How should igneous rocks be grouped?

Abstract

SummaryDespite their importance and frequency of usage, ‘grouping terms’ in petrology such as Association, Family, Lineage, Province, Series and Suite appear to lack established meanings. Partly by analogy with the recently standardized stratigraphic nomenclature, hierarchies of such terms are suggested to cover petrographic classification (rock-names), petrological classification (rock-names extended to magma-types), geographic classification (igneous provinces) and tectono-genetic classification (rock assemblages characteristic of particular tectonic regimes). Used in this way, the exact implications of each term become clearer. Again by analogy with stratigraphy, ‘Super-’ and ‘Sub-’ groupings are suggested in a few cases: ‘Subprovince’ for example can differentiate distinct rock assemblages related to separate parent magmas within one well-defined Province (e.g. nephelinite-carbonatite and basalt-trachyte assemblages in the East African Rift), and ‘Superprovince’ can be used to group coeval Provinces now separated by opened oceans, etc. (e.g. Thulean Superprovince composed of Hebridean, East Greenland, etc., Provinces). The term ‘Province’ itself carries several ambiguities; it is best confined to cases where the ages of the spatially associated igneous rocks fall within a narrow range (e.g. Oslo Province); ‘Belt’ is suggested for those where the age-range is more substantial (e.g. Jurassic-Tertiary magmatism of Southeast Brazil).