The Molar Al2 O3 /(CaO3+Na2O+K2O) Ratios as Discriminant Constraint for Oceanic Plagiogranites and Continental Trondhjemites

The plots of leucocratic, soda-rich siliceous rocks on K₂O-SiO₂ and Rb-Sr diagrams often provide tenuous conclusions with regard to the discrimination of oceanic plagiogranites from continental trondhjemites. Analytical methods for the generation of REE and Sr-isotope data, the effective constraints for discrimination of the above-mentioned lithotypes, are confined to specialized and sophisticated laboratories. The specifically calculated molar Al₂O₃/(CaO+Na₂O+K₂O) ratios (AlCNK), coupled with the SiO₂/Al₂O₃ ratios, of a large number of proven oceanic plagiogranites and continental trondhjemites are assessed to test whether these ratios can be applied as effective discriminant constraints. The major finding of this work is that oceanic plagiogranites are essentially low-alumina and metaluminous types. while continental trondhjemites are exclusively high-alumina and peraluminous types; the separation between the two groups is distinct and clean.