Journal of Petrology

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Journal of Petrology Volume 41 Number 10 Pages 1467-1470 2000
© Oxford University Press 2000

IUGS Reclassification of the High-Mg and Picritic Volcanic Rocks

M. J. LE BAS^,*

SCHOOL OF OCEAN AND EARTH SCIENCE, SOUTHAMPTON UNIVERSITY, SOUTHAMPTON SO14 3ZH, UK

Received March 3, 2000; Revised typescript accepted March 3, 2000

	ABSTRACT

TOP ABSTRACT REVISION OF THE 1989... APPLICATION OF THE REVISION COMMENT REFERENCES

 
The 1989 IUGS classification of the igneous rocks for the high-Mg and picritic volcanic rocks has been revised. Instead of an 18 wt % MgO minimum limit being applied for all high-Mg and picritic volcanic rocks, that is now applicable only to the high-Mg rocks such as komatiite and meimechite. The minimum MgO requirement for picrite is reduced to 12 wt %. The SiO₂ former boundary figure between boninite and komatiite–meimechite–picrite, which was 53 wt %, is reduced to 52 wt %, and the total alkali content for komatiite and meimechite is increased to 2% and for picrite to 3%.

KEY WORDS: picrite; high-Mg; classification

	REVISION OF THE 1989 CLASSIFICATION

TOP ABSTRACT REVISION OF THE 1989... APPLICATION OF THE REVISION COMMENT REFERENCES

 
The classification of the high-Mg and picritic volcanic rocks presented by the IUGS Subcommission in 1989 in the book A Classification of Igneous Rocks and Glossary of Terms by Le Maitre et al. is not satisfactory, and hence it has been reconsidered over the last 3 years by an international Working Group of the Subcommission communicating by e-mail and co-ordinated by the Chairman of the IUGS Subcommission. The Working Group included F. Albarède (Lyons), N. Arndt (Grenoble), W. Baragar (Geological Survey of Canada), M. Cheadle (Liverpool), K. Cox, deceased (Oxford), J. C. Dann (Cape Town), A. Deblond (Tervuren, Brussels), C. H. Donaldson (St Andrews), S. Efremova (Moscow), D. Evans (BHP Minerals, Cape Town), S. A. Gibson (Cambridge), R. Gill (Royal Holloway College, London), T. Grove (MIT), P. Hall (Portsmouth), M. Hole (Aberdeen), D. Hughes (Portsmouth), R. Kent (Coventry), R. Macdonald (Lancaster), J. Malpas (Hong Kong), M. Menzies (Royal Holloway College, London), R. W. Nesbitt (Southampton), T. Nielsen (Copenhagen), D. Presnall (Texas, Dallas), P. Robinson (Dalhousie, Halifax), H. Rollinson (Cheltenham), H. Schmincke (GEOMAR, Kiel), H. Sørensen (Copenhagen), R. N. Thompson (Durham), A. H. Wilson (Zimbabwe) and M. Wilson (Leeds).

This short contribution represents the final agreement reached by the Working Group.

The principal criterion for both the high-Mg and picritic volcanic rocks was formerly that MgO exceeded 18 wt % (Fig. 1). As a result of the revision, the 18 wt % minimum MgO now applies only to the komatiites and meimechites (Fig. 2), and the minimum MgO for picrites is reduced to 12 wt % (Fig. 3) thereby giving the term ‘picrite’ a definition more appropriate to current usage, one poignant example being that by the late Keith Cox (1999). Two other changes were agreed by the Working Group. The upper SiO₂ limit for the high-Mg and picritic volcanic rocks is reduced from 53 to 52 wt % SiO₂, and correspondingly the lower SiO₂ limit for boninites is reduced from 53 to 52 wt % (Fig. 2). The total alkali content for the komatiites and meimechites is increased from 1% to 2%, and for picrite increased from 2% to 3% (Fig. 3).

View larger version (17K): [in this window] [in a new window]   Fig. 1. Classification and nomenclature of the ‘high-Mg’ volcanic rocks before revision (Le Maitre et al., 1989).

 

View larger version (16K): [in this window] [in a new window]   Fig. 2. New classification and nomenclature for the ‘high-Mg’ volcanic rocks: komatiites and meimechites.

 

View larger version (14K): [in this window] [in a new window]   Fig. 3. New classification and nomenclature for the ‘high-Mg’ volcanic rocks: picrites.

 

	APPLICATION OF THE REVISION

TOP ABSTRACT REVISION OF THE 1989... APPLICATION OF THE REVISION COMMENT REFERENCES

 
Strict application of the IUGS classification system means following the flow chart procedure (Fig. 4a and b). Thus the carbonatites, the melilite-bearing, kalsilite-bearing, leucite-bearing rocks, and the kimberlites, lamproites and lamprophyres are classified before any distinction is made of picrites and high-Mg rocks from basanites, nephelinites and picrobasalts. The procedure was detailed by Woolley et al. (1996) and will be included in the second edition of Classification of Igneous Rocks and Glossary of Terms, to be published by Cambridge University Press next year.

View larger version (27K): [in this window] [in a new window]   Fig. 4. (a) Flow chart for the classification of igneous rocks following the IUGS scheme as modified by Woolley et al. (1996). QAPF, quartz–alkali feldspar–plagioclase–feldspathoid modal parameters of the Le Maitre et al. (1989) classification; TAS, total alkalis–silica. (b) Flow chart for the classification of potassic and related igneous rocks following the IUGS scheme as modified by Woolley et al. (1996)

 
Having arrived at the ‘Is it high-Mg’ box in Fig. 4a, the following classification scheme is recommended for use in the order given:

1. if MgO > 8%, SiO₂ > 52% and TiO₂ < 0·5% then it is boninite.
   
2. If MgO > 18%, SiO₂ between 30 and 52% and (Na₂O + K₂O) < 2%, then it is a komatiite or meimechite, the two being distinguished by komatiite <1% TiO₂ and meimechite >1% TiO₂.
   
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 next step in the flow chart would be to enter the TAS classification (Fig. 4a).
   

The above scheme of lowering the MgO figure for picrite to 12% and increasing the alkalis to 3% makes many rocks into picrites that previously were classified as picrobasalt (Fig. 5), but of the 2500 analyses of volcanic rocks with SiO₂ between 41 and 45 wt % taken from the CLAIR database it still leaves about 200 compositions plotting in the picrobasalt field of TAS that are not picrites (Fig. 6). Examination of these 200 analyses in the CLAIR database shows that most have high ‘loss on ignition’ (LOI) contents (>3 wt %) but 81 remain. All 81 have MgO > 4%, FeO + Fe₂O₃ > 7%, CaO > 5%, LOI < 3% and Al₂O₃ averaging 14 ± 4%. Plotting these analyses in Figs 7 and 8 reveals that some correspond to the ferropicrites described by Gibson et al. (2000).

View larger version (30K): [in this window] [in a new window]   Fig. 5. Picrobasalt data taken from the CLAIR database of Le Maitre plotted on TAS before revision (Le Maitre et al., 1989). The term ‘alkali picrite’ proposed during the discussions is not now required.

 

View larger version (22K): [in this window] [in a new window]   Fig. 6. Picrobasalt data taken from the CLAIR database of Le Maitre plotted on TAS after revision, showing that the TAS field of picrobasalts, although reduced, remains a necessity.

 

View larger version (14K): [in this window] [in a new window]   Fig. 7. MgO–(FeO + Fe2O3) plot of the 81 remaining picrobasalts with <3% LOI. All have MgO > 4%, FeO + Fe2O3 > 7%, CaO > 5% and Al2O3 averaging 14 ± 4%.

 

View larger version (13K): [in this window] [in a new window]   Fig. 8. MgO–CaO plot for the same data as in Fig. 7.

 

The term picrobasalt for the remainder therefore continues to be necessary.

	COMMENT

TOP ABSTRACT REVISION OF THE 1989... APPLICATION OF THE REVISION COMMENT REFERENCES

 
Lowering the MgO figure to 12 wt % for picrites still leaves the basanites, nephelinites (especially the melanephelinites) to be classified in or after TAS, the critical boundary between the picrites and the basanites and melanephelinites being the 3% (Na₂O + K₂O) boundary. That basanites and melanephelinites merge into picrites across this boundary is to be expected, bearing in mind that, like the picrites, they carry the image of primary magmas. Had not the melilitites been classified earlier (Fig. 4b), they too would have plotted here, although their low Al₂O₃ contents (typically 8 ± 2%) would have quickly distinguished them.

	ACKNOWLEDGEMENTS

 
The opportunity was taken to present this revised classification at the Komatiites, Norites, Boninites and Basalts meeting at the University of Portsmouth in December 1999 through the co-operation of P. Hall, the conference organizer, where it was discussed positively. The Subcommission is grateful for the opportunity. For the contributions, big and small, by the members of the Working Group, all are sincerely thanked by the Subcommission for allowing a consensus to be achieved.

	FOOTNOTES

 
^*Chairman, IUGS Subcommission on the Systematics of Igneous Rocks, on behalf of the Working Group. e-mail: mjlb{at}soc.soton.ac.uk

	REFERENCES

TOP ABSTRACT REVISION OF THE 1989... APPLICATION OF THE REVISION COMMENT REFERENCES

 
Cox, 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.

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.

Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lameyre, J., Le Bas, M. J., Sabine, P. A., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A. R. & Zanettin, B. (1989). A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Oxford: Blackwell Scientific.

Woolley, A. R., Bergman, S. C., Edgar, A. D., Le Bas, M. J., Mitchell, R. H., Rock, N. M. S. & Scott Smith, B. H. (1996). Classification of lamprophyres, lamproites, kimberlites and the kalsilitic, melilitic, and leucitic rocks: recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks. Canadian Mineralogist, Alkaline Rocks Special Issue 34, 175–186.

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