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 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 andbasaltic 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 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 spinel is pretationof geochemicaldata, even for primitive lavas, presentin a wide range of compositionsthat correlate as will be seenbelow. Neverfheless,by consideringour with rock chemistry(Clynne & Borg 1997).Plagioclase new results, unpublisheddata" and the few published phenocrysts(Anzo-sg) occur in somelavas, and clinopy- results of complete analysesof primitive rocks from roxene (typically diopside with 0.5-1.07oCr2O3) also elsewherein the Cascades,we are able to suggestan- may be present,especially in basaltic andesite.Most swersto thesequestions. samplesin our data-setare comparativelyrich in com- We usedour knowledgeof the eruptivehistories and patible frace elements (Fig. 2; e.g., >1.44 ppm Ni, compositionalvariety of five volcanic centersthat we >200 ppm Cr), havehigh Mg contents(8-10.5 wt.7o have mappedin detail to guide us in choosingrepre- MgO), and a high Mg# l= 100Mg/(Mg + Fe9l sentativeprimitive lavasfor analysis.It is imFortantto (1e., >60). A few evolved sampleswere deliberately appreciatethat each of the five centersis a volcanic includedin orderto assessfractionation and contamina- field with, many vents, even though it may bear the tion. Becauseprimitive lavas are not common among nameof a single large (e.9., Christianset et al. the many monogeneticvents and shield volcanoesof L977, Donnelly-Nolan 1988, Bacon 1990, Clynne any given center, detection of systematicacross-arc 1990, Hildreth & Lanphere 1994). Although we at- compositional variation is hampered by the small tempted to characto'rlz;ethe primitive rocks of each number of samples;exploring larger data-setsof less center at the time this study was initiated, the entire PRIMITTYEMAGMAS AT FIVE CASCADE VOICANIC FIELDS 399

Frc. l. Location rnap showingmajor Cascadevolcanoes (dots), the Cascadevolcanic front Ghort dashes),and plate tectonic features(modified after Muffler & lamanyu 1995). Dept! contourson the top of the slab (1m) dashedwhere known. The number of the narfu1smagnetic anomalies is shownfor oceanicplates (age of anomaly 8 - l1 Ma). The small, young Juan de Fuca and Gorda plates convergeslowly with the North AmericanPlate. Plate convergence is normalto the Cascadearc in northernWashington and British Columtia obliquein southemWashington - northemCalifornia. The vol- canic zone is )1@ h wide at Mount Adams. Mount Shasta- Medicine lake" and Lassen,-30 lxn wide at Crater lake. Volcanoesnot part of this study: M, Meager Mountain; C, Mount Cayley; G, Mount Garibaldi;B, Mount Baker; GP, Glacier Peak; R, Mount Rainieq SII, Mount St. Helens; H, : J, ;TS, ThreeSisters; N, Newberry. range of primitive compositionscannot be represented compensatefor this inadequatecoverage, our interpre- by a small number of samples,and our coverageof tations also consider published data and unpublished somecenters is not comprehensive.In orderto partidly results of analysesin our files. Here, we report a new 400 TI{E CANADIAN MINERALOGIST

TABLE 1. CHEI\1trCAL AND ISOTOPIC ANALYSES OF PRIMIITYE LAVAS

Sample MA:767 MA-696 MA-120 MA-953 MA:322 SM-27 75SH-270 75SV-3 75SH-317 75SH-268 C€nter Mount Mount Mourt Mourt Mormt Simcoe Mount Mount Motmt Morml Adms Adams Adams Adms AdeN Mountains Shasta Shasca Shasta Shasta lYpe* HAOT HAOT HAOT AB AB ALKB HAOT BA BA BAT Iatifirde 46o03.9 6o25.31' 46"t9,31', 46"21.34' M"O2.t4' 45"5E.59' 41"14.58' 4to?3.82' 4to2t.93', 470t5.70' Longtude 121.15.05' 727.45.41' l2l'30.01' t21o42.68'12t 29,83' 120"57.29', t22.Ot.38' 122"t1.39' t22"12.38', t22"13.14 wt To SiO2 48.1 47.5 47.9 49.5 49.0 49.O 49.4 53.1 52.9 55.3 AlzQ 16.3 t6.9 16.7 15.6 15.9 t6.3 17.4 15.1 16.8 16.8 FezQ 1.77 1.96 1.5 2.41 3.07 4.66 t.2l 3.2t 2.15 1.55 FeO 9.30 9.31 9.27 6.40 5.92 6.25 8.00 4.15 5.20 5.ll L{eo 9.62 8.85 E.7E 9.62 8.87 6.28 9.30 9.78 8.94 6.59 CaO 10.1 10.5 10.25 9.81 9.40 7.33 r0.9 9.73 9.62 8.75 Na2O 2,85 2,85 3.02 3.17 3.50 4.77 2,68 2.99 3.06 3.35 Kp o.t73 0.160 0.311 1.480 t.074 2.254 o.2u 0.698 0.468 0.U2 liO2 1.22 t.39 r.42 1.36 1.50 2.28 l.0l 0.70 0.56 0.77 Pzos 0.11 0.16 0.16 0.52 o.39 0.9 0.1I o.2l 0.11 0.18 l{nO 0.t7 0.18 0.18 0.74 0.14 0.15 o.t7 0.13 0.13 0.ll H2O+ 0.31 0.30 0.20 0.16 o.22 0.50 0.27 0.44 0.22 0.28 HzG 0.13 0.ll 0.07 0.07 0.11 0.16 0.09 0.23 0.10 0.18 ca2 o.M 0.01 0.04 0.02 0.04 0,03 0.04 0.05 Toal 100.19 100.1E 99.80 100.26 99.t3 tO0.23 100.79 100.51 100.31 99.81 Mef 6r.2 58.8 59.6 66.7 64.6 51.7 64.6 77.2 69.t U.4 ppn Sc 28.8 32.9 32.7 28.0 25.9 13.8 36.3 n.4 27.2 23.2 V 189 2t7 2t2 180 208 lq 2?r 2fit 198 1E4 Cr 345 A5 3m 347 326 129 389 500 476 226 Co 52.t 53.8 46.8 41.3 42.3 34.4 41.6 34.8 35.6 27.9 Ni t43 t78 t34 2tt t75 103 N 143 t66 67 C1l 58 88 68 92 77 30 28 56 65 5l 7-n 7E 85 838481 92 82 78 78 7l Rb 3.0 3.3 6.9 23.5 7.4 37.6 4.7 8.9 6.7 t2.9 Cs 0.053 0.a42 0.111 0.2?5 0.101 0.359 0.202 0.419 0.159 0.830 Sr 2/13 %:1 289 967 n2 99s n2 M 354 ffi1 Ba 6E 58 8l 547 347 672 t22 225 l4E 257 Pb 0.62 0.42 0.77 4.2tt 3.35 2.06 1.79 3.01 2.21 4.n IA 4.38 5.87 6.94 38.45 22.35 210.90 4.48 10.15 4.53 10.E3 & rr.45 15.29 r7.r2 81.32 50.15 86.53 11.85 '4.00 10.86 25.84 Nd 8.55 9.60 11.78 45.50 27.51 42.96 9.09 13.28 7.M 14.29 Sm 2.55 3.10 3.21 8.59 5.14 8.45 2.77 2.85 1.84 3.09 Eu 1.01 t.2l r.2n 2.42 1.67 2.76 1.05 0.93 0.66 L.O2 Gd 3.18 3.80 3.86 6.53 4.50 7.O2 3.52 2.64 2.04 2.89 ry 3.53 4.?2 4.r9 4.56 3.76 5.33 4.22 2,50 2.23 2.69 Er 2.O3 2.59 2.60 2.il 2.tL 2.42 2.68 r.45 t.37 1.53 Yb 1.79 2.39 2.20 1.90 1.74 1.93 2.50 t.34 1.30 1.38 Y 23 26 u. 24 25 31 23 2t 16 19 Zr 83 98 tt2 2u 165 n5 84 106 70 t22 Hf 1.74 2.31 2.M 4.t9 3.34 5.36 1.90 2.13 1.50 2.72 Nb 4.9 6.0 7.7 15 t4 57 2.5 3.E 2.2 4.2 Ta 0.349 0.46 0.51 0.91 0.91 3.77 0.188 0.n2 0.158 0.n4 Th 0.s9 0.62 0.68 5.09 2.33 3.96 o.il r.74 0.76 2.56 U 0.78 l.3l 0.60 0.90 87Sr/86Sro.702u 0.70292 0.70292 0.70360 0.70329 0.70306 0.7M13 0.70366 0.70362 0.703M l43Ndi/r44Ndo.sr2979 0.513013 0.513013 05r292s 0.512975 0.5t2874 0.51282e 0.512899 0.512887 0.512875 206,Fibr?.MPb19.037 18.925 18.856 18.967 18.872 t8.692 18.803 18.865 18.776 t8.842 2stPbl2uPb rs.s67 15.568 15.540 15.583 15.559 15.525 15.611 15.s87 15.569 15.595 208Pbp04Pb38.528 38.475 38.359 38.630 38.473 38.283 38.541 38.491 38.384 38.509 6180 +5.7 +5.7 +5.7 +5.1 +5.5 +6.0 +4.7 +6.9 +6J +.7.1

8HAOI high-sluminaolivlne tholeiitq AB, arc basalqBA arc basalticandesite; ALKB, alkel.ibasalt I75SH-X8 conteins-40 % phenocrystsofplrqioclase + augite+ orthopyroxene. 4nalyqtli D.F. Siems, major elements;S.T. Pribble FeO, Na2O,H2Ol CO2,Nb, V; T.L. Fries, K2O: J.N. Grossman.Sc. Cr, Co. Zn, Cs, Hf, Ta, Th U; O isotopes,L.H. Aclarni;resL P.E. Brussma[ Seetext foi methods. Ilash indicat€srh41 concentsation was below detectiontinit-oi thatelem€nt was not analyzed

GEoLoGIcALSruwc data-setthat is internally consistent,precise (e.g., con- centrationsof the rare-earthelements, REE,by isotope The Cascadevolcanic arc extends from northem dilution), and reasonablycomplete in termsof the ele- Califomia to southernBritish Columbia (Fig. 1). Arc mentsand isotopes of interestto geochemists. volcanism is associatedhere with subduction of the PRI1VtrTTYEMAGMAS AT FTVE CASCADE VOLCANIC FIEI.DS 401

TABLE I. CHEMICALAND ISOIOPICANALYSES OFPRJMIITYELAVAS Sample 8lc62l 82eA94 84C1143 88C15rf0 88C1530 88C1521 88C1s23 88C1557 8rc354 Centsr Craler Crater &abt Crafer Crater Craler Craler craler Crater lake Id

young Juan de Fuca and Gorda plates. From Glacier compositevolcanoes and abundantmonogenetic volca- Peak northward,convergence is normal to the axis of noesranging from cinder conesto shields.There is no the arc, and volcanism is restricted to comparatively consensusas to whetherfeatures such as the Medicine isolatedcomposite volca[oes and domeclusters (Rogers Lake ,which lies behindthe main axis of 1985, Guffanti & Weaver 1988). Between Mount the arcoshould be considered"Cascade" volcanoes. We Adams and the Lassenvolcanic center, subductionis includethem in the scopeof this paperin orderto obtain oblique, and the arc consistsof widely spacedmajor a completepicture of magmatismin the Cascadearc. 402 THE CANADIAN MINERALOGIST

TABLE I. CHA4ICALAND ISOTOPIC ANALYSES OFPRIMIITVEI,AVAS

Sample L8&1398 LC861046 IrA2-nO rc88-131I LC861005 LM87-13E4IrcEE-1312 rc82-905 rc8G855 rc85-671 Cent€r r assen Lassen L€ssen [.assen I crs€n Lass€n t ssen lassen L,ass€n Lass€n rype AB HAOT HAOT HAOT AB AB AB AB AB AB Ladtude 40,00.90' 40"26.47' No23,44', &o19.20' 4oo40,t2' &.32,16', ff20.46', 40"32.47' 40'31.88' 40038.75' Irngtrde 121.57.77'121.59.15'l2l't7,5L' t22'03.94', t2t"45.M', tzt"tt,7E', 122003,08',t2to09.20' 127"02.93' 121o09.83'

wLVo siQ 49.5 48.2 48.4 48.0 51.3 49.8 50.8 49.7 49.5 51.0 AlzQ t6.4 t7.4 17.4 18.0 t5.7 15.6 16.6 17.5 t6.7 t6.9 FezQ 1.78 l.8l 1.68 2.35 1.7 3.24 1.22 2.33 3.66 2.tu FeO 7.40 7.46 7.76 6.61 6.36 5.39 6.4E 6.22 5.80 6.02 Mp 10.3 10.1 9.79 9.34 1l.l 10.7 9.97 8.32 8.19 8.00 CaO 10.5 tt.4 tt.2 n.2 t0.7 9.81 l1.0 9.66 9.79 9.08 Na2O 2.47 2.55 2.6& 2.52 2.45 3.06 2.48 3.10 3.25 3.r2 Kzo 0.403 0.138 o.t72 0.157 0.377 09U 0.299 0.659 0.830 1.006 Tio2 0.E5 0.80 0.96 0.81 0.52 1.32 0.73 0.98 1.34 l.l8 Pzos 0.11 0.09 0.10 0.09 0.08 0.46 0.10 0.25 0.2E 0.30 IVIDO o.t1 0.17 0.17 0.16 0.14 0.15 0.15 0.16 0.15 0.15 H2O+ o.49 0.11 0.13 0.48 0.13 0.16 o.lE 0.38 0.2 0.42 HzG 0.22 0.05 0.05 0.37 0.05 0.04 0.05 0.36 0.13 0.o2 cn2 0.07 0.02 0.03 0.05 0.04 0.04 0.04 Toal 100.60 100.30 100.52 1m.16 100.55 [email protected] 100.06 99.62 99.86 99.88 Msf 67.1 66.5 65.3 65.6 71.5 69.7 70.r &.t 61.6 62.9 ppm Sc 38.1 40.0 43.0 40.1 34.0 26.3 37.6 32.4 28.7 25.0 v 2t5 218 228 227 223 49 2r4 2t8 237 246 Cr N2 229 323 235 6?A W 501 323 u9 3t7 Co 43.9 46.1 48.1 45.0 43.7 40.0 38.5 38.5 38.1 36.7 Ni 2t3 190 tE4 lEO 2W 752 85 tU tt7 155 Ol 79 93 tzt 94 69 50 32 74 66 7l 7,1 70 70 74 7l 68 9r 70 8l 85 77 Rb 6.2 1{ I ? 3.3 16.l 4.1 7.2 10.9 16.3 C8 0.319 0.032 0.062 0.057 0.107 0.295 0.226 0.229 0.265 0.402 Sr ?37 24r 235 45 294 601 520 405 449 486 Ba 26& 76 89 85 112 4t3 10E n6 343 360 Pb 2.32 t.O2 1.20 1.00 t.40 3.90 1.40 4.30 3.03 3.31 I5 7.94 3.03 4.44 4.00 3.69 20.76 6.99 11.48 t3.o2 13.{l G 17,93 8.43 9.94 8.81 8.96 49.60 16.44 26.52 37.75 31.18 Nd 10.69 6.91 8.03 7.t4 6.09 26.45 9.48 t4.87 18.46 t1.M Sm 2.78 2.17 2.57 2.23 t.69 5.20 2.26 3.35 4.28 3.93 Eu 0.96 0.87 1.00 0.88 0.61 r.47 0.83 1.05 1.36 1.30 Gd 3.21 2.95 3.44 3.03 1.98 4,20 2.58 3.32 4.36 3.99 Dy 3.92 3.95 4.42 3.88 2.22 3.27 3.20 3.46 4.18 3.E5 Er 2.60 2.74 2.97 2.67 t.45 t.71 2.21 2.14 2.4t 2.22 Yb 2.58 2.80 2.95 2.69 1.44 1.50 2.20 2,02 2.2t 2.M Y 2t 19 u2l t4 16 L9 19 2t 18 b 76 60 796/. 59 142 E0 108 rzt t29 Hf t.74 l.3E 1.79 t.47 t.t4 3.10 1.72 2.43 2.68 2.7 Nb 2.O 1.5 2.t 1.8 1.5 ll 2.2 7.8 7.3 9.3 Ta 0.15 0.16 0.t2 0.691 0.162 0.511 0.498 0.63 Th 2.50 0.n 0.45 0.32 0.61 2,34 0.68 t.74 t.75 1.93 U 0.61 0.41 0.62 0.58 0.73 0.31 875r/865r 0.7M20 0.70380 0.70381 0.70379 0.70383 0.70408 0.70317 0.70389 0.70400 0.70392 r43Ndjfl44Nd.ostmo 0.5128750.512891 0.512905 0.512E98 0.512828 0.512970 0.512833 0.512800 0.512780 206Pbt2t4Pb18.715 18.751 18.952 18.947 18.863 18.835 18.814 t9.0t2 18.838 18.858 207Pblzo4Pb15.601 15.581 15.593 15.589 15.608 15.599 t5.579 15.621 15.605 t5.602 2o8Pt/2o4Pb38.438 38.398 38.556 38.558 38.506 38.512 38.388 38.617 38.526 38.523 StEO +7.2 +6.1 +5.8 +6.7 +5.6 +6.9 +6.9 +5.9 +6.8 +7.4

Chemicallyprimitive lavasare present throughout most Cenozoiccrystalline basementof the sub-arccrust is of the arc in the zone of oblique convergence.These widely exposednorth of a point approximatelymidway typically issued from monogenetic cinder cones, between Mounts Adams and Rainier. From Mount shields,or fissure vents. Shastato the south, Mesozoic and Paleozoicoceanic From the Tbree Sisters to Lassen.the Basin and andimmafure continental terranes are exposed adjacent Rarge provincaimFinges upon the arc, so that NWaSE to the arc. The interveningregion, however, lacks base- to N-S normal faults are commonat leastas far west as ment exposures,and the modernarc is constructedon the arc axis. Normal faults in the Cascadescontinue as Tertiary arc volcanic rocks. Tertiary plutons cut these far north as the Columbia River (Walker & Macleod andbasement rocks in the north, andlocally asfar south 1991),but do not occur in southernWashington. Pre- as cental .There may be no pre-Tertiarybase- PRIMITTVE MAGMAS AT FIVE CASCADE VOICANIC FIELDS 403

TABLE I. CHEMCAL AND ISOTOPICANALYSES OF PRIMITTVEIAVAS Sample t.c8clm9 82-72-t 767M 880M 1085M 8slM 1376M 881M 1161M Cerrter Lassen Medicine Medicine Medicine lvledicine Medicine Mdicine Medicine Medicine I qke Irke ldre lake Id1g lgke lake Lake Type BA HAOT IIAOT HAOT HAOT HAC}T IIAOT HAOT HAOT ktitude 40044.03' 41.30.3" 41"47.1" 41030.0" 41"54.5" 47047.2" 41o32.9" 41o30.0" 4l'30.7" Inngitude 121.50.13'l2l'38.7" 121..36.9" 121o37.9" 121.38.6" 121"34.0" 121.14.7' 121038.0"l2l'37.5" wt.Vo Si02 58.2 47,6 47.3 47.9 48.0 47.6 47.9 49.0 52.7 AIzQ 16.0 18.5 18.0 18.3 17.8 18.4 18.2 r7.7 t6.7 Fe2O3 1,.66 2.06 1.5 1.24 t.94 t.75 r.83 1.17 1.34 RO 4.13 6.33 7.2 7.31 7.r5 7.r0 7.29 7.86 6.90 Mso 7.57 10.5 10.4 10.3 10.3 9.91 9.40 8.63 6.85 CaO 7.72 1.2.0 11.8 r2.l 11.9 tt.1 l1.3 1t.4 9.55 Na2O 3.48 2.26 2.38 233 2.43 2.47 2.46 2.78 3.05 Kzo 0.806 0.074 0.062 0.073 0.085 0.094 0.114 0.333 1.092 Tt02 0.s2 0.59 0.71 0.63 0.74 0.11 0.80 0.90 0.92 Pzos o.t2 0.06 0.08 0.05 0.08 0.10 0.07 0.ll 0.13 N4nO 0.10 0.15 0.15 0.15 0.16 0.16 0.16 0.16 0.15 H2O+ 0.2t 0.19 0.23 0.26 0.r7 0.23 0.?0 a.n 0.35 Hzo- 0.09 0.09 0.14 0.13 0.t4 0.10 0.38 0.06 0.24 co2 0.17 0.20 0.03 0.02 0.05 0.02 0.02 Total 100.61 100.57 100.15 100.80 t00.92 100.43 100.60 100.29 99.95 Mgf 70.6 69.6 68.4 68.6 67.4 67.1 65.2 63.3 60.1 ppn Sc 19.2 31.8 35.8 34.8 37.8 35.7 40.8 35.5 32.0 v r49 t78 184 182 196 192 202 205 191 Cr a:2 n6 225 189 2n 201 2t3 158 rn Co 29.5 48.6 50.1 49.7 50.7 48.6 47.3 43.9 37.1 Ni 17t 218 297 205 185 176 t& t23 104 Cu 50 tzt 105 118 74 111 ll0 tt7 t02 7^ 61 55 63 58 60 65 65 76il Rb 10.9 1.5 2l I1.9 3t.4 Cs 0.429 0.078 0.022 0.080 0.029 0.028 0.49t 2.04 Sr 762 178 2tL t77 2t1 2Vt 255 t99 r93 Ba r79 17 4829 40 47 79 87 U9 Pb 3.40 0.35 0.35 0.31 0.38 0.61 1.13 3.2r la 7.53 1,.25 1.66 1.33 1.98 l.8l 2.65 3.87 8.08 Ce 16.04 3.82 4.82 4.04 5.01 5.47 6.35 10.37 19.20 Nd 9.32 3.62 4.79 3.83 5.1,7 5.23 5.74 7.76 11.38 Sm 2.t2 t.34 1.68 1.42 t.75 l.8l 1.90 2.42 3.06 Eu 0.73 0.62 o.73 0.65 o.74 0.78 0.83 094 l.0l Gd 2.0t 1,.96 2.37 2.t0 2.46 2.50 2.65 3.20 3.64 Dy 1.85 2.66 3.15 2.85 3.16 3.26 3.47 4.00 4.46 EI 1.06 1.76 2.t0 1.89 2.08 2.17 234 2.60 2.88 Yb 0.98 t.70 2.06 1.83 1.99 2.07 2.t9 2.50 2.86 Y 13 15 19 22 2319n 2t Jl k 9246 56 5l 61 54 68 94 135 Hf 1.83 l.0l 1.18 0.96 t.21 t.23 t.Al 1.89 3.02 Nb 2.6 1.1 t.2 1.0 1.3 1.8 3.2 Ta 0.155 0.11 0.11 0.20 0.38 Th t.t4 0.n 0.28 0.19 0.36 1.03 4.t7 U 0.80 1.53 8?Sr/86Sr 0.70305 0.7034/. *or* *or* 0.74342 *or* o-r$ 0.7034t 0.70346 r43NdJ/l44Nd0.512869 0.512968 0.512965 0.512932 0.512936 0.512974 0.51.2936 0.512950 0.5t2932 2c6Pbl2MPb 18.674 18.692 18.873 18.819 18.914 18.872 18.881 r8.831 18.911 2olPbpMPb 15.535 15.6n 15.569 1,s.s76 1{ <72 15.565 15.590 15.538 15.582 208Pb204Pb 38.2t7 38.406 38.468 38.446 38.463 38.420 38.536 38.387 38.528 6180 +6.9 +5.9 +5.8 +5.8 +5.9 +5.7 +6.3 +5.2 +5.9

ment beneaththe Cascadearc betweenMount Adams The continentalcrust is ^40 km thick beneaththe and -100 km north of Crater Lake @iddihough et al. Mount Adams volcanic field (Mooney & Weaver 1986).Here, beneaththe arc, the basementis likely to 1989). A reversedseismic refraction profile -30 km be composedof accretedEarly Tertiary oceanicbasalt west-south"i/estof Crater Lake defined a crustalthick- and marine sedimentaryrocks (e.9., Wells & Heller nessof44 km Q,e.averet aL 1984),although it is uncer- 1988"Trehu et aL 1994\. tain if this is the caseunder the volcanicfield. Extensive 4M THE CANADIAN MINERALOGIST seismicrefraction surveys of the Mount Shasta- Medi- MAT 262 variable multicollector mass spectometer. cine Lake areadelineate crustal structurebut not total Measured86SrF8Sr ratios werenormalized to a value of thickness(Fws et al. 1987),which Mooney & Weaver 0.7194.and 146Ndr/144Ndratios. to a value of 0.7219. suggestedis 38-40 km. Crustal thicknessbeneath the Empirical thermal mass-fractionationcorrections of Lassenregion is 38 + 4 km (Mooney & Weaver 1989). o.l|Vooper massunit wereapplied to Pb isotopemea- surements.Oxygen was exfractedwith C1F3,converted Mgrnons to CO2, and analyzed using a modified Nier-type, 6-inch dual-collecting mass spectrometer.Reported Samples whole-rock 6180values are calibratedto a 6180value of +9.6Voofor NBS-28 quartz relative to the SMOW Sampleswere selectedfrom collectionsmade during standard.Further details of analltical proceduresfor geologicalmapping of Mount Adams Qlildreth & Fier- isotopic compositionsare given in Bacon et al. (L994). stein 1995), and Crater Lake caldera @acon,unpubl. map, 1995),Mount Shasta(Christiansen, PRtrvtrflvEMacMa-Tvpss unpubl.map, 1995), the Medicine I akevolcano (Donnelly- Nolan, unpubl. map, 1995), and the Lassenvolcanic We recognizet)uee end-memberprimitive magma- center(Clynne & Muffler, unpubl.map, 1995).Powders groups in fhe Cascades,characterized mainly by their were groundin an aluminashatterbox. Although many tace-element and alkali-metal abundances:high-alu- sampleshad been analyzedpreviously, the entire set mina olivine tholeiite. arc basaltand basaltic andesite, was re-analyzedas a group (exceptfor isotopic compo- and intraplatebasalt. Names of theseend-members re- sitions and someREE abundances)in order to improve flect historical precedengthe subduction-relatedgeo- precision. chemistryof'arc" basaltand basalticandesite, and the fact that tholeiitic and innaplatebasalts are not limited Major ekments in occurrenceto the arc iself. We emphasizethat a continuum of compositions seemsto exist between Major elementswere determined by wavelength- these end-members,although examplesintermediate dispersionX-ray fluorescence(XRF) on fused glass betweenextreme.arc and intraplatevarieties are,scarce, disksin Iakewood, Colorado.The Na2Oand K2O values and that assignmentof some lavas with intermediate given in Table I weredetennined by flame photometry. characteristicsto any one group is somewhatarbitrary. Ferrousiron, H2O1,and CO2contents were determined A fundamental observationis that approximately by standardwet-chemical methods. coeval lavas of different types have erupted from nearbyvents. A corollary is that Quatemaryacross-arc Trace ekm,ents compositionalvariation, present where the volcanic arc is wide, is manifestedonly in the broadestterms by Lead and REE concentrationswere measuredby occrurenceof extremesubduction-component-enriched isotope-dilutionmass spectrometry. Energy-dispersion magmasin the fore-arc and more aftafine, intraplate XRF was used for Rb" Sr, Y, Zr, Bu Ni" and Cu types in the back-arc region (e.9., Lassenarea; Borg concentrations.Concentrations ofNb andV weredeter- et aL.1997). mined by inductively coupledplasma - atomic emis- sion spectrometryGCP-AES) following quantitative High-alumira olivirw tholeiite( IIAOT) chemical separation.Although the Nb values are pre- cise, there is somequestion as to their accuracyat the Primitive HAOT occurs locally throughout the lowest concentationsreported because of poor know- Cascadesfrom southernWashinglon to nortlern Cali- ledge of concentrationsin standards.The concentra- fornia. The field occurrenceof HAOT reflects the low tions of the remainingelements reported in Table 1 (Cs, viscosity of thesenormally phenocryst-poorto aphyric Sc, Cr, Co, Zn,Hf, TA Th, and lI) were measuredby magmas.Vents are markedby low shield summits,pit instrumental neutron-activation analysis (II,{AA) in craters,small spattercones, or spatterramparts along Reston, Virginia. In a few samples,some of these eruptiveflssures. Pahoehoe flow surfacesare common elementsarc presentat concentrationsbelow detection in complex sequencesof many related flows, each of limits. A specialeffort was madeto obtain data for Cs which may be as little as 0.5 m thick, but in their by INAA by counting after approximatelyone year in entirety may total many tens of metersin intacanyon orderto allow interferingnuclides to decayto low levels. settings or topographicdepressions. Tube-fed HAOT flow fields. suchas the Giant Craterflows at Medicine Isotopes Lake @onnelly-Nolane/aL 1991), extendas much as 45 km from their vents. Diktytaxitic texture and Isotopic compositionsof Sr, Nd, Pb, andO (Table l) subophiticgroundmass augite are commonin HAOT. were determined for all 38 samples of this study. The major-elementcompositions of HAOT samples Analyses(Sr, Nd, Pb) wereperformed with a Finnigan are similar to thoseof mid-oceanridge basalt,MORB, PRIMITTqE MAGMAS AT FWE CASCADEVOLCANIC FIELDS 405

W@ 19 12 # *, 18 l1 "4!NAB c) ,9, na t^ L .-Fl c I o 9.,tt -' g,;c A 8r0 aEl WLSM " ^c cc 16 XA@A I L^ AL "" s 15 8

'14 7 si02 5b 50 sio2 12 3.@

11 L i rcMc I 1.00 t, s thM csc I KL 10 L !A@ c l*',-9fi^ e *c.a n$L ur- o ^ A ont $s Aau"M s :< aF aM ?L'L" 0.10 I CL ft* M.c 7 M s SM 6 45 sio2 55 60 si02 60

Lg 70 MxatsLL ^ 'uu A L Hy cc 200 ^ I +65 c s o) c s "&do* ," 2 tso "fu%,\ 's AL c&^ 60 M tcw eAu 1@ L 55 s M. 50

SM 50 0r_ 45 si02 60 100 Cr 600 700

FIc. 2. Selectedvariation diagrams(major-element oxides recalculated to sum to 100wt.7o volatile-free and with all Fe asFeO*, Ni and Cr in ppm). Bold italic letters identify samplesof IIAOT, including differentiatedand contaminatedsample from Medicine Lake. Note log scalefor K2Oin order to show variationin HAOT. Non-italic lettersidentify arc basaltand basaltic andesite,which have higher SiO2, K2O, and Cr, generally lower Al2O3 and CaO contentsthan HAOT; MgO and Mg# tl00MgO/(MgO + FeO*), molarl canbecomparable.,SMismantle-xenolith-bearingalkalibasaltfromthe SirncoeMountains volcanic field" High Ni content and Mg# of most samples,including basaltic ,imply that they are primitive. Representativebasalts in this and other figures plotted for reference:^lAB is Okmok basalt sampleID-16 of Nye & Reid (1986),l7lis KilaueabasaltstandardBHVO (tr.btr;y 1983),andMORBis aFAMOUS meamid-oceanridgebasatt (Langmuir et al.1977). 406

6@ a M 11 lrt.ld1'l.FvF M I C Cralerlake M M Mediche lrke 5@ t l; ih'.sn*" L LassenvF 400 I :J {AA IU '?:" 5 A f; soo ut "h'* w@"&*3 cc 1.0 P 2AO

LL 100

0.9 0 123 20 LalSmp $te 0.10 150 c 125 0.08 W e/ 100 -a *Y i:trifffi,, tr 0.06 €/ " -o M a"gomta/ z at .""/l b7s o o.* t,-sg dl AYgU q Cnd ^ rsL 0.02 ^;"yr A.+- WENW,@B,M

50 100 150 200 250 106x Cs/K LaAlb

Frc. 3. Characteristictrace-element ratios and concentrations.In this and subsequentdiagrams, N-MORB, E-MORB, andOIB are from Sun & McDonough(1989). a) Chondrite-normalized(Sun & McDonough 1989)Lalsm versasEu/Eu*. b) Nb verszs Ba concentrations(ppm). Trend "To SimcoeMountains" refemto samplelabeled SM in other figures (672 ppm Ba 57 ppm Nb). Most samplesplot along the arc trend, but thosefrom the Mount Adams and four from the Lassenvolcanic fields are richer in Nb at comparableBa contents.Average upper and lower crust (Avg U Crwt, Avg L Crwt) are from Taylor & Mclennan (1995). c) CrRb--K relations. Coupled CslRb and Cs/K ratios higher than in oceanicbasalts reflect modem subduction-derivedcomponent in arc basaltand basaltic andesite, ancient(?) in HAoT-source. "{Jppercrustal contamination" refers to Giant Crater lavas, shown by Baker et aI. (1991) to have assimilatedhypabyssal gnnite (three Medicine l*ke sampleswith high Cs/I( ratios).Ar;g Granite is the averageof 8 granitic xenolithsin the Burnt Lava flow at Medicine Lake (Grove et al. 1988).d) IrA{b rersrzsBa/I.{b ratios showinglines of equalBa/La ratio. Higher Ba/I.{bratios Onn lf-MOftB at La./l.{b> 1 for all samplesexcept Oree from Mount Adams and the SimcoeMountains alkali basalt reflect presenc€of a suMuction-derivedcomponent. The low Ba./La value of Mount Adams samplesis in keeping witl a relatively strong intraplategeochemical sipature there.Field for lavas from CraterLake areawith >67oMgp from Bruggmanet al. (1989) and C.R. Bacon (unpubl.d^4 1995).

but are distinguishedby higher Al contents,commonly Medicine Lake suggest that HAOT magmas had >L7VoNzOt Grg. 2). In comparisonto other primitive pre-eruptiveH2O contentsof 30.3Vo(Sisson & Layne Cascadelavas, HAOTs havehigh Ca (most>l l%oCaO; 1993).Concentrations of B& Cs, Rb, Th, Sr, and Pb, Fig. 2) and low Na (^2.5Voversus 87o Na2O).Incom- however, are typically higher in HAOT than in patible elementsare presentat concenhationsneady as N-MORB. SomeHOAT sampleshave Cs/I( andCs/Rb low as, or similar to, those in MORB, as exemplified ratios notablyhigher than valuestypical of OIB, ocean- by K (commonly<0.L0Vo IQO: Fig. 2). Measurements island basalt, or MORB, and they plot along the arc of dissolved H2O in melt inclusions in olivine from trend in Figure 3c. Likewise,HAOT sampleshave high PRIMITTYE MACMAS AT FIVE CASCADE VOLCANIC FIELDS 407

AM I

Eg N (5l d c.K,/ OG ^ t-z; o ". l,-s L

4

3 89 11 12 FeO*

t.cl3 ts,

0l! '*0.5 0.05 0.10 0.15 0.20 0.25 0.0 1.0 1.5 2.0 NbZr Tanfb

F\c. 4. Concentrationsof M and Ta FeO-FeO* relations,and element-ratiodiscrimination diagrams. a) Nb versar Ta (ppm), showing lines of equal NblTa ratio. Most sampleshave NblTa ratios lower than oceanicbasalts. hecision is poor at low abundances,so NblTa ratios for sampleswith Nb < 2 ppm are imprecise.Two Medicine Lake sampleswith NblTa = 9 me HAOTfractionatescontaminatedwith hypabyssal graniterepresentedbyAvg Granite,theaverage composition of 8 xenolitls in the Bumt Lava flow aJMedicine l,ake (Grove et al. 1988).b) Analyzed FeO contentsversus total Fe as FeO* showing lines of equalFeOlFeO* ratio. Arc basattsand basaltic andesites typically aremore oxidized than samplesof HAOT. c) NbZr versusBzl7,r ratos (after Leemanet al, L990,Figure 7b). This diagramsepantes intraplate (hrCh NblZr) from suMuction- related(high BaZr) magmas.Evidence for anintraplate component in thepresent data-set exists for Lassenand Mount Adams volcanic fields (comparewith Fig. 4d). Dotted field enclosescompositions of 85 lavasfrom the Craterl*ke arcawith267o MgO @rugpan et al. 19891,C.R. Bacon, unpubl. dat4 1995). Average uplrer crust (Avs U Crust) is from Taylor & Mclennan (1995),and probably has higher NbZr andTalYb than Cascadearc crust.Pelagic sediment composition (7o .Sadl is from Hole et aL (1984).d) TalYb versusT\Nb ratios. Expandeddata-sets for all but fhe Shastaarea show presenceof a high-(IafYb) intraplatecomponent at Mount Adams,Lassen, and Medicine Lake volcanicfields, but not at CraterLake. Field for lavas from Crater Lake areaas in Figure 4c. Fields for Mount Adams,Lassen, and Medicine Lake volcanic fields from unpublisheddataofW. Hildreth,M.A. Clynne,andJ.M.Donnelly-Nolan, respectively (1995).MORBhthis figurerepresents both N-MORB andE-MORB of Sun & McDonough(1989).

BaA{b,Ba/La, and Lali.{b rafios(many have Lall.{b > 2), enrichmentof the large-ion lithophile elemetts,UI'E, exceptfor thosefrom the Mount Adams volcanic field in HAOT is not relatedto modernsubduction and the @ig. 3d). This subduction-relatedsignature also is presentCascade arc, but is an older subduction-related presentin HAOT from eastof the Cascades(Ilart et al. featurethat is characteristicof the HAOT soulceover a 1984; W.K. Harl unpubl. dat6 L995), suggestingthat larger region. 408 TIIE CANADTAN MINERALOGIST

The lavas from Medicine Lake are the most incom- The flows themselvesare thicker and,where preserved, patible-element-depletedin this study. They include have blocky or aa surfaces. primitive tbrougb ffierentiated and contami- himitive arc basaltsand magnesianbasaltic andesites nated lavas, in part from the Giant Crater lava field have higher SiO2 contents and comparableor even @onnelly-Nolanet al. 1991).The distincttrend of the higher Mg#than found in HAOT @g. 2). Most do not Medicine Lake samplestoward high Cs/I( @g. 3c) reach the extremeCaO and MgO contentsof HAOT reflects assimilation of granite, as documentedin the [Frg. 2; note,however, that somecompositions given in Burnt Lava (Ctroveet al. 1988),the Giant Craterflows Clynne (1993) and Baker et al. (1994) have MgO as @aka et al. 1991),andlakeBasalt flMagner et aL 1995). high 6s in HAOTI. Arc basaltand basalticandesite are Chondrite-normahzedREE patternsfor HAOT are characterizedby theUIE (Cs,Rb, K Ba Sr, Pb, Th, tD IREE-depletedto slightly ZREE-enriched.Heavy REE enrichmentrelative to the I/FSE (Nb, Ta, Zx, W, Ti) concenfrationsare generally)10 X chondrites,consid- typical of arc rnagmas,and greaterthan that in HAOT erably highsl than in most primitive arc and as seen, for example, in a plot of Nb yersasBa basalticandesites of the Cascades.Positive Eu anomaliss (Frg. 3b). Cs/I( and Cs/Rb ratios Grg. 3c) are higher @ulEu* up to -1.2) are negatively correlated with than MORB or OIB values Morris & Hafi 1983,Ben LalSqe Gig. 3a). Othman et al. 1989), as noted abovefor someHAOT The M/Ta ratio varies significantly among our samples,and trend toward primitive Aleutian arc basalt HAOT samples(Fig. 4a). Most have M/Ia ratios of (Nye & Reid 1986). -12-16, significantlylower than MORB, OIB, or chon- Lidht REE enrichmentrelative to HREE andHFSE dritic values of 17-18 given by Sun & McDonough is typical of Cascadeprimitive arc basalt and basaltic (1989). There is a tendencyfor increasingM/Ta with andesite.In theserocks, LaA.{b rafios range from 1.5ts 4, increasingabundances of theseelements in the Cascade far higher than in MORB or intaplate basalts@g. 3d), lavas, althoughthis is somewhatequivocal because of and comparableto island-arcbasalt. There is a corre- potential systematicerror at low concentrations.These spondingrelative enrichment in 84 asindicated by high observationsare not readily explainedby a single-stage BaA.{band Ba/I-a ratios. The arc basaltsand basaltic partial melting processbecause the ratio of crystal/melt andesiteshave comparativelysmall Eu anomalies, partition coefficients,DNb/Dru, is generallySl for vir- which may be either positive or negative@ig. 3a), and tually all phasesthat have beenstudied experimentally low IIREE contents(most <10 X chondrites). (Green1995). Pla::k & White (1995)suggested that the Values of NblTa arc -13-17 for the arc basaltsand causeof sub-chondriticNblTa valuestley measuredfor basaltic andesitesin the presentstudy @g. 4a). As in low-(Nb,Ta) arc basalts and MORB may be source HAOT, many of theseratios are sub-chondritic.Low depletionby prior exfraction of low-degree,relatively M/Ia ratios arenot uniqueto the Cascadearc, asPla* high-(NblTa) partial melt. Judging from the composi- & White (1995) reported NblTa ratios down to 6 in tions of granitic xenoliths presentedby Grove et al, ICP-MS characterizationof arc basalts.The low ratios (1988),the two MedicineLake sampleswith NblTa may reflect prior extraction of melt from the source ratios of^9 have theselow values,and relatively high (Plank & White 1995) andpreferentialretention ofNb Nb and Ta concentrations,because of assimilationof relativeto Ta in rutile duringdehydration of a zubducting granite,as describedby Baker et al. (199L). slab, resulting in lowering of M/Ta in the overlying Compatibletrace-element abundances are somewhat mantle wedge (Green 1995). The latter suggestionis diagnosticof magmatype. Although both HAOT and basedon rutile - aqueousfluid partition coefficients, arc lavas have Ni concentrationsup to ^200 ppm, Cr whereDM/Dru is nominally >1 in resultsof many, but values in HAOT lavas do not exceed ^400 ppm not all, experimentsreported by Brenan et al. (7994); (Ftg. 2). HAOT lavas typicatly have higher Co and Sc note, however,that DM/Dra is generallyindistinguish- concentrations(most >40 and >30 ppm, respectively) able from 1 ifpublished analyticaluncertainty is con- than the arc lavas. sidered. The arc lavas are relatively oxidized, as found else- Arc basalt and basaltic andesite whereby others(e.9., Gill 1981).Values of FeO/FeO* rangefrom 0.9 for the mostprimitive samplesof HAOT Arc ("calc-alkaline")basalt and basalticandesite to 0.6 for someof the arc rocks Grg. 4b). Presumably, lavas are abundantnear the axis of the Quaternary the arc signatureis correlatedwith an increasein pre- Cascadesas far nofih as southernWashington. They eruption H2O content (e.9., Sisson & I,ayrc L993, commonlyhave more abundantolivine andplagioclase Stolper& Newmat1994). The more oxidizednature of phenocryststhan HAOT, andmay alsocontain clino- the arc rocks probably reflects relative pre-eruptive pyroxene; some lack plagioclasephenocrysts. Higher oxidationstate and is unlikely to be relatedto degassing SiO2content and probablelower temperaturesof erup- of themorehydrousmagmas (Carmichael 1991). Water tion translateto higher viscositiesand a different mode transportedto the mantle wedge by subductionprob- of occurrence. These magmns erupted from vents ably is responsiblefor oxidationof the sourceregion of markedby cinder conesor form cone-cappedshields. axcmagmas (Brandon & Draper 1996). PRIMITWE MAGMAS AT FTVE CASCADE VOLCANIC FIELDS 409

15.70

15.65 +6 -o E o[ 15.60 Lower z o cnd Q+5 al .cl ts.ss '4,,,,'.,,,,ltt o$ ;'' +4 N ., i;;

'. .,Ji:,.-J:llF: '15.50 +3 :f;Fi$

15,215 0.7030 0.7035 0.7040 0.7M5 14.4 18.6 18.8 19.0 ,t9.2 19.4 87Sr/863r 2ffiPbF04Pb

+7.5 7.5 L L +7.O s s LS L LL LSL s SL tssLs - +6.5 c c 6.5 G c0 Qll o CM to @ L L +6.0 SM c 6.0 sMc,M c tflM M LLC 'T M L(D ttM ILC MI C c IAA AA CM L L A A

tl A u +5.0 5.0 +2 +3 +4+5+6n+8 20 40 60 80 100 truo Sr/Nd

rSrF6Sner,sa,r FIc. 5. Selectedisotopic parameters.a) €Nd.Most samplesplot in a small region approximatelywithin the conservativelydrawn mantlearray (from l,eemanet al. 1990,Fig. 10),Mount Adams sampteinigh, i,assenand Shastalow. YSrf6Sr T9 !**o samplewith = 0.70305(below the mantle array) is magnesianbasaltic andesite (sample LCSG1009; 587oSio). b) Pb isotopecorrelation diagram. As in most arcs,aata can be interpretedas mixtures of MoRB-like pb andpb from marine sedimentsor, possibly, continentalcrust. Medicine Lake samplewitl highg5f aTPbl2tAPbis most primitive IIAoT' which suggeststh1 this Pb is mainly derivedfrom subductedsediment. Note that the SimcoeMountaini sample, which representsa melt of intraplate-typemantle, plots on the,VI/RL [northernhemisphere regression line for oceanicbasalts fron Hart (1984)1,and thus has an OIB affrnity (nearbyLassen sample is LC8G10{i9). FieLdof NE pacifi.csedbnents fuom Church(1976), Cascade Ores from &vrchet aI. (1986)1,arld, ParificMORB from White et aL (1.987),Inwer Crustrepresents maximum likely values f'0-250 Ma tectonothermalage lower crust" of Rudnick & Goldstein (1990). c) e* ,"rr whole-rock6180. Shasta and, commonly, Iassen sampleshave high 0l8Oand low e1q6values. Crater Lake samplesvrith 6180 > +6.Waoare basaltic andesitesand an evolved HAOT. d) SrA.{dyersas 6rso. Arc magmashave higher Sr/Nd ratios rhan oceanic basalts aad "average" continental crust If high-OtEovalues for Shastaand Iassen sampleswere related to a subduction-derivedcomponen! these rocks shouldhave high SrA.{dratios, which many do not.

Intraplate basalt primitive lavas from the Lassen and Mount Adams volcanicfields containhints of an innaplatecomponent Basaltwith a strongintraplate chemical signature of Gig. 3b; Bullen & Clynne 1989,Leeman et al. 1990). UIE enichment in the absenceof Nb-Ta depletionis The SimcoeMountains sampleis from an ejectaring limited in our data set to the samplefrom the Simcoe that containsmantle xenoliths. Although few lavas are Mountains volcanic field (SM in figures). However, as strongly alkaline as the Simcoe units, Quaternary 4ro TIIB CANADIAN MINERALOGIST alkali basaltvents are not restrictedto the back arc, but clude definitive statementsabout isotopic compositions eSrFSr occurin a 150km east-westbelt acrossthe Cascadearc of end-membermantle components. The lowest in southernWashington. andhigbestla3Nd/aNd ratios were measuredfor samples of IIAOT from the Mount Adams field, but most have MANTLECoupoNnnm IMPLIEDBY values comparableto those for arc basalt and basaltic 875r/865r ETD-MSMBER MAGMAS andesite.Lavas with particuladyhigh andlow 143Ndr/144Ndratios may be contaminated.Borg et al. Compositionsof primitive Cascadelavas suggest (1997) suggestedthat lavas from the Lassenarea that blending of melts from, or mixing of, end-member plot below the mantle array (e.g., magnesianbasaltic geochemicaldomains in their mantle sources.Consid- andesiteLC86-1009, Fig. 5a) contain a large fraction eration of data for moderately evolved lavas @6Vo of fluid-transported Sr, but not Nd, derived from MgO) along with the set of primitive rocks analyzed metabasaltof the subductedslab; this samplealso has here sfiengthensttre casefor a minimum of three end- MORB-like Pb isotopic ratios. Note, however,that the memberprimitive liquids that reflect componentsof the alkali basaltfrom the SimcoeMountains has a similar mantlebeneath the Cascades:depleted sub-arc mantle, isotopic compositlonof Sr, Nd, and Pb @gs. 5a b), as a subductioncomponent, and intraplatemantle. though suchfeatures also may be intrinsic to intraplate mantle. Trace-elementratios The isotopiccomposition of Pb is sensitiveto crustal contamination,particularly in HAOT, where the base- Following Leeman et aI. (I99O), we have plotted level concentrationis <1 ppm. Neady all sampleshave NblZr versw BtZr FiS.4c) to illustrate some of the ln$er MPblMPb ratios than the northernhemisptere effectsof mixing of threeend-member magma E4les or regressionline and Pacific MORB on a 2trPbPa4Pb mantle components.A large number of high-quafity versus 2ffiPbPMPbdiagrarn (Fig. 5b). The Pb-isotopic analysesfor Nb in specimensfrom the CraterLake area frend toward fields for northeastPacific sedimentsand suggestthat there is no intraplatemantle present there, Cascadeores rnay reflect crustal contaminationor Pb as all data trend directly from primitive HAOT toward derivedfrom (not necessarilyrecently) subductedsedi- IAB and pelagic sedimentat nearly constantNbZr. ment. A modem sedimentarycomponent should pro- Note that all samplesfrom the Mount Adamsand a few duce a positive correlation between the Pb isotope from the Lassenvolcanic fields aredisplaced to slightly ratios. Miller et al. (L994) reported a sffong negative higher NbZr values,relative to the CraterLake trend, correlation between 207P1oPg.Pband Ce/Pb ratios for and the SimcoeMountains sample lies at higher NbZr Umnak in the Aleutians, and interpretedthis as evi- than Kilaueabasalt. dence for fluid-transportedmantle Pb. Our Cascade The number of data points available for plotting data have the low Ce/Pb ratios typical of arcs, but increasssconsiderably when INAA resultsare usedin possessonly a weaknegative correlation overall between aplot of Ta,/YbverswThXb (Fig.4d). The large numbers 2olPbPMPband Ce/Pbratios. of datapoints for Medicine Lake, CraterIrke, Lassen, good with and Mount Adams volcanic suites define fields that Oxygenisotope ratios show a correlation show the sametrends as in Figure 4c. The INAA data geologicalsetting, as 6180values 2 +6.5Voowe limited - Sierra confirm the lack of an intraplateOigh TalYb) signature to the Lassenand Shastaareas, where Klamath at Crater Lake, suggestthat one is weakly presentat Nevadabasement is known to be present.That values Medicine Lake, and add a few samplesto the Lassen between+5.6 and +6.1%oalso occur at Lassen,along and Mount Adamsfields that reflect mixing of all three with the negative correlation between 6180 and eNa end-members. Alternatively, crustal contamination (Fig. 5c) in the data set as a whole, suggeststhat the could result in values intermediatebetween arc and higfr 6ts6 andlow g16values may be dueto assimilation intraplate frends, becausethe Ta/Yb ratio of average of crust. lnplications of regional isotopic variation are crust is higher than in arc lavas (Iaylor & Mcknnan discussedfurther below. 1995).However, the slopesof the lower field boundaries for thetbree centers are remarkably consistent in Figure4d Cttsr,trcAl-Cnenacrsr,rsncs oF TIIEFfvE CSNTERS: and cannotbe explainedby additionof crustalmaJerial. Er.sIuEl.tr-ABUNDANCEDIAGRAMS Isotopic composition Element-abundancediagrams (Fig. 6) highlight The primitive lavas of the presentstudy plot within chemicalsimilarities and differencesa:nong the cent€rs. or very closeto the conservativelydrawn mantle array Theseplots alsoprovide for comparisonwith examples (after Leemanet aL 1990) on the 8751165rv€aur 8p6 of primitive lava compositionsfrom oceanridge, island diagram Gig. 5a). Regional differences in isotopic arc,and intraplate setrings (Fig. 6[ N-MORB, E-MORB, composition(discussed below) and the limited number IAB, OIB) that aid in identification of geochemical of primitive sarnplesanalyzed from each center pre- signaturesof mantlecomponents. Also evidentare effects PRIMITTVE MAGMAS AT FTVE CASCADE VOICANIC FIELDS 4Il

Crater Lake volcanicfield

to l0 o = zl z ! ! 6 I t r

CsBaUTa LaF P asmTi Y tsPb P ZrSmTi Y Rb Th Nb K Ce Sr Nd Hf Eu Dy Yb Ge Sr Nd Hf Eu Dy Yb

Mount Shastavolcanic field

dl t z z ! tro

CsBaUTaLaPbP Ztl n tl CaBaUTaLaPbPZSmTiY RbThNbKCeSr Nd HI tsu Dy Yb Rb Th Nb K Ce Sr Nd Hf Eu Dv Yb

Lassenvolcanic field

@,d@R@s Sm & McDonough ,4@ Nw& Reid tD16

CsBaUTa LaPb P CsBa U Ta[aPb P zrSmTi Y RbThNbK C6 Sr Nd Flt Eu Dy Yb Rb Th Nb K Ce Sr Nd Flf Eu Dy Yb

Ftc. 6. Element-abundancediagrams for the five volcanicfields of this snrdyand representative N-MORB, E-MORB, IAB, andOIB (sourcesof dataas in otherfigures). Normalization values from Sun& McDonougb(1989). Filled symbolsare HAOT, andopen symbolsrepresent arc basalt and basaltic andesite, and alkali basalt.SiO2 contents indicated for Cascadelavas (recalculated to sum to lM wtTo volatile-freeand with all Fe asFeO*). The effectsof fractionationof HAOT andcontamination with granitein the uppercmst are shown by four samplesfrom the GiantCrater lava field at MedicineIake (differentiatedand contaminated HAOT plottedwith opensymbols). Base-level FIAOT hasN-MORB-like abundancesof mostincompatible elements, but is enrichedin l"I//, asare sample's of HAOT at the othercenters. Crat€r Laks, Shastaand lassen samplesform a continuumof compositions from HAOT to arcbasaltic arde,site. Magnesian basaltic andesite from Iassen(LC8GI009; dashedpafiem), an exftemeexample of the arc signatre, is stronglydepleted in elementsat the right of the diagram"Alkali basaltfrom the SimcoeMountains has an OIB-like (intraplate)pauem. Arc basaltfromMount Adamsand a basaltfrom Lassenalso have steep pauems and comparatively smallNb-Ta'\re11s" zuggestiveof an infaplaie componenl 4t2 TIIE CANADIAN MINERALOGIST of processessuch as assimilalisl of rocks derivedfrom enhancementof the arc siguature,including relative the upper crust. depletionin.F/REE and Y. Note the high Pb/Srratios of the Shastaarea lavas. Samplesof HAOT and magne- Med.icineInke volcano sian basalticandesite from the Shastaregion described byBaker et aL (1994)have similarN-MORB-normalized Four representativesnnrples from the Giant Crater patternsto thosein Figure 6c. lava field of the Medicine [,ake volcano(Fig. 6a) illus- fratethe effectsof fractionationand crustal assimilation Mount Adans volcanicfield on the trace-elementpattern of primitive HAOT @onnelly-Nolanet al. 1991,Baker et al. 1991).T:he A11 three components are present in the Mount least differentiatedrock (82-:72+ studied by Bartels Adams volcanic field samples(Fig. 6d), as also found et al. 1991\ has HFSE and HREE abundances^O.5X by Leemanet al. (1990) for the southernWashington the Sun & McDonough (1989) N-MORB values,and Cascadestaken as a whole. The Simcoe Mountains is the most incompatible-element-depletedsample in sample has a clear intraplate signature, as stated our data set. Even this primitive rock, however, has previously. ffus high Nb and Ta concenffationsin the elevatedconcentrations of some ULE retanve to the sampleof HAOT plot0edmay result from presenceof other incompatibleelements. The other three samples intraplate-typemantle in its source.Arc basaltshows a display a systematicincrease in incompatibleelements, moderateNo--Ta'\rell". overall low concentrationsof except for Sr. Baker et aL (1997) describedhow the the HFSE, comparativelyhigh Pb/Sr ratjo, andHREE variationin compositionof the Giant Craterlavas is due depletiontypical of other samplesrich in a subduction to fractionationof HAOTplus assimilationof granilein component. the upper crust. This scenariois consistentwith the patternsin Figure 64 whereincreases in Sr, Eu, and Ti Lassenvolcanic field are suppressed.Note also that the fItEE and most elementson the right side of the diagramincrease in an The Lassenvolcanic centerand surroundingregion approximately parallel fashion, and that Ba is not contain many vents and a gteat variety of mafic lavas enrichedrelative to Th andRb in the two most-evolved @ullen & Clynne 1989, Clynne 1993, Borg et aL samplas(cf IAB in Figure 60. L997).The tlree componentsare presentat Lassenas in southemWashington. Primitive HAOT is much lke Crater Lake that at Crater Lake but for its higher Pb/Sr ratio and slight K enricbment.True alkali basaltdoes not occur The four samplesfrom the CraterLake areaillustrate at Lassen,but intaplate mantle is clearly represented the effect of blending TIAOT and axcrnagftN or their Gig. ad), and may be responsiblefor the relatively sourcecomponents @acon 1990,Bacon et al. 7994)n shallow Nb-Ta "well'o and high Ti of sample the absenceof an intraplate componentor contamina- LM87-1384 (Fig. 6e). Thereis no questionof a strong tion with felsic material (Fig. 6b). The most primitive arc signaturein many of the Lastsa vqleanicfield lavas. HAOT has HFSE andREE abundancescomparable to It is moststriking in magnesianbasaltic andesite (dotted N-MORB, exceptthat Nb is lower in the CraterLake line), which hasa pattemvery muchlike tlat of primitive rock. Other than K, the concentrationsof the ULE ue IAB @g. 6f), yet contains587o SiO2 at aaMgt of 71. elevated, as at Medicine Lake. The other samples, which range from differentiatedHAOT to maguesian Cnmncal Cnenecrmrsncs oFTHEFfvE Cmur.ns: basalticandesite, display a systematicenrichment in all REEPATTERNS elementsto the left of Ti and a concomitantdecrease in HREE ard Y ; i. e., asthe lavasbecome richer in incom- Isotope-dilutionanalyses for the REE aresufficiently patible elements, the arc signature becomes more precisethat subtletiesof chondrite-normalizedpatterns pronouncedoand concentrations of the moderalely are meaningful and can be interpreted in terms of incompatible elementsdecrease. This observation processesand mantle sources.Patterns are presented in applies in a general way to Crater Lake, Shasta,and Figure7 for the samesamples as areplotted in Figure6. Lassen. MedicineLake volcarn Mount Shsstaarea The convex-upwardpattern of the most primitive The three samplesfrom the Mount Shastaarea plot- HAOT from Giant Crater @ig. 7a) is similar to that of ted in Figure 6c can be interpretedin the sameway as N-MORB (Frg. 70, althoughREE concentrationsare thosefrom CraterLake. The HAOT is more differenti- lower overallin the Medicinelake rock. The positiveEu atedthan the most primitive rocks from MedicineLake anomalyis pronounced.With fractionation of basaltic or Crater Lake, but still has the relatively flat P-Yb magma and assimilation of granite, the REE abun- pattern. The two samples of basaltic andesiteshow dancesincrease, the LREE become fractionated,the PRIMITIVE MAGMAS AT FIVE CASCADE VOLCANIC FIELDS 413

Crater Lake volcanicfield b

Fracdonadon+ UpD€rCrustal \* Contamlnaflon 950 ot HAOT o- o 52 )o o -tri E10 /*1"t,

c

La Ce Nd SmEu Gd Dv

50 o E E c o o ! I 10 lr

@@,n@@sSm&McDmugtl & Reld(198$ Oknok lDlo

o o E !t o E

=a > o o tr c

SmEu Gd Dy

Ftc. 7. Isotope-dilution,REEdata for samplesplotted in Figure 6 normalizedto the C-l chondritevalues of Sun & McDonough (1989). Syrnbolsand SiO2contents as in Figure 6. Four samplesfrom the Giant Crater Lava fiel4 Medicine Lake, have decreasingEu/Eu*, increasingLREE fractiomtion, and uniform increasesn HREE w:rrhffierentiation and contamination with granite.Negative Ce anomalies,as in 1085M, are presentin other Medicine Lake lavas.REE patternsof CraterLake sampleschange systematically from LREE-depletedHAOT with positive Eu anomaly to sip.oidal pattems in basaltic andesites.Simitarpatterns arefound atlassenandShasta. Low abundancesoftheflREEin the arcbasaltandbasaltic andesite pattemssuggest generally higher degreesof melting than for samplesof IIAOT. The sigmoidal shapeof the REE pauern, which suggestsan eclogitic residue,apparently is inheritedfrom a subduction-derivedcomponent, possibly transported to the mantlewedge by a partial melt of the slab.Although the HAOT patlem at Mount Adamsis like thoseelsewheren arc basalt is similar to alkali basaltfrom the SimcoeMountains. 4t4

HREE show a parallel rise, and the Eu anomaly anomalyat a little more than 10X chondrites(Fig. 7c). decreases,becoming negative in the most evolved The basaltic andesiteshave lower levels of the HREE sample.Fractionation and contaminationof the Giant than the HAOT, a smallpositive Eu anomaly,and slight Craterlavas thus result in I/REE behaviorunlike that in to moderateZJ?EE enrichment. Sample 75SV-3 has a arc basaltsand basalticandesites of the other volcanic sigmoidalpattern like -agnesian basalticandesite from fields. In fact, lavasat the Medicine Lake volcano (and Cratertake, but with lesspronounced lREE emichment" Mount Adams volcanic field) have relatively high abundancesof. the HREE overall: of 168 INAA deter- Mount Adamsvolcanic field minations of REE concentrationsin Medicine Lake samplesvtrft.XVo MgO, only two haveYbly less than 10 The REEpatternfor IIAOT from the Mount Adams (J.M.Donnelly-Nolan, unpubl datq 1995).This expanded volcanicfield (Fig. 7d) is similar to that of HAOT from data-setcontains u 1"w slamFles of "sigmoidal" REE the other centers.Alkali basaltfrom the SimcoeMoun- patterns(see below), but with lower LREE and higher tainsvolcanic field hasa steep,straightpattern. Surpris- HREE tJ:,?nat Crater Lake. Patternswith monotonic ingly, arcbasalt has a nearlyidentical pattern, including slopes,as are common in lavas from Mount Adams, Yb at -10X chondrite,and lacking upwardconcavity in are rare and have comparatively gentle slopes tJreHREE pattem as found in lavas with arc signature (30 < Larv<50). a.tthe othercenters. A searchof 48 INAA resultsof rocks The precisionof the isotope-dilution analysesfor the with MgO 2 6Vo CY'/.Hildreth & J. Fierstein,unpubl. REE andreplication by INAA indicatethat the negative data, 1995) found three with sigmoidal pafterns,and Ce anomalyin sample1085M (and 1376M,Table 1) is only one with Yb1"< 10. real. The INAA data-setfor Medicine Lake contains many sampleswith a negativeCe anomaly.The only Inssen volcanic other samplesin this sflrdywith a negativeCe anomaly field are two specimensof FIAOT from Lassen.The pres- enceof a negativeCe anomalyin island-arclavas has As expected on the basis of element-abundance been attributed to subduction of pelagic sediment patterns, the four representativesamples from the (White & Patcheft1984, Hole et aL 1,984).It may be Lassenvolcanic field in Figure 7e show a wide range that the negative Ce anomaly results from compara- of REE pattems.HAOT is similar to its relativesat the other centers.Sample LC88-1398 is arc basalt on tively ancient @4esozoic?)sediment subduction that locally affected the TIAOT sowce, and that is not the basis of chemical composition,but it has HAOT- (Clynne evidentin arc basaltand basaltic andesite because of an like compositionsof the phenocrysts 1993).It overwhelning contribution from modem subduction, has similar I/RE4 but notably higherZREE concentra- in which pelagic sedimentis not well represented. tions, than primitive IIAOT. This samplealso has high ULE (except Sr) concentrations, high eSrF6Sr, 2il7Pb1ruPb,618O, 143Nd/[44Nd Crater Lalce andlow values.Arc basalt has a sigmoidal pattern similar to magnesianbasaltic T\e REE pattemsfor the four samFlesin Figure 7b andesite from Crater Lake. The magnesianbasaltic from the CraterLake arearotate about a ooint nearTb. andesitefrom the Lassenfield, althoughcharacterized asnoted previously for HAOT andcalc-;lkaline basal- by a similarly shapedpattern, has much lower abun- tic andesites@acon 1990). Primitive IIAOT has an dancesof the KEE, in keeping with its incompatible- *-tr4933-like pattemand also the stongly positiveEu element-poor overall composition. Positive Eu anomalytypical of end-memberHAOT. Differentiated anomalies are small or lacking in the Lassen area HAOT (84C1143)has brgherREE abundancesoverall samples.In the examplesin Figure 7e, and in a larger and, although still convex upward, is slightly LREE- data-setof 42 compositionsof rocks with MgO 26Vo enriched.Arc basalt and magnesianbasaltic andesite (Clynne 1993), there is no sample with the staieht" show a progressiveincrease in LREE and decreasein fractionated pat0ernshown by alkali basalt from the HREE wfu7eretaining a small positive Eu anomaly. SimcoeMountains, as might be anticipatedto be found Thesesamples have a sigmoidalpattem: convex-upward on the basisofthe other trace-elementevidence for an LREE and concave-upwardHREE. The larger Crater intraplatecomponent (Fig. 4d). As at CraterLake, some Lake data-setof 76 sampleswith MgO 2 6Voanalyzed samples from the Lassen area have relatively low by INAA @ruggmanet al. 1989,C.R. Bacon,unpubl. I/REE contents(10 of 42 haveYby< 10). data 1995) contains18 exampleshaving Yb1,< l0 and severalwith a sigmoidalpattern. RrcroNar, DrrFsRm.ICEsINTI{E CoMposITIoN oF PRnvrrTVELAvAS Mount Shastaarea Description of the compositionsof primitive lavas The one sampleof TIAOT from the Mount Shasta from the five centers has touched upon systematic area has z flat REE pattern with small positive Eu regional variations in abundancesof the incompatible PRIMITTYE MAGMAS ATFWE CASCADE VOI-CANIC FIELDS 4r5

48 ,{8 A

46 M /t6 s I o E tu *r f 5 (U ([ J ,rl J42 ,'f N

.,L 38 0.0 O.7t

*- 48 li,,. * l'=: 46 o I rI! NM != 4l d t (u J 421 J42

t 40

38 't "L15.50 1 18.7 18.8 18.9 19.0 9.t 206PbP04.Pb

48 *[ 46 -r o o EM E *l .= f GI 6 J42 J -i

40

38 :L +2 +3 +4 +5 +6 +7 +8 +5.0 +5.5 +6.0 +6.5 +7.O +7.5 €uo 6180

FIc. 8. TiOz contentsand selectedisotope ratios yersaslatitude. Shadedfields indicateranges of literatule data [isotopicdata for Mount Adamsfrom Leemanet al. (1990), CraterLake from Baconet aI. (1994),Iassen from Bullen & Clynne (1990)and unpublisheddata" 1.995: TiO2 contentsfrom Baker et aI. (L994) and sourceslisted for Fig. zldl. Note increasein minimum TiO2 contentfrom southto north, and regional differencesin medianvalues (dots) of isotoperatios for data of this study (letters). 416 TTIE CANADIAN MINERALOGIST elementsand isotopic compositions.Here, we plot and primitive lavas of this study again suggestregional discussselected geochemical parameters as functions differences.In this case,zuPbPA4Eb tends to be higher of latitude in order to focus on the more significant andMPbPMFolower in the south@igs. 8c, d). This crude featuresof regional variation. anti-correlationbetween Pb isotope ratios may reflect Vari*ions in TiO2contentand variousisotopic rafios crustalcontamination of lavaserupted in Californi4 or are plotted versuslaf,tttude in Figure 8. Also shown are it may be an artifact of inadequatecoverage in the fields definedby our own unpublisheddata and litera- present data-set.Lower crust would be expectedto fire valuesfor theMount Adanrs,Craterlake, andIassen have low 238Ulg.F4ooand hence comparatively 1ow areas.Available isotopic data for Shastaand Medicine 206P1oPMPb(as seenat Lassen)if sufficiently old. Lake are insufficient for meaningful definition of Minimum eNdvalues show a generalincrease from ranges of values. Of the others, Crater Lake has the south to north @g. 8e), with Medicine Lake being narrowestrange of isotopic compositions. restrictedto relatively high values,perhaps because of fts limited coverage in this data set. The trend is HFSE and Fe defined by relatively low €N6of the Lassenand Shasta samples. There are the most data for Ti among the HFSE. Minimum TiO2 contentsat MgO 2 6Voincreaseby A clear regional trend is shown by maximum 6180 a factor of two from soutl to north (Fig. 8a). Crater values @ig. 8f). The highest 6180 values in primitive Lake hasthe mostrestricted range of concentrations,in rocks occur in the south,and median valuesgenerally keeping with the lack of an observedf/F,lE-rich in- decreaseto the nortl. A11centers except Shasta" where traplate contibution. Maximum TiO2 values at Medi- data are limiled to four samples,have somevalues in cine Lake (L.97o),Lassen (1.87o), and Mount ddams the nomral mantlerange of +5.5 t 0.47ooQt/Iattey et al. Q.SVo)arc similarto thoseof inhaplatebasalts. Mount 1994'land at or belowthe baselevelof +5.9n +6.2%o Adams also has highsl seaesafiationsof other,FIFSE for arc basaltssuggested by Harmon & Hoefs (1995). andFe than the centersto the south.The relatively high Mattey et aL (L994)suggested that at the lowestsolidus I{FSE contentsof basaltsfrom the Mount St. Helens, temperaturesconsidered likely for peridotite, allowing ,and Mount Adamsvolcanic fields were for the largest mineral-melt fractionations,the maxi- noted by Leemanet al. (1990). mum 6180values of basalticliquids in equilibrium vrith peridotitic mantlewould be in the range+6.0 - +65Vou REE Causesof l8O enrichmentare suggestedbelow.

All five centershave HAOT with LREE-depletedto RScToNAL DFFERENCFS IN BASEMENT RocKs AND slightly LREE-enrichedchondrite-normalized patterns, CRUSTALCoNrarurnerrolq with a positive Eu anomaly.A negativeCe anomalyis present in somesam.ples of HAOT, and is particularly Regionaldifferences in geochemicalparameters that commonaJ Medicine Lake. "Sigmoidal" LREE-eniched are sensitiveto contaminationsuggest that many primi- pattems with convex-upwardLREE, concave-upward tive lavas record some degree of crustal interaction HREE, and Ybrv < L0 are common at Crater Lake, (e.g., Bacon et al. 1994). The most compelling evi- Shasta, and Lassen. Patterns with monotonic steep dencefor this interaction in the data gatheredfor this negative slopes and Ybr,' -10 are common only at studyis the commonlyhigh $trg valuesof samplesfrom Mount Adams. the Shastaand l,assen areas. the hi.gh6iaO values mrght result from surface processes,although Sr, Pb, Nd, and O isotopes this seems unlikely given the restriction of 6180values ) +6.5%oa to the Lassen The rangein Sr isotopic compositionis well defined and Shastavolcanic fields and the laree proportion r8O-rich by the expandeddata-sets for of samplesthere (11 of l5). Chemi- the Mount Adams,Crater 875r/865r Lake, and Lassenvolcanic fields, which include results cally primitive lavas with ratios ) 0.704,only published elsewhere.Of these, Lassen shows the presentat Lassenand Shasta,also have 61802 +6.7%o 2o7PbPsPb greatestvariation. Note that the fields for 87SrP6Srin Thesesamples commonly have high values, Figure 8b contain a few sanrplesof andesitewrth <6Vo coupled with low 2MPbl2MPband eN6values (e.g., MgO and low 87SrF6Srratios; ttrese samplesare LC88-1398), which are consistentwith contamination believedto havehad a low-87sr/86srbasaltic parent not with lower crust characterizedby ancientdepletion in representedamong erupted lavas (e.g., Bacon et al. U. Potential assimilan8 or reactive wallrocks would 1994).The primitive lavasanalyzed in thepresent study have to be altered mafic or ultramafic rocks with havemedi.an 8751165r rados that tend to decreasefrom comparativelyhigh 6180values (+8 - +l2Voo),as may southto north. be present in the Klamath Mountains basement[see Lead isotopic compositionsshow broad overlap summaryin Baconet aI. (1994,p. 1550)1,in orderto among the Cascadecenters. However, values for the produce the required shift in 6180 without notably PRIMITIVE MAGMAS AT FTVE CASCADE VOLCANIC FIELDS 4r7 decreasingM.g# or concentrationg6f semFatible known to be imFortanl but the contaminantis granite trace-elementsin the resultingmagrruN. of theupper crust (Grove et al. 1988,Baker er al. 1991) Regionalgeophysical surveys suggest a causefor the that probably is Cenozoicin age. The most contami- elevated6180 values for the Shastaand Lassen area nated and differentiatedof theseGiant Crater samples volcanicrocks. Isostatic residual gravity highs immedi- (1161M)has a 6180value of +5.970o,whereas one sample ately west of the Shastaand Lassen volcanic fields of basalt(8S1M) has an anomalouslylow 618Ovalue of @lakely et al. 1985,Blakely & Jachens1990) approxi- +5.2Voo.Definitive O-isotopicevidence for a basement- matelycoincide witfu high p-v6ve velocitiesin the crust contaminationeffect is lacking in southemWashington, @enz et al. 1992) and imply that thick sequencesof even though Mount Adams is only ^40 km south of mafic or ultramafic (ophiolitic) rocks (or both) pmject exposuresof pre-Tertiary rocks. One samplefrom the beneathMount Shastaand probably also Lassen.North Mount Adamsarea (MA-953) hasa 6180value of only of Mount Shasta,where increasesin 6180of primitive +S.lVoo.Contamination with a smallamount of low-l8O lavas relative to presumedmantle values are smaller rock from the upper crust may explain the 6180values (Fig. 8; Baconet al. L994),+hecontinuation of the belt of these samples.Alternatively, the low-18Osamples of gravity highs is west of the Cascadesand merges may simply reflect variation within the mantle,because with the Oregon Coast Ranges (Blakely & Jachens these values are within the range defined by mantle 1990),where the rocksresponsible for it would haveno peridotites(Mattey et al. L994). effect on the modernarc. Resultsof seismicrefraction Althotrgl we have presentedisotopic data that are snrveys@ws etal. 1987,Fig. ll) indicatethat the readily interpretedin terms of crustal contamination, OrdovicianTrinity ultramaficsheet is presentin theuppr geochemicalevidence for contaminationis not always crust as deepas -10 km beneathMount Shastaand that separablefrom the effectsof subductionin the Cascade a 7.0 km/s layer,possibly consisting of ophiolitic rocks, province. This is a direct result of the fact that the begins-17 km beneaththe surface.To the east,beneath basementrocks consist of accretedoceanic and arc Medicine Lake, where high-ttQ primitive lavas have terranes stitched together by tonalite-granodiorite not beenreported,Fris et al. (1987)did not identify the plutons, all of which have sufferedthe effects of Late Trinity ultramafic sheetand found that a 7.0 km/s layer Cenozoic magmatism and associatedhydrothermal is not reacheduntil a depthof ^27 km. activity that have addedto and re-processedthe crust. It hasbeen suggested that the subductioncomponent Thusothe age of a subductioncomponent identified in is the carrierof excess18O in somearc lavas(e,g.,Ito the geochemistryof primitive lavas can be somewhat & Stern 1985/86,Woodhead et al. L987). Borg et aI. ambiguous,at least in the HAOT source,as discussed (1997)propose that the high 6180values for lavasfrom below. the Lassenarea reflect their mantle source,which was modified by a high-18Osubduction component. Laser- Souncs Cnanacrunrsrrcs fluorination O-isotopic analysesof mantle minetals sfiongly suggestthat the mantleis quite uniform in its Many authorshave pointed out that IIAOT and arc O-isotopic composition,regardless of the presenceof lavas cannotbe relatedby any reasonablecombination hydrous phases,casfing doubt on subduction-related of fractionation,assimilation, or melting of a common increasesin the 6180value of the mantlewedge (Mattey source (e.g., Hughes & Taylor 1986, Bacon 1990, et al. 1994).The SrA.{dratio, which is likely to be lower Baker et aI. L994).T\e compositionalspectumresults in continental and oceanic crustal rocks than in arc from variation in extent of melting of mantle sources magmas (Rudnick 1995), can be used to test the that are variably enrichedin a subductioncomponent hypothasisthat the zuMuctioncomtr)onent is responsible and have a ftInge in capacityto producebasaltic melt for high-6lao lavas of the Shastaand Lassen areas. (i.e., fertility). Were the subductioncomponent responsible for ele- vated 6180values, sampleg with high 6180would have Temperature,depth and water content high SrA{d ratios. Because there is no correlation between6180 and Sr/I.{d [or Sr/P, used as an index by The origin of HAOT liquids is relatively staigbt- Borg et aL (1997)1,and because many of the high-trQ forward. Barla;lset al. (1991) conductedphase-equilib- lavas also have SrA.{dratios of only ^20-30 Gig. 5d), rium experimentson primitive HAOT from Medicine far lower than samFlesv/ith a strongsubduction-related Lake, including sample 82-:lz-f. They found that geochemicalsignature (Sr/Nd ashigh as 82), a contami- nqminally anhydrousHAOT is in equilibrium with a nation processis the more likely explanationfor the spinel lherzotte assemblageat -129O'C at 11 kbar, high 6taq values. Any plagioclasefractionation that presumablythe point of separationof HAOT magma might accompanycontamination would further reduce from the mantle lrecall that Sisson & Layne (1993) SrA.{dfor a glven increasein 6180. reported S0.37odissolved H2O in melt inclusions in Our data for Medicine Lake include four samples olivine phenocrystsl.These pressure conditions corre- from the Giant Crater lava field" where assimilationis spondto a depthnear the baseof the crust (Mooney & 418 TIIB CANADIAN MINERALOGIST

Weaver 1989). Baker et al. (1994) concludedthat describedby Conrey et al. (1997; sampleRC93-50) HAOT at Shasta(their high-aluminabasalt) represents from the northernOregon fore-arc provides an exteme 6-L07o nearly anhydrousmelting of depletedmantle exampleof a sigmoidal REE paltnm in a subduction- that hadpreviously been enriched with a modestamount component-richprimitive lava (Ce -300x, Yb ^tx of subductioncomponent (e.g., Donnelly-Nolur et al. chondrites). 1991).We acceptthis model, and suggestthat other Generation of sigmoidal REE patterns ca:rnot be HAOT compositions in the northern Catifornia to modeled by melting spinel or garnet lherzolite (e.9., southern Washington Cascadescan be explained by Martin 1987) with either Z.REE-depletedor monoton- combinationsof variationin the amountof a subduction ically decreasing(negative slope) chondrite-normalized component added to the source and differences in REE abundances,such as are characteristicof many degreeof melting [owing primarily to sourcefertility, peridotite xenoliths and massifs(McDonough & Frey as reflected by amount of clinopyroxene (Clynne 1989). Neither can fractional crystallizationof liquids 1993),but also to H2O contentl. PositiveEu anomalies with suchREE patternsproduce the sigmoidal pattem probably result from the reducednature of HAOT and becausebulk distibution-coefficients for the HREE the presenceof re,sidualclinopyroxene @onnelly-Nolan must be significantly greaterthan 1 and must decrease et al. l99l) at the site of last equilibrationin the mantle. with increasingREE atomlcnumber Qimiting any role Higher Sc contentsin HAOT in comparisonwith arc of garnet).Rather, melting and crystallizationmodels basaltsand basaltic andesitesalso may reflect a larger require that the sigmoidal REE pattem be a charac- contribution from clinopyroxenein the IIAOT source, teristic of the sourceperidotite (Stem et al. 1989). relative to the less fertile sourceof the arc magmas,in McCulloch & Gamble (1991) argued that fluid which all clinopyroxenemay be consumedand Sc in extractedfrom the subductedslab would carry LREEblt melts thereafter diluted by high degreesof melting. that,IdREEwould be retainedin the slab.Addition of such Alternatively, residualgarnet, ifpresent in the sourceof a fluid-transported subduction component to de- the arc magmasbut not in that of HAOT, would result pleted mantle could producethe requiredREEpattern in lower Sc contentsof the arc magnas. that would yield melts with a sigmoidalREE pafrern. The origin of arc basalt and basaltic andesite is Infertile peridotite xenoliths with a sigmoidalREE pat- commonlylhked to the effectsof H2O on melting in a tern and LREE +-70x chondritic values have been subduction-component-emichedmantle. Morris & Hart descriM by Ionov et al. (1995),whosuggested thai these (1983),Hickey etal. (1986),Llrbr (7992),Stolper & rocks had experiencedlarge degreesof partial melting Newman (1994), Baker et al. (1994), and othershave and melt extraction, followed by metasomatismby suggestedthat melt fraction during genesisof arc basalt (mEE-bearing)fluids. Calculatedcompositions of patial and basaltic andesiteis a function of the H2O content melt for suchmetasomatized peridotites have a sigmoidal (and relative fertiliry) of the mantle source.High pre- REE patternand abundancessimilar to thoseobserved eruptiveH2O contentsof mafi.carc magmashave been in the Cascadelavas. reporledby Anderson(1974), Sisson & Layne(1993), Alternatively, a slab-derivedmelt might carry a sig- and Sobolev& Chaussidon(1996). For examplg Sisson moidal IREEsignature to the depletedmantle wedge. & Layne(1993) documented up to 3.3Vodissolved H2O Supportfor this mechanismcan be found in rocks ofthe in melt inclusions in olivine from basaltic andesite tonalite - trondhjemite - granodiorite (fIG) suite, eruptednear Mount Shasta.Baker et al. (1994)summa- which commonlyhave sigmoidalREE pattemsand are rized evidencefor the importanceof H2O on extentof believedto be forrned by partial melting of eclogile or melting at upper mantle pressuresand presenteda gamet granulite (metabasalt),leaving a gamet + clino- model for the origin of calc-alkalinebasalt and magne- pyroxenet amphiboleresidue (futh & Hanson L972, sian basaltic andesiteof the Mount Shastaarea. They Ar:thet al. 1978,Drummond & Defant 1990,Rapp & arguedthat thesemagmas contained between 3.5 and Watson 1995). Note that we axe not suggestingthat 6VoH2O,lastequilibrated with harzburgiteat -1200'C primitive Cascadelavas are slab melts, only that such at ca. l0 kbar, and representmelting extentsof up to meltsmay be responsiblefor the sigmoidalcharacter of ^3UVo.Thecomparatively low HREE,Y, andSc contents REE pattems. In either case,fluid or melt transport, of the arc lavas would be a result of high degreesof melting of peridotite in the wedge is incapable of melting of a depleted,relatively infertile sourcethat had producing the sigmoidal REE paltsrn without prior beenemichedin H2O,ULE,andLkEEby additionof a emichmentof that sourceby a slab-derivedfluid or melt subductioncomponent. We favor a model in which variation in concenha- tions of SiO2 and incompatibleelements in arc basalt Origin of sigmaidalREE patterns andbasaltic andesite are tied to tlre amountof suMuction- compone,ntenrichment which, along with sourcefertility, The sigmoidalREZ patternsof arc basaltand basaltic determinesthe extentof melting at a given lemperature andesitefrom Lassen,Shasta and Crater Lake appear and depth. This model is consistentwith the adjacent to be consistentwith subduction-componentenrich- occturenceof both HAOT and lavaswith strongtrace- ment of magma sources.The Quaternaryabsarokite elementsignatures of arc magmas.Note that the most PRIMITTYE MAGMAS AT FTVE CASCADE VOLCANIC FIELDS 419 silica-richbasaltic andesite (LC86-1009) has a sigmoidal young pelagic sediment.Low loBePBeratios and B REE patrem(Fig. 7), similarto arc basalt(I 487-1384), concentrations in four lavas from the southwest but relatively low abundancesof all REE (and Sc), Washington Cascades(Monis & Tera 1989, Leeman consistentwith its origin as a high-degreemelt, presum- et aL 1990) probably reflect slow subductionof the ably leavinga harzburgiticresidue, as suggested by Baker young, hot Juan de Fuca plate plus sedimentderived et al. (1994) for magnesianbasaltic andesite at Shasta. from the continentalmargin (198e-and B-poor), and The steep, straight nEE pattems and btgh HREE consequentloss of fluid-transportedelements during concentrationsof our alkali basalt sample from the dehydration of the slab tenchward of the volcanic Simcoevolcanic field imply a low degreeof melting of front. The proportion of the subduction component enriched mantle. Although the arc basalt from the obtained from altered basaltic rocks of the slab is Mount Adams volcanic field has a similar REE pattern equivocal, as we do not find the negative correlation (Fig. 7), we suggestthat it has lower abundancesof between 2tlPbt2gPband Ce/Pb ratio used by Miller moderatelyincompatible elements because it representsa et al. (L994) to identify mantle Pb derived from a highermelt-fraction, enriched in a subductioncomponenl subductedslab. An intraplatemantle componentis presentin lavas Tm CASCADEUppsn Mar.w.s from the Mount Adams and Lassenvolcanic fields and appearsalso to be presentin somerocks from Medicine (Fig. et al. 1997) and, Composition Lake 4d). At Lassen @org apparently, Medicine Lake, the innaplate signature tendsto increaseto the east whereasthe arc character We havepresented compositional data indicative of decreases.Hughes (1990) found geochemicalevidence at leastthree end-member primitive magmasat the five for both depletedand OlB-source-likemantle beneath Cascadevolcanoes studied. lXese imply three mantle the central Oregon Cascades(approximate lat. componentsin the sourcesof IIAOT, arc basalt and 44o-45oN,north of CraterLake), Conrey et al. (1997) basalticandesite, and intraplatebasah depletedsub-arc reportedwithin-plate basalts fromnorthern Oregonand mantle, mantle enriched by a modern subduction southernWashington, and some analysesof basaltic component,and OlB-source-likeintraplate mantle. The andesitefrom the chain (lat. 44'N) sizes of domainswithin the mantle are unknown, but reported by Gardner (1994) also have an intraplate the spatial associationof vents for all three lava types signature.A thorough searchat Crater Lake failed to indicateseither limited areal extent or a layeredstruc- provide any evidencefor an intraplatesignature ture. It doesnot appearto be possibleto discriminate @acon 1990, and unpubl. data); data availablefor the Shasta betweenmixing of sourcesin the mantle and blending areaare insufficient for any conclusion.The intraplate of melts ftom different sourcesin order to producethe componentdoes not lead to high 87Sr^6srratios or low continuumof lava compositions. Ep6values, as would be expectedof old continental Universal presence of HAOT with minor UIE lithosphere. More likely, it is similar to OlB-source enrichmentrelative to N-MORB requireswidespread mantle, as suggestedby Hughes(1990). A heterogene- depletedsub-arc mantle. The arc signaturepresent in ous mixture of depleted and OlB-source mantle do- the HAOT sourcemay be an old one, as HAOT with mains, enrichedby a subductioncomponent has been (commonly + similar1,11,8 emicbment includingCs Pb)is identified as the source of arc lavas is many studies found far to the eastof the modem Cascadearc" in the (e.g.,Morris & Hart 1983,Gill 19M, Iltckey et aL 1986). OregonPlateau and northern Basin and Rangeregion QIafi et aL.1984;W.K. Harr unpubl.data, 1995). This A possibleanswer to the questionof how OIB- depletedsub-arc mantle corresponds to Carlson's(1984) source-likemantle domains occur beneath the Cascades Cl incompatible-element-depletedmantle reservoir is that asthenospheremay rise locally east of the arc, eastof the Cascades. suchas beneath the Simcoevolcanic field, andbecome We suggestthat the subduction signature,clearly entrainedin the westward flowing upper part of the present in arc basalt and basaltic andesiteof the wedge.Alternatively, OlB-source-likedomains may be Cascades,is linked to modernsubduction because these an integral part of the relatively young continental lavas are restricted to the arc itself and have much margin lithosphere.This lithosphere is composedof higherLlIE abundancesthan samplesof HAOT, which accretedoceanic and island-arc terranes,presumably are consideredto representsimilar 61lower degreesof including a mix of MORB- and OlB-source mantle, melting. The actual source of arc basalt and basaltic variously affectedby Cenozoicand Mesozoicarc mag- andesitemay be less fertile than that of HAOT @aker matism and related fluids. Lack of evidencefor OIB- et al. L994,Clynne & Borg 1997),though it mustbe sourcemantle beneath Crater Lake resultseither from a more hydrous. Clearly, some geochemicaltracers are failure of volcanismto sampleit, perhapsbecause the derived from subductedsediment, such as Cs and Pb. volcaniczone is muchnarrower (-30 km) than in south- However,the few publishedloBePBe data for Cascade em Washinglonor northemCalifornia €1@ km), or to a lavas do not indicate a measurablecontibution from real absenceof OlB-source"plums" there. 420 THE CANADIAN MINERALOGIST

In the part of the arc included in this snrdy, the HAOT hasbeen erupted in the cenffalOregon Cascades sub-arclithosphere probably is Cenozoicin age north from at least7 Ma (Conrey et al. L997; D.R. Shenod, of Crater Lake and definitely containscrustal rocks at written comm., 1995),the uppermostmanfle must have least as old as Paleozoicto the south, beneathShasta beengradually heated owing to relaxationofhorizontal and probably Lassen. It is in the last two areas of thermal gradientsnear local high-temperatureregions. relatively ol4 and thus more "mature" crust, wherewe Althoughsustained temllerafires as high as l2@-13@oC find isotopic evidencesuggestive of contaminationof may not be everywherecharacteristic of the uppermost someprimitive lavasby crustalmaterial. mantle beneaththe southernWashington to northern Califonria Cascades.the manflein this reeion is none- Therrnalstructure thelessanomalouslv hot.

Eruption of HAOT implies that temperaturesin the Acrovowr-eDcElrmr,trs uppermostCascade mantle are higher than indicatedby ftaditional models of subduction zones@aker et al. 1994).High temperaturesat low pressuresfor separa- We are pleasedto acknowledgethe contributionsof tion of HAOT magma from the mantle required by the many chemistswho producedthe major-element phase-equilibriumstudies (*L290'C at 11 kbar: Bart€ls ICP, and INAA data of this study, especially D.F. et al. l99L), reinforced by the low pre-eruptiveH2O Siems, S.T. Pribble, T.L. Fries, and J.N. Grossman. content of HAOT magrnas(Sisson & Layne 1993), Advice and instruction in analysisby massspectrome- demand that the uppermost mantle beneath the try were kindly p16vi4ed to the second author by Cascadesis, at least locally, impressivelyhot. This is T"D. Bullen andJ.L. Wooden We thankL.H. Adami for consistent with the youth of tle subducting slab measuringoxygen isotope ratios. W.K. Hart sharedhis @rg. 1), high heat flow @lackwell et aL 1990),and unpublisheddata with us. Discussionswith T.W. Sisson extensionalenvironment (Rogers 1985) of the soutlem and J.B. Lowensternhelped us to refine our interpreta- Washington to northern California Cascades.The tions. Commentsby Sisson and D.A. Clague on an low l9BePBeratios and B concentrationsin the few early version of the manuscriptled to many improve- publishedcompositions of Cascadelavas also arguefor ments. Journal reviews by M.B. Baker and T. Plank a high-temperaturethermal regime in comparisonto resultedin further clarification of the manuscript.We many arcs (Leeman et al. 1990). As pointed out by thank A.D. Johnstonand G.T. Nixon for organizingthe Baker et al. (1994),magmas that give rise to arc basalt 1995 MAC symposium and for the opportunity to andbasaltic andesite also must havetraversed this same contribute to this thematic issue of The Carwdian thermal regime, and owe their distinctive composition Mineralagist. to melting of hydrous domainswiflin it, althoughthe temperature of last equilibration apparently was RSFERE{cEs *I200"C. They suggestedthat lower temperaturesof last equilibration for arc basalt and basaltic andesite Amrr, S.(1983): Sildies in "standardsamples" of silicaferocks than for HAOT resultfrom the st€€peradiabatic melting andminerals 1969-1982. Geol. Sun. Cot", Pap,8-15. curve for hydrous compositions,assuming onset of melting at a commondepth of ^,60km. ANDERSoN,A.T., JR. (1974): Evidencefor a pioitic, volatile- The model presentedby Baker et aL (L994) for the rich magmabeneatl Ml Shasta California- J, Petrol, 15, origin of magnesianlavas at Mount Shast4 which we 243-267. believe appliesto the primitive lavasof this study,calls upon adiabaticupwelling andmelting of sub-arcmantle ARrH, J.G., Baurn" F., hTERMAN,Z.E. & FhmueN, I. (1978): Geochemistry gabbro- - - in the wedge that is locally enriched ofthe diorite tonalite with variable trondhjemite suite of southwestFinland and its implica- amountsof a modern, hydrous subduction-derived tions for the origin of tonalitic and nondhjemitic magnas. component.Support for this hypothesisis found in the J. Petrol.19.289-316. mdel of Frukawa (1993)for inducedflow in the wedge owing to mechanicalcoupling with the subductingslab & IIANSoN,c.N. (1972):Quartz diorites derived by and in the observationsby hao et al. (1994) of low partial melting of eclogiteor amphiboliteat maatledepths. P-wave velocities in the uppermostmanile beneath Contrib. Mineral. PetroL 37. 161,-174. northernHonshu. Ongoing lithospheric extension in the norttrernCalifomia to southernWashington Cascades is Becou, C.R. (1990): Calc-alkaline,shoshonitic, and primitive consistentwith mantleupwelling and promotesescape tholeiitic lavas from nonogenetic volcanoesnear Crater Iake, Oregon.J. Petrol. 31, 135-L66. of primitive magrnas.The required temperaturesof 1200-1300"C virtually at the base of the Cascade GUNN,S.H., LANprsRB,M.A. & WooosN, J.L. crust must be transient, local phenomena, or the (1994): Multiple isotopic componentsin Quaternary lower crustmust be quite refractory,as otherwisevolu- volcanic rocks of the Cascadearc near Crater Lake. minous crustal melts would be expected. Because Oregon.J. Petrol. 35, l52l-L556. PRIMITIVE MAGMAS AT FTVE CASCADE VOLCANIC FIEI.DS 421

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& PRrcE,R. (1994):Primitive basaltsand & _ (1990):Trace elementand isotopic andesitesfromtle Mt. Shastaregion,N. Califomia:products constraints on magmatic evolution at Lassenvolcanic of varyingmelt fractionand wat€r contont. Contrib, MineraL 19,671-19,69I. center."I. Geophys.Res.95, Perrol. l1.8. lLl-129. Carusox, R.W. (1984): Isotopic constraints on Columbia BARNIES,C.G. (1992): Petrology of monogeneticvolcaaoes, River flood basalt genesisand the natureof the subconti- Mount Bailey area,, Oregon. J. VolcarcL nentalmantle. Geochim. Cosmochirn. Acta 48.2357-2372. Geothzrm. Re s. 52. L41.-l 56. Centvncus,, I.Sn. (1991): The redox states of basic and BAR'rE-s,K.S., Iirz,nn, RJ. & Gnow, T.L. (1991):High silicic magmas:a reflectionof their sowcnre$ons? Contrib. pressurephase relations of primitive high-aluminabasalts Mineral. Petrol. 106. 129-141. from MedicineLake volcano,northem California. Contrib. Mineral. Pe trol. t0E. 25327 0. CnnrsrraNsml,R.L., IC.rnuntrlrpl, F.J., Bur

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