397 The Canadian M ine ralo g i st Vol. 35, pp. 397423 (1997) PRIMITIVEMAGMAS AT FIVECASCADE VOLCANIC FIELDS: MELTSFROM HOT, HETEROGENEOUS SUB.ARC MANTLE CHARLES R. BACONI, PEGGY E. BRUGGMAN, ROBERT L. CHRISTIANSEN, MICHAEL A. CLYNNE" JULIE M. DONNELLY-NOLAN ANDWES HILDRETH U.S.Geological Survey, 345 Mitdl.efield Road" Mmlo Parh Califurnia94025-3591,U.SA. ABSTRACT Major and trace elementconcenftations, including REE by isotopedilution, and Sr, Nd, Pb, and O isotoperatios have been determinedfor 38 mafic lavasfrom the Mount Adams,Crater Lake, Mount ShastaMedicine Lake, and Lassenvolcanic flelds, in the Cascadearc, northwestempart of the United States.Many of the sampleshave a high Mg# tl@Mg/(Mg + FeD > 601and Ni content(>140 ppm) suchthat we considerthem to be primitive. We rccognlzerhree end-member p:,jmillrve magma groups in the Cascades,characterized mainly by their trace-elementand alkali-metal abundances:(1) High-alumina olivine tholeiite (HAOT) hastrace element abundaaces similarto N-MORB, exceptfor slightly elevatedLllE, andhas Eu/Eu* > 1. (2) Arc basalt andbasaltic andesite have notably higher L/lE contents,generally have higher SiO2contents, are more oxidized andhave higher Cr for a given Ni abundancethan HAOT. Theselavas show relative depletioninl/F.i4 havelowerl/ftEE andhigherl,ftEEthan HAOT, andhave smallerEulEu* (0.94-1.06).(3) Alkali basaltfrom the Simcoevolcanic field eastof Mount Adamsreprcsents the third end-membr, which contributesan intraplate geochemicalsigpature to magna compositions.Notable geochemical featuresamong the volcanicflelds are: (1) Mount Adamsrocks are richest in Fe andmost incompatible elements including I/FSE; (2) the mostinconpatible-element depleted lavas occur at MedicineLake; (3) all centershave relatively primitive lavaswith high ULEIHFSE nttos but only the Mount Adams,Lassen, and Medicine Lake volcanic fields also haverelatively primitive rocks with an intraplategeochemical signature; (4) there is a tendencyfor increasing87SrF6Sr, 2r7Pbp4Pb, and 6180 and decreasing 236Pbp@fband 143Nd,f144Ndfrom north to south.The tlree end-memberCascade magma types reflect contributionsfrom three mantlecomponents: depleted sub-arc mantle modestly enriched in ZIZE during ancientsubduction; a moder&hydrous subduction componenl and OlB-source-likedomains. Lavas with arc and intraplate (OIB) geochemicalsignatures were eruptedclose to FIAOT, and many lavas are blends of two or more magma t)?es. pre-eruptive HzO contents of FIAOT, coupted with phase-equilibriumstudies, suggestthat these magmas were relatively dry and last equiJibratedin tle mantle wedge at temperaturesof -1300"C anddepths of^40 km, virtually at the baseofthe crust.Arc basaltand basaltic andesite reprcsent grcater extents of melting than HAOT, presumablyin the same generalthermal regime but at somewhatlower mantle separation temperatures,ofdomains ofsub-arc mantlethat havebeen enriched by a hydroussuMuction componentderived from the young, relatively hot Juande Fucaplate. The primitive magmasoriginated by partial melting in responseto adiabaticupwelling within the mantlewedge. Tectonic extension in this part ofthe Cascadearc, onecharacterized by slow obliqueconvergence, contributes to mantleupwelling and facilitates eruptionof primitive magmas. Keyvords: arc magmatism,primitive basalt,geochemistry, isotopes, tace elements,Cascades, northwestern United States. SoMl\aans Nous avonsd6termind la concentrationdes 6l6mentsmajeurs et des 6l6mentstaces, y comprisles terresrares, par dilution d'isotopes,ainsi queles rapportsdes isotopes de Sr, N4 Pb et O, de 38 6chantillonsde lavesmafiques des suites volc?niques de Mount Adams,Crater Lake, Mount Shasta,Medicine Lake, et Lassen,dans l'arc desCascades, dans le nord-ouestdes Etats-Unis. Plusieursdes 6chantillonsont une valeur 6lev6ede Mg# tl00Me(Mg + FeD > 601et une teneur6lev6e en Ni (>1210ppm), de sorteque nous leurs attribuonsun caractdreprimitif. Nous prdconisousI'existence de trois p6les de rnagmasprimitifs dansles Cascades,que distinguentles concentrationsen 616mentstraces et en alcalins. (l) ks thol6iites i olivine riches en A1 (HAOT) possddentdes concentrations en 6l6mentstraces semblables i cellesdes basaltes normaux des rides oc6aniques(N-MORB), sauf pour un l6ger enrichissementen 6l6mentslithophiles d large rayon (Lll,E), et montrentun rapportEu/Eu* sup6riewh 1. (2) Les basaltesd'arc et les and€sitesbasaltiques possBdent une concentrationnetlsment plus 61ev6een ULE, et sont en g6n6ralplus siliceux, plus oxyd6s, et contiennentplus de Cr porrr une teneur en Ni donndeque H.AOT. Ces laves montrent en plus un appauwissementen 6l6mentsi large rapport de valencei rayon (I/FSE), des teneursen terresrares lourdesplus faibles et en terresrarcs l6gBresplus 6lev6esque HAOT, et un rapport EulEu* plus faible (0.94-1.06). (3) k basattealcalin de la suite de Simcoe,d I'est de Mount Adims, un troisihmepdle, contribueune signatureg6ochimique "intraplaque" aux compositionsde magmas.Parmi les caract6ristiquesg6ochimiques notables de ces suitesvolcaniques, signalons que (l) les rochesde Mount Adamssont les plus richesen Fe et en la plupart des6l6ments incompatibles, y comprisles Ir'F,SE, (2) les lavesles plus appauwies en 6l6mentsincompatibles se trouvent i Medicine Lake, (3) tous les centrescontiennent des laves relativenent primitives, ayant \ E-mail address:[email protected] 398 THE CANADIAN MINERALOGIST desftpprts LIIEIHFSE 61ev6s,mais seulsles centes volcaniquesde Mount Adams,Lassen, et MedicineLake contiennentdes rochesrelativement primitives ayant une signatureg6ochimique "intraplaque", et (4) 875r/865r,2x7Pbt2MPb, et 6180 tendente augmenter,et2MPbPMPb et l43Nd/144Ndtendent i diminuer, du nord vers le sud. Les tois p6les identifi6s parmi les magmas des Cascadest6moignent ds I'imFlication de trois composantesdans le marteau: un manteauappaulri en dessousde I'arc, et modestementenrichi enLIIJ4 pendantun stadede subductionplus ancien,une composalterepr6sentart la plaquecontemporaine, hydrat6e,en subduction,et un mantsauressemblant A la sourcedes basaltes des lles oc6aniques.Des venuesde magmaayant une signaturegdochimique typique d'un milieu d'arc et d'autrestypiques d'un milieu intraplaquesont mises en placeh proximitd de venuesHAOT, et plusieurslaves sont en fait un m6langed'au moins deux sortesde magmabasaltique. D'apGs les teneursen H2Odes venues HAOT avantleur 6ruption,consid6r6es avec les r6sultatsd'6tudes de l'6quilibre desphases, ces magmas 6taient relativementsecs, et ils se sont dquilibr6sen demier lieu dansle coin de manteaupar dessusla zone de subductioni environ 1300"C et d une profondeurd'environ 4O km, donc prds de la basede la cro0te. Irs venuesde basalted'arc et d'and6site basaltiquerepr6senteraient des taux de fusion plus 6lev6sque pour le cas de HAOT, pr6sumdmentdans le m0ne contexte thermiquemais i une temperaturede s6parationplus faible, dansdes domainesdu manteauen dessousde l'arc qui avalent6te enrichis par une composantehydratfe venant de la subductionde la plaque luan de Fuca relativementjeune et chaude.I,es rnagmasprimitifs ont pris naissancepar fusion partielle en r6ponsei une remont6edans le coin du manteau.Une extension tectoniquedans cette partie de l'arc des Cascades,due i une convergencelente et oblique, a contibu6 i la mont6edu manteau et a facilitf l'6ruption desvenues de magmasprimitifs. Clraduit par la R6daction) Mots-cl6s: magmatismed'arc, basalteprimitif, g6ochimie,isotopes, 6ldments traces, Cascades, nord-ouest des Etab-Unis. INTRoDUCiloN primitive lavas is beyond the scopeof this study (cl Botg et al. L997). Many of the aaalyzedsamples are poor in incompat- major goal oosee The of this study is to through' ible elementsand belong to the group variously termed effects of ffierentiation and contaminationin order to high-alumina basalt Gerlach & Grove (1982); establishmantle chaxacteristics melting processes [HAB: and seeDonnelly-Nolan et al. (1991,p. 27,856)for history beneaththe Cascades(Frg. l) by phenocryst- selecting of HAB terminologyl, high-alumina olivine tholeiite poor primitive samplesof chemically lavathat approxi- Hart et al. (1984), Bacon (1990), Barnes primitive [HAOT: mateliquid compositions.We considerrocks (1,992)1,or low-K olivine tholeiite (LKOT: Bullen & if they are rich in Mg relative Fe, high to have concen- Clynne (1989); we will refer to theseas HAOT. Other trationsof compatibletrace elementso and low contents lavas are designatedarc basalt(SiO2 < 52 wt.%d)or ba- of phenocrysts (generally 4Vo). ln extreme cases, saltic andesite(SiO2 > 52 wt.Vo;a sampleof magnesian primitive lavasmay approachcompositions of primary andesitewith 58 wt.VoSiO2 is included with basaltic magmas,in which ferromagnesianphenocrysts have andesitefor simplicity). One sarnplein our set is an mantle-compatiblecompositions, but in most of our basalt from the Simcoe volcanic field, east of elamples, tJte designationprimitive merely indicates alkali Mount Adams in Washington;it representsa melt of that a sampleis oneof a particulareruptive unit that has intraplate-typemantle. been least modified since the magmaleft the mantle. himitive lavas have equilibrium assemblagesof phe- Our study addressesquestions of mantle composi- nocrysts and commonly have olivine or oli- tion, influence of subduction,melting processes,and vine + chromian spinel as their only phenocrysts. thermal structurein the mantle beneaththe arc. Vari- Olivine is typically Fo66s in tholeiitic (seebelow) and ation in the compositionof the crust complicatesinter- Fo36-e6incalc-alkaline lavas. The chromian