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Naming Materials in the Magma/Igneous Rock System

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Naming Materials in the Magma/Igneous Rock System EARTH SCIENCE REVIEWS ELSEVIER Earth-Science Reviews 37 (1994) 215-224 Naming materials in the magma/igneous rock 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 diorite as a plutonic equivalent of basaltic andesite and peridotgabbro as a plutonic equivalent of picrobasalt. Peridotite 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 peridotites. 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 mineral 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 minerals 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 granite. 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 mafic 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 quartz, orthoclase, albite, anorthite, alkali feldspar, such a rock may have the modal leucite, nepheline and/or kalsilite. The amphi- composition of an alkali feldspar granite; but in a boles, particularly the calcic amphiboles, 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 plagioclase, 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 pyroxenes, whereas the that components such as these occur in normal plutonic diorites invariably contain amphibole 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 basalts 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.
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