A-type granites and related rocks: petrogenesis and classification

The problems of identification of A-type granitoids are analyzed. These rocks occur in different geodynamic settings. Owing to their mantle nature, they show distinct REE specialization. These are acid intrusive rocks, whose volcanic products are of crucial scientific and practical significance. However, neither the great number of proposed classification schemes and diagrams, including those based on expensive analytical data, nor hot scientific discussions of their identification and the ambiguity of the term A-granitoids helped to determine their classification features in full measure. A principally new discriminant diagram is proposed for the classification of igneous A-type rocks, based on the analysis of earlier obtained results and the petrochemical composition of these rocks. A comparative analysis of subdivision of granites and related felsic volcanic rocks (SiO2 > 67 wt.%), based on the ternary (Na2O + K2O)-Fe2O3*x5-(CaO + MgO)x5 and other widely applied diagrams, has shown the advantage of the proposed discriminant diagram for the classification of A-type granitoids from different geodynamic settings.