The Calc-Alkaline Series and the Trend of Fractional Crystallization of Basaltic Magma: A New Approach at Graphic Representation

A new variation diagram has been devised in which the ratio {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}Total\ O\ not\ combined\ in\ OH⁻Si⁺⁴+\ (Al⁺³\ in\ 4-co-ordination) × 10{document} is plotted as abscissa. This ratio, here called the "linkage factor," may be calculated from the chemical analysis. Atomic percentages, or the ratios {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}MgMg + Fe × 100\;\;\; or\;\;\; CaCa + Na + K × 100{document} may be plotted along the ordinate. Diagrams showing the first ratio plotted against the linkage factor are especially useful in studying variations of products of fractional crystallization of basaltic magma and in comparing these variations with those exhibited by the calc-alkaline series of igneous rocks. The main variation trend resulting from normal fractionation of basaltic magma is characterized by increasing linkage factor and decreasing {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}MgMg+Fe × 100\ ratio.{document}. In contrast, differentiation in the calc-alkaline series exhibits a trend marked by a decreasing linkage factor and a decreasing {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}MgMg+Fe × 100\ ratio.{document}. This is considered additional evidence that the calc-alkaline series of igneous rocks is not produced by normal fractionation of basaltic magma.