A Model for the Formation and Crystallization of Corundum-Normative Calc-Alkaline Magmas through Amphibole Fractionation

Calc-alkaline intrusive and extrusive suites frequently show a trend from diopside- to corundum-normative with increasing SiO₂ which may be explained by the crystallization of hornblende from hydrous magmas. The extent to which normative corundum increases depends in part upon the Na/(Na + K) ratio of the magma. With a ratio greater than 0.6 a large compositional stability field of hornblende is developed, while for lower values mica may crystallize instead of, or as well as, hornblende. As most naturally occurring micas in granitic rocks are corundum-normative, their crystallization will buffer the increase in normative corundum in residual liquids. Garnets are present in some corundum-normative calc-alkaline intrusive and extrusive rocks, and most are thought to crystallize from highly differentiated granitic magmas under crustal pressures. A reaction relation exists once garnet begins to crystallize involving the disappearance of mica and hornblende. As a result garnet occurs in leucocratic rocks such as granites, aplites and pegmatites which are devoid of other ferromagnesian phases.