Are Geochemical Diagrams Compatible Proxies of the Modal QAP Scheme for Classification/Nomenclature of Granitoid Rocks?

The consistency between geochemical discrimination diagrams and the modal Quartz–Alkali feldspar–Plagioclase (QAP) classification scheme was investigated by evaluating the accuracy of three diagrams—the Quartz–Plagioclase coordinates (P-Q), the SiO2–CaO/(CaO + K2O) (SCK), and the Silica–Total Alkali (TAS) diagrams—for granitoid rocks. A global dataset of 1981 samples, each containing both whole-rock geochemical data and quantitative modal mineralogy, was employed. The results indicate that the P-Q and SCK diagrams have relatively low overall accuracy (~50%–55%) in reproducing the QAP classification. Their accuracy is acceptable for granites but notably lower for basic and intermediate rock types. The SCK diagram achieves higher accuracy (~80%) for tonalite but exhibits considerable dispersion for other lithologies. Despite the IUGS recommending the TAS diagram for volcanic rocks only, it is commonly used for intrusive rocks; this study, however, finds that it yields low accuracy rates for most common plutonic rocks and is therefore unsuitable for their reliable classification. This limited accuracy is attributed to the use of only three or four oxides to estimate major mineral proportions, a practice equivalent to dimensionality reduction that results in substantial information loss.