Naming materials in the magma/igneous rock system

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% (Na2O + K2O) 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.