Chemical subdivision of the A-type granitoids; petrogenetic and tectonic implications

The A-type granitoids can be divided into two chemical groups. The first group (A (sub 1) ) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A (sub 2) ) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A (sub 1) group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A (sub 2) group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.