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Home , Alkali feldspar granite, Foidolite, QAPF diagram, TAS classification

EARTH SCIENCE

REVIEWS ELSEVIER Earth-Science Reviews 37 (1994) 215-224

Naming materials in the magma/ system

Eric A.K. Middlemost Department of Geology and Geophysics, Unit,ersity of Sydney, NSW 2006, Australia Received 22 June, 1993; revised and accepted 25 August, 1994

Abstract

The main aim of igneous petrology is to develop a complete specification of the magma/igneous rock system. This paper is a sequel to an earlier essay (Middlemost, 1991) on the classification of igneous rocks and magmas. It explores the ways and means of developing a single, consistent method of naming all igneous materials. All modal classifications are fettered by problems arising from heteromorphism, extremes in grain size, and the presence of glass. Only chemical parameters can provide a reliable and straightforward method of classifying all the common igneous rocks and their magmas. This is undoubtedly true of glassy rocks and magmas. The potential of the TAS diagram in this new and enlarged role is evaluated, resulting in a modification of some boundaries recommended for the volcanic rocks. A new comprehensive chemical classification of the plutonic rocks is introduced. To keep it and the volcanic classification in tandem, several new terms are proposed. They include gabbroic as a plutonic equivalent of basaltic and peridotgabbro as a plutonic equivalent of picrobasalt. is defined as the plutonic equivalent of picrite and by taking the idea of equivalence a step further, it is defined as a peridotgabbroic or gabbroic rock that contains more than 18% MgO and less than 2% total alkalis. The picrobasaltic and basaltic rocks that contained more than 18% MgO and more than 2% (Na20 + K20) are called alkalic picrites. Their plutonic equivalents are named alkalic . A benefit of this new chemical classification of plutonic rocks is that it enables one to avoid the awkward term ultramafic. A single classification that links magmas, plutonic and volcanic rocks should be appreciated by all geochemists and petrologists who amass, and manipulate, large geochemical databases but are unwilling, or unable, to carry out quantitative modal analyses. This classification also enables geoscientists to focus on magma -- the most important concept in igneous petrology.

1. Introduction nately, the lUGS Subcommission on the System- atics of Igneous Rocks has spent over 20 years The primary aim of igneous petrology is to considering the various problems of igneous rock provide a full specification of the magma/igneous nomenclature (Le Maitre et al., 1989). Their cogi- rock system that operates in and on the surface of tations and recommendations are a valuable re- the Earth. Research into how this intricate sys- source, but these data need to be evaluated and tem functions is presently unnecessarily impeded winnowed before they can provide a systematic by the absence of a single, consistent method of set of names for the common magmas and ig- naming the various magmas and rocks that trans- neous rocks. An important reason why the Sub- fer heat and materials within this system. Fortu- commission's recommendations appear to be

0012-8252/94/$29.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-8252(94)00052-2 216 E.A.K. Middlemost / Earth-Science Re~iews 37 (l 994) 215-224 cumbersome is that their deliberations were methods are refined in the future, the presence guided by nine principles (Le Maitre et al., 1989, of glass will always restrict the use of modal data pp. 2-4; Le Bas and Streckeisen, 1991). These in the classification of many volcanic rocks. Seri- principles not only set their agenda, they also ous problems also arise in determining the modes seem to have ossified some of their deliberations of coarse-grained rocks and rocks with granitic, and conclusions. The Subcommission affirmed poikilitic, banded and directive textures. Chayes that the igneous rocks include a variety of "ig- (1956, p. 1) observed that "the number of reliable neous-looking" materials produced by deuteric, modes obtained by measurements made on pol- metasomatic and even metamorphic processes ished slabs is almost vanishingly small." In mod- and they also proposed that the primary classifi- ern research into the origin and evolution of cation of igneous rocks should be based on their coarse-grained rocks, the trend appears to be content or mode. If petrologists wish to away from quantitative petrographic modal analy- preserve the Subcommission's all-embracing defi- sis, because this process requires "tedious slab- nition of igneous rocks it would be appropriate to bing, messy staining, and mindless point-count- use the expression magmatic rocks to describe ing" (Clarke, 1992, p. 6). Contemporary research those rocks that solidify from a hot, molten, or papers in igneous petrology are much more likely partly molten, condition. This definition hinges to contain major element data than accurately on the inexact term hot. There is clearly a need determined modes. Perhaps in the future re- to assign a lower limit to the temperatures at- search practice will swing the other way as there tained by normal magmatic processes. The singu- is now a growing demand for accurately deter- lar natrocarbonatite lavas of Oldoinyo Lengai mined modes for the development of effective Volcano in Tanzania are reported to erupt at major and trace element models that simulate temperatures as low as 580°C (Dawson, 1989, p. dynamic magma systems. 269). Sulphur flows erupt at even lower tempera- Volcanic and plutonic modal classifications can tures and cryovolcanism, or ice volcanism, has never be identical because such as leucite been described from some of the icy satellites of and pigeonite are characteristic of some volcanic Jupiter and Neptune. A temperature of 650°C rocks, but are unstable under plutonic (sensu would seem an apt lower limit for normal silicate stricto) conditions. The main reason some petrog- magmatic activity. raphers support the retention of the hypabyssal Many problems are encountered when carry- class of igneous rocks is that the textures and ing out petrographic modal analyses. They arise modes of these rocks often differ from those of because of extremes in grain size, complicated comagmatic volcanic or plutonic rocks. In present textures, presence of megacrysts, heteromor- day regional geology there is a tendency to down- phism and the unwillingness of many geoscien- grade the abundance of hypabyssal rocks. For tists to undertake quantitative modal analyses. In example, hypabyssal microgranite is frequently his statistical appraisal of petrographic modal labelled . Rittmann (1973, p. 72) examined analysis Chayes (1956) showed that it is not feasi- the impact of heteromorphism on the modal clas- ble to determine the modal composition of ig- sifications of igneous rocks by classifying various neous rocks of all grain sizes and textures. The pairs of igneous rocks that had similar chemical principal reason for introducing the TAS classifi- compositions but different mineral assemblages. cation was that petrographers were unable to He showed that each rock in these pairs may plot determine the modes of rocks with glassy, or in in a different field on the QAPF diagram. Het- some samples fine-grained, mesostases. At pre- eromorphism is found even among granitic rocks. sent some of the problems created by fine-grained Tuttle and Bowen (1958, pp. 127-129) showed materials can be solved by the application of that samples of these quintessentiaIly plutonic image analysis software to back-scattered elec- rocks may have similar chemical compositions, tron images, and the use of quantitative X-ray but dissimilar modal compositions. Heteromor- powder diffraction methods. Howsoever these phism of this type is usually observed in rocks E.A.K. Middlemost ~Earth-Science Reviews 37 (1994) 215-224 217 that have experienced different cooling histories. normative kalsilite. Many other common If, for example, most of the sodium and potas- minerals contain one, or more, of the normative sium in a hypersolvus granite is distributed into minerals , orthoclase, albite, anorthite, alkali , such a rock may have the modal leucite, and/or kalsilite. The amphi- composition of an ; but in a boles, particularly the calcic , gener- subsolvus granite of the same composition most ally contain normative QAPF minerals. Edenite, of the sodium may join with calcium to form pargasite, magnesio-hastingsite, hastingsite and , and most of the potassium may con- the various types of katophorites, all contain more centrate in biotite thus giving the rock a granodi- than 10% normative nepheline and some norma- oritic mode. The pair andesite/diorite is another tive plagioclase. The pure edenite component excellent example of heteromorphism. These contains 7.9% normative albite and 12.7% nor- rocks usually have similar chemical compositions, mative nepheline, whereas the pure ferro-edenite but frequently contain quite different mafic min- component contains 27.4% normative albite and erals. The high temperature, quickly cooled an- 12.6% normative quartz. It must be emphasised desites typically contain , whereas the that components such as these occur in normal plutonic invariably contain mafic minerals. A typical augite analysis from and/or biotite. The diorites tend to contain Deer et al. (1992, p. 152, No. 8) contains 7.0% quartz, whereas the equivalent excess of silica is normative plagioclase, and a typical biotite analy- usually concealed in the mesostases of the an- sis (p. 285, No. 6) contains 35.3% normative desites. The modal classification of the abundant leucite, 5.9% normative kalsilite and 1.9% nor- mid-oceanic ridge also poses problems mative nepheline. because they have what appear to be vitrophyric textures, but according to Natland (1991, p. 65) they often contain megacrysts that are out of 2. History of the TAS diagram equilibrium with their quickly cooled mesostases. Heteromorphism is essentially what sets the In 1984 the IUGS Subcommission (Zanettin, charnockitic rocks apart from the main sequence 1984) introduced their total alkali-silica, or TAS, of plutonic rocks; and in the lamprophyric rocks classification of volcanic rocks and recommended heteromorphism confounds all simple classifica- that it should only be used to classify volcanic tions. Rittmann (1973) and Streckeisen and Le rocks unsuited to petrographic modal analysis. Maitre (1979) have tried to resolve the problem The long history of the TAS diagram has been of heteromorphism. It is concluded that hetero- recounted by Le Bas et al. (1992). In the 1950's it morphism will always remain a formidable prob- was used to separate the Hawaiian volcanic rocks lem in modal classifications of igneous rock. This into tholeiitic and alkalic suites (Tilley, 1950, pp. statement should not be interpreted as insinuat- 41, 42; Kuno et al., 1957, p. 203; Macdonald and ing that modes are of secondary importance, be- Katsura, 1964, p. 87). Tomkeieff (1953, p. 24) cause in the global magma/igneous rock system promoted it as a valuable aid to classification it is, for example, essential to know whether a because "the relative amount of alkali determines rock is a or an eclogite. the suite (per-alkalic, alkalic, etc.) while the rela- Another problem that needs to be unravelled tive amount of silica determines the class (basic, concerns those igneous rocks that contain minor acid, etc.)." The merits of the TAS diagram were amounts of foids in their norms, but not in their affirmed by Rittmann (1962, p. 109) who com- modes. This paradox is typical of many monzo- bined it with his ingenious sigma index (1957, p. gabbros, monzodiorites, and . 34). This index is equal to (Na20 + K20)2/(SIO2 It is readily understood if one calculates the - 43), and when combined with the TAS diagram norms of the mafic minerals found in these rocks. it provides an elegant method of defining the Phlogopite, for example, contains 52.9% norma- principal igneous lineages. Rocks of the subalka- tive olivine, 27.3% normative leucite and 19.8% line suite have sigma values of less than 2.5, 218 E.A. K Middlemost / Earth-Science Reviews 37 (1994) 215-224 sigma values of between 2.5 and 10 define the TAS diagram and those defined by the QAPF midalkaline suite, whilst the rocks of the normal diagram? (3) What are the normal petrogenetic alkaline suite have sigma values ranging from 10 relationships between data-points in adjoining to 25. In 1964 Saggerson and Williams (p. 75) fields on the TAS diagram? (4) Would present used a TAS diagram to separate the nepheline- day igneous petrology benefit from the introduc- bearing from the nepheline-free volcanic rocks of tion of a chemical classification of plutonic rocks northern Tanzania. Later in 1965 Kuno (p. 310) based on the TAS diagram? attempted to used a TAS diagram to separate the Igneous rocks occupy a restricted area within tholeiitic, high-alumina and alkalic suites of rocks. the whole TAS diagram [i.e. SiO 2 0%-100%, Irvine and Baragar (1971, p. 532) also used the (Na20 + K20) 0%-100%]. This is just as one TAS diagram for separating the alkalic from the would expect because igneous rocks are princi- subalkalic rocks. In the 1970's and early 1980's pally composed of silicate minerals, and the com- several new classifications of the volcanic rocks positions of all these minerals lie within the area were introduced that used the TAS diagram bound by the minerals kalsilite-fayalite-quartz (Frolova and Petrova, 1972; Middlemost, 1972; (see Fig. 1). There is a need to set precise limits Rumyantseva, 1977; Cox et al., 1979; Middlemost, on the fields occupied by the and 1980 and Efremova and Stafeev, 1985). Between , because these rocks are the "attrac- 1984 (Zanettin, p. 19) and 1986 (Le Bas et al., p. tors", or terminuses, in petrogeny's residua sys- 745) slight modifications were made to the lUGS tem (Bowen, 1937). To define them more closely Subcommission's TAS classification of volcanic one has to investigate the petrographic signifi- rocks. cance of the high-silica and high-alkali oxides areas on the TAS diagram. Fig. 1 is an extended TAS diagram that contains the data points for 3. TAS diagram explored several minerals, such as quartz (Qz), orthoclase (Or), leucite (Lc) and kalsilite (Ks). The straight Now that the TAS diagram is received as a line Ks-Qz passes through Or and Lc and de- valid, and valuable, aid in the classification of fines the outer high-silica and high-alkali oxides volcanic rocks, it is important to evaluate it by boundary for igneous rocks on the TAS diagram. considering four interrelated questions. (1) What Rocks that plot on this line would contain only types of igneous rocks, if any, occupy the outer these phases and no mafic minerals. The Ks-Qz areas of the TAS diagram away from the fields line places an outer limit on the field and that have been formally demarcated? (2) What is this boundary can be more closely defined as the relationship between the rocks defined by the occurring between Or40Qz60 and OrsoQz2o. Fig. 1

3(). Ks 25 No -~~

2O Na2Os-K20 15 10 5- Fa 0 ; : t i i -.~,.~z 2O 30 40An 50 60 70 80 90 100 SiO2 Fig. 1. A total alkali versus silica (TAS) diagram showing the positions of the minerals, kalsilite (Ks), nosean (No), fayalite (Fa), nepheline [Ne i.e. (Na3K)AISiO4], anorthite (An), leucite (Lc), orthoclase (Or), albite (Ab) and quartz (Qz). The triangle Ks-Fa-Qz encloses all the rock-forming silicate minerals. E~A.K. Middlemost / Earth-Science Ret,iews 37 (1994) 215-224 219 also provides a rudimentary illustration of the 18. relationship between the QAPF and the TAS 16' systems of classification, where Q = Qz, A = the 14, triangle Or (orthoclase)-Ab (albite)-Abg0Anm, 12, ! F = the triangle Ks-No (nosean)-Lc and P = the 10' Na20+K20 line An (anorthite)-Abg0Anl0 . If NaA1SiO 4 (co- 8' ordinates 42.3 silica and 21.8 total alkalis) and 6' not (Na3K)A1SiO 4 is used to define nepheline 4, point in Fig. 1 then F is represented by the 2 quadrilateral Ks-No-Ne-Lc. When comparing 0 I the QAPF and TAS diagrams it is immediately 30 40 50 60 70 80 90 apparent that field one (> 60% Q) on the QAPF SiO2 Fig. 2. Frequency distribution diagrams for phonolites (Ph), diagram (Le Maitre et al., 1989, p. 15) can be nephelinites (Fn) and leucitites (F/), after Le Bas et al. (1992, readily transposed onto the TAS diagram where figs. 18, 22 and 23), superimposed on the TAS chemical it defines the outer limit to the rhyolite/granite classification of igneous rocks and their magmas as proposed field. On the QAPF plutonic diagram field one is in this paper. shown as containing the and the quartz-rich granitoids, whereas this field remains cedure for classifying magmas. It is considered unnamed on the QAPF volcanic diagram (Le apposite for the magmas of the main sequence Maitre et al., 1989, p. 23). One might call all fine rocks to be classified using the TAS diagram and grained rock that plots in this field silexite (after given the same names as those of the common Miller, 1919, p. 30), however the Subcommission volcanic rocks (e.g. picrobasalt magma or rhyolite is probably correct in implying that there are no magma). known pristine extrusive rocks of this composition The IUGS Subcommission on the Systematics on Earth. of Igneous Rocks (Le Bas et al., 1986) uses the Foidite is the group name (Streckeisen, 1965) presence or absence of normative nepheline to for those volcanic rocks that contain more than subdivide the basalts into alkali or subalkali vari- 60% foids in the QAPF modal system of classifi- eties, it sets a limit of 10% normative olivine as cation. In earlier classifications most of these the boundary between and tephrite, and rocks would have been called either leucitites or nephelinites. When the silica and total alkali ox- ides values of the latter rocks are plotted on a Sodalitite / Nephelinolith / Leucilolilh TAS diagram they fill a broad area that extends 18- from the foidite field into the adjacent tephrite 16" and fields. Fig. 2 designates the 14 fields that contain 90% of the rocks that were 12 10" originally named nephelinite (Fn), leucitite (F1) N~I20+K20 and (Ph) in the Computerised Library 8: of Analysed Igneous Rocks (CLARE) database (Le Bas et al., 1992, pp. 12 and 18). This figure enables one to establish boundaries for the foidite exite and foidolite fields in the low silica and high 30 40 50 60 70 80 90 alkali oxides areas of the TAS diagram (Figs. 3 SiO2 and 4), and it also helps one set limits on the Fig. 3. A modified chemical classification and nomenclature phonolite and foid field in Figs. 3 and 4. of volcanic rocks and magmas using the total alkali versus A noteworthy advantage of a simple chemical silica (TAS) diagram (mainly after Le Bas et al., 1986, fig. 2). Note the trachydacite field between , and rhyo- classification of igneous rocks, such as the TAS lite. The sodalitites, nephelinoliths, leucitoliths and silexites classification, is that it provides a convenient pro- are outside the main sequence of volcanic rocks. 220 E.A.K. Middlemost /Earth-Science Reviews 37 (1994) 215-224

Tawile / Unite / ltalite to take great care to avoid computing chemical I8 parameters that magnify the changes introduced 1b Foid 14 Syenite S by weathering, hydrothermal alteration or sample

ite preparation for chemical analysis. Unfortunately 12 the CIPW norm calculation often magnifies mi- 10 luartz ~;~2()+K?O nor changes in the abundance of Na, K and/or 8 Vlonzonite onzo Granite -diorite Ca (cf. Ragland, 1989, p. 56). If one wishes to 6 Monzogabbm avoid using normative minerals in classification, 4 . iorite 3ranodiorile one should examine the use of major oxides data. 02 ~Pcridotgal~brI )tgal~br~ l Diorile~i°fitel , zolite For example, instead of using normative olivine I i I I I ) 41) 50 60 70 80 90 for separating basanite from tephrite, one could SiO2 use MgO to get a similar separation of these Fig. 4. Proposed chemical classification and nomenclature of rocks; that is, the contain 8% or more plutonic rocks using the total alkali versus silica (TAS) dia- gram. Note the quartz field located between gran- MgO, whereas the tephrites contain less than 8% ite and syenite. The tawites, urtites, italites and quartzolites MgO (cf. Le Maitre 1984, p. 254). As Macdonald are outside the main sequence of plutonic rocks. and Katsura (1964) have shown, the TAS diagram can be used to separate the alkalic basalts from the subalkalic basalts. In Fig. 5 this is done by using a straight line that approximately follows a value of 20% normative quartz to separate the the 2% normative foid isopleth (Le Bas, 1992, p. trachydacites from the . Difficulties 10). The coordinates of this line are 52% SiO2, arises when CIPW norms are used in classifica- 5% (Na20 + K20) and 45% SiO2, 2% (Na20 + tion. This is mainly because there is no standard, K20). valid method of carrying out this calculation or of It is instructive to plot the lunar volcanic rocks adjusting Fe(III)/Fe(II) ratios. Petrological liter- on the TAS diagram. Mare lavas, from the Apollo ature contains many incorrect norms and norm 12, Apollo 15, Luna 16 and Luna 24 sites, all plot calculating programmes that generate bizarre to- within the picrobasalt and basalt fields, whereas tals (Yoshii and Hirano, 1977; Fears, 1985; Mid- mare lavas from the Apollo 11 and Apollo 17 dlemost, 1989). When manipulating chemical data sites usually fall outside these fields (Heiken et from rocks that are even slightly altered one has al., 1991, p. 449). These aberrant lavas contain

52.5,18 57,18 18 16 14 14

12 ~8.4,1~ 10 3711 Na20+K20 6~ 8 35,9 ~4]4~ :,9.~ ¢ 98 \85.9,6.8 6 77~ 87"5'4"7

30 40 50 60 70 80 90 SiO2

The pairs of numbers are the coordinales of the intersections of the lines. Fig. 5. The coordinates (silica:total alkalis) of the intersections of the lines on the proposed TAS classification of igneous rocks and magmas, where Ba =alkalic basalt/alkalic gabbro, Bs=subalkalic basalt/subalkalic gabbro and Td=trachydacite (dacitic trachyte)/quartz monzonite. E.A.K. Middlemost ~Earth-Science Reuiews 37 (1994) 215-224 221 less than 41% SiO 2 and more than 8% TiO 2. On petrologists would regard the QAPF diagram as a Earth iron-titanium oxide ores are usually found modification of petrogeny's residua system in close association with anorthosites and other (NaA1SiO4-KA1SiO4-SiO2; Bowen, 1937). In layered cumulate rocks (Stanton, 1972, pp. 371- both diagrams the silica oversaturated and silica 386). Anorthosites are abundant on the Moon saturated fields are much the same, but in the and usually contain less than 0.15% TiO 2. This QAPF diagram all the folds of markedly different implies that if the lunar anorthosites evolved from chemical compositions are grouped as a single picrobasaltic, or basaltic, magma they must have end member (F). shed considerable amounts of TiO 2 probably as By itself the QAPF classification is unable to armalcolite or . The aberrant Ti-rich lavas classify the rocks in the field surrounding the P probably evolved from a magma contaminated by (plagioclase) end member. This field contains all reaction with, or incorporation of, cumulate rocks the common primitive (i.e. least differentiated) with a high-Ti content. As cumulate rocks, and rocks, such as picrite, picrobasalt, basalt and some cumulate contaminated rocks, are not normal of the rocks called basanite, , main sequence igneous rocks, it is not appropri- andesite, (hawaiite and potassic tra- ate to use the TAS diagram to classify these chybasalt) and basaltic (mugearite rocks. If one wished to assume a neutral position and shoshonite). These rocks congregate in this on the origin of these lavas one might regard all field because the anorthite component of plagio- volcanic rocks that contain less than 52% SiO 2 clase is the only primitive component in the QAPF and more than 7.5% TiO 2 and more than 16% system. In the CLARE database of chemical total iron as FeO, as special volcanic rocks. Such analyses of volcanic rocks used by the Subcom- rocks might be called apolloites. This discussion mission (Le Bas, 1992, p. 8) 73.9% of the rocks in steers ones attention to the problem of the classi- the database plot in the immediate field that fication of the cumulates and it is indisputable borders P, and thus cannot be successfully classi- that the simplest way of classifying rocks of this fied on the QAPF diagram. The TAS diagram is special group is by using modal criteria. the complete antithesis of this as it can easily classify all the common basic to intermediate igneous rocks. This is because the TAS diagram 4. QAPF diagram evaluated is an important variation diagram that readily portrays the normal liquid lines of descent of the The QAPF and TAS classifications might be common igneous rocks. These liquid lines of de- expected to have little in common, because (1) scent follow remarkably stable pathways and con- the TAS classification is essentially a type of nect the primal picrobasalt-basalt domain with ternary diagram with SiO2, (Na20 + K20) and the rhyolite and phonolite terminuses identified (TiO 2 + AlzO 3 q- Fe203 + FeO + MnO + MgO in petrogeny's residua system (see Fig. 6). This is + CaO + P205 ) as the end members, (2) the mafic not meant to imply that the rate of down-temper- and related minerals, and thus the oxides TiO2, ature fractionation portrayed on a TAS diagram Fe20 3, FeO, MnO, MgO and P205, are disre- is constant. For example, during the early stages garded in the QAPF classification and (3) many in the fractional crystallisation of tholeiitic liquids minerals discarded by the QAPF classification the silica content may increase very slowly (Wright are, as has already been noted, covert hosts to and Peck, 1978, p. 18), or even decrease, after variable amounts of the components found in and plagioclase have begun to crys- QAPF phases. The reason the QAPF diagram tallise. This is because the silica contents of these functions as a practical modal classification of phases are similar to, or greater than, their host igneous rocks is that it separates the silica over- magma. In the alkali basalts large volumes of saturated from the silica undersaturated rocks, olivine usually crystallise at an early stage and and in a more equivocal way it separates the this promotes a somewhat rapid rate of down- differentiated rocks from their antecedents. Some temperature fractionation on the TAS diagram. 222 E~4.K. Middlemost ~Earth-Science Reviews 37 (1994) 215-224

5. TAS classification of plutonic rocks and volcanic classifications in tandem, this field has also been tentatively introduced into the vol- canic classification where it may be called either Perhaps the classification of plutonic rocks the dacitic trachyte or trachydacite field (Fig. 3). needs to be approached in a new way (cf. Wilson, The names that have been given to the plutonic 1989, p. 8). A logical first step would be to apply rocks that occupy the same fields on the TAS the successful TAS classification of volcanic rocks diagram as trachybasalt, basaltic trachyandesite to the common plutonic rocks (cf., Nockolds, and trachyandesite, are monzogabbro, monzodi- 1954; Le Maitre, 1976), and see what happens orite and monzonite, respectively. In the silica (see Fig. 4). When this is done one discovers, as undersaturated fields the plutonic equivalents to one might expect, that the following pairs of tephrite, phonotephrite and have rocks, (1) gabbro and basalt, (2) diorite and an- been named foid gabbro, foid monzodiorite and desite, (3) and dacite, (4) foid syen- foid monzosyenite. It is not proposed to ignore ite and phonolite, (5) syenite and trachyte and (6) the textural and modal data that can be acquired granite and rhyolite, plot in the same fields. from these rocks as such data may provide invalu- is particularly interesting because it plots able information about the pressure/temperature astride the 63% silica boundary-line on the TAS environment in which they congealed/ crystal- classification of volcanic rocks. When using a lised. Detailed data on the chemical composition TAS classification of the plutonic rocks one can of the essential phases also may provide decisive either make a special compartment for tonalite petrogenetic information. between 61% and 65% silica, or one can omit Many plutonic rocks of the peridotite group tonalite, and let the classification pass directly (e.g. harzburgite) occupy the picrobasalt field, but from diorite to granodiorite. In the plutonic rocks they also overlap into the low-alkali part of the there is no direct equivalent of basaltic andesite, basalt field (e.g. lherzolite), or into the foidite but it would be apposite to call a plutonic rock of field (i.e. dunite). It is proposed to use the name this composition a gabbroic diorite. A new field peridotgabbro for plutonic rocks that occupy the has been introduced for the quartz monzonites. It same field as the picrobasalts, irrespective of is situated between granite, granodiorite, syenite whether they contain modal plagioclase or not. and monzonite (see Fig. 4). To keep the plutonic Peridotite is regarded as the plutonic equivalent of picrite; taking the idea of equivalence one step further, peridotite may be defined as a peridot- gabbroic or gabbroic rock that contains more 18~ than 18% MgO and less than 2% total alkali 16. i metals. Bristow (1984, p. 117) studied the picritic 14, rocks of the extensive Karoo Petrographic

12- f Province of southern Africa and reported that

10~ many of these rocks contain more than 2% Na20+K20 84 (Na20 + K20). It is thus suggested that the pi- 6J crobasaltic and basaltic rocks that contain > 18% MgO and > 2% (Na20 + K20) should be called 4 ! it, alkalic picrites. The plutonic equivalent of alkalic 2 picrite would be alkalic peridotite. A benefit of 0 I 30 40 50 60 70 80 90 using chemical criteria to classify plutonic rocks is SiO2 that it enables one to avoid using the awkward Fig. 6. A TAS diagram showing the approximate traces of the and equivocal term ultramafic, as this term in- normal liquid lines of descent of the common magmas and the cludes rocks of remarkably diverse chemical com- positions of the rhyolite and phonolite terminuses. Data used to plot the liquid lines of descent was obtained from a position and origin, such as chromitites, dunites, selection of typical igneous rock suites. hornblendites, lherzolites and . E.A.K. Middlemost ~Earth-Science Reviews 37 (1994) 215-224 223

6. Conclusions esis of the Volcanic Rocks of the Karoo Province. Spec. Publ. Geol. Soc. S. Afr., 13: 105-123, Chayes, F., 1956. Petrographic Modal Analysis: An Elemen- The TAS diagram is considered eminently tary Statistical Appraisal. Wiley, New York, 113 pp. suitable for naming and classifying all the solid Clarke, D.B. 1992. Granitoid Rocks. Chapman and Hall, and molten materials in the magma/igneous rock London, 283 pp. system. All geoscientists who collect large geo- Cox, K.G., Bell, J.D. and Pankhurst, R.J., 1979. The Interpre- chemical databases, but are unable, or reluctant, tation of Igneous Rocks. Allen and Unwin, London, 450 Pp. to get the quantitative modal data necessary to Dawson, J.B., 1989. Sodium carbonatite extrusions from classify their igneous materials on a QAPF dia- Oldoinyo Lengai, Tanzania: Implications for carbonatite gram, should regard the new TAS classification complex genesis. In: K. Bell (Editor), Carbonatites, Gene- of plutonic rocks as a convenient additional tool sis and Evolution. Unwin Hyman, London, pp. 255-277. that can help them compare and contrast igneous Deer, W.A., Howie, R.A. and Zussman, J., 1992. An Intro- duction to the Rock-Forming Minerals, 2nd ed. Longman, materials. In the past the absence of quantitative London, 696 pp. modal data has often led to the use of imprecise Efremova, S.V. and Stafeev, K.G., 1985. Petrochemical Meth- and linguistically awkward terms, such as grani- ods of Investigation of Rocks. Nedra, Moscow, 511 pp. toid. In some publications the term granitoid has Fears, D., 1985. A corrected CIPW program for interactive inadvertently been expanded to include rocks that use. Comput. Geosci., 11: 787-797. Frolova, T.I. and Petrova, M.A., 1972. Classification of vol- clearly occupy the syenitic or dioritic fields. All canic basic, intermediate and ultrabasic bodies. Effusive the "oid names", including dacitoid, gabbroid Materials Subcommission of the Terminological Commis- and foiditoid, should remain field terms. Perhaps sion of the Petrographic Committee of the USSR. they may be of some value to the geophysical Heiken, G., Vaniman, D. and French, B.M., 1991. Lunar fraternity who often study inaccessible igneous Source Book. Cambridge University Press, Cambridge, U.K., 736 pp. materials. The TAS diagram should be recog- Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the nised as a valuable vehicle for teaching the basic chemical classification of lhe common volcanic rocks. Can. tenets of igneous petrology, because it combines J. Earth Sci., 8: 523-548. the functions of a practical device for classifying Kuno, H., 1959. Origin of Cenozoic petrographic provinces of igneous rocks and magmas, yet it is also a potent Japan and surrounding areas. Bull. Volcanol., Ser II: 37- 76. variation diagram showing how rocks are related Kuno, H., 1965. Fractionation trends of basaltic magmas in to one another. This paper is dedicated to Termi- lava flows. J. Petrol., 6(2): 302- 321. nus the Roman god of boundaries. Kuno, H., Yamasaki, K., Iida, C. and Nagashima, K., 1957. Jpn. J. Geol. Geogr., 28(4): 179-218. Le Bas, M.J., 1989. Nephelinitic and basanitic rocks. J. Petrol., 30: 1299-1312. Acknowledgements Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. and Zanenin., B 1986. A chemical classification of volcanic rocks based The author wishes to thank Dr. Daniel S. on the total alkali - silica diagram. J. Petrol., 27(3): Barker of the University of Texas at Austin and 745-750. Le Bas, M.J., Le Maitre, R.W. and Woolley, A.R., 1992. The Dr. Mike J. Le Bas of the University of Leicester construction of the total alkali-silica chemical classifica- for their valuable assistance in critically reading tion of volcanic rocks. Mineral. Petrol., 46: 1-22. and improving the manuscript of this paper. Le Maitre, R.W., 1962. Petrology of volcanic rocks, Gough Island, South Atlantic. Geol. Soc. Am. Bull., 73: 1309- 1340. Le Bas, M.J. and Streckeisen, A., 1991. The IUGS systematics References of igneous rocks. J. Geol. Soc. London, 148: 825-833. Le Maitre, R.W., 1976. The chemical variability of some Bowen, N.L., 1937. Recent high-temperature research on common igneous rocks. J. Petrol., 17(4): 589-637. silicates and its significance in igneous geology. Am. J. Le Maitre, R.W., 1984. A proposal by the IUGS Subcommis- Sci., 33: 1-21. sion on the Systematics of Igneous Rocks for a chemical Bristow, J.W., 1984. Picritic rocks of the North Lebombo and classification of volcanic rocks based on the total alkali South-East Zimbabwe. In: A.J. Erlank (Editor), Petrogen- silica (TAS) diagram. Aust. J. Earth Sci., 31(2): 243-255. 224 E.A,K Middlemost / Earth-Science Ret~iews 37 (1994) 215-224

Le Maitre, R.W. (Editor) with Bateman, P., Dubek, A., Keller, Streckeisen, A., 1965. Die Klassifikation der Eruptivegesteine. J., Lameyre, J., Le Bas, M.J., Sabine, P.A., Schmid, R., Geol. Rundsch., 55: 478-491. Sorensen, H., Streckeisen, A,, Woolley, A.R. and Zanet- Streckeisen, A. and Le Maitre, R.W., 1979. A chemical ap- tin, B. 1989. A Classification of Igneous Rocks and Glos- proximation to the modal QAFP classification of the ig- sary of Terms: Recommendations of the International neous rocks. Neues Jahrb. Mineral. Abh., 136(2): 169-206. Union of Geological Sciences Subcommission on the Sys- Strong, D.F., 1972. Petrology of the Island of Moheli Western tematics of Igneous Rocks. Blackwell, Oxford, 193 pp. and Indian Ocean. Geol. Soc. Am. Bull., 83: 389-406. Wall Chart. Tilley, C.E., 1950. Some aspects of magmatic evolution. Q. J. Macdonald. G.A. and Katsura, T. 1964. Chemical composition Geol. Soc. London, 106: 37-61. of Hawaiian lavas. J. Petrol., 5: 82-133. Tomkeieff, S.I. 1953. Petrochemistry and petrogenesis. In: Middlemost, E.A.K., 1972. A simple classification of volcanic The Tectonic Control of Igneous Activity. Inter-University rocks. Bull. Volcanol., 36(2): 382-397. Geological Congress, Dep. Geol., Univ. Leeds, pp. 24-27, Middlemost, E.A.K., 1980. A contribution to the nomencla- Tuttle, O.F. and Bowen, N.L., 1958. Origin of Granite in the ture and classification of volcanic rocks. Geol. Mag., 117: Light of Experimental Studies in the System NaAISi30 s- 51-57. KA1Si308-SiO 2-H20. Geol. Soc. Am., Mem., 74, 153 pp. Middlemost, E.A.K., 1989. Iron oxidation ratios, norms and Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, Lon- the classification of volcanic rocks. Chem. Geol., 77: 19-26. don, 466 pp. Middlemost, E.A.K., 1991. Towards a comprehensive classifi- Wright, T.L. and Peck, D.L., 1978. Crystallisation and Differ- cation of igneous rocks and magmas. Earth-Sci. Rev., 31: entiation of the Alae Magma, Alae Lava Lake, Hawaii. 73-87. U.S. Geol. Surv. Prof. Pap., 935-C, 20 pp. Miller, W.J., 1919. Pegmatite, silexite, and aplite of Northern Yoder, H.S. and Tilley, C.E., 1962. Origin of basaltic magmas: New York. J. Geol., 27: 28- 54. An experimental study of natural and synthetic rock sys- Natland, J., 199l. Mineralogy and crystallisation of oceanic tems. J. Petrol., 3: 342-532. basalts. In: P,A. Floyd (Editor), Oceanic Basalts. Blackie, Yoshii, M. and Hirano, H., 1977. Test data for the normative Glasgow, pp. 63-93. calculation programs. Bull. Geol. Surv. Jpn., 28: 401-412. Nockolds, S.R., 1954. Average chemical composition of some Zanettin, B., 1984. Proposed new chemical classification of igneous rocks. Bull. Geol. Soc. Am., 65: 1007-1032. volcanic rocks. Episodes, 7(4): 19-20. Raglan& P.C., 1989, Basic Analytical Petrology. Oxford Univ. Press, 369 pp. Rittmann, A., 1957. On the serial character of igneous rocks. Egypt. J. Geol., 1(1): 23-48. Rittmann, A., 1962. Volcanoes and their Activity. Inter- , Dr. Eric Middlemost is a senior lec- science, New York, 305 pp. (Translated from Rittmann, turer in igneous petrology at the Uni- A., 1960. Vulkane und ihre T~itigkeit. Ferdinand Enke, versity of Sydney, Australia. He has Stuttgart.) published many papers on the classifi- Rittmann, A., 1973. Stable Mineral Assemblages of Igneous cation and the nomenclature of ig- Rocks: A Method of Calculation. Springer, Berlin, 262 pp. neous rocks and magmas. His princi- Rumyantseva, N.A., 1977. On the classification of the effusive pal field of research is the origin and rocks. Records BMO, l: 53-61. evolution of alkaline igneous rocks. Saggerson, E.P. and Williams, LA.J., 1964. Ngurumanite from Recently he has published papers on southern Kenya and its bearing on the origin of rocks in carbonatites, lamproites, lampro- the northern Tanganyika alkaline district. J. Petrol., 5(1): phyres, phonolites and tephrites. In 4(1- 81. 1985 he published a book on magmas Stanton, R.L., 1972. Ore Petrology. McGraw-Hill, New York, and magmatic rocks. He is currently working on a sequel to 713 pp. this book.