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Petrology and Geochemistry of the GalÁ JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. Bll, PAGES 19,533-19,563, NOVEMBER 10, 1993 Petrologyand Geochemistryof the GaMpagosIslands' Portrait of a PathologicalMantle Plume WILLIAM M. WmT• Departmentof GeologicalSciences, Cornell University, Ithaca, New York ALEXANDER R. McBIRNEY Centerfor Volcanology,University of Oregon,Eugene ROBERT A. DUNCAN Collegeof Oceanography,Oregon State University,Corvallis We reportnew major element,trace element,isotope ratio, and geochronologicaldata on the Galfipagos Archipelago.Magmas erupted from the largewestern volcanos are generallymoderately fractionated tholeiites of uniformcomposition; those erupted on otherislands are compositionallydiverse, ranging from tholeiites to picritic basanitoids.While thesevolcanos do notform a strictlylinear age progressive chain, the agesof the oldestdated flows on anygiven volcano do form a reasonableprogression from youngest in thewest to oldestin theeast, consistent with motionof theNazca plate with respect to thefixed hotspot reference frame. lsotoperatios in theGalfipagos display a considerablerange, from values typical of mid-oceanridge basalt on Genovesa(87Sr/86Sr: 0.70259, end: +9.4, 206pb/204pb:! 8.44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, œNd: +5.2, 206pb/204pb: 20.0). La/SmN rangesfrom 0.45 to 6.7; otherincompatible element abundances and ratios show comparable ranges. Isotope andincompatible element ratios define a horseshoepattern with the mostdepleted signatures in the centerof the GalfipagosArchipelago and the moreenriched signatures on theeastern, northern, and southern periphery. These isotopeand incompatible element patterns appear to reflectthermal entrainment of asthenosphereby the Galfipagos plumeas it experiencesvelocity shear in the uppermostasthenosphere. Both north-south heterogeneity within the plumeitself and regional variations in degreeand depth of meltingalso affect magma compositions. Rare earth systematicsindicate that melting beneath the Galfipagos begins in thegarnet peridotite stability field, except beneath the southernislands, where melting may occurentirely in the spinelperidotite stability field. The greatestdegree of meltingoccurs beneath the central western volcanos and decreases both to theeast and to thenorth and south. Sis. 0, FeB.0, andNaB. 0 values are generally consistent with these inferences. This suggests that interaction between the plume and surroundingasthenosphere results in significantcooling of the plume. Superimposedon thisthermal pattern producedby plume-asthenosphereinteraction is a tendencyfor meltingto be lessextensive and to occurat shallower depthsto thesouth, presumably reflecting a decrease in ambientasthenospheric temperatures away from the Galfipagos SpreadingCenter. INTRODUCTION emergent volcanos in the Galfipagos,nine have been active historically and four others have erupted in the Holocene and Productionof basaltic magmasover stationary,long-lived should be considered still active. By comparison,only six hotspotsin oceanbasins is now generallythought to be a mani- Hawaiian volcanosare active,and theseare alignedin two chains festationof rising convectiveplumes in the mantle[Morgan, with a clear age progression. The GalfipagosIslands are also 1971]. The longevityof suchhotspots, their stationary nature, the exceptionalgeochemically: while mostbasalts have typical OIB simpleage progression of volcanismin the islandchains, and the incompatibleelement concentrationsand isotoperatios, basalts geochemicaldistinctions between oceanic island basalts (OIB) that are essentially indistinguishable from MORB in their and mid-ocean ridge basalts (MORB) are among the most geochemistryare also commonproducts of Galfipagosvolcanos convincingevidence in supportof theconvective plume theory. [Baitis and Swanson,1976; Whiteand Hofmann,1978]. Not all oceanicislands conform to this simplepattern. Among We report the resultsof a petrologicaland geochemicalre- the most exceptionalisland groups is the Galfipagos,which connaissanceof the GalfipagosIslands, including Sr, Nd, and Pb consistsof 10 majorvolcanic islands and a numberof lesserrem- isotopic analysesof 124 samples,as well as trace and major nant volcanosthat emerge from a broad, shallow submarine elementanalyses, and 38 K-Ar age determinations.Our results platform(Figure 1). While two aseismicridges, the Cocosand suggestthat the uniquefeatures of Galfipagosvolcanism reflect Carnegieridges, extend from the Galfipagos Archipelago in thedi- the local plate tectonicenvironment, and thermal and dynamic rectionof Cocosand Nazca plate motions,the emergentvolcanos interaction between the asthenosphereand a mantle plume do not form a linear chain. Nor is there any simple geographic undergoingvelocity shear. Tectonicfactors control the location, patternto the agesof the volcanos,although the oldestvolcanos size, and styleof volcanism. Thermalinteraction between plume tend to occur in the southeastarea of the archipelago. Of the 21 and asthenosphereresults in asthenospherebeing entrainedinto the center of the plume. This, and the consequentplume- asthenospheremixing, producesmuch of the observedgeochemi- Copyright1993 by the AmericanGeophysical Union. cal variation. Superimposedon this mixing are variationsin depth Papernumber 93JB02018. and degreeof meltingwhich alsoresult from thermalinteraction 0148-227/93/93JB-02018505.00 betweenplume and asthenosphere. 19,533 19,534 WHITEET AL.: PETROLOGYAND GEOCHEMISTRY OF THE GAL•PAGOS 93 ø W 92 ø W 91 ø W 90 ø W 89 ø W 88 ø W DARWINOLFI. I. I CocosPlaTe PINTA Q) reDONDA '•ENOvESA •ecuA•,,wøt••••'":•'•ARCHENA 0 0o • ¾'""•"t''" '• '"'"'••""•'•••••'•:•--;'::-'•-:'::•:.:':'-•SEYMOUR •'"'•'••'••'••••••;. '" rABIDA BALtrA ß )•...:..::•:•:.:•:..::::-•'•u;::•::•.... :::.?• sterrA :'•'"••••••}•. '/.:-'•g4 '"R' :-:'-::-:;----:: ........ ::::?-•:.:':.': ' '• ';'........... :'" . .....ß................... : ' ••••'•2.¾;'::•::'::/.':•:::'::'::::;::.?:'.::..:'.:..... SANTA '[E ................... •••'• ............ '••'• '••'•'"-"'-"-'•1 FL ?... m m m m m WHITMER m m m m m SMT. I I 93 ø W 92 ø W 91 ø W 90 ø W 89 ø W 88 ø W Fig. 1. Map of the GalfipagosArchipelago. GSC is theGalfipagos Spreading Center. Boldline showsthe lithospheric faultsystem that separates thin, weak lithosphere to thenorth and east from stronger and thicker lithosphere to the south andwest inferred by M. A. Feighnerand M. A. Richards(submitted manuscript, 1993). Insetshows the regional tectonic setting. GEOLOGICAL BACKGROUND AND PREVIOUS WORK concurrentlyto the productionof the Cocosand Carnegieridges, on the separatingCocos and Nazca plates, much as the Iceland TectonicSetting hotspotgenerates paired volcanic ridges on the Europeanand The GalfipagosIslands rise from a shallowsubmarine volcanic North Americanplates. With the northwardmigration of the ridge platform that is the westernterminus of the east-westtrending 5-6 m.y. ago, magmasupply to the CocosRidge was essentially CarnegieRidge. The GalfipagosSpreading Center (GSC), which severedand the output of the hotspotfocused on the western separatesthe Cocosplate from the Nazca plate andjoins the East CarnegieRidge. Dredgingand SeaBeam studiesof seamountson Pacific Rise in a triple junction some 1000 km west of the the CarnegieRidge revealedthat at least somewere onceislands GalfipagosIslands, bisects the region. The GSC is a moderately [Christie et al., 1992], extendingthe time availablefor evolution fast spreading ridge (•-6 cm/yr) with significantasymmetric of the uniqueGalfipagos biota back to at least9 Ma. The Coiba spreading over the last 5 m.y. along the section nearest the and Malpelo ridges are smaller blocks, dismemberedfrom the Galfipagos(85" to 91øW) with more rapid spreadingto the north eastern end of the Carnegie Ridge and transportednorthward [Hey, 1977]. Spreadingelsewhere along this plateboundary has during Nazca-SouthAmerican plate collision [Malfait and generallybeen symmetrical. North of lsabelaIsland, an ill-defined Dinkerman, 1972; van Andel et al., 1971]. Basementvolcanic ages left-lateraltransform fault at approximately90ø50'W offsetsridge from theseridges support the hotspotmodel [van Andel et al., segmentsby 150-km, and is the boundarybetween symmetric 1973]. spreading ridge segments to the north and west, and the M. A. Feighnerand M. A. Richards(Lithospheric structure and asymmeti'icsegment to the southand east. Thisresults in a 3-5 compensationmechaism of the GalfipagosArchipelago, submitted m.y. age differencein the lithosphereunderlying the westernand to Journal of GeophysicalResearch, 1993) haveanalyzed gravity easternhalves of the archipelago. data from the GalfipagosPlatform and find that the centerof the The GSC has migratednorthward away from the hotspotin the archipelagois underlain by weak lithosphere,with an elastic last5-6 Ma, judgingfrom the volcanic traces of thehotspot, the thicknessof 6 km or less,and is closeto Airy compensation.The Cocos and Carnegie ridges [Hey, 1977]. Before that time, the western and southernportions of the platform are flexurally hotspotlay directly beneaththe GSC and contributedmagmas supportedby a lithospherewith an effectiveelastic thickness of WHITE ET AL.: PETROLOGYAND GEOCHEMISTRY OF THE GAL•PAGOS 19,535 about 12 km. The transition from strong to weak lithosphere central shield volcanos with well-developed calderas,but they appearsto be abrupt and can be modeledas an arcuatefault that differ conspicuouslyfrom the familiar shields of Hawaii and runs beneathEspafiola and Floreanaislands in the south,beneath
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