American Mineralogist, Volume62, pages666471, 1977 Theoceanic basalt-trachyte relation in generaland in the CanaryIslands Fsr-rxCsnvns GeophysicalLaboratory CarnegieInstitution of Washington Washington,D. C.20008 Abstract 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 rock typeswas basedentirely on planimetric mediatein compositionbetween basalt and trachyte estimates of map areas underlain by various pet- weremarkedly less abundant than trachytes.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- islands. 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 island 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 oxides 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 crystal 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"trachybasalts 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 trachyandesites"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 petrology, 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 basalts 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 Hawaii,as he asserts, dullness" and "monumental lack of interestingfea- trachyteswere in fact lessabundant than hawaiites tures," charactersthey somehow do not share with and mugearites.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 =o lrJ!so *40G 30 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