A chemical–mineralogical classification of common plutonic rocks and associations

TLDR

The study proposes a chemical‑mineralogical classification of common plutonic rocks based on major‑element data plotted in three simple diagrams and aims to define their magmatic associations. The method uses a two‑step procedure: first, each sample is named and its chemical and mineralogical characteristics are measured; second, the sample is assigned to one of three association types—cafemic, aluminous, or alumino‑cafemic—using sub‑type criteria such as quartz content, alkali ratio, and color index, and can also classify volcanic equivalents. The resulting classification aligns with most existing typologies, demonstrating that the new criteria are broadly consistent with prior divisions.

Abstract

ABSTRACT A classification is proposed, based mainly on major element analytical data plotted in a coherent set of three simple chemical-mineralogical diagrams. The procedure follows two complementary steps at two different levels. The first is concerned with the individual sample: the sample is given a name (e.g. granite, adamellite, granodiorite) and its chemical and mineralogical characteristics are determined. The second one is more important: it aims at defining the type of magmatic association (or series) to which the studied sample or group of samples belongs. Three main types of association are distinguished: cafemic (from source-material mainly or completely mantle-derived), aluminous (mainly or completely derived by anatexis of continental crust), and alumino-cafemic (intermediate between the other two types). Subtypes are then distinguished among the cafemic and alumino-cafemic associations: calc-alkaline (or granodioritic), subalkaline (or monzonitic), alkaline (and peralkaline), tholeiitic (or gabbroic-trondhjemitic), etc. In the same way, numerous subtypes and variants are also distinguished among the aluminous associations using a set of complementary criteria such as quartz content, colour index, alkali ratio, quartz–alkalies relationships and alumina index. Although involving a new approach using partly new criteria, this classification is consistent with most of the divisions used in previous typologies. The method may also be used in the classification of the volcanic equivalents of common plutonic rocks.