A System of Classification and Nomenclature of Granitic Rocks

Abstract

The paper proposes a system of classification and comprehensive kinematic-descriptive-genetic nomenclature of granitic rocks based on results of detailed petrographic studies of samples from the southern margin of the Canadian shield. Unlike the schemes currently in use that emphasise only the relative proportions of quartz, potassium-feldspar and plagioclase, the proposed classification recognizes the significance of several additional parameters: (a) biotite/amphibole ratios, (b) non-opaque calcic trace mineral contents (e.g., sphene, apatite, epidote), and (c) plagioclase compositions. Using these parameters, a new modal Quartz-Alkali feldspar-Plagioclase ternary diagram is presented. An example of a comprehensive kinematic-descriptive-genetic nomenclature of granitic rocks, derived from the proposed system is: 'late-kinematic, pink, slightly porphyritic, weakly foliated, medium- to coarse-grained, chloritized, muscovite-biotite-albite (An₆)-microcline leucogranitic anatexite', The proposed system should prove useful in mineral exploration programmes aimed at locating rare-mineralic granitic bodies, and in inter-continental petrographic comparisons.