Igneous rocks generated in subduction zone generally belong to subalkaline rock. Plotting of K2O and Na2O in Harker diagram produces for classes of subduction magma series: low-K series, calc-alkaline series, high-K series, and shoshonitic series. Potash content in igneous rock is critical because it can represent the degree of contamination of magma. Low-K series is the same as tholeiitic rock. Calc-alkaline magma series have high alumina content. Shoshonitic series represents alkaline rock. Miyashiro (1974) in Wilson<ref name=Wilson /> reveals the difference between calc-alkaline magma series with tholeiitic magma through Fe content. Calc- alkaline magma series show decreasing content of FeO in increasing SiO2 content. Otherwise, tholeiitic magma series show enrichment of Fe in early stage of fractionation. | Igneous rocks generated in subduction zone generally belong to subalkaline rock. Plotting of K2O and Na2O in Harker diagram produces for classes of subduction magma series: low-K series, calc-alkaline series, high-K series, and shoshonitic series. Potash content in igneous rock is critical because it can represent the degree of contamination of magma. Low-K series is the same as tholeiitic rock. Calc-alkaline magma series have high alumina content. Shoshonitic series represents alkaline rock. Miyashiro (1974) in Wilson<ref name=Wilson /> reveals the difference between calc-alkaline magma series with tholeiitic magma through Fe content. Calc- alkaline magma series show decreasing content of FeO in increasing SiO2 content. Otherwise, tholeiitic magma series show enrichment of Fe in early stage of fractionation. |