3.16 Oceanic Plateaus A.C.Kerr Cardiffuniversity,Wales,UK

3.16 Oceanic Plateaus A.C.Kerr CardiffUniversity,Wales,UK

3.16.1 INTRODUCTION 537 3.16.2 FORMATION OF OCEANIC PLATEAUS 539 3.16.3 PRESERVATIONOFOCEANIC PLATEAUS 540 3.16.4GEOCHEMISTRY OF CRETACEOUSOCEANICPLATEAUS 540 3.16.4.1 GeneralChemicalCharacteristics 540 3.16.4.2 MantlePlumeSource Regions ofOceanic Plateaus 541 3.16.4.3 Caribbean–ColombianOceanic Plateau(, 90 Ma) 544 3.16.4.4OntongJavaPlateau(, 122 and , 90 Ma) 548 3.16.5THE INFLUENCE OF CONTINENTALCRUST ON OCEANIC PLATEAUS 549 3.16.5.1 The NorthAtlantic Igneous Province ( , 60 Ma to Present Day) 549 3.16.5.2 The KerguelenIgneous Province ( , 133 Ma to Present Day) 550 3.16.6 IDENTIFICATION OF OCEANIC PLATEAUS IN THE GEOLOGICAL RECORD 551 3.16.6.1 Diagnostic FeaturesofOceanic Plateaus 552 3.16.6.2 Maﬁc Triassic Accreted Terranesinthe NorthAmericanCordillera 553 3.16.6.3 Carboniferous to CretaceousAccreted Oceanic Plateaus inJapan 554 3.16.7 PRECAMBRIAN OCEANICPLATEAUS 556 3.16.8ENVIRONMENTAL IMPACT OF OCEANICPLATEAU FORMATION557 3.16.8.1 Cenomanian–TuronianBoundary (CTB)Extinction Event 558 3.16.8.2 LinksbetweenCTB Oceanic PlateauVolcanism andEnvironmentalPerturbation 558 3.16.9 CONCLUDING STATEMENTS 560 REFERENCES 561

3.16.1 INTRODUCTION knowledge ofthe oceanbasins hasimproved over the last 25years,many moreoceanic plateaus Although the existence oflarge continentalflood havebeenidentified (Figure1).Coffinand basalt provinceshasbeenknownfor some Eldholm (1992) introduced the term “large igneous considerabletime, e.g.,Holmes(1918),the provinces” (LIPs) asageneric term encompassing recognition thatsimilarfloodbasalt provinces oceanic plateaus,continentalfloodbasalt alsoexist belowthe oceans isrelatively recent. In provinces,andthoseprovinceswhich form at the early 1970s increasingamounts ofevidence the continent–oceanboundary (volcanic rifted fromseismic reflection andrefraction studies margins). revealed thatthe crust inseverallarge portions of LIPsaregenerallybelieved to be formed by the oceanfloorissignificantly thickerthan decompression meltingofupwellinghottermantle, “normal” oceanic crust,which is6–7kmthick. knownasmantleplumes. Although ideasabout Oneofthe first areasofsuch over-thickened crust hotpots andmantleplumeshavebeenaroundfor to be identified wasthe Caribbeanplate(Edgar almost 40years (Wilson,1963),itisonly inthe etal.,1971)which Donnelly (1973) proposed to be past 15years thatLIPshavebecomethe focus of an“oceanic floodbasalt province”. Theterm major research.Oneofthe mainreasons for the oceanic plateauwascoined byKroenke (1974), increased research activity intoLIPsisthe andwasprompted bythe discovery ofalarge area realization thatsignificant proportions ofthese ofthickened crust(. 30 km) inthe western Pacific LIPserupted overarelatively short time, often known asthe OntongJavaplateau(OJP). Asour less than2–3Myr (see reviewinCoffin,1994).

537 538 Oceanic Plateaus Thishasimportant implications for mantlepro- Pacific, whileinthe IndianOceanthe Kerguelen cessesandsource regions (Hart etal.,1992; Stein plateauwasdeveloping.Theareas,volume andHofmann,1994),aswell asenvironmental maximum thicknessesandagesofthe largerof effects on the globalbiosphere(e.g.,Caldeiraand theseplateaus aregiveninTable1. TheOntong Rampino,1990; Courtillot etal.,1996; Kerr, Javaisthe largest ofthe Cretaceous plateaus. 1998). Oceanic plateaus canalsobecomeaccreted Itcovers anarea of1.9 106 km 2 ,andhasan to continentalmargins,andithasbeenproposed estimatedtotalvolum£ eof4.4 107 km 3 thattheseplateaus havebeensignificant contri- (EldholmandCoffin,2000). Althou£ gh early butors to the growthofcontinentalcrust (e.g., seismic refraction datasuggested thatthe OJP Abbott,1996; Albarede, 1998). wasasthick as43km(Furomoto etal .,1976),a The most recent major phaseofoceanic morerecent synthesisbased on existingseismic plateauformation wasinthe Cretaceous when andnewgravity data(Gladczenko etal.,1997) the OntongJava, Manihiki, Hess Rise, andthe hasindicated the averagethickness to Caribbean–Colombianplateaus formed inthe be , 32 km.

Figure1 Mapshowingall major oceanic plateaus,andotherlarge igneous provincesdiscussed inthe text (afterSaunders etal.,1992).

Table1 Agesanddimensions ofJurassic–Cretaceous oceanic plateaus. Oceanic plateauMeanage Area Thickness range Volume (Ma) (106 km 2 ) (km) (106 km 3 ) Hikurangi early-mid Cretaceous 0.7 10–152.7 ShatskyRise1470.2 10–282.5 MagellanRise145 0.510 1.8 Manihiki 123 0.8 . 20 8.8 OntongJava121(90) 1.9 15–32 44.4 Hess Rise99 0.8 . 159.1 Caribbean88 1.1 8–20 4.4 SouthKerguelen110 1.0 , 22 6.0 CentralKerguelen/BrokenRidge 861.0 19–21 9.1 SierraLeoneRise , 73 0.9 . 10 2.5 MaudRise , . 73 0.2 . 10 1.2 AfterEldholm andCoffin(2000). Formation ofOceanic Plateaus 539 3.16.2 FORMATION OF OCEANIC PLATEAUS The production oflarge volumes(. 106 km 3 ) ofmelt inaperiodasshort as2–3Myr implies magmaproduction ratesupto25%higherthan thoseobserved atpresent-daymidoceanridges (Eldholm andCoffin,2000),andisgenerally believed to necessitateahigh flux ofhotter-than- ambient asthenospheric mantlebelowthese provinces(e.g.,McKenzie andBickle, 1988). Numericalandphysicalmodels show thatthis hottermantlecommonly takesthe form ofa mantleplumewhich ascendsbythermalbuoy- ancythrough the overlyingmantle(Loper,1983; McKenzie andBickle, 1988;Campbell etal., 1989; FarnetaniandRichards,1995). Physical constraints demandthatmantleplumesmust ascendfrom aboundary layerwithinthe Earth, eitherthe core–mantleboundary ( D 00)orthe 670 kmdiscontinuity. Large ascendingmantle plumesare, on average, 200 8 Chotterthanthe ambient uppermantle(McKenzie andBickle, 1988)andundergodecompression meltingas theyapproachthe baseofthe lithosphere. PhysicalmodelingexperimentsbyGriffithsand Campbell (1990) haveshown thatmantleplumes arelikely to ascendthrough the mantlefrom their source boundary layerinthe form ofalarge semi-spherical“head”fed from the source region Figure2 Cartoon to illustratehow mantleplumesare byanarrowerplumetail(Figure2). Alterna- believed to (a)risethrough the asthenospherefrom either tively,numericalmodelingbyFarnetaniand the 670 kmdiscontinuity or D 00 and(b)flattenalongthe Richards(1995)suggested thatplumeheads baseofthe lithosphereandundergodecompression startinginthe mantleonly riseabout three meltingto produce aLIP (afterSaunders etal.,1992). plumehead diameters beforespreadingout. In eithercase, asthe plumeapproachesthe baseof the lithosphere, itspreadsout overabroadly circulararea (whichcanbe asmuch as1000 km indiameter) andundergoesadiabatic decompression,producingmelt overmostofthe area covered bythe flattened-out plumehead (Campbell andGriffiths,1990). Theamount of melt producediscritically dependent on the thickness ofthe preexistinglithosphere, since the baseofthe rigid, nonconvectinglithospherewill actasa“lid”onthe upwellingplumemantleand on the extent ofdecompression melting.Thus,a mantleplumeascendingbelow thick continental lithosphere( . 50km) will produce asmaller thickness ofmelt thanaplumewhich ascends Figure3 Schematic diagramshowinghow original beneathoceanic lithosphere( # 7km) (Figure3). lithospheric thickness andmantlepotentialtemperature Anothersignificant factor indeterminingthe affectthe amount ofmelt produced (melt thickness) and amount ofmelt generated byamantleplumeis how thesefactors relateto continentalfloodbasalts the temperatureofthe plume: generallythe (CFB),volcanic rifted margins (VRM),off –ridge and higherthe temperature, the moremelt will be ridge–centered oceanic plateaus (OP),andmidocean produced(Figure3). ridges(MOR). Theinitial 40 Ar/39Arstep-heatingagesfor LIPs supportmodels ofrapid formation anderuption, Caribbeanplateau,95–86Ma: Kerr etal.,1997a; ofteninless than2–3Myr (Richards etal.,1989). Sinton etal.,1998;Hauff etal.,2000b). Never- Asmoreage datahavebecomeavailable, awider theless,itstill appears thatsubstantialproportions age-range hasemergedfor someLIPs(e.g.,the oftheseprovinceswereformed overgeologically 540 Oceanic Plateaus short timeperiods(e.g.,the OntongJavaplateau accreted oceanic plateausections. For example, formed on two occasions:122^ 3and90^ 4Ma; the OntongJavaplateaucollided withthe Neal etal.,1997). westward-dippingSolomon Islandssubduction Theoretically,oceanic plateaus canform any- zoneat10–20 Ma, resultinginareversalinthe whereinthe oceanbasins; however,most oceanic polarity ofsubduction from west to east,and plateaus appeartohaveformed atornear the upliftandexposureofthe deepersections of midoceanridges(e.g.,Kerguelen,Manihiki and the plateauonthe Solomon Islands(Neal etal., OntongJava),i.e.,regions thatareconduciveto 1997; Petterson etal.,1999). Asecondexampleis voluminous decompression melting(Eldholm and the Caribbean–Colombianoceanic plateau,which Coffin,2000). Atfirst glance, itappears somewhat formed inthe Pacific at , 90 Ma (Sinton etal., coincidentalthatplumesofdeepmantleorigin 1998;Hauff etal.,2000a). Within10Myr the reach the baseofthe lithosphereatamidocean eastward-movingFarallon platehad broughtthe ridge.However,aspointed out bySaunders etal. southern portion ofthe plateauinto collision with (1996),alikely explanation for thisobservation continentalnorthwestern SouthAmerica,resulting isthatmantleplumescan“capture”oceanic inthe accretion ofslicesofthe plateauontothe spreadingcenters (cf.present-dayIceland). continentalmargin(Kerr etal.,1997b). This accretion wasaccompanied byback-steppingof the subduction zonewest ofthe accreted plateau 3.16.3 PRESERVATION OF OCEANIC slices. Shortly afterits formation the northern PLATEAUS portion ofthe plateaubegantomoveinto the The oldest insitu oceanic crustisJurassic inage proto-CaribbeanseawaybetweenNorthandSouth (Pringle, 1992),becausewithin200 Myr ofits America (Burke, 1988;Kerr etal.,1999; White formation atamidoceanridge, the oceanic crust etal.,1999). Indoingso the plateauencoun- wasrecycled back intothe asthenosphere. tered the eastward-dipping“GreatArcofthe Although many ofthe CretaceousandlateJurassic Caribbean”. Unableto subduct,the thick plateau oceanic plateaus still form part ofthe ocean clogged the subduction zone, resultingina basins,the preservation potentialofoceanic reversalinthe polarity ofsubduction from east plateaus olderthanJurassic islow. Fortunately, to west. Thisreversalinsubduction polarity in however,oceanic plateaus aremuch morebuoyant conjunction withaback-steppingofsubduction thanoceanic crustof“normal” thickness formed behindthe advancingplateau(Burke,1988), atamidoceanridge (e.g.,Cloos,1993). This effectively isolated the Caribbeanasaseparate excess buoyancyisprimarily dueto the greater plate.Overthe past , 80Ma the northern portion crustalthickness ofoceanic plateaus if the ofthe Caribbeanplateauhasmoved into the gap plateaus arerelatively young;residualheatfrom betweenNorthandSouthAmerica andhasbeen theirformation canalsocontributeto theirbuoy- uplifted andsubaeriallyexposed aroundits ancy. Recent measurements ofuppermantleshear margins (Figure4),thus makingitavailablefor wavesplittingandshearwavevelocity structure detailed study. Itisinways such asthesethat (Klosko etal.,2001)revealthatthe OntongJava remnantsofthesethick, buoyant oceanic plateaus plateauisunderlainbya300kmthick, long-lived, canbe preserved andincorporated intothe rheologically strongandchemically depleted root. continentalcrust.The identification ofthese Klosko etal. (2001) proposethatthisroot olderpreserved plateaus withinthe geological represents the residuefrom mantlemeltingand recordwill be explored inalatersection. thatits consequent lowerdensity contributes significantly to the buoyancyofboththe Ontong Javaandotheroceanic plateaus. Thenetresult of 3.16.4GEOCHEMISTRY OF CRETACEOUS thisexcess buoyancyisthatoceanic plateaus,in OCEANIC PLATEAUS contrast to normaloceanic crust,aremuch less 3.16.4.1 GeneralChemicalCharacteristics easily subducted (Ben-Avraham etal.,1981; Cloos,1993; KimuraandLudden,1995). Instead Table2showsrepresentativeanalysesof insitu ofbeingcompletely recycled back into the mantle, andaccreted portionsofCretaceousoceanic theirupperlayers canbe “peeled off ”(Kimuraand plateaus. Cretaceous oceanic plateaus arepredo- Ludden,1995)andaccreted on to the marginof minantly basaltic ( , 10 wt.% MgO)incompo- the subduction zone.Thisplateauaccretion can sition (Figure5),although more-MgO-rich lava occur eitheratanAndean-typecontinental flowsarefoundinthe Caribbean–Colombian margin,or anislandarc. oceanic plateau(CCOP). Typically,oceanic pla- Although many ofthe insitu Cretaceous teaus possess generally low levels ofcompatible oceanic plateaus havebeendrilled bythe Deep elements (i.e.,Ni , 300 ppm andCr , 1,000 ppm; Sea Drilling(DSDP)andOceanDrillingPrograms see Figure6). Interms ofincompatibletrace (ODP),the insightthatthesedrill holescan elements the majority ofoceanic plateaulavasand provide isrelatively limited compared to the intrusivesheets possess relatively flatrareearth Geochemistry ofCretaceous Oceanic Plateaus 541

Figure4 Maptoshow the mainaccreted outcrops ofthe Caribbean–Colombianoceanic plateaualongwiththe locations ofDSDP/ODP drill holeswhich penetrated the thickened crust ofthe Caribbeanplate. primitivemantle-normalized patternswithabun- typesofenriched mantleEM1andEM2,and dancesvaryingbetween5and10 timesprimitive HIMU,so-called becauseithasahigh 238 U/ 204 Pb mantlevalues(Figure7). ratio,or m value. Theradiogenic isotopecompositions ofoceanic Inrecent years our knowledge ofthe radiogenic plateaus havebeenwell characterized:initial 1 Nd isotopesystematicsofoceanic plateaus hasbeen valuesfor oceanic plateaus generally range from augmented bythe analysisofHfandOsisotopes. 6.0 to 9.0,whereasinitial 87 Sr/86 Srratios fall Although datafor the KerguelenandOntongJava þmostly beþ tween0.703 and0.704(Figure8),i.e., plateaus arestill relatively sparse, moredataexist the 87 Sr/86 Srand 1 . Nd valuesaregenerally less for the CCOP.Initial 1 Hf valuesfor the CCOP depleted interms oftheirradiogenic isotopesthan andOJP range from 10 to 18(Figure9(a)).The “normal” (N)-MORB astypified byMORB from CCOP samplesform atrendbetweenMORB the East Pacific Rise(Figure8(a)).Elevated initial source mantleandthe HIMU component,while 87 Sr/86 Srratios ( . 0.7040) aremostlikely dueto the OJP samplespossess lowerinitial 1 Nd values secondary alteration byhydrothermalfluids. Itis atequivalent 1 Hf thanthe CCOPandappearto noteworthythatthe high-MgO rocksgenerally form atrendtowardsEM2. Themuch lower 1 Hf possess moreextreme( 1 Nd) i valuesthanthe and 1 Nd for the Kerguelenplateau(Figure9(a)) basalts(Figure8(a)).Figure8(b),aplot of will be discussed inSection 3.16.5.2. Initial g Os 207 Pb/ 204 Pb against 206Pb/ 204 Pb,reveals that for high-MgO rocksfrom the CCOP range from 0 mostoceanic plateaurocksrange between18.5 to 18, whereasthe basalts range from 2 7to 10 þ þ and19.5 206Pb/ 204 Pb andbetween15.525and (Figure9(b)).How representativetheserangesare 15.625 207 Pb/ 204 Pb.Aninterestingfeatureof for g Os,requiresthe acquisition ofmoredata Figure8(b)isthatmany ofthe basaltslie on a from otheroceanic plateaus. trendbetweenthe proposed mantlecomponentsof HIMU andDMM (ZindlerandHart,1986) while 3.16.4.2 MantlePlumeSource Regions of the high-MgO lavasform atrendbetweenDMM Oceanic Plateaus andthe enrichedmantlecomponent EM2. Zindler andHart (1986) proposed four mantlecom- Since the pioneeringstudyofHoffmanand ponents:depleted MORB mantle(DMM),two White, ithasbecomewidely accepted thatoneof Table2 RepresentativeanalysesofCretaceous oceanic plateaulavas.

PlateauLocation SampleData ZrNb MoHf Pb Ta Th U 87 Sr/ 86 Sri 1 Ndi 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb sources

KerguelenODP site74716-5,103-6150.90 1.21 17.23 10.620.16 7.479.591.88 2.00 0.16 101.22 3.63 KerguelenODP site748 79-6,90-4 149.242.7417.508.77 0.09 7.54 6.71 4.72 0.561.12 98.997.14 KerguelenODP site74915-5,125-7152.73 1.5215.1811.950.17 7.22 8.31 3.420.54 0.18101.22 1.35 KerguelenODP site75017-3,23-26 149.21 1.17 15.77 14.620.188.36 9.01 1.99 0.19 0.11 100.61 4.31 KerguelenODP site73834-1,88-92 252.17 1.73 15.44 10.820.185.519.17 2.981.950.21 100.16 0.58 OntongJavaODP site807 75-4,46-48 348.751.62 14.16 13.430.21 6.7412.132.380.13 0.1499.690.45 OntongJavaODP site807 88-3,76-79 349.741.13 14.27 13.44 0.22 7.4312.002.06 0.280.09 100.66 2 0.24 OntongJavaSantaIsabelI96 449.281.31 14.20 12.44 0.16 8.03 12.501.890.150.10 100.06 1.61 OntongJavaMaliataSG1550.381.6513.54 14.010.19 7.30 11.642.32 0.150.14101.32 0.30 6.0 OntongJavaMaliataML407 549.67 1.3513.6413.470.19 7.45 11.383.080.26 0.13 100.62 2.145.0 CCOP GorgonaGOR160645.13 0.6411.7812.520.1818.2510.101.30 0.06 0.05100.01 CCOP GorgonaGOR117650.400.77 13.8112.620.23 8.6512.392.430.02 0.06 101.38 CCOP GorgonaGOR94-35746.58 0.37 10.5711.410.17 21.968.420.55 0.19 0.03 100.253.26 CCOP ColombiaSDB18850.63 1.4714.19 13.340.23 6.77 10.942.890.150.12 100.72 1.57 CCOP ColombiaVIJ1851.45 2.02 12.85 15.200.19 5.789.572.29 0.07 0.17 99.590.90 CCOP ColombiaCOL472 948.810.8912.8610.660.17 11.55 11.612.91 0.080.09 99.643.03 CCOP CuraçaoCUR1410 46.420.579.5311.060.17 22.868.20 0.90 0.02 0.0599.792.23 CCOP CuraçaoCUR20 10 52.13 0.7812.85 9.800.20 8.60 14.301.430.03 0.06 100.181.41 CCOP DSDP site15011-2,63-67 11 49.461.27 16.60 10.300.10 8.58 9.872.60 0.13 0.10 99.02 CCOP Ecuador EQ11248.88 1.23 14.39 12.750.20 8.96 10.811.890.72 0.18100.01 4.45 Rb CsSrBa Sc VCrNi CuZnGa La Ce KerguelenODP site74716-5,103-6118.5 , 0.8234229 33.8201 401 72 66 22.0 12.3025.50 KerguelenODP site748 79-6,90-4 17.82.811131 1661 18.7 170 166 1827918.7 105.00224.00 KerguelenODP site74915-5,125-7112.3 , 0.521411434.5271 260 30 11522.7 6.8016.10 KerguelenODP site75017-3,23-26 19.0 , 0.9 193 30 38.9 269 193 120 93 19.84.00 8.90 KerguelenODP site73834-1,88-92 237.4273 336 36.9 267 9528102 20.7 17.1039.30 OntongJavaODP site807 75-4,46-48 31.0 17445.9 313 162 99 6.1514.59 OntongJavaODP site807 88-3,76-79 310.0 107 52.3 349163 872.96 8.07 OntongJavaSantaIsabelI96 42.0 11537 50.0 341238122 3.409.90 OntongJavaMaliataSG151.3 0.01 1082841.0 392 566112493 17.0 4.7512.60 OntongJavaMaliataML407 51.80.01 100 23 40.0 29561 62 229 5717.0 3.8610.60 CCOP GorgonaGOR16061.0 64828.3 227 1373 723 13.2 0.652.17 CCOP GorgonaGOR11760.3 107 21 41.1 371 194112 15.7 1.02 2.99 CCOP GorgonaGOR94-3574.2 3420 27.3 166 829682030 5610.80.22 0.57 CCOP ColombiaSDB1882.51563742.2 331 20897 19.1 3.809.80 CCOP ColombiaVIJ180.4892744.4531 63 58 133 12420.2 6.0516.55 CCOP ColombiaCOL472 91.9 3988530.83341393 26412570 14.3 2.426.33 CCOP CuraçaoCUR1410 1.7 46629.7 1862017 1032 7570 9.1 1.20 4.20 CCOP CuraçaoCUR20 10 0.469 11 43.3 258 552178102 66 12.2 2.84 7.59 CCOP DSDP site15011-2,63-67 11 3.2 117 16 335373 127 15085 17.0 3.16 8.55 CCOP Ecuador EQ1125.2 111 1561.5353285 101 3.509.63 PrNd SmEuGd Tb DyHoErTmYb LuY KerguelenODPsite74716-5,103-61 13.503.501.03 0.00 0.651.890.2823.2 KerguelenODPsite748 79-6,90-4 1103.00 14.203.31 8.90 1.251.800.26 27.7 KerguelenODPsite74915-5,125-71 11.903.641.240.69 2.71 0.39 29.3 KerguelenODPsite75017-3,23-26 16.30 2.32 0.84 3.00 0.58 2.570.3424.6 KerguelenODPsite73834-1,88-92 221.205.281.73 0.942.870.44 28.8 OntongJavaODP site807 75-4,46-48 311.653.39 1.31 4.510.812.71 0.4230.0 OntongJavaODP site807 88-3,76-79 36.502.30 0.93 3.39 0.70 2.400.3824.0 OntongJavaSantaIsabelI96 41.567.84 2.77 1.02 3.500.63 4.20 0.91 2.640.37 2.30 0.3522.0 OntongJavaMaliataSG151.984.61 1.69 1.13 4.140.73 4.561.00 2.90 0.412.830.4325.0 OntongJavaMaliataML407 51.70 5.67 2.11 0.943.54 0.684.22 0.92 2.570.36 2.470.37 23.0 CCOP GorgonaGOR160 60.462.861.250.48 1.58 0.29 2.10 1.430.22 1.30 0.21 13.8 CCOP GorgonaGOR117 60.60 3.531.740.63 2.07 0.413.13 2.180.37 2.20 0.32 22.8 CCOP GorgonaGOR94-3570.13 0.90 0.650.341.500.30 2.180.48 1.44 0.23 1.420.21 15.3 CCOP Colombia SDB188 9.30 2.85 1.11 2.940.802.750.4329.4 CCOP Colombia VIJ1812.0 3.97 1.474.76 0.93 4.250.63 41.7 CCOP Colombia COL472 90.92 5.58 1.76 0.742.67 2.92 0.58 1.44 1.36 0.21 19.0 CCOP CuraçaoCUR1410 3.16 0.380.5011.5 CCOP CuraçaoCUR20 10 0.79 4.801.39 0.55 1.872.11 0.45 1.381.36 0.21 15.8 CCOP DSDP site15011-2,63-67 11 1.36 6.942.32 0.892.870.523.36 0.69 1.890.281.820.26 21.0 CCOP Ecuador EQ1121.568.11 2.570.96 3.460.66 4.400.90 2.60 0.39 2.530.3822.3 ZrNb MoHf Pb Ta Th U 87 Sr/86 Sri 1 Ndi 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb KerguelenODPsite74716-5,103-6197 7.00 2.340.460.20 0.0000.705783 2 4.0 17.6515.5138.16 KerguelenODPsite748 79-6,90-4 1599 121.90 12.408.00 9.0813.202.2000.705319 2 3.3 18.19 15.6438.38 KerguelenODPsite74915-5,125-7191 5.802.16 1.63 0.401.800.148 0.704260 1.818.03 15.55 38.16 KerguelenODPsite75017-3,23-26 1473.33 1.16 0.820.19 1.400.048 0.7061651.417.5315.4938.01 KerguelenODPsite73834-1,88-92 2166 8.90 3.890.54 2.27 0.709730 2 8.2 17.8215.7539.01 OntongJavaODP site807 75-4,46-48 3985.70 2.640.340.560.704330 5.0 18.4015.5338.38 OntongJavaODP site807 88-3,76-79 3643.32 1.740.20 0.340.703560 5.9 18.67 15.55 38.54 OntongJavaSantaIsabelI96 4654.30 0.23 0.27 0.1200.703690 5.418.6415.54 38.60 OntongJavaMaliataSG15814.90 0.592.500.17 0.350.280.0970.7040403.9 17.85 15.4737.91 OntongJavaMaliataML407 5835.00 0.36 2.63 0.150.250.16 0.0470.704130 5.7 18.32 15.5138.23 CCOP GorgonaGOR160 6290.48 1.00 1.30 0.02 0.0110.7030419.518.32 15.5037.82 (continued) 544 Oceanic Plateaus

Pb the principalcontributors to the source regions of 9 1 0 0

20 4 deepmantleplumesaresubducted oceanic slabs 8 .9 7. 84 3 8 . 5 6 3 8 . 4 6 3 8 . 6 3 8 .1 3 8 .9 3 8 . 2 3 8 .7

Pb / andtheirsediments which descended through the

20 8 asthenospheric mantleandponded ateitherthe 670 kmdiscontinuity or the core–mantlebound- Pb

(1991) , ary (D 00). ZindlerandHart (1986) identiﬁed three 20 4 5 . 58 5 . 54 1 5 . 58 1 5 . 58 1 5 . 93 1 5 .60 1 5 . 33 mainmantleplumecomponents:HIMU(pro- Pb / e t a l. posed asbeingderived from subducted oceanic 207 crust); andtwo enrichedcomponents,EM1and o ch um

Pb EM2. Inaddition,the upperasthenospherewas

20 4 proposed asconsistingofdepleted MORB 19.33 1 8 . 61 19.22 19.31 19.0 8 1 5 . 55 19.07 1 8 .16

Pb / mantle(DMM). a (19 84 ) ,J ´ 206 Morerecently,ithasbeenshown thatdepleted

i signaturesfoundinsomeLIPsrepresent a che v e rr ı .2 9. 4 1 8 .6 5 . 54 8 .1 6.6

Nd component which,ratherthanbeingdueto

11.0 10.1 1

a n dE entrainment ofdepleted uppermantlematerial ´ (DMM),isderived from depth, andso isan upr e 197 S r i 32 8 3 .6 4 767 33 8 0 . 5 1 71 3207 2961 321 5 7.0 3200 3 54 67 integralpart the plumeitself(Kerr etal.,1995a,b; 8 6 .70 .70

(1999). Fitton etal.,1997;Kempton etal.,2000). 0.70 0.70 0. 5 13 0.70 0.70 S r/

8 7 Kerr etal. (1995a,b)andWalker etal. (1999) a (19 84 ) ,D ´ e t a l. proposed thatthisdepleted plumecomponent 00 4 0.70 4 0.70 wasultimately derived from subducted oceanic che v e rr ı U 0.21 0.09

Re yn a u d lithosphere, unlike HIMU, which hasits source in more-enriched upperoceanic crust.

.20 Evidence for recycled oceanic crust andlitho- 0.62 0.0 8 0.03 0.30 0.10 0.90 0.26 0.2 5 0.0 8 00 Th sphereinthe mantleplumesource regionsof (2000 b ) , 12 , 6 Ai t ke n a dE oceanic plateaus hasbeenpresented byseveral

e t a l. authors (e.g.,Walker etal.,1999; Hauff etal., (2002) 0.23 Ta 2000a),andmixingtrendsbetweenadepleted Ha u ff

e t a l. component andHIMU areclearly seenonmostof .20 .00 .1 4 0. 58 Pb 2.37 0.23 the radiogenic isotopeplots (Figures8and9). Tejada (1996 b ) , 11 ,5 .0 5 12.10 .92 3.1 4 0. 48 2.0 5 0.30 1. 8 90 e t a l. (1996) 3.16.4.3 Caribbean–ColombianOceanic Plateau Ke rr

e t a l. ( , 90 Ma)

M o Hf TheCaribbean–ColombianOceanic Plateau Tejada

(2002) , 10 (CCOP)isexposed aroundthe margins ofthe 01 . 8 71 . 8 01 .16 . 4 01 Caribbeanandalongthe northwestern continental 8 .00 4 .29 2. 5 01 e t a l. marginofSouthAmerica (Figure4). The (1993 a ) ,4 thickened natureofthe bulkofthe Caribbean

e t a l. plate(8–20 km; Edgar etal.,1971; Mauffretand 90 30 0 4 .16 Z r Nb 70 30 6 8 11.0 4 03 Leroy,1997)testiﬁestoits originasanoceanic

(2000 b ) , 9 Ke rr plateau. Theplateauhasbeendrilled byDSDP 26 63 9 4 02

a t Leg 15andODP Leg 165(Figure4;Bence etal., e t a l. 11 10

sour ce s 1975;Sinton etal.,2000). The accreted plateau a u ff

(199 5 ) , 3 Mah on e y materialinColombia, Ecuador,CostaRica and

eD Hispaniolaconsists offault-boundedslicesof e t a l. 1 4 10 20 4 72 (1997) ,H 1 88 basaltic, andoccasionally picritic lavasandsills J 1 8 119 Sa mpl OR 117 OR 9 4- 3 5 71 1 - 2 , 63 67 withrelatively fewintercalated sediments andash VI SDB COL EQ 11 e t a l. layers (Kerr etal.,1997a). Although theypreserve e rr layered andisotropic gabbrosandultramaﬁc 8K rocks,unlike accreted ophiolitesgenerated at o CUR o CUR s i t e 1 5 01 (1992) , 2 Mah on e y da t a, ¸a ¸a spreadingcenters,theseaccreted sequencesof ac ac om bia om bia om bia DP

e t a l. oceanic plateaudonot possess sheeted dyke G or g on aG G or g on aG C ol C ur C ol C ol C ur DS Ec u ad or complexes(Kerr etal.,1998). ( c ont i nu ed ).

unpu b l i s hed Severalotherexposuresareworthyofspecial

u L o ca t i on mention:ﬁrstly,the 5kmthick section on the Ke rr

b l e 2 islandofCurac¸ao,70 kmnorthofthe coast of CCOP P l a t ea Ta CCOP CCOP CCOP CCOP 7 AC CCOP CCOP CCOP CCOP S our ce s : 1 Sa lt e rs Venezuela(Figure4). The sequence consists of Geochemistry ofCretaceous Oceanic Plateaus 545

Figure5 Plots ofmajor elements (wt.%) against wt.% MgO for lavasfrom throughout the CCOP.All lavaswith . 18wt.% MgO containaccumulated olivine.Datasources:Colombia—Kerr etal. (1997a),Gorgona—Echeverr´ıa (1980),Kerr etal. (1996a),Arndt etal. (1997),CostaRica—Hauff etal. (2000b),Curaçao—Kerr etal. (1996b). Circled fieldsarefrom the OntongJavaplateau(Mahoney etal.,1993a,b). Wherethe AandCGtypesdiffermarkedly incomposition theyareplotted asseparatefields,otherwisethe OJP isplotted asasinglefield.OJP Site1185 unpublished datawerekindly provided byGodfreyFitton. pillowed picrites(MgO . 12 wt.%) low inthe (1996b),itislikely thattheselavascontain succession thatgraduallygivewaytomore substantialaccumulated olivine, andso the basaltic pillow lavasnearerthe top. Thesepillow wholerock compositions ofthesehigh-MgO lavasareintercalated withhyaloclastitehorizons rockscannot represent thoseofparentalmantle andintrusivesheets (Klaver,1987; Kerr etal., melts. Estimatesofthe MgOcontent ofthe 1996b). The secondnoteworthylocality isthe parentalmelts for various parts ofthe province islandofGorgona, 50kmoff the western coast vary from 18wt.% MgO to about 12 wt.% MgO ofColombia(Figure4). Thissmall island (Kerr etal.,1996a,b;Revillon etal.,1999). (2.5 8km) isthe siteofthe youngest komatiites Although picritic lavasaremorecommon thanin (MgO£ -rich lavaflows: . 15wt.%),which possess otherCretaceousoceanic plateaus,basalts areby platy andblade-shaped olivines,givingthe rocks farthe most common rock typepreserved inthe acharacteristic “spinifex” texture(Echeverr´ıa, CCOP.The vast majority ofsamplescontain 1980; Kerr etal.,1996a). Komatiitesare between6and10 wt.% MgO (Figure5). Al 2 O 3 relatively common inthe pre-Cambrian,how- contentsbroadly increasewithdecreasingMgO, ever,the only known Phanerozoic komatiites reflectingthe importance ofthe addition and occur aspart ofthe CCOP on GorgonaIsland. removalofolivineduringthe petrogenetic history Theformation oftheseCretaceouskomatiitesin ofthe CCOPlavas. CaOincreaseswithdecreasing the CCOPhasled to the suggestion thatpre- MgOuntilMgOreaches8–10 wt.% beyond Cambriankomatiitesformed inancient oceanic which the CCOPlavasdisplayascattered but plateaus (Storey etal.,1991). discernibledownwardtrend.Fe2 O 3 (t) andTiO2 The lavasofthe CCOP areclassified as displaybroadly horizontaltrendsuntilabout tholeiitic.The mostmagnesianlavasfoundin 8–10 wt.%MgO,belowwhich bothincrease the province containupto28wt.% MgO markedly. Thesetrendscanbe modeled bythe (Figure5). However,asshown byKerr etal. initialfractionalcrystallization or accumulation of 546 Oceanic Plateaus

Figure6 Plots oftrace elements (ppm) andratios oftrace elements against wt.% MgO for lavasfrom the CCOP. The OJP isplotted asasingleﬁeldexcept wherethe AandCGtypesdiffermarkedly incomposition. Datasourcesare asfor Figure5. Geochemistry ofCretaceous Oceanic Plateaus 547

Figure7 Primitivemantlenormalized multi-element plot showingthe average composition ofhigh-MgO lavas (komatiitesandpicrites) andbasalts from various parts ofthe CCOP, plotted alongwithaverage N-MORB values (dashed line)andacompositionalfieldfor the OJP.Datasourcesareasfor Figure5.Primitivemantlenormalizing valuesandN-MORB from Sun andMcDonough (1989). olivine(plus minor Cr-spinel) followed bythe beingmoredepleted, andsomemoreenriched commencement ofcrystallization ofplagioclase thanthe basalts,particularly for the most highly andclinopyroxenebetween8wt.% MgO and incompatibletrace elements such asTh, Nb, La, 10 wt.% MgO (Kerr etal.,1996b). Ce &Nd(Figure7). Trace element data(Figure6) also supportthe Theheterogeneity ofthe high-MgO rocksis proposed fractionalcrystallization model:Niand also reflected inthe radiogenic isotoperatios, Crcontentsfall withdecreasingMgO contents, particularly 1 Nd (Figures8and9). Virtuallyall andSr,although commonly disturbed bysub- the analyzed basaltsfrom the CCOP possess initial solidus alteration processes,isgenerallyreduced 1 Nd rangingfrom 6to 9,whereasthe high below10wt.% MgO.Despitesomescatter,the MgO lavasgenerallyþ fall ouþ tside thisrange ( 1 Nd content ofincompatibletrace elements,e.g.,Nb . 9and , 6; Figure8). Elevated initial andZr,generally increaseswithdecreasingMgO 87 Sþ r/86 Srratiosþ foundinseveralparts ofthe contents (Figure6). Ratios ofhighly incompatible province havebeenattributed eithertocontami- traceelementsarenotnormallyaffected by nation withaltered oceanic crust(Curaçao:Kerr moderatedegreesoffractionalcrystallization or etal.,1996b)ortosecondary alteration (Gorgona: mantlemelting, andtheythereforehavethe Revillon etal.,1999). potentialtorevealheterogeneitiesinthe mantle The wide range ofisotopic datafor the CCOP source region ofthe plateau. Theseratios are reveals thatthe enrichedanddepleted lavasare plotted against MgO inFigure6(g)–(h). Oneof notsimply formed byvariablemeltingofa the most interestingaspects ofthe trace element homogeneous source region,but ratherreflect datafor the CCOPisthatthe basaltic lavaspossess meltingoflong-termdepleted andenriched anarrowerrange ofincompatibletrace element componentsfromamarkedly heterogeneous ratios thanthe picrites. For instance, well over plumesource region. (Kerr etal.,1996a, 2002; 80% ofthe basaltic samples(, 12 wt.% MgO) Arndt etal.,1997; Hauff etal.,2000a;Thompson from the CCOP possess La/Yratios between0.05 etal.,2004). and0.2 andZr/Nb ratios between7and20 The greaterheterogeneity ofthe high MgO (Figure6(g)–(h)).Incontrast,the picritic and rocksincomparison to the basalts,hasbeen komatiitic lavaspossess much morevariable interpreted to reflectthe formation oftheselower ratios ofincompatibletrace elements,withLa/Y MgOmagmasthrough mixingandfractional rangingfrom 0.05to 0.45 andZr/Nb from 5to 85. crystallisation ofthe high-MgO magmasinlarge Thisisalso shown on primitivemantlenormalized magmachambers.The heterogeneous high-MgO multielement plots,whereitcanbe seenthatthe rocksthus represent magmasthatpassed relatively CCOPbasalts possess broadly flatpatterns quickly through the lithospherewithoutbeing whereasthe high-MgO picritesandkomatiites trapped inmagmachambers (Kerr etal.,1998). aregenerally much morevariable, withsome The extent ofpartialmeltingrequired to produce 548 Oceanic Plateaus

Figure8 Plot of(a)initial 1 Nd against 87 Sr/86 Srfor Figure9 Plots of(a)initial 1 Hf against initial 1 Nd high-MgO lavasandbasalts from the CCOP and(b) and(b)initial g Osagainst initial 1 Nd for Cretaceous 207Pb/ 204 Pb against 206Pb/ 204 Pb.Shown on bothdia- oceanic plateaus. Datasources:Atlantic andIndian grams arefieldsfor the OJP- AandCGtypes(Mahoney MORB—Salters (1996),Pacific—Nowell etal. (1998), etal.,1993a,b)andEast Pacific Rise(EPR)MORB Kerguelenplateau—Salters andHart (1991),CCOP and (Mahoney etal.,1995). Otherdatasourcesareasfor Gorgona—Walker etal. (1999),Thompson etal. Figure5.Mantleend-membercompositions arefrom (2004),OJP—Babbs(1997). ZindlerandHart (1986).

1993a). Additionaldataarealso now becoming the parentalmagmasofthe CCOP hasbeen availablefrom ODP Leg 192 which recently calculated to be ofthe orderof20% (Kerr etal., penetrated the OJP atfour sites(Mahoney etal., 1997a). 2001). OnMaliatathe stratigraphicthickness ofthe accreted plateaureaches3–4 km,andthe succes- 3.16.4.4OntongJavaPlateau( 122 and , sion isdominated bypillowed andmassivebasaltic 90 Ma) , flows(Petterson etal.,1997; Babbs,1997). Like The OntongJavaplateau(OJP)inthe western the CCOP, dykesarevolumetricallyminor. The Pacific (Figure1) hasbeentectonically uplifted DSDP/ODP drill holeshavepenetrated into the andexposed alongits southeastern margin,atthe plateautoadepthof216 m(Site1185B;Mahoney Solomon Islandsarc, mostly on the Islandsof etal.,2001). Thesampled sectionsconsist MaliataandSantaIsabel. Incontrast to the CCOP, predominantly ofpillowed andmassivebasalts which hasnumerous exposed sections,theseare withoccasionalthininterlavasediments (Neal currently the only knownsubaerialexposuresof etal.,1997; Mahoney etal.,2001) the OJP.The rest ofour knowledge ofthe OJP Ingeneral,the OJP lavasaremorehomo- comesfrom aseriesofdrill holes:DSDP Site289 geneousthanthoseofthe CCOP (Figures5, 6 andODP Sites803 and807 (Mahoney etal., and8). All the lavasfrom the province analyzed The Influence ofContinentalCrust on Oceanic Plateaus 549 thus fararebasaltic incomposition,withmostof Theseauthors haveproposed thatthe magma the samplespossessing6–8 wt.% MgO (Figures5 chambers thatfed the plateauarerepresented in and6). Although the lavasofthe OJP possess a the midcrust byolivinegabbros. Thesemodels restricted compositionalrange,theynonetheless alsosuggest thathigh compressionalP-wave fall intotwo compositionallydistinctgroups. velocitiesof . 7.1 kms2 1 deepwithinthe OJP Thesegroups werefirst noted inthe lavasfrom aredueto the presence ofolivineandpyroxene ODPSite807,whereMahoney etal. (1993b) cumulatesproduced bythe fractionation of divided the lavasinto different units (AandC-G). primary picritic melts. Alternatively,the high UnitA(A-Type)ischemically distinctfrom Units P-wavevelocitiescouldbedueto the presence of C-G (C-G-Type)andpossesseshigherlevels of garnetgranulitedeepinthe plateau,which bothincompatibleelements (e.g.,TiO2 ;Sr,ZrNb Gladczenko etal. (1997) suggested mayhave andthe LREE;Figures5–7) andratios ofhighly formed bydeformation andhydrothermal incompatibleto moderately incompatibletrace alteration oflowercrustalcumulates. elements (e.g.,La/Y). The A-Typebasaltsalso havelowerinitial 1 Nd valuesandhigherinitial 87 86 Sr/ Srratios thanthe C-G-Type(Figure8(a)). 3.16.5THE INFLUENCE OF CONTINENTAL However,the totalrange of 1 Nd ( 6.5– 4.9) CRUST ON OCEANIC PLATEAUS and 87 Sr/86 Sr(0.7034–0.7041) isrelaþ tively þ small incomparisonwiththoseinthe CCOP Initially itmayseemodd thatthe composition (Figure8(a)).Thesameistruefor incompatible ofoceanic plateaus shouldbeinfluenced by trace element ratios,withthe OJP basaltsonly continentalcrust,andcertainly for the CCOP varyingbetween0.12 and0.22 for La/Yand andthe OJP, which apparently formed well away between15.5–19.7 for Zr/Nb (Figure6). How- from continentalmargins,thereisnoevidence of ever,recent analysesfrom ODP Leg192 (Sites the involvement ofcontinentalcrust intheir 1185 and1187; on the eastern edge ofthe plateau) petrogenesis. However,LIPscanalso form at haverevealed the occurrence ofmore-MgO-rich the continent–oceanboundary aswell aserupting lavas(up to 11 wt.%) withhigherNi andCr onto eitheroceanic or continentallithosphere, and contents (Mahoney etal.,2001;G.Fitton unpub- the formation ofaLIP insuch atectonic settingis lished data;Figures5and6). Inaddition to their oftenrelated to continentalbreak-up. The role higherMgO, preliminary geochemicaldata played bymantleplumesincontinentalbreak-up reveals thatthesebasaltspossess lowerlevels of (causalorconsequential) remains controversial (WhiteandMcKenzie, 1989; Hill,1991; Coffin incompatibleelements (e.g.,TiO2 :0.72–0.77 and Zr:36–43),thanthe AandC-G Types(Figures5 andEldholm,1992; Saunders etal.,1992; Barton and6). andWhite, 1995). However,whethermantle Neal etal. (1997) haveconcluded on the basis plumesarethe reason for or aresult ofcontinental ofgeochemicalmodelingthatthe major andtrace break-up,the associated erupted lavasand element compositions ofthe A- andC-G-type intrudedsills form thick magmatic sequenceson lavasofthe OJP areconsistent with20–30% the margins ofthe rifted continents:the so-called partialmeltingofaperidotitesource.The more seaward-dippingreflectorsequences(SDRS). enriched natureofthe A-Typelavasimplies TheseLIPsmayalsoerupt on the adjacent derivation form aslightly moreenriched source continents to form continentalfloodbasalt pro- region,possibly inconjunction withsmaller vinces. Furthermore, continuingplume-related degreesofmelting.Mahoney etal. (2001) have magmatism combined withfurtherseparation of proposed thatthe moreMgO-rich, incompatible the continents ultimately results inthe formation element-poor lavasdiscovered duringLeg 192 ofoceanic plateaus. Two examplesofprovinces represent moreextensivemeltingofthe plume such astheseareexplored below:the North source region. However,analternativeexpla- Atlantic IgneousProvince (NAIP)andthe nation isthattheselavaswerederived from amore Kerguelenplateau. depleted mantlesource region,andradiogenic isotopedataarerequired inordertoresolvethis 3.16.5.1 The NorthAtlantic Igneous Province issue.Noneofthe compositions sampled thus far ( , 60 Ma to Present Day) aremagnesianenough to represent possible parentalmelts,andso arebelieved to have The openingofthe NorthAtlantic , 60 Ma is undergone30–45%fractionalcrystallization, closely associated withmagmatism from the involvingolivine, plagioclaseandclinopyroxene “head”phaseofthe Icelandicplume.(For a (Neal etal.,1997). comprehensivereviewofthe NAIP see Saunders Although the deepercrustalandlithospheric etal.,1997). Much ofthe initialvolcanism levels ofthe OJP arenot exposed,seismic velocity (Phase1:62–58Ma;Saunders etal.,1997)was datahasbeenused to modelthe crustalstructure confined to the continentalmargins,i.e.,the on- (Farnetani etal.,1996; Gladczenko etal.,1997). landsequencesinwestern Britain,the Faroe 550 Oceanic Plateaus Islandsandeast andwest Greenland, aswell 3.16.5.2 The KerguelenIgneous Province asthe seaward-dippingreflector sequencesof ( , 133 Ma to Present Day) the southeast Greenlandmarginandthe Hatton Bank(Figure10). Most oftheselavasareconta- The initialvolcanism ofthe Kerguelenplume minated withArchean-age continentalcrust and isclosely associated withthe break-up of thus possess low 1 Nd andhigh Ba/Nb (Figure11), Gondwanainthe early–mid-Cretaceous,i.e., alongwithlow 206Pb/ 204 Pb.Asthe NorthAtlantic the separation ofIndia, Australia andAntarctica continued to open,asecondintenseburst ofmag- (Morgan,1981; RoyerandCoffin,1992). Like matism occurred (beginningat56Ma;Phase2, the NAIP, much initialvolcanism isfoundon Saunders etal.,1997). The lavasfrom this the margins ofthe rifted continents (Figure12): magmatism arepreserved inthe upperportions the Rajmahalbasaltsinnortheastern India (Kent ofthe SDRS, off the coast ofsoutheast Greenland etal.,1997)andthe Bunbury basalts inwestern andWestern Europeandhavebeendrilled by Australia (Frey etal.,1996). Not surprisingly, ODP Legs104, 152and163 (Viereck etal., thesebasaltsareextensively contaminated by 1988;Fitton etal.,2000). Incontrast to the continentallithosphereandyieldaninitial Phase1lavas,theselavasshow fewsigns of 87 Sr/86 Srratioof . 0.7042and 1 Nd, 4.0 contamination bycontinentalcrust (low Ba/Nb; (Figure13). 1 Nd . 6; (Figure11) 206Pb/ 204 Pb . 17),indicat- The geographicalcomponentsofthe plateau ingthatbythistimethe NAIP wasanentirely (Figure12)andthe geochronologyarebriefly oceanic LIP.TheIcelandicplumehasbeen outlined below; however,amoredetailed review producingmelt overmost ofthe past 60 Myr, canbe foundinFrey etal. (2000); Coffin etal. asevidencedby55–15Mavolcanism alongthe (2002).The first massivepulseofKerguelen Greenland–Icelandridge andthe Faroes–Iceland plumemagmatism created the Southern ridge,andthe 15Ma–present volcanism on Kerguelenplateau(118–110Ma;Figure12). Iceland. Latermeltingofthe plumewasresponsiblefor

Figure10 Mapshowingthe locations ofthe principalon-landexposuresofthe NorthAtlantic Igneous Province and the seaward-dippingreﬂector sequences. Identiﬁcation ofOceanic Plateaus inthe GeologicalRecord 551

Figure11 Plot ofBa/Nb vs. initial 1 Nd lavasfrom the NAIP.Shown on the diagramarelavasfrom Skye (Thompson etal.,1982; Dickin etal.,1987),Mull (Kerr etal.,1995),Iceland(He´mond etal.,1993)andboth pre- andpost-continentalbreak-up lavasfrom ODP Leg 152(Fitton etal.,1998a). the formation ofthe ElanBank(108–107 Ma), the CentralKerguelenPlateau(101–100 Ma), BrokenRidge (95–94Ma),the Ninetyeast Ridge (82–37 Ma),andthe Northern Kerguelenplateau (35–34Ma)(Figure12). Volcanism continuesto the present-dayandhasproduced the Kerguelen ArchipelagoandHeardandMacDonaldIslands. The lavasfrom the Southern Kerguelenplateau 87 86 andBrokenRidge haveinitial Sr/ Srratios and Figure12 Mapshowingthe maincomponents of 1 Nd values(Figure11)which range from 0.7037 the Kerguelenplateau(KP)discussed inthe text (after to 0.7102 and 4.0 to 2 9.4, respectively (Salters Frey etal.,1996). etal.,1992; Mahþ oney etal.,1995). Someofthis variation canbe interpreted asmixingbetween intercalated withbasalt flows (Frey etal.,2000). Southeast IndianRidge MORB andthe Kerguelen Thisdiscovery hasconfirmed the presence ofpre- plume(WeisandFrey,1996). However,elevated Cambriancrustalrockswithinthe Kerguelen La/Nb ratios (Figures13and14(a)) andthe plateau(Nicolaysen etal.,2001),thus supporting extremeisotopic compositions ofbasaltsdrilled at the lithospheric contamination modelfor the ODPSite738anddredgesamplesfrom the eastern 87 86 high La/Nb, low 1 Nd basaltsofthe Kerguelen BrokenRidge( Sr/ Sr0.710; 1 Nd 2 9.0; plateau. Figures8and13)cannot be explained bysuch mixingprocesses. Ithasbeenproposed thatthese signaturesaredueto contamination bycontinental lithosphere(Storey etal.,1989; Mahoney etal., 3.16.6 IDENTIFICATION OF OCEANIC PLATEAUS IN THE GEOLOGICAL 1995;HasslerandShimizu,1998). Operto and RECORD Charvis,1996 haveimaged aseismicallyreflec- tivetransition zonebeneaththe crust/mantle The rationalefor thissection issummed up by interfaceofthe Southern Kerguelenplateau, thisquestion:Ifthe CCOP or OJP wereaccreted interpreted asfragmentsofcontinentalcrust. on to acontinentalmarginandpreserved inthe Thiscrustappears to haveisotopic similaritiesto geologic recordfor 1billion years,whatfeatures Archeancrustfoundon the margins ofGondwana, couldweuseto identifythemasoceanic plateaus? which raisesthe possibility thatfragments ofsuch Thissection will reviewdiagnostic geochemical crusthavebecomeincorporated into the Indian andgeologicalcharacteristicsofoceanic plateaus, Oceanbasinduringcontinentalbreak-up. andthenwill show,illustrated byexamples,how Recently,duringdrillingatSite1137 on the thesecriteria canbe used to identifyplateau ElanBank(part ofthe Kerguelenplateau; sequencesinthe geologicalrecord.Table3 Figure12)clastsofgarnet-biotitegneiss providesasummary ofthe diagnostic featuresof havebeendiscovered inafluvialconglomerate Cretaceousoceanic plateaus andmafic sequences 552 Oceanic Plateaus

Figure13 Plots to show the geochemicalvariation oflavasfrom the early Cretaceous lavasderived from the Kerguelenplume.(a)Initial 87 Sr/86 Srvs. initial 1 Nd and(b)primitivemantlenormalized multi-element plots showingaveraged datafor ODP drill sites. Acompositionalfieldfor the OJP (Mahoney etal.,1993a,b)isshown on bothdiagrams. Datasources:Rajmahal—Kent etal. (1997);Bunbury—Frey etal. (1996);ODP sites—Salters etal. (1992); Mahoney etal. (1995). withinthe continentalcrust,which havebeen oceanic plateaus donot possess the abundant interpreted asoceanic plateaus. Details ofaccreted volcanic ashlayers present involcanic arc oceanic plateaus thus faridentified inthe sequences. However,asFigure14shows,alow geologicalrecordaresummarized inTable4. (La/Nb) pmn ratioisnot anentirely robustsignature ofanoceanic plateausequence, since samplesof the Kerguelenoceanic plateauoftenpossess high 3.16.6.1 Diagnostic FeaturesofOceanic Plateaus (La/Nb) n values,dueto magmainteraction with Bothchemicalandgeologicalfeaturescanbe fragments ofcontinentallithospherebeneaththe useful inthe identification ofoceanic plateaus. plateau. Thisexamplehighlights the importance Condie(1999)andKerr etal. (2000) have ofnot relyingsolely on chemicaldiscriminantsof discussed the diagnostic featuresofoceanic tectonic environment,withoutalso considering plateaus indetail,andonly abriefaccount will the geologicalevidence.Inthe caseofKerguelen, be givenhere.Table3summarizesthe character- the lack ofvolcaniclastic horizons helps confirm isticswhich areuseful indistinguishingigneous its oceanic plateauaffinity. Asdiscussed byKerr rocksformed inanoceanic plateaufrom those etal. (2000),many ofthe geologicaldiscriminants which originated inothertectonic settings. betweenoceanic plateaus andmidoceanridges Igneous rocksproduced inanislandarc, or a maybe ambiguous (Table3).Geochemical continentalsubduction zonesetting, arerelatively characteristicsmust,therefore, be used to dis- easilydistinguishedfromoceanic plateau tinguishlavasfrom thesetwo tectonic settings. sequences(Table3),becausearcsgenerally Most Cretaceous oceanic plateaulavaspossess possess moreevolved lavas,withubiquitous relatively flatnormalized REE patterns (Figure7), high (La/Nb) pmn ratios (Figure14(b)),andonly whereasmostmidoceanridge basalts possess light very rarely containhigh-MgO lavas. Additionally, REE-depleted patterns reflectingamoredepleted Identification ofOceanic Plateaus inthe GeologicalRecord 553 recordsimply becausetheyareeasily eroded away,unless theyareburied bysediments. Quite oftenthe only remainingindications ofacon- tinentalfloodbasalt province arethe dykesand ventsthrough which the lavaserupted.The 200Ma CentralAtlantic MagmaticProvince, which formed duringthe break-up ofSouth America, Africa, andNorthAmerica hasbeen identified largely on the basisofits remnant dyke swarms (e.g.,Marzoli etal.,1999).

3.16.6.2 Mafic Triassic Accreted Terranesin the NorthAmericanCordillera Significant proportions ofthe NorthAmerican Cordilleraconsist ofmafic sequencesofaccreted oceanic terranes(Figure15). Someofthesehave beenidentified asoceanic plateaumaterialranging inage from PermiantoEocene(see reviewin Condie,2001). Atleast three oftheseoceanic plateauterranesarepredominantly Triassic inage (Wrangellia, CacheCreek andAngayucham; Pallister etal.,1989; Lassiter etal.,1995;Tardy etal.,2001)andobviously represent amajor phase ofoceanic plateauvolcanism atthistime.These plateausequencesarecharacterized bypillow basaltsandintrusivesheets,withoccasional Figure14 (a)Frequencydiagramshowingthe range intercalated tephraandhyaloclastitelayers,indi- in(La/Nb) pmn for lavasfrom the CCOP–OJP, the Kerguelenplateau,EPR MORB, andarclavas. (Arc catingformation inshallow water,or bysubaerial datafrom Thirlwall etal.,1996.) Otherdatasourcesas eruption.Inthe Wrangellia terranethereis inFigures5, 8and12). (b)Plot ofNivs. Mg–number considerableevidence for rapid upliftofthe sea for the CCOP–OJP andlavasfrom back arcbasins. floor (presumably bythe plumehead)immediately Datafrom Wood etal. (1980),Woodhead etal. (1998), prior to eruption (Richards etal.,1991). Leat etal. (2000).Bothdiagrams modified from Kerr The basaltsofthe Cache Creek andAngayuc- etal. (2000). hamterranesdisplayarestricted range inMgO withmostofthe basaltsrangingfrom 5.0 wt.% to 8.5wt.%. Thesebasaltspossess low (La/Nb) pmn mantlesource region. Furthermore, high-MgO ratios ( , 1.2),essentially flatREEpatterns lavascanbe foundinoceanic plateaus,but are (Pallister etal.,1989; Tardy etal.,2001)and largely absent from oceanic crustgenerated at 1 Nd valuesthatrange mostly from 9.9 to 4.5 midoceanridges. (Figure16). AsFigure16 shows,all tþ hesefeaþtures Incompatibletrace elements areonly oflimited aresimilartothe OJP.However,someofthe useindistinguishingbetweenvolcanic succes- basaltsfromthe Wrangellia Terrane, despite sionsformed inback-arcbasins andthoseformed showingasimilarrange inMgO content,have inoceanic plateaus (Table3). However,the lower (La/Nb) pmn ratios . 1andsteeperREE patterns mantletemperaturebelowaback-arcbasin (Figure16)thanthosefrom Cache Creek and ( T p , 1,280 8 C)compared to amantleplume Angayucham. Lassiter etal. (1995)suggested that ( T p . 1,400 8 C)results inthe eruption offew thisisdueto the magmaseruptingthrough, and high-MgO lavas. Anadditionalconsequence of beingcontaminated bypreexistingisland-arc thislowermantletemperatureisthatback-arc lithosphere.However,itisalsopossiblethat,as basinlavasgenerally possess lowerNi andCr inthe caseofthe Kerguelenplateau,large contents for agivenMg numberthanoceanic fragments ofancient continentallithospherewere plateaulavas(Figure14(b)). Furthermore, incorporated inthe proto-Pacific Ocean,andthe becauseoftheirproximity to activesubduction lavasofthe Wrangellia oceanic plateauwere sites,back-arcbasinsequencesarealso more contaminated bythislithosphere.The contami- likely to containabundant volcaniclastic horizons nation ofthe mostevolved Wrangellia basaltswith thanoceanic plateaus. eitherarccrustorancient continentallithosphereis Continentalfloodbasaltsarenoteasy to also supported byabroadly negativecorrelation preservefor longperiodsoftimeinthe geological between 1 Nd and(La/Nb) pmn (Figure16). 554 Oceanic Plateaus 3.16.6.3 Carboniferous to Cretaceous Accreted Oceanic Plateaus inJapan l a t ed s edi m e nts o o e s

no The Japaneseislandsareessentially composed r a e Ra r e ofaseriesofterranesthathavebeenaccreted to I nt e r ca

p e l agic the continentalmarginofthe Eurasianplate duringthe past 400 Myr. Theseterranesconsist oftrench-ﬁllingterrigenoussediments withvari- ablequantitiesofaccreted oceanic crust thatare e nt e nt e nt e r ia l on e sn s i on a ly intrudedandpartly overlainbythe products of r e qu e rupt i on S u ba subsequent subduction-related volcanism. Within Japanthe agesofthe accreted complexesbecome youngerfrom northto southandfrom west to east a l f r e qu a ly s e tt i n g s. (Kimura etal.,1994). However,relatively littleis e sn a r ef a r e o cca a r e no y e s f r qu knownabout the trace element chemistry ofthese o cca s i on o cca s i on oceanic accreted terranes. t ec ton ic The ChugokuandChichibubelts insouthwest Japancontainupto30% basaltic material(green- fe r e nt ed ed stones) inthrust contactwithlimestones,cherts dif l a v s Te p h r y e rs l a v s l a v s e sr e sy andmudstones(Tatsumi etal.,2000). This no y e sr a ll a ll lithologicalassociation,combined withprelimi- a r e p i llow a r e p i llow Pi llow not not nary major element dataandasmall range oftrace element data, suggeststhatthesebasaltic assem-

d y e sr blagesareremnants ofaplume-derived oceanic s e qu n ce f rom

iche plateau. Tatsumi etal. (2000)proposed that thisoceanic plateauformed inthe Panthalassan Ocean(proto-Pacific)inthe Carboniferous nr iched nr iched LREE-e nr fla ty fla ty

f vol ca n ic (350–300 Ma). p a tt e rn ly ly ly The PermianYakuno ophiolitecomplexin REE e pl t ed nr iched i n a nt i n a nt i n a nt southern HonshuIslandisasequence ofsub- on d r i t e norm a l z ed o LREE-e o LREE-e

Ch marinebasalts,gabbrosandultramaﬁcrocks (Isozaki,1997). The presence ofpelagic sedi- F l a tt F l a tt ments,the lack ofasheeted dyke complexandthe factthatthe sequence isofconsiderablethickness, 10%

1 all suggest thatitispart ofanoceanic plateau , .. 1

a pmn (Isozaki,1997). Incontrast to the otheraccre- ) 1 1 LREE-e # e olo gica l cha r ac t i st ic so 1 P r ed om 1 P r ed om 1 LREE-d 1 P r ed om o tionary belts inJapan,the Sorachi-Anivia terrane # # # # .. .. 1t ows a r ie s f rom # inHokkaidoandSakhalinisdominated by ( La / Nb stly

o fﬂ oceanic crust andlithosphere(Kimura etal.,

mo 1994;Tatsumi etal.,1998). Itcomprisespillow lavasanddoleritesills withintercalated pelagic sediments containingTithonian(150–145 Ma) 3%) e nt e nt ge o che m ica l a n dg , radiolaria, alongwithalowerunitinwhich -MgO r a e r a eV

r e qu ultramaﬁcrocks,includingserpentinite, harzbur- f r e qu L ow ). l a v s( giteanddunite, aremorecommon (Kimura etal.,

2000 1994). Major element datahaveshownthatsome Diag nost ic picrites(. 12 wt.% MgO)arefoundwithinthe e t a l. gO 1 4 %) succession. This,combined withhigh CaO/Al 2 O 3 . a r e a r ef r a e r a e ratios (indicativeofahigh degree ofmantle igh-M f r e qu nt

Tab l e 3 melting),led Kimura etal. (1994)andTatsumi ( af t e r Ke rr l a v s( etal. (1998)toproposeanoceanic plateauorigin for the Sorachi-Anivia terrane.ThisJurassic–

n ic )r earlyCretaceous plateau(named the Sorachi plateau; Kimura etal.,1994)isofthe sameage a s lt

a s lt asthe ShatskyRise(Figure1,Table1). In db m a r gi n f e qu nt u f r e qu nt combination withpaleomagneticdata, this idge a l ﬂ oo

r if t ed suggeststhatthe Sorachi plateauandthe Shatsky s e tt i n gH pl a t ea i sl a n db Risewereoriginallyasingleplateauwhich formed nearthe Kula–Paciﬁc–Farallon triple pmn — pr i m t v e a ntl norm l z ed a Ocea n ic Tec ton ic Ma r gi n a l ba s i nr Ocea n ic V ol ca n ic Mid o cea nr A r c ( ont i n e nt a l & o cea C ont i n e nt junction , 150Ma (Kimura etal.,1994). Table4 Proposed accreted oceanic plateaus foundwithincontinents. NameLocation Age (Ga)References

Coonterunah andWarrawoonaGroups PilbaraCraton,Australia , 3.5Green etal. (2000) Southern Barberton Belt KaapvaalCraton,SAfrica 3.5–3.2 De Wit etal. (1987) PietersbergBelt KaapvaalCraton,SAfrica , 3.4DeWit etal. (1987) a Opapimiskan-Markop Unit,NorthCaribou Belt Superior Province , 3.0 HollingsandWyman(1999) OlondoBelt AldanShield, Siberia 3.0 PuchtelandZhuravlev(1993); Bruguier(1996) SouthRimUnit,NorthCaribou Belt Superior Province , 3.0 HollingsandWyman(1999) Sumozero-Kenozero Belt Baltic Shield3.0–2.8Puchtel etal. (1999) SteepRock &LumbyLake Beltsa Superior Province 3.0–2.9 Tomlinson etal. (1999) BalmerAssemblage, Red Lake Superior Province 2.99–2.96 Tomlinson etal. (1998) GreenstoneBelta Kostomuksha Belt Baltic Shield2.8Puchtel etal. (1998b) VizienBelt Superior Province 2.79 Skulski andPercival(1996) Malartic-Vald’OrArea Superior Province 2.7 Kimura etal. (1993); Desrochers etal. (1993) TisdaleGroup,Abitibi Belt Superior Province , 2.7 FanandKerrich (1997) Schreiber-Hemlo-WhiteRiverDayohessarah Superior Province 2.8–2.7 Polat etal. (1998) Vetreny Belta Baltic Shield2.44 Puchtel etal. (1997) BirimianProvince West Africa 2.2 Abouchami etal. (1990); Boher etal. (1992) Povungnituk&ChukotatGroupsa CapeSmithFoldBelt 2.04Francis etal. (1983); Dunphy etal. (1995) Northern Que´bec Onega Plateau a Baltic Shield1.98Puchtel etal. (1998a) JormuaOphiolite a NE Finland1.95Peltonen etal. (1996) FlinFlon Belt CentralCanada 1.92–1.90 Lucas etal. (1996); Stern etal. (1995) Arabian-NubianShieldNEAfrica-MiddleEast 0.90-0.87SteinandGoldstein(1996) Chichibu&ChugokuBelts SW JapanCarboniferous Tatsumi etal. (2000) Yakuno OphioliteSWJapan0.285 Isozaki (1997) Mino TerraneCentralJapanLPermianJones etal. (1993) Cache Creek TerraneCanadianCordilleraTriassic Tardy etal. (2001) AngayuchamTerraneAlaska Triassic Pallister etal. (1989) Wrangellia TerraneWestern NorthAmerica 0.227 Lassiter etal. (1995). Sorachi PlateauNorthern Japan0.152–0.145 Kimura etal. (1994); Tatsumi etal. (1998) a Thesesequencesdisplayevidence ofcontamination bycontinentalcrust andareinterpreted ashavingformed duringcontinentalbreak-up or,closeto acontinentalmargin(see text). 556 Oceanic Plateaus TheSorachipart ofthe plateauwascarried northwestwardsonthe Kulaplateandcollided withJapan , 110 Ma.Limited trace element data for the Sorachi plateaulavassupport acommon plumesource for thesetwo plateaus. The data coverthe samecompositionalrange asthatof dredgedsamplesfromthe ShatskyRise (Figure17). Furthermore, the datarevealthatthe plumesource region ofthe Sorachi plateauwas markedly heterogeneousandcontained both enrichedanddepleted components(Kimura etal., 1994;Tatsumi etal.,1998)(Figure17).

3.16.7 PRECAMBRIAN OCEANIC PLATEAUS Theidentiﬁcation ofaccreted pre-Cambrian Figure15 MapshowingNorthAmericanaccreted oceanic plateaus,particularly ingreenstonebelts, oceanic terranesincludingthe oceanic plateau hasimportant implications for the generation of sequencesdiscussed inthe text (afterTardy etal., continentalcrust(Abbott,1996; Albarede, 1998; 2001; Condie, 2001). Condie, 1999). Kerr etal. (2000) havepresented

Figure16 Plots of(a)(La/Nb) pmn against initial 1 Nd for Cache Creek, Wrangellia, OJP andthe Kerguelenplateau and(b)chondritenormalized (Sun andMcDonough, 1989) REE plot showingaveragesfor Wrangellia, Angayucham, Cache Creek, andthe Kerguelenplateau,withthe range for the OJP.DatasourcesareasinFigures5and12; NorthAmericandatafrom Pallister etal. (1989); Lassiter etal. (1995); andTardy etal. (2001). EnvironmentalImpactofOceanic PlateauFormation 557

Figure17 Plot ofNb/Yagainst Nb/Zrshowingdata from dredge samplesfrom the ShatskyRiseandthe accreted Sorachi plateau(datafrom Tatsumi etal., Figure18 Plot of(La/Nb) vs. initial 1 Nd for data 1998). Also shown arefieldsfor the CCOP andOJP. pmn from ArcheanandProterozoic accreted oceanic plateaus DatasourcesareasinFigure5. andprovincesproposed to haveformed duringcon- tinentalrifting.Datasources:Abouchami etal. (1990); Skulski andPercival(1996); Puchtel etal. (1998a); asummary ofaccreted pre-Cambrianoceanic Tomlinson etal. (1998); HollingsandWyman(1999). plateaus andthe readerisreferred to theirpaper for moredetailed information. include the Balmerassemblage,Red Lake belt Someofthe oldest preserved oceanic plateau (Tomlinson etal.,1998),SteepRock-LumbyLake sequencesarethosefoundin , 3.5GaBarberton belts (Tomlinson etal.,1999),andOpapimiskan- andPietersbergbelts ofthe KaapvaalShieldof Markop unit,NorthCaribou belt (Hollingsand southern Africa (De Wit etal.,1987; Smithand Wyman,1999).Thesesequenceshavebeen Erlank, 1982).Thesebelts containpillow basalts interpreted ashavingformed intectonic settings andkomatiites,withchemicalsignatures(Lahaye related to continentalbreak-up,similartothe etal.,1995)suggestingalikely originaspart NorthAtlantic Tertiary igneous province andparts ofanoceanic plateau. The Pilbaracraton of ofthe Cretaceous Kerguelenplateau. Australia appears to possess someofthe oldest Puchtel etal. (1998b,1999)haveproposed that oceanic plateaumaterialsofaridentified (Green the 3.0–2.8GaKostomuksha andSumozero- etal.,2000)inthe , 3.5GaCoonterunah and Kenozero greenstonebelts ofthe Baltic Shield WarrawoonaGroups. represent remnantsofoceanic plateaus.This Greenstonebeltsofthe CanadianSuperior interpretation isbased on the occurrence of province, ranginginage from 3.0 to 2.7 Ga also crustally uncontaminated pillowbasalts containlavagroups thathavebeeninterpreted to (Figure18)andkomatiiteswithout terrestrial be remnantsofaccreted oceanic plateaus. These sedimentary intercalations. Incontrast the 2.4Ga belts include the SouthRimunitofthe North Vetreny greenstonebelt andthe 1.98GaOnega Caribou belt (HollingsandWyman,1999),the plateau,also part ofthe Baltic Shield, display Vizienbelt (Skulski andPercival,1996), chemicalevidence ofcrustalcontamination the Malartic-Vald’Or(Desrochers etal.,1993), (negative 1 Nd;Figure18). Thesesequences the TisdaleGroupofthe Abitibi belt (Fanand wereinterpreted byPuchtel etal. (1997,1998a) Kerrich, 1997),andthe Schreiber-Hemloandthe ashavingformed duringcontinentalbreak-up. WhiteRiver-Dayohessarah belts (Polat etal., OtherProterozoic oceanic plateauterraneshave 1998). The evidence for anoceanic plateauorigin beenidentified inthe Birimianprovince of isbased on the occurrence ofpillow basaltsand western Africa (Figure18)(Abouchami etal. komatiiteswithoutterrestrialsedimentary inter- 1990; Boher etal.,1992),the Arabian-Nubian calations or sheeted dyke swarms,possessinglow Shield(SteinandGoldstein,1996),andthe Flin Flon belt inCanada (Stern etal.,1995). (La/Nb) pmn andthe low positive 1 Nd (Figure18) thatarecharacteristic ofCretaceousoceanic plateaus. Severalofthesesequenceswithinthe 3.16.8ENVIRONMENTAL IMPACT OF Superior province areinstratigraphic contactwith OCEANIC PLATEAU FORMATION basaltsandkomatiitesthatpossess asignatureof continentallithospherecontamination,i.e.,nega- Although the potentialenvironmentalimpactof tive 1 Nd and(La/Nb) pmn . 1(Figure18). These continentalfloodbasalt provinceshasbeen 558 Oceanic Plateaus documented bymany authors(e.g.,Hallam, 1987a;McLean,1985;RenneandBasu,1991; Courtillot etal.,1996),the possibleeffects of oceanic plateaueruptionsonthe atmosphere, biosphereandhydrospherehavereceived com- paratively littleattention (see,for example, Courtillot,1994). Thisomission issurprising since the inclusion ofoceanic plateauevents actually strengthens the correlation betweenLIP eruptionsandmass extinction events (Kerr,1997), byprovidingafeasibleterrestrialcausalmecha- nism for severalsecondorderextinction events (Sepkoski,1986).

3.16.8.1 Cenomanian–TuronianBoundary (CTB) Extinction Event Severalofthesesecondorderextinction events occurred inthe mid-Cretaceous.Oneofthese, the CTB event ( , 93 Ma)hasbeenlinked byseveral authorstothe formation ofoceanic plateaus (Sinton andDuncan,1997; Kerr,1998). The CTB event ischaracterized bythe world-wide deposition oforganic-rich blackshales(Jenkyns, 1980; Schlanger etal.,1987).The formation of blackshaleimpliesawidespread reducing environment (“anoxia”) inthe oceans atthis time(Figure19). Inaddition to this,the CTB wasatimeofmajor sea leveltransgression (Hallam,1989) andismarked byapositive carbon isotopic anomaly ( d 13Cexcursion) ofup to 4–5‰(Arthur etal.,1987),indicatingan incrþ easeinorganic carbon burialrate(Figure19). Sea water 87 Sr/86 Sr(Figure19)reachesamaxi- mum of0.70753inthe late-Cenomanian,and drops steadily to avalueof0.70737 inthe mid- Turonian,beforestartingto riseagain. Average globalsurface temperatures(includingoceanic temperatures) aroundthe CTB were6–14 8 C higherthanpresent (Kaiho,1994),andthisis Figure19 Graphsshowinghow various parameters most likely dueto anincreaseinglobalatmos- discussed inthe text vary from 110 Ma to 80Ma.The pheric CO2 content which mayhavebeen . 10 dotted verticallinerepresentsthe Cenomanian– timespresent-daylevels (Figure19)(Arthur etal., Turonianboundary (afterKerr,1998). 1987). Thesephenomenawereaccompanied byan extinction event thatresulted inthe demiseof26% andratios foundinCTB blackshalesaresimilarto ofall knowngenera(Sepkoski,1986).Although plume-derived volcanic rocksandmidoceanridge the overall extinction rateismuch lowerthanthat basalts. For example, inmafic volcanic rocks Ni/Ir 104 valuesrange from 70 to 190,and atthe Cretaceous–Tertiary boundary,deepwater £ marineinvertebratesfared much worseinthe CTB inCTBsedimentsthisratioaverages180. event (Kaiho,1994). Thisdifference supports the Incontrast,average sedimentary rockspossess Ni/Ir 104 ratios of , 100 (Orth etal.,1993). viewthatanomalous oceanic volcanism around £ the CTB mayhaveplayed asignificant rolein the environmentalandbiotic crisisatthistime (Kerr,1998). 3.16.8.2 LinksbetweenCTB Oceanic Plateau Siderophileandcompatiblelithophiletrace Volcanism andEnvironmental elementssuch asSc, Ti, V, Cr,Mn,Co,Ni, Pt, Perturbation IrandAuareenrichedinCTB black shales(Leary andRampino,1990; Orth etal.,1993). Kerr The mostextensiveplume-related volcanism (1998)hasshown thattrace element abundances aroundthe CTB occurred inthe oceans,with EnvironmentalImpactofOceanic PlateauFormation 559 the formation ofthe CCOPalongwithportions of displacingseawater(CourtneyandWhite, 1986) the OJP andKerguelenplateau. Inaddition to this andbythe thermalexpansion ofseawaterdueto oceanic volcanism,acontinentalfloodbasalt heating.Thesteadyriseinglobalsea level province related to the Marion hotspot also throughoutthe LateAlbianandCenomanian erupted atthistime, asMadagascarrifted from (Figure19)mayreflectthe arrivalofthe India(Storey etal.,1995). The estimated erupted Caribbean,OntongJavaandKerguelenplume volumeofoceanic plateaulavasaroundthis headsbelow the oceanic lithosphere, prior to timeis , 1.0 107 km 3 andmaybe much higher extensivevolcanism (Vogt,1989; Larson,1991). (Kerr,1998). The£ potentialphysicalandchemical Thisplume-related upliftofoceanic lithosphere effects ofoceanic plateauvolcanism on the global mayalso havecaused the disruption ofimportant environment aresummarized inFigure20 and oceanic circulation systems such thatcool,polar discussed below. (oxygenated)waters werenot circulated to lower Anobvious physicaleffectofoceanic mantle latitudes,resultinginincreased oceanic anoxia. plumevolcanism istoraisesea levelbylava Additionally,hydrothermalfluidsfrom oceanic extrusion onto the oceanfloor through the buoyant plateauvolcanism couldhavecontributed to plumehead upliftingthe oceanic lithosphereand warmeroceans,andthus to anoxia, since oxygen

Figure20 Flow diagramsummarizingthe possiblephysicalandchemicalenvironmentaleffects ofthe formation of large igneous provincesaroundthe Cenomanian–Turonianboundary (afterKerr,1998). 560 Oceanic Plateaus solubility isconsiderably reduced inwarmer nutrients mayresult inthe enlargement ofthe seawater(Sinton andDuncan,1997). trace metal-restricted habitatofdeeperdwelling Positive d 13Canomaliesatthe CTB reflect organisms(Wilde etal.,1990),leadingto increased ratesoforganic carbon burialasaresult increased predation bydeeperdwellingcreatures ofhigh productivity andmoreeffectivepreser- on thoselivinginshallowerwater. vation oforganic material(Arthur etal.,1987). Throughout the past 250Myr significant black Such increased productivity means the supply of shaledeposits occur duringseveralotherperiods deepoceannutrients,such asphosphates,into and, like the CTB, theseotherblack shalesare surface waters must also increaseandthisprocess associated with, sometimessevere, environmental mayhavebeeninducedbyoceanic plateau disruption (Jenkyns,1980; Hallam,1987b;Arthur volcanism (Vogt,1989; Sinton andDuncan, andSageman,1994). Itisinteresting, and 1997). probably highly significant,thattheseblack Elevated CO2 levels atthe CTB mayalsobe shaleeventscorrelatewiththe formation of dueto increased volcanic activity. Kerr (1998)has oceanic plateaus or plume-related volcanic rifted 17 margins (Table5). The Aptian–Albian(121– calculated thatapproximately 10 kg ofCO 2 wouldhavebeenreleased asaresult ofoceanic 99 Ma)appears to havebeenaperiodofpersistent plateauvolcanism aroundthe CTB (Arthur etal., environmentaldisturbance withthree distinct 1987).Additionally,LIP volcanism also releases oceanic anoxia events(withassociated black substantialamounts ofSO ,chlorine, fluorineand shales) duringthisperiod(Bralower etal., 2 1993). Acausallinkbetweenblack shalefor- H 2 Swhich, whenreleased into seawater,would havemade the CTB oceans much moreacidic mation,environmentalperturbation andoceanic (Kerr,1998). The lack ofcarbonateatthe CTB volcanism isgivenfurthercredence bythe fact maybe the result ofincreased dissolution bymore thatamajor periodofoceanic plateauformation acidic seawater,which wouldalso releasemore occurred duringthe Aptian–Albian(121–98Ma) (see above). CO2 to the atmosphere(Arthur etal.,1987). Theseadditions ofCO to the atmosphere Finally,Condie etal. (2001) havepresented 2 evidence thatthe correlation betweenblack shale wouldhaveresulted insignificant globalwarming. deposition,paleoclimatic disturbance andmantle The solubility ofCO inseawaterdecreasesasthe 2 superplumeevents canbe extended back to the temperaturesrise;sothe warmerthe oceans get, pre-Cambrian. Particularlygoodcorrelations the less CO will dissolveinthem. Thus,withthis 2 betweenenvironmentaldisturbance andmantle positiveCO feedback mechanism itispossible 2 plumeactivity occur at1.9 Ga and2.7 Ga thata“runawaygreenhouse”climatemayhave developed quiterapidly (Figure20)(Kerr,1998). Increased weatheringofcontinentalsilicatescan reduce CO2 levels. However,the rateofCO 2 releaseatthe CTB wasmuch greaterthanits 3.16.9 CONCLUDING STATEMENTS uptake byslow weatheringprocesses. Increased Oceanic plateaus represent overthickened areas atmospheric CO2 andthe upwellingofnutrients ofoceanic crust(10–35km),which appearto fromthe deepoceancouldhaveresulted in haveformed asaresult ofdecompression melting increased productivity inoceansurface waters ofalarge mantleplumehead, often(although not (Figure20),leadingto the widespread deposition always)within1–2Ma.Geologicalandgeochem- ofblack shalesandthus areduction inCO2 levels. icalevidence suggeststhatoceanic plateaus have Increased concentration oftoxic trace metals in formed throughout aconsiderableperiodof the oceans,liberated byhydrothermalfluidsfrom Earth’s history. oceanic plateaulavapiles,maywell havebeena The thickness ofthe crustalsections ofoceanic contributory factor to the demiseofsomemarine plateaus impliesthattheyarenot easily subducted. organisms aroundthe CTB (Wilde etal.,1990). Thus,whentheseplateaus encounterasubduction The upwellingofdeepoceantrace metals and zone, theirtop-most portions tendto become

Table5 Correlations betweenblack shaleevents andoceanic plateauvolcanism overthe last 250Ma. Age Black shalesOceanic plateauorvolcanic rifted margin Aptian–Albian(121–99 Ma)Extensiveworld-wide deposits OntongJava&otherPaciﬁc plateaus, Kerguelen Tithonian(150–144 Ma)Extensivedeposits inEurope Sorachi plateau&ShatskyRise andwest Asia Toarcian(190–180Ma)Extensiveworld-wide deposits Karoo,Ferrar&Weddell Sea Carnian(227–220 Ma)Fewdeposits Wrangellia

Datasources:Hallam(1987b); Jenkyns(1980); RileyandKnight(2001); HergtandBrauns (2001). References 561 accreted to acontinent or anislandarc.Inthis Bence A.E.,Papike J.J.,andAyuso R.A.(1975)Petrologyof wayfragments ofoceanic plateaus havebecome submarinebasalts from the CentralCaribbean:DSDP Leg 15. J.Geophys. Res. 80 ,4775–4804. incorporated intocontinentalmargins and BoherM.,AbouchamiW.,MichardA.,Albarede F.,andArndt preserved inthe geologicalrecord. N.T.(1992) CrustalGrowthinWest Africa at2.1 Ga. Two recent examplesofsuch accreted oceanic J.Geophys. Res.-Solid Earth 97,345–369. plateaus arethe CCOP andthe OJP.All ofthe BralowerT.J.,SliterW.V.,Arthur T.J.,Lekie R.M.,Allard lavassampled from the OJP andmostofthe lavas D.J.,andSchlangerS.O.(1993) Dysoxic/anoxic events in the Aptian–Albian(Early Cretaceous). In The Mesozoic fromthe CCOP arerelativelyhomogeneous Pacific: Geology,Tectonics,andVolcanism, American basaltswithinitial 1 Nd valuesbetween 5and GeophysicalUnion Monograph77 (eds. 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