American Mineralogist, Volume62, pages666471, 1977

Theoceanic - relation in generaland in the CanaryIslands

Fsr-rxCsnvns GeophysicalLaboratory CarnegieInstitution of Washington,D. C.20008


Abundant data from the Canary archipelagoconfirm an earlier suggestionbased on scattereddata from volcanicislands as a wholethat, amongpublished analyses of specimens from the oceanicbasalt-trachyte association, trachytic materials are considerablymore nu- merousthan thoseof compositionsintermediate between basalt and trachyte.Frequency distributionsof SiOr,CaO, and Thornton-Tuttleindex for the Canariesdata are strikingly similarto thosefor the earlierworldwide collection. In the Canaries,as in mostoceanic and many otherexamples of the basalt-trachyteassociation, trachyandesitic materials are prob- ably considerablyless abundant than trachyticmaterials. The relationis not readilyex- plainedby crystallractionation but seemsreadily explicable by fractionalmelting.

In hismemoir on Ascension,Daly (1925)suggested relative abundancesof granitic plutonic and basaltic that, in oceanicisland assemblages,rocks inter- volcanic typeswas basedentirely on planimetric mediatein compositionbetween basalt and trachyte estimates of map areas underlain by various pet- weremarkedly less abundant than .Unlike rographic units (for a review, seeChayes, 1975),he most of his petrologicalgeneralizations, this one nowherespecifies the basisfor his inferenceabout the seemsto have attractedlittle immediateattention. relative abundance of intermediate and trachytic Evensuch devoted contemporary students of the al- materials in the basalt-trachyte association of the kalinerocks as Bowen and Shand had nothingto say oceanicislands. In view of the amount and quality aboutit; indeed,Daly himselfdoes not mentionit in of information then available about the oceanic his own text (Daly, 1933).It wastaken up and con- . it could not have been based on such esti- siderablystrengthened by Barth et al. (1939),who mates; even now, despite extensivestudies of many alleged,even then not quite unexceptionably,that groups in the past two decades,this would lavaswith siliqacontent in the range53-58 percent be possible for only a few islands. were "entirely lacking" in the Pacific basin; this In 1963I recordedthe resultsof an extensiveliter- marks its first appearancein a major text. Barth's ature searchindicating that materialsintermediate in positionis accepted4nd generalizedby Turner and composition betweenbasalt and trachytewere indeed Verhoogen(1960, p. 199),who go sofar asto saythat much less abundant than trachyte among published "in the oceanicbasalt association there is complete chemical analyses of specimens from all the known chemicalgradation from olivinebasalt to oligoclase basalt-trachyte associationsof the oceanic islands. basalt but transitionql types betweenthe latter and This point could be made from frequency distribu- trachyteare rere or altogetherabsent" (italics added). tions either of raw or of appropriate norma- In the successorvolume (Carmichael et al., 1974,p. tive components, whether prepared from the whole 423), however,the whole questionis mooted,and data array or from subsetsfiltered in various ways to Daly is creditedonly with drawingattention to "the eliminate hydrated or oxidized materials. The rela- intimatefield association between basalt and trachyte tion also seemedto hold in numerous marginal and on Ascensionand other islands,"a rather curious nonoceanicoccurrences of the association. caseof throwingthe baby out while preservingthat There was of course no binding assurancethat "somewhatpuzzling feature," the bath water. what was true of the literature was also true of the Although Daly's betterknown dictum about the lithosphere. From what seemedto me a fair and

666 CHAYES: OCEANIC BASALT-TRACHYTE RELATION 667 thorough review of conventional objections, how- counts comparableto relative areal abundances ever, I concluded there was no persuasivereason to would be desirable,Cann thinksthat eventhe latter suspect persistent systematic biases toward under- mightgive "a verypoor representation of therelative representationof materialsof intermediatecomposi- volumeproportions of therock typespresent." Baker tion among analyzed specimens.The burden of evi- (1968)agrees heartily with Cann's positionre the dence was-and is-to the effect that these materials misleadingcharacter of both specimencounts and are indeed less abundant than trachyte in the ba- areal abundancesand with both Cann and Harris salt-trachyte associationof the oceanicislands. aboutthe nonexistenceof the Daly gap.Unlike them' The conclusionwas disconcertingboth intrinsically however,he findsthat on ftrsisland (St. Helena)the and becauseof the way in which it was reached. "distinctivefield characters(coordinated with pet- Among proponents of fractionation it is rographicand geochemicalstudies)" of intermediate rather generally agreed (per contra see Mukherjee, rock types"permitted a detailedinvestigation of their 1967) that the amount of residual liquid decreases distributionto be made."The decisivefield charac- monotonically in the course of fractionation. One ters are, mirabiledictu, that (l) trachyandesitetends would not expect that trachytes generatedin this to occurin thick massiveflows that "weatherto flat fashion would be more abundant than the trachy- discsof fresh rock in a characteristicorange mate- andesitesregarded as their immediate parents. And rial" whereas" lorm numerousthin if one were sufficiently persuaded of the overriding flowsthat . . . couldbe mistakenfor basaltsin many importance of crystal fractionation in the oceanic casesbut not for "and (2) "trachy- volcanic dispensation,one would not welcomesuch a andesitesare mediumto dark greenin color with a conclusion,however it was reached.Recent develop- distinctiveshimmer or platiness"whereas "trachyba- ments in , discussedbelow, should greatly salts are fine-grainedblack, purple, dark grey or weaken objectionsof this kind. rarely very dark greenrocks often with a distinct Nothing has happened, however, to lessen the sheen(nol a platiness)"!' force of objections based on the way in which the Read separatelythese notes are rather lessthan conclusionwas reached.The procedurewas bound to persuasive.Considered together they have about affront some petrologists,for it ignored entirely such thema distinctair of specialpleading; despite marked matters as rock identification, mode of occurrence, and quite evidentlyirreconcilable differences about areal distribution, stratigraphicposition-in sum, the the field characteristicsof the all-importanttrachy- whole complex of conbeptsand operationsinvolved andesitesand even about the possibility of identifying in making and interpreting geological maps. Reac- them,as well as aboutappropriate procedures to be tion was not long in forthcoming, and there have usedin estimatingtheir abundancein the eventthey been numeious notes restating objections I thought could be identified,the authors are nevertheless had been fairly discussedand dispassionatelydis- firmly agreedthat trachyandesitesare indeedmore missedin the original paper. abundantthan trachyte.How this agreementhas Harris (1963), for instance,argued that trachytes beenreached is not explainedand seemsquite in- are characteristically"oversampled" becausethey are explicable. "unusual" and frequently intrusive, that "trachy- Thereis, of course,no questionat all that basaltis andesiteswere probably much less overcollectedas by a very large margin the dominantrock of the they more closelyresemble in the field and in oceanicislands, and in many islandsand island hand specimen," and that ratios of specimenscol- groupsrocks of anyother kind areeither very rare or lected are likely to "represent" relative areal abun- lacking.Such groups can hardly be expectedto pro- dances "only if an island has been subjectedto a vide decisiveinformation about the relativeabun- thorough field study and detailed niapping." Cann danceof trachyteurs-i-uls rocks intermediate in com- (1968) doubled this in spades, finding that inter- position betweentrachyte and basalt.Macdonald mediate rocks are overlooked not only becausethey (1963)points out, for instance,that trachytesare look like other rocks but more importantly because exceedinglyrare in the Hawaiianarchipelago, and it of their "extreme petrographic(and hand specimen) would hardlybe surprisingif in ,as he asserts, dullness" and "monumental lack of interestingfea- trachyteswere in fact lessabundant than tures," charactersthey somehow do not share with and .It is to be recalled,however, that the rocks from which they cannot be distinguished. Whereas Harris evidently presumes that specimen 'Italics and parenthesesin original. 668 CHAYES: OCEANIC BASALT-TRACHYTE RELATION


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5t 4b 55 65 75 E5 0 5 t0 t5 20 25 0 25 50 75 t00 Fig. L Distribution of SiO, (A), CaO (B), and Thornton-Tuttleindex (C) in analysesof 551 specimensof the basalt-trachyte associationof the oceanicislands, worldwide. (Redrawn from Figs. lA-3A of chayes, 1963.) in SiOzcontent and Thornton-Tuttle index. Hawaiian firstAtlantic island described that showsthe bimodal hawaiitesare basaltic,and so too aremost Hawaiian distribution of rock typesthat was postulatedfrom mugearites;that is to say,they are essentiallyalka- chemicaldata by Chayes."One would hardlyguess line basaltsor trachybasalts,not trachyandesites.from this that Gran Canariaand Tenerifeare neigh- Trachytesdo seemto be exceedinglyscarce in the boring islandsin a small archipelago,or that the 'lBunsen-Daly ,but so too are rocks composi- gap" is the veryone "postulated from tionallyintermediate, in any realisticsense, between chemicaldata by Chayes."Without mention either of basaltand trachyte;published evidence suggests that Schminke'sconclusion that Gran Canariais an ex- on the whole the Hawaiianarchipelago, like some cellentexample of the Daly gapin the Canariesor of other islandgroups, is not a favorablesite for study Ridley's later assertionthat the nearby island of of thisproblem. Tenerifeis the first exampleof it found in the whole The situationis quitedifferent in the Canaries,in Atlantic,Fuster (1975) maintains that the Daly gapis whichtrachytes and phonolitesabound, and thereis iiitot well defined" in either Gran Canariaor Tene- evena considerabledevelopment of highly siliceous {-ft, ot in La Palma,but "is frequentlyfound" in the peralkalineignimbrites. Despite the abundanceand rdcksof Fuertaventura,Gomera, Hierro, and Lanza- notorietyof thesematerials, however, opinion about rote. the existenceand significanceof the Daly gap in the- In viewof thisconfusion, now twice confounded, it Canariesis dividedand confused.In 1969Schminke may be helpfulto summarizeCanaries data currently concludedthat "the Bunsen-Dalygap is very realin in the publicdomain, for thisarchipelago is by far the Gran Canaria. . . . There are no flows of inter- mostclosely studied and denselysampled of the At- mediatecomposition between the underlying'basalts' lanticoceanic island groups, and perhapsof all such (up to 547oSiO) and the rocks of the lower and grqups,and hasthe furtheradvantage of containing middleunits (SiO, above67Vo), despite flow-by-flow considerableamounts both of trachyteand of mate- samplingin manyprofiles across the contact." Never- rial intermediatein compositionbetween basalt and theless,in 1970Ridley, though concerned to stresshis trachyte. fundamentalagreement with the positionof Harris, The literaturesearch leading to my own articleon Cann, and Baker,announced that "Tenerifeis the the Daly gap unearthed551 publishedanalyses of CHAYES: OCEANIC BASALT-TRACHYTE RELATION 669




2-- U I roo e, r 80

.-t I lr I llL-- ll I '-r 40 t-t J' -r-i j- 20 l_i 0L 35 r00

Fig. 2. Distribution of SiO, (A), CaO (B), and Thornton-Tuttle index (C) in analysesof volcanic rocks of the Canary Islands.Solid line-979 specimensfrom all islands; broken line-556 specimens lrom Gran Canaria, Tenerife, and La Palma only' specimensfrom all known clearly oceanicexamples incidenceof is extraordinarily high. It nev- of the basalt-trachyteassociation.2 Thanks largely to erthelessseems to be true that even here,as shown in the efforts of Fuster,his students,and his colleagues, Figure 2, the number of published analysesof vol- there are now considerably more than that many canic rocks intermediatein composition betweenba- from only the three Canary islandsin which he con- salt and trachyte is markedly lessthan the number of siders that the Daly gap is not well defined. Fuster published analysesof trachyte. cites 610 of thesein his current discussion,all made It is sometimessaid that trachytestend to be sam- in the laboratories of his department, and 105 still pled more frequently than trachyandesitessimply be- unpublishedas of this writing. causethey are more alkaline and petrologistsprefer A literature scan,kindly undertakentwo yearsago to study alkalinerocks, but phonolite-the character- by Fuster's coworker J. Briindle, yielded 1094 pub' istic "intermediate" of the Canaries assemblage-is lished analysesof rocks from the Canaries.These are richer in alkaliesthan is the associatedtrachyte. It is now includedin the data basecurrently maintainedat frequently assertedthat trachytes are oversampled the Geophysical Laboratory. As shown in Figures I relative to trachyandesitesbecause trachytes tend to and 2, the distributions of key variablesin the Can- form conspicuous, well-defined plugs or sills and ariesdata are remarkably similar to those found ear- trachyandesites characteristically form flows not lier in the "world" sample of oceanic i'sland ba- readily distinguishable from basalt. However, de- salt-trachyteassociations. tailed studiesof the islandsof the archipelago,by the Salic rocks as a whole seemfar more abundant in Madrid group and others, do not suggest that the central Canariesthan in any other known oceanic trachyte intrusivesare,more common than phonolite islands,and among the salicrocks of the Canariesthe intrusives,that nonintrusive trachytesare less com- mon than nonintrusivephonolites, or that look more like basaltsthan do trachytes.It is some- 'zNinety-one of these were from the Canaries, all taken from an times allegedthat trachytesare oversampledrelative early tabulation by Fuster. The tally for the Canaries should have to trachyandesitesbecause thri former and been higher even then; the papers of Hausen were overlooked in 'the the latter are interlayeredwith the dominant basalts, initial search, but his data are included in reductions reported but publishedaccounts record very little differencein here. Most of the Canaries analysesused in the original com- phono- pilation are very old and have not been retained in the current data fhe rnodes of occurrence of the trachytes and base. ) lites of the Canaries,and there seemsto be almost no 670 CHAYES.' OCEANIC BASALT-TRACHYTE RELATION

trachyandesitein the archipelago.sFinally, it is often fractional crystallizationof such a basaltwere indeed said that trachytes have been oversampled simply of trachytic composition the same might be true of becausethey are extremelyrare, but most of the work the first liquid generatedin the remelting of an in- in the Canaries has been done very recently by pe- completely fractionated basalt of this type. This trologists thoroughly familiar with the controversy would of courserequire that a rather deviousthermal over the Daly gap and fully aware that, among care- history be repeatedas often as required. There must fully described and analyzedspecimens of the oceanic first be enough heat to generatebasalt ; at basalt-trachyteassociation, materials intermediate in some level in the lithosphereenough heat must then composition between trachyte and basalt are con- be dissipatedso that this magma may cool and crys- siderably rarer than trachytes. tallize1'the crystallized magma must next be reheated In sum, there is no reason to suppose that this sufficientlyto produce a "first" or early melt. Finally, particular sampling is biased,by either geologicalor melting must be checkedbefore reaching the stageat psychological factors, in such fashion as to under- which the composition of the liquid fraction is no estimatethe relativeabundance of intermediates.The longer trachytic, yet the temperature must remain relativepaucity of analysesof such specimensin pub- high enough so that the melt fraction can be sepa- lished information about the Canariesstrongly sug- rated from the solid residue before it crystallizes,a geststhat materialsof this kind are indeedless abun- condition which then seemedarbitrary and very diffi- dant than trachytes in that archipelago, as they cult to satisfy. usually seem to be, though often less markedly so, A much better understanding of the remelting throughout the basalt-trachyte association of the processnow indicatesthat this last step is not nearly oceanicislands and in many of its nonoceanicoccur- so difficult as it then seemed,but may also eliminate rences. the need for the whole procedure. Basing his argu- From this discussion,as from its predecessor,it ment on the elegantgraphical analysisof partial fu- seems reasonableto conclude that where trachyte sion by Presnall(1969) and experimentaldata on the occurs at all in the oceanicislands it usually occursin melting of hydrous silicatecharges and rocks, Yoder amounts greater than would be anticipated as a (1973, especiallyp. 165-167)has recently suggested consequenceof fractional crystallizationof a basaltic that basalt and might be generatedfrom a parent. Is it neverthelesspossible that crystal frac- common parent in a singlefractional melting episode. tionation of basaltis indeedthe only processessential Rhyolite would be formed first and expelledfrom the to the formation of trachyte, and that its anomalous chamber; whether or not rocks intermediatein com- abundance is a consequenceof independent con- position betweenbasalt and rhyolite were generated trols-presumably tectonic-over the eruptive pro- would dependon whetheror not the crystallizationof cess, controls which operate to insure that more the subsequently formed basalt magma was frac- trachyte than trachyandesiteis brought to the sur- tional. Yoder argues that similar relations probably face?This is perhapsnot impossible,but it is surelya hold for undersaturatedmaterials. slender and unlikely reed to lean upon. For why, Indeed,Presnall (1969, p. ll93) had alreadysug- throughout Cenozoic time and at sites widely scat- gested, albeit rather gingerly and with appropriate tered over all three ,should tectonicprocesses obeisanceto the Anglican establishmentview dis- unrelatedto fractionation operateon magma cham- cussedabove, that the Daly gap might be explained bers in about the same way and at about the same by fractional melting.. The parsimony of the scheme poinl in the fractionation process? is very attractive, for it is hardly to be doubted that It seemsmuch more likely that some processother there is indeed one fractional melting episodein the than crystal fractionation is responsible.In 1963 I history of every basalt, but it is difficult and usually suggestedthat trachyte might be generated by partial impossibleto establishthat any particular basalthas fusion of previouslycrystallized ,suppos- undergonemore than one. In this schemesalic alka- ing, perhaps incorrectly, that if the final residue of line would be generated at and perhaps above the lowest "invariant point," and expelled from the magma chamber before the second such sThe base ( contains l16 analysesof volcanic rocks with 53 point was reached.If sufficientearly-formed magma SiO, < 57; of these, 16 are called trachybasalt, 63 phonolite, 25 were expelled in this fashion, melting trachyte, 3 basalt, 4 tahitite, 3 trachybasanite, I . One of the solid analysis is recorded in the file as a trachyteandesite and none as a residue would cease until the second point was reached; further melting would then generate CHAYES: OCEANIC BASALT.TRACHYTE RELATION 671 magma(s) of basaltic composition, which, in turn, Chayes, F. (1963) Relative abundance of intermediate members of would be expelledfrom the chamber.Whether or not the oceanic basalt-trachyte association. J. Geophys Res., 68, t5r9-1534 of were generated - (1975)Distribution of major oxidesin Cenozoicvolcanics: would depend on whether, and to what extent, the SiO2. Carnegie Inst. Wash. Year Book,74, 542-545. crystallization of these later-formed magmas was Daly, R. A. (1925) The geology of Ascension lsland. Proc. Am. fractional. The probability of a central minimum in Acad. Arrs Sct.,60, l-80. the frequencydistribution of rock compositionwould - (1933) lgneousRocks and the Depths of the . McGraw- New York. be very high, for even if the crystallization of the Hill Book Co., Fuster, J. M. (1975) Las Islas Canarias:un ejemplo de evoluci6n later-formed basalt magma(s) was perfectly frac- espacial y temporal del vulcanismo oceanico Estud. Geol. tional, the amount of trachytegenerated by the whole (Madrid), 31, 439-463. processwould be greater than could be yielded by Harris, P. G. ( 1963)Comments on a paper by F. Chayes,"Relative fractional crystallizationalone, unless,of course,the abundance of intermediate members of the oceanic ba- salt-trachyteassociation." J. Geophys.Res., 68, 5103-5107. early-formed trachyte magma did not escapefrom Macdonald, G A. (1963) Relative abundance of intermediate the magma chamber until the second "invariant members of the oceanic basalt-trachyte association-a dis- point" was reached. cussion.J Geophys.Res., 68, 5100-5102. Whether this apparently reasonableexplanation is Mukherjee, A. (1967) Role of fractional crystallizationin the de- indeed correct only time, and perhapsnot even that, scent:basalt - trachyte Contrib . Petrol., 16,139-148. D. (1969) The geometricalanalysis of partial fusion. will tell. The evidencereviewed here, however, seems Presnall, C. Am J. Sci,267, 1178-1194. to me to establishbeyond reasonabledoubt that an Ridley, I. (1970)The abundanceofrock typeson Tenerife,Canary explanation is in fact required; the Daly gap can Islands, and its petrogenetic significance. Bull Volcanol, 34, neither be explainedaway in the fashion proposedby 196-205. its recent detractors nor satisfactorilyaccounted for Schminke, H U (1969) sequenceon Gran Canaria. Bull. Volcanol.. 33. ll99-1219. as a consequenceof pure crystal fractionation of Turner, F. J. and J. Verhoogen (1960) Igneous and Metamorphic basalticmagma. Petrology, 2nd ed McGraw-Hill Book Company, New York Yoder, H. S., Jr. (1973) Contemporaneousbasaltic and rhyolitic References magmas.Am Mineral., 58, 153-171

Baker, I. ( I 968) Intermediate oceanicvolcanic rocks and the "Daly (Readers from which data gap." Earth Planet Sci. Lett.,4, 103-106. wishing a bibliography of sources were are invited to request it via Barth, T. F. W., C. W Correns and P. Eskola (1939) Die Entste- summarized in Figure 2 obtained a Manual for hung der Gesteine.Springer Verlag, Berlin. the rock information system RKNFSYS, User's Cann, J. R. (1968) Bimodal distribution of rocks from volcanic which is availablefrom the author.) islands. Earth Planet. Sci. Lett.,4, 479-480. Carmichael, I., F. J. Turner and J Verhoogen (1974) Igneous Manuscripl receiued, February 9, 1977; accepted Petrology. McGraw-Hill Book Company, New York. for publication, March 16, 1977.