstudyof northernmost body of anorthosite (the “northerndome”ofFrostet al.1993),whichhenamedthePoeMountainAnorthosite.HeshowedthatthePoeMoun-tainAnorthositehasadistinctivemagmaticstratigraphyandconcludedthatithadmainlycrystallizedin situinamid-crustal-levelmagmachamber (Scoates&Frost1996, Scoates 2000, Scoates et al. 2010).This is incontrasttothewidelyacceptednotionthatmassifanor-thositesareemplacedasdiapiriccrystalmushes(e.g.,Emslie 1985,Longhi&Ashwal 1985,Ashwal 1993,Barnichonet al.1999).Scoates&Chamberlain(1995,1997) usedU–Pb ages from zircon and baddeleyiteto show that the southern anorthosite (the “southerndome”ofFrostet al.1993)isdistinctlyolderthanthenorthernanorthosites(1756Macomparedto1434–1436Ma).Accordingly,theynamedtheolderbodytheHorseCreekAnorthosite (see also Frost et al. 2000), andsuggestedthatthetermLaramieAnorthositeComplexberestrictedtotheyoungerrockstothenorth(seealsoFrostet al.2000).
ArealExtentandRelationshiptoOtherUnits
TheChugwaterAnorthositeisnamedforitsexpo-surealongthebanksofMiddleandSouthChugwatercreeks.Approximately 160 km2 in areal extent, itoutcropsinpartsoftheBaldyMountain,KingMoun-tain, GoatMountain, and Ragged TopMountain1:24,000USGSquadrangles. Itssoutherncontact liesmainlyon landclosed tous,but itappears to intrudeacomplexseriesofProterozoicsupracrustalrocksandpossiblymonzonitic rocks associatedwith theHorseCreekAnorthositetothesouth(Fig.2).Tothesoutheast,itiscutbythemonzonitesandgranitesoftheMaloinRanchPluton(Kolker&Lindsley1989). Its foliationisalsocutbythesmallCoyoteFlatsAnorthosite.Thesouthern portion of itswestmargin is cut by poorlyexposed rocks that are similar to theMaloinRanchPlutonandtheSybilleintrusion,whereasfarthernorth,alligneousrocksarecoveredbyPhanerozoicsediments.Tothenorthwest,theChugwaterAnorthositeiscutbytheSnowCreekAnorthosite(the“SnowCreekdome”ofFrostet al.1993)andbytheStrongCreekPluton.Bothcontactsarepoorlyexposed,butthereappearsto
havebeenconsiderableminglingbetweentheseunits,as indicated by abundant inclusions of ChugwaterAnorthositewithin theyoungerunits.Wesuspectbutcannotprovethattherewasalsoback-intrusionoftheyounger units into theChugwaterAnorthosite.Thenortherncontact isproblematic,asbothexposureandsampledensityarepoor;wetentativelyplaceitmainlyalongStrongCreek.The eastern contacts aremainlyLaramidefaultsalongwhichtheChugwaterAnorthositehasbeen thrustoverPhanerozoic sediments.There ispermissiveevidencefrompetrographyandPbisotopesthatatleastpartofthe“anorthositeundivided”eastofthethrustsinFigure1maybeChugwaterAnorthosite(Frostet al.2010).
Exposures, outcrop, and sampling
Outcropsof theChugwaterAnorthositearenotallone couldwish for.Although exposure is commonlyadequate for reconnaissancemapping, rarely canonetraceunitsindetailorfindclearcontacts.Weatheringhas been sufficiently intense that fresh rockoutcropsmainlyonlyinstreambeds,andthereitmaybeimpos-sibletosampleadequatelyexceptbydrilling.Mostofthesamplesreportedinthispaper,andnearlyallthoseforwhichwhole-rockcompositionsarereported,weresampled by drill.We sampled virtually every freshoutcropinthe~160km2areaoftheChugwaterAnortho-site;gapsinoursamplebasearenotforwantoftrying!
TheChugwaterAnorthosite is characterized bydistinctivelayering,bothmodalandtextural,foliationof tabularplagioclase,whichismainlyparallel to thelayering, presenceof tabular andmegacrystic plagio-clase,much ofwhich shows labradorescence and istypically deformed to varyingdegrees, and a distinc-tivemagmatic stratigraphy in itswestern portion. Itshares someof these characteristicswith eachof thenearbyPoeMountain andSnowCreek anorthosites,but isdistinctive in its totality.BothSnowCreekandChugwateranorthositeshaveabundant labradorescentplagioclase(~An52–57Or3–4);thePoeMountainAnortho-siteismainlytoosodictoshowthatfeature.BoththePoeMountain andChugwater anorthosites havebothtitaniferousmagnetiteandilmenite,whereastheSnowCreeklacksmagnetitebuthasilmenite,typicallywithinclusionsofrutile.
In this paper,we present field, petrographic, andgeochemicalevidencethatleadsustoconcludethattheChugwaterAnorthositeisalayeredfeldspathicintrusionthatwas emplaced at approximately10–12kmdepthbyinjectionofseveralmajorpulses(atleastthree)ofmagmaladenwithplagioclasecrystals(probably40%crystalsof~An52).Thesilicaactivityofthesemagmaswas high (0.7 to 1.0 relative to quartz), and thus themainChugwaterAnorthositeisolivine-free.Thismainserieswas repeatedly intrudedby, and locallymixedwith, at least two different leucotroctoliticmagmas,one having normativeAn very close to that of the
main-seriesanorthosite,theotherdistinctlymorecalcic(normativeAn58–64).Themixedrocksshownormativeandmodalolivine.Thereisanoverallupwardincreaseof incompatible trace elements, suggesting that therewas at least some communication of residualmeltsthroughouttheintrusion.
Layering,Stratigraphy,andStructure
Layering: size and morphology of plagioclase
By definition, anorthosites contain 90%ormoremodal plagioclase.Becausemany of the features of
the ChugwaterAnorthosite to be described reflectthenatureofitsplagioclase,itisusefultoprovideanoverview,with details to follow in the petrographysection. Somewhat arbitrarily,we call tabular grainswider than approximately 5 cmmegacrysts.Typicalmegacrystsrangeupto10cminwidth,butlargeronesoccuraswell.Outcropswithabundant,orientedmega-crystsglintbrightlyinsunlight,earningthefieldname“silver-dollaranorthosite”.Weargueinalatersectionthatthemegacrystsandsomebutnotallofthesmallertabularplagioclasewerepresent in themagmaswhentheChugwaterAnorthositewasemplaced.
Fig. 2. Geologicalmap of theChugwaterAnorthosite and surrounding rocks.Thickdashedlinesshowthestrikeof layeringandfoliation.Thecentralandsouthwesternportions of theChugwaterAnorthosite havewell-constrained stratigraphic relationsandarecalledthe“StratigraphicChugwaterAnorthosite”.Thatstratigraphyhasbeenconstructed across the linesA–A’ andA’–A”; thicknesses have not beenmeasureddirectly, but are estimated fromdip and horizontal extent. East of the “structuralboundary”, the ChugwaterAnorthosite has a distinctly different foliation, andstratigraphic positions cannot be assigned.Similarly, theChugwaterAnorthosite tothenortheastiscutbynumerousfaultsofLaramideage(onlythemostprominentareshown)andbygraniticdikes;accordinglyitwasnotpossibletoassignstratigraphicpositionstothisportionaswell.Thehigh-Caanorthositesarethelargestofnumerousinclusionsofhigh-Ananorthosite thatdecorate the“structuralboundary”within theChugwaterAnorthosite, aswell as parts of its boundary betweenwith theMaloinRanchPluton.Abbreviations:GI:Greaserintrusion,SnCA:easternportionofSnowCreekAnorthosite,SCC:StrongCreekComplex,CFA:CoyoteFlatsAnorthosite,Ltr:leucotroctolite intrusions (only the largestare shown), fran: fracturedanorthositeofNewhouse&Hagner (1957),Gr: granite, including theMuleCreek lobe (northernportion) andShermanGranite (southern portion).Locations ofmajor Fe–Ti oxidedepositsaregivenbyI:IronMountain,G:GoatMountain,S:Shanton.
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TheChugwaterAnorthositeislayeredonscalesthatrange fromcentimeters to kilometers.Much layeringreflects variations inmodal abundanceof plagioclaserelative to ferromagnesianminerals (Figs. 3a, b);typically the range is fromapproximately 85 to 96%plagioclase.Rarelycanlayersbetracedoverdistancesgreaterthan10–20timestheirthickness.Sometermi-nateabruptly,butmostdieoutgradually,asalong-strikevariations inmode diminish the difference betweenadjacentlayers.Inadditiontoanorthositeandgabbroicanorthosite, the ChugwaterAnorthosite containsnumerousbodiesoftroctoliteandleucotroctolite.Thelargerbodiescross-cuttheanorthositicunits,butsmallerones (dimensions from a fewmeters to several tensofmeters) typically lie parallel or subparallel to theregional layering.Boundaries tend tobe sharpwhereleucotroctolitecutsanorthosite,butmorediffuseagainstgabbroicanorthosite.
Lamination resulting from the subparallel align-mentoftabularandmegacrysticplagioclaseproducesafoliationthatisnearlyalwaysparalleltocompositionallayering.Althoughwehavenotmadeadetailedstudyin theChugwaterAnorthosite,we infer that there isno orientation of plagioclasewithin the foliation andthusnolineation,similartothefindingsofLafranceet al.(1996)forthePoeMountainAnorthosite.Texturallayeringisalsomainlyparalleltomodallayeringandplagioclase lamination, and can take several forms.Mostcommonlyit isexpressedbyvariationsingrainsize,usuallyofplagioclase,andthusmaycomplementmodal layering. But textural layering also reflectsacross-strike variations in the degree of deformationthroughgrain-boundarymigrationinplagioclase.Typi-cally,themostplagioclase-richlayersshowthegreatestdeformation,expressedby thepresenceofbent twinsandabundantneoblasticrecrystallizedplagioclase.Suchlayerstypicallyrangefromcentimeterstotensofmetersinthickness,andlikemodallayers,rarelycanbetracedfordistancesgreaterthan10–20timestheirthickness;manycontainangularrelicsofmegacrysts.
Stratigraphy
Wecallthewesternportionsthe“stratigraphicChug-waterAnorthosite”, because the largest-scale layers(Figs.2,4)defineadistinctivestratigraphyconsistingofthreedominantlyanorthositeunits(frombasetotop,An1,An2,An3)andthreeslightlymoremaficunitswith80–90%plagioclase.AccordingtoStreckeisen(1967),themoremafic units are leucogabbros.However, toemphasize their close affinitywith the true anortho-sites of theChugwaterAnorthosite,we here followBuddington(1939)andcallthemgabbroicanorthosites,atermalsoacceptedbyStreckeisen(1976).Anadvan-tage of this usage is that “anorthositic” can then beusedasageneraltermfortheentireseries.Wecallthemoremaficunitsthelower,middle,anduppergabbroicanorthosites (Gan1,Gan2,Gan3), respectively.Eachpair(e.g.,An1+Gan1)isconsideredamagmaticunit,so thethreepairscompriseUnits1,2,and3.CriticalsamplelocalitiesaregiveninFigure4.Weemphasizethateachof thesestratigraphicunitscontainscompo-sitionallayeringonsuccessivelyfinerscales,sothatitiscommontofindtrueanorthositewithineachof thegabbroicanorthositeunitsandvice versa.Acompositesection(A–A’andA’A’’inFig.2)totalsjustover10,000m (Fig. 5).We are confident regarding the top 8000mofsection; theneedtocorrelatenearly5kmalongstrike across possible offsets byLaramide-age faults,combinedwith gentler dips,make the lower 2000mofthesectionsomewhatlesscertain.Furthermore,thebaseforthesectionisarbitrarilychosennearthetraceofprominentLaramidefaults;moreanorthositeoutcropsto the east (i.e., below thebase chosen for the strati-graphicChugwaterAnorthosite),butacombinationof
poorexposure,late-magmaticalteration,andextensiveLaramide faulting in that area preclude stratigraphicassignmentinthisregion(Fig.2),soweexcludeitfromthestratigraphicChugwaterAnorthosite.Weemphasizethat the stratigraphic section has not beenmeasureddirectly,buthasbeenconstructedusingmapdistancesandaveragedipsineacharea.
UnitAn1(0–4750m;Fig.2)isthethickestandleastwelldefinedoftheunits.Inadditiontotheuncertaintiesincorrelationmentionedabove,itiscutbyseverallargeleucotroctolitebodies(Fig.6).UnitGan1(4750–5590m) locally contains the bestmodal layering on the10–100cmscale,butitsgenerallysubduedtopographyyields fewgood outcrops.UnitAn2 (5590–6240m)is the thinnestof themainunits; it standsoutclearlyon satellite imagesandas localhighson topographicmaps.Italsocontainsadistinctiveseriesofleucotroc-tolitepods,2to20macross,thatoccurthroughoutitsexposedlengthofnearly10km.Gan2(6240–8300m)is the thickest of the gabbroic anorthosite units.An3(8300–9520m),theuppermostanorthosite,isthemostdistinctiveof the sixunits: it consists almost entirelyofextremelywell-laminatedmedium-grained(typically
2–3 cmacross, 2–3mm thick) tabular anorthosite. Itranges in thickness from700meters in the southeastto 1220m in the vicinity ofKingMountain (section3,T17N,R72W).Megacrysts, although present, aremuchrarerthaninlowerunits.Similarmedium-grainedtabular anorthosite alsooccurs locally4000m lower,nearthetopofAn1,buttypicalthicknessesthereare1–5meters,andtheselayerscannotbetracedmorethan100malongstrike.Thetopmostunit,Gan3(9520–10,400m),isirregularinthickness,endingagainstcountryrockto thesouthandbeing truncatedbyapoorlyexposedmonzosyenite unit to the southwest.The sequencedescribedherestandsincontrasttothelayeredunitsofMichikamauIntrusion,Labrador(Emslie1970),whereanorthositeoverliesmoremaficlayers,arelationshiptobeexpectedifbuoyancyalonecontrolledtheformationoflayering.JohnLonghi(writtencommun.,2009)hassuggested the possibility that ourGan1mightmorelogically be pairedwithAn2, andGan2withAn3.Unfortunately, none of our data, field, petrographic,orgeochemical,permitsustotestthatidea.Weretainour field-based pairings in part by analogywith thenearbyPoeMountainAnorthosite,wheremoremafic
Fig.4. Sample-locationmapfortheChugwaterAnorthositeandnearbyrocks.Samplelocations not shownon thismap are indicated inFigures 1 and 6.To save space,quadrangle designators are abbreviated:B:BM (BaldyMountain),G:GM (GoatMountain),K:KM(KingMountain),R:RTM(RaggedTopMountain).
Thewell-developed layering and foliation of theChugwaterAnorthositemake it possible to defineseveralstructuralfeatures.ThestratigraphicChugwaterAnorthositeconsistentlydipstothewestandsouthwest,
withdipsgenerallyincreasingup-section;itcorrespondstothesouthwesternportionofthe“anticline”proposedbyNewhouse&Hagner(1957).Intheeasternportionsof the stratigraphic ChugwaterAnorthosite, we doin fact see somehintsof an “anticlinal axis” (centralportionofFig. 2).However, thedetails are consider-ablymore complicated.To thewest andnorth of theMaloinRanchPluton(thusclosetothe“anticlinalaxis”ofNewhouse&Hagner),thereareabruptdiscontinui-ties in structure between the stratigraphicChugwaterAnorthositeandverysimilaranorthosites lyingto theeastandsoutheast(Fig.2).Thediscontinuitiesinstrikeare evident on themap, but the dips are discordantaswell:most of the dips in the nearby stratigraphicChugwaterAnorthositeare30°orless,whereasthoseofthenorth–southstrikinganorthositestotheeastareall greater,withmost being at least 70°.We definetheregionofabruptstructuralchangeasa“structuralboundary”. In a number of places, this boundary isdecoratedbyinclusionsofhighlydeformedanorthositethatismuchmorecalcic(An67–75)thantheChugwaterAnorthosite.MostoftheseinclusionsaretoosmalltoappearonFigures2and6,buttwolargerinclusionsareshown.InthesoutheasternChugwaterAnorthosite,the“structuralboundary”separatesrocksonthewestthathavedistinct,consistentlayeringandlaminationfromonesontheeastwheresuchfeaturesareeitherlackingorchaotic.Petrographicallyandcompositionally,therearenodifferencesbetweentherocksoneithersideofthe“structuralboundary”, soweconsider themall tobepartsoftheChugwaterAnorthosite.Evidently,theyhave experienced distinctly different deformationalhistories.Itisnoteworthythatwiththeexceptionoftheinclusionsnotedabove,noneof theChugwaterAnor-thositeshowsthehighlydeformedandgranulatedrocksseenatthebordersoftheEgersund–Ognaanorthosite,Norway (Barnichonet al. 1999), and similar plutonsbelievedtohavebeenemplaceddiapirically.
Laramide faulting. TheChugwaterAnorthosite,in commonwithmost of theLAC, has been cut bynumerousfaultsandshearzonesofpresumedLaramideage.Ingeneral,theseincreaseinabundancefromwestto east, and, indeed, the northeasternmost portion oftheChugwaterAnorthosite has been thrust eastwardoverPhanerozoiclimestones,someofwhichhavebeenlocallyoverturned.Inmostplaceswithintheanortho-site,itisimpossibletodetermineeitherthedirectionorextentofdisplacementalongtheotherfaults.Protero-zoic granite dikes showing little or no offset suggestthatmanyofthesefeaturesmayhavebeenshearzoneswith little displacement. In any event, theymake itextremelydifficulttotracelayersandotherstructuresinthenortheasternportionoftheChugwaterAnorthosite.This is themajor reason thatwe arbitrarily start thebaseofthe“stratigraphicChugwaterAnorthosite”westoftheregionofabundantfaults.JusttotheeastoftheregionofFigure6(seeFig.4),somanyofthesefeaturesintersect that the anorthositewas highly comminuted
Fig. 5. Stratigraphic column for the southwesternportionsof theChugwaterAnorthosite,constructedusingprofilesalongA–A’ andA’–A” in Figure 2. Some key samplenumbers are shown. Light grey: units dominated byanorthosite; dark grey: units dominated by gabbroicanorthositeandmixedrocks.Verticallines:intrusionsandinclusions of leucotroctolite and troctolite.Dot pattern:monzosyenite.
Possibility of a detachment thrust beneath the Chug-water Anorthosite.Geophysicalmeasurements, bothgravity(Hodgeet al.1970,1973,Smithet al.1970)andseismicreflection(Allmendingeret al.1982,Breweret al.1982),consistentlyshowthattheanorthositebodiesin the LAC at present are quite thin (1.6 to 4 km)comparedtotheirarealextent(~650km2)andinferredstratigraphicthickness.Thisismostsurprisinginviewofthefactthatstratigraphicthicknessesof6000–10,000m have been documented in both the northern (PoeMountainAnorthosite: Scoates 1994, Scoates et al.2010)andsouthern(ChugwaterAnorthosite;thispaper)portionsoftheLAC.OnepossibleexplanationisthatthepresentexposuresoftheLAClieaboveaLaramidedetachmentthrust.Besidesthegeophysicalstudies,theonly evidence for this interpretation is a report thata diamond-drill hole at IronMountain (I in Fig. 2)
Significance of the “anticlinal axis” of Newhouse & Hagner.OurmappingintheChugwaterAnorthositeshows that the “anticlinal axis” reported in the areabyNewhouse&Hagner (1957) actually reflects thecombinationoftwoandpossiblythreedistinctfeatures:(1)theveryrealandregularwesterlytosouthwesterlydipofthestratigraphicChugwaterAnorthosite,togetherwithlocalcurvatureinthesouthernandcentralportionsof Figure 4, and (2) the abrupt change tomainlynorth–south-striking,nearlyvertical fabric to theeastand south of the “structural boundary”. In addition,there is the very real possibility, especially near theeasternboundary thrust, thatdisparateportionsof theChugwaterAnorthositemay have been juxtaposedbyLaramide-age faulting, and that any appearanceof an antiform theremaybe accidental and thereforemisleading.
TheChugwaterAnorthosite is cut by a variety ofgraniticdikes,noneofwhichislargeenoughtoshowon the scale of Figures 1 and 2.Their thickness isvariable,rangingfrom1to15m.MostofthesedikesstrikeN–StoNW–SE,althoughallotherorientationsoccur.Ingeneral,boththesizeandabundanceofthesedikes increase fromwest to east. Some dikes havenegligible effects on the host anorthosite andwereprobably relatively dry.Other dikes, probablywithhighH2Ocontents,pervasivelyalteredtheanorthositethroughwhichtheypassed:theplagioclaseisbleachedwhite,whereastheferromagnesianmineralshavebeenconvertedtohornblendeoractinolite.Somedikesaltertheanorthositetodistancestentimestheirownthick-ness.TheconcentrationofsuchalterationintheeasternportionsoftheChugwaterAnorthositeaddsgreatlytothe difficulty in recognizing and tracing stratigraphicvariationsinthatarea.TogetherwiththegreatereffectsofLaramidefaultingintheeast,thisledustoconcen-trateoureffortsinthemoretractablewesternpartsoftheChugwaterAnorthosite (“stratigraphicChugwaterAnorthosite”).A composite ferrodiorite–monzodioritedike(Mitchell1993)cutspartoftheChugwaterAnor-thosite(Fig.6).Smallerdikesofferrodioritealsooccur.
Fe–Ti oxide bodies
TheChugwaterAnorthositeishosttofourdepositsofmassive Fe–Ti oxides; from northeast to south-west,thesearethewell-knownIronMountaindeposit(Hayden1871,Ball1907,Singewald1913,Frey1946a,Eberle1983),theGoatMountainprospect,theShantondeposit (Hild 1953, Frey 1946b), and an unnamedprospectinsection6,T17N,R71W(I,G,S,and“6”inFig.2).Newhouse&Hagner(1957)notedthatthesealllieonoradjacent to their“anticlinalaxis”.Ourmoredetailedmapping raises serious doubts regarding thisapparentcorrelation.
PetrographyandMineralCompositions
Analytical
Mineralcompositionsweredeterminedbyelectron-probemicroanalysis.Most analysesweremade ontheStonyBrookCameca“Camebax”probe,usinganacceleratingpotentialof15kVandsamplecurrentsof15nAforplagioclaseand20nAforotherphases.Weused, as standards for plagioclase,microcline forK,albiteforNa,anorthiteforCa,Al,andSi,ilmeniteforTi,andfayaliteforFe.Forferromagnesianphases,weusedenstatiteforMgandSi,anorthiteforCaandAl,ilmeniteforTi,fayaliteforFe,excepthematiteforFeinFe–Tioxides,andrhodoniteforMn.Someferromagne-sianandoxidemineralsweremeasuredattheUniversity
ofWyomingusing15kVand20nAandthefollowingstandards: for augite, Si,Al,Ti,Mg,Fe,Na, andCaonaugite,Cronchromite,NionsyntheticNi-olivine,Mnonmagnesianfayalite.Fororthopyroxene,weusedSi,Mg,Feonferroanenstatite,Al,Ti,Ca,andNaonaugite,Cronchromite,NionsyntheticNi-olivine,Mnonmagnesianfayalite.Forolivine,weusedSi,Fe,andMn onmagnesian fayalite,Mg on ferroan enstatite,Al,Ti,andCaonhornblende,Cronchromite,NionsyntheticNi-olivine.Forilmenite,weusedFe,Ti,andOonilmenite,Mnonmagnesianfayalite,Mg,Al,andCronachromium-richulvöspinel,andZnongahnite.
Textures
The dominant feature of almost all samples ofanorthositeandgabbroicanorthositeintheChugwaterAnorthositeisthepresenceofplagioclasemegacrysts.Inpreparingsamplesforthin-sectioning,onemustbecareful to ensure that phases other than plagioclaseare present on the surface chosen.One result of thisselection process is that point-countedmodes arenearlymeaningless,as they tend tounderestimate thetrue volumeof plagioclase in the sample. In thePoeMountainAnorthosite,poikilitictextures(withaugite,orthopyroxene, olivine, and even Fe–Ti oxides allservingasoikocrysts)arecommon,suggestingsimul-taneouscrystallizationofthosephasesandplagioclase(Scoates 1994, Scoates et al. 2010). In contrast, apoikilitic texture is extremely rare in theChugwaterAnorthosite; all ferromagnesianphases are interstitialtoplagioclase, suggesting that they formedonlyafterthemajorityofplagioclasehadalreadycrystallized.TheChugwaterAnorthositetexturesareconsistentwiththebody having been emplaced as amagma ladenwithplagioclase crystals. InTable 1,we list themineralspresentandtherangeofplagioclasecompositionsforkey samplesof theChugwaterAnorthosite.Availableaverage compositions of plagioclase, clinopyroxene,orthopyroxene,andolivinearegiveninTables2a–2d.
Overview of mineralogy
Plagioclase.The singlemost distinctive featureof theChugwaterAnorthosite is its abundant coarse,tabularplagioclase,muchofwhichshowslabradores-cence.The tabular face is [010],with the larger axessubequal,andthethicknessdistinctlylessthanthatoftheotherdimensions.Rarelyarefacesotherthan[010]preserved.Typicallytabulargrainsupto~5cmacrosshavethicknessesof0.5cmorless.Megacrysts(tabularplagioclasewithwidths greater than 5 cm) typicallyhaveproportionallygreaterthicknesses,oftheorderof0.2 thewidth.Plagioclase compositions inmost oftheChugwaterAnorthositeandsomeoftheleucotroc-tolitesaretypicallyAn50toAn56;whereastherangeofcompositionsinagivensampleincreasestowardthetop
ofthestratigraphicsection,overallthereappearstobea1–2%upwarddecreaseinAncontent(Figs.7a,b,c).ThemostcommoncompositionisAn52–54Or3–4,whichleads to iridescence in the blue to blue-green range.Many anorthosite samples contain a fewplagioclasegrainsthataredistinctlymorecalcicthantherest.Weinterpret theseasxenocrysts,probablyfromtheoldercalcicanorthositesthatarefoundasinclusionswithinpartsoftheChugwaterAnorthosite.Twosetsofleuco-troctolites,atthe5960–6075mand9740mlevels,arealsodistinctlymorecalcic(Fig.7c).Somecalcicplagio-claseintheChugwaterAnorthositemayreflectmixingbetweentheseleucotroctolitesandthehostanorthositicunits(tobediscussedlater).
Deformation.Nearly all plagioclase in theChug-waterAnorthosite showsvarying extentsofdeforma-tion,mostofithigh-temperature.Intheleast-deformedrocks, megacrystic and other tabular plagioclasegrainshavesuturedgrain-boundariesandarebent,ascan be seen both in hand sample and in thin section(Figs. 8a–c).As the extent of deformation increases,the tabular grainsmay become polygonalized, andneoblasts form on their grain boundaries, a processdocumentedbyLafranceet al. (1996).Theneoblastslack the Fe–Ti oxide inclusions that give themega-crysts their distinctive black color.As the populationofneoblastsincreases,therelictmegacrystsremainasblackporphyroclastsisolatedbygraytowhiteneoblasts.
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Wheremost of theoriginal plagioclasehas recrystal-lized,theresultisafine-grainedwhitetograyrockwithagranoblastictexture.Eveninthemostrecrystallizedrocks, however, onemayfind local black, iridescentfragmentsoftheoriginalmagmaticplagioclase.
Inclusions in plagioclase.Allmegacrystsandmostother tabular plagioclase grains in the ChugwaterAnorthosite are dark grey to black as the result ofnumerousinclusions(Fig.8a).Therearetwomaintypesof inclusions,bothofwhichareoriented inpreferreddirections: black opaque rods (mainlymagnetite andilmenite)andhoney-coloredtored-brownplatesthatarerichinTiandFe(mainlyilmeniteandrutile,butwithsomebiotite).Manyoftheplatesaresothin(<1mm)thattheyarenon-opaque(Fig.8d).Plagioclasewiththeinclusions typicallycontains0.3–0.4wt%TiO2.Bothtypesof inclusionsareabsentfromplagioclaseof thecross-cuttingtroctolitesandleucotroctolites.Theyarealsoabsentfromneoblasticplagioclaseandfromrimsonmegacrysts. Ina later section,wesuggest that theinclusion-bearing regionsmay indicate crystals that
formed at greater depth andwere presentwhen theChugwaterAnorthositemagmawasemplaced.
Pyroxenes. Some pyroxene grains in gabbroicanorthosite are subhedral, suggesting that theymaybe cumulus;most, however, are clearly interstitial toplagioclase,asisalwaysthecaseforanorthositeunits. Oikocrystic pyroxene, common in the PoeMoun-tainAnorthosite to the north (Scoates 1994, Scoateset al. 2010), is essentially absent in theChugwaterAnorthosite.
Orthopyroxene contains thin lamellae of augitealong(100).Someorthopyroxeneformsanovergrowthon olivine.Orthopyroxene from leucotroctolite hasthecompositionWo1–2En69Fs29,whereasthatfromthegabbroic anorthosites ranges fromWo1–2En68Fs30 toWo1–2En60Fs38, and that from the anorthosites, fromWo1–2En60Fs38 toWo1–2En51Fs47.Aswith the augite,non-quadrilateral components are low,withAl2O3ranging from1.0 to 2.0wt%,MnO, from0.3 to 0.5wt%, andTiO2, from0.2 to0.3wt%.Orthopyroxenefromtheleuctotroctolitestendstohaveminor-elementcompositionsthatfallonthehighsideoftheseranges,whereas that from themain serieshavecompositionsthatfallonthelowside.
Opx[i.e.,Fs/(En+Fs)]formain-seriesrockslies near 0.5 in the lower portions of theChugwaterAnorthosite,dropsto0.3to0.4inthemiddleportions,andthenincreasesfairlyuniformlyupto0.56atthetop.
Opxmayberesetuponcooling(OpxgainsFefromCpxbutlosesittoolivinebyFe–Mgexchange),weconsideritamorereliablemeasureofthemg#ofanorthositicrocksthanthat derived frombulk-rock analysis. [In rockswith~90%plagioclase,smallvariationsinthemodalpropor-tionsofFe–Tioxidesand ferromagnesianphasescanmakeverylargerelativedifferencesinbulkMgandFe.]Our data suggest that onemay approximate the bulkmg# in theChugwaterAnorthosite through the rela-tionshipmg#=0.92–XFe
Opx.However,thatempiricalrelationshipdoesnotholdfortheleucotroctolites,anditmaynot hold for anorthositic rocks other than theChugwaterAnorthosite.
Pigeoniteispresentonlyintheupper2000metersof theChugwaterAnorthosite and has “inverted” tocoarse lamellae of augite broadly parallel to (001)in the orthopyroxenehost. It has a bulk compositionnearWo10–12En45Fs43 (Fig. 9).Minor elements in the
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hostOpx and augite lamellae are similar to those intheprimaryphases.Invertedpigeonitealsooccursinasmall iron-enrichedbody that intrudes theChugwaterAnorthositeabouthalfwayupthestratigraphy.
Olivine.Olivineisabsentfromnearlyallsamplesofanorthositesensu strictobutispresentinmanygabbroicanorthosites and by definition in all leucotroctolitesand troctolites. In a later section,we argue that theabsenceofolivinefromthetrueanorthositesisrealandsignificant,ratherthananartifactofthelowcontentofferromagnesianmineralsintheserocks.OlivinehasacompositionnearFo65intheleucotroctolitesandtrocto-litesandrangesfromFo65toFo54intheolivine-bearinggabbroicanorthosites(Fig.9).TheCaO,MnO,andNiOcontentsofolivinearelow,lessthan0.03,0.5,and0.2wt%,respectively.Inapproximatelyhalfoursamples,olivine has partially or completely altered to chloriteplusotherphases.
Biotite. Biotite typicallymakes up 1–2 vol.% ofleucotroctolites,occurs insomegabbroicanorthosites
(typicallythosecontainingolivine),andisabsentfromanorthosites(exceptasoccasionalorientedlamellaeinplagioclasemegacrysts).IttypicallyrimsFe–Tioxideand has the deep-red color associatedwith elevatedcontentofTi.
Quartz.Primaryquartzisabsentfromtheleucotroc-tolites andmost of theChugwaterAnorthosite.Mostsamplesthatcontaininvertedpigeoniteintheuppermost2000moftheChugwaterAnorthositealsocontaintracetominoramountsofprimaryquartz.
Fe–Ti oxide.Both theChugwaterAnorthosite andthe associated leucotroctolites contain ilmenite andtitaniferousmagnetite(Ti–Mgt).Typically,thelatterismoreabundant,appearingasgrainsand(111)lamellaeofilmeniteinarelativelylow-Timagnetitehost.Rarely,isitpossibletoreconstructtheoriginalspinelcompo-sition by re-integrating the oxy-“exsolved” lamellaewiththehost;QUILFcalculations(seealatersection)suggest original compositions ofUsp40–60. In rockswheretitaniferousmagnetitedominates,theilmeniteis
homogeneous,withcompositionstypicallyintherangeHem3–6.However, in the fewsamples forwhich suchtitaniferousmagnetiteisscarceorabsent,theilmeniteshowsfineexsolution-inducedlamellaeofhematite,andthe bulk composition isHem12–13. Ilmenitewith 3–6wt%Fe2O3 is faroutofequilibriumwith titaniferousmagnetite and the coexisting ferromagnesian silicatesforanyreasonablerangeoftotalpressureandtempera-ture.We conclude that the oxide inmostChugwaterAnorthositesamplescooledalongamagnetiteisopleth(Frostet al. 1988,Fig. 4),whichwould have forcedreductionoftheilmenite.Itissignificantthattherareilmenitethathasexsolvedhematiteandbulkcomposi-tionsofHem12–13occursonlyinsampleshavingaveryhighilmenite:magnetiteratioandwouldthereforehavecooledmorenearlyalonganilmeniteisopleth.WeinferthatHem12–13 is close to the composition of all theChugwaterAnorthositeilmenitepriortocooling.InourQUILFcalculations,wetookHem13asanupperlimitontheilmenitecomposition.
Sulfides. Pyrrhotite with lesser pentlandite andchalcopyriteoccur as traceamounts in somesamplesofanorthosite,andaremoreabundantinsomeleuco-troctolites andgabbroic anorthosites, especially thosethatoccuratthe~4000mlevel.
Threemainmagma-types evidently contributed totheformationoftheChugwaterAnorthosite:theparentto the anorthosites and gabbroic anorthosites,whichapparentlylackedolivine,andatleasttwovariantsoftroctolitic or leucotroctoliticmagma, both ofwhichweremagnesianrelativetotheanorthositicrocks.ThetroctoliticmagmasassociatedwiththelowerportionofthestratigraphicChugwaterAnorthositewererelativelysodic, having normativeAn contents very similar tothose of the anorthositic rocks,whereas those in theupper portionwere distinctlymore calcic (Fig. 8c).There is strong petrographic evidence formixing ofthetroctoliticmagmaswiththegabbroicanorthosites.
Themainpetrographicevidenceformixingbetweenthe anorthositicmagma and the associated leucotroc-toliticmagma ormagmas is zoning in theminerals,especially plagioclase. In some cases, the zoning isobvious, as in theAn2 andGan3 units, where the
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invadingleucotroctoliteisAn-rich,andplagioclaseinthenearbygabbroicanorthositeshowsstrongreversedzoning. In other cases,where the plagioclase in thetroctolitedifferslittlefromthatoftheanorthositicunits,thezoningismuchmoresubtle.Heretheevidenceformixingstemslargelyfromoutermostzonesofplagio-clasemegacrysts that are optically continuouswith,but lack the oriented ilmenite lamellae of, the cores.Inclusion-free plagioclase in gabbroic anorthosite isespeciallyprominentnearolivine. In favorablecases,thezoningalsoshowsupinoneormoreoftheferro-magnesianminerals.Overall, thesepatternsofzoningsuggestanabruptchangeinthenatureoftheremainingliquid,consistentwithmagmamixing.Troctolitesandleucotroctolites usually have sharp boundarieswheretheyinjectedtrueanorthosite,presumablybecausethelatterhadlittleremainingliquidformixing.Incontrast,the contactswith gabbroic anorthosite are diffuse,
Importantly,whenthesetexturalcriteriaareappliedtoseparatemixedfromnon-mixedsamples, the latter(91intotal)areallfoundtobefreeofolivine,aphasethatisdistinctiveofthetroctoliticrocks.Of51anortho-siticsamplesclassifiedasmixed,incontrast,40containolivine, andmost of the rest are richer inOpx thannon-mixedsamplesnearby,suggestingthatanyaddedolivinecomponentmayhavereactedwithSiO2toformadditionalOpx.Thepetrographicevidenceformixingisfurthersupportedbygeochemicalevidence(presentedlater).TheChugwaterAnorthositecomprisesalltheseunits.Forconvenience,werefertotheanorthositesandgabbroicanorthositesthatshownoevidenceofmixingasthe“mainseries”,andthosethatshowsuchevidenceas“mixedrocks”.
Fig. 7. Plagioclase compositionsversus stratigraphic height in theChugwaterAnorthosite.The thickfilledbar shows therangeofmostanalyses,and the trianglesshow theextremevalues found ineachsample;boxesshownormativeAnforanalyzedsamples,calculatedfrommajor-elementcompositions.a.Main-seriesChugwaterAnorthosite.b.Mixedrocks.c.Leucotroctolites.
Anoutcropofgabbroicanorthositenearthenorth-westernendofAn2(BM289,5935mlevel)containsabundantmultiply-zonedcrystalsofiridescentplagio-clasethatshowresorbedhorizons,includingtheonethatadornedthecoveroftheJournalofPetrologyin1998.ThiszonediridescentplagioclaseissoabundantinthevicinityofBM289thatweinformallynamedthearea“ZIPCity”(Fig.10).Weinterpretthesecrystalstobetheresultof repeated injectionsandmixingofhotter,more calcic leucotroctoliticmagma into a gabbroicanorthositelayeroftheChugwaterAnorthosite.SimilarrelationshipsoccurintheNainPlutonicSuiteonPaulIsland(Wiebe1990,p.9)andinthePortManversRunIntrusion(MichaelHamilton,writtencommun.,2009).A true anorthosite layer immediately aboveBM289(An2), in contrast, contains pods of calcic troctoliteandleucotroctoliteatthe5960–6075mlevel(samples
KM36,5960m;BM52,6000m;BM269,6075m);herethecontactsaresharpanddistinct,withnoevidenceofmixing.Theseleucotroctoliteinclusionsareparticularlyinteresting.We havemapped eight of them, rangingin size fromapproximately3 tomore than10metersacross, over an along-strike distanceof 8 km.Modalplagioclasedecreases systematically from89% in thenorthwest (BM52) to 71% in the southeast (KM36),whereas theAn content decreases from 64 to 59%.Grainsizealsodecreasesfromnorthwesttosoutheast.Weconsider itpossible that thesenow-isolated inclu-sionswere once part of a continuous intrusion thatwasbrokenapartduringlate-stagedeformationoftheenclosingChugwaterAnorthosite.Ifthisinterpretationiscorrect,thenthefeeder,whichhasnotbeennotfound,mayhavebeenneartheregionofBM52,whereplagio-clasephenocrystsaccumulatedbyflowdifferentiation.
ThedecreaseinAncontenttothesoutheastmayhaveresulted either from a decrease in the abundance ofcalcicphenocrystsorfromasmalladmixtureofresidualliquidfromthehostanorthosite.
IntensiveParameters
Mineral compositions in plutonic rocks like theChugwaterAnorthosite typically undergo significantchangesduringcooling,bothbyexsolutionandthroughexchangewith other phases. In an attempt to “seethrough”thesechanges,wehaveappliedQUILFequi-libria(Lindsley&Frost1992,Frost&Lindsley1992,Andersenet al.1993)totheChugwaterAnorthositeandsummarizetheresultshere.
Pressure
The ChugwaterAnorthosite lacks a definitivegeobarometer, sowe estimate pressure from nearbyrocks.PressureforthenorthernmarginoftheLaramieAnorthositeComplexhasbeenestimatedtobenear3kilobars(SybillePluton,Fuhrmanet al.1988;contactaureole,Grant&Frost1990),whereaspressuresforthesouthernmargin are closer 3.5 to 4 kilobars (MaloinRanchPluton,Kolker&Lindsley1989;contactaureole,Xirouchakis1996).BecausetheChugwaterAnorthositeliesinthesouthernportionofthecomplex,weadoptanemplacementpressureofcloseto3.5kilobars.Clearly,however, if our reconstructed stratigraphy is correct,
Themost robust thermometer in theChugwaterAnorthosite is the pigeonite thermometer,which isbasedontheXFeoftheassemblageOpx+Cpx+Pig(Davidson&Lindsley1989,Lindsley&Frost 1992,Fig.2).Onlyafewsamplesshowevidenceforcoexis-tenceofallthreepyroxenes,butfortheremainder,thegap between pigeonite-bearing rocks (fictiveXFe
Because the compositions of the Fe–Ti oxideshave been significantly reset on cooling,we use theQUIlF equilibria (Lindsley&Frost 1992) to calcu-late the oxygen fugacity and silica activity for theChugwaterAnorthosite.To facilitate comparison,wefix the temperatureat1050°Cand thepressureat3.5kbar.Forthef(O2)ofolivine-bearingrocks,weusethelocation of the assemblageOl–Opx–Cpx–Mgt–Ilm,which is dependent only onXFe of the silicates. Forassemblageswithoutolivine,wemustcalculatearangeof possible oxygen fugacities, the lower limit being
that of olivine saturation,whereas the highest is thatdefinedby ilmenitewithXhem= 0.13.We chose thisvaluebecauseitisclosetothecompositionofilmeniteinChugwaterAnorthositerocksthathavecooledalonganilmeniteisopleth(i.e.,thosethatoriginallyhadaveryhighilmenite:magnetiteratio)(Frostet al.1988,Fig.4).
Fromthesecalculations,wedeterminethatthemainseriescrystallizedatoxygenfugacitiesbetweenFMQand FMQ+ 0.5, the leucotroctolites crystallized atoxygen fugacities aroundFMQ+1.0, and themixedrocks,atoxygenfugacitiesbetweenFMQandFMQ+1.0.Thecalculatedactivityofsilica(relativetoquartz)ofthemainseriesvariesfrom0.7atthebaseupto1.0atthetop,wherefreequartzislocallyfound.Incontrast,thesilicaactivityofthemixedandtroctoliticrocksliesinarestrictedrange,definedbyolivinesaturation,near0.7(Fig.11).
Whole-RockChemistry
We selected 37 samples from the ChugwaterAnorthosite formajor-elementanalysis, togetherwitha limited number of trace elements; for 32 of these,wealsohavemoreextensiveinformationonthetraceelements,includingtheREE(Tables3aand3b).Theseinclude15samplesofthemainseriesand15ofmixedandtroctoliticrocksfromthestratigraphicChugwaterAnorthosite.LocationsofanalyzedsamplesaregiveninFigures4,5,and6.
Sample preparation and analytical details
TheweatheringprofileinanorthositicrocksoftheLACrangesfromseveralcentimeterstoseveralmetersin thickness.Themajority of samples collected forthisstudywereobtainedusingawater-cooledportablediamonddrill,withcoretubes2.5cmindiameterand
30cmlong.Dependingonthegrainsize,wecollectedfourto10coresforsamplesthatweretobeanalyzed.The surface of each corewas abradedwith tungstencarbide(WC)papertoremoveresiduefromtheinsideofthecoretube.CoarsecrushingwasdoneinaRocklabshydraulicpressbetweentwoflatWCplates(percussionmethod).Asinglealiquot(75to100g)ofhomogenizedcoarselycrushedsamplewaspowderedinaWCshat-terboxforthreeminutes.Allreportedgeochemicaldatainthisstudyarefromthisaliquot.
Concentrations of themajor elementswere estab-lished byX-ray fluorescence spectrometry (XRALLaboratories,DonMills,Ontario)andbytheICP–MSmethod(DukeUniversity,NorthCarolina).
Major elements
Themajor-element data hold few surprises, ascompositions are dominated by the components ofintermediate plagioclase.Normativemineralogywascalculatedassumingthat15%oftotalFeisferric.Verylow(cation)normativecontentsof candne forsomesamplesprobablyarenotrealandmayreflectuncertain-tiesintheanalyses.Asexpected,normativeolcontents
ofmostmixedandtroctoliticrocksaredistinctlyhigherthanthosereportedforthemainseries,reaching25%fortroctoliteBM273(4180mlevel).Figure12containsplotsofnormativeqandol asafunctionofstratigraphiclevel,witholexpressedasnegativeqforconvenience.Mostmain-series rocks plot at or just above zero,whereas allmixed rocks and leucotroctolites shownormative olivine.Themodest contents of normativeolivineformain-seriesrocksat4900to6790mmirrorsthemainlylowervaluesofXFe
Opx(Fig.13)andhighermg#inthatinterval.
Whole-rockmg numbers (Table 1) do not varysystematicallywith stratigraphic height. In anortho-sitic rocks,mg numbers are problematical, as theyare extremely sensitive to the relative proportions offerromagnesian silicate andFe–Ti oxide crystals thatwere present at the stagewhen the residual liquidwasexpelled,aswellastoerrorsinchemicalanalysisresultingfromverylowabundancesofMgandFe.Insamples classified as of themain series (non-mixed)onpetrographiccriteria,mg#rangesfrom0.41to0.57.Formixedrocks,Mg#rangesmainlybetween0.57and0.62,withthevaluesdroppingto0.53(7256mlevel)and0.49(7585mlevel).Thisdropiscloselyreflected
Fig.11. Valuesofsilicaactivity(relativetoquartz)(Andersenet al.1993),asafunctionof stratigraphic height.Rectangles: range of values formain-series (olivine-free)ChugwaterAnorthosite.Upperlimitofa(SiO2)isgivenbyquartzsaturation[a(SiO2)= 1] or by assumingXHem= 0.13 in ilmenite (see text); the lower limit is that ofolivine saturation.Grey ellipses:mixed rocks that contain olivine, black ellipses:leucotroctolites;forbothofthese,therangeincalculateda(SiO2)islimitedbytheFe/Mgvaluesinthepyroxenes.
valuesforthemixedrocksalmostcertainlyreflecttheadmixture ofMg-rich leucotroctolitematerial,whichhasmg# ranging from0.63 to 0.69. SamplesKM44andKM46(4900mlevel)werecollectedlessthan1mapart,yettheirmg#are0.49and0.56.Thedistributionofmg#approximatelyreflectsthatofXFe
Opx(0.39–0.41inKM44; 0.35–0.38 inKM46).We suspect that the
highermg#ofKM46,aswellasitsrangeofXFeOpx,may
reflectsomemixingofleucotroctoliteintothissample.Cationnorms for the rocksof themain series are
plagioclase-rich(asexpected):normativefeldspartotalsfrom80.1 to 95.6%.The normative content ofOr isquiteclosetothemeasuredOrcontentsofChugwaterAnorthositeplagioclase,consistentwiththefactthatnoseparateK-feldsparphaseisseen.Thusitisappropriate
908 thecanadianmineralogist
touse total normative feldspar as an estimate for theamountofmodalplagioclase.NormativeAncontents[an/(an+ab+or)]tendtolieattheupperendorevenabove therangeof themostabundantmodal feldsparcompositionsmeasuredbyelectronmicroprobe(Figs.7a,b, c).Thereareprobably severalexplanations forthisphenomenon.First,becauseofthegeometricrela-tionshipbetweenthevolumeofasolidandthesurface
exposedbyarandomsectionthroughit,themostcalcicportion of a normally zoned plagioclasemay not beexposedatthethin-sectionsurface,orifexposed,maynothavebeenanalyzed.Second,thenormcalculationignoresthesmallbutrealCa-Tschermakscomponentofthepyroxenes,whichisslightlydominantovertheirJdcomponent.Third,xenocrystsofcalcicplagioclasemaybe underrepresented in the population of plagioclase
Formost analyzed samples,we have data for 21traceelementsplustherare-earthelements;analytical
detailsfortheICP–MSmethodaregiveninMeureret al.(1999,p.147-148).Asonemightexpect,Srishighlycorrelatedwith total feldspar in each rock.ElementssuchasRb,V,Y,Sc,Hf,Nb,Ta,Tl,ThandU,whicharelargelyincompatibleinplagioclase,showconsider-ablevariation.Allincreasebyfactorsof3to10frombottomtotopoftheChugwaterAnorthosite(Fig.14).The increase is far from smooth; it is punctuated by
greaterthanone(Fig.15a).ThevalueofEu/Eu*andtheslopeofthepatternsdecreasewithoverallabundance;both features alsomainly decreasewith increasingstratigraphic height, but there are excursions thatapproximatelymirrorthevariationsin otherincompat-ibletraceelements(e.g.,Fig.14).Ofparticularinterestis thenear-absenceofaeuropiumanomalyat the topof theChugwaterAnorthosite; under the reasonableassumptionthattheplagioclasemegacrystshereretainapositiveEuanomaly,thentheremainderofthesamplemusthaveacorrespondingnegativeanomaly.TheREEpatternsforthemedium-Anleucotroctolitesaresimilartothoseofthemain-seriesChugwaterAnorthosite(butwithlessvariationinabundance)(Fig.15b).
Discussion
Significance of Fe–Ti oxide inclusions in plagioclase megacrysts
We believe that the Fe–Ti oxide inclusions inplagioclase place important constraints on the evolu-tionoftheChugwaterAnorthosite;itisthusnecessarytounderstandtheirorigin.Weconsider threepossibleorigins:(1)entrapmentofearlyFe–Tioxidebygrowing
plagioclase, (2) late-stage deposition by circulatingfluids, and (3) exsolution from the plagioclase.Werejectcase(1)onfourverydifferentgrounds.First,itismostimplausiblethattheparentalliquidwouldhavebecomesaturatedwithFe–Tioxidebeforeplagioclase,and thepreferredorientationof the inclusions is alsohard to explain if they had simply been engulfed bygrowingplagioclase.Second,favorablesectionsshowthatdifferentsetsofalbite-twinlamellaehavedifferentorientationsoftheinclusions,stronglysuggestingthatboth theplagioclase and its albite twins existedpriorto formationof the inclusions.Third,wehaveshownexperimentally that the inclusions cannot have coex-istedwith their host plagioclase at 1100°C at 1 barorat5kbar.Heatingexperimentsonplagioclasewithinclusions,intendedtohomogenizetheinclusionsandplagioclase, insteadproducedminute pockets ofmeltwithin thehost plagioclase.Thus clearly the separatecrystallinephaseswerenotstablewithrespecttomeltingatthattemperature,andmeltformedintheexperimentsuntilalltheoxidewasusedup.Fourth,magnetiteandrutile,which are found touching each other in someinclusions, are not compatible at temperatures above~400°C(Lindsley1991,p.85).Weconclude that theinclusions couldonlyhave formed in the plagioclase
Fig. 12. Normative olivine or quartz (mol.%) formain-series rocks (open circles),mixedrocks(grey-filledcircles),andleucotroctolites(blackcircles)intheChugwaterAnorthosite.Positiveandnegativenumbersgivepercentagesofnormativequartzorolivine,respectively.Normativecalculationsassumethat15%oftotalironisferric;ifthisvaluevariedby±5%,thepositionofthezeropoint(nonormativequartzorolivine)wouldvarybyapproximately±1%.Main-seriesrockshavenomodalandlittleornonormativeolivine, suggesting that themainChugwateranorthositicmagmawasnotolivine-normative, and thatolivinewascontributedbyadmixtureof leucotroctoliticmagmawiththeChugwaterAnorthosite.
Origin(2),late-stagedepositionbyfluids,cannotbecompletelyruledout,butseemsunlikely.Insofarasonecantellinthinsection,theinclusionsoccupyvolumeswithintheplagioclase;thereisabsolutelynoevidencethat they are associatedwith healed cracks or otherplanarfeatures.Furthermore,theabsenceofinclusionsfromtherimsofmegacrysts(evenwherethecoreandrimcompositionsofplagioclasearevirtuallyidentical)and fromplagioclaseofnearby leucotroctolite arguesagainstdepositionfromapervasivefluid.Byelimina-tion,weconcludethatcase(3),originof theorientedinclusionsbyexsolution,isthebestexplanation.Suchanorigin,alsosuggestedbyAnderson(1966),ismostcompatible with the preferred orientation and theextremethinnessoftheplatyinclusions.It isalsotheonlyplausibleexplanationforthedistributionofinclu-sionsinthespectacularlyzonediridescentplagioclasecrystalsinsampleBM289(5935mlevel).Plagioclase
compositionsrangefromAn52toAn57,correspondingto interference colors fromblue to red.Oxide inclu-sionsoccuronly in the blue resorbed cores(whichareinterpreted to represent the plagioclase compositionpriortomixingwithhigh-Antroctolite);theyareabsentfromallthemanyzonedovergrowthlayers,whichareinterpreted to have formed following repeated injec-tionsofthetroctoliteattheleveloffinalemplacement.OxidelamellaeareabsentevenfromzoneshavingthesameAncontentasthecores,sothiscannotbeasimplebulk-composition effect.Thus it seems clear that theFe–TioxidelamellaeintheChugwaterAnorthositedonotforminplagioclasethatcrystallized at the depth of emplacement in the middle crust.AndastheplagioclaseinBM289 cooled slowly enough to permit feldsparexsolution (hence the iridescence), the absence ofFe–Tioxidelamellaeisunlikelytobeakineticeffect.We conclude that feldspar that crystallized at greaterpressure (hence also higher temperature inH2O-poormelts)mayhaveincorporatedgreateramountsofTiO2
ThatinterpretationissupportedbytheexperimentsreportedbyScoates&Lindsley (2000).Theymeltedacompositioncontaining2.9wt%TiO2and16.7wt%FeO(T)at10kbarand1235°Candcrystallizeditatthesame temperature (1130°C)but at twodifferentpres-sures:10kbarand5kbar(thelatterpressuretosimulate
decompressionofanupwellingmagma).Bothexperi-mentsproducednearlyidenticalplagioclaseintermsofmajorelements(~An53),buttheplagioclaseformedat10kbarcontained0.32–0.37wt%TiO2,approximatelytwiceasmuchasthatformedat5kbar(0.13–0.20wt%).Incontrast,ironcontentsofplagioclasewereessentiallyindependent of pressure: 0.69–1.04wt%FeO at 10kbar, 0.67–1.13wt%at 5 kbar.TheTiO2 contents oftheplagioclasecrystallizedat10kbarareclosetothe
Fig. 15. Rare-earth elements (chondrite-normalized;Boynton1984) in themain series andmixed rocksof theChugwaterAnorthosite.a.Mainseries:Unit1(An1+Gan1),Unit2(An2+Gan2),andUnit3(An3+Gan3);b.leucotroctolites.
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bulk compositions of ilmenite-bearing plagioclase intheChugwaterAnorthosite.
If one accepts the exsolution origin of the Fe–Tioxide lamellae, then several interesting implicationsemerge.Therestrictionoftheinclusionstoplagioclasecoresstronglysuggeststhattheconditionsatwhichthecores formed favored sufficient solubility ofFe- andespeciallyofTi-bearingcomponentstoallowthelaterexsolution.Most likely, these conditionswere thoseatwhichmegacrysts and other plagioclase entrainedin theChugwaterAnorthosite originally crystallized,presumablyadeepmagmachamber.Ifthisisso,thenthepresenceoftheilmenitelamellaeprovidesasimplepetrographicmeanstodistinguish“intratelluric”plagio-clasefromthatwhichcrystallizedin situ,eitherdirectlyfrommeltorduringneoblasticrecrystallization.
Blackplagioclaseisfairlycommoninavarietyofigneousrocks,butwesuggestthatinmanysuchcases,theplagioclasegetsitscolorfrominclusionsofTi-poormagnetite.WefurthersuggestthatitistheTi-richinclu-sionsthatmaybediagnosticofplagioclasethatformedat considerable depths. If that criterion is valid, thenwe conclude that very approximately one-half of theplagioclasenowpresentinthemain-seriesChugwaterAnorthositewasbroughtintothelevelofemplacementasentrainedcrystals;theremaininghalfmusttheneitherhavecrystallized from themelt that accompanied thecrystalsintothelevelofemplacement,orhavelosttheoxide inclusions during deformation and neoblasticrecrystallization.
Origin and nature of the parent melt
Therearethreemainwaysinwhichrockswithhighcontentsofplagioclase,i.e.,anorthosites,couldpoten-tiallyform:(1)crystallizationfromahyperfeldspathicmagma,(2)formationfromamelthavingahighpropor-tionofentrainedplagioclasecrystals,and(3)removalofmostoftheresidualmeltaftermuchplagioclasebutrelatively small amounts of ferromagnesianmineralshadcrystallized.WebelievethatallthreemechanismsmayhavebeenimportantintheformationoftheChug-waterAnorthosite.
Theoriginoffeldspar-richmeltsthroughatwo-stageprocess is awell-recognizedmodel for anorthositegenesis(Morse1968,Emslie1985,Longhi&Ashwal1985,Longhiet al.1993).Thefirststageiscrystalliza-tionofamantle-derivedmeltat thebaseofthecrust.Thehigh-pressure conditions at the base of the crusthavetwomajoreffectsonthecrystallizationofbasalticmelt. First, the crystallization fields for olivine andespeciallypyroxenesexpandwith increasingpressureat the expense of plagioclase (Fram&Longhi 1992,Scoates&Lindsley2000),allowingtheproportionsofplagioclasecomponents tobuildup in the liquid,andsecond,theinitialplagioclasetocrystallizeisrelativelysodic(Longhiet al.1993,Scoates&Lindsley2000).Inthis“standardmodel”ofanorthositegenesis,thedense
olivineandpyroxenesettle to thebottomof the low-levelmagmachamber,effectivelyremovingthempriortoemplacement into thecrust.The secondadvantageis that,owingtoitscompressibilityat thesepressures(seeFig.12ofScoates2000),theresidualbasalticmeltismuch denser than plagioclase.Thuswhen plagio-claseeventuallycrystallizes, itfloatstothetopofthechamber.Finally,thisplagioclase-richmushatthetopof the chamber becomesgravitationally unstable andrisesdiapiricallyintothecrust,ultimatelyforminganor-thositeasmuchoftheresidualliquidisexpelledthroughavagueprocesscalled“filter-pressing”.However,thismodelalonecannotfullyexplainlayeredbodieslikethePoeMountainAnorthositeandtheChugwaterAnortho-site,whichappeartohavecrystallizedanddifferentiatedinmid-crustal-levelmagmachambers.
Wiebe (1990, 1992) has proposed a variation onthis “standardmodel” to explain the production ofhyperfeldspathicmelts,magmas containing greaterproportions of plagioclase components than could beproducedbycotecticcrystallizationalone.Inhismodel,theplagioclase-richportionofthedeepmagmachamberis reheated, probably through emplacement of freshmantle-derivedmelt that provides heat but relativelylittlemass.Aportionof thefloatingplagioclasecrys-talsisresorbed,andsincemuchoftheferromagnesianmineralsthathadcrystallizedalongwiththeplagioclasehadsettledtothebottomofthechamberandwouldbeunavailableforresorption,theresultingmeltwouldbehyperfeldspathic.Afurtheroutcomeofthismechanismisthattheresultingmeltcouldhaveapositiveeuropiumanomaly,acharacteristicofmanyhigh-Algabbrosthathavebeenproposedaspossibleparentsforanorthosites(e.g.,Mitchellet al.1995,Scoates&Mitchell2000).Note that adiabatic decompressionof themushuponascentintothecrustcouldalsoresultinminorresorp-tionofplagioclase;Longhiet al.(1999)calculatedthata pressure release from13 to 4 kbar could remelt asmuchas4%ofthesuspendedplagioclase.
Inanearliersection,wesuggestedonthebasisofTi-rich inclusions that approximately one-half of theplagioclasenowpresentintheChugwaterAnorthositecame in as entrained crystals that had formed at aconsiderablygreaterdepththanthelevelatwhichtheintrusionwasemplaced.Withthisfractioninmind,wecanmakesomeeducatedguessesaboutboththefrac-tionofcrystalsinthemagmathatproducedtheChug-waterAnorthosite,aswellasitsnormativeplagioclasecontent.Thetotalfeldsparinananorthositemustreflectthe contributions from entrained plagioclase crystalsplus normative plagioclase in themelt,minus thefeldsparcontentofinterstitialliquidthatwasremoved.However,thefinalproportionsofentrainedplagioclasedependonly on thefirst and last of these. Figure 16shows the total feldspar that could be produced bymeltshaving60,65,and70%normativefeldsparandcarrying varying amounts of entrained plagioclasecrystals. Consider amagma having 67%normative
feldsparandcontaining40%entrainedplagioclase (ainFig.16);itstotalfeldsparcontentis80%(binFig.16)priortofilterpressing.Suchamagmacouldcrys-tallize directly to a gabbroic anorthositewithout anyremoval of liquid.Assume that the interstitial liquidhas the composition of ferrodiorite (Mitchell et al.1996),which contains approximately 46%normativefeldspar.SincethepresentaveragefeldsparcontentofthesolidifiedChugwaterAnorthosite isclose to90%,thiswouldrequireremovalofapproximately15–18%interstitial liquid,whichwould yield an anorthositecontaining50%entrainedplagioclaseand40%plagio-clasethatcrystallizedin situ.Whereassmallvariationsintheproportionsofentrainedplagioclaseandnorma-tive feldspar content of the originalmelt could yielda similar result, other considerations suggest that theanswercannotvarygreatlyfromthisestimate.
Smallerproportionsofentrainedplagioclasewouldrequire either correspondingly greater normativefeldspar contents of themagma (thus an increasinglygreater hyperfeldspathic character) or removal ofcorrespondinglygreater amountsof interstitial liquid.
Neitherisattractive.Stronglyhyperfeldspathicmagmawould require a greater contribution from theWiebemechanismdescribedabove.Sincethatmodelrequiresanextrastep(reheating),Ockham’srazorsuggeststhatit should be invoked asminimally as possible.Andremovingmoreinterstitialliquidraisesseriousproblemsaboutwhere that liquid has gone, since onlyminoramountsof ferrodioriteareexposed in thevicinityoftheChugwaterAnorthosite.
Higherproportionsofentrainedplagioclasearealsoproblematical,astheviscosityofthemagmaincreasesexponentiallywith the crystal content (Marsh 1996).This leads to a conundrum: tomake anorthosite, onewants amaximum fraction of entrained plagioclase(this ispartof theattractionof theMorse–Emslie–Longhi –Ashwalmodel of anorthosite petrogenesis;the anorthosite is emplaced as a crystal-richmush),but formovement and production of layerswithin amagmachamber,asinterpretedfortheChugwaterandPoeMountainanorthosites, that fractionshouldbeassmallaspossiblesoastokeeptheviscosityrelativelylow!Marsh(1996)pointedoutthatthereisaprofound
Fig.16. Nomogramshowingthetotalfeldsparcontentthatwouldbecontributed(priortoremovalofresidualliquid)bymagmascontaining60,65,and70%normativefeldspar,plusvaryingamountsofentrainedplagioclasecrystals.Therelationshipsholdforbothweightandvolumepercent(ifpossibledifferencesinthermalexpansionareignored).Touse the diagram, one chooses an amount of entrainedplagioclase (for example,40%asina),movesverticallytointersectthedesiredpercentageofnormativefeldsparinthemelt,andmoveshorizontally(b)todeterminethetotalfeldsparcontent(80%in theexamplechosen).Note thatbecause thenormativeOrcontentsofChugwaterAnorthositerocksandmeasuredOrcontentsoftheirplagioclasearenearlyidentical,itisappropriatetotreatallnormativefeldsparas“plagioclase”.
916 thecanadianmineralogist
changeinthebehaviorofmeltsat~55%crystallinity;atandabovethatproportion,theybehavemainlylikeweaksolids.TheparentstoPoeMountainAnorthositeandChugwaterAnorthositemusthavehadaconsider-ably smaller percentageof entrainedplagioclase than55%.As a compromise,we adopt the estimate thattheaveragemagmaemplaced to form theChugwaterAnorthosite contained~40%entrainedplagioclase inamelthaving~67%normativefeldspar.Thisassumedcomposition ofmelt is onlymildly hyperfeldspathic,approximately4–8%abovethecotecticvalue.Approxi-mately 4% hyperfeldspathic character could haveresultedsimplyfromadiabaticresorptionofplagioclase,asthecotecticwouldshiftawayfromplagioclaseupondecompressionduringascentintothecrust(Lindsley&Emslie1968,Fram&Longhi1992,Longhiet al.1999,Scoates2000).ThusthecontributionrequiredfromtheWiebereheatingmechanismneednotbegreat.
Small volumes of fine-grained high-Al gabbrosoccur as dikes throughout the LaramieAnorthositeComplexandhavebeenproposedasexamplesofmeltsthat could have produced anorthosite plus residualferrodiorite throughpolybaric fractionation (Mitchellet al. 1995).Althoughmost of the high-Al gabbrosarestronglyolivine-normative,thereareafewhigh-Algabbrosingroup1ofScoates&Mitchell(2000)thataresilica-saturatedornearlysoandthathavecompositionssimilar to themagma thatmayhavebeen the sourcefor themainseriesof theChugwaterAnorthosite.Anexperimentalstudyofstepwisefractionalcrystallizationof a high-Algabbro from theLAC strongly supportsthishypothesis(Scoates&Lindsley2000).
Relatively high silica activity and f(O2) of the Chugwater Anorthosite
TwopuzzlingaspectsoftheChugwaterAnorthositeareitsrelativelyhighsilicaactivity(0.7–1.0basedonaquartz standard state;Fig.10A)and relativelyhighf(O2); neither is a featurewemight expect in a rockderived directly from themantle.One possibility isthattheparentwasderivedbyremeltingofProterozoiccrust, as advocated byLonghiet al. (1999). Frostet al. (2003,2010)havepointedout that theChugwaterAnorthositehasthemostmantle-likeinitialNdandSrisotopicratiosoftheLaramieanorthosites(seealsoFig.15ofScoates&Chamberlain 2003), but these ratioscould also reflect remelting of Proterozoic crust thatwasonlyslightlyolderthantheanorthositeitself.Whiletheisotopicdataalonecannotruleoutacrustalorigin,wepreferthesimplerinterpretationofadirect-mantlesource.Anexplanationforbothfeaturescouldbe theadditionofSiO2totheChugwaterAnorthositefromthecountry rockduring its ascent and fractionation.Thisadditionalsilicawouldreactoutanyolivinepresent,andthroughtheQUILFreactions(Lindsley&Frost1992),couldalsoraisethef(O2).Unfortunately,theChugwaterAnorthositeoccursmainlysouthoftheCheyennebelt,
wherethedominantcountry-rockisProterozoicmate-rial~300Myrolder than theChugwaterAnorthosite;even substantial amounts of contaminationmight notshowup in theNd andSr isotopic ratios (Scoates&Chamberlain2003,Frostet al.2010).Indirectevidencefavoringcrustalcontaminationofmantle-derivedmate-rialisthefactthatthehighestvaluescalculatedforsilicaactivity inparts of themain series arenear 0.75, notgreatly above those for olivine-saturation (0.67–0.70;Fig.11).
Emplacement of the Chugwater Anorthosite and magmatic processes: crystallization in situ
ThereareseveraltexturalandgeochemicalfeaturessuggestingthattheChugwateranorthositeaccumulatedessentiallyinplace.First,theplutoncontainslayeringonallscalesfromkilometertodecimeteranddisplaysfabrics and textures that are dominated by tabularplagioclase, a typically igneousmorphology.Second,we see enrichments in incompatible elementswithincreasingstratigraphiclevelthatarelocallyinterruptedbyexcursions toward lowervalues.These excursionsaremost readily interpreted as recording replenish-mentofnewmagmawithinamagmachamber.Thesefeatureswouldhavebeendestroyedhadtheplutonbeenemplacedeitherinthesolidstateorasamushwithahighproportionofcrystals.
We propose that the ChugwaterAnorthositecrystallizedwithin amagma chamber at a depth ofapproximately 10–12 km, as based on the availablegeobarometry.Themagma chamberwas filled by aseriesofinjectionsofamildlyhyperfeldspathicmagmawithapproximately40%entrainedplagioclasecrystals,as outlined above.Therewere at least threemajorepisodes of injection, corresponding to stratigraphicunits1,2,and3,buttheremaywellhavebeenmore.ExcursionsinXFe
Opx that themagma that fedUnit2mayhavebeenmarginallymoremagnesiancomparedtotheunitsaboveandbelow.Figure17isasetofschematicdiagramsillustratingtheemplacementoftheChugwaterAnorthositeasinterpretedhere.
TheU–Pbradiometricdatesonsamplesfromnearthebottom(BM136,1404mlevel,1435.4±0.5Ma;baddeleyite; Scoates&Chamberlain 1995) and top(KM13,8860mlevel,1436.0±0.5Ma;zircon;Frostet al. 2010) of theChugwaterAnorthosite are indis-tinguishablewithin analytical uncertainty, suggestingthat all 10,000+mmay have been emplaced (or atleast cooled through the closure temperatures forPbdiffusion in zircon andbaddeleyite, ~1000°C)withinamillionyears.Wehavenoreasontosuspectthatall10,000+mexistedasanopenmagmachamberatanygiventime,butitcertainlyappearsthattheentirebody
Fig.17. SchematicdiagramsshowingstagesinthedevelopmentoftheChugwaterAnorthosite.1.Injectionofatleastthreepulsesofmegacryst-richmagma,resultinginthethreeunitsofthemainseries.Silicaactivityinthemagmaswas0.70–0.75,solittletonoolivineformedfromthem.Theinjectionsneednothavebeensimultaneous,butsomeresidualliquidmusthaveremainedinolderunitsasnewpulseswereemplaced.2.Plagioclaseaccumulatesnearthebottomofeachunit.Someresidualliquidisremoved(notshown),somemigratesupward,producinganoverallupwardenrichmentinincompatibleelements.3.Atleasttwodifferentleucotroctoliticmagmas(onehigh-An,onemedium-An)areinjectedintotheChugwaterAnorthosite.Contactswithanorthositelayersaremainlysharp,suggestingthatthoselayersbehavedassolidsatthatstage.However,therewas extensivemixingbetween leucotroctolite and the residualmelts in gabbroic anorthosite layers,producingolivine-bearingmixedrocks.Not shown:Doming,whichmaywellhavebeenoccurringduringandafterstages2and3.AlltrueanorthositesintheChugwaterAnorthositeshowevidenceofhigh-temperaturedeformation,suggestingthat theyweremainlysolidat thetimeofthedoming.Incontrast,mostgabbroicanorthositesshowsome,butlesser,degreesofdeformation,suggestingthatsufficientresidualmeltremainedtoaccommodatemuchof thedeformation in those layersbyflow.Some leucotroctolites,especially thoseintrudingUnit1,aredeformed,butmostarenot,suggestingthattheywereemplaced,oratleastcrystallized,relativelylateinthedomingprocess.Domingprobablywascontemporaneouswith“filterpressing”toremovemuchresidualmelt.DrainingawayofdenseresidualliquidwouldhaveincreasedthegravitationalinstabilityoftheChugwaterAnorthositeandthuscouldhavecontributedtotheformationofdoming.
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was emplaced during that short an interval.Each ofthe threemain units likely crystallized and fraction-atedindependently,butseverallinesofevidenceshowthat the lower units had not fully crystallized beforetheoverlyingonewasemplaced.Themostcompellingevidenceistheoverallupwardincreaseinincompatibletraceelements (Figs.14–15),which isbestexplainedbyescapeofatleastsomeresidualliquidsfromlowerlevels into the units above. In addition, the fact thatthroughouttheChugwaterAnorthosite,theanorthositelayersaredistinctlymoredeformed than theadjacentlayersofgabbroicanorthositestronglysuggeststhatatthe timeof deformation (presumablyduringdoming;seebelow),themoremaficlayersineachofthethreemainunitsstillretainedsufficientmelttoaccommodatethestrainwithoutmajordeformationoftheplagioclase.Thisinterpretationisfurthersupportedbytheobserva-tionthattheintrudingleucotroctolitescrosscutandhavesharpboundariesagainstanorthositelayersbutclearlymixedwiththeslightlymoremaficlayers.
Origin of the layering
As stated above,we attribute the existenceof thethree stratigraphic units 1, 2, and 3 to threemajorinjections ofmagma. In particular, unitsAn2 andAn3canreadilybetracedthroughouttheoutcropareaof the stratigraphicChugwaterAnorthosite and aredistinctive in their thickness (km scale).UnitAn1 isless distinctive, in part because it contains numerousregions of gabbroic anorthosite thatmay result frommultiple smaller injections ofmagma. It is possiblethat decameter-scale layers also reflect episodes ofmagma recharge, at least in part.However, our datasimplydonotpermit testsof thishypothesis.Aswasthe case for the PoeMountainAnorthosite (Scoates1994,2000,Scoateset al.2010),plagioclasemusthaveaccumulated as a network of cumulus grains on thefloorofthemagmachambertoformthevariousscalesoflayering.Sincethehostmeltwashyperfeldspathic,only plagioclasewas initially stable, and each layerwould gain further plagioclase through adcumulusgrowth,therebyproducingthetrueanorthositelayers.Eventuallythemeltwouldbecomepyroxene-saturatedaswell,givingrisetolayersofgabbroicanorthositeaspyroxenescoprecipitatedwithplagioclase(Fig.17,step2).Theoriginofdecimeter-scalelayeringintheChug-waterAnorthositeremainsenigmatic.Deepweatheringandthelackoffreshoutcropsmakedetailedobserva-tiondifficult,butmodallygradedlayering,ascribedtodensitycurrentsintheSkaergaardIntrusionandDukeIslandComplexbyIrvine(1987),forexample,appearsto be lacking.One possibilitymay beflowdifferen-tiationduring emplacement of themagma,which, asdiscussed earlier, probably contained~40%entrainedplagioclasecrystals.FlowduringpenecontemporaneousdomalupliftoftheChugwaterAnorthosite(seealatersection)mayhaveaccentuatedlayerdevelopment.
Mixing with leucotroctolites
Themain-series ChugwaterAnorthosite, whichmainly lacksmodal olivine,was injected repeatedlybyat least twotypesof leucotroctolites(Fig.17,step3).Bothtypesaredistinctlymoremagnesianthanthemain-seriesChugwaterAnorthosite;onehasnormativeandmodalcompositionsofplagioclasethatarecloselysimilartotheChugwaterAnorthosite,whereastheotherisclearlymorecalcic.Thefirsttypecouldmixinsmallproportionswith theChugwaterAnorthosite,with theonly obvious evidence being an increase in orthopy-roxeneasolivine reactedwith silica;once theexcesssilicawasexhausted,smallamountsofmodalolivinewouldthenappear.Weseethisinalmosteveryinstancewhereleucotroctoliteintrudedandmixedwithgabbroicanorthosite.Plagioclaseislittleaffected,exceptthatitdevelopsarimlackingtheFe–Ti-richlamellaetypicalofmegacrysticplagioclase.Incontrast,wherethemorecalcicleucotroctoliteshavemixed,theplagioclaseoftheanorthositeshowsreverseoroscillatoryzoning.
PerhapsfivepercentofthemassoftheChugwaterAnorthositeconsistsoftheseintrudingleucotroctolites;another10to15%hasbeendemonstrablyaffectedbymixingwiththem,althoughtheoverallmassfractionofleucotroctolitewithinthemixedrocksisprobablysmall.Onesample(BM273,4180mlevel)isatruetroctolite;with 25%normative olivine and 63% feldspar, it isclosetothecotecticproportions.Itisequigranularandfinegrained(1–2mm),andalmostcertainlyrepresentsaliquid.Alltheotherleucotroctoliteshave70%norma-tive feldsparormore,and thusprobably reflect somedegreeofplagioclaseaccumulation,althoughmostlackobviousplagioclasephenocrysts.AnexceptionisBM52(6000m level),which has 89% normative feldsparandisthusnearlyananorthosite.Weascribethehighfeldsparcontentofthissampletoflowdifferentiation,withplagioclasephenocrystsconcentratedinaregionofpresumablyrestrictedflow.Thegeneticrelationshipof the leucotroctolites to themain-seriesChugwaterAnorthosite, if any, is unclear, although it is possiblethattheyreflectless-evolvedsamplesofthemagmathatgaverisetothemain-stageanorthosites.
Origin and timing of the doming
ThestratigraphicChugwaterAnorthosite formsananticline plunging to the south and southwest, and ismost likelya relicofadomal structure.Far too littlecountryrockisexposedtobecertain,butintheabsenceofknownregionaldeformationatthetimeofemplace-ment of theChugwaterAnorthosite,we assume thatthedomingwasproducedduring late-stageprocessesassociatedwithemplacementoftheChugwaterAnor-thositeitself.Alikelyexplanationisthattherelativelylight, plagioclase-rich cumulates becamegravitation-ally unstable andbegan to rise.Muchof the domingevidentlytookplacewhiletherewasstillresidualliquid
presentinthegabbroicanorthositelayers,wheremostofthedeformationassociatedwiththedomingcouldhavebeenaccommodatedbyflowofmeltbetweencrystals.Indeed,thisprocessmayhaveaidedinthedevelopmentof decimeter-scale layering. In the plagioclase-richlayers, however, the deformationwas accomplishedthroughrecrystallizationofplagioclasevia fastgrain-boundarymigration(Lafranceet al.1996).Thatprocessoccurs only at near-magmatic temperatures,which iscompatiblewiththenotionthatsomemeltremainedinmoregabbroiclayers.Someleucotroctolites intrudingtheChugwaterAnorthosite also show deformation,suggestingthattheyhadbeeninjectedpriortotheendofthedeformation.MotionassociatedwiththedomingmayexplainwhywefindnoobviousrooftotheChug-waterAnorthosite,forexampleahorizonofrelativelyfine-grained leucogabbro; in the oneplacewhere theexposuresaregood,gabbroicanorthositeofUnit3 isindirectcontactwithpartiallymeltedpeliticrocks.Wehavenowayofestimatinghowmuchverticaldisplace-mentwasassociatedwiththedoming.
Significance of the “structural boundary”
Asnotedinanearliersection,theanorthositicrockstothesouthandsoutheastofthe“structuralboundary”(Fig. 2) have distinctly different orientations and aremore strongly deformed, even though they are indis-tinguishablepetrographicallyandchemicallyfromthe“stratigraphicChugwaterAnorthosite”.We concludethattheserockswereapartoftheChugwaterAnortho-sitethathadadifferentdeformationalhistory.Wecanonlyspeculateaboutwhatledtothedifferentdeforma-tionalhistories,butonepossibilityisthattheChugwaterAnorthositeeastandsouthofthe“structuralboundary”simply underwent an earlier and possibly strongerdomingevent.AnotheristhatitwasstronglyaffectedbytheemplacementoftheMaloinRanchPluton,abasinalstructureplungingtothesoutheast(Kolker&Lindsley1989).Permissivesupportforthisnotionisprovidedbythepresenceofinclusionsofhighlydeformed,high-AnanorthositeneartheboundarybetweentheChugwaterAnorthosite and theMaloinRanchPluton aswell asalong the structural boundarywithin theChugwaterAnorthosite. In the southeast, the structural boundaryappears to involve truncation of theAn1,Gan1, andAn2units by the broadly curvingupper units (Gan2,An3,Gan3);however,thepictureisobscuredbylater(Laramide?)faulting,Whateverthecauses,thesedefor-mationsappeartohavebeenmainlylate-magmatic,astheassociateddeformationoftheplagioclaseoccurredat a high temperature.However, local concentrationsofepidotenearthe“structuralboundary”suggestthatdeformation theremust have continueddown to lowtemperatures.
Clearly,thehigh-Aninclusionsassociatedwiththeboundariesareolder than theChugwaterAnorthosite,
butwehavenoideajusthowmucholder.Theytypi-callyaresodeformedthattheyappearlikegranulatedsugar.Oureffort toextractdatablematerial fromonewasunsuccessful.Onepossibilityisthattheyarecoevalwiththe1.76GaHorseCreekanorthosite,whichalsoishighinAn.AnotheristhattheyarebarelyolderthantheChugwaterAnorthosite,butthatliketheChugwaterAnorthositerockssouthandsoutheastofthe“structuralboundary”, they underwent strong deformation justpriortoincorporationalongtheboundary.
Extraction of residual liquid; nature of “filter-pressing”
Weestimatedabovethattoachieveitspresentbulkcomposition,theChugwaterAnorthositemusthavelostapproximately15–18%ofitsmass,theresidualliquid,whichispresumedtohaveacompositionclosetothatofsomeoftheferrodioritesfoundintheLaramieAnor-thositeComplex.Thisraisestwoimportantquestions:(1)whereisthatmassnow,and(2)bywhatmechanismwas it removed?Some residualmelt from the lowerportionsoftheChugwaterAnorthositemovedupward,as evidencedby theup-section increase in incompat-ibleelements.Althoughferrodioritesareabundantbothas dikes and pods in theChugwaterAnorthosite andadjacentrocks,theirexposedmassistrivialcomparedto the amount needed to achievemass balance.Thusmuchresidualmeltmusthavebeenremovedfromthepresent outcrop area of theChugwaterAnorthosite,leavingonlysmall,volumetricallyinsignificantpocketsandintrusionsofferrodioriteasevidenceofthismuchlargervolume.Oneveryrealpossibilityisthatthedenseresidual liquidsdraineddown-dipduring thedoming;indeed, theremovalofdensematerialwould increasethe gravitational instability believed to have causedthedomingandwouldaccelerateit.Thusmuchoftheresidualliquidmayliebelowthepresentlevelsofexpo-sure. In addition, a concept called “filter-pressing” iscommonlyinvokedtoexplaintheexpulsionoftrappedliquid,buttoourknowledgenoonehasyetproposedaviablemechanism to accomplish it.Onepossibilitywouldbethenucleationandexpansionofalatevaporphasethatmightdrivethetrappedliquidstoregionsoflowerpressure.Suchamechanismhasbeeninvokedfortheformationofdiktytaxitic texture inbasalts (Fuller1931, 1938). In diktytaxitic basalts, the gas-filledangular interstices remain, producing the distinctive“net-like”textureoftheserocks.Inananorthositeat3–4kbarpressure,however,thecavitieswouldcollapseasthegasescaped,therebycontributingtothedeformationsotypicalofanorthosites.Ifthismechanismhadbeenoperative in theChugwaterAnorthosite,quiteclearly,given thehigh temperatures and relatively anhydrousassemblage inferred for it, the vapor could not havebeenwater-rich.Although space precludes a detaileddiscussion here, there is substantial direct (Frost&
The ChugwaterAnorthosite has a mappablemagmatic stratigraphy totaling at least 8,000meters,andprobablymorethan10,000meters.Itsmainseriescomprises threemajor units, eachwith a dominantlyanorthosite(>90vol.%plagioclase)baseandgabbroicanorthosite (80–90vol.%plagioclase) upper portion.Mostof theplagioclase in themain series isAn50–55;olivineisessentiallyabsent.Layering,mostlymodalbutinplacestexturalaswell,occursonscalesfromdeci-meters to decameters.Themain serieswas producedby threemajor injections ofmildly hyperfeldspathicmagma containing approximately 40vol.%entrainedplagioclasemegacrysts.Themain serieswas repeat-edlyintrudedbyatleasttwoleucotroctoliticmagmas.Contactsofleucotroctolitearesharpagainstanorthositebutdiffuse against gabbroic anorthosite; petrographicevidence showsmixing between leucotroctolite andthe residual liquidof gabbroic anorthosite, indicatingthatthelatterhadnotcompletelysolidifiedatthetimeofintrusion.Priortofinalsolidification,theChugwaterAnorthositewasdomed,probablyasaresultofgravita-tionalinstabilityofplagioclase-richmaterial.Removalofdenseresidualliquid(ferrodiorite)byfilterpressingprobablycontributedtothatinstability.
Althoughmassifanorthositesaregenerallyconsid-ered to have been emplaced as crystal-laden diapirs(Ashwal1993),theChugwateranorthosite,likeitssisterthePoeMountainanorthosite, isbestconsideredasaplagioclase-richlayeredintrusion.Thedirectevidencefor crystallization from an openmagma chamber ismore subtle in theChugwater anorthosite than in thePoeMountainanorthosite,whichrecordsfeaturessuchas scourmarksandmagmatic “drop stones” (Scoates1994, 2000, 2002,Scoateset al. 2010).Nonetheless,theevidenceisstrongenoughtosupportthecontentionthat theChugwater anorthosite formed by processesthat are not very different from those found in othermafic–ultramaficlayeredigneousintrusions,exceptforthefactthatthemajorcumulusmineralwasplagioclase,ratherthanolivineorpyroxene.
Acknowledgements
We thank Jon Philipp,GoeffRawling, and PattiBleifus for assistance during thefieldwork,WilliamMeurer,JeremyMitchell,andAllanKolkerforchemicalanalyses, andGregory Symmes, Susan Swapp, andBobRapp for electron-probe analyses.DHL thankstheUniversité Libre deBruxelles for a productivesabbaticalin1997–98,undertheauspicesofaUniver-sity InternationalChair, duringwhichmuch of thisreportwasprepared.ThisworkwassupportedbyNSF
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Received December 15, 2008, revised manuscript accepted July 6, 2010.
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