JOURNAL OF PETROLOGY VOLUME 41 NUMBER 10 PAGES 1467–1470 2000

IUGS Reclassification of the High-Mg andPicritic Volcanic Rocks

M. J. LE BAS1∗SCHOOL OF OCEAN AND EARTH SCIENCE, SOUTHAMPTON UNIVERSITY, SOUTHAMPTON SO14 3ZH, UK

RECEIVED MARCH 3, 2000; REVISED TYPESCRIPT ACCEPTED MARCH 3, 2000

The 1989 IUGS classification of the igneous rocks for the high-Mg Kong), M. Menzies (Royal Holloway College, London),R. W. Nesbitt (Southampton), T. Nielsen (Copenhagen),and picritic volcanic rocks has been revised. Instead of an 18 wt %

MgO minimum limit being applied for all high-Mg and picritic D. Presnall (Texas, Dallas), P. Robinson (Dalhousie,Halifax), H. Rollinson (Cheltenham), H. Schminckevolcanic rocks, that is now applicable only to the high-Mg rocks such

as komatiite and meimechite. The minimum MgO requirement for (GEOMAR, Kiel), H. Sørensen (Copenhagen), R. N.Thompson (Durham), A. H. Wilson (Zimbabwe) and M.picrite is reduced to 12wt %.TheSiO2 former boundary figure between

boninite and komatiite–meimechite–picrite, which was 53 wt %, is Wilson (Leeds).reduced to 52 wt %, and the total alkali content for komatiite and This short contribution represents the final agreementmeimechite is increased to 2% and for picrite to 3%. reached by the Working Group.

The principal criterion for both the high-Mg and picriticvolcanic rocks was formerly that MgO exceeded 18 wt %(Fig. 1). As a result of the revision, the 18 wt % minimum

KEY WORDS: picrite; high-Mg; classification MgO now applies only to the komatiites and meimechites(Fig. 2), and the minimum MgO for picrites is reduced to12wt %(Fig. 3) therebygiving the term‘picrite’ adefinitionmore appropriate to current usage, one poignant examplebeing that by the late Keith Cox (1999). Two other changesREVISION OF THE 1989were agreed by the Working Group. The upper SiO2 limitCLASSIFICATIONfor the high-Mg and picritic volcanic rocks is reduced from

The classification of the high-Mg and picritic volcanic 53 to 52 wt % SiO2, and correspondingly the lower SiO2rocks presented by the IUGS Subcommission in 1989 in limit for boninites is reduced from 53 to 52 wt % (Fig. 2).the book A Classification of Igneous Rocks and Glossary of Terms The total alkali content for the komatiites and meimechitesby Le Maitre et al. is not satisfactory, and hence it has is increased from 1% to 2%, and for picrite increased frombeen reconsidered over the last 3 years by an international 2% to 3% (Fig. 3).Working Group of the Subcommission communicating bye-mail and co-ordinated by the Chairman of the IUGSSubcommission. The Working Group included F. Al-barede (Lyons), N. Arndt (Grenoble), W. Baragar (Geo-

APPLICATION OF THE REVISIONlogical Survey of Canada), M. Cheadle (Liverpool), K.Strict application of the IUGS classification system meansCox, deceased (Oxford), J. C. Dann (Cape Town), A. De-following the flow chart procedure (Fig. 4a and b). Thusblond (Tervuren, Brussels), C.H. Donaldson (St Andrews),the carbonatites, the melilite-bearing, kalsilite-bearing,S. Efremova (Moscow), D. Evans (BHP Minerals, Capeleucite-bearing rocks, and the kimberlites, lamproites andTown),S.A.Gibson (Cambridge),R.Gill (RoyalHollowaylamprophyres are classified before any distinction is madeCollege, London), T. Grove (MIT), P. Hall (Portsmouth),of picrites and high-Mg rocks from basanites, nephelinitesM. Hole (Aberdeen), D. Hughes (Portsmouth), R. Kent

(Coventry), R. Macdonald (Lancaster), J. Malpas (Hong and picrobasalts. The procedure was detailed by Woolley

∗Chairman, IUGS Subcommission on the Systematics of IgneousRocks, on behalf of the Working Group. e-mail: mjlb@soc.soton.ac.uk Oxford University Press 2000

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 10 OCTOBER 2000

Fig. 1. Classification and nomenclature of the ‘high-Mg’ volcanic rocks before revision (Le Maitre et al., 1989).

Fig. 2. New classification and nomenclature for the ‘high-Mg’ volcanic rocks: komatiites and meimechites.

Fig. 3. New classification and nomenclature for the ‘high-Mg’ volcanic rocks: picrites.

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LE BAS CLASSIFICATION OF PICRITIC VOLCANIC ROCKS

Fig. 5. Picrobasalt data taken from the CLAIR database of Le Maitreplotted on TAS before revision (Le Maitre et al., 1989). The term ‘alkalipicrite’ proposed during the discussions is not now required.

Fig. 6. Picrobasalt data taken from the CLAIR database of Le Maitreplotted on TAS after revision, showing that the TAS field of picrobasalts,although reduced, remains a necessity.

(1) if MgO >8%, SiO2 >52% and TiO2 <0·5% thenit is boninite.

Fig. 4. (a) Flow chart for the classification of igneous rocks following the (2) If MgO >18%, SiO2 between 30 and 52% andIUGS scheme as modified by Woolley et al. (1996). QAPF, quartz–alkali (Na2O+K2O) <2%, then it is a komatiite or meimechite,feldspar–plagioclase–feldspathoid modal parameters of the Le Maitre

the two being distinguished by komatiite <1% TiO2 andet al. (1989) classification; TAS, total alkalis–silica. (b) Flow chart forthe classification of potassic and related igneous rocks following the meimechite >1% TiO2.IUGS scheme as modified by Woolley et al. (1996) (3) If MgO >12%, SiO2 between 30 and 52% and

(Na2O + K2O) <3%, then it is a picrite.(4) If these conditions are not fulfilled, then the nextet al. (1996) and will be included in the second edition

of Classification of Igneous Rocks and Glossary of Terms, to be step in the flow chart would be to enter the TASclassification (Fig. 4a).published by Cambridge University Press next year.

Having arrived at the ‘Is it high-Mg’ box in Fig. 4a, The above scheme of lowering the MgO figure forpicrite to 12% and increasing the alkalis to 3% makesthe following classification scheme is recommended for

use in the order given: many rocks into picrites that previously were classified

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The term picrobasalt for the remainder therefore con-tinues to be necessary.

COMMENTLowering the MgO figure to 12 wt % for picrites stillleaves the basanites, nephelinites (especially the me-lanephelinites) to be classified in or after TAS, the criticalboundary between the picrites and the basanites andmelanephelinites being the 3% (Na2O+K2O) boundary.That basanites and melanephelinites merge into picritesacross this boundary is to be expected, bearing in mindthat, like the picrites, they carry the image of primarymagmas. Had not the melilitites been classified earlier(Fig. 4b), they too would have plotted here, althoughtheir low Al2O3 contents (typically 8 ± 2%) would haveFig. 7. MgO–(FeO + Fe2O3) plot of the 81 remaining picrobasalts

with <3% LOI. All have MgO >4%, FeO+ Fe2O3 >7%, CaO >5% quickly distinguished them.and Al2O3 averaging 14 ± 4%.

ACKNOWLEDGEMENTSThe opportunity was taken to present this revised clas-sification at the Komatiites, Norites, Boninites and Basaltsmeeting at the University of Portsmouth in December1999 through the co-operation of P. Hall, the conferenceorganizer, where it was discussed positively. The Sub-commission is grateful for the opportunity. For the con-tributions, big and small, by the members of the WorkingGroup, all are sincerely thanked by the Subcommissionfor allowing a consensus to be achieved.

REFERENCESCox, K. G. (1999). Inaugural Address: The Picrite Basalt Controversy.

In: Subbarao, K. V. (ed.) Deccan Volcanic Province. Geological Society of

India, Memoir 43(1), xxxi–xxxx.Fig. 8. MgO–CaO plot for the same data as in Fig. 7. Gibson, S. A., Thompson, R. N. & Dickin, A. P. (2000). Ferropicrites:

geochemical evidence for Fe-rich streaks in upwelling mantle plumes.Earth and Planetary Science Letters 174, 355–374.as picrobasalt (Fig. 5), but of the 2500 analyses of volcanic

Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lameyre, J., Lerocks with SiO2 between 41 and 45 wt % taken from Bas, M. J., Sabine, P. A., Schmid, R., Sorensen, H., Streckeisen,the CLAIR database it still leaves about 200 compositions A., Woolley, A. R. & Zanettin, B. (1989). A Classification of Igneousplotting in the picrobasalt field of TAS that are not Rocks and Glossary of Terms: Recommendations of the International Union of

Geological Sciences Subcommission on the Systematics of Igneous Rocks. Oxford:picrites (Fig. 6). Examination of these 200 analyses inBlackwell Scientific.the CLAIR database shows that most have high ‘loss on

Woolley, A. R., Bergman, S. C., Edgar, A. D., Le Bas, M. J., Mitchell,ignition’ (LOI) contents (>3 wt %) but 81 remain. AllR. H., Rock, N. M. S. & Scott Smith, B. H. (1996). Classification81 have MgO >4%, FeO + Fe2O3 >7%, CaO >5%, of lamprophyres, lamproites, kimberlites and the kalsilitic, melilitic,

LOI <3% and Al2O3 averaging 14± 4%. Plotting these and leucitic rocks: recommendations of the IUGS Subcommissionanalyses in Figs 7 and 8 reveals that some correspond to on the Systematics of Igneous Rocks. Canadian Mineralogist, Alkaline

Rocks Special Issue 34, 175–186.the ferropicrites described by Gibson et al. (2000).

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