| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology Volume 42 Number 11 Pages 2169-2171 2001
© Oxford University Press 2001
A Note on the IUGS Reclassification of the High-Mg and Picritic Volcanic Rocks
1DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PO BOX 914, MAIN BUILDING, PARK PLACE, CARDIFF CF10 3YE, UK
2LGCA, UNIVERSITÉ DE GRENOBLE, BP 53, 1381 RUE DE LA PISCINE, 38031 ST MARTIN D’HERES, FRANCE
Received April 30, 2001; Revised typescript accepted May 23, 2001
 |
INTRODUCTION |
|---|
 TOP INTRODUCTION KOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
The purpose of this short note is to offer some comments and observations on the new IUGS classification of high-Mg rocks (Le Bas, 2000
), particularly its use of the terms ‘komatiite’ and ‘picrite’ with no reference to texture or the original usage of these terms. At the outset of this discussion we would like to place on record that we are in agreement with the need to redefine the nomenclature of high-Mg rocks. There are, however, important issues relating to both the use of the term ‘komatiite’ for all extrusive volcanic rocks with >18 wt % MgO and the definition and use of the term ‘picrite’, neither of which are discussed by Le Bas (2000). |
KOMATIITES—A DEFINITION |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
Komatiites were first recognized by Viljoen & Viljoen (1969)
in the Barberton greenstone belt, South Africa. The features that marked these rocks out as distinctive were not only their high Mg content but also their large, skeletal, platy, bladed or acicular grains of olivine, which Viljoen & Viljoen (1969) called ‘crystalline quench texture’. Thus, komatiites from the type locality were first distinguished on the basis of their chemistry and texture. The term ‘spinifex texture’ was first introduced by Nesbitt (1971).
The next serious attempt to define komatiite was by Arndt & Nisbet (1982), who pointed out that a komatiite is not simply an ultramafic lava, but it also possesses a well-developed spinifex texture. A recent overview of komatiite lava flows and spinifex textures has been given by Arndt (1994) and the reader is referred to that work for further information.
|
THE 1991 IUGS CLASSIFICATION |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
The new classification of high-MgO rocks stems in part from the 1991 IUGS classification proposal (Le Bas & Streckeisen, 1991
), which defines picrites as possessing >18 wt % MgO with between 1 and 2% total alkalis. Komatiites, on the other hand, were defined as having >18 wt % MgO with <1 wt % total alkalis. Although this classification permitted slightly more latitude in the use of the terms picrite and komatiite, the 2000 IUGS classification still uses both these terms in a purely chemical sense and ignores the distinctive textural features of picrites and komatiites. |
TESTING THE NEW IUGS CLASSIFICATION SYSTEM: LATE CRETACEOUS KOMATIITES AND PICRITES FROM GORGONA ISLAND, COLOMBIA |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
The only known post-Cambrian komatiites, from Gorgona Island, Colombia (Echeverría, 1980
; Kerr et al., 1996a; Arndt et al., 1997) largely preserve their original mineralogy and chemistry. In addition to spinifex-textured komatiites (Fig. 1a), picritic dykes and breccias with polyhedral and some skeletal olivines, but no spinifex textures (Fig. 1b), are also present. This suite of rocks represents ideal samples on which to assess the applicability of the new classification.
|
Figure 2 shows all available analyses of Gorgona komatiites and picrites plotted in fig. 6 of Le Bas (2000)
. The Gorgona data are taken from Echeverría (1980), Kerr et al. (1996a) and Arndt et al. (1997). This diagram reveals that all non-spinifex-textured (Fig. 1b) picrites from Gorgona now classify as komatiites, as they all possess >18 wt % MgO. Furthermore, and perhaps more worrying, 67% of available analyses of spinifex-textured Gorgona komatiites are, according to the new classification system, renamed ‘picrites’. This is despite the fact that they possess the widely recognized textural characteristics of komatiites, namely spinifex-textured olivines (often up to 5 cm long).
|
More confusion arises when one realizes that within many individual differentiated komatiite flows, samples from the non-spinifex cumulate zone classify as komatiites under the new definition (>18 wt % MgO) whereas the spinifex-textured portion of the flow classifies as a ‘picrite’. In fact, many worldwide occurrences of high-MgO lavas with polyhedral olivines now classify as ‘komatiites’, without possessing the main textural feature of Precambrian and Gorgona komatiites—spinifex olivines. A case in point are the thick high-Mg lava flows of the Curaçao lava succession, which, like the Gorgona lavas, are a part of the Caribbean oceanic plateau (Kerr et al., 1996b). These lavas possess polyhedral, skeletal olivines, and individual flows commonly display some degree of crystal settling. Using the new classification, the olivine cumulates in the lower parts of Curaçao flows classify as ‘komatiites’, and those in the upper parts as ‘picrites’. Similarly, the lower cumulates of olivine-rich flows in continental flood volcanic sequences such as those of Baffin Bay, Karoo and Noril’sk all become ‘komatiite’ if texture and petrography are ignored! We believe that all these cases graphically illustrate the very real problems of a nomenclature system for igneous rocks that fails to take account of texture.
|
PICRITES AND THE NEW IUGS CLASSIFICATION |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
The problem with the use of the term ‘picrite’ stems from the way the term is employed by different groups of geologists. Traditional petrologists, trained to name a rock using criteria that can be applied in the field, are normally happy with a definition like that normally found in petrological texts, which emphasizes an abundance of olivine phenocrysts. The problem arises when other groups use picrite to imply formation of the rock from a highly magnesian ‘picritic’ liquid. The IUGS criteria—MgO between 12 and 18% and 1–2% alkalis—include a variety of rocks that are very different from the traditional concept of picrite.
Let us consider four examples, all volcanic rocks, all of which conform to the IUGS criteria:
- an olivine porphyritic lava formed by minor crystal accumulation from a silicate liquid magma with 11% MgO;
- an olivine porphyritic lava formed by partial crystallization of a liquid with 15% MgO;
- the upper pyroxene spinifex-textured part of a differentiated komatiitic basaltic flow; such rocks commonly contain large acicular grains of augite or pigeonite, but no olivine;
- a pyroxene–plagioclase cumulate in the lower part of the same flow.
Only the first two are picrites in the petrological sense—the last two do not even contain olivine! Examples (2), (3) and (4) form through the crystallization of a highly magnesian ‘picritic’ liquid, but not example (1).
The only real solution is to insist that the meaning implied by the use of the term picrite be clearly spelt out. For most purposes an objective definition that incorporates both textural and chemical criteria is probably best. Even in the IUGS classification scheme (Le Bas 2000), texture is not completely ignored—it is used to identify the rock as volcanic and not plutonic. For highly magnesian lavas in which rocks with similar bulk composition can be produced by radically different processes, texture must also be incorporated into any classification scheme. The issue of whether a rock forms from highly magnesian liquid, or not, is a subject of discussion and interpretation and it should be considered separately, not as part of a general classification scheme.
|
CONCLUDING REMARKS |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
In summary, there are several issues that need to be addressed. First, the classification of two-thirds of Gorgona spinifex-textured komatiites as picrites obviously calls into question the stringent use of a purely chemical classification system for highly magnesian rocks. Second, the unqualified use of the term ‘komatiite’ for all volcanic rocks with >18 wt % MgO and <1 wt % TiO2 will undoubtedly lead to confusion in the literature. Indeed, the confusion has commenced with the non-spinifex Gorgona picrites having already been classified as ‘komatiites’ (Thompson & Gibson, 2000
). Such indiscriminate usage of the term komatiite may well result in names such as ‘petrographic’ komatiite, ‘non-spinifex-textured’ komatiite, or even (heaven forbid) ‘real’ komatiite or ‘pseudo-’ komatiite being used to help differentiate between picrites and spinifex-textured komatiites. In regard to a resolution of these problems we would strongly recommend that the term komatiite be reserved solely for lavas with characteristic spinifex-textured olivines, or lavas that can be related directly, using field or petrological criteria, to lavas with this texture. This would resolve much of the ambiguity in nomenclature that exists within the new IUGS classification scheme. We do not recommend the introduction of a new name for lavas with MgO >18 wt % that do not possess spinifex-textured olivines. It is usually sufficient simply to describe such rocks as ‘the basal olivine-rich parts of differentiated flows’ or some such similar phrase.
We wish to stress that this contribution is not written in a spirit of criticism, rather its purpose is to highlight potential problems that may arise through the use of this classification system and that other geologists, less familiar with komatiites, and picrites, may not be aware of. It is our belief that the unqualified use of the current IUGS classification scheme will create more problems than it solves. We trust that this note will stimulate much debate and discussion on the nomenclature of high-MgO volcanic rocks in the wider petrological community. We hope that it will allow the issues raised here to be satisfactorily resolved, and a workable consensus to be reached on the classification of these rocks.
|
FOOTNOTES |
|---|
*Corresponding author. E-mail: kerra{at}cardiff.ac.uk.
|
REFERENCES |
|---|
|
TOPINTRODUCTIONKOMATIITES—A DEFINITIONTHE 1991 IUGS CLASSIFICATIONTESTING THE NEW IUGS...PICRITES AND THE NEW...CONCLUDING REMARKSREFERENCES |
|---|
Arndt, N. T. (1994). Komatiites. In: Condie, K. C. (ed.) Archean Crustal Evolution. Amsterdam: Elsevier, pp. 11–44.
Arndt, N. T. & Nisbet, E. G. (1982). What is a komatiite? In: Arndt, N. T. & Nisbet, E. G. (eds) Komatiites. London: George Allen and Unwin, pp. 19–27.
Arndt, N. T., Kerr, A. C. & Tarney, J. (1997). Dynamic melting in plume heads: the formation of Gorgona komatiites and basalts. Earth and Planetary Science Letters 146, 289–301.
Echeverría, L. M. (1980). Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: field relations and geochemistry. Contributions to Mineralogy and Petrology 73, 253–266.
Kerr, A. C., Marriner, G. F., Arndt, N. T., Tarney, J., Nivia, A., Saunders, A. D. & Duncan, R. A. (1996a). The petrogenesis of komatiites, picrites and basalts from the Isle of Gorgona, Colombia; new field, petrographic and geochemical constraints. Lithos 37, 245–260.
Kerr, A. C., Tarney, J., Marriner, G. F., Klaver, G. Th., Saunders, A. D. & Thirlwall, M. F. (1996b). The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curaçao, Netherlands Antilles: a remnant of an oceanic plateau. Contributions to Mineralogy and Petrology 124, 29–43.
Le Bas, M. J. (2000). IUGS reclassification of the high-Mg and picritic volcanic rocks. Journal of Petrology, 41, 1467–1470.
Le Bas, M. J. & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society, London 148, 825–833.
Nesbitt, R. W. (1971). Skeletal crystal forms in the ultramafic rocks of the Yilgarn Block, Western Australia: evidence for an Archaean ultramafic liquid. Geological Society of Australia Special Publication 3, 331–347.
Thompson, R. N. & Gibson, S.A. (2000). Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites. Nature 407, 502–506.
Viljoen, M. J. & Viljoen, R. P. (1969). The geology and geochemistry of the Lower Ultramafic Unit of the Onverwacht Group and a proposed new class of igneous rocks. In: Upper Mantle Project. Geological Society of South Africa, Special Publication 2, 55–85.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  M. Kahoui, Y. Mahdjoub, and F. V. Kaminsky Possible primary sources of diamond in the North African diamondiferous province Geological Society, London, Special Publications, January 1, 2008; 297(1): 77 - 109. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. C. KERR, J. TARNEY, P. D. KEMPTON, M. PRINGLE, and A. NIVIA Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolivar, Colombia: Evidence for a 'Wet' Mantle Plume? J. Petrology, September 1, 2004; 45(9): 1877 - 1906. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. FANG and Y. NIU Late Palaeozoic Ultramafic Lavas in Yunnan, SW China, and their Geodynamic Significance J. Petrology, January 1, 2003; 44(1): 141 - 158. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. HERZBERG and M. J. O'HARA Plume-Associated Ultramafic Magmas of Phanerozoic Age J. Petrology, October 1, 2002; 43(10): 1857 - 1883. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. C. Kerr, A. C. KERR, J. A. ASPDEN, J. TARNEY, and L. F. PILATASIG The nature and provenance of accreted oceanic terranes in western Ecuador: geochemical and tectonic constraints Journal of the Geological Society, September 1, 2002; 159(5): 577 - 594. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. LE BAS Reply to Comment by Kerr and Arndt J. Petrology, November 1, 2001; 42(11): 2173 - 2174. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||