The nomenclature and classification of granitoids and the petrochemical interpretation of geotectonic settings of granitoids: A case study of the Q′-ANOR diagram

<p indent="0mm">Granitoids are the most common felsic intrusive rocks occurred in the continental crust. The Q′-ANOR diagram is widely used in the classification and nomenclature of felsic intrusive rocks. However, little attention has been paid to the accuracy of the classification of rocks by using the Q′-ANOR diagram. In this study, based on a global dataset of 1952 pairs of major element contents and actual mineral modals of felsic intrusive rocks, we have compared the discriminant results of the Q′-ANOR diagram with those of the modal QAP classification diagram of actual mineral contents, and have found that only 55% of samples from the dataset can be “correctly” classified by using the Q′-ANOR diagram. However, the further examinations show that the Q′-ANOR diagram can be used to successfully distinguish the alkali-feldspar-rich flesic intrusive rocks from the plagioclase-rich ones. The projections of different granitoid groups that are defined by their petrochemical and mineralogical features display distinguished distribution patterns in the Q′-ANOR diagram. The trajectories of granitoids reveal that the calcic and the calc-alkalic granitoids in the Q′-ANOR diagram changed their compositions along the cotectic crystallization line of quartz and feldspars during the whole path, but the alkali and the alkali-calcic granitoids changed their compositions along the cotectic crystallization path of alkali-feldspar and plagioclase in the early stage and along the cotectic crystallization path of quartz-feldspars in the late stage. Furthermore, the projection points of granitoids from different tectonic environments in the Q′-ANOR diagram are clearly distinguishable, so the Q′-ANOR diagram can be used as a tool for discriminating the tectono-magmatic characteristics of granitoid suites.