n5 Thz Camdian M ineralogist Vol. 35,pp. n5-290 (1997)
ARG: KERMADEC K.RICHMAGMASFROMCLARKVOLCANO,SOUTHERN PRIMITIVE A PARADOXIN THEK- DEPTHRELATIONSHIP JOHNA. GAMBLE' ANDROB H.K. CHRISTIE of WellingtottP.O.Box600,Wellingto\Newfual'an'd Departmznt of Geology, VictoriaUniversity IANC. WRIGTTI P.O.Box14-90LKilbimie,WellingtoaNewkalmtd Research, NatiornlInstitateof WaterandAtmospherir RICHARD J. WYSOCZANSKI ResearchSchoolof Eanh Scierces,Aa.stralianNational University,GPOBox 4, Canberrq ACT' 2601,Awtralia ABSTRACT Clark volcanois the last significantvolcanic edifice in the oceanicsegmentof the nearly 3000-km-longTonga- Kermadec - New Zealandvolcanic arc system,which becomescontinentalin the TaupoVolcanic Zone ofNew Zealand.Clark volcanois a submarinebasalticandesite- daciteedi8ce with broadly similar petrochemicalaffinity to the rest of the Kermadecisla:rdarc (i.e.,lavasaremostly of basalt-basaltic andesitecomposition,v/ith A,2VoKzO at507oSiOz,MgO in therange5 ta7Vo,(Ce/Yb), between100ppm) andCr (>200ppm), and5 < (Ce/Yb)r < 7; thesebasaltsareuniquewithin the Kermadec- NewZealand which contast assemblages in theK-richbasaltsat Clarkareolivine(t chromianqpinel)+ clino,pyroxene, regime.Phenocryst subduction in "typical' KermadecArc basalts.In addition fe high lgv6ls 6f K with tle plagioclase+ olivine t clinopgoxene assemblages and the iignt rare-earthelements,the K-rich suite showshigb concentrationsof Ba (^6@ ppm), Rb (40 - 60 ppm) and Cs (-1 ppm). Radiogenicisotqres of Sr, Nd and Pb in the K-rich basaltsare enrichedrelative to other maguas of the oceanic KermadecArc, including thoseof the more 'typical" Clark basalts,andoverlapwith thoseof basaltsfrom thecontinentalTaupo Volcanic Zone of New Zealand.We interpretthesepdmitive K-rich lavasasa rare exampleof near-slab,small-volumemelts of a mantle wedge sourceenrichedby sedimentand fluid transferfrom the descendingslab of lithosphere.The nature of this subductingslabis significan! becauseit conprisescrustofthe Hikuangr Plateau,an anomalouslythick sequenceofcretaceous basaltbasementand sedimentarybasin fill, which is subductingbeneathtle present-daycontinentalTaupoVolcanic Tnne and the oceanicso$hernKermadecArc. Keywords:arc geochemistry,primitive K-rich maguas, sediments,KermadecArc, southwestemPacific. SoMMaRs Le volcan Clark est Ie demier 6difice volcaniquedu secteuroc6aniquede l'arc Tonga - Kermadec- Nouvelle-Z6lande, presque3@0 km de long, avantqu'il ne deviennecontinentaldansla zonevolcaniquede Taupo,en Nouvelle-Zdlande.tr s'agit d'un-volcan sous-marind and6sie basaltique- dacite, dont le spectrep6trochimiqueressembleen g6n6rali celui des aufes volcansde I'arc insulafuede Kermadec.Plus pr6cis6ment,la plupart descoul6esont une compositiondansI'intervalle basalted teneuren SiO2de 507o,entre5 et 77odeMgO, et un rapport(Ce/Yb)oallant anddsitebasaltique,avecenviron0.2VodeK2Od:urcle de 100ppm) et Cr (>200ppm), avec(Ce/Yb)zentre5 et 7. Cettesuite Les basaltesriches en K contiennent, dtange sembleunique dansle systdmede subductionKermadec- Nouvelle-Z61ande. de phdnoqistaux,olivine (t spinellechromifbre)+ clinopyroxdne,ce qui dilfdre nettementde l'assemblage commeassemblage plagioclase+ olivine I clinopyroxBnedansles basaltesplus typiquesde I'arc de Kermadec.En plus d'un enrichissementen K et en Grresraresl6gbres,les basaltesrichesen K possadentdesteneurs6lev6esen Ba (environ600 ppm)' Rb (entre40 et 60 ppm), et Cs (environ I ppm). Irurs teneursen isotopesradiog6niquesde Sr, Nd et Pb sont plus 6lev&s que dansles autresvenues basaltiquesdu secteurocdaniquede l'arc de Kermadec,par exemple,les basaltesnormau( du m6mevolcan, et ressemblent tlavantagei celles desbasatteidu secteurcontinentalde l'arc, dansla zonevolcaniquede Taupo,en Nouvelle-Zelande-Nous interpr6tonsceslavesprimitives richesen K commeunemanifestationassezrare de magnasissuspar firsion partielle restreinte du coin demanteauprdsde la zonede subduction,dont la sourcea 6tf enrichiepar un apportde s6dimentet unecontributionpar 1 E-mail address:
[email protected]
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TIIE CANADIAN MINERALOGIST
transfertvia unephasefluide issuedela plaquelithosph6riqueen subduction.La naturede cetteplaquerevetici unesignification particulibre;il s'agit de la cro0te anormalement6paissedu plateaude Hikurangi, faite d'un soclede basaltescr6tac€set d'un remplissagede bassins6dimentaire,qui estpr6sentement en situationde subductionsousla partie continentalede I'arc, et donc sousIa zonevolcaniquede Taupo,ainsi que soussapartie ocdanique,dansle secte[f sudde l'arc de Kermadec. (Traduit par la Rddaction) Mots'clds: g6ochimiedesrochesd'arc, magmasprimitifs riches en I(, s6diments,arc insulairede Kermadec,secteursud-ouest du Pacifique.
INTRoDUc'noN For many ye:trs,the Tonga- Kermadecsubduction systemhas beenrecognizedas an archeqryalexample of a primitive oceanicisland-arc @wart et al. L977, Ewart & Hawkesworth1987).This regionhasplayeda major role in the developmentof modelsand ideason marginalbasinsandbackarcbasins(Karig 1970,lgTl), and was the focus of lrg 135 of the OceanDriling Program in 1990 - 1997 (Hawkins et aL L994). "ftte southernextensionof the Tonga- KermadecIslandArc toward New Zealand is representedby submarine volcanoes,which have only recently beensurveyedin detail (Gambleet al. 1993b,1994,Wright 1994,Gamble & Wright 1995). These volcanoes,from the north, include the Silent II and Rumble (tr, III and IV) seamounts(Smith & Brothers 1988, Gamble et al. 1993b) and tlree more recently surveyededifices namedRumbleV, Tangaroaand Clark (Wright 1994). Southwestof Clark volcano,a major changein slopeof the seafloor delineatesthe transitionfrom oceanicarc, withvolcanoesconstructed on oceaniccrust,to continenhl arc,wherevolcanoesareconstuctedon continentalcrusr Whakatane volcano, a basaltic andesite - andesite effice with local relief of some 1100meters(Fig. l), marksthe northernlimit of the offshoresegmentof the continentalTaupoVolcanicZone (fYZ) (Wrighr 1992, Gamble et al. 1993b). It is also significant that the Hikurangi Plateau(,ewis 1994,Wood & Davy 1994, Mortimer & Parkinson 1996) comprises the upper portion of the lithospheric slab subductingpresently beneaththe continental(f,/Z) wrd theoceanic(southem KermadecArc) portions of the plate boundary(Fig. 1) alongthe Hikurangi Trough.The Hikurangi Plateauhas a crustal thickness of 10-15 km (as deterrninedby gravity modeling: Davy & Wood 1994) comparedto 5-7 km for norrnaloceaniccrustto the northeast,which is subducting beneath the northern KermadecArc. Flrthennore, seismicreflectionprofiling indicates that the Hikurangi Plateau basementhas a Mesozoic ro Cenozoicsedimentarycover up to severalkilometers thick (Wood & Davy 1994).T\e Hikurangi Trough is partialy infllled by sediment. Clarkvolcanois a lage volcanicstructure,constucted on oceaniccrustandwhoselocal relief is about 1600m (Wright 1994).During dredgesirnrplingof the volcano, an unusualsuiteof K-rich (1.5 - 2.25VoK2O at SAVo SiO2)basalticrockswererecovered,in additionto lavas (basalticandesite,andesiteand dacite)of more,typical"
KermadecArc (I(A) character(43Vo K2O at 52.6Vo SiO2in basalticandesite). Elsewhere,the presenceof K-rich volcanic rocks in island-arcsuiteshas beenassociatedwith depth to the Wadati - Benioff Zone (Hatherton& Dickiruon 1969, Marsh & CarmichaelI974),lelLgh of the mantlemelting column @laxk & Langmuir 1988), or enrichment processesin the sub-arcmantle @dwardset al. l99l). In this paper,we presentnew whole-rockgeochemical and mineral data on suites from Clark volcano, and otheryolcanoestowardthe southemextremityof the KA system.We comparethesedatato results of previous studies from the KA @wart & Hawkesworth 1987, Smith& Brothers1988,Gambleet al. L990,1.993ub), and developa model to accountfor the petrogenesisof the K-rich suiteof basalticrocks. ANALYTCAL
TECHIIQUES
All samplesreportedin this work were collectedby dredgrng on the submarinevolcanic edifices during researchcruisesin 1988- 1992;seeWright (1994) for details. Samplesused for penological study were selectedon the basis of freshness,as determinedby study of broken dredge-haulsamples.Many contain freshglass,which wascarefirllyprisedfrompillow rind surfaces.Prior to crushing,all sampleswerethoroughly washedwith distilled water in an ultrasonic bath and oven-driedat 110"C. SamFleswere then reducedto cm-sizedchips, rewashedand dried, and crushedto a powder in a tungstencarbidering mill. This powder was usedfor standardX-ray fluorescenceQ(RF) techniquesin theAnalyticalFacilityof VictoriaUniversityand InstrumentalNeutron Activation Analysis (INAA); dl isotopeanalyses wereundertakenon selectedrock-chips. Mineral analysesweremadeby electronmicroprobe in the Analytical Facilify of YUW, using a JEOL733 Superprobeand firl ZAF-correctionprocedures.Concentrationsof major element (oxides) and trace elements(Sc,V, Cr, Ni, Cu,Zn, Ga"Rb, Sr, Y, Zr, Nb, Ba Pb) were determinedby standard)(RF methodsin the Anaiytical Facility, YUW (Palmer 1990). In most casqs,our precisionfor the traceelementsis betterthan SVo(relatle).Increasedcountingtimeswereemployed in the caseof Nb, which extendedthe theoreticallimit of detectionto
