Sedimentary and igneous trends on AFM diagrams

Summary It is pointed out that some chemically analysed sedimentary rock series when plotted on AFM diagrams (A = Na2O + K2O; F = total iron as FeO; M = MgO) partly mimic trends generally considered characteristic of igneous variation, especially the calc-alkali trend. Although the scatter in sedimentary rock plots is generally greater than that given by igneous rocks this is not necessarily so for calc-alkali suites often show considerable scatter. These observations should be considered when deducing the origin of highly metamorphosed gneisses and quartzo-feldspathic rocks that might be meta-arkoses, meta-granites or meta-volcanic rocks.