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International Geology Review, Vol. 44, 2002, p. 686701.Copyright 2002 by V. H. Winston & Son, Inc. All rights reserved.

0020-6814/02/610/686-16 $10.00 686

What Is the Calc-alkaline Rock Series?

HETU C. SHETH,1 IGNACIO S. TORRES-ALVARADO,2AND SURENDRA P. VERMA

Centro de Investigacin en Energa, Universidad Nacional Autnoma de Mxico, Priv. Xochicalco s/n, Col. Centro, A. P. 34,

Temixco, Morelos 62580, Mexico

Abstract

The calc-alkaline series of rocks was originally defined in the early 1930s on the basis of thealkali-lime index on a combined SiO2 versus (Na2O + K2O) and SiO2 versus CaO plot. The usageof the term has evolved considerably since, and today it is used variably for the subalkalic basalt-andesite-dacite-rhyolite suite, or any rock suite containing andesite, or island-arc rocks, or rockswith high ratios of large-ion-lithophile elements (LILE) to high-field-strength elements (HFSE), or

simply rocks with negative HFSE anomalies (e.g., Nb-Ta) in primitive mantlenormalized multiele-ment diagrams. Although such variable usage is normal in science, the use of certain geochemicalvariation diagrams to define and depict the calc-alkaline series is not strictly appropriate. Two ofthese widely used diagrams are the total alkalies-silica (TAS) diagram and the (Na2O + K2O)FeO*MgO (AFM) triangular diagram, neither of which has calcium as one of the plotting parameters. TheTAS diagram can be used to depict alkaline, subalkaline, and probably transitional rocks, butnot calc-alkaline or high-alumina rocks. Care must be taken while using such diagrams for rockclassification, inasmuch as some of them are inherently unsuitable and several together may classifyeven a single rock into widely different associations. Association or suite are probably betterterms than series, because they imply neither comagmatic relationships nor linear trends. Thecalc-alkaline suite of rocks is abundant along destructive plate margins, but calc-alkaline geochem-istry is not a 100% foolproof indicator of subduction processes, inasmuch as calc-alkaline rocks are

also known from regions undergoing extension, such as the Basin and Range province and the Gulfof California. Therefore, caution must be exercised in interpreting ancient terrains with complicatedgeology and calc-alkaline rocks as former subduction zones. Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic.

... we cannot improve the language of any science without at the same time improving the science itself;neither can we, on the other hand, improve a science, without improving the language or nomenclaturewhich belongs to it.

Abb de Condillac (cited in Anderson, 1999)

Introduction

CALC-ALKALINE (syn. calc-alkalic) is a verywell known and widely used term in igneous petrol-ogy today, although its usage is not consistent, theterm being used in widely different senses by vari-ous workers. As Middlemost (1985, p. 118) noted,There is no generally accepted succinct definition

of the calc-alkalic rock association. Most petrolo-gists would, however, agree that the volcanic rocksof this association consist of a comagmatic suite ofsubalkalic silica-oversaturated rocks, and that: (a)they tend to contain more Al2O3 than the normalrocks of the tholeiitic association; (b) their interme-diate members do not normally develop any signifi-cant enrichment in iron; and (c) orogenic andesite isthe most characteristic member of the association.

However, even this agreement is difficult toimplement in actual case studies. An examination of

the petrological literature, including researchpapers and textbooks, shows many case studies inwhich the use of the term is not justified: the termhas often been incorrectly used for particular rocksuites on the basis of their geochemical parameters

1Present address: Department of Geology and Geophysics,School of Ocean and Earth Science and Technology (SOEST),

University of Hawaii at Manoa, Honolulu, HI 96822. Addressafter November 1, 2002: Homi Bhabha Centre for ScienceEducation (HBCSE), Tata Institute of Fundamental Research(TIFR), V. N. Purav Marg, Mankhurd, Bombay, 400 088 India.2Corresponding author: Fax: +52-55-5622-9791; Phone: +52-55-5622-9726; e-mail: [email protected]


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CALC-ALKALINE ROCK SERIES 687

that have no bearing on the nomenclature. Also,rocks belonging to this association are identified,often without sufficient rationalization, with a par-ticular tectonic setting (subduction). We discussbelow the progressive evolution of the term through

time, the various geochemical parameters that havebeen routinely used to identify rocks as calc-alka-line, and the problems arising from the use of someof these parameters.

Alkaline, Subalkaline

and Calc-alkaline Rocks

The TAS diagram: Alkaline and subalkaline rocks

The total alkalies-silica (TAS) diagram (Harker,1909; Le Maitre, 1984), in which the wt% Na2O +K

2

O contents of volcanic rocks are plotted againsttheir wt% SiO2 contents, enables the assignment ofvolcanic rock names (e.g., Cox et al., 1979;Kremenetskiy et al., 1980; Middlemost, 1980; LeMaitre, 1984; Le Bas et al., 1986; Rickwood, 1989)and also distinguishes two rock seriesthe alka-line (alkalic) and subalkaline (subalkalic)(Fig. 1). Various investigators (Macdonald and Kat-sura, 1964; Macdonald, 1968; Irvine and Baragar,1971; Miyashiro, 1978) have proposed somewhatdifferent coordinates for the dividing line betweenthe two series (see Rickwood, 1989 for numerical

values). The dividing line by Macdonald and Kat-sura (1964), for example, is the one that best dividesHawaiian lavas into alkaline and tholeiitic

groups, as identified based on their normative min-eral assemblage (Kuno et al., 1957). Rocks that fallroughly on the dividing line itself, i.e., are neitherstrongly alkaline nor subalkaline, could be calledtransitional (Fig. 1).

According to Tatsumi and Eggins (1995), thestrength of the alkaline-subalkaline divisoryscheme is that the alkaline and subalkaline magmaseries tend to remain separated during differentia-tion, since the early crystallization of olivine, cli-nopyroxene, and plagioclase from basaltic magmasdrives derivative magmas farther away from thecritical plane of silica undersaturation (Fo-Di-Ab) that divides the two magma series in thebasalt tetrahedron (Yoder and Tilley, 1962) (Fig.2). Also, alkaline magmas have normative olivineand feldspathoids (e.g., nepheline), and subalka-line magmas have normative orthopyroxene and

plagioclase, and can be further divided into oliv-ine- and quartz-normative types (Tatsumi andEggins, 1995). Middlemost (1975) has proposed

FIG. 1. Total alkalies-silica (TAS) plot (Harker, 1909)

showing classification of igneous rocks into the alkaline and

sub-alkaline or tholeiitic series. Boundary lines proposed

by Macdonald and Katsura (1964) and Irvine and Baragar

(1971) are shown.

FIG. 2. The basalt tetrahedron Di-Fo-Ne-Qz (Yoder and

Tilley, 1962), with the critical plane of silica undersatura-

tion (Fo-Di-Ab) and critical plane of silica saturation (En-

Di-Ab), where Di is diopside, Fo is forsterite, Ne is nepheline,

Qz is quartz, Ab is albite, and En is enstatite. This is helpful

in classifying basalts based on their normative mineral compo-

sitions. Quartz tholeiites have normative quartz, olivine tholei-

ites have normative hypersthene and olivine, and alkalibasalts have normative olivine and nepheline (Tatsumi and

Eggins, 1995).


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688 SHETH ET AL.

separate diagrams of wt% K2O versus wt% SiO2and of wt% Na2O vs. wt% SiO2 to classify basaltsinto alkaline and subalkaline types, and the subal-kaline basalts are themselves further divided intonormal subalkaline basalts and low-K subalka-line basalts (Fig. 3).

Calc-alkaline rocks: Definitions through time

Peacock (1931) and Kennedy (1933) firstdivided the subalkaline series into tholeiitic andcalc-alkaline magma types (see discussion in Tat-sumi and Eggins, 1995, pp. 5455). Kennedy(1933) proposed the existence of two different pri-mary magma series: (1) the olivine-basalt magmatype, comprising olivine basalts containing phe-nocrysts of olivine, augite, plagioclase, and magne-tite, and differentiating to silica-undersaturatedalkaline rocks; and (2) the tholeiitic magma type,containing phenocrysts of pyroxene, plagioclase,

and magnetite, and differentiating to silica-satu-rated and silica-oversaturated rocks.

Peacock (1931) provided a quantitative defini-tion of four rock series, including the calc-alkalineseries, by superimposing two Harker plots (SiO2 ver-sus Na2O + K2O, and SiO2 versus CaO) on the same

graph (i.e., using the same scale). The criticalparameter for the division was the alkali-limeindex, which is the SiO2 value at which the best-fitcurves through the two trends for the given rocksuite intersected (Fig. 4). His four rock series werealkalic (with alkali-lime index 61 wt%)(Fig. 4). This scheme designates series with lowalkali contents as calcic, and those with high alkalicontents as alkalic. The point of intersection of thetwo best-fit curves in Peacocks graph, whichdecides the critical SiO2 value (the alkali-limeindex), is related more to changes in alkalies than tochanges in CaO, because the CaO curves of mostrock series are roughly similar, but the alkali con-tents and alkali curves of various rock series can behighly variable (Ragland, 1989, p. 322). Tatsumiand Eggins (1995) felt that this scheme has becomelargely redundant. In any case, with the availabilityof large data sets today and inherent spread on thesediagrams, the use of Peacocks definition of the calc-

alkaline series will depend largely on the correcttype of regression method to be used for estimating

FIG. 3. wt% K2O-SiO2 (A) and Na2O-SiO2 (B) plots (Mid-

dlemost, 1975) showing classification of igneous rocks into

alkalic and sub-alkalic (including normal sub-alkalicand low-K subalkalic) series.

FIG. 4. Peacocks (1931) diagram showing how the alkali-

lime index is defined. The continuous line marked lime

shows the variation of lime with silica, and the broken linemarked alkalies shows the variation of alkalies with silica,

for the given rock suite. The silica value at which these two

lines intersect is the alkali-lime index. If it is between 56 and

61% SiO2, the concerned rock suite is designated calc-alka-

line.


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the alkali-lime index (e.g., Baumann and Wtzig,1997; Drapper and Smith, 1998).

Nockolds and Allen (1953, 1954, 1956) sug-gested the use of the terms tholeiitic and calc-

alkaline for magma series with and without ironenrichment during progressive differentiation(increasing SiO2 contents), respectively. Miyashiro(1974) quantitatively defined the two series, propos-ing that the calc-alkaline and tholeiitic rock seriesshow steeper and gentler slopes, respectively, thanthe straight line

SiO2 (wt%) = 6.4 FeO*/MgO + 42.8.

He also suggested that if the magmas are differenti-

ated to intermediate degrees (such that 2.0 < FeO*/MgO < 5.0), the calc-alkaline series would lie abovethe line, and the tholeiitic below it. According toTatsumi and Eggins (1995), Miyashiros discrimi-nant line is often misused and applied as a singlecompositional discriminant outside this composi-tional range, rather than as a trend-slope compar-ison for which it was primarily intended. Again,appropriate regression methods and ANOVA-typestatistical tests may have to be used for this purpose.

Calc-alkaline rocks: Current usage of the termToday, the term calc-alkaline is applied vari-

ably to island-arc or subduction-zone rocks rangingfrom basalt to rhyolite (e.g., Basaltic VolcanismStudy Project, 1981), or to a magma series interme-diate between the low-K and high-K series (e.g.,Wilson, 1996). Some petrologists use the term torepresent the subalkalic basalt-andesite-dacite-rhy-olite suite and their intrusive equivalents, whereasothers use the term to describe any series of rocks ormagmas that contain andesite or its intrusive equiv-

alent (see Middlemost, 1985, p. 117). Some con-sider high ratios of the large-ion-lithophile elements(LILE) to the high-field-strength elements (HFSE)(e.g., high Ba/Nb, high Rb/Ti) as an important char-acteristic of calc-alkaline rocks. For example,Hooper (1994) stated that high LILE/HFSE ratiosare characteristic of the calc-alkaline suite of rocksdirectly associated with subduction. The Encyclope-dia of the Solid Earth Sciences (Kearey, 1993, p. 78)described calc-alkaline rocks as characterized byhigher concentrations of CaO in relation to alkalies

in comparison with alkaline igneous rocks andtherefore as rocks in which the dominant feldsparis plagioclase rather than alkali feldspar. Notably,none of these characteristics are in any way related

to how the suite was originally defined by Peacock(1931). Middlemost (1985, p. 117) has made theinteresting observation that if one determines thealkali-lime index of a typical suite of rocks that con-

tain orogenic andesites, one discovers that they usu-ally belong to the calcic series of Peacock, not calc-alkalic. On the contrary, Middlemost (p. 118) foundthat rocks of the tholeiitic basalt-icelandite suiteusually plot within the calc-alkalic field on Pea-cocks diagram. To some extent, the varible usage ofthe term calc-alkaline is natural, and the termmay continue to be used meaningfully and withoutambiguity as long as each worker specifies his basisfor using it. However, a matter of more concernarises in cases where a calc-alkaline series may be

defined with, and depicted on, an inappropriatemultielement-chemical variation diagram, and thenused as unequivocal evidence for tectonic setting.Both issues are discussed below.

Identification of Calc-alkaline Trends

on Geochemical Diagrams

The TAS diagram

Wilkinson (1968, p. 171) included both thetholeiitic and calc-alkaline series under the

heading subalkaline. This has been followed bymany. For example, Wilson (1996, p. 9) stated that,in general, the subalkaline magma series can besubdivided into a high-alumina or calc-alkalineseries, and a low-K tholeiitic series, correspond-ing to the subalkaline and low-K subalkalinefields of Middlemost (1975). However, the represen-tation of the high-alumina or calc-alkaline series onthe total alkalies-silica (TAS) diagram is somewhatpuzzling, as the diagram has neither Al2O3 nor CaOas one of its plotting parameters.

Kuno (1960, 1966) described basalts from theFar East (Japan, Manchuria, and Korea), and classi-fied them into tholeiitic (with low Al2O3 and alka-lies), alkaline (with variable Al2O3 and higheralkalies), and high-alumina (with higher Al2O3and intermediate alkalies) series, on a TAS diagram(Fig. 5). However, Kunos (1960, 1966) high-alu-mina series has significance, as far as the TAS dia-gram is concerned, only as the one intermediatebetween the other two in terms ofalkali contents, inhis study area. The critical point is this: Kuno plot-

ted his high-alumina rocks in the TAS diagrambecause they had intermediate alkali contentsbetween his tholeiitic and alkaline rocks. A suite ofsamples from another region, with the same interme-


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690 SHETH ET AL.

diate alkali contents, and plotted on that diagram,does not deserve the name high-aluminaand theTAS diagram cannot anyway depict alumina con-tents. The appropriate term for these rocks, if theywere represented on the TAS diagram, would be sub-alkalineinasmuch as the classifying power of theTAS diagram is inherently limited to alkaline,subalkaline, and probably transitional rocks.

Let us assume that we have a suite of samplesfrom an intraplate ocean island, or an intraplatecontinental flood basalt province, and we plot therelevant data for these on a TAS diagram usingKunos (1966) classification so that some samplesplot in the tholeiitic field, others in the high-alu-mina field, and the rest in the alkalic field.Would this be sufficient to enable us to interpret,without any hesitation, the tholeiitic and high-alu-mina lavas as marking a volcanic arc and the alkaliclavas an associated backarc? We are aware, ofcourse, that no petrologist would do this, but that isonly because the tectonic setting of the rocks inquestion is known. What do we do when we are deal-ing with ancient rocks exposed in deformed terrainsof complex geology and dubious tectonic setting?How can we be more confident with ancient rocksthan with modern rocks?

In fact, Kuno (1966) also used the terms high-alumina and calc-alkaline differently, statingthat the calc-alkaline rock series could be derivedfrom the tholeiitic, alkalic, or high-alumina seriesby fractionation under high oxygen pressure. He

also used an AFM (Na2O + K2OFeO*MgO) dia-gram, Fe/Mg ratio, and phenocryst assemblage todistinguish the calc-alkaline series from the tholei-

itic series (see Ragland, 1989, p. 323). Primaryhigh-alumina basalts can be produced at depthsintermediate between tholeiitic and alkaline mag-mas (e.g., Green et al., 1967; Tatsumi et al., 1983).

Recently the term has been applied loosely to por-phyritic magmas, in particular to subduction zonemagmas rich in plagioclase phenocrysts (Tatsumiand Eggins, 1995). Workers like Brophy and Marsh(1986) and Brophy (1987) considered these magmasto be the product of melting of subducting oceaniccrust, but Crawford et al. (1987) demonstratedthat they are derived by the differentiation ofmore magnesian parent basalts with plagioclaseaccumulation.

Thus, calc-alkaline and high-alumina are

not synonymous, but Middlemost (1985, p. 95), Wil-son (1996) and Philpotts (1990) have considered thehigh-alumina and calc-alkaline series as essentiallythe same. These are undoubtedly very complexrocks petrogenetically. In any case, our main argu-ment standsthat neither a high-alumina nor acalc-alkaline series can be defined, or even repre-sented accurately, on a TAS diagram.

The Alkali Index versus wt% Al2O3 plot

Wilson (1996) found a plot of Alkali Index (A.I.)

vs. wt% Al2O3 (Middlemost, 1975) useful in distin-guishing tholeiitic basalts from calc-alkaline basalts(Fig. 6). She reported that very few low-K basaltsplot in the high-alumina field in this diagram, butopined that this is usually readily explained in termsof accumulation of plagioclase crystals. The majorchemical difference between the more mafic mem-bers of the typical tholeiitic and calc-alkalineseries, Wilson stated, is in their Al2O3 content: calc-alkaline basalts and andesites contain 1620%Al2O3, whereas tholeiitic ones contained only 1216% (also Philpotts, 1990, p. 100). However, thecalc-alkaline rocks themselves may owe their highAl2O3 to phenocryst accumulation, because a char-acteristic feature of island arc volcanics is theirhighly porphyritic nature, magmas of the tholeiiticseries being in general the least porphyritic (Wil-son, 1996, p. 9) and plagioclase being the most com-mon phenocryst phase (Ewart, 1982; Wilson, 1996,p. 169170). This could mean that this plot may notbe very useful in distinguishing tholeiitic basaltsfrom calc-alkaline basalts.

The AFM diagramFollowing Irvine and Baragar (1971), several

workers have used the AFM diagram to divide the

FIG. 5. Total alkalies-silica plot (Kuno, 1966) showing the

fields for his tholeiitic (TH), high-alumina (HI-AL), and

alkalic (ALK) series, into which he divided basalts from

eastern Asia.


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subalkaline rocks into calc-alkaline and tholeiiticseries on the basis of their iron contents, where A =

Na2O + K2O, F = total Fe expressed as FeO, and M =MgO (all in wt%). F approximately equals FeO +0.8998 Fe2O3 (Irvine and Baragar, 1971; Johnson etal., 1985). Authors like Pearce et al. (1975) did notrecalculate Fe2O3 as FeO but designated F = (wt%FeO + wt% Fe2O3) taken as such. Rickwood (1989)noted that this practice is unlikely to result in seri-ous misplotting, with which we do not agree becauseof error amplification in ternary diagrams (see,e.g., Butler, 1979). The proponents of the discrimi-nant lines did intend that the iron adjustment shouldbe done to compensate for oxidation. Also, all oxidesare calculated to 100% on an H2O- and CO2-freebasis (e.g., Ewart, 1982), although this anhydrousadjustment would not matter significantly.

In the AFM diagram, tholeiitic suites commonlyshow a strong trend of iron enrichment in the earlystages of differentiation, whereas calc-alkalinesuites do not undergo iron enrichment due to theearly crystallization of Fe-Ti oxides and trendstraight across the diagram toward alkali enrich-ment (Fig. 7). The tholeiitic trend is exhibited bylavas of Thingmuli volcano in Iceland, and the calc-

alkaline trend by the Cascades lavas (Carmichael,1964). The former are the so-called icelanditesandesites poorer in alumina and richer in iron than

the typical orogenic andesites (see Middlemost,1985, p. 121). The following problems exist,however.

1. The AFM diagram involves neither calciumnor alumina. A calc-alkaline or high-alumina seriesof rocks, so defined by other appropriate criteria,may be plotted on it, but the diagram cannot be usedto define a rock series as calc-alkaline. Of course, itis the lack of iron enrichment during fractionationthat is characteristic of the calc-alkaline series, andit is because of this lack of iron enrichment that acalc-alkaline series is usually shown trending

straight down across the AFM diagram. But what thediagram is really showing is the lack of iron enrich-ment, not calc-alkalinity.

FIG. 6. Plot of Alkali Index (A.I.) vs. wt% Al2O3 separating

tholeiitic basalts from high-alumina and calc-alkaline basalts

(Middlemost, 1975).

FIG . 7. The AFM diagram, showing the boundaries

between the tholeiitic (TH) and calc-alkaline (CA) fields

(heavy lines) as proposed by Kuno (1968) and Irvine and

Baragar (1971). Kunos boundary is based on Japanese rocks,

and Irvine and Baragars boundary on rocks from many loca-

tions worldwide. Kunos boundary yields a smaller area for the

tholeiite suite than Irvine and Baragars. Also shown are the

typical tholeiitic trend for lavas of Thingmuli volcano, Iceland

(Carmichael, 1964) and the typical calc-alkaline trend exhib-

ited by the average compositions of Cascades lavas (Car-

michael, 1964). Abbreviations: A = (Na2O + K2O) wt%; M =

MgO wt%. For both Kuno and Irvine and Baragar plots, F iscalculated as (FeO) + (Fe2O3 expressed as FeO) in wt%. The

coordinates for Kunos boundary are A,F,M: 72.0, 24.0, 4.0;

50.0, 39.5, 10.5; 34.5, 50.0, 15.5; 21.5, 57.0, 21.5; 16.5, 58.0,

25.5; 12.5, 55.5, 32.0; and 9.5, 50.5, 40.0. The coordinates for

Irvine and Baragars boundary are A, F, M: 58.8, 36.2, 5.0;

47.6, 42.4, 10.0; 29.6, 52.6, 17.8; 25.4, 54.6, 20.0; 21.4, 54.6,

24.0; 19.4, 52.8, 27.8; 18.9, 51.1, 30.0; 16.6, 43.4, 40.4; and

15.0, 35.0, 50.0 (from Rickwood, 1989; Rollinson, 1993).


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692 SHETH ET AL.

2. The AFM plot distinguishes intermediatemembers of the series very well, but considerableoverlap exists at the mafic and felsic ends. At theextreme felsic end there is no satisfactory way of

distinguishing calc-alkalic and tholeiitic members,and all granitic rocks are simply assigned to thecalc-alkaline series (Philpotts, 1990, p. 100), rea-sons for which are not clear.

3. Miyashiro (1974) observed that the calc-alka-line and tholeiite series do not represent two dis-crete trends of magmatic evolution but are twoartificially defined divisions of continuously vari-able and diverse trends; he felt that the use of alkalielements in the distinction could be misleading.Jensen (1976) found that the AFM diagram was

sometimes misleading in discerning rock chemicaltrends.

4. The A-F-M parameters make up less than50% of the oxide weight percentages and thereforecannot fully represent the rock chemistry (Rollin-son, 1993).

5. Only four components (Na + K, Fe, and Mg) ofthe whole rock are rounded up to 100%, leading todistortion of actual trends, and unequaldistortion forrock suites having members ranging from felsic tomafic. For example, if the volcanic rock series var-

ies in composition from basalt to dacite, about 40%of the basalt is used when plotting onto an AFMdiagram, whereas only about 15% of the dacite isused (Rollinson, 1993).

6. The problem of closure or constant sum effectresults in forced correlations (Chayes, 1960; Skala,1979; Butler, 1986; Ragland, 1989; Rollinson,1993), such that an unknown amount of the exhib-ited variability is an artifact of closure.

7. Because of percentage formation, the subcom-positions do not reflect the variations present in the

overall dataset (Aitchison, 1986; Rollinson, 1993),inasmuch as percentage formation may drasticallychange the statistical properties of the data (the rankorder of the means, variances, and correlation coef-ficients may change or reverse). This results in atleast part of any trend being artificial (Butler, 1979).

8. A series, according to widespread conven-tion, is not restricted to rocks related by simple frac-tional crystallization, or melting. Many petrologistswould use the term series for magmas havingundergone variable fractionation and mixing and

contamination. In fact, Rollinson (1993, p. 66)argued that most trends on variation diagrams arethe result ofmixing. However, trends can also existbetween completely unrelated rocks, and it is there-

fore preferable to use the terms association orsuite over series.

9. Fractionation under reducing conditions gen-erally suppresses magnetite crystallization, which

leads to iron enrichment in the early stages. Thisgenerates the tholeiitic trend. Under oxidizing con-ditions, however, magnetite crystallizes from theoutset, and quickly depletes the residual liquids iniron, which generates the calc-alkaline trend(Osborn, 1962; Miyashiro, 1974). These trends aretherefore probably more closely related to differingactivities of oxygen and water during high-levelfractionation than to any fundamental differences inthe chemistry of the parent magmas (Wilson, 1996,p. 173).

The Calc-alkaline Suite:

Tectonic Connotations

A note is in order concerning the tectonic conno-tations of the calc-alkaline suite. This suite of rocksis widespread in island arcs and active continentalmargins, with tholeiitic lavas dominating the volca-nic front, and alkaline lavas dominating the backarcregions. This has sometimes led to the identificationof ancient terrains with calc-alkaline rocks as

former subduction zones. However, there are severalregions undergoing extension and rifting wherecalc-alkaline lavas have been erupted. Theseinclude the Basin and Range province in the west-ern United States (Gans et al., 1989; Hawkesworthet al., 1995), the Gulf of California (Martin-Barajaset al., 1995; Paz Moreno and Demant, 1999), andthe Rio Grande rift (McMillan and Dungan, 1986).For this reason, caution needs to be exercised ininterpreting ancient terrains with calc-alkalinerocks as former subduction zones. We discuss below

a specific instructive modern case study from theMexican Volcanic Belt (MVB).

The MVB is a 1000 km long, E-Wtrending beltbetween Puerto Vallarta and Veracruz (Fig. 8). Itcomprises more than 8000 individual structures(Robin, 1982), including towering stratovolcanoes,calderas, domes, and monogenetic cone fields (Fig.8). Several of its large stratovolcanoes are presentlyactive (Colima, Popocatpetl), or have been histori-cally active (Ceboruco, Citlaltpetl). The SierraChichinautzin (SCN) is a volcanic field of Pleis-

tocene to Recent age (


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220 Quaternary monogenetic volcanic centers(Mrquez et al., 1999b and references therein).

The MVB has traditionally been considered as acomplex arc resulting from the subduction of theCocos plate under the North American plate (e.g.,Molnar and Sykes, 1969; Thorpe, 1977; Menard,1978; Negendank et al., 1985). The reason it hasbeen described as complex is that numerous geo-logical, geophysical, and geochemical anomalies inthe subduction scenario exist. For example, theMVB is not parallel to the Middle America trenchbut makes an angle of ~20 to it (Molnar and Sykes,1969). The Wadati-Benioff zone beneath the belt isseismically very poorly defined, and is completelyabsent beneath the central MVB (Singh and Pardo,1993; Pardo and Surez, 1995). The seismicitythroughout the MVB is shallower than 60 km, withmost earthquake foci less than 20 km deep andextensional (Suter et al., 1992; Singh and Pardo,1993). A pronounced gravity low is observed allover the MVB, and especially over its central part,which suggests a low-density, low-seismic-velocity

anomalous mantle layer at the base of the crust (~40km depth) (Molina-Garza and Urrutia-Fucugauchi,1993; Campos-Enrquez and Snchez-Zamora,

2000). The Sierra Chichinautzin monogenetic volca-nic field is located precisely in this area. Such geo-physical characteristics of the MVB as a whole arein marked contrast to what are observed alongBenioff zones like the Japan, Aleutian, Philippine,or Central American (Gill, 1981). There is also nogeochemical evidence for any role of the subductingCocos plate in the petrogenesis of either the maficmagmas or the evolved magmas at the volcanic frontin the central part of the MVB (Verma, 1999, 2000a;Velasco-Tapia and Verma, 2001), or in the LosHumeros caldera in the eastern part of the MVB(Verma, 1983, 2000b). It is due to these observa-tions contrary to the subduction explanation thatseveral workers have interpreted the MVB not as anarc but as a rift-like structure undergoing extension(see Sheth et al., 2000; Mrquez et al., 2001, andreferences therein).

Verma (2000a) analyzed major and trace ele-ments (including the rare-earth elements) and radio-genic isotopes in basalt and sediment samples fromthe Cocos plate and the accretionary wedge.

Besides, Verma (1999) and Velasco-Tapia andVerma (2001) have provided a complete geochemi-cal and Nd-Sr isotopic data set for mafic rocks from

FIG. 8. The Mexican Volcanic Belt (MVB) (shaded region) and other important geological provinces of Mexico.

Abbreviations: SMO = Sierra Madre Occidental; SMOr = Sierra Madre Oriental; SMS = Sierra Madre del Sur. Major MVB

volcanoes (filled triangles) shown are: C = Colima; Ce = Ceboruco; P = Paricutn; NV = Nevado de Toluca; Po = Popoc-

atpetl; Iz = Iztacchuatl; Ci = Citlaltpetl (Pico de Orizaba). Black box just south of Mexico City is the Sierra Chich-

inautzin monogenetic volcanic field. Modified after Verma (1999).


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694 SHETH ET AL.

the SCN. The most important geochemical charac-teristics of the SCN rocks according to Verma (1999)

are: (1) the evolved (andesitic and dacitic) rocksgenerally have lower REE concentrations than themafic rocks; (2) the evolved and mafic rocks have

similar Pb isotopic compositions, but the evolvedrocks have slightly higher 87Sr/86Sr and somewhatlower143Nd/144Nd than the mafic rocks; and (3) theevolved rocks have lower Nb concentrations and

higher Ba/Nb ratios than the mafic ones. Verma(1999) modeled the petrogenesis of the evolvedrocks and found that they could not be explained bypartial melting of the subducted slab, simple differ-entiation of mafic magmas (which would lead to, forexample, isotopic ratios being the same for bothgroups of rocks, and the REE contents being higherin the evolved rocks than the mafic rocks, the oppo-site of what is observed), or melting of the mantle inthe presence of fluids released by the subductedslab. Verma (1999) found that the partial melting of

lower continental crust and magma mixing were theonly plausible mechanisms. His main conclusionsare as follows: (1) the SCN mafic magmas were pro-duced by partial melting of a heterogeneous uppermantle; (2) the SCN evolved magmas were generatedby melting of a more heterogeneous lower continen-tal crust; and (3) mixing of mantle-derived basalticmagmas and lower crust-derived andesitic and dac-itic magmas, prior to eruption, can explain SCNlavas of intermediate compositions. Based on a par-tial melting inversion model, Velasco-Tapia and

Verma (2001) concluded that SCN mafic magmatismwas derived from partial melting of an enrichedlithospheric mantle in an extensional tectonic set-ting. They found a mantle source comparable to thatinferred from mantle xenoliths of central Mexico,with a range of 7 to 16% partial melting.

If the SCN evolved and mafic magmas are plot-ted together on a TAS diagram using Kunos (1966)classification (tholeiitic, high-alumina, andalkalic) (Fig. 9), some of them would be desig-nated high-alumina. On an Alkali Index versus wt%Al

2

O3

diagram, however, only two of the samplesplot in the high-alumina/calc-alkaline basalt field,with all the rest lying in the tholeiitic basalt field. Inthe AFM diagram (Fig. 10), however, the data forthese rocks define a typical calc-alkaline trend (cf.the Cascades trend in Fig. 7). But does this calc-alkaline trend indicate a subduction origin of theSCN and thereby the MVB? Any unrelated maficand evolved samples plotted together may form alinear trend simply by virtue of their compositions.The dominant calc-alkaline chemistry of the MVBmagmas (e.g., Verma and Aguilar-Y-Vargas, 1988)

usually has been cited as strong evidence for theirorigin from subduction (e.g., Negendank et al.,1985; Nixon et al., 1987; Suter et al., 1995). For

FIG. 9. A. TAS diagram showing the positions of lavas from

the Sierra Chichinautzin volcanic field (Mexican Volcanic

Belt). Thick continuous line is the boundary between the alka-

line and tholeiitic fields of Macdonald and Katsura (1964).

The two thick broken lines are Kunos (1966) boundaries sep-

arating alkaline (ALK), high-alumina (HI-AL), and tholeiitic

(TH) fields. Boundary lines and fields for the common volca-

nic rocks, proposed by the IUGS (e.g. Le Bas et al., 1986), are

also shown in the background. B. Plot of Alkali Index versus

wt% Al2O3. Alkali Index is defined as [wt% (Na2O + K2O)]

divided by [(wt% SiO2 43) 0.17]. Data sources for both

plots are Verma (1999, 2000a), 21 samples. All analyses have

been recalculated to 100% on a volatile-free basis, following,

for example, Ewart (1982) and iron oxidation ratios calculated

following Middlemost (1989), using SINCLAS (Verma et al.,

2002).


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example, Delgado et al. (1998) studied basalts fromXitle volcano in the Sierra Chichinautzin volcanicfield, found that these rocks plotted in the calc-alka-line field on the AFM diagram, and thereby con-

cluded that Xitle volcano lavas show calc-alkalineaffinity characteristic of subduction-related envi-ronments, and that the data do not supportVermas (2000a) suggestion for a rifting process toexplain the origin of Xitle volcano basalts due totheir calc-alkaline affinity (Delgado et al., 1998, p.128). However, we find various anomalies in thesubduction scenario sufficiently compelling, andwould instead continue to argue that calc-alkalinityis not evidence for subduction. As noted, there areseveral regions undergoing crustal extension where

calc-alkaline lavas have erupted. In fact, calc-alka-linity may not be a suitable indicator of tectonic set-ting at all, but may instead reflect the mantlesources and petrogenetic processes. Recently, Shethet al. (2000) have interpreted the calc-alkalinechemistry of the MVB magmas as simply reflectingthe process of crustal contamination experienced bymantle-derived magmas, and their fractionationmechanisms, rather than subduction (see also Mor-ris et al., 2000). Melting of the continental crust,crustal contamination of mafic magmas, and mixing

between mafic and evolved magmas are commonprocesses throughout the MVB (e.g., Nixon, 1988;Luhr, 1997; Verma, 1999, 2000b, 2001; Verma andNelson, 1989; Verma et al., 1991; Mrquez et al.,1999b). A volcanic suite cannot be characterized assubduction-related merely because it has calc-alka-line characteristics or because it includes andesitesor dacites (Mrquez et al., 1999a; Morris et al., 2000).

Ferrari and Rosas-Elguera (1999) and Ferrari etal. (2000) have defined MVB lavas with negative Nbanomalies in mantle-normalized incompatible-ele-ment diagrams as calc-alkaline, and in turn as sub-duction related. We opine that the negative Nbanomalies, like calc-alkaline geochemistry, reflectcontamination of mantle-derived magmas by gra-nitic or granulitic continental crust depleted in Nband similar high-field-strength elements (e.g., Ta,Ti), as shown by Verma (1999, 2000a, 2000b, 2001)for several areas of the MVB. Negative Nb anoma-lies are widespread in lavas contaminated by conti-nental crust, and are shown not only by arc lavas butalso by continental flood basalt lavas, such as thoseof the Deccan Traps, India (e.g., Mahoney et al.,

2000) and the Central Atlantic Magmatic Provincecomprising parts of South America, North America,Africa, and Europe (Marzoli et al., 1999).

Calc-alkaline and High-Alumina Rocks

on the TAS Diagram

We return to problems related to the depiction ofcalc-alkaline rocks on geochemical variation dia-grams. Ragland (1989) discussed the current con-fused state of nomenclature of igneous rock series.Discussing various classification schemes, whichtend to classify the same rock as belonging to whollydifferent series, he asked (p. 326), Must a rock ful-fill all these various criteria to be classified as beingin a particular series? If this were true, a consider-able number of rocks could never be placed in anyseries. As an example, Morrison (1980) found thatshoshonitic rocks usually plot in the alkaline fieldon the TAS diagram but in the calc-alkaline fieldon the AFM diagram.

Macdonald and Katsura (1964), who used theTAS diagram to classify basaltic rocks in Hawaii,proposed only two suites, tholeiitic and alkalic.As Ragland (1989, p. 323) noted, the boundarybetween Kunos high-alumina and alkaline

basalts is quite close to the boundary between Mac-donald and Katsuras tholeiitic and alkalinebasalts, which results in mafic rocks of a certain

FIG. 10. The AFM diagram with Irvine and Baragars

(1971) boundary between the tholeiitic (TH) and calc-alkaline

(CA) fields, with data for lavas of the Sierra Chichinautzin vol-

canic field (Mexican Volcanic Belt). Data sources are the same

as for Figure 9. All analyses recalculated to 100% on an H2O-

and CO2-free basis. Abbreviations: A = (Na2O + K2O); F =

FeO* = total Fe as FeO; and M = MgO, all in wt%. Total Fe as

FeO = FeO + 0.8998 Fe2O3 (following Irvine and Baragar,

1971; Johnson et al., 1985; see Ragland, 1989; Rickwood,

1989).


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696 SHETH ET AL.

composition being classified as tholeiitic by Mac-donald and Katsuras diagram, but as high-aluminaby Kunos diagram. Ragland asked which of thesetwo is correct, answering that Kunos scheme is pref-

erable if one is working on subduction-relatedbasaltic rocks at a convergent margin, but Mac-donald and Katsuras scheme is preferable if one isworking on intraplate or ocean-ridge basalts.

We ask, therefore, which scheme should be usedwhen one is studying ancient rocks whose tectonicsetting is unknown? Or modern rocks in a complextectonic setting? The use of one diagram or the otherin that case would lead to widely varying terminol-ogy, and if the objective were to infer the tectonicsetting based on the geochemical characteristics,

two tectonic settings not even remotely resemblingeach other would be inferred for the same rock: thetholeiitic classification might lead to the inferenceof an ocean-ridge or marginal-basin setting, and thehigh-alumina classification for the same rock wouldlead to the inference of an island-arc setting. Whichof these two would be correct? For that matter, theBushveld Layered Complex of South Africa andsome other igneous masses contain some rocks thatshow a trend towards iron enrichment, and othersthat do not (Middlemost, 1985, p. 118)what would

be the tectonic setting of the Bushveld Complexbased on this? As noted earlier, subalkalic basalticmagmas might be identified on the basis of either ofthese trends to be a result of, for example, local dif-ferences in oxygen fugacity of the parent magma(Middlemost, 1985, p. 118) and different fraction-ation assemblages (Rogers and Hawkesworth,1999).

The above confusion is completely unwarranted,and the solution is as follows. The Macdonald andKatsura classification is the correct one in this case,

and the depiction of the high-alumina series onthe TAS diagram, following Kuno (1966), is incor-rect in the first place, so his classification should notbe used, and there is no paradox regarding whetherthese mafic rocks are tholeiitic or high-alumina.With the correct classification, they are tholeiiticand only tholeiitic. We emphasize once again, thatKuno plotted his high-alumina series between thealkalic and tholeiitic series because the high-alu-mina series had intermediate alkali contentsbetween the other two. Although the real name for

this intermediate group of rocks would probably betransitional, Kuno was led to call it high-alu-mina because of their relatively high alumina con-tents. The cause of the entire problem is obvious

namely, the high-alumina series was depicted on,and defined with, the TAS diagram, a diagram onwhich it cannot strictly be defined or represented.

An exampleRagland (1989, p. 329) provided an example of

the current confusion. The basaltic rock with the fol-lowing composition (his Table 1.1)wt% SiO248.70, TiO2 1.29, Al2O3 16.60, Fe2O3 2.05, FeO8.29, MnO 0.16, MgO 6.63, CaO 10.70, Na2O 2.83,K2O 0.47, H2O

+ 0.81, H2O 0.67, P2O5 0.20, CO2

0.09, and Total 99.49is classified into differentrock series on the basis of different classifications,as follows: high-alumina (TAS plot, Kuno, 1966),tholeiitic (TAS plot, Irvine and Baragar, 1971),

tholeiitic (TAS plot, Macdonald and Katsura, 1964),tholeiitic (K2O-SiO2 plot, Gill, 1970), calc-alkaline(AFM plot, Kuno, 1968), tholeiitic (FeO*/MgO ver-sus SiO2 plot, Miyashiro, 1978), subalkaline (amolecular norm plot, Irvine and Baragar, 1971),tholeiitic (Al2O3 versus plagioclase compositionplot, Irvine and Baragar, 1971), and tholeiitic (AFMplot, Irvine and Baragar, 1971). Thus variousschemes classify this single rock sample variably,and according to Ragland (1989), the rock wouldhave to be elected tholeiitic by a 6-2 vote. But this is

no solution; more diagrams with other classificationcriteria, which would classify this rock as calc-alka-line, could easily be constructed by aficionadosof that term. Thus, the rock could easily be madetholeiitic or calc-alkaline by devotees of one classi-fication scheme or the other.

We think that the aforementioned rock is indeedtholeiitic, or rather subalkaline, for another, stron-ger, simpler reason. The depiction of high-aluminabasalts being inappropriate on the TAS plot, theclassification of this rock as high-alumina based on

Kunos (1966) TAS plot is incorrect. Likewise, thedepiction of calc-alkaline basalts being inappropri-ate on the AFM diagram, the classification of theabove-listed rock as calc-alkaline based on KunosAFM plot is also incorrect. The rest of the diagramsare unanimous (tholeiitic is also subalkaline).

Therefore, we conclude that it is the use of inap-propriate geochemical discrimination diagrams thatis the cause of particular rock suites, or even singlerocks, being classified into widely different associa-tions. To avoid all the unwarranted confusion, we

suggest the use of simple rock names (basalt, andes-ite, etc.) defined by the IUGS Subcommission on theSystematics of Igneous Rocks (Le Maitre, 1984; LeBas et al., 1986; Le Bas, 1989; Le Bas, 2000) based


12/16


on the TAS diagram. An inherent feature of thisclassification, attractive in the present context, isthat it is non-genetic. The classification is purelydescriptive, independent of field location or field

association, except that the rock in question is vol-canic. To these rock names can be attached thequalifier terms alkalic and subalkalic (e.g.,alkalic basalt), but not calc-alkaline or high-alu-mina. Verma et al. (2002) have developed a com-puter program SINCLAS that classifies volcanicrocks on the basis of their total alkalies-silica con-tents and iron oxidation ratios, provides rock androot names (following Le Bas et al., 1986; Le Bas,1989, 2000), and also calculates CIPW norms.

Conclusions

Since originally defined in 1931, the term calc-alkaline has come to mean various things to variouspeople. It is variably used for (1) island arc rocks,(2) andesites, (3) rocks with high LILE/HFSE ratios,(4) rocks with negative Nb anomalies, etc. This vari-able usage may not be problematic as long as eachpetrologist employing the term specifies his or herbasis for using it. However, calc-alkaline rocks mustbe identified with, or depicted on, appropriategeochemical variation diagrams, and neither theTAS diagram nor the AFM diagram seem suitablefor this, although both have been used extensively(the AFM particularly so). The TAS diagram canclassify rock suites into alkaline, subalkaline,and transitional, but not calc-alkaline norhigh-alumina. The confused situation in whicheven a single rock can be classified into completelydifferent series using different diagrams (Ragland,1989, pp. 320329), is largely due to some of thesediagrams being irrelevant and unsuitable for defin-ing certain rock suites. Association and suiteare terms preferable to series, because they neednot imply comagmatic relationships, and need notrequire linear trends on geochemical variation dia-grams. A calc-alkaline suite of rocks, even whenidentified with the right criteria, should have no tec-tonic connotations. Not all orogenic andesites arecalc-alkaline, and not all calc-alkaline andesites areorogenic.

Does the term calc-alkaline deserve expunge-

ment from the petrological literature? Perhaps itdoes. Chayes (1964) gave pertinent reasons why theterm tholeiite should be expunged from the liter-ature, and substituted by subalkaline. It is rele-

vant to recall that the IUGS Subcommission onSystematics of Igneous Rocks (Le Bas et al., 1986;Le Bas, 2000) has eliminated both the terms calc-alkaline and tholeiite, and apparently replaced

them by subalkaline. Use of a computer programsuch as SINCLAS (Verma et al., 2002) could facili-tate automatic rock nomenclature, and thus reducethe present confusion prevailing in geologicalliterature.

Acknowledgments

This work was partly supported by DGAPA-UNAM project IN-106199. The manuscript waswritten while H.C. Sheth was supported by a post-

doctoral fellowship from CIE-UNAM, and muchrevised and improved while he was supported by aSOEST Young Investigator award of the Universityof Hawaii.

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