The 2Q-(Or + Ab)-4An (QUORAA) diagram: poor for classification but good at deciphering the evolution of granitoids

Abstract

The 2Q-(Or + Ab)-4An (QUORAA) diagram (Enrique 2018) is proposed as an approximation of the modal QAP naming scheme for silica-saturated plutonic rocks. However, whether this scheme can reasonably reproduce the modal QAP classification results still needs to be tested. Through the inspection based on a dataset of 955 pairs of the mineral mode and whole-rock chemical compositions from the literature, it was found that the ratio of consistency between the two schemes is only 63.56 %. The consistency ratio is higher for discrimination of the granites (s.s.), but the inconsistency increases significantly for the rocks containing more mafic minerals (M > 25 %). As a section of the An-Ab-Or-Q (haplogranodiorite) tetrahedron, the QUORAA diagram is very helpful to illustrate the evolutionary paths of granitoid melts. The typical arc-related (ACG + ATG), the collision-related peraluminous (CPG + MPG), the rift-related peralkaline (PAG) as well as the potassic KCG granitoids show different trajectories in the QUORAA diagram. It is concluded that the QUORAA diagram would be applicable as a petrogenetic tool rather than the classification scheme.