This chapter is a summary of all the published recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks together with some other decisions agreed to since the last Subcommission meeting in Prague in 1999.

PRINCIPLES

Throughout its deliberations on the problems of classification the Subcommission has been guided by the following principles, most of which have been detailed by Streckeisen (1973, 1976) and Le Bas & Streckeisen (1991).

1. For the purposes of classification and nomenclature the term “igneous rock” is taken to mean “Massige Gesteine” in the sense of Rosenbusch, which in English can be translated as “igneous or igneous-looking”. Igneous rocks may have crystallized from magmas or may have been formed by cumulate, deuteric, metasomatic or metamorphic processes. Arguments as to whether charnockites are igneous or metamorphic rocks are, therefore, irrelevant in this context.

2. The primary classification of igneous rocks should be based on their mineral content or mode. If a mineral mode is impossible to determine, because of the presence of glass, or because of the fine-grained nature of the rock, then other criteria may be used, e.g. chemical composition, as in the TAS classification.

3. The term plutonic rock is taken to mean an igneous rock with a phaneritic texture, i.e. a relatively coarse-grained (> 3 mm) rock in which the individual crystals can be distinguished with the naked eye and which is presumed to have formed by slow cooling. Many rocks that occur in orogenic belts have suffered some metamorphic overprinting, so that it is left to the discretion of the user to decide whether to use an igneous or metamorphic term to describe the rock (e.g. whether to use gneissose granite or granitic gneiss).

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